Conse-quently, over the past 20 years, governments have imposed limits either for maximum heavy-metal loads in soils or for amounts of sewage sludge and heavy metal concentrations in sew
Trang 1Heavy-Metal Uptake by Agricultural Crops from Sewage-Sludge Treated Soils of the Upper Swiss Rhine Valley and the Effect of Time
Catherine Keller, Achim Kayser, Armin Keller, and Rainer Schulin
CONTENTS
14.1 Introduction 273
14.2 Material and Methods 275
14.2.1 Geographic and Climatic Conditions at the Experimental Site 275
14.2.2 Experimental Setup and Crop Chronology 275
14.2.3 Soil and Plant Analysis 277
14.3 Results 278
14.3.1 Heavy Metal Distribution and Migration in Soil 278
14.3.1.1 Effects of Sewage Sludge Treatments on Soil Properties – Aging Effect 278
14.3.1.2 Effects of Sewage Sludge Treatments on Heavy Metal Concentrations and Binding – Aging Effect 278
14.3.1.3 Migration of Heavy Metals through the Soil Profile 279
14.3.2 Plant Uptake of Heavy Metals 281
14.3.2.1 Plant Uptake of Heavy Metals and Effects on Crop Production 281
14.3.2.2 Spatial Variability of Heavy Metal Contents in Plants 284
14.3.2.3 Changes Over Time 284
14.3.2.4 Plant-Soil Interactions: Influence of Soil Factors on Heavy Metal Uptake by Crops 285
14.4 Discussion and Conclusion 286
14.4.1 Impact of the Waste and Sludge Applications on the Soil 286
14.4.2 Impact on Plants 288
Acknowledgment 289
References 289
14.1 Introduction
Application on agricultural lands is a popular method for the disposal of sewage sludge, as
it represents at the same time a low-cost fertilizer However, if excessive loads of pollutants are introduced with the application of low-quality sludges, this practice may adversely
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affect soil fertility, threaten groundwater quality, and lead to food chain poisoning Conse-quently, over the past 20 years, governments have imposed limits either for maximum heavy-metal loads in soils or for amounts of sewage sludge and heavy metal concentrations
in sewage sludge applied to soils
In Switzerland, the first regulations concerning the use and the quality of sewage sludge were issued in 1981 (sewage sludge ordinance) and revised in 1992 (Table 14.1) Though the total amounts and heavy metals concentrations of sewage sludges have decreased consid-erably after these regulations were enforced (SFSO, 1997) (Table 14.1), mass flux analyses show that heavy metals still accumulate in agricultural soils when the tolerance limits for sludge quality and application rates are fully exploited Moreover, distribution on fields is not uniform and local areas may have received excessive loads In total, 55% of the sewage sludge produced in 1994 (4 million cubic meters) was used in agriculture, leading to yearly total addition of ca 200 t of heavy metals (nearly 10% of the total heavy metals added to these soils) (SFSO, 1997) Keller and Desaules (1997) calculated that if the maximum con-centrations allowed by the ordinance were applied at the maximum rates tolerated, sludge treated would reach the Swiss guide values for Pb and Cu within 100 years They estimated that almost 44,000 ha have concentrations above the Swiss guide values for Cu and Zn and almost 65,000 ha for Cd due to application of sludges Together with the other sources of pollution, contaminated areas could amount to as much as 200,000 ha (Häberli et al., 1991), that is, 15% of the surface used for agriculture and settlements
Considerable uncertainty exists about the long-term fate of polluting heavy metals One possibility is that the mobility and bioavailability of soil-polluting heavy metals stabilize or even decrease with time (the so-called “plateau effect”) (Dowdy et al., 1994; Smith, 1997; Brown et al., 1998) On the other hand, it is also possible that metals become more mobile, e.g., because of the mineralization of sewage sludge organic matter (“time bomb effect”) (Zhao et al., 1997) Field studies covering several decades have produced ambiguous results (Chang et al., 1997; Logan et al., 1997) and led to contradictory conclusions (Chaney
TABLE 14.1
Quantities of Heavy Metal Present in Sewage Sludge and Their Transfer to Agriculture in 1989 and
1994 and Average Heavy Metal Concentrations Measured in 1989 in Switzerland
Metal
Guide Values (g·t –1 DM)
%used in agriculture
Weighted Mean (g·t –1 DM)
Limit Values (g·t –1 DM)
From Candinas, T and A Siegenthaler, Grundlagen des Düngung: Klärschlamm und Kompost in des
SAEFL (Swiss Agency for the Environment, Forests and Landscape), The Environment in Switzerland 1997, EDMZ, Bern, Switzerland, 1997, 372; Keller, T and A Desaules, Flächenbezogene Boden-belastung mit Schw-ermetallen durch Klärschlamm, Schriftenreihe des FAL, 23, 1997 With permission.
