Inorganic arsenic oxyanions

Inorganic arsenic oxyanions, frequently present as environmental pollutants, are very toxic for most micro-organisms. Many microbial strains possess genetic determinants that confer resistance. In bacteria, these determinants are often found on plasmids, which has facilitated their smdy to the molecular level. Bacterial plasmids conferring arsenic resistance encode speciflc efflux pumps able to extrude arsenic from the cell cytoplasm, thus lowering the intracellular concentration of the toxic ions. Recently, apparently similar arsenic membrane transport proteins have been found with yeast, plants, and animals (8,9) (see Sec. V, below). 

A 0.5-gram mass of either the organo-treated or inorganic cation exchanged zeolite and 50 mL of 10 mM/L arsenate or chromate aqueous solutions were placed into Erlenmeyer flasks and mechanically shaken in reciprocating mode to attain equilibrium. Different equilibrium periods for individual zeolite modifications and both aqueous oxyanions species have been established. The adsorption isotherm experiments were conducted using above mass/ volume ratio of samples with an initial metal concentrations ranged from 0.5 to 100 mM/L at laboratory temperature. The... 

Some differences in arsenate and chromate adsorption on ODA-clinoptilolite and Pb-(Ag-linoptilolites) as well were recorded (Figs. 5 and 6). ODA-clinoptilolite exhibited more efficient arsenate and chromate removal from aqueous solutions than the inorganically exchanged modifications. However, silver exchanged clinoptilolite revealed higher capacity values for both oxyanions uptake than lead exchanged clinoptilolite did. This phenomenon supports preferred silver treated clinoptilolite utilization for specific water purification process even on the base of environmental acceptability. 

By batch description trials Organo- and inorganically- modified zealot was subjected up to 24 hr in distilled water, tap water and 2% Nalco aqueous solutions in laboratory shaken machine to demonstrate how strongly the examined oxyanions are bound on the modified zeolite. While only slightly chromate desorption in the maximum extent about 20 mg/L was observed, approximately one order higher arsenate desorption was found, corresponding to increased ionic strength in waters. However, in both cases ODA-clinoptilolite exhibited the lowest desorption characteristics. Here, the... 

Phosphorus and arsenic are both group 15 elements. The main phosphorus oxyanion, phosphate (PO43- ), has the same charge and tetrahedral configuration as inorganic As(V), which allow phosphate to out compete As(V) for adsorption sites on clay minerals and various iron and aluminum compounds over a wide range of pH conditions (Stollenwerk, 2003), 85 (Gao and Mucci, 2001) (Su and Puls, 2003), 2582. Phosphate is also capable of desorbing arsenic from humic acids and some mineral surfaces (Stollenwerk, 2003), 91 (Mok and Wai, 1994), 111 (Lafferty and Loeppert, 2005), 2120. 

Arsenic consists of two major oxyanions, As(III) and As(V) in water (Smedley et al, 2002). These two As species exhibit very different affinities to the mineral surfaces, and the retention of both As(V) and As(III) is strongly pH dependent (Meng et al, 2000) Both inorganic and organic states of the As(III) tend to be more toxic to humans than those of the As(V) forms. The efficiency of most separation methods for As(III) removal is low. To obtain an effective and efficient separation, an oxidation process, which can convert As(III) to As(V), is necessary. Upon efficient oxidation, as a pretreatment, the total removal of As can be effectively improved (Bissen et al, 2003). Different oxidants have been investigated in the As oxidation process, e.g. O2, O3, H2O2, zero-valent aluminum and iron, activated carbon and manganese (Bissen et al, 2003). 

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