Development of novel polysaccharide based adsorbents for precious metal recovery

Gao, Xiangpeng (2018) Development of novel polysaccharide based adsorbents for precious metal recovery. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Biosorption is a green, efficient, and low cost method to recover precious metals from aqueous solutions compared to conventional methods. Over the past two decades, great efforts have been made to fabricate biosorbents to recover precious metals from natural polysaccharides in batch adsorption process. However, most of the studies were focused on mono-metallic solutions, while biosorption from multi-metallic solutions should be addressed as it is more closely parallels the real life scenario. In addition, the adsorption mechanism is not clearly defined, especially for the competitive adsorption of multi metal ions. Moreover, most of the polysaccharide based adsorbents synthesized are not suitable to be packed into fixed-bed columns for large-scale processes due to their poor mechanical strength, improper particle sizes, and extremely slow mass transfer rates. To fill these research gaps, this study mainly focuses on fabrication of polysaccharide based adsorbents with good selectivity as well as physical structure and mechanical strength, understanding the selective adsorption mechanism, and dynamic adsorption in fixed-bed column. In this work, five polysaccharide-based adsorbents have been synthesized for selective adsorption of precious metals from multi-metallic solutions. Three adsorbents have been synthesized by cross-linking (or gelatinization) the polysaccharides (cellulose or sodium alginate), and then grafting functional groups onto the surface of the adsorbents. These adsorbents have exhibited outstanding selectivity towards precious metals as well as high capacities. Biosorption of Au (III) in acidic media are mediated by the electrostatic and covalent interactions between Au (III) and hydroxyl, carboxyl, amino, and –C=S functional groups. Porous epichlorohydrin/thiourea modified alginate adsorbent (PETA) has been synthesized by direct templating method. PETA has shown great capacities and selectivity as well as remarkable improvement in adsorption kinetics than the nonporous adsorbent. In order to achieve a smaller diameter for the purpose of fixed-bed adsorption test, a microsphere adsorbent (ETA) has been synthesized by facile emulsion method of a water-in-oil emulsion of modified alginate. ETA microspheres were packed into a fixed-bed column and a mathematical model was applied to describe the breakthrough curves of Pd (ІІ) and Cu (ІІ) ions under different experimental conditions. Several characterization methods were applied to investigate the adsorption mechanism. The polysaccharides, synthesized adsorbents, and the adsorbents after adsorption were characterized by FT-IR spectroscopy to identify the chemical bonds and functional groups. Scanning electron microscope (SEM) images were recorded to observe the surfaces and structures of the adsorbents. X-ray photoelectron spectroscopy (XPS) was used to determine the electron donors and acceptors from the shift of their binding energies. X-ray diffraction (XRD) was used to detect the reduced gold particles. A transport-dispersive model with a linear driving force kinetics equation was utilized to predict breakthrough curves of Pd (ІІ) and Cu (ІІ) in the fixed-bed packed with ETA microspheres. A pH-dependent competitive Langmuir isotherm was developed and used in the simulation. Effects of flow rates, feed concentrations, and pH values were studied and experimental data were used to validate the mathematical model. The good agreement between the simulated and experimental breakthrough curves confirms that transport–dispersive model is capable of predicting the dynamic adsorption performance over wide operating range.

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