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Published online 31 May 2006
Published in J Environ Qual 35:1026-1031 (2006)
DOI: 10.2134/jeq2005.0354
© 2006 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
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TECHNICAL REPORTS

Heavy Metals in the Environment

Effect of Organic Amendment and Plant Roots on the Solubility and Mobilization of Lead in Soils at a Shooting Range

M. Levonmäkia,*, H. Hartikainena and T. Kairesalob

a Department of Applied Chemistry and Microbiology, University of Helsinki, P.O. Box 27 (Latokartanonkaari 11), FIN-00014 University of Helsinki, Finland
b Department of Ecological and Environmental Science, University of Helsinki, Niemenkatu 73, FIN-15140 Lahti, Finland

* Corresponding author (mirva.levonmaki{at}helsinki.fi)

Received for publication September 15, 2005.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Lead (Pb) dissolving gradually from spent pellets constitutes a serious environmental risk in and near shooting ranges, and remediation measures are necessary to prevent its movement to deeper soil layers and ground water. In this study, the effectiveness of organic amendment and plant roots in stabilizing Pb was assessed in a microcosm experiment. Planted (Scots pine, Pinus sylvestris L.) and unplanted microcosms consisting of coarse-textured mineral soil covered with Pb-contaminated humic topsoil were coated with uncontaminated peat layers of 1 to 3 cm and incubated for 77 d. In a percolation test, the microcosms were washed with ultra pure water to simulate heavy rain so as to rinse water-soluble lead (Pbw) from the topsoil layer. Although Pbw remained below detection limits in the mineral soils in all test units, acid-soluble lead (Pba) increased. Peat amendment diminished Pba in the mineral soil layer, this effect being more pronounced in planted soils, indicating that Pb was taken up by the plants. The percolation test showed that the effect of Scots pine seedlings on Pb movement was minor when peat was added. A long-term dissolution test revealed that considerably more Pb was released from old pellets into soil extracts than from new ones, whereas only traces of Pb, if any, were dissolved in sterilized pure water.

Abbreviations: DOC, dissolved organic carbon • ICP–MS, inductively coupled plasma–mass spectrometry • Pba, acid-soluble lead • Pbw, water-soluble lead


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
ON SHOOTING RANGES thousands of kilograms of pellets of various sizes and chemical composition may end up in the soil each year. According to Mukherjee (1993), about 563 Mg of Pb were discharged in Finland in 1990. Heavy metals such as Pb can be released from the pellets and have detrimental effects on biological systems. Because the pellets, which have accumulated over many years, have dissolved and oxidized to various extents (for reactions see Cao et al., 2003), shooting range soils are complex systems to estimate the solubility and mobility of Pb. The dissolution is controlled by contact time, climate, and soil conditions. Total Pb in soil is a parameter that describes the long-term potential environmental risk in the worst case. However, total Pb is not necessarily a relevant parameter for biological functions in soil especially, because it is not fully bioavailable. In chemical terms, the concept of the bioavailable fraction is difficult to apply because plants can promote bioavailability themselves via mechanisms such as root exudates and the acidifying effects of CO2 produced in root respiration and H+ released in connection with cationic nutrient intake.

The chemistry and mobility of metals are regulated via their fractionation between soil and soil solution (Evans, 1989). Lead can occur as free metal cations, exchangeable cations, soluble and insoluble organic and inorganic complexes, insoluble precipitates such as oxides, carbonates, and hydroxides, or as a constituents of mineral lattices (indigenous soil Pb). These fractions differ markedly in their bioavailability. In boreal forest soils, the retention mechanisms of Pb may vary within the soil profile between different horizons because various layers differ in their chemical and mineralogical properties. For instance, in topsoils rich in organic matter, the main retention mechanism is usually the binding of Pb to humic substances via complexation, whereas in deep mineral soil the retention mechanism is usually specific adsorption on oxide surfaces (see Ettler et al., 2005). According to Sauvé et al. (2000) Pb has a much higher affinity for certain mineral surfaces (Fe oxides and phosphates in particular) than for organic matter. Because organic anions tend to be sorbed on oxide surfaces, they may indirectly increase Pb solubility by blocking the reactive sites of oxides. Chemisorption on silicate clays and precipitation as Pb carbonate, hydroxide, or phosphate are all favored by high pH (McBride, 1994).

