Degradation of phthalate and di-(2-ethylhexyl)phthalate by indigenous and inoculated microorganisms in sludge-amended soil

The metabolism of phthalic acid (PA) and di-(2-ethylhexyl)phthalate (DEHP) in sludge-amended agricultural soil was studied with radiotracer techniques . The initial rates of metabolism of PA and DEHP (4.1 nmol/g [dry weight]) were estimated to be 731.8 and 25.6 pmol/g (dry weight) per day, respective ly. Indigenous microorganisms assimilated 28 and 17% of the carbon in [C-14 ]PA and [C-14] DEHP, respectively, into microbial biomass. The rates of DEH P metabolism were much greater in sludge assays without soil than in assays with sludge-amended soil. Mineralization of [C-14]DEHP to (CO2)-C-14 incre ased fourfold after inoculation of sludge and soil samples with DEHP-degrad ing strain SDE 2. The elevated mineralization potential was maintained for more than 27 days. Experiments performed with strain SDE 2 suggested that t he bioavailability and mineralization of DEHP decreased substantially in th e presence of soil and sludge components. The microorganisms metabolizing P A and DEHP in sludge and sludge-amended soil were characterized by substrat e-specific radiolabelling, followed by analysis of C-14-labelled phospholip id ester-linked fatty acids (C-14-PLFAs). This assay provided a radioactive fingerprint of the organisms actively metabolizing [C-14]PA and [C-14]DEHP . The C-14-PLFA fingerprints showed that organisms with different PLEA comp ositions metabolized PA and DEHP in sludge-amended soil. In contrast, micro organisms with comparable C-14-PLFA fingerprints were found to dominate DEH P metabolism in sludge and sludge-amended soil. Our results suggested that indigenous sludge microorganisms dominated PEEP degradation in sludge-amend ed soil. Mineralization of PEEP and PA followed complex kinetics that could not be described by simple first-order equations. The initial mineralizati on activity was described by an exponential function; this was followed by a second phase that was described best by a fractional power function. In t he initial phase, the half times for PA and DEHP in sludge-amended soil wer e 2 and 58 days, respectively. In the late phase of incubation, the apparen t half times for PA and DEEP increased to 15 and 147 days, respectively. In the second phase (after more than 28 days), the half time for DEEP was 2.9 times longer in sludge-amended soil assays than in sludge assays without s oil. Experiments with radiolabelled DEHP degraders suggested that a signifi cant fraction of the (CO2)-C-14 produced in long-term degradation assays ma y have originated from turnover of labelled microbial biomass rather than m ineralization of [14C]PA Or [14C]DEHP. It was estimated that a significant amount of DEHP with poor biodegradability and extractability remains in slu dge-amended soil for extended periods of time despite the presence of micro organisms capable of degrading the compound (e.g., more than 40% of the DEH P added is not mineralized after 1 year).