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Trang 3Heavy-Metal Uptake by Agricultural Crops from Sewage-Sludge Treated Soils 275
and Ryan, 1993; McBride, 1995) The new USEPA (1993) regulations in the United States have induced scientists to reevaluate the results obtained from long-term field experiments and to assess the phytotoxicity and bioavailability of heavy metals added to soils through repeated applications of biosolids (McBride, 1995; Schmidt, 1997) Results of long-term experiments have recently been summarized by Berti and Jacobs (1996), Barbarick et al (1997), Miner et al (1997), Sloan et al (1997), and Zhao et al (1997) In Switzerland, Krebs
et al (1998) found that after 15 years, heavy metals extracted by 0.1 M NaNO3 (so-called
“bioavailable fraction,” OIS [1998]) increased with time in soils that had been amended between 1976 and 1984 with sewage sludge This increase was correlated with a pH decrease and raises the question of stability with time of soil characteristics and sludge residuals including the organic matter content Indeed, McBride (1995) found that soil char-acteristics and sludges’ inorganic constituents seem to exert an increasing control with time
on metal solubility
The available evidence indicates that the fate of heavy metals in soils and the associated risks may vary considerably, depending on soil properties, cultivation practices, and cli-matic factors This means that an extensive data set covering a wide range of conditions is necessary to enable predictions of the metal availability in the long term
In this chapter we present the results of an experiment which was started in 1969 In the first years, massive doses of sewage sludges from various origins were applied repeatedly
on plots of conventionally farmed arable land We were interested in the effects of these treatments on plant uptake of the polluting metals and the development of phytoavailabil-ity over time
14.2 Materials and Methods
14.2.1 Geographic and Climatic Conditions at the Experimental Site
The experimental site was located at the leveled floor of the Rhine Valley of eastern Swit-zerland The valley descends smoothly in a north-northeasterly direction and repeatedly broadens up to 12 km The climate is relatively mild, permitting productive agricultural activities Salez is situated at an altitude of 430 m Mean average temperature is 8.6°C and mean rainfall is 1300 mm with a maximum during summer (stations Vaduz and Saxerriet, respectively [SMA, 1995]) The valley bottom is covered by alluvial deposits, mainly car-bonatic clays lying on top of sand or gravel (de Quervain et al., 1963) Soils are generally rich in mineral nutrients Fluvisols and cambisols are most common and some histosols can
be found in former wetlands
14.2.2 Experimental Setup and Crop Chronology
The experimental plots were first set up in the Rhine Valley in Buchs, northeast of Switzer-land, in 1969 Parcels (four treatments, four replicates each) of soils were artificially contam-inated with heavy metals from biosolids over a period of 7 years (von Hirschheydt, 1987) Apart from controls with no waste or sludge application, treatments consisted of (a) appli-cation of composted municipal waste from a nearby incineration plant; (b) same as (a), but
in addition application of various types of highly contaminated sewage sludges; (c) same
as (b), but with a double dose of sewage sludges (Table 14.2)
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The original design was a 4 × 4 Latin square with plot sizes of 12.5 m2 In 1987, the plots were moved to their present location in Salez, approximately 15 km to the north, because the Buchs site was claimed for construction purposes (Stenz, 1995) The topsoil (25 cm depth) of each plot was translocated separately to Salez, where the experiment was re-established In addition to the soils originating from Buchs, a set of four replicate plots with local topsoil from Salez was installed The Salez soil, which has different characteristics with respect to some soil parameters, was included in the experiment, as it was also used
as subsoil in the plot setup
The experimental setup of Salez represented a fully balanced factorial design with four replicates of each of the following five “treatments” of soil and waste/sludge applications:
