Publication

Arsenic adsorption and oxidation at manganite surfaces. 1. Method for simultaneous of determination of adsorbed and dissolved arsenic species

Résumé

Arsenic occurs in the +III oxidation state as a metastable species in oxic waters. Under oxic conditions, As(III) is both more mobile in natural waters and less efficiently removed by water treatment processes than As(V). Other oxidants, however, can react with As(III) more rapidly than oxygen. The oxidation of As(III) by manganite occurs on the time scale of hours. Here, a method is introduced for the rapid determination of the total and dissolved concentrations of arsenic species in this heterogeneous system; adsorbed arsenic concentrations are calculated by difference. The oxidation reaction is quenched by the addition of ascorbic acid to effect the reductive dissolution of manganite and concomitant release of adsorbed As(III) and As(V) into solution. Once in solution, As(III) and As(V) are separated using anion-exchange chromatography. Comparison of dissolved and total concentrations of As(III) and As(V) clearly illustrates that the overall conversion rate of As(III) to As(V) in this system would be overpredicted based solely on dissolved As(III) concentrations and underpredicted based solely on dissolved As(V) concentrations. The overall conversion of As(III) to As(V) was more rapid at pH 4 than at pH 6.3 and was unaffected by the presence of boric acid at 95 μM or 3 mM. However, the presence of 200 μM phosphate (at pH 4) decreased the overall rate of conversion of As(III) to As(V). Comparison of total and dissolved As(III) concentrations during the reaction time course demonstrates that the effects of pH and phosphate on adsorbed As(III) concentrations are generally consistent with these kinetic observations. | Arsenic occurs in the +III oxidation state as a metastable species in oxic waters. Under oxic conditions, As(III) is both more mobile in natural waters and less efficiently removed by water treatment processes than As(V). Other oxidants, however, can react with As(III) more rapidly than oxygen. The oxidation of As(III) by manganite occurs on the time scale of hours. Here, a method is introduced for the rapid determination of the total and dissolved concentrations of arsenic species in this heterogeneous system; adsorbed arsenic concentrations are calculated by difference. The oxidation reaction is quenched by the addition of ascorbic acid to effect the reductive dissolution of manganite and concomitant release of adsorbed As(III) and As(V) into solution. Once in solution, As(III) and As(V) are separated using anion-exchange chromatography. Comparison of dissolved and total concentrations of As(III) and As(V) clearly illustrates that the overall conversion rate of As(III) to As(V) in this system would be overpredicted based solely on dissolved As(III) concentrations and underpredicted based solely on dissolved As(V) concentrations. The overall conversion of As(III) to As(V) was more rapid at pH 4 than at pH 6.3 and was unaffected by the presence of boric acid at 95 μM or 3 mM. However, the presence of 200 μM phosphate (at pH 4) decreased the overall rate of conversion of As(III) to As(V). Comparison of total and dissolved As(III) concentrations during the reaction time course demonstrates that the effects of pH and phosphate on adsorbed As(III) concentrations are generally consistent with these kinetic observations.

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Concepts associés (36)
Arsenic
L'arsenic est l'élément chimique de numéro atomique 33, noté par le symbole As. Son corps simple se présente sous la forme d'un solide cristallin argenté. L'arsenic appartient au groupe des pnictogènes () avec l'azote (N), le phosphore (P), l'antimoine (Sb), le bismuth (Bi) et le moscovium (Mc). Il a des propriétés intermédiaires entre celles des métaux et des non-métaux, comme l'antimoine dont il est proche. Il est généralement considéré comme un métalloïde.
Arsenic trisulfide
Arsenic trisulfide is the inorganic compound with the formula . It is a dark yellow solid that is insoluble in water. It also occurs as the mineral orpiment (Latin: auripigmentum), which has been used as a pigment called King's yellow. It is produced in the analysis of arsenic compounds. It is a group V/VI, intrinsic p-type semiconductor and exhibits photo-induced phase-change properties. occurs both in crystalline and amorphous forms. Both forms feature polymeric structures consisting of trigonal pyramidal As(III) centres linked by sulfide centres.
Arsenic contamination of groundwater
Arsenic contamination of groundwater is a form of groundwater pollution which is often due to naturally occurring high concentrations of arsenic in deeper levels of groundwater. It is a high-profile problem due to the use of deep tube wells for water supply in the Ganges Delta, causing serious arsenic poisoning to large numbers of people. A 2007 study found that over 137 million people in more than 70 countries are probably affected by arsenic poisoning of drinking water.
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