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Review

Selenium Removal Using Nanomaterials and Biosorbents Functionalized with Metal Oxides: A Review

by
Vesna M. Marjanović
1,*,
Dragana Božić
1,* and
Bernd Friedrich
2
1
Mining and Metallurgy Institute Bor, Alberta Ajštajna 1, 19210 Bor, Serbia
2
IME Process Metallurgy and Metal Recycling, Intzestraße 3, 52056 Aachen, Germany
*
Authors to whom correspondence should be addressed.
Metals 2026, 16(5), 490; https://doi.org/10.3390/met16050490
Submission received: 24 February 2026 / Revised: 8 April 2026 / Accepted: 14 April 2026 / Published: 30 April 2026
Versions Notes

Abstract

Water pollution, caused by selenium contamination, is a significant global issue due to its toxic effects on humans and animals. Selenium occurs in several oxidation states, among which selenite and selenate are the most mobile and bioavailable forms. Traditional water treatment methods are often limited in efficiency, whereas adsorption offers a simple, cost-effective, and efficient solution. Various adsorbents, including metal and mineral oxides, carbon-based materials (activated carbon, graphene oxide), biosorbents, and nanocomposites, have shown high potential for Se removal. Adsorbent modifications—physical, chemical, or composite—significantly enhance adsorption capacity, selectivity, and material stability. Studies have demonstrated that nanomaterials and nanocomposites, such as MnFe2O4, PAA-MGO, magnetic MOFs, and magnetite-based biochars, enable rapid removal of Se(IV) and Se(VI) with high adsorption capacities. Se(IV) is primarily adsorbed through innersphere complexation, while Se(VI) forms weaker outer-sphere interactions, explaining differences in removal efficiency. Factors such as pH, the presence of surface hydroxyl and amino groups, surface charge, and competing ions strongly influence the adsorption process. Multivalent ions reduce Se adsorption efficiency, whereas monovalent ions (NO3− and Cl−) have minimal impact. Modified adsorbents, nanomaterials, and nanocomposites provide sustainable and practical solutions for selenium removal from water, combining high efficiency, selectivity, and reusability, making them suitable for real-world water treatment applications.

1. Introduction

Wastewater quality is a major global concern, driven by rapid industrialization and agricultural activities. Increasing pollution of water resources, especially from contaminants such as selenium, requires focused treatment innovations to protect human and environmental health.
Elevated selenium levels in water have become a significant environmental issue. Their occurrence emphasizes a need to evaluate and advance removal methods, as selenium’s toxic effects demand effective action.
Increased selenium content in wastewater is primarily due to anthropogenic activities, including mining and metallurgical activities, coal combustion, oil refining, pesticide production, and the application of selenium-containing compounds in agriculture. Natural sources of selenium mobilization include geological processes such as rock weathering and volcanic activity [1,2]. Increased concentrations of selenite represent a major environmental concern because of its greater solubility and enhanced bioavailability compared to other selenium species [3]. Numerous countries worldwide have reported elevated levels of selenium in drinking water sources. The presence of selenium in drinking water has been shown to adversely affect human and animal health. Conventional materials, methods, and existing technologies fail to meet stringent water quality criteria and regulatory standards. Consequently, increased awareness of selenium toxicity in drinking water has stimulated extensive research into novel materials and approaches for selenium removal [4]. Most technologies employed for the removal of pollutants from water exhibit high removal efficiencies; however, each is associated with specific limitations regarding practical application. For instance, membrane filtration offers high removal efficiency due to the uniformity of the membrane pore size. Nevertheless, its widespread application is constrained by membrane fouling and the operational complexity of the process [5]. Biological wastewater treatment technologies are generally considered environmentally sustainable, yet they typically require longer treatment times and may result in incomplete pollutant removal in certain cases. These limitations highlight the need for careful selection and optimization of treatment methods depending on the target pollutants and specific wastewater characteristics [6]. Based on these considerations, adsorption has emerged as a promising alternative for the removal of pollutants from wastewater. It is widely regarded as a simple, cost-effective, and highly efficient method for eliminating heavy metals and metalloids from water. In the scientific literature, particular attention has been given to the removal of selenium using adsorption, with a focus on the development and application of various types of adsorbents. Adsorption materials can be classified into several categories, including organic resins; metal and mineral oxides (both single and mixed oxides and hydroxides); carbon-based adsorbents such as activated carbon and graphene; bio-sorbents; and adsorbents derived from natural waste. Despite these advantages, adsorption also presents certain limitations, including sensitivity to pH and temperature, interference from competing ions (notably sulfate), and costs associated with regeneration and disposal of spent adsorbents [7,8].
Extensive research has demonstrated that agricultural by-products and waste materials from the food and wood industries can serve as effective low-cost biosorbents for the removal of heavy metal ions from wastewater [9,10,11,12]. Examples include walnut and nut shells, spent grain, olive stones, peanut skins, onion and orange peels, rice husks, leaves, coffee and tea waste, and other plant-based residues. Particular attention has been given to lignocellulosic biosorbents, such as bark, leaves, and wood-derived materials.
Biosorbents contain different functional groups, including hydroxyl-, carboxyl-, sulfhydryl-, amino-, and amide- groups, which can be potentially active sites for attracting and bonding of metal ions [13].
Selenium (Se) is a metalloid with atomic number 34 and is a ubiquitous element in the Earth’s crust, where it ranks 69th in abundance, with an average concentration of approximately 0.004 μg g−1 [2,9]. Selenium occurs in several oxidation states, including selenate (SeO42−, Se(VI)), selenite (SeO32−, Se(IV)), selenide (Se2−, Se(II)), and elemental selenium (Se0). Among these species, selenate and selenite are the most mobile and bioavailable forms and are typically the dominant selenium species present in contaminated waters.
The oxidation state and chemical speciation of selenium (Se) represent key parameters governing its environmental behavior, distribution, and biogeochemical cycling Selenium exhibits a wide range of oxidation states (−II to +VI), and its transformation between these states is strongly controlled by prevailing redox conditions. Under reducing conditions, selenium predominantly occurs in its lower oxidation states, primarily as metal selenides (Se(−I) and Se(−II)) and as elemental selenium (Se0). These reduced forms are generally characterized by low solubility and limited mobility, leading to their accumulation in solid phases such as sediments and soils.
In contrast, under (sub)oxic conditions, selenium is mainly present in its higher oxidation states, Se(IV) and Se(VI), in the form of oxyanions. Specifically, these include selenite (SeO32−) and selenate (SeO42−), which exhibit significantly higher aqueous solubility and mobility compared to reduced selenium species. As a result, these oxidized forms are more readily transported in aquatic systems and are more bioavailable to living organisms, thereby playing a crucial role in both nutrient cycling and toxicity pathways.
Moreover, the speciation of Se(IV) and Se(VI) oxyanions is highly dependent on pH, which regulates their protonation–deprotonation equilibria and, consequently, their dominant aqueous forms. Variations in pH can substantially influence selenium reactivity, including its adsorption onto mineral surfaces, complexation with organic matter, and participation in redox transformations. These processes ultimately determine the environmental fate, persistence, and bioavailability of selenium across different geochemical settings [14,15].
The removal of selenium from wastewater is particularly challenging due to the coexistence of multiple oxidation states, differences in solubility and chemical behavior, and interactions with other dissolved constituents and competing pollutants, which can significantly affect treatment efficiency [16]. A variety of treatment technologies have been applied for the removal of selenium from water, including reverse osmosis [16,17], nanofiltration [1,10,18,19], ion exchanging [20,21], co-precipitation, biological treatment [1,22,23,24,25,26,27], chemical reduction [17,28,29], coagulation [18], and adsorption using different types of adsorbents.
The World Health Organization (WHO) standard for drinking water is 40 μg/L, while the US EPA upper limit is 50 μg/L. The highest permissible selenium concentration in drinking water set forth in Directive (EU) is 20 µg/L. Unlike many heavy metals, selenium is not consistently regulated with fixed emission limit values in EU wastewater legislation. Senium is generally not included among standard regulated parameters, and its discharge limits are typically defined on a case-by-case basis through permitting procedures. In practice, permissible concentrations of selenium in effluents are commonly within the range of 0.01–0.05 mg/L, depending on environmental sensitivity and regulatory framework [30].
In order to provide an overview of recent advancements and trends in this field, a systematic review of relevant scientific papers published between 2010 and 2024 was conducted. A systematic literature review was conducted using databases including ScienceDirect, Web of Science, and Scopus. The search covered the period from 2010 to 2024 using keywords such as “selenium removal”, “adsorption”, “nanomaterials”, and “biosorbents”.
A total of approximately 250 publications were initially identified. After applying inclusion criteria (peer-reviewed articles, relevance to adsorption-based selenium removal, and availability of quantitative data), 120 studies were selected for detailed analysis.

