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Immobilization and phytoavailability of cadmium in variable charge soils effect of lime addition
Plant and Soil 251: 187–198, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands. 187 Immobilization and phytoavailability of cadmium in variable charge soils. II. Effect of lime addition N.S. Bolan 1,4 ,D.C.Adriano 2 ,P.A.Mani 3 &A.Duraisamy 3 1 Institute of Natural Resources, Massey University, Palmerston North, New Zealand. 2 Savannah River Ecology Laboratory, Drawer E, Aiken, SC 29802, USA. 3 Tamil Nadu Agricultural University, India 4 Corresponding author ∗ Received 12 March 2002. Accepted in revised form 9 October 2002 Key words: cadmium, calcium, heavy metal, immobilization, pH, phytoavailability, precipitation, surface charge Abstract The effect of pH-increases due to Ca(OH) 2 and KOH addition on the adsorption of cadmium (Cd) was examined in two soils which varied in their variable-charge components. The effect of Ca(OH) 2 on immobilization and phytoavailability of Cd from one of the soils, treated with various levels of Cd (0–10 mg Cd kg −1 soil), was further evaluated using mustard (Brassica juncea L.) plants. Cadmium immobilization in soil was evaluated by a chemical fractionation scheme. The addition of Ca(OH) 2 and KOH increased the soil pH, thereby increasing the adsorption of Cd, the effect being more pronounced in the soil dominated by variable charge components. There was a greater increase in Cd 2+ adsorption in the KOH-treated than the Ca(OH) 2 -treated soil, which is attributed to the greater competition of Ca 2+ for adsorption. Increasing addition of Cd enhanced Cd concentration in plants, resulting in decreased plant growth (i.e., phytotoxicity). Although addition of Ca(OH) 2 effectively reduced Cd phytotoxicity, Cd uptake increased at the highest level, probably due to decreased Cd 2+ adsorption resulting from increased Ca 2+ competition. There was a significant inverse relationship between dry matter yield and Cd concentration in soil solution. Addition of Ca(OH) 2 decreased the concentration of the soluble + exchangeable Cd fraction but increased the concentration of inorganic-bound Cd fractions in soil. Since there was no direct evidence for CdCO 3 or Cd(OH) 2 precipitation in the variable charge soil used for the plant growth experiment, alleviation of phytotoxicity can be attributed primarily to immobilization of Cd by enhanced pH-induced increases in negative charge. Introduction In many countries, cadmium (Cd) has been identi- fied as a major toxic heavy metal reaching the food chain, directly through crop uptake and indirectly through animal transfer (Adriano, 2001). This is a main reason why this element has been studied ex- tensively in relation to soil and plant factors affecting its bioavailability (Table 1). Cadmium accumulation in cropping and pasture soils is derived primarily from impurities in phosphate fertilizers, and biosolids added ∗ FAX No: 1-803-725-3309. E-mail: bolan@srel.edu during normal farming practice (Roberts et al., 1994; Williams and David, 1976). Health authorities in many parts of the world are becoming increasingly concerned about the effects of heavy metals on environmental and human health and its potential implications to international trade (Adri- ano, 2001). For example, the Cd accumulating in the offal (mainly kidney and liver) of grazing animals not only makes it unsuitable for human consumption but also imperils its suitability for manufacturing pet food (Roberts et al., 1994). Similarly, bioaccumulation of Cd in wheat and rice crops has serious implications to animal and human health, and to local and interna- tional cereal marketing (Nogawa and Kido, 1996). For these reasons, there is urgency to ensure that the heavy 188 Table 1. Selected references on the immobilization and phytoavailability of cadmium by liming materials Liming material Cadmium source Observation Reference CaCO 3 Fertilizer Decreased Cd concentration in straw Andersson and Siman (1991) Ca(OH) 2 (8, 15 and 22 Mg ha −1 ) Limed biosolids (spiked Decreased soil solution Cd and plant Basta and Sloan (1999) with Cd(NO 3 ) 2 ) uptake of Cd CaCO 3 (10gkg −1 ) Cd-enriched sewage sludge Decreased Cd-phytotoxicity in