The Removal of Arsenic from Contaminated Water: A Critical Review of Adsorbent Materials from Agricultural Wastes to Advanced Metal–Organic Frameworks

Abstract

Arsenic pollution in potable water is a significant worldwide health concern. This study systematically evaluates current progress in adsorption technology, the most promising restorative approach, to provide a definitive framework for future research and use. The methodology entailed a rigorous evaluation of 91 peer-reviewed studies (2012–2025), classifying adsorbents into three generations: (1) Natural adsorbents (e.g., agricultural/industrial wastes), characterized by cost-effectiveness but limited capacities (0.1–5 mg/g); (2) Engineered materials (e.g., metal oxides, activated alumina), which provide dependable performance (84–97% removal); and (3) Advanced hybrids (e.g., MOFs, polymer composites), demonstrating remarkable capacities (60–300 mg/g). The primary mechanisms of removal are confirmed to be surface complexation, electrostatic interactions, and redox precipitation. Nevertheless, the critical analysis indicates that despite significant laboratory efficacy, substantial obstacles to field implementation persist, including scalability limitations (approximately 15% of materials are evaluated beyond laboratory scale), stability concerns (e.g., structural collapse of MOFs at extreme pH levels), and elevated costs (e.g., MOFs priced at approximately $230/kg compared to $5/kg for alumina). The research indicates that the discipline must transition from only materials innovation to application science. Primary objectives include the development of economical hybrids (about $50/kg), the establishment of uniform WHO testing standards, and the implementation of AI-optimized systems. The primary objective is to attain sustainable solutions costing less than $0.10 per cubic meter that satisfy worldwide deployment standards via multidisciplinary cooperation.

1. Introduction

Over 100 million people throughout the world are exposed to arsenic (As) levels that are too high for safe drinking water [1]. Arsenic is a naturally occurring element in groundwater caused by geological weathering. Groundwater gets contaminated further by activities such as mining, smelting, and agriculture [2]. Documented cases illustrate the seriousness of anthropogenic arsenic pollution. Mining operations significantly contribute to environmental contamination; specifically, tailings and effluents from gold mines in Ghana and South Africa have resulted in groundwater arsenic concentrations surpassing 5000 μg/L. Industrial processes such as smelting have led to soil and water contamination near copper smelters in Chile and India, with recorded concentrations ranging from hundreds to thousands of μg/L. The application of arsenic-based pesticides, such as lead arsenate, in historical cotton-growing areas of the United States, along with the utilization of arsenic-contaminated groundwater for irrigation in Bangladesh and Vietnam, has resulted in considerable soil accumulation and subsequent leaching into aquifers, establishing a cyclical contamination pattern [3]. A strict 10 μg/L limit was recommended by the WHO since skin lesions, cardiovascular illnesses, and numerous malignancies may be caused by chronic exposure to inorganic arsenic (As(III) and As(V)) [3]. The aqueous speciation of arsenic fundamentally influences its behavior and removal efficacy. In natural waters, inorganic arsenic mostly occurs as oxyanions: arsenite (As(III)) and arsenate (As(V)). The distribution of these species is determined by redox potential (Eh) and pH. In reducing circumstances (e.g., deep groundwater), neutral arsenious acid (H₃AsO₃, pKa = 9.2) predominates, rendering As(III) challenging to eliminate owing to its neutral charge. Conversely, in oxidizing environments, arsenate manifests as negatively charged oxyanions (H₂AsO₄⁻ at pH 2.2–6.9, and HAsO₄²⁻ at pH 6.9–11.5), hence promoting its extraction by electrostatic attraction [4]. Remediation attempts are made more difficult by the fact that As(III) is both more mobile in water systems and sixty times more poisonous than As(V) [4]. The variation in toxicity and mobility reflects a fundamental distinction in their affinity for functional groups on adsorbent surfaces. The neutral H₃AsO₃ species exhibits a reduced affinity for various surfaces, primarily engaging through weaker non-specific interactions such as van der Waals forces or hydrogen bonding. The anionic As(V) species (H₂AsO₄⁻, HAsO₄²⁻) engage in stronger, specific interactions. They demonstrate a strong affinity for metal oxide/hydroxide surfaces (e.g., Fe-O, Al-O, Zn-O) via ligand exchange, resulting in the formation of stable inner-sphere complexes. As(V) oxyanions exhibit a strong affinity for positively charged functional groups, including protonated amines (-NH₃⁺) and quaternary ammonium cations (-N⁺(CH₃)₃), through electrostatic interactions. This review critically analyzes the design of various adsorbent materials, ranging from biosorbents rich in hydroxyl groups to metal–organic frameworks (MOFs) with tailored metal clusters. It focuses on how these materials exploit differing affinities, with many advanced options incorporating elements that simultaneously oxidize As(III) to As(V) and sequester the resulting oxyanions. Conventional arsenic removal methods encounter considerable obstacles. For instance, coagulation-filtration produces hazardous sludge necessitating meticulous disposal [5]. Reverse osmosis has substantial operating expenses and energy requirements [6]. Ion exchange is adversely affected by competitive anion interference, such as phosphate and sulfate [7]. These techniques are often less effective for removing uncharged As(III) species, frequently requiring a pre-oxidation step, which increases complexity and cost. Although these approaches achieve 70–90% elimination under optimal circumstances, their practical use in underdeveloped locations is constrained by infrastructural prerequisites, thus necessitating an immediate need for resilient, cost-effective solutions [8]. The expanding number of scholarly articles on adsorption as a principal solution demonstrates the ongoing worldwide research effort in this area. As illustrated in Figure 1, the yearly publication count on arsenic adsorption has skyrocketed over the last two decades, indicating its essential relevance in the water treatment industry.

