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Solubility of lead, zinc and copper added to mineral soils
Environmental Pollution 107 (2000) 153±158
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Solubility of lead, zinc and copper added to mineral soils C.E. MartõÂnez a,*, H.L. Motto b a
Department of Soil, Crop, and Atmospheric Sciences, Cornell University, Ithaca, NY 14853, USA b Department of Environmental Science, Rutgers University, New Brunswick, NJ 08903, USA Received 30 December 1998; accepted 3 May 1999
Abstract Elevated levels of heavy metals in soils are a result of industrial activities, atmospheric deposition, and the land application of sewage sludges and industrial by-products. Their persistence in the soil environment has created interest in the possible changes in solubility. In this study, total dissolved concentrations of Pb, Zn, and Cu were monitored in seven metal-amended soils (a calcareous and six acid mineral soils). Single metal solutions were added to soils and equilibrated (aged) for 40 days. During the 40 days the soil was allowed to air-dry and was rewetted in cycles of about 5 days. At the end of this reaction period, metal solubility was measured (by atomic absorption spectrometry and direct current plasma spectrometry) at the initial soil pH and at decreased pH values which were induced by addition of small aliquots of acid. As expected, solubility of added Pb, Zn, and Cu increased with a decrease in pH. Furthermore, the results showed that the solubility relationship with pH was similar in all non-calcareous soils. This suggests that metal solubility may be controlled by similar soil components, presumably involving soil characteristics such as pH, organic matter content, and soil mineralogy. For each metal, an approximate pH value was found at which solubility deviated from the solubility of metals when they occur in soils at typical (natural) values. This pH was about (pH0.2): 5.2 for Pb, 6.2 for Zn, and 5.5 for Cu. Thus, pH values below these thresholds may enhance metal mobility, biological availability and toxicity in soils. Metals dissolved at higher pH in the calcareous soil (18.8 g kgÿ1 inorganic carbon, initial pH 8.2). In a calcareous soil, a signi®cant fraction of these metals react with carbonates, and decreased pH results in much higher metal dissolution. Yet, metal solubility in soils is not determined by the formation and dissolution of single metal compounds. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Metal solubility; Threshold pH value; Added metals; Soils; Eect of soil carbonates
1. Introduction Interest continues in the fate of metals in polluted soils because of direct potential toxicities to biota and indirect threats to human health from contamination of groundwater and accumulation in food crops. Numerous investigations have been conducted on the chemistry of metals in soils (e.g. McBride and Blasiak, 1979; Harter, 1983). These studies have examined the amounts and retention of metals in soils, and the eect of various properties on their adsorption and solubility. Results have shown a large degree of retention and low solubility of metals from soils, with the retention or solubility aected by soil parameters such as pH (McBride and Blasiak, 1979; Cavallaro and McBride, 1980; Harter, 1983), amount of metal (Garcia-Miragaya, 1984; Basta * Corresponding author. Tel.: +1-607-255-1728; fax: +1-607-2558615. E-mail address: [email protected] (C.E. MartõÂnez).
and Tabatabai, 1992), cation exchange capacity (Ziper et al., 1988), organic matter content (Elliot et al., 1986), and soil mineralogy (Tiller et al., 1963; Jenne, 1968; Kinniburgh et al., 1976; Cavallaro and McBride, 1984; Kuo, 1986; Ziper et al., 1988). In general, soil pH seems to have the greatest eect of any single factor on the solubility or retention of metals in soils, with a greater retention and lower solubility of metal cations occurring at high soil pH (Cavallaro and McBride, 1980, 1984; Harter, 1983; Garcia-Miragaya, 1984; Stahl and James, 1991; Basta et al., 1993). Of the soil constituents, soil clays have shown a highly pH-dependent sorption of Cu and Zn (Cavallaro and McBride, 1984). The presence of carbonates in soils also aects metal retention. For example, Saeed and Fox (1977) studied Zn adsorption on four acid and three calcareous soils and observed a linear relationship between increased pH and decreased solubility in acid soils; the calcareous soils presented a non-linear relationship.
