Novel Synthesis Pathways for Highly Oxidative Iron Species: Generation, Stability, and Treatment Applications of Ferrate(IV/V/VI)

Difficulties arise related to the economy-of-scale and practicability in applying conventional water treatment technologies to small and remote systems. A promising oxidation technology better suited for these applications is that of electro-oxidation (EO), whereby contaminants are degraded via direct, advanced, and/or electrosynthesized oxidant-mediated reactions. One species of oxidants of particular interest includes ferrates (Fe(VI)/(V)/(IV)), where only recently has their circumneutral synthesis been demonstrated, using high oxygen overpotential (HOP) electrodes, namely boron-doped diamond (BDD). In this study, the generation of ferrates using various HOP electrodes (BDD, NAT/Ni-Sb-SnO2, and AT/Sb-SnO2) was investigated. Ferrate synthesis was pursued in a current density range of 5-15 mA cm-2 and initial Fe3+ concentrations of 10-15 mM. Faradaic efficiencies ranged from 11-23%, depending on operating conditions, with BDD and NAT significantly outperforming AT electrodes. Speciation tests revealed that NAT synthesizes both ferrate(IV/V) and ferrate(VI), while the BDD and AT electrodes synthesized only ferrate(IV/V) species. A number of organic scavenger probes were used to test the relative reactivity, including nitrobenzene, carbamazepine, and fluconazole, whereby ferrate(IV/V) was significantly more oxidative than ferrate(VI). Finally, the ferrate(VI) synthesis mechanism by NAT electrolysis was elucidated, where coproduction of ozone was found to be a key phenomenon for Fe3+ oxidation to ferrate(VI).

Analytical quantification of aqueous permanganate: Direct and indirect spectrophotometric determination for water treatment processes

Three spectrophotometric methods have been developed and compared for the quantification of low concentrations (0.03-63 μM) of aqueous permanganate in neutral pH conditions. Although permanganate is a widely used oxidant in drinking water and wastewater treatment, no widely accepted method of quantification has been reported to date. While one method presented does not require the need for any reagent chemicals (direct spectrophotometric analysis), it yielded a relatively low molar absorption coefficient of 3340 M^-1 cm^-1 at 525 nm and a level of detection (LOD) and quantification (LOQ) of 0.45 and 1.51 μM, respectively. Some instability of permanganate species during direct quantification was found to occur over 60 min, with a total decrease of 0.002 (arbitrary units) of absorbance, equivalent to a decrease in concentration of 0.6 μM. Beyond 60 min, no further degradation was observed. Indirect spectrophotometric analyses using 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) and sodium iodide (NaI) provided a significantly more sensitive method for permanganate quantification, yielding molar absorption coefficients of 140,030 and 61,130 M^-1 cm^-1, respectively. The LOD and LOQ were determined to be 0.01 and 0.03 μM for the ABTS method and 0.02 and 0.08 μM for the NaI method, respectively. Although conservative and accurate limits of quantification for both the ABTS and NaI methods are presented, which should be sufficient of most practical applications, lower limits may be possible with further refinement of the methods.

The role of mixing in potassium ferrate(VI) consumption kinetics and disinfection of bypass wastewater

Bypass wastewaters need an appropriate auxiliary treatment to address their broad range of chemical and bacterial characteristics. The dual capacity of potassium ferrate(VI) as disinfectant/oxidant and coagulant may be useful in a sustainable process retrofit to provide adequate treatment to such wastewaters. However, the engineering aspects of potassium ferrate(VI) based technology to retrofit within existing coagulation-flocculation-sedimentation basins have not been studied. This study investigated, for the first time, the role of rapid mixing on the rate of potassium ferrate(VI) decay and disinfection in bypass wastewaters from extreme wet weather flow events. First-order, second-order, and double exponential models were fit to the potassium ferrate(VI) consumption data, and the double exponential model was able to represent the potassium ferrate(VI) decay in all conditions with a high coefficient of determination and low mean square error. In addition, Chick-Watson and Hom models were tested in this study, and both fit the E. coli disinfection results. The rates of potassium ferrate(VI) consumption and disinfection derived from the models were higher using 500-1000 rpm rapid mixing speeds than they were when magnetic stirrer mixing was used for the same initial dosage and wastewater sample. There was no significant increase in the potassium ferrate(VI) consumption or disinfection rates with the increase of the rapid mixing speeds from 500 to 1000 rpm which revealed that the reactions were kinetically controlled. The coagulation capability of potassium ferrate(VI) enhanced the sedimentation ability and contributed almost the same as the chemical disinfection capability to the overall E. coli removal. This study suggests that potassium ferrate(VI) can be implemented in existing facilities that mix coagulants to enhance primary sedimentation, yet potassium ferrate(VI) can provide both disinfection and coagulation at lower mixing speeds.

A membrane-based electrochemical flow reactor for generation of ferrates at near neutral pH conditions

We report the electrosynthesis of Fe(vi) in a flow reactor operating in batch recirculation mode at neutral conditions using boron doped diamond (BDD) and Fe(iii).

Mitigation of algal organic matter released from Chaetoceros affinis and Hymenomonas by in situ generated ferrate

This study demonstrates the application of in situ ferrate (Fe(VI)) for the efficient removal of dissolved algal organic matter (AOM) from seawater. Sodium hypochlorite (NaOCl) and ferric (Fe(III)) were used to produce in situ Fe(VI) by wet chemical oxidation. First, the removal efficiencies of two model AOM compounds, humic acid (HA) and sodium alginate (SA), were evaluated in the presence of sodium chloride with an initial influent dissolved organic carbon (DOC) concentration of 5.0 mg C L^-1 at different pH levels to establish the optimal doses for in situ Fe(VI) generation. The concentration of Fe(VI) was determined by the 2,2-Azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) ultraviolet-visible spectrophotometry method. In the case of HA, 72% DOC removal was recorded when applied with 1.5 mg L^-1 of Fe(III) and 1.5 mg L^-1 of NaOCl (in situ Fe(VI) concentration of 1.46 mg L^-1) while 42% DOC removal was observed for SA. Subsequently, the removal of AOM extracted from two bloom-forming algal species, Chaetoceros affinis (CA) and Hymenomonas (Hym), cultivated in seawater from the Red Sea, were tested with in situ generated Fe(VI) at the established optimum condition. In situ Fe(VI) recorded superior performance in removing AOM extracted from CA and Hym, showing 83% and 92% DOC removal when the influent DOC concentrations were 2.48 and 2.63 mg L^-1, respectively. A detailed AOM characterization was conducted using liquid chromatography-organic carbon detection.