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Electrocoagulation (EC) has been known for more than a century. Applications in industries such as water and wastewater treatment processes have been adapted for the removal of metals, nonmetals, suspended solids, organic compounds, COD (chemical oxygen demand), and BOD (biological oxygen demand). Iron electrodes have been preferred over aluminum due to their durability and cost. However, the electrochemical reactions occurring with EC using iron electrodes have not been systematically studied. For a better understanding of the mechanism and reactions for EC using iron electrodes, we present a review of the concept of green rust (GR) and its relationship to the current theory of EC. Experimental results obtained by measuring pH at different zones near the iron electrodes during the EC process are detailed, and are used to illustrate the mechanism and reactions that occur at both the anode and the cathode. The mechanism and reactions presented explain phenomena associated with EC and are congruent with solubility and Pourbaix diagrams.

Green rusts are unstable compounds containing a mixture of ferrous and ferric hydroxides that belong to a family of minerals known as layered double hydroxides (LDH). They can be represented with the general formula [FeII(6-x) FeIIIx (OH)12] x+[Ax/n-*yH2O] x-, where A is an n-valent anion mainly Cl-, CO3-2 and SO4-2, and in which either the bivalent or the trivalent iron can be replaced for other trivalent or bivalent metal ions. Green rust was first identified and studied as a corrosion product later it was identified in soils and related to interactions between microbes and metals in soils. Green rusts have been shown to be highly reactive compounds that are useful in the reduction of organic compounds such as methachlor, of inorganic elements such as Chromium, Selenium and Uranium; Arsenic removal, and in the treatment of Acid mine drainage (AMD). Green rust has been synthesized combining Fe(II) and Fe(III), by air oxidation of FeCl2 and other methods. In this paper, the electrochemical generation of green rust, its characterization by X-ray Diffraction (XRD), and implications to electrocoagulation will be discussed