Control of lead solubility in soil contaminated with lead shot: effect of soil moisture and temperature.
Extremely high loadings of lead (Pb) commonly occur in soils at recreational shooting ranges. The deposition of large amounts of lead shot ammunition has resulted in soil Pb concentrations >10 000 mg/kg (Manninen and Tanskanen 1993; Mellor and McCartney 1994; Stansley and Roscoe 1996; Rooney et al. 1999; Chen et al. 2001). In a previous paper (Rooney et al. 2007), we established the significance of the interaction between the corrosion products of Pb shot and the soil solid phase in controlling [Pb.sup.2+] activity in the soil solution. Lead shot corrosion products were observed to develop rapidly into crusts surrounding individual Pb pellets, with concurrent increases in Pb in the soil solution and in the soil fine earth fraction. These processes were also strongly influenced by soil pH (Rooney et al. 2007).
The Pb shot corrosion reaction in soil, be it caused by a flow of energy from metal to soil or between electrolytic cells of differing potential on the surface of Pb pellets, requires the presence of soil solution to act as the electrolyte (Chandler and Bayliss 1985). However, the extent to which soil moisture content affects the rate of Pb shot corrosion and subsequent transfer to the soil is unknown. Similarly, the influence of temperature on the Pb shot-soil system is unclear, although both reaction and transport kinetics (Jurinak and Tanji 1993) and the solubility of C[O.sub.2] will be affected by temperature. C[O.sub.2] solubility will influence the Pb shot--soil system because Pb carbonates are important components of such systems (Rooney et al. 2007).
The objective of the current study is to therefore assess the extent to which both soil moisture and temperature can influence the rate and nature of Pb shot corrosion in the soil.
Materials and methods
Two bulk samples of uncontaminated (<20 mg Pb/kg) topsoil (a Waimakariri sandy loam and a Temuka silt loam) were sampled, sieved (<4 mm), homogenised, and air-dried. Some key soil physical and chemical properties are given in Table 1. Soil pH was determined in a 1:2 soil:deionised water mixture that had been stirred intermittently for 30min and then left to equilibrate overnight. Cation exchange capacity (CEC) was determined by ammonium acetate leaching at pH 7.0 (Blakemore et al. 1987). Organic soil carbon was determined using a LECO CNS-2000 analyser (neither soil contained significant amounts of inorganic C). Amorphous iron (Fe) was determined by ammonium oxalate extraction (Blakemore et al. 1987) and particle size analysis by the pipette method (Day 1965). Portions of the soils were treated with lime (Ca[(OH).sub.2]) or HCl to adjust soil pH to ~4.5, 5.5, or 6.5. The soils were adjusted to 75% of field moisture capacity and maintained at 25[degrees]C for 4 weeks, after which they were air-dried and sieved to <2mm, and the final pH values checked. To prepare the soils for incubation, 200-g samples of the pH-adjusted soils were mixed with 10.00 [+ or -] 0.05 g of Pb shot (5% Pb; No. 8 size; 2.3mm diameter). For each treatment there were 3 replicates (Pb shot added) and a control (no Pb shot).
The moisture and temperature treatments used were as follows. For soil moisture, samples were prepared using the 2 soils at the 3 different soil pH values, preparing sufficient replicate samples to allow samples to be removed for analysis every 6 months throughout the 24-month experiment. Deionised water was added to all jars to moisten soil samples to either field capacity (FC) or 70% of FC as required, and all samples were then placed in an incubator maintained in an aerobic environment at 25[degrees]C. Soil moisture content was checked regularly by weighing and maintained at FC, or 70% of FC, with deionised water. For soil temperature, samples were prepared using the two soils, each adjusted to a nominal soil pH of 5.5. Sufficient replicate samples were prepared to allow samples to be removed for analysis after 12 and 24 months of incubation. Deionised water was added to all jars to moisten soil samples to FC, and samples were then placed in incubators maintained in an aerobic environment at 10, 25, or 30[degrees]C. Soil moisture content was checked regularly by weighing and maintained at FC with deionised water.
