Arsenic and antimony in water and wastewater: overview of removal techniques with special reference to latest advances in adsorption

Investigation of antimony adsorption on a zirconium-porphyrin-based metal-organic framework

A zirconium-porphyrin based organic framework PCN-222 was employed for investigating the adsorption performance of Sb(III) in aqueous solution. It is proved that the adsorbent has the advantages of rapid adsorption and high capacity. Interestingly, we discover that PCN-222 shows pH-dependent adsorption performance, with higher capacity at pH = 2 and 8 than at pH = 5. According to XPS and FT-IR analyses, an adsorption model of PCN-222 with pH = 2, 5, and 8 is proposed, that is, zirconium clusters combine with antimony at different pH values with bidentate complexes, monodentate complexes, and alkaline monodentate complexes, thus producing an excellent adsorption effect. Moreover, the porphyrin ring is also beneficial for the adsorption of antimony. In addition, PCN-222 shows good regeneration and recycling performance, and it is a promising adsorbent as well as a platform for investigating the removal of Sb(III) in water treatment.

Removal of Arsenic Oxyanions from Water by Ferric Chloride-Optimization of Process Conditions and Implications for Improving Coagulation Performance

The chronic ingestion of arsenic (As) contaminated water has raised significant health concerns worldwide. Iron-based coagulants have been widely used to remove As oxyanions from drinking water sources. In addition, the system’s ability to lower As within the maximum acceptable contamination level (MCL) is critical for protecting human health from its detrimental effects. Accordingly, the current study comprehensively investigates the performance of As removal under various influencing factors including pH, contact time, temperature, As (III, V) concentration, ferric chloride (FC) dose, and interfering ions. The optimum pH for As (V) removal with FC was found to be pH 6–7, and it gradually decreased as the pH increased. In contrast, As (III) removal increased with an increase in pH with an optimum pH range of 7–10. The adsorption of As on precipitated iron hydroxide (FHO) was better fitted with pseudo-second order and modified Langmuir–Freundlich models. The antagonistic effect of temperature on As removal with FC was observed, with optimum temperature of 15–25 °C. After critically evaluating the optimum operating conditions, the uptake indices of both As species were developed to select appropriate an FC dose for achieving the MCL level. The results show that the relationship between residual concentration, FC dose, and adsorption affinity of the system was well represented by uptake indices. The higher FC dose was required for suspensions containing greater concentration of As species to achieve MCL level. The As (V) species with a greater adsorption affinity towards FHO require a relatively smaller FC dose than As (III) ions. Moreover, the significant influence of interfering species on As removal was observed in simulated natural water. The author hopes that this study may help researchers and the drinking water industry to develop uptake indices of other targeted pollutants in achieving MCL level during water treatment operations in order to ensure public health safety.

A review on sources, identification and treatment strategies for the removal of toxic Arsenic from water system

Arsenic liberation and accumulation in the groundwater environment are both affected by the presence of primary ions and soluble organic matter. The most important influencing role in the co-occurrence is caused by human activity, which includes logging, agricultural runoff stream, food, tobacco, and fertilizers. Furthermore, it covers a wide range of developed and emerging technologies for removing arsenic impurities from the ecosystem, including adsorption, ion exchangers, bio sorption, coagulation and flocculation, membrane technology and electrochemical methods. This review thoroughly explores various arsenic toxicity to the atmosphere and the removal methods involved with them. To begin, the analysis focuses on the general context of arsenic outbreaks in the area, health risks associated with arsenic, and measuring techniques. The utilization of innovative functional substances such as graphite oxides, metal organic structures, carbon nanotubes, and other emerging types of composite materials, as well as the ease, reduced price, and simple operating method of the adsorbent material, are better potential alternatives for arsenic removal. The aim of this article is to examine the origins of arsenic, as well as identification and treatment methods. It also addressed recent advancements in Arsenic removal using graphite oxides, carbon nanotubes, metal organic structures, magnetic nano composites, and other novel types of usable materials. Under ideal conditions for the above methods, the arsenic removal will achieve nearly 99% in lab scale.

Exposure characteristics of antimony and coexisting arsenic from multi-path exposure in typical antimony mine area

In this study, samples of daily foods, drinking waters, surface waters, and soils were collected and screened to investigate the external exposure of Sb and As from various intake pathways in typical Sb mining area. Biomarker samples of residents were analyzed to monitor internal exposure characteristic of Sb and As in human body. Exposure dosages of As and Sb and transfer of Sb and As from environment to human body were estimated based on the external and internal exposure. The following results were obtained: daily intakes of food accounted for major intakes of both Sb and As, and highlighted the significance of foods intakes from rice and vegetable. The results of Monte Carlo simulations showed that total daily intake of Sb(n = 1444)and As(n = 1131) approximately reached 1.08 × 10^-2 mg/kg/d and 1.19 × 10^-3 mg/kg/d, in which 98.82% and 63.07% of residents have exceeded the threshold dosages of Sb and As. The contaminants contents in biomarkers indicated that Sb exhibited the similar internal exposure as As, while the total transfer rate of Sb from environment to human were estimated as approximately 2.04-2.40 times lower than As. This study also suggested that drinking water is another important pathway with high bioavailability and male resident may present higher priority than female in uptake of Sb and As. The paper suggested the similarity and difference on bioavailability existed in Sb and its group V elements, As, that would provide the essential information on exposure of Sb and As in the typical Sb mine area.

