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Among the most promising novel methods for treating heavy metal contamination in fine soils is electrokinetic soil remediation - passing a low-voltage current through the soil that causes the target contaminants to migrate to the electrodes where they can be captured, broken down or immobilized. However, the widespread in-situ application of this process as a creative solution for the problem of metal contamination has been prevented because of a variety of problems, some of which are linked to the available electrodes. The limitations related to the available electrodes include: corrosion of the anodes difficulties in removing contaminated water and gases from the electrodes poor electrical contact of electrodes with the soil, cost of producing functioning electrodes, and limitations related to the physical form of the electrodes. Many of these historical limitations have been eliminated by the introduction of Electrokinetic Geosynthetics (EKGs) which combine electrokinetics with geosynthetics technology to create geosynthetice lectrodes. In this investigation, the use of an EKG system to capture and dispose of zinc ions from kaolin soil is evaluated by conducting extensive labor4tOry tests using specially designed bench scale tanks. A comprehensive test programme was established to include two electrokinetic remediation removal approaches: One Process Approach and Two Process Approach. The former was achieved by electro, migration of metal contaminants towards the cathode electrode without any external enhancement agent while the latter combined soil flushing with the electrokinetic process. The testing technique enabled the study of effects of. water flushing, soil depth, intermittent currents and replenishing draining chambers along the soil profile with deionised water, on the efficiency of cation removal. The respective responses were primarily presented in terms of electrical current/test duration, water content/test duration, zinc concentration/normalised distance from the anode, zinc concentration/ test duration and pW normalised distance from the anode. Results indicated that the EKG system has a lot of potential for cleaning up soil at sites contaminated by heavy metals and other harmful compounds. The in-depth analysis also showed clearly that combining the technique with soil flushing improved the effectiveness of the treatment operation. The study considerably extended the understandinogf the performance of electrokinetic technology particularly in fine grained soil and are as where further research would improve this understanding were highlighted.

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.

Weathered petroleum hydrocarbons are a highly complex, important soil contaminant. After forty years of petroleum research, weathered hydrocarbons are still not sufficiently understood or appropriately accounted for in contaminated land risk assessments or the associated analytical methods that inform them. Improved insights into these contaminants potential for biotransformation and their residual toxicity are essential for improving risk assessments, bioremediation strategies and effective regeneration of previously contaminated land. This thesis explores the biotransformation of weathered hydrocarbons in the context of risk assessment and management. The research includes a critical review and synthesis of six in-house historical pilot studies, implementation of a novel ultrasonic solvent extraction method for petroleum hydrocarbons and development of analytical tools, providing new insights for human and environmental risk assessments. The biotransformation potential and subsequent effect on the toxicity of two weathered hydrocarbon contaminated soils were investigated using soil microcosms. The use of a previously remediated soil provided novel insight into extended bioremediation potential for petroleum hydrocarbon residues to undergo further biotransformation. The novel ultrasonic extraction method developed collaboratively is a preferred alternative to traditional Soxhlet methods with very high precision (RSD ≤ 10%) and extraction efficiencies. Key benefits of the technique include reduced costs, shorter extraction times (1 h. vs. 8 h.) lower solvent consumption (40 ml vs. 150 ml) and improved extraction efficiencies (recovery ≥ 95 %). Ecotoxicological responses (using mustard seed germination and Microtox® assays) showed that a reduction in total petroleum hydrocarbon (TPH) load within soils could not necessarily be linked to a reduction in residual toxicity, thus reductions in TPH alone is not a suitable indicator of risk reduction. The residues in the previously remediated soil underwent further biotransformation with losses of up to 86 and 92 % in the aliphatic and aromatic fractions respectively. Grinding of this soil was shown to reduce the effectiveness of a nutrient treatment on the extent of biotransformation possible by up to 25% and 20% for the aliphatic and aromatic hydrocarbon fractions, respectively. Toxicity assays confirmed that biotransformation is not physically driven by surface area limitations, contrary to expectation, as responses of ground and un-ground soils were not significantly different (P>0.05). This may have implications for future studies using grinding as a pre-treatment, where biotransformation may be limited by grinding rather than other factors. Both the soils showed significant biotransformation (P<0.05) after 16 weeks of treatment. However, although the soil not previously treated had significantly less TPH losses, a loss of up to 92% shows that further degradation of this soil is possible even though previous investigations had suggested biotransformation had stopped. This has implications for bioremediation practitioners in that it questions whether bioremediation could be restarted and lower concentrations achieved, and warrants further investigation.

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Remediation of contaminated land needs to be carried out using methods that are both cost effective and minimise environmental pollution. However, the remediation option currently chosen by practitioners is often based upon limited economic information with the true environmental costs not being considered. This can result in the least sustainable option being chosen. This study has developed a methodology to evaluate the sustainability, in terms of economic and environmental costs, for a range of treatments available for the remediation of diesel contaminated land, including bioremediation (with and without the addition of brewery spent grain), disposal to landfill and thermal treatment. Initial laboratory investigations indicated that the use of brewery spent grain decreased the time taken for the clean-up of soil contaminated with diesel, suggesting that bioremediation augmented by the addition of this organic material was a viable option. A costing model was then developed that included all of the costs associated with the remediation options chosen. This included both direct and indirect costs. The results show that considering the indirect costs of remediation such as costs associated with delayed development the land, make bioremediation in this study an economically feasible option. Finally environmental costs were considered with a focus on the release of carbon dioxide a known greenhouse gas. Respirometry was used to determine the volume of carbon dioxide released during the bioremediation process. This information was then combined with data collected from a range of other sources and the impact of the chosen remediation options on atmospheric greenhouse gas release was evaluated. Other environmental impacts were also determined including land and water pollution. The results indicate that bioremediation with brewery spent grain has one of the lowest environmental costs and showed that emission from pollutants such as NOx, PM1.0, PM2.5, NH3 and SO2 could contribute to the limit values in the area covered by remediation work. The model developed in this study has indicated that the use of bioremediation with and without the use of brewery spent grain is a sustainable remediation option providing both direct and indirect economic costs are included. The results have indicated that, the strategy of using brewery spent grain to augment bioremediation v process promotes the re-use of by-product material, reduces waste and conserve resources. There is a need for the remediation industry to adopt similar models in order that decisions made, as to the remediation option chosen, are based upon accurate costings of their sustainability.

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