Adsorption of humic acids onto goethite: Effects of molar mass, pH and ionic strength

• Published in: Journal of colloid and interface science Vol. 314; no. 1; pp. 107 - 118
• Main Authors: Weng, Liping, Van Riemsdijk, Willem H., Hiemstra, Tjisse
• Format: Journal Article
• Language: English
• Published: San Diego, CA Elsevier Inc 01.10.2007; Elsevier
• Subjects: ADAPT; CD-MUSIC; charge-distribution; Chemistry; Colloidal state and disperse state; competitive adsorption; electrostatic interactions; Exact sciences and technology; Fulvic; fulvic-acid; General and physical chemistry; Humic; Humic acid adsorption; Iron (hydr)oxides; iron-oxide; LCD; mineral particles; natural organic-matter; NICA; nica-donnan model; Surface physical chemistry; variable-charge; Variable-charge particles; water interface; ADAPT; Iron (hydr)oxides; Humic acid adsorption; Fulvic; Variable-charge particles; Humic; LCD; CD-MUSIC; NICA; Iron oxide; Molar mass; Transition element compounds; Particle; Adsorption; Ionic strength; pH; Goethite; Humic acid
• ISSN: 0021-9797; 1095-7103
• DOI: 10.1016/j.jcis.2007.05.039

In this paper, the LCD (ligand charge distribution) model is applied to describe the adsorption of (Tongbersven) humic acid (HA) to goethite. The model considers both electrostatic interactions and chemical binding between HA and goethite. The large size of HA particles limits their close access to the surface. Part of the adsorbed HA particles is located in the compact part at the goethite surface (Stern layers) and the rest in the less structured diffuse double layer (DDL). The model can describe the effects of pH, ionic strength, and loading on the adsorption. Compared to fulvic acid (FA), adsorption of HA is stronger and more pH- and ionic-strength-dependent. The larger number of reactive groups on each HA particle than on a FA particle results in the stronger HA adsorption observed. The stronger pH dependency in HA adsorption is related to the larger number of protons that are coadsorbed with HA due to the higher charge carried by a HA particle than by a FA particle. The positive ionic-strength dependency of HA adsorption can be explained by the conformational change of HA particles with ionic strength. At a higher ionic strength, the decrease of the particle size favors closer contact between the particles and the surface, leading to stronger competition with electrolyte ions for surface charge neutralization and therefore leading to more HA adsorption. Using the LCD model, the effects of molar mass, pH, and ionic strength on the adsorption of humic substances onto mineral surfaces can be better understood.