Bioavailability and heavy metals speciation assessment in the contaminated soils of Doustbaglu mineralized area, NW Iran

The behavior, bioavailability and toxicity of heavy metals in soil essentially relies on their chemical spices. This paper attempts to quantify the heavy metals spices bound to various soil fractions at five sampling stations (S1-S5) from the Doustbaglu area, NW Iran, to ascertain their bioavailability, using sequential extraction. The results revealed that more than 90% of Cr and Cu were retained in the residual fraction, signifying the geogenic origin due to acid rock drainage of the surrounding mineralized zones. Ni, Pb and Zn; were assigned in a group presenting mostly in the residual (ranging 60 to 90%) and Fe/Mn oxy-hydroxide fractions, so reflecting a limited toxicity. The mean concentration of As and Sb at the non-residual fractions was significant (57.4 and 67.3%, respectively), primarily in the exchangeable as well as carbonate fractions, and thus they have high potential to be released into pore-water if conditions become more acidic. The highest contamination factors (Cf) were obtained for As and Sb (at S4-S5), while the lowest Cf were found for Cr, Cu, Pb and Zn. The mean risk assessment code (RAC) of As (37.8%) and Sb (39.5%) suggest the high risk situation, meanwhile, As at S2 (50.4%), S5 (51.6%); and Sb at S2 (53.1%) seems to be crossing threshold into the very high risk condition. The order of bioavailability based on RAC was (As > Sb) > > > Ni > Pb > Zn > Cr > Cu.