Journal of Environmental Quality 31:162-167 (2002)
© 2002 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
TECHNICAL REPORT
Heavy Metals in the Environment
Soil Solid-Phase Controls Lead Activity in Soil Solution
S. H. Badawy*,a,b,
M. I. D. Helala,
A. M. Chaudrib,
K. Lawlorb and
S. P. McGrathb
a Soil Science Dep., Faculty of Agriculture, Cairo Univ., Giza, Egypt
b Agriculture and Environment Division, IACR-Rothamsted, Harpenden, Hertfordshire, AL5 2JQ, UK
* Corresponding author (sbadawy4{at}hotmail.com)
Received for publication September 8, 2000.
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ABSTRACT
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Lead pollution of the environment is synonymous with civilization. It has no known biological function, and is naturally present in soil, but its presence in food crops is deemed undesirable. The concern regarding Pb is mostly due to chronic human and animal health effects, rather then phytotoxicity. However, not much is known about the chemistry and speciation of Pb in soils. We determined the activity of Pb2+, in near neutral and alkaline soils, representative of alluvial, desertic and calcareous soils of Egypt, using the competitive chelation method. Lead activity ranged from 10-6.73 to 10-4.83 M, and was negatively correlated with soil and soil solution pH (R2 = -0.92, P < 0.01 and R2 = -0.89, P < 0.01, respectively). It could be predicted in soil solution from the equation: log
= 9.9 - 2pH. A solubility diagram for the various Pb minerals found in soil was constructed using published thermodynamic data obtained from the literature, and our measured Pb2+ activities compared with this information. The measured Pb2+ activities were undersaturated with regard to the solubility of PbSiO3 in equilibrium with SiO2 (soil). However, they were supersaturated with regard to the solubilities of the Pb carbonate minerals PbCO3 (cerussite) and Pb3(CO3)2(OH)2 in equilibrium with atmospheric CO2 and hydroxide Pb(OH)2. They were also supersaturated with regard to the solubilities of the Pb phosphate minerals Pb3(PO4)2, Pb5(PO4)3OH, and Pb4O(PO4)2 in equilibrium with tri-calcium phosphate and CaCO3. The activity of Pb2+ was not regulated by any mineral of known solubility in our soils, but possibly by a mixture of Pb carbonate and phosphate minerals.
Abbreviations: GF–AAS, graphite furnace atomic absorption spectrometry • DTPA, diethylenetriaminepentaacetic acid
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INTRODUCTION
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LEAD pollution of the environment occurs through anthropogenic activities such as mining and smelting, use of biosolids in agriculture, and the past use of anti-knock gasoline additives such as tetramethyl and tetraethyl lead. Although a move to unleaded gasoline was made in many countries a decade or so ago, leaded gasoline is still available and used in many developing countries. Smelting and gasoline combustion primarily cause air pollution with the lead particles reaching the soil through dry and wet deposition (Bolt and Bruggenwert, 1978; Milberg et al., 1980). Lead pollution, irrespective of source, is of major concern because of its long residence time in the soil and its possible association with cognitive development in children. It has no known biological function and is one of a small group of heavy metals whose increase both in soils and in the food chain is deemed undesirable. Because of this, many countries in the industrialized world have set limits for Pb in both soils and food crops to reduce its buildup in the environment and the food chain (Commission for the European Communities, 1986; USEPA, 1993).
Lead in soil is thought to be mostly bound to the solid phase and, therefore, insoluble. Hence, not much is known about the chemistry and speciation of Pb in soils, and as with other heavy metals, Pb reaching the soil may undergo several reactions with different soil constituents, which affect its solubility, mobility, and availability to plants. Therefore, the chemistry of Pb in soil can be affected by (i) specific adsorption reactions at mineral interfaces, (ii) precipitation of sparingly soluble compounds, of which they are constituents, and (iii) the formation of relatively stable complex ions or chelates, due to interactions with soil organic matter and inorganic constituents (Santillan-Medrano and Jurinak, 1975; Davis, 1995). These interactions may either occur separately, or in combination with each other, either successively, simultaneously, or even alternating (Bolt and Bruggenwert, 1978). The relative importance of these interactions in the retention and/or release of Pb in soils is poorly understood. Elliott et al. (1986) reported that under acidic conditions adsorption onto the solid phase was more important than precipitation in removing metal ions from solution. For near neutral and alkaline soils, complexation reactions may be superimposed on adsorption processes, thereby complicating the prediction of the relative solubility of heavy metals in soil solution (Elliott and Denney, 1982). For example, loss of Pb from soil solution above pH 6.0 has been attributed to the precipitation of a separate solid phase (Harter, 1983). Hence, Pb entering the soil may become partitioned among several soil compartments. The main compartments are the soil solution, the adsorption surfaces of the clay–humus exchange complex, precipitated forms, secondary Fe and Mn oxides, alkaline earth carbonates, the soil humus, and silicate lattices (Davis, 1995).