2. Adsorbent Modification

Adsorption is a process in which pollutants are transferred from the liquid phase to the surface of a solid adsorbent through physical and/or chemical interactions, resulting in the formation of interfacial bonds between the adsorbate and the adsorbent [31].
The main advantages of adsorption technologies include simple operation, economic feasibility, the availability of a wide range of adsorbents, and the possibility of adsorbent regeneration. Moreover, the application of various adsorbent modification strategies plays a crucial role in improving adsorption performance and adapting materials to specific treatment requirements [32,33,34].
Methods are commonly classified into physical, chemical, or composite. Physical modification involves altering the physical structure of the adsorbent without changing its chemical composition. Thermal treatment can modify the pore structure and increase the specific surface area, thereby enhancing the adsorption capacity. Another physical modification technique is particle size reduction (grinding or milling), which increases the external surface area and improves adsorption performance.
This approach is commonly applied to zeolites, as it increases the number of accessible active sites for adsorption.
Chemical modification represents a more complex but highly effective approach for improving the adsorption properties of adsorbents. One of the most common chemical modification methods is surface functionalization, which involves the attachment of specific functional groups to the adsorbent surface. For instance, the affinity of adsorbents toward metal ions can be enhanced by introducing amino functional groups, which form strong chemical interactions with ions in solution. This results in improved selectivity and increased adsorption capacity.
Another chemical modification approach is impregnation, in which adsorbents are treated with selected chemical substances that are deposited on the surface or within the pore structure. This process alters the chemical properties of the adsorbent and enhances adsorption efficiency and capacity toward specific target ions in solution.
Ion exchange is also a widely used method for adsorbent modification. This process involves replacing the original exchangeable ions of the adsorbent with other ions. For example, in zeolites, sodium ions can be exchanged with calcium or magnesium ions, thereby modifying the surface charge and affinity toward different ionic species. Ion exchange modification is particularly effective for the removal of heavy metal ions from water. By selecting appropriate exchange ions, targeted removal of specific metal ions can be achieved with high efficiency.
Composite adsorbents often exhibit enhanced adsorption capacity, selectivity, and structural stability compared with single-component adsorbents. They are particularly effective in complex systems where the simultaneous removal of multiple pollutants is required.
Through the careful integration of physical and chemical modification methods, adsorbents with large specific surface areas and tailored functional groups can be developed, resulting in excellent adsorption performance. Overall, modified adsorbents demonstrate improved adsorption efficiency, selectivity, and stability, enabling the effective removal of heavy metals and diverse classes of pollutants. Different modification approaches therefore enhance adsorbent properties and make them especially suitable for the simultaneous removal of multiple types of contaminants.
Activation of surface sites with iron oxides is achieved using various modification methods, such as hydrothermal synthesis, co-precipitation, copolymerization, sol–gel techniques, and others. In addition, adsorbents modified with magnetite can be efficiently separated from the solution using an external magnetic field.
A key factor in adsorption applications is the efficiency and effectiveness of the adsorbent, which depends on its specific surface area, the presence of functional groups on the surface, and strong affinity toward the adsorbate. Magnetite exhibits a strong affinity for selenate, which is crucial for the overall adsorption performance and effectiveness of the material.

3. Adsorbents for Selenium Removal

3.1. Metal Oxides

Among various adsorbent materials, metal oxides have attracted significant attention due to their favorable physicochemical properties, which imply high surface activity due to their unique properties.
Metal oxides, particularly those of aluminum, iron, and silicon dioxide, in both natural and synthetic forms, are among the most widely studied adsorbents for removing selenium from water. These materials are characterized by large specific surface areas and relatively high points of zero charge (pHpzc), resulting in positively charged surfaces over a broad pH range and favorable electrostatic interactions with selenium oxyanions.
Studies on the interactions of selenite (SeO32−) and selenate (SeO42−) with mineral oxides have shown that selenate forms a weakly bound outer-sphere complex, whereas selenite forms a strongly bound inner-sphere complex. This distinction in binding strength explains the generally higher adsorption affinity of selenite compared with selenate on mineral oxide surfaces [7,35].
Research has confirmed that activated alumina, which primarily consists of aluminum oxide (Al2O3), is an effective adsorbent for water treatment applications due to its high surface area, porosity, and strong affinity for target contaminants [36,37,38]. The preparation is carried out by dehydrating aluminum hydroxide at high temperatures. Activated alumina is typically prepared by dehydrating aluminum hydroxide at high temperatures.
It exhibits favorable physical properties, including a well-developed macroporous and mesoporous structure and a large specific surface area. Its point of zero charge (pHpzc) ranges from 8.4 to 9.1, which contributes to a positively charged surface over a wide pH range, enhancing its affinity for anionic species such as selenite and selenate [39,40]. Studies have shown that activated alumina is largely ineffective for the adsorption of Se(VI). Its adsorption performance for Se(IV), however, is highly dependent on the solution pH as well as the presence of SiO2 and arsenic, which can influence the availability of active sites and the overall adsorption efficiency [41].
Iron oxyhydroxides and oxides, including magnetite (Fe3O4) [42,43,44,45], hematite (α-Fe2O3) [37,42], maghemite (γ-Fe2O3) [46], commercial FeOOH adsorbents [11], and iron oxide/hydroxide nanoparticles [12] have been extensively studied for selenium adsorption. In addition, binary metal oxides, such as Al(III)/SiO2 and Fe(III)/SiO2, have been synthesized to enhance the adsorption capacity of SiO2 for both Se(IV) and Se(VI) [47].
Titanium dioxide (TiO2), particularly in its anatase form, is another effective adsorbent for various selenium species [36,48,49]. Among its polymorphs, anatase is a non-toxic mineral characterized by high chemical stability, a large specific surface area, and the ability to participate in both oxidation and reduction reactions. The point of zero charge (pHpzc) of anatase is 6.3 (at 25 °C). The adsorption of selenium on anatase decreases with increasing pH, with selenate adsorption becoming negligible at pH values above 6, whereas selenite adsorption persists to a higher pH range [36,46,48,49].