wheat Bingham et al. (1979) Ca(OH) 2 (8, 15 and 22 Mg ha −1 ) Sewage sludge Decreased the solution Cd; increased Brallier et al. (1996) residual fraction and plant uptake of Cd CaCO 3 ) (2.1–45 Mg ha −1 ) Sewage sludge Less movement of Cd than Cu and Zn in Brown et al. (1997) limed soil Ca(OH) 2 and CaCO 3 (0–1120 kg ha −1 ) Sand Reduced Cd phytotoxicity Chaney et al. (1977) CaCO 3 ) (0–4.5 g kg −1 )CdSO 4 Decreased CaCl 2 and NH 4 OAc extractable Fernandes et al. (1999) Cd in soil and plant tissue Cd – Limed soil Increased Cd 2+ sorption Filius et al. (1998) Ca(OH) 2 Arable soil – fertilizer Cd Decreased Cd in chemical extractants and Gray et al. (1999) plant tissue CaCO 3 ) (0–20 g kg −1 )Cd(NO 3 ) 2 (1.5 mg Cd kg −1 ) Decreased Cd concentration in plant tissue Han and Lee (1996) CaCO 3 (0–5.226 g kg −1 ) Arable soil fertilizer Cd Decreased Cd concentration in plant tissue He and Singh (1994) CaCO 3 (to pH 7.4) Arable soil/sewage sludge Decreased Cd 2+ adsorption Hooda and Alloway (1996) CaCO 3 (17.92 Mg ha −1 ) Sewage sludge and Decreased uptake of Cd by plants John and van Laerhoven (1976) Milorganite resulting in Cd attenuation CaCO 3 (0–1000 mg kg −1 ) Arable soil Increased plant Cd at low level of CaCO 3 ; John et al. (1972) decreased at high levels CaMgCO 3 (4 Mg ha −1 ) Forest soil Decreased Cd concentration in soil Kreutzer (1995) solution CaCO 3 (0–10 g kg −1 ) Arable soil Decreased Cd uptake by lettuce Lehoczky et al. (2000) Limestone (83% CaCO 3 Arable soil No affect on Cd uptake by sunflower plants Li et al. (1996) and 12% MgCO 3 ) CaCO 3 (3000 kg ha −1 ) Arable soil Decreased DTPA extractable Cd in soils Maclean (1976) andCdinplanttissue CaCO 3 (0–20 Mg ha −1 ) Arable soil Increased Cd concentration in potato tuber Maier et al. (1997) CaCO 3 (0–2.5 Mg ha −1 ) Arable soil Decreased Cd concentration in barley grain Oliver et al. (1996) CaCO 3 NPK fertilizer Decreased DTPA and NH 4 NO 3 Singh and Myhr (1998) extractable Cd;increased plant tissue Cd CaCO 3 (1–5gkg −1 ) Arable soil Decreased DTPA and NH 4 NO 3 Singh et al. (1995) extractable Cd, and plant tissue Cd CaCO 3 P fertilizer No effect on Cd concentration of potato tuber Sparrow et al. (1993) CaCO 3 Pasture soil Decreased Cd concentration in plant tissue Tyler and Olsson (2001) CaO Limed sewage sludge Less uptake of Cd by plants Vasseur et al. (1998) CaCO 3 ,MgCO 3 ,CaSO 4 Arable soil Decreased Cd concentration with CaCO 3 Williams and David (1976) and MgCO 3 , but increased with CaSO 4 189 metal content of foodstuffs produced complies with regulatory standards and is comparable to that from other countries. A range of soil amendments, such as lime, phos- phate compounds and alkaline-stabilized biosolids have been found to be effective in immobilizing metals, thereby reducing their bioavailability in soils (Basta et al., 2001; Knox et al., 2000). Since avail- ability of metals to plants (i.e., phytoavailability) is typically greater in acidic soils than alkaline soils, neutralizing agents in the form of lime are commonly added to acidic soils. Although the primary incent- ive in liming acidic arable soils is the suppression of toxic bioavailable aluminum and manganese to plants, it may also limit the uptake of certain critical metals such as Cd. Liming is increasingly being practiced as a management tool to immobilize metals in soils, as well as in biosolids and mine tailings, thereby reducing their phytoavailability and transport to groundwater (Table 1). Several reasons have been attributed to the lime- induced immobilization of metals (Bolan et al., 1999b): increases in negative charge (CEC) in variable charge soils; formation of strongly bound hydroxy metal species; precipitation of metals as hydroxides; and sequestration due to enhanced microbial activ- ity. Calcium addition in the form of lime also causes an inhibition of the translocation of metal from root to shoot. However, in some soils, addition of Ca- containing compounds such as lime and gypsum has been shown to increase the plant availability of metals (John et al., 1972; Williams and David, 1976). This is attributed to the exchange of Ca 2+ with the metal ions and the subsequent increase in the concentration of metal ions in soil solution. In a series of laboratory and