Magnetic zeolitic imidazolate frameworks composite as an efficient adsorbent for arsenic removal from aqueous solution

In this study, magnetic zeolitic imidazolate frameworks (ZIF-8) was prepared by a one-step method, where its evolution involved the coprecipitation reactions concomitant with the self-assembly reactions. Structural characterizations indicated that magnetic ZIF-8 showed irregular polyhedral morphology with a large specific surface area (696.5 m²/g) and saturation magnetization (4.31 emu/g). The as-prepared magnetic ZIF-8 enhanced the adsorption performance of As(III) and As(V), compared with bare Fe₃O₄. The pseudo second-order kinetic model (R² = 0.9627 and 0.9893 for As(III) and As(V), respectively) and the Langmuir model (R² = 0.9441 for As(III) and 0.9851 for As(V)) can fit the adsorption process well, confirming the nature of single-layer homogeneous chemisorption. The adsorption capacity was 30.87 and 17.51 mg/g, and their corresponding values of PC were 2.664 and 1.286 L/g, for As(III) and As(V), respectively. Solution pH showed an adverse effect on As(V) adsorption whereas no obvious effect on As(III). The ionic strength and coexisting ions had not obvious influence on adsorption of As(III) and As(V). The adsorption mechanism was explored and discussed based on the detailed spectroscopy analysis. This adsorbent can be recovered magnetically after use, which is promising for the practical application.

A Study of the Adsorption and Removal of Sb(III) from Aqueous Solution by Fe(III) Modified Proteus cibarius with Mechanistic Insights Using Response Surface Methodology

Environmental pollution caused by excessive Sb(III) in the water environment is a global issue. We investigated the effect of processing parameters, their interaction and mechanistic details for the removal of Sb(III) using an iron salt-modified biosorbent (Fe(III)-modified Proteus cibarius (FMPAs)). Our study evaluated the optimisation of the adsorption time, adsorbent dose, pH, temperature and the initial concentration of Sb(III). We use response surface methodology to optimize this process, determining optimal processing conditions and the adsorption mechanism evaluated based on isotherm model and adsorption kinetics. The results showed that—(1) the optimal conditions for the adsorption of Sb(III) by FMPAs were an adsorption time of 2.2 h, adsorbent dose of 3430 mg/L, at pH 6.0 and temperature 44.0 °C. For the optimum initial concentration of Sb(III) 27.70 mg/L, the removal efficiency of Sb(III) reached 97.60%. (2) The adsorption process for Sb(III) removal by FMPAs conforms to the Langmuir adsorption isotherm model, and its maximum adsorption capacity (qmax) is as high as 30.612 mg/g. A pseudo-first-order kinetic model provided the best fit to the adsorption process, classified as single layer adsorption and chemisorption mechanism. (3) The adsorption of Sb(III) takes place via the hydroxyl group in Fe–O–OH and EPS–Polyose–O–Fe(OH)2, which forms a new complex Fe–O–Sb and X≡Fe–OH. The study showed that FMPAs have higher adsorption capacity for Sb(III) than other previously studied sorbents and with low environmental impact, it has a great potential as a green adsorbent for Sb(III) in water.

Experimental and statistical analysis of As(III) adsorption from contaminated water using activated red mud doped calcium-alginate beads

Arsenic present in water bodies causes devastating effects on aquatic organisms and indirectly poses a hazardous threat to human existence. There is an urgent need to develop potential and convincing technologies to troubleshoot this problem. In the present study, an adsorbent has been prepared using the waste red mud from hazardous aluminium industry and doping it with calcium-alginate beads (ARMCB) for the effective removal of As(III) from wastewater. The concentration of As(III) was reduced from 0.101 mg/L to 0.008 mg/L after adsorption which effectively met the economic and environmental conditions imposed by WHO (>0.01 mg/L). Further, the statistical Response Surface Methodology (RSM) is adopted to analyze the combined effects of four operational parameters namely: pH, sorbent dosage, contact time and initial concentration on the adsorption of As(III) from the synthetic contaminated water samples. A high correlation coefficient (R2) value of 0.9672 projected by ANOVA confirmed the satisfactory regression of the developed model. The maximum adsorption capacity is found to be 1.807 mg/g at optimum operating conditions. The surface characterization of the adsorbent before and after adsorption by SEM, EDX, XRD, and FTIR confirms the potentiality of the adsorbent towards As(III) ions. Thermodynamic, adsorption isotherms and kinetic analysis respectively projected the endothermic Langmuir model adsorption of As(III) and the pseudo-second-order rate kinetics of the sorption mechanism. The current study aids the implementation of the developed robust technique for the successful removal of As(III) from industrial and domestic polluted water samples.

Antimonate sequestration from aqueous solution using zirconium, iron and zirconium-iron modified biochars

Antimony (Sb) is increasingly being recognized as an important contaminant due to its various industrial applications and mining operations. Environmental remediation approaches for Sb are still lacking, as is the understanding of Sb environmental chemistry. In this study, biosolid biochar (BSBC) was produced and utilized to remove antimonate (Sb(V)) from aqueous solution. Zirconium (Zr), Zirconium-iron (Zr-Fe) and Fe-O coated BSBC were synthesized for enhancing Sb(V) sorption capacities of BSBC. The combined results of specific surface area, FTIR, SEM-EDS, TEM-EDS, and XPS confirmed that Zr and/or Zr-Fe were successfully coated onto BSBC. The effects of reaction time, pH, initial Sb(V) concentration, adsorbate doses, ionic strength, temperature, and the influence of major competitive co-existing anions and cations on the adsorption of Sb(V) were investigated. The maximum sorption capacity of Zr-O, Zr-Fe, Zr-FeCl3, Fe-O, and FeCl3 coated BSBC were 66.67, 98.04, 85.47, 39.68, and 31.54 mg/g respectively under acidic conditions. The XPS results revealed redox transformation of Sb(V) species to Sb(III) occurred under oxic conditions, demonstrating the biochar's ability to behave as an electron shuttle during sorption. The sorption study suggests that Zr-O and Zr-O-Fe coated BSBC could perform as favourable adsorbents for mitigating Sb(V) contaminated waters.