3.2. Carbon-Based Adsorbents

Activated carbon is widely recognized as a leading commercial adsorbent in water treatment applications, primarily due to its highly porous structure, large surface area, and ability to effectively remove diverse pollutants.
Activated carbon is the most widely used commercial adsorbent for water treatment. It is highly effective for the removal of various organic pollutants, although its adsorption capacity for metal ions and metalloids is sometimes limited. Granular activated carbon (GAC) is typically employed in continuous treatment processes. To enhance the removal of oxyanions, including selenium species, activated carbon can be modified with iron, which improves its affinity for anionic contaminants.
However, such modifications often lead to a reduction in specific surface area and pore volume, which can affect overall adsorption performance [50,51,52,53]. Iron-coated granular activated carbon (Fe-GAC) synthesized by Zhang et al. demonstrated effective selenite removal over a wide pH range (2–8). The adsorption process followed pseudo-second-order kinetics and was well described by the Langmuir isotherm, indicating monolayer adsorption [53].

3.3. Biosorbents

The increasing demand for cost-effective, sustainable, and environmentally friendly water treatment solutions has driven extensive research into non-conventional adsorbents derived from natural resources and waste materials, which offer the potential to reduce preparation costs, utilize readily available by-products, and efficiently remove a wide range of pollutants from aqueous systems.
The main goal of research on biosorbents and adsorbents derived from natural waste is to identify low-cost alternatives to conventional adsorbents, minimizing expenses associated with material preparation, regeneration, and disposal, while utilizing readily available waste or natural materials.
Natural-material-based adsorbents, including biomass-derived polymers, hemicellulose, and lignin, are increasingly used in wastewater treatment due to their cost-effectiveness, high availability—particularly as by-products of the paper industry—and their ability to efficiently remove various pollutants from water.
Agricultural wastes have been utilized to prepare carbon-based sorbents after treatment with hot sulfuric acid, including peanut husk [54] and rice husk [55]. This treatment partially oxidizes cellulose and hemicellulose, generating surface functional groups such as –COOH and –OH, which enhance adsorption. SEM and XRD analyses revealed the presence of elemental selenium particles on the adsorbent surface, indicating the reduction of Se(IV) to Se(0) occurring directly on the surface.
Physical and chemical tests confirmed that carbon oxidation had taken place on the surfaces of peanut and rice husks treated with sulfuric acid [54,55]. Various biological materials, including fungi, algae, aquatic plants, yeast, and fish scales, have also been successfully applied for selenium removal from aqueous systems [56,57,58,59,60,61,62].

