Coagulation–Sedimentation in Water and Wastewater Treatment: Removal of Pesticides, Pharmaceuticals, PFAS, Microplastics, and Natural Organic Matter

Abstract

Coagulation–sedimentation remains a widely used process in drinking and wastewater treatment, yet its performance for emerging contaminants requires further evaluation. This review summarizes recent advances in conventional and novel coagulant systems for the removal of pesticides, pharmaceuticals, per- and polyfluoroalkyl substances (PFAS), natural organic matter (NOM), and micro- and nanoplastics (MNPs). The efficiency of conventional aluminum- and iron-based coagulants typically ranges from 30–90% for NOM and pesticides, 10–60% for pharmaceuticals, <20% for PFAS, and up to 95% for microplastics. Modified and hybrid materials, including titanium-based and bio-derived coagulants, demonstrate superior performance through combined mechanisms of charge neutralization, adsorption, and complexation. The zeta potential of particles was identified as a key factor in optimizing MNP removal. The ability of iron and titanium to form complexes with organic ligands significantly influences the removal of organic pollutants and metal–organic interactions in water matrices. While most research remains at the laboratory scale, promising developments in hybrid and electrocoagulation systems indicate potential for field-scale application. The review highlights that coagulation is best applied as a pretreatment step in integrated systems, enhancing subsequent adsorption, oxidation, or membrane processes. Future studies should focus on large-scale validation, energy efficiency, and the recovery of metal oxides (e.g., TiO₂) from residual sludge to improve sustainability.

1. Introduction

For decades, conventional coagulation–sedimentation processes have been widely applied for the removal of turbidity, iron, and other suspended or colloidal particles from water [1,2,3,4,5,6,7]. These processes are based on destabilization and aggregation of particles through charge neutralization, sweep flocculation, and polymer bridging [8]. The performance of coagulation is influenced by pH, alkalinity, temperature, and the presence of competing ions, all of which control floc size, density, and stability [9]. While these classical applications remain important, they represent only part of the broader role of coagulation in water and wastewater treatment.

In recent years, the scope of coagulation has expanded significantly due to the increasing occurrence of emerging contaminants in aquatic environments. These include natural organic matter (NOM), which acts as a precursor to toxic disinfection by-products (DBPs) [10,11,12], pesticides [13,14,15], pharmaceuticals such as diclofenac, ibuprofen, and carbamazepine [16,17,18,19], per- and polyfluoroalkyl substances (PFAS) [20,21,22,23], and micro- and nanoplastics [24,25,26,27,28,29]. Many of these compounds occur at trace concentrations (ng/L–µg/L) yet raise concerns due to persistence, bioaccumulation, and potential chronic toxicity. Conventional treatment plants are often not optimized for their removal, and coagulation–flocculation processes have therefore been revisited as either direct removal techniques or essential pretreatment steps to enhance subsequent processes such as adsorption, advanced oxidation, and membrane filtration [30,31,32,33,34,35,36,37,38,39].

Recent innovations also include the development of bio-based coagulants (e.g., chitosan, Moringa oleifera) and nanostructured or composite materials, which aim to reduce sludge production, improve removal efficiency, and enhance environmental sustainability [40,41]. Hybrid systems that integrate coagulation with complementary technologies—such as ultrafiltration, ozonation, or biological treatments—have shown synergistic effects, particularly for NOM, pharmaceuticals, and pesticides [42,43,44,45,46,47].

Taken together, these developments highlight the transition of coagulation–sedimentation from a conventional turbidity and iron control method to a versatile, adaptive technology for tackling a wide range of emerging pollutants [48,49,50]. This review therefore synthesizes current knowledge on coagulation–sedimentation processes for the removal of pesticides, pharmaceuticals, PFAS, microplastics, and NOM, with particular emphasis on novel coagulant materials, hybrid treatment strategies, and sustainable approaches in line with circular economy principles. Given these developments, there is a growing need to critically evaluate the role of coagulation–sedimentation in addressing not only traditional contaminants such as iron and turbidity, but also a wide spectrum of emerging pollutants. These include natural organic matter (NOM) and its role as a precursor of disinfection by-products, pesticides, pharmaceuticals, per- and polyfluoroalkyl substances (PFAS), and micro- and nanoplastics [46,51,52,53,54,55,56]. While conventional coagulation remains the cornerstone of drinking water and wastewater treatment, its effectiveness against such contaminants varies widely and is strongly influenced by water matrix characteristics, coagulant type, and operational conditions. Moreover, the development of bio-based coagulants and hybrid systems offers new opportunities to enhance performance while reducing chemical demand and sludge generation [57,58].

The aim of this review is therefore to provide a comprehensive synthesis of current knowledge on coagulation–sedimentation for the removal of emerging pollutants from water and wastewater. Special emphasis is placed on the removal mechanisms and influencing factors, the role of novel and natural coagulants, the integration of coagulation with complementary processes such as adsorption, ozonation, and membrane filtration, and the sustainability dimension, including sludge reuse and circular economy approaches. By identifying knowledge gaps and future research directions, this review seeks to outline the potential of coagulation–sedimentation as a versatile and sustainable technology for tackling the evolving challenges of water quality management. Integration of coagulation–sedimentation with advanced membrane filtration, flotation, and biological methods also remains underexplored, though it has potential to significantly improve performance and cost-effectiveness [59].

Practically, there is a lack of advanced automation and real-time monitoring systems, which limits operational optimization and rapid response to changing water quality [59]. Furthermore, the growing importance of circular economy approaches calls for development of technologies, for example, iron recovery and reuse from treatment by-products. Current methods produce substantial sludge volumes, posing ecological and economic disposal challenges. Future research should focus on sustainable solutions that not only remove contaminants effectively but also enable recycling and waste minimization [60].

Addressing these gaps is essential for advancing innovative, efficient, and sustainable removal technologies that meet increasing quality and environmental demands. Previous reviews have addressed individual contaminant groups or single coagulant types, but few have provided a comprehensive cross-comparison of conventional and novel coagulants across pesticides, pharmaceuticals, PFAS, micro- and nanoplastics, and NOM. This review uniquely integrates a lot of data from studies to identify performance trends, operational gaps, and transition pathways from laboratory to field-scale applications, with particular emphasis on titanium-based and bio-derived coagulants.

2. Development of Technology and New Coagulation Materials

The past decade has witnessed an explosion of novel coagulant materials engineered not only for conventional targets such as turbidity and iron, but increasingly for the removal of emerging contaminants, including pesticides, pharmaceuticals, per- and polyfluoroalkyl substances (PFAS), and micro- and nanoplastics. Among these, nanoscale zero-valent iron (nZVI) has attracted particular attention due to its exceptionally high specific surface area and strong reducing power. Wang et al. (2022) reviewed the synthesis, surface functionalization, and environmental fate of nZVI, showing that unmodified particles can remove a wide range of inorganic and organic contaminants with efficiencies exceeding 90% in bench-scale tests [61]. More recent work has focused on stabilizing nZVI against rapid oxidation and aggregation by supporting it on substrates such as clays, carbon materials, or polymers. These composites prolong particle reactivity, improve sedimentation behavior, and reduce the risk of nanoparticle release into the environment [61,62,63].

Parallel to nanomaterials, bio-based coagulants have re-emerged as eco-friendly alternatives to inorganic salts. Chitosan—a deacetylated derivative of chitin—is biodegradable, non-toxic, and exhibits strong affinity for colloidal particles through charge neutralization and bridging. Pontius (2016) demonstrated that chitosan can achieve turbidity and contaminant removal comparable to ferric chloride, while generating denser flocs and producing sludge with lower metal content [64]. Equally promising is Moringa oleifera seed extract, whose cationic proteins effectively neutralize negatively charged colloids. A recent study by Silva et al. (2024) reported over 90% turbidity reduction at pH 6–7 using defatted Moringa powders, with negligible impacts on treated-water pH, although some increase in residual organics was observed [57,58].

