Physicochemical Treatment of Electroplating Wastewater: Efficiency Evaluation and Process Optimization

Abstract

Electroplating wastewater poses a serious environmental threat due to its high concentrations of heavy metals and persistent organic pollutants. This study evaluated the efficiency of a combined coagulation and activated carbon filtration process for the treatment of real electroplating wastewater containing Ni²⁺, Zn²⁺, Cu²⁺, and Cr⁶⁺ ions. The research was conducted in two stages. In the first stage, laboratory-scale experiments were performed to determine the optimal coagulant type (Fe- and Al-based), dosage, and pH (5.0–10.0) for contaminant removal. In the second stage, the selected operating conditions were applied and validated under real industrial plant conditions at a pilot scale. The laboratory studies demonstrated that the highest Cr removal efficiency was achieved using an iron-based coagulant (PIX), while polyaluminum chloride (PAX) proved most effective for the removal of Ni and Zn. Subsequent filtration through activated carbon further enhanced heavy metal removal, increasing overall efficiencies to above 90%. The reported removal efficiencies represent the overall performance of the integrated treatment process. The results confirm that the integration of chemical coagulation and activated carbon filtration is an effective, environmentally friendly, and economically viable approach for treating real electroplating wastewater, enabling compliance with current environmental standards.

1. Introduction

Electroplating is an essential surface treatment process based on electrochemical deposition of metallic coatings on conductive substrates. The process enhances the mechanical, chemical, and aesthetic properties of materials by improving corrosion resistance, electrical conductivity, hardness, and appearance [1]. It is widely applied in various industrial sectors, including electronics, automotive, aerospace, and household equipment manufacturing [2].

The electroplating industry is one of the major sources of environmental pollution due to the generation of highly contaminated wastewater. Electroplating wastewater (EPW) typically contains elevated concentrations of heavy metals such as chromium (Cr), nickel (Ni), copper (Cu), zinc (Zn), cadmium (Cd), lead (Pb), and mercury (Hg)—all of which are toxic, non-biodegradable, and prone to bioaccumulation in the environment [3]. The discharge of untreated or insufficiently treated EPW can cause severe ecological impacts, contaminating surface waters and soils, disrupting aquatic ecosystems, and posing risks to human health through long-term exposure. For instance, Cr⁶⁺ compounds are carcinogenic, while Pb and Cd affect the nervous system and renal function [4].

Electroplating wastewater is a complex mixture containing both inorganic and organic pollutants. In addition to heavy metals, these effluents often contain surfactants, brighteners, stabilizers, and leveling agents—organic additives that increase chemical oxygen demand (COD) and are resistant to biodegradation [1]. Electrolytes and dissolved metal salts present in these solutions result in high conductivity and solubility, further complicating treatment by conventional biological methods [5].

Depending on the technological process, EPW can be classified into several categories: (i) acidic and alkaline wastewaters from pickling and degreasing; (ii) organic wastewaters from cleaning and polishing operations; (iii) cyanide-containing streams from plating of Cu, Sn, Ag, or Zn; (iv) chromium-containing effluents from passivation and anodizing; and (v) heavy-metal-containing rinses, mainly with Ni and Cu [6]. Such wastewater often exhibits fluctuating pH and contaminant loads, high COD, and low biodegradability (low BOD/COD ratio), making biological treatment insufficient and requiring physicochemical processes.

Treatment technologies for EPW include chemical precipitation, ion exchange, adsorption, membrane filtration, flotation and electrochemical processes [6,7]. Among these, adsorption and electrochemical methods have gained attention due to their efficiency and potential for metal recovery [8,9]. Nevertheless, these methods often suffer from drawbacks such as high operational costs, chemical reagent consumption, and difficulties in handling the generated sludge. Moreover, most studies have been conducted on synthetic wastewater, which fails to reflect the complexity of real industrial effluents [10].

Chemical coagulation remains one of the simplest and most effective processes for EPW treatment, converting dissolved contaminants into insoluble precipitates that can be removed by sedimentation or filtration. It is also compatible with existing wastewater treatment infrastructure and relatively cost-effective [11].

