Potential degradation rates of contrasting plastic additives in agricultural soils

Plastic additives associated with plastic products are essential to their function. However, a number of compounds, which are included at relatively high abundances, and frequency in plastic products, have known associated hazards. These include phthalates, which are endocrine disruptors, as well as antioxidants and UV stabilisers, which may bioaccumulate. As additives are not chemically bound, they are susceptible to leaching to the wider environment, including into soils. However, the potential abiotic controls on degradation of additives in soil, such as soil type, pH and nutrient availability, following leaching remains unknown. This study investigates the degradation of three contrasting, high production plastic additives in soil to elucidate potential controls on the bioavailability of the plastic additives.

Three additives of contrasting function were selected: bis(2-ethylhexyl) phthalate (DEHP), a plasticiser, which is an endocrine disruptor; tris(2,4-di-tert-butylphenyl)phosphite (Irgafos® 168, an antioxidant under assessment as bio accumulative under EU REACH, and octabenzone, a UV stabiliser. An agricultural soil from North Wales with no previous history of plastic use was sieved (2 mm) and maintained at constant moisture (30% WHC). Additives were dried onto sand then homogenised in soil to yield a concentration of 500 ng g⁻¹ soil. The degradation of additives was monitored over a 21 d time course with light exclusion. In the UK soil, Irgafos® 168 was not detected after t=0 d, due to rapid conversion to its oxidation product, tris(2,4-di-tert-butylphenyl)phosphate (Irgafos® 168ox), which also occurs abiotically during plastic production. However, no further microbial degradation of this antioxidant was observed over the 21 d period. DEHP and octabenzone both exhibited rapid degradation within 4 d, yet remained at 223 ng g⁻¹ and 51 ng g⁻¹, respectively, for the remainder of the 21 d experiment. The degradation of DEHP and octabenzone is proposed to be microbial, with 49% and 78% removed over 21 d, and the relative bioavailability of the additives was octabenzone>DEHP>>Irgafos® 168(ox). This will be expanded to include soils from across climatic zones (India, Vietnam, Australia, Brazil, Egypt), to elucidate controls on additive degradation linked to soil properties, including pH, soil type and nutrient availability, which are hypothesised to influence the bioavailability and preference for additive degradation.