Speciation of mobile arsenic in soil samples as a function of pH

Arsenic (as As2O5)-contaminated soil samples collected from wood-preservative plants were extracted at pH 1–13 and analyzed by ion chromatography-inductively coupled plasma mass spectrometry. The extraction conditions were chosen to simulate the pH changes occurring in the environmental waters or pH changes due to acidic or basic accidental leaks, in order to provide information about the mobile arsenic species in the environment. Only the mobile fraction of toxic metals in the soil can cause contamination and health risks. The highest amount of arsenic was extracted at the highest pH, i.e. pH 13. At pH 3–9, in the case of most samples, less than 2% of the total arsenic was extracted, which indicates that pH changes in the environment would have to be very dramatic if they were to release high amounts of arsenic bound to the soil. Only inorganic species, As(III) and As(V), were found in extractions and much higher concentrations of As(V) than As(III) were extracted in all cases. However, some species conversion had taken place, since also As(III) was found in extractions and in the original solution in wood-preservative plant arsenic was present as As2O5. Also the effects of different types of solutions and the ionic strength of the carbonate buffer in arsenic extraction were studied.