Study On The Impact Of Two Water Environment Factors On The Interaction Between Polycyclic Aromatic Hydrocarbons And Humic Acids

Water environment factors can change the structure of humic acids, which occurs abundantly in water and have important impact on the interaction between polycyclic aromatic hydrocarbons (PAHs) and humic acids(HAs).Metal ions and surface acid as two environment factors aroused widely attention. In this study, some typical metal ions (Al3+,Cu2+,Zn2+) and surface acids (SDS,SDBS,Brij-35) were selected. Zeta potential,colloidal particle size and fluorescence spectrum of HAs were measured to find out the change of the structure of humic acid aroused by metal ions and surface acids(SAs).Fluorescence quenching method was used to determine the association constants between HAs and PAHs under the action of metal ions or SAs. And comprehensive analysis of the data above was done in order to find the relationship between the structural change of HAs and the change of association constants.Changes on the structure of humic acids and the association constants between HAs and PAHs aroused by metal ions can be described as Three-stage model.First stage, when the concentration of metal ions is low, chelating coordination reaction took place between metal ions and negatively charged functional groups of humic acid. The negative charge on the surface of humic acid was neutralized as a result, intermolecular repulsion between the molecules of humic acid declined and HA molecules curl up. In this stage, Zeta potential of HA increased and the colloidal particle size of HA declined. Because of this, the opportunity for the PAH to contact the hydrophobic region of HA molecules declined. As a result, the association constants between HAs and PAHs declined.Second stage, as the concentration of metal ions increase, under the action of metal ions, molecular coagulation between the HA molecules took place. In this stage, Zeta potential of HA change little and the colloidal particle size of HA inceased. Because of this, the opportunity for the PAH to contact the hydrophobic region of HA molecules inceased. As a result, the association constants between HAs and PAHs inceased.Third stage, as the concentration of metal ions came to a certain degree, the formation of hard spherical polymer between the HA molecules took place. In this stage, Zeta potential and the colloidal particle size of HA became constant. The structure of HA molecules remained the same. As a result, the association constants between HAs and PAHs became constant.In addition, the comprehensive analysis revealed that, the complex capacity to HA molecules of the three metal ions is different:Al3+> Cu2+>Zn2+.Changes on the structure of humic acids and the association constants between HAs and PAHs aroused by surface acids can be described as new three-stage model.First stage, when the concentration of surface acids is low, hydrogen bonds were formed between the functional group of HA and the polar head group of surface acids.In this stage, Zeta potential of HA declined and the colloidal particle size of HA increased. Because of this, more hydrophobic cavities were formed in HA molecules. As a result, the association constants between HAs and PAHs increased.Second stage, as the concentration of surface acids increase more and more surface acids molecules were combined to the HA molecules, HA molecules became denser and denser. Because of this, the hydrophobic cavities in HA molecules became smaller and smaller. So, the opportunity for the PAH to contact the hydrophobic region of HA molecules deceased. As a result, the association constant between HAs and PAHs deceased.Third stage, as the concentration of surface acids came to a certain degree, the sites on the HA molecules that can bind with surface acid were limited, as the concentration of surface acids increase, the size of hydrophobic cavities on the HA molecules remained the same. As a result, the association constants between HAs and PAHs became constant.