[FIGURE 1 OMITTED]
Sampling and analyses
Samples were removed from the incubator every 6 months (moisture study) or 12 months (temperature study). The soil solution was removed from the moist soil (Pb shot included) for analysis, by centrifuging for 30min at a relative centrifugal force of 1800G (based on the method of Elkhatib et al. 1987). Solution pH was determined immediately after centrifugation, then the solution was filtered (0.45 [micro]m) and analysed for Pb by flame or graphite furnace atomic absorption spectrophotometry (FAAS or GFAAS), and for dissolved organic carbon (DOC) using a Shimadzu TOC 5000A total organic carbon analyser. Concentrations of Ca, Mg, K, and K were determined by FAAS, and anions (N[O.sub.3.sup.-], [Cl.sup.-] , S[O.sub.4.sup.2-], P[O.sub.4.sup.3-]) by ion exchange chromatography.
Following centrifugation, the soil was air-dried. All visible Pb shot was removed from the air-dried soil samples by hand, then the soil was gently crushed and sieved to <1 mm to recover the remaining Pb pellets. Within the 24 months of the experiment, no pellet cores (consisting of non-corroded Pb) were reduced in diameter by <1 mm; thus, all Pb pellets were eliminated from subsequent soil analysis.
Total fine earth (<1mm) soil Pb concentrations were determined by microwave digestion in concentrated HN[O.sub.3] (USEPA Method 3051). Three certified reference soil materials (Standard Reference Materials 2709, 2710, and 2711; National Institute of Standards and Technology) were also analysed using this method. The concentrations determined using the USEPA Method 3051 microwave method were within the published ranges for acid-leachable Pb concentrations for these materials.
Sequential fractionation of fine earth Pb was carried out using a modified Tessier (Tessier et al. 1979) method (exchangeable fraction: 1 M Mg[(N[O.sub.3]).sub.2] (pH 7.0) for l h; carbonate fraction: 1 M NaOAc (pH 5.0) for 5 h; Fe-Mn oxide fraction: 0.04 M N[H.sub.2]OH.HCl in 25% HOAc at 96[degrees]C for 6h; organic fraction: 0.02 M HN[O.sub.3] + 30% [H.sub.2][O.sub.2] (pH 2.0) at 85[degrees]C for 5h; residual fraction: microwave digestion in concentrated HN[O.sub.3], as for USEPA Method 3051). For logistical reasons, from each treatment block of 3 replicate samples, only 1 replicate (nearest to median soil pH) was selected for sequential fractionation of Pb. Lead concentrations in digests and extracts was determined by FAAS.
Lead chemical speciation calculations
The chemical speciation model used in this study is WHAM Version 6.0 (www.windermere.ceh.ac.uk/aquatic_processes/ wham). Briefly WHAM 6 consists of a discrete site description of the cation-complexing properties of fulvic acid (FA) and humic acid (HA), combined with a sub-model for electrostatic effects on specific binding and a Donnan sub-model for counter ion accumulation (Tipping 1998). Complexation reactions with inorganic ligands are also accounted for. WHAM 6 also includes a surface complexation model with parameters for ion-binding to oxides of iron, aluminium, manganese, and silicon, and a model for cation exchange on clay.
Results and discussion
Development of crust material
Corrosion crusts developed on Pb shot in soil incubated at both high and low moisture contents. Visual comparison clearly showed that the amounts of crust material present at corresponding pH values were much less for the lower moisture treatment (Fig. 1). The amount of metal removed by corrosion is directly proportional to the amount of current flow (Evans 1981); therefore, the smaller amounts of corrosion products observed at lower soil moisture content may be due to reduced presence of electrolyte for current flow.