Pellets remain mainly in the top layer of the soil on shooting ranges and are exposed to organic acids released by the decomposition of organic matter. Plowing has reduced the number of pellets in the top layer of some shooting range soils (Jørgensen and Willems, 1987). However, plowing is problematic in forest soils rich in roots, and even hazardous because the fragile, oxidized crust on the surface of pellets can be crushed, thereby enhancing the solubility of Pb. Addition of a stabilizing agent to the soil would be an ideal way to control Pb leaching, since the vegetation is not disturbed by the spreading operation, and breakable pellets remain intact. Immobilization studies of Pb have recently dealt with zeolite, compost, and Ca(OH)2 (Castaldi et al., 2005). The mechanisms of Pb retention by phosphate rock have also been researched (Cao et al., 2004). There are few, if any, studies dealing with peat as an amendment agent on shooting range soil. Organic amendments may precipitate Pb in soil (McBride, 1994) but their dissolved components may enhance its mobility (Jordan et al., 1997; Geebelen et al., 2002) and contribute to its uptake by plant roots (Albasel and Cottenie, 1985; Jin et al., 2005). The role of plants can be significant because low-molecular-weight root exudates are able to promote the mobility of metals by acidifying the rhizosphere (Mench et al., 1988).

The primary aims of this preliminary study were to examine the dissolution of Pb from pellets, investigate the mobility of pellet-derived Pb in different soil layers, and assess the impact of organic amendment and plant roots on Pb mobility in a laboratory microcosm under controlled conditions. Furthermore, a percolation test was performed to simulate heavy rain and monitor Pb transport from the upper humic soil layer to deeper mineral soil layers.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Soil Samples
Soil samples were collected from a shotgun shooting range area in use for 15 yr in Hollola, southern Finland. The sampling site was located in a typical Finnish dry peaty forest with thick moss and lichen cover. Heavily contaminated samples (humic layer, 0–5 cm) were taken directly from the shooting sector where most of the pellets fall to the ground. A large number of Pb pellets were found in the sampling area (about 5 pellets cm–2). The uncontaminated mineral samples (underneath the humic soil layer at a depth of 5–20 cm) were taken 150 m from the area where pellets had fallen. All soil samples were sieved (4-mm mesh), homogenized, and stored at 4°C for 1 wk. Their characteristics are given in Table 1.


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  		Table 1. Properties of the soil samples (mean ± SD).

 
The moisture of soil samples was determined as weight loss during drying in an oven at 105°C for 6 h. After drying, the organic matter content was determined as loss on ignition (at 550°C for 2 h). The pH was measured in a soil-water suspension at a ratio of 1:2.5. Dissolved organic carbon (DOC) was extracted by ultra pure water at a soil-to-solution ratio of 1:10 w/v and determined by carbon analyzer (TOC-5000; Shimadzu, Kyoto, Japan). Acid-soluble Pb was extracted in an autoclave with 8 M HNO3 (for details, see Turpeinen et al., 2000) and analyzed for Pb with inductively coupled plasma–mass spectrometry (ICP–MS) (Sciex Elan6000; PerkinElmer, Wellesley, MA).

Dissolution Experiment with Pellets
Dissolution of Pb from pellets into extracted soil water (pH 4.05) and distilled water (pH 6.15) was determined in an incubation test in the laboratory. The test was performed with new unused pellets and with pellets between 5 mo and 15 yr old gathered from the humic topsoil of a shooting range. Old pellets were oxidized, as indicated by their white crust. To obtain a soil water for incubation test, uncontaminated humus soil sample was extracted with water at a water-to-soil ratio of 1:50 (w/v). Distilled water was sterilized before use. In the experiment, 1 g of both types of pellets was incubated with 50 mL of soil water or distilled water in polyethylene bottles with 30 replicates. During the incubation, five subreplicates were sampled from each treatment at Days 1, 36, 71, 106, and 141, and 3.5 yr after the beginning of the experiment. Subreplicates were filtered through 5893 filter paper (Schleicher & Schuell, Dassel, Germany) and analyzed for Pb with ICP–MS. The total number of bottles was 120.