0269-7491/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0269-7491(99)00111-6
C.E. MartõÂnez, H.L. Motto / Environmental Pollution 107 (2000) 153±158
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(surface 0±8 inches), air-dried, and passed though a 2mm stainless steel screen prior to experimentation. The soils include ®ve soils from Puerto Rico and two soils from New Jersey. The soils from Puerto Rico were the Mucara (Vertic Eutropepts), Tanama (Lithic Tropudalfs), Naranjito (Typic Tropohumults), Guanabano (Typic Argiustolls), and (E)Mucara (Vertic Eutropepts, collected at a dierent location than Mucara) series, and the soils from New Jersey were the Freehold (Typic Hapludults) and Quakertown (Typic Hapludults) series. Soil characteristics are given in Table 1. In addition, metal solubility was measured on the Tanama (calcareous) soil after treatment to remove carbonates (Kunze and Dixon, 1986). This material is referred to as the HAc-Tanama soil. Soil pH, organic matter, cation exchange capacity (CEC), and texture analyses were performed in the Soil Testing and Plant Analysis Laboratory, Rutgers University. Soil pH was measured in a 1:1 soil:water ratio (by volume) suspension. Organic matter was determined by the Walkey±Black (chromic acid) method (Nelson and Sommers, 1982), CEC by NH+ 4 saturation at pH 7 (Rhodes, 1982), and sand, silt, and clay content by the Bouyoucos (hydrometer) method (Gee and Bauder, 1986). Inorganic carbon was determined using the methodology described by Bundy and Bremner (1972). Mineral composition of the clay fraction was determined by X-ray diraction of oriented samples following Mg and K saturation and heat treatments. Free iron oxides (nonsilicate or easily reducible Fe) were determined using a citrate-bicarbonate-dithionate (CBD) method described by Jackson et al. (1986). Acid ammonium oxalate (noncrystalline inorganic and organiccomplexed forms of Fe and Al) and sodium pyrophosphate (organic-complexed Fe and Al) extractable Fe and Al were determined by methods described by Ross and Wang (1993). The extracts were analyzed for
It has been suggested (e.g. Schultz et al., 1987) that the magnitude of metal retention and dissolution depends on the metal±soil reaction time and on the system pH (pH at which the adsorption process takes place). Metal adsorption and solubility from soils is often measured after the metal's equilibration with the soil for 16±24 h. This laboratory approach does not permit metals to react with the soil for long enough time to reach a steadystate. In this study, we measure the solubility of important trace metals (Cu, Pb, Zn) after a metal±soil reaction time (aging) of 40 days, thus allowing slow reactions to occur and providing more realistic conditions. Little information is available on the soil pH at which the solubility of added metals increases and on the eects that soil properties (e.g. carbonates) have on metal solubility. Knowing the pH below which metals are more soluble is important if metal accumulation by plants is to be minimized, if ground and surface water pollution is to be avoided, and for a permissible (safe) use of metalcontaining wastes as agricultural amendments. This study involves the addition and long-term aging of Pb, Zn, or Cu to soils at levels commonly found in metal-contaminated soils having a variety of properties. Since the tendency of soil pH is to decrease with time (due to weathering reactions), the solubility of these metals in extracts at pH values lower than the initial pH is measured and a speci®c pH value below which Pb, Zn, and Cu solubility increases determined for a calcareous and non-calcareous soils. 2. Materials and methods 2.1. Soils, soil properties, and soil characterization Soils with native concentrations of metals (Pb, Zn, Cu) were used in this study. The soils were collected Table 1 Chemical and physical characteristics of soils Soil series
pHw
CEC (cmol kgÿ1)
Mucara Tanama Naranjito Guanabano (E)Mucara Freehold Quakertown
7.7 8.2 5.9 8.0 6.4 5.1 6.4
53.3 21.8 14.5 27.1 27.9 6.9 11.4
a
Texturea
SCL SCL CL CL SL SL SiCL
ICb
OMc
Sand
Silt
Clay (g kgÿ1)
Fedd
Feod
Fepd
Alod
Alpd
1.0 18.8 0.4 3.8 0.2 0.1 0.7
5.8 3.3 5.9 16.4 3.9 18.4 35.7
520 500 370 420 680 640 200
260 190 300 240 160 200 520
220 310 330 340 160 160 280
7.9 8.3 36.7 21.4 8.4 17.4 16.2
2.6 0.5 2.7 0.5 1.9 3.7 2.5
0.2 0.1 1.5 0.2 0.7 4.8 1.7
2.4 0.6 0.9 3.0 2.4 1.9 2.4
0.3 0.2 1.8 0.4 1.0 2.8 1.7
Major minerals
Smectite, chlorite Smectite Kaolinite, smectite Chlorite±vermiculite Smectite±chlorite Smectite±kaolinitee Vermiculite±kaolinitee
SCL, sandy clay loam; CL, clay loam; SL, sandy loam; SiCL, silty clay loam. IC, inorganic carbon (carbonates). c OM, organic matter. d Fed, citrate-bicarbonate-dithionate extractable Fe; Feo, oxalate extractable Fe; Fep, pyrophosphate extractable Fe; Alo, oxalate extractable Al; Alp, pyrophosphate extractable Al. e Hydroxy-Al interlayer 2:1 clay. b
C.E. MartõÂnez, H.L. Motto / Environmental Pollution 107 (2000) 153±158
Fe and Al by atomic absorption spectrometry (AAS; Perkin-Elmer 603 Atomic Absorption Spectrophotometer) or direct current plasma spectrometry (DCP; Fisons Instruments, Spectra Span 7, Direct Current Plasma). Carbonates were removed from Tanama soil using a modi®cation of the method described by Kunze and Dixon (1986). The soil was treated with 1 M sodium acetate (pH 5) buer solution after which the presence of carbonates was checked with strong acid, and inorganic carbon was determined (Bundy and Bremner, 1972). Inorganic carbon content was 0.4 g kgÿ1 in HActreated Tanama soil (versus 18.8 g kgÿ1 in Tanama soil). Metals present in the native and amended soils were dissolved by boiling 2 g of soil in 15 ml of 70% perchloric acid for 3 h. The metals were determined in the diluted digestate by AAS (Table 2). Copper in the native Mucara and Naranjito soils is somewhat higher than typical, but other metal levels are within the typical range (Bohn et al., 1985). 2.2. Metal addition to the soils and pH adjustment The soils were spiked with individual metals by adding metal solutions (Pb(NO3)2, Zn(C2H3O2)2, or CuSO4) to air-dry soil, mixing, and allowing the soil and metal to react for a 40-day period. Total concentrations in metal-amended soils are presented in Table 2. During this equilibration (aging) period, the soil was allowed to air-dry, was mixed, and was rewetted in cycles of about