The adsorption technique has become a leader owing to its efficacy over a range of arsenic concentrations (μg/L to mg/L) [9], ability to employ both natural and synthetic adsorbents [10], and reusability [11]. The approach consists of two primary steps [12]: physisorption, characterized by weak van der Waals interactions (ΔH = 5–40 kJ/mol) [13], and chemisorption, which entails the creation of stronger chemical bonds (ΔH > 40 kJ/mol) [14]. Although recent studies have thoroughly addressed certain material classes, such as iron-based adsorbents [10] and biochar [15], a comprehensive study that critically evaluates the progression from low-cost wastes to sophisticated hybrids remains absent. The field is progressing swiftly with innovative functional materials such as layered double hydroxides (LDHs) [16] and MXenes, which exhibit significant potential for anion exchange and adjustable surface chemistry; however, their investigation for arsenic remediation remains in its early phases relative to metal–organic frameworks (MOFs) and metal oxides [14]. This review methodically assesses three generations of materials. These include first-generation (natural) materials such as agricultural byproducts (e.g., rice husks [17], soybean hulls [18], industrial byproducts (such as red mud [19], blast furnace slag [20], and bio-sorbents (e.g., watermelon peel and fungal biomass) [21,22]. The second-generation (engineered) materials include metal oxides (e.g., TiO₂ nanoparticles [23], ZnO [24], activated alumina [25], and zeolites (composites treated with Fe₃O₄) [26]. On the other hand, third-generation (advanced) materials include MOFs (e.g., Fe−BTC, MIL-53(Al) [27,28], polymer hybrids such as Rubber tire-P(APTAC) [29], and cryogels (Aluminum-doped polyacrylamide) [30]. Laboratory investigations provide encouraging outcomes [31]; however, substantial obstacles persist in the scalability of nanomaterial fabrication [32], long-term stability of bio-adsorbents [33], and regeneration efficacy after several cycles [34]. Recent reviews have effectively addressed specific material classes, including iron-based adsorbents [10] and biochar [15]. However, there remains a lack of comprehensive analysis that critically examines the progression from low-cost wastes to advanced hybrids. Moreover, numerous reviews emphasize the adsorption capacities attained under optimal laboratory conditions. This review aims to address that gap by offering a distinct generational framework to critically assess both performance and, crucially, the practical viability and scalability of these materials. This study examines the “implementation trilemma,” which describes the inverse relationship among scalability, stability, and cost that obstructs the transition from laboratory success to field deployment. This study seeks to address the existing gap by: (1) conducting a comparative evaluation of three generations of adsorbents in terms of performance, mechanism, and scalability; (2) pinpointing the fundamental and technological barriers that impede the transition from laboratory research to practical application; and (3) delineating a roadmap for future research priorities centered on implementation science and economic viability. This study connects basic research with practical implementation by critically evaluating 91 papers (2012–2025) to inform future material development and field applications.

This review presents a concise overview of the three generations of adsorbents, summarized in

Table 1. Comparative summary of the three generations of arsenic adsorbents.

Characteristic	1st Generation (Natural/Waste)	2nd Generation (Engineered)	3rd Generation (Advanced Hybrids)
Example Materials	Rice husks, red mud, watermelon rind, soybean hulls	Activated alumina, TiO2, ZnO, Fe3O4 nanoparticles	MOFs (e.g., MIL-53, ZIF-8), polymer composites, MXenes
Primary Mechanism	Physisorption, electrostatic attraction, ion exchange	Chemisorption, surface complexation, ligand exchange	Synergistic mechanisms: size exclusion, redox, complexation
Avg. Capacity (Q_max)	Low (0.1–5 mg/g)	Moderate (5–50 mg/g)	High (50–300+ mg/g)
Key Advantage	Very low cost, sustainable, waste valorization	High reliability, proven efficacy, commercial availability	Exceptional capacity & selectivity, tunable properties
Key Limitation	Low capacity, low reusability, variable composition	Sensitive to water chemistry (pH, competing ions)	High synthesis cost, poor stability, scalability challenges

, which emphasizes the evolution in design, performance, and related challenges.

2. Natural and Waste-Based Adsorbents

The use of natural and waste-derived materials for arsenic adsorption has garnered considerable interest owing to its cost-effectiveness, sustainability, and prospective uses in a circular economy. These materials use plentiful functional groups (e.g., hydroxyl, carboxyl, amine) and mineral constituents (e.g., Fe, Al, Ca) to immobilize arsenic species via various methods [18,19]. This section rigorously analyzes three subcategories of these adsorbents, substantiated by recent experimental evidence [20,21,22].

2.1. Agricultural Byproducts

Agricultural wastes serve as potential adsorbents due to their lignocellulosic compositions and widespread availability. This is achieved by ligand exchange (Arsenate substitutes hydroxyl groups on cellulose/hemicellulose) [17,21], electrostatic attraction (protonated amine (–NH₃⁺) groups attract AsO₄³⁻ when pH < 7) [18], and surface precipitation (Iron/calcium minerals in ash constituents generate insoluble arsenates) [19]. Shabbir et al. [17] found that rice husks may attain 98% arsenic removal from groundwater in Pakistan’s Vehari area (starting concentration: 200 µg/L), attributing this efficacy to synergistic interactions between arsenic and silica-cellulose matrices. The adsorption process adhered to pseudo-second-order kinetics, indicating that chemisorption was primarily governed by surface complexation with oxygen-containing groups. Likewise, soybean hulls demonstrated a 71% removal effectiveness [18], with Fourier-transform infrared (FTIR) spectroscopy validating the critical function of carboxyl groups, likely through ligand exchange mechanisms in which arsenate ions substitute surface hydroxyl groups on the biosorbent matrix, or through coordination with protonated functional groups in acidic environments [18,31]. A significant development is the citric acid-modified watermelon peel biosorbent created by Letechipia et al. [31]. Figure 2 illustrates that the treated biosorbent (TB) attained a 99.99% arsenic removal rate (65 µg/L to 0.01 µg/L) in a pH range of 5.5–7.5, surpassing the performance of natural biosorbents (NB, 81.53%). Scanning electron microscopy (SEM) demonstrated that acid treatment enhanced surface porosity by 40%, whilst energy-dispersive X-ray spectroscopy (EDS) validated arsenic deposition on TB surfaces. This corresponds with the research by Letechipia et al. [31], which indicated that watermelon peel had a maximal adsorption capacity of 2.42 µg/g via ligand exchange processes.

