Emerging Approaches for Dechlorination of Plastic Waste Prior to Thermochemical Recycling: A Comprehensive Review

Abstract

The increasing generation of plastic waste has intensified the search for sustainable recycling technologies capable of recovering valuable resources while minimizing environmental impacts. Among emerging solutions, thermochemical recycling processes such as pyrolysis, gasification, and hydrothermal liquefaction have gained significant attention. However, the presence of chlorine-containing polymers, particularly poly(vinyl chloride) (PVC), represents a major challenge due to the release of hydrogen chloride (HCl), catalyst deactivation, equipment corrosion, and contamination of valuable products. Consequently, effective dechlorination pretreatments are essential for improving process performance and product quality. This review provides a comprehensive overview of current pretreatment technologies for chlorine management in plastic solid waste. Mechanical sorting, density-based separation, chemical extraction, alkaline treatments, hydrothermal processing, thermal dehydrochlorination, catalytic methods, and emerging photocatalytic approaches are critically analyzed and compared. The mechanisms, operating conditions, chlorine removal efficiencies, technological readiness levels, and industrial applicability of each method are discussed. Moreover, particular attention will be devoted to integrating different pretreatment technologies to achieve an overall enhanced chlorine removal efficiency before thermochemical technologies. Finally, reported techno-economic, environmental, and safety indicators are compared where quantitative data are available, while the principal data gaps and technological barriers to large-scale implementation are identified.

1. Introduction

Plastic materials have become indispensable in modern society due to their low cost, versatility, lightweight nature, and durability [1]. Global plastic production has exceeded 400 million tons annually and it is expected to continue increasing over the coming decades [2]. Despite their numerous benefits, end-of-life plastic management remains a major environmental challenge. Mechanical recycling currently represents the most widely adopted plastic waste recovery route. Nevertheless, its effectiveness is limited by contamination, polymer degradation, additives, and the heterogeneous composition of post-consumer waste streams [3]. Consequently, thermochemical recycling technologies, including pyrolysis, gasification, hydrothermal liquefaction, and catalytic cracking, are increasingly being investigated as complementary solutions capable of converting mixed plastic waste into fuels and chemical feedstocks [4]. Nevertheless, the presence of chlorine in plastic waste streams represents one of the major challenges for both mechanical and thermochemical recycling technologies [5]. Chlorine-containing materials are widely distributed across municipal, commercial, and industrial waste streams, resulting in heterogeneous feedstocks with highly variable chlorine concentrations [6]. During thermal conversion processes, chlorine is primarily released as hydrogen chloride (HCl), leading to various technical issues such as equipment corrosion, catalyst deactivation, contamination of pyrolysis oils, and the formation of undesirable chlorinated compounds. Consequently, understanding the origin and distribution of chlorine within plastic waste streams is essential for selecting appropriate pretreatment and dechlorination strategies [7].

Poly(vinyl chloride) (PVC) is by far the most important contributor to chlorine in plastic waste streams. Owing to its molecular structure, PVC contains approximately 56–57 wt.% chlorine, making it the most chlorine-rich commodity plastic currently produced on a large scale. Global PVC production exceeds 50 million tons annually and accounts for a significant fraction of plastics used in construction, infrastructure, healthcare, packaging, and electrical applications [8]. The thermal degradation of PVC is characterized by an initial dehydrochlorination stage occurring between approximately 250 and 350 °C, during which large amounts of HCl are released [7]. Even relatively small PVC concentrations in mixed plastic waste can therefore substantially increase the chlorine content of thermochemical recycling feedstocks. Studies have shown that PVC fractions below 1 wt.% may already cause operational problems in pyrolysis and gasification systems, highlighting the importance of effective chlorine management [9].

As a result, chlorine removal before thermochemical conversion has become an essential processing step. Various pretreatment technologies have been proposed, ranging from mechanical sorting and density separation to chemical, hydrothermal, catalytic, and photocatalytic approaches [10]. Kakuta et al. [11] melted plastic wastes with a mixture of converter dust and heavy oil bottom component of coal tar to detach the chlorine from PVC. It was found that the addition of calcium oxide, calcium hydroxide, and oyster shells into the feedstock could reduce the chlorine from 350 to around 50 ppm. Park et al. [12] used a continuous two-stage reactor to produce oil with low levels of chlorine; the reactor consists of an auger followed by pyrolysis in a fluidized bed device. The auger operates at 300–400 °C, and the HCl produced during the process is removed by a lime hot filter before the pyrolysis zone. They used blends with 4% PVC and 96% low-density polyethylene (LDPE), reducing chlorine content of the final oil product to 9.25 ppm. Also, hydrothermal treatment has emerged as a promising strategy for the dechlorination of PVC-containing waste streams because it enables chlorine removal under relatively mild conditions while transferring most chlorine into the aqueous phase as chloride ions [13,14,15]. In addition, several studies have demonstrated that subcritical and supercritical water can achieve high dechlorination efficiencies, making hydrothermal processing an attractive pretreatment prior to thermochemical recycling [16]. More recently, Zhao et al. [17] showed that Ni²⁺ ions generated by corrosion of Ni-containing reactors catalyze hydrothermal dechlorination by promoting C-Cl bond cleavage and hydroxyl substitution, significantly improving chlorine removal under mild hydrothermal conditions.

This review critically evaluates these technologies and discusses their future role in integrated recycling systems. Although considerable research has been devoted to individual dechlorination technologies, including alkaline treatment, hydrothermal processing, catalytic dechlorination, and mechanical separation, the available literature remains largely fragmented, with most studies focusing on specific processes or model PVC materials. To date, comprehensive reviews critically comparing the physical and chemical pretreatment strategies applicable to heterogeneous and realistic plastic waste streams prior to thermochemical recycling are still scarce [5,9,10]. In particular, limited attention has been paid to evaluating the advantages, limitations, technological maturity, and industrial applicability of the various pretreatment routes within an integrated chlorine-management framework.

In this context, the present review aims at filling the aforementioned research gap giving a comprehensive insight into the subject. First, the fundamental mechanisms governing chlorine release during thermochemical conversion are discussed. Subsequently, the most relevant pretreatment technologies—including physical separation, solvent-assisted recovery, mechanochemical activation, alkaline dechlorination, hydrothermal treatment, catalytic dechlorination, thermal pretreatment, and emerging photocatalytic approaches—are critically evaluated with particular emphasis on their dechlorination efficiency, scalability, environmental impact, and suitability for processing heterogeneous post-consumer plastic waste. After reviewing individual dechlorination technologies, this work aims at proposing an integrated chlorine-management perspective in which pretreatment is considered a key enabling step for improving the efficiency, safety, and sustainability of subsequent thermochemical plastic waste recycling.

2. Chemistry of Chlorine Release During Thermochemical Conversion

2.1. Thermal Degradation Behavior of PVC

As already mentioned, among the polymers present in municipal, industrial, and post-consumer plastic waste streams, poly(vinyl chloride) (PVC) represents the dominant source of chlorine entering thermochemical recycling facilities. Owing to its chemical composition, PVC contains approximately 56–57 wt.% chlorine, making it significantly different from polyolefins such as polyethylene (PE) and polypropylene (PP), which are entirely composed of carbon and hydrogen. Other chlorine-containing polymers include poly(vinylidene chloride) (PVDC), chlorinated polyethylene (CPE), chlorinated rubber, and several specialty materials; however, their overall contribution to chlorine emissions is generally much lower due to their more limited production volumes and narrower range of applications.

The thermal degradation of PVC differs substantially from that of most commodity plastics. While polyolefins primarily degrade through random chain scission reactions occurring above 400 °C, PVC begins to decompose at considerably lower temperatures through a dehydrochlorination mechanism. This behavior is mainly attributed to the relatively weak carbon–chlorine bonds present along the polymer backbone and to the presence of structural defects that act as initiation sites [18].

PVC degradation generally proceeds through two major stages. The first stage, occurring between approximately 250 and 350 °C, involves the progressive elimination of hydrogen chloride from the polymer chain. During this phase, carbon–chlorine bonds are cleaved, generating conjugated polyene structures and releasing substantial quantities of HCl. The second stage occurs at higher temperatures, typically above 400 °C, when the unsaturated polyene structures undergo cyclization, aromatization, cross-linking, and thermal cracking reactions, leading to the formation of gases, liquids, aromatic hydrocarbons, and carbonaceous residues [7].

The overall dehydrochlorination process may be represented by the simplified reaction:

PVC → Polyene sequences + HCl

(1)

Equation (1) provides an overall representation of PVC dehydrochlorination but does not describe the sequence of elimination reactions. Thermally labile structures, particularly allylic and tertiary chlorides, can act as preferential initiation sites for HCl elimination [18].

