Ferrate and Permanganate

Project Overview

While numerous water and wastewater treatment technologies exist to remediate even the most polluted waters, conventional processes can often be unsuitable for small, remote and decentralised system applications. The impracticality of conventional, centralised technologies and processes is associated with the lack of economy-of-scale, availability of trained operators, and/or the inaccessibility and cost to transport necessary process supplies, such as coagulant and chemical oxidants. Electrochemical water treatment technologies, although not currently widely adopted, present an interesting and promising alternative to conventional processes for these niche applications, as they can eliminate the chemical supply chain associated with conventional treatment processes through the electrochemical generation of process chemicals, on-site and on-demand. While technologies like electrocoagulation have been shown to be promising for remote drinking water system applications for both organic and inorganic contaminants, the process of electrochemical oxidation (electro-oxidation, EO) has not yet received the same degree of attention. One particular electrode material, which has been widely observed to effectively treat a number of common raw water pollutants through electro-oxidation, is boron-doped diamond (BDD), and this may be a suitable option for remote and small system applications.

An ever-growing concern for vulnerable communities with limited treatment processes, like those typical of small and remote systems, is the prevalence of organic micro-pollutants, such as pesticides and industrial chemicals, in raw water sources. These contaminants have been found to be ubiquitous in natural waterways, accumulating in both ground and surface water supplies, originating from many sources including agricultural runoff, road runoff, wastewater effluents and atmospheric fallout, amongst other routes. Due to a sharp increase in production of new organic chemicals since the mid-twentieth century, many drinking water treatment processes were not adequately designed to address this influx of micro-pollutants. As a consequence, some 300+ organic pollutants were found to be present in drinking water samples in the United Kingdom in the 1970s. As these micro-pollutants continue to accumulate in surface and groundwater used as drinking water supplies, appropriate treatment technologies need to be considered. This is particularly true for small, remote and under-developed communities lacking sophisticated drinking water treatment processes, which can adequately address this issue of micro-pollutants.

While EO has traditionally proceeded via direct electron transfer at the electrode surface, the development of powerful high oxygen overpotential (HOP), non-active electrode materials like BDD has enabled the technology to better address many of the aforementioned ubiquitous micro-pollutant contaminants. In particular, materials like BDD have enabled the EO process to become vastly more efficient due to the increased potential range (reduction and oxidation) of water stability: -1.25–2.3 V_SHE. This minimises the oxygen evolution reaction (OER) and facilitates the generation of hydroxyl radicals (•OH) in circumneutral pH conditions. Although the primary mechanism of micro-pollutant degradation has been associated with •OH during BDD EO, oxidation is limited to the electrode surface where radicals are generated and weakly adsorbed, rendering the technology a point- source application for oxidation/disinfection. However, a potentially important component yet to be fully exploited is the •OH-mediated oxidation and simultaneous synthesis of chemical oxidants from dissolved species in solution (e.g. Fe²⁺, Mn²⁺, Cl^–) for downstream oxidation/disinfection throughout the distribution system. While the generation of oxidant species like persulfate (E₀ = +1.96 V_SHE), peroxodiphosphate (E₀ = +2.07 V_SHE) and various reactive chlorine species have been observed to increase micro-pollutant degradation rates, some drawbacks include a decrease in mineralisation rates due to the formation of potentially harmful recalcitrant chlorinated disinfection by-products.

A particularly novel consideration yet to receive the same attention is the circumneutral generation of powerful iron and manganese-based oxidants, namely ferrate (typically Fe⁶⁺/FeO₄^2-, but can exist as Fe⁵⁺ and Fe⁴⁺) and permanganate(VII) (Mn⁷⁺, MnO₄^–). Both ferrate and permanganate are known to have high redox potentials (E₀: Fe(VI) = +2.2 V_SHE, E₀: Mn(VII) = +1.5 V_SHE), with non-toxic reduction products (e.g., hydrolysis species of Fe³⁺ and Mn³⁺), whereby the former has been observed to function as an effective coagulant chemical for further treatment. Ferrate and permanganate are already both used in water and wastewater treatment applications, dosed in their salt form of potassium ferrate (K₂FeO₄) and potassium permanganate (KMnO₄), respectively. Both K₂FeO₄ and KMnO₄ have not been widely applied to water and wastewater treatment processes, however, due to their expensive synthesis methods and instability.

While the conventional synthesis processes and stability of ferrate and permanganate have limited their widescale adoption in the water and wastewater industry, the use of HOP materials like BDD may provide a novel reaction pathway for their circumneutral electrosynthesis from low concentration, low oxidation state iron (Fe²⁺) and manganese (Mn²⁺), typical of concentrations found in raw groundwater sources. In doing so, an opportunity to exploit aqueous iron and manganese species already present in a raw water matrix during conventional EO processes can be realised. This novel simultaneous process would involve both the conventional direct, and hydroxyl radical mediated, oxidation at the electrode surface, as well as the synthesis of highly oxidative ferrate and permanganate species for further residual oxidation downstream and within the bulk water solution.