glasshouse trials, the potential value of phosphate, lime and biosolids on the immobilization and the consequent reduction in the phytoavailability of Cd in variable charge soils was examined in relation to the mechanisms mentioned above. The effect of phosphate has been reported in an earlier paper (Bolan et al., 2002). Lime is discussed in this paper and biosolids is addressed in a subsequent paper. Materials and methods Soils Two surface (0–30 cm) pasture soils (Egmont and Tokomaru) which vary in their variable charge char- acteristics were used to examine the effects of pH and Ca 2+ on surface charge and subsequent adsorp- tion/precipitation of Cd. In pasture soils, surface layer generally refers to 0–15 cm depth, but in this study soil from 0–30 cm depth was used mainly to get a sample with relatively low pH in order to justify liming. The Egmont soil contains higher amounts of variable charge components such as allophanic clays and organic matter than the Tokomaru soil, which is dominated by vermiculite. The specific characterist- ics of the soils used in this study are given elsewhere (Bolan et al., 2002). The soils were treated with four levels of calcium hydroxide (Ca(OH) 2 ) or potassium hydroxide (KOH) to achieve a pH range of 5.2 (control) to 7.9. Cal- cium hydroxide (Ca(OH) 2 ) was used instead of the most commonly used liming material, calcium car- bonate (CaCO 3 ), due to the quick action of Ca(OH) 2 compared to CaCO 3 . Potassium hydroxide was in- cluded in order to delineate the effects of pH and Ca 2+ concentration on the adsorption of Cd 2+ .These samples were incubated in a glasshouse for 4 weeks and subsequently used for surface charge and Cd 2+ adsorption measurements. The Egmont soil was also used to examine the effect of Ca(OH) 2 treatment on the phytoavailability of Cd in a glasshouse experiment. Surface charge and cadmium adsorption The surface charge of the Ca(OH) 2 and KOH-treated soil samples was measured using 0.1 M NaCl follow- ing the ion retention method. Cadmium adsorption was measured at a Cd concentration of 0.001 M us- ing Cd(NO 3 ) 2 and the amount of Cd 2+ adsorbed was calculated from the difference between the amount ad- ded and that remaining in solution after equilibration. Details of the surface charge and Cd 2+ adsorption measurements are given in the earlier paper in this series (Bolan et al., 2002). Plant growth experiment A glasshouse plant growth experiment was set up to investigate the effect of Ca(OH) 2 treatment on the plant uptake of Cd. Previously incubated Ca(OH) 2 - 190 amended Egmont soil samples were subsequently treated with increasing levels of Cd (0–10 mg kg −1 soil) using Cd(NO 3 ) 2 and further incubated for 4 weeks. The incubated soil samples were transferred to plastic pots. Indian mustard (Brassica juncea L.) was used as a test plant due to its ability to tolerate high levels of heavy metals in soils (Anderson et al., 2001). Eight seeds were sown in each pot and after about 2 weeks of growth the seedlings were thinned to four plants per pot. During the germination period the moisture content of the soil was maintained at 80% of field capacity and after thinning the moisture content was raised to field capacity. Complete Hoagland nutri- ent solution (Hoagland and Arnon, 1950) was added twice per week. The plants were harvested 12 weeks after seed- ing and dried to constant weight at 70 ◦ Cusinga forced draught oven. The dry weights were recorded and the plant materials were ground using a stainless steel grinder. The plant materials were digested us- ing concentrated HNO 3 (Robinson et al., 2000) and the concentration of Cd in the plant digest was ana- lysed using a graphite-furnace AAS (GBC 909AA, Melbourne, Australia). Extractable cadmium and fractionation of cadmium The concentrations of exchangeable-Cd and soil solution-Cd were measured at all levels of Cd addition in the plant growth experiment. The exchangeable-Cd was measured by extracting with 1 M NH 4 OAc at a soil:solution ratio of 1:10 for 1 h. The soil solution was obtained by the centrifugation method and the concentration of Cd in the soil solution was measured. A simple sequential extraction procedure (Sposito et al., 1982) was used to fractionate soil Cd into differ- ent