Adsorption is also a commonly used method for removing heavy metals from galvanic wastewater. The effectiveness of adsorption depends on various factors, including the properties of the adsorbent (e.g., specific surface area, porosity, and presence of functional groups), the characteristics of the metal ions themselves, and the process conditions (e.g., pH, contact time, dosage of adsorbent, and initial metal concentration). Of the many materials investigated for heavy metal adsorption, activated carbon is one of the most effective and widely used thanks to its high surface area, well-developed pore structure and strong chemical stability. Activated carbons, including granular activated carbon (GAC), demonstrate substantial capacity for metal ion removal due to the combined effect of physical adsorption and chemisorption. Consequently, they are widely used as efficient sorbents in wastewater treatment, including in hybrid systems that combine coagulation and filtration [12].

Recent reviews highlight significant progress in the development of advanced wastewater treatment technologies for electroplating effluents. Costa et al. (2021) analyzed recent advances (2017–2021) in nickel-rich wastewater treatment, classifying the methods into physicochemical, electrochemical, and bioremediation approaches [9]. Yu et al. (2020) emphasized integrated technologies that enable both pollution control and metal recovery [13], while Rout et al. (2023) explored graphene-based nanomaterials as high-performance adsorbents for heavy and noble metals [14].

Despite these advances, systematic bibliometric analyses focusing on electroplating wastewater remain limited. Bibliometric approaches can quantitatively assess scientific literature to reveal research trends, knowledge gaps, and collaboration networks [15]. They have been successfully applied to different fields but comprehensive analyses for real galvanic wastewater are still scarce. This creates a gap in understanding the evolution of treatment technologies and research priorities in this area.

The increasing environmental stringency and variability in electroplating wastewater composition necessitate optimized, integrated treatment systems. Previous studies investigated the application of integrated physical, chemical, and electrochemical technologies on the treatment of heavy metal-laden wastewater. For example, some recent studies applied the series of coagulation and membrane filtration (as a polishing step), in which significant improvements in the removal of metals including Ni²⁺, Zn²⁺, Cu²⁺, and total chromium from electroplating wastewater were observed [16,17]. However, the use of membrane filtration also presented challenges such as higher energy consumption and membrane fouling. The integration of coagulation (including electrocoagulation) and adsorption has also been investigated for the treatment of different types of wastewaters, landfill leachates, and aqueous solutions added with contaminants such as heavy metals [18,19]. It is worth noting, however, that studies on the application of coupled coagulation and adsorption for real electroplating wastewater treatment are still limited. Combining coagulation with sorption can enhance removal efficiencies and reduce residual metal concentrations to meet regulatory discharge limits. Such hybrid approaches can also improve system robustness against fluctuating wastewater quality. Coagulation is still widely implemented in many industrial wastewater treatment systems due to its robustness, operational simplicity, and cost-effectiveness. Despite the emergence of advanced treatment technologies, coagulation remains a cornerstone process for the removal of suspended solids, organic contaminants, and a broad range of inorganic pollutants across diverse industrial sectors [11]. Recent studies have also demonstrated that the performance of coagulation strongly depends on the physicochemical characteristics of the coagulant itself. For example, increasing the basicity of pre-hydrolyzed PACl coagulants markedly enhanced the removal of polycyclic aromatic hydrocarbons (PAHs), achieving up to 83–91% elimination of target compounds, together with substantial reductions in turbidity, color, and TOC [20].

The objective of the present study is to evaluate and optimize the efficiency of conventional coagulation and coagulation followed by sorption, for the treatment of real electroplating wastewater. The research aims to compare the removal performance of different streams for key heavy metals (Ni, Cu, and Cr), and identify optimal operating parameters, in order to assess their feasibility for industrial application.

This study addresses the gap in knowledge related to the performance of hybrid systems created as a result of combining specific individual processes namely coagulation-based technologies and GAC filtration under real operating conditions and provides insights for developing sustainable treatment strategies for the electroplating industry. Most existing studies focus on either coagulation or GAC adsorption applied separately and under laboratory conditions, while the performance of combined coagulation–GAC systems treating real electroplating wastewater under practical operating conditions remains insufficiently explored.

2. Materials and Methods

2.1. Source and Characteristics of Electroplating Wastewater

The investigated wastewater originated from a local electroplating facility operating zinc, nickel, and chromium plating lines. The samples were collected directly from various sections of the plant, resulting from different galvanic processes carried out under real industrial operating conditions. The wastewater therefore represents actual industrial effluents rather than synthetic or model solutions, and its composition reflects typical electroplating wastewater streams containing mixed heavy metals and process-related additives.

