Simultaneous determination of arsenic(III,V), selenium(IV,VI), and antimony(III,V) in natural water by coprecipitation and neutron activation analysis

Simultaneous determination of the chemical speciation of arsenic, selenium, and antimony in water samples has been studied and two simple and rapid differentiation methods were developed. In the first method, arsenic(III), selenium(IV), and antimony(III) in aqueous samples can be simultaneously coprecipitated with Pb(PDC)(2) (PDC=pyrrolidinedithiocarbamate) from pH 2 to 4, while under the same conditions the higher oxidation state forms of these elements remain in solution. Quantitative coprecipitation of arsenic(V), selenium(VI), and antimony(V) can subsequently be achieved by reduction with titanium(III) chloride followed by Pb(PDC)2 coprecipitation at pH 4. The second method is based on the variation of the coprecipitation yield of arsenic(III) and arsenic(V) with Pb(PDC)2 with the pH of the solution. Arsenic(III) was coprecipitated at pH 2, followed by coprecipitation of arsenic(V) at pH 9 allowing both species to be separated and determined individually. All of the precipitates, both of higher and lower oxidation states of the elements, were collected on membrane filters for determination by neutron activation analysis. The detection limits are 1 ng l(-1) for arsenic and selenium and 0.1 ng l(-1) for antimony. The behavior of monomethylarsonic and dimethylarsonic acid in the sequential coprecipitation technique was also investigated. Through a series of validations by analyzing water, standard reference materials (NASS-2, NASS-3, SLRS-1 and SLRS-2) and simulated samples, it was found that the proposed methods can be applied satisfactorily to the determination of arsenic(III,V), antimony(III,V) and selenium(IV,VI) in various aqueous systems. (C) 1999 Elsevier Science B.V. All rights reserved.