Crust material also developed on Pb shot in both soils at all soil temperatures; however, the amount of crust material on Pb shot was substantially less at 10[degrees]C than at the 2 higher temperatures (Fig. 2). Change in temperature is most likely to affect the Pb shot-soil system through reaction kinetics or the solubility of C[O.sub.2] and carbonate minerals. However, Essington et al. (2004) calculated that a temperature change in the range 10-30[degrees]C does not significantly affect the solubility of Pb-carbonate minerals which dominate the corrosion crust. Therefore differences in crust solubility seem unlikely to have caused the variation in volume of corrosion crust material. However, the rate of surface reactions and/or transport of reactants and solutes will be slower at lower temperature and appears to have reduced the rate of Pb shot corrosion and production of corrosion crust material.
Accumulation of Pb in the soil fine earth fraction
Despite a large proportion of the added Pb remaining as intact Pb shot, fine earth (<1 mm) Pb concentrations rose rapidly in all moisture and temperature treatments (Figs 3 and 4). Figure 3 indicates that anomalously high soil Pb concentrations were evident in some of the 12- and 18-month data. It is suspected that the accuracy of fine earth Pb concentrations determined for some of these 12- and 18-month samples was compromised by changes in the methodology used in sieving of dried soil + Pb shot samples. These changes most likely resulted in a greater than average removal of crust material from the shot during sieving, the crust material ending up in the <1 mm soil sample. Thus, measured fine earth Pb concentrations are most likely to overestimate the true concentration for these samples. After taking this into account, the temporal trends indicate that fine earth Pb concentrations became steady after 12 months incubation in the Waimakariri soil. This trend was less apparent for fine earth Pb concentrations in the Temuka, most likely as a consequence of the greater sorption capacity of this soil.
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
For both soils, there were clear differences between the 2 moisture treatments in the amounts of Pb associated with the fine earth soil fraction at all samplings. Lead concentrations were much higher in the samples maintained at the higher moisture content, particularly in the Temuka soil (Fig. 3). Similarly, temperature had a marked effect on the concentrations of Pb associated with the fine earth soil fraction. At both the 12- and 24-month samplings, fine earth Pb concentrations were much higher at 25 and 30[degrees]C than 10[degrees]C (Fig. 4). However, there were no consistent differences between 25 and 30[degrees]C.
Since there were only 2 data collection periods, it is more difficult to interpret temporal trends for the temperature treatments (Fig. 4). Two possible effects of temperature on the Pb shot-soil system, i.e. kinetics and mineral solubility, were discussed above. If the dominant effect of temperature was on mineral solubility, higher Pb-carbonate solubility at 10[degrees]C would be expected to result in higher solution and soil Pb concentrations. Instead, the relatively low solution Pb concentrations suggest that the dominant effect of temperature is on reaction kinetics, as described by the Arrhenius equation (k=[Ae.sup.-E/RT]), i.e. the rate of Pb shot corrosion appears to have increased with temperature.
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
Fractionation of Pb in the soil fine earth fraction
The fractionation of Pb in the 2 soils after 24 months incubation at 70 or 100% FC moisture content is shown in Fig. 5. For the 2 lowest pH treatments for each soil, differences between the 2 moisture contents are fairly clear and supported by ANOVA. For both soils, at the higher moisture content, there are greater (P < 0.001) proportions of the total Pb present in potentially labile (exchangeable plus carbonate) forms. In particular, the proportions of total Pb present in the exchangeable fraction are significantly greater at the higher moisture content (P < 0.001 for the Waimakariri and P < 0.05 for the Temuka soil). There is no consistent pattern for the highest pH treatment. The increase in exchangeable Pb with decreasing pH is consistent with observations made previously (Rooney et al. 2007).
