Arsenate Species Research Articles

Unexpected enhancement effects of phosphate on co-immobilization of uranyl arsenate under acidic conditions

Understanding the formation of uranyl-arsenate compounds (M(UO2)2(AsO4)2·nH2O, M represents divalent metallic cations) is of great importance in controlling the mobility and fate of uranium (U) and arsenic (As) in the natural environment. Phosphate is a ubiquitous coexisting ion with properties similar to arsenate and can complex with uranyl to form uranyl phosphate compounds, thus potentially affecting the formation of uranyl-arsenate compounds. Herein, the effect of phosphate, on the immobilization of uranyl, arsenate and divalent metallic cations (e.g., Fe2+, Mg2+, Ni2+, Cu2+) was explored. Surprisingly, the presence of phosphate promoted the immobilization of uranyl arsenate with the divalent metallic cations under acidic conditions. For example, at pH 3.0, the immobilization efficiency of uranyl and arsenate in the uranyl-arsenate-Fe2+ system increased from 20.6 % and 18.6 % to 82.6 % and 52.2 %, respectively. Moreover, the chemical stability of the formed uranyl arsenate-bearing compounds in the presence of phosphate is significantly improved, and the TCLP leachability of As and U decreased by 81 % and 97 %, respectively. The experimental results illustrated that with PO43- served as a proton acceptor, uranyl and arsenate species were changed and more metallic cations participate in immobilization. Gibbs free energy and Gibbs free energies change also illustrated that the formation of Fe(UO2)2(AsO4)2·5H2O was relatively thermodynamically favorable in the presence of phosphate. The results of thermodynamic calculations could account for the enhancement effects of phosphate on immobilization of uranyl arsenate. The findings have important implications for the mobility and fate of U and As and the solution of their co-contamination, especially in acidic environments.

Metal-Organic Framework Functionalized Chitosan/Pectin Membranes for Solar-Driven Photo-Oxidation and Adsorption of Arsenic

The presence of arsenic as neutral arsenite and oxoanionic arsenate species are a worldwide issue that requires an effective remediation technology capable to remove both AsIII and AsV from water. In this research we propose the assembly of a membrane build up from chitosan/pectin (CHIPEC) composite biopolymer and the UiO-66-NH2 porous and photo-active metal–organic framework. CHIPEC@UiO-66-NH2 membrane integrates porosity, adsorptive capacity, and activity to photo-oxidize and adsorb AsIII and AsV species. Nanometric-sized UiO-66-NH2 was synthetized and homogeneously distributed within the CHIPEC matrix, achieving a composite membrane with optical physico-chemical and optical properties (band gap of 2.91 eV) suitable to be applied on photooxidation of AsIII and adsorption of AsV. Under optimal conditions, maximum adsorption capacities (Qmax) values of 254 and 168 mg g−1 were reached for AsIII adsorption by MOF and membrane, respectively. Notably, for AsV adsorption, Qmax values of 268 and 335 mg g−1 were reached for powdered and immobilized MOF, respectively. Moreover, the membrane keeps most of its activity for AsV adsorption when employed in surface and groundwater matrixes. Further, the efficiently of UiO-66-NH2 to photo-oxidize AsIII to AsV, subsequently adsorbing the arsenate oxyanions, is improved by a factor of five when is integrated into the polymer. A detailed multi-step mechanism for the arsenic transformation and adsorption in CHIPEC@UiO-66-NH2 has been proposed on the basis of the anionic exchange at Zr hexa-nuclear clusters and amino groups in the MOF. Overall, MOF-biopolymeric composite has been proved as a straightforward, efficient, low-cost, and environmentally friendly solution for detoxifying arsenic in surface and underground water.

Metal-Organic Frameworks for Dual Photo-Oxidation and Capture of Arsenic from Water.