5 days. Solubility experiments were performed after completion of this reaction period. After the 40 days of reaction, metal solubility was measured at the original pH and after decreasing the soil pH by addition of HCl. The amounts and concentrations of HCl necessary to obtain the desired pH range for each soil were determined prior to the study. Preliminary studies (plots of concentration and pH versus time) demonstrated that by 16 h after addition of acid the pH of the soil suspension and the metal concentration in solution had reached a steady-state (MartõÂnez, 1995). The steady-state condition was used as the criteria for achievement of equilibrium. The pH values varied from the initial soil pH to approximately pH 3. Two grams of soil was weighted into a 50-ml polypropylene centrifuge tube; 20 ml of 0.01 M CaCl2 was added, HCl was added to obtain the desired pH, and the tubes shaken for 16 h in an end-over-end shaker at 15 cycles minÿ1. The addition of acid (concentration and amount) was selected so that the total volume was not signi®cantly changed. After shaking, the ®nal pH was measured. The ®nal pH was used to interpret metal solubility. Three replications of each pH were prepared. However, the ®nal pH varied slightly between the replications (probably due to heterogeneity of the soil), so each replication was treated as a unique pH. After measuring pH, the samples were centrifuged for 15 min at 15,000 rpm, and the supernatants analyzed for metals by AAS or DCP. 3. Results and discussion
Table 2 Metal concentration in native and metal-amended soilsa
3.1. Solubility from metal-amended soils
Concentration Zn (mmol kgÿ1)
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Soil series
Pb
Cu
Native soils Mucara Tanama Naranjito Guanabano (E)Mucara Freehold Quakertown
0.11 0.08 0.14 0.12 0.09 0.21 0.24
(1.4) (3.1) (4.4) (1.1) (2.6) (8.7) (7.5)
1.28 0.77 2.19 1.33 1.24 0.96 1.30
(2.4) (4.7) (2.7) (3.7) (1.2) (6.2) (0.0)
2.16 0.59 2.04 1.01 1.51 0.13 1.26
(1.9) (4.8) (1.4) (2.2) (2.0) (6.9) (4.6)
Metal-amended soils Mucara Tanama Naranjito Guanabano (E)Mucara Freehold Quakertown HAc-Tanamab
4.93 5.83 3.62 5.25 5.39 4.82 6.06 4.97
(2.7) (3.5) (3.1) (3.8) (4.6) (0.8) (0.8) (1.4)
9.82 10.41 6.93 8.32 7.73 7.05 8.84 7.15
(2.2) (1.8) (0.6) (1.6) (5.1) (0.3) (0.2) (1.5)
6.27 4.07 8.05 4.83 6.22 3.52 5.90 4.51
(3.9) (3.5) (2.0) (5.2) (0.7) (2.3) (0.8) (2.7)
a Values represent mean concentrations of three replicates. Relative standard deviations (%) are shown in parentheses. b Na-acetate buer-treated Tanama soil.
The metal present in solution came mostly from the added metal as evidenced by the small amounts of metals dissolved from the control (no metal added) soils as compared to metal-amended soils (MartõÂnez, 1995), thus indicating that the added metal was more soluble than the original even after a long aging period. As expected, the solubility of added Pb, Zn, or Cu increased with a decrease in soil pH for all soils (Fig. 1) in agreement with research on metal adsorption on soils and soil constituents (Cavallaro and McBride, 1980, 1984; Harter, 1983; Garcia-Miragaya, 1984; Stahl and James, 1991). Each metal, however, exhibits an approximate pH value at which solubility increases markedly, independent of soil, except for the calcareous Tanama soil. The pH at which metal solubility from amended soils deviates from the solubility in control soils was the criterion for selection of an approximate pH value for increased solubility (MartõÂnez, 1995). All soils were evaluated individually, and the pH at which metal solubility increased was determined. This pH was about 5.20.2 for Pb, 6.20.2 for Zn, and 5.50.2 for Cu
156
C.E. MartõÂnez, H.L. Motto / Environmental Pollution 107 (2000) 153±158
for all noncalcareous soils. The dierence among these pH values indicate dierent anities for metals in soils, and suggest a weaker reaction with soil constituents of Zn than of Pb and Cu. These pH values agree with the order of metal solubility reported by Sauerbeck and Rietz (1983) (Zn>Pb), and with the order of adsorption reported by Harter (1983) (Pb>Cu>Zn). In the calcareous Tanama soil (18.8 g kgÿ1 inorganic carbon), metals dissolved at higher pH values than in the other soils. In the Tanama soil, the pH at which metal solubility readily increased was about 6.00.2 for Pb, 6.20.2 for Cu, and 6.80.2 for Zn. Carbonate minerals in soils can have direct or indirect eects on metal solubility: direct through their surface interactions and indirect through their pH eect on other soil constituents (Papadopoulos and Rowell, 1988). Carbonates were removed from the Tanama soil, metals added, and their solubility determined with decreased pH after a 40-day reaction period (Fig. 2). There was an increase in metal solubility from the carbonatefree (HAc-Tanama) soil as the pH went down, but not as pronounced as in the Tanama soil containing carbonates. The solubility curves of the Na-acetate buer-treated Tanama soil resemble the solubility curves of noncalcareous soils, and solubility values are within those found for noncalcareous soils (cf. Fig. 1).