The wastewater was divided into two main streams: chromium-containing wastewater (Cr⁶⁺) and non-chromium electroplating wastewater containing variable amounts of heavy metals (Cu, Ni, and Zn). This classification is consistent with standard industrial wastewater management practices and enabled a technically relevant evaluation of treatment performance for distinct electroplating wastewater streams.

Raw wastewater was taken from the equalization tank before any chemical treatment. Wastewater samples were collected before the neutralization stage and stored at 4 °C until analysis. Because wastewater generation in the plant is irregular, three types of process effluents streams (S) were identified: S1—chromium wastewater, S2—chromium—free galvanic that contain variable amounts of heavy metals (Cu, Ni, Zn) and S3 spent wastewater solution. For laboratory studies, a composite “worst-case” sample (S3) was prepared by mixing S1 and S2, in proportions reflecting their typical occurrence during simultaneous discharge events (

Table 1. Characteristic of plating industrial wastewater before treatment.

Parameter	Unit	S1	S2	S3
pH	-	4.1 ± 0.7	7.0 ± 0.4	6.7 ± 0.7
Conductivity	mS/cm	26.0 ± 6.4	39.0 ± 8.3	25.0 ± 7.2
COD	mg/L	440 ± 56	505 ± 78	494 ± 53
Chromium	mg/L	2.50 ± 0.17	–	2.00 ± 0.16
Nickel	mg/L	0.10 ± 0.02	0.90 ± 0.42	1.00 ± 0.35
Copper	mg/L	–	0.78 ± 0.36	0.80 ± 0.34
Zinc	mg/L	–	2.90 ± 0.21	2.90 ± 0.20

).

The average daily flow is Q_d = 300 m³/day. The in-plant treatment system is designed to reduce heavy metal concentrations to levels compliant with Polish discharge standards: Cr⁶⁺ ≤ 0.1 mg/L, total Cr ≤ 0.5 mg/L, Zn ≤ 2.0 mg/L [21,22].

The wastewater was characterized by high turbidity, variable pH, and elevated concentrations of heavy metals. The initial pH ranged from 4 to 7.5, COD from 400 to 800 mg/L, and conductivity from 25.5 to 60.0 mS/cm. Cr⁶⁺ was the dominant contaminant in S1, while Zn²⁺, Ni²⁺, and Cu²⁺ prevailed in S2. All streams exhibited low pH and elevated electrical conductivity.

2.2. Treatment Processes

The study was conducted in two stages. First, laboratory-scale experiments were carried out to determine the optimal coagulant dosages and pH values for treating the investigated electroplating wastewater. Based on these results, the most effective operating conditions were selected. In the second stage, these operating conditions were applied and validated under real industrial plant conditions at pilot scale, as shown schematically in Figure 1. This two-stage approach enabled the optimization of process parameters under controlled laboratory conditions and their verification under industrially relevant operating conditions.

Initial experiments involve screening different coagulants to determine the optimal type, dosage, and pH for treatment efficiency. The best-performing coagulant under specific operating conditions is selected for further continuous-flow studies.

The selected coagulant is applied in a continuous-flow system, followed by post-treatment using granular activated carbon (GAC). The process performance is monitored through the breakthrough curve to assess the adsorption capacity and stability of the GAC filter over time.

The following criteria are used to assess the treatment efficiency and process performance:

-

Removal efficiency of heavy metals: chromium (Cr), nickel (Ni), copper (Cu), and zinc (Zn).

-

Overall reagent balance and operating conditions, including pH correction and chemical dosages.

-

Operational stability of the GAC filter, expressed as the time to breakthrough.

All experiments are performed in triplicate and data are presented as mean values.

2.2.1. Coagulation Process

The metal removal experiments were carried out on a laboratory scale at a temperature of 20 °C, using a volume of 500 mL. The coagulation experiments were carried out in a laboratory-scale jar test. Coagulants based on aluminum sulfate (Al₂(SO₄)₃·18H₂O) (A and B) and ferric chloride (FeCl₃·6H₂O) (C) were used as coagulants due to their widespread industrial application and proven efficiency in heavy metal removal. Three commercial coagulants (Dempol/Scanpol) were examined: PIX (A), FLOKOR 1,2/PAC (B), PAX XL19H (C).

For each test, 500 mL of raw wastewater was placed in a jar 1 dm³ and subjected to rapid mixing at 200 rpm for 2 min, followed by slow mixing at 40 rpm for 15 min. The suspension was allowed to settle for 30 min. The supernatant was then carefully decanted for further analysis.