–CH₂–CHCl–CH₂–CHCl– → –CH=CH–CH₂–CHCl– + HCl

(2)

–CH=CH–CH₂–CHCl– → –CH=CH–CH=CH– + HCl

(3)

Equations (2) and (3) are simplified consecutive elimination steps rather than a complete elementary mechanism. Starnes and Ge [19] linked the acceleration after initiation to autocatalytic pathways involving polyene sequences, HCl, and radical intermediates. Krongauz [20] distinguished an initial direct HCl-elimination regime from a later HCl-induced autocatalytic regime for the specific plasticized PVC formulation investigated, while Cao et al. [21] showed that heating rate shifts and broadens the HCl-release interval.

One of the most distinctive characteristics of PVC degradation is its strongly autocatalytic nature. The process is generally described through initiation, propagation, and autocatalytic stages [22].

The degradation is initially triggered at thermally unstable sites, including allylic chlorides, tertiary chlorides, branching points, and structural defects generated during polymer synthesis or processing. These weak points facilitate the first elimination reactions, resulting in the release of small amounts of HCl.

Once HCl is formed, neighboring carbon–chlorine bonds become increasingly susceptible to elimination. Successive dehydrochlorination reactions generate extended conjugated double-bond sequences along the polymer backbone. The released HCl acts as a catalyst, promoting further elimination reactions and accelerating degradation rates. This autocatalytic behaviour explains the rapid increase in chlorine release once decomposition has begun and it is responsible for the relatively narrow temperature window in which most chlorine is removed [19].

Following dehydrochlorination, the polymer backbone becomes enriched in conjugated polyene sequences. These unsaturated structures are thermodynamically unstable and undergo a variety of secondary reactions as temperature increases.

The principal pathways include intramolecular cyclization, intermolecular cross-linking, aromatization, and thermal cracking. Through these reactions, the conjugated polyenes are progressively converted into monocyclic and polycyclic aromatic compounds such as benzene, toluene, xylene, styrene, naphthalene derivatives, and other aromatic hydrocarbons. Simultaneously, extensive cross-linking reactions contribute to char formation and increase the yield of solid residues [7,19,23].

The formation of conjugated polyenes also causes the characteristic discoloration observed during PVC degradation, where the material progressively changes from white to yellow, brown, and eventually black as the concentration of unsaturated structures increases.

Vinyl chloride-containing copolymers and mechanically mixed plastics can exhibit additional degradation behavior. Grassie et al. [24] detected both HCl and acetic acid during degradation of vinyl chloride–vinyl acetate copolymers, whereas Gao et al. showed that mixing PVC with PS, PE, or PET can alter chlorine speciation and phase distribution even when the overall chlorine-release ratio changes only slightly [25]. A simplified conceptual network is therefore used below to distinguish PVC chlorine release from interactions with the products of the other polymers.

PVC → HCl + chlorine-containing degradation products
PE/PS/PET → polymer-specific pyrolysis products
Interaction of product streams → changes in chlorine speciation and phase distribution

(4)

Equation (4) is a conceptual reaction network rather than a single stoichiometric reaction. Metals and metal oxides may further modify chlorine chemistry: Fe₂O₃ and TiO₂ have been reported to affect the onset of PVC dehydrochlorination and volatile-product distribution, while alkali and alkaline-earth metals can promote chlorine fixation during co-pyrolysis [26,27]. Molecular calculations for Fe₃O₄ describe dissociative adsorption of already formed HCl and chlorinated products and should not be interpreted as the primary PVC-degradation mechanism [28].

PVC is not the only chlorine-containing material that may enter plastic-waste streams. Polyvinylidene chloride (PVDC), chlorinated polyethylene, chlorinated natural rubber, and chlorinated paraffin additives show distinct thermal behavior and should not be represented using a single PVC degradation scheme [29,30,31,32].

These materials nevertheless require separate consideration because their polymer backbones, chlorine distribution, and formulation differ from those of PVC. PVDC and chlorinated polyethylene should therefore not be treated as simple PVC analogues, while chlorinated natural rubber and chlorinated paraffins represent chemically modified materials or additives with their own chlorine-release behavior. Such structural and compositional differences can affect the onset and rate of chlorine release and the relative formation of HCl and other chlorine-containing products during heating. Accordingly, kinetic parameters and dechlorination efficiencies derived for PVC are not assumed to be directly transferable to these materials, and material-specific characterization remains necessary [29,30,31,32].

2.2. Evolution, Fate of Hydrogen Chloride and Its Implications During Thermochemical Processes

The generated HCl may follow several pathways depending on process conditions. It can be released into the gas phase, adsorbed onto solid surfaces, captured by alkaline sorbents, dissolved in aqueous condensates, or participate in secondary chlorination reactions. Because HCl is highly corrosive and environmentally hazardous, its effective removal is a critical requirement in industrial recycling facilities [33]. The early release of HCl compared with the degradation of most hydrocarbon polymers provides the scientific basis for selective thermal dehydrochlorination technologies, in which chlorine is removed prior to the main pyrolysis stage.

Although hydrogen chloride is the primary chlorine-containing product generated during PVC degradation, secondary organochlorine compounds may also form during thermochemical conversion. These compounds include chlorinated aliphatics, chlorobenzenes, chlorophenols, and a variety of other chlorine-substituted hydrocarbons [34]. In the pyrolysis of pure PVC, chlorinated organic species may originate from reactions involving chlorine radicals, unsaturated polyene intermediates, and aromatic compounds generated during secondary degradation reactions. As the polyene structures undergo cyclization and aromatization, the resulting aromatic intermediates can be further chlorinated under suitable conditions, leading to the formation of chlorobenzenes and chlorophenols [34].

The formation of organochlorine compounds becomes even more relevant during the thermochemical treatment of mixed plastic waste streams [29]. In these systems, PVC releases HCl while polymers such as PE and PP generate olefins, paraffins, and hydrocarbon radicals. Chlorine-containing species may subsequently react with these intermediates, promoting secondary chlorination reactions and generating a wider range of chlorinated products. Such reactions are favored by elevated chlorine concentrations, prolonged residence times, and the presence of catalytic surfaces capable of activating hydrocarbon molecules [29,35]. The generation of organochlorine compounds is undesirable because it lowers the quality of pyrolysis oils, complicates downstream upgrading processes, contributes to catalyst deactivation, and increases environmental concerns associated with product utilization [35].

Catalyst deactivation represents one of the most important challenges associated with chlorine-containing feedstocks. Hydrogen chloride and organochlorine compounds can indeed adversely affect catalysts employed in catalytic pyrolysis, hydrocracking, steam reforming, and upgrading processes [36]. Several deactivation mechanisms have been reported, including the loss of active sites, framework degradation, pore blockage, metal chlorination, and accelerated coke formation. Zeolitic catalysts, particularly H-ZSM-5, may experience changes in the distribution of Brønsted and Lewis acid sites following prolonged exposure to chlorine-containing vapors. Similarly, transition-metal catalysts can undergo chlorination reactions that significantly reduce catalytic activity and alter product selectivity [37]. As catalyst replacement and regeneration contribute substantially to operating costs, chlorine removal prior to catalytic upgrading is generally considered essential for maintaining process performance and economic viability.

In addition to catalyst poisoning, chlorine significantly accelerates corrosion throughout thermochemical recycling systems. Hydrogen chloride readily reacts with metallic surfaces present in reactors, heat exchangers, condensers, pipelines, and gas-cleaning equipment [38,39]. A simplified corrosion reaction can be represented as:

Fe + 2HCl → FeCl₂ + H₂

(5)

The metal chlorides formed may subsequently volatilize and undergo oxidation, resulting in continuous material degradation. Corrosion rates are generally enhanced by high temperatures, elevated chlorine concentrations, and the presence of moisture [5]. These phenomena increase maintenance requirements, shorten equipment lifetime, and substantially raise operational costs. Consequently, corrosion control remains one of the principal motivations for implementing chlorine-removal pretreatments before thermochemical conversion.

Because HCl is both corrosive and acutely hazardous, gas containment and scrubbing are important process-safety requirements. National Institute for Occupational Safety and Health (NIOSH) reports a ceiling recommended exposure limit of 5 ppm and an IDLH value of 45 ppm for HCl [40,41].

3. Need for Pretreatment Technologies

The chemical pathways described above clearly illustrate why chlorine management is a critical prerequisite for the successful thermochemical recycling of plastic waste. The release of hydrogen chloride, the formation of organochlorine compounds, catalyst deactivation, and corrosion collectively reduce process efficiency and product quality while increasing operating costs.

For comparison of the technologies reviewed below, polymer dechlorination denotes cleavage of chlorine from the polymer, chlorine transfer denotes redistribution of chlorine to another process phase, HCl capture denotes removal of HCl from a gas stream, and chlorine recovery denotes collection of chlorine in a defined recoverable stream. These terms are not used interchangeably.

