Adsorption Technologies in Wastewater Treatment Processes

1. Introduction

The escalating challenge of wastewater purification, driven by industrial expansion and population growth, seeks more effective solutions for removing persistent and emerging contaminants such as heavy metals [1], antibiotics [2], and microplastics [3]. Conventional methods, including biological treatment and advanced oxidation, often face limitations in efficiency, cost, or adaptability when confronting complex wastewater matrices [4]. In this context, adsorption technology emerges as a fundamentally important and versatile approach for wastewater remediation, offering distinct advantages that warrant focused scientific exploration. This Special Issue, “Adsorption Technologies in Wastewater Treatment Processes”, is dedicated to highlighting the research progress in this field.

Adsorption technology holds enduring prominence due to its inherent operational simplicity, economic viability, and exceptional versatility [5]. Its critical advantage lies in the potential to develop effective adsorbents from abundant, low-cost, and often waste-derived materials. It not only reduces reliance on expensive raw materials but also contributes to significant economic benefits through resource recovery [6,7,8]. Compared with many energy-intensive alternatives, adsorption technology can work effectively under ambient conditions with relatively simple infrastructure, leading to the significant decrease in both capital investment and operational costs. The core mechanism of adsorption relies heavily on specific physicochemical interactions between adsorbent surfaces and target pollutants, usually resulting in the selective adsorption and, thereby, the effective removal of target pollutants from complex water matrices or at trace concentrations [9]. In most cases, other methods may prove inadequate.

Presently, the continuous innovation in material synthesis, functionalization, and regeneration strategies is essential to overcome the historical limitations and enhance the performance for pollutants removal. In view of the critical roles of adsorption technology in addressing global water quality safety, the organization of this Special Issue is both timely and necessary. Although major research has made progress, knowledge still remains dispersed. This work serves a vital purpose: to consolidate current understanding, highlight the importance of adsorption technology, and provide a platform showcasing the latest significant advancements that are driving the field forward. The main objective of this Special Issue is to underscore its indispensable contribution to developing more efficient, economical, and sustainable solutions for water purification. It is our hope that this compilation can provide a valuable reference, stimulate further interdisciplinary research, and accelerate the practical implementation of advanced adsorption technologies to safeguard water resources globally.

2. An Overview of Published Articles

Contribution 1: Engineered bamboo charcoals (BCs) through magnesium chloride impregnation (Mg-BC) and sodium alginate hydrogel encapsulation (Gel-Mg-BC), respectively, were used to inhibit nitrate leaching from fertilized soils. The adsorption performance of Mg-BC and Gel-Mg-BC for nitrate was examined in the batch experiments. Compared with raw BC, the maximum adsorption capacities of these two materials increased to 99.09 and 25–30 mg/g, respectively. Although hydrogelation reduced the adsorption capacity of Gel-Mg-BC, the nitrate leaching decreased by 15–60% in the soil column experiment. The advantage of this study lies in its preventive approach using waste-derived materials for in situ groundwater protection.

Contribution 2: This work designed an integrated process that combined electrocoagulation (Al electrodes) with adsorption (activated carbon) to decontaminate textile-dyeing wastewater containing high concentrations of total nitrogen (TN) and total organic carbon (TOC). The effects of applied voltage (5, 10, and 15 V), electrochemical configuration (bipolar and packed bipolar electrolysis), electrolyte types and their concentrations (NaCl and Na₂SO₄, 5 and 10 mM), and electrolysis time (0–60 min) on TN and TOC removal were investigated. The results indicated that the addition of electrolytes improved initial reaction kinetics but exacerbated metal dissolution. The integrated process (15 V + AC) achieved high removal efficiencies (86.04% TN and 77.98% TOC) without the addition of NaCl and Na₂SO₄, which was superior to electrocoagulation or adsorption alone. Compared with pure electrocoagulation, aluminum leaching decreased by 40.12%. The synergy of electrocoagulation and adsorption achieved a good compromise between treatment efficiency and environmental impact, providing an attractive paradigm for the development of sustainable technologies in textile-dyeing wastewater treatment.

