Sorption of L-methionine-sulphoximine on humic acids and clay minerals

The sorption of L-methionine-sulphoximine (MSX) on soil-extracted humic fra ctions (MW>20,000 Da), and mined clays (kaolinite and montmorillonite) was studied by a reversed-phase high-performance liquid chromatography method. The molecule, acting as an inhibitor of glutamine synthetase activity, was recently used in soil incubation assays for the determination of gross rate s of N mineralization. Equilibrium data of MSX adsorbed on humic fractions and clay minerals were described by both the Freundlich and Langmuir sorpti on isotherms. It was observed that humic fractions were more effective than mined clay minerals in removing MSX from water. Most isotherms were of Lan gmuir type, indicating a higher affinity of MSX for sorbing sites which bec ome increasingly saturated at higher equilibrium concentrations. Analysis o f Langmuir empirical constants revealed that different adsorbing mechanisms took place. Results showed that when MSX is applied in soil incubation ass ays, sorption of MSX on soil colloids considerably reduces the availability of the inhibitor to target microorganisms, and renders the method unsuitab le for the determination of the gross rate of N mineralization, especially when high levels of both organic matter and clay minerals are present in th e soil.