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Evaluation of Ferrate Preoxidation for Drinking Water Treatment

Ferrate (Fe(IV)) has been proposed as a viable alternative for pre-oxidation in drinking water treatment (Jiang & Lloyd, 2002; Sharma, Kazama, Jiangyong, & Ray, 2005). The primary advantages of ferrate include a strong oxidation potential without the formation of halogenated by-products. In addition, the by-product of ferrate oxidation, ferric iron (Fe(III)), may have beneficial impacts on downstream particle destabilization and removal processes. Also, ferrate has disinfectant properties and may also provide pathogen inactivation in drinking water (Sharma et al., 2005). However, despite these advantages, there is a dearth of research experience that examines the implications of using ferrate for treating actual drinking water sources for potable water production. Studies were conducted evaluating the nature of particles that result from ferrate reduction in a laboratory water matrix and in a natural surface water with a moderate amount of dissolved organic carbon. Particle characterization included size, surface charge, morphology, X-ray photoelectron spectroscopy and transmission Fourier transform infrared spectroscopy. Characteristics of ferrate resultant particles were compared to particles formed from dosing ferric chloride, a common water treatment coagulant. In natural water, ferrate addition produced significantly more nanoparticles than ferric addition. These particles had a negative surface charge, resulting in a stable colloidal suspension. In natural and laboratory matrix waters, the ferrate resultant particles had a similar charge versus pH relationship as particles resulting from ferric addition. Particles resulting from ferrate had morphology that differed from particles resulting from ferric iron, with ferrate resultant particles appearing smoother and more granular. X-ray photoelectron spectroscopy results show ferrate resultant particles contained Fe2O3, while ferric resultant particles did not. Results also indicate potential differences in the mechanisms leading to particle formation between ferrate reduction and ferric hydrolysis. An analysis of soluble manganese oxidation by ferrate (Fe(VI)) was executed at the bench-scale, in a laboratory water matrix, both with and without the presence of natural organic matter (NOM). In the laboratory water matrix without NOM, the oxidation of Mn(II) by Fe(VI) was found to follow a stoichiometry of 2 moles Fe(VI) to 3 moles Mn(II), resulting in reduced, particulate Fe(III) and oxidized, particulate Mn(IV). The size distribution of resulting particles included significant amounts of nanoparticles. The observed stoichiometric ratio held for multiple initial Mn(II) concentrations and pH values. The presence of NOM did not significantly affect the stoichiometry, indicating limited competitive oxidant demand. Fe(VI) dosages above the stoichiometric ratio produced Mn(VII). The rate of the Mn(II) oxidation reaction was fast relative to typical time scales in drinking water treatment, with an estimated second order rate constant of approximately 1.0×104 M-1 s-1 at pH 9.2. A laboratory assessment of ferrate for drinking water treatment was conducted, including batch and continuous flow experiments on several different natural water samples. In batch experiments, ferrate preoxidation enhanced the removal of ultraviolet light absorbing compounds (UV254) by subsequent coagulation in a minority of water samples, while the majority of samples showed no improvement. In continuous flow experiments, ferrate was incorporated into small-scale models of existing treatment plants. In general, ferrate preoxidation improved finished water turbidity, UV254 absorbance and disinfection by-product formation as compared to no preoxidation and preoxidation with Mn(VII). However, for one natural water, improvements were similar in magnitude to those achieved by adding the same mass of Fe(III) in place of Fe(VI) prior to a formal coagulation step. Particulate iron resulting from Fe(VI) reduction was effectively destabilized and removed via coagulation and filtration. Ferrate may be a viable technology for drinking water treatment systems; and present a better alternative to existing oxidants by providing disinfection and oxidation of inorganics without negative impacts to downstream stream processes. The benefits of adding ferrate are likely to vary based on raw water quality and treatment goals
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