Adsorption of heavy metal ions on humic acid-kaolinite complexes

This paper studied the adsorption of heavy metal ions on kaolinite in the presence of humic acid(HA).Three adding sequences,composed of one co-adsorption process and two pre-adsorption processes,were compared with each other in this study.When Cu2+ equilibrium concentrations were lower than 10 mg/L,no significant difference in the amount of Cu2+ adsorption was observed in the three adding sequences.When Cu2+ equilibrium concentrations were higher than 10 mg/L,the Cu2+ adsorption amount for co-adsorption and kaolinite pre-equilibrated with two Cu2+ adding sequences were slightly higher than that for kaolinite pre-equilibrated with HA.In order to investigate the effect of HA on heavy metal retention,the authors first prepared the HA-kaolinite complexes by adding HA solutions of varied proportions to kaolinite,and then used the HA-kaolinite complexes to study Cu2+ adsorption at pH 5.The results show that complexes have higher Cu2+ adsorption capacity than pure kaolinite.When HA contains carboxylic and phenolic active groups,the adsorption sites on kaolinite increased by HA adsorbed on kaolinite and form the S—HA—Cu ternary structure on the complex surface.The Cu2+ adsorption amount was related positively to the proportion of HA in the complexes.The formation of the S—HA—Cu ternary structure is the main adsorption mechanism on HA-kaolinite complexes at pH 5.Several mechanisms coexist in the complex system when the pH increases from 5 to 7.In the solution con-taining Cu2+ and Cd2+ at 1∶1 molar ratio,the HA-kaolinite complex displays higher selectivity to Cu2+ as compared to Cd2+.