Doses of coagulant (50, 100, 150, 200, 250 mg/L) were added to the samples. After settling, the metal concentrations in the supernatant of the treated samples were measured. The effects of coagulant dose (50–250 mg/L) and pH (6–10) (adjusted with H₂SO₄ or NaOH) were evaluated. The optimal conditions were determined based on the highest removal efficiency of metals and the lowest residual turbidity.

Supernatant samples were collected after sedimentation for the determination of metal concentrations, COD, TSS, pH, and conductivity.

2.2.2. Filtration Process

The clarified effluent was subjected to filtration through a fixed bed column packed with granular activated carbon (GAC) (BET surface area: 980 m²/g, particle size: 1–2 mm). The column (height 50 cm, internal diameter 3 cm) was operated in downflow mode at a constant flow rate of 10 mL/min. The filtration process was continued until breakthrough occurred, defined as the effluent metal concentration exceeding 5% of its influent value. Samples were collected periodically to determine metal concentrations and evaluate removal kinetics.

For Cr-containing wastewater, mainly iron-based coagulants were used in the preceding stage to enhance Cr(VI) reduction to Cr(III), facilitating adsorption and precipitation.

2.3. Analytical Methods

The procedures for wastewater sampling and the instrumentation used for wastewater quality parameter measurements were determined according to recommendations of Polish State Sanitary Inspection and ISO standards. The temperature was monitored by using an electronic thermometer PT-411 (Wroclaw, Poland). Turbidity was measured using a turbidity meter (Hach 2100N, Wroclaw, Poland), and pH and conductivity were determined using a multiparameter meter (WTW Inolab Multi 9420, Wroclaw, Poland) [23,24]. COD was analyzed using the dichromate reflux method [25,26]. The concentrations of Ni, Cu, Zn and Cr were quantified by atomic absorption spectrometry (AAS) (Hatch Lange DR 3900) (Wroclaw, Poland) [27]. All analyses were performed in triplicate, and results are presented as mean values ± standard deviation.

The percentage removal efficiency (R, %) for each contaminant was calculated as:

R = (C₀ − C) C₀ × 100%

(1)

where C₀ is the initial concentration and C is the concentration after treatment (mg/L).

3. Results and Discussion

3.1. Influent Quality (Raw Wastewater)

Raw Wastewater was taken from different points of industrial installation.

Table 2. Heavy metals in raw electroplating wastewater from technological streams.

WW Stream	pH	Cr [mg/L]	Cu [mg/L]	Ni [mg/L]	Zn [mg/L]
WWS 1	6.0	2.66	0.06	0.10	0.02
WWS 2	8.7	0.02	0.80	0.98	2.90
WWS 3	7.6	2.20	0.70	0.99	2.94

summarizes the heavy metal composition of samples collected from the industrial facility for laboratory studies of treatment processes. Given the low pH, reagent-based chemical treatment (alkali, coagulant, polymer) was selected as the first step, followed by polishing on granular activated carbon (GAC).

The dominant contaminants were Cr(VI) in Stream S1 and Zn²⁺, Ni²⁺, and Cu²⁺ in Stream S2, while all of these metals were present in Stream S3. The pH values and the concentrations of Cu, Ni, and Zn were determined in wastewater that had been neutralized under laboratory conditions.

3.2. Effect of Coagulant Dose and pH

Coagulant dose significantly influenced metal removal (Figure 2). In this study, an increase in the coagulant dose from 50 to 200–250 mg/L resulted in a significant enhancement in the removal of dissolved Cu, Ni, Zn, and Cr, followed by a slight decline at higher dosages. The maximum removal efficiencies were commonly achieved with the 200mg/L dose. A common trend was observed: performance increased up to an optimum around 200–250 mg/L, then plateaued or slightly decreased. This characteristic “optimum dose” behaviour has been extensively documented in the extant literature. The prevailing wisdom of classical coagulation studies is that initial dose increments promote charge neutralization and sweep-floc formation, leading to improved aggregation and sedimentation of metal-bearing colloids [28,29]. However, it is important to note that exceeding the optimum dose can result in overdosing, which can induce colloid restabilization, increased ionic strength, and the formation of fragmented or overly dense flocs with diminished adsorption capacity. Such effects are consistent with the slight performance reduction observed at 250 mg/L and have also been reported for pre-hydrolyzed aluminum species in real industrial wastewaters [30].