In this study, the electrochemical generation of ferrate and permanganate from low oxidation, low concentration iron and manganese in circumneutral conditions was investigated. The effect of operating variables, like current density and initial iron and manganese concentration, on ferrate and permanganate generation is investigated, respectively. To assess the efficacy of the oxidant generation process for water treatment applications, the subsequent treatment of two relevant and problematic target contaminants, namely atrazine and perfluorooctane sulfonate (PFOS), was pursued. The separate effects on pollutant degradation by EO, electro-ferrate and electro-permanganate oxidation, and the simultaneous EO and electro-ferrate/permanganate process, were evaluated.

Two high level research goals were accomplished: (1) development of a novel electrochemical reaction pathway for ferrate and permanganate oxidant synthesis, in circumneutral pH conditions, using low oxidation state, low concentration aqueous iron and manganese via BDD electrochemical oxidation; (2) evaluation of the capability of a combined EO and oxidant generation system, in a water treatment context, to degrade two model organic pollutants, atrazine and PFOS. In doing so, several key research outcomes were yielded, and the following will summarise the principal conclusions arising from the study.

Summary of Results

The research has provided the first example available in the scientific literature of in-situ electrochemical ferrate production in neutral pH conditions, using ambient iron (Fe²⁺) concentrations typical of raw (low oxygen) surface and ground water sources. The ferrate generation efficiency and effect of various operating variables were investigated, as well as the mechanism of iron oxidation. Some key conclusions which arose from this work include:

Ferrate generation was mass transport limited, with no significant differences in synthesis at the three current density conditions (10, 40 and 80 mA cm^-2) when the initial Fe²⁺ concentration was 54 μM and below.
The rate-limiting step for ferrate generation at low initial iron concentrations is the diffusion of the iron cation reactant to the electrode surface through the Nernst diffusion layer.
At increased initial Fe²⁺ concentration (179 μM), current density was observed to effect ferrate synthesis, thereby indicating that the process was no longer solely diffusion limited.
A developed mathematical model describing ferrate generation under mass transport limitations correlated well with experimentally yielded data, helping to confirm that the lab-derived results agreed with those predicted by theory, as well as providing insights into reactor design considerations which could increase ferrate yield (such as inter-electrode gap and reactor volume).
A mixed reaction mechanism of both direct- and hydroxyl radical mediated oxidation was observed to be responsible for the synthesis of ferrate, from tests using cyclic voltammetry and hydroxyl radical scavengers.

In addition to ferrate generation, this study provided the first example of in-situ electrochemical permanganate generation in neutral pH conditions, using low concentrations of ambient manganese (Mn²⁺). While the circumneutral generation of permanganate was observed to proceed less readily as that observed for ferrate generation, several important findings were yielded:

Although relatively low permanganate concentrations were generated under the test conditions, meaningful amounts were yielded demonstrating the first example of the reaction pathway.
The oxidation of Mn²⁺ progressed principally to an intermediate oxidation state in the form of insoluble manganese (Mn^3,4,5+), such as thermodynamically stable species MnO₂, Mn₂O₃ and/or Mn₃O₄.
A decrease in the concentration of aqueous manganese (Mn²⁺) occurred at a greater rate as the operating current density increased, irrespective of the initial concentration (Mn²⁺ = 55 and 182 μM), indicating greater oxidation due to both increased hydroxyl radical generation and direct oxidation.
A mathematical model was developed to describe manganese oxidation to insoluble oxide species, which correlated well with experimental results.

Having established a thorough understanding of the electrochemical generation of both ferrate and permanganate, the effectiveness of a simultaneous electro-oxidation and oxidant generation system for treating organic micro-pollutants was evaluated. The first pollutant explored was atrazine, which functioned as a suitable performance standard for comparison to other oxidation systems. From this work, several research conclusions were established:

The EO-only process (no iron or manganese addition) facilitated significant degradation of atrazine and was well represented by pseudo-first-order reaction kinetics, due to the constant regeneration of hydroxyl radical species at the electrode surface.
Atrazine degradation increased with an increase in current density due to the increased hydroxyl radical formation.
Atrazine pseudo-first-order degradation rates were observed to increase when Fe2+ was added to the water matrix, due to the generation and subsequent oxidation of atrazine by ferrate. Furthermore, degradation rate constants increased with an increasing initial Fe²⁺ concentration (9, 18 and 54 μM) due to the increased production of ferrate.
Although only a small concentration of permanganate (≤ 1 μM) could be generated by neutral BDD electro-synthesis, it was observed to be adequate for enhanced degradation of atrazine, as pseudo- first-order reaction rates increased when Mn²⁺ was added to the water matrix.
Atrazine degradation pathways were determined for all oxidation processes, by monitoring oxidation by-product formation. The main by-products identified included DEDIA, DEA, DIA and hydroxyatrazine.