operationally defined forms that include soluble + exchangeable fraction (F1), organic-bound fraction (F2), inorganic-bound fraction (F3) and residual frac- tion (F4). For Cd measurements the soil samples from the pot experiment were used at the end of the glass- house trial and the details of the extractable Cd and fractionation measurements are given in Bolan et al. (2002). Results Surface charge Negative charge, as indicated by Na + adsorption, in- creased with increasing pH due to Ca(OH) 2 and KOH additions. The pH-induced increase in negative charge was higher for the Egmont than the Tokomaru soil, which is attributed to the difference in the variable charge components between the soils. At similar pH values, the Ca(OH) 2 -treated soils contained slightly higher amount of negative charge than the KOH- treated soils, the difference being more pronounced in the Egmont soil (Figure 1A). An increase in soil pH has often been shown to enhance the solubliliz- ation of organic matter, resulting in an increase in the concentration of dissolved organic carbon (DOC) (Temminghoff, 1998). In the present study the con- centration of DOC increased with increasing pH, and the pH-induced increase in DOC was higher in the KOH-treated than the Ca(OH)-treated soil (Table 2). Temminghoff (1998) has shown that DOC concen- tration in limed soils is partly controlled by Ca 2+ concentration in soil solution. Calcium can act as a bridge between the negatively charged DOC and soil particles and also helps in the coagulation of DOC. The greater loss of organic matter in the form of DOC in the KOH-treated soil may be one of the reasons for the smaller increase in pH-induced negative charge. Cadmium adsorption As expected, the Egmont soil adsorbed higher amounts of Cd 2+ than did the Tokomaru soil. Ad- sorption of Cd 2+ increased with increasing pH, the effect being more pronounced in the Egmont than in the Tokomaru soil (Figure 1B). There was a signi- ficant relationship between increases in pH-induced surface charge and Cd 2+ adsorption. However, only a small fraction of the pH-induced surface charge (7–11%) was occupied by the adsorbed Cd 2+ ,and the ratio of pH-induced increases in Cd 2+ adsorp- tion:negative charge was slightly less (0.07:1.0) for the Ca(OH) 2 - than the KOH-treated (0.11:1.0) soil. Cal- cium has often been shown to compete strongly with Cd 2+ for adsorption (Boekhold et al., 1983), resulting in decreased Cd 2+ adsorption in the Ca(OH) 2 -treated soil. It is necessary to point out that CaCO 3 is the most commonly used liming material, which dissolves very slowly thereby resulting in less competition from Ca 2+ for Cd 2+ adsorption under field conditions. 191 Table 2. Effect of pH on dissolved organic carbon (DOC) in the soil treated with various levels of Ca(OH) 2 or KOH (within a column, means followed by the same letter are not significantly different at the 10% level) Tokomaru soil Egmont soil Ca(OH) 2 KOH Ca(OH) 2 KOH pH DOC (mg kg −1 ) pH DOC (mg kg −1 ) pH DOC (mg kg −1 ) pH DOC (mg kg −1 ) 5.32 5.62a 5.32 5.62a 5.23 10.2a 5.23 10.2a 5.73 8.01ab 5.91 12.5b 5.92 12.5ab 5.75 18.9b 6.76 10.4bc 6.83 18.5c 6.71 17.6b 6.81 36.9c 7.91 14.3cd 7.82 35.2d 7.85 23.1c 7.71 58.2d Plant growth and cadmium uptake The dry matter yield decreased with increasing level of Cd application, indicating the phytotoxic effect of Cd (Figure 2A). In general, the inhibitory effect of Cd on plant growth decreased with increasing pH. How- ever, at all levels of Cd addition, the dry matter yield decreased at the highest pH value. As expected, the plant tissue concentration of Cd increased with in- creasing level of Cd addition (Figure 2B) which was the main reason for the inhibition of plant growth with increasing level of added Cd. However, except for the highest pH value, Cd concentration in the plant tissue decreased with increasing pH. The dry matter yield decreased with increasing concentration of Cd in the plant tissue (Figure 3A). The phytotoxicity threshold concentration of Cd in the plant tissue, as defined by the concentration of Cd in plant tissue correspond- ing to 50% growth decrement (PT 50 ) was found to be 110.6 mg kg −1 (Figure 3A). The PT 50 value is often found to vary between plant and metal species. PT 