Planteoptag af miljøfremmede, organiske stoffer fra slam

Summary

The uptake in barley and carrot of the substances di(2-ethylhexyl)phthalate (DEHP) and linear alkylbenzenesulfonates (LAS) from sewage sludge was investigated in greenhouse experiments. The applicability of three mathematical models for estimating uptake of the substances in plants was evaluated.

Methods for conducting controlled greenhouse experiments with uptake of xenobiotics (DEHP and LAS) in young barley plants and carrots from sludge/soil mixtures have been established. The methods comprise measurements of the degradation of the substances and remaining concentrations as well as parameters regarding growth, yield and plant characteristics, which are necessary for modelling the uptake in plants of xenobiotics in plants.

The plants were grown in field soil, which was mixed uniformly with sewage sludge in varying proportions equivalent to dosages of sewage sludge of 0.4 to 90 t/ha (sludge dry weight) or 1/15 - 15 times the maximum allowed dosing of sewage sludge on agricultural soil in Denmark. The concentrations of LAS and DEHP achieved were up to approximately 230 and 9 mg/kg dry weight soil/sludge mixture, respectively. Carrots were grown to harvest (12 weeks), while barley plants were used for examining the uptake and translocation of the substances in young plants (after three weeks). Furthermore, experiments with barley plants were conducted in which the sludge was layered into the soil as clumps and experiments in which the uncontaminated soil was spiked with the pure substances before sowing. The concentrations of LAS and DEHP in soil and plant samples were measured by specific chemical analysis.

The mathematical models used, comprised the calculations recom-mended in the EU Technical Guidance Document (TGD) (EU, 1996) for assessment of human exposure to organic substances via food. These comprise a model (a simple version of the more complicated PlantX model) by which only the concentration in leaves is estimated. In the TGD, calculations are also included for estimating the concentration in roots, which are based on the assumption that there is equilibrium between the substance in the pore water of the soil and the plant tissue. Furthermore, the more complicated PlantX model (Trapp, 1995) and a model, developed in the USA (Paterson et al., 1994, Hung & Mackay, 1998), were used.

LAS were found in barley plants and DEHP was found in barley plants as well as in carrots and carrot top. In general, the content of LAS found in the plants could not be related to the concentrations in the soil/sludge mixtures and the concentrations measured were very low and close to the detection limit. The only exception from this was elevated concentrations of LAS in barley roots at the highest sludge concentrations.

Similar results were obtained for the content of DEHP in roots and stems/leaves. The concentrations in carrot leaves and roots were considerably above the detection limit. The concentrations of DEHP were highest in peel of the carrots but were not related to the concentrations in the soil/sludge mixtures.

In spite of careful cleaning of all root samples before measurements, it cannot be excluded that the measured LAS and DEHP originate from single sludge particles remaining adhered to the root samples.

In leaves/stems of barley, the DEHP content was just above the detection limit in all samples, while the content in carrot leaves was higher. The concentrations were varying independently of the DEHP concentrations in the soil/sludge mixtures. The origin of the DEHP measured in stems/leaves is therefore considered to be atmospheric deposition. No indication of a translocation of DEHP from roots to leaves could be found.

During the growth period, LAS were partly degraded in the soil/sludge mixtures but at the end of the experiments, considerable amounts were still found in samples from the highest sludge dosages. The degradation of DEHP, having a half-life in soil of approx. 50 days, was lower than that of LAS having a half-life in soil of approx. 15 days. Thus, the lack of measurable uptake of the two substances in plants was not due to degradation of the substances in the soil. However, the bioavailability of the substances measured in the soil at the end of the experiments may have been reduced and it is possible that the substances were degraded at a rate comparable to the release rate from the sludge during the experiment. Both substances were degraded more rapidly in spiked soil than when added in sewage sludge, and degradation proceeded faster in the presence of plants than in soil without plants. When sludge was administered as clumps, degradation of LAS was slower than in pots with the usual uniform mixture of sludge and soil.

In summary, it can be concluded that after administration of sludge dosages equivalent to 15 times the maximum allowed dosage in Danish agriculture, LAS is not translocated to plant shoots at concentrations above 1 mg/kg (total LAS in plant dry weight) in young barley and in carrot plants, while there is a possibility of adsorption to/uptake in the roots of barley plants (up to 20 mg/kg (total LAS in plant dry weight)). DEHP is not translocated to shoots of young barley plants at concentrations above 1 mg/kg (DEHP in plant dry weight), not to concentrations above 10 mg/kg in carrot plants and the adsorption to/uptake in barley roots is limited.

All the models overestimated the concentration of LAS in roots as well as in shoots/leaves. The concentration of DEHP was underestimated by the models. It should however be mentioned that the models are highly sensitive to the applied values for the sludge-soil-water partitioning coefficient, which was not measured in the present project. The concentration of DEHP in leaves was underestimated by calculations not considering uptake from the air, whereas calculations with the three mathematical models supported the theory that the DEHP found in leaves was due to uptake from the atmosphere. PlantX and Patersons & Mackays model gave comparable results.

The results of the experiments and the model calculations show that in order to improve the predictive power of the models, there is a need for separate studies of the bioavailability of the substances in soil, e.g. the sorption of the substances to sludge and to soil and soil/sludge mixtures as well as studies regarding the correlation between the rate of biodegradation and the rate of release of the substances from the sludge particles. Furthermore, there is a need for laboratory experiments to investigate parameters, characteristic of the plants; like, how much substance is actually penetrating the endodermis to reach the vascular tissues, whether substances that are taken up are metabolised in the plants and to which degree the substances are adsorbed to the outer layers of the roots.