Environ. Sci. Technol. 1994, 28, 1260-1267
Uptake of Chlorobenzenes by Carrots from Spiked and Sewage Sludge-Amended Soil Min-Jian Wang and Kevin C. Jones’
Institute of Environmental and Biological Sciences, Lancaster University, Lancaster, LA1 4YQ, U.K. ~~
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Uptake of chlorobenzenes (CBs) by carrots grown in soil treated with different rates of sewage sludge or spiked with the compounds was investigated. Sludge-amended soil tended to retain more CBs than spiked soil. A relatively lower rate sludge application (19.4 t ha-1, dry wt) resulted in a higher proportion of “residual” CBs in soil and a higher proportion being taken up by carrots. Both carrot foliage and roots took up CBs from all the soil treatments. There was no evidence of significant transport of CBs between carrot roots and tops. Off-take of CBs in carrot foliage was affected by the yields of the aboveground parts, implyingthat the better growing leaves might take up CBs more effectively. Off-take of CBs by roots increased with soil CB concentrations. Penetration of CBs through the carrot peel to the core was restricted, except for DCBs, and the resistance increased with the CB molecular weight. Generally, among the CBs, tetrachlorobenzenes (TeCBs) were taken up most effectively by carrot foliage and root peel, while trichlorobenzenes (TCBs) were taken up most effectively by carrot cores. However, uptake of CBs by carrots only accounted for a very small proportion of CBs applied into soil.
Introduction As part of a larger project investigating the significance of organic chemicals in sewage sludges applied to agricultural soils, this study is concerned with the transfer of one class of compounds-the chlorobenzenes (CBs)-into a crop plant. CBs are of interest in this context, because they are ubiquitous in sludge and possess a broad range of physicochemical properties that may enable various fate mechanisms in soils and uptake pathways to plants to be investigated. As discussed previously (I), chlorobenzenes (CBs) are lipophilic and volatile compounds, which can be taken up by plants via both root and foliar pathways (2-4). Most of the work on plant uptake of CBs has been done using compound-spiked soils (5,6). Overcash et al. (7)reported that among the priority pollutants tested, monochlorobenzene (MCB) and 1,4-dichlorobenzene (DCB) gave the highest bioconcentration factors (BCFs) into crops grown in sludge-amended soil spiked with 14C-labeled compounds, but they did not verify the intact compounds. Furthermore, the plant uptake of “native” CBs added to soil in sewage sludge may be different from that spiked to soil and/or sludge. In the light of these problems, an experiment was designed to highlight (1) the probable “worst case” of plant uptake of CBs concerning the potential human exposure to a food crop, (2) the significance of plant uptake as a fate of CBs applied to soil in sewage sludge; (3) the possible differences between the compound-spiked soil and sludge-amended soil; and (4)
differences between CB compounds in their transfer into the above- and below-ground portions of a crop plant. A root crop growing in a sandy soil that had received fresh applications of sewage sludge was considered as representative of the probable worst case. Carrots (Daucus carota) were chosen as the test plant since they have a high lipid content and oil channels in the root, which have been reported to give greater potential for the uptake of nonpolar chemicals (8). After soil treatment with pentachloronitrobenzene, carrots had the highest concentrations of hexachlorobenzene (HCB) among the seven crops tested in an experiment (9). In a study of plant uptake of sludge-borne PCBs by three crops, only carrot peels were contaminated with measureable amounts of PCBs (IO). Using a laboratory-spiked soil system for determining separately the uptake of 14C-labeledHCB by plant roots and by leaves after volatilization, both carrot roots and leaves gave the highest bioaccumulation factors in six test plants (11). A sandysoilwithaloworganicmattercontent was selected for this study, since organic chemicals are more available for volatilization and plant uptake from this type of soil compared to the more absorbent loam or peat soils (12,13). Volatilization of organics can result in plant foliar uptake. Sewage sludge was applied at relatively lower and high rates separately to investigate its potential influence under “routine” and extreme situations. Spiked soil was also tested for the purpose of comparison.
* Address correspondence to this author; e-mail address:
[email protected].
