Cu and Zn mobilization in soil columns percolated by different ...
Environmental Pollution 157 (2009) 823–833
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Environmental Pollution journal homepage: www.elsevier.com/locate/envpol
Cu and Zn mobilization in soil columns percolated by different irrigation solutions Lu Y.L. Zhao a, b, Rainer Schulin a, Bernd Nowack a, c, * a
Institute of Terrestrial Ecosystems, ETH Zurich, Universitaetstrasse 16, 8092 Zurich, Switzerland Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong c Empa – Swiss Federal Laboratories for Materials Testing and Research, Lerchenfeldstrasse 5, CH-9014 St. Gallen, Switzerland b
Fertilizer addition to soils leads to strong mobilization of Cu and Zn and to changes in metal speciation in the top layer of metal-polluted soils.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 July 2008 Received in revised form 10 November 2008 Accepted 12 November 2008
We investigated the effects of different concentrations of nitrate and ammonium in irrigation water on the mobilization of Zn and Cu in repacked soil columns with a metal-polluted topsoil and unpolluted subsoils over two and a half years. Soil solution samples were collected by suction cups installed at vertical distances of a few centimeters and analyzed for dissolved organic carbon (DOC), Cu, and Zn (total and labile). During high N treatments the pH decreased and the presence of exchangeable cations resulted in Zn mobilization from the surface soil. The nitrogen input stimulated the biological activity, which affected both concentration and characteristics of DOC and consequently Cu speciation. Metal leaching through the boundary between the polluted topsoil and the unpolluted subsoils increased soilbound and dissolved metals within the uppermost 2 cm in the subsoils. Our study shows that agricultural activities involving nitrogen fertilization can have a strong influence on metal leaching and speciation. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Soil Heavy metals Fertilizer Nitrogen Mobilization
1. Introduction The potential mobilization of metals is a concern in the risk assessment of metal-polluted soils. Metal mobilization depends on soil characteristics and is controlled by the soil matrix and the composition of the soil solution. Soil organic matter content, pH and total metal burden determine soil–liquid partitioning coefficients of metals (Sauve´ et al., 2000). DOC complexation and major cation concentrations are major factors controlling metal concentrations in soil solution (Sauve´ et al., 2003). Their effects can, however, differ considerably between different metals. For example, Cu was leached as metal–organic complex when the pH increased (Temminghoff et al., 1997), while Zn was mobilized when the pH decreased and the ionic strength increased in the infiltration water (Sauve´ et al., 2000; He et al., 2006). DOC, pH and Ca concentrations in soil solution depend on the soil type and are influenced by microbial and plant processes (Guggenberger et al., 1994; Zhao et al., 2007a). DOC solubility is also promoted by high soil solution pH (Godde et al., 1996) and low Ca concentrations (Ro¨mkens and Dolfing, 1998). Studies in
* Correspondence to: Bernd Nowack, Empa – Swiss Federal Laboratories for Materials Testing and Research, Lerchenfeldstrasse 5, CH-9014 St. Gallen, Switzerland. Tel.: þ41 (0)71 274 76 92; fax: þ41 (0)71 274 78 62. E-mail address: [email protected] (B. Nowack). 0269-7491/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2008.11.011
large lysimeters (Vance and David, 1991; Camobreco et al., 1996; Antoniadis and Alloway, 2002; Rais, 2005) showed that DOC values in the topsoil were greater in the subsoil and that DOC leached from the topsoil was immobilized by adsorption in the subsoil together with metals leached from the topsoil. This immobilization was related to the different properties of the soil horizons. The amount and the composition of the infiltration water influence the composition of the soil solution. Experiments in soil columns showed that changing the rate of infiltration had little effect on pH, whereas DOC and Ca concentrations were strongly affected and thus in turn also Cu and Zn, the latter more by competition with Ca for binding sites on the solid phase than by complexation with DOC (Zhao et al., 2007b). Similar effects may result from changes in the composition of the infiltration water, e.g. by fertilizer application. For example, adding DOC to the infiltration water increased the extractable and bioavailable metals (Antoniadis and Alloway, 2002), while sewage sludge application limited metal solubility in contaminated soils (McBride et al., 2000). Nitrogen as a nutrient element and an important component of inorganic fertilizers is known to cause acidification when added as ammonium (Bouman et al., 1995; He et al., 1998) and to decrease the aggregate stability (Graham et al., 2002). Abundant supply of nitrogen also stimulates microbial biodegradation of soil organic matter. Whereas the influence of organic manure on metal mobility is well studied, there are only a few studies on the effects of nitrogen
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fertilization on metal mobility. A pot experiment showed that heavy metals and major cations were increased and pH was decreased in soil solution after a cumulative addition of NH4NO3 and KNO3 (Lorenz et al., 1994). Annual application of urea or inorganic fertilizer reduced extractable Ca and Mg and pH but increased soluble Al in both forest and agriculture soils (Graham et al., 2002; Fox, 2004). Metals mobilized in one soil horizon may be immobilized again in a deeper horizon. Therefore, the processes of metal mobilization and immobilization cannot be inferred only by analysis of leachates. It is necessary to study changes in soil solution over depth and time in response to varying inputs. In previous studies we employed a new column set-up to study the effects of different irrigation rates and plants on metal mobilization (Zhao et al., 2007a,b). Soil solution was collected using suction cups installed at centimeter intervals. This set-up proved to be well suited to study metal leaching processes over depth with high spatial resolution. These studies showed that the concentrations of DOC, dissolved metals, major cations and anions increased steadily with depth in a metal-polluted topsoil but showed abrupt changes across the boundary between polluted topsoil and unpolluted subsoil. In particular dissolved Cu and Zn concentrations dropped to background levels within 2 cm in the unpolluted subsoils. The aim of this study was to investigate the effect of the composition of the irrigation water on the mobilization of Cu and Zn in metal-polluted topsoil and on the subsequent immobilization in unpolluted subsoil. Synthetic rainwater, high and low concentrations of nitrate and ammonium were used with the same column design as described before (Zhao et al., 2007b). Concentrations and speciation of Cu and Zn in soil solution were recorded as a function of depth and time and related to general soil solution characteristics. To assess metal leaching, total soil metal content was analyzed with high spatial resolution at the end of the experiment. Our hypotheses were (i) that the nitrogen application reduces the soil solution pH and thus results in the increased mobilization of Zn; (ii) that the decrease in pH leads to low DOC concentrations and thus low Cu concentrations in the presence of high nitrogen concentration; and (iii) that the mobilized metals are adsorbed in the upper layer of the unpolluted subsoil.
