Behavior of Lead in Soil Robert L. Zimdahl Department of Botany and Plant Pathology, Colorado State University, Fort Collins, Colo. 80521
Rodney K. Skogerboe" Department of Chemistry, Colorado State University, Fort Collins, Colo. 8052 1
The behavior of ionic lead in soils of diverse characteristics is investigated. Soils have rather large capacities for immobilization of lead, and these may be reasonably predicted by a correlation function involving cation-exchange capacity and pH. Results demonstrate that the fixation of lead is principally caused by reactions involving essentially insoluble organic materials. Precipitation by carbonate and sorption by hydrous metal oxides appear to be of secondary importance. The conclusions are indirectly confirmed by previous reports involving plant uptake studies. Lead occurs naturally in all soils (1).Its terrestrial abundance in soils ranges from 1to 200 pg/g with a mean of 15 (2). Studies intended to define the levels of lead contamination from auto exhaust all reach the following general conclusions ( 3 ) :the contamination of soil by lead is inversely correlated with distance from the highway and depth in the soil profile, and directly correlated with traffic volume. Lead from autos is exhausted primarily as particulates composed primarily of halide compounds (4-6). These particles are often converted to other compounds, particularly lead sulfate (7), in the atmosphere or the soil. During residence in the soil, the particulates may remain relatively immobile due to the low solubilities of the compounds involved. Alternatively, the movement of lead in the soil profile and its ultimate fate may be determined by one or more of several processes. These depend largely on the dissolution of the lead particles in the ground water. The lead dissolved may be leached through the soil profile if it remains in a soluble form. I t may be immobilized by soil microorganisms ( 8 ) ,precipitation, sorption or ionexchange interactions with soil entities (e.g., clays), or fixation by materials such as organic matter. It may also be taken up by plants, thereby entering the food chain. Data are available which imply that the latter possibility may be of real significance. Page and Ganje ( 9 ) ,for example, have reported that lead is toxic to corn, beans, lettuce, and radishes a t lower concentrations in slightly acid soil. No consistent trends were observed in foliar concentrations, but increasing soil pH decreased lead concentrations found in the plant roots. MacLean et al. (10) suggested the pH effect was due to repression of lead solubility at higher pH values. John and Van Laerhoven ( I I ) , as well as Cox and Rains (12 ), reported that raising the pH of lead-contaminated soils with lime had little effect on root uptake but reduced the amount translocated to foliar portions of the plants. John and Van Laerhoven (11) found that lead carbonate was nearly as available to oat roots (Auena satiua L.) as the chloride or nitrate. Thus, they discounted the possibility that the lower availability at higher pH was due to the formation of insoluble lead carbonate. In essence, these reports strongly suggest that lead in soil can reach the soil-plant root interface and be taken up by plants. Very little definitive information is available, however, concerning the factors that govern the availability (mobility) of lead in soil. Included among the factors that may be important in ascertaining the mobility of lead in soil are: pH, soil texture, clay mineral type and concentration, percent organic matter, concentrations and identities of soil cations and anions, and soil drainage. To define the capacity of soils to immobilize (or 1202
Environmental Science 8 Technology
transfer) lead, experiments have been carried out with soils covering a range of the above characteristics. The behavior of ionic lead in several soils has been examined to determine the capacities of the different soils for lead and the soil parameters most important in determining lead capacities. The experiments have also provided information regarding the primary operative processes which allows postulation of the general mechanism(s) of immobilization.