2.2. Industrial Waste

Industrial byproducts have the twin benefits of waste valorization and a strong affinity for arsenic owing to their metalliferous nature. Lu et al. [23] documented that red mud (a bauxite byproduct) attained a significant 153-fold decrease in arsenic content (from 6100 to 40 mg/L) by AlAsO₄ precipitation.

Table 2. List of the major chemical components of red mud [1].

Element	Fe	Ca	Al	Si	Ti	O	Others
Concentration (%)	20.56	11.52	10.51	7.71	3.40	37.48	8.82

indicates that the material’s composition of 20.56% Fe and 10.51% Al (by weight) enabled super-adsorption, with X-ray diffraction (XRD) validating the creation of crystalline aluminum arsenate. The adsorption process exhibited pH dependence, demonstrating best efficacy in alkaline conditions (pH 9–11), when Fe/Al hydroxides acquire positive surface charges [19].

Blast furnace slag (BFS), a kind of metallurgical waste, exhibited remarkable As(V) absorption (4040 µg/g) in column experiments conducted by Lekić et al. [24]. It has an optimal performance for column systems with 90% regeneration efficiency [24]. The calcium oxide in the material (11.52% Ca) facilitated electrostatic adsorption, while manganese oxides facilitated the oxidation of As(III). Fly ash had a comparatively lower substantial capacity (≈800 µg/g) attributable to its unburned carbon and iron oxide content [21], while its efficacy fluctuated based on the combustion source. However, fly ash needs activation (e.g., acid washing) for reliable performance [21]. Red mud: Maximum documented capacity (101.5 mg/g) necessitates pH modification [23].

2.3. Bio-Sorbents

In addition to plant-derived materials, microbial biomass has emerged as an effective arsenic scavenger. Asare et al. [35] isolated Bacillus subtilis ATCC13952 from soils of Ghanaian gold mines, achieving 99.97% clearance of As(III) by extracellular polymeric substance (EPS)-mediated biosorption. Thermodynamic investigation indicated exothermic adsorption (ΔH = −159.05 kJ/mol), whereas FTIR identified carboxyl, phosphoryl, and sulfhydryl groups as primary binding sites. The Elovich kinetic model demonstrated the most accurate fit, indicating diverse surface interactions. Fungal biosorbents such as Aspergillus niger have significant potential, with chitin-melanin complexes attaining 85–90% As(V) absorption throughout the pH range of 3.0–8.0 [22]. Nonetheless, biomass processing (drying, pelletization) continues to pose an economic challenge for large-scale implementations. The economic viability of biochar significantly hinges on its capacity for regeneration and reuse. Regeneration techniques commonly employed involve chemical desorption with alkaline solutions, such as 0.1–1.0 M NaOH, which effectively displace adsorbed arsenic anions via ion exchange and restore the surface functionality of biochar [15,36]. Regeneration efficiency may decline after several cycles as a result of pore blockage or irreversible chemical alterations to the biochar’s surface. The thermal regeneration of spent biochar is effective but necessitates considerable energy input (200–300 °C), which influences its overall life-cycle cost and environmental impact [15]. The selection of a regeneration method involves a trade-off between operational costs and the recovery of adsorption capacity.

2.4. Polysaccharide-Based Adsorbents

Polysaccharides, including chitosan, cellulose, and alginate, are increasingly recognized as sustainable scaffold materials for arsenic adsorption. Their natural abundance, biodegradability, and diverse surface chemistry, characterized by functional groups such as hydroxyl (-OH) and amine (-NH₂), provide effective coordination sites for arsenic species [37,38]. Chitosan, a deacetylated form of chitin, demonstrates notable efficacy owing to its elevated amine content, which protonates to -NH₃⁺ under acidic conditions, facilitating the electrostatic attraction of As(V) oxyanions. Chitosan beads functionalized with molybdate ions exhibit high selectivity for As(V), achieving capacities of up to 40 mg/g; however, their performance is significantly influenced by pH levels [39,40]. Cellulose, the predominant natural polymer, is frequently modified or integrated into composites to improve its stability and functionality. Cellulose-based materials exhibit significant mechanical strength, rendering them appropriate for column applications [41]. Alginate, sourced from brown algae, is recognized for its capacity to create stable hydrogels and beads, frequently utilized for encapsulating reactive materials such as nano-sized metal oxides. Polysaccharide-based composites integrate a sustainable matrix with a high affinity for the dispersed phase, addressing issues such as aggregation and separation difficulties [42]. Alginate beads loaded with iron nanoparticles demonstrate high removal efficiencies for both As(III) and As(V) and can be regenerated for multiple cycles. The primary challenge for pure polysaccharide sorbents lies in their restricted capacity for As(III) and the competition posed by other anions. Nevertheless, through strategic design methods such as chemical grafting, cross-linking, or the creation of hybrid composites, their efficacy can be significantly improved, establishing them as a flexible and environmentally sustainable option for water remediation [37,42].

