Effects of the Active Components of Humic Acids and Their Proportions on the Dynamics of Lead Transformation and Availability in Purple Alluvial Soil

Abstract

Humic acids (HAs) play important roles in mediating the occurring forms and activity of Lead (Pb) in soils. Simulating experiments together with chemical sequential extraction were carried out to understand the dynamics of lead transformation and availability in purple alluvial soil as influenced by humic acid (HA) and fulvic acid (FA) and their ratios (HA/FA). The main results obtained are as following:1 The distributions of Pb forms bound by Fe-Mn oxides and organic matter increased, while the proportions of exchangeable Pb and residual Pb fraction decreased after the establishment of transformation equilibrium in 60 d. 2The exchangeable-Pb and carbonate bound Pb showed highest positive contribution to the availability of Pb in the tested soil; and HA passivated soil Pb activity by decreasing the proportion of carbonate bound Pb and FA activated soil Pb by increasing the proportions of exchangeable Pb and carbonate bound Pb. The passivation or activation roles increased with increasing amount of HA or FA; A sharp increase of soil Pb availability occurred at FA dosage higher than 1% as C basis, suggesting that 1% (C) exogenous FA was the change-point for soil Pb activation;3The overall effects of HAs depended on its ratio of HA/FA at a constant addition of HAs (1% C). HAs suppressed soil Pb activity at a ratio of HA/FA ≥7/3, while they activated soil Pb activity at a ratio of HA/FA≤5/5. The transformation extent and direction of Pb in soils differed at various dosage and ratios of HA/FA, resulting in different overall effects on the Pb availability in soil.