Materials and Procedures Soils. Although all soils were not used in all phases of this work, a total of 17 mineral and one organic soil was included. Seven Colorado soils were selected primarily because of their different cation-exchange capacities and their agricultural importance. Some soils were provided by scientists interested in the lead question, and others were selected at random. The soils were characterized using standard techniques for determination of particle size distribution (13).Organic matter (Table I) was determined by wet combustion at room temperature in potassium dichromate and sulfuric acid (14). Cation-exchange capacity (CEC) was measured by exchange with sodium acetate at pH 8.2 ( 1 5 ) ,and the pH was determined by the saturated paste method (15). Carbonate concentrations were determined by the addition of acid to evolve COz with collection of the COz on Ascaride for gravimetric analysis (16).Total organic carbon (Table 111) was determined by the dry combustion method ( I 7). Amorphic oxide iron was determined by the method described by McKeague and Day ( I S ) , and oxide manganese was determined by the acidified hydroxylamine hydrochloride method (19). The general characteristics of the soils studied are shown in Table I. Capacity Studies. All soils were passed through a 20-mesh screen to remove rocks, pebbles, and litter. To assure uniform samples in subsequent studies, larger amounts of each soil were passed through 32,60,100, and 200 mesh screens on a mechanical shaker. The weight percent retained in each size range was determined, and smaller composite samples of each soil were prepared by recombining the screened fractions in the weight proportions obtained. Structuring of the composites in this manner was found essential to assure sample uniformity in terms of general composition and particle size characteristics between experiments. The rate of immobilization studies was conducted using 10 g of soil and 150 mL of a lead nitrate solution (860 pg Pb/mL). The solution was agitated continuously in a closed flask, and aliquots of the supernatant were periodically withdrawn for analysis. To determine the capacities of the soils for ionic lead, 25 g of soil was placed in a flask with 100 mL of a lead nitrate solution. Such samples of each soil were equilibrated with solutions in which the lead concentration was varied in seven increments ranging from 0 to 8000 pg/mL. The flasks were sealed with parafilm, and the slurries agitated in a 30 "C water bath for 48 h. The supernatant was then removed by centrifugation and decantation. The supernatant was acidified with HNOBto prevent loss of lead prior to analysis. Samples were analyzed for total lead by atomic absorption spectrophotometry using a Varian-Techtron Model AA-5 unit and an air-acetylene flame. Solution samples were nebulized into the flame under standard operating conditions, and lead absorbance was measured at the 217.0-nm resonance line. Soil samples were digested in hot "03, washed with "03, di-
Table 1. Characteristics of Soils
pH
Sand
Sllt
Clay
Organlc matter
CEC, rneq/100 g
Natlve lead content (ppmw)
7.4 7.1 6.4 8.2 5.4 7.0 8.1 7.8 8.0 4.1 6.8 6.0 5.3 6.3 7.9 6.7 6.2 5.7
86 67 51 67 28
1 12 25 20 21 57 17 20 16 32 28 43 9 46 39 41 43 25
13 21 25 13 51 43 27 12 38 34 54 36 8 44 45 43 43 22
0.8 1.1 0.7 0.5 1.9 1.9 1.4 1.4 0.5 1.8 1.8 4.9 5.0 2.5 1.5 2.2 3.4 23.8
2.4 7.0 7.4 8.5 9.2 9.9 10.3 10.8 11.0 13.0 13.4 13.9 15.4 16.0 19.6 30.5 38.5 125.0
1.9 5.9 6.4 6.2 17.0 14.0 7.8 7.2 7.4 26.0 16.0 23.0 19.3 14.4 9.5 15.2 16.0 33.0
YO
Sol1
1. Lakeland 2. 3. Ascalon 4. San Luis 5. Cecil 6. 7. 8. 9. 10. Troy 11. Weld 12. Chester 13. 14. 15. 16. Heldt 17. Drummer 18.
SOll a type
LS SCL SCL SL C Sic SCL SL
sc
CL C SiCl LS Sic C Sic Sic M
Source
Fla. COlO. COlO. COlO. Mo. Mo. COlO. COlO. Mich. N.Y. COlO. Mo. Ind. Mich. Colo. COlO. Ill. Ohio
0 56 68 46 34 18 18 83 10 16 16 14 53
'LS, loamy sand; SCL, sandy clay loam; SL, sandy loam: C, clay; SiCI, silty clay loam: M, muck.
luted to volume, and analyzed in the same way (20).Corrections for light scattering and molecular absorption, due to the presence of species other than lead in the analytical solutions, were made using a hydrogen hollow cathode a t the same wavelength (21-23). The corrected absorbance values were applied to analytical curves obtained by analysis of standard solutions prepared by dissolving high-purity lead in 3 M "03. In instances where the lead concentration of the analytical solutions was too high for analysis via the above procedure, the 10-cm slot burner was rotated 90 degrees to decrease the absorption path length, or a less sensitive absorption wavelength was used, e.g., 283.3 or 405.8 nm. Replicate measurements made on a particular analytical solution were typically reproducible to better than f 2 % relative standard deviation (20).
