Chromium(VI) Removal from Aqueous Solutions by Mg−Al−CO3 Hydrotalcite: Sorption−Desorption Kinetic and Equilibrium Studies

Structured LDH/Bentonite Composites for Chromium Removal and Recovery from Aqueous Solutions

This study focuses on chromium removal through adsorption and ion exchange using structured calcined layered double hydroxide (LDH) (MgAl)-bentonite composites. Firstly, the powders were structured into granulates to study the effect on Cr sorption kinetics to circumvent the limitations of working with powders in real-life applications. Secondly, the regeneration of the structured composites was optimized to enable multi-cycling operation, which is the key for their applicability beyond laboratory scale. Firstly, the LDH/bentonite ratio was optimized to obtain the best performance for the removal of Cr³⁺ and Cr⁶⁺ species. In powder form, the calcined adsorbent containing 80 wt% LDH and 20 wt% bentonite performed best with an adsorption capacity of 48 and 40 mg/g for Cr³⁺ and Cr⁶⁺, respectively. The desorption was optimized by studying the effect of the NaCl concentration and pH, with a 2 M NaCl solution without pH modification being optimal. The kinetic data of the adsorption and desorption steps were modelled, revealing a pseudo-second order model for both. This was also demonstrated using XRD and Raman measurements after the Cr³⁺ and Cr⁶⁺ adsorption tests, indicating successful uptake and revealing the adsorption mechanism. Finally, five consecutive adsorption-desorption cycles were performed, each showing nearly 100% adsorption and desorption.

Cr(VI) removal from contaminated waters using ultra-thin layered meixnerite

Cr(VI) is of great concern to public health and environmental safety due to its high toxicity. Here, we report a low-cost yet highly efficient method to prepare a novel LDH, ultra-thin layered meixnerite, which performed superiorly in treatment of aqueous Cr(VI) with little secondary pollution being induced. The produced ultra-thin layered meixnerite was composed of nanoparticles with a thickness of around 7 nm, less than 9 times the thickness of a single LDH layer. The XRD patterns of the ultra-thin layered meixnerite, in which the characteristic diffraction peaks of a typical LDH were weakened or even disappeared, confirmed the successful delamination. This special morphology of the ultra-thin layered meixnerite was not only helpful to its full dispersion in the Cr(VI)-bearing solutions but also facilitated the formation of more active sorption sites on its external surface. As a result, the maximum sorption capacity of UTLM for Cr(VI) removal was 480.9 mg g^-1, far higher than that of OM (196.9 mg g^-1). In addition to electrostatic attraction and anion exchange, the ultra-thin layered meixnerite could also become restacked during removal of aqueous Cr(VI) to generate inner-sphere complexation, finally inducing an enhanced Cr(VI) uptake. Furthermore, XPS analysis characterized the promotion of the break of Al-OH bond with the increase in temperature, and the Cr-O peak increased correspondingly from 29.69% at 25 °C to 48.77% at 85 °C, resulting that the ultra-thin layered meixnerite could remove Cr(VI) more effectively at higher reaction temperatures. Therefore, ultra-thin layered meixnerite is very suitable for future application in treatment of industrial wastewaters with elevated temperatures.

Comparative study of low-cost fluoride removal by layered double hydroxides, geopolymers, softening pellets and struvite

Excessive F^- in drinking water due to natural and anthropogenic activities is a serious health hazard affecting humans worldwide. In this study, a comparative assessment was made of eight mineral-based materials with advantageous structural properties for F^- uptake: layered-double-hydroxides (LDHs), geopolymers, softening pellets and struvite. These materials are considered low-cost, for being either a waste or by-product, or can be locally-sourced. It can be concluded that Ca-based materials showed the strongest affinity for F^- (Ca-Al-CO₃ LDHs, slag-based geopolymer, softening pellets). The Langmuir adsorption capacity of Ca-Al-CO₃ LDHs, slag-based geopolymer and softening pellets was observed to be 20.83, 5.23 and 1.20 mg/g, respectively. The main mechanism of F^- uptake on Ca-Al-CO₃ LDHs, Mg-Al-Cl LDHs, slag-based geopolymers and softening pellets was found to be sorption at low initial F^- concentrations (<10 mg/L) whereas precipitation as CaF₂ is proposed to play a major role at higher initial F^- concentrations (>20 mg/L). Although the softening pellets had the highest Ca-content (96-97%; XRF), their dense structure and consequent low BET surface area (2-3 m²/g), resulted in poorer performance than the Ca-based LDHs and slag-based geopolymers. Nevertheless, geopolymers, as well as struvite, were not considered to be of interest for application in water treatment, as they would need modification due to their poor stability and/or F^- leaching.