Experimental Design of the Microcosm Experiment
In the laboratory experiment, 24 microcosms, as illustrated in Fig. 1 (polyethylene, diameter = 12 cm, height = 25 cm) were filled with growth medium as follows: 2.9 kg of uncontaminated coarse-textured mineral soil (moisture 35%) was covered with 0.46 kg of contaminated humic soil (moisture 15%) including pellets, and the pots were wrapped in black plastic. To monitor the leaching of Pb with water, 30 holes (diameter = 1 mm) had been drilled through the bottom of each microcosm. Soil material was sampled during the experiment through two holes (diameter = 6 mm) drilled in the wall of each microcosm at depths of 4 cm (humus layer) and 12 cm (mineral layer).


Figure 1
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  		Fig. 1. Microcosm experimental setup.

 
The microcosms were divided into two blocks. The first was incubated without any plants, while the second block was planted with Scots pine seedlings grown in a plant nursery on peat substratum. The roots of the seedlings were washed with ultra pure water before planting in mineral soil. No visible mycorrhiza were observed on the root surfaces at the beginning of the experiment. The contaminated humic soil was added to the microcosms after planting the seedlings. Three different treatments with organic amendment (peat, moisture 50%, Pba = 22 mg kg–1 and Pbw = 0.005 mg kg–1 dry matter, loss on ignition 89%) and a control with three replicates were tested in both blocks. In three treatments, peat was added to the top of the growth medium in layers that were 1, 2, and 3 cm thick, while the control was left without peat addition. The moisture was adjusted to remain constant daily during the experiment by watering with ultra pure water. The microcosms were kept for 77 d under constant conditions (temperature 16°C, 18 h light 350 µmol m–2 s–1 and 6 h dark).

Once the experiment started, soil samples were taken from both soil layers five times (Days 1, 14, 35, 56, and 77) with a polyethylene corer (diameter = 0.5 cm, length = 10 cm) through the holes in the walls of the microcosms. The holes were closed with plastic tape between samplings. The pellets in the sample were removed before chemical analyses. Water-soluble Pb and DOC were extracted from the soil samples with ultra pure water using a solution ratio of 1:10 w/v. The soil slurry was shaken (250 rpm) for 2 h and filtered through a 0.45-µm filter (Nalgene syringe filters; Nalge Nunc International, Rochester, NY). Acid-soluble Pb was determined at the outset and at the end of the experiment with 8 M HNO3. All soil extracts were analyzed for Pb with ICP–MS, and water extracts were also analyzed for DOC with a carbon analyzer.

Each microcosm was leached with 150 mL of ultra pure water twice during the experiment (Days 34 and 76). Percolated water was collected on plates (covered with polyethylene plastic film) below each microcosm. The percolates were analyzed for Pb with ICP–MS and for DOC with a carbon analyzer.

Statistical Analysis
Water-soluble Pb and Pba in soil samples and Pb in percolates were analyzed statistically by one-way analysis of variance comparing control and peat treatments. Differences between treatments were determined by Tukey's test. Concentrations of Pbw, Pba, and Pb in percolates were related to organic matter and DOC content. The statistical significance was defined as P < 0.05.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Dissolution of Lead from Pellets
Dissolution of Pb from both new and old pellets was markedly higher into soil water than into sterilized distilled water (Fig. 2). In both incubation solutions, the dissolution was largely dependent on the age of the pellets, being 100% higher for old pellets than for new. The dissolution rates into soil water increased after the second sampling but then appeared to remain constant. During the 141 d of the experiment, soluble Pb concentrations in the soil extract more than doubled. After 3.5 yr, the Pb amount dissolved into soil water was 10 to 20 times higher than that into distilled water. Assuming that the dissolution of Pb from pellets proceeds linearly, the relationship between the amount of Pb dissolved (y) and contact time (x) follows the equations:

Formula

Formula

Formula

Formula


Figure 2
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  		Fig. 2. Lead dissolved from new and old pellets in sterilized distilled water and in soil water as a function of incubation time (mean ± SD). Note the scales.