S Salez soil with no waste or sludge application
B Buchs soil with no waste or sludge application
BW Buchs soil with only composted municipal waste application
BWS1 Buchs soil with composted municipal waste + single dose of sewage sludge
application BWS2 Buchs soil with composted municipal waste + double dose of sewage
sludge application Plot size was 1.8 m2, totalling an experimental area of 36 m2
Between 1989 and 1993, the crops listed in Table 14.3 were grown In 1994 and 1995 the site lay fallow In 1996 beets were grown once more: this time two cultivars were tested, all plots were divided into two halves, and each half was planted with one cultivar Plots were treated uniformly with respect to fertilization and application of pesticides, regardless of the crop Until 1993 they were fertilized with NH4NO3 + Mg, Colzador, and Tresan Bor
TABLE 14.2
Composted Waste and Sludges Characteristics
a) Amounts of composted waste and sludges applied during contamination period
From von Hirschheydt, A., Zur Wirksamkeit von Schwermetallen aus Müllkomposten auf Ertrag und Zusam-mensetzung von Kulturpflanzen Teil I und II Studienreihe Abfall-Now Abfalltechnisches Labor mit Anhang
am Institut für Siedlungswasserbau, Wassergüte- und Abfallwirtschaft der Universität Stuttgart, Bandtäle 1, Stuttgart, 1987 With permission.
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Trang 5Heavy-Metal Uptake by Agricultural Crops from Sewage-Sludge Treated Soils 277
In 1996, NH4NO3 + Mg and (NH2)2OC were used for N-fertilization The herbicides used were Gesaprim® and Alipur® Ridomil-Fortex® was applied to avoid fungal infections
14.2.3 Soil and Plant Analysis
Topsoils (0 to 20 cm) were sampled in spring 1989, summer 1990, and fall 1990, 1993, and
1996 In 1989 samples from replicate plots of the same treatment were bulked on site In all other sampling campaigns, composite replicate samples were taken per plot In 1996 sam-ples were taken from one plot of each treatment every 10 cm along the soil profile UFAG Laboratories (Sursee, Switzerland) carried out soil analysis for 1989–1993; the samples of
1996 were analyzed in our lab Selected soil properties and total heavy metal contents of the topsoils are listed in Tables 14.4 and 14.5
Soil samples were oven dried at 40°C, crushed, and sieved to 2 mm with a nylon sieve Soil pH was measured in 0.01 M CaCl2 (FAC, 1989) Carbonate content was determined with a Poisson apparatus by measuring the CO2 volume produced (FAC, 1989) Organic
TABLE 14.3
Crop Rotation from 1989 to 1996
TABLE 14.4
Physical and Chemical Parameters of Soil Samples Collected in July 1990
TABLE 14.5
Total Heavy Metal Concentrations (Average of Four Replicates ± Standard Deviations) in Soil
( mg ·kg –1 )
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carbon content was determined using a modified version of the K2Cr2O7-method (UFAG, internal method) Total N was measured after Kjeldahl digestion following the DIN 19684 procedure, and total P was determined colorimetrically after smelting in KNO3/NaNO3 and digestion in boiling HNO3/H2SO4 (FAC, 1989) Cation exchange capacity and base sat-uration were determined by BaCl2-triethanolamine extraction at pH 8.1 (FAC, 1989) Iron-and Al-oxides were determined after extraction with cold (amorphous forms) or boiling (amorphous + crystallized forms) NH4-oxalate (FAC, 1989) “Total” heavy metal concentra-tions were determined in duplicate after digestion in HNO3/HCLO4/HF (Ruppert, 1987),
“pseudo-total” heavy metal concentrations with boiling 2 M HNO3 (FAC, 1989), and
“soluble” heavy metals were extracted with 0.1 M NaNO3 (FAC, 1989) The distinctions between “total,” “pseudo-total,” and “soluble” were made after Gupta et al (1996) and according to their biological relevance (Gupta and Aten, 1993)
Plant samples were rinsed thoroughly under tap water, oven dried, preground in an ultra centrifuge, and ground in an agate ball mill For heavy metal analysis 1-g samples were either oven-digested in a 1:1 mixture of boiling HNO3 (65%) and H2O2 (30%) or 0.5-g samples were microwave-digested in 2 mL HNO3 (65%), 2 mL HF (48%), and 1 mL H2O2 (30%) Flame and graphite furnace atomic absorption spectrometry (AAS) and inductively coupled plasma atomic emission spectrometry (ICP-AES) were used for the chemical analysis of extracts