50 values of >10 mg kg −1 for soybean (Miller et al., 1976), > 500 mg kg −1 for radish tops and > 300 mg kg −1 for radish roots (John et al., 1972) grown in CdCl 2 treated soils, and respectively 2.5, 2.0, 150 and 158 mg kg −1 for beet root, carrot, Swiss chard and tomato grown in nutrient solution (Turner, 1973) were obtained. Extractable cadmium The concentrations of both NH 4 OAc extractable-Cd and solution-Cd increased with increasing level of Cd addition, but decreased with increasing pH (Fig- ure 4). As in the case of phosphate addition (Bolan et al., 2002), there was a significant inverse relation- ship between pH-induced increase in negative charge and the concentration of NH 4 OAc extractable-Cd and solution-Cd. The dry matter yield decreased with in- creasing concentration of either NH 4 OAc extractable- Cd or soil solution-Cd (Figure 3B). The soil solution- Cd explained a greater variation (47%) in the dry matter yield and the plant tissue concentration than did the NH 4 OAc extractable-Cd (26%). This indicates that in short-term experiments, plants take up Cd predom- inantly from soil solution, while most of the adsorbed Cd 2+ extracted by NH 4 OAc is not phytoavailable. Cadmium fractionation Metal fractionations using the sequential extraction techniques have primarily been used to identify the fate of the metals applied in sewage sludges and in soils contaminated by smelters and mine drainage wastes (Sposito et al., 1982). In the present study, the sequential fraction procedure achieved almost com- plete (between 96 and 108%) recovery of the added Cd in the soil used for the plant growth experiment. The concentration of Cd in all fractions increased with in- creasing level of Cd addition, the concentration being higher in the organic-bound (F2), oxide-bound (F3), and residual fractions than the soluble plus exchange- able fraction (F1) (Table 3). With increasing pH the concentration of Cd in the F1 fraction decreased with a corresponding increase in the other fractions. This is similar to the observations made by others for both Cd and other metals in the presence of lime (Table 1) and other inorganic amendments, such as apatite and flyash (Knox et al., 2000; Pierzynski and Schwab, 1993). These studies suggest that treating the soils with inorganic wastes shifts the solid phases of the metals away from mobile fractions to forms that are immobile and less bioavailable. Plants derive most of their nutrients from F1 fraction (Adriano, 2001). This 192 Figure 1. Relationships between pH and increases in surface charge (A) and Cd 2+ adsorption (B): (—-—-) Egmont Ca(OH) 2 ;(—-—-) Egmont KOH; (—- —-) Tokomaru Ca(OH) 2 ;(—-—-) Tokomaru KOH. Figure 2. Dry matter yield of Brassica juncea (A) and the concentration of Cd in plant tissue (B) at various pH levels due to Ca(OH) 2 addition: ()5.2;()5.9;()6.7;()7.8). 193 Figure 3. Effect of pH on (A) NH 4 OAc extractable Cd and (B) soil solution Cd: ()0Cd;()0.3mgCdkg −1 ;()3.0mgCdkg −1 ;() 10.0mgCdkg −1 ). Figure 4. Relationships between dry matter yield and plant tissue Cd concentration (A) and NH 4 OAc extractable Cd () or soil solution Cd ()(B). 194 Table 3. Effect of pH on the fractionation of Cd in the soil treated with various levels of Ca(OH) 2 in the plant growth experiment (within a column, means followed by the same letter are not significantly different at the 10% level) pH Soil Cd level Soil fraction ∗ (mg Cd kg −1 )(mgCdkg −1 ) F1 F2 F3 F4 5.2 0 0a 0.01a 0.013a 0.015a 0.3 0.11b 0.056a 0.121b 0.039a 3.0 0.85d 0.57b 0.92c 0.72b 10 3.56f 1.87c 2.36d 2.34c 5.9 0 0a 0.012a 0.017a 0.013a 0.3 0.042a 0.061a 0.152b 0.056a 3.0 0.42c 0.65b 1.24c 0.89b 10 1.61e 1.95c 3.02e 3.38d 6.7 0 0a 0.011a 0.016a 0.014a 0.3 0.027a 0.068a 0.162b 0.068a 3.0 0.14b 0.65b 1.32c 1.01b 10 1.02d 2.24c 3.35e 3.56d 7.8 0 0a 0.012a 0.023a 0.023a 0.3 0.008a 0.067a 0.165b 0.078a 3.0 0.008a 0.56b 1.43c 1.25b 10 0.81d 2.37c 3.56e 3.54d ∗ F1 – soluble + exchangeable; F2 – organic-bound; F3 – inorganic- bound; F4 – residual. indicates that increasing soil pH resulted in a decrease in the phytoavailability of Cd. Discussion The data from the laboratory and glasshouse exper- iments clearly demonstrated that Cd in soils can be immobilized by increasing the soil pH through ad- dition