Anoxic oxidation of arsenite linked to denitrification in sludges and sediments

In this study, denitrification linked to the oxidation of arsenite (As(III)) to arsenate (As(V)) was shown to be a widespread microbial activity in anaerobic sludge and sediment samples that were not previously exposed to arsenic contamination. When incubated with 0.5 mM As(III) and 10 mM NO₃⁻, the anoxic oxidation of As(III) commenced within a few days, achieving specific activities of up to 1.24 mmol As(V) formed g⁻¹ volatile suspended solids d⁻¹ due to growth (doubling times of 0.74-1.4 d). The anoxic oxidation of As(III) was partially to completely inhibited by 1.5 and 5 mM As(III), respectively. Inhibition was minimized by adding As(III) adsorbed onto activated aluminum (AA). The oxidation of As(III) was shown to be linked to the complete denitrification of NO₃⁻ to N₂ by demonstrating a significantly enhanced production of N₂ beyond the background endogenous production as a result of adding As(III)-AA to the cultures. The N₂ production corresponded closely the expected stoichiometry of the reaction, 2.5 mol As(III) mol⁻¹ N₂-N. The oxidation of As(III) linked to the use of common-occurring nitrate as an electron acceptor may be an important missing link in the biogeochemical cycling of arsenic.     

A comparative study of As(III) and As(V) in aqueous solutions and adsorbed on iron oxy-hydroxides by Raman spectroscopy

The sorption of the arsenite (AsO₃³⁻) and the arsenate (AsO₄³⁻) ions and their conjugate acids onto iron oxides is one of main processes controlling the distribution of arsenic in the environment. The present work intends to provide a large vibrational spectroscopic database for comparison of As(III) and As(V) speciation in aqueous solutions and at the iron oxide - solution interface. With this purpose, ferrihydrite, feroxyhyte, goethite and hematite were firstly synthesized, characterized in detail and used for adsorption experiments. Raman spectra were recorded from As(III) and As(V) aqueous solutions at various pH conditions selected in order to highlight arsenic speciation. Raman Scattering and Diffuse Reflectance Infrared Fourier Transform (DRIFT) studies were carried out to examine the respective As-bonding mechanisms. The collected data were curve-fitted and discussed according to molecular symmetry concepts. X-ray Absorption Near Edge Spectroscopy (XANES) was applied to confirm the oxidation state of the sorbed species. The comprehensive spectroscopic investigation contributes to a better understanding of arsenic complexation by iron oxides.     

Processes releasing arsenic to groundwater in the Caldes de Malavella geothermal area, NE Spain

Arsenic concentrations exceeding the World Health Organization drinking water guideline (10 μg/L) have been measured in thermal and non-thermal groundwaters from the Caldes de Malavella geothermal area (La Selva graben, NE Spain). The CO₂-rich Na-HCO₃ thermal waters (up to 60 °C at the spring) have elevated arsenic concentrations ([As_T] from 50 to 80 μg/L). The non-thermal waters are of Ca-Na-HCO₃-Cl type and have [As_T] between <1 and 200 μg/L, defining a hot-spot distribution. The present-day contribution of As from CO₂-rich thermal waters to non-thermal aquifers is very limited, as shown by the concentration of geothermal tracers such as Li and B. Redox-controlling processes appear to govern the mobility of As in the non-thermal waters. Arsenate is clearly predominant in most oxidizing groundwaters (>85% of As(V) over total As), whereas reducing, high-As groundwater reaches up to 100% in arsenite. The reductive dissolution of Fe(III) oxyhydroxides and the coupled release and reduction of adsorbed As explain the elevated dissolved arsenite (up to 190 μg/L) and Fe (up to 14 mg/L) content in the more reducing non-thermal groundwater. Conversely, the high levels of nitrate (up to 136 mg/L) ensure an oxidizing environment in most non-thermal groundwaters ([As_T] between <1 and 60 μg/L). Under these conditions, Fe(III) oxyhydroxides are stable and As release to groundwater is not related to their dissolution. Instead, dissolved arsenate concentrations up to 60 μg/L are explained by a competition for sorption sites with other species, mainly bicarbonate and silicic acid, while arsenate desorption due to pH increase is not considered a major process.     

