Application of the BCR fractionation procedure to inform the remediation of contaminated soils.

The potential of the modified BCR fractionation (three-step) procedure as an ecological risk assessment tool and to predict the effectiveness of various soil remediation procedures was investigated. Contaminated soil samples were collected from an Industrial Age lead smelting site. Stable isotope analysis for Pb showed that lead contamination in the soil was not due to petroleum. Data obtained from mineralogical techniques (XRD, XRF, ESEM/EDX, FT-IR) and the BCR procedure showed that contaminant metals in the soil were mainly present in reducible and oxidisable forms or associated with Fe/Mn oxyhydroxides and silicates. However, XRD analysis of the residues from each step of the sequential extraction process did not reflect these changes in concentration of the metals.Batch extraction experiments using pH adjusted deionised water showed that approximately 50% of the total Pb in the soil was extracted in the pH ranges 2-3 and 8-10. This raised questions about the actual fractionation of Pb in the soil, as metal release in the first pH range would have been due to dissolution of calcite while in the latter it would been due to the dissolution of goethite. Batch extraction experiments using magnesium and calcium chloride solutions at pH 5 showed that a higher proportion of Pb was in an ion-exchangeable form than that predicted by the BCR procedure. Batch extractions experiments using ethanoic acid showed that more than 50% of the total lead in soils was extracted into 3M ethanoic acid solutions at pH 5.In order to understand this phenomenon further, the molarity and pH of the ethanoic acid used in the first step of the modified BCR procedure were changed to 3.5M and 5 respectively, and fractionation experiments were conducted. This is a three-step procedure customised for the fractionation of metals in soils heavily contaminated with lead. The data suggested that a majority of the Pb in the soil samples was in an ion-exchangeable or weak-acid soluble form. The relatively high dissolution of Pb in comparison with other metals in dilute ethanoic acid at pH 5 suggested that the lead compounds in the soil would have existed as independent mineral phases. This also showed that approximately 80-100% of the metal could be potentially bioavailable and available for extractions using complexing reagents.In order to test these predictions, phytoextraction and soil washing experiments were conducted. Phytoextraction data showed that all the essential metals were hyperaccumulated and approximately 10-15% of the total Pb in the soil was extracted from the soil. The concentration of bioaccumulated Pb was almost equal to that extracted in the first step of the modified BCR process. This showed that the modified BCR process could effectively predict the bioavailability of non-essential elements.Soil washing data showed that approximately 70%, 60% and 90% of the total lead in the soil was extracted using citric acid, L-cysteine and EDTA (all three reagents at pH 5) respectively. XRF analysis of the residues from the extractions showed that although there were analysable changes in the concentration of lead, there were no significant changes in the concentration of sulfur and phosphorus in the soil. This provided further evidence for the observation that a majority of the lead in the soil would have existed as independent mineral phases, most likely as PbO, which is ion-exchangeable and weak acid soluble. The data also showed that the customised BCRsequential extraction procedure could effectively predict the concentration of metals desorbed during soil washing. Additionally, column washing experiments using EDTA and ethanoic acid showed that the basic mineral composition of the soils does not alter under the influence of the complexing agents. These experiments also showed that the rate of metal desorption from soil surfaces is not dependent on the soil-solution contact time, rather it depends on metal speciation in soils.Helianthus annuus plants were grown hydroponically in deionised water and phosphate free nutrient solutions spiked with Pb(NO3)2 to study the effect of soil mineralogy on biological metal uptake. The data show that a majority of the lead was hyperaccumulated in the roots of the plants as thermodynamically stable chloropyromorphite and Ca pyromorphite. Additional hydroponics experiments were conducted to study the uptake of Au by H. annuus. These experiments showed that Au forms nanoparticles on the surface of roots.