 
Lead and Dissolved Organic Carbon in Soil Samples in the Microcosm Experiment
Lead pellets did not sink from the surface humic soil layer to the mineral soil layer below during the experiment. The Pbw in mineral soil was under the detection limit of ICP–MS in all samples. During the early stages of the experiment, Pbw increased in the humic soil layer in all treatments in the planted block (Fig. 3B), while there was a contrasting steady increase in Pbw in the control units, up to the end of the experiment and, with the 1-cm peat layer treatment, Pbw remained at a constant level after Day 34. However, Pbw in the units with 2- and 3-cm peat layers decreased markedly after watering at Day 34. In contrast to the Pbw in the planted block, Pbw in the non-planted block decreased in the control and in the 1-cm peat layer treatment at the third sampling after watering (Fig. 3A). The tests on the units with the thicker peat layers (2 and 3 cm), however, revealed a trend toward lower Pbw levels after the third sampling in both blocks.


Figure 3
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  		Fig. 3. Water-soluble Pb (mean ± SD) in the surface humic soil layer (A) in non-planted units and (B) in planted units with peat layers of 1, 2, and 3 cm, plus a control unit with no added peat. Leaching treatments were performed at Days 34 and 76. The sampling day is given in the bar and leaching tests are marked with arrows.

 
Acid-soluble Pb in the mineral layer in the non-planted block was higher in the control unit than in the peat treatments at the end of the microcosm experiment (Table 2), while the planted block showed no differences between the treatments. The Pba concentration had dropped to about half that observed at the beginning of the experiment.


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  		Table 2. Concentration of acid-soluble lead (Pba) in mineral soil in various peat treatments at a depth of 12 cm at both the beginning and the end of the experiment.

 
Dissolved organic carbon in the humic soil layer increased in both blocks during the experiment (Fig. 4), the concentration being higher in the planted block than in the non-planted block, especially in the 1- and 3-cm peat treatments. Dissolved organic carbon increased in the mineral soil in both blocks during the experiment, but no differences between the blocks were found (data not shown). No correlations between Pbw and DOC were found in either the humus or mineral soil layers. In the non-planted block, pH increased from 4.0 to 4.3 during the experiment while the pH was slightly decreased at 3.9 in the planted block.


Figure 4
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  		Fig. 4. Dissolved organic carbon (DOC) in the surface humic soil layer of (A) the non-planted and (B) planted units (mean ± SD). The sampling day is given in the bar and leaching tests are marked with arrows.

 
Lead and Carbon in Percolated Water in the Microcosm Experiment
The concentration of DOC was almost equal in all percolates and no significant differences between the treatments were found (data not shown). In contrast, the peat addition clearly diminished Pb in percolates, but it was irrespective of peat layer thickness (Table 3). In both blocks, the peat addition clearly diminished Pb in the percolates. The first percolate in the planted block from the control units was markedly higher in Pb (0.070 mg L–1 Pb) than that from the units with peat addition (0.013–0.036 mg L–1). Surprisingly, the Pb concentrations of percolates from the units with the 2-cm peat layer were slightly higher than in those from other peat treatments. The Pb concentration in the second percolate was higher than in the first in all units, the increase in the control unit being 17%, and up to 100% with peat additions. It is noteworthy that the percolates from the control units were markedly higher in Pb in the non-planted block than in the planted one.


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  		Table 3. Lead concentrations in the microcosm percolates (mean ± SD).