Rapid column-mode removal of arsenate from water by crosslinked poly(allylamine) resin

Performances of crosslinked poly(allylamine) resin (PAA) as arsenate (As(V)) adsorbent were studied using a column packed with PAA in hydrochloride form. PAA has a high amino group content of 14.6 mmol/g in free amine form and a high chloride content of 10.2 mmol/g in hydrochloride form. Its wet volumes in water were 4.5 and 3.1 mL/g in hydrochloride and free amine forms, respectively, indicating its high hydrophilicity. Breakthrough capacities for As(V) were evaluated changing conditions of adsorption operations: pH of feeds from 2.2 to 7.0, concentration of As(V) in feeds from 0.020 to 2.0 mM, and feed flow rate from 250 to 4000 h⁻¹ in space velocity. Breakthrough capacities increased from 2.6 to 3.4 mmol/g with a decrease in pH from 7.0 and 2.2, and also from 0.81 to 2.8 mmol/g with an increase in As(V) concentration from 0.020 to 2.0 mM. When feed flow rate increased from 250 to 4000 h⁻¹, breakthrough capacities changed form 3.5 to 0.81 mmol/g. Because of non-Hofmeister anion selectivity behavior of PAA, the interference of chloride and nitrate was minor. Although PAA slightly preferred As(V) to sulfate, the latter more markedly interfered with uptake of As(V) than chloride and nitrate. Competitive uptake of As(V) and phosphate revealed that PAA slightly preferred phosphate to As(V). The adsorbed As(V) was quantitatively eluted with 2 M HCl and PAA was simultaneously regenerated into hydrochloride form. All results were obtained using the same column without change of the packed PAA and any deterioration in column performances for 4 months.     

Adsorption of arsenate on synthetic goethite from aqueous solutions

Goethite was synthesized from the oxidation of ferrous carbonate precipitated from the double decomposition of ferrous sulfate doped with sodium lauryl sulfate (an anionic surfactant) and sodium carbonate in aqueous medium. The specific surface area and pore volume of goethite were 103 m(2) g(-1) and 0.50 cm(3) g(-1). Batch experiments were conducted to study the efficacy of removal of arsenic(V) using this goethite as adsorbent for solutions with 5-25 mg l(-1) of arsenic(V). The nature of adsorption was studied by zeta-potential measurements. The adsorption process followed by Langmuir isotherm and diffusion coefficient of arsenate was determined to be 3.84 x 10(11)cm(2)s(-1). The optimum pH of adsorption was found to be 5.0. The kinetics of adsorption was evaluated with 10 mg l(-1) and 20 mg l(-1) of As(V) solutions and activation energy of adsorption, as calculated from isoconversional method was in the range of 20 kJ mol(-1) to 43 kJ mol(-1). This suggests that the adsorption process is by diffusion at the initial phase and later through chemical control. FT-IR characterization of arsenic treated goethite indicated the presence of both AsOFe and AsO groups and supported the concept of surface complex formation.     

Effects of pH and competing anions on the speciation of arsenic in fixed ionic strength solutions by solid phase extraction cartridges

Anion exchange resins (AERs) separate As(V) and As(III) in solution by retaining As(V) and allowing As(III) to pass through. Anion exchange resins offer several advantages including cost, portability, and ease of use. The use of AERs for the instantaneous speciation of As minimizes the effects of preservatives on As species analysis. The aims of this study were to: (1) Examine the effects of pH and competing anions on the efficacy of solid phase extraction cartridges (SPECs) for speciation of As in a 0.01 molL(-1) NaNO(3) background electrolyte. (2) Identify optimal conditions (e.g. flow rates) for As speciation. (3) Calculate method detection limits (MDLs) from spiked background electrolyte and percent recoveries of As species from spiked extracts of mine wastes. The most effective SPEC retained As(V) through a range of environmentally relevant pH values (4-8). The mass loading capacity for As(V) was reduced in the background electrolyte (0.006 mg) compared with As(V) in deionized H(2)O (0.75 mg). Some retention (10-20%) of As(III) occurred on pre-wetted cartridges. Approximately 98% of spiked As(III) passed through dry cartridges. The recommended flow rate (0.5 mL min(-1)) was increased to 5 mL min(-1) without significant effect on As(V) retention. The presence of anions decreased the retention of As(V) with sulfate and phosphate having the greatest impact. MDLs were 0.004 mg L(-1) for both inorganic species. Spike recoveries in 0.01 M NaNO(3) mine waste extracts averaged 94% for As(III) and 107% for As(V).     

