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:
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.
197
Acknowledgements
This project was supported by Agricultural Human
Resource Development Programme (AHRDP) funded
by FAO. The U.S. Department of Energy contract
number DE-FC-09-96SR18546 with the University of
Georgia’s Savannah River Ecology Laboratory sup-
ported Drs. Bolan and Adriano’s writing/editing time.
We would like to thank Dr. Chris Anderson for his help
in analysing the soil and plant samples for Cd using the
graphite furnace AAS.
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Section editor: A.J.M. Baker