Arsenic adsorption by iron-aluminium hydroxide coated onto macroporous supports: Insights from X-ray absorption spectroscopy and comparison with granular ferric hydroxides

Enhanced Arsenic Removal from Aqueous Solution by Fe/Mn-C Layered Double Hydroxide Composite

A novel material named Fe/Mn-C layered double hydroxide composite (Fe/Mn-C-LDH) was synthesized to remove arsenic from an aqueous solution. The removal performance of the composite toward arsenic ions was studied through the batch experiments. The experiment results showed that Fe/Mn-C-LDH exhibited a high adsorption capacity of 46.47 mg/g for As(III) and 37.84 mg/g for As(V) at 318 K, respectively. In addition, the investigation of the release of Fe3+ and Mn2+ in the process of arsenic adsorption revealed that the Fe/Mn-C-LDH exhibited better stability than Fe/Mn-layer double hydroxide (Fe/Mn-LDH) with fewer Mn2+ and Fe3+ releasing under the same condition. The BET results showed that the specific surface area of Fe/Mn-C-LDH decreased after adsorption of As (III) and As (V). Furthermore, the Density Functional Theory (DFT) calculation results proved that the adsorbent combining arsenic by T-site to produce a better adsorption effect for arsenic. Possessing better stability and adsorption capacity, Fe/Mn-C-LDH could potentially serve as a perfect adsorbent for arsenic removal from an aqueous environment. It would provide a promising approach for removing heavy metal from the aquatic environment in the future.

Phosphate competition with arsenate on poorly crystalline iron and aluminum (hydr)oxide mixtures

Phosphate competes with arsenate for sorption sites on poorly crystalline iron and aluminum (hydr)oxides. The competition has implications e.g. for the management of arsenic-contaminated soil and water. Phosphate competition with arsenate on mixed phases containing both iron and aluminum (hydr)oxides has rarely been investigated. Here, the phosphate competition with arsenate on mixtures of poorly crystalline aluminum hydroxide (Alhox) and ferrihydrite (Fh), was investigated in batch experiments at pH 6.5. X-ray absorption spectroscopy (XAS) was performed on the phosphorus and arsenic K edges, which offered a unique insight in the partitioning of arsenate and phosphate on mixed Alhox-Fh sorbents. Under the studied conditions the sorption capacity of the mixed sorbents (per mol Al or Fe) increased at higher Alhox to Fh ratios. The XAS measurements provided direct evidence that phosphate competed more effectively with arsenate for sorption sites on Alhox than on Fh. For example, in a mixture with 50% of both sorbents and with similar additions of arsenate and phosphate, 71% of the oxyanions adsorbed on Fh and 46% on Alhox were arsenate. Consequently, phosphate may mobilize arsenate more easily from mixed iron-aluminum matrices that are rich in aluminum.

Preparation, characterization, and binding profile of imprinted semi-IPN cryogel composite for aluminum

Human body is greatly exposed to aluminum due to its high abundance in the environment. This nonessential metal is a threat to the patients of chronic renal disorders, as it is easily retained in their plasma and quickly accumulates in different tissues. Thus, there is great need to remove it from the aqueous environment. In this study, Al3+ imprinted semiinterpenetrating polymer network (semi-IPN)-based cryogel composite was prepared and applied for the purification of environmental and drinking water samples from aluminum. Poly (2-hydroxyethyl methacrylate) (pHEMA) discs were produced via cryogenic treatment and imprinted semi-IPN was introduced to the 3-(trimethoxysilyl) propyl acrylatemodified macroporous cryogel discs. The adsorption properties and selectivity of the aluminum (III) imprinted semi-IPN cryogel composite were studied in detail. The imprinted semi-IPN cryogel composite showed good selectivity towards aluminum (III) ions with the imprinting factor (IF) of 76.4 in the presence of competing copper (II), nickle (II), and iron (III) ions. The maximum adsorption capacity of 271 μmol g-1 was obtained for aluminum (III) at pH 7.0 within 10 min using imprinted semi-IPN cryogel composite. The good selectivity and reusability of aluminum (III)-imprinted semi-IPN cryogel composite makes this material an eligible candidate for the purification of drinking water from aluminum (III) leaving important minerals remained in the water.

