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

Sewage sludge is frequently used as a soil fertilizer although it may conta in elevated concentrations of priority pollutants including di-(2-ethylhexy l)phthalate (DEHP). In the present study, the kinetics of microbial [C-14]D EHP mineralization was studied in laboratory microcosms with sewage sludge and agricultural soil. A biphasic model with two independent kinetic expres sions was used to fit the mineralization data. The initial mineralization a ctivity was described well by first-order kinetics (r(2) > 0.97), whereas m ineralization in long-term incubations (>40 days) was described better by f ractional power kinetics (r(2) > 0.95). The mineralization activity was muc h lower in the late phase presumably due to a decline in the bioavailabilit y of DEHP caused by diffusion-limited desorption. The initial DEHP minerali zation 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 mineraliza tion was 4-5 times faster than anaerobic mineralization. DEHP mineralizatio n in sludge-amended soil was much more temperature sensitive than was DEHP mineralization in soil without sludge. Indigenous microorganisms in the sew age sludge appeared to dominate DEHP degradation in sludge-amended soil. It was estimated that >41% of the DEHP in sludge-amended soil will have escap ed 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 miner alization under in situ conditions.