When the required chlorine masses are reported, polymer dechlorination and phase-specific chlorine yield are expressed as:

η_deCl(%) = [(m_Cl,feed − m_Cl,residual)/m_Cl,feed] × 100

(6)

Y_Cl,i(%) = (m_Cl,i/m_Cl,feed) × 100

(7)

Chlorine mass balance should be considered separately from polymer dechlorination. Gao et al. [25] reported that low-temperature PVC pyrolysis transferred most chlorine to the gas phase, with smaller fractions in liquid and char, and that co-pyrolysis with other polymers changed this partitioning. Tito et al. [16] similarly showed that high PVC dechlorination can retain most carbon in the solid while transferring chlorine to an HCl-containing aqueous stream. Complete balances are not reconstructed where the original studies do not quantify all chlorine-containing streams.

To mitigate these issues, numerous pretreatment strategies have been developed, including physical separation, density-based sorting, chemical extraction, mechanochemical activation, hydrothermal processing, thermal dehydrochlorination, catalytic sorption, and emerging photocatalytic approaches. The following sections critically review these technologies, comparing their mechanisms, effectiveness, technological readiness levels, and potential for industrial implementation.

4. Mechanical and Physical Pretreatment Technologies

Mechanical and physical pretreatment technologies represent the first line of intervention for chlorine management in plastic waste recycling systems. Unlike chemical or thermal dechlorination processes, these approaches aim to remove chlorine-containing fractions before further processing by exploiting differences in physical, optical, electrical, or density-related properties among polymers. Such methods are particularly attractive because they generally require lower energy inputs, generate limited secondary waste streams, and can be readily integrated into existing recycling infrastructures [42]. In many industrial facilities, physical separation constitutes the most economically viable strategy for reducing chlorine concentrations prior to thermochemical conversion. The effectiveness of these pretreatment technologies is especially important considering that even small amounts of PVC contamination may significantly affect pyrolysis oil quality, catalyst performance, and equipment lifetime [43].

Figure 1 summarizes the classification of the main physical pretreatment technologies currently employed for chlorine management in mixed plastic waste streams. These approaches can be broadly divided into separation technologies, which reduce the chlorine load by selectively isolating PVC-rich fractions, and structure-modification technologies, which directly alter the polymer matrix to facilitate chlorine removal or selective recovery. Although all these methods contribute to reducing the chlorine burden prior to thermochemical conversion, they differ considerably in terms of technological maturity, scalability, and industrial implementation, as later discussed.

4.1. Sorting Technologies

Sensor-based sorting technologies, as near-infrared spectroscopy, X-Ray fluorescence, laser-induced breakdown spectroscopy, and hyperspectral imaging and artificial intelligence-assisted sorting, have become increasingly important for the identification and separation of chlorine-containing plastics from mixed waste streams. Their primary objective is to selectively remove PVC-rich fractions before thermal processing, thereby minimizing hydrogen chloride generation and reducing the burden on downstream dechlorination systems.

Near-infrared spectroscopy (NIR) is currently one of the most widely implemented technologies in industrial recycling facilities [42,44]. NIR systems identify polymers based on their characteristic vibrational absorption bands in the near-infrared region. During operation, plastic particles are illuminated and their reflected spectra are compared with reference databases to enable rapid polymer classification. NIR sorting is particularly effective for separating polyolefins, PET, and PVC from relatively clean waste streams and can achieve high throughput rates suitable for industrial applications. However, its performance may decrease in the presence of dark-colored plastics, multilayer materials, and heavily contaminated surfaces, which can interfere with spectral interpretation.

As an alternative, X-ray fluorescence (XRF) sorting has emerged as one of the most effective technologies for chlorine detection because it directly identifies elemental chlorine rather than relying on polymer-specific spectral signatures [45,46]. When exposed to X-ray radiation, chlorine-containing materials emit characteristic fluorescence signals that can be rapidly detected and quantified. Consequently, XRF systems are particularly effective for PVC removal from mixed plastic waste streams and are frequently employed in advanced recycling facilities where strict chlorine limits must be respected. Compared with NIR systems, XRF offers superior selectivity for chlorine-bearing materials, although equipment costs and radiation safety requirements may increase operational complexity.

Moreover, laser-induced breakdown spectroscopy (LIBS) represents a more recent development in sensor-based sorting [47,48]. In LIBS systems, a high-energy laser pulse generates a localized plasma on the material surface, and the emitted light is analyzed to determine elemental composition. Because chlorine can be directly detected, LIBS offers considerable potential for identifying PVC and other halogenated materials. Furthermore, LIBS provides rapid analysis and can be combined with automated sorting systems. Nevertheless, industrial implementation remains limited due to equipment costs and the complexity of data processing.

The principal safety requirements are method-specific: XRF systems require controls for ionizing-radiation exposure, whereas LIBS requires appropriate laser-safety procedures and controlled access to the beam path [49,50].

Finally, recent advances in hyperspectral imaging and artificial intelligence-assisted sorting are further improving polymer separation performance [51,52]. Hyperspectral systems combine imaging and spectroscopy to generate spatially resolved chemical information, while machine-learning algorithms enable real-time identification of complex waste streams. Such approaches are expected to play an increasingly important role in future recycling facilities, particularly as waste compositions become more heterogeneous and quality requirements for recycled feedstocks become more stringent.

4.2. Gravity-Based and Surface-Based Separation of Plastic Polymers

Gravity-based separation, commonly referred to as sink–float separation, is one of the most mature and widely adopted physical pretreatment technologies for reducing the chlorine content of mixed plastic waste streams [53]. The process exploits density differences among polymers, allowing low-density polyolefins such as polyethylene (PE) and polypropylene (PP) to float in water, while denser polymers including PVC and PET settle [54]. Consequently, sink–float separation provides an effective first step for reducing the chlorine load prior to thermochemical conversion. When higher separation selectivity is required, dense media such as calcium chloride or zinc chloride solutions can be employed to separate polymers with similar densities, although complete separation of PVC and PET remains challenging because of their overlapping density ranges [55]. In industrial recycling plants, gravity separation is often combined with hydrocyclones or sensor-based sorting technologies to improve polymer purity and increase process throughput [56]. Recent studies have reported PVC purities above 90% using hydrocyclone-assisted density separation, confirming its potential as an efficient pretreatment step. However, gravity separation should be regarded as a PVC concentration technology rather than a true dechlorination process, since chlorine remains chemically bound within the recovered polymer [57,58].

Froth flotation represents an attractive complementary technique for separating PVC from polymers with similar densities, particularly PET. Unlike sink–float separation, flotation exploits differences in surface wettability by selectively modifying polymer surfaces using surfactants or wetting agents, enabling PVC-rich particles to be separated through bubble attachment [59]. Laboratory studies have demonstrated high separation efficiencies under controlled conditions, highlighting the potential of flotation to reduce chlorine contamination before pyrolysis [55]. Nevertheless, large-scale implementation remains challenging because flotation performance is highly sensitive to particle size, surface contamination, reagent dosage, and hydrodynamic conditions [60]. In addition, the need for surfactants increases process complexity and generates wastewater requiring treatment [61]. Consequently, froth flotation is generally considered a complementary technology that is most effective when integrated with density separation and sensor-based sorting rather than employed as a stand-alone solution.

4.3. Mechanochemical Dechlorination

Mechanochemical dechlorination occupies an intermediate position between pure physical pretreatment and conventional chemical dechlorination. Unlike sorting or density separation, which only remove PVC-rich fractions from mixed plastics, mechanochemical treatment directly modifies the chemical structure of chlorine-containing polymers through mechanical activation [62]. The process is generally carried out by high-energy ball milling, where repeated impact, compression, shear, and friction forces induce particle size reduction, polymer chain scission, amorphization, and the formation of reactive defect sites. These effects weaken carbon–chlorine bonds in PVC and promote dehydrochlorination under relatively mild or even ambient-temperature conditions [63].

The fundamental advantage of mechanochemical dechlorination is that mechanical energy can be used to drive solid-state reactions without large amounts of solvent. In the presence of alkaline or metal oxide additives, the chlorine released from PVC can be converted into stable inorganic chlorides rather than emitted as gaseous HCl. Typical additives include CaO, Ca(OH)₂, NaOH, KOH, Fe₂O₃, and other oxides [63]. The role of these additives is twofold: they enhance the mechanochemical cleavage of C-Cl bonds and simultaneously capture chlorine as chloride salts. For example, co-grinding PVC with CaO or Fe₂O₃ can lead to the formation of chloride-containing products such as calcium or iron chlorides, whereas less reactive oxides such as SiO₂ and Al₂O₃ tend to promote PVC degradation with lower chlorine fixation efficiency [64,65].