Furthermore, in the HAc-treated Tanama soil, the pH at which metal solubility deviates from solubility in the control (no metals added) soil is the same as with other noncalcareous soils. This result gives a strong indication that a portion of the metal added to the calcareous soil is associated with the calcium carbonate phase following the 40 days equilibration time. It is generally agreed that CaCO3 surfaces provide sites for metal±surface interactions via speci®c adsorption or precipitation reactions (McBride, 1979, 1980; Papadopoulos and Rowell, 1988; Zachara et al., 1988). If calcium carbonate is removed from the soil the adsorption capacity decreases (Madrid and Diaz-Barrientos, 1992). Carbonate compounds dissolve easily by lowering the pH of extraction so that metal solubility from calcareous soils is more susceptible to decreases in pH. Zn was completely desorbed after adsorption on pure CaCO3 surfaces by lowering the pH of the solution (Zachara et al., 1988), and differences in the solubility curves of Zn from calcareous and noncalcareous soils have been demonstrated (Saeed and Fox, 1977). Although the soils used in this study have a wide range of characteristics (Table 1), the metals behave the same with respect to pH decreases as evidenced by similar solubility curves (Fig. 1), thus suggesting the formation of similar reaction products (or reaction with
Fig. 1. Solubility of Pb, Zn, and Cu as a function of pH of extraction in metal-amended soils.
Fig. 2. Metal solubility from HAc-treated Tanama soil (circles) compared to Tanama soil (squares).
C.E. MartõÂnez, H.L. Motto / Environmental Pollution 107 (2000) 153±158
similar soil components) in all noncalcareous soils. This behavior may result if speci®c adsorption reactions are the predominant mechanism controlling metal solubility. Speci®c adsorption depends on the identity of the metal and the reactive site (McBride, 1994); thus, a unique threshold pH for solubility would be expected for each metal cation if it reacts with a similar soil constituent or constituents in dierent soils. 3.2. Solubility from metal-amended soils and from various pure compounds Solubility studies cannot dierentiate among ion exchange, speci®c adsorption, and precipitation reactions. Solubility measurements represent a total in metal dissolution and cannot be used in the study of individual reaction products between metals and soils. Solubility diagrams of individual pure compounds of divalent metals show a uniform pH eect on metal solubility, and in most cases a slope of two (Lindsay, 1979). Several authors have rejected the precipitation of pure compounds as the mechanism controlling metal solubility in soil systems (McBride and Blasiak, 1979; Cavallaro and McBride, 1980; Basta and Tabatabai, 1992) while others argue for its control at high metal concentrations and high pH values (Udo et al., 1970; Santillan-Medrano and Jurinak, 1975; El-Falaky et al., 1991). In this study, plots of the logarithm (base 10) of the total metal concentration in the soil extracts versus pH indicate that the slopes are lower than for pure compounds, less than 2 (Fig. 3). The slopes range from 0.08 to 0.85 for Pb, 0.06 to 1.51 for Zn, and 0.08 to 1.66 for Cu; thus indicating that precipitation of a single pure compound is not controlling metal solubility in these soils. The Tanama soil showed the highest slope, which corresponds closest to the pure metal carbonates (Fig. 3). Cavallaro and McBride (1978) suggested the precipitation of Cd and Cu carbonates in calcareous subsoils. The properties of Tanama soil are dominated by carbonate mineralogy and probably represent the
157
greatest potential for the formation of pure compounds (carbonates). The use of solubility diagrams to indicate the presence of individual pure compounds requires equilibrium conditions. In a mixed system where equilibrium has not been established (e.g. soils), the amount of metal in solution may not correspond to the solubility relationship of a single pure compound. Metals may be dissolving from more soluble compounds (or weaker adsorption sites) and precipitating as less soluble forms. Until equilibrium is obtained (if it would ever happen in soils), this will result in solubilities that do not correspond to any single compound and in slopes other than two. A mixture of reaction products probably form in soils and one must question if 40 days of reaction time is sucient for metal±soil reactions to reach equilibrium. The lack of correspondence of metal solubility from these soils to that of pure compounds does not indicate that these compounds are not formed in soils, but indicates that an individual compound is not controlling metal dissolution in these soils as measured. Two other points should be considered regarding the solubility diagrams. First, metal concentrations and activities are plotted as equal, although it is known that concentrations give higher values than activities. Determination of activities from concentrations will shift the curves to lower values. Second, precipitation of pure compounds in soil systems may not happen at the same pH as precipitation from homogeneous solutions. Soil solids aect surface reactions and are involved in heterogeneous catalysis. The presence of a surface may enhance reaction rates relative to rates in pure solution. For example, the hydrolysis of Al was reported to occur more extensively and at lower pH in soils than in aqueous solution (Ragland and Coleman, 1960). This phenomenon was explained as the result of sorption of the hydrolysis product (Al(OH)+ 2 ) by soil surfaces. Speci®c adsorption of metals and solid solution formation are possibilities when considering metal solubilities below those predicted by the solubility product of pure compounds.
Fig. 3. Logarithm (base 10) of soluble Pb, Zn, and Cu from amended soils and from various pure compounds. Equilibrium lines for mineral phases were calculated based on the Ksp values reported by Lindsay (1979). An ionic strength (I) of 0.01 M was assumed. Symbols as in Fig. 1.
