| Summary: | A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg,
in fulfilment of the requirements for the degree of
Doctor of Philosophy
Johannesburg 2012 === Wastes from mining operations usually contain a suite of pollutants, among them cyanide and its complexes; heavy metals; metalloids and radionuclides. The pollution plume can affect public health through contamination of drinking water supplies, aquatic ecosystems and agricultural soils. As such, waste management and remediation has become an important integral component of mining. Conventional chemical and physical methods are often expensive and ineffective when the pollutant concentrations are very high, so the challenge of developing cost-effective materials with high adsorption efficiencies for pollutants still remains.
This research was dedicated to the development of biosorbents with high metal loading capacity for the remediation of mine wastewater, namely: zeolite/bentonite functionalised with microbial components such as histidine, cysteine, sorbitol and mannitol; zeolite/bentonite functionalised with Penicillium-simplicissimum and zeolite-alginate complex generated by impregnating natural zeolite into alginate gel beads. The ability of the fresh water algae, Oedogonium sp. to remove heavy metals from aqueous solutions in batch systems was also assessed.
Optimum biosorption conditions for the removal of Co, Cu, Cr, Fe, Hg, Ni, Zn and U (in a single-ion and multi-ion systems) were determined as a function of pH, initial concentration, contact time, temperature, and mass of biosorbent. An increase of adsorption capacity was observed following modification of natural zeolite/bentonite by microbial components with a maximum adsorption capacity obtained at low pH. The FTIR results of the developed biosorbents showed that the biomass has different functional groups that are able to react with metal ions in aqueous solution.
Immobilisation of fungi (Penicillium-simplicissimum) on zeolite/bentonite yielded biomass of 600 mg g-1 (10-fold higher than the non-immobilised one) at a pH 4, showing the potential of this sorbent towards remediation of AMD-polluted mine sites. The maximum uptake of metals ions (in a multi-ion system) was higher and constant (40-50 mg g-1) in the inactive fungal biomass (heat-killed) from pH 2 to 7. The uptake of U and Hg increased significantly in the zeolite/bentonite-P.simplicissimum compared to their natural forms due to the presence of the N-H, S-H and COO- groups.
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The pseudo second-order adsorption model was found to be more suitable in describing the adsorption kinetics of metal ions onto biomasses in single- and multi-ion systems with the sorption of nickel being controlled by film diffusion processes (with the coefficient values of 10-7 cm2 s-1). The thermodynamic parameters showed that the adsorption onto developed biosorbents was feasible and spontaneous under the studied conditions.
The calculated values of the loading capacities in column adsorption for the natural zeolite/bentonite as well as zeolite/bentonite-P.simplicissimum were close to those obtained in the batch tests, mainly for U and Ni. The bed depth service time model (BDST) was used successfully to fit the experimental data for Ni and U adsorbed on the natural zeolite. This suggested a linear relationship between bed depth and service time, which could be used for scale-up purpose.
The developed biosorbents could be regenerated using 1 mol L-1 HNO3 solution for potential re-use. The total decrease in biosorption efficiency of zeolite-Penicillium simplicissimum after five cycles of adsorption-desorption was ≤ 5% which showed that zeolite/bentonite-Penicillium simplicissimum had good potential to adsorb metal ions repeatedly from aqueous solution. On applying it to real wastewater samples, the zeolite-P. simplicissimum biosorbent removed 97% of the metals. Penicillium sp. immobilisation enhanced the potential and makes it an attractive bioremediation agent.
The zeolite-alginate sorbent exhibited elevated adsorption capacities for metals. This showed potential for use of such a system for remediation purposes. It also provides a platform to explore the possibility of using zeolite in conjunction with other polysaccharide-containing materials for heavy metal removal from wastewaters.
The results obtained in this study have shown that zeolite and bentonite are good supports for biomass. The biofunctionalised zeolite/bentonite systems have potential in removal of heavy metals from wastewaters.
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