Materials and Methods Soil samples were collected adjacent to the Woburn Market Garden Experiment managed by the Rothamsted Experimental Station. The soil was very similar to that used for previous work (14,15),taken from an area which has been covered with grass for many years and has never received sewage sludge or any other organic amendment. After collection, the soil was air-dried, sieved through a 2-mm mesh size, and thoroughly mixed. Sewagesludge was collected from the same sewageworks mentioned previously (15),which serves both a municipal and an industrial catchment. The dry matter content of the liquid sludge was only 3 5% ,of which 54 3’ 6 was organic matter. After being centrifuged, the dry matter content was increased to 10.75% . The centrifuged sewage sludge was also mixed thoroughly and freshly analyzed using the method described previously (16). Plastic pots (20-Lvolume) were prepared for the experiment by lining with aluminum foil, placing 1 kg of gravel in the pot bottom, and overlying with 1 kg of sand to aid drainage and avoid soil loss. Four soil treatments were adopted; the sludge application rates are listed in Table 1. The original organic matter content of the soil was higher than that used in previous work (15),probably because it had been covered with grass and had not been cultivated for some time. Two of the soils were applied with centrifuged sewage sludge at low (19.4 t ha-’, dry wt) and high (165 t ha-’, dry
1280 Environ. Sci. Technol., Vol. 28, No. 7, 1994
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Table 1. Soil Treatment with Sludge for Experiments
sludge applied (g kg*,dry wt)
treatment control CB standard spiked low rate sludge amended high rate sludge amended
0 0 6.35 59.5
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sludged soils were calculated on the basis of the analytical results of the sludge and the application rates. The spiked soil was treated with CB standards to make the soil CB concentrations similar to those in the high rate sludge treatment. Control soil was not amended with sludge or CB standards. For each treatment, 40 kg of soil was taken, treated, and mixed thoroughly before being divided into triplicate. The soil treated at the high rate was mixed with the wet sludge directly, while the soil for the low rate treatment was wetted with about 8 L of distilled and deionized water before the addition of sludge. For the spiked soil, a specially made stock solution in 6.7 mL of hexane was diluted with 1 L of acetone, and then the diluted solution was further diluted with 9 L of distilled and deionized water and added to the soil. To ensure the homogenity of the treatments, soils were then partially air-dried and sieved again through a 2-mm mesh. Soils were then put into the pots over the grit and sand. A standard N:P:K fertilizer was applied into all the soils (1g/kg of soil) at the time of the thorough mixing of the soils, before the treatment with sludge or CB solution. After being put into the pots, the soils were watered normally, and 1week later, carrot seeds (Daucus carota) were sown 1 cm below the soil surface in a grid pattern, covered with soil, and lightly watered. The pots were arranged in a glasshouse so that no pot was next to a pot with the same soil treatment. By thinning out the seedlings, about 20 carrots were left in each pot. During the experimental period, temperatures in the glasshouse ranged between 20 and 30 "C. Carrots were harvested after 100 days and separated into root and above-ground parts. Soil samples from all pots were taken at both the time of sowing and harvest and analyzed fresh (15). Recoveries of the CBs from soil were 82-97 % , with coefficients of variation (% CV) less than 10%. Plant yields and wet
weights were measured. The top foliage samples of the carrots were not washed during the sample preparation, but the stem bases were cleaned to remove the trace amounts of soil, Carrot roots were meticulously cleaned with distilled and deionized water. All the samples were bagged and stored at 4 "C before analytical treatment. Root samples were separated into peel (1,except for the DCBs, indicating that the CB concentrations in peels were generallyhigher than those in the corresponding soils. Some of the TCBs and TeCBs in the cores (dry wt) had BCFs of >1 too, implying that these compounds had a greater tendency to be taken up by carrot roots. The BCFs of HCB in the whole roots were 0.57-3.5 on a dry wt basis and 0.068-0.39 for fresh mass, much lower than those grown under field conditions by Smelt and Leistra (9) (12-19, dry wt) and under special laboratory conditions by Schroll and Scheunert (11) (32, fresh wt). Discussion of CB Uptake. The order of CCB concentration in the carrot foliage was as follows: the high rate sludge amended > the low rate sludge amended > the compound spiked > the control (see Table 4), but that in the carrot cores was as follows: the spiked > the high rate > the control > the low rate (see Table 5). They were totally different. If the translocation of CBs between the carrot roots and shoots were significant, the sequence would be expected to be the same or somewhat similar. Therefore, the translocation of CBs between the belowand above-ground parts appears to be seriously restricted, and the contamination of CBs in the two parts of the plant can basically be viewed as being due to root and foliar uptake separately. McCrady et al. (24) reported that translocation of TCDD from contaminated roots to shoot tissues was undetectable for both soybean and corn. Topp et al. (21) pointed out that translocation of CBs taken up by barley and cress roots to the shoots could not be determined since this pathway was greatly exceeded by foliar uptake of CBs present in air. This may also have been true in this study. CB off-take by carrot foliage correlated to the foliar yield (see Figure lc,d), supporting the role of the foliar uptake pathway. This also implies that the better growing shoots might sorb more CBs than the worse, although a dilution effect was reported to be due to an increase in plant mass (26). For carrot shoots, better growth produces more new leaves and a larger surface area, which may sorb the organic compounds more effectively. However, the concentrations of CBs in the carrot foliagewas still affected by the soil CB concentrations (see Figure la,b). When CBs volatilized from individual soils, it is possible that the foliage grown in the soils with higher CB concentrations could have been exposed to these compounds more before the CB concentrations in the air became homogeneous.
