Biodegradation of DDT and DDE

Abstract

Soil borne DDT residues contaminate New Zealand milk and meat products imposing a serious threat of export restrictions on these goods. Various approaches to rapidly reduce DDT and DDE contamination in soil were investigated. It also was intended to design a bioassay to measure bioavailability of DDT residues.

Supplementing a Waimakiriri sandy loam and a Tempelton silt loam, which were contaminated with DDT residues, with 18 different organic amendments resulted in high reduction of DDT (up to 100%) in both soils. Soils were kept in the dark at 25°C for a period of 98 days. DDT primarily was converted to DDD which was subjected to further degradation in the Waimakiriri sandy loam only.

The amount of DDE did not decline in the Tempelton silt loam, but in the Waimakiriri sandy loam 25% and 27% of initial DDE disappeared over 98 days in soil enriched with saw dust + glucose and wheat straw + glucose, respectively.

An unsterilized Taitapu silt loam, which was amended with a range of wheat straw concentrations, was inoculated with DDT or DDE and a spore suspension of the American white rot fungus Phanerochaete chrysosporium. Up to 80% of DDT and 45% of DDE dissappeared from' the soil during the incubation period of 98 days at 30°C in the dark. In soil enriched with spores of P. chrysosporium significantly more DDT and DDE declined during incubation than in soil without the fungus.

Whether in absence or presence of P. chrysosporium DDT decreased linearly with respect to the amount of straw present. DDE decline occurred under the presence of straw only, and if the straw concentration was ≥5% the concentration of DDE decreased first order with respect to the amount of straw amended. DDE decline was further significantly influenced by moisture level of the soil-straw matrix. More DDE was degraded at a WHC of 60% than at a WHC of 30%.

Nutrient nitrogen deficient medium, which was supplemented with DDT or DDE, was inoculated with mycelial plugs of P. chrysoporium or P. cordylines, a white rot fungus native to New Zealand. Significantly more DDT and DDE decreased in liquid medium containing P. cordy lines than in medium inoculated with P. chrysosporium. In flasks containing the New Zealand fungus approximately 90% of initial DDT and 45% of initial DDE decreased during 35 days of incubation in the dark at 30°C.

The bioavailability of DDT, DDD and DDE was measured using Spirodela oligorrhiza plants as a test organism. It was found that the plants were not sensitive enough to detect DDT residues at a level they occur in nature.