77

4

Determinants of Metal Retention to and Release

from Soils

Yujun Yin, Suen-Zone Lee, Sun-Jae You, and Herbert E. Allen
CONTENTS

4.1 Introduction 77
4.2 Experimental Approaches 78
4.3 Results and Discussion 79
4.3.1 Sorption of Metals to Soils 79
4.3.1.1 Sorption of Cadmium 80
4.3.1.2 Sorption of Mercury 82
4.3.1.3 Soil Sorption Phases 87
4.3.2 Metal Desorption from Soils 88
4.4 Conclusions 89
References 90

Introduction

Fate and bioavailability of metals in soils are governed by many environmental processes.
Without consideration of transport and rhizosphere reactions, the processes influencing a
trace metal uptake by a plant are depicted in Figure 4.1. All plants take up metals in soils
via pore water; therefore, the partitioning of metals between soil solution and solids is the
primary factor determining metal bioavailability. Strong retention affinities for soil solid
surfaces would reduce the risk of a metal species to the ecosystem, whereas poor retention

later in this chapter, pH also affects the soil-water partitioning of natural organic matter in
soils, which in turn affects metal sorption and desorption. At a fixed pH, sorption and des-
orption of metals in soils depend on both particulate and aqueous composition and char-
acteristics. For example, a larger surface area could result in a greater sorption. The high
valent, or even low valent major cations which are present at high concentrations in soil
solution, could compete with heavy metals for available surface sites and consequently
decrease sorption (Zachara et al., 1993). The presence of inorganic and organic ligands in
soil solution, on the other hand, could reduce metal sorption by forming soluble complexes
with metals (Huang and Lin, 1981; Yin et al., 1996). The combined effects of all of these fac-
tors determine metal sorption and desorption in soils.
This report summarizes some of the results on metal sorption and desorption on soils
obtained in our lab over the last several years. The focus of this summary is on the com-
bined effects of major environmental variables on metal sorption and desorption. Sorption
of eight metal species on soils are compared. The effects of soil pH and surface properties
as well as of soluble organic matter, a major cation, and an anion on metal sorption are elu-
cidated. Likewise, desorption of metals from soils as a function of pH and dissolved
organic matter is discussed.

4.2 Experimental Approaches

We have employed 15 representative noncontaminated soils collected from the state of
New Jersey to conduct our experiments. The soils were air dried and sieved through a

FIGURE 4.1

Metal speciation and bioavailability in soils. M

2+

is the metal of interest.

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Determinants of Metal Retention to and Release from Soils

79
2 mm screen. Soil aggregates were broken by hand and a wooden mallet before sieving. All
experiments were conducted using soil fractions less than 2 mm which were thoroughly
mixed before use. The soils were characterized in terms of surface properties and chemical
composition. Detailed soil characteristics have been reported elsewhere (Yin et al., 1996).
The texture of these soils varies from sand to silt-clay loam with sand (630 µm to 2 mm)
content ranging from 20 to 92%, silt (2 to 630 µm) from 3 to 49%, and clay (< 2 µm) from 6
to 37%. Soil pH ranges from 4.2 to 6.4, CEC from 800 to 9500 mmol/kg, organic matter from
0.2 to 8.6%, and surface area from 1100 to 11,590 m

2

/kg.
Sorption experiments were conducted by adding metal nitrates to soil-water suspensions
which were mixed at a ratio of 1 g/100 mL in 125 ml Erlenmeyer flasks or polyethylene plas-
tic bottles. Sodium nitrate or Ca(NO

3

)

2

(or a mixture of both) was used to maintain an ionic
strength of 0.01


and 4% HCl following the method described by Shi (1996).
Desorption of metals from soils was investigated by mixing each soil sample with 0.01

M

NaNO

3

at a ratio of 25 g/20 ml in 50 ml polyethylene centrifuge tubes. Five parallel mix-
tures were prepared for each soil, and the pH values of the mixtures were adjusted with
1