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There is no evidence of a direct impact of the sludge applications on the foliar CB content, except for the influence through the increased foliage yields. CB off-take by carrot roots correlated to soil concentrations (see Figure 2d), indicating that root uptake made the dominant contribution to the root CB content. The better growth of roots in sludged pots resulted in lower CB concentrations (see Figure 2a-c), presumably because of a "dilution effect" with increased root diameters restricting the penetration the CBs. Compared with the corresponding soil CB contents, even the concentrations of the CBs in the carrot peels were relatively low in the sludge-amended soils. The increased organic matter content of the sludge-amended soils might have made the CBs in these soils less available for root uptake. However, Overcash et al. (7)reported that plant 14Cuptake by fescue, corn, wheat, and soybeans showed no alteration with the presence of municipal sludge for 14C-labeledMCB, 1,4DCB, and di-n-butyl phthalate ester. To be taken up by carrot roots, organic chemicals have to be sorbed on the root peels first, and then they penetrate through the peels to reach the cores. Chemicals with a high tendency to sorb will presumably be held effectively, at least by the peels. Those with a low sorption tendency will not be taken up very efficiently because the first stage of the uptake would be limited. The concentrations of DCBs in the carrot cores were not lower than those in the peels, while all the other CBs had higher concentrations in the peels, showing that DCBs might penetrate through the carrot peels easily. However, DCBs were not the most abundant compounds in the carrot roots, even though all soils had the higher concentrations of DCBs than other CBs, probably because of their low sorption tendency.
Penetration of CBs, except DCBs, through the carrot peels to the cores appeared to be hindered, shown by the significant differences between their concentrations in the peels and cores. Generally, the data in Table 5 imply that the more highly chlorinated CBs are retarded more effectively by the carrot peels with the intermediate CBs, i.e., the TCBs having the highest BCFs in the root cores (see Table 5). It is also interesting that HCB and PeCB did not have the highest BCFs, even in the root peels. This is probably because they are the least mobile compounds among CBs, which may have restricted their movement from the soil to the root peel surfaces. The TeCBs gave the highest BCFs for CBs into the root peels. Significance of Plant Uptake. Obviously,both carrot shoots and roots did take up the CBs, so clearly there is the potential for human exposure to CBs from crop plants (17). However, carrot uptake makes an insignificant contribution to the total loss of CBs added to the soils. During this experiment, only0.03-0.72 % of the compounds were taken up by the carrots, which is negligible compared with the other loss pathways, principally volatilization, as discussed previously (14, 15). There was a greater loss of spiked CBs from the soil than those in the sludge-amended soils during the soil treatment procedures, although similar amounts of CBs were added to the spiked and high rate sludge-amended soils. An interesting observation is that the low rate of sludge amendment gave the smallest proportion of CB loss during soil treatment but the biggest during the carrot growth. The final soil residues of CBs in the low rate soil accounted for the biggest percentage of the total CBs among the treated soils. This implies that the potential influence of the low rate sludge application may not be Environ. Sci. Technol., Vol. 28, No. 7, 1994
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correctly predicted by the research on high rate applications. Relationships between CB BCFs and Properties. Volatilization strongly influenced the foliar uptake of CBs by the carrots because (a) CBs had to evaporate from soils before being sorbed by the leaves and (b) CBs might revolatilize from the foliage to the atmosphere. Figure 3a gives the correlation between averageBCFs of carrot foliage and the volatization potential [log(V,/K,,); (1, 15)1, showing that the CB BCF into the foliage generally decreased with increasing log(Vp/Kow),except for HCB. Presumably HCB has relatively little potential to volatilize from soils and to provide sufficient compound in the air for uptake (see Figure 3a). In fact, only 5.9-21% of HCB was lost from the soils during carrot growth, while between 24 and 41 % of the CCBs volatilized (see Table 2). This observation is consistent with that made in a detailed study of CB fate in soils (14,15).Figure 3a suggests that foliar uptake may occur most effectively for the “intermediate” CBs. Compounds with high volatilization potential may also revolatilize from foliar surfaces. Those with a very low volatilizationpotential [e.g., log(V,,/Kow)value of