Figure 6 shows the effect of temperature on the fractionation of Pb in the 2 soils after 24 months of incubation. In both soils there is a significant increase in potentially labile Pb (exchangeable plus carbonate fractions) with increasing temperature (Waimakariri P < 0.001, Temuka P<0.01), with significant increases in exchangeable Pb in particular (Waimakariri P < 0.01, Temuka P<0.001). By comparison with the data shown in Fig. 5, the increase in the proportion of exchangeable Pb with increasing temperature is likely to be related mainly to the decrease in pH associated with increasing temperature. However, the increases in proportions of Pb in the exchangeable and carbonate fractions with both increasing moisture and temperature are also an effect of increasing Pb concentrations in the soil fine earth fraction (Figs 3 and 4). These are the main forms of Pb accumulated from the corroding Pb shot (Rooney et al. 2007).
[FIGURE 6 OMITTED]
Pb in the soil solution
Soil solution Pb concentrations increased over the 24-month period for both moisture treatments in both soils at all pH levels (Fig. 7). Mean solution Pb concentrations in the uncontaminated soils were <40 [micro]g/L. After 24 months of incubation, mean soil solution Pb concentrations for the 70% FC moisture treatment were generally <40% of the corresponding means at 100% FC. This suggests that low moisture limits the production of potentially soluble corrosion products.
[FIGURE 7 OMITTED]
In general, incubation temperature also had a strong effect on soil solution Pb concentrations (Fig. 8). At 10[degrees]C, the solution Pb concentrations in the Waimakariri and Temuka soils after 24 months of incubation were <5% and 12% of those at 30[degrees]C, respectively. The decrease in soil solution pH with increasing temperature may have contributed to this effect, but in the main, the differences are related to increases with temperature in Pb shot dissolution and accumulation of Pb in the fine earth fraction.
Chemical speciation of Pb in soil solution
Chemical speciation of Pb in soil solution was modelled with WHAM 6 using the same inputs and assumptions as described by Rooney et al. (2007). As in that study on the effects of soil pH, the results of the speciation modelling indicated that in both soils, at all 3 temperatures and both soil moisture contents, the dominant form of Pb in soil solution was organically complexed Pb. The remaining Pb in solution was mainly in the form of the simple [Pb.sup.2+] ion. Also, as in Rooney et al. (2007), modelled [Pb.sup.2+] activities in the soil solution were clearly much lower than would be the case if they were controlled by the solubility of the dominant Pb compounds present in the soils (i.e. [Pb.sub.3][(C[O.sub.3]).sub.2][(OH).sub.2] and PbC[O.sub.3]). Thus, the current data supports our hypothesis that, at least during the 24 months of the study, soil solution Pb concentrations were more likely to be controlled by sorption of Pb by the soil solid phase. Because of the similarity of the current speciation data to those of our previous study (Rooney et al. 2007), we do not present the detailed data here. However, both soil moisture and temperature did have some effect on the proportions of soil solution Pb that were present as organically complexed Pb.
[FIGURE 8 OMITTED]
In general, the proportion of soluble Pb complexed with organic matter tended to decrease with increasing temperature (Fig. 9), and with increasing moisture content (Fig. 10). These trends were closely related to the changes in pH, soluble organic C, and total soluble Pb resulting from the various treatments. This is demonstrated clearly by the following multiple regression equation calculated from the data for both soils, all 3 temperatures, and both soil moisture treatments:
Pb-org (%) = 64.7 - 3.26 (total soluble Pb) + 6.45 pH + 0.00828 (DOC) [R.sup.2] = 0.82
where Pb-org (%) is the proportion of soil solution Pb complexed by soluble organic matter as calculated by WHAM 6, and the units for total soluble Pb and DOC are mg/L.
The above equation helps to explain the trends observed in Figs 9 and 10. Although soluble organic C concentrations tended to increase with time in both soils, thus increasing the potential for complexing Pb, solution pH tended to decrease and total soluble Pb to increase at the same time, more than cancelling out the effect of soluble organic C. Decreases in solution pH with time were more marked at the higher temperature and moisture treatments, as were increases in total solution Pb (Figs 7 and 8). These 2 factors are probably related, since it is known that higher metal loadings on soils can lead to decreases in soil pH due to displacement of [H.sup.+] ions from sorption sites by [Pb.sup.2+] ions, and/or release of [H.sup.+] ions during [Pb.sup.2+] hydrolysis (e.g. Basta and Tabatabai 1992).