3. Engineered Adsorbents

The advancement of engineered adsorbents has transformed arsenic removal by allowing meticulous regulation of material characteristics, including surface area, porosity, and active site density [23,25,43]. In contrast to natural adsorbents, these synthetic or modified materials provide consistent performance and superior adsorption capabilities, making them essential for the treatment of extremely polluted water sources. This section analyzes three main kinds of designed adsorbents, using 32 experimental investigations [15,16,23,24,25,26,27,28,29,30,31,32,33,34,36,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65] to evaluate their effectiveness, mechanisms, and practical constraints.

3.1. Metal Oxide Nanoparticles

Metal oxide nanoparticles have been shown to be very efficient arsenic scavengers owing to their elevated surface-to-volume ratios and plentiful surface hydroxyl groups (Figure 3). Muensri et al. [50] performed systematic investigations contrasting titanium dioxide (TiO₂) and zinc oxide (ZnO) nanoparticles under uniform settings (25 mL sample volume, 0.05 g adsorbent dosage, pH 6.0). Their study, conducted with As(V), demonstrated that TiO₂ attained a 97% arsenic removal rate across a wide concentration spectrum (200–2000 ppb), markedly surpassing ZnO’s 84% removal efficacy (Figure 3). The mismatch was ascribed to TiO₂’s greater specific surface area (about 150 m²/g compared to ZnO’s approximately 50 m²/g) and its enhanced affinity for arsenate molecules, as corroborated by X-ray photoelectron spectroscopy (XPS) study [50,51]. The adsorption process adhered to Langmuir isotherms, signifying monolayer coverage, with FTIR spectra exhibiting distinctive changes at 830 cm⁻¹ (As–O stretching) and 420 cm⁻¹ (Ti–O–As bending) [51]. Chandan et al. [43] revealed a significant advancement in sustainable nanoparticle synthesis by creating an eco-friendly method for ZnO production with Acacia catechu leaf extract. Figure 4 illustrates the synthesis of ZnO nanoparticles. The phytochemical-mediated synthesis produced stable particles measuring 20–40 nm with remarkable crystallinity, shown by XRD peaks at 2θ = 31.7°, 34.4°, and 36.2°. The nanoparticles exhibited an arsenic removal effectiveness of 90.3% at neutral pH 7.0, with an adsorption capacity of 0.85 mg/g. Residual flavonoids on nanoparticle surfaces, validated by thermogravimetric analysis (TGA), were seen to augment arsenic binding by chemisorption and electrostatic interactions [43,55]. Long-term performance evaluations indicated a 15–20% drop in capacity after five adsorption–desorption cycles, presumably attributable to partial nanoparticle disintegration in acidic environments [55].

The study by Rajni et al. utilizes published research from the past decade to elucidate synthetic methods and the adsorption characteristics of versatile Polymer Metal Oxide Nanocomposites (PMONCs), assessing their efficacy in arsenic metal removal, which peaked at 83.65% efficiency at lower pH values and 4 bar pressure [66].

Iron oxide-based adsorbents have shown significant efficacy in the removal of arsenite. Salem Attia et al. [61] created a magnetic γ-Fe₂O₃-coated zeolite composite that accomplished 95.6% arsenic (III) removal in under 15 min at pH 3.0. The material’s superparamagnetic characteristics (saturation magnetization = 38 emu/g) facilitated straightforward separation with external magnets, while X-ray absorption near-edge structure (XANES) investigation verified the oxidation of As(III) to the less hazardous As(V) during adsorption. The researchers suggested a twofold mechanism: (1) an initial electrostatic attraction between protonated ≡Fe–OH₂⁺ groups and H₃AsO₃, followed by (2) ligand exchange resulting in the formation of ≡Fe–O–AsO₂ complexes [26]. Regeneration tests using 0.1 M NaOH demonstrated a 93.95% preservation of efficiency after five cycles, while considerable iron leaching (about 2.1 mg/L) was seen [26,60]. Metal oxide nanoparticles have the greatest capacities (TiO₂: 0.99 mg/g; Fe₃O₄: 1.2 mg/g); however, they need regular regeneration [23,26].

3.2. Activated Alumina Systems

Activated alumina (AA) is one of the most extensively used synthetic adsorbents for arsenic removal, especially in community-scale water treatment facilities. Activated alumina systems provide the most reliable performance across various water chemistries (±5% efficiency variance) but exhibit lower capacity (0.1–0.3 mg/g) [25,59]. Majumder et al. [58] used response surface methodology (RSM) to optimize AA performance, determining pH 6.26 as the optimal setting for As(V) removal, achieving 94.06% efficiency at a starting concentration of 0.213 mg/L. Their central composite design investigations indicated that the adsorption capacity reached its maximum with 3.29 g of AA and a contact period of 3.16 h. Extended X-ray absorption fine structure (EXAFS) analyses revealed that arsenate ions formed bidentate binuclear complexes with the alumina surface (Al–O–As bond distance = 3.12 Å), with the adsorption energy computed at −28.6 kJ/mol using density functional theory (DFT) simulations [25]. Recent advancements have concentrated on hybrid AA systems to address intrinsic constraints. Dhanasekaran and Sahu [59] created a sawdust-ferric hydroxide-activated alumina (SFAA) composite that integrated the mechanical stability of activated alumina with the improved arsenic affinity of iron oxides. The SFAA adsorbent demonstrated a maximum capacity of 0.10 mg/g at pH 6.5, with breakthrough column experiments indicating 1200 bed volumes were processed before arsenic concentration above 10 μg/L. The researchers observed that sulfate and phosphate ions decreased removal effectiveness by 15–20%, underscoring the need for pre-treatment for practical applications [59].