Despite rapid technological progress, heavy metal water pollution, and particularly arsenic contamination, remains a significant global challenge. In addition, the stabilization of trivalent arsenic as neutral arsenite (AsIII) species hinders its removal by conventional sorbents. While adsorption of anionic arsenate (AsV) species is in principle more feasible, there are only few adsorbents capable of adsorbing both forms of arsenic. In this work, we explore the potential of two well-known families of Metal-Organic Frameworks (MOFs), UiO-66 and MIL-125, to simultaneously adsorb and photo-oxidize arsenic species from water. Our results demonstrate that the formation of AsV ions upon light irradiation promotes the subsequent adsorption of AsIII species. Thus, we propose the combined utilization of photocatalysis and adsorption with Metal-Organic Framework photocatalysts for water remediation purposes.

Simultaneous Removal of Arsenate and Fluoride Using Magnesium-Based Adsorbents

In this study, arsenate, As(V), and fluoride (F) were simultaneously removed from contaminated water using MgO, Mg(OH)2, and MgCO3 as Mg-based adsorbents, as existing studies only focus on their individual removal. The removal performance of As(V) and F followed the order MgCO3 < Mg(OH)2 < MgO. Under the test conditions, MgO and Mg(OH)2 met the environmental standards for As and F (0.01 and 0.8 mg/L, respectively), but MgCO3 did not. The As(V) removal performance was not significantly affected by an increase in the initial F concentration. It was concluded that As(V) was adsorbed and removed more preferentially than F by Mg-based adsorbents because a considerable amount of F remained even when the majority of As(V) was removed. Most arsenic (As)-adsorption data for MgO fit the Langmuir and Freundlich models, whereas those for Mg(OH)2 did not fit either model well. Additionally, the As-adsorption data for MgCO3 fit the Freundlich model but not the Langmuir model. Most of the F-adsorption data for the Mg-based adsorbents fit the Langmuir and Freundlich models. The removal mechanisms of As(V) and F using Mg-based adsorbents were assumed to be predominantly caused by ion-exchange and chemical-adsorption reactions on the adsorbent surface because no magnesium arsenate, magnesium fluoride, or magnesium hydroxide fluoride species were observed in the X-ray diffraction analysis. This research advances the sustainable As–F simultaneous treatment method using inexpensive adsorbents.

Pantoea conspicua promoted sunflower growth and engulfed rhizospheric arsenate by secreting exopolysaccharide

A rhizobacterium, Pantoea conspicua, was examined against sunflower seedlings' growth under arsenate stress. Sunflower upon exposure to arsenate resulted in compromised growth that might be due to the accumulation of higher concentrations of arsenate and reactive oxygen species (ROS) in seedlings' tissues. The deposited arsenate led to oxidative damage and electrolyte leakage, making the sunflower seedlings vulnerable to compromise its growth and development. However, inoculation of sunflower seedlings with P. conspicua alleviated arsenate stress in host by initiating a multilayered defence mechanism. In fact, P. conspicua filtered out 75.1% of the arsenate from growth medium that were available to the plant roots in the absence of the said strain. To accomplish such activity, P. conspicua secreted exopolysaccharides as well as altered lignification in host roots. The arsenate (24.9%) that made its way to plant tissues was countered by helping the host seedlings to produce higher levels of indole acetic acid, non enzymatic antioxidants (phenolics and flavonoids) and antioxidant enzymes (catalase, ascorbte peroxidase, peroxidase, superoxide dismutase). As a result, ROS accumulation and electrolyte leakage were brought back to normal levels as observed in control seedlings. Hence, the rhizobacterium associated host seedlings achieved higher net assimilation (127.7%) and relative growth rate (113.5%) under 100 ppm of arsenate stress. The work concluded that P. conspicua alleviated arsenate stress in the host plants by imposing physical barrier as well as improving host seedlings’ physiology and biochemistry.

