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Journal of Environmental Quality 30:1667-1674 (2001)
© 2001 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORT
Waste Management

Distribution and Movement of Sludge-Derived Trace Metals in Selected Nigerian Soils

Monday O. Mbilaa, Michael L. Thompson*,a, Joe S. C. Mbagwub and David A. Lairdc

a Agronomy Dep., Iowa State Univ., Ames, IA 50011-1010
b Univ. of Nigeria at Nsukka, Nsukka, Nigeria
c USDA-ARS, National Soil Tilth Lab., Ames, IA 50011

* Corresponding author (mlthomps{at}iastate.edu)

Received for publication May 26, 2000.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES
 
Use of metal-rich sewage sludge as soil fertilizer may result in trace-metal contamination of soils. This study was conducted to evaluate the effects of long-term sludge application on trace-metal (Zn, Cu, Pb, and Ni) distribution and potential bioavailability in Nigerian soils under a tropical wet–dry climate. Total metal analyses, sequential chemical fractionation, and DTPA extractions were carried out on samples of control and sludge-amended pedons in Nigeria (a Rhodic Kandiustult and two Rhodic Kandiustalfs from Nigeria, respectively). The sewage sludge applied to the soils contained higher levels of Zn and Cu than Pb and Ni. The control pedon contained low levels of all four metals. Soil enrichment factors (EF) were calculated for each metal in the sludge-amended pedons. Compared with the control soil, the sludge-amended pedons showed elevated levels of Zn and Cu, reflecting the trace-metal composition of the sewage sludge. Zinc and Cu in the sludge-amended soils were strongly enriched at all depths in the profile, indicating that they had moved below the zone of sludge application. The sequential extraction and DTPA analyses indicated that the sludge-amended soils contained more readily extractable and bioavailable metal ions than the unamended soil.

Abbreviations: DTPA, diethylenetriamine pentaacetic acid • ICAP-AES, inductively coupled Ar plasma–atomic emission spectroscopy • UNN, University of Nigeria at Nsukka • CBD, citrate–bicarbonate–dithionite • EF, enrichment factor


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES
 
LIMITED information exists that specifically addresses the fate of trace metals added by sewage sludge application to soils in tropical settings or that addresses the methodology of investigating such concerns. Application of wastes to soils is a very common practice in Nigeria and has raised some concerns regarding the increasing concentrations of heavy metals in soils. General concerns have been raised by Sridhar and Bammeke (1986) and Ntekim et al. (1993) about the increasing deterioration of natural waters, soils, and air in Nigeria by solid waste dumping, but no detailed study has been conducted to investigate the fate of trace metals when incorporated with sewage sludge in these highly weathered tropical soils.

A number of trace-metal studies on temperate soils have reported some metal mobility after sludge addition (Baxter et al., 1983; Darmody et al., 1983). However, most studies have concluded that sludge-borne metals remained near the zone of sewage sludge incorporation (Emmerich et al., 1982a,b; Williams et al., 1980, 1984; Juste and Mench, 1992). In any case, conclusions about the fate of trace metals in temperate soils may not be broadly applicable to tropical soils. The high rainfall and commonly low pH of soils in many tropical ecosystems provide optimum conditions for metal transport. On the other hand, intense weathering and leaching of soil parent materials results in residual accumulation of oxides and hydroxides of Fe and Al in the soil, which can specifically sorb trace metals (Kalbasi et al., 1978).

Identifying the chemical forms in which the metals are retained in the soil is helpful to predict their potential mobility to water sources, their plant availability, and the amount of metal cycling through the food chain. Sequential extraction techniques have been commonly used to extract different forms of metals from soils (Lake et al., 1984; Shuman, 1985, 1991; van Valin and Morse, 1982; Berti and Jacobs, 1996). A typical sequential extraction approach uses progressively stronger chemical reagents to sequentially solubilize various chemical fractions of the total metal content of the soil, such as readily exchangeable, carbonate-bound, sesquioxides-bound, organic matter–bound, and residual chemical fractions (primarily metal ions that are incorporated in silicate structures). Such fractions are operationally defined and are based on assumptions about the ability of the extracting reagents to remove each form of the element from the soil without altering other forms of the element.

Sequential extraction techniques have been criticized (Miller and McFee, 1983; Sloan, 1997; Sheppard and Stevenson, 1995). An extracting solution may dissolve less of the target fraction and more of a nontarget fraction than desired. Also, elements extracted from one component may not remain in solution but may precipitate or resorb onto other components. Such problems suggest that interpretations of sequential extraction data must be made cautiously.

