The effect of pH on the adsorption of arsenic(III) and arsenic(V) at the TiO 2 anatase [1 0 1] surface

Biosorption of arsenic (III) from aqueous solution using calcium alginate immobilized dead biomass of Acinetobacter sp. strain Sp2b

This study presents a novel biosorbent developed by immobilizing dead Sp2b bacterial biomass into calcium alginate (CASp2b) to efficiently remove arsenic (AsIII) from contaminated water. The bacterium Sp2b was isolated from arsenic-contaminated industrial soil of Punjab, a state in India. The strain was designated Acinetobacter sp. strain Sp2b as per the 16S rDNA sequencing, GenBank accession number -OP010048.The CASp2b was used for the biosorption studies after an initial screening for the biosorption capacity of Sp2b biomass with immobilized biomass in both live and dead states. The optimum biosorption conditions were examined in batch experimentations with contact time, pH, biomass, temperature, and AsIII concentration variables. The maximum biosorption capacity (qmax = 20.1 ± 0.76 mg/g of CA Sp2b) was obtained at pH9, 35 ̊ C, 20 min contact time, and 120 rpm agitation speed. The isotherm, kinetic and thermodynamic modeling of the experimental data favored Freundlich isotherm (R2 = 0.941) and pseudo-2nd-order kinetics (R2 = 0.968) with endothermic nature (ΔH° = 27.42) and high randomness (ΔS° = 58.1).The scanning electron microscopy with energy dispersive X-ray (SEM-EDX) analysis indicated the As surface binding. The reusability study revealed the reasonable usage of beads up to 5 cycles. In conclusion, CASp2b is a promising, efficient, eco-friendly biosorbent for AsIII removal from contaminated water.

Nanoconfinement boosts affinity of hydrated zirconium oxides to arsenate: Surface complexation modeling study

Nanoscale hydrated zirconium oxide (HZO) holds great potential in groundwater purification due to its ability to form inner-sphere coordination with arsenate. Despite being frequently used, especially as encapsulations in host materials for practical application in water treatment, the adsorption mechanisms of solutes on HZO are not appropriately explored, in particular for arsenate adsorption. In this study, we investigated the Zr-As coordination configuration and identified the most credible Zr-As configuration using surface complexation modeling (SCM), XPS and FT-IR analysis. The corresponding intrinsic coordination constants (Kintr) values was calculated by SCM, and the nanoconfinement effects were distinguished by comparing bare HZO with the HZO nanoparticles (NPs) encapsulated inside the strongly basic anion exchanger D201. Potentiometric titration suggests that the surface Zirconium hydroxyl groups (≡ZrOH) mainly exist in protonated form (≡ZrOH2+). Batch adsorption experiments demonstrate that the D201 hosts could adsorb As(V) through ion exchange by the quaternary ammonium groups under the low ionic strength (≤0.01 M NaNO3) and at pH > 6. The nanocomposite (HZO@D201) exhibits a higher adsorption capacity in a wide range of pH (3-10) and ionic strength (0.001-0.1 M NaNO3) than bare HZO. SCM simulations reveal that the coordination configuration of diprotonated monodentate mononuclear (MM-H2) dominates at pH 3-6, while deprotonated bidentate binuclear (BB-H0) dominates at pH > 7. For each configuration, the intrinsic coordination constants (Kintr) of HZO@D201 (10-0.66 and 10-16.10, respectively) are significantly higher than those of bare HZO (10-12.24 and 10-44.42, respectively), indicating a superior chemical bonding affinity caused by nanoconfinement. The obtained Kintr values are used to predict arsenate adsorption isotherms in pH 3 and 9, and the results align with the SCM simulation outcomes. This study may offer a feasible method for investigating the nanoconfinement effect of emerging nanocomposite adsorbents from a thermodynamic perspective, and provide reference coordination equilibrium constants of HZO for research and practical application.

Facile synthesis of ZnO/Hal nanocomposite for arsenite (As(III)) removal from aqueous media

Arsenite (As(III)) is the most toxic form of arsenic that is a serious concern for water contamination worldwide. Herein a ZnO/Halloysite (Hal) nanocomposite was prepared by the chemical bath deposition method (CBD) through seed-mediated ZnO growth on the halloysite for eliminating As(III) from the aqueous solution. The growth of ZnO on seeded halloysite was investigated based on the HMTA: Zn2+ molar ratio in the solution. An optimum molar ratio of HMTA:Zn for nucleation and growth of ZnO upon halloysite was obtained 1:2 based on morphological analysis. The TGA results confirmed that thermal stability of HNT was enhanced by ZnO decoration. The prepared ZnO/Hal nanocomposite at optimal conditions was employed for arsenite (As(III)) removal from aqueous solutions. Experimental data were evaluated with different isothermal, thermodynamic, and kinetic models. Based on the zeta potential results, Hal nanocomposites had a greater negative value than pure Hal. Therefore, the ZnO/Hal nanocomposite exhibited efficient As(III) adsorption with a removal efficiency of 76% compared to pure Hal with a removal efficiency of 5%. Adsorption isotherm was well correlated by both non-linear Langmuir and Sips models, exhibiting maximum adsorption capacity of As(III) at 42.07 mg/g, and 42.5 mg/g, respectively. As a result of the study, it was found that the fabricated Hal nanocomposite with low toxicity can be used effectively in water treatment.