Use of LDH- chromate adsorption co-product as an air purification photocatalyst

This work deals with the use of layered double hydroxides for a double environmental remediation. The residue obtained in the use of these materials as a chromate sorbent in water, was subsequently studied as a photocatalyst for the removal of NO_x gases. With this aim, MgAl-CO₃ layered double hydroxides were synthesized by the coprecipitation method with a divalent/trivalent metal ratio of 3. After its calcination at 500 °C, the mixed oxide was obtained and MgAl-CrO₄ were synthesized by the reconstruction method. A complete chemical, morphological and photochemical study of the samples was carried out with techniques such as XRD, FT-IR, TGA, XRF, PL, DRIFTS and UV-Vis spectroscopy. Results showed that LDH materials presented no significant changes in their structure after their use as a sorbent. Photocatalytic tests of the samples showed a very good NO removal efficiency, as well as a high selectivity (low NO₂ emissions) through complete oxidation of these oxides to nitrate. The incorporation of chromate into the LDH structure improved the absorption of light in the visible region of the spectra, producing an improvement of 20% in the NO elimination compared with the LDH without chromate.

Synchronous reduction-fixation of reducible heavy metals from aqueous solutions: Application of novel mesoporous MFT/SBA-15 composite materials

The usual treatment for Cr(VI)-contaminated wastewater primarily included reduction, adsorption, and the subsequent separation of the Cr-laden adsorbent. Among these factors, the adsorbent is the most critical factor in determining Cr removal efficiency. In this study, a novel melamine-formaldehyde-thiourea (MFT) chelating resin/mesoporous silica composite material (MFT/SBA-15) was synthesized via a co-condensation method and used for the reduction and fixation of Cr(VI)-contaminated water. Cr(VI) adsorption onto MFT/SBA-15 obeyed the pseudo-second-order model, and the chemical adsorption was the rate-limiting step in the adsorption process. Also it followed the Langmuir adsorption model, with single molecular layer adsorption characteristics. The organic components within MFT/SBA-15 were the core functional groups for Cr(VI) adsorption, and the formation of a coordination bond (CS→Cr) between the lone electron pairs of the S atom and Cr during the adsorption process led to the synchronous reduction-fixation processes of Cr(VI). These synchronous effects were further demonstrated for other reducible heavy metals, including As(V) and Cu(II), but negligibly observed in chemically stable elements, such as Zn(II), Ni(II), Pb(II), Cd(II), and As(III). The novel mesoporous MFT/SBA-15 materials combine the advantages of the chelating resin and mesoporous silica and have excellent potential for the wastewater treatment of reducible heavy metals through synchronous reduction-fixation.

Synthesis of nickel-iron layered double hydroxide via topochemical approach: Enhanced surface charge density for rapid hexavalent chromium removal

Hexavalent chromium (Cr(VI)) is considered to be a potential metal contaminant because of its toxicity and carcinogenicity. In this work, the surface charge density of nickel-iron layered double hydroxide (NiFe LDH) is tuned through iron valence change to improve the performance in adsorption of Cr(VI). The addition of iron divalent in the precursor enhances the surface positivity and reducibility of Fe²⁺-NiFe LDH, resulting in a nearly 150% Cr(VI) maximum adsorption capacity improvement. The increase of hydroxyl groups and charge density on the surface of NiFe LDH is due to the topological chemical transition from Ni²⁺-Fe²⁺ LDH to Ni²⁺-Fe³⁺ LDH. The adsorption of Cr(VI) onto Fe²⁺-NiFe LDH prepared via topochemical approach is highly pH-dependent. The adsorption dynamics and isotherms results may be clearly elucidated by the pseudo-second-order model and Langmuir isotherm model. Electrostatic attraction, interlayer anion exchange and adsorption-coupled reduction are proven to be the main Cr(VI) removal mechanisms for Fe²⁺-NiFe LDH. This finding demonstrates that Fe²⁺-NiFe LDH adsorbents have potential application for efficient removal of Cr(VI) pollutants.

Innovative Magnetite Based Polymeric Nanocomposite for Simultaneous Removal of Methyl Orange and Hexavalent Chromium from Water

One of the most important directions for environmental remediation is the effective removal of dyes and toxic heavy metals from water using newly fabricated nanoadsorbents. Here, magnetic Fe3O4 nanoparticles were combined with nitrogen-containing functional group polymers chitosan (CS) and polypyrrole (ppy) to synthesize a nanocomposite (polypyrrole@magnetic chitosan) useful for removing methyl orange (MO) and hexavalent chromium (Cr (VI)) from water. The physicochemical properties of the nanocomposite were determined using SEM, TEM, XRD, FT–IR, and TGA techniques. The effect of different factors on the adsorption system was studied including the contact time, pH, and the effect of co-existed ions. The kinetic study illustrated that the adsorption fit well with Langmuir isotherm. The maximum adsorption capacity of MO and Cr (VI) was found to be 95 and 105 mg/g, respectively. The reusability of the nanocomposite was studied for up to five cycles using 0.1 M NaOH as eluent with a slight decrease of adsorbent efficiency. Furthermore, the removal mechanism studied suggested the removal of MO via adsorption and Cr (VI) via chemical reduction and adsorption. This study suggests that a ppy@magnetic chitosan nanocomposite is a promising nanoadsorbent for removing MO and Cr (VI) from water.