Beyond single-component systems, composite and hybrid coagulants leverage synergistic interactions among metals, polymers, and porous supports. Ferric–polymer hybrids combine the rapid hydrolysis of Fe³⁺ salts with the bridging action of synthetic polymers such as polyacrylamide, yielding flocs that are both dense and deformable—ideal for compact treatment units. A recent review highlighted “one-plus-one-greater-than-two” effects when these hybrids were optimized, achieving up to 95% removal of suspended solids with lower coagulant doses [65]. Similarly, iron–zeolite composites embed Fe(III) species within natural or synthetic zeolites, producing adsorptive–coagulant matrices capable of simultaneously removing colloids, dissolved metals, and organic micropollutants, with removal capacities for Fe³⁺ exceeding 15 mg/g in batch studies [66].

While these innovations hold great promise, their environmental footprints must be carefully assessed. Life-cycle analyses of nZVI production reveal that raw-material extraction and nanoparticle synthesis can contribute substantially to energy use and greenhouse-gas emissions, although these impacts may be offset by reduced sludge generation and higher contaminant removal rates in operation. Bio-based coagulants typically exhibit lower cradle-to-grave impacts, but they may introduce biodegradable organics that require further treatment. Composite materials provide a balance by reducing overall chemical use, but often involve more complex manufacturing routes. Standardized protocols for ecotoxicity testing and life-cycle impact assessment are therefore needed to ensure that next-generation coagulants are not only effective but also truly sustainable. In summary, nanoscale, bio-based, and hybrid coagulants represent a new generation of materials capable of addressing both conventional pollutants and emerging contaminants [8]. Laboratory studies have shown promising results, but future research must focus on their long-term stability, large-scale applicability, and integration with advanced treatment technologies to realize their full potential in sustainable water and wastewater purification [44,67,68,69].

Titanium-Based Coagulants and Hybrid Reagents

In recent decades, titanium-based coagulants have attracted growing attention as an alternative to conventional aluminum- and iron-based salts. Their main advantage lies in the low toxicity of titanium, which is not regulated by most water-quality standards. Because Ti(IV) undergoes complete hydrolysis and forms poorly soluble Ti(OH)₄, the residual metal concentrations in treated effluents are considerably lower than those obtained with Al- or Fe-based coagulants. Moreover, the sludge generated during titanium coagulation can be reused for producing high-value materials such as TiO₂ photocatalysts. The use of titanium coagulants may therefore help mitigate health concerns associated with aluminum residues and the operational problems caused by excess iron in subsequent treatment stages [70]. Despite their high treatment efficiency and the absence of many limitations typical of conventional coagulants, the widespread application of titanium reagents is constrained by the relatively high cost of titanium compounds. In a recent study [71] a concept was proposed for synthesizing complex titanium-containing reagents using chemical dehydration processes. The optimal coagulant composition was determined to include a mixture of aluminum sulfate (50–87 wt%), aluminum chloride (10–40 wt%), and oxysulfate titanate (2.5–10 wt%), with an optimal titanium content of 5.0–7.5 wt%. Comparison of these newly synthesized reagents with conventional aluminum and iron coagulants showed that the use of complex titanium coagulants not only reduced residual contaminant concentrations (phosphates, turbidity, and organic matter) but also lowered reagent consumption by 20–25% and enhanced sludge sedimentation and filtration performance [71]. Titanium-based coagulants, such as TiCl₄ and polymerized titanium compounds (PTC/Ti-polymers), have emerged as promising alternatives to traditional Fe- and Al-based salts. Several studies have reported that they performed better in terms of turbidity, UV₂₅₄, and DOC removal than FeCl₃ at equivalent doses [72]. In marine waters, TiCl₄ more effectively removes hydrophilic fractions of dissolved organic matter (DOM)—including humic substances and low-molecular-weight neutrals—than FeCl₃, while the resulting sludge can be valorized to recover TiO₂ with an anatase phase [73].

Research on algal bloom-affected waters has shown that both TiCl₄ and PTC effectively reduce algal organic matter (AOM) and improve the performance of downstream ultrafiltration (UF) and seawater RO systems [74]. In model humic acid solutions, newly developed PTC coagulants demonstrated favorable floc formation kinetics and mechanical stability compared to polyaluminum chloride (PAC), confirming their potential for enhanced natural organic matter removal.

The efficiency of TiCl₄ coagulation and the resulting floc characteristics are significantly influenced by general water hardness and ionic strength—factors that should be considered when designing optimal operating conditions [72]. A notable advantage of the Ti-salt approach is the potential for sludge utilization: after incineration, the residues can yield valuable TiO₂. It has been estimated that a medium-sized wastewater treatment plant could produce ~446.5 kg TiO₂ per year from recovered sludge [75].

Under conditions of low alkalinity and high dissolved organic carbon, titanium coagulants (including TiCl₃) can outperform conventional salts in removing hydrophobic compounds and suspended solids. Furthermore, complex Al–Ti coagulants combining aluminum sulfate (50–87 wt%), aluminum chloride (10–40 wt%), and oxysulfate titanate (2.5–10 wt%)—with an optimal titanium addition of 5.0–7.5 wt%—have been developed, merging the advantages of both aluminum and titanium systems [71]. Spectroscopic analyses (FTIR/XPS) revealed pH-dependent binding interactions between Al–Ti coagulants and humic acids, enabling process optimization through adjustment of solution chemistry [76].

Additionally, TiCl₄ has proven effective for removing composite pollutants, such as AgNP–humic acid systems, in combined coagulation–UF setups, substantially reducing membrane fouling [73]. Overall, the literature indicates that titanium salts can combine high purification efficiency with valuable sludge reuse through TiO₂ recovery, while their performance strongly depends on pH, water or wastewater composition, and coagulant formulation [70]. Titanium-based reagents constitute a significant class of modern coagulants in recent water treatment research.

Table 1. Comparison of coagulant types and key properties.

Coagulant Type	Main Composition/Examples	Key Mechanism	Typical Removal Efficiency	Advantages	Limitations
Aluminum-based	Al2(SO4)3, PACl	Charge neutralization, sweep flocculation	60–90% (NOM, turbidity)	Widely used, cost-effective	Limited removal of emerging pollutants
Iron-based	FeCl3, Fe2(SO4)3	Complexation with organics, charge neutralization	50–85% (pesticides, NOM)	Effective for organic-rich waters	Produces more sludge
Titanium-based	TiCl4, TiCl3	Hydrolysis → Ti(OH)4, adsorption	70–95% (organics, microplastics)	High activity, potential sludge reuse	Cost, limited full-scale use
Bio-based	Chitosan, Moringa oleifera	Bridging, hydrogen bonding	50–90% (dyes, NOM, microplastics)	Renewable, biodegradable	Variable efficiency
Composite/hybrid	Fe–Ti, Al–bio	Combined mechanisms	70–98%	High adaptability	More complex synthesis

presents a comparison of coagulant types and key properties.

3. Organic Pollutants Removed by Coagulation

While coagulation was traditionally developed for the removal of turbidity and iron, its role has progressively expanded to address a broad spectrum of organic pollutants present in water and wastewater. Many of these contaminants—including natural organic matter (NOM), pesticides, pharmaceuticals, per- and polyfluoroalkyl substances (PFAS), and micro- and nanoplastics (MNPs)—occur at trace levels (ng/L–µg/L), yet pose significant risks due to persistence, bioaccumulation, and potential chronic toxicity [77,78,79,80,81]. Conventional treatment plants are often not optimized for their elimination, which has prompted renewed interest in coagulation–flocculation as either a direct removal mechanism or as a pretreatment step that enhances subsequent technologies such as adsorption, membrane filtration, or advanced oxidation. Recent advances in coagulant development—ranging from nanomaterials and hybrid composites to bio-based alternatives—open new opportunities to improve the efficiency and sustainability of coagulation for these pollutants. However, the effectiveness of the process remains highly dependent on pollutant characteristics (e.g., hydrophobicity, molecular size, charge), water matrix composition, and operational parameters such as pH, coagulant type, and dosage. This chapter synthesizes the current state of knowledge on coagulation–sedimentation for the removal of emerging organic contaminants. Special focus is placed on five critical groups: natural organic matter as a precursor of disinfection by-products (DBPs), pesticides, pharmaceuticals, per- and polyfluoroalkyl substances, and micro- and nanoplastics. For each group, removal mechanisms, influencing factors, and recent innovations—including hybrid processes and novel coagulant materials—are discussed to provide a comprehensive picture of both challenges and opportunities.

