An investigation into soil pollution and remediation of selected polluted sites around the globe

Mgbeahuruike, Leonard Udochi (2018) An investigation into soil pollution and remediation of selected polluted sites around the globe. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

In this study, simulated soil (SS), as well as its constituents (quartz sand [QS], bentonite clay [BC] and peat moss [PM]) were used as a model to study the leaching behaviour of potential toxic elements (PTEs) in a batch process using EDTA as an extractant and comparing it with the extraction capacities of the other extractants EDDS, 2, 4-pentanedione [Hacac], citric acid [CA] and tartaric acid [TA]). Prior to leaching, SS and its constituents QS, BC, and PM were contaminated with single metals (SM) and multi-metals (MM) at different concentrations. EDTA was the most efficient and TA the least efficient extractant. Hacac showed preferential recovery for Cu and Ni. Metal recovery by EDDS in SS was significantly diminished compared to EDTA. Once the leaching behaviour in the batch process had been established, other process configurations (column and heap) were investigated. The effect of three different leaching processes and 4 different leaching agents on the extraction of five metals showed that total metal extraction of up to 50% efficiency could be achieved. EDTA proved to be the best overall extractant when used in a batch leaching process. Different metals showed preferential recoveries with specific processes and extractants. The results obtained from the multi-metal leaching profile of the SS was used to predict the leaching capacities of the three environmental soils sourced from Nigeria (Nig-S) and Romania (Ro-SB and Ro-PH), using column and heap leaching processes. The behaviour of SS compared in practice with the leaching behaviour exhibited by the Ro-SB. The leaching profiles of the base metals from the different soils (both natural and simulated) was studied by using first-order, power function and Elovich equations. The Elovich equation provided the best fit of the experimental data. The rate of metal release was higher for the SS and Ro-SB soils than the Nig-S and Ro-PH soils. Subsequently, the data were analysed to determine the B/C Ratio, NPV, and payback period of the study. The analysis gave a B/C Ratio of 0.2, negative NPV of £ 1797.70 and £ 351.77 (for 50% and 90% recovery of EDTA-Heap process respectively) and payback period of 4 years. These indicators are suggestively below the decision criteria. Thus, the process is not economically viable. However, a sensitivity analysis performed based on 25% and 50% changes in the prices of the various metals and extractants shows that when 90% of the extractants could be recycled, a significant change in the process economics could be achieved. Overall the results suggest that the design of a contaminant-specific leaching process performed in a sequential manner could practically leach all the metals, and offer a potential separation of the metals from the leach liqour.