The present study examined the dependency of pH on the coagulation process, utilizing a wide pH range from 6 to 10 (see Figure 3). The metal removal efficiency exhibited an increase with an increase in pH, reaching maximum values at pH ≥ 8. This phenomenon can be attributed to the pH-dependent stability of the complexes. It is evident that an increase in pH results in an increase in the affinity and stability of metal complexes. The highest efficiencies were observed at a pH of 8 and with the use of a coagulant known as PAX. These efficiencies were found to be 80% for chromium (Cr), 90% for copper, 89% nickel, and 84% for zinc (Zn). The findings of this study are analogous to those reported in a prior investigation employing pre-hydrolyzed alumina coagulants and aluminum sulfate [19]. The concentration of copper and zinc decreased to approximately 80% with PIX coagulation, the values were 80% and 82%, respectively. The chromium removal efficiencies with PIX in the pH ranges of 8–10 were comparable. A mild decline above 250 mg/L is consistent with restabilization/charge reversal and sweep-floc interference reported in the coagulation literature, where overdosing can re-disperse colloids and re-release complexed metals from weak flocs [29,31,32]. The maximum observed removals were recorded at the optimum dosage of Cr: 94% Cu, Ni, Zn: ~80%. With the dosage set at the optimal level, a range of pH values from 6 to 10 was observed, thereby confirming a discernible pH-dependent response (see Figure 2 and Figure 3). In the alkaline pH range (8.5–10), a significant fraction of heavy metal removal is expected to result from chemical precipitation governed by metal hydroxide solubility. Nevertheless, the experimental data indicate that pH adjustment alone does not fully account for the observed removal behaviour, as clear differences in removal efficiency were observed at comparable pH values depending on coagulant type and dose. The presence of a distinct optimum coagulant dose and the consistently higher performance of PAX compared to PIX suggest an additional contribution from coagulation-related mechanisms, including charge neutralization, sweep flocculation, and adsorption onto hydrolysed Al/Fe species. Therefore, heavy metal removal in the proposed system should be interpreted as the combined effect of pH-driven precipitation and coagulant-assisted physicochemical processes. This finding is consistent with the hypothesis that speciation-controlled precipitation/adsorption, hydroxo-complex formation, and the minimum solubility of Me(OH)₂/Me(OH)₃ near neutral-alkaline pH, as well as enhanced polymeric Al/Fe hydrolysis products that bridge/adsorb metals and organics, are all contributing factors [1,2,7,33].

3.3. Combined Coagulation–GAC Filtration

The integration of coagulation (PIX and PAX, 200 mg/L) with subsequent granular activated carbon (GAC) filtration resulted in a substantial improvement in the removal of residual metals compared with coagulation alone (see

Table 3. Comparative Assessment of Metal Removal Efficiencies (Coagulation vs. hybrid process).

Metal	Coagulation PIX/PAX	Coagulation PIX or PAX + GAC	Dominant Removal Mechanisms	Literature Data with Key References
Ni	70/89%	91%	Precipitation as Ni(OH)2; adsorption on GAC via surface complexation	90% [7,28,29]
Zn	83/89%	92%	Zn(OH)2 precipitation; strong affinity to activated carbon; micropore uptake	90% [7,30,31]
Cu	80/92%	93%	Hydroxo-complex adsorption; interaction with oxygen groups on GAC	75% [16,27,30]
Cr	94/80%	95%	Precipitation of Cr(OH)3; sweep flocculation; minimal gain after GAC	80–95% [18,26,30]

). Following the implementation of the combined treatment, removal efficiencies of 91% for Ni, 90% for Zn, 82% for Cu, and up to 95% for Cr were achieved. The enhancement was particularly notable for Ni, Zn, and Cu, whereas Cr exhibited only marginal improvement, indicating that coagulation alone was already close to its maximum achievable removal for chromium species.

A detailed comparison of percentage removals confirms that the combined process outperforms the single coagulation step for most metals (Table 3). The high removal efficiency for zinc can be attributed to a dual mechanism: The precipitation of sparingly soluble Zn(OH)₂ during the process of coagulation, occurring at a near-neutral pH, and the subsequent microprecipitation and adsorption within the microporous structure of GAC, are the two primary mechanisms under investigation. This behaviour is consistent with the well-established sorption affinity of activated carbon towards metal ions, especially when partial hydrolysis or complexation enhances surface reactivity.