3.4. Nanomaterials and Nanocomposites

In recent years, nanomaterials have attracted considerable scientific attention owing to their promising applications across various fields, particularly in water purification technologies. The unique physicochemical properties of nanomaterials and nanoscale particles, such as high specific surface area and enhanced surface reactivity, result in superior adsorption performance. Numerous studies have demonstrated the effectiveness of nanomaterials in the removal of a wide range of contaminants from aqueous systems. Selenium is recognized as a particularly challenging pollutant to remove from water due to its high solubility and the coexistence of multiple oxidation states. Consequently, the application of nanomaterials for selenium removal has emerged as a topic of significant research interest, driven by their high removal efficiency and adsorption capacity [1].
Nanomaterials represent an emerging class of adsorbents in the field of water treatment and have demonstrated high efficiency for selenium removal. Their application as adsorbents has gained increasing attention due to distinctive physicochemical properties, including high specific surface area, readily accessible pore structures, and favorable adsorption–desorption behavior. Among various nanomaterials, iron oxide-based nanomaterials have been extensively investigated for selenium removal because of their strong affinity toward selenium species. Given that selenate in soils and sediments predominantly interacts with Fe(III) oxides and iron hydroxides, both synthetic and natural iron-based nano-adsorbents have been widely applied for the removal of selenium from aqueous systems. Depending on their composition and surface functionality, these nanomaterials can be classified into single- or multi-metal-based nanoparticles, magnetic nanoparticles, and nanocomposites [7,63].
Graphene-based nanomaterials have also been widely explored for pollutant removal owing to their exceptionally high surface area and tunable surface chemistry [64,65,66]. Hydrophilic graphene oxide (GO), rich in surface hydroxyl and carboxyl groups, can be readily functionalized with iron oxide nanoparticles to form magnetic graphene oxide nanocomposites. These materials have demonstrated high adsorption efficiency for both Se(IV) and Se(VI), rapid adsorption kinetics, and facile separation from water using an external magnetic field [67].
Single iron nanoparticles as adsorbents were investigated by Gonzales et al. for selenium adsorption using nanocrystalline iron (III) oxide (Fe3O4) synthesized both via microwave-assisted and conventional methods [68]. The selenium removal efficiency was further evaluated using colloidal iron (Fe3+) oxide/hydroxide nanoparticles (NanoFe), which demonstrated not only high removal efficiency but also good adsorption capacity [12]. Amrani et al. synthesized goethite nanotubes (α-FeOOHNRs) via a rapid hydrolysis method for investigating selenium adsorption [69].
Mixed-metal-based nanoparticles have been investigated for selenium adsorption. The nano-binary oxide MnFe2O4 (synthetic jacobsite), synthesized via a hydrothermal method, has proven to be an efficient adsorbent for selenium oxyanions [70].
Nanocomposite materials can be defined as materials in which nanoparticles are incorporated into a support matrix through modification or functionalization. By nature, these materials are heterogeneous in composition. The advantages of nanocomposite materials include long-term stability, cost-effectiveness, and the combined properties of both nanoparticles and porous supports [71]. Encapsulation of various nanomaterials within macromolecules (polymers) represents a promising strategy for nanocomposite preparation, as different polymers provide distinct benefits that contribute to enhanced adsorption efficiency [7,72]. The removal of selenium using various organic and inorganic materials has proven effective for wastewater treatment.
Many carbon- and graphene-based nanocomposites have shown high efficiency for selenium removal from water. Continuous research aimed at developing effective adsorbents for selenium has led to the improvement of modified graphene oxide (GO) nanocomposites, combining graphene and iron oxides to enhance surface properties through the incorporation of magnetic iron oxide nanoparticles (MGO) [1]. Evans et al. introduced a novel mesoporous carbon framework for magnetic iron oxide nanoparticles (MNA) that selectively adsorbs selenite from wastewater [73].
Metal–organic frameworks (MOFs) consist of organic ligands coordinated to metal centers, providing a high surface area, excellent chemical and thermal stability, and a tunable structure. This adsorbent has also been successfully applied for the simultaneous removal of inorganic selenium and antimony species [74].
Table 1 provides an overview of nanocomposite materials, including combinations of commercial Fe3O4 [75], multi-walled carbon nanotubes with magnetic iron oxide nanoparticles (MIO-MWCNTs) [76], metal–organic frameworks (MOFs) [77,78], polymer-based adsorbents [79] and modified biochars [79,80,81]. Distilled water samples that were boned to remove selenium from the water were spiced with Se at various concentrations.
For example, poly(allyl trimethylammonium)-grafted chitosan/biochar composites (PATMAC-CTS-BC) have shown effective SeO42− removal over a wide pH range (2–10). The dominant removal mechanism involves electrostatic attraction between selenium oxyanions and permanently charged quaternary ammonium groups (=N+), as well as interactions with protonated amino groups and redox–complexation involving amino and hydroxyl functionalities [79].
González et al. (2010) [70] investigated the potential adsorption of selenite and selenate from aqueous solutions using nano-synthesized MnFe2O4 via the batch technique. Nanosynthesized MnFe2O4 (NM) and Jacobsite were synthesized in the laboratory by slow titration of a mixture of Fe2+ and Mn2+ ions. The effects of pH, contact time, and the presence of competitive anions on the adsorption capacity of the synthesized NM were evaluated. The Langmuir isotherm was applied to determine the adsorption capacity of NM. The study demonstrated that the synthetic nano-Jacobsite phase material, with an average grain size of 27.4nm, was capable of removing both selenite and selenate from aqueous solutions across a pH range of 2 to 6.
Optimal adsorption for both selenite and selenate occurred within 5 min of contact time at pH 4 [31].
The research by Lee et al. (2016) demonstrated that magnetic iron oxide nanoparticles/multi-walled carbon nanotubes (MIO– MWCNTs) are effective in removing selenium species (Se(IV) and Se(VI)) from aqueous solutions. Batch experiments were conducted to evaluate the effects of contact time, temperature, solution pH, initial pollutant concentration, and the presence of interfering anions. Adsorption kinetics were best described by a pseudo-second-order model for both Se(IV) and Se(VI). The adsorption capacity for Se(IV) decreased from 9.45 to 4.65 mg/g as the pH increased from 1.8 to 7.1, while for Se(VI) it gradually decreased from 6.09 to 0.11 mg/g over a pH range of 1.9 to 7.0. Equilibrium studies indicated that the Redlich–Peterson model provided the best fit for the adsorption data of both selenium species. According to the Langmuir isotherm, the maximum adsorption capacities were 13.08 mg/g for Se(IV) and 6.13 mg/g for Se(VI). Additionally, this adsorbent was evaluated for arsenic (As) removal from aqueous solutions [76].
Sun et al. (2022) investigated a Zr-based mercapto-functionalized magnetic metal–organic framework, Fe3O4@SiO2@UiO-66- (SH)2 (MUS), for the simultaneous removal of inorganic antimony (Sb) and selenium (Se) species via adsorption. Kinetic studies indicated that the adsorption process followed a pseudo-second-order model, with adsorption rate constants ranging from 0.188 to 0.591 g/mg−1 min−1. The adsorption isotherm was well described by the Langmuir model, with maximum adsorption capacities of 49.0 mg/g for Se(IV) and 27.3 mg/g for Se(VI) at 298 K. The MUS magnetic nanocomposite demonstrates rapid pollutant removal, fast adsorption kinetics, good stability, and high adsorption capacity, making it a promising material for the simultaneous removal of inorganic Sb and Se [77]. Furthermore, the mercapto-functionalized magnetic metal–organic framework (MUS, containing Fe3O4, SiO2, UiO-66-(SH)2) demonstrated efficient SeO42− removal due to its high specific surface area and abundant active adsorption sites. Compared to Cu/Co/Mn-Fe2O4 and Fe3O4/multi-walled carbon nanotubes, MUS exhibited a higher adsorption capacity for inorganic selenium. Moreover, the adsorption capacities of the MUS nanocomposite have been confirmed for the simultaneous removal of multiple anions (primarily Se and Sb), highlighting its strong potential for practical applications [77].
