CADMIUM
AVAILABILITY IN REMEDIATED SOIL: AVAILABILITY INDICES
Anna Sophia Knox* (formerly A. Chlopecka) and
Domy Adriano (The University of Georgia, Savannah River Ecology Laboratory,
Drawer E, Aiken, SC, 29802, USA, Phone: 803 725 2752, Fx: 803 725 3309, E-mail:
achlopecka@srel.edu)
ABSTRACT
A
goal of in situ remediation is to
reduce the fraction of toxic elements which are potentially mobile or
bioavailable, i.e., reduce risk. This study evaluated the efficacy of various
soil amendments in reducing Cd availability in contaminated soil. Amendments
reduced metal mobility by decreasing the mobile fraction of Cd and increasing
its value in the stable fractions. The effectiveness of applied amendments was
evaluated by the availability indices such as:
the modified distribution coefficient (Kmd), bioavailability factor
(BF), recalcitrant factor (RF), and the
transfer factor (TF). Results from this study indicate that the
amendment application to metal contaminated soils resulted in increased values
for Kmd and RF and decreased values for BF and TF. Such changes
among the soil quality indices indicate success of the remediation technique
and may have relevance in risk assessment and monitoring.
Several
researchers tested various amendments such as zeolite, apatite, Fe oxides, and
biosolids as in situ Cd stabilizing
agents (Knox et. al., 2000 a). Such
amendments are inexpensive, readily available, and can be economically applied
to large tracts of contaminated soil using standard agronomic practices without
the need for expensive technical oversight and long-term management.
Immobilization of contaminants in
remediated soil can be evaluated by plant growth, plant yield, and the metal
concentration in plant tissues. Additionally, stabilization of metals in
contaminated soils can be expressed by stabilization indices such as: the
modified distribution coefficient (Kmd), bioavailability factor
(BF), recalcitrant factor (RF), and the transfer factor (TF) (Knox, 1998, and Knox et al., 2000 b). The
modified distribution coefficient (Kmd) is defined as the ratio of
metal in soil ([M]s) to its concentration in the soil solution ([M]ss).
The bioavailability factor (BF) is defined as the ratio of the metal content in
the exchangeable phase ([M]ex) to total metal concentration in the
soil ([M]T), BF = [M]ex/[M]T. This index
indicates the fraction of the total concentration of a metal in the soil that
is considered readily available to plants. The recalcitrant factor (RF) is the
ratio of the metal in the residual fraction as estimated by aqua regia and HF
digestion during sequential extraction, ([M]R), to the total content
of metal in the soil ([M]T), RF =
[M]R/[M]T. This index indicates the virtual
irreversible retention of metal by the solid phase. Typically, the RF index can
be expected to be lower in soils with low pH and low clay content. The transfer
factor (TF) is the ratio of the metal content in plant tissue, ([M]P),
to the total concentration of metal in the soil ([M]T), TF = [M]P/[M]T.
It is normally considered as a measure of plant uptake by the roots and
subsequent translocation to the aerial portion of the plant. Of usual concern
is the transfer and accumulation in the edible portion of the plant. It is widely
recognized that metals accumulate in roots with less transfer to grain and
other edible tissues; therefore, TF values may be expected to be highest for
roots and lower for the edible tissues.
A
greenhouse pot experiment was conducted using 7 kg of top soil from an Appling
silt loam with the following properties: pH, 5.4; particles<0.02 mm
diameter, 21.5%; organic matter (OM), 25 g kg-1. This
soil contained 27, 15, and 0.2 mg kg-1 of
total Zn, Pb, and Cd, respectively, which are considered as background
levels. Cadmium was added to the soil
at 20 and 40mg kg-1 as a defined mixture of
various metal sources (40% as sulfate, 25% as carbonate, 20% as oxide and 15%
as chloride). After equilibration, natural zeolite, apatite and Fe oxide were
added to the soil at a rate of 25 g kg-1.There
were four replicates in each treatment and all pots were arranged in a
completely randomized design. The
natural zeolite, phillipsite, from Colorado had the following properties pH
9.1, calcium carbonate equivalence (CCE) 4.9%, Si/Al ratio 2.1, CEC
1.65cmol/kg. Apatite was from North Carolina and had pH 7.8 and CCE 21.8%. Fe
oxide (trademark name Fe-rich) was a
by-product from the processing of TiO2
pigment at E.I. du Pont de Nemours, Wilmington, DE. The iron-oxide contained
31.7%, 1.2%, 2.4%, and 0.35% of Fe, Mn,
Ca and Mg, respectively.
Three plant species were grown on
the potted soil: rye (Secale cereale L.),
maize (Zea mays L.) and oats (Avena sativa). Rye and maize were harvested after 6 weeks but oats
was harvested at the mature stage. Aboveground parts of plants were taken for
chemical analysis. Sequential
extraction was used to partition metals into five fractions (exchangeable,
carbonate, Fe-Mn oxides, organic and residual) (Tessier et al., 1979). Metal concentrations
in plant tissues were determined by wet chemical method (HNO3 and H2O2).
