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Sorption of polar and nonpolar aromatic compounds to two humic acids with varied structural heterogeneity

The major objective of the present study was to evaluate the correlation between structural nature of humic acids (HAs) and sorption affinity of organic compounds with varied polarity. We compared the sorption behavior of three aromatic compounds—nonpolar phenanthrene (PHEN) and 1,2,4,5‐tetrachlorobenzene (TeCB) and highly polar 2,4‐dichlorophenol (DCP)—to a solid‐phase coal humic acid (CHA) and a soil humic acid (SHA) suspended in aqueous solution. The structural nature of HAs was characterized using elemental analysis, ultraviolet absorbance, diffusive reflectance Fourier‐transform infrared, and solid‐state 13C nuclear magnetic resonance. The two tested HAs have very different structural properties: CHA consists primarily of poly(methylene)‐rich aliphatics with high aromatic content and some COO/N–C=O but low polarity, while SHA consists of young materials of lignin, carbohydrates, and peptides with high polarity. In response to the structural heterogeneity of HAs, sorption of nonpolar and more hydrophobic solutes (PHEN, TeCB) to CHA is much greater than that to SHA because of the predominance of hydrophobic effects; however, disparities in sorption affinity between the two HAs become smaller for polar and less hydrophobic DCP because of the major role played by polar interactions. The influence of pH on the sorption of different solutes to the two HAs was also discussed. The results of the present work highlight the importance of structural heterogeneity of both solutes and HAs in the sorption process.