Kinetics of di-(2-ethylhexyl)phthalate mineralization in sludge-amended soil

Sewage sludge is frequently used as a soil fertilizer although it may contain elevated concentrations of priority pollutants including di-(2- ethylhexyl)phthalate (DEHP). In the present study, the kinetics of microbial [¹⁴C]DEHP mineralization was studied in laboratory microcosms with sewage sludge and agricultural soil. A biphasic model with two independent kinetic expressions was used to fit the mineralization data. The initial mineralization activity was described well by first-order kinetics (r² > 0.97), whereas mineralization in long-term incubations (>40 days) was described better by fractional power kinetics (r² > 0.95). The mineralization activity was much lower in the late phase presumably due to a decline in the bioavailability of DEHP caused by diffusion-limited desorption. The initial DEHP mineralization rate in sludge-amended soil varied between 3.7 and 20.3 ng of DEHP (g dw)^-1 d^-1 depending on incubation conditions. Aerobic DEHP mineralization was 4-5 times faster than anaerobic mineralization. DEHP mineralization in sludge-amended soil was much more temperature sensitive than was DEHP mineralization in soil without sludge. Indigenous microorganisms in the sewage sludge appeared to dominate DEHP degradation in sludge-amended soil. It was estimated that >41% of the DEHP in sludge-amended soil will have escaped mineralization after 1 year. In the absence of oxygen, >68% of the DEHP will not be mineralized within 1 year. Collectively, the data suggest that a significant fraction of the DEHP in sludge-amended soils may escape mineralization under in situ conditions.