Distribution of a nonylphenol isomer and the herbicide MCPA in soil derived organo-clay complexes

Organo-clay complexes in soil play an important role in the immobilization and persistence of xenobiotics. In this study, the fate of 14 C-labeled 4-(3,5-dimethylhept-3-yl)phenol (NP) and 4-chloro-2mehtylphenoxyacetic acid (MCPA) in soil was examined using incubation periods from 1d to 180d (NP) and 1d to 120d (MCPA). Special emphasis was on the distributions of the compounds and its degradation products among the soil sub-fractions sand, silt and clay, which were obtained by particle size fractionation. Additionally the soil organic matter complexed to the clay fraction was fractionated by a conventional procedure into an humic acids (HA), fulvic acids (FA) and humin (HU) in order to localize the site of binding of the xenobiotics within this important soil fraction. During the experiments, which were performed with two concentrations of the xenobiotics, the microbial activity in the assays was examined by determination of the DMSO reductase activity. A considerable reduction of the microbial activity was observed with the high concentrations of MCPA and NP (1 mg /g) as compared to the low concentrations (MCPA 559 μg per 100 g, NP: 121 μg per 100 g of soil). A different incorporation behavior was observed for NP and MCPA which resulted in distinctly different distributions of radioactivity within soil sub-fractions. MCPA exhibited a lower tendency to interact with soil particles, although the portions associated with soil particles were non-extractable with organic solvents. In contrast, the high affinity of NP to particles was accompanied by a lower formation of non-extractable residues. The total distribution of 14 C seemed to be influenced significantly by the physico-chemical properties of the contaminants and the microbial activity in soil.