or acidic subsoil. The subsoil was covered with a polyamide mesh (mesh size 60 mm), fixed by tape and glue, and 23 cm polluted topsoil was filled in, followed by a final layer of 2 cm quartz sand. In each column 15 suction cups (50 cm long Rhizon samplers, Rhizosphere Research Products, the Netherlands) were installed at 4, 10, 13, 16, 18, 20 and 22 cm depth in the topsoil, and 24, 26, 28, 30, 33, 38, 44 and 50 cm depth in the subsoil. A depth of 0 cm corresponds to the surface of the topsoil layer. To investigate the changes of soil solution across the soil layer boundary, the distance between the suction cups was smaller above and below the mesh than in the rest of the column. Two 100 cm long suction cups (Rhizon samplers, Rhizosphere Research Products, the Netherlands) were installed in the sand layers at the surface of the column for irrigation (1 cm) and at the bottom of the column (56 cm) to continuously collect the leachate by applying a constant suction by the peristaltic pump. No water drained through the bottom of the column. Suction cups were acidcleaned (0.01 M HNO3) prior to installation. The suction cups are known to only weakly sorb metals and major cations and anions are not affected (Rais et al., 2006). The suction cups were bent into a spiral and fixed by a plastic stick to hold it in place and shape. Thus the samples taken from these suction cups can be considered as an average soil solution at a certain depth. The columns were saturated from the bottom to the top with reverse-osmosis water by increasing the water table to 10 cm below the soil surface for about 4 h each time that 10 cm of soil had been filled in. Afterwards the columns were allowed to drain. Four columns were built, two with calcareous subsoil and two with acidic subsoil and all with the same polluted topsoil. They were set up in a climate chamber with the conditions of 50% humidity, 22 C and 14 h illumination. 2.3. Column experiments The columns were irrigated with synthetic rainwater of different composition through a peristaltic pump for two and a half years (Table 2). The sequence of the different irrigation conditions, the sampling frequency and the purposes of each procedure are shown in Table 3. The high application of nitrogen (100 mg N l1) represents a one-time nitrogen application of 250 kg ha1 needed for one season of cultivation. The first 20 days at a flow rate of 33.3 mm day1 were used to equilibrate the soil and flush out the high initial concentrations of DOC and nutrients that were initially mobilized by rewetting the repacked soil. A bromide tracer (60 mg l1) was applied during the flow-rate experiment. At the end of the experiment the four columns were individually irrigated with rates of 20, 10, 5, 2.5 mm day1 for 7 days. Brilliant blue was applied continuously for 4 days before sampling (concentration 6 g l1). The soil was then removed from the top to the bottom in 2 cm layers. Each exposed layer was photographed to document the distribution of the dye tracer. After weighing, the soil samples were oven-dried at 105 C and weighed again to obtain the water content. 2.4. Chemical analyses
2.2. Column set-up
2.4.1. Dissolved metal concentrations, speciation and pH Each soil solution sample was divided immediately after sampling into three subsamples. One subsample was used to measure Cu2þ and labile Zn concentrations within one week after sampling. Before measurement, the samples were sealed in plastic tubes and stored at 4 C. Cu2þ activity was measured using a cupric ion selective electrode (Model 9429, Thermo Orion, MA) coupled with an Ag/AgCl reference electrode (in 3 M KCl) (Model 6.0733.100, Metrohm, Switzerland). Solution pH was measured at the same time by a Metrohm 713 pH meter. The electrodes were calibrated according to the method of Sauve´ et al. (1995). The pCu2þ in the calibration solution was calculated with ChemEQL (Mu¨ller, 1996). Electrode potential and pCu2þ were linearly correlated from pCu2þ 11.5 to 4.5 and the detection limit was 1012 M Cu2þ. After measuring Cu2þ activity, one drop of 65% HNO3 was added to the solution sample and dissolved Cu and Zn were measured by Flame-AAS (Varian, SpectraAA 220FS, USA) or ICP-OES (VISTA – MPX CCD Simultaneous ICP – OES, USA). Labile Zn was measured by differential pulse anodic stripping voltammetry (DPASV) with a hanging mercury drop electrode (HMDE) on a 693 VA Processor and a 694 VA Stand, Metrohm, Switzerland. The detection limit was 0.2 mM.