Enhanced adsorption of hexavalent chromium and the microbial effect on quartz sand modified with Al-layered double hydroxides

To enhance the hexavalent chromium (Cr(VI)) removal performance of simulated constructed rapid infiltration systems (CRIS) with quartz sand (QS) substrate, QS coated with Al-layered double hydroxides (Al-LDHs@QS) was prepared by the co-precipitation method under alkaline conditions. A scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and X-ray diffractometer (XRD) were used to characterize QS before and after modification. The result showed that the Al-LDHs were successfully coated on the surface of the QS. The isotherm adsorption experiment indicated that compared with the original QS, the adsorption property of the modified QS changed from monolayered chemical adsorption to multilayered physical adsorption, perhaps because of different types of adsorption forces. Moreover, the adsorption capacity of modified QS was significantly enhanced and ZnAl-LDHs@QS had a maximum adsorption capacity (1428.57 mg·kg^-1) nearly 6 times greater than that of the original QS (232.56 mg·kg^-1). Adsorption experiments at different pH showed that the adsorption capacity of ZnAl-LDHs@QS gradually increased as acidity decreased. High-throughput sequencing revealed that the relative abundance of chrome-tolerant microorganisms at the phylum and family levels were increased in modified QS compared with original QS. Hemocytometer counting revealed enhanced microbial quantity on the surface of QS after modification. The content of extracellular polymeric substances (EPS) and the enzymatic activity of the microorganisms adhered to the surface of modified and original QS were detected, results showed that Al-LDHs had an obvious influence on the promotion of EPS secretion and enhanced the enzymatic activity of microorganisms. These changes indicated that the modified QS created better conditions for microorganism growth, and the improved microbial effect caused strong biosorption, resulting in greatly enhanced Cr(VI) removal. Thus, ZnAl-LDHs@QS is a better choice for CRIS to remove Cr(VI).

A Revised Pseudo-Second-Order Kinetic Model for Adsorption, Sensitive to Changes in Adsorbate and Adsorbent Concentrations

The development of new adsorbent materials for the removal of toxic contaminants from drinking water is crucial toward achieving the United Nations Sustainable Development Goal 6 (clean water and sanitation). The characterization of these materials includes fitting models of adsorption kinetics to experimental data, most commonly the pseudo-second-order (PSO) model. The PSO model, however, is not sensitive to parameters such as adsorbate and adsorbent concentrations (C₀ and C_s) and consequently is not able to predict changes in performance as a function of operating conditions. Furthermore, the experimental conditionality of the PSO rate constant, k₂, can lead to erroneous conclusions when comparing literature results. In this study, we analyze 103 kinetic experiments from 47 literature sources to develop a relatively simple modification of the PSO rate equation, yielding dqtdt=k'Ct(1-qtqe)2. Unlike the original PSO model, this revised rate equation (rPSO) provides the first-order and zero-order dependencies upon C₀ and C_s that we observe empirically. Our new model reduces the residual sum of squares by 66% when using a single rate constant to model multiple adsorption experiments with varying initial conditions. Furthermore, we demonstrate how the rPSO rate constant k' is more appropriate for comparing literature studies, highlighting faster kinetics in the adsorption of arsenic onto alumina versus iron oxides. This revised rate equation should find applications in engineering studies, especially since the rPSO rate constant k' does not show a counter-intuitive inverse relationship with increasing reaction rates when C₀ is increased, unlike the PSO rate constant k₂.

Evaluation of Adsorption Mechanism of Chromium(VI) Ion Using Ni-Al Type and Ni-Al-Zr Type Hydroxides