Identification of the sparingly soluble compounds controlling Pb solubility in soil is of great importance for maintaining and improving food crop quality, and chemical remediation of polluted environments. In this respect, the determination of Pb2+ activity in soil solution is essential for availability and toxicity studies. However, direct measurement of Pb2+ activity in soils is difficult using most commonly available methods. Also, the concentration of Pb in soil solution is often below the detection limits of most instruments. In our study, we used the competitive chelation method (Workman and Lindsay, 1990) to measure Pb2+ activity in near neutral and alkaline soils representative of Egyptian soils. Furthermore, we compared our measured Pb2+ activities with the solubilities of various Pb minerals expected to regulate lead activity in soil.
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MATERIALS AND METHODS
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Eleven soil samples (0–30 cm depth) were collected from different locations to represent the three main soil types found in Egypt: alluvial, desertic, and calcareous soils (Table 1). Soils 1, 3, 4, and 11 were agricultural soils close to urban areas and highways and are, therefore, predominantly polluted with Pb from car exhaust fumes. Soil 2 was an agricultural soil next to an industrial site and Soil 10 was an agricultural soil that has been receiving raw sewage effluent from domestic and industrial sources from Cairo since 1912. Soils 5 to 9 were agricultural soils with no previous history of heavy metal contamination. Each sample was a thoroughly homogenized composite sample consisting of 10 subsamples. The soil samples were air-dried and sieved through a 2-mm sieve prior to analysis. The physical properties, clay and organic matter contents, total carbonate, and pH of the soils were determined using the standard methods reported by Dewis and Freitas (1970). Total Pb in soil was determined by graphite furnace atomic absorption spectrometry (GF–AAS) after digestion with concentrated HF–HClO4 acids following the procedure of Dewis and Freitas (1970). Extractable Pb was determined by shaking 10 g soil with 20 mL solution containing 5 mM diethylenetriaminepentaacetic acid (DTPA), 0.01 M CaCl2, and 0.1 M triethanolamine (TEA) adjusted to pH 7.3 (Lindsay and Norvell, 1978) for 2 h. The filtrates were analyzed for Pb using GF–AAS.
Lead activity in soils was determined using the competitive chelation method described by Workman and Lindsay (1990) for the determination of Cd activity. Briefly, the competitive chelation method involves competition between the metal ion of interest with another metal ion for a given chelating ligand. Hence, the method consists of shaking a known amount of soil with a series of chelating solutions containing different mole fractions of the studied metal and a competing one. Based on the work of Workman and Lindsay (1990), we used Cd as the competing metal ion for Pb. Hence, the mole fraction here is represented by the ligand-bound Pb concentration over the ligand-bound Pb concentration plus the ligand-bound Cd concentration. The ligand used was 10 µM DTPA in all cases and is represented in the text by the letter L. A series of chelating solutions were prepared containing different mole fractions [i.e., PbL/(PbL + CdL)] ranging from 0 to 1. To each mole fraction solution, 0.01 M CaCl2 was added to maintain Ca2+ activity in solution. The pHs of these solutions were initially 6.1, but were adjusted to those of the soils under investigation using 0.5 M NaOH and/or HCl, which also served to maintain the soil suspension pHs near their original soil values throughout the experiment.
Fifteen grams, in triplicate, of each soil were weighed into 125-mL Erlenmeyer flasks, and 30 mL solution with a known PbL/(PbL + CdL) mole fraction was added together with 10 mg of CdCO3 to maintain Cd2+ activity in solution. Blanks were also prepared by adding 30 mL solution, without the chelating agent, but with 0.01 M CaCl2, to 15 g of each soil, in triplicate. The Erlenmeyer flasks were covered with parafilm, which was perforated to allow gaseous exchange with the atmosphere, before shaking on a rotary shaker at 120 rpm for 5 d. The suspensions were then transferred to centrifuge tubes, and the pH determined before centrifuging at 15000 rpm for 20 min. The supernatants were filtered through a 0.45-µm filter, and the filtrates analyzed for Pb and Cd using GF–AAS.
Calculations
Lead and Cd concentrations in the filtrates were used to calculate the final mole fractions [i.e., PbL/(PbL + CdL)] for each soil after subtracting the blanks. Graphs of initial versus final mole fractions for each soil were plotted and the equilibrium mole fractions obtained, as shown in Fig. 1
for Soil 2. The equilibrium mole fraction value for each soil was then used to calculate Pb2+ activity in the soil solution as shown below.