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
As expected, the dissolution of Pb from old pellets was much higher than from new ones. Pellets that end up in soil are readily oxidized, and CO2 and H2O allow the further formation of Pb carbonates [predominantly PbCO3 and Pb(CO3)2(OH)2] from PbO (Jørgensen and Willems, 1987). Lead carbonates are quite insoluble in weakly acid solutions (Lindsay, 1979). However, Murray et al. (1997) and Lin et al. (1995) have suggested that Pb moving through the soil profile originates from Pb carbonates or sulfates solubilized in the acid soil. It is probable that the transformation of Pb into soluble forms is strongly affected not only by the solution pH but also by the chemical composition and microbiological activity in the solution. In the present study, the significantly higher dissolution of Pb in soil water as compared to distilled water can be attributed to the presence of organic acids and microbes extracted from soil. Lin et al. (1995) estimated that on average 4.8% of metallic Pb in pellets is transformed into soluble form within 20 to 25 yr. Complete transformation is estimated to take 100 to 300 yr (Jørgensen and Willems, 1987). Our calculations indicated that complete transformation takes over 200 yr with old pellets in soil water, and about 350 yr with new pellets. This difference in the age shortened the transformation time by 150 yr, an outcome which indicates that the dissolution does not proceed linearly, because of the weathering of the pellets. However, it is likely that the slopes of the lines describing the dissolution of new pellets will increase in the long run. The old pellets were actually a maximum of 15 yr older than the new ones, a time during which the new pellets will oxidize and their dissolution rate will thus increase. Furthermore, the transformation process would occur much faster if the pellets are fragmented when striking the target.

The finding that Pb was leached from the microcosms with percolating water agrees with the results of the dissolution test with pellets. The fact that Pb and DOC concentrations were higher in the second percolates provides evidence that Pb was continuously dissolved from the pellets during the experiment and leached in the form of DOC complexes. Several authors have reported that complexation with organic ligands increases total soluble Pb and hence the potential of Pb leaching, assuming that the complex does not become saturated and precipitate (DeConinck, 1980; Sposito, 1986; Sauvé et al., 1998). The chemical form of compounds can change during transportation, as was shown in the study by Courchesne and Hendershot (1997), where most dissolved organic matter precipitated in the upper layer of the mineral soil with increasing complexation with Fe and Al ions. It is likely that Pb was leached as a complex with DOC derived from the humic soil layer. The finding that Pba increased in mineral soil whereas Pbw remained under the detection limit suggests that Pb complexes precipitated on oxide surfaces.

Lead solubility was assumed to be high because low soil pH (4) has been reported to enhance the solubility of several Pb compounds (Lindsay, 1979). However, pellet-derived Pb remained mainly in the surface humic soil layer, which agrees with the findings of Johnson and Petras (1998). Even though the humic surface soil was highly contaminated with Pb (Table 1), only 0.02 to 0.08% of acid-soluble Pb was found in water-soluble form during the experiment. Lead in the percolates was also quite low, regardless of the high Pba in the humic soil layer. However, it is possible that over time, free adsorption sites available in humic material are occupied and the mobility of excess Pb increases.

Accumulation of Pb in the organic horizon depends not only on the quantity but also on the quality of organic matter (Fujikawa and Fukui, 2001). Jordan et al. (1997) found that peat humic acids (HAs) have a higher binding affinity for Pb than peat fulvic acids (FAs). In the present study, peat addition decreased the Pb concentration in percolates, an effect which was more apparent in the non-planted block (74–84%) than in the planted one (16–86%). Some Pb–DOC complexes probably precipitated on the way to deeper mineral soil layers, because a relatively small amount of Pb ended up at the depth of 12 cm in the mineral soil. It is likely that peat addition increased the proportion of HAs in the topsoil, effectively retarding Pb mobility by complexation. It is probable that the topsoil in the control units, representing a typical moss and lichen layer of boreal forest soil, contained a higher proportion of soluble organic compounds which transported Pb readily. The effect of peat addition on the Pb in the percolates and the Pba in mineral soil layer was not unambiguous, as peat addition had no effect on total DOC in the percolates. Furthermore, Pbw in the humic layer did not correlate with total DOC, although only total concentration of DOC in soil was measured in this study. It is possible that fractionation of DOC would have revealed a positive correlation between Pb and some molecular size of DOC. Metal–DOC complexation is not well understood because the chemical structure of DOC is highly heterogeneous. The fact that no Pbw was detected in the mineral soil layer in any treatment, but elevated concentrations of Pba were found in the unplanted control provides evidence that the transported Pb was retained on cation exchange sites or specifically sorbed on oxide surfaces.