Preparation of an As(V) solution for scorodite synthesis and a proposal for an integrated As fixation process in a Zn refinery

A method for producing an As(V) solution from an As-bearing material obtained from a nonferrous hydrometallurgical process was investigated. Preparation of the As(V) solution included oxidative leaching of As with a NaOH solution, elimination of As as a calcium arsenate precipitate (johnbaumite: Ca₅(AsO₄)₃(OH)) with a Ca(OH)₂ secondary salt, washing the Ca–As compounds, and reaction of the Ca–As precipitate with H₂SO₄. This process was shown to be industrially applicable. The As ions and Cu ions were effectively separated by oxidative leaching with O₂ gas injection under strongly basic conditions. In this system, As dissolved in the NaOH solution and Cu precipitated with the residue. The dissolved As in this highly concentrated NaOH solution was then effectively precipitated from the solution by addition of a surplus amount of CaO, which allowed recycling of the NaOH solution. The addition of surplus Ca precipitated Ca₅(AsO₄)₃(OH) and Ca(OH)₂, which inhibited the leaching of As but did leach Ca and Na. When the Ca–As compounds were dissolved with H₂SO₄, Ca ions precipitated in the form of gypsum from the As-bearing solution. The gypsum produced by this process is likely to give rise to a number of As-related issues and the As level, therefore, needs to be reduced. This process is advantageous for the treatment of As since it is stabilized as scorodite. The production of an As(V) solution could be applied to hydrometallurgical operations as it necessary for the removal of As. This process is shown to be practically useful to As removal in Zn refining and a closed flow circuit is proposed for integration of this process into a Zn hydrometallurgical operation.     

Atmospheric heavy metal deposition in Finland during 1985-2000 using mosses as bioindicators

Heavy metal deposition and changes in the deposition patterns were investigated on the basis of surveys carried out in 1985, 1990, 1995 and 2000. The concentrations of 10 elements (Cd, Cr, Cu, Fe, Ni, Pb, Zn and V; and As and Hg since 1995) were determined on moss (Hylocomium splendens, Pleurozium schreberi) samples collected from the same permanent sample plots in each survey. The heavy metal concentrations, apart from those in southernmost Finland and close to a number of major emission sources, were relatively low. The mean concentrations of all the heavy metals decreased during the period covered by the surveys. The metals that showed the strongest decrease in concentration since 1985 were Pb (78%), V (70%) and Cd (67%). The concentrations of the other heavy metals decreased by 16-34%. The concentrations of Cr, Cu and Ni were clearly associated with local emission point sources and changes in emission levels. The concentrations of As and Hg, which were measured for the first time during the 1995 survey, decreased on the average by 26% and 10%, respectively.     

Speciation of arsenite and arsenate in rice grain – Verification of nitric acid based extraction method and mass sample survey

Arsenite and arsenate speciation was performed in 121 commercially purchased samples of 12 rice types to understand their relative relevance in rice grain. General effectiveness of a recently developed extraction protocol based on 0.28 M nitric acid at 95 °C was verified by checking the recovery of total grain arsenic and by comparing arsenic speciation in NIST-CRM-1568a, NMIJ-CRM-7503a and IMEP-107 with certificated and literature values. The arsenic speciation highlights the predominance of arsenite in 115 and dimethylarsinic acid in six samples and common minor components including arsenate, monomethylarsonic acid and two unknown arsenical species. Our data also indicate potential influences of other trace elements on As speciation in rice grain. Averagely, arsenite accounts for 90% of inorganic grain arsenic regardless of geographic origin, rice type, grain size, cultural practice and polish treatment. The high arsenite predominance indicates an elevated toxic effect of arsenic in rice than is perceived to date.     

Multifunctional use of magnetite-coated tuff grains in water treatment: Removal of arsenates and phosphates

Natural filtration material tuff (T) was modified by coating with nano-sized magnetite. The grain fraction of 0.6–1.9 mm was submitted to hydrothermal synthesis of magnetite. Thus formed magnetite modified tuff (MMT) was characterized in terms of Fe-content, N2 adsorption- desorption isotherm, SEM, zeta potential-pH analyses and adsorption behavior towards phosphates/arsenates in batch and column conditions. Elemental analysis showed that 36.54 mg g^?1 of magnetite was attached to the porous tuff grains. This modification changed pore structure and specific surface area. An increase of cca 35% in Sp value was obtained. Batch experiments proved that MMT was 4-5 times more efficient in removal of phosphates/arsenates than non-modified T. The maximum sorption capacities of phosphates calculated based on Langmuir equation were 0.45 and 1.91 mg g^?1, while those for arsenate were 0.551 m g^?1 and 2.36 mg g^?1 for T and MMT, respectively.The intra-particle diffusion model was the most suited for describing the adsorption process of phosphate and arsenate onto MMT.Fixed-bed column data corroborated batch results, i.e. MMT was 6 times superior in contaminant adsorption than T. Modification with magnetite improved T potential for usage in water treatment applications: its filtration ability remained unchanged, while adsorption capacity for phosphates/arsenates removal was improved.