Mechanisms and Efficiency of the Simultaneous Removal of Metals and Cyanides by Using Ferrate(VI): Crucial Roles of Nanocrystalline Iron(III) Oxyhydroxides and Metal Carbonates

Abstract

The reaction of potassium ferrate(VI), K(2)FeO(4), with weak-acid dissociable cyanides--namely, K(2)[Zn(CN)(4)], K(2)[Cd(CN)(4)], K(2)[Ni(CN)(4)], and K(3)[Cu(CN)(4)]--results in the formation of iron(III) oxyhydroxide nanoparticles that differ in size, crystal structure, and surface area. During cyanide oxidation and the simultaneous reduction of iron(VI), zinc(II), copper(II), and cadmium(II), metallic ions are almost completely removed from solution due to their coprecipitation with the iron(III) oxyhydroxides including 2-line ferrihydrite, 7-line ferrihydrite, and/or goethite. Based on the results of XRD, Mössbauer and IR spectroscopies, as well as TEM, X-ray photoelectron emission spectroscopy, and Brunauer-Emmett-Teller measurements, we suggest three scavenging mechanisms for the removal of metals including their incorporation into the ferrihydrite crystal structure, the formation of a separate phase, and their adsorption onto the precipitate surface. Zn and Cu are preferentially and almost completely incorporated into the crystal structure of the iron(III) oxyhydroxides; the formation of the Cd-bearing, X-ray amorphous phase, together with Cd carbonate is the principal mechanism of Cd removal. Interestingly, Ni remains predominantly in solution due to the key role of nickel(II) carbonate, which exhibits a solubility product constant several orders of magnitude higher than the carbonates of the other metals. Traces of Ni, identified in the iron(III) precipitate, are exclusively adsorbed onto the large surface area of nanoparticles. We discuss the relationship between the crystal structure of iron(III) oxyhydroxides and the mechanism of metal removal, as well as the linear relationship observed between the rate constant and the surface area of precipitates.

Ferryl and Ferrate Species: Mössbauer Spectroscopy Investigation

“…[25,26] The encapsulation of arsenic and heavy metals by iron(III) oxide nanoparticles (γ-Fe2O3), generated from Fe(VI), was investigated in detail by in-field Mössbauer spectroscopy. [27][28][29] In the case of arsenic, the results clearly demonstrated that a significant portion of arsenic was embedded in the tetrahedral sites of the γ-Fe2O3 spinel structure.…”

Redox Properties of Iron-Based Materials in Water Treatment Technologies: An Overview of Laboratory Versus Field Experiences

“…Degradation of various pollutants was successfully tested involving pharmaceuticals [18], endocrine disruptors [19], organosulfur compounds [20], pesticides [21], chemical warfare agents [22]. Furthermore, inorganic compounds such as amines [23], cyanides [4] and heavy metals [24]; were investigated with respect to removal by ferrate(VI). In addition to the high efficient removal of pollutants, ferrate(VI) can effectively eliminate water pathogens, bacteria and viruses and acts as disinfection agent [25].…”

“…The innovative nanomaterials are beneficial to this purpose because of their high reactivity (especially metallic nanoparticles [1]) and sorption capacity (metal oxide nanoparticles [2]). The iron based materials proved to be promising material due to the availability of iron-bearing precursors and versatility of Fe in its various oxidation states while Fe0/II is a reducing agent [3], FeIV/V/VI is a strong oxidant [4]. Furthermore, the non-toxic iron oxyhydroxides are the final reaction products of Fe-based (nano) materials [5], [6]; these minerals show the similar properties as their naturally occurring analogues [7].…”

Formation and toxicity of brominated disinfection byproducts during chlorination and chloramination of water: A review

“…[155,156] Examples include removal of arsenic and metal cyanides by ferrate. [157][158][159][160] When As(III) was oxidized to As(V) by ferrate, Fe(III) oxides/hydroxides formed, which subsequently remove As (V). Importantly, this study demonstrated that the significant portion of arsenic embedded in tetrahedral sites of the γ -Fe 2 O 3 spinel structure.…”