Gold Ions Recovery by Self-Flocculating Microalga and Fungi-Algae Pellets with Simultaneous Microalgal Bioflocculation

Abstract

The challenge of economical separation of tiny microalgal cells from dilute solution restricts their industry commercialization as promising biosorbents. In the fundamental work of this thesis, self-flocculating microalga Tetradesmus obliquus AS-6-1, a freshwater microalga with the ability to aggregate together and facilitate its gravity sedimentation, was freeze-dried and used as biosorbent to recover gold ions from low concentrated wastewater. Optimization of adsorption conditions was carried out to obtain the higher maximum gold(III) adsorption capacity ( ) of 181.82 mg/g from Langmuir model, indicating its better potential as biosorbent for gold ions recovery than the non-flocculating algae Tetradesmus obliquus. The optimization of desorption process was also conducted and it was found that gold ions-loaded biomass of T. obliquus AS-6-1 was effectively regenerated by 0.2 M thiourea at pH 2.0 within 15 min. Although the regenerated microalgae separated from wastewater using their self-flocculating sedimentation in the funnel reactor retained high adsorption efficiency of > 97 %, further studies are still required to improve the flocculating property of alga adsorbent without loss in binding capacity to gold ions. The second goal of this study is to improve the flocculating property of T. obliquus AS-6-1 by supplement and comparison of crude algal extracts and fungal Aspergillus niger pellets as bioflocculating agents. Compared to the algal bioflocculants, fungal pellets as flocculants cost less and performed better in bioflocculation of microalgae. In fungal pellet-assisted bioflocculation, 91.38 % of algal cells were immobilized by the live fungal pellets and formed the 9-10 mm fungi-algae pellets within 3 h at 25 °C, 200 rpm, pH 5.0 with the fungi:algae ratio of 1:2. With the ease of storage and achieving comparable flocculating performance (98.24 % within 6 h) as the live fungal pellets, the lyophilized fungal pellets are also considered as efficient and promising bioflocculants for microalgae harvest. Finally, this study aimed to evaluate the potential of the pre-formed fungi-algae pellets above and the new combos of microalgae T. obliquus AS-6-1 and fungal A. niger pellets as biosorbents for gold ions recovery, respectively. The results found that the lyophilized co-pellets achieved adsorption capacity of 112.36 mg/g under the optimum conditions, which performed better than the live co-pellets due to more cell-wall polysaccharides involved in gold interaction. More than 97 % of gold ions were selectively absorbed by the lyophilized fungi-algae pellets and almost 98 % of gold ions recovery achieved from multi-metal wastewater in the column reactor. On the other hand, to save the production time of fungi-algae pellets as biosorbents, the improved gold adsorption processes by the new combos of microalgae and live or lyophilized fungal pellets were conducted. The acidic-preferred A. niger pellets and alkaline-preferred T. obliquus AS-6-1 could co-exist at pH 3.5 and achieved 196.08 mg/g of maximum gold(III) adsorption capacity according to the Langmuir fitting, simultaneously more than 95 % of algal cells were bioflocculated by the live/lyophilized A. niger pellets. The fungi-algae combos performed better than the individual algae and fungi alone in gold(III) adsorption, which may be due to more exopolysaccharides produced in the microalgae-fungi system and provoked by the presence of high-level gold(III). More than 98 % gold(III) were selectively absorbed by the combo of algae and live fungal pellets and 92.12 % of gold ions recovery achieved from multi-metal wastewater in the column reactor with higher ratio of height to diameter (H/D). This study may provide new insights into the in situ resource recovery of precious metals and reduction of their hazardous effects on water resources by microalgae immobilized on fungal pellets with the ease of biosorbent separation from wastewater.