A review of cryogels synthesis, characterization and applications on the removal of heavy metals from aqueous solutions

The physical and chemical attributes of cryogels, such as the macroporosity, elasticity, water permeability and ease of chemical modification have attracted strong research interest in a variety of areas, such as water purification, catalysis, regenerative medicine, biotechnology, bioremediation and biosensors. Cryogels have shown high removal efficiency and selectivity for heavy metals, nutrients, and toxic dyes from aqueous solutions but there are challenges when scaling up from lab to commercial scale applications. This paper represents an overview of the most recent advances in the use of cryogels for the removal of heavy metals from water and attempts to fill the gap in the literature by deepening the understanding on the mechanisms involved, which strongly depend on the initial monomer composition and post-modification agent precursors used in synthesis. The review also describes the advantages of cryogels over other adsorbents and covers synthesis and characterization methods such as SEM/EDS, TEM, FTIR, zeta potential measurements, porosimetry, swelling and mechanical properties.

Arsenic surface complexation behavior in aqueous systems onto Al substituted Ni, Co, Mn, and Cu based ferrite nano adsorbents

The present study is about surface complexation behavior of arsenic species adsorbed onto ternary metal oxide adsorbents (Ni-Al-Fe, Co-Al-Fe, Mn-Al-Fe, and Cu-Al-Fe). The analysis is carried out by X-ray absorption spectroscopy (XAS) tool. XANES (μ(E) vs. E) spectra close to the absorption edge (i.e., As K-edge) of all samples are observed along with the As(III) and As(V) standards. The first derivative of XANES for Ni-As(V), and Cu-As(V) samples agree with that of As(V) standards, respectively. Whereas, As(III) adsorbed adsorbent systems (i.e., Ni, Co, Mn, and Cu) are observed with mixed oxidation state of arsenic. A total of 65-85 % is observed with initial oxidation state (As(III) or As(V)), and remaining 15-35 % is observed with modified oxidation state (As(V) or As(III)) that explains the occurrence of possible charge transfer. EXAFS analysis shows the As-O bond distances in the range of 1.7-1.8 Å. The corresponding As-M bond distances are around 2.7, 3.2, and 3.6 Å which confirms the formation various edge sharing (²E), and corner sharing (²C, ¹V) surface complexes. Surface coverage is understood as an important parameter as bidentate attachments (²E, ²C) are evident in As(III), and As(V), but monodentate attachments (¹V) are only observed in As(V).

As(III) and Cr(VI) oxyanion removal from water by advanced oxidation/reduction processes-a review

Water pollution by human activities is a global environmental problem that requires innovative solutions. Arsenic and chromium oxyanions are toxic compounds, introduced in the environment by both natural and anthropogenic activities. In this review, the speciation diagrams of arsenic and chromium oxyanions in aqueous solutions and the analytical methods used for their detection and quantification are presented. Current and potential treatment methods for As and Cr removal, such as adsorption, coagulation/flocculation, electrochemical, ion exchange, membrane separation, phyto- and bioremediation, biosorption, biofiltration, and oxidative/reductive processes, are presented with discussion of their advantages, drawbacks, and the main recent achievements. In the last years, advanced oxidation processes (AOPs) have been acquiring high relevance for the treatment of water contaminated with organic compounds. However, these processes are also able to deal with inorganic contaminants, mainly by changing metal/metalloid oxidation state, turning these compounds less toxic or soluble. An overview of advanced oxidation/reduction processes (AO/RPs) used for As and Cr removal was carried out, focusing mainly on H₂O₂/UVC, iron-based and heterogeneous photocatalytic processes. Some aspects related to AO/RP experimental conditions, comparison criteria, redox mechanisms, catalyst immobilization, and process intensification through implementation of innovative reactors designs are also discussed. Nevertheless, further research is needed to assess the effectiveness of those processes in order to improve some existing limitations. On the other hand, the validation of those treatment methods needs to be deepened, namely with the use of real wastewaters for their future full-scale application. Graphical abstract ᅟ.