A particularly interesting example of mechanochemical chlorine utilization was reported by Wang et al. [64], who co-ground spent Li-ion battery cathode material with waste PVC and iron. In that system, PVC was not only dechlorinated but also acted as a chlorine donor: chlorine atoms from PVC were transformed into chloride ions that reacted with lithium to form recoverable LiCl, while cobalt was converted into a magnetic cobalt–iron oxide phase. This study is important from a circular-economy perspective because it demonstrates that the chlorine contained in PVC can be redirected toward useful chloride formation rather than treated exclusively as a contaminant. However, this approach is more appropriately interpreted as mechanochemical co-processing or upcycling than as a conventional dechlorination pretreatment for plastic pyrolysis.

From a mechanistic perspective, mechanochemical dechlorination proceeds through a combination of physical activation and chemical reaction. Milling reduces the particle size of PVC and increases the interfacial contact between the polymer and the additive. At the same time, repeated mechanical stress induces chain scission and structural disorder, generating unsaturated sequences and promoting C–Cl bond cleavage [66]. Earlier studies showed that the dechlorination rate increases with milling time and can be correlated with the impact energy transferred by the balls, confirming that process efficiency depends strongly on milling intensity, ball-to-powder ratio, rotational speed, additive loading, and reactor geometry [62,63,66].

Mechanochemical processing should not be assumed to be strictly isothermal. In situ studies of model mechanochemical systems show that frictional energy dissipation can raise the measurable bulk temperature and alter reaction kinetics, although these results do not quantify transient local “hot spots” during PVC treatment [67,68]. For PVC wastes, dechlorination has instead been correlated directly with impact energy, particle fracture, and generation of fresh reactive surface; radical pathways proposed for Zn-assisted milling remain mechanistic interpretations rather than direct measurements of local temperature [69,70].

When waste or cutoff material is milled with waste PVC, they are converted into chloride, which can be collected separately by water leaching the milled product [66]. The hydrocarbon and the chloride collected after water leaching can be sources for feedstock recycling. Nevertheless, these results should be interpreted carefully. Most mechanochemical experiments are performed using finely powdered PVC, model mixtures, or relatively homogeneous waste fractions [71]. Real plastic waste streams are far more complex and contain plasticizers, fillers, pigments, stabilizers, multilayer materials, dirt, and polymers with very different mechanical properties [3]. These features can reduce milling efficiency, limit contact between PVC and alkaline additives, and increase the energy demand required to achieve high dechlorination yields.

Mechanochemical treatment also offers important advantages for downstream thermochemical recycling. The reduction in particle size and increase in surface area can improve heat and mass transfer during pyrolysis, gasification, or hydrothermal treatment [72]. In addition, the partial removal or stabilization of chlorine before thermal conversion reduces the risk of HCl evolution, corrosion, catalyst poisoning, and chlorinated oil formation [72]. Therefore, mechanochemical pretreatment may be especially useful as part of an integrated process, for example, followed by alkaline washing, water leaching, low-temperature dehydrochlorination, or catalytic pyrolysis [72].

However, despite its scientific potential, mechanochemical dechlorination still presents several limitations. First, high-energy milling can be electricity-intensive, and the overall energy balance is rarely reported in sufficient detail [66]. Second, scale-up from planetary ball mills to continuous industrial mills is not straightforward because energy transfer, residence time distribution, heat generation, equipment wear, and throughput change substantially with reactor size [73]. Third, large excesses of additives are often required to reach high chlorine removal, which generates additional solid residues and chloride-rich aqueous streams after washing. Finally, milling media abrasion may introduce metallic contamination into the treated plastic fraction, which could affect downstream catalytic processes [74].

When NaOH or KOH is used, reagent handling also introduces a caustic hazard; NIOSH reports ceiling recommended exposure limits of 2 mg m⁻³ for both substances [75,76].

For these reasons, mechanochemical dechlorination should not yet be considered a mature stand-alone solution for large-scale chlorine removal from heterogeneous plastic waste. Its strongest potential lies in niche applications involving PVC-rich streams, relatively dry industrial residues, cable waste, Waste Electrical and Electronic Equipment (WEEE plastics), or co-processing strategies where chlorine can be valorized as a useful chloride rather than simply removed. Future research should focus on continuous milling systems, reactive extrusion, energy-consumption assessment, additive recycling, life-cycle analysis, and validation using real mixed plastic waste rather than pure PVC powders. If these challenges are addressed, mechanochemical dechlorination could become a valuable component of integrated chlorine-management strategies for plastic waste recycling.

4.4. Solvent-Based Recovery and Swelling-Assisted Separation

In addition to conventional mechanical recycling technologies, several solvent-based approaches have been developed for the recovery and purification of PVC from complex waste streams [77]. These processes occupy an intermediate position between mechanical and chemical recycling. Although chemical solvents are employed to soften, swell, or selectively dissolve the polymer, the PVC macromolecular structure remains largely intact throughout the treatment. Consequently, these technologies are generally classified as advanced or non-conventional mechanical recycling methods rather than true chemical recycling processes [78].

Among the most well-known industrial examples is the VinyLoop^® process, developed by Solvay for the recovery of PVC from complex waste streams including cables, flooring materials, coated fabrics, and multilayer products [79]. The process is based on the selective dissolution of PVC in a proprietary solvent within a closed-loop system. After preliminary cleaning, shredding, and size reduction, the waste is contacted with the solvent, allowing PVC to dissolve while non-soluble contaminants are removed through filtration, centrifugation, and sedimentation steps. The dissolved PVC is subsequently precipitated through solvent evaporation and recovered as a fine powder suitable for the manufacture of new products such as waterproof membranes, floor coverings, technical coatings, and footwear components. One of the main advantages of VinyLoop is its ability to process highly contaminated waste streams while simultaneously recovering a relatively pure PVC fraction. Furthermore, solvent recycling within the closed loop reduces solvent consumption and minimizes environmental emissions.

Despite these advantages, solvent-dissolution processes present several challenges. The technology requires multiple separation, filtration, precipitation, and solvent-recovery steps, leading to relatively high capital and operating costs. In addition, solvent management becomes increasingly complex when processing heterogeneous waste streams containing plasticizers, stabilizers, fillers, pigments, and other additives. While VinyLoop demonstrated the technical feasibility of large-scale PVC purification, the economic viability of such systems remains strongly dependent on feedstock quality, solvent recovery efficiency, and market demand for recycled PVC.

More recently, alternative approaches based on polymer swelling rather than complete dissolution have been proposed, particularly for the treatment of PVC-coated electrical cables [80,81]. In these systems, organic solvents are used to selectively swell the PVC insulation layer, creating mechanical separation between the polymer coating and the metallic conductor. Following swelling, mechanical treatment using ball mills or rod mills facilitates the detachment of the softened PVC from the embedded copper wire.

Kumar et al. [82] demonstrated that solvents such as acetone and n-butyl acetate can effectively swell PVC while simultaneously extracting plasticizers, particularly diisononyl phthalate (DINP). Under optimized conditions, plasticizer extraction efficiencies approaching 100% were achieved within approximately one hour of treatment, while subsequent ball milling enabled complete separation of PVC and copper even for relatively long cable sections. Solvent recovery through distillation further improved process sustainability, with recovery efficiencies exceeding 80–90%.

Compared with complete dissolution technologies, swelling-assisted separation offers several potential advantages. Solvent consumption is generally lower, polymer recovery is simplified, and the simultaneous extraction of plasticizers enhances the quality of the recovered PVC fraction [80,83].

Nevertheless, important challenges remain before large-scale implementation can be considered. The process still relies on significant quantities of organic solvents, requiring efficient solvent recovery systems and strict safety measures due to flammability concerns. Furthermore, treatment times remain relatively long, and the mechanical separation efficiency may be strongly influenced by cable geometry, additive composition, and the degree of weathering of the waste material [80]. The need for milling equipment also introduces additional energy consumption and equipment wear considerations.

Solvent-specific hazards must also be considered. Acetone, for example, is highly flammable and occupational exposure may cause irritation and central-nervous-system effects [84].

From the specific perspective of chlorine management, both dissolution-based and swelling-assisted technologies should be viewed primarily as PVC recovery and purification processes rather than dechlorination methods. The chlorine remains chemically bound within the recovered PVC and is therefore not removed from the polymer matrix. However, these technologies may still contribute indirectly to chlorine management by concentrating PVC into dedicated streams that can subsequently be recycled separately or subjected to targeted dechlorination treatments. Consequently, solvent-based recovery approaches can be considered complementary pretreatment technologies that facilitate downstream chlorine management rather than standalone solutions for chlorine removal.

4.5. Comparison Between Physical Separation Technologies

Table 1. Advantage and disadvantage of physical separations.

