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PubMed2004

Geochemistry of inorganic arsenic and selenium in a tropical soil: effect of reaction time, pH, and competitive anions on arsenic and selenium adsorption.

Factors that can affect As and Se adsorption by soils influence the bioavailability and mobility of these elements in the subsurface. This research attempted to compare the adsorption capacities of As(III), As(V), Se(IV), and Se(VI) on a tropical soil commonly found in Singapore in a single-species system. The effect of reaction time, pH, and competitive anions at different concentrations on the adsorption of both As and Se species were investigated. The As and Se adsorption isotherm were also obtained under different background electrolytes. The batch adsorption experiments showed that the sequence of the As and Se adsorption capacities in the soil was As(V) > Se(IV) > As(III) > Se(VI). The adsorption kinetics could be best described by the Elovich equation. The adsorption of As(V), Se(IV), and Se(VI) appeared to be influenced by the variable pH-dependent charges developed on the soil particle surfaces. Phosphate had more profound effect than SO4(2-) on As and Se adsorption in the soil. The competition between PO4(3-) and As or Se oxyanions on adsorption sites was presumably due to the formation of surface complexes and the surface accumulation or precipitation involving PO4(3-). The thermodynamic adsorption data for As(V) and Se(IV) adsorption followed the Langmuir equation, while the As(III) and Se(VI) adsorption data appeared to be best-represented by the Freundlich equation.

Adsorption↗
PubMed2006

Arsenic in soils and plants of woodland regenerated on an arsenic-contaminated substrate: a sustainable natural remediation?

Plant As accumulation at three As-polluted sites where spontaneous re-vegetation has taken place is examined. Each site had a different source of soil As (coal fly ash, LeBlanc process waste, canal dredging). Plant analysis indicates that soil-plant As transfer is poor at each site. Any mobile As is retained in root tissues, with little transfer to shoots. Bryophytes, pteridophytes, herbaceous and woody plants sampled at each site predominantly showed As concentrations of <3 mg kg(-1) dry wt, whilst total soil As ranged between 50 and 220 mg kg(-1) dry wt. Risk associated with food chain transfer at these sites is low when compared to other routes such as direct ingestion/inhalation of As-contaminated particulates re-entrained from an unvegetated or unstable substrate.

Arsenic↗
PubMed2002

Arsenic(III) oxidation and arsenic(V) adsorption reactions on synthetic birnessite.

The oxidation of arsenite (As(III)) by manganese oxide is an important reaction in both the natural cycling of As and the development of remediation technology for lowering the concentration of dissolved As(III) in drinking water. This study used both a conventional stirred reaction apparatus and extended X-ray absorption fine structure (EXAFS) spectroscopy to investigate the reactions of As(III) and As(V) with synthetic birnessite (MnO2). Stirred reactor experiments indicate that As(III) is oxidized by MnO2 followed by the adsorption of the As(V) reaction product on the MnO2 solid phase. The As(V)-Mn interatomic distance determined by EXAFS analysis for both As(III)- and As(V)-treated MnO2 was 3.22 A, giving evidence for the formation of As(V) adsorption complexes on MnO2 crystallite surfaces. The most likely As(V)-MnO2 complex is a bidentate binuclear corner sharing (bridged) complex occurring at MnO2 crystallite edges and interlayer domains. In the As(III)-treated MnO2 systems, reductive dissolution of the MnO2 solid during the oxidation of As(III) caused an increase in the adsorption of As(V) when compared with As(V)-treated MnO2. This suggested that As(III) oxidation caused a surface alteration, creating fresh reaction sites for As(V) on MnO2 surfaces.

Absorptiometry, Photon↗
PubMed2006

Contrasting effects of dissimilatory iron (III) and arsenic (V) reduction on arsenic retention and transport.

