The Research On Adsorption And Desorption Of Natural Organic Matter On Hematite

Natural organic matter （NOM） is composed of a series of large organicmolecules with complex structures. It is widespread in the natural environment, andits presence may affect the environmental behaviors of many pollutants. NOM in theenvironment can easily interact with metal oxides. This research used a humic acid（HA） to represent NOM and studied the adsorption and desorption behavior of the HAon hematites in different solutions, thus providing a theoretical basis for theinteraction between NOM and metal oxides.In this paper, batch experiments of adsorption kinetics and isotherm anddesorption kinetics in different background solutions were conducted. Meanwhile, anew molecular weight characterization method, HPSEC, was applied in this study.The results showed as follows:（1） The HPSEC method was established in our laboratory. By optimizing themethod’s parameters, we confirmed that this molecular weight testing method couldreach good reproducibility and that results obtained by this method in our laboratorycould be comparable to those obtained by other laboratories.（2） The adsorption of HA on hematites could reach equilibrium in24h. Thedynamic process could be divided into two stages. In the first stage, the adsorptionwas fast, and mainly the relative small HA molecules were adsorbed. The adsorptionin the second stage was much slower, with mainly the relative large moleculesadsorbed. In this stage, the larger HA molecules would replace the smaller ones.Although the adsorption amount was constant in the48th hour, the molecule replacingprocess was still ongoing. The dynamic process could be well described by Lagergrenpseudo-second kinetic equation.（3） The adsorption isotherm showed that when the initial concentration of HAreached about30mg/L, saturated adsorption amount was obtained. The presence ofsulfate ions and phosphate ions, especially the phosphate, could significantly reduceHA’s adsorption amount on hematites because of the competitive adsorption between HA and the sulfate or phosphate. In different background solutions, the adsorptionbehavior of HA with different molecular weights may be different. The adsorptionisotherm could be well described by both Freundlich and Langmuir models. And theadsorption mechanisms may include electrostatic interaction, ligand exchange, spatialinteraction etc.（4） In neutral solution conditions, none of Na⁺, Ca²⁺, Cl-or SO₄^2-could inducethe desorption of HA from hematites, even extended the desorption time. Under thesame pH and ionic strength condition, the phosphate solution could obviously promptHA to desorb, for phosphate’s adsorption energy is larger than HA and phosphate iseasier to combine with hematitie. The desorption process was slow. Desorptionamount increased with the ionic strength increasing. Phosphate desorbed relativesmall HA molecules first and then macromolecular HA.（5） When the solution pH was higher than the PZC of hematite, desorption tookplace immediately. It changed the hematite’s surface potential from positive tonegative and thus changing the interaction between the HA and hematite.