3.3. Zeolite-Based Adsorbents

Zeolites have undergone significant modifications to improve their arsenic adsorption efficiency by framework substitution and surface functionalization [67,68,69,70]. Khatamian et al. [60] conducted a comprehensive comparison of Fe₃O₄- and Fe₂O₃-modified zeolite A, revealing that the latter exhibited enhanced removal efficiency (98.52% vs. 95.39%) attributable to more robust Fe–O–As bonding, as verified by Mössbauer spectroscopy. The oxidation state of iron was essential, as Fe(III) exhibited a greater affinity for arsenate than Fe(II) (binding constants: 1.8 × 10⁴ vs. 6.3 × 10³ M⁻¹) [60]. Zeolitic imidazolate frameworks (ZIFs) signify a significant advancement in the design of adsorbents [64,71,72,73]. Jian et al. [64] revealed that ZIF-8 could concurrently eliminate both As(III) and As(V), although via distinct mechanisms: As(V) generated Zn–O–As complexes (EXAFS coordination number = 3.2), while As(III) mostly adhered via van der Waals interactions inside the hydrophobic ZIF-8 pores. The material exhibited exceptional stability in performance throughout extensive pH (2–9) and temperature (10–40 °C) ranges; nevertheless, phosphate competition reduced capacity by 40% [64]. Yin et al. [65] further enhanced this system by integrating zero-valent iron (ZVI) into ZIF-8, resulting in a composite that accomplished 99.9% removal of As(III) and 71.2% removal of As(V) from real acid mine drainage (pH 2.8, 34 mg/L total arsenic). X-ray diffraction examination verified the synthesis of solid FeAsO₄·2H₂O during the adsorption process, facilitating prolonged arsenic immobilization [65]. Advanced zeolites (ZIF-8, Fe-ZIF) exhibit remarkable selectivity but encounter scaling issues [64,65].

4. Advanced Hybrid Materials

The forefront of arsenic adsorption research is characterized by sophisticated hybrid materials that integrate the selectivity of synthetic frameworks with the sustainability of natural elements. These third-generation adsorbents use synergistic effects to attain exceptional removal efficiencies (exceeding 99% in some instances) while tackling scaling issues associated with traditional materials. This section examines two transformational subcategories—metal–organic frameworks (MOFs) and polymer/gel composites.

4.1. Metal–Organic Frameworks (MOFs)

Metal–organic frameworks (MOFs) signify a transformative advancement in adsorbent engineering, characterized by their exceptional porosity (up to 7000 m²/g) and adjustable functionality [74,75]. The remediation of arsenic by metal–organic frameworks (MOFs) presents a potential method for arsenic elimination due to their structural adaptability, customizable pore dimensions, and extensive surface area [76]. Zhu et al. [77] illustrated this using Fe−BTC MOF, wherein iron clusters coordinated with 1,3,5-benzenetricarboxylic acid ligands attained an As(V) capacity of 12.258 mg/g at pH 4.0. X-ray absorption spectroscopy indicated bidentate bridging between AsO₄³⁻ and the Fe₃O clusters (Fe–O–As bond angle = 123.7°), with the adsorption process conforming to the Redlich-Peterson isotherm (R² = 0.998) [77]. The Al-based MIL-53(Al) MOF shown enhanced potential, with Li et al. [78] documenting a capacity of 105.6 mg/g at pH 8.0. This remarkable performance resulted from “breathing effects”—reversible pore enlargement, from 8.5 Å to 13 Å, following hydration, which facilitated deeper arsenic infiltration. Density functional theory (DFT) simulations showed that As(V) binds more strongly to the μ₂-OH groups of MIL-53 (–42.3 kJ/mol) than to Fe−BTC (–38.1 kJ/mol) [78]. Yang et al. [79] evaluated the magnetic CoFe₂O₄@MIL-100(Fe) hybrid in real groundwater (1 mg/L As), attaining: −143.6 mg/g As(III) and 114.8 mg/g As(V) adsorption—Over 90% regeneration efficiency over 12 cycles using 0.1M NaOH. Nonetheless, the obstacles include pH Sensitivity and phosphate Competition. For instance, the capacity of SUM-8 MOF decreases from 152.52 mg/g at pH 2.0 to 28.4 mg/g at pH 10.0 as a result of ligand protonation [80]. The phosphate competition decreases the adsorption of La-MOF-808 by 35% at equimolar concentrations [81].

4.2. Polymer and Gel Adsorbents

4.2.1. Innovative Composites

Polymer hybrids mitigate the mechanical fragility of conventional materials while incorporating multifunctionality [82]. These include waste rubber tire-P(APTAC) and iron hydroxide-PolyHIPE. Remarkably, Imyim et al. [83] grafted poly(3-acrylamidopropyl)trimethylammonium chloride onto tire granules, resulting in 99% removal of As(III) and 92% removal of As(V). Elution with 0.10 M HCl facilitated an 85% recovery of arsenic. In addition, Katsoyiannis et al. [84] applied a coating of Fe(OH)₃ on polymeric high-internal-phase emulsions, achieving arsenic reduction to <10 μg/L (EU standard) via ligand-assisted oxidation of As(III) to As(V).

4.2.2. Cryogel Advancements

Önnby et al. [85] synthesized polyacrylamide cryogels including Al nanoparticles, exhibiting a capacity of 20.3 ± 0.8 mg/g compared to 7.9 ± 0.7 mg/g for the unmodified gel [86]. pH-Responsive released 92% arsenic desorption at pH 12.0. However, the significant challenges lie in the synthesis costs, long-term stability, and selectivity. The manufacturing of MOFs is 3–5 times more costly than that of activated alumina [58,78]. Polymeric adsorbents exhibit around 15% capacity degradation per year in constant flow conditions [83,84]. The majority of hybrids continue to exhibit suboptimal performance at elevated salinity conditions (e.g., >1000 mg/L Cl⁻) [79,85].

