Effect and Mechanism of Humic Acid with Different Molecular Weight on Adsorption of Cadmium on Montmorillonite

Industrial and agricultural activities lead to increasing soil cadmium (Cd) pollution. As an important functional component of soil colloid, both montmorillonite (Me) and humic acid (HA) have good adsorption properties for Cd. Four molecular weight grades of HA were obtained by tangential flow ultrafiltration. FTIR, element determination, functional group titration were used to study the effect of HA with different molecular weight on the adsorption of Cd on montmorillonite and the mechanism of action. The results showed that with the increase of molecular weight of HA, the content of hydroxyl and carboxyl groups decreased while the phenolic hydroxyl group and methyl group increased, the aromaticity increased, the hydrophobicity became stronger, and the pH value increased. The contents of Cd and HA adsorbed by montmorillonite in the ternary system increased with the increase of molecular weight. When the initial concentration of Cd was 50 mg/L, the adsorption capacity of HA, HA1, HA2, HA3, HA4 montmorillonite system for Cd was 1.99, 2.11, 2.46, 4.12, 4.88 mg/g, respectirely. And the Cd adsorbed under the action of each molecular weight HA was mainly reducible state. The smaller HA was easier to compete for the adsorption of the Cd already bound on the surface of montmorillonite, and due to its strong hydrophilicity, the Cd adsorbed by small HA would be brought into the solution, reducing the content of Cd absorbed by the system. With the increase of molecular weight, its competition effect with montmorillonite was reduced, and the HA was difficult to enter into the montmorillonite layers. At this time, under the hydrophobic effect, the large molecule HA adsorbed Cd would cover the surface of montmorillonite, thus increasing the adsorption amount of Cd in the system. © 2021, Editorial Department of Advanced Engineering Sciences. All right reserved.