Chloride converts lead slag into a bifunctional material to remove heavy metals

Efficient and safe removal of arsenic and lead from industrial wastewater is essential for ecological protection. In this study, we developed a novel method using lead slag as a purifying agent and sodium chloride as a reinforcing agent to remove arsenic and lead from industrial wastewater. Through a combination of experiments and simulations, we elucidated the mechanisms involved in this reaction. The initial concentrations of As and Pb ions in the industrial wastewater were 4333 and 188 mg/L, respectively. After the reaction at 25 °C and a pH ranging from 9.7 to 10, the concentrations of arsenic and lead were reduced to 4.9 mg/L and 0.008 mg/L, respectively, achieving a removal rate of 99.9%. Our experimental results demonstrated that Pb2+ and AsO43- ions released from the lead slag and industrial wastewater reacted with Cl- ions to form Pb5(AsO4)3Cl precipitates, thus effectively eliminating a significant amount of As and Pb species. Simulation studies indicated that Pb5(AsO4)3Cl exhibited exceptional stability below 400 °C and could be directly stored. Additionally, the lead slag, which is rich in silica, played a crucial role in removing and stabilizing As and Pb ions. Under alkaline conditions, silica encapsulated the As and Pb species, adhering to the surface of the Pb-As co-precipitates and forming dense, irregular, small particles with internal and external structures that impeded the efflux of As and Pb ions. This phenomenon was confirmed through scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The kinetics of As and Pb ion removal was consistent with the pseudo-second-order kinetic model, indicating that the removal process was primarily governed by chemical interactions. Lead slag exhibits significant potential and advantages in the removal of As and Pb. This innovative method offers an effective approach to address heavy metal contamination in industrial wastewater, thus contributing to ecological protection.

Self-Competitive Adsorption Behavior of Arsenic on the TiO2 Surface

TiO2 is a commonly used material to remove arsenic from drinking water by adsorption as well as photocatalytic oxidation (PCO). In the present paper, arsenic adsorption and PCO at different pH environments are studied on the (1 1 0) facet of rutile TiO2 (r-TiO2). A self-competitive adsorption (SCA) behavior of arsenic is observed; i.e., arsenic species compete to adsorb on the surface. Related DFT calculations are carried out to simulate adsorption. SCA behavior is the key to connecting calculation results with experimental results. Furthermore, PCO of arsenite is performed at different pH values. Of note, PCO is related to adsorption; namely, the adsorption process determines the whole PCO reaction speed. Therefore, SCA is also helpful for the PCO reaction. The SCA behavior is useful not only for arsenic on r-TiO2 but also for arsenic on anatase TiO2 (a-TiO2). It may be helpful to further study arsenic adsorption and PCO on other materials such as Fe2O3 and MnO2. The SCA behavior extends our understanding of arsenic and provides new insights into arsenic removal and its cycle in nature.

In situ prepared Chlorella vulgaris-supported nanoscale zero-valent iron to remove arsenic (III)

Nanoscale zero-valent iron (nZVI) has a high removal affinity toward arsenic (As). However, the agglomeration of nZVI reduces the removal efficiency of As and, thus, limit its application. In this study, we report an environmentally friendly novel composite of Chlorella vulgaris-supported nanoscale zero-valent iron (abbreviated as CV-nZVI) that exhibits a fast and efficient removal of As(III) from As-contaminated water. Scanning electron microscopy-energy-dispersive spectroscopy (SEM-EDS), X-ray diffractometry (XRD), attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR), and X-ray photoelectron spectroscopy (XPS) were used to characterize and analyze the CV-nZVI. These results indicated that the stabilization effect of C. vulgaris reduced the nZVI agglomeration and enhanced the reactivity of nZVI. The experiments showed a removal efficiency of 99.11% for As(III) at an optimum pH of 7.0. The adsorption kinetics and isotherms followed the pseudo-second-order kinetic model and Langmuir adsorption isotherm with the superior maximum adsorption capacities of 34.11 mg/g for As(III). The FTIR showed that the As(III) was adsorbed on the CV-nZVI surface by complexation reaction, and XPS indicated that oxidation reaction was also involved. After five reuse cycles, the removal efficiency of As(III) by CV-nZVI was 32.93%, suggesting that the CV-nZVI had some reusability and regeneration. Overall, this work provides a practical and highly efficient approach for As remediation in As-contaminated water, and simultaneously resolves the agglomeration problems of nZVI nanoparticles.

Repurposing of steel rolling sludge: Solvent-free preparation of α-Fe2O3 nanoparticles and its application for As(III/V)-containing wastewater treatment

Steel rolling sludge (SRS) is the by-product of metallurgical industry with abundant iron content, which needs to be utilized for producing high value-added products. Herein, cost-effective and highly adsorbent α-Fe₂O₃ nanoparticles were prepared from SRS via a novel solvent-free method and applied to treat As(III/V)-containing wastewater. The structure of the prepared nanoparticles was observed to be spherical with a small crystal size (12.58 nm) and high specific surface area (145.03 m²/g). The nucleation mechanism of α-Fe₂O₃ nanoparticles and the effect of crystal water were investigated. More importantly, compared with the traditional methods of preparation cost and yield, this study was found to have excellent economic benefits. The adsorption results indicated that the adsorbent could effectively remove arsenic over a wide pH range, and the optimal performance of nano adsorbent for As(III) and As(V) removal was observed at pH 4.0-9.0 and 2.0-4.0, respectively. The adsorption process was consistent with pseudo-second-order kinetic and Langmuir isothermal model. The maximum adsorption capacity (q_m) of adsorbent for As(III) and As(V) was 75.67 mg/g and 56.07 mg/g, respectively. Furthermore, α-Fe₂O₃ nanoparticles exhibited great stability, and q_m remained at 64.43 mg/g and 42.39 mg/g after five cycles. Particularly, the As(III) was removed by forming inner-sphere complexes with the adsorbent, and it partially oxidized to As(V) during this process. In contrast, the As(V) was removed by electrostatic adsorption and reaction with -OH on the adsorbent surface. Overall, resource utilization of SRS and the treatment of As(III)/(V)-containing wastewater in this study are in line with the current developments in the environmental and waste-to-value research.