5. Adsorption Mechanisms and Modeling

The elimination of arsenic from aqueous systems by adsorption is dictated by intricate interfacial interactions between adsorbent surfaces and arsenic species. Comprehending these mechanisms—from electrostatic attraction to chemical complexation—is essential for enhancing adsorbent design and forecasting performance in practical scenarios. This section consolidates data from 27 studies in this review [7,8,12,13,14,25,31,35,58,60,64,65,77,78,83,85,87] to clarify the predominant adsorption processes, kinetic models, and isotherm behaviors that characterize arsenic absorption across various material classes. To provide a clear conceptual framework, Figure 5 illustrates the primary mechanisms governing arsenic adsorption, which are then discussed in detail below.

5.1. Physicochemical Mechanisms Regulating Arsenic Adsorption

5.1.1. Surface Complexation

The formation of inner-sphere complexes between arsenic oxyanions and metal oxide/hydroxide surfaces constitutes the most resilient adsorption process. EXAFS examination of iron-based adsorbents, such as γ-Fe₂O₃-coated zeolite, verifies the bidentate binuclear coordination of As(V) with Fe(III) sites, with a Fe–O–As bond distance of 3.18 Å and a coordination number of 2.1 [26]. This particular bonding arrangement exemplifies robust, selective chemisorption and is the principal cause for the high effectiveness of iron-based compounds. In MIL-53(Al) MOF [78], arsenate preferentially associates with μ₂-OH bridging sites, exhibiting a binding energy of −42.3 kJ/mol, as determined by density functional theory (DFT). The increased (more negative) binding energy relative to other materials elucidates the enhanced capacity of this MOF, as the reaction is more thermodynamically advantageous.

5.1.2. Electrostatic Interactions

The pH-dependent surface charge of adsorbents significantly affects arsenic removal effectiveness. Activated alumina (AA) demonstrates optimal As(V) adsorption at pH 6.26 [58], characterized by a surface mostly consisting of ≡Al–OH₂⁺ (point of zero charge, PZC = 8.1). This phenomenon arises because the main As(V) species at this pH (H₂AsO4⁻) has a considerable affinity for the positively charged alumina surface. In contrast, the removal of As(III) reaches its maximum at pH 9–10 for iron oxides, attributed to the generation of ≡Fe–O⁻ sites that draw H₃AsO₃ [26,60]. The change in optimal pH indicates the distinct speciation and interaction mechanisms associated with As(III) compared to As(V). Polymer adsorbents such as poly(APTAC)-modified rubber tire use permanent quaternary ammonium groups (–N⁺(CH₃)₃) to facilitate pH-independent electrostatic attraction of As(V). This presents a considerable benefit for treating water with fluctuating pH, since efficacy stays consistent.

5.1.3. Oxidation-Reduction Reactions

Nanoscale zero-valent iron (nZVI) and Fe(II)-containing materials facilitate arsenic immobilization via combined adsorption and reduction processes. X-ray photoelectron spectroscopy (XPS) of Fe-ZIF-8 composites [65] indicates that 68% of adsorbed As(III) is oxidized to As(V), which then precipitates as ferric arsenate (FeAsO₄·2H₂O). This remediation process consists of two stages: initially, toxic As(III) is oxidized to the less toxic As(V), followed by the immobilization of As(V) in a highly stable mineral phase, which significantly decreases leaching potential. This dual process attains a 99.9% elimination of As(III) even in oxygen-deficient groundwater [65].

5.1.4. Supplementary Mechanisms

Physical encapsulation and microbial interaction represent alternative pathways that also play a role in the removal processes, in addition to the primary mechanisms. Physical encapsulation takes place in highly porous materials such as cryogels and metal–organic frameworks (MOFs), wherein arsenic species are confined within nanopores [88]. Ion exchange predominates in materials such as zeolites, where arsenate anions can substitute for other anions (e.g., Cl⁻) within the framework [60]. Microbial interaction is essential for bio-sorbents; bacteria such as Bacillus subtilis adsorb arsenic through functional groups and can actively transform it via enzymatic reduction or oxidation, sequestering it intracellularly [35]. The Elovich kinetic model’s strong correlation with biosorption [35] suggests a heterogeneous surface characterized by multiple active sites engaged in complex biochemical interactions, extending beyond mere physisorption.

5.2. Kinetic and Isothermal Modeling

The examination of kinetic, isotherm, and thermodynamic data is essential for understanding the adsorption mechanism, assessing efficiency, and determining the practical applicability of the process in real-world scenarios.

5.2.1. Kinetics of Adsorption

Kinetic models assess the adsorption rate and identify potential rate-controlling steps. The Elovich model effectively characterizes arsenic absorption on heterogeneous biosorbents like Bacillus subtilis (R² = 0.991), indicating multi-site chemisorption with an initial adsorption rate of 0.11 mg/(g·min). This phenomenon is typical of materials exhibiting a broad distribution of active site energies, commonly found in unmodified biological surfaces.

Pseudo-second-order kinetics are prevalent in the majority of engineered and advanced materials, indicating that chemisorption serves as the main rate-limiting step. The rate constants (k₂) serve as a quantitative metric for assessing the rate of adsorption.

Metal–organic frameworks (MOFs): Fe−BTC exhibits a rate constant (k₂) of 2.3 × 10⁻³ g/(mg·min) [77].

Polymer gels: Al-cryogel exhibits a permeability constant (k₂) of 1.8 × 10⁻³ g/(mg·min) [85].

The comparable k₂ values among these varied materials suggest that the binding event, such as complexation, typically occurs at a similar rate, whereas diffusion to the site, particularly in porous frameworks like MOFs, may play a crucial role.

5.2.2. Adsorption Isotherms

Isotherm models characterize the equilibrium distribution of arsenic between liquid and solid phases, delineating the maximum capacity.