158
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Solubility of lead, zinc and copper added to mineral soils C.E. MartõÂnez a,*, H.L. Motto b a
Department of Soil, Crop, and Atmospheric Sciences, Cornell University, Ithaca, NY 14853, USA b Department of Environmental Science, Rutgers University, New Brunswick, NJ 08903, USA Received 30 December 1998; accepted 3 May 1999
Abstract Elevated levels of heavy metals in soils are a result of industrial activities, atmospheric deposition, and the land application of sewage sludges and industrial by-products. Their persistence in the soil environment has created interest in the possible changes in solubility. In this study, total dissolved concentrations of Pb, Zn, and Cu were monitored in seven metal-amended soils (a calcareous and six acid mineral soils). Single metal solutions were added to soils and equilibrated (aged) for 40 days. During the 40 days the soil was allowed to air-dry and was rewetted in cycles of about 5 days. At the end of this reaction period, metal solubility was measured (by atomic absorption spectrometry and direct current plasma spectrometry) at the initial soil pH and at decreased pH values which were induced by addition of small aliquots of acid. As expected, solubility of added Pb, Zn, and Cu increased with a decrease in pH. Furthermore, the results showed that the solubility relationship with pH was similar in all non-calcareous soils. This suggests that metal solubility may be controlled by similar soil components, presumably involving soil characteristics such as pH, organic matter content, and soil mineralogy. For each metal, an approximate pH value was found at which solubility deviated from the solubility of metals when they occur in soils at typical (natural) values. This pH was about (pH0.2): 5.2 for Pb, 6.2 for Zn, and 5.5 for Cu. Thus, pH values below these thresholds may enhance metal mobility, biological availability and toxicity in soils. Metals dissolved at higher pH in the calcareous soil (18.8 g kgÿ1 inorganic carbon, initial pH 8.2). In a calcareous soil, a signi®cant fraction of these metals react with carbonates, and decreased pH results in much higher metal dissolution. Yet, metal solubility in soils is not determined by the formation and dissolution of single metal compounds. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Metal solubility; Threshold pH value; Added metals; Soils; Eect of soil carbonates
1. Introduction Interest continues in the fate of metals in polluted soils because of direct potential toxicities to biota and indirect threats to human health from contamination of groundwater and accumulation in food crops. Numerous investigations have been conducted on the chemistry of metals in soils (e.g. McBride and Blasiak, 1979; Harter, 1983). These studies have examined the amounts and retention of metals in soils, and the eect of various properties on their adsorption and solubility. Results have shown a large degree of retention and low solubility of metals from soils, with the retention or solubility aected by soil parameters such as pH (McBride and Blasiak, 1979; Cavallaro and McBride, 1980; Harter, 1983), amount of metal (Garcia-Miragaya, 1984; Basta * Corresponding author. Tel.: +1-607-255-1728; fax: +1-607-2558615. E-mail address: [email protected] (C.E. MartõÂnez).
and Tabatabai, 1992), cation exchange capacity (Ziper et al., 1988), organic matter content (Elliot et al., 1986), and soil mineralogy (Tiller et al., 1963; Jenne, 1968; Kinniburgh et al., 1976; Cavallaro and McBride, 1984; Kuo, 1986; Ziper et al., 1988). In general, soil pH seems to have the greatest eect of any single factor on the solubility or retention of metals in soils, with a greater retention and lower solubility of metal cations occurring at high soil pH (Cavallaro and McBride, 1980, 1984; Harter, 1983; Garcia-Miragaya, 1984; Stahl and James, 1991; Basta et al., 1993). Of the soil constituents, soil clays have shown a highly pH-dependent sorption of Cu and Zn (Cavallaro and McBride, 1984). The presence of carbonates in soils also aects metal retention. For example, Saeed and Fox (1977) studied Zn adsorption on four acid and three calcareous soils and observed a linear relationship between increased pH and decreased solubility in acid soils; the calcareous soils presented a non-linear relationship.