In the non-planted block, leaching treatment performed at Day 34 just before the soil sampling resulted in decreased Pbw in the humic soil layer in the control and 1-cm peat treatment (Fig. 3A). Percolating water had leached most of the Pb transferred in water-soluble form and Pbw tended to decrease toward the end of the experiment. In the planted block, by contrast, Pbw increased gradually in the control unit, while in the 1-cm peat treatment reached a constant high level after the first leaching treatment. Probably the regular watering of the planted block explains the finding that Pbw in the control at the beginning was much lower than in the non-planted block. The planted block needed more water than the non-planted block since the moisture was kept constant during the experiment. High Pbw in the planted block at the latter stage of the experiment may partly be a result of CO2 produced by root respiration. Elevated partial pressure of CO2 favors the formation of PbCO3 over Pb3(CO3)2(OH)2 (Lindsay, 1979; Essington et al., 2004) in the weathered crust of the pellets. The solubility of PbCO3 is, however, enhanced by a further increase in partial pressure of CO2 (Badawy et al., 2002). The role of plants in soil CO2 production is significant; root respiration in boreal forest ecosystems being estimated to contribute 62 to 89% of total observed soil respiration efflux (Bonan, 1993; Ryan et al., 1997). The higher Pbw in the humic soil layer in the planted block can be partly explained by pH being lower than in the non-planted block. The formation of more soluble PbHCO3+ increases at low pH (Evangelou, 1998). Obviously Pb was taken up by plants in the planted block, since in the mineral soil (sampling depth 12 cm) Pba did not increase during the experiment, while it did in the non-planted block. In fact, the total Pb uptake of the plants at 1, 2, and 3 cm was 5.72, 6.49, and 3.65, respectively, and 7.14 mg in the control (Levonmäki et al., unpublished data, 2005).

In both blocks, the decrease in Pbw in the humic soil layer in the treatments with 2- and 3-cm peat additions was somewhat delayed beginning at the fourth sampling when considering the first leaching treatment. It is noteworthy that Pbw in the humic soil layer did not follow the increasing trend of the DOC. It is likely that the thick peat layer produced DOC which was able to complex Pb in the humic soil layer. Without peat or with a thin peat layer, in contrast, the small-sized molecular fraction of DOC derived from needles and lichen was dominant.

The results of this short-term study showed that concern over Pb mobility in shooting range soils is justified, even though the amount of leached Pb in relation to total Pb is not very great. Leaching tests revealed that heavy rain is able to transfer Pbw to deeper soil layers in humic soil and to increase the risk of leaching. Plants can retard the leaching of Pb into deeper soil layers by root uptake but they can also enhance the solubility of Pb in soils via root exudates and acidification of the rhizosphere (Smith, 1969; Kabata-Pendias, 2004). It is common knowledge that the stability of metal–DOC complexes is pH-dependent and, as shown by Reddy et al. (1995), free ionic forms (Pb2+) followed by ion pairs are dominant at low pH. In addition, the environmental hazard on shooting ranges is complicated because of progressive dissolution of pellets of various ages and sizes. Continuous dissolution of pellets is also a long-term problem, as soil water dissolves them more rapidly than distilled water does. Microcosm experimentation should be repeated as a long-term study to evaluate the efficiency of peat addition as a measure to alleviate environmental risk of Pb in the long run.


   ACKNOWLEDGMENTS
 
This work was supported by the Ministry of Education and the Marjatta and Eino Kolli Foundation. We thank the Hälvälä Shooting Range Association for their cooperation and administrative help during the project.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 





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