Potential of joyweed Alternanthera sessilis for rapid treatment of domestic sewage in SHEFROL® bioreactor

In a first-ever report on this subject, it is shown that a common amphibious plant joyweed (Alternanthera sessilis) can be used in rapid and efficient treatment of biodegradable wastewaters, typified by domestic sewage. The plant was effective when used indoors under artificial lighting, as well as outdoors. It enabled treatment of sewage, varying widely in strength (from 300 mg/L to 1800 mg/L in chemical oxygen demand), to the extent of 78.9-83.9%. It was also able to remove biological oxygen demand, suspended solids, phosphorous, nitrogen, and the heavy metal copper to the extent of 87%, 93%, 45%, and 43%, respectively. Over 99% of total coliforms, faecal coliforms, and faecal streptococci were also removed. The treatment was very swiftly achieved, at a hydraulic retention time of just 6 h, in the "sheet flow root level" (SHEFROL^®) bioreactor developed earlier by us and of which a patent claim has been registered. The findings indicate that A. sessilis has the potential to affect primary, secondary, and tertiary treatment of domestic sewage along with significant pathogen removal in a single process step when used in SHEFROL^® bioreactors.

Effects of ionic strength on removal of toxic pollutants from aqueous media with multifarious adsorbents: A review

Adsorption is one of the most widely used and effective wastewater treatment methods. The role of ionic strength (IS) in shaping the adsorption performances is much necessary due to the ubiquity of electrolyte ions in water body and industrial effluents. The influences of IS on adsorption are rather complex, because electrolyte ions affect both adsorption kinetics and thermodynamics by changing the basic characteristics of adsorbents and adsorbates. For a given adsorption system, multiple or even contradictory effects of IS may coexist under identical experimental conditions, rendering the dominant mechanism recognition and net effect prediction complicated. We herein reviewed the key advancement on the interaction and mechanisms of IS, including change in number of active sites for adsorbents, ion pair for metal ions, molecular aggregation and salting-out effect for organic compounds, site competition for both inorganic and organic adsorbates, and charge compensation for adsorbent-adsorbate reciprocal interactions. The corresponding fundamental theory was thoroughly described, and the efforts made by various researchers were explicated. The structural optimization of adsorbents affected by IS was detailed, also highlighting polyamine materials with exciting "salt-promotion" effects on heavy metal removal from high salinity wastewater. In addition, the research trends and prospects were briefly discussed.

A critical review on arsenic removal from water using iron-based adsorbents

Intensive research efforts have been pursued to remove arsenic (As) contamination from water with an intention to provide potable water to millions of people living in different countries. Recent studies have revealed that iron-based adsorbents, which are non-toxic, low cost, and easily accessible in large quantities, offer promising results for arsenic removal from water. This review is focused on the removal of arsenic from water using iron-based materials such as iron-based nanoparticles, iron-based layered double hydroxides (LDHs), zero-valent iron (ZVI), iron-doped activated carbon, iron-doped polymer/biomass materials, iron-doped inorganic minerals, and iron-containing combined metal oxides. This review also discusses readily available low-cost adsorbents such as natural cellulose materials, bio-wastes, and soils enriched with iron. Details on mathematical models dealing with adsorption, including thermodynamics, kinetics, and mass transfer process, are also discussed. For elucidating the adsorption mechanisms of specific adsorption of arsenic on the iron-based adsorbent, X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) are frequently used. Overall, iron-based adsorbents offer significant potential towards developing adsorbents for arsenic removal from water.

