Adsorption Of Heavy Metal On Clay Mineral-Humic Acid Complexes

Humus such as humic acid (HA) and fulvic acid (FA) is a kind of natural organicmaterial that widely exists in soil, sediments and aquatic environment. It can bedivided into free-state humus and mineral-bound humus by its existing condition.The free-state humus is very little in soil, and an overwhelming majority of humus isadsorbed on clay mineral surfaces to form stable clay mineral-humus complexes.The sorption behavior alters the surface physicochemical characteristics of clayminerals, changing the hydrophilic surface to somewhat hydrophobic. At the sametime, humus effects the interaction between clay mineral and heavy metal bychelating heavy metal ion, promoting or retarding the adsorption of heavy metal onclay mineral, and controlling the migration and transformation of heavy metal in theenvironment.The HA used in this study was extracted from a paddy soil. Kaolinite andmontmorillonite were selected as prototype minerals for this study. In order touncover the mechanism of heavy metal sorption on clay mineral-HA complexes, wefirst prepared complexes by coating HA onto kaolinite and montmorillonite in abatch adsorption experiment, and then used the complexes as sorbent to investigatethe adsorption of heavy metal on clay mineral-HA complexes.Adsorption isotherm of HA on kaolinite fit the Freundlich adsorption modelaround experimental concentrations, and the R² is 0.960. Ligand exchange wasmajor mechanism in the interaction between HA and kaolinite. The adsorptionbehavior mainly took place on the hydroxyl site of aluminum. The percentage of HAsorbed on kaolinite decreased with an increase in pH values. The pH value could \-^,affect the surface charge of HA molecules and kaolinite. So the adsorptionmechanism of HA on kaolinite was different as a result of the change of pHconditions. At pH5.0, the results of adsorption showed that kaolinite-HA complexes had ahigher Cu2 adsorption capacity than pure kaolinite. Because HA contains carboxylic, phenolic active groups, the adsorption sites on kaolinite increased by HAsorbed on kaolinite and formed the S-HA-Cu ternary structure on the complexsurface. The amount of Cu²⁺ adsorption was positively related to the proportion ofHA in complexes. The formation of the S-HA-Cu ternary structure was the mainadsorption mechanism on kaolinite-HA complexes at pH＜5. Several mechanismscoexisted in the complex system when pH increased from 5 to 7. In the solutioncontaining Cu²⁺ and Cd²⁺ at 1:1 molar ratio, the kaolinite-HA complex displayedhigher selectivity to Cu²⁺ compared as Cd²⁺.The adsorption of HA on montmorillonite was much lower than on kaolinite. Theinterlayer surfaces do not participate in the uptake of HA. There were three possiblemechanisms governing the adsorption of HA on montmorillonite. They were cationbridging, water bridging, and H-bond complexation, and the ligand exchange waslimited. Interlayer surfaces of montmorillonite might not participate in the uptake ofHA. The sorbed HA on montmorillonite could desorb into solution with increasingpH value.The enhancement of Cu²⁺ adsorption on montmorillonite by the presence of HAwas insignificant. This phenomenon was attributed to the fact that montmorillonitehad much higher adsorption capacity for Cu²⁺, and the contribution of HA for Cu²⁺complexation was neglected comparing to montmorillonite. Another explanation tojustify the experimental behavior is the amount of HA on montmorillonite is low, sothe effect of HA was limited.In all, HA is found to enhance the metal adsorption capacity of mineral surfaces,in particular kaolinite. The extent of enhancement is positively related to the amountof HA adsorption on clay minerals, and the enhancement mechanism lies on themodel of binding between HA and clay minerals.