Adsorption of Humic Substances by Hydroxyaluminum‐ and Hydroxyaluminosilicate‐Montmorillonite Complexes

Abstract

AbstractThe interaction between humic substances purified from an Andisol and montmorillonite (Mt) complexed with hydroxyaluminosilicate (HAS) or hydroxyaluminum (HyA) ions was studied. The HAS ions were formed from solutions containing HyA ions and orthosilicic acid at an Al concentration of 4 mM; Si/Al atomic ratios of 0, 0.25, 0.5, 0.75, and 1.0; and a NaOH/Al molar ratio of 2.0. The adsorption of humic substances by HAS‐Mt and HyA‐Mt complexes increased as pH decreased in the adsorption systems. At concentrations of 10 to 1000 mg humic substance L−1 and pH 5.5, the adsorption of humic acid by these complexes decreased with an increase in the Si/Al atomic ratio of hydroxy materials fixed in interlamellar spaces of Mt, whereas the adsorption of fulvic acid followed almost the opposite trend. When the adsorption data of humic substances by these complexes were evaluated using the Eadie‐Hofstee plot of the Langmuir adsorption equation, multiple linear relationships were obtained, indicating the presence of multiple populations of sites that have a differing affinity for humic substances. A ligand exchange reaction, in which humic substances displaced OH or OH2 groups from the coordination shells of structural Al on the edges and HAS or HyA polymers fixed on the edges, external planar surfaces, and/or the interlamellar spaces of Mt, may be involved in the adsorption processes. The results indicate that besides HyA ions, the reactivity of HAS ions in humus accumulation and the pedogenic implication in soil environments merit attention.