Selenium removal from water using adsorbents: A critical review

Directional Freeze-Drying-Mediated Chitosan Aerogel with Ordered Structure Allowing High-Efficient Removal of Se(IV) from Wastewater

The progress of industrialization has led to a notable elevation in selenium (Se) levels within aquatic environments, surpassing established thresholds and posing significant risks to both human health and ecological equilibrium. Chitosan (CS) exhibits considerable potential in mitigating waterborne pollutants owing to its nontoxic nature, cost-effectiveness, and the presence of abundant hydroxyl and amino functional groups along its backbone. However, its subpar mechanical and thermal stability, susceptibility to acidic dissolution, and challenges in recycling impede its widespread use in water pollution mitigation. To address the aforementioned issues, this study employs a liquid nitrogen-directed freezing process to synthesize chitosan aerogel, aiming to enhance the adsorption efficiency of Se(IV). Morphological and adsorption tests demonstrate that the compact and closely interconnected porous structure facilitates diffusion of Se(IV) into the aerogel, thereby enhancing its adsorption efficiency. The theoretical adsorption capacity of the CS aerogel for Se(IV) is 56.45 mg/g, surpassing that of numerous natural and composite adsorbents, with adsorption equilibrium achieved within 2.5 h. Moreover, the CS aerogel demonstrates substantial potential in remediating Se(IV)-contaminated wastewater and improving circulation stability. A series of characterization results demonstrate that the primary adsorption mechanism of the CS aerogel onto Se(IV) involves electrostatic interactions, complemented by hydrogen bonding between the amino and hydroxyl groups of the CS aerogel and Se(IV), thereby augmenting the adsorption efficacy. This study introduces innovative avenues for tailoring the functionality of 3D macroscopic materials to address the remediation of heavy metals in aquatic environments.

Treatment of aqueous per- and poly-fluoroalkyl substances: A review of biochar adsorbent preparation methods

Per- and poly-fluoroalkyl substances (PFAS) are synthetic chemicals widely used in everyday products, causing elevated concentrations in drinking water and posing a global challenge. While adsorption methods are commonly employed for PFAS removal, the substantial cost and environmental footprint of commercial adsorbents highlight the need for more cost-effective alternatives. Additionally, existing adsorbents exhibit limited effectiveness, particularly against diverse PFAS types, such as short-chain PFAS, necessitating modifications to enhance adsorption capacity. Biochar can be considered a cost-effective and eco-friendly alternative to conventional adsorbents. With abundant feedstocks and favorable physicochemical properties, biochar shows significant potential to be applied as an adsorbent for removing contaminants from water. Despite its effectiveness in adsorbing different inorganic and organic contaminants from water environments, some factors restrict its effective application for PFAS adsorption. These factors are related to the biochar properties, and characteristics of PFAS, as well as water chemistry. Therefore, some modifications have been introduced to overcome these limitations and improve biochar's adsorption capacity. This review explores the preparation conditions, including the pyrolysis process, activation, and modification techniques applied to biochar to enhance its adsorption capacity for different types of PFAS. It addresses critical questions about the adsorption performance of biochar and its composites, mechanisms governing PFAS adsorption, challenges, and future perspectives in this field. The surge in research on biochar for PFAS adsorption indicates a growing interest, making this timely review a valuable resource for future research and an in-depth exploration of biochar's potential in PFAS remediation.

Se(IV)/Se(VI) adsorption mechanisms on natural and on Ca-modified zeolite for Mediterranean soils amended with the modified zeolite: prospects for agronomic applications

In the present study, the ability of a modified CaCl2 zeolite (Ca-Z) to both increase Se(IV) availability and restrict Se(VI) mobility in soils is examined. As it was resulted from batch experiments and verified by X-ray absorption fine structure (XAFS) and X-ray fluorescence (XRF) spectroscopies, higher amounts of both Se species adsorbed on Ca-Z compared to natural zeolite (Z-N) forming outer-sphere complexes while the oxidation state did not alter during agitation of samples. Thereafter, Ca-Z was incorporated in six Greek soils, divided into acid and alkaline, at a 20% (w/w) rate and a series of equilibrium batch experiments were performed with soils alone and soils-Ca-Z mixtures to investigate sorption and desorption processes and mechanisms. The acid soils, either treated with Ca-Z or not, adsorbed higher amounts of Se(IV) than alkaline ones, whereas soils alone did not adsorb Se(VI) but impressively high adsorption of Se(VI) occurred in the Ca-Z-treated soils. Desorption of Se(IV) was higher from the Ca-Z-treated soils and especially from the acid soils. Higher distribution coefficients of desorption than the distribution coefficients of sorption were observed, clearly pointing to a hysteresis mechanism. The experimental data fitted with Langmuir and Freundlich isotherms. In the presence of Ca-Z, the Langmuir qm values increased indicating higher Se(IV) retention while Langmuir bL values decreased suggesting lower bonding strength and higher Se(IV) mobility. Overall, treating the soils with Ca-Z increased Se(IV) adsorption and mobility whereas it provided sites for Se(VI) adsorption that did not exist in the studied soils.