Facile synthesis of nanosheet-assembled γ-Fe2O3 magnetic microspheres and enhanced Sb(III) removal

The development and utilization of magnetic nanoadsorption materials with large adsorption capacity and easy separation are the research hotspot nowadays. In this study, nanosheet-assembled maghemite (γ-Fe₂O₃) magnetic microspheres were successfully synthesized by an environmental friendly, quick, and simple method, for enhanced Sb(III) removal from aqueous solution. Scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), vibrating sample magnetometer (VSM), and Brunauer-Emmett-Teller (BET) were used to characterize the material. The results showed that the product contained flower-like γ-Fe₂O₃ microspheres composed of petal-shaped nanosheets interspersed with each other. The specific surface area and pore volume were 69.23 m²/g and 0.15 cm³/g, respectively. The material has a strong magnetic response, which allows rapid solid-liquid separation under the action of an external magnetic field. The effects of different dosages, solution pH, and contact time on the adsorption effect were studied by batch adsorption experiments, and the reusability of the materials was evaluated. Both Freundlich isothermal adsorption model and pseudo-second-order kinetic model were able to describe the uptake of Sb(III). The maximum adsorption capacity of the material was 47.48 mg/g under optimal conditions. The adsorption mechanism is mainly that Sb and lattice oxygen (O_X^2-) form Fe-O-Sb coordination bonds, which is incorporated into the crystal structure of γ-Fe₂O₃ as inner-sphere surface complexes. The synthetic material has the advantage of simple preparation process, good adsorption capacity, operation over a wide range of pH, and easy physical separation from treatment systems with good potential for future application to treat polluted wastewater.

Insights into conventional and recent technologies for arsenic bioremediation: A systematic review

Arsenic (As) bioremediation has been an economical and sustainable approach, being practiced widely under several As-contaminated environments. Bioremediation of As involves the use of bacteria, fungi, yeast, plants, and genetically modified organisms for detoxification/removal of As from the contaminated site. The understanding of multi-factorial biological components involved in these approaches is complex and more and more efforts are on their way to make As bioremediation economical and efficient. In this regard, we systematically reviewed the recent literature (n=200) from the last two decades regarding As bioremediation potential of conventional and recent technologies including genetically modified plants for phytoremediation and integrated approaches. Also, the responsible mechanisms behind different approaches have been identified. From the literature, it was found that As bioremediation through biosorption, bioaccumulation, phytoextraction, and volatilization involving As-resistant microbes has proved a very successful technology. However, there are various pathways of As tolerance of which the mechanisms have not been fully understood. Recently, phytosuction separation technology has been introduced and needs further exploration. Also, integrated approaches like phytobial, constructed wetlands using As-resistant bacteria with plant growth-promoting activities have not been extensively studied. It is speculated that the integrated bioremediation approaches with practical applicability and reliability would prove most promising for As remediation. Further technological advancements would help explore the identified research gaps in different approaches and lead us toward sustainability and perfection in As bioremediation.

Multicomponent adsorption of pentavalent As, Sb and P onto iron-coated cork granulates

The assessment of multicomponent adsorption of pentavalent metalloids is important since they are often found together in groundwaters and mining runoff. For this purpose, adsorption of As(V), Sb(V) and P(V) onto iron-coated cork granulates was studied in binary and ternary systems. Data from equilibrium and kinetic studies revealed that uptake of these contaminants is a multilayer, heterogeneous process well described by Freundlich, extended Freundlich and Elovich models. Most of the observed interactions are competitive and were related to the chemical structure and aqueous behaviour of each anion. Sb(V) adsorption was found to be most impaired and P(V) uptake the least affected by the presence of other pentavalent anions. The aggravation in the reduction of adsorbed amount from binary to ternary solution was more prominent for As(V) than Sb(V). Sb(V) adsorption outweighed that of the other pnictogens in acidic solutions, but in neutral conditions As(V) or P(V) adsorption may predominate instead. P(V) adsorption was the most sensitive to electrolyte addition, namely Ca salts, which may promote precipitation of calcium phosphates. This work provides useful insights regarding the design of adequate adsorption treatment systems for the simultaneous treatment of pentavalent metalloids.

Antimonate removal by diatomite modified with Fe-Mn oxides: application and mechanism study

In this study, diatomite coated with Fe-Mn oxides (DFMO) was synthesized through calcination. The adsorption of antimonate (Sb(V)) by DFMO was studied, and environmental factors affecting the adsorption were investigated. The components of DFMO were identified as γ-Fe₂O₃, γ-MnO₂, and SiO₂, in the presence of diatomite covered with nanoscale metal oxides. Batch experiments were carried out to evaluate the antimonate adsorption performance in aqueous solution. Results showed that maximum Sb(V) adsorption capacity of DFMO reached 10.7 mg/g at pH 4, corresponding to 22.2 mg/g per unit metal oxides. Antimonate adsorption occurred on heterogenous surface, following the Freundlich and Pseudo-second order model. Overall, antimonate adsorption was favored at acidic condition due to low point of zero charge. However, when treating electroplating wastewater, neutral pH condition exhibited a higher efficiency than acidic pH, because co-existing ions in electroplating wastewater significantly affects antimony adsorption. Further investigation showed that among different potential co-existing ions, fluoride can strongly inhibit the adsorption of antimonate at 5 mg/L under pH 4. Density functional theory (DFT) analysis confirmed that adsorption energy on DFMO follows: HF < F^- < Sb(OH)₆^-, indicating that fluoride is easier to bind with DFMO compared to antimonate, especially under pH 3.5 at which fluoride exists as HF. Moreover, the competitive adsorption of fluoride toward antimonate indicated the necessity of pre-treatment like neutralization and precipitation before adsorption process.

