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Environmental fate & pathways

Biodegradation in soil

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Description of key information

Results of simulation studies for degradation of DEHP are highly variabale. A very conservative value for DT50 of 300 days at 12 °C is used for chemical safety assessment. Overall data and especially more recent ones argue however

Key value for chemical safety assessment

Half-life in soil:
300 d
at the temperature of:
12 °C

Additional information

The results from studies on the degradation rate of DEHP in soil are very variable and are difficult to interpret. The range is from 92% degraded after 30 days at 30°C to 3% mineralisation after 100 days at 20°C. Since the main degradation product MEHP is of toxicological relevance it is not appropriate to calculate the environmental half-life based on primary degradation rates.

The two mineralization studies from Madsen et al. 1999 and Gejlsberg et al. 2001 are considered as reliable studies with acceptable restrictions. It monitored the biodegradation of DEHP in soil and sludge ameneded soil in foreseeable natural conditions. Therefore these references are used as key studies.

- Madsen et al., 1999, studied the mineralisation of DEHP in a sandy loam soil with and without amendment of sewage sludge. DEHP and [ring-UL- 14C] DEHP was added to the soils to concentrations of 1.6, 3.2, 9.9 or 35.1 mg DEHP/kg dwt. Soil and sludge amended soil (soil sludge ratio 58:1 (dwt: dwt)) was incubated in the dark at 5,10 or 20°C under aerobic conditions. The water content of the soils was held at 75% of the field capacity. The DEHP mineralisation in sludge-amended soil under anaerobic conditions at 20°C was also studied.

Mineralisation of DEHP could be divided into two distinct phases: an initial phase that followed first order kinetics and a slower second phase that was better described by fractional power kinetics. Mineralisation did not reach 50% during the initial phase in any of the measurements. The authors of the study therefore suggest that the effective half time should be based on the kinetics of the second phase.

Authors therefore highlighted that mineralization of DEHP in soil and sludge-amended soil was regulated strongly by temperature: The half times for mineralisation of DEHP in soil was 224 days at 5°C, 187 days at 10°C and 73 days at 20°C.

The half times for mineralisation of DEHP in sludge amended soil was >> 365 days at 5°C, 337 days at 10°C and 150 days at 20°C.

Moreover, mineralisation of DEHP increased with increasing initial DEHP concentrations in sludge-amended soil. The half time and the duration of the two phases were however not affected by the DEHP concentration.

At least, like demonstrated in water and sediment, mineralization of DEHP at 20°C in sludge-amended soil was strongly limited with oxygen conditions (approximately 5 times slower in anaerobiosis than in aerobiosis) and was comparable to the aerobic mineralisation at 5°C.

At long term incubation (>91days) the anaerobic mineralization decreased to very low level (T0.5 >>365 days).

- Gejlsbjerg et al. (2001) compared the mineralization of uniformly ring-labelled 14C-DEHP with both sludge amended soil (sludge: soil ratio 1:20 (dwt: dwt) or 1:100 (dwt: dwt)) and unamended soil considering the water content (40 or 80% of the water holding capacity).

A coarse sandy soil was used for these experiments. The soil samples were incubated in the dark at 15°C for two months. In addition the effects of soil type on the mineralisation was studied using the coarse sandy soil and two other soils: a sandy soil and a more clayey soil.

The mineralisation of DEHP in the coarse soil during two months amounted to approximately 20% of added 14C in all treatments regardless of sludge: soil ratio and water content. The mineralisation in the two other soils was lower and was 5.8 and 6.8% of added 14C in sludge amended soil and 8.5 and 9.4 added 14C in unamended soil. This indicate that bioavailability due to adsorption on various types of minerals affect the biodegradation of DEHP in soils.

All other available data indicate variable per cent of degradation in soils from negligible at low temperature to 58% at room temperature (Dörfler et al. 1996) and from half lifes of 95 days (Schmitzer et al. 1988), or from 14 days to more than 200 days (Rudel et al 1993).

More recently, new field studies were published. The most relevant ones were performed on long term exposure and under conditions of composting to estimate the impact when used for fertilization application.

Peterson et al. (2003) studied the behavior of DEHP during fertilization process (see section 5.2.4 in iuclid) and potential effect on plant by 3 different methods under field and greenhouse conditions.

The first experiment is a 3-year field study using different types of soils: a sandy loam and a loamy sand. Both were supplemented with 4 types of wastes: Two sewage sludges (SS) from a municipal WWTP, a household compost from a municipal composting facility containing kitchen wastes mixed with shredded straw. (compost) and a solid pig manure from a local pig operation (manure).

The wastes were amended once a year during 3 years. Assuming no biodegradation would have occurred, according to the input the total amount at the end of the study would have been 0.238, 0.092, 0.290 and 0.04 mg/kg in SShigh, SSlow, compost and Manure trials respectively. However, after 3 years, actual DEHP concentration ranged from below detection limit (<0.05mg/kg) to 0.103 mg/kg without significant differences among the treatment applied.

The second experiment was a 1 year experiment consisting in a plot experiment with banded sewage sludges. Sludge turnover and toxicity to crop was studied in details under semi-realistic conditions with the loamy sand soil. Each treatment was controlled for the DEHP concentration in the sludge soil matrix.