Development of a New Method to Estimate the Water Purification Efficiency of Bulk-Supported Nanosorbents under Realistic Conditions

The direct use of nanosorbents for water purification is limited due to their aggregation and the lack of techniques for their recovery from natural waters. To overcome these problems, the affixation of nanomaterials onto bulk, non-mobile supports has been proposed. However, a method to simulate the efficiency of these sorbents under realistic conditions is still not available. To address this need, this work describes a method for evaluating the sorption efficiency of nanosorbent materials incorporated on bulk supports under non-equilibrium conditions. The method combines the principles of passive sampling, an environmental monitoring technique that is based on passive diffusion of dissolved contaminants from water to a sorbent, with batch sorption experiments that measure sorption under equilibrium conditions, to determine the parameters associated with water purification. These parameters are the maximum sorption capacity of the sorbent and the sampling rate, which is the volume of contaminated water treated per unit of time. From these variables, the deployment time of the sorbent until reaching saturation is proposed as an alternative indicator of sorbent efficiency. As proof-of-principle, the removal of oxyanions from a Zr-metal-organic framework (MOR−1) immobilized on cotton textiles was investigated. The results show that the sorption capacity under passive diffusion uptake conditions, is approximately 20 mg/g for As(VI) and 36 mg/g Se(IV), which is 10 to 30 times lower compared to that determined in batch sorption studies, indicating that conventional equilibrium sorption overestimates the efficiency of the sorbents under realistic conditions. The application of the method to a worst-case scenario, involving the severe contamination of freshwaters with arsenate species, is also demonstrated.

The effect of electrocoagulation (EC) on total arsenic, arsenite (As3+) and arsenate (As5+) species removal from model groundwater investigating toxicity and sludge characteristic

This study showed that the model groundwater containing As3+ and As5+ species was successfully treated with electrocoagulation (EC) first time in the literature investigating toxicity reduction, inorganic arsenic species and detailed sludge characterization. The arsenic removal from model groundwater with 1000 µg/l total arsenic containing equal arsenite (As3+) and arsenate (As5+) concentration was examined by the EC treatment optimized with following parameters; current density (5.0, 7.5 and 10.0 mA/cm2) supporting sodium sulfate electrolyte amount (10, 20 and 30 mM Na2SO4) and initial water pH (3, 6 and 9). In EC treatment, the 99.87% arsenic removal was obtained with 10 mA/cm2, 10 mM Na2SO4 at pH 3 after 40 min supplying 1.44 μg/l effluent As concentration lower than the WHO limit for drinking water. Through transformation mechanisms of more toxic As3+ to less toxic and easily settled As5+ according to As speciation analysis, the toxicity of the model groundwater was successfully decreased in parallel with total arsenic, As3+ and As5+ removal during EC. The precipitated Al(OH)3 and Al2O3 coagulants were the main peaks in the FTIR-ATR spectrum as well as As(III)–O vibration observed between 717 and 721 cm−1 peaks and As(V)–O vibration dominated 899 and 972 cm−1 peaks were detected in the produced sludge after the EC. The SEM–EDS morphological analysis was demonstrated that the sludge was consisted of mostly amorphous structure aggregated size range of 200 μm–2 mm, relatively uniform cake including O, Al, As, Na, and S.

Comparative evaluation of Fe-, Zr-, and La-based metal-organic frameworks derived from recycled PET plastic bottles for arsenate removal

Metal-organic frameworks (MOFs) derived from recycled polyester (polyethylene terephthalate, PET) bottles were investigated in both batch and column studies for the removal of arsenate. As-synthesized Fe-MOF, Zr-MOF, and La-MOF were systematically analyzed by SEM, PXRD, FTIR, BET, and XPS techniques. The obtained MOFs showed high crystallinity with the specific surface areas of 128.3, 290.4, and 61.8 m2/g for Fe-MOF, Zr-MOF, and La-MOF, respectively. The Langmuir isotherm and pseudo-second-order kinetic model simulated arsenate adsorption on MOF materials well, which can be explained by electrostatic interactions, surface complexation, and ligand exchange mechanisms. The maximum adsorption capacities of arsenate onto Fe-MOF, Zr-MOF, and La-MOF were found to be 70.02, 85.72, and 114.28 mg/g at pH 7, respectively. The effect of pH and co-existing anions on the arsenate adsorption on MOF materials was also evaluated for practical applications. The MOF materials showed reduced adsorption capacity for arsenate by less than 10% up to four cycles of regeneration and did not induce any significant (p > 0.05) acute toxicity (<2.5% mortality) in Daphnia magna. In a flow-through system, Fe-MOF, Zr-MOF, and La-MOF were used to treat 176, 255, and 398 mL bed volumes of arsenate contaminated water, respectively, and consistently reduced the concentration of arsenate ions from 500 to 10 μg/L. This study clearly demonstrated that MOF materials derived from waste PET bottles are economically promising adsorbents for the successful elimination of arsenate species from aqueous environments.