N

HNO

3

or NaOH to cover a range of 4 to 8 so that the effect of pH on metal desorption
could be evaluated. The soil mixtures were equilibrated by shaking for 24 h under the same
conditions described previously. After equilibration, the samples were centrifuged and the
supernatants were filtered through a 0.45 µm pore size membrane filter. The metal concen-
trations in the filtrates were then determined by an ICP.
Clean techniques were employed throughout the experimental process. All glassware
and plastic containers were cleaned by acid-soaking overnight followed by thorough rinse
with distilled and then deionized water. Metal-free ultrapure water generated by a NANO
pure system (Burnstead) was used for all experiments. Trace metal grade or ACS certified
chemicals were used. All persons conducting experiments wore non-talc, class-100 plastic


–

, but also for the sur-
face sites (-

Ο

–

) that can be considered as bases. Thus, at low pH values, while the sorption
of metals of weaker acids was small due to the competition of protons, the sorption of met-
als of stronger acids was large because of their stronger binding affinity for the surfaces.
Increases in pH increased sorption principally because the surface potential decreased.
Further increases in pH beyond about pH 7 decreased sorption slightly, except for Hg
which decreased substantially. The pH value at which the sorption of a metal started to
decrease mainly depended on the acidity of a metal. Generally speaking, for metals that are
stronger Lewis acids, sorption reached maximum at lower pH values and then decreased.
As discussed later in detail, decreases in metal sorption at higher pH values mainly
resulted from the complexation of metals by soluble ligands. The metals that are stronger
Lewis acids tended to be affected to a greater extent by soluble ligands; thus, the sorption
of these metals decreased to a greater extent as pH increased. Cadmium(II) and Hg(II) rep-
resented the two ends in terms of metal sorption behavior; the rest of the metals of this
study fell in-between. Cadmium sorbed the least to the soil at low pH and was also affected
the least by soluble ligands, while Hg(II) sorbed the most to the soil at low pH and was also
affected the most by soluble ligands at high pH. In this report, we use Cd(II) and Hg(II) as
examples to illustrate the effects of metal nature and soil properties on sorption of metals.

4.3.1.1 Sorption of Cadmium


80
60
40
20
0
50
40
30
20
10
0
2345678910
% Adsorbed
Dissolved organic C (mg/L)
pH
Hg
Pb
Cu
Zn
Ni
Co
Cd
DOC

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Determinants of Metal Retention to and Release from Soils

81


K

d

value and the soil organic matter content was obtained at each fixed pH value. As an
example, Figure 4.3b shows the linear relationship for pH 6.
The close correlation of Cd(II) sorption to the organic matter content suggests that soil
organic matter is the most important component determining Cd(II) sorption. It is expected
that a normalization of the

K

d

values with the soil organic matter content would reduce the
variance of the

K

d

s at fixed pH shown in Figure 4.3a. As shown in Figure 4.3c, the normal-
ization of

K

d

values indeed shrank the data to a line. The regression coefficient R


As shown in Figure 4.2, sorption of Hg(II) reached maximum at pH around 4 and then
decreased. The sorption isotherms of Hg(II) on these soils at the natural pH followed an
S-shape rather than the L-shape usually observed for sorption of metals on mineral sur-
faces. The S-shaped isotherm has been ascribed to the complexation of soluble organic mat-
ter for metals (Sposito, 1989; Yin et al., 1997a). As shown in Figure 4.4, at low Hg(II) loading,
a large amount of Hg(II) was complexed by soluble organic matter; thus, sorption was
small. Increases in Hg(II) loading level increased the amount of Hg(II) available for sorp-
tion, and therefore increased sorption. When the soluble organic matter was saturated, fur-
ther increases in Hg(II) loading resulted in rapid increases in Hg(II) sorption, and
eventually saturated the surface binding sites.
Based on Hg(II) sorption isotherms, we speculated that decreases in Hg(II) sorption at
higher pH might also result from complexation of Hg(II) by soluble organic matter. This
was supported by the fact that the dissolved organic matter increased as pH increased
(Figure 4.2). We further studied the effect of dissolved organic matter on Hg(II) sorption by
adding varying amount of organic matter extracted from soil to soil suspensions at fixed
pH 6.5. When the dissolved organic matter (measured as organic C) increased from 1.4 to
61.1 mg/L, sorption of Hg(II) decreased from near 60 to 28% at pH 6.5, implying strong
complexation of Hg(II) by the dissolved organic matter.
When Ca(NO

