Arsenic release from river sediments in a gold-mining area (Anllons River basin, Spain); effect of time, pH and phosphorous concentration

The release of arsenic (As) from As-rich riverbed sediments (Anllons River, Spain) was studied. In this area As is associated with pyrite and arsenopyrite and its mobilization has been favoured by past gold-mining activities. The effect of pH and phosphorous (P) concentrations, as well as the kinetics of the As release process were studied. Also, As species in solution were determined as a function of pH and time. The release of As was greatly influenced by pH and it was between 10 and 45 times higher at pH (sub stat) 10 than at pH (sub stat) 4 for all the sediments studied. The percentages of As released were between 0.1 and 0.8% of total As at pH (sub stat) 4 but increased to values between approximately 7 and 11% of total As at pH (sub stat) 10. The sediment containing the highest concentration of As released the highest amount of As (approximately 18 mg/kg at pH (sub stat) 10). The As release kinetics was also greatly influenced by pH, mostly at alkaline pH. At pH (sub stat) 10, the amount of released As increased progressively with time. In contrast, at pH (sub stat) 4 only small differences were observed after 24 h in the amounts of As released. At alkaline pH As was released along with Fe and Al, and accompanied by organic matter dissolution. This suggests a determining role of organic matter in the As release behaviour in alkaline media. In contrast, at weakly acid pH, the release of these components was very low, suggesting that As release is controlled mainly by desorption. The speciation of As was influenced by the pH and the release time. At pH (sub stat) 4 the major species in solution were As(III) and As(V) (representing 45-69% and 51-29% of As released, respectively). In contrast, at pH (sub stat) 10 almost all the released As was As(V) (approximately 99%). The amount of As released from sediments increased with the increase in initial P concentration in solution and it experienced a dramatic increase at added P concentrations above 1 mM. Namely, the percentage of As released by 10 mM P reached approximately 6% of total As for the sediments richer in As. The results obtained let us conclude that changes in pH, especially alkalinization, and inputs of P to the river markedly enhanced As mobilization from the Anllons River sediments, thus increasing health risks.