Arsenic Oxidation Assisted by Carbon-based Catalysts in Dynamic and Continuous Conditions
Arsenic , Oxidation , Continuous conditions , Carbon catalysis
Arsenic is a toxic element and has become a major element of concern in the mining industry. Severe environmental issues may arise if the soluble arsenic species released by mining and extractive metallurgy activities were not responsibly and appropriately managed. Most arsenic immobilization techniques are not efficient for treating trivalent arsenic in solutions. In the cases where soluble trivalent arsenic is predominant in the waters, a first step of oxidation is usually required to yield a desirable overall arsenic removal efficiency. This thesis presents the results of an investigation on the carbon catalyst assisted arsenic oxidation process with emphasis on the impact of the key process parameters on oxidation efficiency. Rapid small-scale continuous column testing method was employed to simulate dynamic and continuous operations. The process residence time, pH and initial arsenic concentration are demonstrated to be the most crucial operational parameters that have significant impacts on arsenic oxidation. The arsenic oxidation efficiency is proven to be highly sensitive to the change of aqueous pH due to the shift of reaction pathways at various pH scenarios, which is correlated to the specific behaviors of different aqueous arsenic species. Both preloaded and recycled activated carbon are validated for competent capacity to catalyze arsenic oxidation. The reliable long-term performance of activated carbon is demonstrated in a 27 days-long test with a steady arsenic oxidation efficiency of above 97%. Another carbon-based catalyst, Lewatit® AF 5, which has a higher surface area and more active sites than activated carbon, is used in a column test to verify that the functionality of carbon, instead of the specific surface area, has the most catalytic ability of carbon to oxidize arsenic. This is further supported by tests with nitric acid treated activated and AF5, in which the arsenic oxidation is greatly improved at the initial stage, though the efficiency eventually declines to the same level of that of the original carbons. The initial improvement of arsenic oxidation is attributed to the nitric acid exploitation of oxidative function groups on the carbon surfaces.