Authors	Feedstock	Process	Advantage	Disadvantage
Saito et al. [66]	Waste PVC resin	Dry milling (Mechanochemical)	High dechlorination efficiency (up to 99%) under mild operating conditions	Additional washing or separation steps are required to remove the resulting chloride salts
Inoue et al. [63]	PVC + metal oxides	Ball milling (Mechanochemical)	Simultaneous C–Cl bond cleavage and chlorine immobilization as stable chloride salts (CaO, Fe2O3, etc.)	High additive-to-PVC ratios are often required to achieve complete chlorine fixation
Baláz et al. [62]	PVC + eggshell waste	Co-milling (Mechanochemical)	Simultaneous valorization of PVC and biogenic waste; more than 55% of chlorine converted into stable compounds	Demonstrated only at laboratory scale using homogeneous feedstocks; applicability to heterogeneous plastic waste remains uncertain
Jiang et al. [79]	Mixed PVC wastes	Selective dissolution (VinyLoop® process)	High-purity PVC recovery through closed-loop solvent recycling	High capital cost, complex solvent management, and no direct chlorine removal
Kumar et al. [80]	Electronic cables	Swelling + ball milling	Simultaneous recovery of high-purity copper, PVC, and plasticizers under ambient conditions	Applicable mainly to cable waste; efficient solvent recovery is essential for process sustainability
Xu et al. [81]	Thin cables	Swelling + centrifugation	Complete recovery of copper, PVC, and plasticizers with limited polymer degradation	Process applicability is largely restricted to cable waste with relatively uniform geometry
Kumar et al. [82]	Cables	Wet milling (Mechanochemical)	One-pot separation with reduced processing time compared with dry milling	Solvent recovery and process scale-up remain major challenges
Pita et al. [85]	Mixed Plastic Flakes	Froth flotation	Effective separation of polymers with similar densities; relatively low thermal energy demand	Performance strongly depends on particle size, surfactant dosage, and wastewater management; limited industrial implementation
Pongstabodee et al. [55]	Mixed post-consumer plastics	Three-stage sink–float + selective flotation	Combines density and surface-property differences to improve separation	Requires multiple separation steps and flotation reagents

summarizes the main relevant studies on physical separation technologies, highlighting the principal advantages and disadvantages of each approach.

Quantitative efficiencies in Table 1 should be interpreted together with the operating conditions and analytical basis reported in the cited studies; values are not extrapolated beyond those experimental systems.

Therefore,

Table 2. Technology Readiness Level of the physical pre-treatments methods.

Technology	Industrial Maturity
Sensor-based sorting (NIR)	Commercially established [56]
XRF sorting	Commercially available [56]
Sink–float separation	Commercially established [55]
Hydrocyclones	Commercially established [57]
Froth flotation	Pilot/limited industrial implementation [53]
Solvent dissolution (VinyLoop®)	Commercially demonstrated [79]
Swelling-assisted separation	Laboratory/pilot scale [82]
Mechanochemical dechlorination	Laboratory scale [69]

compares the current implementation status of the principal physical pretreatment technologies, highlighting the gap between commercially established processes and emerging laboratory-scale approaches.

5. Chemical Pretreatment Technologies for Chlorine Removal

Chemical pretreatment technologies constitute one of the most extensively investigated approaches for chlorine removal from plastic waste streams [5]. Unlike physical separation methods, which aim to isolate chlorine-containing polymers, chemical treatments directly target chlorine-bearing species through hydrolysis, dehydrochlorination, or substitution reactions [86]. These methods are particularly relevant when PVC-containing materials cannot be efficiently separated from mixed waste streams or when residual chlorine concentrations remain above the acceptable limits for thermochemical conversion processes [87].

The effectiveness of chemical dechlorination is strongly influenced by polymer composition, particle size, chlorine distribution, reaction temperature, solvent properties, and reagent concentration. Depending on the selected process, chlorine removal efficiencies may range from moderate values to more than 95% in optimized alkaline dechlorination systems [88,89]. The main chemical pre-treatment technologies will be investigated and analyzed in the following sections, such as alkaline dechlorination, catalytic dechlorination, metal oxides as reactive sorbents, red mud, zeolite-based catalysts and bifunctional systems.

Figure 2 summarizes the main chemical pretreatment strategies currently investigated for chlorine removal from plastic waste prior to thermochemical recycling. The schematic highlights the different operating principles of alkaline dechlorination, reactive sorbents, and catalytic systems, together with their common objective of reducing chlorine content, mitigating HCl emissions, and improving the quality of thermochemical conversion products.

5.1. Alkaline Dechlorination and Calcium-Based Sorbents

Alkaline dechlorination is one of the most effective and extensively studied methods for chlorine removal from PVC-containing waste streams. The process relies on the reaction between chlorine-bearing polymers and alkaline reagents, which promote dehydrochlorination and simultaneously neutralize the released hydrogen chloride [89]. The most investigated alkaline reagents include sodium hydroxide (NaOH), potassium hydroxide (KOH), and calcium hydroxide [Ca(OH)₂] [90]. During treatment, hydroxide ions attack the polymer structure and facilitate the elimination of HCl from the PVC backbone [90]. This reaction generates conjugated polyene sequences while preventing the accumulation of free HCl within the reaction medium [90]. Subsequently, the HCl generated reacts with the alkaline species to form stable chloride salts. This neutralization step is particularly important because it shifts the reaction equilibrium toward further chlorine removal and suppresses secondary degradation reactions [89].

Among the alkaline reagents investigated, sodium hydroxide (NaOH) and potassium hydroxide (KOH) generally provide the highest dechlorination efficiencies owing to their strong basicity and high solubility, enabling rapid chlorine removal. Nevertheless, their widespread industrial application is limited by the generation of highly alkaline wastewater and the associated costs of reagent consumption and effluent treatment. In contrast, calcium hydroxide [Ca(OH)₂] exhibits lower intrinsic reactivity but offers significant economic and environmental advantages, including lower reagent cost, easier handling, and the possibility of recovering chlorine as stable calcium chloride [91,92]. In addition, calcium-based sorbents reduce corrosion risks and suppress secondary chlorination reactions during thermochemical processes [93]. For these reasons, calcium-based systems are often considered more attractive for large-scale pretreatment of PVC-containing waste streams [7,27,94].

Despite its high dechlorination efficiency, alkaline treatment remains a reagent-intensive process. The management of chloride-rich aqueous effluents, reagent recovery, and the treatment or valorization of by-products continue to represent significant challenges for industrial implementation. Consequently, future research should focus on process intensification, reagent recycling, and the integration of alkaline dechlorination with complementary pretreatment and thermochemical recycling technologies to improve overall process sustainability. On the contrary, calcium-based sorbents reduce corrosion risks, suppress secondary chlorination reactions during pyrolysis and, at the same time, include low cost, widespread availability, and compatibility with existing industrial processes. However, significant sorbent consumption is often required, leading to the generation of chloride-rich solid residues that require subsequent management or disposal. Furthermore, pore blockage caused by CaCl₂ formation may progressively reduce sorbent activity during prolonged operation. Strong alkalis also require appropriate occupational controls [75,76]. Formation of CaCl₂ or other chloride products is therefore described as chlorine fixation or transfer to a solid phase rather than disappearance of chlorine from the overall process inventory.

5.2. Catalytic Dechlorination

Catalytic dechlorination represents an intermediate strategy between conventional thermal dehydrochlorination and chemical chlorine removal processes. Unlike purely thermal treatments, which rely exclusively on temperature to release hydrogen chloride from PVC, catalytic systems employ reactive solids capable of accelerating dechlorination reactions, capturing released chlorine species, or simultaneously upgrading pyrolysis products [95,96].

Catalytic dechlorination can occur through two principal mechanisms. The first involves the promotion of dehydrochlorination reactions within chlorine-containing polymers, facilitating the cleavage of carbon–chlorine bonds at lower temperatures. The second relies on the adsorption or chemical fixation of chlorine-containing compounds onto reactive catalyst surfaces, thereby preventing chlorine transfer into liquid or gaseous products [95,97].

From an industrial perspective, catalytic dechlorination is particularly attractive because it can be integrated directly into pyrolysis reactors, secondary catalytic beds, vapor upgrading units, or downstream polishing systems. Consequently, catalyst-based approaches are increasingly considered an effective intermediate step to mechanical sorting and thermal pretreatment technologies.

5.2.1. Metal Oxides as Reactive Sorbents for Dechlorination

Metal oxides have attracted considerable attention because they combine chlorine capture capability with catalytic activity. Among the materials investigated, iron oxides, magnesium oxide, zinc oxide, and mixed-metal oxide systems have shown promising performance [98].