-

Langmuir Model: Relevant for monolayer adsorption on uniform surfaces (e.g., activated alumina, Q_ₘₐₓ = 0.318 mg/g [58]). A high R² value for the Langmuir model indicates a uniform surface characterized by specific, identical sites. This is frequently an idealization, yet it is applicable to numerous synthetic materials.

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The Freundlich Model accounts for multilayer adsorption on heterogeneous substrates, exemplified by red mud (n = 2.14, K_F = 1.83 L/mg) [23]. The Freundlich exponent *n* greater than 1 signifies a favorable adsorption process, with its value serving as an indicator of heterogeneity.

-

The Sips Model presents a hybrid isotherm applicable to advanced materials such as Metal–Organic Frameworks (MOFs). For instance, MIL-53(Al) exhibits a maximum adsorption capacity (Q_ₘₐₓ) of 105.6 mg/g and a heterogeneity index of 1.2 [78]. The Sips model is significant because it simplifies to the Langmuir model under conditions of low heterogeneity and to the Freundlich model under high heterogeneity, thus proving suitable for complex adsorbents with multiple mechanisms.

5.2.3. Thermodynamic Parameters

The thermodynamic analysis elucidates the spontaneity, characteristics, and energy variations associated with the adsorption process.

ΔG° values range from −28.6 kJ/mol for activated alumina to −42.3 kJ/mol for MIL-53(Al), indicating that the adsorption process is spontaneous. The more negative ΔG° for MIL-53(Al) indicates a stronger driving force and a more efficient adsorption reaction relative to activated alumina.

ΔH°: The exothermic characteristic (e.g., −159 kJ/mol for B. subtilis [35]) of most processes indicates the stability of the arsenic-adsorbent complex. A high negative value for biosorption signifies the formation of a robust chemical bond.

ΔS° values that are positive, such as 213 J/(mol·K) for Fe−BTC [77], suggest an enhancement in disorder at the solid–liquid interface during the adsorption process. The release of solvated water molecules from the hydrated arsenate ion and the hydrated adsorbent surface is commonly linked to the formation of an inner-sphere complex, a process that is entropically favorable.

The applicability and appropriateness of these models frequently rely on the characteristics of the adsorbent material.

Table 3. Primary kinetic and isotherm models used to describe arsenic adsorption across different material classes.

Model Type	Model Name	Primary Application	Mechanistic Insight	Selected Refs.
Kinetic	Pseudo-First-Order (PFO)	All types, but often poor fit	Physisorption; pore diffusion	[50,58]
	Pseudo-Second-Order (PSO)	Engineered & Advanced Materials	Chemisorption is rate-limiting	[26,77,83]
	Elovich	Heterogeneous Surfaces (Biosorbents)	Multi-site chemisorption on irregular surfaces	[35]
Isotherm	Langmuir	Homogeneous surfaces (* MOs, * MOFs)	Monolayer coverage on a surface with identical sites	[58,77,78]
	Freundlich	Heterogeneous surfaces (Biosorbents, Waste)	Multilayer adsorption on a surface with sites of different energies	[21,23]
	Sips (Langmuir-Freundlich)	Advanced Materials (MOFs, Composites)	Hybrid model; describes heterogeneous surfaces that approach monolayer capacity	[78]

* Note: MOs: Metal Oxides; MOFs: Metal–Organic Frameworks.

summarizes and compares the predominant models for each adsorbent class, along with their underlying mechanisms.

6. Challenges and Perspectives

Progress in the creation of adsorbents for arsenic removal has been substantial; nonetheless, considerable obstacles remain in translating laboratory-scale achievements to practical applications. This section assesses three primary obstacles—scalability, material stability, and economic viability—based on 22 studies from this review [6,23,25,26,32,35,43,58,59,60,64,65,78,79,83,84,85,87,89], while suggesting practical solutions via emerging technologies and innovative material design. The present obstacles in adsorbent deployment are highlighted below.

(i)

Scalability Limitations

Green-synthesized ZnO nanoparticles [43] exhibit remarkable adsorption (90.3% removal) but encounter batch-to-batch variability (>15% capacity variation) attributed to phytochemical inconsistencies in Acacia catechu extracts. Metal–organic frameworks (MOFs) such as MIL-53(Al) need solvothermal synthesis (24–72 h at 100–120 °C), rendering large-scale manufacture economically unfeasible (~$230/kg compared to $5/kg for activated alumina). Column Reactor Design: Activated alumina systems [58,59] demonstrate fast breakthrough (<1000 bed volumes) in field testing, attributable to (i) channeling effects (15–20% reduced contact efficiency), (ii) fouling caused by natural organic matter (NOM) [59].

(ii)

Concerns regarding Material Stability

Iron Extraction via Leaching: γ-Fe₂O₃-coated zeolite [26] experiences a loss of 2.1 mg Fe/L every cycle, posing a danger of secondary contamination. Polymeric Decomposition: The Poly(APTAC)-modified rubber tire [83] has an annual capacity reduction of 12% due to: Oxidative cleavage of C–N bonds (FTIR evidence at 1650 cm⁻¹), Swelling-induced fracturing in acidic environments (pH < 4).

(iii)

Economic and Logistical Obstacles

NaOH elution (0.1–1.0M) constitutes 60–70% of the operating costs in MOF/polymer systems [79,83]. Thermal regeneration of biochar (200–300 °C) requires 15–20 kWh/m³ of treated water. Spent adsorbents often exceed toxicity thresholds (e.g., 5 mg As/kg for landfill [87]), necessitating stabilization as glassy slag (~$120/ton [23]).