Enhanced arsenate removal by Fe-impregnated canola straw: assessment of XANES solid-phase speciation, impacts of solution properties, sorption mechanisms, and evolutionary polynomial regression (EPR) models

The impact of arsenic (As) contamination of water is an ongoing concern worldwide with As released from anthropogenic activities including mining and agriculture. Biosorption is a promising As treatment methodology used currently for arsenate (As(V)) sorption from water. The biosorbent was developed by a simple and inexpensive treatment of coating of canola straw particles with iron hydroxides. The modification procedure was optimized with consideration of the concentration of iron solution, pH of modification process, and sonication time. A higher concentration of iron and lower pH led to an improved sorption capacity of the iron-loaded canola straw (ICS), while impacts of sonication time were not conclusive. Pareto analyses indicated that the magnitude of the effect of the pH was higher than that of the iron concentration. Overall, the maximum As(V) sorption capacity of the ICS was 5.5 mg/g for an 0.25 M FeCl₃ solution concentration at pH 3. Analysis of kinetic data showed that the sorption processes of As(V) followed pseudo-second order and Elovich mechanisms, while sorption isotherm data were best represented by Freundlich and Temkin isotherm models. Studying the effect of ionic strength using NaCl suggested that the inner-sphere complex was the probable sorption mechanism. The thermodynamic parameters including ΔS°, ΔH°, and ΔG° showed that the As(V) sorption was thermodynamically favorable and spontaneous. Arsenic K-edge X-ray absorption near edge structure (XANES) spectroscopy indicated that no reaction to As(III) occurred during the sorption of As(V) using the optimum ICS biosorbent. The evolutionary polynomial regression (EPR) approach was able to closely match predicted vs. experimental sorption capacities (R² = 0.95). Overall, the improved understanding of the biosorbent's capability for removal of As(V) will be beneficial for assessment of its use for treatment of various water and wastewater matrices. In addition, knowledge gained from this research can assist in the understanding of sorption capacities of a variety of other biosorbents.

Enhanced removal of As(III) and As(V) from water by a novel zirconium-chitosan modified spherical sodium alginate composite

Most nano-scaled adsorbents have trouble in separating from aqueous solution, thus, a need for new materials of facile separation and predominant adsorption performance has arisen. This present study focused on a novel segregative zirconium-chitosan modified sodium alginate (Zr-CTS/SA) composite preparation and its performance for As(III/V) removal from aqueous solution. The obtained composite presented a spherical structure with a diameter of 2.0-3.0 mm and favorable thermal stability. Experimental data showed that Zr-CTS/SA had considerable adsorbability for As(III) and As(V), the adsorption capacities were enhanced about at least 20 and 6 times separately compared with pristine SA beads. The adsorption processes of As(III) and As(V) could both be described with Langmuir isotherm model and the maximum adsorption capacities reached 43.19 and 76.78 mg g^-1, respectively. The kinetic data of As(III) followed the intra-particle diffusion model while As(V) fitted the pseudo-first-order model. Moreover, the adsorption mechanisms of As(III/V) involved ligand exchange with Cl on the surface of Zr-CTS/SA, another reaction pathway for As(V) was the electrostatic attraction with protonated -OH and -NH₂ groups. Note that the employment of Zr-CTS/SA in low-concentration arsenic solution exhibited a residue concentration as low as the 10 μg L^-1 WHO guideline for drinking water.

Bioinspired synthesis of fiber-shaped silk fibroin-ferric oxide nanohybrid for superior elimination of antimonite

Bioinspired fibrous materials have emerged as a unique class of matrix for fabrication of fiber-shaped nanomaterial assemblies. Here, we report a novel functional fiber-shaped nanohybrid for efficient removal of antimonite via in situ synthesis of ferric oxides anchored to silk nanofibril. The silk nanofibril matrix played important roles in the growth of ferric oxides via metal-ligand interactions. The achieved nanocomposites had high surface areas and activity with more functional groups, contributing to superior antimonite elimination. The nanocomposite achieved a maximum removal capacity of 159.9 mg/g toward antimonite. And the common interfering ions of SO₄^2-, NO₃-, CO₃^2-, PO₄^3- and SiO₃^2- exhibited negligible influence on antimonite removal. The mechanism study point that two factors are closely involved: surface complexation and hydrogen bonding. Benefiting from the low cost and environmental-friendly nature of silk fibroin as well as excellent removal capacity and high selectivity, it suggests that the nanohybrids might be promising for antimonite extraction from contaminated water.

Current Trends of Arsenic Adsorption in Continuous Mode: Literature Review and Future Perspectives

Arsenic is a toxic element for humans and a major pollutant in drinking water. Natural and anthropogenic sources can release As into water bodies. The countries with the greatest arsenic contamination issues lack the affordable technology to attain the maximum permitted concentrations. Adsorption can be a highly efficient and low-cost option for advanced water treatment, and the development of new cheap adsorbents is essential to expand access to water with a safe concentration of arsenic. This paper aims to review the state of the art of arsenic adsorption from water in continuous mode and the latest progress in the regeneration and recovery of arsenic. The disposal of the exhausted bed is also discussed. Fixed-bed column tests conducted with novel adsorbents like binary metal oxides and biosorbents achieved the highest adsorption capacities of 28.95 mg/g and 74.8 mg/g, respectively. Iron-coated materials presented the best results compared to adsorbents under other treatments. High recovery rates of 99% and several cycles of bed regeneration were achieved, which can aggregate economic value for the process. Overall, further pilot-scale research is recommended to evaluate the feasibility of novel adsorbents for industrial purposes.

