Speciation of molybdenum in river water by size fractionation and catalytic determination.

A speciation scheme of trace molybdenum was proposed for river water based on size fractionation by filtration and ultrafiltration and the catalytic spectrophotometric determination of the reactive molybdenum concentration (CR). The total concentration (CT) of molybdenum was determined by the same method after acid decomposition to obtain the concentration (CT - CR) of unreactive molybdenum. Most molybdenum in natural river-water samples was found to be reactive species. A large part of the molybdenum was found in the fraction of molecular weight (MW) < 10(3), and was estimated to be MoO4(2-) from the chemical equilibria of molybdate ions. The residual part of molybdenum was found in the colloidal and particle fractions (MW > or = 10(4)), and was characterized as reactive molybdenum adsorbed or complexed on humic iron aggregates. The coexistence of silicate contributed to a decrease of the particle size of humic iron aggregates associated with molybdenum. The above-mentioned speciation results were confirmed by an analysis of artificial samples. The changes in the fractionation results by acidification (0.1 M HCl) were also used to characterize molybdenum in natural water.