Monitoring heavy metal toxicity in polluted soil environments
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.