To evaluate the feasibility of nickel–aluminum (the Ni2+:Al3+ molar ratios of 1.0:1.0 and 1.0:2.0 are denoted as NA11 and NA12, respectively) and nickel–aluminum–zirconium type (the Ni2+:Al3+:Zr4+ molar ratios of 0.9:1.0:0.09 and 0.9:2.0:0.09 are denoted as NAZ1 and NAZ2, respectively) hydroxides for Cr(VI) removal from aqueous media, the adsorption capability and adsorption mechanism of Cr(VI) using the above-mentioned adsorbents were investigated in this study. The quantity of Cr(VI) adsorbed onto NA11, NA12, NAZ1, and NAZ2 was 25.5, 25.6, 24.1, and 24.6 mg g−1, respectively. However, the quantity of aluminum (base metal) released from NA11 (approximately 0.14 mg g−1) was higher than that from NAZ1 (approximately 1.0 µg g−1), indicating that NAZ1 was more suitable for Cr(VI) removal than NA11. In addition, the effects of pH, contact time, and temperature on the adsorption of Cr(VI) were evaluated. Moreover, to elucidate the adsorption mechanism of Cr(VI) using NA11 and NAZ1, the elemental distribution, X-ray photoelectron spectrometry spectra, and ion exchange capability were also determined. Cr(VI) adsorbed onto the NAZ1 surface was easily desorbed using a sodium hydroxide solution under our experimental conditions. The information regarding this study can be useful for removing Cr(VI) from aqueous media.

Nd(III) and Gd(III) Sorption on Mesoporous Amine-Functionalized Polymer/SiO2 Composite

The strong demand for rare-earth elements (REEs) is driven by their wide use in high-tech devices. New processes have to be developed for valorizing low-grade ores or alternative metal sources (such as wastes and spent materials). The present work contributed to the development of new sorbents for the recovery of rare earth ions from aqueous solutions. Functionalized mesoporous silica composite was synthesized by grafting diethylenetriamine onto composite support. The physical and chemical properties of the new sorbent are characterized using BET, TGA, elemental analysis, titration, FTIR, and XPS spectroscopies to identify the reactive groups (amine groups: 3.25 mmol N g-1 and 3.41 by EA and titration, respectively) and their mode of interaction with Nd(III) and Gd(III). The sorption capacity at the optimum pH (i.e., 4) reaches 0.9 mmol Nd g-1 and 1 mmol Gd g-1. Uptake kinetics are modeled by the pseudo-first-order rate equation (equilibrium time: 30-40 min). At pH close to 4-5, the sorbent shows high selectivity for rare-earth elements against alkali-earth elements. This selectivity is confirmed by the efficient recovery of REEs from acidic leachates of gibbsite ore. After elution (using 0.5 M HCl solutions), selective precipitation (using oxalate solutions), and calcination, pure rare earth oxides were obtained. The sorbent shows promising perspective due to its high and fast sorption properties for REEs, good recycling, and high selectivity.

Investigation of Fractal-like Characteristics According to New Kinetic Equation of Desorption

Most of the adsorbents have porous structures and a suitable kinetic model is essential for studying these systems. The kinetic Langmuir model is one of the first theoretical models, which can be used for desorption studies. In the present research, the fractal-like concept was added to the kinetic Langmuir model of desorption. A new integrated kinetic Langmuir equation was provided to investigate the rate of desorption from a solid surface. The preferred characteristic of the provided rate equation is the application of the fractal concept for the kinetic study of the desorption process from porous surfaces. The derivation of a new equation was confirmed using the generated data. The fractal-like concept for some experimental desorption studies was obtained. This parameter can show how the porous structure of an adsorbent can affect the desorption kinetics.

Application of Zn/Al layered double hydroxides for the removal of nano-scale plastic debris from aqueous systems

Nano-scale plastic debris (NPDs) are emerging as potential contaminants as they can be easily ingested by aquatic organisms and carry many pollutants in the environment. This study is aimed to remove NPDs from aqueous environment for the first time by using eco-friendly adsorption techniques. Initially, the interaction between NPDs and synthesized Zn-Al layered double hydroxide (LDH) was confirmed by pH titration of Zn-Al LDH against NPDs at varying mass ratio (50:1 to 50:7) and FTIR analysis for both before and after 2 h of contact time. Fast removal was observed in deionized water and synthetic freshwater with maximum sorption capacity (Q_max) of 164.49 mg/g,162.62 mg/g, respectively, according to Sips isotherm. Whereas, removal was least in synthetic hard water having a Q_max value of 53 mg/g. For 2 mM concentration of SO₄^2- and PO₄^3-, the adsorption capacity significantly decreased to 2%. The removal efficiency was found 100 % at pH 4, while at pH 9, it reached 37 % due to increased competitive binding and destabilization of LDH under alkaline conditions. The process of sorption was spontaneous in different types of water studied. The study reveals that Zn-Al LDH can be used as potential adsorbent for the removal of NPDs from freshwater systems.

