Characteristics of MCPA sorption in ferralsol using high performance liquid chromatography

4-Chloro-2-methylphenoxyacetic acid (MCPA) is a widely used herbicide with high biological toxicity in ferralsol. Sorption is a key process that regulates the behavior of MCPA migration and conversion in soil. In order to evaluate the sorption behavior of MCPA in ferralsol, a simple and sensitive high performance liquid chromatographic method (HPLC) for the determination of MCPA in solution was developed with C₁₈ column and PDA detector set at 280 nm. The mobile phase consisted of a mixture of 80 : 20 (V/V) methanol and distilled water (adjusted to pH value 2.5 with acetic acid). The sorption isotherm experiment was also conducted. The results showed that good linear relationships (R²= 0.999 9) between HPLC peak area and the concentration of MCPA was obtained in the range from 0.5 to 40 mg/L. Detection limits (LOD) and quantitation Limit (LOQ) were 0.2 mg/L and 0.5 mg/L, respectively. Satisfactory relative recoveries ranging from 98% to 103% were achieved with relative standard deviations of 3.5%-4.0%. Those results demonstrated that the HPLC method was suitable for the evaluation of MCPA sorption. The isotherm data of MCPA sorption in ferralsols were fitted well by Freundlich model with △G<0 and |△G| =9.34-14.73 kJ/mol, which suggested that MCPA sorption on the ferralsols was an spontaneous, exothermic and anisotropic process dominated by physical forces. The value of K_f for MCPA ranged from 0.85 to 4.24 L/kg, which revealed that MCPA was difficult to be adsorbed by soils. It may further cause a potential environmental risk to groundwater. Moreover, diverse sorption capacity of MCPA was observed among three types of ferralsols in the following order: paddy soil > rhodic ferralsol > haplic ferralsol, which was closely related to the soil pH, organic matter and mineral. The sorption process was dominated by hydrogen bonding and dipole interaction.