Simulation of adsorption equilibrium of heavy metal cations on soils in circumneutral aqueous solution: Influences of solution pH and dissolved humus substances

Adsorption isotherms are often required for understanding mobility, fate and bioavailability of contaminants in soils. Those about Pb (II) and Cd (II) on Andosols and Kanto loam were investigated in this work. Results show that adsorption increased with cation equilibrium concentration (10^-4 - 1 meq L^-1) and solution pH (5 - 7), and also that most adsorption isotherms can be simulated precisely with the ion-exchange-based Urano model. The applicability of model in predicting adsorption equilibriums of cations on soils in circumneutral aqueous solution (pH 5 - 7) were experimentally confirmed. There is an exception as to the Pb (II) adsorption on Andosols with the solid-to-liquid phase ratio higher than 1:100, there adsorption isotherms at pH 5 and pH 7 crossed, and the measurement precision decreased. The Urano model equation becomes inapplicable. Further experiment where humus substances (HS) were added indicated that the abnormal phenomenon can be attributed to dissolved HS and their complexation with Pb (II). The dissolution of HS with solution pH has enhanced Pb (II) concentration in solution but hardly reduced the total amount of adsorption on soil.