Journal of Environmental Quality 30:1266-1270 (2001)
© 2001 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
TECHNICAL REPORT
Organic Compounds in the Environment
Influence of Plant Growth on Degradation of Linear Alkylbenzene Sulfonate in Sludge-Amended Soil
G.K. Mortensen*,a,
H. Egsgaardb,
P. Ambusb,
E.S. Jensenb and
C. Grønc
a Risø National Lab., Plant Biology and Biogeochemistry Dep., Building PBK-124, Post Office Box 49, DK-4000 Roskilde, Denmarkl
b Dep. of Agricultural Sciences, The Royal Veterinary and Agricultural Univ., Agrovej 10, DK-2630 Taastrup, Denmark
c DHI-Water and Environment, Agern Allé 11, DK-2970 Hørsholm, Denmark
* Corresponding author (gerda.krog.mortensen{at}risoe.dk)
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ABSTRACT
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Widespread application of sewage sludge to agricultural soils in Denmark has led to concern about the possible accumulation and effects of linear alkylbenzene sulfonate (LAS) in the soil ecosystem. Therefore, we have studied the uptake and degradation of LAS in greenhouse pot experiments. Sewage sludge was incorporated into a sandy soil to give a range from very low to very high applications (0.4 to 90 Mg dry wt. ha-1). In addition, LAS was added as water solutions. The soil was transferred to pots and sown with barley (Hordeum vulgare L. cv. Apex), rape (Brassica napus L. cv. Hyola 401), or carrot (Daucus carota L.). Also, plant-free controls were established. For all additions there was no plant uptake above the detection limit at 0.5 mg LAS kg-1 d.w, but plant growth stimulated the degradation. With a growth period of 30 d, LAS concentrations in soil from pots with rape had dropped from 27 to 1.4 mg kg-1 dry wt., but in plant-free pots the concentration decreased only to 2.4 mg kg-1 dry wt. When LAS was added as a spike, the final concentration in soil from planted pots was 0.7 mg kg-1 dry wt., but in pots without plants the final concentration was much higher (2.5 mg kg-1 dry wt.). During degradation, the relative fraction of homologues C10, C11, and C12 decreased, while C13 increased.
Abbreviations: LAS, linear alkylbenzene sulfonate
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INTRODUCTION
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SEWAGE SLUDGE is used in agriculture as a fertilizer and an organic amendment to improve physical and chemical soil properties. In addition, this application is an ecologically sound way to dispose of the sludge compared with burning or landfill disposal. In Denmark, 70% of sewage sludge was recycled to agriculture in 1993 (Tørslev et al., 1997). However, sludge contains high concentrations of organic compounds, such as surfactants, where concentrations of LAS in anaerobically digested sludge are from 1.3 to 30 g kg-1 dry wt., and in aerobically stabilized sludge are from 0.1 to 0.5 g kg-1 dry wt. (McAvoy et al., 1993). Because of these high concentrations and the use of sewage sludge in agriculture, the Danish regulation for the application of waste products has introduced provisional maximum contaminant levels (PMCLs) for organic contaminants in sludge for application on agricultural land (Ministry of Environment and Energy, 1996).
Linear alkylbenzene sulfonates are a group of anionic surfactants, which are widely used in detergent products. Linear alkylbenzene sulfonate is a mixture of homologues with linear alkyl chains in the range C10 to C13. In addition, all conceivable isomers are present with the exception of the terminal isomer (C1). During wastewater treatment there is a shift in the relative homologue composition toward higher homologues and more external isomers in the sludge compared with the commercial products (Prats et al., 1993). The commercial LAS is primary the Na salt, but occurrence of Ca and Mg salts in the environment might cause precipitation of Ca and Mg salts, which have lower bioavailability and thereby influence the toxicity and biodegradation (Berna et al., 1989; de Wolf and Feijtel, 1998).
It has been shown that LAS was readily degradable in aerobic, aqueous systems with half-lives of a few days and with preferential biodegradation of the longer alkyl chain LAS homologues and more external isomers (Terzic et al., 1992). Half-lives of a few days were also determined in aerobic river water and sediments, but showed little variation among different homologues and isomers (Larson, 1990). When sludge is added to agricultural soils, a complex system is established with a mixture of LAS homologues and isomers, and a large amount of organic material of varying composition and different minerals contained in both the sludge and the soil. Both the chemical composition and mineralogy of the soil and the application method of LAS (for example, in sewage sludge) influence the bioavailability (Wolf and Feijtel, 1998).
