Arsenic Mobility in As-Containing Soils from Geogenic Origin: Fractionation and Leachability

Abstract

The lithogenic arsenic in soils of the Anllóns River basin (Spain) was quantified, its chemical fractions were determined, and its leachability characteristics were compared under various experimental conditions. Fifty soil samples of C horizons, sampling an area of 50 km2 along the river course, were submitted to an exploratory analysis. Pseudo-total arsenic ranged between 2 and 489 mg kg−1. Arsenic leachability using the standard methods DIN 38414-S4 and TCLP was less than 0.25% of pseudo-total As. Then the effect of pH (3, 6, and 9), solid : liquid (S : L) ratio (1 : 10 and 1 : 50), phosphate (10 mM), and contact time (24 and 240 h) on arsenic mobilization was studied in nine soils with the highest As concentration. Arsenic mobilization increased at alkaline pH and lower S : L ratio; the most decisive factor was the addition of phosphate, increasing arsenic leachability up to 1,000 times, and this increased even 2.3 times when the contact time was extended from 24 hours to 240 hours. The results suggest that the mobilization of arsenic may be underestimated in short-term water leaching tests and that the environmental conditions favouring arsenic mobilization should be taken into account for a sound evaluation of the transfer risk of arsenic towards aquatic ecosystems.