Characteristics and mechanisms of aluminum salts on arsenate removal by coagulation: Significance of aluminum speciation distribution and transformation

It is well known that the hydrolyzed aluminum species of Al-based coagulants play a critical role in coagulation process. So far, however, little information has been collected on arsenate removal from the perspectives of aluminum speciation distribution and transformation. In this study, three different Al-based coagulants have been applied to investigate the distribution and transformation of hydrolyzed aluminum species and their coagulation behaviors for arsenate removal. High levels of in situ Al13 were produced from AlCl3 under weakly acidic conditions. Lab-prefabricated polyaluminum chloride PACl1 was almost entirely preformed Al13 under acidic conditions while large polymeric aluminum species Al14 and Al15 became dominated under alkaline conditions. While for commercial polyaluminum chloride PACl2, large polymeric aluminum species were the dominated in the pH range of 4–9. Results showed that the best coagulation effect occurred at pH 6 for three coagulants with arsenate removal were all above 95%. The distribution and transformation of Al species were variant in different Al-based coagulants, which results in difference of mechanisms on arsenate removal. AlCl3 had synergistic effects on arsenate removal based on charge neutralization of in situ Al13 and sweep flocculation of Al(OH)3. Charge neutralization of preformed Al13 generated by PACl1 and the bridging adsorption of Al13 aggregates have a key role to play in arsenate removal. The sweep flocculation by Al(OH)3 during PACl2 coagulation process was proved to be the main function for arsenate removal. This study provides a new insight into the interaction and mechanism between arsenate and various aluminum species of Al-based coagulants.

Reusable Nanocomposite Membranes for Highly Efficient Arsenite and Arsenate Dual Removal from Water

AbstractNanocomposite membranes (NCMs) of poly(vinylidene fluoride‐hexafluoropropylene), PVDF‐HFP, with different yttrium carbonate and magnetite loadings, are prepared, and their dual adsorption capacity over neutral arsenite and anionic arsenate species is evaluated. The nanoparticles (NPs) and the corresponding NCMs are fully characterized in morphology, microstructure, thermal, and surface properties. The nanocomposite membranes present a micrometric porous structure with a homogeneous distribution of the active nanoparticles. Chemical, thermal, and water‐contact angle characteristics of the NCMs point out that they maintain the chemical and thermal stability of the polymer while improving the wettability. Arsenic removal depends on NP loading and pH of the media. For instance, efficiencies close to 100% are achieved for arsenate species under acidic conditions, while adsorption capacity over arsenite is also incremented above 80%. Fe3O4/PVDF‐HFP nanocomposite shows a dual affinity for the adsorption of As(III) and As(V) species, with the maximum adsorption capacities of 92.82 and 137.08 mg g−1, respectively. In addition, both NCMs are easily activated and reused without significant efficiency loss. Consequently, the nanocomposite membranes represent low‐cost, reusable, and efficient water remediation systems suitable for the long‐term removal of As(III) and As(V) under conditions mimicking real polluted surface and groundwater.

Joint effect of urinary arsenic species and serum one-carbon metabolism nutrients on gestational diabetes mellitus: A cross-sectional study of Chinese pregnant women.