In their study, Sun et al. (2015) investigated three types of spinel ferrite nanoparticles (NPs), MFe2O4 (M = Mn, Cu, Co), synthesized via the hydrothermal method. The results indicated that the adsorption capacities for Se(IV) and Se(VI) followed the order CuFe2O4 > CoFe2O4 >> MnFe2O4. An increase in pH led to decreased adsorption of both Se(IV) and Se(VI). The adsorption capacity of Se(IV) was primarily determined by the hydroxyl group content on the MFe2O4 surface, whereas Se(VI) adsorption largely depended on the surface charge of MFe2O4. Following Se(IV) adsorption, the pHpzc of MFe2O4 NPs shifted to a lower pH, indicating the formation of an inner-sphere complex. After Se(VI) adsorption, the surface charges of MFe2O4 NPs decreased, particularly at pH values below the pHpzc, while the pHpzc remained unchanged, suggesting the formation of outersphere Se(VI) complexes. Attenuated total reflection infrared spectroscopy further confirmed the formation of inner-sphere and outer-sphere complexes for the adsorption of Se(IV) and Se(VI), respectively [82].
Yu et al. (2017) [83] prepared a polyamine-modified magnetic graphene oxide nanocomposite (PAA-MGO) with high adsorption capacity for selenium oxyanions. This adsorbent was easily synthesized via the coprecipitation method at room temperature.
The adsorption capacities of the PAA-MGO nanocomposite at pH 5.8 were 120.1 mg/g for Se(IV) and 83.7 mg/g for Se(VI).
Adsorption isotherms were well described by the Freundlich model for both Se(IV) and Se(VI), indicating multilayer adsorption on the rough surface of PAA-MGO. The maximum adsorption capacities for Se(IV) and Se(VI) were observed at pH 3.1.
According to the Langmuir model, the adsorption capacities were 114.7 mg/g for Se(IV) and 70.9 mg/g for Se(VI) [83].
For the removal of Se(IV) selenite and Se(VI) selenate from aqueous solutions, Guo et al. (2022) employed a newly synthesized binary MOFs, UiO-66(Fe/Zr), with a high specific surface area of 467.52 m2 g−1. The material demonstrated high adsorption efficiency for both Se(IV) and Se(VI) over a wide pH range (2–11). Maximum adsorption capacities of 196.77 mg g−1 for Se(IV) at pH 3 and 258.81 mg g−1 for Se(VI) at pH 5 were reported. The adsorption process followed a pseudo-second-order kinetic model and was well described by the Langmuir isotherm, highlighting the strong potential of binary MOFs for selenium removal from aqueous solutions [78].
Zhang et al. (2020) developed a novel poly(allyltrimethylammonium)-grafted chitosan–biochar composite (PATMAC-CTS-BC) for selenate (SeO42−) removal from water. Batch experiments were conducted following the procedures used in adsorption kinetic studies. The results demonstrated that PATMAC-CTS-BC rapidly and effectively removes selenate, achieving over 97% removal within 10 min. The adsorption kinetics were best described by a pseudo-second-order model. According to the Langmuir isotherm, the maximum adsorption capacity was 98.99 mg/g. This adsorbent exhibits high selenate removal efficiency across a broad pH range (2–10), attributed to the permanent positive charge of quaternary ammonium groups (=N+–) [79].
Hong et al. (2020) [81] investigated iron-impregnated waste biochar (Fe-FWB) for Se(VI) adsorption at selenium concentrations of 100–300 mg/L, pH 7, and 25 °C. Iron-impregnated biochar (Fe-FWB) was synthesized for the removal of Se(VI) from aqueous solutions to achieve high Se(VI) removal efficiency [79]. Adsorption equilibrium was reached within 6 h, and both pseudo-first-order and pseudo-second-order kinetic models adequately described the adsorption process. The adsorption isotherm was well fitted by the Freundlich model, indicating multilayer adsorption of Se(VI) on Fe-FWB with heterogeneous binding energies.
The maximum adsorption capacity of Fe-FWB for Se(VI) was 11.7 mg/g. Increasing the solution pH from 3 to 11 decreased the adsorption capacity from 19.2 to 7.4 mg/g, due to electrostatic repulsion and competition with hydroxyl ions at higher pH. The effect of interfering anions on Se(VI) adsorption followed the order: HCO3− > HPO42− > SO42− > NO3− [81].
Manoko et al. (2022) investigated the application of biochar with magnetite nanoparticles (MBC-SPS-450) at a selenium concentration of 183 mg L−1, pH values of 5, 7, and 9, and a temperature of 23 °C. In their study, they focused on the production of a magnetic biochar composite derived from wastewater treatment sludge as a material for pollutant removal. Both pure, non-demineralized biochar and its magnetized variant were produced from paper waste sludge via co-pyrolysis. The magnetized variant was synthesized using Fe3+ and Fe2+ salts. The resulting biochar exhibited favorable structural and chemical properties suitable for adsorption. The adsorbent MBC-SPS-450 was employed to remove phosphorus (P), selenate (Se), and methylene blue (MB) from wastewater, achieving adsorption capacities of 48.83, 58.43, and 5.92 mg/g, respectively [80]. The highest adsorption capacity was observed at pH 9, reaching 333.33 mg/g according to the Freundlich model [80].
A hybrid adsorbent (ER/DETA/FO/FD), based on a cross-linked copolymer impregnated with iron oxide in the form of goethite, was first developed by Marjanovic et al. (2020) [87] and applied for the adsorption of Se(VI) ions from synthetic and drinking water samples. The effects of initial solution pH, selenate concentration, and contact time on adsorption capacity were investigated.
Equilibrium data were described using the Langmuir model, while adsorption kinetics followed a pseudo-first-order model. The maximum adsorption capacity was 28.8 mg/g. The results indicate that the adsorption process strongly depended on pH, with optimal performance at pH 4. The experimentally determined maximum adsorption capacity was 22.5 mg/g, while the Langmuir model predicted a capacity of 28.8 mg/g, demonstrating the adsorbent’s significant adsorption potential. Based on the pseudo-first-order and pseudo-second-order kinetic models, the adsorption capacities were 7.24 mg/g and 13.99 mg/g, respectively [88].
Abbasi et al. (2024) investigated the synthesis and application of a nano-zerovalent copper biochar composite (nCu0-BC) for the removal of selenium oxyanions from water. The nCu0-BC adsorbent demonstrated high removal efficiency, achieving over 98% removal of both selenite and selenate within 12 h. Adsorption kinetics studies indicated that the process followed a pseudo-second-order model for both Se(IV) and Se(VI). The adsorption isotherm was well described by the Langmuir model, with maximum adsorption capacities of 13.33 mg/g for Se(IV) and 27.66 mg/g for Se(VI) [86].
For the first time, highly porous lignin-based microspheres modified with magnetite nanoparticles (A-LMS Fe3O4) were employed for the removal of Se(VI) selenate from both synthetic water samples and real drinking water. The effects of experimental conditions, including selenate concentration, adsorbent dose, and contact time, on adsorption capacity were investigated using batch experiments. FTIR, XRD, and SEM techniques were applied to analyze the structural and morphological properties of the adsorbent before and after adsorption. The maximum adsorption capacity was determined to be 69.9 mg/g for Se(VI) at pH 6.46 in synthetic water samples. Based on adsorption/desorption results, the Se(VI) removal efficiency reached 99%. Adsorption kinetics were best described by the pseudo-second-order model, with a determined adsorption capacity of 34.94 mg/g for Se(VI). Based on the pseudo-first-order and pseudo-second-order kinetic models, the adsorption capacities were 29.64 mg/g and 41.56 mg/g, respectively, while the Langmuir model indicated a significantly higher capacity of 302.11 mg/g [85].