Soil pH determines arsenic-related functional gene and bacterial diversity in natural forests on the Taibai Mountain

Arsenic-related functional genes are ubiquitous in microbes, and their distribution and abundance are influenced by edaphic factors. In arsenic-contaminated soils, soil arsenic content and pH determine the distribution of arsenic metabolizing microorganisms. In the uncontaminated natural ecosystems, however, it remains understudied for the key variable factor in determining the variation of bacterial assembly and mediating the arsenic biogeographical cycles. Here, we selected natural forest soils from southern and northern slopes along the altitudinal gradient of Taibai Mountain, China. The arsenic-related functional genes and soil bacterial community was examined using GeoChip 5.0 and high-throughput sequencing of 16S rRNA genes, respectively. It was found that arsenic-related functional genes were ubiquitous in tested forest soils. The gene arsB has the highest relative abundance, followed by arsC, aoxB, arrA, arsM, and arxA. The arsenic-related functional genes distribution on two slopes were decoupled from their corresponding bacterial community. Though there are higher abundance of bacterial communities on the northern slope than that on the southern slope, for arsenic-related functional genes, the abundance has the contrary trend which showing the more arsenic-related functional genes on the southern slope. In the top ten phyla, Proteobacteria and Actinobacteria were dominant phyla which affected the abundance of arsenic-related functional genes. Redundancy analysis and variance partitioning analysis indicated that soil pH, organic matter and altitude jointly determined the arsenic-related functional genes diversity in the two slopes of Taibai Mountain, and soil pH was a key factor. This indicates that the lower pH may shape more microbes with arsenic metabolic capacity. These findings suggested that soil pH plays a significant role in regulating the distribution of arsenic-related functional microorganisms, even for a forest ecosystem with an altitudinal gradient, and remind us the importance of pH in microbe mediated arsenic transformation.

Adsorption removal of arsenic from aqueous solutions and groundwater by isomeric FeOOH

FeOOH as a naturally abundant, relatively low-cost and effective adsorbent have been gradually valued in wastewater field rich in arsenic pollution, which can make for environmental remediation. In this study, FeOOH samples included Gth1/Gth2 as goethite, Aka1/Aka2 as akaganéite, and Lep as lepidocrocite, were prepared and used as adsorbents, and adsorption kinetic and isotherm experiments of As(III) were analyzed. Meanwhile, the effects of pH, adsorbent content, arsenic initial concentration and electrolyte solutions on adsorption processes were also discussed in detail to study adsorption behaviors and mechanism. The results showed that As(III) could be effectively adsorbed on goethite, akaganéite and lepidocrocite, the adsorption equilibrium achieved after 24 h. When As(III) concentration ranged in 40 mg/L, the saturated adsorption amounts (mg/g) calculated by the Langmuir equation were 12.3 (Gth1), 7.50 (Gth2), 6.29 (Aka1), 23.4 (Aka2), and 17.7 (Lep). The increase of adsorbent and adsorbate levels was favorable to improve the adsorption capacities of As(III) within a certain range. Removal efficiency of As(III) with Na₂SO₄ and NaH₂PO₄ as electrolyte reduced by about 10% and 30%, respectively. Therefore, the appropriate parameters in the adsorption process for investigation were isomeric FeOOH of 1.0 g/L, pH 7.0 and NaNO₃ as electrolyte. In simulated groundwater filter system initially with 200 μg/L of arsenic species at about pH 7.0, arsenic removal strength for five FeOOH adsorbents (0.8 g) was Aka2 > Aka1 and Gth1 > Lep and Gth2. Some differences were present in the infrared (IR) spectra of arsenic-loaded and original isomeric FeOOH. These outcomes could give the aim at seeking high efficient materials for the purification of arsenic contaminated groundwater and put out the suggestion.

Biochar Adsorbents for Arsenic Removal from Water Environment: A Review

Arsenic intake can cause human health disorders to the lungs, urinary tract, kidney, liver, hyper-pigmentation, muscles, neurological and even cancer. Biochar is potent, economical and ecologically sound adsorbents for water purification. After surface modifications, adsorption capacity of biochar significantly increased due to high porosity and reactivity. Adsorption capacities of the biochar derived from the municipal solid waste and KOH mixed municipal solid waste were increased from 24.49 and 30.98 mg/g for arsenic adsorption. Complex formation, electrostatic behavior and ion exchange are important mechanisms for arsenic adsorption. Organic arsenic removal using biochar is a major challenge. Hence, more innovative research should be conducted to achieve one of the 17 sustainable development goals of the United Nations i.e. "providing safe drinking water for all". This review is focused on the arsenic removal from water using pristine and modified biochar adsorbents. Recent advances in production methods of biochar adsorbents and mechanisms of arsenic removal from water are also illustrated.

Novel design of amine and metal hydroxide functional group modified onto sludge biochar for arsenic removal

This study involved novel-designed sludge biochar (SB) adsorbed for arsenic removal with lower operating costs and higher adsorption efficiency properties. Generally, biochar only relies on micropores for pollutant adsorption, but physical adsorption is not highly efficient for arsenic removal. Therefore, in order to improve the removal efficiency of arsenic by SB, diethylenetriamine (DETA) and FeCl₃ were used in this study to modify the surface of SB by an immersion method. The objectives of this research are to obtain optimum operation conditions by assessing the effect of different Fe content, pH and initial concentration on adsorbing arsenic. This study is the first to use Density Functional Theory (DFT) to simulate and verify the adsorption mechanism of arsenic by SB. Results showed the presence of amine/iron oxyhydroxides functional groups greatly promoted SB surface activity and its arsenic adsorption potential. The surface area, pore volume and pore size of the SB were estimated to be 525 m² g^-1, 0.35 cm³ g^-1 and 8.71 nm, respectively. The DFT model result is the same as the result of arsenic adsorption performance with high adsorption energy (-246.3 kJmol^-1) and shorter bond distances (1.42 Å), indicating strong chemical adsorption between arsenic and material. The reaction mechanism is divided into four pathways, including oxidation-reduction, complexation, electrostatic adsorption and pore adsorption.