Table 2. Summary of removal efficiencies for emerging pollutants.

Contaminant Group	Representative Compounds	Typical Coagulants Studied	Removal Efficiency Range	Notes
PFAS	PFOA, PFOS	FeCl3, TiCl4, PAC	<20–60%	Improved with electrocoagulation
NOM/DOM	Humic acids, fulvic acids	Al2(SO4)3, PACl, TiCl4	50–90%	Strong dependence on pH and dose
Micro- & nanoplastics	PE, PS, PET	FeCl3, chitosan, TiCl4	70–99%	Removal enhanced at zeta ≈ 0 mV
Pesticides	Atrazine, glyphosate	FeCl3, PAC, TiCl4	40–95%	Electrocoagulation most effective
Pharmaceuticals	Diclofenac, carbamazepine	FeCl3, PAC, bio-based	10–80%	Strongly compound dependent

presented summary of removal efficiencies for emerging pollutants.

3.1. PFOS and PFOA

Per- and polyfluoroalkyl substances (PFAS; also PFASs, colloquially the “forever chemicals”) are synthetic organofluorine compounds with multiple fluorine atoms attached to an alkyl chain [82,83]. According to PubChem, roughly 7 million such substances have been described [84]. Since the advent of Teflon (1938), PFAS have been used to make fluoropolymer coatings and materials resistant to heat, grease, stains, and water; today they occur in water-repellent apparel, carpets, cosmetics and personal-care products, paints, furniture, adhesives, food packaging, firefighting foams, electronics, and consumer coatings [84,85]. Many PFAS—especially PFOS and PFOA—are persistent environmental contaminants; the media often refer to them as “forever chemicals”. Long human half-lives (even > 8 years) stem from the exceptionally strong C–F bond [86]. These compounds migrate through soils, bioaccumulate in fish and wildlife, and residues are now detected in rainwater, drinking water, and wastewater [87]. Because of their high mobility, they can enter via the skin and the lacrimal ducts; lip-applied products may be inadvertently ingested. The sheer number and diversity of PFAS hinder comprehensive risk assessment, underscoring the need for continued, targeted research [85,87]. Exposure to PFAS—some classified as carcinogenic and/or endocrine-active—has been linked to kidney, prostate, and testicular cancers; ulcerative colitis; thyroid disease; reduced antibody response/immune suppression; decreased fertility; pregnancy-related hypertension disorders; impaired fetal and infant growth; developmental problems; obesity; and dyslipidemia [88]. Regulatory action proceeds under the Stockholm Convention (since 2009), and jurisdictions such as China and the EU have announced further restrictions and phase-outs. At the same time, several major producers/users (including the United States, Israel, and Malaysia) have not ratified the agreement; the chemical industry has lobbied to relax provisions or shifted production to countries with less stringent rules (e.g., Thailand) [20,22,89].

Coagulation is the first unit process that encounters the mixture of contaminants—including PFAS on WTPs. Although coagulation is not purpose-built for micropollutant removal, understanding the fate of PFAS during coagulation is essential to judge whether treatment residuals can act as a significant sink for these compounds. Researchers [90] compared the removal of PFOA and PFOS from real waters using four metal coagulants (Al, Fe, Zr, Zn) and found a clear performance order: Al > Fe > Zr > Zn. For aluminum, at pH < 5.5 and doses > 5 mg·Al·L⁻¹, measurable decreases were observed (≈>15% for PFOA and >30% for PFOS), with sorption/hydrophobic attachment of PFAS to forming flocs as the dominant pathway. To probe matrix effects, the authors dosed five representative organic compounds. High-molecular-weight (HMW) organics enhanced PFAS capture—first binding PFAS and then co-removing them with the flocs—whereas low-molecular-weight (LMW) DOM, which coagulates poorly, suppressed PFAS removal. Under optimized aluminum conditions, maximum reductions reached about 23% (PFOA) and 56% (PFOS) [90].

Xiao et al. [91,92] highlighted that PFOS (perfluorooctane sulfonate) and PFOA (perfluorooctanoic acid) are persistent, widely detected contaminants. In their study, they charted the “removal domains” of these polar compounds on a coagulation phase map that links operating chemistry to dominant coagulation mechanisms. The variables examined included solution pH, coagulant dose, coagulant type (alum and ferric chloride), natural organic matter (NOM), initial turbidity, and flocculation time. Jar tests showed that conventional coagulation—alum at 10–60 mg/L with a final pH of 6.5–8.0—achieved at most ~20% removal of PFOS and PFOA. Better performance occurred under enhanced coagulation, i.e., higher alum doses (>60 mg/L) with lower terminal pH (4.5–6.5). From these results, the authors proposed a dose–pH coagulation map indicating the conditions under which PFOS/PFOA removal is most likely. Mechanistically, the dominant pathway was sorption of PFOS/PFOA onto nascent Al(OH)₃ microflocs formed during the initial hydrolysis of alum; extending flocculation from 2 to 90 min did not yield additional removal [91,93].

Researchers Chen et al. [94] became interested in whether PFAS can indirectly alter the performance of conventional drinking-water treatment by affecting algae present in source waters. They collected raw water from a representative drinking-water source in Beijing (China) and spiked two representative PFAS at environmentally relevant levels. Bench-scale tests coupling coagulation with ultrafiltration (UF) showed that PFAS-amended waters formed larger flocs and experienced more severe UF fouling than PFAS-free controls. The strongest response appeared at the lower dose (0.1 μg/L), where mean floc size increased ~1.6× and membrane flux declined by >10%. Mechanistic probes indicated that these effects were driven not by direct PFAS–coagulant interactions, but by an algal stress response that boosted secretion of extracellular biopolymers—chiefly polysaccharides—promoting both floc growth and cake buildup on the membrane. Overall, the team concluded that PFAS in surface waters can exert dual, indirect impacts on treatment trains: enhancing clarification via larger flocs while worsening UF fouling, a trade-off operators should factor into process control and optimization [94,95].

Researchers Kim et al. [96] conducted a systematic investigation of electrocoagulation (EC) for removing perfluorooctanoic acid (PFOA) from water using an iron (Fe) electrode. They evaluated how key operating variables—current density, stirring speed, and electrolyte concentration (NaCl)—governed PFOA removal kinetics and efficiency. Increasing the current density from 2.4 to 80.0 mA·cm⁻² raised the 6-h removal from roughly 10% to ~100%. During EC, the team detected formate (HCOO⁻) and three short-chain perfluorocarboxylates—PFPeA, PFHxA, and PFHpA—as organic products. This pattern is consistent with stepwise C–C scission of the perfluoroalkyl chain and subsequent degradation to shorter carbon-chain species, with defluorination occurring in parallel. After 6 h, about 65% of fluorine was recovered (as F⁻ and as organic fluorine in the shorter PFCAs), alongside a ~60% reduction in TOC, while PFOA was completely removed from solution [96,97]. Overall, the results show that Fe-electrode electrocoagulation can efficiently transform PFOA into shorter perfluorinated by-products with substantial mineralization, and that performance is strongly controlled by electrical and hydraulic operating conditions [96]. Researchers Maroli et al. [98] investigated whether modifying conventional coagulation with a cationic surfactant could overcome the well-known limitations of the process for PFAS removal. Although coagulation/flocculation is widely applied in water and wastewater treatment because of its simplicity and low cost, it performs poorly for PFAS. The team conducted jar tests with two short-chain and two long-chain PFAS (each at 10 μg·L⁻¹), using cetyltrimethylammonium chloride (CTAC) as a coagulation aid. They also evaluated elevated coagulant doses: 60 mg·L⁻¹ alum and 100 mg·L⁻¹ ferric chloride (FeCl₃).