Following the filtration/sorption step, metal removal efficiencies increased, thus confirming that the secondary sorption process played a significant role in reducing dissolved metal concentrations. The slight improvement observed for Cr (from approximately 94% after coagulation to approximately 95% after filtration) indicates that chromium—likely existing predominantly as Cr(III) after coagulation—was effectively removed via precipitation and sweep-floc mechanisms during the coagulation step. This finding is consistent with previous research that demonstrated the limited additional efficacy of carbon adsorption for trivalent chromium once precipitation-based removal has reached a state of near-completion [34,35].

The coagulation–GAC hybrid system exhibited the optimal performance for Zn and Ni, with a synergistic enhancement of removal resulting from precipitation and sorptive interactions. The limited gain for Cr indicates that coagulation alone may be sufficient when chromium is the predominant target pollutant. However, combined treatment is more advantageous for mixed-metal wastewaters, where metals exhibit different sorption affinities and precipitation behaviours.

The removal efficiencies achieved in this study are consistent with reported values for hybrid coagulation–adsorption systems applied to industrial wastewaters containing heavy metals. For instance, Qasem et al. (2021) observed similarly high removals (>90%) for Zn and Ni using coagulation followed by activated carbon treatment, attributing the improvement to the complementary mechanisms of adsorption, surface complexation, and microprecipitation within carbon pores [7]. Moreover, the synergistic effects observed in this study are consistent with the established principles of combined physicochemical treatment systems, which underscore the merits of sequential removal mechanisms for complex wastewater matrices [29,36].

In summary, the results indicate that coagulation followed by GAC filtration provides a robust and efficient strategy for the removal of Ni, Zn, and Cu, whereas its additional effect on Cr removal is minimal. These findings lend support to the implementation of hybrid treatment trains in industrial wastewater management, particularly in scenarios where achieving low residual metal concentrations is imperative for regulatory compliance.

The incremental gain on Ni/Zn/Cu is attributed to adsorption of dissolved residuals (free ions, weak complexes) and microsedimentation within GAC pores, while Cr shows limited additional removal because most Cr(VI) is reduced/precipitated as Cr(III) hydroxide during coagulation, leaving little dissolved fraction for adsorption—behavior widely reported for coagulation-led Cr control, with GAC serving primarily as a polishing step for organics and trace metals [37].

The removal behaviour exhibited in this study can be elucidated by well-established coagulation and adsorption mechanisms. During the coagulation process with PIX, PAC, or PAX, three predominant pathways were identified as contributing to metal elimination:

The process of charge neutralization in colloids and metal–organic complexes is of particular interest [38]. The second phenomenon under consideration is sweep flocculation driven by the formation of amorphous Al/Fe hydroxide precipitates. Thirdly, there is a reduction in Cr(VI) to Cr(III), a process which is particularly pronounced when Fe-based coagulants are used.

Following reduction, Cr(III) readily precipitates as Cr(OH)₃ and is efficiently removed through settling or media filtration. These mechanisms have been comprehensively delineated in the extant literature on coagulation and are consistent with prior findings for electroplating wastewaters.

Subsequent to the process of coagulation, the implementation of GAC filtration serves as an efficacious polishing step, with the objective of targeting residual constituents that have not been adequately removed by coagulation alone. The primary function of GAC is the removal of the dissolution of organic compounds (for example, surfactants, brighteners and complexants) is facilitated through a combination of π–π interactions, hydrophobic attractions and hydrogen bonds [39].

It has been demonstrated that a portion of dissolved metal species is formed through surface complexation with oxygen-containing functional groups.

Metal sorption on GAC increases with pH due to both deprotonation of surface sites and changes in metal ion speciation, which enhance hydrolysis and favour the formation of surface-reactive metal hydroxo complexes [40].

For the effluents investigated here, an operating point of approximately 200 mg/L of PAX and a pH of approximately 8 provided an optimal balance between chemical consumption and removal performance. This ensured that precipitation and charge neutralization were maximized, while the effects of overdosing were minimized.

In wastewater streams where heavy metal concentrations remain elevated at lower pH levels, GAC functions as an efficient “second barrier,” thereby improving effluent stability and buffering operational variability. While GAC alone is not a primary metal-removal technology under neutral pH unless chemically modified, its synergistic role following coagulation significantly enhances removal efficiency and effluent reliability. GAC, due to its highly developed porous structure, large specific surface area, and the presence of surface functional groups, provides effective adsorption of heavy metal complexes. Compared to other carbon-based materials, such as biochar or advanced carbon nanomaterials, GAC offers a favorable balance between adsorption capacity, operational stability, and applicability at the industrial scale [41].