0269-7491/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0269-7491(99)00111-6
C.E. MartõÂnez, H.L. Motto / Environmental Pollution 107 (2000) 153±158
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(surface 0±8 inches), air-dried, and passed though a 2mm stainless steel screen prior to experimentation. The soils include ®ve soils from Puerto Rico and two soils from New Jersey. The soils from Puerto Rico were the Mucara (Vertic Eutropepts), Tanama (Lithic Tropudalfs), Naranjito (Typic Tropohumults), Guanabano (Typic Argiustolls), and (E)Mucara (Vertic Eutropepts, collected at a dierent location than Mucara) series, and the soils from New Jersey were the Freehold (Typic Hapludults) and Quakertown (Typic Hapludults) series. Soil characteristics are given in Table 1. In addition, metal solubility was measured on the Tanama (calcareous) soil after treatment to remove carbonates (Kunze and Dixon, 1986). This material is referred to as the HAc-Tanama soil. Soil pH, organic matter, cation exchange capacity (CEC), and texture analyses were performed in the Soil Testing and Plant Analysis Laboratory, Rutgers University. Soil pH was measured in a 1:1 soil:water ratio (by volume) suspension. Organic matter was determined by the Walkey±Black (chromic acid) method (Nelson and Sommers, 1982), CEC by NH+ 4 saturation at pH 7 (Rhodes, 1982), and sand, silt, and clay content by the Bouyoucos (hydrometer) method (Gee and Bauder, 1986). Inorganic carbon was determined using the methodology described by Bundy and Bremner (1972). Mineral composition of the clay fraction was determined by X-ray diraction of oriented samples following Mg and K saturation and heat treatments. Free iron oxides (nonsilicate or easily reducible Fe) were determined using a citrate-bicarbonate-dithionate (CBD) method described by Jackson et al. (1986). Acid ammonium oxalate (noncrystalline inorganic and organiccomplexed forms of Fe and Al) and sodium pyrophosphate (organic-complexed Fe and Al) extractable Fe and Al were determined by methods described by Ross and Wang (1993). The extracts were analyzed for
It has been suggested (e.g. Schultz et al., 1987) that the magnitude of metal retention and dissolution depends on the metal±soil reaction time and on the system pH (pH at which the adsorption process takes place). Metal adsorption and solubility from soils is often measured after the metal's equilibration with the soil for 16±24 h. This laboratory approach does not permit metals to react with the soil for long enough time to reach a steadystate. In this study, we measure the solubility of important trace metals (Cu, Pb, Zn) after a metal±soil reaction time (aging) of 40 days, thus allowing slow reactions to occur and providing more realistic conditions. Little information is available on the soil pH at which the solubility of added metals increases and on the eects that soil properties (e.g. carbonates) have on metal solubility. Knowing the pH below which metals are more soluble is important if metal accumulation by plants is to be minimized, if ground and surface water pollution is to be avoided, and for a permissible (safe) use of metalcontaining wastes as agricultural amendments. This study involves the addition and long-term aging of Pb, Zn, or Cu to soils at levels commonly found in metal-contaminated soils having a variety of properties. Since the tendency of soil pH is to decrease with time (due to weathering reactions), the solubility of these metals in extracts at pH values lower than the initial pH is measured and a speci®c pH value below which Pb, Zn, and Cu solubility increases determined for a calcareous and non-calcareous soils. 2. Materials and methods 2.1. Soils, soil properties, and soil characterization Soils with native concentrations of metals (Pb, Zn, Cu) were used in this study. The soils were collected Table 1 Chemical and physical characteristics of soils Soil series
pHw
CEC (cmol kgÿ1)
Mucara Tanama Naranjito Guanabano (E)Mucara Freehold Quakertown
7.7 8.2 5.9 8.0 6.4 5.1 6.4
53.3 21.8 14.5 27.1 27.9 6.9 11.4
a
Texturea
SCL SCL CL CL SL SL SiCL
ICb
OMc
Sand
Silt
Clay (g kgÿ1)
Fedd
Feod
Fepd
Alod
Alpd
1.0 18.8 0.4 3.8 0.2 0.1 0.7
5.8 3.3 5.9 16.4 3.9 18.4 35.7
520 500 370 420 680 640 200
260 190 300 240 160 200 520
220 310 330 340 160 160 280
7.9 8.3 36.7 21.4 8.4 17.4 16.2
2.6 0.5 2.7 0.5 1.9 3.7 2.5
0.2 0.1 1.5 0.2 0.7 4.8 1.7
2.4 0.6 0.9 3.0 2.4 1.9 2.4
0.3 0.2 1.8 0.4 1.0 2.8 1.7
Major minerals
Smectite, chlorite Smectite Kaolinite, smectite Chlorite±vermiculite Smectite±chlorite Smectite±kaolinitee Vermiculite±kaolinitee
SCL, sandy clay loam; CL, clay loam; SL, sandy loam; SiCL, silty clay loam. IC, inorganic carbon (carbonates). c OM, organic matter. d Fed, citrate-bicarbonate-dithionate extractable Fe; Feo, oxalate extractable Fe; Fep, pyrophosphate extractable Fe; Alo, oxalate extractable Al; Alp, pyrophosphate extractable Al. e Hydroxy-Al interlayer 2:1 clay. b
C.E. MartõÂnez, H.L. Motto / Environmental Pollution 107 (2000) 153±158
Fe and Al by atomic absorption spectrometry (AAS; Perkin-Elmer 603 Atomic Absorption Spectrophotometer) or direct current plasma spectrometry (DCP; Fisons Instruments, Spectra Span 7, Direct Current Plasma). Carbonates were removed from Tanama soil using a modi®cation of the method described by Kunze and Dixon (1986). The soil was treated with 1 M sodium acetate (pH 5) buer solution after which the presence of carbonates was checked with strong acid, and inorganic carbon was determined (Bundy and Bremner, 1972). Inorganic carbon content was 0.4 g kgÿ1 in HActreated Tanama soil (versus 18.8 g kgÿ1 in Tanama soil). Metals present in the native and amended soils were dissolved by boiling 2 g of soil in 15 ml of 70% perchloric acid for 3 h. The metals were determined in the diluted digestate by AAS (Table 2). Copper in the native Mucara and Naranjito soils is somewhat higher than typical, but other metal levels are within the typical range (Bohn et al., 1985). 2.2. Metal addition to the soils and pH adjustment The soils were spiked with individual metals by adding metal solutions (Pb(NO3)2, Zn(C2H3O2)2, or CuSO4) to air-dry soil, mixing, and allowing the soil and metal to react for a 40-day period. Total concentrations in metal-amended soils are presented in Table 2. During this equilibration (aging) period, the soil was allowed to air-dry, was mixed, and was rewetted in cycles of about