of liming materials. Decreases in Cd uptake arise from increased Cd 2+ adsorption caused by pH- induced increases in negative charge (Bolan et al., 1999b). However, adsorption may decrease with in- creasing Ca 2+ concentration due to a decrease in activity coefficient, increase of inorganic complex- ation and increase in Ca 2+ competition. Additional benefit arises from the antagonistic effect from Ca 2+ added through liming, which may suppress Cd uptake by competing for exchange sites at the root surface. Liming, as part of the normal cultural practices, has often been shown to reduce the concentration of Cd and other metals in the edible parts of a number of crops. Addition of other alkaline waste materials such as coal fly ash has also been shown to decrease Cd content of plants (Table 1). In these cases, the effect of liming materials in decreasing Cd uptake has been attributed to both decreased mobility of Cd in soils and to competition between Ca 2+ and Cd 2+ ions on the root surface. In general, Cd uptake by plants de- creases with increasing pH. For example, higher Cd concentrations were obtained for lettuce and Swiss chard on acid soils (pH 4.8–5.7) than on calcareous soils (pH 7.4–7.8) (Mahler et al., 1978). Consequently, it is recommended that soil pH be maintained at pH 6.5 or greater in land receiving biosolids containing Cd (Adriano, 2001). However, it is also possible that in alkaline soils, solubility and uptake of Cd can be enhanced due to facilitated complexation of Cd with humic or organic acids (Harter and Naidu, 1995). Thus the resultant effect of liming on Cd (im)mobilization and subsequent phytoavailability depends on the re- lative changes in pH and Ca 2+ concentration in soil solution. It has often been observed that the adsorption of Cd 2+ increases with increasing pH (Bolan et al., 1999a; Naidu et al., 1994), resulting in low phytoavail- ability of Cd in alkaline soils. Filius et al. (1998) observed that the equilibrium solution concentration at which zero Cd 2+ sorption–desorption occurred (called 195 null point) decreased with increasing pH, indicating that even at low solution concentration adsorption con- tinued to occur at high pH. For example, at the lowest pH (4.68) the soil sample released 50 µmol Cd per kg soil at an equilibrium Cd concentration of 0.1 µ M, but at the same concentration, the soil with the highest pH (6.81) was still adsorbing Cd 2+ from the solution. Generally with an increasing pH, increasing amount of irreversibly bound Cd 2+ occupies specific sorption sites whereby the proportion of Cd 2+ bound reversibly to non-specific exchange sites becomes insignificant (Tiller, 1989). Various reasons have been advanced for pH- induced immobilization of metals in soils. Firstly, an increase in pH in variable-charge soils causes an increase in surface negative charge resulting in an increase in cation adsorption (Naidu et al., 1994). Secondly, an increase in soil pH is likely to result in the formation of hydroxy species of metal cations which are adsorbed preferentially over the metal cation. Naidu et al. (1994) observed that CdOH + spe- cies are formed above pH 8 which have a greater af- finity for adsorption sites than just Cd 2+ . And thirdly, precipitation of Cd as Cd(OH) 2 is likely to result in greater retention at pH above 10 (Naidu et al., 1994). Evidences for these mechanisms in the present study are given below. It is to be pointed out that the highest soil pH obtained in this experiment was only 7.91. Soil solution pH is one of the major factors con- trolling surface properties of variable charge compon- ents (Barrow, 1985). An increase in pH increases the net negative charge which is attributed to the disso- ciation of H + from weakly acidic functional groups of organic matter and some clay minerals (Curtin et al., 1996; Thomas and Hargrove, 1984). In the present study the increases in negative charge per unit in- crease in pH ranged from 11.5 to 15.7 mmol kg −1 for the Tokomaru soil and from 63.1 to 64.2 mmol kg −1 for the Egmont soil. The amount of surface charge acquired through an increase in pH depends on the amount and nature of variable charge components (Bolan et al., 1999b). It has been estimated that rais- ing pH by one unit increases