Impact of inorganic ions and natural organic matter on arsenates removal by ferrate(VI): Understanding a complex effect of phosphates ions

Arsenic compounds are carcinogenic to humans and are typically removed from contaminated water using various sorbents. The ionic composition plays a significant role in arsenate removal efficiency during the process of water remediation. Here, we quantify the effects of natural ions (chlorides, nitrates, carbonates, sulfates, and phosphates) and humic acid on the removal of arsenates by ferrate(VI) at pH = 6.6. In the experiments, the initial concentration of arsenates was 10 mg L^-1 (as As) and the concentrations of ions varied in the range from 5 to 100 mg L^-1 of element in ionic form and humic acid. The achieved results show that only phosphate ions had principle influence on the efficiency of arsenate removal by ferrate(VI). The effect of phosphates was elucidated by applying transmission electron microscopy, energy-dispersive X-ray spectroscopy, and low temperature in-field ⁵⁷Fe Mössbauer spectroscopy to solid samples, prepared under different weight ratios of ferrate(VI), arsenates, and phosphates. These results show three crucial effects of phosphates on the arsenate removal mechanisms. At low P:As weight ratio (up to 1:1), the incorporation of arsenate ions into the crystalline structure of γ-Fe₂O₃/γ-FeOOH nanoparticles was found to be suppressed by the presence of phosphates. Thus, arsenates were mainly adsorbed onto the surface of γ-Fe₂O₃/γ-FeOOH nanoparticles. Further increase in the P:As weight ratio (more than 1:1) resulted in the competition between arsenates and phosphates sorption. With the increased concentration of phosphate ions, the number of arsenates on the surface of γ-Fe₂O₃/γ-FeOOH nanoparticles was reduced. Finally, the complexation of iron(III) ions with phosphate ions occurred, leading to a decrease in the arsenates removal efficiency, which resulted from a lower content of precipitated γ-Fe₂O₃/γ-FeOOH nanoparticles. All these aspects need to be considered prior to application of ferrate(VI) for arsenates removal in real natural waters.

Nanofibers of resorcinol-formaldehyde for effective adsorption of As (III) ions from mimicked effluents

In the present study, the core-shell structured RF/PVA nanofibers have been developed and used for the adsorption of As³⁺ ions from the mimicked liquid effluents. Efficient-facile fabrication of the structured nanofibers (300-417 nm diameter) was accomplished using facile electrospinning technique. Chi parameter (χ = 25.56) and free energy of mixing (E_mix = 17.19 kcal/mol) calculated via molecular dynamics simulations depicted compatibility of the polymeric system resulting supermolecular core-shell nanofibers, whose adsorption results were also supported by the FE-SEM, FT-IR, and UV-VIS spectroscopy analysis. The adsorption analysis was performed using both linear and non-linear regression methods, for kinetic models and adsorption isotherms. The developed nanofibers demonstrated an adsorption capacity of 11.09 mg/g at a pH of 7, and an adsorption efficiency of 97.46% on protracted exposure, which is even adaptable at high temperatures with 93.1% reclamation. FE-SEM analysis and FT-IR spectra confirm the adsorption of As (III) ions on RF/PVA nanofibers and the presence of embedded hydrophilic oxygen sites for metal ion adsorption. The developed RF/PVA nanofibers demonstrate scalability in fabrication, low-cost, recycling, and less solid waste generation, depicting the large-scale applicability in removing arsenic ions from effluent waste. Graphical Abstract ᅟ.

