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Assessing Micronutrient Status of Soil and Plant Tissue Using Ion Exchange Membranes

Tejowulan, S. 1994. M.Sc. Thesis. Dept. Soil Science, University of Saskatchewan, Saskatoon, SK

Abstract

A series of laboratory experiments was conducted with the objectives of: determining the feasibility of a soil washing technique with chelating agents as a means of removing trace metal contaminants hm soils, investigating exchange resins as a means of removing metals from the resulting extracts, applying the techniques for reducing trace metal concentrations in contaminated urban soils, and estimating the metal speciation and retention in the soils through computer modeling. The chelating agent ethylene diamine tetraacetic acid (EDTA) is promising. The ligand is more effective in removing the metal contaminants from soils than HCI. In a batch experiment EDTA released a large portion of metals from soils to soil extracts. A batch method was also developed for recovering the metal contaminants in the resulting WA soil-extracts. Using an anion exchange resin (Am) AG 3-x4, up to 99% of the Cd, Cu, Pb, and Zn were removed hm the leachates. To increase the efficacy of the extraction method we tested different reagent, amount, and technical application strategies using soil columns. Mixing the reagent with the whole soil column was found to be the most efficient method for the application of the chelating agent for the purpose of metal extraction from soils. It was found that WA was superior to citric acid in removing Cd, Cu, Pb, and Zn from the soils investigated. Up to 27% more Cd, 23% more Cu, 42% more Pb. and 21% more Zn were removed by the EDTA and the removal was further increased with the addition of an acid/salt solution (1.0 M HCI and 1.0 M KCI) to the columns. Other promising results are that the AER in columns is an effective method for extracting metals from EDTA soil-extracts with different properties. The resin column method was found to be more efficient than the min batch procedure. The most efficient metal extraction technique that was developed in the preceding experiment was tested in the laboratory using larger soi1 samples. The results of the simulated field experiment confirmed that the remediation of contaminate. soils can be achieved using EDTA. The majority of the contaminated soils were decreased to the point that they could be used for commercial and or residential use. The results also indicated that no metal enrichment was observed in the lower sections of the soil column after leaching the column with an EDTA solution. The fractionation of the soils before and after leaching revealed that much of the metals in the soils was retained in the carbonate and oxide fractions and much of the metals mobilized by the leaching come from these two fractions. Although an increase in the amount of metals was observed in the exchangeable fraction of some of the cleaned soils. in general, less metals are found in every fraction after the washing. Metal speciation and retention by soi1 surfaces influence the fate of the freed metals in the soils, and therefore. they determine the degrees of success of metal leaching in soils. Computer models can k used to predict the possible metal speciation and interaction with solid surfaces in soils. Using the MINEQL program, the models estimate that most of the metal species in the soil extracts (in the presence of EDTA, chloro, hydroxo, and carbonate ligands and humate and hydrous ferric oxide (HFO) solid phases) were present as soluble metal-EDTA complexes. The findings were in close agreement with the results obtained hm the experiment using the AER. It appears that the metal-EDTA complexes found in the systems are very stable over a wide range of soil pH. The results indicated that computer modeling, in part. can be a valuable tool to describe the potential chemical reactions in the soil solution. Our surface complexation models also indicated that the EDTA is much stronger than the humate and the HF0 solid phases in forming complexes with the metals. As a consequence, the adsorption of the Cd, Cu Pb, and Zn by the solid surfaces was negligible in the presence of EDTA.