3

)

2

was used as the background electrolyte and the ionic strength was still
maintained at 0.01



OM

is the soil organic matter normalized partitioning coefficient. The regression equation for
the middle graph is log K

om

= 1.084 + 0.477 pH. (Top and bottom graphs are from Lee et al.,

Environ. Sci. Technol.

,
3418–3424, 1996.)

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Determinants of Metal Retention to and Release from Soils

83
cantly decreased the solubility of organic matter (Figure 4.5b). Calcium has been suggested
to be able to decrease the solubility of organic matter either by coagulation or complexation
in which Ca serves as a bridge between solid surfaces and organic matter (Schnitzer, 1986).
Because the dissolved organic matter decreased in Ca electrolyte, more Hg(II) became avail-
able for surface sorption; therefore, Hg(II) sorption increased.
In the presence of high concentrations of Cl

–


Consquently, an increase in Cl

–

concentration had almost no or only slight effect on Hg(II)
sorption on the soils with high organic matter contents (Figure 4.6). At the high pH range
in which the calculated Hg-OH complexes become predominant over Hg-Cl complexes,
addition of Cl

–

had almost no effect on Hg(II) sorption (Yin et al., 1996).
Because of the significant effects of soluble ligands on Hg(II) sorption, both surface and
aqueous reactions have to be considered in modeling Hg(II) sorption on soils. In this study,
we developed a model to describe these reactions. In the solution phase of this study with-
out addition of extra Cl

–

, the concentration of Cl

–

ranged from nearly 1

×

10

–6


; T =
25 ± 2°C.

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84

Environmental Restoration of Metals–Contaminated Soils

The reactions for protons and Hg(II) binding to the dissolved organic matter are
expressed by Equations 1 to 4 in which charges are omitted for simplicity:
(1)
(2)

FIGURE 4.5

Effect of Ca

2+

on Hg(II) sorption onto the Booton Union loam as a function of pH. (Top) Fractional Hg sorption;
(bottom) solubility of organic matter. Soil solution = 1 g/100 mL; I = 0.01

M

; T = 25 ± 2°C.
LHg+ HgL=
K

(3)
(4)
where

L

is the organic ligand,

Hg

is the inorganic Hg which includes both free and hydroxo
Hg species,

HgL

is the organically bound Hg,

K

HgL

and

K

HL

are the stability constants for
Hg and proton binding with ligand (L/



×

10

–4

mol/L), the third term in Equation 5
is negligible, which yields:
(6)
The reactions for proton and Hg binding with surface sites are expressed by Equations 7
to 10:

≡

SO

–
+

Η

g

= ≡

SOHg

DOC L[] HL[]HgL[]++=
DOC L[] HL[]+≅
K
=SOHg
≡SOHg[]
≡SO
–
[]Hg[]

T
Hg
C
w
–
≡SO
–
[]Hg[]
==
4131/frame/C04 Page 85 Wednesday, August 9, 2000 3:13 PM
© 2001 by CRC Press LLC
86 Environmental Restoration of Metals–Contaminated Soils
≡ SO
−
+
Η
+
= ≡ SOH (9)
(10)
where ≡SO
–

≡SOHg
Γ
t
= A, Equation 13 becomes
(14)
We fit the experimental data for Hg(II) sorption on each soil with Equation 14 using a
multivariant nonlinear regression method (Wilkinson, 1993). The predicted soluble Hg
concentration based on the model agreed well with the measured value for all soils with a
regression coefficient R
2
ranging from 0.911 to 0.981. We correlated the model fitting param-
eter A with soil properties and found a good linear relationship between A and soil organic
C content (Figure 4.7):
A = 0.81 SOC (15)
Equation 15 again suggests that soil organic matter is the major binding component. The
model fitting also indicated that the competition of protons with Hg for the surface sites
was unimportant under the experimental conditions where the pH ranged from 3 to 10.
Therefore, the term K
≡SOHg
[H] in Equation 14 is negligible. Based on the model fitting result
for each soil, the average values for proton and Hg binding with dissolved organic matter
were calculated. Substitution of these average values and Equation 15 into Equation 14
gives a universal equation:
K
≡SOH
≡SOH[]
≡SO
–
[]H
+