Their results show that combining a higher coagulant dose with 1 mg·L⁻¹ CTAC markedly enhanced PFAS capture: PFBS (short-chain sulfonate) exceeded 40% removal, while PFOA/PFOS (long-chain) achieved >80%, with FeCl₃ outperforming alum. A clear functional-class effect emerged: sulfonates (PFBS, PFOS) were removed more efficiently than carboxylates (PFBA, PFOA), consistent with greater hydrophobicity and stronger interactions with forming flocs. Moreover, pairing CTAC with powdered activated carbon (PAC) produced a synergistic response, increasing the removal of PFBS, PFOA, and PFOS to >98%. Taken together, the findings indicate that a cost-effective, cationic-surfactant pre-dose can substantially improve PFAS treatment in existing coagulation/flocculation trains—an approach that is readily retrofittable to conventional plants.

Table 3. PFAS removal by coagulation and process modifications—literature snapshot.

No.	Water Type	Coagulant & Dose	pH/Conditions	Mechanism/Notes	Removal Efficiency	Ref.
1	Surface water (intake)	PACl ≈ 10 mg/L	Optimized near-neutral (jar tests)	Adsorption on freshly formed Al(OH)3; charge neutralization; PACl pre-hydrolyzed cations	PFOA > 90%	[99]
2	Drinking water matrix (jar tests with NOM)	Alum 10–60 mg/L (conventional); >60 mg/L (enhanced)	Final pH 6.5–8.0 (conventional) vs. 4.5–6.5 (enhanced)	Sorption to nascent Al(OH)3 microflocs during early hydrolysis; flocculation time (2–90 min) not rate-limiting	≤20% (conventional); higher under enhanced coagulation	[91]
3	Surface water (bench-scale)	FeCl3 100 mg/L + CTAC 1 mg/L; optional PAC	Near-neutral; cationic surfactant as aid; PAC co-dosing	Cationic-surfactant pairing & hydrophobic association; enhanced charge reversal; PAC synergy	PFOA/PFOS > 80%; with PAC + CTAC: PFBS/PFOA/PFOS > 98%	[98]
4	Synthetic & natural waters	FeCl3·6H2O 50 mg/L; bio-coagulant	Typical pH 5–8; jar tests	Fe(OH)3 sweep/co-adsorption; cationic proteins (bio-coagulant) enable bridging	FeCl3: PFOS ~32%, PFOA ~12%; Moringa: PFOS ~65%, PFOA ~72%	[100]
5	Coagulation with additives	Alum or FeCl3 + CTAB 0.58–0.87 mg/L; or PAC ≥ 40 mg/L	Near-neutral; additive-assisted coagulation	Cationic surfactant–PFOS association improves capture by flocs; PAC provides adsorptive sites	PFOS > 90–95% (CTAB ≥ 0.58–0.87 mg/L); with PAC ≥ 40 mg/L also >90%	[101]

presents few examples from literature review for PFAS removal.

3.2. Natural and Dissolved Organic Matter (NOM/DOM)

Natural organic matter (NOM) is a complex mixture of organic compounds commonly found in surface and groundwater. It consists mainly of two fractions: non-humic substances (e.g., amino acids, polysaccharides, hydrocarbons, lipids, and low-molecular-weight acids) and heterogeneous humic substances. The composition and concentration of NOM depend on watershed characteristics, seasonal changes, weather events, and anthropogenic activity.

NOM properties are closely linked to its origin and the biogeochemical processes occurring in aquatic systems. It reacts with oxidants and disinfectants such as chlorine, chlorine dioxide, and ozone, contributing to the formation of disinfection by-products (DBPs). Therefore, effective NOM removal is a key objective in water treatment. However, its molecular diversity, colloidal stability, and variable hydrophobic–hydrophilic balance make it difficult to eliminate using conventional processes such as coagulation and adsorption [102,103,104]. Natural organic matter (NOM) is commonly found in soils, waters, and sediments. Its main sources are terrestrial plant residues as well as metabolic products of bacteria, algae, and aquatic plants. NOM is highly diverse, including humic substances, carbohydrates, proteins, lipids, and organic acids. Its amount and properties depend on origin, degree of degradation, and environmental factors such as climate, geology, and topography. In surface waters, NOM consists of hydrophobic fractions (rich in aromatic structures) and hydrophilic fractions (aliphatic and nitrogenous compounds such as sugars and amino acids). The largest share of dissolved organic carbon comes from humic substances—humic acids, fulvic acids, and humins. The presence of NOM in drinking water causes various issues: it gives raw water color, taste, and odor, increases chemical demand for oxidation, coagulation, and disinfection, and promotes the formation of disinfection by-products (DBPs) such as trihalomethanes and haloacetic acids. Some brominated DBPs are particularly toxic. NOM also clogs filtration membranes, stimulates microbial growth in distribution systems, and facilitates the transport of heavy metals and organic pollutants [105,106].

The literature provides numerous methods for removing NOM from water and wastewater with varying degrees of effectiveness [105]. These include adsorption, activated carbon [107], modified flu ash waste [108], bentonite as an adsorbent, and natural agricultural waste materials as adsorbents [109,110]. From 30 to 90% of NOM can be removed from drinking water by ion exchange treatment [111]. The most widely used approach to remove natural organic matter (NOM) in drinking water treatment is coagulation–flocculation, typically followed by sedimentation or flotation, filtration, and disinfection. While primarily aimed at reducing turbidity, this process also eliminates part of NOM, especially hydrophobic fractions, and is considered the standard method worldwide. There is no single process dedicated solely to removing TOC or DOC; instead, their reduction occurs incidentally through different treatment steps. More advanced or alternative options include adsorption, membrane processes, ion exchange, biofiltration, advanced oxidation processes (AOPs), or combined methods such as AOP coupled with biologically activated carbon (BAC) [106,112]. Wang et al. [45] investigated the formation and speciation of disinfection by-products (DBPs) during chlorination of different treated effluents, including raw water (RW), ultrafiltration (UF), coagulation water (CW), and combined coagulation–ultrafiltration (CW-UF). Results showed that advanced treatments substantially reduced DBP formation, with CW-UF achieving the highest overall removal efficiency (69.5%). Among carbonaceous DBPs (C-DBPs), trichloromethane (TCM) was the dominant species (~40% of total THMs). DBP concentrations followed the order RW > UF > CW > CW-UF, consistent with residual SUVA values and the removal of fulvic- and humic-like components, indicating that lower aromaticity decreases DBP yields. For nitrogenous DBPs (N-DBPs), dichloroacetonitrile (DCAN) was the most abundant species, while trichloronitromethane (TCNM) was poorly removed (<10%) by either coagulation or ultrafiltration alone. CW-UF reduced halogenated N-DBPs by 52–62% compared with RW and enhanced precursor removal by 29.4% and 15.5% relative to UF and CW alone, respectively. The combined process showed higher DOC and UV254 removal efficiencies due to synergistic effects between coagulation (removing high-MW hydrophobic fractions) and ultrafiltration (eliminating residual low-MW organics < 5 kDa). Overall, CW-UF improved water safety by decreasing DBP formation potential by 30.8% and 16.9% compared with UF and CW, respectively. It should also be noted that iron can form stable complexes with organic ligands, including pesticides and pharmaceutical residues, which may influence their speciation, mobility, and removal efficiency during coagulation. TCM and DCAN were identified as the dominant C-DBP and N-DBP species. DBP yields increased with contact time, whereas chlorine dosage had only minor influence. These findings highlight CW-UF as an effective strategy to mitigate DBP risks in municipal water treatment [45].

