3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Silica and maghemite nanoparticles for the remediation of acid mine drainage-contaminated waters and Nanoparticle modification of metal uptake by a freshwater alga-Scenedesmus sp(2015-01-30) Etale, AnitaAims: The adsorptive removal of Cu, Mn, Hg and U by silica and maghemite nanoparticles (NPs) under acid mine drainage (AMD) conditions was investigated with the aim of assessing the applicability of NPs for remediation of AMD-contaminated water. The effect of NPs on metal uptake by algae, an increasingly popular remediation alternative, was also investigated. Methods: NP and algal metal removal were quantified by batch experiments using commercially prepared, bare and amine-functionalised silica-carbon hybrid NPs characterised for size, surface area, porosity, crystallinity, elemental composition and hydrodynamic size. Metal uptake by algae was quantified in the presence and absence of NPs. Results: Silica and maghemite NPs can be used for the adsorptive removal of Cu, Mn, Hg and U from AMD-contaminated surface and ground water. NP metal uptake was rapid and equilibria were attained within 5 minutes with silica and maghemite NPs, and within 45 minutes with amine-functionalised hybrid NPs. Adsorption efficiencies for Cu, Mn, Hg and U at pH 3 were 52, 56, 56 and 49%, respectively with silica and 56, 52, 75 and 50%, respectively, with maghemite NPs. Metal removal was enhanced by >10% in solutions containing ferric, manganese or sulphate ions, although Cu removal was inhibited in solutions with a >1 Mn:Cu ratio. Despite the presence of high affinity amine groups in hybrid NPs, Cu removal was only 52% due to the low surface area of the adsorbent. The comparative study with Hg, however, showed that surface area was not the only determinant of adsorption efficiency: maghemite NPs with a specific surface area ~15 times less than silica adsorbed 21% more Hg. Metal removal by Scenedesmus sp. was enhanced by 12-27% in solutions containing NPs due to the greater sorption surface areas. NPs also modified metal partitioning in algal cells: intracellular concentrations were lower and extracellular concentrations higher in solutions containing NPs relative to controls (no NPs). Conclusion: Silica and maghemite NPs can be applied for the adsorptive removal of Cu, Mn, Hg and U from AMD-contaminated water and to improve the efficiency of phycoremediation by Scenedesmus sp. These findings also point to the possibility of retardation of metals by NPs during their transportation from tailings and contaminated sites. Their partitioning to NPs and the strength of the interactions thereof can determine the prevalence of the metals in solution or in the solid phase.Item Risks of urban agriculture: lead and cadmium intake by Kigali residents from locally grown produce(2011-07-07) Etale, AnitaThis study determined the concentrations of lead and cadmium in edible parts of Colocasia esculenta, Amaranthus spp.and Ipomoea batata cultivated on farms in industrially polluted sections of Nyabugogo Marsh in Kigali, Rwanda. The concentrations in all three crops exceeded European Union (EU) standards for metal concentration in food crops. C. esculenta roots (Taro) contained the highest concentration of lead (1.02 mg kg-1) and cadmium (0.56 mg kg-1), approximately ten and six times over the EU limits, respectively. Even though I. batata (sweet potato) contained the lowest concentrations of lead (0.75 mg kg-1), this is almost eight times the upper limit. The highest bioaccumulation factors (the ratio of plant metal concentration to that of the soil in which it is found growing) for both metals were observed in amaranth plants. The concentrations of lead and cadmium in the farm soils were all acceptable based on EU standards (300mg kg-1 for lead and 3mg kg-1 respectively. The average daily consumption by an adult in the community living around the Marsh and where some of the produce is sold is 50g of amaranth, 120g of taro and 180g of sweet potato. Based on the metal concentration and these rates of consumption, the daily dietary intake of lead by an adult in the community from amaranth, taro and sweet potato is 1 x 10-4, 3 x 10-4 and 4 x 10-4 mg kg-1 respectively. The daily intake of cadmium is 4 x 10-4, 1.7 x 10-4 and 1.2 x 10-4 mg kg-1 for amaranth, taro and sweet potato respectively. These metal intakes are well within the recommendations set forth by the World Health Organisation. The community also has access to multiple sources of dietary and non dietary zinc such as beans, milk and rain water collected from zinc coated roofing sheets, which serves to ameliorate the effects of cadmium. It is however worth noting that survey data may have yielded overestimates of these zinc sources, due to the conditions under which the surveys were conducted i.e. in the hearing of neighbors due to the cramped nature of housing, which may have prompted respondents to inflate consumption quantities of expensive food items. The calculated maximum recommended quantities for daily intake of the crops are very large and are unlikely to be consumed by the population i.e. >2kg of amaranth, >2 kg of taro and 3 kg of sweet potato per day for an adult. Additionally, because this is a poor community, access to such quantities of food on a daily basis is not likely. The community is therefore not exposed to health risks from consuming metal contaminated crops, largely because of the small quantities consumed. The local population is therefore at no immediate risk to exceeding metal consumption limits by consuming vegetables grown in the Nyabugogo Marsh, but the threats will likely increase if the pollution of the Marsh is not addressed.