7. Emerging Solutions and Future Directions

Although there is a mechanistic understanding, a notable gap persists in translating these mechanisms into predictable, large-scale performance. The assumption that hybrid materials will inherently demonstrate synergistic effects frequently lacks support from long-term column studies or pilot-scale tests that replicate actual water matrices containing competing ions. The advancement of adsorbent design depends on the application of mechanistic insights to deliberately develop next-generation materials. Embedding photocatalytic TiO₂ within an MOF [90] integrates the oxidation mechanism (TiO₂ converting As(III) to As(V)) with the MOF’s enhanced adsorption capability for As(V), resulting in a synergistic system. Designing composites that incorporate complementary mechanisms, such as a Redox-active component (ZVI) alongside a complexation-heavy component (FeOOH), can effectively address both arsenic species concurrently.

7.1. Advanced Material Engineering

δ-MnO₂@Fe/Co-MOF-74 combined with polyaniline attains a capacity of 300.5 mg/g [90], concurrently enhancing mechanical stability (Young’s modulus = 2.7 GPa compared to 0.3 GPa for pure MOF). Photocatalytic TiO₂-MIL-101(Cr) composites facilitate UV-induced in situ oxidation of As(III) and allow for adsorbent regeneration, exhibiting a 5% capacity reduction after 20 cycles.

7.2. Strategies for Process Intensification

Fe-loaded alginate gels fabricated with triply periodic minimum surfaces (TPMS) demonstrate:

(i) 2.6 times greater throughput compared to packed beds. (ii) 90% reduction in pressure drop (ΔP = 0.3 bar at 10 mL/min) [91]. Neural networks trained on 1240 datasets predict arsenic (As(V)) removal efficiency (R² = 0.94) utilizing pH, surface area, ionic strength, and optimal dosage/contact duration [87].

7.3. Circular Economy Strategies

Electrodialysis of wasted regenerant produces 98% pure As₂O₅ crystals, which are marketable to the semiconductor industry [32]. Spent algal biosorbents converted to biochar have a 40% greater capacity than fresh material owing to mesopore formation [22]. Despite advancements in material innovations that have elevated arsenic adsorption capabilities to remarkable levels (e.g., 300.5 mg/g for MOF hybrids [90]), their practical use is constrained by challenges in manufacturing scalability, long-term stability, and cost-effectiveness. The amalgamation of sophisticated manufacturing (3D printing), artificial intelligence, and circular design principles offers a feasible solution to these difficulties, possibly decreasing treatment costs by 50–70% over the next decade. Subsequent research should emphasize field validations in accordance with WHO-recommended testing settings [6] to close the laboratory-to-community divide.

8. Conclusions

This review systematically analyzes the evolution of adsorbent materials for arsenic removal, outlining a clear progression from first-generation natural wastes to third-generation advanced hybrids. This work’s primary scientific contribution is its comprehensive evaluation of more than 80 studies, establishing a clear framework for material selection according to specific application requirements, whether focused on cost-effective remediation or high-performance treatment. The analysis demonstrates that material innovation has achieved adsorption capacities exceeding 300 mg/g in advanced metal–organic frameworks; however, a significant gap remains between laboratory results and field-scale application.

This review’s major findings can be summarized into three key insights:

There is a distinct inverse relationship between adsorption capacity and practical viability. Natural materials provide a cost-effective solution with over 99% removal efficiency; however, they are characterized by low capacity and limited reusability. In contrast, advanced hybrids such as metal–organic frameworks (MOFs) and composites demonstrate outstanding performance but are constrained by high costs, approximately $230 per kilogram, and intricate synthesis processes.

The Centrality of Mechanisms: Arsenic removal is primarily determined by distinct mechanisms, including surface complexation for metal oxides, electrostatic attraction for polymers, and redox precipitation for iron-based materials. The effectiveness of an adsorbent is fundamentally influenced by the extent to which its design utilizes these mechanisms in the specific water chemistry being targeted.

The Scalability Trilemma: The journey toward commercialization is hindered by an interconnected set of challenges involving scalability, stability, and cost. Addressing one challenge frequently intensifies another; for instance, improving stability via complex synthesis raises costs, whereas lowering costs may undermine performance and stability. This review identifies a primary limitation in the current research landscape: the predominant focus on idealized laboratory conditions, which frequently does not accurately predict performance in real-world environments characterized by complex matrices and variable operating conditions. The primary advantage of adsorption technology is its versatility and potential for sustainable application, especially when utilizing waste-derived materials.

Future efforts must shift focus from solely materials discovery to the field of implementation science. The most promising approach involves the development of intentionally designed hybrid materials that combine the scalability of low-cost supports with the high selectivity of advanced functionalities. The effectiveness of arsenic adsorption technology will ultimately be assessed not by laboratory measurements in milligrams per gram but by the attainment of a sustainable solution costing less than $0.10 per cubic meter, which complies with WHO standards, achieved through interdisciplinary collaboration that addresses economic and engineering constraints.

This review delineates the progression in adsorbent development: beginning with first-generation natural materials (e.g., modified watermelon peel, Q_max ≈ 2.42 µg/g) that enable cost-effective remediation under certain conditions, advancing to second-generation engineered oxides (e.g., TiO₂, Q_max ≈ 0.99 mg/g) that ensure dependable performance, and culminating in third-generation advanced hybrids (e.g., MIL-53(Al), Q_max ≈ 105.6 mg/g) that demonstrate remarkable capacity yet encounter scalability issues.

The optimal application of each adsorbent class depends on a balance of critical factors, including the material’s intrinsic capacity, its optimal pH range, selectivity in the presence of competing ions, and its regeneration potential, all of which have been discussed in detail for the representative materials herein.

The choice of adsorbent is a trade-off between intrinsic performance (capacity) and practical viability (cost, stability); low-cost natural materials can achieve regulatory limits in many real-world waters, while advanced hybrids offer solutions for high-concentration industrial wastes where cost is a secondary factor.