As(V) sorption from aqueous solutions using quaternized algal/polyethyleneimine composite beads

Composite beads (APEI*), obtained by the controlled interaction of algal biomass with PEI, followed by ionotropic gelation and crosslinking processes using CaCl₂/glutaraldehyde solution, constitute efficient supports for metal binding. The quaternization of algal/PEI beads (Q-APEI*) significantly increases the sorption properties of the composite beads (APEI*) for As(V). The materials are characterized by SEM/EDX, TGA, BET, elemental analysis, FTIR, XPS, and titration. The sorption of As(V) is studied in function of pH while sorption mechanism is discussed in function of metal speciation and surface characteristics of the sorbent. Optimum sorption occurs at pH close to 7. Fast uptake kinetics, correlated to textural properties are successfully fitted by pseudo-first order rate equation and the Crank equation (for resistance to intraparticle diffusion); equilibrium is reached with 45-60 min. The Langmuir equation finely fits sorption isotherms; maximum sorption capacity reaches 1.34 mmol As g^-1. Arsenic can be completely eluted using 0.5 M CaCl₂/0.5 M HCl solutions; the sorbent maintains high sorption and desorption efficiencies for a minimum of 5 cycles. The sorbent is tested for the removal of As(V) from mining effluents containing high concentration of iron and traces of zinc. At pH 3, the sorbent shows remarkable selectivity for As(V) over Fe. After controlling the initial pH to 5, a sorbent dosage of 2 g L^-1 is sufficient for achieving the complete recovery of As(V) from mining effluent (corresponding to initial concentration of 1.295 mmol As L^-1).

Quaternization of Composite Algal/PEI Beads for Enhanced Uranium Sorption-Application to Ore Acidic Leachate

The necessity to recover uranium from dilute solutions (for environmental/safety and resource management) is driving research towards developing new sorbents. This study focuses on the enhancement of U(VI) sorption properties of composite algal/Polyethylenimine beads through the quaternization of the support (by reaction with glycidyltrimethylammonium chloride). The sorbent is fully characterized by FTIR, XPS for confirming the contribution of protonated amine and quaternary ammonium groups on U(VI) binding (with possible contribution of hydroxyl and carboxyl groups, depending on the pH). The sorption properties are investigated in batch with reference to pH effect (optimum value: pH 4), uptake kinetics (equilibrium: 40 min) and sorption isotherms (maximum sorption capacity: 0.86 mmol U g-1). Metal desorption (with 0.5 M NaCl/0.5 M HCl) is highly efficient and the sorbent can be reused for five cycles with limited decrease in performance. The sorbent is successfully applied to the selective recovery of U(VI) from acidic leachate of uranium ore, after pre-treatment (cementation of copper, precipitation of rare earth elements with oxalate, and precipitation of iron). A pure yellow cake is obtained after precipitation of the eluate.

Fluoride removal by Ca-Al-CO3 layered double hydroxides at environmentally-relevant concentrations

In this study, F^- removal by Ca-Al-CO₃ layered double hydroxides (LDHs) was investigated at environmentally-relevant concentration ranges (2-12 mg/L) to below the WHO guideline, with an emphasis on the effect of LDHs' modification, as well as the effects of initial F^- concentration, adsorbent dose, pH, temperature and co-existing ions. Ca-Al-CO₃ LDHs, either untreated, calcined or microwave treated, showed affinity for the removal of F^- from synthetic groundwater with capacities of 6.7-8.4 mg F^-/g LDHs at groundwater-relevant pH, with a higher F^- removal capacity at lower pH (<8) and lower temperature (12 °C, as compared to 25 °C & 35 °C). Since calcination and microwave treatment resulted in only marginal defluorination improvements, using untreated LDHs appears the practically most feasible option. For the untreated LDHs, competition with Cl^- and NO₃^- was not observed, whereas at higher HCO₃^- and SO₄^2- concentrations (>250 mg/L) a slight reduction in F^- removal was observed. This study indicates the potential of Ca-Al-CO₃ LDHs as a cost-effective F^- removal technology, particularly when locally sourced and in combination with low-cost pH correction.

Sol-Gel Synthesis and Characterization of Coatings of Mg-Al Layered Double Hydroxides

In this study, new synthetic approaches for the preparation of thin films of Mg-Al layered double hydroxides (LDHs) have been developed. The LDHs were fabricated by reconstruction of mixed-metal oxides (MMOs) in deionized water. The MMOs were obtained by calcination of the precursor gels. Thin films of sol-gel-derived Mg-Al LDHs were deposited on silicon and stainless-steel substrates using the dip-coating technique by a single dipping process, and the deposited film was dried before the new layer was added. Each layer in the preparation of the Mg-Al LDH multilayers was separately annealed at 70 °C or 300 °C in air. Fabricated Mg-Al LDH coatings were characterized by X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), and atomic force microscopy (AFM). It was discovered that the diffraction lines of Mg3Al LDH thin films are sharper and more intensive in the sample obtained on the silicon substrate, confirming a higher crystallinity of synthesized Mg3Al LDH. However, in both cases the single-phase crystalline Mg-Al LDHs have formed. To enhance the sol-gel processing, the viscosity of the precursor gel was increased by adding polyvinyl alcohol (PVA) solution. The LDH coatings could be used to protect different substrates from corrosion, as catalyst supports, and as drug-delivery systems in medicine.