The total concentrations of trace metals in soil do not indicate the amounts that are available for plant uptake (Srikanth and Reddy, 1991). For instance, Chukuma (1993) used a plant/soil ratio (P/S) as an index of bioaccumulation of metals (Zn, Pb, and Cd) in a Nigerian soil. He observed that the total metal concentrations in the leaves of bilinga (Nauclea popeeguinei L.) and cogongrass [Imperata cylinderica (L.) Beauv.] did not reflect total concentrations of the elements in the soil, suggesting a gap between bioavailable forms and total soil concentrations of the elements. In addition to sequential extraction of metals, the diethylenetriamine pentaacetic acid (DTPA) extraction has been used in bioavailability studies. The DTPA extraction was originally used to identify soils requiring Zn, Cu, Fe, and Mn supplements (Lindsay and Norvell, 1978). Some studies have strongly correlated DTPA-extractable metals to plant-available metals (Lindsay and Cox, 1985; Kelling et al., 1977).

The present study was initiated to evaluate the effects of long-term sewage sludge application on trace metal distribution and movement in soils amended with sewage sludge under a tropical wet–dry climate in Nigeria. The objectives were: (i) to compare the concentrations of Pb, Zn, Cu, and Ni in the sludge-amended soils with those of unamended soils, (ii) to determine the relative potential bioavailability of the metals, and (iii) to assess the degree to which applied metals may have moved within the soil profile.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES
 
Three pedons were sampled at the University of Nigeria, Nsukka (UNN), sewage farms. Pedon 1 was the control soil that had not been amended with sewage sludge. Pedons 2 and 3 were sludge-amended sites, and they had received an estimated 45 Mg ha-1 yr-1 of sewage sludge for about 37 yr. The three pedons were adjacent to one another on the landscape and had similar pedogenic histories. Pedon 1 was a fine-loamy, kaolinitic, isohyperthermic Rhodic Kandiustult; Pedons 2 and 3 were fine-loamy, kaolinitic, isohyperthermic Rhodic Kandiustalfs. The wet season in Nigeria is between April and October, and the dry season is between November and March. The rainfall in the Nsukka area averages 1600 mm yr-1 and is bimodally distributed with peaks in July and September (Agboola, 1979). The physical and chemical characteristics of the soils are presented in Table 1.


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  		Table 1. Physical and chemical characteristics of soil in the three studied pedons.

 
The applied sludge, derived from sewage from the residential and administrative areas of the University of Nigeria at Nsukka, had been processed by settling and air-drying at the university sewage treatment plant. For this study, the sewage sludge and soil samples were air-dried at room temperature for about 1 wk and were ground to pass a 10-mesh sieve. Moisture content was determined by drying subsamples at 105°C to a constant weight. Total Zn, Pb, Cu, Ni, and Al were solubilized according to the method of Miller and McFee (1983) by digestion with trace-metal-grade 16 M HNO3 using an Al digesting block. Metals were determined by inductively coupled Ar plasma–atomic emission spectroscopy (ICAP-AES). All values were corrected for spectral interference from Fe, Al, and P following the instrument manufacturer's guidelines. Samples were run in triplicate, and coefficients of variation for the trace metals were 7 to 22% Pb, 5 to 9% Zn, 3 to 4% Cu, and 3 to 4% Ni.

A sequential extraction scheme was used to fractionate trace metals in the samples (Table 2). We modified the method of Tessier et al. (1979) by using Mg(NO3)2 to extract exchangeable metals (Shuman, 1985) and by using the citrate–bicarbonate–dithionite extraction method (Jackson et al., 1986) to extract sesquioxide-bound metals. The concern of Shuman (1985) that Na dithionite is sometimes contaminated with Zn was addressed by preparing standard solutions with blanks that received exactly the same treatment as the soil samples. The sums of the sequentially solubilized metal fractions were similar to but not equal to the total metal released in the 16 M HNO3 digest, probably due to losses resulting from washing the sample after each extraction and possible reprecipitation of dissolved minerals during the sequence of extractions. The DTPA-extractable metals in the soil samples were determined by ICAP-AES after extraction with 0.005 M DTPA, 0.01 M triethanol amine, and 0.01 M CaCl2, adjusted to pH 7.3 (Lindsay and Norvell, 1978).


