Adsorptive fractionation of humic acid at air-water interfaces

By using a simple bubble column, the adsorption behavior of a commercial soil-humic acid (CHA) at air-water interfaces was investigated. At pH 4.0, the concentrations of the CHA exhibited clear gradients in the bubble column, and increased significantly along the column height; smaller concentration gradients were also observed at pH 6.0. These concentration profiles demonstrate the surface activity of humic acid and pH-dependent affinity toward air-water interfaces. Taking advantage of the bubble column method, we interestingly found that the adsorptive fractionation of the CHA at air-water interfaces did occur. The components with higher molecular weight and stronger UV absorptivity showed greater affinity toward air-water interfaces, despite that the fractionation pattern was reduced to a certain extent as solution pH increased. The organic carbon-normalized pyrene partition coefficient Koc values deviated from the corresponding values of original bulk solutions at both pH 4.0 and 6.0, and increased along the height of the column. Our results demonstrate the usefulness of the simple bubble column, and suggest that the adsorptive fractionation of humic acid at air-water interfaces might have implications for some natural environments and engineered systems where air-water interfaces exist extensively.