A Comparative Study on Removal of Hazardous Anions from Water by Adsorption: A Review

This paper presents a comparative review of arsenite (As(III)), arsenate (As(V)), and fluoride (F−) for a better understanding of the conditions and factors during their adsorption with focus on (i) the isotherm adsorption models, (ii) effects of pH, (iii) effects of ionic strength, and (iv) effects of coexisting substances such as anions, cations, and natural organics matter. It provides an in-depth analysis of various methods of arsenite (As(III)), arsenate (As(V)), and fluoride (F-) removal by adsorption and the anions’ characteristics during the adsorption process. The surface area of the adsorbents does not contribute to the adsorption capacity of these anions but rather a combination of other physical and chemical properties. The adsorption capacity for the anions depends on the combination of all the factors: pH, ionic strength, coexisting substances, pore volume and particles size, surface modification, pretreatment of the adsorbents, and so forth. Extreme higher adsorption capacity can be obtained by the modification of the adsorbents. In general, pH has a greater influence on adsorption capacity at large, since it affects the ionic strength, coexisting anions such as bicarbonate, sulfate, and silica, the surface charges of the adsorbents, and the ionic species which can be present in the solution.

Dynamic arsenic aging processes and their mechanisms in nine types of Chinese soils

Although specific soil properties controlling the arsenic (As) aging process have been studied extensively, few investigations have attempted to determine how soil types influence As bioavailability and fractionations in soils. Nine types of soil were selected from typical grain producing areas in China, and the bioavailability and fractionations of As during aging were measured. Results showed that available As in all soils rapidly decreased in the first 30 days and slowly declined thereafter. In spiked soils, As easily became less available and less toxic in low pH soils compared to high pH soils, demonstrating the importance of soil pH on As availability. Results from fitting kinetic equations revealed that the pseudo-second-order model described the As aging processes well in all soils (R² = 0.945-0.999, P < 0.01, SE = 0.09-4.25), implying that the mechanism for As aging combined adsorption, external diffusion, and internal diffusion. Fe oxides were more important than Al oxides for determining the As aging rate (|k|). Based on these results, we are the first to propose the approximate aging equilibrium time (T) for As, which was mainly influenced by soil clay content. The shortest time for approximate stabilization of As aging was 28 d in latosol soils (LS), while the longest approximate equilibrium time was 169 d in cinnamon soils (CS). Individual soil properties controlling the variation in different As fractionations further confirmed that the influences of soil types on As aging were the result of the combined effects of soil properties and a time-consuming redistribution process.

Adsorption of As(III) versus As(V) from aqueous solutions by cerium-loaded volcanic rocks

Contamination of drinking water with arsenic causes severe health problems in various world regions. Arsenic exists predominantly as As(III) and As(V) depending on the prevailing redox conditions of the environment. Most of the techniques developed for treating As(V) are not very effective for As(III), which is more toxic and mobile than As(V). In this study, novel cerium-loaded pumice (Ce-Pu) and red scoria (Ce-Rs) adsorbents were developed to remove both As(III) and As(V) ions from water. The Ce-Pu and Ce-Rs adsorbents were characterized using ICP-OES, EDX, and SEM. The experimental equilibrium sorption data fitted well Freundlich and Dubinin-Radushkevich (D-R) isotherms. The adsorption was very fast and reached an equilibrium within 2 h. Both Ce-Rs and Ce-Pu showed high As(III) and As(V) removal efficiency in a wide pH range between 3 and 9, which is an important asset for practical applications. The Ce-Pu and Ce-Rs adsorbents can be recycled and used up to three adsorption cycles without significant loss of their original efficiency. Accordingly, Ce-Pu and Ce-Rs seem to be suitable for removal of arsenic from aqueous systems.

Bushy sphere dendrites with husk-shaped branches axially spreading out from the core for photo-catalytic oxidation/remediation of toxins