The scheme of the column set-up is shown in Fig. 1. PVC columns of 62 cm length and 20 cm inside diameter were closed at the bottom by a PVC plate with holes (diameter 6 mm). A polyamide mesh (mesh size 200 mm) was placed on the plate, and 2-cm quartz sand (size < 0.5 mm) was filled in, followed by 35 cm of calcareous
2.4.2. Other soil solution analyses A second subsample of soil solution was used to determine anions. Aliquots of 1 mL were taken immediately after sampling and fixed by adding 50 mL of 37% formaldehyde (Merck, for analysis). The samples were sealed in a 2 mL glass vial and
2. Materials and methods 2.1. Soils The soils were sampled in 2004 from a model ecosystem experiment (details in Nowack et al., 2006). The topsoil was a weakly acid loam originally taken from an agricultural field (Luvisol) and the subsoil was either an acidic loamy sand from a Haplic Alisol or a calcareous sandy loam from a Calcaric Fluvisol. Part of the topsoil had been pretreated with filter dust from a non-ferrous metal smelter in 2000. Total Cd, Cu, Pb and Zn were 10, 640, 90 and 3000 mg kg1, respectively. For our experiment, the polluted topsoil was mixed with unpolluted topsoil (ratio 1:4) to lower the metal concentrations. The soil properties are given in Table 1. The soils were sieved through 2 mm and kept moist at a temperature of 15 C.
Table 1 The properties of soils used in the columns. Soil type
Sanda/%
Silta/%
Claya/%
Corga/%
Carbonatea/%
pHa
Zn/mg kg1
Cu/mg kg1
Polluted topsoil Calcareous subsoil Acidic subsoil
35.5 74.2 87.3
49.4 16 7.8
15.1 9.8 4.9
1.51 1.12 0.32
A: 100 mg N l1 average of 2 column replicates; 6:: 6 mg N l1, average of 2 column replicates; 7;: synthetic rainwater, average of 2 column replicates and 3 measurements) from acidic subsoil columns (solid symbols) and calcareous subsoil columns (open symbols). The error bars show 95% confidence interval of the measurements (n ¼ 6 for synthetic rainwater; n ¼ 2 with 3 replicates each) for 100 mg N l1 and 6 mg N l1. The dashed line indicates the location of the mesh – the boundary between topsoil and subsoil.
current work. In addition to Ca ammonium and K could also have displaced Zn in this study. The positive nitrate term may relate to ammonium, which was applied during the nitrogen treatment and therefore incorporate ion exchange reactions. The nitrogen application not only resulted in the acidification of the surface soil but also provided ammonium and other cations to exchange metals from the binding sites of solid phase. Together with the decrease in pH and ion exchange by ammonium, this resulted in high dissolved Zn concentrations and consequently the leaching to lower parts of soil profile. Immobilization of Zn by adsorption or precipitation happened when (1) the pH increased again and (2) ammonium was removed by ion exchange or nitrification. When the high Cu and Zn concentrations in equilibrium with the high metal contents in the topsoil leached into the unpolluted subsoil, a new equilibrium between solution and solid phase was established. This resulted in an almost immediate immobilization of Cu and Zn in the uppermost layer of the subsoil. Over the period of the experiment, the leached metals continuously
increased the metal content in the uppermost subsoil layer, which resulted in a higher equilibrium Zn concentration in solution with time, visible in the constant increase in dissolved Zn towards the end of the experiment in the uppermost subsoil solution samples. The leaching of Zn from the lowest subsoil layer was not affected by the irrigation water; however, for Cu there were significant changes during the time course of the experiment. DOC in the effluent also changed significantly over the 470 days of the experiment, more pronounced in the calcareous subsoil than in the acidic subsoil. The reduction of DOC resulted in less Cu leached from the soil towards the end of the experiment. Time was also a factor that influenced the metal concentrations in solution. In the multi-linear regressions the coefficient for time was always negative, indicating that less metals were mobilized with time. Metals added into a soil will redistribute between the solid-phase components of the soil with time (Han and Banin, 1997, 1999). In general, shortly after addition, the metals are found in
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Fig. 7. Total heavy metal content in the soil at the beginning (dashed line) and at the end of the experiment. (B: the column with calcareous subsoil; : the column with acidic subsoil). The shown data are the average of two columns. The inset shows an enlargement of the uppermost subsoil section. The horizontal dashed line indicates the location of the mesh – the boundary between topsoil and subsoil.
Fig. 8. Extractable Cu and Zn (0.1 M NaNO3) in soil samples along the column after the experiment. (B: the column with calcareous subsoil; : the column with acidic subsoil). The inset shows an enlargement of the uppermost subsoil section. The dashed line indicates the location of the mesh – the boundary between topsoil and subsoil.
