An Investigation Into the Activated Carbon-Catalyzed Arsenic Oxidation Process

Abstract

Arsenic is a highly toxic element that is becoming an increasingly more prevalent issue for mining and metallurgical operations. Arsenic is an abundant element in the earth’s crust and is commonly associated with metals such as gold, silver, copper, nickel and many others. Arsenic is often oxidized prior to its removal from process streams because the higher oxidation state of arsenic typically offers a more stable solid product. A novel and robust process to oxidize arsenic is activated carbon-catalyzed arsenic oxidation which is a process which uses oxygen, as the only consumable reagent, and activated carbon. The arsenic oxidation mechanism was explored via cyclic voltammetry, zeta potential measurements, X-ray photoelectric spectroscopy, shake tests, batch tests and continuous column tests. It was determined that hydrogen peroxide is formed on the surface of activated carbon and this aids in the oxidation of arsenic. Adsorption of arsenic species on the activated carbon was a large factor in the investigation. Two mechanisms of arsenic oxidation were theorized and it is believed that they occur simultaneously. The first hypothesized reaction pathway is that As (III) is oxidized when it contacts hydrogen peroxide on the activated carbon surface and is then instantly adsorbed as an As (V) compound. The second hypothesized reaction pathway is that As (III) adsorbs onto the surface of activated carbon and is then oxidized by hydrogen peroxide present in close proximity in the solution to the activated carbon surface. Activated alumina was used instead of carbon during a batch test experiment in order to verify that it is the functional groups of carbon that perform the catalytic ability of activated carbon to oxidize arsenic and not the porous nature of its surface. It was concluded that activated alumina had no ability to oxidize arsenic and therefore it is the functional groups on the carbon surface that provide the oxidative capabilities.