Sorption-controlled degradation kinetics of MCPA in soil

The relationship between sorption strength and degradation kinetics has been studied for the pesticide MCPA in a sandy top- and subsoil. After adding two types of sorbents (crushed peat and activated carbon) in various amounts to the sandy soils, sorption, desorption, and mineralization of C-14-MCPA were measured. The obtained Freundlich constants (K-F) varied between 0.7 and 27.2 mg((1-nF).)L(nF)/kg, and the first-order mineralization rate constants varied between 0.001 and 0.128 d(-1). The results showed an inverse relationship between sorption strength and mineralization. A higher K-F value corresponded to a smaller mineralization rate and less mineralization. A correlation coefficient of r(2) = 0.934 between the log-transformed Freundlich desorption coefficient (KF,de,) and the log-transformed mineralization rate constant W was obtained. After 7, 14, 22, and 35 days of incubation, soil samples were consecutively extracted by water, methanol, and 5 M NaOH to separate the remaining C-14 into 3 different pools. The extractions showed that the mineralization only proceeded from the water extractable pool of MCPA. Thin-layer chromatography revealed a formation of small amounts of metabolites; <7% of initially added C-14 was present as other compounds than C-14- MCPA in the water and methanol extractable pools. The study showed mineralization to be strongly correlated with sorption, represented by the desorption coefficient, and hence stresses the significance of bonding strength for estimating pesticide degradation in soil.