Table 4 Leaching of metals and DOC from the topsoil into the subsoil and out of the subsoil during 32 months (values in the brackets represent the errors derived from the measured leachate volume). Subsoil type
Water/l DOC/mg Cu/mmol Zn/mmol
Leaching from topsoil
Leaching from subsoil
Acidic
Calcareous
Acidic
Calcareous
756(20) 220(19) 1179(83)
122.2(8.4) 785(31) 234(20) 1299(70)
470(44) 13.3(1.3) 84.6(7.3)
117.1(9.2) 1651(78) 29.7(1.9) 38.1(2.8)
exchangeable or easily mobilizable fractions and tend to redistribute within months or years into more crystalline or even residual fractions (Sposito et al., 1983; Mandal and Mandal, 1986). Although the metals in this experiment had been added a few years before the experiment, long-term changes in metal sequestration were still affecting the binding of metals to the solid phase. In addition the most soluble metals were always removed and the remaining metals were therefore stronger bound.
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Table 5 Multi-linear regression analysis for dissolved Cu and Zn, Cu2þ and labile Zn in soil solution of the topsoil (n ¼ 336), the acidic (n ¼ 192) and the calcareous subsoil (n ¼ 192). The numbers in parentheses represent standard errors. log(DOC)/kg l1
pH
1 log(NO 3 )/mol l
log(time)/day
Constant
R2
Topsoil
log Cu log(Cu2þ) log Zn log(labile Zn)
0.539(0.037) 0.617(0.082) 0.319(0.046) 0.491(0.057)
– 0.772(0.056) 0.649(0.041) 0.685(0.039)
0.231(0.019) 0.541(0.055) 0.415(0.031) 0.517(0.039)
0.201(0.018) – – –
3.006(0.161) 4.063(0.578) 0.689(0.327) 1.240(0.405)
0.729c 0.601c 0.748c 0.714c
Acidic subsoil
log Cu log(Cu2þ) log Zn log(labile Zn)
0.753(0.173) – 0.684(0.179) 0.779(0.253)
– 1.206(0.043) 0.265(0.040) 0.302(0.059)
0.772(0.146) 0.694(0.149) 0.384(0.111) –
0.650(0.088) – 0.512(0.079) 0.315(0.0.113)
3.520(0.857) 0.065(0.422) 5.824(0.0.957) 7.538(1.389)
0.348c 0.826c 0.570c 0.336b
Calcareous subsoil
log Cu log(Cu2þ) log Zn log(labile Zn)
0.323(0.109) – 0.709(0.165 0.396(0.219)
0.826(0.077) 1.141(0.082) 0.267(0.108) –
–
– – 0.461(0.097) –
1.417(0.602) 0.972(0.577) 6.633(1.045) 9.527(0.852)
0.485b 0.580b 0.277a 0.131a
a b c
0.247(0.079) – 0.428(0.147)
Statistical significance (2-tailed) for p 0.1. Statistical significance (2-tailed) for p 0.01. Statistical significance (2-tailed) for p 0.001.
Fig. 9. Measured Cu (a), Cu2þ (b), Zn (c) and labile Zn (d) in soil solution of polluted topsoil vs. predicted values obtained from the multi-linear regressions (Table 5). (B: synthetic rainwater; >: 100 mg N l1; 6: 6 mg N l1; 7: synthetic rainwater).
5. Conclusions Our investigation showed that the exchangeable cations and the low pH by high nitrogen treatment in the irrigation water resulted in the mobilization of large amounts of Zn from the surface soil layer. High nitrogen input stimulated the microbial degradation
and decreased DOC concentrations and consequently dissolved Cu in the solution while the low pH was not a controlling factor for DOC and Cu mobilization. Metal mobilization described by DOC, pH, nitrate and time indicates that the composition of soil solution is the key to assess metal leaching and long-term investigation was necessary. Leaching of metals through the boundary between the
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polluted topsoil and the unpolluted subsoils resulted in an increase of soil-bound metals within the uppermost 2 cm subsoil. However, long time leaching needs to be considered in risk assessment because of a slight increase in dissolved metals at both the uppermost topsoil and subsoils over time. The study shows that agricultural activities involving fertilization with nitrogen can have strong influence on metal leaching. Acknowledgements The authors are grateful to Martin Keller for constructing the columns and to M. Gu¨nthard-Goerg from Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) for soil materials. The project was funded by Swiss National Science Foundation Grant No. 200020-105229. References Antoniadis, V., Alloway, B.J., 2002. The role of dissolved organic carbon in the mobility of Cd, Ni and Zn in sewage sludge-amended soils. Environmental Pollution 117, 515–521. Bouman, O.T., Curtin, D., Campbell, C.A., Biederbeck, V.O., Ukrainetz, H., 1995. Soil acidification from long-term use of anhydrous ammonia and urea. Soil Science Society of America Journal 59, 1488–1494. Camobreco, V.J., Richards, B.K., Steenhuis, T.S., Peverly, J.H., McBride, M.B., 1996. Movement of heavy metals through undisturbed and homogenized soil columns. Soil Science 161, 740–750. Fox, T.R., 2004. Nitrogen mineralization following fertilization of Douglas-fir forests with urea in western Washington. Soil Science Society of America Journal 68, 1720–1728. Godde, M., David, M.B., Christ, M.J., Kaupenjohann, M., Vance, G.F., 1996. Carbon mobilization from the forest floor under red spruce in the northeastern USA. Soil Biology & Biochemistry 28, 1181–1189. Graham, M.H., Haynes, R.J., Meyer, J.H., 2002. Changes in soil chemistry and aggregate stability induced by fertilizer applications, burning and trash retention on a long-term sugarcane experiment in South Africa. European Journal of Soil Science 53, 589–598. Guggenberger, G., Zech, W., Schulten, H.R., 1994. Formation and mobilization pathways of dissolved organic-matter – evidence from chemical structural studies of organic-matter fractions in acid forest floor solutions. Organic Geochemistry 21, 51–66. Han, F.X., Banin, A., 1997. Long-term transformations and redistribution of potentially toxic heavy metals in arid-zone soils incubated. 1. Under saturated conditions. Water, Air, and Soil Pollution 95, 399–423. Han, F.X., Banin, A., 1999. Long-term transformation and redistribution of potentially toxic heavy metals in arid-zone soils: II. Incubation at the field capacity moisture content. Water, Air, and Soil Pollution 114, 221–250.