The combined coagulation-GAC treatment train exhibited performance for Cr, Ni, Zn, and Cu removal that is consistent with the ranges reported for other physicochemical systems treating electroplating and mixed-metal industrial effluents [42]. Across the studies under consideration, coagulation consistently provides the dominant removal step, particularly for chromium (Cr), whereas granular activated carbon (GAC) contributes through residual metal polishing and organics removal, improving compliance margins and stabilizing effluent quality.

As indicated by preceding studies, dose and pH emerge as the predominant operational factors in metal removal processes within coagulation-based systems [43]. The consequences of exceeding recommended dosages are particularly salient, as they necessitate additional polishing steps to ensure compliance with stringent discharge criteria. For chromium specifically, several works demonstrate that the addition of GAC can further enhance removal, particularly at higher pH, due to concurrent co-precipitation, surface complexation and adsorption. This allows shorter operating times and improved process kinetics compared with single-step electrocoagulation or chemical precipitation.

The mechanistic evidence, when considered in its entirety, lends support to the conclusion that the coagulation–GAC hybrid approach offers a robust and synergistic solution for the treatment of metal-laden wastewater. This approach has been demonstrated to provide both high primary removal and effective polishing of residual contaminants.

The results obtained in this study are highly consistent with previously reported findings for metal-laden industrial effluents and align with mechanistic models describing coagulant hydrolysis, polymerization, metal speciation, and flocculation dynamics. The correlation between empirical findings and prevailing theory serves to substantiate the dependability of the observed optima for both dose and pH, thereby validating the efficacy of PIX and PAX as coagulants for the treatment of real plating wastewater.

The results confirm that the integrated coagulation and activated carbon filtration process is an effective approach for the treatment of real electroplating wastewater containing heavy metals. The obtained removal efficiencies are consistent with recent review studies identifying coagulation as a robust core technology in industrial wastewater treatment, particularly when combined with adsorption-based polishing steps [39,44]. From a practical perspective, the process performance supports compliance with the requirements of the Polish Water Law and regulations governing wastewater discharge to sewerage systems, while the use of real industrial wastewater enhances the relevance of the findings for full-scale applications.

Therefore, considering the findings of other researchers [45,46,47,48,49,50] who have investigated galvanic wastewater treatment processes, our study addresses the existing research gap and contributes to the development of hybrid treatment systems combining coagulation with GAC filtration, evaluated using real galvanic wastewater. The discussion provides an in-depth analysis of the influence of key process parameters, offering a comprehensive assessment of the performance of this treatment configuration.

4. Conclusions

The combined coagulation–filtration with GAC adsorbent system demonstrated high efficiency in the treatment of electroplating wastewater containing both inorganic and organic contaminants. Coagulation using PIX at an optimal dose of around 200 mg/L and a pH of approximately 7 achieved a chromium (Cr) removal efficiency of up to 94% and a copper (Cu), nickel (Ni) and zinc (Zn) removal efficiency of 76–80%. By comparison, PAX achieved higher removal efficiencies for Cu, Ni and Zn, reaching almost 90% for all three metals. Subsequent granular activated carbon (GAC) filtration acted as an effective polishing step, increasing Ni and Zn removal to around 90–91% and Cu to approximately 90%. Cr showed only marginal improvement as it had already been efficiently removed during coagulation. These results demonstrate that combining coagulation, particularly with PAX, and post-treatment sorption significantly improves the overall performance of metal removal, especially for Ni, Zn and Cu. They also highlight the importance of hybrid treatment strategies in achieving high-quality effluent. The results confirm that the integrated PAX–GAC process effectively targets both metal ions and organic constituents. This dual treatment strategy enhances the stability and robustness of the overall system against influent variability, ensuring consistent compliance with regulatory discharge standards. Consequently, the proposed approach represents a technically feasible and environmentally sound solution for advanced treatment of electroplating effluents. Future research should focus on applying statistical optimization techniques to further refine process parameters, evaluating the robustness of the treatment under variable wastewater compositions, and assessing long-term operational performance at full scale. In addition, studies on sludge valorization, activated carbon regeneration, and comprehensive cost–benefit and environmental impact analyses would provide valuable insights for large-scale implementation.