Performance of Halloysite-Mg/Al LDH Materials for Aqueous As(V) and Cr(VI) Removal

This research focused on the investigation of layered double hydroxide (LDH)/halloysite materials’ adsorption efficiency and mechanisms in reactions with aqueous As(V) and Cr(VI) in a broad pH range. The materials consisting of Mg/Al LDH and halloysite were synthesized using both direct precipitation and physical mixing methods. The XRD, FTIR, DTA, SEM and XPS methods were used to evaluate the quality of the obtained materials and get insight into removal mechanisms. The XRD, FTIR and DTA confirmed LDH formation and showed the dominating presence of intercalated carbonates in the LDH structure. The SEM of the materials revealed characteristic agglomerates of layered LDH particles deposited on halloysite tubular forms. The raw LDH phases showed high removal efficiency of both As(V) and Cr (VI) for initial pH in the range of 3–7. In the studied concentration range the materials containing 25 wt % of LDH exhibited a removal efficiency very similar to the raw LDH. In particular, the halloysite presence in the materials’ mass had a positive effect in the reactions with As(V), which was removed by chemisorption. At a low pH the LDH component underwent partial dissolution, which lowered the adsorption efficiency. Apart from the anion exchange mechanism at a low pH the Cr(VI) was removed via formation of MgCrO₄ with Mg (II) being released from the LDH structure. The XPS spectra for As(V) did not show changes in oxidation state in the reactions. In turn, a partial reduction of Cr(VI) to Cr(III) was observed, especially at a high pH. The use of materials composed of two different minerals is promising due to reduction of costs as well as prevention of adsorbent swelling. This opens the possibility of its use in dynamic adsorption flow through systems.

Antimonate Removal from Polluted Mining Water by Calcined Layered Double Hydroxides

Calcined layered double hydroxides (LDHs) can be used to remove Sb(V), in the Sb(OH)6− form, from aqueous solutions. Sorption batch experiments showed that the mixed MgAlFe oxides, obtained from calcined hydrotalcite-like compound (3HT-cal), removed Sb(OH)6− through the formation of a non-LDH brandholzite-like compound, whereas the mixed ZnAl oxides, resulting from calcined zaccagnaite-like compound (2ZC-cal), trapped Sb(OH)6− in the interlayer during the formation of a Sb(V)-bearing LDH (the zincalstibite-like compound). The competition effect of coexistent anions on Sb(OH)6− removal was HAsO42− >> HCO3− ≥ SO42− for 2ZC-cal and HAsO42− >> HCO3− >> SO42− for 3HT-cal. Considering the importance of assessing the practical use of calcined LDHs, batch experiments were also carried out with a slag drainage affected by serious Sb(V) pollution (Sb = 9900 μg/L) sampled at the abandoned Su Suergiu mine (Sardinia, Italy). Results showed that, due to the complex chemical composition of the slag drainage, dissolved Sb(OH)6− was removed by intercalation in the interlayer of carbonate LDHs rather than through the formation of brandholzite-like or zincalstibite-like compounds. Both 2ZC-cal and 3HT-cal efficiently removed very high percentages (up to 90–99%) of Sb(V) from the Su Suergiu mine drainage, and thus can have a potential application for real polluted waters.

Adsorption mechanism of hexavalent chromium onto layered double hydroxides-based adsorbents: A systematic in-depth review

An attempt has been made in this review to provide some insights into the possible adsorption mechanisms of hexavalent chromium onto layered double hydroxides-based adsorbents by critically examining the past and present literature. Layered double hydroxides (LDH) nanomaterials are typical dual-electronic adsorbents because they exhibit positively charged external surfaces and abundant interlayer anions. A high positive zeta potential value indicates that LDH has a high affinity to Cr(VI) anions in solution through electrostatic attraction. The host interlayer anions (i.e., Cl^-, NO₃^-, SO₄^2-, and CO₃^2-) provide a high anion exchange capacity (53-520 meq/100 g) which is expected to have an excellent exchangeable capacity to Cr(VI) oxyanions in water. Regarding the adsorption-coupled reduction mechanism, when Cr(VI) anions make contact with the electron-donor groups in the LDH, they are partly reduced to Cr(III) cations. The reduced Cr(III) cations are then adsorbed by LDH via numerous interactions, such as isomorphic substitution and complexation. Nonetheless, the adsorption-coupled reduction mechanism is greatly dependent on: (1) the nature of divalent and trivalent salts utilized in LDH preparation, and the types of interlayer anions (i.e., guest intercalated organic anions), and (3) the adsorption experiment conditions. The low Brunauer-Emmett-Teller specific surface area of LDH (1.80-179 m²/g) suggests that pore filling played an insignificant role in Cr(VI) adsorption. The Langmuir maximum adsorption capacity of LDH (Q^o_max) toward Cr(VI) was significantly affected by the natures of used inorganic salts and synthetic methods of LDH. The Q^o_max values range from 16.3 mg/g to 726 mg/g. Almost all adsorption processes of Cr(VI) by LDH-based adsorbent occur spontaneously (ΔG° <0) and endothermically (ΔH° >0) and increase the randomness (ΔS° >0) in the system. Thus, LDH has much potential as a promising material that can effectively remove anion pollutants, especially Cr(VI) anions in industrial wastewater.