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  		Table 2. Extracts and conditions used to sequentially extract trace metals from soil samples.{dagger}

 

   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSION
 REFERENCES
 
Metals in the Sewage Sludge
The total metal concentrations in the UNN sewage sludge sample, collected in 1997, were 787 mg Zn kg-1, 126 mg Cu kg-1, 47 mg Pb kg-1, and 22 mg Ni kg-1. These concentrations were low with respect to current regulations of the USEPA, and the sludge would be considered suitable for land application in the USA (USEPA, 1993). The low concentrations of the metals, especially Pb and Ni, were expected due to the domestic origin of the sludge.

No historical records were available on the frequency or rate of sewage sludge application at the study sites. Therefore, in 1997, we estimated the application rate by mapping 1 m2 on the ground near our sampling sites, then collecting and weighing all the dry sludge that had been recently applied within the marked area. On this basis, the sludge application rate was roughly estimated to be 15 dry Mg ha-1 during each cropping season. If sewage sludge were applied at that rate three times per year for 37 yr, the total sludge application would have been 1665 Mg ha-1. If the metal concentrations had been constant during 37 yr, the cumulative inputs of Zn and Cu would have been 1310 and 210 kg ha-1, respectively.

Sequential chemical fractionation of the sludge sample showed that extractable Zn was equally distributed among Mg(NO3)2-extractable, NaOAc-extractable, CBD-extractable, and H2O2-extractable fractions (Fig. 1). Copper (about 80%) was predominantly in the H2O2-extractable fraction. Lead occurred mainly in the H2O2-extractable fraction (about 40%), as well as in the CBD-extractable and HNO3-extractable fractions. Nickel occurred in either the CBD-extractable fraction or the HNO3-extractable fraction. Thus, the distributions of the metals in the sludge varied, and it was not clear to what extent their occurrence in the sludge might influence their behavior after incorporation in soils. Repeated sampling of this sludge has not been done, and thus, there is uncertainty about variations in the distribution of forms of the metals over time. Some reports (e.g., Sposito et al., 1982; Berti and Jacobs, 1998) have shown variations in trace metal contents of sludges when sampled at different seasons. Such differences may exist in the UNN sludge, but the variations are probably small because of its domestic origin. However, this characterization is the first of its kind for the UNN sludge, and future research is needed to monitor periodic changes in trace metal concentrations.



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  		Fig. 1. Forms of Zn, Cu, Pb, and Ni in UNN sewage sludge as determined by sequential extractions.

 
Distribution of Trace Metals in the Control Pedon
Morphological, chemical, and mineralogical properties of the study pedons have been discussed in detail by Mbila (2000). Pedogenesis led to the development of soil with three distinct layers. The A horizons were a thin layer of sandy loam to loamy sand, containing small to moderate amounts of organic matter. The B horizons consisted of an upper, less ferruginous, Bt horizon and a lower, more ferruginous, Bt horizon.

There was an increase in Fe oxide (CBD-extractable Fe) concentration from the A horizons to the B horizons of the three pedons (Fig. 2). Without exception, the highest concentration of Fe oxide was in the lower Bt horizons. The control pedon was strongly to very strongly acid (pH 3.9–5.3), and the organic C content ranged from 0.4 to 8.4 g kg-1, decreasing with soil depth (Table 1).



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  		Fig. 2. Distribution of citrate–bicarbonate–dithionite–extractable Fe distribution in the three pedons.

 
Total concentrations of trace metals in the control pedon (Fig. 3) were comparable to those of most uncontaminated soils worldwide (Kabata-Pendias and Pendias, 1984). Total concentrations were low at the surface of the control pedon and increased with soil depth. In the Bt horizons, the concentration of Cu increased with depth and was greatest in the horizon with the highest Fe-oxide concentration (Bt3). Nickel exhibited a similar pattern, except for a slight increase near the base of the Bt1 horizon that continued to the Bt3 horizon. Zinc and Pb concentrations gradually increased with soil depth to the base of the Bt2 horizon (80–110 cm), but then increased in the Fe-rich Bt3 horizon. These observations suggest that trace metal distribution in the control pedon was related to the accumulation of free Fe oxides.



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  		Fig. 3. Trace metal distribution with soil depth in Pedon 1, Pedon 2, and Pedon 3.

 
Estimating Trace Metal Distribution and Enrichment of Sludge-Amended Pedons
Concentrations of all trace metals were greater in the sludge-amended pedons than in the control pedon (Fig. 3). Moreover, the trace metal contents of the sludge-amended pedons (i.e., elevated levels of Zn and Cu and low levels of Pb and Ni) reflected the trace metal composition of the UNN sewage sludge, consistent with sludge being the source of metals in the sludge-amended pedons.

