Mechanism of selenite removal by a mixed adsorbent based on Fe-Mn hydrous oxides studied using X-ray absorption spectroscopy

Layered Double Hydroxides as the Unique Product of Target Ionic Construction for Energy, Chemical, Foods, Cosmetics, Medicine and Ecology Applications

Layered Double Hydroxide (LDH) is an α-modification of the M-host (M2+ ) hydroxide, in which some part of the M-host cations is replaced by M-guest cations (M3+ or M4+ ). The emerging excess positive charge is compensated by the intercalation of anions into the interlayer space, which also contains water molecules. LDHs exhibit anion exchange properties. Targeted ionic design of LDHs via combining three components (M-host, M-guest cations, intercalated anions) allows the creation of a very wide range of highly efficient electrochemical, electrocatalytic, electrochromic substances, catalysts, ion exchangers, sorbents, color pigments, pharmacological drugs, food, and cosmetic additives. In this review, the structure and areas of application of LDHs are considered from the perspective of the targeted ionic design of a substance for a specific application.

Extended X-ray Absorption Fine Structure Revealed the Mechanism of Arsenate Removal by the Fe/Mn Oxide-Based Composite under Conditions of Fully Saturated Sorption Sites

Molecular mechanism of arsenate removal by a promising inorganic composite based on Fe/Mn oxides and MnCO3 was studied under the rarely investigated conditions of fully saturated sorption sites (characteristic of dynamic sorption, such as water treatment plants) at the pH of 4/6/7/8 using As K-edge extended X-ray absorption fine structure (EXAFS)/X-ray absorption near-edge structure (XANES), X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared spectroscopy (FTIR). Comparison of arsenic speciation in the initial adsorbate solution (calculated by Visual MINTEQ) and after sorption (determined by As 3d XPS) allowed the interpretation of the initializing forces of the interfacial processes. Contribution of various solid phases of this composite anion exchanger to the removal of arsenate was disclosed by examining the Fe 2p3/2 and Mn 2p3/2 XPS spectra supported by FTIR. As K-edge EXAFS simulation not only proved the chemisorptive binding of aqueous As(V) anions to the Fe/Mn oxide-based adsorbent but also demonstrated the presence of a variety of sorption sites in this complex structured porous material, which became available step-wise upon an increasing pressure on the interface with high arsenate loading during the long-term sorption process. The type of inner-sphere complexation of As(V) on the saturated surface discovered by As K-edge EXAFS modeling was a function of pH. Analysis of EXAFS fitting data resulted in suggestion of a methodological idea on how the EXAFS-derived coordination numbers can be used to distinguish the localization of adsorbed ions (surface versus structure emptiness). This work also provides more insights into the superiority of composite adsorbents (compared to the materials based on individual compounds) in terms of their capability to adapt/change the molecular sorption mechanism in order to inactivate (remove) more toxic aqueous anions.

Selenium removal from water using adsorbents: A critical review

Selenium (Se) has become an increasingly serious water contamination concern worldwide. It is an essential micronutrient for humans and animals, however, can be extremely toxic if taken in excess. Sorption can be an effective treatment for Se removal from a wide range of water matrices. However, despite the synthesis and application of numerous adsorbents for remediation of aqueous Se, there has been no comprehensive review of the sorption capacities of various natural and synthesized sorbents. Herein, literature from 2010 to 2021 considering Se remediation using 112 adsorbents has been critically reviewed and presented in several comprehensive tables including: clay minerals and waste materials (presented in Table 1); zero-valent iron, iron oxides, and binary iron-based adsorbents (Table 2); other metals-based adsorbents (Table 3); carbon-based adsorbents (Table 4); and other adsorbents (Table 5). Each of these tables, and their relevant sections, summarizes preparation/modification methods, sorption capacities of various Se adsorbents, and proposed model/mechanisms of adsorption. Furthermore, future perspectives have been provided to assist in filling noted research gaps for the development of efficient Se adsorbents for real-world applications. This review will help in preliminary screening of various sorbent media to set up Se treatment technologies for a variety of end-users worldwide.

Porous Nanobimetallic Fe-Mn Cubes with High Valent Mn and Highly Efficient Removal of Arsenic(III)

Iron (Fe) oxides are the most commonly used adsorbent materials for the aqueous removal of Arsenic (As), but they have deficiencies, including low uptake and poor removal of the relatively higher toxicity As(III). Introduction of transition metals into Fe-containing adsorbents, an inexpensive method of altering Fe chemical states, is likewise of interest for removing As(III) from water by means of adsorption. Porous cubic Fe-Mn structures with BET surface area of 450 m² g^-1 were herein prepared via chemical etching of Mn-substituted Prussian Blue analogues (PBAs). Cyclic voltammetry showed a "protective" role of polyvinylpyrrolidone (PVP) during the preparation process, so that Mn with high valence state was readily preserved in this binuclear corner-sharing structure. The calculated reaction Gibbs free energy, which was the most negative of the studied adsorbents, indicated that the adsorption was promoted in the presence of high valence Mn. The structures were capable of directly capturing As(III) oxyanions to below the acceptable limits in drinking water standards, and the saturation uptake capacity of 460 mg g^-1 was substantially higher than comparable materials under consideration. The work therefore presents a new benchmark for As-adsorbent materials and demonstrates the promise of the porous Fe-Mn structure for rapid removal of other metal ions from contaminated water for environmental remediation.

Immobilization of selenite from aqueous solution by structural ferrous hydroxide complexes

Selenium species before and after reaction with FHCs were studied and classified into five groups.