Adsorption mechanisms of chromate and phosphate on hydrotalcite: A combination of macroscopic and spectroscopic studies

Hydrotalcite (HT) is a layered double hydroxide (LDH), which is considered as a potential adsorbent to remove anion contaminants. In this study, adsorption of chromate (CrO₄) and phosphate (PO₄) on HT was conducted at various pH and temperatures. Related adsorption mechanisms were determined via the isotherm, kinetic, and competitive adsorption studies as well as the Cr K-edge X-ray absorption fine-structure (XAFS) spectroscopy. The maximum adsorption capacities for CrO₄ and PO₄ on HT were 0.16 and 0.23 mmol g^-1. Regarding adsorption kinetics, CrO₄ and PO₄ adsorption on HT could be well described by the second order model, and the rate coefficient of CrO₄ and PO₄ on HT decreased significantly with the increasing pH from 5 to 9. The adsorption kinetics for CrO₄ and PO₄ were divided into fast and slow stages with the boundary at 15 min. This biphasic adsorption behavior might be partially attributed to multiple reactive pathways including anion exchange and surface complexation. Fitting results of Cr K-edge EXAFS analysis showed a direct bonding between CrO₄ and Al on HT surfaces. Such a surface complexation appeared to be the rate-limiting step for CrO₄ adsorption on HT. By contrast, the diffusion through the hydrated interlayer space of HT was the major rate-limiting step for PO₄. This study determined the adsorption behaviors of CrO₄ and PO₄ on HT, including the initial transfer process and the subsequent adsorption mechanisms. Such information could improve the strategy to use HT as the potential adsorbent for the remediation of anionic pollutants.

Hydrothermal synthesis of hydrocalumite assisted biopolymeric hybrid composites for efficient Cr(vi) removal from water

Hydrocalumite (HC) incorporated biopolymer (alginate and chitosan) based hybrid composite materials were developed for the selective removal of chromium.

Uranium in well drinking water of Kabul, Afghanistan and its effective, low-cost depuration using Mg-Fe based hydrotalcite-like compounds

Toxic elements in drinking water have great effects on human health. However, there is very limited information about toxic elements in drinking water in Afghanistan. In this study, levels of 10 elements (chromium, nickel, copper, arsenic, cadmium, antimony, barium, mercury, lead and uranium) in 227 well drinking water samples in Kabul, Afghanistan were examined for the first time. Chromium (in 0.9% of the 227 samples), arsenic (7.0%) and uranium (19.4%) exceeded the values in WHO health-based guidelines for drinking-water quality. Maximum chromium, arsenic and uranium levels in the water samples were 1.3-, 10.4- and 17.2-fold higher than the values in the guidelines, respectively. We next focused on uranium, which is the most seriously polluted element among the 10 elements. Mean ± SD (138.0 ± 1.4) of the ²³⁸U/²³⁵U isotopic ratio in the water samples was in the range of previously reported ratios for natural source uranium. We then examined the effect of our originally developed magnesium (Mg)-iron (Fe)-based hydrotalcite-like compounds (MF-HT) on adsorption for uranium. All of the uranium-polluted well water samples from Kabul (mean ± SD = 190.4 ± 113.9 μg/L; n = 11) could be remediated up to 1.2 ± 1.7 μg/L by 1% weight of our MF-HT within 60 s at very low cost (<0.001 cents/day/family) in theory. Thus, we demonstrated not only elevated levels of some toxic elements including natural source uranium but also an effective depurative for uranium in well drinking water from Kabul. Since our depurative is effective for remediation of arsenic as shown in our previous studies, its practical use in Kabul may be encouraged.

