Study On Mechanism Of The Noncovalent Interaction Between Humic Acid And Polycyclic Aromatic Hydrocarbon

Humic acids (HAs) are the main components of the dissolved organic carbon in aquatic systems and correspond probably to the most abundant naturally occurring organic molecules. They are usually considered to be able to affect solvability, transformation, bioavailability and final fate of the hydrophobic organic contaminants such as polycyclic aromatic hydrocarbon, PAH. PAHs are often produced by the combustion of fossil fuel and the petrochemical industry and difficult to be effectively degradated by the normal biological treatment so that some of them would be discharged to the aquatic systems. HAs can interact with PAHs by the the noncovalent bond and play a key role in controlling the speciation and mobility of them in waters. According to the previous researches, the influence of HAs on the fate of PAHs often reveals drastic differences and this variability raises questions about the real mechanisms of the interaction on molecular-level between the two kinds of compounds. However, so far the question has not been satisfactorily solved. Using instrumental and chemical analysis, this paper mainly studied the crucial functional groups on HAs binding anthracene as a typical PAHs, the interaction mechanism and the effect of some coexisting metal ions on it.The experimental study was conducted using two commercial HAs with different origins and anthracene. The structures of the two HAs were examined by infrared (IR) spectrometry and 3D fluorescence spectrometry. It was identified that aromatic hydrocarbons with carboxyl and hydroxyl substituent are the main HA structures of them and there also exist certain aliphatic acids for HAI and nitrogen heteroaromatic rings for HA∏By using a series of anionic and nonionic resins for fractionation and concentration,10 fractions were obtained from the each of HAs according to their ionicity and aromaticity. Each fraction was characterized by specific UV absorbance at 254 nm (UV254/TOC), and carboxyl and hydroxyl contents by potentiometric titration. In order to evaluate the affinity of different HA fractions for anthracene, the apparent binding constant Koc was obtained by fluorescence quenching. As a result of Pearson correlation analysis, a strong correlative relationship was found between Koc and UV254/TOC for most HA fractions, indicating that the crucial functional groups on HA interacting with anthracene would be the aromatic moieties. Koc tended to increase as acidity increased but the content of acidic groups showed little effect on Koc, showing that there is no direct interaction between the acidic groups and anthracene.In order to gain understanding on the mechanism of the HA-PAH inetraction process and the reason for its pH dependence, the three kinds of compounds were selected as the model HA segments:aromatic acids (such as benzoic acid, hydroxybenzoic acids), heteroaromatic compounds (such as phenanthroline, thymine) and the complicated compound with both heteroaromatic and aliphatic carboxylic acid groups (such as folic acid). The core structures of the above-mentioned species are all found in humic acid. The UV difference spectra of the mixture of each segment and anthracene showed the appearance of the charge-transfer bonds that were ascribed to the disturbance of the interaction on theπconjugated systems of the reagents. The Fourier transform infrared spectra (FTIR) of the model HA segments also indicated that the vibration bonds shifted a little before and after reacting with anthracene. These results confirmed that aπ-πelectron donor-acceptor (EDA) interaction was the main reaction between anthracene and the planar aromaticπ-electron systems of the model segments, in which anthracene acted asπdonor and the model segments asπacceptor. A comparison of the binding constants Koc of different model segments with anthracene determined by the fluorescence quenching method showed that the protonated nitrogen heterocyclic ring might be a strongerπ-acceptor than the benzene ring, indicating that nitrogen heterocyclic segment would play the main role for the interaction of HA molecules with anthracene. Besides, the results of the fluorescence quenching also displayed that Koc of P-hydroxybenzoic acid and anthracene was larger 2-3 orders than that of other hydroxybenzoic acids, proving that the position of the acidic groups on the benzene ring also greatly affected the interaction of HA and anthracene, and this would be the main reason caused the poor correlativity between the conten of the acidic groups and the values of Koc.The dependence of the interaction between the model HA segments and anthracene on pH was also investigated by FTIR and UV difference spectra, and it was found that the spectra changing of benzoic ancid and O-hydroxybenzoic acid before and after interaction with anthracene was distinctly different from that of the others when pH varied, indicating that the mechanisms of the pH effect on them were distinct. As for aromatic acids, the monosubstituted and orthosubstituted benzene ring, theπ-H hydrogen bond played a main role in the interaction between anthracene and the aromatic carboxylic proton on the benzene ring when pH≤pKa, and theπ-πelectron donor-acceptor (EDA) interaction increasingly became the main binding mode when pH >pKa-However, the orthosubstituted benzene ring, p-hydroxylbenzoic acid, which is easy to form the planar multi-molecule congeries because of its special D-π-A structure, could interact with anthracene by theπ-πEDA model no matter the solution acidity, and the binding intensity was higher than that between other benzoic acids and anthracene because of the larger contacting surface produced by the formation of the multi-molecule congeries. The decreasing pH of solution could also strengthen the interaction between the N-heterocycle and anthracene because of the protonization of N atom and the resulting enhancement of the electron-attracting ability of the N-heterocyclicπsystem. Additionally, it was confirmed that the aliphatic carboxylic acid could supply the proton to the N-heterocycle and be helpful for its affinity for anthracene although there is no strong non-specific intermolecular interaction between the aliphatic carboxylic acid and anthracene. Based on the above findings, it could be concluded that the differences of the mechanism of pH dependence would be the essential reason for the complicated relationship between the conten of the acidic groups and the values of Koc.Using fluorescence, UV apectra and Zeta potential meter, the effect of three divalent metal ions Ca2+, Zn2+and Cu2+, and one trivalent metal ion Al3+ on the HA structure and the apparent binding constant Koc between HA molecules and anthracene was investigated. The experimental results displayed that the either Ca2+, Zn2+or Cu2+coordinated with the caboxyl and hydroxyl groups attached on the aromatic moieties and the interaction made the colloidal partical of HA firstly condensed and then dissociated with the concentration of the metal ions increasing. The configurational reaction of humic acid with copper ion was proposed. It was also found that these coordination reactions could strengthen the electron-attracting ability of the aromatic ring on HA and enhance the HA-anthracene binding constant Koc. Al3+ion seemed to interact with the carboxylic acid groups on the aliphatic moieties of HA for its largest hydrated ion radius and induced the aggregation of the HA molecule. On the contrary, Al3+ ion hinder the HA-anthracene binding for Koc decreased a litter with increasing Al3+ concentration.