Iron oxide-based catalysts are generally considered among the most effective materials for chlorine removal. Studies on municipal plastic waste-derived oils demonstrated that Fe₂O₃ promotes the dehydrochlorination of chlorinated hydrocarbons while simultaneously fixing chlorine through the formation of iron chlorides [99]. Importantly, iron oxide catalysts exhibit greater stability than ZnO or MgO, which often suffer rapid deactivation after exposure to hydrogen chloride.

The dechlorination mechanism involves both catalytic activation of organochlorine compounds and adsorption of chlorine-containing intermediates on the catalyst surface. As a result, iron oxide systems can reduce chlorine concentrations in pyrolysis oils from several hundred ppm under optimized conditions [98,99].

Magnesium oxide and zinc oxide also exhibit high initial chlorine-capture efficiencies because of their strong basicity [100]. However, their performance often decreases with operating time due to the formation of stable chlorides that block active sites and reduce catalyst regeneration potential. Consequently, these materials are generally considered less suitable for long-term industrial operation.

5.2.2. Red Mud as a Low-Cost Dechlorination Catalyst

Red mud, a by-product of alumina production, has emerged as an attractive low-cost catalyst and sorbent for chlorine management. Its composition typically includes Fe₂O₃, Al₂O₃, SiO₂, TiO₂, and alkaline oxides, providing both catalytic and adsorption functionalities [101,102,103]. Several studies have demonstrated that red mud can simultaneously promote cracking reactions and remove chlorine-containing species during plastic pyrolysis [101]. The iron oxides contained within red mud play a particularly important role by adsorbing chlorine species and converting them into stable iron chlorides. In addition, the porous structure and acid–base properties of red mud contribute to hydrocarbon upgrading reactions. Recent investigations have shown chlorine reductions exceeding 45–75% depending on catalyst formulation, operating conditions, and feedstock composition [101]. Furthermore, red mud offers significant economic advantages because it valorizes an abundant industrial residue that would otherwise require disposal. Nevertheless, red mud presents some limitations. Its composition varies significantly according to bauxite origin and processing conditions, leading to variability in catalytic performance. Additionally, the accumulation of chlorides and heavy metals may complicate catalyst regeneration and long-term reuse.

5.2.3. Zeolite-Based Catalysts and Bifunctional Systems

Zeolitic catalysts such as HZSM-5, HY, and modified aluminosilicates are widely employed in catalytic pyrolysis due to their strong acidity and cracking activity [104]. Although zeolites alone exhibit limited chlorine-capture capacity, they can be combined with metal oxides to create bifunctional catalysts capable of simultaneously upgrading hydrocarbons and removing chlorine [97,104]. Iron-impregnated zeolite represents one of the most promising examples. In these systems, the zeolite framework provides cracking and aromatization activity, while iron oxide acts as a chlorine scavenger. Such bifunctional catalysts have been reported to produce upgraded liquid fuels with chlorine contents below 100 ppm while maintaining high liquid yields [104]. However, catalyst deactivation remains an important challenge. Chlorine-containing species may alter the distribution of Brønsted acid sites, promote coke formation, and gradually reduce catalyst activity. Therefore, efficient regeneration strategies are required for long-term industrial operation.

5.3. Critical Assessment and Industrial Perspective

Despite the significant progress achieved during the last two decades, catalytic dechlorination remains less mature than mechanical separation or conventional thermal dehydrochlorination. Most published studies have been conducted at laboratory scale using model feedstocks, synthetic chlorinated oils, or PVC-rich streams rather than realistic heterogeneous municipal plastic waste. A further challenge concerns catalyst lifetime. Many materials exhibit excellent initial chlorine-capture performance but progressively deactivate due to chloride accumulation, pore blockage, sintering, or coke deposition. Regeneration strategies remain insufficiently investigated, and long-term stability data are rarely available.

From an industrial perspective, iron oxide-based materials and red mud currently appear among the most promising options because they combine relatively high chlorine-removal efficiencies with low cost and acceptable stability. Conversely, highly active materials such as ZnO or MgO often suffer rapid deactivation and may not be economically viable at large scale. Consequently, future developments should focus on multifunctional catalysts capable of simultaneously achieving chlorine capture, hydrocarbon upgrading, and catalyst regeneration. Hybrid systems combining sorbents, catalytic cracking catalysts, and thermal pretreatment stages are likely to represent the most realistic pathway toward large-scale chlorine management in advanced plastic recycling facilities.

The critical assessment of the chemical pre-treatments is summarized in

Table 3. Advantages and disadvantages of chemical pre-treatments.

Authors	Feedstock	Process	Advantages	Disadvantages
Kumagai et al. [92]	Individual plastics (PVC, PE, PP, PS) and mixed plastics	Thermal decomposition in the presence of CaO or Ca(OH)2	Efficient in situ HCl capture; inexpensive and readily available sorbents; easily integrated into thermochemical recycling	Performance strongly depends on feedstock composition and polymer type; high sorbent consumption and CaCl2-rich residues require further management
Meng et al. [100]	PVC resin + CaO, ZnO, CuO, MgO, Fe2O3, Al2O3	Low-temperature catalytic pyrolysis with different metal oxides	Systematic comparison of metal oxides; identifies the influence of oxide basicity and reactivity on PVC dechlorination and chlorine fixation	Laboratory study using model PVC; catalyst regeneration and performance with heterogeneous plastic waste were not investigated
Yang et al. [105]	Mixed PVC/PE waste	Coupled catalytic dechlorination and catalytic pyrolysis for CNT production	Simultaneous chlorine removal and production of high-value carbon nanotubes; integrated waste valorization	Demonstrated only on model PVC/PE mixtures; catalyst stability, regeneration and process economics remain uncertain
Ding et al. [98]	PVC	Catalytic pyrolysis with natural iron oxide minerals	Simultaneous catalytic dechlorination, chlorine capture and carbon sequestration; low-cost and naturally abundant catalysts	Catalyst regeneration and long-term stability require further investigation; performance with heterogeneous plastic waste remains unknown
Jiang et al. [101]	Chlorine-containing organic solid waste	Chemical looping conversion using red mud as multifunctional oxygen carrier and chlorine sorbent	Simultaneous oxygen transfer and HCl capture; valorization of industrial waste; low catalyst cost	Catalytic performance depends on red mud composition; chloride accumulation and regeneration remain challenging
Hu et al. [97]	PVC-containing mixed plastic waste	Stepwise catalytic pyrolysis using Fe2O3/HZSM-5	Simultaneous dechlorination and catalytic upgrading; reduced chlorine transfer to pyrolysis oil; enhanced aromatic production	Catalyst deactivation due to coke and chloride accumulation; validated under laboratory conditions
Sophonrat et al. [106]	PVC/cellulose mixture	Ex situ catalytic pyrolysis with CaO adsorption followed by catalytic reforming	Efficient HCl removal before catalytic upgrading; protects downstream catalysts from chlorine poisoning; improved vapor quality	Evaluated using simplified binary mixtures; finite CaO adsorption capacity; regeneration and applicability to real waste require further investigation
Inoue et al. [63]	PVC + metal oxides	Mechanochemical treatment with metal oxides	Simultaneous mechanochemical dechlorination and chlorine immobilization; comparison of different oxide additives	High oxide-to-PVC ratios required; evaluated using model PVC under laboratory conditions

.

Reported dechlorination percentages should be interpreted together with feed composition, temperature, residence time, chlorine basis, reagent or catalyst loading, and analytical method. Where such information is unavailable in the original source, it is not reconstructed in this review.

To avoid presenting dechlorination percentages as stand-alone performance metrics, quantitative values are discussed in the main text only when sufficient experimental context is available. For example, the 3.07–82.51% range reported by Yang et al. [107] is explicitly linked to the PVC examined in that study, a temperature range of 175–280 °C, and a residence time of 30 min. Values reported in the comparative tables should therefore be interpreted as study-specific results rather than as directly transferable process efficiencies.

Catalyst and sorbent lifetime should be evaluated using distinct metrics rather than a single stability descriptor. Available studies report different quantities, including HCl capacity, deactivation mechanism, regeneration, or reuse cycles; therefore, only directly reported values are compared in

Table 4. Representative catalyst and sorbent lifetime metrics reported for chlorine management.

Material/Study	Capacity or Performance	Deactivation Mechanism	Regeneration/Reuse	Cycles/Retention
Hydrated lime (Ravina et al.) [33]	3.59 mg HCl g−1 after 30 min for the best sample	Not reported as cyclic deactivation	Not reported	Not reported
Iron oxide (Lingaiah et al.) [99]	Dechlorination activity in PVC-derived oil	HCl adsorption followed by iron-chloride formation	Reversible adsorbed HCl removed using He	No multi-cycle retention series
Ga liquid metal (Polo-Garzon et al.) [108]	Approximately 90% reduction of PVC chlorine at 200 °C	Initial decrease after first run	Catalyst reused	Four cycles; relatively stable thereafter

, and unavailable metrics are indicated as not reported [33,99,108].

