A comparative study of the humic acid adsorption capability between kaolinite and montmorillonite

Abstract：Kaolinite and montmorillonite are representative minerals of the warm-humid and cold-arid climate, respectively. A comparative study of the humic acid adsorption capability between kaolinite and montmorillonite might provide important evidence for the environmental behaviors of heavy metals in soils which are developed in different climate zones. In this study, batch adsorption experiments for humic acid were carried out with two species of clay minerals (kaolinite and montmorillonite) under different solution conditions of pH values, ionic strengths and initial concentrations of the humic acid. The aim of this study was to explore the causes responsible for the generality and diversity between kaolinite and montmorillonite in the adsorption of humic acid. Some conclusions have been reached: 1) humic acid adsorbed on both kaolinite and montorillonite gradually increases with increasing initial concentrations of humic acid and ionic strength under the condition pH=5; 2) the adsorption amount of humic acid on kaolinite and montorillonite decreases with the increasing pH value; 3) the humic acid adsorbed on kaolinite is higher than that adsorbed on montorillonite under the condition pH＜6, and things are just the opposite under the condition pH＞6, which may be related to different adsorption mechanisms of kaolinite and montmorillonite for humic acid. When pH＜6, electrostatic interaction between kaolinite and humic acid dominantes, and the ligand exchange and hydrogen bonds seem to play a minor role, while hydrogen bonds are the main adsorption mechanism for montmorillonite. When pH＞6, the main adsorption mechanisms of kaolinite and montmorillonite are hydrophobic interaction and cationic bridge, respectively.