Activated Iron Media for Wastewater Treatment: Mechanisms for Anionic Contaminants Immobilization and Transformation

Abstract

Activated iron media has been proven effective for treating a variety of environmental contaminants with more strengths and less drawbacks comparing to conventional zero-valent iron (ZVI) technology. Despite its success, some of the detailed mechanistic understandings of the activated iron media are still in mystery. This study embarks on laboratory experiments and analyses mainly focusing on the roles of green rusts as part of the effective ingredients of the activated iron media for immobilization and transformation of anionic contaminants from water. The investigation centers on the anionic exchanging capacity of the green rust enabled by the unique intercalation layer of the green rust structure.

In the Chapter II and III, green rusts are found to exist as a precursor/intermedia towards the formation of the activated iron media. A model is proposed where green rust as a coating on iron is oxidized by nitrate but also regenerated on the ZVI particle surface under certain conditions. Externally-added ferrous ion participates in nitrate reduction directly as an electron donor; in excess, ferrous ion could further facilitate the transformation of the final products to green rust instead of magnetite. Roles of green rust coating are postulated as a combination of two models, semiconductor and coordinating surface: on one hand, green rust concatenates zero-valent iron with the outer surface and delivers electrons to where nitrate is reduced; on the other hand, green rust as a reactive layered double hydroxide provides substantial redox-active sites through its anion exchanging capacity.

Chapter IV of the study focused on the potential mechanism of selenocyanate transformation and immobilization by an activated iron media system. A series of screening tests were conducted, and the results accentuate the importance of dissolved oxygen on selenocyanate removal by ZVI. Radicals that generated from oxidation of ZVI by dissolved oxygen might play an important role in SeCN⁻ removal. A stoichiometry relationship between ferrous ion and selenocyanate has been established, and the fates of selenium and cyanide group are discussed.

This study generates new knowledge that might be useful for developing a novel reactive media with enhanced ion exchanging capacity for removing selected contaminants, and a more robust AIM treatment system that is not only capable of reductive treatment of target contaminants, but also advanced oxidative degradation of certain contaminants.