Optimization of Antimony Removal by Coagulation-Flocculation-Sedimentation Process Using Response Surface Methodology

Coprecipitation-adsorption plays a significant role during coagulation-flocculation-sedimentation (C/F/S) of antimony (Sb) in water. This work uses a Box–Behnken statistical experiment design (BBD) and response surface methodology (RSM) to investigate the effects of major operating variables such as initial Sb(III, V) concentration (100–1000 µg/L), ferric chloride (FC) dose (5–50 mg/L), and pH (4–10) on redox Sb species. Experimental data of Sb(III, V) removal were used to determine response function coefficients. The model response value (Sb removal) showed good agreement with the experimental results. FC showed promising coagulation behavior of both Sb species under optimum pH (6.5–7.5) due to its high affinity towards Sb species and low residual Fe concentration. However, a high dose of 50 mg/L of FC is required for the maximum (88–93%) removal of Sb(V), but also for the highest (92–98%) removal of low initial concentrations of Sb(III). Furthermore, BBD and RSM were found to be reliable and feasible for determining the optimum conditions for Sb removal from environmental water samples by a C/F/S process. This work may contribute to a better understanding and prediction of the C/F/S behavior of Sb(III, V) species in aqueous environments, to reduce potential risks to humans.

Efficient extraction of antimony(III) by titanate nanosheets: Study on adsorption behavior and mechanism

Antimony has been listed as a critical pollutant in many countries because of its toxic effects on earth organisms. In this study, titanate nanosheets (TNS) were prepared with a high specific surface area by alkaline hydrothermal method. The adsorption mechanism and adsorption capacity of removing Sb(III) from aqueous solutions with TNS as an adsorbent were investigated for the first time. The FTIR and XPS analysis indicated that the interlayer sodium ions of TNS were responsible for Sb(III) adsorption. The batch experiments were conducted on solution pH, adsorbent dosage, initial concentration and reaction time. The results exhibited that when pH was 2, the removal rate was about 90% with the dosage of TNS was 0.1 g/L. The adsorption reaction was exceedingly rapid in the initial 5 min, and then the reaction was in equilibrium after about 30 min. The experimental data were better fitted with Langmuir isotherm model, and the maximum adsorption amount could attain 232.56 mg/g. The experiments showed that TNS had outstanding anti-interference performance to common cations. Therefore, TNS were considered to be an excellent material for removing Sb(III) from aqueous solutions.

Environmental applications of a biodegradable cysteine-based surfactant

An improved method has been developed for the efficient synthesis of octanoyl-cysteine in single-chain form (N-octanoyl-cys) which operates as a surfactant over a wide pH range, is easily decomposed into natural products and has a high product yield. The compound offers an environmentally acceptable agent for the adsorption of a range of heavy metals from contaminated waters/soils, and it could also be used in general household detergents or personnel-cleaner formulations, and even in toothpastes. The surfactant was used as a co-surfactant for flotation of perfluorooctanoic acid (PFOA), a per/poly-fluoroalkyl substance (PFAS). The new surfactant produced significant foaming and removed 70% of the PFOA after 30min of foam fractionation. The compound is also potentially useful in facilitating the release of PFAS compounds; these are negatively charged and often bound to charged particles in sand, clay, and humic-acid-coated materials and microorganisms via bridging multivalent ions, such as Ca²⁺, Mg²⁺, Al³⁺ and Fe³⁺, as well as a range of other heavy-metal ions present in soil to varying degrees. In soils (and also in contaminated water), the common chelating agent EDTA is often used to encapsulate these ions (e.g. Ca²⁺, the dominant ion in soil) at moderately high pH to aid in the release of the bound PFAS compounds. However, it would be more environmentally acceptable to use this new biodegradable surfactant, which could combine chelation with foam-fractionation separation of surface-active (e.g. PFAS) components in soils.

Recent Advances of Graphene-Based Strategies for Arsenic Remediation

The decontamination of water containing toxic metals is a challenging problem, and in the last years many efforts have been undertaken to discover efficient, cost-effective, robust, and handy technology for the decontamination of downstream water without endangering human health. According to the World Health Organization (WHO), 180 million people in the world have been exposed to toxic levels of arsenic from potable water. To date, a variety of techniques has been developed to maintain the arsenic concentration in potable water below the limit recommended by WHO (10 μg/L). Recently, a series of technological advancements in water remediation has been obtained from the rapid development of nanotechnology-based strategies that provide a remarkable control over nanoparticle design, allowing the tailoring of their properties toward specific applications. Among the plethora of nanomaterials and nanostructures proposed in the remediation field, graphene-based materials (G), due to their unique physico-chemical properties, surface area, size, shape, ionic mobility, and mechanical flexibility, are proposed for the development of reliable tools for water decontamination treatments. Moreover, an emerging class of 3D carbon materials characterized by the intrinsic properties of G together with new interesting physicochemical properties, such as high porosity, low density, unique electrochemical performance, has been recently proposed for water decontamination. The main design criteria used to develop remediation nanotechnology-based strategies have been reviewed, and special attention has been reserved for the advances of magnetic G and for nanostructures employed in the fabrication of membrane filtration.

A critical review on recent developments in MOF adsorbents for the elimination of toxic heavy metals from aqueous solutions

Effective and substantial remediation of contaminants especially heavy metals from water is still a big challenge in terms of both environmental and biological perspectives because of their adverse effects on the human health. Many techniques including adsorption, ion exchange, co-precipitation, chemical reduction, ultrafiltration, etc. are reported for eliminating heavy metal ions from the water. However, adsorption has preferred because of its simple and easy handlings. Several types of adsorbents are observed and documented well for the purpose. Recently, highly porous metal-organic frameworks (MOFs) were developed by incorporating metals and organic ligands together and claimed as potent adsorbents for the remediation of highly toxic heavy metals from the aqueous solutions due to their unique features like greater surface area, high chemical stability, green and reuse material, etc. In this review, the authors discussed systematically some recent developments about secure MOFs to eliminate the toxic metals such as arsenic (both arsenite and arsenate), chromium(VI), cadmium (Cd), mercury (Hg) and lead (Pb). MOFs are observed as the most efficient adsorbents with greater selectivity as well as high adsorption capacity for metallic contamination. Graphical abstract.