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Fig. 1. Changes in PbL/(PbL + CdL) mole fraction after shaking solutions with Soil 2, where L represents the chelating ligand.
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The equilibrium equation required to calculate Pb2+ activity in soil solution can be derived as follows:
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 | [3] |
Rearranging Eq. [3] gives:
 | [4] |
where Km0.01 is the mixed equilibrium constant at solution ionic strengths of 0.01 M, L is the concentration of the ligand DTPA and [ ] indicates molar concentrations. Since the activity coefficients of Pb2+ and Cd2+ are equal and activity coefficients for PbL3- and CdL3- are equal, Eq. [4] can be written in terms of activity, expressed as (), as follows:
 | [5] |
In Eq. [5] the (PbL3-)/(CdL3-) value can be obtained from the equilibrium mole fraction ([PbL/PbL + CdL]), which is determined experimentally for each soil. Since CdCO3 was added to the soil suspensions to control Cd2+ activity throughout the experiment and the solutions were at equilibrium with atmospheric CO2, Cd2+ activity was calculated as follows:
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 | [7] |
At 0.03 KPa CO2:
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Substituting Eq. [8] into Eq. [5] gives the calculated Pb2+ activity:
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The pH value used in Eq. [9] was the pH of the soil suspension closest to the equilibrium point in the initial and final mole fraction plot.
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RESULTS AND DISCUSSION
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Total Pb concentrations in agricultural soils with no previous history of metal contamination (e.g., Soils 5 to 9) ranged from 23 to 42 mg kg-1, and were similar to those reported in uncontaminated soils of about 20 to 40 mg kg-1 (Davis, 1995; Table 1). Agricultural soils next to major roadways (e.g., Soils 1, 3, 4, and 11) contained approximately two to three times the concentration of total Pb compared with soils with no history of contamination. Soil 2, next to a large industrial complex, surprisingly contained less Pb than soils next to roadways. However, the highest total soil Pb concentration was found in the agricultural soil (Soil 10) receiving raw sewage effluent from Cairo for 88 yr up to the present time. The concentration of Pb in this soil was about five times that of soils with no previous history of contamination, and about twice that of soils from along major roadways. The difference in total soil Pb concentration between the soils was also reflected in the amount of DTPA-extractable Pb (Table 1).
Table 2 shows the concentrations of the initial and final chelated Pb and Cd, the initial and final solution PbL/(PbL + CdL) mole fraction, and the equilibrium soil solution pH, after shaking Soil 2 with the chelating solutions for 5 d. Similar tables were constructed for the other 10 soils, but are not shown here for brevity. For all soils, at low initial mole fractions, the chelating solutions solubilized Pb from the soil, whereas at higher values the chelates lost Pb. The sum of PbL and CdL in all solutions from the 11 soils, after equilibrium, was always >70% of the initial sum of PbL and CdL before addition, indicating that Pb and Cd were the principal metals being chelated.
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Table 2. Changes in the solution mole fraction [MF = PbL/(PbL + CdL), where L represents the chelating ligand] before and after shaking with Soil 2.
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A plot of the final versus initial PbL/(PbL + CdL) mole fraction was made for each of the 11 soils studied; however, the plot for Soil 2 is presented in Fig. 1 only for brevity. Similar relationships have been found for Cd (El-Falaky et al., 1991) and Zn (Norvell et al., 1987; Singh et al., 1982) using the competitive chelation method. Figure 1 shows that the line of the mole fraction after 5 d shaking intersected the initial mole fraction line at a certain point that reflected neither gain nor loss of Pb (i.e., the equilibrium mole fraction). The equilibrium mole fraction values were obtained for all 11 soils and ranged from 0.21 to 0.52. These values were significantly correlated with soil clay (R2 = 0.60, P < 0.05) and organic matter (R2 = 0.65, P < 0.05; Soil 10 excluded from the regression analysis) contents, and increased with increasing clay and/or organic matter content.
The equilibrium mole fraction value (PbL/CdL) for each soil was then used in Eq. [9] to calculate Pb2+ activities in the 11 soils (Table 3). Lead activities were found to range from 10-6.73 to 10-4.83 M (1.9 x 10-7 to 1.5 x 10-5 M), and were negatively correlated with soil pH (R2 = -0.92, P < 0.01; Fig. 2)
. Lead activity was highest in Soil 10 of pH 6.8, from El-Gabal Al Asfar, receiving raw sewage effluent, and lowest in soil with no previous history of contamination and an average pH of 7.7. The pHs of soils from next to roadways and the industrial site were similar to those of soils with no previous history of metal contamination, as were the Pb2+ activities (Table 3). Gregson and Alloway (1984) found that the soil solutions from contaminated soils contained about 10-6 M Pb. It is well known that Pb2+ and Cd2+ activities increase as soil pH decreases (Lindsay, 1979; Sauvé et al., 1997; Santillan-Medrano and Jurinak, 1975; El-Falaky et al., 1991; Japony and Young, 1994). The Pb2+ activities in our soils as a function of soil pH (Fig. 2) could be predicted by the equation:
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Table 3. Equilibrium pH, mole fractions [PbL/(PbL + CdL), where L represents the chelating ligand], and Pb2+ activities in the soils studied.