Growing evidence indicates that arsenic (As) exposure can increase the risk of gestational diabetes mellitus (GDM). However, little is known about As species and GDM and the combined effect of As and one-carbon metabolism (OCM) on GDM. We aimed to examine the associations between As species and GDM and evaluate the potential interactions of folate, vitamin B12, and homocysteine (Hcy) with As species on GDM prevalence. We measured levels of arsenite (As3+), arsenate (As5+), dimethylarsinic acid (DMA), and arsenobetaine (AsB) species in urine and folate, vitamin B12, and Hcy in serum from 396 pregnant women in Tianjin, China. The diagnosis of GDM was based on an oral glucose tolerance test. Associations of As species in urine with GDM were evaluated using generalized linear models (GLMs) and Bayesian kernel machine regression (BKMR). Additive interactions of As and OCM with GDM were estimated by determining the relative excess risk due to interaction (RERI). Of the 396 pregnant women, 89 were diagnosed with GDM. Continuous increases in urinary inorganic As were associated with GDM in the GLMs, with adjusted odds ratios of 2.12 (95% CI: 0.96, 4.71) for As3+, and 0.27 (95% CI: 0.07, 0.98) for As5+. The BKMR in estimating the exposure-response functions showed that As3+ and AsB were positively associated with GDM. However, As5+ showed a negative relationship with GDM. Although the additive interactions between As exposure and OCM indicators were not significant, we found that pregnant women with higher urinary As3+ and total As accompanied by lower serum vitamin B12 were more likely to have higher odds of GDM (3.12, 95% CI: 1.32, 7.38 and 3.10, 95% CI: 1.30, 7.38, respectively). Our data suggest a positive relation between As3+ and GDM but a negative relation between As5+ and GDM. Potential additive interaction of As and OCM with GDM requires further investigation.

Isotopic exchange of oxygen, sulfur, hydrogen and copper between aqueous phase and the copper minerals brochantite, libethenite and olivenite

AbstractFractionation factors for the isotopes of O, H, S, or Cu (as appropriate) were determined for the minerals brochantite [Cu4(SO4)(OH)6], libethenite [Cu2(PO4)(OH)] and olivenite [Cu2(AsO4)(OH)] and corresponding aqueous solutions at temperatures between 30 and 70°C. All samples used for this determination were synthetic and the degree of fractionation was expressed as 1000 ln α = (A × 106/T2) + B, where A and B are empirical parameters. A few natural libethenite samples from its type locality Ľubietová-Podlipa were also analysed and compared to the prediction based on the isotopic composition of meteoric water and our fractionation factors. The hydrogen fractionation factors agreed with the prediction well, whereas those for oxygen did not. A possible explanation is the disequilibrium of aqueous phosphate (and also arsenate) species and the solution in our experiments or the interaction of meteoric fluids with the isotopically heavy (in terms of oxygen) country rocks. Because the effects of isotopic disequilibrium in our experiments cannot be ruled out, the oxygen fractionation factors should be used with caution. The determined fractionation factors can be used as an isotope geothermometer, given that it can be proven that the phases of interest precipitated from the same fluid in equilibrium. Libethenite is predicted to have slightly lower δ65Cu values than its parental solution, but brochantite slightly higher δ65Cu values than its parental solution. Simple forward models, simulating neutralisation or reduction of mine drainage, show that precipitation of these minerals and removal of the co-existing fluid, could cause isotopic variations (in δ65Cu) on the order of 1‰ or more.

Development of Eu-based metal-organic frameworks (MOFs) for luminescence sensing and entrapping of arsenate ion

A new type of Eu-based metal organic framework (Eu-MOF) characterized with ability of arsenate ion luminescence sensing and entrapping was constructed via solvothermal method based on the optimization of the organic ligand. The Eu-MOF aqueous dispersion sample displayed arsenate-triggered augment in luminescence emission intensity with appreciable contrast and detection sensitivity with limit of detection down to 17.8 nM toward arsenate ion, which is less than one-fourth of the upper limit concentration of arsenic in drinking water that the World Health Organization (WHO) set for public health, 72 nM. Additionally, the excellent recognition specificity Eu-MOF toward arsenate species over various potential interfering anions and cations was also confirmed. The rapid adsorption ability of Eu-MOF nanostructures toward arsenate in aqueous milieu that plausibly originated from its nature of chemical adsorption and the adsorption feature conforming to the Langmuir model was also verified. The advantages that Eu-MOF nanostructure displays in terms of high sensitivity and recognition specificity in luminescence sensing of arsenate ion and the desired arsenate entrapping ability in aqueous milieu is expected to impart it potential for sensing and entrapping trace amount of arsenate in water in practical applications.