Reduction of arsenate As(V) and As-bearing Fe (hydr)- oxides have been proposed as dominant pathways of As release within soils and aquifers. Here we examine As elution from columns loaded with ferrihydrite-coated sand presorbed with As(V) or As(III) at circumneutral pH upon Fe and/or As reduction; biotic stimulated reduction is then compared to abiotic elution. Columns were inoculated with Shewanella putrefaciens strain CN-32 or Sulfurospirillum barnesii strain SES-3, organisms capable of As (V) and Fe (III) reduction, or Bacillus benzoevorans strain HT-1, an organism capable of As(V) but not Fe(III) reduction. On the basis of equal surface coverages, As(III) elution from abiotic columns exceeded As(V) elution by a factor of 2; thus, As(III) is more readily released from ferrihydrite under the imposed reaction conditions. Biologically mediated Asreduction induced by B. benzoevorans enhances the release of total As relative to As (V) under abiotic conditions. However, under Fe reducing conditions invoked by either S. barnesii or S. putrefaciens, approximately three times more As (V or III) was retained within column solids relative to the abiotic experiments, despite appreciable decreases in surface area due to biotransformation of solid phases. Enhanced As sequestration upon ferrihydrite reduction is consistent with adsorption or incorporation of As into biotransformed solids. Our observations indicate that As retention and release from Fe (hydr)oxide(s) is controlled by complex pathways of Fe biotransformation and that reductive dissolution of As-bearing ferrihydrite can promote As sequestration rather than desorption under conditions examined here.

Adsorption↗
PubMed1999

Enzymatic reduction of arsenic compounds in mammalian systems: the rate-limiting enzyme of rabbit liver arsenic biotransformation is MMA(V) reductase.

A unique enzyme, MMA(V) reductase, has been partially purified from rabbit liver by using DEAE-cellulose, carboxymethylcellulose, and red dye ligand chromatography. The enzyme is unique since it is the rate-limiting enzyme in the biotransformation of inorganic arsenite in rabbit liver. The K(m) and V(max) values were 2.16 x 10(-)(3) M and 10.3 micromol h(-)(1) (mg of protein)(-)(1). When DMA(V) or arsenate was tested as a substrate, the K(m) was 20.9 x 10(-)(3) or 109 x 10(-)(3) M, respectively. The enzyme has an absolute requirement for GSH. Other thiols such as DTT or L-cysteine were inactive alone. At a pH below the physiological pH, GSH carried out this reduction, but this GSH reduction in the absence of the enzyme had little if any value at pH 7.4. When the K(m) values of rabbit liver arsenite methyltransferase (5.5 x 10(-)(6) M) and MMA(III) methyltransferase (9.2 x 10(-)(6)) were compared to that of MMA(V) reductase (2.16 x 10(-)(3) M), it can be concluded that MMA(V) reductase was the rate-limiting enzyme of inorganic arsenite biotransformation. MMA(V) reductase was also present in surgically removed human liver.

Animals↗
PubMed2005

Comparison of arsenic levels in fingernails with urinary As species as biomarkers of arsenic exposure in residents living close to a coal-burning power plant in Prievidza District, Slovakia.

The associations between As levels in fingernails with both As concentrations in urine and environmental samples are reported. The participants (aged 20-80 years, mean 66 years) lived in the vicinity of a coal-burning power plant with high As emissions in the Prievidza District, Slovakia. Samples were taken in 1999 and 2000. The As levels in fingernails (n = 524) were measured after washing and digestion with microwave heating by hydride generation atomic absorption spectrometry. The spot urine samples (n = 436) were speciated for inorganic As (As(inorg)), monomethylarsonic (MMA) and dimethylarsinic acid (DMA) by hydride-cryogenic trap-atomic absorption spectrometry. The geometric mean As level in fingernails was 0.10 mug/g (range, <0.01-2.94 microg/g). There was a clear association between As in fingernails and the distance of the home to the power plant (P<0.001). Geometric mean As levels were: 0.17 microg/g distance < or = 5 km, 0.10 microg/g 6-10 km and 0.08 microg/g > 10 km. The association between the distance to the power plant and total urinary As (As(sum)) (n = 436, no fish consumption during the last 3 days before sample collection) was less pronounced (P = 0.018). The As levels in fingernails were positively correlated to As in soil (n = 207, r = 0.23, P<0.001) and to As in house dust (n = 209, r = 0.30, P<0.001). The associations between urinary As(sum) and As concentrations in soil (n = 159, r = 0.13, P<0.105) and in house dust (n = 162, r = 0.14, P<0.081) were quite similar. As levels in fingernails were associated with urinary As(sum) and with the different As species in urine. It is concluded that As levels in fingernails are a reliable marker of environmental As exposure, and that As concentrations in fingernails reflect the As exposure in a similar manner compared with urinary As(sum) and As species.

Adult↗