Soil pH has a stronger effect than arsenic content on shaping plastisphere bacterial communities in soil

Microplastic (MP) pollution is widespread in various ecosystems and is colonized by microbes that form biofilms with compositions and functions. However, compared with aquatic environments, the soil environment has been poorly studied in terms of the taxonomic composition of microbial communities and the factors influencing the community structure of microbes in the plastisphere. In the present study, a microcosm experiment was conducted to investigate the plastisphere bacterial communities of MP (polyvinyl chloride, PVC) in soils with different pH (4.62, 6.5, and 7.46) and arsenic (As) contents (13 and 74 mg kg^-1). Bacterial communities in the plastisphere were dominated by Proteobacteria and Firmicutes, with distinct compositions and structures compared with soil bacterial communities. Soil pH and As content significantly affected the plastisphere bacterial communities. Constrained analysis of principal coordinates and a structural equation model demonstrated that soil pH had a stronger influence on the dissimilarity and diversity of bacterial communities than did soil As content. Soil pH affected As speciation in soil and on MP. The concentration of dimethylarsinic acid (DMA) was significantly higher on MP than that in soil, indicating that As methylation occurred on MP. These results suggest that environmental fluctuations govern plastisphere bacterial communities with cascading effects on biogeochemical cycling of As in the soil ecosystems.

Enhanced adsorption of aqueous Pb(II) by modified biochar produced through pyrolysis of watermelon seeds

A biochar (BC) was obtained by the pyrolysis of watermelon seeds (WM) in nitrogen environment. In addition, a modified biochar (HP-BC) was obtained by means of H₂O₂ treatment of BC. Later on, both kinds of biochar (BC and HP-BC) were characterized and compared as regards their potential for Pb(II) adsorption from wastewater. Characterization was performed by using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), energy dispersive spectroscopy (EDS), Zeta potential analysis, elemental mapping, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Pb(II) adsorption characteristics for HP-BC and BC as were evaluated as a function of solution pH, contact time and Pb(II) equilibrium concentration, using kinetic and thermodynamic studies, as well as adsorption isotherms. Regarding kinetics, the pseudo-second order model showed good fitting to experimental data. Based on the Langmuir model, the maximum Pb(II) adsorption capacities were calculated as 44.32 mg g^-1 and 60.87 mg g^-1 for BC and HP-BC, respectively. Thermodynamic study indicated that Pb(II) adsorption onto BC and HP-BC was spontaneous and primarily governed by chemisorption and surface complexation. In view of the results, the H₂O₂ modification of the watermelon seeds biochar can be considered as a promising and cost effective approach as regards Pb(II) removal from water/wastewater, which would not cause adverse impacts on the surrounding environments. Overall, it can be seen as a procedure promoting the effective recycling of a waste/by-product, in line of the precepts of the circular economy, aiding to protect human and environmental health.

Adsorption of arsenic (III) from aqueous solution by a novel phosphorus-modified biochar obtained from Taraxacum mongolicum Hand-Mazz: Adsorption behavior and mechanistic analysis

A novel phosphorus (P) modified biochar (PLBC) was produced by pyrolyzing biomass of the dietic herb Taraxacum mongolicum Hand-Mazz (TMHM) and treating it with monopotassium phosphate (KH₂PO₄). This phosphorous loaded biochar was then assessed as adsorbent for As(III) removal from contaminated water. In the current research, the adsorbent was characterized before and after P loading by means of SEM-EDX, TEM, FTIR and XRD techniques. It was evidenced that the presence of P on the surface of the biochar (BC) could improve its efficiency to remove As(III) from contaminated environments. Adsorption kinetics were evaluated by performing batch-type experiments at varied times and pH values (5, 7 and 9). The kinetic study revealed that a contact time of 24 h was required to attain equilibrium and the experimental data were best fitted to the pseudo-second-order kinetic model (q_e = 17.1 mg g^-1). In addition, several batch experiments were conducted with varied arsenic concentrations. During the adsorption tests, the maximum adsorption of As(III) was found at pH 5. The adsorption study further showed that compared to BC, PLBC depicted increased removal of As(III) from contaminated solutions. The adsorption experimental data showed the best fit to the Langmuir isotherm model (with R² = 0.84), with maximum As(III) adsorption capacity reaching 30.76 mg g^-1 for PLBC.

Testing of Chemically Activated Cellulose Fibers as Adsorbents for Treatment of Arsenic Contaminated Water

Exposure to different arsenic concentrations (higher than 10 μg/L), either due to the direct consumption of contaminated drinking water or indirectly by using contaminated food is harmful for human health. Therefore, it is important to remove arsenic from aqueous solutions. Among many arsenic removal technologies, adsorption offers a promising solution with a good efficiency, however the material used as adsorbent play a very vital role. The present investigation evaluated the behavior of two cellulose-based adsorbent materials, i.e., viscose fibers (V) and its TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) derivative, obtained by using the well-established TEMPO-mediated protocol (VF). Due to the known arsenic affinity for Fe ions the two materials were later doped with it. This was done after a preliminary functionalization with di-2-ethylhexyl phosphoric acid (DEHPA), to obtain two materials: V-DEHPA-Fe and VF-DEHPA-Fe. Arsenic adsorption is known to be pH dependent (between 6 and 8); therefore, the optimal pH range for As(V) adsorption has been established. In order to evaluate the adsorption mechanism for both the synthesized materials, the influence of contact time, temperature and initial concentration was evaluated. Langmuir, Freundlich and Sips equilibrium isotherm models were used in order to determine the ability of the model to describe As(V) adsorption process. The maximum adsorption capacity of the material V-DEHPA-Fe was 247.5 µg As(V)/g with an As(V) initial concentration of 5 mg/L and for the material VF-DEHPA-Fe it was 171.2 µg As(V)/g with initial concentration of 5 mg/L.

Synthesis and Characterization of Magnetic Xerogel Monolith as an Adsorbent for As(V) Removal from Groundwater

Arsenic contamination of groundwater is still a global problem due to the toxicity at low dose on human health confirmed by epidemiological studies. Magnetic xerogel monoliths (MXs) were synthesized by the sol-gel polymerization using resorcinol, formaldehyde, alkaline catalyst and magnetite. The varying molar ratios of magnetite and resorcinol (M/R) in the gel were evaluated for As(V) removal from groundwater. The surface chemistry, structure and morphology of MXs related to arsenic adsorption were characterized by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy and point of zero charge. Batch adsorption experiments were carried out to investigate the effects of Fe contents, initial pH and adsorbent dose on As(V) removal performance. The MXs with molar ratio of M/R at 0.15 gave the maximum As(V) adsorption capacity and removal with values of 62.8 µg/g and 86.7%, respectively. The adsorption data were well described by the Elovich equation of the kinetic model and the Freundlich isotherm. The thermodynamic studies demonstrated that the adsorption process was endothermic and spontaneous in nature. MXs showed to be a good alternative for As(V) removal from groundwater and achieving the efficient desorption, and thus fulfilled the Mexican standard for drinking water.