Contribution 3: Peeled mulberry stems were converted to biochar through calcination at 700 °C and functionalized with magnesium-doped hydroxyapatite via the sol–gel synthesis method to obtain an adsorbent (Mg_0.1-HMp) for Pb(II) removal from aqueous solutions. The BET result indicated that the specific surface area and total pore volume of Mg_0.1-HMp increased to 316.3 m²/g and 0.55 cm³/g, respectively. The doping of magnesium in hydroxyapatite created lattice defects, enhancing accessibility of –OH/PO₄³⁻ sites. Various characterization techniques revealed that the Pb(II) removal mechanisms mainly included electrostatic interaction, surface complexation, chemical precipitation, and ion exchange. At a pH of 5, Mg_0.1-HMp achieved a maximum adsorption capacity of 312.5 mg/g, which was much more than raw biochar (123.5 mg/g) based on the Langmuir isotherm model. This study converted agricultural residues to an efficient adsorbent through appropriate modification, contributing to mitigating the hazards of heavy metal-contaminated water bodies.

Contribution 4: This work prepared a cost-efficient powdered activated coke (PAC) through an integrated one-step carbonization-activation process using low-rank lignite, targeting phenolic wastewater remediation. The experimental results demonstrated rapid adsorption kinetics (equilibrium time: 20 min) and high removal efficiency (99.9% phenol elimination at an adsorbent dosage of 4 g/L), with no significant interference by coexisting salts (NaCl and Na₂SO₄). The phenol adsorption process followed the pseudo-second-order kinetic model (R² > 0.9892) and the Langmuir isotherm model (R² > 0.9779). The maximum adsorption capacity achieved was 52.21 mg/g at 20 °C. It is worth noting that the hydrothermal regeneration still preserved 86.1% capacity after five cycles, leveraging the unique properties of subcritical water for desorption and pore reactivation. The hierarchical pore structure of PAC (mesopore-dominated, 514.4 m²/g of specific surface area) and its production simplicity (1/3 cost of commercial carbon) underscored its viability as a sustainable adsorbent for phenol-containing wastewater.

Contribution 5: This study presented the carbon/hydroxyapatite composite (CHAP) from waste animal bones as permeable reactive barrier (PRB) media for the remediation of groundwater contaminated by mining leachates. The static experiments revealed exceptional adsorption capacities for Cu (80 mg/g), Zn (67.86 mg/g), and Mn (49.29 mg/g), with competitive adsorption experiments showing preferential uptake (Cu > Zn > Mn). The dynamic experiments validated the field applicability, treating 613.60 (Cu), 458.65 (Zn), and 204.14 (Mn) bed volumes before breakthrough. The waste-derived synthesis, coupled with extended service life and PRB compatibility, positioned it as an eco-efficient solution for in situ immobilization of polymetallic ions in flowing systems.

Contribution 6: This research designed a synergistic adsorbent by embedding sulfur/nitrogen-enriched powdered porous organic polymer (POP, 3 wt.%) into electrospun chitosan/polyvinyl alcohol (CS/PVA) nanofibers, crosslinked for stability. The composite achieved 92.9% of Hg (II) removal (116.1 mg/g) within 120 min, driven by chemisorption via thiocarbonyl (–C=S) and amine (–NH₂) groups. The Hg (II) adsorption followed the Langmuir model and was controlled by film diffusion. The 3D porous structure of the membrane enabled efficient Hg (II) access, while the adsorbent retained 91% capacity after five cycles with the aid of HCl/thiourea elution. This strategy synergized POP’s high affinity with CS/PVA’s mechanical robustness, offering a recyclable platform for toxic metal recovery.