The maximum concentration of Zn in the sludge-amended pedons was in the surface horizons, and that concentration remained relatively high in the Fe-rich Bt horizons (Fig. 3). Copper concentration was also greatest in the surface horizons but declined sharply in the upper part of the Bt horizon, and was nearly constant at depths below 90 cm. Comparable concentrations of Pb and Ni were observed in the control pedon and the sludge-amended pedons.

To estimate the degree of trace-metal enrichment due to sewage-sludge amendment, a trace-metal enrichment factor (EF) was calculated (Sinex and Helz, 1981; Rule, 1986; Covelli and Fontolan, 1997). The EF assesses the degree to which the amended pedons were enriched by comparing normalized metal concentrations with those of the control pedon at similar depths. The EF was calculated as follows:

where

Metals were normalized against Al because Al is considered to be a relatively immobile element in the soil. Depth increments in Pedons 2 and 3 were recalculated to reflect the depth increments in Pedon 1 by weighting factors based on horizon thicknesses. An EF value of unity denotes no enrichment or depletion relative to the control, whereas an EF > 1 denotes enrichment relative to the control (Sinex and Helz, 1981; Covelli and Fontoalan, 1997) (Fig. 4).



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  		Fig. 4. Metal enrichment of Pedon 2 and Pedon 3.

 
Relative to the control pedon, the sludge-amended soils were clearly enriched with Zn and Cu but not with Pb or Ni (Fig. 4). Enrichment factors were 1.2 to 5.2 and 1.3–5.9 for Zn and to Cu, respectively. The EF was highest in the surface horizons and decreased with depth, due to the surface application of the sludge. Also, Zn and Cu are essential micronutrients to plants and plant uptake tends to recycle labile Zn and Cu back to the upper soil horizons.

Enrichment of Zn and Cu in the subsurface horizons of the sludge-amended pedons indicates the mobility of these metals within the pedon. Mobility of sludge-derived metals has been reported by other authors (e.g., Williams et al., 1984; Darmody et al., 1983; Dowdy et al., 1984), but not to a degree as pronounced as in our observations. However, none of those studies involved field sites with long-term (>30 yr) sludge application on soils with a very deep solum under a truly tropical setting. Trace-metal mobility in the sludge-amended pedons may be a function of the rate and number of applications of sewage sludge (Williams et al., 1984) as well as factors external to the soil such as annual precipitation. It may also be a function of soil properties (e.g., soil pH, Korte et al., 1976) and the individual metal properties (Williams et al., 1984). Because of the low pHs (3.8–5.3) (Table 1), variable-charge surfaces (such as Fe oxide minerals) will carry a net positive charge and provide limited sites for adsorption of trace metals, thus promoting metal mobility.

Mobility and Potential Bioavailability of Trace Metals
The sequential extraction indicated that about 94% of total Zn in the control soil was either in the HNO3-extractable, CBD-extractable, and H2O2-extractable fractions (Fig. 5). Zinc in these fractions is likely to be less bioavailable and less mobile than those in Mg(NO3)2-extractable and NaOAc-extractable fractions. In comparison with the control pedon, there was less Zn in the HNO3-extractable, CBD-extractable, and H2O2-extractable fractions of the sludge-amended soils (Pedons 2 and 3), whereas the Mg(NO3)2-extractable and NaOAc-extractable Zn fractions were four to sixfold higher. The Mg(NO3)2-extractable and NaOAc-extractable Zn fractions are considered to be exchangeable and weakly bound, and they may be readily available for plant uptake or leaching (McLaren and Crawford, 1973; Mathews, 1984). Less than 7% of the total Zn (pedon average) in the control soil was Mg(NO3)2- and NaOAc-extractable, whereas 26 to 36% of Zn in the sludge-amended soils were Mg(NO3)2-extractable and NaOAc-extractable. Lindsay (1972) has reported that below pH 7.7 the predominant soluble species of Zn is Zn2+, so the large partitioning of Zn in the exchangeable fraction is probably due to the presence of Zn2+ on exchange sites of soil clays and organic matter.



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  		Fig. 5. Forms of (a) Zn and (b) Cu in the three pedons as determined by sequential extractions.

 
The DTPA-extractable Zn was greater in the sludge-amended pedons than in the control pedon (Table 1). The DTPA-extractable Zn was nearly proportional to the total (HNO3-extractable) Zn, and their distribution with soil depth was similar. Judged by the critical value of 0.6 to 0.8 mg DTPA-extractable Zn kg-1 soil suggested by Lindsay and Norvell (1978), some plants growing in the control soil (0.2–0.9 mg Zn kg-1 soil) may be Zn deficient. But DTPA-extractable Zn in Pedon 2 ranged from 11.3 to 18.8 mg Zn kg-1 soil and in Pedon 3 from 0.7 to 8.1 mg Zn kg-1. The levels of DTPA-extractable Zn in the sludge-amended soils indicate more than enough plant-available Zn for crop production.