These findings underline the dual importance of carbonaceous and nitrogenous DBPs in drinking water safety. While C-DBPs such as trichloromethane (TCM) are typically more abundant and have been long regulated, N-DBPs including dichloroacetonitrile (DCAN) are often more toxic and remain less well controlled. To illustrate these distinctions, Figure 1 provides a comparative overview of C-DBPs and N-DBPs, highlighting their representative species, typical precursors, and relative health implications [106,113].

Wang et al. [114] investigated the effects of ozonation and powdered activated carbon (PAC) on the removal of dissolved organic matter (DOM) and the formation of disinfection by-products (DBPs) in reservoir water. They reported that trihalomethanes (THMs) and haloacetonitriles (HANs) were the dominant carbonaceous and nitrogenous DBPs, respectively. The integrated coagulation–PAC process removed more than 70% of THM precursors and 93% of HAN precursors, compared with only 10.5% and 45% removal achieved by coagulation alone. PAC was particularly effective in adsorbing low-molecular-weight aromatic proteins, and its performance improved by ~10% when combined with pre-ozonation. Ozonation decreased the formation of HANs but simultaneously increased the yields of more toxic brominated THMs (from 78.5 to 128.1 μg/L). Kinetic analysis showed that THM precursors with high reactivity were readily removed by both coagulation and PAC adsorption. Pre-ozonation effectively oxidized hydrophobic NOM with aromatic structures into more hydrophilic substances, enhancing subsequent removal but also favoring bromine substitution during chlorination. Overall, Wang et al. demonstrated that combined ozonation and PAC treatment substantially reduces HAN formation, while careful control is needed to mitigate the simultaneous increase in brominated THMs [114]. Because of its significant impact on water quality and treatment processes, NOM removal has become a major challenge in modern water purification technologies. Zhao et al. [72] investigated the effects of total hardness and ionic strength on TiCl₄ coagulation efficiency and floc properties during the removal of natural organic matter (NOM). Experiments with model waters containing humic acid (HA) and fulvic acid (FA) showed that increasing hardness and ionic strength enhanced both particle and NOM removal, as well as improved floc characteristics such as size, compactness, and mechanical strength. In the combined coagulation–ultrafiltration (C–UF) process, TiCl₄ pre-coagulation significantly reduced membrane fouling, while higher hardness and ionic strength further increased permeate flux, particularly in HA-rich waters [72]. Following the demonstrated efficiency of TiCl₄ in improving floc characteristics and reducing membrane fouling, attention has also turned to the control of residual dissolved organic matter (DOM), which contributes to the formation of disinfection by-products (DBPs) and to secondary microbial regrowth within water distribution systems. Conventional coagulation processes employing aluminum or iron salts enable only partial removal of dissolved organic matter.

To address this limitation, the performance of titanium(III) chloride (TiCl₃) was evaluated for dissolved organic carbon (DOC) removal and compared with that of aluminum sulfate (alum). Hussain et al. [115] jar-test experiments demonstrated that TiCl₃ achieved the highest DOC removal efficiency at approximately pH 3, where charge neutralization was the dominant coagulation mechanism, as confirmed by the formation of larger and denser flocs. At pH 4.5, particle destabilization proceeded mainly through adsorption and enmeshment within the precipitate matrix. Fluorescence analysis revealed that TiCl₃ removed humic-like fractions more effectively than alum. Moreover, the application of a two-stage system combining aluminum and TiCl₃ coagulants further improved overall DOC removal. These results confirm the potential of TiCl₃ as an alternative coagulant, particularly for low-alkalinity waters with high DOC concentrations and low pH values [115]. Building on these observations, researchers [73] investigated the comparative performance of titanium(IV) chloride (TiCl₄) and ferric chloride (FeCl₃) in removing different fractions of natural organic matter (NOM) from seawater. The characterization of organic fractions was carried out using liquid chromatography coupled with an organic carbon detector (LC–OCD). Both coagulants effectively removed hydrophobic compounds; however, TiCl₄ exhibited significantly higher efficiency in eliminating hydrophilic fractions, such as humic substances and low-molecular-weight neutrals. The removal efficiency of these fractions increased with higher TiCl₄ doses, achieving complete elimination of biopolymers and a 63.6% reduction in humic substances, compared to 26.5% for FeCl₃. Both coagulants showed limited efficiency in removing the so-called “building blocks” of NOM, yet TiCl₄ more effectively eliminated the LMW neutral fractions responsible for membrane fouling. The sludge obtained after TiCl₄ coagulation was calcined to produce TiO₂ nanoparticles with an anatase structure doped with silicon, as confirmed by XRD and SEM/EDS analyses. These findings demonstrate that TiCl₄ is a highly effective coagulant for removing hydrophilic organic fractions from seawater while also enabling the recovery of a valuable by-product—TiO₂ nanomaterials [73].

3.3. Micro- and Nanoplastics (MNPs)

Microplastics (MPs, <5 mm) and nanoplastics (NPs, <1000 nm) are small plastic particles formed through the degradation and fragmentation of larger plastic materials or directly released as primary particles from industrial and household sources [116,117]. They consist mainly of synthetic polymers such as polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), and polyethylene terephthalate (PET), often containing additives like plasticizers, dyes, and flame retardants. Due to their small size, large surface area, and chemical stability, MPs and NPs persist in aquatic environments and are difficult to remove during conventional water and wastewater treatment processes. They can adsorb heavy metals, hydrophobic organic compounds, and pathogens, acting as vectors that facilitate the transport of pollutants through water systems. Moreover, their presence in treated effluents and sludge may lead to secondary contamination, posing risks to aquatic organisms and potentially entering the food chain [118].