Contribution 7: This work synthesized a hybrid nanocomposite (rGO-BC@ZrO₂) by integrating zirconia nanoparticles (ZrO₂ NPs, 5–20 nm) within a carbon matrix formed from reduced graphene oxide (rGO) and black cumin (BC) seed powder. Its point of zero charge was 6.2. Systematic batch tests identified optimal conditions (e.g., 2 g/L adsorbent dosage, pH 10) for methylene blue (MB) adsorption, achieving almost complete removal (96–100%). The measured adsorption isotherm followed the Freundlich model, indicating the multilayer adsorption on heterogeneous surfaces, further supported by the Sips isotherm. The thermodynamic studies revealed the spontaneous, exothermic adsorption dominated by physisorption. The kinetic studies favored the pseudo-first-order model. The composite maintained significant efficacy (>60% removal) after five adsorption–desorption cycles. The key innovations were the synergistic use of sustainable biomass (BC) with rGO to host ZrO₂ NPs, achieving high MB uptake through multilayer binding, and demonstrating robust reusability.

Contribution 8: This study developed an adsorbent (Eu-Fe₂O₃-ZrO₂) by modifying iron-zirconia binary oxide nanoparticles (Fe₂O₃-ZrO₂) with eugenol (Eu), a natural phenolic compound extracted from cloves. Scanning electron microscopy (SEM) confirmed the successful surface functionalization and the formation of quasi-spherical nanoparticles (4–5 nm). The adsorption performance for eosin yellow (EY) dye was optimized, showing a high removal efficiency of more than 90% (10–70 mg/L of EY, 1.0 g/L of adsorbent dosage, pH 7, and 27 °C of temperature). The substantial adsorption capacity predicted by the Langmuir model was 91.0 mg/g. The EY adsorption was a spontaneous, exothermic process and followed the pseudo-second-order kinetic model. Interference tests revealed pronounced selectivity limitations against smaller anionic dyes like bromophenol blue (BB). The core advances of this study were the eco-friendly functionalization using a natural, non-toxic agent (eugenol), achieving high EY uptake primarily via chemisorption.

Contribution 9: This work presented cement kiln dust (CKD), an abundant industrial residue, as a cost-efficient adsorbent for heavy metals (Pb, Zn, Cu, and Cd) in simulated wastewater. Batch experiments systematically optimized operational parameters. CKD demonstrated high removal efficiencies: 98% (Pb), 94% (Zn), 92% (Cu), and 90% (Cd). The removal mechanism involved combined effects: chemical precipitation, electrostatic attraction, ion exchange, and surface complexation. The measured equilibrium data followed the Langmuir model well, and the predicted maximum adsorption capacities of Zn, Cu, and Cd were 1.9, 1.9, and 1.8 mg/g. Compared with conventional treatments, a major operational benefit of this work was the significant reduction in hazardous sludge volume. The principal contribution was the effective valorization of waste CKD into a practical, sustainable adsorbent for multi-metal remediation.

3. Concluding Remarks

This Special Issue strongly demonstrated the key role of the adsorption technique in promoting sustainable wastewater treatment. It indicated significant progress made in three aspects:

(i)

Material innovation: Converting waste biomass (e.g., mulberry stems, bamboo charcoal) and industrial byproducts (e.g., cement kiln dust) into high-efficiency, low-cost adsorbents through strategic modification (e.g., Mg-doping, hydroxyapatite functionalization).

(ii)

Process enhancement: Synergistically coupling adsorption with electrocoagulation processes to boost contaminant removal (e.g., dyeing wastewater treatment).

(iii)

Engineering applicability: Designing field-deployable solutions such as permeable reactive barriers and regenerable nanocomposites.

In summary, these advances validated adsorption as a versatile and economically viable cornerstone for water purification. Looking ahead, the field must accelerate efforts towards precision, integration, and circularity:

(i)

Precision design: Developing smart adsorbents with the target-specific affinity for emerging contaminants (e.g., per- and polyfluoroalkyl substances (PFASs), microplastics (MPs), disinfection by products (DBPs)) via atomic-level engineering.

(ii)

Hybrid systems: Integrating adsorption with AI-driven process control and complementary technologies (e.g., catalytic oxidation) to address complex wastewater matrices.

(iii)

Full-cycle sustainability: Establishing closed-loop frameworks—from waste-derived adsorbent synthesis to spent material valorization (e.g., resource recovery, adsorbent reuse)—ensuring minimal environmental footprint.