Copper in the control pedon was predominantly in the HNO3-extractable fraction (75% of total Cu content) (Fig. 5). However, the distribution of total Cu among chemical fractions differed significantly from that of Zn. In the sludge-amended pedons, Cu was predominantly in the H2O2-extractable fraction (pedon average of 52 and 42% of the total content in Pedons 2 and 3, respectively).

The distribution of soil Cu among chemical fractions strongly reflected the distribution of Cu in the UNN sewage sludge (Fig. 1), supporting the hypothesis that the chemical forms of metals in sludge influence their behavior when applied to the soil. That the largest proportion of Cu is in the organic fraction agrees with many studies that have shown that Cu forms strong specific (covalent) bonds with electron-rich functional groups in organic matter (McLaren and Crawford, 1973; Tyler and McBride, 1982). Unlike Zn, only a small proportion of total Cu was present in the exchangeable and weakly bound fractions. Thus, mobility and bioavailability of Cu may be controlled by the binding of Cu to soluble organic matter (Kabata-Pendias and Pendias, 1984). High levels of total and extractable Cu at all depths in the sludge-amended soils suggests that leaching of Cu bound to soluble organic compounds (e.g., humic and fulvic acids) has facilitated redistribution of Cu in these pedons (e.g., Tyler, 1981).

Copper extractable by DTPA increased in the sludge-amended pedons over the control pedon (Table 1). Lindsay and Norvell (1978) proposed <0.2 mg DTPA-extractable Cu kg-1 soil as the critical Cu level in soils for some field crops. On this basis, field crops growing in the control soil would experience near-optimum Cu levels (0.1–0.3 mg Cu kg-1 soil), and those growing in the sludge-amended soils (0.6–7.4 mg Cu kg-1 soil) will most likely encounter high levels of Cu in the soil solution.

Lead and Ni distributions in the five fractions were similar in the control and sludge-amended pedons. For Pb, 91 to 97% of the total Pb was in the HNO3-extractable, CBD-extractable, and H2O2-extractable fractions, and <6% was in the Mg(NO3)2-extractable fraction for all pedons (Fig. 6a). Similarly, 98% of the total Ni content was in the HNO3-extractable, CBD-extractable, and H2O2-extractable fractions, and <2% was in the Mg(NO3)2-extractable fraction for all pedons (Fig. 6b). The total contents of Pb and Ni and their chemical partitioning into unreactive forms in the UNN sewage sludge, the control pedon, and the sludge-amended pedons indicate that their bioavailability is not a major concern for these soils. There were no large differences in the DTPA-extractable Pb and Ni between the control pedon and the sludge-amended pedons (Table 1). Therefore, sewage-sludge amendment probably did not increase the bioavailability of Pb and Ni.



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  		Fig. 6. Forms of (a) Pb and (b) Ni in the three pedons as determined by sequential extractions. See legend in Fig. 5 for explanation of fractions.

 

   CONCLUSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSION
REFERENCES
 
Long-term amendment of agricultural soils in Nigeria with sewage sludge from a residential area resulted in Zn and Cu enrichment in the soils. Lead and Ni occurred at relatively low concentrations in the sewage sludge and were not enriched in the soils. Enrichment of Zn and Cu at all depths of the sludge-amended pedons suggests that the metals had moved downward from the zone of sludge incorporation (0–25 cm). Sequential extraction of the sludge-amended pedons indicated that a large proportion of total Zn was in exchangeable Mg(NO3)2- or NaOAc-extractable fractions. Copper, on the other hand, was mainly associated with the H2O2-extractable fraction. In addition, a greater proportion of the Zn and Cu in the sludge-amended pedons was in DTPA-extractable forms than in the control pedon. These findings suggest that Zn and Cu were more mobile and bioavailable in the sludge-amended pedons than in the unamended pedon.


   ACKNOWLEDGMENTS
 
We are grateful to the Rockefeller Foundation (African Dissertation Fellowship Program) and the Iowa Agriculture and Home Economics Experiment Station for funding that supported this research. In addition, we thank Pierce Fleming for assistance with ICP analyses. Journal Paper no. J-18888 of the Iowa Agriculture and Home Economics Experiment Station, Ames, IA. Project no. 3359, supported by Hatch Act and State of Iowa funds.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES
 





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