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Environmental fate & pathways

Biodegradation in soil

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Studies in sludge-amended soils suggest that NPEO degrades in soil under aerobic conditions, partly mineralising and partly resulting in formation of NP. Anaerobic conditions slow down or impede breakdown in the soil. Studies on NP show that it may persist in landfills under anaerobic conditions; however, it does not appear to be persistent in soil under aerobic conditions. The half-life of NP in soil depends on parameters such as environmental conditions, isomer profile and the type of soil (Environment Canada, 2001; JRC, 2002).

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Concentrations of NP, NPE-1 and NPE-2 were followed in sludge-amended soil. The soil samples were collected from the upper 5 cm of planted grassland from an experimental plot in Liebefeld, Switzerland. This site was part of a long-term field study and had received anaerobically digested sludge at an average application rate of 13.5 tonnes/ha year (dry weight) since 1976. The sludge was applied to the surface soil as a liquid spread, 4-6 times per year. During the morning of 16 May 1986, 2.5 t/ha of sludge were spread over the experimental plot. A few hours after application, the first samples were collected. The initial concentrations of NP, NPE-1 and NPE-2 in the amended soil were 4.7, 1.1 and 0.1 mg/kg dry weight (dw), respectively. These rapidly decreased to 20% of their initial levels during the first 3 weeks. In an additional 70 days, a level was reached that remained constant during the next 130 days. The residual mean concentrations of NP, NPE-1 and NPE-2 in soil 320 days after test start were 0.5, 0.12 and 0.01 mg/kg dw, respectively (Marcomini et al.,1989).

The mineralization of 14C-labelled NPE-2 was investigated in different domestic wastewater treatment plant sludge-soil mixtures and soils (coarse-sandy / sandy / clayey) collected in Denmark. Experiments were conducted with 6 g wet weight samples in glass tubes maintained at 40 – 80% water holding capacity (WHC) and 15°C in the dark for 2 months. The presence of oxygen was the most important parameter determining mineralisation of the substances. NP and NPE-2 were rapidly mineralised in aerobic sludge-soil mixtures while only a minor part was mineralised in sludge dominated by anaerobic conditions. Mineralization of NP and NPE-2 was not affected by the soil type since the percentage of compound mineralized (49.7 – 63.7 and 55.2 – 64.4%, respectively) after two months was not different between any of the test mixtures (Gejlsbjerg et al., 2001).

The degradation of an NPE-1/NPE-2 mixture (2, 60 and 308 mg/L) in landfilled sludge receiving anaerobic digestor sludge from a pulp plant and landfilled municipal solid waste (Sweden) was determined under methanogenic conditions for 150 days. The inocula were incubated in 123 ml bottles at 30 or 37°C. In both inocula, at a concentration of 2 mg/L, the added NPE-1/NPE-2 was transformed to NP by anaerobic microorganisms. The background level of NP in the landfilled municipal solid waste was so high that a transformation of NPE-1/NPE-2 only increased the indigenous NP concentration by 5-10%. Significant decrease of NPE-1 and NPE-2 was observed within 22 days. An increase to 81% during 53 days was observed in samples with landfilled sludge. At a concentration of 60 mg/L NPE-1/NPE-2, approximately 20% NP was formed during 40 (landfilled municipal solid waste) and 80 days (landfilled sludge). The concentration of formed NP remained constant until Day 150. At 308 mg/L, less than 1% of the added NPE-1/NPE-2 was transformed into NP (Ejlertsson et al., 1999).

Microcosm experiments were conducted to evaluate the mineralization of 14C-labelled NP, NPE-4 and NPE-9 in a soil/biosolids (99.5:0.5 w/w) environment planted with crested wheatgrass (Agropyron crestatum). The column microcosms (7.5 x 36 cm) were located in a greenhouse with a 18:6-h light:dark photoperiod and a day/night temperature of 20±1/16±1°C. Soil moisture was maintained at 60 – 80% of field capacity. Experimental duration was 150 days. The biosolids, containing approximately 1,000 mg/kg NP, were obtained from a treatment plant receiving industrial, commercial and domestic wastewater. Three inital nominal concentrations (6, 24, 47 mg/kg dw) of NP, NPE-4 and NPE-9 were tested. Unplanted and soil-poisoned controls were run in parallel. In viable soils, 6-10% of NP, 12-29% of NPE-4 and 17-28% of NPE-9 mineralized to14CO2 within 150 days. The final percent of mineralisation was not significantly affected by difference in the initial concentrations of the substances in the soil/biosolids mixture. No statistical difference was shown between planted and unplanted systems (Dettenmaier E and Doucette WJ, 2007).