Sorption kinetics and leachability of heavy metal from the contaminated soil amended with immobilizing agent (humus soil and hydroxyapatite)

Release of heavy metals onto the soil as a result of agricultural and industrial activities may pose a serious threat to the environment. This study investigated the kinetics of sorption of heavy metals on the non-humus soil amended with (1:3) humus soil and 1% hydroxyapatite used for in situ immobilization and leachability of heavy metals from these soils. For this, a batch equilibrium experiment was performed to evaluate metal sorption in the presence of 0.05 M KNO3 background electrolyte solutions. The Langmuir isotherms applied for sorption studies showed that the amount of metal sorbed on the amended soil decreased in the order of Pb2+ > Zn2+ > Cd2+. The data suggested the possibility of immobilization of Pb due to sorption process and immobilization of Zn and Cd by other processes like co-precipitation and ion exchange. The sorption kinetics data showed the pseudo-second-order reaction kinetics rather than pseudo-first-order kinetics. Leachability study was performed at various pHs (ranging from 3 to 10). Leachability rate was slowest for the Pb2+ followed by Zn2+ and Cd2+. Out of the metal adsorbed on the soil only 6.1–21.6% of Pb, 7.3–39% of Zn and 9.3–44.3% of Cd leached out from the amended soil.