14.3 Results
14.3.1 Heavy Metal Distribution and Migration in Soil
The Salez soil differs markedly in most of the investigated soil properties from the Buchs soil (control and treatments) No major differences were found on the soil properties of the Buchs plots, except for a slight increase in organic matter and a decrease in pH with increas-ing load of biosolids (B<BW<BWS1<BWS2) (Figure 14.1) While the organic carbon and carbonate contents and the texture remained constant, pH increased between 1990 and
1993 in all soils, in particular in the soils treated with compost and sewage sludge Soil pH was always lower in the soils treated with biosolids than in the controls
Binding — Aging Effect
In topsoils, 2 M HNO3 concentrations of all heavy metals increased with increasing load of biosolids Consequently, metal concentrations were highly correlated with each other For example, a strong correlation was found between total Cd and Zn concentrations (r = 0.88)
In order to assess the significance of the differences observed between treatments, system-atic replications of sampling within plots, soil extractions, and measurements were made
in 1996 and compared to the variation between treatments The coefficients of variation for
Cd and Zn are shown in Table 14.5 and Figures 14.2a and 14.2b It was about 7% for repli-cate analysis of the soluble zinc concentrations (including replirepli-cate extractions) Spatial variability was 25% between single four replicate cores within a plot Bulked soil samples showed about 28% of variation in average between replicate plots of the same treatment Although only four replicate plots were available for each treatment, treatment effects were significant in spite of this large background variability due to spatial and analytical effects: the coefficient of variation pooled for all treatments was about 90%
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Trang 7Heavy-Metal Uptake by Agricultural Crops from Sewage-Sludge Treated Soils 279
In comparison to the high total metal concentrations measured in the treated soils, the
NaNO3-extractable Cd and Zn concentrations were low, which can be attributed to the high
soil pH Again, the different metals showed high correlation, e.g., the coefficient of
correla-tion between concentracorrela-tions of NaNO3-extractable Cd and Zn was r = 0.89 Moreover, when
soluble metal concentrations from 1990 were considered, the log-transformed soluble and
total concentrations for cadmium and zinc were correlated with correlation coefficients of
0.78, resp 0.75 But these differences in NaNO3-extractable Cd and Zn concentrations were
solely due to the difference in doses added in form of biosolids because the ratio between
the NaNO3- and HNO3-extractable metal was similar for the three treatments (after
subtrac-tion of the respective concentrasubtrac-tion of the untreated Buchs soil)
Total heavy metal concentrations did not change during the whole period after the end of
the biosolids application (data not shown) But the analysis of variance revealed significant
time effects on NaNO3-extractable Cd and Zn concentrations: the “soluble” concentrations of
both elements pooled over all sewage treatments decreased significantly (P value < 0.001)
between 1990 and 93 for Cd and 90 and 96 for Zn (Figures 14.2a and 14.2b) NaNO3-extractable
Cd and Zn concentrations decreased in the same proportions in all treatments, but Zn and Cd
decreased more rapidly in sewage sludge treated soils than in the waste treatment (BW) and
controls (S and B) (Figure 14.3) Thus there was a reduction of the differences between
treat-ments with time
Figure 14.2 also shows the NaNO3-extractable Cd and Zn concentrations of the samples
from 1987: opposite to the trend described above, NaNO3-extractable Cd and Zn
concen-trations were higher in 1990 than in 1987 In 1987 the soil samples were collected just prior
to the translocation on the Salez site
Heavy metals profiles were sampled in 1996 to evaluate any possible vertical transfer As
shown by the total content, the contaminated layer was on average restricted to the first
30 cm, which corresponds to the original establishment of the plots All plots have similar low
FIGURE 14.1
Soil pH for the two controls and the three treatments in the samplings of 1990, 1993, and 1996.