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Contents lists available at ScienceDirect
Environmental Pollution journal homepage: www.elsevier.com/locate/envpol
Cu and Zn mobilization in soil columns percolated by different irrigation solutions Lu Y.L. Zhao a, b, Rainer Schulin a, Bernd Nowack a, c, * a
Institute of Terrestrial Ecosystems, ETH Zurich, Universitaetstrasse 16, 8092 Zurich, Switzerland Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong c Empa – Swiss Federal Laboratories for Materials Testing and Research, Lerchenfeldstrasse 5, CH-9014 St. Gallen, Switzerland b
Fertilizer addition to soils leads to strong mobilization of Cu and Zn and to changes in metal speciation in the top layer of metal-polluted soils.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 July 2008 Received in revised form 10 November 2008 Accepted 12 November 2008
We investigated the effects of different concentrations of nitrate and ammonium in irrigation water on the mobilization of Zn and Cu in repacked soil columns with a metal-polluted topsoil and unpolluted subsoils over two and a half years. Soil solution samples were collected by suction cups installed at vertical distances of a few centimeters and analyzed for dissolved organic carbon (DOC), Cu, and Zn (total and labile). During high N treatments the pH decreased and the presence of exchangeable cations resulted in Zn mobilization from the surface soil. The nitrogen input stimulated the biological activity, which affected both concentration and characteristics of DOC and consequently Cu speciation. Metal leaching through the boundary between the polluted topsoil and the unpolluted subsoils increased soilbound and dissolved metals within the uppermost 2 cm in the subsoils. Our study shows that agricultural activities involving nitrogen fertilization can have a strong influence on metal leaching and speciation. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Soil Heavy metals Fertilizer Nitrogen Mobilization
1. Introduction The potential mobilization of metals is a concern in the risk assessment of metal-polluted soils. Metal mobilization depends on soil characteristics and is controlled by the soil matrix and the composition of the soil solution. Soil organic matter content, pH and total metal burden determine soil–liquid partitioning coefficients of metals (Sauve´ et al., 2000). DOC complexation and major cation concentrations are major factors controlling metal concentrations in soil solution (Sauve´ et al., 2003). Their effects can, however, differ considerably between different metals. For example, Cu was leached as metal–organic complex when the pH increased (Temminghoff et al., 1997), while Zn was mobilized when the pH decreased and the ionic strength increased in the infiltration water (Sauve´ et al., 2000; He et al., 2006). DOC, pH and Ca concentrations in soil solution depend on the soil type and are influenced by microbial and plant processes (Guggenberger et al., 1994; Zhao et al., 2007a). DOC solubility is also promoted by high soil solution pH (Godde et al., 1996) and low Ca concentrations (Ro¨mkens and Dolfing, 1998). Studies in
* Correspondence to: Bernd Nowack, Empa – Swiss Federal Laboratories for Materials Testing and Research, Lerchenfeldstrasse 5, CH-9014 St. Gallen, Switzerland. Tel.: þ41 (0)71 274 76 92; fax: þ41 (0)71 274 78 62. E-mail address: [email protected] (B. Nowack). 0269-7491/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2008.11.011
large lysimeters (Vance and David, 1991; Camobreco et al., 1996; Antoniadis and Alloway, 2002; Rais, 2005) showed that DOC values in the topsoil were greater in the subsoil and that DOC leached from the topsoil was immobilized by adsorption in the subsoil together with metals leached from the topsoil. This immobilization was related to the different properties of the soil horizons. The amount and the composition of the infiltration water influence the composition of the soil solution. Experiments in soil columns showed that changing the rate of infiltration had little effect on pH, whereas DOC and Ca concentrations were strongly affected and thus in turn also Cu and Zn, the latter more by competition with Ca for binding sites on the solid phase than by complexation with DOC (Zhao et al., 2007b). Similar effects may result from changes in the composition of the infiltration water, e.g. by fertilizer application. For example, adding DOC to the infiltration water increased the extractable and bioavailable metals (Antoniadis and Alloway, 2002), while sewage sludge application limited metal solubility in contaminated soils (McBride et al., 2000). Nitrogen as a nutrient element and an important component of inorganic fertilizers is known to cause acidification when added as ammonium (Bouman et al., 1995; He et al., 1998) and to decrease the aggregate stability (Graham et al., 2002). Abundant supply of nitrogen also stimulates microbial biodegradation of soil organic matter. Whereas the influence of organic manure on metal mobility is well studied, there are only a few studies on the effects of nitrogen