Effects of environmental factors on selenite volatilization by freshwater microalgae

The accumulation and volatilization of Se by algae in surface water are important parts of the biogeochemical cycle of selenium but are also variable and complex. Experiments with 5-8 day of exposure under various temperatures, solution pH values, lighting regimes, and different initial Se concentrations were carried out to study the change in Se accumulation and volatilization behavior of algae. The study showed that algae accumulated and volatilized more Se under harsher environments, such as a lower pH, a shorter lighting time, and a higher Se load. The maximum average daily volatilization rate of Se was 234 ± 23 μg Se (g algae·d)-1, much greater than the values of previous studies. Therefore, in some Se-polluted water environments, when the pH of lakes is acidic, Se emissions to the atmosphere are much higher than currently estimated. Both the accumulation rate (Raccu) and volatilization rate (Rvol) of Se by algae were significantly negatively correlated with final pH, final OD, and residual Se in solution (Cres). Moreover, multiple linear regression equations were used to estimate the rates of Se accumulation and volatilization. This study provides theoretical basis data to quantify the contribution of selenium metabolism by algae to selenium biogeochemistry and a technical reference for the treatment of Se-containing wastewater.

Lignin Microspheres Modified with Magnetite Nanoparticles as a Selenate Highly Porous Adsorbent

Highly porous lignin-based microspheres, modified by magnetite nanoparticles, were used for the first time for the removal of selenate anions, Se(VI), from spiked and real water samples. The influence of experimental conditions: selenate concentration, adsorbent dosage and contact time on the adsorption capacity was investigated in a batch experimental mode. The FTIR, XRD, SEM techniques were used to analyze the structural and morphological properties of the native and exhausted adsorbent. The maximum adsorption capacity was found to be 69.9 mg/g for Se(VI) anions at pH 6.46 from the simulated water samples. The modified natural polymer was efficient in Se(VI) removal from the real (potable) water samples, originated from six cities in the Republic of Serbia, with an overage efficacy of 20%. The regeneration capacity of 61% in one cycle of desorption (0.5 M NaOH as desorption solution) of bio-based adsorbent was gained in this investigation. The examined material demonstrated a significant affinity for Se(VI) oxyanion, but a low potential for multi-cycle material application; consequently, the loaded sorbent could be proposed to be used as a Se fertilizer.

Exceptional removal and immobilization of selenium species by bimetal-organic frameworks

Binary metallic organic frameworks can always play excellent functions for pollutants removal. One binary MOFs, UiO-66(Fe/Zr)), was newly synthesized and applied to remove aquatic selenite (SeIV) and selenate (SeVI). The adsorption behaviors and mechanisms were investigated using batch experiments, spectroscopic analyses, and theoretical calculations (DFT). The characterization results showed that the material inherited the topological structure of UiO-66 and excellent thermal stability. The large specific surface area (467.52 m²/g) and uniform mesoporous structures of the synthesized MOFs resulted in fast adsorption efficiency and high adsorption capacity for selenium species. The adsorbent kept high adsorption efficiency in a wide pH range from 2 to 11 with good anti-interference ability. The maximum adsorption capacity for Se(IV) and Se(VI) reached as high as 196 mg/g at pH 3 and 258 mg/g at pH 5, respectively. The process was conformed to fit pseudo-second-order kinetics and Langmuir isotherm, and could be explained by the formation of Fe/Zr-O-Se bond on the material surface, which was interpreted by the results of XPS, FTIR and DFT calculation. The regeneration and TCLP experiments demonstrated that UiO-66(Fe/Zr) could be regenerated for five cycles without obvious decrease of efficiencies, and the leaching rate of the adsorbed Se(IV) and Se(VI) in the spent adsorbent were only 4.8% and 2.3%. More than 99% of original Se(IV) and Se(VI) in the lake and tap water samples (1.0 mg/L of Se) could be removed in 2.0 h.

Research Progress on Adsorption of Arsenic from Water by Modified Biochar and Its Mechanism: A Review

Arsenic (As) is a non-metallic element, which is widely distributed in nature. Due to its toxicity, arsenic is seriously harmful to human health and the environment. Therefore, it is particularly important to effectively remove arsenic from water. Biochar is a carbon-rich adsorption material with advantages such as large specific surface area, high porosity, and abundant functional groups, but the original biochar has limitations in application, such as limited adsorption capacity and adsorption range. The modified biochar materials have largely enhanced the adsorption capacity of As in water due to their improved physicochemical properties. In this review, the changes in the physicochemical properties of biochar before and after modification were compared by SEM, XRD, XPS, FT-IR, TG, and other characterization techniques. Through the analysis, it was found that the adsorbent dosage and pH are the major factors that influence the As adsorption capacity of the modified biochar. The adsorption process of As by biochar is endothermic, and increasing the reaction temperature is conducive to the progress of adsorption. Results showed that the main mechanisms include complexation, electrostatic interaction, and precipitation for the As removal by the modified biochar. Research in the field of biochar is progressing rapidly, with numerous achievements and new types of biochar-based materials prepared with super-strong adsorption capacity for As. There is still much space for in-depth research in this field. Therefore, the future research interests and applications are put forward in this review.