Production of an iron-coated adsorbent for arsenic removal by hydrothermal carbonization of olive pomace: Effect of the feedwater pH

The removal of arsenic from water by adsorption is currently hindered by the elevated cost of conventional adsorbent materials. To overcome this limit, an innovative iron-coated adsorbent was produced by hydrothermal carbonization (170 °C, 30 min) of olive pomace, an inexpensive byproduct of the olive oil production. Hydrothermal carbonization experiments were performed starting from olive pomace dispersions in solutions with acidic, neutral and alkaline pH, in presence and absence of FeCl₃. Acidic conditions improved the carbonization, ensuring reduced H/C and O/C ratios, and increased the adsorbent stability. However, acidic pH yielded unsatisfactory iron coating, with only 32% of the iron dissolved in the initial solution transferred to the produced hydrochar. Under alkaline pH, 96% of the iron in the feedwater was, in contrast, stably dispersed over the hydrochar surface, giving the highest maximum arsenic adsorption capacity (4.1 mg/g). However, alkaline pH promoted biomass hydrolysis, causing the loss of 60% and 87% of the total C and N, respectively, and reducing the stability of the produced hydrochar. A two-stage process was tested to overcome these issues, including hydrothermal carbonization under acidic pH with FeCl₃, followed by the addition of NaOH. This process prevented biomass hydrolysis yielding a stable hydrochar. However, as compared to the one-stage alkaline synthesis, the two-stage process produced an hydrochar with reduced arsenic adsorption capacity (1.4 mg/g), indicating that biomass hydrolysis could positively influence hydrochar adsorption characteristics, possibly by increasing the specific surface area. Indications are then provided on how to optimize the two-stage process in order to produce a hydrochar with both satisfactory stability and arsenic adsorption capacity.

Zeolite-Rich Composite Materials for Environmental Remediation: Arsenic Removal from Water

Natural zeolites are used as adsorbents in purification processes due to their cation-exchange ability and molecular sieve properties. Surface modified natural zeolites (SMNZs), produced by attaching cationic organic surfactants to the external surface, can simultaneously act as ionic exchangers and organic molecule adsorbents. In this paper, SMNZs were produced and investigated as adsorbents for As(V) removal from wastewater: two natural zeolites, clinoptilolite and phillipsite, were modified using HDTMA-Br and HDTMA-Cl as surfactants. The obtained samples were then characterized under static and dynamic conditions. Results showed that As(V) removal follows a pseudo-second order kinetic, with fast adsorption rates: every sample reached 100 % removal in 2 h, while equilibrium data showed a Langmuir-like behavior, with a greater anion uptake by the HDTMA-Br modified SMNZs due to the formation of a compact and complete micellar structure. Finally, fixed-bed tests were performed to characterize the samples under dynamic conditions assessing the effect of severe operating parameters on the dynamic exchange capacity, selectivity and efficiency of the process. The obtained results demonstrate a good ability of the tested materials to adsorb As(V) from wastewater, confirming the effectiveness of the proposed surface modification technique in expanding the possibility of using natural zeolites in these processes.

Trophodynamics of arsenic for different species in coastal regions of the Northwest Pacific Ocean: In situ evidence and a meta-analysis

China has been the major fishery producer in the Northwest Pacific Ocean for decades and the seafood safety deserves continuous concern. In this study, 188 organism and 27 sediment samples were collected from the Jiaozhou Bay, a typical semi-enclosed bay adjacent to the Northwest Pacific Ocean, to study the arsenic (As) pollution level and trophodynamics in the coastal regions of China combined with a meta-analysis. Results showed that arsenic was the most abundant in crustaceans with the average of 28.84 ± 4.95 mg/kg in dry weight, in comparison with molluscs (18.68 ± 2.51 mg/kg) and fish (9.31 ± 1.45 mg/kg). Additionally, based on a meta-analysis, arsenic in coastal organisms generally decreased from north to south in China. With increasing values of δ¹⁵N, arsenic was significantly biomagnified in the molluscs but bio-diluted in the groups of crustaceans and fish. When all the species were taken into consideration, overall bio-dilution of As was observed through the simplified food chain in the Jiaozhou Bay. Based on the target hazard quotient (THQ), the health risk of consuming seafood from the Jiaozhou Bay was not significant except for several kinds of crustaceans. The smaller THQs indicated lower health risk of eating molluscs and fish than crustaceans. Besides, urban households tended to undertake much higher risk than rural households. Based on our results, it is recommended for urban citizens to reduce the frequency of consuming crustaceans and give preference to fish when choosing seafood.

Arsenic and cadmium removal from water by a calcium-modified and starch-stabilized ferromanganese binary oxide

A new calcium-modified and starch-stabilized ferromanganese binary oxide (Ca-SFMBO) sorbent was fabricated with different Ca concentrations for the adsorption of arsenic (As) and cadmium (Cd) in water. The maximum As(III) and Cd(II) adsorption capacities of 1% Ca-SFMBO were 156.25 mg/g and 107.53 mg/g respectively in single-adsorption systems. The adsorption of As and Cd by the Ca-SFMBO sorbent was pH-dependent at values from 1 to 7, with an optimal adsorption pH of 6. In the dual-adsorbate system, the presence of Cd(II) at low concentrations enhanced As(III) adsorption by 33.3%, while the adsorption of As(III) was inhibited with the increase of Cd(II) concentration. Moreover, the addition of As(III) increased the adsorption capacity for Cd(II) up to two-fold. Through analysis by X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FTIR), it was inferred that the mechanism for the co-adsorption of Cd(II) and As(III) included both competitive and synergistic effects, which resulted from the formation of ternary complexes. The results indicate that the Ca-SFMBO material developed here could be used for the simultaneous removal of As(III) and Cd(II) from contaminated water.