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fertilization on metal mobility. A pot experiment showed that heavy metals and major cations were increased and pH was decreased in soil solution after a cumulative addition of NH4NO3 and KNO3 (Lorenz et al., 1994). Annual application of urea or inorganic fertilizer reduced extractable Ca and Mg and pH but increased soluble Al in both forest and agriculture soils (Graham et al., 2002; Fox, 2004). Metals mobilized in one soil horizon may be immobilized again in a deeper horizon. Therefore, the processes of metal mobilization and immobilization cannot be inferred only by analysis of leachates. It is necessary to study changes in soil solution over depth and time in response to varying inputs. In previous studies we employed a new column set-up to study the effects of different irrigation rates and plants on metal mobilization (Zhao et al., 2007a,b). Soil solution was collected using suction cups installed at centimeter intervals. This set-up proved to be well suited to study metal leaching processes over depth with high spatial resolution. These studies showed that the concentrations of DOC, dissolved metals, major cations and anions increased steadily with depth in a metal-polluted topsoil but showed abrupt changes across the boundary between polluted topsoil and unpolluted subsoil. In particular dissolved Cu and Zn concentrations dropped to background levels within 2 cm in the unpolluted subsoils. The aim of this study was to investigate the effect of the composition of the irrigation water on the mobilization of Cu and Zn in metal-polluted topsoil and on the subsequent immobilization in unpolluted subsoil. Synthetic rainwater, high and low concentrations of nitrate and ammonium were used with the same column design as described before (Zhao et al., 2007b). Concentrations and speciation of Cu and Zn in soil solution were recorded as a function of depth and time and related to general soil solution characteristics. To assess metal leaching, total soil metal content was analyzed with high spatial resolution at the end of the experiment. Our hypotheses were (i) that the nitrogen application reduces the soil solution pH and thus results in the increased mobilization of Zn; (ii) that the decrease in pH leads to low DOC concentrations and thus low Cu concentrations in the presence of high nitrogen concentration; and (iii) that the mobilized metals are adsorbed in the upper layer of the unpolluted subsoil.
or acidic subsoil. The subsoil was covered with a polyamide mesh (mesh size 60 mm), fixed by tape and glue, and 23 cm polluted topsoil was filled in, followed by a final layer of 2 cm quartz sand. In each column 15 suction cups (50 cm long Rhizon samplers, Rhizosphere Research Products, the Netherlands) were installed at 4, 10, 13, 16, 18, 20 and 22 cm depth in the topsoil, and 24, 26, 28, 30, 33, 38, 44 and 50 cm depth in the subsoil. A depth of 0 cm corresponds to the surface of the topsoil layer. To investigate the changes of soil solution across the soil layer boundary, the distance between the suction cups was smaller above and below the mesh than in the rest of the column. Two 100 cm long suction cups (Rhizon samplers, Rhizosphere Research Products, the Netherlands) were installed in the sand layers at the surface of the column for irrigation (1 cm) and at the bottom of the column (56 cm) to continuously collect the leachate by applying a constant suction by the peristaltic pump. No water drained through the bottom of the column. Suction cups were acidcleaned (0.01 M HNO3) prior to installation. The suction cups are known to only weakly sorb metals and major cations and anions are not affected (Rais et al., 2006). The suction cups were bent into a spiral and fixed by a plastic stick to hold it in place and shape. Thus the samples taken from these suction cups can be considered as an average soil solution at a certain depth. The columns were saturated from the bottom to the top with reverse-osmosis water by increasing the water table to 10 cm below the soil surface for about 4 h each time that 10 cm of soil had been filled in. Afterwards the columns were allowed to drain. Four columns were built, two with calcareous subsoil and two with acidic subsoil and all with the same polluted topsoil. They were set up in a climate chamber with the conditions of 50% humidity, 22 C and 14 h illumination. 2.3. Column experiments The columns were irrigated with synthetic rainwater of different composition through a peristaltic pump for two and a half years (Table 2). The sequence of the different irrigation conditions, the sampling frequency and the purposes of each procedure are shown in Table 3. The high application of nitrogen (100 mg N l1) represents a one-time nitrogen application of 250 kg ha1 needed for one season of cultivation. The first 20 days at a flow rate of 33.3 mm day1 were used to equilibrate the soil and flush out the high initial concentrations of DOC and nutrients that were initially mobilized by rewetting the repacked soil. A bromide tracer (60 mg l1) was applied during the flow-rate experiment. At the end of the experiment the four columns were individually irrigated with rates of 20, 10, 5, 2.5 mm day1 for 7 days. Brilliant blue was applied continuously for 4 days before sampling (concentration 6 g l1). The soil was then removed from the top to the bottom in 2 cm layers. Each exposed layer was photographed to document the distribution of the dye tracer. After weighing, the soil samples were oven-dried at 105 C and weighed again to obtain the water content. 2.4. Chemical analyses
2.2. Column set-up