This work describes densely interlinked bushy "tree-like chains" characterized by neatly branched sphere dendrites (bushy sphere dendrites, BSD) with long fan-like, husk-shaped branching paths that extend longitudinally from the core axis of the {110}-exposed plane. We confirmed that the hierarchical dendrite surfaces created bowls of swirled caves along the tree-tube in the mat-like branches. These surfaces had high-index catalytic site facets associated with the formation of ridges/defects on the dominant {110}-top-cover surface. These swirled caves along the branches were completely filled with 50-100 nm poly-CN nano-sphere-fossils with orb-like appearance. Such structural features are key issues of the inherent surface reactivity of a powerful catalyst/trapper, enabling photocatalytic oxidation and trapping of extremely toxic arsenite (AsO₃^3-) species and photo-induced recovery of arsenate (AsO₄^3-) products from catalyst surfaces. The light-induced release of produced AsO₄^3- from BSD indicates (i) highly controlled waste collection/management (i.e., recovery), (ii) low cost and ecofriendly photo-adsorbent, (iii) selective trapping of real sample water to produce water-free arsenite species; (iv) multiple reuse cycles of catalysts (i.e., reduced waste volume). Matrixed dendrites, covered with 3D microscopic sphere cores that capture solar-light, trap toxins, and are triggered by light, were designed. These dendrites can withstand indoor and outdoor recovery of toxins from water sources.

Bacterial iron-oxide nanowires from biofilm waste as a new adsorbent for the removal of arsenic from water

Biofilm waste material generated by the bacteria in the groundwater pipelines was found is composed of amorphous twisted iron-oxide nanowires which are shown to have considerable adsorption properties for removal As(iii) and As(v) ions from waters.

Adsorptive Removal of As(III) from Aqueous Solution by Raw Coconut Husk and Iron Impregnated Coconut Husk: Kinetics and Equilibrium Analyses

As(III) removal from aqueous solution was conducted using low-cost adsorbents like unmodified raw coconut husk (RCH) and modified iron impregnated coconut husk (IICH). Prepared both adsorbents was characterisation by using elemental analyses, FTIR, TGA, SEM and EDX. The analysis behaviour indicates, both adsorbents are highly suitability for As(III) removal. The effects of operational parameters, such as pH, adsorbent dose and initial concentration on these adsorbents were investigated and compared with other agriculture based adsorbent. The result reveals that the As(III) removal capacity is effective in the pH range of 6.2–7.8 and the optimum pH and adsorbents dose was found as 7.0 and 40 g l−1, for RCH and IICH, respectively. Kinetic and equilibrium studies over a wide range of operating conditions are tested to evaluate the effectiveness of RCH and IICH to remove As(III) from water. The values of both k f1 and k s2 values are found to be nearly same and same trend was observed at higher 50 mg l−1 and lower arsenic concentration 25 mg l−1 for RCH and IICH. But the kinetic data is fitted better in the pseudo-second-order kinetic model than the pseudo-first order model. The effective intraparticle diffusion coefficient of As(III) ions in RCH and IICH is observed to be 2.145×10−9 cm2 s−1 and 1.838×10−10 cm2 s−1, which indicates that the overall As(III) adsorption on both adsorbents are intraparticle diffusion control. Equilibrium isotherms for the adsorption of As(III) on RCH and IICH were analyzed at different dose and different initial concentration. At different concentration system, Freundlich isotherm and Redlich-Peterson are best fitted followed by Langmuir and Temkin isotherm models and for varying doses, all equilibrium models give almost similar fitness.

Fe3O4/Reduced Graphene Oxide Nanocomposite: Synthesis and Its Application for Toxic Metal Ion Removal

The synthesis of reduced graphene oxide modified by magnetic iron oxide (Fe3O4/rGO) and its application for heavy metals removal were demonstrated. The obtained samples were characterized by X-ray diffraction (XRD), nitrogen adsorption/desorption isotherms, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), and magnetic measurement. The results showed that the obtained graphene oxide (GO) contains a small part of initial graphite as well as reduced oxide graphene. GO exhibits very high surface area in comparison with initial graphite. The morphology of Fe3O4/rGO consists of very fine spherical iron nanooxide particles in nanoscale. The formal kinetics and adsorption isotherms of As(V), Ni(II), and Pb(II) over obtained Fe3O4/rGO have been investigated. Fe3O4/rGO exhibits excellent heavy metal ions adsorption indicating that it is a potential adsorbent for water sources contaminated by heavy metals.