Metal contents in all solutions were determined by atomic adsorption
spectrometry (AAS) or inductively coupled plasma-mass spectrometry (ICP-MS).
It is
generally contended that the availability of metals, and thus uptake by plants,
is related both to their total concentrations and to their forms and
associations in the soil, and to a number of geochemical factors operating at
the soil-root interface. The influence of plant species on metal uptake may
also be considerable. Different species, and indeed different cultivars,
regulate metal uptake at both the soil-root and root-shoot interfaces to
varying degrees. In situ
stabilization techniques reduce the fraction of potentially toxic elements
which are mobile or bioavailable. In these techniques simple approaches have to
be developed to determine mobility or bioavailability. Mobility can be
evaluated using specific chemical reagents that are used to extract amounts of
elements as close as possible to what plants may take up. Another method
commonly adopted in soil and environmental studies is sequential extraction
where chemical reagents with increasing strength are employed. Based on results
from sequential extraction the effectiveness of remediation techniques can be
evaluated by three indices: Kmd, BF and RF.
Data from this study are showing that all tested amendments
significantly increased values of Kmd but decreased BF values for
Cd, i.e., decreased the bioavialability of Cd (Fig. 1). Factor RF indicates
retention of Cd by the solid phases. All amendments increased the value of RF
for both Cd levels with Fe oxide and apatite being the most effective. For example, apatite addition increased RT
values from 35% in the control (i.e., no apatite) to 50% in the soils spiked
with 40 mg kg-1 Cd (Fig. 1).
In this experiment three plants were tested and yield data are
indicated in (Fig. 2). Yield reduction was observed in all tested plants with
both doses of Cd in the soil. The highest reduction of yield was obtained in
the treatment with 40 mg Cd/kg for rye and oats (Fig. 2). Yields of plants in
the treatments with zeolite, apatite,
and Fe oxide were higher than yields
obtained from control treatments or blank treatments (Fig. 2). However, plants
responded differently to each amendment, for example, Fe oxide had the greatest
effect on the yield of rye and oats but
apatite had the greatest effect on the maize yield.
The highest Cd concentrations in plant tissues (rye, maize and oats)
were observed in blank treatments (Fig. 3). For example, in 6-week old rye Cd
concentrations was 42.8 and 79.6 mg/kg, respectively, for first and second dose
of Cd. All amendments significantly reduced Cd uptake in tested plants,
however, in rye the reduction of Cd uptake was the lowest (Fig. 3). Fe oxide
was the most effective amendment in reducing Cd uptake; over 90% of the
reduction of Cd concentrations in the maize and oats was observed for both
levels of Cd. The next most effective amendment was apatite, which decreased Cd
concentration in oat leaves tissues by 90% and in maize tissues by over 85%.
Transfer factor (TF) values in this
study indicate the following order of amendment effectiveness: Fe
oxide>apatite >zeolite. The TF values indicate that Cd was the most
available to plants in blank treatments and its concentrations in plant tissues
were high and exceed permissible level of Cd in plant tissues (Fig. 1).
Results from this study indicate
that Fe oxide, apatite and zeolite significantly decreased Cd mobility in soil,
as indicated by values of the stabilization indices, Kmd, BF, RF,
and TF. Additionally the yield of each tested plant was enhanced by applied
amendments. Significant reduction of Cd uptake by plants indicates the
effectiveness of these stabilizing materials in protecting the quality of the
food chain.
Knox
AS, Adriano DC (1998), In: Proc. 4th Int. Symp. Exhib. Environmental
Contamination in Central and Eastern Europe, Published by: Institute for
International Cooperative Environmental Research Florida State University,
Tallahassee, FL, pp. 355-362, CD.
Knox AS, Seaman JC, Mench MJ, Vangronsveld J, (2000 a), In: Environmental Restoration of Metals Contaminated Soils (IK Iskandar, Editor), CRC Press, Boca Raton, FL.
Knox AS, Seaman J,
Pierzynski G, Adriano DC (2000), In: Bioremediation of Contaminated Soils. (DL
Wise, DJ Trantolo, EJ Cichon, HI Inyang, U Stottmeister, Editors), New York,
Marcel Dekker, Inc., pp. 811-836.
Tessier A, Campbell PGC,
Bisson M, (1979), Anal. Chem. 51: 844-850.
Figure captions
Figure 1. The availability
indices of Cd in remediated soil; means of four replicates followed by letters
a, b, c, and d are significantly different at P<0.05.
Figure 2. Yield of tested plants
(DW, g/pot); means of four replicates followed by letters a, b, c, and d are
significantly different at P<0.05.
Figure 3. Cadmium
concentrations in plant tissues (mg/kg); means of four replicates followed by
letters a, b, c, and d are significantly different at P<0.05.