Complexation of trace metals with humic acids from soil,sediment and sewage

ABSTRACT

Metal—humate interactions in aqueous and terrestrial ecosystems control the speciation of trace metals and hence, their bioavailability and toxicity. The present study investigated the complexation interactions of copper, cadmium and lead, in their divalent states, with humic acids extracted from three different sources: the treated sewage from a treatment plant, Yamuna river bed sediment, and Yamuna river flood plain soil, all at Okhla, Delhi, employing ion selective electrode potentiometry. The conditional stability constants of the complexes were computed from Scatchard plots. The influences of the two important metal speciation factors, viz., pH and ionic strength of the reaction medium on the conditional stability constants were ascertained by investigating the reactions under three different pH: 5.0, 6.0 and 7.0 and three different ionic strengths: 0.01, 0.03 and 0.1. Stabilities of metal—humic acid (HA) complexes follow the order: Cu-HA > Pb-HA > Cd-HA for humic acids from any single source and are found to increase with a rise in pH and fall in a ionic strength of the medium. The humic acids extracted from the soil and the sediment emerged as stronger complexing agents, as compared to that extracted from sewage.