Kinetic and Isotherm Study of As(III) Removal from Aqueous Solution by PET Track-Etched Membranes Loaded with Copper Microtubes

This paper reports on the synthesis and structure elucidation of track-etched membranes (TeMs) with electrolessly deposited copper microtubes (prepared in etched-only and oxidized polyethylene terephthalate (PET) TeMs), as well as on the comparative testing of arsenic (III) ion removal capacities through bath adsorption experiments. The structure and composition of composites were investigated by X-ray diffraction technique and scanning electron and atomic force microscopies. It was determined that adsorption followed pseudo-second-order kinetics, and the adsorption rate constants were calculated. A comparative study of the applicability of the adsorption models of Langmuir, Freundlich, and Dubinin-Radushkevich was carried out in order to describe the experimental isotherms of the prepared composite TeMs. The constants and parameters of all of the above equations were determined. By comparing the regression coefficients R2, it was shown that the Freundlich model describes the experimental data on the adsorption of arsenic through the studied samples better than others. Free energy of As(III) adsorption on the samples was determined using the Dubinin-Radushkevich isotherm model and was found to be 17.2 and 31.6 kJ/mol for Cu/PET and Cu/Ox_PET samples, respectively. The high EDr value observed for the Cu/Ox_PET composite indicates that the interaction between the adsorbate and the composite is based on chemisorption.

Modelling of palladium(II) adsorption onto amine-functionalised zeolite using a generalised surface complexation approach

The many uses of palladium in medicine, catalysts and other industries make it a very important precious element. Many industries using palladium discharge process wastewaters that may release elevated concentrations of palladium into the environment. This study focused on the recovery of palladium from aqueous solutions by zeolite functionalised with spent brewer's yeast. Batch experimental results were used to calibrate a generalised surface complexation model based on coupling parameter estimation (PEST) to the PHREEQC geochemical modelling code. PHREEQC is an acronym which stands for pH, redox, equilibrium and C programming language. Calibration was based on the determination of sorption constants for the reactions of palladium with the adsorbent. The generalised amine surface groups (derived from yeast), the moles of adsorption sites and surface area were specified. The recovery of palladium was assessed as a function of solution pH, adsorbent dosage and initial concentration of palladium in the presence of other cations and anions at different concentrations. The highest recovery of palladium (>97%) was observed at pH 2 and 10 g L^-1 adsorbent dosage which, decreased with increasing solution pH. The amount of palladium removed increased in the presence of competing ions and anions. There was no significant difference (p > 0.05) between the modelled and measured data, which indicated that PHREEQC modelling code coupled with PEST can accurately determine the recovery of palladium using amine-based adsorbents when all the required information is specified. This is very useful in instances where limited experimental data is available for non-conventional and novel surfaces to make accurate predictions of sorption processes involving them.

Agro-Waste Derived Biomass Impregnated with TiO2 as a Potential Adsorbent for Removal of As(III) from Water

A novel type of adsorbent, TiO2 impregnated pomegranate peels (PP@TiO2) was successfully synthesized and its efficacy was investigated based on the removal of As(III) from water. The adsorbent was characterized using Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectrometer (EDS), X-ray Diffraction (XRD) analysis, and Fourier Transform Infrared (FTIR) Spectroscopy, to evaluate its morphology, elemental analysis, crystallinity, and functional groups, respectively. Batch experiments were conducted on PP@TiO2 for As(III) adsorption to assess the adsorption isotherm, effect of pH, and adsorption kinetics. Characterization data suggested that TiO2 was successfully impregnated on the biomass substrate. The equilibrium data better fitted to the Langmuir isotherm model having a maximum adsorption capacity of 76.92 mg/g and better distribution coefficients (KD) in the order of ~103 mL/g. The highest percentage of adsorption was found at neutral pH. The adsorption kinetics followed the pseudo-2nd-order model. X-ray Photoelectron Spectroscopy (XPS) of the adsorption product exhibited that arsenic was present as As(III) and partially oxidized to As(V). PP@TiO2 can work effectively in the presence of coexisting anions and could be regenerated and reused. Overall, these findings suggested that the as-prepared PP@TiO2 could provide a better and efficient alternative for the synergistic removal of As(III) from water.

Treatment of olive mill effluent by adsorption on titanium oxide nanoparticles

Olive mills wastewater (OMW) causes a serious environmental problem in the olive oil producing countries. This is due to its high organic matter content (COD), acidic pH values, suspended solids and high content of phytotoxic and antibacterial phenolic compounds. In this study, titanium dioxide (TiO₂) as an adsorbent to reduce the COD value of the olive mill wastewater was investigated. Several variables were studied including the removal efficiency, effect of the initial COD value, amount of TiO₂, temperature and pH value. The results revealed that the adsorption reached equilibrium within <120 min. Isotherm studies showed that the adsorption equilibrium data is in agreement with Freundlich isotherm. In addition, the results showed that the adsorption process was spontaneous and exothermic. The kinetic study indicated that adsorption did follow a pseudo-second order reaction. Variation of the amount of the TiO₂ showed that using of 1.5 and 2 g/L of TiO₂ caused the COD to drop from 1000 ppm to about 100 ppm (equilibrium concentration) in about 120 min. However, the use of 1 g/L of TiO₂ exhibited almost the same effect on the COD-uptake, and the equilibrium concentration was about 400 ppm. The COD uptake was found to be inversely proportional with the temperature, pH value and the addition of salts such as sodium chloride (NaCl) and potassium chloride (KCl).