4. Adsorption Mechanisms

The adsorption of selenite (SeO32−) and selenate (SeO42−) on solid surfaces, such as metal oxides, nanocomposites, or biochars, occurs through different mechanisms due to their distinct chemical structures and charge distributions. Selenite ions (Se(IV)) typically form inner-sphere complexes with adsorbent surfaces. In inner-sphere complexation, the selenite directly binds to surface hydroxyl groups (–OH) of the adsorbent via covalent bonds. This strong binding results in higher adsorption affinity for Se(IV) compared with Se(VI) under similar conditions. The adsorption is often sensitive to pH, as protonation of surface groups enhances electrostatic attraction, while high pH can reduce adsorption due to deprotonation and electrostatic repulsion.
Selenate ions (Se(VI)) usually form outer-sphere complexes with adsorbents. Outer-sphere complexation involves weaker electrostatic interactions between negatively charged selenate ions and positively charged adsorbent surfaces, without direct covalent bonding.
Adsorption of Se(VI) is generally less strong than Se(IV), and its efficiency decreases significantly at higher pH due to increased electrostatic repulsion. The factors influencing binding include surface charge and the point of zero charge (pHpzc). Adsorbents with positively charged surfaces (pH < pHpzc) attract both Se(IV) and Se(VI) anions. Additionally, surface functional groups such as hydroxyl, carboxyl, amino, and mercapto groups enhance complex formation and overall adsorption efficiency.
Sun et al. (2015) reported that the adsorption capacity of Se(IV) was primarily determined by the content of surface hydroxyl groups on MFe2O4, whereas the adsorption capacity of Se(VI) mainly depended on the surface charge of MFe2O4. Increasing the pH resulted in a decrease in the amount of adsorbed Se(IV) and Se(VI). During Se(IV) adsorption on spinel ferrite nanoparticles (MFe2O4 NPs; M = Mn, Cu, Co), the point of zero charge (pHpzc) shifted to lower values. Specifically, the pHpzc decreased from 6.0 to 4.0 for MnFe2O4 NPs, from 6.0 to 4.0 for CuFe2O4 NPs, and from 8.1 to 6.0 for CoFe2O4 NPs, indicating the formation of inner-sphere complexes. In contrast, during Se(VI) adsorption, the surface charge of MFe2O4 NPs decreased, particularly at pH values below pHpzc, while the pHpzc itself remained unchanged, indicating the formation of outer-sphere complexes, as confirmed by XPS spectra [82].
Lu et al. (2017) reported that the enhanced selenium removal by poly(allylamine)-modified magnetic graphene oxide (PAAMGO) is attributed to dual interactions: ligand exchange between surface hydroxyl groups of iron oxide and Se oxyanions, and electrostatic interactions between amine groups and Se oxyanions [83].
Guo et al. (2022) determined that iron and zirconium were involved in the adsorption process through the potential formation of Fe/Zr–O–Se bonds on the surface of the binary MOF material UiO-66(Fe/Zr), as supported by XPS, FTIR, and DFT calculations [78]. Zhang et al. (2021) investigated the adsorption of selenite (Se(IV)) and selenate (Se(VI)) on the poly(allyltrimethylammonium)-grafted chitosan and biochar composite (PATMAC-CTS-BC). The adsorption process followed pseudo-second-order kinetics and the Langmuir isotherm. The removal mechanisms of SeO42− were mainly attributed to electrostatic interactions with quaternary ammonium groups (=N+–) and protonated –NH3+ groups, as well as redox complexation interactions with –NH2, –NH–, and –OH functional groups [79]. Hong et al. (2020) reported that the adsorption mechanism of Se(VI) on Fe-impregnated biochar (Fe-FWB) was analyzed using XPS, and it was mainly observed that outer-sphere complex formation occurred [81].
Manoko et al. (2022) reported that the adsorption mechanism of Se on magnetite biochar composite (MBC-SPS-450) can be classified as involving both outer- and inner-sphere complexation [80]. The dominant adsorption mechanism is outer-sphere complex formation on the surface of Al, Si, and Fe oxides. At pH values above 8, outer-sphere complexation prevails, while at pH values below 7, inner-sphere complexation is dominant [2,81]. The removal of SeO42− anions from solution also occurs through precipitation and inner-sphere complex formation with Ca2+ and Mg2+ ions available on the surface of MBC-SPS-450 [80,88].
For Se, the adsorption mechanism on magnetic biochar composites involves surface complexation, which can be classified as both outer- and inner-sphere. Al, Si, and Fe oxides on the material surface predominantly mediate Se sorption through outer-sphere complex formation. The availability of free Ca2+ and Mg2+ ions also contributes to the efficient removal of SeO42− via precipitation and inner-sphere complexation [80,88].
Spectroscopic analyses were employed to characterize both the unused and spent nCu0-BC. XRD analysis revealed that nCu0- BC contains copper in two crystalline phases: zero-valent copper (Cu0) and copper oxide. XPS analysis indicated that Cu0 serves as the main electron donor for the reduction in Se oxyanions; Se(IV) is reduced to Se0, while Se(VI) is reduced to both Se(IV) and Se0. Density Functional Theory (DFT) simulations demonstrated that both Se(VI) and Se(IV) form stable complexes with nCu0-BC. Molecular dynamics simulations revealed a high adsorption affinity of Se oxyanions toward nCu0-BC due to the presence of Cu0 on its surface, resulting in the formation of stable bidentate binuclear complexes between Se oxyanions and Cu0 [86].
Molecular dynamics simulations revealed a high adsorption affinity of Se oxyanions toward nCu0-BC due to the presence of Cu0 on its surface, resulting in the formation of stable bidentate binuclear complexes between Se oxyanions and Cu0. In this study, the adsorption mechanism of selenate by the investigated adsorbent likely involves both outer- and inner-sphere surface complexation, with the rate-limiting step corresponding to the formation of inner-sphere complexes [87].

5. Effect of Water Matrix and Competing Ions

The presence of coexisting anions in aqueous solutions can significantly influence the adsorption of selenium species. Common anions, such as carbonate (CO32−), phosphate (PO43−), sulfate (SO42−), and nitrate (NO3−), may compete with selenite (SeO32−) and selenate (SeO42−) for available adsorption sites on the adsorbent surface. The extent of interference depends on the charge, concentration, and affinity of the competing ions, as well as the surface properties and functional groups of the adsorbent. Understanding these effects is crucial for evaluating the practical applicability of adsorbents in real water treatment systems.
Gonzales et al. (2010) investigated the effect of adding potential interfering anions (Cl−, NO3−, SO42−, PO43−) on the binding of selenite and selenate to the nano-Jacobsite [70]. The introduction of individual anions Cl− and NO3− into the solution did not significantly affect the adsorption of selenite and selenate. The addition of SO42− had a competitive effect mainly on selenate adsorption, at an initial concentration of 10 ppm, with the effect increasing at 100 ppm. The presence of PO43− reduced selenate adsorption to 87%, while selenite adsorption decreased to 20% [70].
Lee et al. (2016) determined that interfering anions, such as phosphate and carbonate, significantly reduced the removal of arsenic and selenium from solution. In the case of Se(IV), the influence of interfering anions followed the order bicarbonate > phosphate > sulfate. For Se(VI), the effect was in the order bicarbonate > sulfate > phosphate. It should be noted that nitrate did not interfere with selenium removal [76].
Sun et al. (2022) tested the influence of common cations (K+, Na+, Ca2+, Mg2+, Fe3+, Al3+, Cu2+, and Zn2+) and anions (Cl−, Br−, NO3−, SO42−, H2PO4−, HPO42−, and CO32−) on the adsorption behavior of inorganic Sb and Se species on the Zr-based MUS matrix. The presence of anions, particularly H2PO4−, HPO42−, and CO32−, led to a decrease in the adsorbed amounts of inorganic Sb and Se species [77].
Zhang et al. (2021) investigated the influence of different coexisting anions (Cl−, NO3−, HCO3−, and SO42−) on the adsorption of SeO42− onto the PATMAC-CTS-BC adsorbent. The presence of coexisting anions affected the removal efficiency of SeO42− in the following order: SO42− > HCO3− > NO3− > Cl−. Among them, SO42− exhibited the strongest inhibitory effect on SeO42− adsorption, which was attributed to its higher negative charge density. Moreover, a pronounced decrease in SeO42− adsorption capacity was observed with increasing concentrations of coexisting anions from 0 to 10 mmol L−1, indicating a significant influence of ionic strength on the adsorption process. These findings suggest that electrostatic interactions play a major role in the adsorption of SeO42− onto PATMAC-CTS-BC. The effects of coexisting anions were evaluated through batch experiments using solutions containing SeO42− (100 mg L−1) and varying concentrations of coexisting anions (0, 1, 5, and 10 mmol L−1) [79].
The study Kalaitzidou et al. presents a composite material consisting of amorphous iron oxyhydroxides with the addition of Fe3O4 nanoparticles, aimed at achieving a high adsorption capacity for Se(IV) in water. The synthesis involves the separate preparation of both components by oxidative precipitation, followed by their combination (up to 10% Fe3O4) through high-energy wet mixing. The results show that the composite with 5% Fe3O4 at pH 7 achieves an adsorption capacity for Se(IV) of 1.48 mg/g, with only a slight decrease of about 5% compared to pure iron oxyhydroxide. Modification of amorphous schwertmannite with Fe3O4 nanoparticles of approximately 40 nm in size, at loadings up to 10 wt%, resulted in the preservation of Se(IV) adsorption efficiency. At a maximum residual concentration of 20 μg/L, relevant for drinking water applications, an adsorption capacity of 1.48 mg/g for Se(IV) was obtained at pH 7 in water with a natural-like composition. It is important to note that the addition of magnetic Fe3O4 imparts the nanocomposite with the ability to respond to an external magnetic field, enabling easy handling and separation of the adsorbent at the end of the process [84].
Hong et al. (2020) investigated the effects of coexisting anions—bicarbonate, sulfate, nitrate, and phosphate—on the adsorption of Se(VI). The inhibitory effects of these anions followed the order HCO3− > HPO42− > SO42− > NO3−. A decrease in Se(VI) adsorption capacity was observed with increasing concentrations of coexisting anions. The influence of interfering anions on Se(VI) adsorption can be explained by the concept of shared charge of oxyanions. Shared charge is defined as the ratio of the valence of the central atom to the number of coordinated oxygen atoms. A higher shared charge indicates that fewer negative charges are distributed on the oxygen atoms. Consequently, oxyanions with lower shared charge can form stronger bonds with metal sites due to stronger electrostatic interactions [81].
Manoko et al. (2022) analyzed the influence of coexisting phosphorus (P), selenate (Se), and methylene blue (MB) ions on adsorption in order to evaluate the practical applicability of the adsorbent. The results indicated that P exhibited the highest affinity for the adsorbent surface, followed by MB, while Se showed the lowest affinity [80].
Abbasi et al. (2024) [86] reported that minerals present in water can be adsorbed onto the surface of a nano zero-valent copper biochar composite (nCu0–BC), thereby limiting the interaction between selenium oxyanions and the adsorbent. It was found that common water constituents (Ca2+, Mg2+, Na+, K+, Cl−, NO3−, HCO3−, and SO42−) had a minimal effect (<5%) on Se(VI) removal. In contrast, the presence of PO43− significantly reduced the removal efficiency of Se(VI) from 90% to 32%.
Additionally, the presence of Ca2+, Na+, Cl−, NO3−, and HCO3− reduced Se(IV) removal by 7–10% [87].
Overall, the presence of coexisting ions can markedly influence the adsorption of selenium species by competing for active adsorption sites and altering electrostatic interactions. Oxyanions with higher charge density and stronger affinity, particularly phosphate and carbonate species, generally exert the strongest inhibitory effects on selenium adsorption, while monovalent anions such as nitrate and chloride show minimal interference. These findings highlight that the effectiveness of selenium adsorbents under real water conditions strongly depends on water chemistry, emphasizing the importance of considering competing ions when evaluating the practical applicability of adsorption-based treatment systems.
In real wastewater systems, high concentrations of sulfate, phosphate, and carbonate significantly reduce selenium adsorption efficiency. Phosphate ions typically exhibit the strongest competition due to their high affinity for adsorption sites, while nitrate and chloride have minimal effects.