the negative charge of soil organic matter by about 300 mmol kg −1 (Helling et al., 1964). The surface charge of the soil mineral component is generally far less pH-dependent than that of soil organic matter. For example, the negative charge of soil clay may only increase by 30–40 mmol kg −1 per pH unit (Curtin et al., 1996; Helling et al., 1964). However, the pH-dependence of mineral sur- face charge can vary considerably depending on the nature of the component minerals. Mineral constitu- ents that dissociate H + when pH is increased through liming include hydroxy-Al polymers associated with the surfaces of phyllosilicate minerals, amorphous and short-range ordered aluminosilicates, and rup- tured surfaces of silicates and oxides (Thomas and Hargrove, 1984). In the present study, although there was a positive relationship between increases in pH-induced surface charge and Cd 2+ adsorption, only a small fraction of the surface charge was occupied by Cd 2+ .Othershave also made similar attempts relating the pH-induced increases in surface charge to Cd 2+ adsorption by vari- able charge soils (Boekhold et al., 1993; Bolan et al., 1999a; Naidu et al., 1994). For example, Bolan et al. (1999a) observed that approximately 50% of the pH- induced increase in surface negative charge in variable charge soils was occupied by Cd. The remaining sur- face negative charge was presumed to be occupied by the H + and K + ions, added in acid and alkali solu- tions to alter the soil pH. Similarly, Naidu et al. (1994) and Bolan et al. (2002) demonstrated that the effects of ionic strength and specifically adsorbed anions on Cd 2+ adsorption operate partly through their effects on surface charge. The effect of pH on metal sorption has also been related to the exchange of H + for the metal ions. On this basis, Boekhold et al. (1993) modified the Freund- lich equation to account for the effect of pH on Cd 2+ sorption in soils (Eq. (1). S = K f C n (H + ) m (1) The exponent m is considered as a stoichiometric coef- ficient indicating relative replacement ratio of H + by Cd 2+ (number of moles H + replaced by one mole of Cd). A range of m values ranging from 0.5 to 1.8 have been obtained for Cd 2+ adsorption in soils (Boekhold et al., 1993; Filius et al., 1998; Naidu et al., 1994), indicating that depending on the soil and solution com- position, varying amounts of H + are released per unit Cd 2+ sorbed. Precipitation as metal hydroxides or carbonates is considered to be one of the mechanisms for the immobilization of metals, such as Pb, Zn and Cd by liming materials (Pierzynski and Schwab, 1993; Street et al., 1978). The formation of the new solid phase (i.e., precipitate) occurs when the ionic product in the solution exceeds the solubility product of that phase. In normal soils, precipitation of metals is un- likely, but in highly metal contaminated soils, this process can play a major role in the immobilization of 196 metals, especially under alkaline pH. Using the solu- bility product (pK sp ) values for metal hydroxy species, Sillen and Martell (1971) calculated the minimum pH range of 8.8–9.8, 7.4–8.5, 6.1–6.9 and 6.1–9.1 for the precipitation of Cd, Zn, Cu and Pb hydroxides, respectively, in soil systems. For a given mineral com- position, the stability sequence is Pb > Cu > Zn > Cd. In limed soil, the activities of free Cd 2+ and OH − ions, and CO 2 partial pressure control the precipitation of Cd as CdCO 3 (octavite) and Cd(OH) 2 (Street et al., 1978). From the solubility product (pK sp )values of these precipitates (CdCO 3 , 11.3; Cd(OH) 2 , 14.7) it is possible to estimate the minimum concentration of free Cd 2+ in soil solution required for the onset of precipitation. This value decreases with increas- ing pH, and in the present experiment, the calculated values were 0.0112 mg L −1 and 0.0123 mg L −1 for CdCO 3 and Cd(OH) 2 , respectively, for the limed soil at the highest pH (7.9). The measured concentration of Cd in the soil solution exceeded the above cal- culated concentration only at the highest level of Cd addition (10 mg kg −1 ) which may provide some evid- ence for precipitation. However, it is important to note that the measured soil solution concentration gives the total Cd concentration which includes