Competitive adsorption geometries for the arsenate As(V) and phosphate P(V) oxyanions on magnetite surfaces: Experiments and theory

Abstract In the present study, the competitive adsorption geometries for arsenate and phosphate on magnetite surfaces over a pH range of 4–9 were investigated using in situ attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR) and two-dimensional correlation analysis (2D-COS). The adsorption energies and infrared vibrational frequencies of these surface complexes were also calculated by first-principles simulations. Arsenate and phosphate have different preferences for the magnetite surface in the presence of aqueous solvent at both acid and alkaline pH. For the adsorption of phosphate, mono-protonated monodentate mononuclear (MMM) complexes dominated at acid pH, while non-protonated bidentate binuclear (NBB) complexes were dominant at alkaline pH. Arsenate mainly formed bidentate binuclear (BB) complexes with some outer-sphere species, both of which were more prevalent at acid pH. The pre-absorbed inner-sphere arsenate species were scarcely affected by the introduction of phosphate. However, the pre-absorbed phosphate oxyanions, especially the MMM complexes, were significantly substituted by BB arsenate at the magnetite surfaces. The adsorption affinity of phosphate and arsenate species for magnetite surface was found to increase in the following order: MMM phosphate complex &amp;lt; NBB phosphate complex &amp;lt; BB arsenate complex, which was consistent with the calculated adsorption energies. The simulated infrared vibrational frequencies for the most favorable adsorption modes of each oxyanion display distinctive patterns, and their trends are in excellent agreement with experimental data. The effects of pH, adsorption sequence, and mineral species on the competitive adsorption between arsenate and phosphate oxyanions are also discussed, and their different competing ability and stability on the magnetite surfaces can be ascribed to the variations in adsorption geometry and strength of binding. To the best of our knowledge, this is the first study aiming to distinguish the stability of the different phosphate and arsenate complexes on magnetite by employing a combined approach of in situ spectroscopy and DFT simulations. Our results provide molecular-level insight into the geometries and relative stabilities of the adsorption of phosphate and arsenate on magnetite surfaces, which is useful for interpretation of the mobility and bioavailability of these anions.

N-Alkylated chitosan coupled to the liquid-phase polymer-based retention (LPR) technique to remove arsenic (V) from aqueous systems

Water-soluble polymer based on alkylated chitosan with a quaternary ammonium group (Ch-QAG) was prepared, characterized, and applied to remove arsenate ions from aqueous solution by LPR technique. The arsenic removal was performed by the washing method (WM) and enrichment method (EM). Through the WM, studies of the pH and variation in the concentrations of interferents and arsenate ions were carried out. The effect of the removal of arsenate ions in simulated water was determined from the Camarones River in northern Chile. Ch-QAG showed high affinity for binding arsenate species (99% of removal) at pH 11.0 at a molar ratio of 20:1 polymer: As(V). High selectivity was also observed in the presence of interfering ions such as Cl−, SO42-, and PO43-, resulting in a removal rate over 80% at percentages over 95% for a concentration of 100 mg L-1 of As (V). The maximum retention capacity obtained was 112, 105, and 98 mg g-1 for three load cycles. The retention percentage for simulated water was 46.3% at a concentration of 1300 μ g L-1. In conclusion, the results presented in this study show that using Ch-QAG with ultrafiltration membranes is a great alternative to remove As (V) at high removal rates.