6. Conclusions

Modified adsorbents—through physical, chemical, or composite approaches—demonstrate enhanced efficiency, selectivity, and stability, making them highly effective for the removal of heavy metals and diverse pollutants from water. Metal oxide-, carbon-, and biosorbent-based materials, including those derived from natural waste, have demonstrated high efficiency and versatility for selenium removal from water, offering sustainable and practical solutions for environmental remediation. Nanomaterials are highly promising adsorbents for selenium removal from water due to their large surface area, tunable surface chemistry, and high adsorption efficiency. Iron oxide-based, graphene-based, and mixed-metal nanoparticles all exhibit excellent performance, combining rapid adsorption, selectivity, and ease of recovery. These materials offer versatile and sustainable solutions for environmental remediation, underscoring their potential for practical water purification applications.
Nanocomposite adsorbents efficiently remove Se(IV) and Se(VI) from water by providing high surface area, active sites, and functional groups, with Se(IV) forming strong inner-sphere and Se(VI) weaker outer-sphere complexes.
Selenite (Se(IV)) is predominantly adsorbed through inner-sphere complexation involving ligand exchange with surface functional groups, resulting in stronger binding and higher adsorption affinity. In contrast, selenate (Se(VI)) is mainly retained via outer-sphere electrostatic interactions, although inner-sphere complexation, precipitation, and redox processes may also contribute depending on adsorbent composition and solution chemistry.
Coexisting anions, particularly multivalent species such as CO32−, PO43−, and SO42−, compete with selenium oxyanions for available adsorption sites, thereby reducing removal efficiency. The extent of this interference depends on ion concentration, solution pH, and adsorbent surface properties, highlighting the importance of water chemistry in evaluating the performance and practical applicability of selenium adsorbents in real water treatment systems.
The regeneration and reuse of adsorbents are critical factors for practical application. Most adsorbents retain acceptable efficiency over 3–10 cycles, although gradual performance loss occurs due to surface fouling and structural degradation. Chemical regeneration using alkaline or acidic solutions is commonly applied, but it may lead to secondary waste generation.
Although nanomaterials exhibit superior adsorption performance, their synthesis and operational costs are significantly higher compared to conventional adsorbents such as activated carbon. The economic feasibility of large-scale application depends on material cost, regeneration efficiency and lifespan.
The potential leaching of metal ions and nanoparticles from adsorbents represents an environmental concern. Improper disposal of spent adsorbents may lead to secondary contamination, emphasizing the need for stabilization and safe handling strategies.
Most adsorption studies are conducted under batch conditions. However, real-world applications require continuous flow systems such as fixed-bed columns. Scaling up remains a major challenge due to differences in hydrodynamics and mass transfer.
Future research should focus on the development of bifunctional materials capable of simultaneous adsorption and reduction of selenium species, improved selectivity in complex water matrices, and long-term stability under real conditions.
Despite significant progress, challenges related to cost, regeneration, and environmental safety must be addressed to enable large-scale implementation.

Author Contributions

Conceptualization, methodology, software, validation, formal analysis, investigation, resources, data curation, V.M.M. and D.B.; writing—original draft preparation V.M.M.; writing—review and editing D.B.; visualization, supervision B.F., project administration V.M.M. All authors have read and agreed to the published version of the manuscript.

Funding

This work is financially supported by the Ministry of Science, Technological Development and Innovation of the Republic of Serbia, Agreement on the implementation and financing of scientific research work of the National Research Institute in 2026, No. 451-03-33/2026-03/200052, and the EU under Program 2nd EIT-HEI call: Building Ecosystem Integration Labs at HEI to foster Smart Specialization and Innovation on Sustainable Raw Materials—HEI4S3-RM.