Results and Discussion The objectives of these studies were twofold. Initial experiments were designed to determine the effects of soil characteristics on the ability of soils to immobilize ionic lead which might result from the dissolution of particulate species from auto exhaust. In this context, the capacity of soils for immobilization and the rate, extent, and duration of immobilization are important aspects of the question. Later experiments were designed to elucidate the nature of the phenomena responsible for the lead-soil interactions observed. For convenience, the experiments are presented in the sequence in which they were conducted. R a t e of Immobilization. The rate a t which ionic lead is depleted from soil solutions was measured for four different soil types. Lead nitrate solutions (150 mL of 860 pg Pb/mL) were added to 10 g of each soil, and the slurries agitated a t constant temperature. Aliquots (5 mL) of the supernatant were periodically withdrawn and analyzed for lead. In each case the amount of lead available for uptake was a t least twice that actually removed by interaction with the soil. Thus, the excess was sufficient to avoid a concentration effect on the equilibrium time dependency. The example results shown in Figure 1 indicate that a time of a t least 24 h was required to establish 90% of the equilibrium condition attained after 48 h. Because of the relatively long equilibration time required, caution must be exercised when ionic lead solutions are added to soil for plant uptake studies. Moreover, careful mixing of soils doped in this manner is mandated t o ensure the uniform
distribution of lead throughout. For these reasons, plant uptake experiments in which lead solutions are simply poured on the surface of soils with plants present must be considered of questionable validity (24).All subsequent measurements in the present study allowed 48 h to ensure the close approach of equilibrium. T e m p e r a t u r e Effect. Experiments like those above were run on four distinct soils a t 20,30, and 40 "C to determine the effect of temperature. The amount of ionic lead immobilized was independent of temperature a t least for this range. Thus, all subsequent experiments were performed a t 30 "C. Determination of Soil Immobilization Capacity. T o determine the amount of ionic lead immobilized by the soils, several 25-g samples of each soil were equilibrated with lead nitrate solutions as described above. The lead concentrations with which each series of soil samples was equilibrated were systematically varied from zero to 8000 pg Pb/mL in 7-8 increments. The amount of P b fixed by the soil a t each concentration could thus be determined by analysis of the amount left in solution a t equilibrium. The soil samples were also analyzed to verify the results. These results were used to determine N L ,the number of moles of lead taken up per gram
f 4
b-
1'
,-
~E.G.
I
; , , , , , I T l l l e (hours,
Flgure 1. Uptake of lead from solution by two soils '100
[
Solution concentration at 48 h Solution concentration at time t
3
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of soil for the ith concentration level, and N*, the moles of lead per gram of soil a t saturation (maximum capacity). Evaluation of these results indicated that the relation between uptake and solution concentrations was of the form:
where Ci is the equilibrium concentration for the ith initial concentration, and b is a constant determined empirically from the data. Equation 1 may be modified to obtain: -ci =-
1
+-ci
Ni N*b N* Use of this form permits a linear least-squares fit of Ci/Ni vs. Ci for each set of data. The value of N* is determined by the reciprocal of the slope of the least-squares fit. Given N * , the value of b is determined from the intercept. The capacity values and the associated correlation coefficients determined for the 18 soils are summarized in Table 11.That the correlation coefficients were consistently greater than 0.98 indicates the general validity of Equation 2. The values of N* indicate that soils have relatively high capacities for immobilization of ionic lead. The reader will recognize that Equations 1 and 2 are of the form associated with the well-known Langmuir adsorption isotherm. I t should be emphasized, however, that chemical and physical processes other than adsorption may be described by equations of the same form. The use of the Langmuir equation was based on selection of the function that fit the data best and which could be easily transformed to a linear relation to simplify data analysis. It will be shown below that processes other than adsorption determine the soil capacity. It was recognized that use of ionic solutions might result in errors in the capacity determination due to anion effects. T o check this, experiments were also conducted with solutions prepared from lead acetate, chloride, and chlorobromide. Although solubility limitations for the latter two salts precluded measurements a t high concentrations, the results were found to reliably fit Equation 2 so that capacity values could be determined from the slopes a t lower concentrations. The capacities determined were equivalent (&IO%)to those found
Table II. Linear Correlation Coefficients and Comparison of Experimental and Calculated Soil Capacities Soil no. a
1 2 3 4 5 6 7
a 9 10 11 12 13 14 15 16 17 18
Correlation coenb
N*(X Exptl
0.972 0.980 0.929 0.996 0.996 0.985 0.997 0.993 0.992 0.978 0.991 0.994 0.990 0.994 0.995 0.995 0.997 0.992
1.5 4.5 3.6 5.2 3.0 4.9 8.4 5.8 5.2 3.1 6.4 6.0 5.9 7.6 12.6 12.0 14.0 35.0
io5)
rnol/g Calcd
3.6 4.7 4.0 6.3 3.5 5.4 6.7 6.5 6.3 3.1 6.1 5.4 5.1 6.7 9.1 10.8 12.5 36.3
% Dlff
*
+140 +4.4
fll.1 +21.2 4-16.7 +10.2 -20.2 +12.1 +21.2 0 -4.7 -10.0 -13.5 -11.8 -27.8 -10.0 -10.7 +3.7
%ee Table I for description. bObtainedby least-squares fit of experimental results to Equation 2. CCalculatedfrom Equation 3. d l O O (N, - NaN;).
