Dicarboxylic acid transport through hematite-coated sand

Abstract

To better understand the behavior of low molecular weight organic acids in subsurface environments, the transport of three dicarboxylic acids (phthalic, maleic, and fumaric acid) in water-saturated columns packed with a hematite-coated sand was investigated in single and binary organic acid systems. Experiments were conducted at a single ionic strength (0.1 M) and at two pH values (4.1 and 5.3). In single-acid systems, the order of breakthrough at both pH values was fumaric acid, followed by maleic acid, and then phthalic acid. The shape of the breakthrough curves for the acids at the two pH values were similar except at pH 5.3 phthalic acid showed two adsorption fronts. The initial front only partially broke through, whereas the second front proceeded to complete breakthrough. This behavior resulted from a marked pH increase during phthalic acid adsorption and suggests that the single-acid systems behaved as dual-component systems with the organic acid and hydrogen ion as variables. The breakthrough curves for the binary organic acid systems showed organic acids with a higher adsorption affinity (e.g., phthalic acid) competitively displace organic acids with a lower adsorption affinity (e.g., fumaric acid). The dual-component effect observed for phthalic acid in the single-acid systems was suppressed in the mixed acid systems, perhaps reflecting pH changes that accompanied the desorption of the weakly-binding acids. These results may provide an important step toward further elucidating the processes controlling organic acid fractionation in the subsurface.