5 days. Solubility experiments were performed after completion of this reaction period. After the 40 days of reaction, metal solubility was measured at the original pH and after decreasing the soil pH by addition of HCl. The amounts and concentrations of HCl necessary to obtain the desired pH range for each soil were determined prior to the study. Preliminary studies (plots of concentration and pH versus time) demonstrated that by 16 h after addition of acid the pH of the soil suspension and the metal concentration in solution had reached a steady-state (MartõÂnez, 1995). The steady-state condition was used as the criteria for achievement of equilibrium. The pH values varied from the initial soil pH to approximately pH 3. Two grams of soil was weighted into a 50-ml polypropylene centrifuge tube; 20 ml of 0.01 M CaCl2 was added, HCl was added to obtain the desired pH, and the tubes shaken for 16 h in an end-over-end shaker at 15 cycles minÿ1. The addition of acid (concentration and amount) was selected so that the total volume was not signi®cantly changed. After shaking, the ®nal pH was measured. The ®nal pH was used to interpret metal solubility. Three replications of each pH were prepared. However, the ®nal pH varied slightly between the replications (probably due to heterogeneity of the soil), so each replication was treated as a unique pH. After measuring pH, the samples were centrifuged for 15 min at 15,000 rpm, and the supernatants analyzed for metals by AAS or DCP. 3. Results and discussion
Table 2 Metal concentration in native and metal-amended soilsa
3.1. Solubility from metal-amended soils
Concentration Zn (mmol kgÿ1)
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Soil series
Pb
Cu
Native soils Mucara Tanama Naranjito Guanabano (E)Mucara Freehold Quakertown
0.11 0.08 0.14 0.12 0.09 0.21 0.24
(1.4) (3.1) (4.4) (1.1) (2.6) (8.7) (7.5)
1.28 0.77 2.19 1.33 1.24 0.96 1.30
(2.4) (4.7) (2.7) (3.7) (1.2) (6.2) (0.0)
2.16 0.59 2.04 1.01 1.51 0.13 1.26
(1.9) (4.8) (1.4) (2.2) (2.0) (6.9) (4.6)
Metal-amended soils Mucara Tanama Naranjito Guanabano (E)Mucara Freehold Quakertown HAc-Tanamab
4.93 5.83 3.62 5.25 5.39 4.82 6.06 4.97
(2.7) (3.5) (3.1) (3.8) (4.6) (0.8) (0.8) (1.4)
9.82 10.41 6.93 8.32 7.73 7.05 8.84 7.15
(2.2) (1.8) (0.6) (1.6) (5.1) (0.3) (0.2) (1.5)
6.27 4.07 8.05 4.83 6.22 3.52 5.90 4.51
(3.9) (3.5) (2.0) (5.2) (0.7) (2.3) (0.8) (2.7)
a Values represent mean concentrations of three replicates. Relative standard deviations (%) are shown in parentheses. b Na-acetate buer-treated Tanama soil.
The metal present in solution came mostly from the added metal as evidenced by the small amounts of metals dissolved from the control (no metal added) soils as compared to metal-amended soils (MartõÂnez, 1995), thus indicating that the added metal was more soluble than the original even after a long aging period. As expected, the solubility of added Pb, Zn, or Cu increased with a decrease in soil pH for all soils (Fig. 1) in agreement with research on metal adsorption on soils and soil constituents (Cavallaro and McBride, 1980, 1984; Harter, 1983; Garcia-Miragaya, 1984; Stahl and James, 1991). Each metal, however, exhibits an approximate pH value at which solubility increases markedly, independent of soil, except for the calcareous Tanama soil. The pH at which metal solubility from amended soils deviates from the solubility in control soils was the criterion for selection of an approximate pH value for increased solubility (MartõÂnez, 1995). All soils were evaluated individually, and the pH at which metal solubility increased was determined. This pH was about 5.20.2 for Pb, 6.20.2 for Zn, and 5.50.2 for Cu
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for all noncalcareous soils. The dierence among these pH values indicate dierent anities for metals in soils, and suggest a weaker reaction with soil constituents of Zn than of Pb and Cu. These pH values agree with the order of metal solubility reported by Sauerbeck and Rietz (1983) (Zn>Pb), and with the order of adsorption reported by Harter (1983) (Pb>Cu>Zn). In the calcareous Tanama soil (18.8 g kgÿ1 inorganic carbon), metals dissolved at higher pH values than in the other soils. In the Tanama soil, the pH at which metal solubility readily increased was about 6.00.2 for Pb, 6.20.2 for Cu, and 6.80.2 for Zn. Carbonate minerals in soils can have direct or indirect eects on metal solubility: direct through their surface interactions and indirect through their pH eect on other soil constituents (Papadopoulos and Rowell, 1988). Carbonates were removed from the Tanama soil, metals added, and their solubility determined with decreased pH after a 40-day reaction period (Fig. 2). There was an increase in metal solubility from the carbonatefree (HAc-Tanama) soil as the pH went down, but not as pronounced as in the Tanama soil containing carbonates. The solubility curves of the Na-acetate buer-treated Tanama soil resemble the solubility curves of noncalcareous soils, and solubility values are within those found for noncalcareous soils (cf. Fig. 1).