[FIGURE 9 OMITTED]
[FIGURE 10 OMITTED]
Although substantial proportions of solution Pb were complexed by soluble organic matter, free ion ([Pb.sup.2+]) concentrations in the soils treated with Pb shot, as calculated with WHAM 6, were increased by up to several orders of magnitude relative to the controls. Free ion ([Pb.sup.2+]) concentrations in the control soils ranged from approximately 5 x [10.sup.-11] to 5 x [10.sup.-12]M, whereas in the soils treated with Pb shot, they increased up to a maximum value of 2.05 x [10.sup.-5] M in the Waimakariri soil or 3.51 x [10.sup.-6] M in the Temuka soil. Lofts et al. (2004) derived toxicity-based critical limits for free ion [Pb.sup.2+] concentrations involving the following simple relationship with pH:
Log [Pb.sub.free, toxic] = -8.3 pH - 4.83
Based on this equation, nearly all of the soil samples treated with Pb shot in the current study have free ion [Pb.sup.2+] concentrations well above the critical limit for Pb toxicity as derived by Lofts et al. (2004). Indeed, the main samples below the limit were the control or time zero samples. Even after just 6 months of incubation with Pb shot, free ion [Pb.sup.2+] concentrations in most cases exceeded the critical limit.
Our previous study (Rooney et al. 2007) showed that soil pH has a major effect on the rate of Pb shot dissolution in soils. The current study shows that soil moisture content and temperature will also have significant effects on Pb shot transformation rates. The results from this study also demonstrate substantial effects of pH, soluble organic C, and total soluble Pb on the speciation of Pb in soil solutions, and thus potential effects of Pb bioavailability. Soils subject to inputs of Pb shot can become highly contaminated with Pb, and our studies indicate that the rate of build-up of Pb associated with the soil solid phase and the soil solution, and the risk associated with this build-up, are likely to vary considerably from site to site depending on both soil and environmental conditions. The rate of approach to equilibrium of the Pb shot-soil-soil solution system will be much slower where soil moisture and temperature limit Pb shot corrosion. However, based on previously published data on soil critical limits, an examination of free ion [Pb.sup.2+] concentrations suggests that after 6 months, almost all samples contaminated with Pb shot exceeded toxicity criteria for a range of soil biological endpoints.
This work was partially funded through AGMARDT and Lincoln University through the award of scholarships to one of the co-authors (C. P. Rooney).
Manuscript received 28 August 2008, accepted 1 December 2008
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R. G. McLaren (A,C), C. P. Rooney (A,B), and L. M. Condron (A)
(A) Soil and Physical Sciences Group, Agriculture and Life Sciences Division, PO Box 84, Lincoln University, Lincoln 7647, Canterbury, New Zealand.
(B) present address: Department of Earth Sciences, Open University, Milton Keynes MK7 6AA, Bucks, England.
(C) Corresponding author. Email: Ron.McLaren@lincoln.ac.nz
Table 1. Selected physical and chemical characteristics of the two uncontaminated soils Waimakariri Temuka Soil sandy loam silt loam New Zealand soil Immature Pallic Gley soil classification soil pH ([H.sub.2]O) 5.8 5.8 CEC ([cmol.sub.c]/kg) 11 31 Carbon (%) 2.5 6.7 Oxalate-extractable Fe (%) 0.48 0.63 Sand (%) 61 14 Silt (%) 49 81 Clay (%) 9 9
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|Author:||McLaren, R.G.; Rooney, C.P.; Condron, L.M.|
|Publication:||Australian Journal of Soil Research|
|Date:||May 1, 2009|
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