Degradation of 2,4-D, 2,4-dichlorophenol, and 4-chlorophenol in soil after sorption on humified and nonhumified organic matter

Soil organic matter (SOM) primarily governs sorption processes and therefore affects the availability of organic chemicals to degrading microorganisms. Transformations of C-14-ring labeled 2,4-dichloro-phenoxyacetic acid (2,4-D); 2,4-dichlorophenol (2,4-DCP); and 4-chlorophenol (4-CP) sorbed on organic materials with increasing degrees of humification (wood, fresh straw, composted straw, Lignin, and humic acid) and on a reference mineral sorbent (Al-oxide) were studied during soil incubation experiments. Chemicals previously sorbed on the different sorbents were applied to the soil, Mineralization kinetics, analysis of water and methanol extracts and measurements of the nonextractable radioactivity before and after soil incubation were done to characterize changes in the availability of sorbed C-14 chemicals. Mineralization of 2,4-D was higher than mineralization of 2,4-DCP and 4-CP: depending on the nature of the sorbent, 47 to 87% of 2,4-D, 10 to 68% of sorbed 2,4-DCP and 12 to 65% of sorbed 4-CP were mineralized. Specific metabolic transformations and formation of bound residues occurred principally with the less humified organic sorbents. The formation of bound residues from the three chemicals was high after sorption on composted straw, where bound residues accounted for 20, 23, and 78% of applied 2,4-D; 2,4-DCP; and 4-CP, respectively. This was attributed partially to the initial sorption-desorption characteristics but was also explained by transformations that had occurred on this decomposing organic material. Our results showed that protection of chemicals against biodegradation by sorption processes can not be generalized since in most of the cases studied sorption induced an increase of mineralization rate.