Furthermore, in the HAc-treated Tanama soil, the pH at which metal solubility deviates from solubility in the control (no metals added) soil is the same as with other noncalcareous soils. This result gives a strong indication that a portion of the metal added to the calcareous soil is associated with the calcium carbonate phase following the 40 days equilibration time. It is generally agreed that CaCO3 surfaces provide sites for metal±surface interactions via speci®c adsorption or precipitation reactions (McBride, 1979, 1980; Papadopoulos and Rowell, 1988; Zachara et al., 1988). If calcium carbonate is removed from the soil the adsorption capacity decreases (Madrid and Diaz-Barrientos, 1992). Carbonate compounds dissolve easily by lowering the pH of extraction so that metal solubility from calcareous soils is more susceptible to decreases in pH. Zn was completely desorbed after adsorption on pure CaCO3 surfaces by lowering the pH of the solution (Zachara et al., 1988), and differences in the solubility curves of Zn from calcareous and noncalcareous soils have been demonstrated (Saeed and Fox, 1977). Although the soils used in this study have a wide range of characteristics (Table 1), the metals behave the same with respect to pH decreases as evidenced by similar solubility curves (Fig. 1), thus suggesting the formation of similar reaction products (or reaction with
Fig. 1. Solubility of Pb, Zn, and Cu as a function of pH of extraction in metal-amended soils.
Fig. 2. Metal solubility from HAc-treated Tanama soil (circles) compared to Tanama soil (squares).
C.E. MartõÂnez, H.L. Motto / Environmental Pollution 107 (2000) 153±158
similar soil components) in all noncalcareous soils. This behavior may result if speci®c adsorption reactions are the predominant mechanism controlling metal solubility. Speci®c adsorption depends on the identity of the metal and the reactive site (McBride, 1994); thus, a unique threshold pH for solubility would be expected for each metal cation if it reacts with a similar soil constituent or constituents in dierent soils. 3.2. Solubility from metal-amended soils and from various pure compounds Solubility studies cannot dierentiate among ion exchange, speci®c adsorption, and precipitation reactions. Solubility measurements represent a total in metal dissolution and cannot be used in the study of individual reaction products between metals and soils. Solubility diagrams of individual pure compounds of divalent metals show a uniform pH eect on metal solubility, and in most cases a slope of two (Lindsay, 1979). Several authors have rejected the precipitation of pure compounds as the mechanism controlling metal solubility in soil systems (McBride and Blasiak, 1979; Cavallaro and McBride, 1980; Basta and Tabatabai, 1992) while others argue for its control at high metal concentrations and high pH values (Udo et al., 1970; Santillan-Medrano and Jurinak, 1975; El-Falaky et al., 1991). In this study, plots of the logarithm (base 10) of the total metal concentration in the soil extracts versus pH indicate that the slopes are lower than for pure compounds, less than 2 (Fig. 3). The slopes range from 0.08 to 0.85 for Pb, 0.06 to 1.51 for Zn, and 0.08 to 1.66 for Cu; thus indicating that precipitation of a single pure compound is not controlling metal solubility in these soils. The Tanama soil showed the highest slope, which corresponds closest to the pure metal carbonates (Fig. 3). Cavallaro and McBride (1978) suggested the precipitation of Cd and Cu carbonates in calcareous subsoils. The properties of Tanama soil are dominated by carbonate mineralogy and probably represent the
157
greatest potential for the formation of pure compounds (carbonates). The use of solubility diagrams to indicate the presence of individual pure compounds requires equilibrium conditions. In a mixed system where equilibrium has not been established (e.g. soils), the amount of metal in solution may not correspond to the solubility relationship of a single pure compound. Metals may be dissolving from more soluble compounds (or weaker adsorption sites) and precipitating as less soluble forms. Until equilibrium is obtained (if it would ever happen in soils), this will result in solubilities that do not correspond to any single compound and in slopes other than two. A mixture of reaction products probably form in soils and one must question if 40 days of reaction time is sucient for metal±soil reactions to reach equilibrium. The lack of correspondence of metal solubility from these soils to that of pure compounds does not indicate that these compounds are not formed in soils, but indicates that an individual compound is not controlling metal dissolution in these soils as measured. Two other points should be considered regarding the solubility diagrams. First, metal concentrations and activities are plotted as equal, although it is known that concentrations give higher values than activities. Determination of activities from concentrations will shift the curves to lower values. Second, precipitation of pure compounds in soil systems may not happen at the same pH as precipitation from homogeneous solutions. Soil solids aect surface reactions and are involved in heterogeneous catalysis. The presence of a surface may enhance reaction rates relative to rates in pure solution. For example, the hydrolysis of Al was reported to occur more extensively and at lower pH in soils than in aqueous solution (Ragland and Coleman, 1960). This phenomenon was explained as the result of sorption of the hydrolysis product (Al(OH)+ 2 ) by soil surfaces. Speci®c adsorption of metals and solid solution formation are possibilities when considering metal solubilities below those predicted by the solubility product of pure compounds.
Fig. 3. Logarithm (base 10) of soluble Pb, Zn, and Cu from amended soils and from various pure compounds. Equilibrium lines for mineral phases were calculated based on the Ksp values reported by Lindsay (1979). An ionic strength (I) of 0.01 M was assumed. Symbols as in Fig. 1.
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