Influence of dissolved organic matter components on arsenate adsorption/desorption by TiO2

The influences of different dissolved organic matter (DOM) components and ionic matters on As(V) adsorption/desorption behavior on the TiO₂ surface were investigated. The results demonstrated that the characteristics and involving order of DOM significantly affected the As(V) adsorption/desorption behavior. The presence of DOM decreased the As(V) adsorption quantity. Fulvic acid (FA) exhibited the most negative effect, and followed by the order of alginate ≈ BSA > SDBS. The precomplexation DOM prevented more As(V) adsorption. While, the presence of DOM caused more As(V) release when the surrounding changed and FA exhibited the strongest effect. The results indicated that the site competition and electrostatic repulsion were the major mechanisms to resist As(V) adsorption. The presence of Fe³⁺ and Ca²⁺ increased As(V) adsorption by bridge effect, while PO₄^3- and CO₃^2- decreased As(V) adsorption owing to the competition.

Adsorption and oxidation of arsenic by two kinds of β-MnO2

MnO₂ is one of the most widespread and cheapest materials in nature that can both adsorb arsenic and oxidize arsenite [As(III)] to arsenate [As(V)]. In this study, column β-MnO₂ [CM] with the main facet of {110} and pincer β-MnO₂ [PM] with the facets of {110} and {101} are synthesized and used to remove arsenic in water under different conditions. For the adsorption process, the experimental data are fitted well with the pseudo second-order kinetic model; the Langmuir model is better than the Freundlich model to describe the adsorption equilibrium isotherms. Furthermore, the As(III) oxidation rate can be denoted by the pseudo zero-order kinetic model and is related to the O₂ concentration, the pH value, the light source and the initial concentration of As(III). Finally, the oxidation mechanism is investigated, and the oxidant should be related to O₂. It is interesting to find that these two kinds of β-MnO₂ exhibit different pH effects for both adsorption and oxidation. For As(III), the adsorption and oxidation abilities of CM follow the order pH 9 > pH 7 > pH 4, whereas the adsorption and oxidation orders of PM are pH 4 > pH 7 > pH 9.

Preparation of AgCl/TNTs nanocomposites for organic dyes and inorganic heavy metal removal

In this study, TiO₂ nanotubes (TNTs) and AgCl-modified TNTs nanocomposites with multiple crystal phases were synthesized through a hydrothermal method without calcination. The resultant samples had a large Brunauer-Emmett-Teller surface area. Additionally, the Ag modification process reduced the recombination rate of electron-hole pairs in the synthesized sample and possessed more oxygen vacancy sites. The surface area of the AgCl-modified TNTs was smaller than that of non-modified TNTs sample; however, the nanocomposites exhibited outstanding photocatalytic performance and adsorption properties. AgCl compounds present on the TNTs surface effectively interacted with Hg⁰, improving the dye photodegradation efficiency. The Hg⁰ removal efficiencies of the TNTs and AgCl-modified TNTs samples were about 63% and 86%, respectively. The crystal violet (CV) and malachite green (MG) removal efficiencies of the AgCl-modified TNTs sample were around 57% and 72%, respectively. Both dyes photodecomposition efficiencies for AgCl-modified TNTs sample are higher than those of TNTs sample. The oxygen vacancy on the AgCl-modified TNTs surface was determined to be advantageous for OH^- and arsenate adsorption through ligand exchange. The maximum adsorption quantity of As⁵⁺ calculated by Langmuir equation was 15.38 mg g^-1 (TNTs) and 21.10 mg g^-1 (AgCl-modified TNTs).

pH effects of the arsenite photocatalytic oxidation reaction on different anatase TiO2 facets

TiO₂ is one of the most cheap materials which can both adsorb arsenic and oxidize arsenite [As(III)] to arsenate [As(V)]. In this study, anatase TiO₂ crystals with different main facets such as {101}, {001} and {100} are synthesized and used to investigate arsenic adsorption kinetics, adsorption isotherms, photocatalytic oxidation (PCO) process and the pH effects. The adsorption kinetics of arsenic on TiO₂ crystals can be described by the pseudo second-order kinetic model. For the adsorption isotherms, the Langmuir model is better than the Freundlich model for arsenic on these TiO₂ crystals. For the PCO process, the rate of As(III) oxidation can be denoted by the pseudo first-order kinetic model. It should be noted that at neutral condition the adsorption and PCO rates of the three kinds of TiO₂ crystals follow the order of {101} > {001} > {100}. The pH effect is above all important for both the arsenic adsorption and its PCO. The highest PCO speed appears at high pH values such as at pH 11 or 12.

Effect of pH to the surface precipitation mechanisms of arsenate and cadmium on TiO2

Much attention has focused on the mutual effect of oxyanions and cations on surfaces, however, there is still a lack of knowledge on the molecular-level surface precipitation mechanisms for As(V) and Cd(II) on surfaces under different pH conditions in acid metallurgical industrial wastewater. The results of in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and extended X-ray absorption fine structure (EXAFS) spectroscopy show that different As(V) and Cd(II) surface precipitation mechanisms take effect at pH 5.0 and 7.0. Under acidic conditions, As(V) was preferentially adsorbed on TiO₂ rather than Cd(II). Thus, a Cd(II)-As(V)-TiO₂ ternary surface complex was formed with the adsorbed As(V) as the bridging molecule, and the subsequent layer-by-layer surface precipitate was generated. Under neutral conditions, the Cd(II)-As(V) surface precipitates were directly formed by using the aqueous Cd(II)-As(V) complex as a "seed", which took approximately 10 times longer than the acidic conditions to reach the adsorption equilibrium. Our findings provide spectroscopic evidence and elucidate the different simultaneous removal mechanisms of As(V) and Cd(II) on TiO₂ under acidic and neutral conditions, which will further our understanding and application of the immobilization of multiple pollutants in industrial wastewaters.