2.4.1. Dissolved metal concentrations, speciation and pH Each soil solution sample was divided immediately after sampling into three subsamples. One subsample was used to measure Cu2þ and labile Zn concentrations within one week after sampling. Before measurement, the samples were sealed in plastic tubes and stored at 4 C. Cu2þ activity was measured using a cupric ion selective electrode (Model 9429, Thermo Orion, MA) coupled with an Ag/AgCl reference electrode (in 3 M KCl) (Model 6.0733.100, Metrohm, Switzerland). Solution pH was measured at the same time by a Metrohm 713 pH meter. The electrodes were calibrated according to the method of Sauve´ et al. (1995). The pCu2þ in the calibration solution was calculated with ChemEQL (Mu¨ller, 1996). Electrode potential and pCu2þ were linearly correlated from pCu2þ 11.5 to 4.5 and the detection limit was 1012 M Cu2þ. After measuring Cu2þ activity, one drop of 65% HNO3 was added to the solution sample and dissolved Cu and Zn were measured by Flame-AAS (Varian, SpectraAA 220FS, USA) or ICP-OES (VISTA – MPX CCD Simultaneous ICP – OES, USA). Labile Zn was measured by differential pulse anodic stripping voltammetry (DPASV) with a hanging mercury drop electrode (HMDE) on a 693 VA Processor and a 694 VA Stand, Metrohm, Switzerland. The detection limit was 0.2 mM.
The scheme of the column set-up is shown in Fig. 1. PVC columns of 62 cm length and 20 cm inside diameter were closed at the bottom by a PVC plate with holes (diameter 6 mm). A polyamide mesh (mesh size 200 mm) was placed on the plate, and 2-cm quartz sand (size < 0.5 mm) was filled in, followed by 35 cm of calcareous
2.4.2. Other soil solution analyses A second subsample of soil solution was used to determine anions. Aliquots of 1 mL were taken immediately after sampling and fixed by adding 50 mL of 37% formaldehyde (Merck, for analysis). The samples were sealed in a 2 mL glass vial and
2. Materials and methods 2.1. Soils The soils were sampled in 2004 from a model ecosystem experiment (details in Nowack et al., 2006). The topsoil was a weakly acid loam originally taken from an agricultural field (Luvisol) and the subsoil was either an acidic loamy sand from a Haplic Alisol or a calcareous sandy loam from a Calcaric Fluvisol. Part of the topsoil had been pretreated with filter dust from a non-ferrous metal smelter in 2000. Total Cd, Cu, Pb and Zn were 10, 640, 90 and 3000 mg kg1, respectively. For our experiment, the polluted topsoil was mixed with unpolluted topsoil (ratio 1:4) to lower the metal concentrations. The soil properties are given in Table 1. The soils were sieved through 2 mm and kept moist at a temperature of 15 C.
Table 1 The properties of soils used in the columns. Soil type
Sanda/%
Silta/%
Claya/%
Corga/%
Carbonatea/%
pHa
Zn/mg kg1
Cu/mg kg1
Polluted topsoil Calcareous subsoil Acidic subsoil
35.5 74.2 87.3
49.4 16 7.8
15.1 9.8 4.9
1.51 1.12 0.32
A: 100 mg N l1 average of 2 column replicates; 6:: 6 mg N l1, average of 2 column replicates; 7;: synthetic rainwater, average of 2 column replicates and 3 measurements) from acidic subsoil columns (solid symbols) and calcareous subsoil columns (open symbols). The error bars show 95% confidence interval of the measurements (n ¼ 6 for synthetic rainwater; n ¼ 2 with 3 replicates each) for 100 mg N l1 and 6 mg N l1. The dashed line indicates the location of the mesh – the boundary between topsoil and subsoil.
current work. In addition to Ca ammonium and K could also have displaced Zn in this study. The positive nitrate term may relate to ammonium, which was applied during the nitrogen treatment and therefore incorporate ion exchange reactions. The nitrogen application not only resulted in the acidification of the surface soil but also provided ammonium and other cations to exchange metals from the binding sites of solid phase. Together with the decrease in pH and ion exchange by ammonium, this resulted in high dissolved Zn concentrations and consequently the leaching to lower parts of soil profile. Immobilization of Zn by adsorption or precipitation happened when (1) the pH increased again and (2) ammonium was removed by ion exchange or nitrification. When the high Cu and Zn concentrations in equilibrium with the high metal contents in the topsoil leached into the unpolluted subsoil, a new equilibrium between solution and solid phase was established. This resulted in an almost immediate immobilization of Cu and Zn in the uppermost layer of the subsoil. Over the period of the experiment, the leached metals continuously
increased the metal content in the uppermost subsoil layer, which resulted in a higher equilibrium Zn concentration in solution with time, visible in the constant increase in dissolved Zn towards the end of the experiment in the uppermost subsoil solution samples. The leaching of Zn from the lowest subsoil layer was not affected by the irrigation water; however, for Cu there were significant changes during the time course of the experiment. DOC in the effluent also changed significantly over the 470 days of the experiment, more pronounced in the calcareous subsoil than in the acidic subsoil. The reduction of DOC resulted in less Cu leached from the soil towards the end of the experiment. Time was also a factor that influenced the metal concentrations in solution. In the multi-linear regressions the coefficient for time was always negative, indicating that less metals were mobilized with time. Metals added into a soil will redistribute between the solid-phase components of the soil with time (Han and Banin, 1997, 1999). In general, shortly after addition, the metals are found in
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Fig. 7. Total heavy metal content in the soil at the beginning (dashed line) and at the end of the experiment. (B: the column with calcareous subsoil; : the column with acidic subsoil). The shown data are the average of two columns. The inset shows an enlargement of the uppermost subsoil section. The horizontal dashed line indicates the location of the mesh – the boundary between topsoil and subsoil.