Sorption and degradation are important processes in relation to the fate of LAS in sludge-amended soils where higher affinity for the environmental matrix lowers the bioavailability for degradation (Knaebel et al., 1994, 1996). Also, the characteristics of the chemical are important. For a mixed chemical like LAS, sorption studies have shown increased adsorption with increased chain lengths and more external isomers (Hand and Williams, 1987). Adsorption and desorption studies have shown the importance of both organic material and clay minerals on the sorption processes (Fytianos et al., 1998; Matthijs and De Henau, 1985; Ou et al., 1996). An experiment with degradation of LAS in spiked soils showed half-lives of 2 d with pretreatments like wetting and drying the soil, stimulating LAS mineralization, but the ultimate fate of LAS was not observed in this study (Knaebel et al., 1990). In sludge-amended soils, half-lives of 7 to 22 d for biodegradation have been reported, but although the first degradation was fast, the residual concentrations were 70 to 99% of the initial concentrations (Holt et al., 1989; Ward and Larson, 1989; Comellas et al., 1993).
At present, however, knowledge is still lacking of the degradation of many organic chemicals in the heterogenous and complex mixtures resulting from mixing sludge and soil to allow reliable risk assessments. The objective of this study was to provide more accurate data for risk assessment of LAS, including plant uptake and realistic degradation rates, especially in relation to plant growth and the effect of application with sludge or as a spike. Further, the chemical composition changes of LAS in the soil–sludge–plant system were studied.
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MATERIALS AND METHODS
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Soil and Sludge
The soil used in the experiments was a sandy soil from Jyndevad, an experimental station located in southwestern Jutland, Denmark. Since 1987, the soil has been kept in an organic rotation and has consequently not received any pesticides, inorganic fertilizers (nitrogen, phosphorous, and potassium), sludge, or other domestic waste products. The soil was taken from the upper 20 cm. Selected soil characteristics are shown in Table 1.
Anaerobic digested sludge from a sewage treatment plant near Copenhagen was used in the experiments. The plant receives wastewater from households and small industries. The sludge was collected immediately before the start of the three experiments and the concentrations of LAS were between 3.7 and 5.1 g kg-1 dry wt.
Soil–Sludge Mixtures
Before the experiments, N, P, K, Mg, Cu, and Mn were added to the soil to avoid deficiency of these elements. In order to obtain homogeneous mixtures, the soil was air-dried and sieved through a 5-mm sieve before addition of sludge. A realistic average sludge application in Denmark is about 4 Mg ha-1 dry wt., but sludge applications of 6 Mg sludge ha-1 dry wt. have been used as a maximum value (Tørslev et al., 1997). Therefore, the sludge additions were factors of four up and down this dosage—about 0.4 to 90 Mg sludge ha-1 dry wt. for the first two experiments, but 10 Mg ha-1 dry wt. for the third experiment, which is the maximum allowable dosage per year as a mean during 10 yr. For each addition, soil and sludge were mixed in a blender, which resulted in reasonably homogenous mixtures.
Greenhouse Experiments
The experiments were performed in a greenhouse under controlled environmental conditions. The soil water content was kept at 60 to 75% of the water holding capacity of the soil. To avoid dry deposition onto leaf surfaces from the air, the pots were shielded by a glass plate above the plants and by rinsed cotton linen at the sides. There was no drainage from the pots.
Experiment 1
Barley was sown in glass pots with a diameter of 25 cm. Foil was wrapped around the pots to protect the soil and roots from light. The pots contained 3.5 kg of the soil–sludge mixture and 27 seeds were sown in each pot. Table 2 shows the experimental setup. In addition to the homogenous mixtures, sludge equal to the highest addition (90 Mg ha-1 dry wt.) was added to two pots, but in a layer with soil above and below the sludge. The growth period was 19 d.
Experiment 2
Carrot seeds were sown in stainless steel pots (diameter 25 cm) containing 20 kg of the soil–sludge mixture. Thirty seeds were sown in each pot, but after 10 d the number of plants was reduced to 10. The experimental setup is shown in Table 2. The growth period was 85 d.
Experiment 3
Rape was sown in glass pots with a diameter of 25 cm. Foil was wrapped around the pots to protect the soil and roots from light. The pots contained 3.5 kg of the soil–sludge mixture and 49 seeds were sown in each pot. Table 3 shows the experimental setup. In addition to the sludge mixtures, LAS was added as water solutions in order to get a concentration comparable to the addition with 10 Mg sludge ha-1 dry wt. The technical product Isorchem (Condea Augusta, Padermo Dugnano, Italy) was used to spike the soil. The growth period was 30 d.
Sampling and Analytical Methods
Green plants were cut 1 cm above the soil surface. The remaining soil in the pots was thoroughly mixed and sampled for analysis. From Experiments 1 and 2, soil from duplicate pots with plants was mixed and results are reported as the mean of two determinations. There was only one plant-free pot as a control. In the experiment with rape, three pots with plants and three plant-free pots were set up in each addition. Results are reported as the mean of the three determinations.
