Complexation and release isotherm of arsenic in arsenic-humic/fulvic equilibrium study

Abstract

Soil organic fractions were extracted from 2 surface soils of West Bengal, one of which was arsenic-contaminated, and were fractionated into fulvic and humic acid fractions following standard procedures. These, and one humic acid sample synthesised in the laboratory, were characterised by pH-potentiometric titrations, viscometric measurements, visible spectrophotometry, and surface tension determinations. The results have been correlated with coiling–decoiling behaviour, as well as aliphatic/aromatic balance of these acids. The stability constant (logK) of the complexes formed by these natural humic/fulvic and synthetic humic acids with arsenic in aqueous phase was evaluated by the ion-exchange method. These logK values tend to suggest that the organo-arsenic complexes were quite stable. Further, the logK values displayed an approximately inverse relationship with the viscosity-average molecular weight, excess time needed to attain a stable higher pH during the pH-potentiometric titration, and also the surface excess (in water) of the given humic/fulvic acid samples. These trends were explained in terms of the relative hydrophobic character of the acids, and their resultant tendencies to complex arsenic in the aqueous phase. The release potential of arsenic from the arsenic–humic/fulvic complexes by soluble phophate and nitrate salts was also examined in terms of the appropriate exchange isotherms. In general, phosphate demonstrated a greater degree of exchangeability with arsenic than did nitrate, at both the lower and higher concentrations.