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The Pb2+ activities in soils as a function of equilibrium soil solution pH (Fig. 2) could be predicted by the equation:
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To identify the Pb minerals that may control the level of Pb in the soils studied, we plotted an equilibrium stability diagram in terms of Pb2+ activity and pH (Fig. 3)
. The thermodynamic data shown in Table 4 for the various Pb minerals were taken from Lindsay (1979) and used to calculate the Pb2+ activities of the minerals at different soil pHs as follows:
 | [12] |
 | [13] |
at a CO2 pressure of 0.03 KPa, Eq. [12] becomes
 | [14] |
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Fig. 3. Measured Pb2+ activities of this study ( ) compared with the solubilities of various Pb minerals.
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The solubility of Pb from PbCO3 (cerussite) and some of the other minerals depicted in Table 4 is plotted in terms of soil pH in Fig. 3. The mineral PbSiO3 is depicted in equilibrium with SiO2 (soil) and SiO2 (quartz), and lead phosphate minerals such as Pb3(PO4)2 and Pb5(PO4)3-OH are depicted in equilibrium with various other phosphate minerals to cover a range of possible solid phase P minerals. The level of Pb2+ activity maintained by Pb-CO3 (cerussite) and Pb3(CO3)2(OH)2 depends on the partial pressure of CO2, and that maintained by lead phosphate minerals depends on the phosphate minerals. Calcium activity was considered to be controlled by CaCO3.
The measured Pb2+ activities of the soils used in our study were superimposed on Fig. 3, and compared with those of the various Pb minerals. The measured Pb2+ activities of our soils were undersaturated with respect to those maintained by PbSiO3 in equilibrium with SiO2 (soil), at the pH values of our soils. This is in agreement with the statement of Lindsay (1979) that lead silicates, in equilibrium with quartz or soil Si, are too soluble to be expected in soils. On the other hand, the measured Pb2+ activities were supersaturated with regard to the lead carbonate minerals PbCO3 and Pb3(CO3)2(OH)2 in equilibrium with atmospheric CO2 and hydroxide, Pb(OH)2. Similarly, the measured Pb2+ activities were supersaturated with respect to lead phosphate minerals Pb3(PO4)2, Pb5(PO4)3(OH), and Pb4O(PO4)2 in equilibrium with tri-calcium phosphate (TCP) and CaCO3 (Fig. 3). The minerals Pb(OH)2, PbCO3, and Pb3(CO3)2(OH)2 have identical Pb activities at a CO2 partial pressure of 0.03 KPa (Fig. 3). The hydroxide Pb(OH)2 is considerably more stable at pH 8, maintaining a Pb2+ activity of approximately 10-8 M, whereas PbCO3 is more soluble at lower pH. Increasing the partial pressure of CO2 increases the solubility of PbCO3, whereas reducing it results in Pb(OH)2 being the stable phase. Lead in "uncontaminated" soils is present at concentrations of <20 mg kg-1 (Davis, 1995), whereas phosphates are generally present at much higher concentrations. It is therefore highly possible that phosphate may control Pb solubility (Nriagu, 1974). According to Fig. 3, Pb phosphate mineral solubilities decreased in the order: Pb4O(PO4)2 > Pb5(PO4)3OH > Pb3(PO4)2.
In conclusion, it is difficult to identify any one Pb mineral that may regulate Pb2+ activities in the soils studied, and it is more likely that a mixture of lead minerals, comprising of lead carbonates and phosphates, may be regulating Pb2+ activities in our soils. A similar conclusion was drawn by Santillan-Medrano and Jurinak (1975), who found difficulty in determining the predominant solid phase regulating Pb solubility in calcareous soils of pH 7.5 to 8.0. They concluded that a mixture of some of the Pb minerals PbCO3, Pb(OH)2, Pb3(PO4)2, Pb5(PO4)3(OH), and Pb4O(PO4)2 were controlling Pb solubility in their soils.
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- Davis, B.E. 1995. Lead. p. 208–223. In B.J. Alloway (ed.) Heavy metals in soil. Blackie Academic and Professional, Glasgow, UK.
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INTRODUCTION