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Environmental Science & Technology
with lead nitrate solutions. Thus, anion identity and concentration did not affect the results even though these salts are of rather diverse character. That solutions of relatively insoluble lead salts produce capacity results equivalent to those of soluble salts and the general applicability of Equations 1and 2 indicates the general validity of the experiments described above. In essence, the results indicate that as lead particulates of low solubility dissolve in ground water, a fraction of that dissolved will be immobilized by interaction(s) with species responsible for fixation. Although the fraction of ionic lead immobilized is concentration dependent, these may be quite high in the soil solution in the immediate vicinity of the dissolving lead particulates. Characterization of Lead Fixation by Soils. Having determined the 48-h lead saturation capacities of 18 distinct soils, subsequent efforts concentrated on defining the soil characteristics responsible for the extent and the nature of the fixation. Initial interpretive efforts were based on attempting to relate the bulk soil characteristics given in Table I to the saturation capacities. A stepwise multiple regression analysis involving these soil parameters indicated that the pH, the cation-exchange capacity (CEC), clay, and organic matter levels were important. Inclusion of bulk clay, organic matter, oxidic iron, and oxidic manganese levels in the regression analysis, however, resulted in very little improvement in the regression (correlation) coefficient. In view of this, because the mechanical analysis methods for determination of clay and organic matter provide only semiquantitative results and because CEC is determined by clay and organic matter, pH and CEC were determined to be of primary importance. The regression equation obtained was: N*(mol/g) =
2.81 X lop6CEC(meq/lOO g)
+ 1.07 X
pH
- 4.93 x 10-5 (3) A value of 0.971 for the regression coefficient of this equation indicates that it should provide reasonably reliable estimates of the soil capacities. Values calculated from the equation are compared with those experimentally determined in Table 11. Because the two sets of values generally agree within 10-2096, the prediction accuracy of Equation 3 approximates the same range. Fixation Mechanisms. Having established the primary importance of pH and CEC and the (apparently) secondary importance of the clay, organic matter, and oxidic iron and manganese contents, subsequent experiments were conducted to determine the nature of the operative fixation phenomena. These experiments were designed to help delineate the significance of fixation processes such as surface adsorption, ion-exchange reactions (e.g., with clays or insoluble organic constituents), and precipitation. Experimental results associated with each of these categorical possibilities are discussed below. Surface Dependence. Two approaches were used as means of evaluating the dependence of P b fixation on surface processes. The bulk surface areas of eight soils were determined by nitrogen BET methods; the surface areas for the eight soils ranged from 0.8 to 74 m2/g. Although several functional relationships were examined, the best correlation between surface area and soil capacity was linear in nature. The correlation coefficient determined, however, was less than 0.60 and not significant at the 90% confidence level. Thus, although BET surface areas are not necessarily accurate estimates of the true surface area, the correlation results were considered indicative of a low degree of significance for surface dependent phenomena. The eight soils (containing their respective capacities of lead) were also size separated into size fractions with me-
Table 111. Concentrations (Weight . - % 1 of Lead. Carbonate, Organic Carbon, Oxidic Iron, and Oxidic Manganese in Size Fractions of Soils Size range, I*
420-840 250-420 177-250 149-1 77 125-149 74-125 99%) polystyrene filters (Delbag 98/99). 222Rnis collected by passing air through 250 g of 6-10 mesh coconut charcoal, cooled to -80 "C with a dry ice slurry. Prior to sample collection the charcoal is heated to 400 "C and outgassed to less than 15 1 pressure. All intake lines are flushed with air a t altitude through a bypass before sampling. Flow through the system is maintained within the range 25-100 L/min STP by aircraft ram pressure. Before radiochemical analysis each polystrene filter is compacted in a laboratory press and placed in a distillation flask. Stable Pb, Bi, and, when desired, Sr carriers and a calibrated 208Poor 209P0spike are added. The filter material is removed by distillation, followed by digestion with sulfuric and nitric acids. Bismuth is subsequently separated from the mixture as BiOCl and redissolved. A t this point, Po is separated from the Bi by autoplating on silver disks. After plating, the Bi is converted to BiP04 for 210Bicounting. The centrifugate remaining after the BiOCl separation contains the P b carrier and is reserved for P b analysis. After conversion of PbS04, to determine the P b yield, the PbS04 is stored to allow complete in-growth of 210Bi.A second Bi carrier is added, and the aloPb content of the sample is determined by 210Bi daughter assay as before. The Z2ZRnsamples are outgassed a t -80 "C to 750 fi pressure and then allowed to come to room temperature. Each sample is transferred by heating the charcoal to 400 "C and passing the gases successively through a trap cooled to -80 "C to remove water, through ascarite to remove COz, and through a second small charcoal trap cooled to -80 "C to reVolume 11, Number 13, December 1977
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