Data Availability Statement

No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Table 1. Experimental Conditions and Adsorption Capacities of Various Fe-Containing Nanocomposite Adsorbents for Selenium Oxyanion Removal from Water.
Table 1. Experimental Conditions and Adsorption Capacities of Various Fe-Containing Nanocomposite Adsorbents for Selenium Oxyanion Removal from Water.
No.Adsorbent Type
Sorbent		Preparation/
Modification Method		Experimental ConditionsAdsorption IsothermKinetic Model, qmax (mg/g)qmax (mg/g)Ref.
1.Nanosynthesized MnFe2O4 (NM), JacobsiteSlow titration of mixture Fe2+ i Mn2+pH = 2–6, T = 25 °C,
cSe = 100 ppb,
t = 5–60 min		Langmuir isotherm/Se(IV) 6.574 ± 0.121Se(VI) 0.769 ± 0.043(Gonzalez et al., 2010) [70]
2.Magnetic iron oxide nanoparticles/multi-walled
carbon nanotubes
(MIO–MWCNTs)		Incipient wetness methodSe(IV)Langmuir isothermT (°C)qmax (mg g−1)Se(IV) 13.08(Lee & Kim, 2016) [76]
PFO
pH = 1.7–7.9
T = 15, 30, 45 °C,
cSe = 5–100 mg/L,
t = 15–240 min		157.987
307.776
457.771
PSO
158.525
308.315
458.149
Se(VI)Langmuir isothermPFOSe(VI) 6.13
pH = 1.9–7.0
T = 15, 30, 45 °C,
cSe = 5–100 mg/L,
t = 15–240 min		153.799
303.757
453.640
PSO
153.928
303.843
453.779
3.Mercapto-functionalized magnetic metal–organic framework based on Zr MUS:
Fe3O4@SiO2@UiO-66-(SH)2		Coprecipitation and sol–gel methodpH = 2, T = 25 °C,
cSe = 10–360 mg/L,		Langmuir isothermPSOSe(IV)Se(VI)Se(IV)
49.0		Se(VI)
27.3		(N. Sun et al., 2022) [77]
 0.3330.591
4.Spinel ferrite nanoparticles MFe2O4NP
(M = Mn, Cu, Co)		Hydrothermal methodpH = 2–11, T = 25 °C,
cSe = 1–25 mg/L,
t = 15–300 min		 PFOSe(IV)Se(VI)Se(IV)
3.90
11.6
14.1		Se(VI)
5.27
5.55
5.97		(W. Sun et al., 2015) [82]
MnFe4
CoFeO4
CuFeO4		3.31
5.05
7.58		2.19
3.03
3.92
PSO  
MnFeO4
CoFeO4
CuFeO4		2.96
13.8
14.2		3.45
3.82
3.85
5.Poly (allylamine)-modified magnetic graphene oxide (PAA-MGO)CoprecipitationpH = 5.8, T = 25 °C,
cSe = 4 mg/L		Langmuir isotherm-Se(IV)
120.1		Se(VI)
83.7		(Lu et al., 2017) [83]
Se(IV)
114.7 mg/g		Se(VI)
70.9 mg/g		    
6.Binary MOFs material
UiO-66(Fe/Zr)		Hydrothermal methodpH = 2–11
T = 25, 35, 45 °C,
cSe = 10–200 mg/L,
t = 10–250 min		Langmuir isothermPSOSe(IV)
196.77		Se(VI)
258.81		(Guo et al., 2022) [78]
7.Poly(cetyltrimethylammonium) grafted chitosan and biochar composite (PATMAC-CTS-BC)Polymerization/precipitation processpH = 1–10,
T = 25, 30, 35, 40 °C,
cSe = 0–500 mg/L,
t = 10–1200 min		Langmuir isotherm
98.99 mg/g		PFOSe(IV)
36.97		/(Zhang et al., 2021) [79]
PSOSe(VI)
37.39
8.Iron oxyhydroxide (FeOOH) + magnetite (Fe3O4)Oxidative precipitationConcentration of Se 100 mg/L
Contact time of 20–60 min,
pH = 7,
Room temperature		/Freundlich parameter
5%
10%
FeOOH
 
Langmuir parameters
5%
10%
FeOOH		Q20 (mg/g)
 
1.48
1.56
1.48
 
Qmax (mg/g)
 
12.8
8.5
10.8
 
1.48 mg/g[84]
9.Waste biochar impregnated with iron (Fe-FWB)Pyrolysis processpH = 3–11,
t = 15, 25, 35 °C,
cSe = 100–300 mg/L		Langmir isotherm/Se(VI)
11.7		(Hong et al., 2020) [81]
10.Biochar with nanoparticles of magnetite MBC-SPS-450/pH = 5, 7, 9 T = 23 °C,
cSe = 183 mg/L, t = 15 min–24 h		Langmir isotherm
38,462 mg/g		PSOSe(VI)
58.43		(Manoko et al., 2022) [80]
11.Lignin Microspheres Modified with Magnetite Nanoparticles: A-LMS Fe3O4Coprecipitation process/
copolymerization		pH = 6.45, T = 22 °C,
cSe = 7.75 mg/L, t = 30–300 min		Langmir isotherm
30,211 mg/g		PFOSe(VI)
29.64		Se(VI)
69.9		(Marjanovic et al., 2022) [85]
PSOSe(VI)
41.56
12.Nano-zerovalent copper biochar composite
(nCu0-BC)		/pH = 2–10,
t = 0–12 h,
cSe = 1–20 mg/L		Langmir isothermPFO mg/g
nCu0-BC
(2 g/L)		Se(IV) 4.95
Se(VI) 4.91		/(Abbasi et al., 2024) [86]
 
Se(IV)
13.33 mg/g
nCu0-BC
(3 g/L)		Se(IV) 4.94
Se(VI) 5.05
 PSO mg/g
nCu0-BC
(2 g/L)		Se(IV) 3.17
Se(VI) 3.18
Se(VI)
27.66 mg/g
nCu0-BC
(3 g/L)		Se(IV) 3.19
Se(VI) 3.14
13.Iron oxide impregnated
hybrid polymer ER/DETA/FO/FD		PolymerizationpH = 2–11,
t = 15–500 min
T = 22 °C,
cSe = 0.1–5 mg/L		Langmuir model
28.8 mg/g		PFO Se(VI)
7.24		/Se(VI)
22.5		(Marjanovic et al., 2020) [87]
PSO Se(VI)
13.99		/
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Marjanović, V.M.; Božić, D.; Friedrich, B. Selenium Removal Using Nanomaterials and Biosorbents Functionalized with Metal Oxides: A Review. Metals 2026, 16, 490. https://doi.org/10.3390/met16050490

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Marjanović VM, Božić D, Friedrich B. Selenium Removal Using Nanomaterials and Biosorbents Functionalized with Metal Oxides: A Review. Metals. 2026; 16(5):490. https://doi.org/10.3390/met16050490

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Marjanović, Vesna M., Dragana Božić, and Bernd Friedrich. 2026. "Selenium Removal Using Nanomaterials and Biosorbents Functionalized with Metal Oxides: A Review" Metals 16, no. 5: 490. https://doi.org/10.3390/met16050490

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Marjanović, V. M., Božić, D., & Friedrich, B. (2026). Selenium Removal Using Nanomaterials and Biosorbents Functionalized with Metal Oxides: A Review. Metals, 16(5), 490. https://doi.org/10.3390/met16050490

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