Effects of inorganic and organic amendments on the mobility (leachability) of heavy metals in contaminated soil: A sequential extraction study

Six kinds of amendments were added to contaminated soil with the goal to alter the mobility of heavy metals and to modify potential environmental impacts. The following amendments were used at doses ranging from 1 to 5%: two kinds of young brown coals (lignite, oxihumolite) with a high content of humic acids, two commercially available derivatives of humic acids (potassium humate and iron humate), and two inorganic materials (fly ash and natural zeolite). Leachability of heavy metals (Cd, Cu, Pb, and Zn) from untreated and amended soils was studied with the aid of a standardized sequential extraction test (BCR test). Cd was present predominantly in the readily mobilizable (acid extractable) form in the soil; its mobility was reduced effectively after an addition of potassium humate. Cu, Pb and Zn were bound more strongly to the soil matrix. The mobility of Cu was reduced after an addition of inorganic amendments (zeolite and fly ash), whereas potassium humate was effective in the immobilization of Pb. Most of Zn (more than 60%) was bound rather strongly in the residual fraction and its mobility was affected only slightly by the presence of amendments at the concentration levels used in this work. Nevertheless, it was shown that even relatively low doses of amendments may cause a significant re-distribution of heavy metals in soil, which could be employed in the soil remediation.

Research Highlights
► Cd is the most mobile element in soil; Cu, Pb and Zn were bound more strongly.
► Even low concentrations of amendments affect a mobility of heavy metals in soils.
► Stabilizing/mobilizing effect of humates relates to their complex-forming ability.
► Key role of soil pH on the heavy metal mobility was confirmed.