Bayarkhuu et al. [54] investigated the application of coagulation–sedimentation for nano- and microplastic removal from drinking water. Instead of conventional jar tests, which require large volumes of plastic specimens and generate secondary waste, they employed a small-scale turbidity monitoring approach (~15 mL). This method enabled rapid screening of coagulant type, dosage, settling time, and water chemistry. Their results showed that hydrophobic interactions play a key role in the coagulation of nano-sized plastics, and that turbidity changes correlate linearly with plastic concentration. Optimal conditions identified through turbidity monitoring were successfully transferred to jar tests, achieving high removal efficiencies. Overall, authors demonstrated that turbidity-based monitoring provides an efficient tool for optimizing treatment processes aimed at mitigating nano- and microplastic pollution in drinking water [54]. Zhang et al. [119] investigated the removal of polystyrene nanoplastics (50–1000 nm) using coagulation with polyaluminum chloride (PAC) and polyacrylamide (PAM). They reported a maximum removal efficiency of ~98% under pH 8.0, 0.4 g·L⁻¹ PAC, and 20 mg·L⁻¹ PAM. Humic acid was found to inhibit removal through competitive adsorption, while XDLVO analysis indicated that charge neutralization and Al–O bond formation governed particle destabilization. Excessive PAM increased cluster size and solution viscosity, reducing settling efficiency. Overall, the study highlighted that chemical coagulation is an effective pathway for nanoplastic removal, provided that coagulant type and water chemistry are carefully optimized [119]. Kouchakipour et al. [29] investigated the use of an innovative Fe₃O₄/C nano-adsorbent, synthesized from waste printer toner, for removing polystyrene nanoplastics (PS-NPs) via magnetic adsorption coagulation (MAC) with polyaluminum chloride (PACl). Under optimized conditions (pH 8, 2.5 mg adsorbent, 17.5 mg/L PACl), removal efficiencies reached ~99%, with adsorption governed by bridging and electrostatic interactions. The process followed pseudo-second-order kinetics and Langmuir isotherm behavior, with a maximum adsorption capacity of 523 mg/g and endothermic characteristics. Importantly, the Fe₃O₄/C adsorbent showed good reusability and stability, highlighting its potential to lower coagulant demand and enhance micro/nanoplastic removal efficiency [29]. Azizi et al. [41] investigated microplastic removal by coagulation through a two-phase study combining a systematic review and bench-scale experiments. In the review phase, databases such as Web of Science, Scopus, and PubMed were searched up to March 2021, yielding 104 articles, of which 14 were analyzed to identify key variables for experimental design. The laboratory phase assessed the removal of polyethylene (PE), polystyrene (PS), and polyamide (PA) using five coagulants (PAC, FeCl₃, AlCl₃, Al(OH)₃, and Al₂(SO₄)₃). Statistical analyses (ANOVA and Kruskal–Wallis) showed significant differences in removal efficiencies among microplastic types, averaging 65% for PA, 22% for PS, and 12% for PE—substantially lower than values reported in the reviewed literature (78% for PS and 52% for PE). No significant differences were found between coagulants, suggesting that the most efficient option is the one requiring the lowest dose, identified as Al(OH)₃ in this study [41]. Ojha et al. [120] investigated coagulation and flocculation as important physico-chemical techniques for nanoplastic and microplastic (NMP) removal from water. Conventional Fe- and Al-based coagulants (e.g., Al₂(SO₄)₃, FeCl₃) combined with flocculants such as polyacrylamide (PAM) promote particle destabilization, charge neutralization, and aggregation into flocs that can be removed by sedimentation or flotation. In addition to classical coagulants, recent studies tested novel materials such as magnetic MgO, tannic acid, and protein amyloid fibrils, which can enhance particle bridging and offer greener alternatives to synthetic chemicals [121,122]. Padervand et al. [123] reported that Al salts removed polyethylene MPs < 0.5 mm more effectively than Fe salts, with PAM substantially boosting performance. Notably, anionic PAM increased removal by >35% compared to cationic PAM under high Al dosage, underscoring the role of polymer type, pH, and coagulant concentration. Electrocoagulation (EC) has also shown high potential. Perren et al. [124] achieved >90% removal of PE MPs (up to 99% at pH 7.5 with NaCl addition), with sacrificial Al/Fe electrodes generating in situ hydroxide coagulants. EC offers advantages such as reduced sludge, lower energy demand (11 A/m² yielding highest efficiency), and compatibility with anodic oxidation to minimize fouling. Overall, both classical coagulation–flocculation and emerging alternatives—including EC and novel coagulants—represent effective NMP remediation strategies, provided that operational factors (coagulant type, pH, dosage, mixing conditions) are carefully optimized. Among chemical methods, coagulation–flocculation achieved promising results through charge neutralization and particle aggregation, though removal efficiency varied with particle size, concentration, and coagulant dosage. He et al. [125] investigated the removal efficiency and mechanisms of polyethylene microplastics (PE) and norfloxacin (NOR) during coagulation using polyaluminum chloride (PAC) and anionic polyacrylamide (APAM). Complex pollutants in water—particularly those involving microplastics (MPs)—pose a significant challenge for modern water treatment technologies. Microplastics, as an emerging class of contaminants, have attracted increasing attention due to their widespread occurrence and persistent environmental impact. In the combined PAC–APAM system, a substantial improvement in PE removal (>99%) was observed, while NOR elimination slightly decreased to approximately 42%, regardless of the presence of APAM. SEM, FTIR, zeta potential measurements, and ANOVA analyses revealed that single-contaminant removal occurred mainly through charge neutralization and sweep flocculation (for PE) and adsorption of Al–NOR complexes. In mixed systems, the enhanced PE removal efficiency was attributed to strengthened charge neutralization and the formation of ternary PE–NOR–Al complexes. The highest removal efficiencies for both PE and NOR were achieved under neutral to mildly alkaline conditions, whereas the presence of metal ions and humic acid exhibited inhibitory and stimulatory effects, respectively. These findings provide new insights into the coagulation mechanisms governing the removal of complex pollutants involving microplastics [125]. These findings highlight the importance of understanding how different coagulant systems interact with complex contaminant mixtures containing microplastics.

For inorganic and polymeric coagulants, a wide range of removal efficiencies has been reported: aluminum and iron salts (30–95%), alum (up to 99%), polyaluminum chloride (13–97%), magnesium hydroxide (~84%), polyamines (~99%), organosilanes (>95%), and polyacrylamide (85–98%). Increasing attention has also been given to natural coagulants such as chitosan, protein amyloid fibrils, and starch, which can achieve high microplastic removal efficiencies (>90%) while offering improved environmental compatibility. The dominant removal mechanisms for these materials involve surface charge neutralization and polymer bridging between microplastic particles and coagulant flocs. Operational parameters such as pH, ionic strength, and mixing intensity play a crucial role in determining treatment efficiency. In particular, pH controls the surface charge (zeta potential) of microplastic particles, influencing their destabilization and aggregation behavior. The best results are generally obtained when the zeta potential approaches zero, indicating effective charge neutralization [126]. These findings confirm that zeta potential measurements remain a valuable diagnostic tool for optimizing microplastic coagulation, although their application at full scale remains challenging. Overall, natural and hybrid coagulants represent promising and more sustainable alternatives for microplastic removal, although further research is needed to scale these solutions from laboratory experiments to full-scale treatment systems [41]. While the performance of various coagulant types has been widely examined, the aggregation behavior of micro- and nanoplastics remains a major environmental concern that requires further mechanistic investigation. Despite their extensive industrial use, these particles exhibit complex surface-charge interactions in aquatic environments, which strongly influence their stability and removal efficiency. In conventional water treatment processes, the zeta potential (ζ) is commonly used to determine optimal coagulation conditions; however, its application in field operations is often challenging. To address this limitation, researchers [127] evaluated the applicability of the zero point of charge (ZPC), associated with the isoelectric point (IEP; ψpI), as an alternative indicator for microplastic aggregation. Under controlled laboratory conditions, ψpI values were determined for polyethylene (PE) and polyvinyl chloride (PVC) at pH 6.59 and 6.43, respectively. The removal efficiency (r) of microplastics was found to depend on the initial pH, polyaluminum chloride (PAC) dosage, and particle size. The highest attachment efficiency (αE = 0.14–0.4) and removal rates (r = 0.04–0.84) were observed at pH values close to the isoelectric point (6–8). Model simulations confirmed a strong correlation (>95%) between αE and r, indicating that ψpI can serve as a reliable indicator of aggregation behavior governed by pH and ionic strength. The use of IEP as an alternative to zeta potential measurements simplifies the assessment of microplastic coagulation processes, although its accuracy may still be affected by pH, coagulant dosage, and particle characteristics [127].

