Sorption of cadmium on humic acid: Mechanistic and kinetic studies with atomic force microscopy and X-ray absorption fine structure spectroscopy

Humic acids (HA) constitute a major fraction of the organic matter in soils and sediments. Little is known about the kinetics and mechanisms of Cd sorption by HAs, especially those pertaining to the surface features and structure of the Cd-humate complexes. We investigated Cd sorption by HA using the conventional batch method, N₂-BET method, atomic force microscopy (AFM), and extended X-ray absorption fine structure (EXAFS) spectroscopy to gain a better understanding of the kinetics and mechanisms. Cadmium sorption by HA can be described by the parabolic diffusion equation. The increase in the apparent diffusion coefficient with an increase in the initial Cd concentration was less pronounced at higher initial Cd concentrations, apparently due to an enhancement in aggregation of the Cd-humate complexes. The HA before and after Cd sorption at Cd concentrations ≤ 10^-6 M in different reaction times was spheroidal in shape. The Cd-humate complexes formed at 10^-4 and 10^-3 M Cd gradually changed to resemble the shape of a bean with increasing reaction time, thereby substantially decreasing their specific surface. Cadmium ions raveled the spheroidal HA particles into a bean-like structure by coordinating to six O atoms from the functional groups of HA, with a Cd-O distance of 2.297(2) Å. The Cd-O bond strength decreased with increasing initial Cd concentration.