The available data therefore do not support a uniform lifetime ranking across metal oxides, sorbents, and liquid-metal systems.

Although numerous chemical pretreatment technologies have been proposed for chlorine removal, their level of technological development varies considerably.

Table 5. Technology Readiness Level of the main chemical pre-treatments.

Technology	Typical Reagents/Catalysts	Primary Mechanism	Industrial Maturity
Alkaline dechlorination	NaOH, KOH, Ca(OH)2	Chemical dehydrochlorination and HCl neutralization	Pilot scale/Industrially demonstrated [89]
Reactive sorbents	CaO, limestone, dolomite	In situ HCl capture (chloride formation)	Commercially demonstrated [33]
Metal oxide catalysts	Fe2O3, ZnO, MgO, CuO	Catalytic C–Cl bond cleavage and/or HCl adsorption	Laboratory–pilot scale [99,100]
Waste-derived catalysts	Red mud	Catalytic dechlorination coupled with HCl capture	Laboratory scale [101]
Modified zeolites	HZSM-5, metal-modified ZSM-5	Catalytic cracking and hydrocarbon upgrading with limited dechlorination	Laboratory scale [104]
Bifunctional catalysts	Fe/HZSM-5, metal oxide–zeolite composites	Simultaneous dechlorination and catalytic upgrading	Laboratory scale [97]

compares the current industrial maturity of the principal chemical dechlorination strategies together with the main barriers that still hinder their large-scale implementation.

6. Emerging Photocatalytic Dechlorination Technologies

Photocatalytic dechlorination has recently emerged as a promising and environmentally friendly strategy for the removal of chlorine from chlorinated polymers and organochlorine compounds [109,110]. Unlike conventional thermal or chemical treatments, photocatalytic processes utilize light energy to generate highly reactive charge carriers capable of promoting oxidation, reduction, and bond-cleavage reactions under relatively mild operating conditions [111]. The possibility of employing solar energy, reducing reagent consumption, and operating at near-ambient temperatures has generated increasing interest in photocatalytic technologies as potential pretreatment methods for plastic waste recycling [112].

Although photocatalytic dechlorination is still at an early stage of development compared with thermal or hydrothermal technologies, recent advances in semiconductor materials, nanostructured catalysts, and visible-light-responsive photocatalysts have significantly expanded its potential applications [112]. In the context of chlorine management, photocatalysis may contribute to the weakening of carbon–chlorine bonds, partial dechlorination of polymer surfaces, degradation of chlorinated additives, and pretreatment of feedstocks prior to thermochemical conversion [112].

Photocatalytic reactions are initiated when a semiconductor material absorbs photons with energy equal to or greater than its band-gap energy. Upon light absorption, electrons are promoted from the valence band to the conduction band, generating electron–hole pairs [113]. The efficiency of photocatalytic dechlorination depends on several factors, including light intensity, wavelength, catalyst surface area, charge-carrier recombination rate, and accessibility of chlorine-containing functional groups.

Titanium dioxide (TiO₂) remains the most extensively investigated photocatalyst because of its chemical stability, low toxicity, abundance, and strong oxidative capability [114]. Under ultraviolet irradiation, TiO₂ generates electron–hole pairs capable of initiating dechlorination reactions and promoting the degradation of chlorinated organic compounds [115,116].

Several studies have demonstrated that TiO₂ photocatalysts can effectively degrade chlorinated hydrocarbons, chlorophenols, chlorobenzenes, and PVC-derived degradation products. Most investigations have been conducted using model chlorinated compounds or direct contact between TiO₂ powders and PVC particles [117,118]. While suitable for mechanistic studies, such solid–solid configurations are difficult to scale up because they require intimate contact between the photocatalyst and highly heterogeneous plastic waste. Furthermore, catalyst recovery, particle agglomeration, dust generation, and limited light penetration significantly reduce the feasibility of slurry-based systems under industrial conditions [119]. Future developments should therefore focus on immobilized photocatalysts and liquid-mediated reactor configurations [120]. In particular, TiO₂-loaded polymeric membranes or structured photocatalytic supports may overcome catalyst recovery issues while enabling continuous operation and easier integration with aqueous or hydrothermal pretreatments [120]. Such systems could simultaneously enhance chlorine extraction and polymer activation before subsequent thermochemical conversion.

Process safety should be considered separately from photocatalytic performance. Artificial UV sources require protection of skin and eyes, while fine or nanoscale TiO₂ requires appropriate occupational controls; ecotoxicological effects of engineered TiO₂ nanoparticles have also been reported under specific experimental conditions [121,122,123]. Immobilization can reduce handling and post-treatment separation of suspended catalyst particles, but does not by itself establish reactor safety.

Overall, photocatalytic dechlorination should currently be regarded as a complementary pretreatment rather than a stand-alone dechlorination technology. Although its technological readiness remains low, advances in visible-light-responsive photocatalysts, catalyst immobilization, membrane reactors, and hybrid photocatalytic–hydrothermal processes may significantly improve its future applicability for sustainable plastic waste recycling.

7. Hydrothermal and Liquefaction Dechlorination

Hydrothermal dechlorination (HTD) has emerged as one of the most promising pretreatment technologies for chlorine-rich plastic waste because it removes chlorine under relatively mild temperatures while avoiding the direct release of hydrogen chloride into the gas phase. Unlike conventional thermal dehydrochlorination, hydrothermal processing exploits the unique physicochemical properties of water under subcritical or supercritical conditions. As temperature and pressure increase, the dielectric constant of water decreases, its ionic product increases, and mass-transfer limitations are significantly reduced, thereby promoting both dehydrochlorination and nucleophilic substitution reactions [124,125]. Two principal reaction pathways have been identified. The first involves the elimination of HCl from adjacent C-Cl and C-H bonds, leading to the formation of conjugated polyene structures. The second consists of nucleophilic substitution, in which hydroxyl groups replace chlorine atoms along the polymer backbone. Both mechanisms contribute to chlorine removal, although their relative importance depends strongly on the operating conditions and the composition of the plastic waste [124].

Among the main process variables, temperature and residence time strongly influence hydrothermal PVC dechlorination. Yang et al. [107] reported that, for the PVC examined in their study, dechlorination increased from 3.07% at 175 °C to 82.51% at 280 °C at a residence time of 30 min. These values are specific to that material and reactor conditions and should not be extrapolated directly to heterogeneous post-consumer plastics. The same study represented the data using first-order kinetics and reported an apparent activation energy of 217.43 kJ mol⁻¹, but these parameters are not treated as universal constants. More recent analysis by Hungwe et al. [126] describes non-catalytic hydrothermal PVC dechlorination as a two-stage process in which the relative importance of chemical reaction and mass transfer changes during conversion. Other variables, including particle size, liquid-to-solid ratio, mixing, and reaction time, can therefore affect the observed performance. Hydrothermal treatment remains attractive because wet feedstocks can be processed without a separate drying step and because chlorine released from PVC can be transferred to an aqueous phase before downstream thermochemical conversion.

Compared with conventional dechlorination technologies, hydrothermal pretreatment offers several distinctive advantages. High chlorine-removal efficiencies can be achieved at relatively moderate temperatures, chlorine is mainly transferred into the aqueous phase rather than released as gaseous HCl, and wet feedstocks can be processed without energy-intensive drying steps. In addition, the process can be naturally integrated with hydrothermal liquefaction, making it particularly attractive for future circular plastic recycling systems. Despite these advantages, several challenges still limit large-scale deployment. High-pressure reactors and corrosion-resistant construction materials considerably increase capital costs, while chloride-rich aqueous streams require appropriate downstream treatment. Furthermore, the energy demand associated with pressurized operation and the limited availability of continuous industrial reactors remain important barriers to commercialization. Future research should therefore focus on process intensification, catalyst-assisted hydrothermal systems, heat integration, and continuous reactor design to improve both the economic and environmental sustainability of hydrothermal dechlorination. Corrosion and pressure containment are additional scale-up constraints. Salimi et al. [127] showed that dechlorination and corrosion can occur concurrently in an SS-316 reactor and that reactor configuration and metal chemistry affect both outcomes. Most dedicated PVC hydrothermal-dechlorination studies remain laboratory-scale; pilot-scale operation has been reported for a specific co-hydrothermal-carbonization system treating PVC-containing medical waste with lignocellulosic biomass, rather than for hydrothermal dechlorination of heterogeneous plastic waste in general [128].

8. Towards an Integrated Pretreatment Strategy for Chlorine Management

The literature reviewed in the previous sections clearly demonstrates that no single pretreatment technology is capable of universally addressing the wide variability of chlorine-containing plastic waste streams. Each approach exhibits distinct advantages and limitations depending on the feedstock characteristics, chlorine concentration, operating conditions, process economics, and the final objectives of thermochemical recycling. Consequently, the available technologies should not be regarded as competing alternatives but rather as complementary strategies that may be integrated into future recycling schemes.

