Immobilization and phytoavailability of cadmium in variable charge soils effect of lime addition

Immobilization and phytoavailability of cadmium in variable charge soils effect of lime addition

Plant and Soil 251: 187–198, 2003.

Immobilization and phytoavailability of cadmium in variable charge soils.

II Effect of lime addition

N.S Bolan1,4, D.C Adriano2, P.A Mani3& A Duraisamy3

1Institute of Natural Resources, Massey University, Palmerston North, New Zealand. 2Savannah River Ecology Laboratory, Drawer E, Aiken, SC 29802, USA.3Tamil Nadu Agricultural University, India

4Corresponding 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)2and 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−1soil), 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)2and 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 Cd2+adsorption in the KOH-treated than the Ca(OH)2-treated soil, which is attributed to the greater competition of Ca2+for adsorption Increasing addition of Cd enhanced Cd concentration in plants, resulting in decreased plant growth (i.e., phytotoxicity) Although addition of Ca(OH)2effectively reduced Cd phytotoxicity,

Cd uptake increased at the highest level, probably due to decreased Cd2+ adsorption resulting from increased

Ca2+ 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 CdCO3or Cd(OH)2precipitation 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

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

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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., phytoavailavail-ability) 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 Ca2+ 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 Ca2+ 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 (CaCO3), due to the quick action of Ca(OH)2 compared to CaCO3 Potassium hydroxide was in-cluded in order to delineate the effects of pH and

Ca2+concentration on the adsorption of Cd2 + These samples were incubated in a glasshouse for 4 weeks and subsequently used for surface charge and Cd2+ 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)2and 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(NO3)2and the amount of Cd2+adsorbed was calculated from the difference between the amount ad-ded and that remaining in solution after equilibration Details of the surface charge and Cd2+ 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-amended Egmont soil samples were subsequently

treated with increasing levels of Cd (0–10 mg kg−1

soil) using Cd(NO3)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◦C using a

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 HNO3 (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 NH4OAc 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)2and 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 Ca2+ 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 Cd2+ than did the Tokomaru soil Ad-sorption of Cd2+ 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 Cd2+ adsorption However, only

a small fraction of the pH-induced surface charge (7–11%) was occupied by the adsorbed Cd2+, and the ratio of pH-induced increases in Cd2+ 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

Cd2+for adsorption (Boekhold et al., 1983), resulting

in decreased Cd2+adsorption in the Ca(OH)2-treated soil It is necessary to point out that CaCO3 is the most commonly used liming material, which dissolves very slowly thereby resulting in less competition from

Ca2+for Cd2 +adsorption under field conditions.

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)

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 (PT50) was found to be

110.6 mg kg−1 (Figure 3A) The PT50 value is often

found to vary between plant and metal species PT50

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

CdCl2treated 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 NH4OAc 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 NH4OAc extractable-Cd and solution-Cd The dry matter yield decreased with in-creasing concentration of either NH4OAc

extractable-Cd or soil extractable-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 NH4OAc extractable-Cd (26%) This indicates that

in short-term experiments, plants take up Cd predom-inantly from soil solution, while most of the adsorbed

Cd2+extracted by NH4OAc 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) (Texchange-able 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

Figure 1 Relationships between pH and increases in surface charge (A) and Cd2+adsorption (B): (—-
—-) Egmont Ca(OH) 2 ; (—-
—-)

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)2addition:

Figure 3 Effect of pH on (A) NH4OAc extractable Cd and (B) soil solution Cd: (
) 0 Cd; (
) 0.3 mg Cd kg −1; () 3.0 mg Cd kg −1; ()

Figure 4 Relationships between dry matter yield and plant tissue Cd concentration (A) and NH4OAc extractable Cd (
) or soil solution Cd

Table 3 Effect of pH on the fractionation of Cd in the soil treated

(within a column, means followed by the same letter are not significantly different at the 10% level)

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 Cd2+adsorption caused by

pH-induced increases in negative charge (Bolan et al.,

1999b) However, adsorption may decrease with

in-creasing Ca2+ concentration due to a decrease in

activity coefficient, increase of inorganic

complex-ation and increase in Ca2+ competition Additional

benefit arises from the antagonistic effect from Ca2+

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 Ca2+and Cd2 + 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 Ca2+ concentration in soil solution

It has often been observed that the adsorption

of Cd2+ 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 Cd2+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 µM,

but at the same concentration, the soil with the highest

pH (6.81) was still adsorbing Cd2+from the solution.

Generally with an increasing pH, increasing amount

of irreversibly bound Cd2+occupies specific sorption

sites whereby the proportion of Cd2+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 Cd2+ 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 Cd2+ adsorption, only a small fraction of the surface charge was occupied by Cd2+ Others have also made similar attempts relating the pH-induced increases in surface charge to Cd2+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

Cd2+ 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 Cd2 + sorption in soils (Eq (1)

S = Kf C n (H+) m (1)

The exponent m is considered as a stoichiometric

coef-ficient indicating relative replacement ratio of H+by

Cd2+(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 Cd2+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

Cd2+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

metals, especially under alkaline pH Using the

solu-bility product (pKsp) 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 Cd2+and OH−

ions, and CO2partial pressure control the precipitation

of Cd as CdCO3 (octavite) and Cd(OH)2 (Street et

al., 1978) From the solubility product (pKsp) values

of these precipitates (CdCO3, 11.3; Cd(OH)2, 14.7)

it is possible to estimate the minimum concentration

of free Cd2+ 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

CdCO3and 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

Cd2+and the complexed Cd The concentration of free

Cd2+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 CdCO3◦ 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 CdCO3 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, Al2(SO4)3or FeCl3to

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 Cd3(PO4)2and

CdCO3which 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 Ca2+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 Ca2+ in-creased the concentration of Cd2+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 CdCO3or 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