Linear alkylbenzene sulfonate was analyzed using a method that was developed for soil analyses (Matthijs and De Henau, 1987), but modified especially for analyzing plant material. The modified method consists of two soxhlet extractions, first with pure water, and then extraction with methanol (super gradient purity). About 5 g of soil or plant material and 225 mL of each solvent was used. The extracts were cleaned using Supelco (Bellefonte, PA) SPE columns: C8 (0.5 g, 6 mL) and SAX (0.5 g, 3 mL). The water extract was cleaned using C8, SAX, and finally C8. The methanol extract was cleaned using SAX and C8. The final extracts were mixed, concentrated and analyzed using high performance liquid chromatography (HPLC) with fluorescence detection (Em: 225 nm; Ex: 295 nm). A Supelcosil LC-8DB column (25 cm x 4.6 mm, 5 µm) and a gradient of water and acetonitrile, super gradient purity (B), was used with a flow at 1 mL min-1. The gradient program was: B, 30%; 10 min followed by a change in B to 99% during 40 min. The different homologues, but not the isomers, were separated. Six samples were analyzed in one series together with a blank, one double determination, and one control sample spiked with LAS to a total LAS concentration of 2 mg kg-1. From the experiment with rape the coefficient of variation (CV) was 7.7% and the recovery 99.1% with a standard deviation of 13% (14 determinations). The detection limits were 0.5 mg kg-1 dry wt. for plant material and 0.2 mg kg-1 dry wt. for soil samples.
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RESULTS
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Concentrations in Soil in Relation to Sludge Additions
In the experiments with barley and carrot, different amounts of sludge were mixed into the soil as described in Table 2. Two sludge samples with different concentrations were used for the two experiments. The initial concentrations measured in the soil at the start and the concentrations at harvest for the two experiments are shown in Table 4.
Barley was grown for 19 d, and after such a short growth period, there were still considerable amounts of LAS in the soil. The concentrations in the soil at harvest were in the range of 18 to 58% of the initial concentrations. The concentration at harvest was higher in the soil where sludge was added in lumps (99 mg kg-1 dry wt.) in comparison with soil, where homogenous mixtures were made (47 mg kg-1 dry wt.). Compared with the experiment with barley, the experiment with carrot shows results with a longer growth period. After 85 d, LAS still remained in the soil from pots with sludge additions above 1.5 Mg sludge ha-1 dry wt., although the concentrations were lower (3.3 and 4.7% of the initial concentrations were still in the soil with additions of 6 and 23 Mg sludge ha-1 dry wt., respectively). In the pots with the highest sludge addition, water saturated conditions arose and partially anaerobic conditions developed.
Concentrations in Soil in Relation to Plant Growth
In all experiments, plant-free controls were established, and in the experiment with rape, an equal number of pots with plants and plant-free pots were set up. In this experiment, LAS was added as spike solutions, also. The measured initial concentrations and the concentrations at harvest are given in Table 5. In all experiments both with sludge and with LAS added as a spike there was a general trend showing that plant growth stimulated the degradation of LAS. In the experiment with rape, where three replicates were established, a Student's t-test (p = 0.05) showed a significant difference between the soil concentrations at harvest.
Distribution of Homologues
The total LAS concentrations were calculated as the sum of the homologues C10, C11, C12, and C13. Therefore, it was possible to follow the changes in the distribution of the homologues. The contents of the homologues in the technical product Isorchem used in the spike experiments differed from that in the sludge used with a higher content of especially C13 in the sludge (38%) compared with the technical product (21%). The changes in the concentrations of the homologues seen in the experiment with rape are shown in Fig. 1 and 2.
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Fig. 1. Distribution of homologues in soil at the start of the experiment and at harvest. Sludge amendment = 10 Mg ha-1 dry wt.
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Fig. 2. Distribution of homologues in soil at the start of the experiment and at harvest. The spike addition is comparable with 10 Mg ha-1 dry wt.
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At the harvest of rape from sludge-amended soil, after 30 d, the concentrations showed a relative decrease in C10, C11, and C12 and an increase for C13, from 35 to 64%. In the pots without rape, the same trend could be seen but not to the same extent. The changes in relative contributions of the homologues in the spiked soils were not so obvious as in sludge-amended soils, but still the same trend with a relative decrease of C10 and C11 and a relative increase of C13 could be observed in the soil with plant growth.
Plant Uptake
The plant uptake of LAS has previously been comprehensively discussed (Grøn et al., 2000). In spite of sludge additions from 0.4 to 90 Mg ha-1 dry wt. and additions of LAS as water solutions with concentrations comparable with the highest sludge application, concentrations in barley and carrot did not exceed the detection limit at 0.5 mg kg-1 dry wt. Similarly, LAS in rape plants was below the detection limit for both sludge application and for LAS added as a spike.
