Preparation of Activated Carbon from Waste Oil Sands Coke by Microwave and Conventional Chemical Activation for Removal of Mercury (II) from Aqueous Solution

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Mercury is a toxic element existing in nature. After mercury enters the aqueous system, it will be transformed into methylmercury and then go through a biomagnification process. If the release of mercury by human activities was not well controlled, it may result in the regional disaster. Activated carbon is the common sorbent used in removal of different kinds of substances in various phases. It has also been proven that activated carbon impregnated with different forms of sulfur is capable of improving the efficiency on adsorption of mercury in aqueous system. Oil sands coke is a byproduct in the process of upgrading the crude oil from oil sands. Moreover, it has high carbon content and sulfur content (approximately 5-6 wt%), which results into that oil sands coke is a good precursor of activated carbon used in removal of mercury from aqueous solution. In this study, activated carbons from oil sands coke were successfully prepared by microwave and conventional chemical activation under a series of activation conditions designed by response surface methodology in combination with central composite design (CCD-RSM). By doing so, the change of physical and chemical characteristics were able to be observed by the CCD-RSM analysis. Compared with conventional chemical activation, microwave chemical activation can develop the surface area and pore volume of activated carbon in shorter time and with a higher production yield and similar SBET. In contrast, the physical and chemical properties of activated carbon from conventional chemical activation is more predictable than activated carbon from microwave chemical activation, in terms of their variation on the activation parameters. The mercury (II) adsorption data for activated carbons from both microwave chemical activation and conventional chemical activation were best fitted with the pseudo-second order model in adsorption kinetics and Freundlich model in adsorption isotherm. The mercury (II) adsorption capacity and removal efficiency of original fluid coke were 12.58 mg-Hg2+/g-AC and 14.44% respectively. The mercury (II) adsorption capacity and removal efficiency of activated carbon from microwave activation were 82.26 mg-Hg2+/g-AC and 94.83% respectively. The mercury (II) adsorption capacity and removal efficiency of activated carbon from conventional activation were 92.89 mg-Hg2+/g-AC and 97.81% respectively. These results suggest that both conventional activation and microwave activation are able to transform the fluid coke into a suitable sorbent for mercury removal from aqueous phase. Pearson correlation analysis shows that the oxygen content and hydrogen content may be the main factors of determining mercury adsorption capacity of the resulting activated carbon. Through XPS analysis, the improvement of mercury (II) adsorption capacities can be attributed to that the formation of the phenolic, alcoholic, etheric functional groups (C-O-) and Carboxyl or ester functional groups (COO). The optimized operating condition of microwave chemical activation can be observed at the power level of 750-1000 W and time of 4-5 min and the removal efficiency would achieve 94%. The optimized operating condition of conventional chemical activation can be observed at the temperature of 700-800℃ and time of 45-85 min and the removal efficiency would achieve 94%.