[]+

++ +

T
Hg
=
C
w
1 K
HL
H
+
[]K
HgL
DOC++
1 K
HL
H
+
[]K
HgL
DOC A
1 K
HL
H
+
[]+
1 K
≡SOH

–2
DOC 0.81SOC 1 0.32 H
+
[]+()+×++

T
Hg
=
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© 2001 by CRC Press LLC
88 Environmental Restoration of Metals–Contaminated Soils
4.3.1.3 Soil Sorption Phases
Sorption of metals by soil components has been extensively documented. Almost all soil
components, especially natural organic matter, oxides, and 2:1 clay minerals, were effective
sorbents for metals. In natural systems, not all minerals are necessarily important in metal
sorption. Some of the mineral surfaces may be coated by other components and become
unavailable. For example, clay minerals are generally coated by metal oxides and organic
matter (Hart, 1982; Davis, 1984; Jenne, 1988), while the metal oxides can also be coated by
organic matter (Stumm, 1992). Our studies have indicated that sorption of both Cd(II) and
Hg(II) correlated very well with the soil organic matter content. This suggests that a large
quantity of the inorganic minerals was probably coated with organic matter in the soils of
this study as hypothesized in Figure 4.9. As a result, the soil-water partitioning of organic
matter as well as the affinity of a metal for both particulate and dissolved organic matter
determined the partitioning of the metal in soils.
Based on this study, it seems that the organic functional groups that dissolved were dif-
ferent from those remaining in the particulate phase. Both particulate and dissolved
organic matter dominated metal sorption. The maximum amount of organic matter dis-
solved accounted for less than 30% of the total soil organic matter content; however, the
dissolved organic matter could reduce Hg(II) sorption by 60 to 70%, indicating strong affin-
ity of Hg(II) to the dissolved organic matter. In case of Cd(II), the effect of the dissolved

Zn was examined. As shown in Figure 4.10 for the Freehold subsurface sandy loam, des-
orption of both metals followed a similar trend as that obtained by sorption experiments.
At pH around 4, the partitioning coefficient (ratio of metal sorbed/soluble metal concen-
tration) of Cu was greater than that of Zn because of the stronger Lewis acid nature of Cu
than Zn as explained previously. As pH increased, the K
d
value of Cu slightly increased
until pH 6.7 and then abruptly decreased with increasing pH. For Zn, the K
d
value initially
increased until pH 6.7 and then abruptly decreased with further increases in pH.
We determined the dissolved organic C concentration and found that the change of DOC
concentration as a function of pH matched the desorption change of both metals. Because Cu
has stronger affinity for the dissolved organic matter, desorption of Cu slightly increased as
the DOC concentration increased. Zinc, however, has relatively weaker affinity for the dis-
solved organic matter (Tipping and Hurley, 1992); thus slight increases in DOC concentration
did not cause noticeable effect on its desorption. Consequently, as the proton competition
decreased and the surface potential increased with increasing pH, Zn desorption decreased.
At pH above 6.7, the significant increases in DOC concentration caused desorption of both
metals to abruptly increase due to the complexation of metals by the dissolved organic matter.
4.4 Conclusions
This review indicates that metal retention to and release from soils was affected by many
interdependent environmental factors. Among these factors, soil pH and organic matter
FIGURE 4.10
Desorption of Cu and Zn from the Freehold subsurface sandy loam. DOC is dissolved organic C. Soil solution
= 25 g/20 mL; I = 0.01 M NaNO
3
; T = 25 ± 2°C.
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© 2001 by CRC Press LLC

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