Two-generational effects and recovery of arsenic and arsenate on Daphnia magna in the presence of nano-TiO2

The toxicity of arsenic (As) can be influenced by many environmental factors. Among them, nanomaterials can adsorb arsenic and alter its bioavailability in organisms. However, the studies on long-term effects of arsenic in the presence of nanoparticles are limited. Thus, the 21-d effect of titanium dioxide nanoparticles (nano-TiO₂) on chronic toxicity of arsenic (arsenate and arsenite) was investigated in two generations of Daphnia magna. The exposed concentration of nano-TiO₂ was 1 mg/L and the concentration of As(Ⅲ) or As(Ⅴ) was 0.2 mg/L which was lower than the 48 h-NOEC (no observed effect concentration). The survival, body length, average number of offspring and time of first brood were determined. Our results indicated that the exposure to nano-TiO₂ and As during the parental generation can affect the health of offspring. Nano-TiO₂ was found to significantly alleviate the mortality and reproduction inhibition of As on D. magna, and the alleviation of As(Ⅴ) was more prominent than that of As(Ⅲ). It is likely that nano-TiO₂ alters the metabolism and adsorption condition of arsenic in the gastrointestinal tract of D. magna. Overall, these results indicate that the increase of arsenic adsorption onto nano-TiO₂ in the gut of D. magna could alleviate the toxicity of arsenic. Nonetheless, further research should be conducted to study the influence of arsenic on the multi-generations of aquatic organisms, especially when it is coexisted with other substances.

A New Hollow-Fiber Adsorbent Material for Removing Arsenic from Groundwater

To optimize the arsenic-adsorption efficiency and the mechanical strength of a hollow-fiber-type adsorbent, the optimal condition of polymeric solution was determined as 32 wt.% TiO2 and 17 wt.% polymer. A micropore-sponge form was developed at the slurry-extrusion speed of 2.500 ml/min and the internal coagulant-solution-extrusion speed of 1.250 ml/min, and the arsenic-adsorption efficiency improved. Given the result under conditions in natural groundwater containing various ions, the hollow-fiber-type adsorbent can be applied to real groundwater purification processes.

As(III) Removal from Aqueous Solution by Calcium Titanate Nanoparticles Prepared by the Sol Gel Method

Arsenic (As) contamination of water is a serious problem in developing countries. In water streams, arsenic can be as As(V) and As(III), the latter being the most toxic species. In this work, an innovative adsorbent based on CaTiO₃ nanoparticles (CTO) was prepared by the sol-gel technique for the removal of As(III) from aqueous solution. X-ray diffraction of the CTO nanoparticles powders confirmed the CTO phase. Transmission electron microscopy observations indicated an average particle size of 27 nm, while energy dispersive X-ray spectroscopy analysis showed the presence of Ca, Ti, and O in the expected stoichiometric amounts. The surface specific area measured by Brunauer, Emmett, and Teller (BET) isotherm was 43.9 m²/g, whereas the isoelectric point determined by Zeta Potential measurements was at pH 3.5. Batch adsorption experiments were used to study the effect of pH on the equilibrium adsorption of As(III), using an arsenite solution with 15 mg/L as initial concentration. The highest removal was achieved at pH 3, reaching an efficiency of up to 73%, determined by X-ray fluorescence from the residual As(III) in the solution. Time dependent adsorption experiments at different pHs exhibited a pseudo-second order kinetics with an equilibrium adsorption capacity of 11.12 mg/g at pH 3. Moreover, CTO nanoparticles were regenerated and evaluated for four cycles, decreasing their arsenic removal efficiency by 10% without affecting their chemical structure. X-ray photoelectron spectroscopy analysis of the CTO surface after removal experiments, showed that arsenic was present as As(III) and partially oxidized to As(V).

Adsorption of Aqueous As (III) in Presence of Coexisting Ions by a Green Fe-Modified W Zeolite

The high toxicity of arsenite and the difficulty to remove it is one of the main challenges for water treatment. In the present work the surface of a low cost zeolite was modified by chemical treatment with a ferrous chloride to enhance its arsenite adsorption capacity. The effect of pH, ions coexistence, concentration, temperature and dosage was studied on the adsorption process. Additionally, the Fe-modified W zeolite was aged by an accelerated procedure and the regeneration of the exhausted zeolite was demonstrated. The Fe-modified W zeolite was stable in the pH range of 3 to 8 and no detriment to its arsenite removal capacity was observed in the presence of coexisting ions commonly found in underground water. The studies showed that the adsorption of As (III) on Fe-modified W zeolite is a feasible, spontaneous and endothermic process and it takes place by chemical bonding. The exhausting process proved the adsorption of 0.20 mg g−1 of As (III) by the Fe-modified W zeolite and this withstand at least five aging cycles without significant changes of its arsenite adsorption capacity. Fe-modified W zeolite prepared from fly ash might be a green and low-cost alternative for removal of As (III) from groundwater.

High efficiency removal of As(III) from waters using a new and friendly adsorbent based on sugarcane bagasse and corncob husk Fe-coated biochars

Water contamination of As is a big issue in many areas around the globe. Therefore, cheap and efficient techniques are essential facing traditional treatment methods. Then, biochars (BC) emerged recently as material that can be used for As removal. However, research about efficiency of BC produced from local feedstock is still needed. The purpose of this study is to assess the efficiency of BC produced from sugarcane bagasse (SB) together with corncob husk (CH) with and without Fe(III) (BC_Fe) modification to be used for removal of As(III) from waters. The BC and BC_Fe produced at different pyrolysis temperatures were characterised using FTIR and SEM/EDS. Adsorption capacities of BC and BC_Fe were evaluated via batch adsorption, desorption and column tests and their performance was compared with adsorption using activated carbon. The results showed that Fe modification improve substantially the As(III) adsorption in a way that both BC_Fe-SB and BC_Fe-CH removed from 85% to 99.9% from 1000 µg/L As(III) solutions. Both materials fitted well in Langmuir model and the maximum adsorption capacity was 20 mg/g for BC_Fe-SB and 50 mg/g for BC_Fe-CH. The adsorption kinetics of BC_Fe was fast (≤ 30 min) and it had a better performance than activated carbon. The column tests showed that the process is efficient even at high As(III) concentrations. The fast removal process and good removal results make the BC_Fe-SB and BC_Fe-CH attractive for in situ and commercial (filters) use, since time and efficiency are required in new technologies.