Distribution behavior of arsenate into α-calcium sulfate hemihydrate transformed from gypsum in solution

Transforming gypsum into α-calcium sulfate hemihydrate (α-HH) provides a promising utilization pathway for the abundant amount of chemical gypsum. The transformation follows the route of “dissolution-recrystallization”, during which the arsenic pollutant in gypsum is released into the solution, and hence raises the possibility of being distributed into the product of α-HH, a potential harm that has always been neglected. Investigation of the transformation process at neutral pH revealed that the arsenate ions in solution were distributed into α-HH and generated an enrichment of arsenic by 4–6 times. Arsenate ions distributed into α-HH by substitution for lattice sulfate, adsorption on α-HH facets and occupation for surface sulfate sites. While at higher concentrations, calcium arsenate coprecipitated with α-HH or even crystallized independently. Increasing temperature accelerated the phase transformation and restrained arsenate migration into α-HH due to the lag of distribution balance. The pH of solution modulated the dominant arsenate species and decreasing pH weakened arsenate substitution capacity for sulfate in α-HH. This work uncovers arsenate distribution mechanism during the transformation of gypsum into α-HH and provides a feasible method to restrain arsenate distribution into product, which helps to understand arsenate behavior in hydrothermal solution with high concentration of sulfate minerals and provides a guidance for controlling pollutants distribution into product.

Simultaneous removal and oxidation of arsenic from water by δ-MnO2 modified activated carbon

The ubiquitous arsenic in groundwater poses a great risk to human health due to its environmental toxicity and carcinogenicity. In the present work, a new adsorbent, δ-MnO2 modified activated carbon, was prepared, and its performance for the uptake of arsenate and arsenite species from aqueous solutions was investigated by batch experiments. Various techniques, including FESEM-EDX, p-XRD, XPS and BET surface area analysis, were employed to characterize the properties of the adsorbent and the arsenic adsorption mechanisms. The results showed that δ-MnO2 covered on the surface and padded in the pores of the activated carbon. Adsorption kinetic studies revealed that approximately 90.1% and 76.8% of As(III) and As(V), respectively, were removed by the adsorbent in the first 9 hr, and adsorption achieved equilibrium within 48 hr. The maximum adsorption capacities of As(V) and As(III) at pH 4.0 calculated from Langmuir adsorption isotherms were 13.30 and 12.56 mg/g, respectively. The effect of pH on As(V) and As(III) removal was similar, and the removal efficiency significantly reduced with the increase of solution pH. Arsenite oxidation and adsorption kinetics showed that the As(V) concentration in solution due to As(III) oxidation and reductive dissolution of MnO2 increased rapidly during the first 12 min, and then gradually decreased. Based on the XPS analysis, nearly 93.3% of As(III) had been oxidized to As(V) on the adsorbent surface and around 38.9% of Mn(IV) had been reduced to Mn(II) after As(III) adsorption. This approach provides a possible method for the purification of arsenic-contaminated groundwater.

Tracing source and mobility of arsenic and trace elements in a hydrosystem impacted by past mining activities (Morelos state, Mexico)

The Sierra Huautla (Morelos State, Mexico) is a biological reserve with historical mines of Ag and Pb. In this area, waters used by inhabitants are contaminated by arsenic (As). An integrated environmental survey was realized both in waters and sediments to better constrain the source and the mobility of As and other trace elements. Two areas of interest were selected: (1) the Nexpa River ecosystem to determine the local geochemical background, and (2) the Huautla area, affected by past mining activities. This study allowed the definition of the local geochemical baseline in sediments or in waters, demonstrated uncontaminated by TE in the Nexpa area, except for As in the dissolved phase or for Cd in Suspended Particulate Matters (SPM). In the Huautla area, TE contents in water were higher than the World Health Organization (WHO) limits for Al, As and Mn in unfiltered waters, and only for As in the dissolved phase. Speciation analyses revealed arsenic to be present only as the toxic-inorganic arsenate species, As(+V). In SPM, Ag, As, Cd and Zn concentrations were higher than Sediment Quality Guidelines (SQG). The different geochemical indice (EF: 5, PLI: 3, EF: Igeo: 5–3) demonstrated that SPM were significantly contaminated and consistute an health risk for Huautla inhabitants exposed to As-contaminated waters and TE-rich SPM. The combination of mineralogy, chemistry, C and S stable isotopes with thermodynamic modeling indicate dissolutions of minerals from local geological formations, sorption-desorption phenomena from clays and oxy-hydroxides, and the weathering responsible for the transport of the TE-rich SPM (1.8 wt% for 17% of the total TE concentration). Moreover, the past mining activity would be a source of the contamination only for As in waters from flooded mines.

