Adsorption of single-ring model naphthenic acids on soils

The adsorption of single-ring model naphthenic acids on soils was investigated using a batch partitioning method. The influence of the molecular structure of the sorbate, the organic carbon content of the sorbent, the temperature, the solution salt (calcium chloride) concentration, and the pH on the adsorption isotherms was determined. The adsorption coefficients (K d ) for two structurally related model naphthenic acids, 4-methycyclohexaneacetic acid (4MACH) and 4-methylcyclohexanecarboxylic acid (4MCCH), were 0.18 and 0.11 mL/g, respectively. The K d values determined for the individual cis and trans isomers were equal for a given model naphthenic acid within the experimental precision of the data. In contrast to reported differences observed for the biodegradation of geometric isomers of model naphthenic acids, the structure of the cis and trans isomers investigated had negligible effects on the measured adsorption isotherms. The adsorption coefficients of 4MACH for two soils with organic carbon contents of 1.6 and 2.7% were 0.18 and 0.22 mL/g, respectively. For the two temperatures studied, similar K d values were obtained for 4MACH (0.18 mL/g at 23°C and 0.21 mL/g at 4°C), indicating a predominantly physical adsorption process. Inorganic salt solutions of calcium chloride (CaCl 2 ) had significant effects on adsorption as the K d increased from 0.10 to 0.22 mL/g within the range of pore fluid chemistry investigated (0-30 mM CaCl 2 ). Likewise, the adsorption of 4MACH was significantly influenced by pH with the K d being 0.20, 0.18, and 0.11 mL/g in acidic (pH 2.9), neutral (pH 6.8), and alkaline solutions (pH 10), respectively. The low values of K d indicated that there would be little adsorption of the single-ring model naphthenic acids by soils. This will result in high partitioning to the water column in aquatic environments.Key words: model naphthenic acids, adsorption coefficients, soils, batch partitioning method.