Fig. 8. Extractable Cu and Zn (0.1 M NaNO3) in soil samples along the column after the experiment. (B: the column with calcareous subsoil; : the column with acidic subsoil). The inset shows an enlargement of the uppermost subsoil section. The dashed line indicates the location of the mesh – the boundary between topsoil and subsoil.
Table 4 Leaching of metals and DOC from the topsoil into the subsoil and out of the subsoil during 32 months (values in the brackets represent the errors derived from the measured leachate volume). Subsoil type
Water/l DOC/mg Cu/mmol Zn/mmol
Leaching from topsoil
Leaching from subsoil
Acidic
Calcareous
Acidic
Calcareous
756(20) 220(19) 1179(83)
122.2(8.4) 785(31) 234(20) 1299(70)
470(44) 13.3(1.3) 84.6(7.3)
117.1(9.2) 1651(78) 29.7(1.9) 38.1(2.8)
exchangeable or easily mobilizable fractions and tend to redistribute within months or years into more crystalline or even residual fractions (Sposito et al., 1983; Mandal and Mandal, 1986). Although the metals in this experiment had been added a few years before the experiment, long-term changes in metal sequestration were still affecting the binding of metals to the solid phase. In addition the most soluble metals were always removed and the remaining metals were therefore stronger bound.
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Table 5 Multi-linear regression analysis for dissolved Cu and Zn, Cu2þ and labile Zn in soil solution of the topsoil (n ¼ 336), the acidic (n ¼ 192) and the calcareous subsoil (n ¼ 192). The numbers in parentheses represent standard errors. log(DOC)/kg l1
pH
1 log(NO 3 )/mol l
log(time)/day
Constant
R2
Topsoil
log Cu log(Cu2þ) log Zn log(labile Zn)
0.539(0.037) 0.617(0.082) 0.319(0.046) 0.491(0.057)
– 0.772(0.056) 0.649(0.041) 0.685(0.039)
0.231(0.019) 0.541(0.055) 0.415(0.031) 0.517(0.039)
0.201(0.018) – – –
3.006(0.161) 4.063(0.578) 0.689(0.327) 1.240(0.405)
0.729c 0.601c 0.748c 0.714c
Acidic subsoil
log Cu log(Cu2þ) log Zn log(labile Zn)
0.753(0.173) – 0.684(0.179) 0.779(0.253)
– 1.206(0.043) 0.265(0.040) 0.302(0.059)
0.772(0.146) 0.694(0.149) 0.384(0.111) –
0.650(0.088) – 0.512(0.079) 0.315(0.0.113)
3.520(0.857) 0.065(0.422) 5.824(0.0.957) 7.538(1.389)
0.348c 0.826c 0.570c 0.336b
Calcareous subsoil
log Cu log(Cu2þ) log Zn log(labile Zn)
0.323(0.109) – 0.709(0.165 0.396(0.219)
0.826(0.077) 1.141(0.082) 0.267(0.108) –
–
– – 0.461(0.097) –
1.417(0.602) 0.972(0.577) 6.633(1.045) 9.527(0.852)
0.485b 0.580b 0.277a 0.131a
a b c
0.247(0.079) – 0.428(0.147)
Statistical significance (2-tailed) for p 0.1. Statistical significance (2-tailed) for p 0.01. Statistical significance (2-tailed) for p 0.001.
Fig. 9. Measured Cu (a), Cu2þ (b), Zn (c) and labile Zn (d) in soil solution of polluted topsoil vs. predicted values obtained from the multi-linear regressions (Table 5). (B: synthetic rainwater; >: 100 mg N l1; 6: 6 mg N l1; 7: synthetic rainwater).
5. Conclusions Our investigation showed that the exchangeable cations and the low pH by high nitrogen treatment in the irrigation water resulted in the mobilization of large amounts of Zn from the surface soil layer. High nitrogen input stimulated the microbial degradation
and decreased DOC concentrations and consequently dissolved Cu in the solution while the low pH was not a controlling factor for DOC and Cu mobilization. Metal mobilization described by DOC, pH, nitrate and time indicates that the composition of soil solution is the key to assess metal leaching and long-term investigation was necessary. Leaching of metals through the boundary between the
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