The regular sampling and analysis of DEHP content showed a degradation of DEHP slow during the 6 first weeks, and approximately 40% of the initial concentration was still present after 6 months. Degradation continued leaving 5 to 6% of initial DEHP at the end of the 12 months. The related half-life can reasonably be assumed to be below 6 months (i.e. about 90 days)

In another study Gibson et al. (2007) (see section 5.2.4 in iuclid) conducted a pilot-scale composting of sludge containing DEHP during 143 days in exterior (UK) from march to July. The experiment was started with a mix of mesophilic anaerobically digested dewatered sludge cake and dry wheat straw in a 10:1 ratio thoroughly mixed. The final heap was turned weekly at the beginning of compositing, then reduced to every two weeks and stopped at the final stages. During the compositing process the concentration in DEHP decreased from 43.4 mg/kg ash on day 7 to less than 13.15 mg/kg ash on day 143 which correspond to a total loss of 64% of the initial content and when fitted with first order kinetics, a half-life of 94 days.

These experiments clearly show that under environmental conditions and conditions relevant for actual use, DEHP can be removed from soil in aerobic/anaerobic conditions with a half-life of around 90 days.

These conclusions are supported by other studies (Cheng et al. 2008 in section 5.2.4; Chang et al. 2009 in section 5.2.3; Amir et al. 2005 in section 5.2.4) showing even lower half–lives. However, conditions in these studies may have been more favourable for biodegradation compared to the previous studies.

Chang et al. (2009) studied the biodegradation of DEHP and DBP in two types of compost or in compost amended soil. Sludge was spiked with test items in combination or alone and mixed with mushroom-straw mixture or animal manure.

For each of these conditions biodegradation was followed in batch experiment and bioreactors under various condition of pH, temperature, soil-to-compost ratios and different concentration in DEHP and DBP (50, 100, 200 mg/kg alone or in combination).

For DEHP in combination with DBP, half-lives of DEHP in soil alone were varying from 69 days at 5°C and pH 7 to 3.5 days at 30°C and pH7. For DEHP alone in soil, half-life of DEHP was 7.7 days at 30°C and pH 7. For DEHP in soil amended with compost, half-lives of DEHP were varying from 6.9 to 4.6 days.

Degradation of DEHP and organic matter of sewage sludge by composting was investigated by Cheng et al. (2008).

After acclimation of the sludge, degradation was studied in a bioreactor under aerobic conditions and 3 different operation conditions (varying sludge/sawdust ratio).

Most of DEHP was degraded under thermophilic phase (66% for E-1, 78% for E-2 and 60% for E-3), which has persisted for 3 days. At the end of this period, DEHP concentration was reduced to less than 100 mg/kg (EU standard for the land application of DEHP containing sewage sludge).

After the second phase total degradation of DEHP varied from 85 to 88 %.

The first order and fractional power kinetics were used in this study to describe the behavior of DEHP biodegradation in thermophilic phase (including initial mesophilic phases) (Phase-I) and the phase thereafter (Phase-II), respectively.

In the test condition DEHP from contaminated sewage sludge, is degraded between 85 to 88% depending on test conditions (sludge:sawdust ratio, VS, TC and TN) when composting is applied in a bioreactor.

Therefore from results of these experiments it can be concluded that DEHP in sludge will not have adverse effects on crops growth if used as a fertilizer.

Then Amir et al. (2005) studied the fate of phthalic acid esters during composting of lagooning sludge and activated sludge. Lagooning sludge was tested in a semi-industrial trial over 180 days in a mixture with straw in a 1:0.12 ratio. Activated sludge was tested in an industrial trial over 135 days with grass in a 1:0.5 ratio. Initial concentration of DEHP in the sludge was 28.67 mg/kg dw and 6.26 mg/kg dw in lagooning and activated sludge respectively. In both case the mixtures were turned over every 15 days to ensure aerobic conditions. pH, temperature, carbon content and Phthalic acid esters content were monitored. Two phases were recorded:

- a phase of stabilization during about 30 days with maximum temperature was 52°C (day 7) in lagooning sludge compost and 72°C (day 4) in activated sludge compost.

- a phase of maturation characterized by a decrease in temperature to level ambient temperature.

The level of decomposition (based on C/N ratio) reached 41.5% and 60.8% in respective case. DEHP showed a continuous decrease during composting. In a first order of kinetic model, the kinetic constant of biodegradation (K) and half-lives of DEHP were 1.53E-2 and 45.5 days for lagooning sludge compost and 2.4E-2 and 28.9 days in activated sludge compost.

These data support the analysis made by Peterson and Staples (2003) who established from the literature a half-life in sediment and soil under anaerobiosis between 69 and 116 days and in soil under aerobic condition between 7 and 69 days.

Thus, recent studies prove that DEHP is not persistent in soil even under partially anaerobic conditions. Therefore the half-life set in the previous EU-RAR (2008) for DEHP (300 d at 12°C) is considered to be very conservative. However, taking account of the high variability observed with simulation test data for biodegradation of DEHP in soil, this conservative approach will be followed and for CSA a half-life of 300 days for agricultural soil at 12 degree C will be assumed.

Value used for CSA: Half-life in soil: 300 d at 12 °C