Isolation and Characterization of Facultative-Anaerobic Antimonate-Reducing Bacteria

Microbial antimonate (Sb(V)) reduction is a promising approach to remove Sb(V) from wastewater. However, current knowledge regarding microbial Sb(V) reduction is limited to strictly anaerobic conditions. This study was the first to isolate three facultative-anaerobic Sb(V)-reducing bacterial strains from the sludge collected from a wastewater treatment facility in an antimony products plant. Two of the isolated strains, designated Dechloromonas sp. AR-2 and Propionivibrio sp. AR-3, were characterized based on their Sb(V)-reducing abilities. When cultivated under anaerobic conditions with Sb(V) and acetate as the electron acceptor and donor, respectively, both strains could efficiently reduce 5.0 mM Sb(V), removing most of it from the water phase within 7 d. Along with Sb(V) reduction by the strains, white precipitates, which were likely amorphous Sb(OH)₃ solids, were formed with a minor generation of soluble antimonite. Additionally, respiratory Sb(V) reduction by both strains occurred not only under anaerobic but also microaerobic conditions. It was suggested that Sb(V) reduction and the growth abilities of the strains under microaerobic conditions presented a substantial advantage of the use of strains AR-2 and AR-3 for practical applications to Sb(V)-containing wastewater treatment.

Application of oxidation-reduction potential (ORP) as a controlling parameter during the synthesis of Fe3O4@PVA nanocomposites from industrial waste (raffinate)

The objective of this study was to develop a process for the synthesis of magnetite nanocomposites from raffinate of hydrometallurgical plants containing iron ions. Towards this goal, a three-step process was developed. After the reduction of Fe(III) to Fe(II) in the first stage, Al3+ in the raffinate was precipitated as Al(OH)3 and finally magnetite NPs were synthesized by chemical co-precipitation method via controlling the oxidation-reduction potentials (ORPs). The produced nanomaterials were analyzed using XRD, TEM, VSM, TGA, and FTIR. The TG, XPS, and FTIR results affirmed the existence of PVA while TEM images illustrated the spherical nanoparticles with an average size of about 19 ± 4 nm. Evaluation of VSM data indicated the highly ferromagnetic behavior for Fe3O4 and Fe3O4@PVA products at the room temperature with a saturation magnetization of 46.98 and 35.78 emu/g, respectively.

A Biosorption-Pyrolysis Process for Removal of Pb from Aqueous Solution and Subsequent Immobilization of Pb in the Char

The application of biosorption in the removal of heavy metals from water faces a challenge of safe disposal of contaminated biomass. In this study, a potential solution for this problem was proposed by using a biosorption-pyrolysis process featured by pretreatment of biomass with phosphoric acid (PA). The PA pretreatment of biomass increased the removal efficiency of heavy metal Pb from water by sorption, and subsequent pyrolysis helped immobilize Pb in the residual char. The results indicate that most (>95%) of the Pb adsorbed by the PA-pretreated biomass was retained in the char, and that the lower pyrolysis temperature (350 °C) is more favorable for Pb immobilization. In this way, the bioavailable Pb in the char was hardly detected, while the Pb leachable in acidic solution decreased to <3% of total Pb in the char. However, higher pyrolysis temperature (450 °C) is unfavorable for Pb immobilization, as both the leachable and bioavailable Pb increased to >28%. The reason should be related to the formation of elemental Pb and unstable Pb compounds during pyrolysis at 450 °C, according to the X-ray diffraction study.

Efficient antimony removal by self-assembled core-shell nanocomposite of Co3O4@rGO and the analysis of its adsorption mechanism

Co₃O₄@rGO were facilely prepared by template free self-assemble in this study. The morphology of Co₃O₄@rGO was actiniaria-like core-shell structural nanocomposites. The formation mechanism of Co₃O₄@rGO core-shell nanocomposite was discussed according to its significant time-dependent morphology evolution course. To evaluate the application potential of Co₃O₄@rGO, its adsorption performance toward highly toxic antimony ions were studied. The Co₃O₄@rGO nanocomposite exhibit high anti-interference ability and high adsorption ability. The maximum adsorption capacities towards Sb(III) and Sb(V) are 151.04 and 165.51 mg/g, respectively. River water samples containing antimony violating the limit were used to evaluate the practical application of Co₃O₄@rGO, and high performance was achieved. The EU and China limits for antimony in drinking water can be met by using mesoporous Co₃O₄@rGO treating the actual river water samples with original antimony concentration lower than 50 μg/L. Adsorption isotherm, adsorption kinetics, pH and co-existing ions effects were also studied in details. The results indicate that mesoporous Co₃O₄@rGO is an excellent adsorbent for antimony removal. Mesoporous Co₃O₄@rGO nanocomposite is a potential candidate for antimony removal from waste water.

