Distribution of Organic Arsenic(methylarsonic acid, dimethylarsinic acid) and Inorganic Arsenic(As(III), As(V)) in River Water

Abstract

A method was investigated for the selective determination of organic arsenic compounds such as methylarsonic acid (MMAA) and dimethylarsinic acid (DMAA) in river water by using hydride generation atomic absorption spectrometry. Inorganic arsenic, MMAA and DMAA in a sample are reduced to arsine, methylarsine and dimethylarsine, respectively, by an addition of NaBH₄. The arsines generated from the sample are collected on a liquid nitrogen cold trap, and separated by heating the trap due to their different boiling point. The arsines vaporized from the trap are carried to a quartz furnace and analyzed by atomic absorption spectrometry.
The good separation of methylarsine and dimethylarsine was accomplished by using 15% OV-3 on Chromosorb W 60/80 mesh as a packing material. The elimination of interferenc es due to foreign ions was investigated and the use of ethylenediaminetetraacetic acid (EDTA)and tartaric acid was very effective for the MMAA and DMAA determination. The detection limits by this method were 3 ppt for MMAA and 6 ppt for DMAA with a 200 ml sample solution. The coefficient variation for replicate determinations of 10 ng MMAA and DMAA were 3.5% and 3.0%, respectively. Therefore this method is suitable for the determination of MMAA and DMAA at trace levels in river water.
The concentrations of arsenic compounds(As(M), As(V), MMAA and DMAA) at 29 sampling sites in Tone river were determined. The average concentration was 1.57 (0.18-5.88) ppb for As(V), O.61 (0.16-3.31) ppb for, As(III), 21 (8-77) ppt for MMAA and 40 (11-140)ppt for DMAA. It was found that more than 97% of arsenic in the river water was inorganic arsenic and a few organic arsenic existed as MMAA and DMAA. However, high concentration of organic arsenic (77 ppt MMAA and 140 ppt DMAA) was observed at Kasumigaura, and organic arsenic occupied 10% of total arsenic. Thus the increase of organic arsenic observed at Kasumigaura might be due to the methylation of inorganic arsenic by the eutrophication of lake.