The toxic and geochemical behaviour of arsenic is strongly influenced by its speciation. Therefore a progress in understanding of the geochemical cycle of arsenic is depending on the development of analytical methods to screen the different arsenic species. A fast and highly automated method is presented in the following paragraphs.
The hydride generation technique (HG) is a well
developed method to determine total concentrations of arsenic. It has reached a highly sophisticated level of automation and sensitivity by introduction of flow injection analysis (FI). The chemical reaction, which is necessary to reduce the analyte to the gaseous hydride, is also a usefull mean to analyse the arsenic speciation.
Hereto the reduction of arsenic by sodium tetrahydridoborate is carried out in appropriate acids: 4.0 mol L-1 hydrochloric acid for the determination of arsenious acid, 0.16 mol L-1 hydrochloric acid for methylated arsenic, and 1.0 mol L-1 tartaric acid for the total arsenic content. Arsenic acid could be calculated from the last result by substracting the two former ones. The detection limits are 100 to 200 ng L-1 depending on the acid medium used.
The commercial available FI-HG system FIAS (Perkin-Elmer) is the instrument used for the described procedure. The conventional configuration is one pump to suck the sample in the FI-valve and a second pump to carry the reductant and a acid to push the sample plug out of the valve into the reaction chamber. For speciation analysis this has to be changed in the following way. A further pumping tube is put onto pump two and connected to the reaction chamber sited outlet of the FI-valve. The acid used to push the sample out of the valve is also transported by this further tubing. This arrangement enables a complete on-line mixing of the sample plug and the acid and permits to skip any off-site sample preparations. The appropriate acid could be chosen solely by changing the acid storage bottle.
Using FI-HG the speciation of arsenic has been determined in hot mineral waters of Baden-Baden (SW-Germany). These waters have more or less similar total arsenic concentrations of 200 µg L-1. Despite of this similarity the shares of arsenious acid on the total contents show a wide range from 1 to 95 percent. The knowledge of speciation enables the calculation of pE of the waters, which shows a systematic dependance on temperature. Oxidation of the waters is accompanied by cooling and vice versa.
A second example is the remobilization of arsenic adsorbed on iron hydroxides in contaminated soils. This process is induced by a change in speciation from the adsorbed arsenate to the lesser adsorbable arsenious acid, which becomes the dominant species in the soil solution under reducing pE. Detailed examinations show that the mobilization of arsenic is not a secondary reaction influenced by the dissolution of the adsorbent, i.e. the iron hydroxides.
These two examples are highlighting how the analysis of speciation could promote our understanding of environmental processes.