Monitoring heavy metal toxicity in polluted soil environments
Date
1995-03
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Journal ISSN
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Type
Degree Level
Doctoral
Abstract
Heavy metal contamination of soil is a concern in society today. However, there is
a lack of suitable methods to predict consistently the bioavailability and mobility of heavy
metals in the soil environment. A series of laboratory and growth chamber experiments
were conducted to investigate the use of ion exchange membranes as heavy metal
accumulators, to monitor heavy metal bioavailability and mobility in soils, and to speciate
heavy metals into "labile" and "nonlabile" pools in contaminated soils.
A method based on the use of ion exchange resin membranes, impregnated with the
chelating agent DTPA, was developed and assessed. Anion exchange membranes were
treated with disodium-DTPA to form a chelating cation exchange membrane referred to as
AEM-DTPA. The chelate-impregnated membrane has high selectivity to complex and
exchange with polyvalent metal cations in soil environments. A burial procedure was
developed in which an AEM-DTPA membrane strip is buried directly in soil at saturated
moisture condition for 60 minutes. Removal of the adsorbed metal ions from the resin
membrane was accomplished by elution with 20 mL of 1 N HCI for 60 minutes. Metal
concentration in the eluent was then determined by atomic absorption spectrophotometry
(AAS).
Four plant non-essential metals: Cd, Cr, Ni and Pb, which are of great concern as
metal contaminants in sewage sludge and other waste products applied to land, were
studied in a growth chamber experiment with three representative crops and two soils,
using the AEM-DTPA as a biotoxicity indicator. Correlation analysis between Cd and Ni
availability as given by the newly developed membrane burial method and Cd and Ni
concentration in the three representative crops (radish, lettuce and oats) indicated that buried
AEM-DTPA membrane could be used as a sensitive indicator of Cd and Ni toxicity in
polluted soil environments. The Cr and Pb availability predicted by the membrane burial
method was also significantly correlated to Cr and Pb concentration in radish and lettuce
leaves and was significantly correlated with heavy metal spike rate and the conventional
DTPA soil test. The critical levels for heavy metal toxicity varied widely from crop to crop,
and soil to soil. Lettuce was more sensitive to heavy metal toxicity than radish and oats.
The critical levels ofDTPA-extractable Cd and membrane-adsorbed Cd corresponding to a
10% reduction in dry matter yield were 1.1 mg kg! and 0.02 µg cm-2, respectively for
lettuce grown on the Asquith sandy loam soil and 2.3 mg kg-l and 0.03 µg cm-2,
respectively for lettuce grown on the Keatley clay loam soil.
The AEM-DTPA membrane was also tested for its suitability in routine plant tissue
testing. The resin method successfully predicted Cd and Cr concentration in radish leaves
and was correlated with Cd and Cr spike rate, but failed to predict Ni and Pb concentration
in radish tissue and all four metals in lettuce and oat tissue.
The AEM-DTPA membrane accumulates Cd, Cr and Pb cations from soil in a
manner indicative of diffusion-controlled phenomena. The quantities of the four metals
adsorbed by the resin membrane from standard solutions indicates that the AEM-DTPA
resin membrane has the highest affinity for Pb. However, when the AEM-DTPA
membrane was burled in the soil, the membrane showed the highest uptake of Ni. Overall,
the AEM-DTPA membrane accumulates heavy metals as a sink or a dynamic exchanger
during long-term burial, as regulated by properties controlling metal diffusion through soils
and the solubility of the metal in the soil.
The potential advantages of using AEM impregnated with DTPA in a soil-burial
application are twofold: (1) the method is based on fundamental chemical and kinetic
principles that are operative in metal movement and adsorption in the rhizosphere; and (2)
the method can eliminate soil sampling, drying, grinding, and the associated
physicochemical changes resulting from sample handling. The disadvantages of the AEMD1PA
method are: 1) a fmite adsorption capacity which may lead to resin saturation in
highly contaminated soils; 2) the exchange sites could be blocked by hydrophobic
compounds; and 3) the release and/or degradation of the chelating agent could be a problem
for long term burial.
Ion exchange membrane was also evaluated as a tool in assessing heavy metal
speciation in soils. By using a sequence of strong acid and weak-acid cation-exchangers
(H+ and Na+ form) and chelating agent, extractable metals can be determined at pH values
ranging from 3 to 9. The total soluble metal content of Cd, Cr, Ni and Pb in the
contaminated soils was subdivided into (i) low-pH labile, (ii) weak-acid labile, (iii) weakbase
labile, (iv) high-pH labile and (v) non-adsorbable forms using cation and anion
exchange membranes. In the procedure developed, soil suspensions are mixed for 12 h
with different types of resin membranes and the cations transferred from the soil are
subsequently eluted from the membranes using 1 N RCI. The HCI extracts are then
analyzed for Cd, Cr, Ni and Pb. Analysis of the aqueous phase left in contact with the soil
residue gives the amount of non-labile species released. Low pH labile fraction constituted
the largest proportion of the added metal in poorly buffered (sandy) soils. Weak acid and
base labile fractions were typically highest in highly buffered soils. Clearly, metal
contaminated soils most likely to cause environmental damage are sandy textured soils
subject to acidification, although the production of chelating substances by roots and
microorganisms may also mobilize considerable quantities of metal in soils of high clay
content.
The cation exchange membrane and AEM-DTPA membrane were tested to see
whether they can be used as heavy metal collectors and determine the extent of metal
leaching. In contaminated soils, neither CEM nor AEM-DTPA membrane could be used as
heavy metal collectors in long-term burial trials. Anion exchange membrane pretreated with
DTPA increased the leaching of Cd, Ni and Zn from the soil, probably due to the release of
chelating agent from the membrane when buried longer than 24 h. Small amounts of
chelating agent released from the membrane slightly acidify the test medium, and increase
the solubility and mobility of these metals in the soil.
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Degree
Doctor of Philosophy (Ph.D.)
Department
Soil Science
Program
Agriculture