The Adsorption/Desorption of Metal Ions, Phosphate, Organic Substances on Goethite and Influence Factors.
Feng-lin Xu and Xue-yuan Li. Huazhong Agricultural Univ, Faculty of Resources and Environment, Wuhan, 430070, China
Goethite widely existed in different kinds of soils in China. The surface character and its interreaction with inorganic or organic cations and anions, enzyme, microorganism differed with aluminosilicates clay minerals, aluminium oxides etc. in soils. Over two decades, our research group studied the adsorption/desorption of metal ions, phosphate, organic substances (Low-molecular-weight-organic acid, herbicide, enzyme, microorganism,) and made some headways for clarifying the adsorption/desorption mechanism of soils with variable charge and soils with permanent charge. This paper will briefly summarize these of follows. 1. The adsorption/desorption of metal ions on goethite and their mechanism (1) The adsorption amount and desorption percentage of Cu2+ on goethite increased and reduced remarkably with increasing electrolyte (KCl) concentration in equilibrium solution respectively. The mechanism for the latter may be that most sorbed on goethite surface were nonspecific adsorption and also different with general exchange adsorption and the mechanism for the former may be: (1) the surface charge density and positive potential of the Stern layer increased as KCl concentration raised, which resulted in the reduce of specific adsorption; (2) it favored the Cu2+ to inter into the diffusion layer from bulk solution; (3) the Cu2+ sorbed in the diffusion layer could form ion pairs with NO3- or ion association substances. (2) Low-Molecular-Weight-Organic-Acid (LMWOA) with low concentration would promote Cd2+ adsorption and inhibit Cd2+ adsorption when LMWOA concentration was over a certain level. The reason probably is a sheet of “organic film” formed on the interface by LMWOA sorbed on goethite, which resulted in produce stronger adsorption sites and reduce the IEP of goethite surface and drop the competition of LMWOA in solution with Cd2+ to adsorb on goethite. The desorption ratio of Cd2+ reduced and rose with rising pH solution containing LMWOA for KNO3 and DTPA used as desorbent respectively. This is on account of the former being electrostatic adsorption but the former being coordination adsorption. (3) The adsorbing capacity and adsorption ratio of Cd2+, Cu2+ on goethite increased with rising the amount of Rhizobium fredii (G6) added and with rising the pH value from 2.5 to 8.0 (for Cd2+) or from 3.4 to 7.4 (for Cu2+) in equilibrium solution respectively. G6 added would promote the desorption of Cd2+ sorbed on goethite in the initial 30 min. and inhibit the desorption with prolongation the desorbing time. (2) The amount of P absorbed on goethite increased with rising P concentration and the desorbing ration rose with rising P adsorption. The maximum desorbing percentage of P sorbed was <2%, indicating that >98% P sorbed was specifically sorbed. The results estimated by X-ray Photoelectron Spectroscopy (XPS) illustrated the amount of H2PO4- species increased and HPO42- decreased with rising the amount of P sorbed. In solution with pH4.4-4.8, most phosphate appeared as H2PO4- species when they were sorbed on goethite and would form to monodentate coordinate compound. In solution of pH 6.2-6.5, goethite surface would emerge binuclear coordinate compound as HPO42- species with the release of hydroxyl. (3) The FTIR spectrum before and after the adsorption of Bensulfuron-methyl on goethite indicated the hydrated groups of goethite surface and the carbonyl, sulfonyl groups of Bensulfuron-methyl took main effect in the adsorption of Bensulfuron-methyl by goethite. (4) Effect of phosphate and LMWOA on the adsorption of acid phosphatase by goethite. The adsorption isotherm curve of acid phosphatase on goethite in acetate solution belongs to L-type and the curve shape was not remarkably influenced by rising the acetate concentration. The adsorption percentage of acid phosphatase by goethite varied with various pH solution and the maximum amount of adsorption (86%) presented at pH 8.6, the reasons were analyzed.