Stabilization mechanism of arsenic in mine waste using basic oxygen furnace slag: The role of water contents on stabilization efficiency

Arsenic stabilization mechanism in a mine waste was investigated using a basic oxygen furnace (BOF) slag. A lab-scale batch test was carried out to stabilize As in the mine waste samples for 1 h, where various amounts of the BOF slag and distilled water were introduced. Different stabilization efficiencies were observed depending on the stabilizing conditions (i.e., BOF slag content and water to mine waste (L/S) ratio). The stabilization efficiencies ranged 75-92% and 92-95% for 5% (w-slag/w-mine waste) and 10% BOF slag treated mine waste samples, respectively. Interestingly, a notable effect of the L/S ratio on the stabilization efficiency was observed (78% at 0.05 L/kg, and 23% at 1.0 L/kg) at the 3% BOF slag treatment. The point of zero charge and the stabilizing pH indicated that the BOF slag surface was negatively charged. Based on the comparison of fresh and Ca-reduced BOF slags, As stabilization mechanism was determined to be adsorption through cation bridges by Ca²⁺. The Surface analysis using X-ray photoelectron spectroscopy (XPS) and the stabilization experiment conducted at lower pH provided evidence that the hindrance of As adsorption resulted from Ca(OH)₂ precipitation on the BOF slag surface when excess water (1.0 L/kg) was added. Such effect of water content seemed to be overcome by providing an excessive amount of the BOF slag. When an ample amount of Ca²⁺ is provided and pH is maintained around 11, not only As adsorption but also calcium arsenate precipitation occur, and both contributed to the stabilization mechanisms of As.

Multifunctional photoactive and selective adsorbent for arsenite and arsenate: Evaluation of nano titanium dioxide-enabled chitosan cross-linked with copper

A novel multifunctional sorbent material of nano-titanium dioxide-enabled chitosan beads cross-linked with copper (CuTICB) is capable of photo-oxidation of As(III) to the less-toxic and more easily adsorbed As(V) in UV light and selective adsorption of arsenite (As(III)) and arsenate (As(V)) in the presence of phosphate, a strong adsorptive competitor and inhibitor of arsenic removal performance. CuTICB is an attractive sorbent as simultaneous photo-oxidation and adsorption reduces treatment time and cost while selective adsorption improves removal efficiency of arsenic in typical environmental conditions where competitive ions are predominant. In CuTICB, nano-titanium dioxide (n-TiO₂) anatase photo-oxidizes As(III) to As(V) through generation of reactive oxygen species. Additionally, Cu-chitosan bidentate crosslinkers form through Lewis acid-base coordinate bonding between Cu(II) and chitosan amine groups resulting in cationic behavior that electrostatically favors As(V) chelation even when phosphate concentrations are orders of magnitude higher. The influence of copper and n-TiO₂ loading on arsenic photo-oxidation and selective removal over phosphate was explored to optimize CuTICB design using batch experiments under varying systems conditions. For a system requiring both photo-oxidation and selective adsorption, it was found that copper and n-TiO₂ act non-linearly and synergistically, where maximum loadings of both does not yield the optimal selectivity or removal efficacy.

Impacts of biochar and oyster shells waste on the immobilization of arsenic in highly contaminated soils

Soil contamination is a serious problem with deleterious impacts on global sustainability. Readily available, economic, and highly effective technologies are therefore urgently needed for the rehabilitation of contaminated sites. In this study, two readily available materials prepared from bio-wastes, namely biochar and oyster shell waste, were evaluated as soil amendments to immobilize arsenic in a highly As-contaminated soil (up to 15,000 mgAs/kg). Both biochar and oyster shell waste can effectively reduce arsenic leachability in acid soils. After application of the amendments (2-4% addition, w/w), the exchangeable arsenic fraction decreased from 105.8 to 54.0 mg/kg. The application of 2%biochar +2% oyster shell waste most effectively reduced As levels in the column leaching test by reducing the arsenic concentration in the porewater by 62.3% compared with the treatment without amendments. Biochar and oyster shell waste also reduced soluble As(III) from 374.9 ± 18.8 μg/L to 185.9 ± 16.8 μg/L and As(V) from 119.8 ± 13.0 μg/L to 56.4 ± 2.6 μg/L at a pH value of 4-5. The treatment using 4% (w/w) amendments did not result in sufficient As immobilization in highly contaminated soils; high soluble arsenic concentrations (upto193.0 μg/L)were found in the soil leachate, particularly in the form of As(III), indicating a significant potential to pollute shallow groundwater aquifers. This study provides valuable insights into the use of cost-effective and readily available materials for soil remediation and investigates the mechanisms underlying arsenic immobilization in acidic soils.

Microscopic insight into precipitation and adsorption of As(V) species by Fe-based materials in aqueous phase

The mechanism of As(V) removal from the drinking water and industrial effluents by iron materials remains unclear at the molecular level. In this work, the association of Fe-based materials with As(V) species was explored using density functional theory and ab initio calculations. Solvent separated ion pair structures of [FeH₂AsO₄]²⁺_aq species may be dominant in an acidic solution of FeAs complex. The association trend of H₂AsO₄^- species by Fe³⁺_aq is found to be quite weak in the aqueous solution, which may be attributed to the strong hydration of Fe³⁺_aq and [FeH₂AsO₄]²⁺ species. However, the association of H₂AsO₄^- species by colloidal clusters is quite strong, due to the weakened hydration of Fe(III) in colloidal structures. The hydrophobicity of Fe-based materials may be one of the key factors for their As(V) removal efficiency in an aqueous phase. When the number of OH^- coordinated with Fe(III) increases, the association trend of As(V) by colloidal ferric hydroxides weakens accordingly. This study provides insights into understanding the coprecipitation and adsorption mechanisms of arsenate removal and revealing the high efficiency of arsenate removal by colloidal ferric hydroxides or iron salts under moderate pH conditions.

Magnetic mesoporous TiO2 microspheres for sustainable arsenate removal from acidic environments

Unique magnetic mesoporous TiO₂ microspheres exhibit superior arsenate removal performance and high stability in acidic environments.