Enhanced removal of antimony in dyeing wastewater by mixing Fe3 O4 with manganese sand filter material

Wastewaters from the printing and dyeing industries contain many contaminants in particular antimony (Sb) that end up in the environment. Both manganese sand filter and Fe₃ O₄ have good removal effect on Sb, and are cheap and easy to obtain. We made a filter material by mechanically mixing the manganese sand filter material and ferro-ferric oxide (Fe₃ O₄ ). The Fe-Mn oxide filter material was analyzed by X-ray diffraction. We studied the filtration of real wastewater from a dyeing wastewater resource recovery facility in Suzhou, China, containing Sb at high concentration of 410 μg/L, using dynamic tests in adsorption columns during 7 days. We tested the effects of filter material volume filling ratio, the empty bed contact time (EBCT), pH, and back washing on the removal of Sb. Results show that the addition of Fe₃ O₄ enhanced the removal of Sb, reaching 85% of initial Sb. When the initial influent pH of the raw water is 3.0, the volume filling ratio of filter material is 60%, the EBCT is 20 min, and the developed dynamic Fe-Mn oxide filter has the best removal effect on Sb. Daily back washing of the filter keeps a Sb removal rate of about 80%. PRACTITIONER POINTS: A novel and cheap Fe-Mn oxide was developed for Sb removal from dyeing wastewater. A self-designed filter device was designed to verify performance of the low-cost material. Optimal design and operational parameters of the filtration process were determined.

Removal of aqueous As(III) Sb(III) by potassium ferrate (K2FeO4): The function of oxidation and flocculation

This study investigated the effects of potassium ferrate (K₂FeO₄) dosage, pH, and reaction time on the removal of aqueous As(III) and Sb(III), and revealed the oxidation and flocculation mechanism of K₂FeO₄. The results show that the removal efficiencies of As(III) and Sb(III) were highly related to the hydrolysate of K₂FeO₄ under acidic conditions, while the efficiencies were low under alkaline condition, owning to the electrostatic repulsion between iron nanoparticles and charged As/Sb species. The increased dosage and reaction time improved the adsorption performance. Based on the comparative experiments with FeCl₃, the simultaneous removal of As(III) and Sb(III) by K₂FeO₄ suggested that As(III) was eliminated due to the processes of oxidation, flocculation, and chemical precipitation, while Sb(III) was removed mostly by oxidation and flocculation. The generated precipitates were characterized with surface analysis and the results support that the oxidization property of K₂FeO₄ was essential during the removal of As(III) and Sb(III), and removal mechanisms between both elements were different.

Nanofiltration for Arsenic Removal: Challenges, Recent Developments, and Perspectives

Arsenic (As) removal is of major significance because inorganic arsenic is highly toxic to all life forms, is a confirmed carcinogen, and is of significant environmental concern. As contamination in drinking water alone threatens more than 150 million people all over the world. Therefore, several conventional methods such as oxidation, coagulation, adsorption, etc., have been implemented for As removal, but due to their cost-maintenance limitations; there is a drive for advanced, low cost nanofiltration membrane-based technology. Thus, in order to address the increasing demand of fresh and drinking water, this review focuses on advanced nanofiltration (NF) strategy for As removal to safeguard water security. The review concentrates on different types of NF membranes, membrane fabrication processes, and their mechanism and efficiency of performance for removing As from contaminated water. The article provides an overview of the current status of polymer-, polymer composite-, and polymer nanocomposite-based NF membranes, to assess the status of nanomaterial-facilitated NF membranes and to incite progress in this area. Finally, future perspectives and future trends are highlighted.

The antimony sorption and transport mechanisms in removal experiment by Mn-coated biochar

A method of Mn-coated biochar production was developed, which showed great removal ability of trivalent antimony (Sb(III)) (0.94 mg g^-1) and pentavalent antimony (Sb(V)) (0.73 mg g^-1), and the adsorption capacity was stable under different pH. According to the adsorption kinetics and isotherm, the adsorption process of both Sb(III) and Sb(V) was chemisorption, which was both monolayer and poly layers heterogeneous chemisorption process. X-ray photoelectron spectroscopy (XPS) and attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy analyses indicated that the main oxides and functional groups involved in the adsorption were manganese oxides (MnO_x), carboxyl and hydroxyl groups and Sb(V) was combined with Mn-coated biochar by inner-sphere complexation. Sb(III) was oxidized by oxygen and MnO_x which was both on the surface of biochar and dissolved in solution. Furthermore, X-ray absorption near-edge structure (XANES) showed that Sb(V) was the main species after Sb(III) and Sb(V) adsorbed on the Mn-coated biochar. Extended X-ray absorption fine structure (EXAFS) analysis indicated that Sb(III) and MnO_x formed the monodentate mononuclear and corner-sharing complexes with a structure of Mn-O-Sb on Mn-coated biochar. While Sb(V) and MnO_x formed inner-sphere complexes including edge-sharing and corner-sharing complexes. The new synthetic material can contribute to develop new remediation strategies for treating Sb-contaminated water.

Removal of Sb(III) from wastewater by magnesium oxide and the related mechanisms

Pollution and remediation of antimony in aquatic ecosystems have been paid increasing attention. In the present work, environmentally friendly nano-MgO was used for the first time to remove Sb(III) from the water system. The batch experiments indicated that the nano-MgO calcined at 400 °C (named 400-MgO) exhibited superior adsorption capacity for Sb(III). The adsorption isotherm was fitted well with the Freundlich model, and especially when the initial concentration is 15 mg‧L^-1, the adsorption capacity is as high as 140.1 mg‧g^-1. Researches through X-ray diffraction (XRD), Transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS) showed that the main removal mechanisms lie in the generation of Mg active sites, which is capable of coordinating Sb(III), during the hydration process of MgO, providing a nucleation center for the progressive production of MgSb₂O₄. As a whole, precipitation is the predominant mechanism for MgO to remove Sb(III). Over time, a part of MgO is hydrated to Mg(OH)₂, and consequent chemisorption also helps to remove Sb(III). Our work has demonstrated that nano-MgO is a promising adsorbent for Sb(III) removal from contaminated water and provided new insights into the interaction mechanism between MgO and Sb(III).