3.4. Pesticides

Pesticides comprise a broad group of chemical compounds, including organophosphorus and organochlorine insecticides, carbamates, dithiocarbamates, arylalkanoic acid derivatives, thiazine and nitrophenol derivatives, urea and uracil derivatives, as well as organomercury, tin, and copper compounds, and pyrethroids. These substances are characterized by high persistence, toxicity, and bioaccumulation potential, leading to long-term contamination of surface and groundwater resources and interference with biological wastewater treatment processes. Even trace concentrations of pesticides can adversely affect activated sludge microorganisms, reducing biodegradation efficiency and promoting the formation of toxic transformation products [14,128]. Biswas et al. reviewed pesticide removal using electrocoagulation (EC) and electrooxidation (EO). They reported that both techniques can achieve removal efficiencies exceeding 99% when Fe, Al, or boron-doped diamond (BDD) electrodes are employed. EC removes pesticides mainly through the in situ generation of metal hydroxide complexes from sacrificial anodes, combined with electro-flotation caused by hydrogen evolution at the cathode. EO, in contrast, provides strong mineralization capacity with relatively low operating costs. The authors emphasized that most studies were conducted in batch reactors using distilled or ultrapure water, while research on continuous-flow systems and real wastewater remains limited. Key operational factors affecting performance include electrode type, cell configuration, current density, pH, and electrolyte concentration. Overall, Biswas et al. highlighted electrochemical methods as efficient, low-sludge, and cost-effective alternatives for pesticide remediation, while stressing the need for further studies under practical treatment conditions [13]. In addition to physico-chemical and electrochemical approaches, several projects have explored biological and nature-based methods for pesticide removal. The ARTWET project (EU LIFE program), coordinated by ENGEES (France) in collaboration with German and Italian research centers, demonstrated the effectiveness of low-cost constructed wetlands for mitigating diffuse pesticide pollution. Pilot- and full-scale experiments achieved retention efficiencies between 40% and 88%, with near-complete removal of glyphosate and up to 99.8% reduction in metalaxyl, penconazole, and chlorpyrifos. The findings confirmed that constructed wetlands can serve as sustainable, adaptable systems not only for pesticide removal but also for nutrient and municipal wastewater treatment [129]. Olfat et al. [15] conducted two pilot-scale studies combining powdered activated carbon (PAC), coagulation, and ceramic microfiltration to assess pesticide removal. They compared inline PAC dosing (10–12 mg/L) with dosing to a 2-h contact tank (8–10 mg/L) in low-turbidity surface waters (<2 mg C/L total organic carbon) spiked with 7.2–10.3 µg/L total pesticides. For waters with low A254-absorbing NOM and pesticides readily adsorbed to PAC, both dosing strategies achieved ≥93% removal with no significant differences. In contrast, when NOM absorbance and less adsorbable pesticides (e.g., dimethoate, bentazone) were present, higher inline PAC doses were required, and even larger doses than those tested might be necessary to meet Drinking Water Directive limits. Cost analysis indicated that inline dosing is generally more economical, particularly in small plants, when PAC demand is low or when identical doses are applied [15]. Coagulation is also suitable as a pre-treatment step for highly polluted pesticide-bearing industrial effluents, particularly when integrated with advanced oxidation and biological processes. Akinapally [14] et al. investigated a sequential treatment combining coagulation, Fenton oxidation, electro-oxidation, and anaerobic digestion for pesticide intermediate wastewater (initial COD ≈ 90,000 mg/L). Alum coagulation (0.125–0.375 g, pH 7) achieved 41% COD removal at an optimal dose of 0.25 g. Subsequent Fenton treatment (FeSO₄·7H₂O 0.04 g, H₂O₂ 1.4 mL, 180 min) provided an additional 35.6% COD reduction, raising the overall efficiency to 62%. The effluent was further treated by electro-oxidation with SS–SS electrodes at 8 V and 4–6 A for 10–60 min, followed by anaerobic digestion with 5-day retention. This multi-stage approach demonstrated the advantage of combining coagulation with chemical, electrochemical, and biological treatments to achieve significant COD removal in highly contaminated pesticide wastewater. Saini et al. [56] investigated the removal of the pesticides methyl parathion and chlorpyrifos using coagulation–flocculation with alum [Al₂(SO₄)₃·18H₂O] and ferric chloride (FeCl₃). Jar tests were designed using response surface methodology (RSM) with central composite design (CCD), varying pH (4–9) and coagulant dosage (20–120 mg/L). Optimal doses were identified as 80 mg/L for alum and 60 mg/L for FeCl₃. The maximum removals reached 79% for methyl parathion and 82% for chlorpyrifos, with FeCl₃ consistently outperforming alum. These findings highlight the importance of coagulant selection and process optimization in pesticide-contaminated water treatment. Ankang [130] et al. investigated coagulation coupled with ozone catalytic oxidation for the treatment of secondary biochemical effluent from phosphorus-containing pesticide wastewater, since conventional pretreatment and biological processes are ineffective in removing phosphorus. During coagulation, process variables such as coagulant type, dosage, concentration, pH, stirring rate, and mixing time were optimized. Using PAFS at 100 mg/L (5 wt%, pH 8, 100 rpm, 5 min), removal efficiencies reached 17.6% for COD, 86.8% for total phosphorus (TP), and 50% for chroma. The subsequent ozone catalytic oxidation step (ozone 120 mg/L, H₂O₂ 0.1 ‰, catalyst 10 wt%, 90 min) further reduced COD to 10.3 mg/L and chromaticity to 8. The study demonstrated that the combined coagulation–ozone catalytic oxidation process offers high efficiency and practical potential for advanced treatment of phosphorus-rich pesticide wastewater [130]. Narayanan et al. [131] investigated a biopolymer–nanoorganoclay composite based on carboxymethyl cellulose and two modified nanobentonites, DMDA (35–45 wt% dimethyl dialkyl amine, C14–C18) and ODAAPS (15–35 wt% octadecylamine, 0.5–5 wt% aminopropyltriethoxysilane), blended in a 1:2.5:2.5 ratio, for the removal of nine pesticides (atrazine, butachlor, carbendazim, carbofuran, imidacloprid, isoproturon, pendimethalin, thiophanate methyl, thiamethoxam) from aqueous solutions. The optimal dosage was determined by jar tests using alum as a coagulation–flocculation aid. The composite achieved 57–100% removal from spiked water and 63–91% from real pesticide effluents, with >90% efficiency observed for butachlor, isoproturon, pendimethalin, and thiophanate methyl. Furthermore, the spent material could be regenerated by acetone washing followed by thermal treatment at 200 °C, retaining 57–97% of its initial performance after two regeneration cycles. These results indicate that the composite is a highly effective and reusable adsorbent for pesticide-contaminated waters [131].

3.5. Pharmaceuticals

Pharmaceutical residues and their metabolites are increasingly recognized as emerging contaminants in aquatic environments. They occur in effluents from wastewater treatment plants and are frequently detected in surface and groundwater at ng·L⁻¹–µg·L⁻¹ levels, sometimes even appearing in drinking water. Non-steroidal anti-inflammatory drugs such as aspirin, ibuprofen, naproxen, and diclofenac are among the most common compounds of concern [132,133,134]. Although not yet regulated, their persistence, hydrophilic properties, and potential chronic toxicity raise concerns regarding both removal efficiency in drinking water treatment plants and the formation of transformation by-products. In this context, coagulation has attracted attention as one of the possible techniques for mitigating pharmaceutical contamination in water [16,17].

