Study On The Mechanisms And Influence Factors Of The Interaction Between Humic Acid And Heavy Metal Based On The Different Methods

Heavy metal and natural organic matter (NOM) are widely coexist in aquaticenvironment. NOM can stable combine with heavy metals via their high content ofoxygen-containing functional groups, have an impact on the environmental behaviorand ecological risks of heavy metal. Since there are a lot of uncertainty about theinteraction between NOM and heavy metal due to the diverse types, structures andproperties of NOM, nowadays more and more advanced methods were used toresearch the interaction. Previous studies found that the the kind of NOM and metal,pH and ion strength of aquatic environment would influence the interaction betweenNOM and heavy metal. Because the interaction between NOM and heavy metal isvery complex, the mechanisms and influence factors of the interaction between NOMand heavy metal need more research. In this study, based on the different researchmethods, four metals (Ag, Pb, Cd, Cr) and two humid acids (HAs, as a model NOM)were selected to explore the interaction between HA and heavy metal, with a focus onthe interaction mechanism and the effects of HA, metal and solution properties. Themain results are as follows:(1) Because of the release of H+, the pH of solution decreased after the additionof metal. When initial solution pH is4.0, there are no obvious change after interaction.Initial solution pH is5.0or6.0, the more metal addition, the more pH decrease.According to the fluorescence quenching of HA after the addition of metal, theinteraction process can be divided into three stages. The first two stages can bedescribed by Stern-Volmer model, and the Stern-Volmer quenching constant (Ksv) instage I is higher than the Ksvin stage II. With the addition of metal, the zeta potentialand hydraulic radius of HA does not have obvious change at first. Then with theincrease of metal concentration, the zeta potential of HA increased, and the hydraulic radius of HA became bigger, too (except Cr, the addition of Cr caused the zetapotential of HA change from negative to positive, the hydraulic radius of HA getbigger before they are smaller). With the addition of metal, there are do not generateprecipitation at first, then the precipitated HA amount increase quickly, finally theprecipitated HA amount becomes more balance. The interaction progress betweenmetal and HA can be described as follows: at first, the interaction between metal andHA mainly via functional groups. Then with the increase of metal concentration, alloxygen-containing functional groups are occupied and the metal interact with HA viaelectrostatic adsorption, the precipitation begin to form.(2) The solution pH has much effect on the interaction between HA and metal,the binding ability of HA increased with the increase of solution pH. The main reasonis at high pH conditions, H+can be released from HA easily, the competition betweenH+and metal ions is weak. And under the condition of high pH, the zeta potential ofHAis lower, metal can interact with HA by electrostatic adsorption easier.(3) Compare the two HAs, the adsorption constant of HA purchased from JuFengis higher than the adsorption constant of HA purchased from Aldrich obviously. Themain reason is that the HA purchased from JuFeng has more functional groups, andthe electronegativity is stronger than the HA purchased from Aldrich. The influence ofHA properties on the interaction between HA and metal are much smaller than theinfluence of pH.(4) Compare the four metals, the order of adsorption constant is Cr> Pb> Cd>Ag, the difference is mainly caused by the different valence state of metals. Thedifference between Pb and Cd is mainly caused by the different electronegativity andcompete adsorption ability.