Combined Micro- and Macro scale X-ray powder diffraction mapping of degraded Orpiment paint in a 17th century still life painting by Martinus Nellius

The spontaneous chemical alteration of artists’ pigment materials may be caused by several degradation processes. Some of these are well known while others are still in need of more detailed investigation and documentation. These changes often become apparent as color modifications, either caused by a change in the oxidation state in the original material or the formation of degradation products or salts, via simple or more complex, multistep reactions. Arsenic-based pigments such as orpiment (As2S3) or realgar (α-As4S4) are prone to such alterations and are often described as easily oxidizing upon exposure to light. Macroscopic X-ray powder diffraction (MA-XRPD) imaging on a sub area of a still life painting by the 17th century Dutch painter Martinus Nellius was employed in combination with microscopic (μ-) XRPD imaging of a paint cross section taken in the area imaged by MA-XRPD. In this way, the in situ formation of secondary metal arsenate and sulfate species and their migration through the paint layer stack they originate from could be visualized. In the areas originally painted with orpiment, it could be shown that several secondary minerals such as schultenite (PbHAsO4), mimetite (Pb5(AsO4)3Cl), palmierite (K2Pb(SO4)2) and syngenite (K2Ca(SO4)2∙H2O) have formed. Closer inspection of the cross-sectioned paint layer stack with μ-XRPD illustrates that the arsenate minerals schultenite and mimetite have precipitated at the interface between the orpiment layer and the layer below that is rich in lead white, i.e. close to the depth of formation of the arsenate ions. The sulfate palmierite has mostly precipitated at the surface and upper layers of the painting.

Recyclable high-affinity arsenate sorbents based on porous Fe2O3/La2O2CO3 composites derived from Fe-La-C frameworks

Efficient removal of aqueous arsenic is still a great challenge in particular for producing clean drinking water and for waste water treatment. To this end, a novel magnetic porous Fe-La composite, comprising La2O2CO3 and Fe2O3, was fabricated via a self-sacrificing template method based on bimetallic (Fe, La)-MOFs calcined at 550 °C. Batch adsorption results demonstrated that Fe-LaXY (X and Y represent the feeding mole ratio of Fe and La precursors) exhibited a maximum adsorption capacity of As(V) (241, 251 and 410 mg g−1 for Fe-La51, Fe-La21 and Fe-La11, respectively), and they also showed satisfactory adsorption kinetics, which can be ascribed to a strong coordination between La and arsenate species. Coexisting sorbates including carbonate, silicate, sulfate, and humic acid had a slight influence on adsorption performance of arsenate, while phosphate isostructural to arsenate gave severe interference. More importantly, the Fe-La11 could be easily separated from water due to its magnetism (7.0 emu g−1), and it also exhibited excellent recyclability (above 80% of removal efficiency at the fifth cycle) as well as stability in terms of release of Fe and La ions (< 0.5 mg L−1 at pH 4.0–10.0). Based on results from X-ray photoelectron spectroscopy (XPS), Powder X-ray diffraction (PXRD) and Raman spectroscopy, it is demonstrated that ligand exchange of surface hydroxyl groups and the formation of inner-sphere surface complexes are main responsible for the As(V) removal mechanism. Combining the magnetic properties, the absorption properties and the recyclability make Fe-LaXY derived from bimetallic MOFs promising sorbents for arsenate removal.