Rigobello [135] et al. evaluated diclofenac (DCF) removal in conventional drinking water treatment processes, with and without pre-oxidation by chlorine or chlorine dioxide, and in combination with granular activated carbon (GAC) filtration. Jar tests using model water (20 HU true color, 70 NTU turbidity, 1 mg/L DCF) showed that coagulation with aluminum sulfate (3.47 mg Al/L, pH 6.5), flocculation, sedimentation, and sand filtration achieved no DCF removal. Pre-oxidation and disinfection partially degraded DCF, with chlorine dioxide outperforming chlorine, though dissolved organic carbon remained unaffected and transformation by-products were detected. LC–MS/MS analysis identified three by-products from chlorination (hydroxylation, aromatic substitution, decarboxylation/hydroxylation) and one hydroxylation product from chlorine dioxide. In contrast, conventional treatment followed by GAC filtration achieved nearly complete DCF removal (≥99.7%), underscoring the importance of adsorption as a polishing step [135]. Erkurt et al. [136] investigated diclofenac removal from drinking water using four coagulants (FeCl₃·6H₂O, MgCl₂·6H₂O, Al₂(SO₄)₃·18H₂O, FeSO₄·7H₂O) in jar tests under varying pH and temperature conditions. The highest efficiency (57.6%) was achieved with FeSO₄·7H₂O at pH 5 and 20 °C, where diclofenac was less ionized due to proximity to its pKa, enhancing adsorption. Removal efficiency increased with rising temperature, consistent with literature reports that higher temperatures promote floc growth, faster precipitation, and improved turbidity reduction. These results confirm that both coagulant type and operational conditions, particularly pH relative to pKa and temperature, critically influence diclofenac removal by coagulation [136]. Sheng et al. [46] investigated the removal of 16 pharmaceuticals, including acetaminophen, diclofenac, ibuprofen, naproxen, carbamazepine, and several sulfonamides, using ultrafiltration (UF), powdered activated carbon (PAC), coagulation (COA), and their combinations. Average removal efficiencies were 29% for UF, 50% for PAC (50 ppm), and only 7% for COA (10 ppm). When PAC dosage was increased to 100 ppm in a combined PAC/UF system, removal efficiency reached 90.3%, whereas UF combined with COA achieved only 33% (a modest 4% improvement over UF alone). The study highlighted that adsorption by PAC coupled with UF separation represents the most effective strategy for pharmaceutical removal from wastewater [46]. Although coagulation is a cornerstone of drinking water treatment, its effectiveness in removing trace pharmaceuticals is often questioned. Several studies indicate that conventional coagulation–sedimentation achieves little or no removal of these compounds, especially in the presence of natural organic matter. Simazaki et al. [137] examined the removal of nine pharmaceuticals (clofibric acid, diclofenac, fenoprofen, gemfibrozil, ibuprofen, indomethacin, ketoprofen, naproxen, and propyphenazone) through chlorination, coagulation–sedimentation with polyaluminium chloride, and powdered activated carbon (PAC) under conditions simulating drinking water treatment. Indomethacin and propyphenazone were fully degraded by chlorination within 30 min, while naproxen and diclofenac persisted at ~30% and most other compounds exceeded 80% after 24 h. Chromatographic analysis indicated the formation of unknown chlorination by-products. PAC adsorption showed competitive effects, with clofibric acid and ibuprofen persisting at ~60% after 3 h due to lower hydrophobicity. In contrast, coagulation–sedimentation hardly removed any of the target pharmaceuticals, even at the optimum turbidity dose, suggesting that most pharmaceuticals in raw water may persist through conventional treatment processes [137]. Mir-Tutusaus et al. [138] evaluated a fungal fluidized bed bioreactor preceded by coagulation–flocculation pretreatment for the continuous treatment of real hospital wastewater (HWW) over 56 days. A parallel control reactor without fungi was also operated. The fungal system achieved effective removal of most pharmaceutical active compounds, including analgesics, anti-inflammatories, antibiotics, and psychiatric drugs, even those considered highly recalcitrant. Denaturing gradient gel electrophoresis (DGGE) and sequencing revealed distinct microbial communities and suggested potential interspecies interactions contributing to pharmaceutical degradation. These results demonstrate that fungal treatment, when coupled with coagulation pretreatment, can provide a promising strategy for mitigating pharmaceutical pollution in hospital effluents [138]. Yang et al. [139] examined the coagulation behavior of five pharmaceuticals—acetaminophen (ACE), carbamazepine (CBZ), 17β-estradiol (E2), naproxen (NAP), and diclofenac (DCF)—using aluminum sulfate in different water matrices (deionized, tap, kaolin-supplemented, and humic acid-supplemented). Compounds with low hydrophobicity (log Kow < 3), such as ACE and CBZ, showed negligible removal (<10%). In contrast, DCF displayed the highest removal in deionized water, while humic acids markedly enhanced the removal of acidic compounds (NAP and DCF), achieving up to 61% and 59% removal, respectively. Kaolin addition also improved the removal of E2, NAP, and DCF by facilitating adsorption [139]. Pharmaceutical contamination poses risks to both human health and ecosystems, and coagulation–flocculation has been studied as a feasible removal method. However, Alazaiza et al. [18] highlighted that conventional chemical coagulation is limited by high costs and excessive, often toxic sludge generation. To address these drawbacks, several researchers have investigated natural coagulants, valued for their biodegradability, safety, and availability. Plant-derived coagulants are more commonly studied than animal-based ones due to lower cost, and their main removal mechanisms involve charge neutralization and polymer bridging. Recent studies demonstrate that natural coagulants can be competitive with chemical ones in terms of performance while offering greater environmental sustainability. Nonetheless, further work is required to optimize extraction methods, assess the impact of environmental factors, and ensure reliable large-scale application as part of green water and wastewater treatment technologies. Overall, the literature demonstrates that conventional coagulation alone is often insufficient for removing pharmaceuticals, especially neutral and hydrophilic compounds. Removal efficiency strongly depends on water matrix characteristics, with natural organic matter and suspended solids influencing adsorption and floc formation. Therefore, while coagulation can partially contribute to pharmaceutical removal, it is best considered as a supportive step requiring integration with advanced or hybrid treatment technologies [137,140,141]. A comparative summary of removal efficiencies across different pharmaceutical classes and treatment methods could further clarify these relationships in future studies.

4. Discussion

This review shows that coagulation–sedimentation, although originally designed for turbidity and iron control, has an expanding role in mitigating diverse emerging pollutants in water and wastewater. Its efficiency is strongly dependent on contaminant type, water matrix, and operational conditions.

When comparing contaminant categories, coagulation proved most effective for suspended and hydrophobic pollutants such as microplastics and humic substances, while showing limited efficiency for polar and persistent compounds like PFAS and pharmaceuticals. Electrocoagulation and surfactant-aided coagulation demonstrated significantly higher removal rates for PFAS (>90%) than conventional salts (<20%), while hybrid systems with PAC or titanium coagulants enhanced pharmaceutical and pesticide removal efficiency by 25–50%.

For pesticides, optimized coagulation and electrocoagulation can achieve high removal, particularly when coupled with advanced oxidation or biological treatments, confirming the importance of hybrid strategies. In contrast, pharmaceuticals are only partially removed, with acidic and hydrophobic compounds showing better performance than neutral or hydrophilic ones [139,142,143,144,145,146]. This indicates that coagulation should be regarded mainly as a pretreatment step to enhance adsorption or membrane processes. In the case of PFAS, conventional coagulation is largely ineffective, but process modifications such as surfactant addition, PAC integration, or electrocoagulation considerably improve removal [81,147,148,149,150,151,152]. Micro- and nanoplastics are more responsive, with removal efficiencies often exceeding 90%, though strongly influenced by polymer type, particle size, and coagulant chemistry [118,153,154]. For NOM, coagulation remains essential as it reduces DBP precursors, though combined methods (PAC, UF, ozonation) are required for comprehensive control. Recent developments in natural and composite coagulants highlight the potential for greener and more sustainable alternatives, while life-cycle aspects, sludge reuse, and automation remain underexplored [104,155].

Overall, coagulation–sedimentation should be viewed not as a stand-alone solution but as a versatile platform that enhances multi-barrier treatment trains. Future work should emphasize real-matrix studies, sustainable coagulant development, and integration with advanced processes to address the complexity of emerging contaminants [155,156]. These comparative findings emphasize that coagulation alone is rarely sufficient for complete removal of emerging contaminants and should primarily serve as a pretreatment step for advanced oxidation or membrane filtration.

5. Conclusions

Coagulation–sedimentation has evolved from a conventional method for turbidity and iron removal into a flexible and adaptable process for managing a wide range of emerging contaminants in water and wastewater. Its performance, however, is highly pollutant specific: while pesticides and micro/nanoplastics can often be efficiently removed, pharmaceuticals and PFAS remain more resistant and require integration with adsorption, advanced oxidation, or membrane processes. Natural and composite coagulants provide promising, more sustainable alternatives to conventional chemicals, though optimization and large-scale validation are still needed.

Overall, coagulation should be considered a key pretreatment and enabling technology within multi-barrier treatment trains, capable of enhancing downstream processes and contributing to safer and more sustainable water management. Future work should focus on green coagulant development, hybrid process design, and circular economy approaches to fully realize its potential in addressing current and emerging water quality challenges. Future research should focus on optimizing coagulant composition and dosage for real wastewater matrices, standardizing performance metrics, and integrating sustainability assessments such as life-cycle analysis and sludge valorization. The transition from laboratory-scale findings to pilot- and full-scale implementations remains a critical step for advancing next-generation coagulant technologies. However, most studies are still confined to laboratory-scale conditions; field-scale validation and long-term monitoring remain essential to confirm their effectiveness under real environmental conditions.