Physical pretreatment technologies constitute the first barrier for chlorine management by selectively removing PVC-rich fractions before thermochemical conversion. Their industrial maturity and relatively low operating costs make them attractive for the preliminary treatment of mixed plastic waste, although complete chlorine removal is rarely achieved because of the heterogeneous nature of post-consumer feedstocks. Chemical and hydrothermal pretreatments, in contrast, directly remove chlorine from the polymer matrix and can therefore substantially reduce the residual chlorine content when physical separation alone is insufficient. Catalytic systems further complement these approaches by capturing hydrogen chloride and limiting chlorine transfer into liquid and gaseous products during thermochemical conversion. Finally, photocatalytic dechlorination, despite its low technological readiness, represents an emerging research direction that may eventually be integrated with other pretreatment technologies through immobilized photocatalysts and hybrid reactor configurations.

Rather than identifying a single “best” dechlorination technology, the present review suggests that future chlorine-management strategies will likely rely on multistage pretreatment schemes, in which different technologies are sequentially combined according to the characteristics of the incoming waste stream. Such an integrated approach has the potential to maximize chlorine removal while minimizing reagent consumption, catalyst deactivation, corrosion, and the formation of chlorinated by-products. A conceptual framework illustrating the possible integration of the pretreatment technologies discussed throughout this review is proposed in Figure 3.

Figure 3 is intended as a conceptual process framework rather than a sequence in which every technology is applied. Physical separation is favored when a PVC-rich fraction can be identified and removed; direct chemical or hydrothermal dechlorination becomes relevant when chlorine remains bound to a PVC-rich stream, whereas reactive sorbents and catalysts are better suited to residual-chlorine polishing or in-process HCl capture. Selection therefore depends on chlorine concentration and chemical form, feedstock heterogeneity, moisture content, the residual-chlorine specification of the downstream process, and process feasibility.

From a chemical-engineering perspective, scale-up also requires adequate heat transfer, mixing, mass transfer, and downstream phase separation. Hydrothermal treatment requires controlled heating and cooling of pressurized solid–liquid systems followed by solid–liquid separation of the dechlorinated material from the chloride-rich aqueous phase; the two-stage kinetic behavior reported by Hungwe et al. [126] further indicates that transport effects may become relevant during conversion. For dry feeds, staged thermal dehydrochlorination followed by Ca-based HCl capture provides an alternative configuration demonstrated in continuous processing studies [12,33]. Sequential hydrothermal dechlorination and downstream liquefaction has likewise been demonstrated as separate processing stages [16].

To further support this integrated perspective,

Table 6. Recommended pretreatment strategies for different plastic waste feedstock characteristics.

Feedstock Characteristics	Recommended Pretreatment	Main Objective	Industrial Maturity
Clean polyolefin-rich waste	NIR sorting + sink–float [55,56]	Separate PVC-rich contamination	Commercial
Mixed municipal plastic waste	NIR + XRF + density separation [56]	Reduce chlorine content of the selected feed stream	Commercial
PVC-rich waste	Alkaline dechlorination [89]	Direct chlorine removal	Pilot
Wet plastic waste	Hydrothermal dechlorination [107]	Polymer dechlorination with chlorine transfer to the aqueous phase	Laboratory; specific pilot case [128]
WEEE plastics	Sensor sorting + mechanochemical treatment [69]	PVC concentration and stabilization	Laboratory–Pilot
Residual chlorine after pretreatment	Catalytic sorbents (CaO, Fe2O3, red mud) [33,101]	HCl capture during pyrolysis	Pilot–Commercial
High-quality fuel production	Fe/HZSM-5 or bifunctional catalysts [97,104]	Simultaneous dechlorination and upgrading	Laboratory–Pilot

summarizes the main characteristics of the pretreatment technologies discussed in this review, highlighting their primary objective, the type of feedstock for which they are most suitable, their principal limitations, and their current level of technological maturity. The comparison emphasizes that each technology addresses a different aspect of chlorine management, reinforcing the need for integrated rather than stand-alone pretreatment strategies.

Quantitative Techno-Economic, Environmental and Safety Assessment

Published techno-economic and life-cycle studies use different feedstocks, plant capacities, products, functional units, and system boundaries, so their numerical results should not be combined into a universal ranking. Hernández et al. [129] compared several chlorine-management routes and found, within their model assumptions, that basic aqueous absorption was mainly applicable at low PVC contents, while hydrogenolysis gave the most favorable economic result and catalytic pyrolysis the lowest global-warming potential among the configurations considered. Yaish et al. [130] likewise showed that selective-dissolution recycling can improve some environmental indicators while worsening others, with distillation-related steam demand representing an important contribution.

Other assessments address different process boundaries: Lu et al. [131] showed that the environmental performance of mechanochemical chlorine recovery is sensitive to milling and process-design conditions, while Salimi et al. [127] quantified the link between hydrothermal dechlorination and reactor corrosion. Han et al. [132] reported substantial modeled energy and carbon-footprint reductions for a photothermal PVC process, but those results should not be transferred to conventional TiO₂ photocatalysis.

For safety context, complete dehydrochlorination of the PVC repeat unit corresponds stoichiometrically to 0.583 kg HCl per kg PVC; therefore, a feed containing 1 wt.% PVC has a theoretical maximum HCl-generation potential of about 5.83 kg per tonne of feed. This is a stoichiometric calculation rather than an experimental emission factor.

Representative quantitative indicators from these studies are summarized in

Table 7. Representative quantitative techno-economic, environmental, and safety indicators reported in the literature.

Technology/Study	Representative Quantitative Result	Assessment Basis	Main Comparison Limitation
Mixed-plastic routes (Hernández et al.) [129]	Basic absorption mainly below ~0.1 wt.% PVC under model assumptions; hydrogenolysis most favorable economics; catalytic pyrolysis lowest GWP	TEA + LCA of modeled configurations	Specific feed, products, and process assumptions
Selective dissolution (Yaish et al.) [130]	Mixed environmental trade-offs relative to virgin PVC; no price parity under the assessed conditions	LCA + techno-economic assessment	System-specific solvent/distillation configuration
Mechanochemical Cl recovery (Lu et al.) [131]	Environmental performance sensitive to milling and design conditions	Ex ante LCA	No universal energy burden can be inferred
Hydrothermal (Salimi et al.) [127]	89.42% dechlorination accompanied by measurable Fe release under the high-severity conditions studied	Experimental dechlorination + corrosion	Specific SS-316 reactor and operating conditions
Photothermal (Han et al.) [132]	Modeled energy and carbon-footprint reductions relative to the thermal reference process	Process model	Not conventional TiO2 photocatalysis

; missing values are not estimated solely to complete the comparison.

These results reinforce the need to compare dechlorination routes within clearly defined process boundaries rather than using isolated performance values as universal technology rankings.

9. Conclusions

The increasing implementation of thermochemical recycling technologies has highlighted the need for efficient chlorine management strategies capable of ensuring high-quality products, minimizing equipment corrosion, and preventing catalyst deactivation. As discussed throughout this review, no single pretreatment technology can universally address the wide variability of chlorine-containing plastic waste streams. Instead, each approach offers specific advantages depending on the feedstock characteristics, chlorine content, and the requirements of the downstream conversion process. Physical pretreatments, including sensor-based sorting and density separation, currently represent the most mature and economically viable solutions for reducing chlorine contamination in mixed plastic waste. However, their effectiveness is limited by the heterogeneous composition of post-consumer plastics and by the persistence of residual chlorine within the polymer matrix. Chemical and hydrothermal pretreatments provide substantially higher dechlorination efficiencies by directly removing chlorine from PVC, while catalytic systems offer the additional advantage of simultaneously capturing hydrogen chloride and improving the quality of thermochemical products. Emerging photocatalytic technologies remain at an early stage of development but may become attractive complementary pretreatments as more efficient visible-light-responsive catalysts and continuous reactor configurations become available.

Overall, the literature clearly indicates that future industrial implementation will not rely on a single dechlorination technology but rather on integrated pretreatment strategies combining physical separation, selective chemical or hydrothermal dechlorination, and catalytic polishing according to the characteristics of the incoming waste stream. Such an integrated approach has the potential to maximize chlorine removal while minimizing reagent consumption, catalyst deactivation, corrosion, and the formation of chlorinated by-products, thereby improving the sustainability and economic viability of thermochemical plastic recycling.

Rather than being considered a secondary pretreatment step, chlorine management should be regarded as a key enabling technology for the transition from conventional plastic disposal to high-value thermochemical recycling within a circular economy.