High performance NiFe layered double hydroxide for methyl orange dye and Cr(VI) adsorption

The NiFe layered double hydroxides (LDHs) with different mole ratio of Ni/Fe (4:1, 3:1, 7:3 and 1:1) were prepared by a simple coprecipitation method. The adsorption performance were evaluated by the removal of methyl orange (MO) dye and hexavalent chromium(VI) heavy metal ion. It is found that Ni4Fe1-LDH can remove more than 92% of MO in 10 min at the 10 mg/L MO initial concentration, and 97% of Cr(VI) in 1 h at 4 mg/L Cr2O7(2-) initial concentration. The saturated adsorption capacity of Ni4Fe1-LDH is found to be as large as 205.76 mg/g for MO and 26.78 mg/g for Cr(VI). The adsorption behavior of this new adsorbent is fitted well with Langmuir isotherm and the pseudo-second-order kinetic model, indicative of a monolayer and chemical adsorption that synergistically originates from exchangeable anions mechanism and layer charge density. Due to the excellent removal capacity of MO and Cr(VI), the NiFe-LDHs could be a promising adsorbent for wastewater treatment.

Arsenate removal by layered double hydroxides embedded into spherical polymer beads: Batch and column studies

In this study, the performance of poly(layered double hydroxides) [poly(LDHs)] beads as an adsorbent for arsenate removal from aqueous solution was investigated. The poly(LDHs) beads were prepared by immobilizing LDHs into spherical alginate/polyvinyl alcohol (PVA)-glutaraldehyde beads (spherical polymer beads). Batch adsorption studies were conducted to assess the effect of contact time, solution pH, initial arsenate concentrations and co-existing anions on arsenate removal performance. The potential reuse of these poly(LDHs) beads was also investigated. Approximately 79.1 to 91.2% of arsenic was removed from an arsenate solution (50 mg As L(-1)) by poly(LDHs). The adsorption data were well described by the pseudo-second-order kinetics model and the Langmuir isotherm model, and the adsorption capacities of these poly(LDHs) beads at pH 8 were from 1.64 to 1.73 mg As g(-1), as calculated from the Langmuir adsorption isotherm. The adsorption ability of the poly(LDHs) beads decreased by approximately 5-6% after 5 adsorption-desorption cycles. Phosphates markedly decreased arsenate removal. The effect of co-existing anions on the adsorption capacity declined in the following order: HPO4 (2-) >> HCO3 (-) > SO4 (2-) > Cl(-). A fixed-bed column study was conducted with real-life arsenic-containing water. The breakthrough time was found to be from 7 to 10 h. Under optimized conditions, the poly(LDHs) removed more than 82% of total arsenic. The results obtained in this study will be useful for further extending the adsorbents to the field scale or for designing pilot plants in future studies. From the viewpoint of environmental friendliness, the poly(LDHs) beads are a potential cost-effective adsorbent for arsenate removal in water treatment.

Immobilization of imidazolium ionic liquids on hydrotalcites using silane linkers: retardation of memory effect

We report a new covalent surface immobilization of silane-modified imidazolium ionic liquids on hydrotalcite-like materials (HTs) and provide detailed characterization of the resulting surface chemistry using PXRD, CP-MAS, TGA and FT-IR.

Preparation and evaluation of a layered double hydroxide film on a nanoporous anodic aluminum oxide/aluminum wire as a highly thermal-resistant solid-phase microextraction fiber

A hierarchical layered double hydroxide framework on a nonporous anodic aluminum oxide/aluminum wire was used as a solid phase microextraction fiber for separation and determination of phenolic compounds.

Layered double hydroxide-alginate/polyvinyl alcohol beads: fabrication and phosphate removal from aqueous solution

In the water treatment field, powder form of layered double hydroxides (LDHs) has wide applications in adsorptions. However, its applications are limited because of low hydraulic conductivity. Here, LDH-alginate/polyvinyl alcohol (PVA) beads were fabricated by entrapment of the Mg-Al LDH powder into alginate/PVA beads. The obtained Mg-Al LDH-alginate/PVA beads were characterized by X-ray diffraction scanning electron microscopy. Their performance for phosphate removal by batch and column adsorption mode was evaluated. The Mg-Al LDH-alginate/PVA beads were found to be efficient adsorbents for phosphate removal. Batch adsorption experiment showed that the phosphate sorption process on the Mg-Al LDH-alginate/PVA beads followed pseudo-second-order reaction order kinetic model and the adsorption isotherm date could be simulated using both Langmiur and Freundlich models. In the column study, the flow rate and inlet phosphate concentration were maintained at 29.62 m³/m² h and 10 mgP/L, respectively. Using 20 cm column depth, the breakthrough and exhaust time were found to be 5 and 31 h, respectively. The percentage of phosphate removal by column was 80.09%. The values of adsorption rate coefficient (K) and the adsorption capacity coefficient (N) were 0.0125 L/mg h and 258.32 mg/L, respectively.