both the free Cd 2+ and the complexed Cd. The concentration of free Cd 2+ which controls precipitation is likely to be much less than the total concentration in the organic matter- rich soil used in the present experiment. For example, Street et al. (1978) and Sauve et al. (2000) noticed that more than 75% Cd remained as organically complexed Cd in soils containing high levels of organic matter. Further it is possible to form inorganic complexes such as CdCO 3 ◦ and CdOH + in limed soils (Street et al. 1978). The plant availability of these complexes is not well established. Street et al. (1978) obtained evidence for precip- itation of Cd as CdCO 3 only in a sandy soil having low organic matter and low CEC. In another instance, Soon (1981) examined the effect on the solubility of Cd in two soils of a number of sewage sludges that had been treated with Ca(OH) 2 ,Al 2 (SO 4 ) 3 or FeCl 3 to precipitate phosphate from effluent water. The sludge samples varied in their lime equivalents and phosphate content. At low levels of Cd addition, the solubility of Cd was controlled by adsorption that was enhanced by increasing pH resulting from the sludge addition. At high levels of Cd addition, however, there was evidence for the precipitation of Cd as Cd 3 (PO 4 ) 2 and CdCO 3 which controlled the solubility. Krishnamurti et al. (1996) observed that compared with bulk soils, solid phase speciation of Cd dif- fers substantially in phosphate fertilizer-treated rhizo- sphere soils. The amounts of Cd species associated with adsorbed and metal–organic complexes of the rhizosphere soils were appreciably higher than those of the corresponding bulk soils. The increase was at- tributed to precipitation by bicarbonate, a product of plant respiration, and the organic acids released as root exudates, present in soil–root interface. Conclusions Liming increased both the pH and Ca 2+ concentration in soil solution. In soils dominated by variable charge components, pH-induced increases in surface charge resulted in an increase in the sequestration of added Cd, thereby reducing its phytoavailability. However, at the highest rate of liming, the lime-borne Ca 2+ in- creased the concentration of Cd 2+ in soil solution due to competition for adsorption sites. This resulted in an increase in the plant uptake of Cd. There was no direct evidence for lime-induced precipitation of Cd as CdCO 3 or Cd(OH) 2 . Lime addition enhanced the transformation of readily bioavailable Cd fraction to less mobile frac- tions. It is important to emphasize that there is a dynamic equilibrium between these fractions, and any depletion of the bioavailable pool due to plant uptake or leaching losses will result in the continuous release from other fractions to replenish the available ‘pool’. This is one of the main reasons why there is some reluctance towards using ‘bioavailable’ pool in soils for regulatory purposes by environmental agencies in monitoring contaminated sites. Liming materials low in heavy metal content may offer a promising option for the in situ immobiliz- ation of metal-contaminated soils. Lime stabilized biosolids are increasingly being used to immobilize heavy metals in soils, thereby reducing their bioavail- ability for plant uptake. But the use of alkaline biosolids may result in the generation of DOC to form soluble complexes with the metals, thereby facilitating their transport. Another major inherent problem asso- ciated with lime-enhanced immobilization in soils is that regular application of lime is necessary to neutral- ize the acid released continuously through plant and microbial processes. [...]... and Merry R H 1996 Effectiveness of liming to minimise uptake of cadmium by wheat and barley grain grown in the field Aust J Agric Res 47, 1181–1193 Pierzynski G M and Schwab A P 1993 Bioavailability of zinc, cadmium and lead in a metal contaminated alluvial soil J Environ Qual 22, 247–254 Roberts A H C, Longhurst R D and Brown M W 1994 Cadmium status of soils, plant and grazing animals in New Zealand... 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Ca 2+ concentration in soil solution. In soils dominated by variable charge components, pH-induced increases in surface charge resulted in an increase in the sequestration