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DISCUSSION
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Plant analyses of LAS in barley, rape, and carrot showed no plant uptake for all three species, neither with sludge applied to soil, with LAS added as a spike solution, nor with barley roots penetrating a sludge layer, as seen with the "lump" setup. In earlier experiments with radiolabeled LAS in growth chambers (Figge and Schöberl, 1989), about 6% of the radiolabeled added LAS to sludge was found in plant biomass, distributed both in the green plant parts of grass, radish, and potato tops as well as in potato tubers, but there was no distinction between parent compound and metabolites. Consequently, plant uptake and translocation of LAS could not be the reason for enhanced removal of LAS from the soil, but an uptake of LAS metabolites could not be excluded. The initial concentrations in this study were comparable with the concentrations in the study of Figge and Schöberl (1989) at 16.2 and 27.2 mg kg-1 dry wt. In both studies these concentrations had no adverse effect on the biomass yield.
The importance of rhizosphere microbial communities for the mineralization of LAS has been investigated in mineralization chambers (Knaebel and Vestal, 1992), where experiments were carried out with corn and soybean. For both species the rhizosphere treatments increased the initial rates of mineralization by a factor of 1.1 to 1.9. Although the presence of rhizosphere microbial populations increased the rate of degradation, no effects were seen in the yields of 14CO2, indicating no effect upon the total mineralization. The soil microbial community was larger and more active in the presence of a rhizosphere. In the present study we have also observed that plant growth stimulated the degradation when LAS was added to the soil as a component in sludge. In the experiment with rape, the concentration in soil from plant-free pots was 8.9% of the initial concentration (27 mg kg-1 dry wt.), while the concentration in soil from pots planted with rape was only 5.2% of the initial concentration. When LAS was added as a spike solution, and therefore in a more available form to the plant and microorganisms, the concentration at harvest was lower and only 2.6% remained in pots with plants, compared with 9.3% in plant-free pots. The effect of plants on the biodegradation of organic compounds applied to soil shows that vegetation may be an important factor influencing biological remediation of contaminated soils. The present experiments also show a greater degradation of LAS, when homogenous soil–sludge mixtures were made compared with soil, where sludge was added in lumps.
Adsorption and biodegradation in soil are important removal mechanisms and the interactions between LAS and the soil compartment may reduce the bioavailability for degradation.
Adsorption experiments with sediments by Hand and Williams (1987) showed an increased adsorption with increasing alkyl chain length and also an increased adsorption when the phenyl position approached the end of the chain. They showed that the sorption partition coefficient increased by a factor of 2.8 for each additional methylene group, in good agreement with the octanol–water partitioning coefficients. Already during the waste treatment there is a shift in the distribution of the homologues of the LAS in sludge compared with the technical products used (Prats et al., 1993). When sludge is applied to agricultural soil, adsorption of LAS to the soil compartments will reduce the availability for degradation. As the strength of adsorption is highest for the homologues with the largest alkyl chain lengths, the shift in alkyl chain lengths observed in this study is in good agreement with these conditions. The change in homologue distribution is a continuous process, where C13 in sludge increased from 35 to 46% after 19 d (barley experiment), 64% after 30 d (rape experiment), and 68% after 85 d (carrot experiment), all results from pots with plants. Without plants, the LAS degradation was slower and at the same time C13 increased but not to the same extent (41, 58, and 53%, respectively). When LAS was added in a more available form as water solutions, and without addition of the complex sludge matrix with a high organic content, the same trend was observed but not to the same extent.
Berna et al. (1989) observed that LAS elimination correlated with water hardness, which is related to the content of calcium and magnesium. These Ca and Mg salts may have a lower bioavailability and hence influence the biodegradation. The sandy soil used in the present study has low concentrations of calcium and magnesium compared with the more clayish soils in other parts of Denmark.
If the varying degree of degradation observed in this study is considered a measure of bioavailability, the distribution of LAS among a sludge–organic particle bound pool and a "free" pool will also affect the real exposure to LAS in the soil. Also, the exposure concentration will vary with time, as there will be periods with high LAS concentrations immediately after application of sludge to the soil. The concentrations will, however, decrease during the first few weeks. Also, the fact that the different homologues are degraded to a different extent might induce a shift in specific toxicity.
The present study has shown that LAS from sewage sludge applied to agricultural soils is not taken up by plants, and that the presence of crop plants increased the microbial degradation of LAS. During the degradation the relative content of C13 increased, while C10 and C11 decreased due to the greater adsorption of higher homologues to the soil. When LAS was added as spike solutions, the degradation was greater than when LAS was applied in sludge. The degradation of LAS in homogenous soil–sludge mixtures was faster than when sludge was added in a layer of sludge lumps.
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ACKNOWLEDGMENTS
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This work was supported by the Danish Environmental Protection Agency and by the Centre for Sustainable Land Use and Management of Contaminants, Carbon and Nitrogen under the Danish Environmental Research Programme.
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REFERENCES
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