Fossil fuels contain naturally occurring selenium, which exists in several oxidation states, including selenide (−2), elemental selenium (0), selenite (+4), and selenate (+6). Refining of oils and processing of coals containing selenium can generate process water with amounts of selenium-containing compounds in excess of limits allowed by governmental standards for discharge of the water into the environment. These aqueous streams often include the selenium in soluble forms, such as selenocyanate (SeCN) in zero oxidative states and selenite (SeO3) and selenate (SeO4) as oxidized species.
Various treatment techniques for the process water add to costs and may not even enable achieving selenium limits. For example, some techniques rely on adsorption. However, sorbent materials previously utilized for treating the process water limit effectiveness and efficiency in such prior adsorption processes.
Therefore, a need exists for improved methods and systems for removal of selenium-containing compounds from a fluid.