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concentrations below 40 cm (Table 14.6) All metals follow the same pattern However, the
depth of the contaminated layer which was not always exactly 30 cm, combined with a
sys-tematic 10-cm sampling procedure, could explain the abrupt decrease in Cd and Zn
concen-trations along the profile of treatment BWS1 (pattern different from the other profiles)
Although NaNO3-extractable Zn concentrations decreased with depth, they were still
higher in the waste and sludge-treated soils than without these treatments Also, there was
no correlation between the total and the NaNO3-extractable Zn concentrations over depth for
the biosolids-treated soils Whereas the organic carbon content was approximately constant
over the soil profiles, the pH showed a tendency to increase with depth in all treatments in
positive correlation with decreasing NaNO3-extractable Zn concentrations (r2 = 0.62),
indicat-ing that zinc availability was controlled by pH
FIGURE 14.2
1987 are shown for comparison.
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Trang 9Heavy-Metal Uptake by Agricultural Crops from Sewage-Sludge Treated Soils 281
14.3.2 Plant Uptake of Heavy Metals
The heavy metal concentrations found in the crops generally reflected different levels of soil pollution However, variability between replicates was high in all treatments and heavy metal uptake differed greatly between plant species (Table 14.7) For the Salez soil,
Cd and Zn concentrations in plant tissues were always lowest, compared to the nontreated and treated Buchs soils
Beanstalks contained low to normal concentrations of Cd and Zn when planted on the Buchs and Salez soils, but concentrations in plant tissues increased significantly with higher levels of soil contamination The most pronounced increase was observed for Cd in the sewage sludge-treated plots BWS1 (10-fold) and BWS2 (40-fold) The concentrations did not differ significantly between plants grown on reference B and treatment BW because the municipal waste (W) did not add significant amounts of Cd to the soil Zinc concentra-tions varied less (1.3-fold and 1.8-fold increase, respectively) but the increase was still con-sistent In contrast to the stalks, concentrations of both Cd and Zn in bean pods were much lower, especially in the sewage sludge-amended soils For Zn, no response to the total con-centrations in soils was found, whereas for Cd, concon-centrations in the plant tissues increased more than 14-fold from Buchs soil to treatment BWS2, while still remaining in the range of normal content (Sauerbeck, 1989)
Like in the beans, heavy metal concentrations in maize were different in the different plant tissues studied Both Cd and Zn concentrations were higher in the leaves As in beans,
an increase was observed with higher soil heavy metal concentrations, but this effect was less pronounced (max 5-fold for Cd) Nevertheless, concentrations of both metals in all tis-sues were in a normal range
In sugar beet, concentrations of Zn and Cd were highest of all plants used in the experi-ment In the leaves, Cd content was elevated even in the reference Buchs soil and increased drastically in the BWS1 and BWS2 treatments (9-fold) The concentrations measured were well above critical levels (Sauerbeck, 1989) The same pattern, but to a lesser extent, was
FIGURE 14.3
Zn from the BWS2 treatment set to 100 The three points of each treatment correspond to the 3 years 1990, 1993, and 1996 with decreasing concentrations.
Trang 10282 Environmental Restoration of Metals–Contaminated Soils
observed for Zn in the leaves However, in the 1991 planting season, Zn concentrations showed no difference between BWS1 and BWS2, whereas in 1996 concentrations were gen-erally lower and were significantly different In sugar beet roots, Zn and Cd contents were generally much lower, but revealed the same pattern as the one seen for the leaves In 1996,
no difference in the Zn concentrations was observed, regardless of the levels of soil contam-ination This means that the plants had a lower heavy metal transfer efficiency to the leaves with increasing heavy metal in the soil
Almost the same uptake pattern was observed for potato plants In the leaves, both Cd and
Zn concentrations increased from nontreated Buchs soil to BWS2 treatment, whereas in the tubers, concentrations were both much lower and did not relate as closely to the soil treat-ment levels For Zn, an excluder-type uptake pattern was observed, as no significant change
in plant concentration was measured from Buchs to BWS2 treatment Concentrations of both metals were generally in a low to normal range in the tubers, and were elevated in the leaves
TABLE 14.6
Heavy Metals, pH, and Organic Matter Profiles Measured in October 1996 (after 7 Years of Compost and Sludge Application Followed by 22 years of Conventional Agriculture) for the Two Controls and the Three Treatments
Depth
BWS1
n.d.: not detected