Brian P. Chaplin

Critical review of electrochemical advanced oxidation processes for water treatment applications

Electrochemical advanced oxidation processes (EAOPs) have emerged as novel water treatment technologies for the elimination of a broad-range of organic contaminants. Considerable validation of this technology has been performed at both the bench-scale and pilot-scale, which has been facilitated by the development of stable electrode materials that efficiently generate high yields of hydroxyl radicals (OH˙) (e.g., boron-doped diamond (BDD), doped-SnO2, PbO2, and substoichiometic- and doped-TiO2). Although a promising new technology, the mechanisms involved in the oxidation of organic compounds during EAOPs and the corresponding environmental impacts of their use have not been fully addressed. In order to unify the state of knowledge, identify research gaps, and stimulate new research in these areas, this review critically analyses published research pertaining to EAOPs. Specific topics covered in this review include (1) EAOP electrode types, (2) oxidation pathways of select classes of contaminants, (3) rate limitations in applied settings, and (4) long-term sustainability. Key challenges facing EAOP technologies are related to toxic byproduct formation (e.g., ClO4(-) and halogenated organic compounds) and low electro-active surface areas. These challenges must be addressed in future research in order for EAOPs to realize their full potential for water treatment.

Critical review of Pd-based catalytic treatment of priority contaminants in water

Catalytic reduction of water contaminants using palladium (Pd)-based catalysts and hydrogen gas as a reductant has been extensively studied at the bench-scale, but due to technical challenges it has only been limitedly applied at the field-scale. To motivate research that can overcome these technical challenges, this review critically analyzes the published research in the area of Pd-based catalytic reduction of priority drinking water contaminants (i.e., halogenated organics, oxyanions, and nitrosamines), and identifies key research areas that should be addressed. Specifically, the review summarizes the state of knowledge related to (1) proposed reaction pathways for important classes of contaminants, (2) rates of contaminant reduction with different catalyst formulations, (3) long-term sustainability of catalyst activity with respect to natural water foulants and regeneration strategies, and (4) technology applications. Critical barriers hindering implementation of the technology are related to catalyst activity (for some contaminants), stability, fouling, and regeneration. New developments overcoming these limitations will be needed for more extensive field-scale application of this technology.

Porous Substoichiometric TiO2 Anodes as Reactive Electrochemical Membranes for Water Treatment

This research investigates the characterization and testing of an anodic reactive electrochemical membrane (REM) for water treatment. The REM consists of a porous substoichiometric titanium dioxide (Ti4O7) tubular, ceramic electrode operated in cross-flow filtration mode. Advection-enhanced mass transfer rates, on the order of a 10-fold increase, are obtained when the REM is operated in filtration-mode, relative to a traditional flow-through mode. Oxidation experiments with model organic compounds showed that the REM was active for both direct oxidation reactions and formation of hydroxyl radicals (OH(•)). Electrochemical impedance spectroscopy data interpreted by transmission line modeling determined that the electro-active surface area was 619 times the nominal geometric surface area. Results from filtration-mode experiments with p-methoxyphenol indicate that compound removal occurred by electro-assisted adsorption and subsequent oxidation. Electro-assisted adsorption was the primary removal mechanism at potentials where OH(•) did not form. At higher potentials (>2.0 V), where OH(•) concentrations were significant, p-methoxyphenol removal occurred by a combination of electro-assisted adsorption and OH(•) oxidation. These removal mechanisms resulted in 99.9% p-methoxyphenol removal in the permeate, with calculated current efficiencies >73% at applied current densities of 0.5-1.0 mA cm(-2). These results illustrate the extreme promise of the REM for water treatment.

Electrochemical destruction of N -Nitrosodimethylamine in reverse osmosis concentrates using boron-doped diamond film electrodes

Boron-doped diamond (BDD) film electrodes were use to electrochemically destroy N-nitrosodimethylamine (NDMA) in reverse osmosis (RO) concentrates. Batch experiments were conducted ito investigate the effects of dissolved organic carbon (DOC), chloride (Cl(-)), bicarbonate (HCO(3-) and hardness on rates of NDMA destruction via both oxidation and reduction. Experimental results showed that NDMA oxidation rates were not affected by DOC, Cl(-), or HCO(3-) at concentrations present in RO concentrates. However, hydroxyl radical scavenging at 100 mM concentrations of HCO(3-) and Cl(-) shifted the reaction mechanism of NDMA oxidation from hydroxyl radical mediated to direct electron transfer oxidation. In the 100 mM Cl(-) electrolyte experimental evidence suggests that the in situ production of ClO(3)(.)also contributes to NDMA oxidation. Density functional theory calculations support a reaction mechanism between ClO(3)(.) and NDMA, with an activation barrier of 7.2 kJ/mol. Flow-through experiments with RO concentrate yielded surface area normalized first-order rate constants for NDMA (40.6 +/- 3.7 L/m(2) h) and DOC (as C) (38.3 +/- 2.2 L/m(2) h) removal that were mass transfer limited at a 2 mA/cm(2) current density. This research shows that electrochemical oxidation using BDD electrodes has an advantage over other advanced oxidation processes, as organics were readily oxidized in the presence of high HCO(3-) concentrations.

Mechanism of p-Substituted Phenol Oxidation at a Ti4O7 Reactive Electrochemical Membrane

This research investigated the removal mechanisms of p-nitrophenol, p-methoxyphenol, and p-benzoquinone at a porous Ti4O7 reactive electrochemical membrane (REM) under anodic polarization. Cross-flow filtration experiments and density functional theory (DFT) calculations indicated that p-benzoquinone removal was primarily due to reaction with electrochemically formed OH(•), while the dominant removal mechanism of p-nitrophenol and p-methoxyphenol was a function of the anodic potential. At low anodic potentials (1.7-1.8 V/SHE), p-nitrophenol and p-methoxyphenol were removed primarily by an electrochemical adsorption/polymerization mechanism on the REM. Increasing anodic potentials (1.9-3.2 V/SHE) resulted in the electroassisted adsorption mechanism contributing far less to p-methoxyphenol removal compared to p-nitrophenol. DFT calculations indicated that an increase in anodic potential resulted in a shift in p-methoxyphenol removal from a 1e(-) direct electron transfer (DET) reaction that resulted in radical formation and significant adsorption/polymerization, to a 2e(-) DET reaction that formed nonadsorbing products (i.e., p-benzoquinone). However, the anodic potentials were too low for the 2e(-) DET reaction to be thermodynamically favorable for p-nitrophenol. The decreased COD adsorption for p-nitrophenol at higher anodic potentials was attributed to reaction of soluble/adsorbed organics with OH(•). These results provide the first mechanistic explanation for p-substituted phenolic compound removal during advanced electrochemical oxidation processes.

Long-Term Evolution of Biodegradation and Volatilization Rates in a Crude Oil-Contaminated Aquifer

Volatilization and subsequent biodegradation near the water Table make up a coupled natural attenuation pathway that results in significant mass loss of hydrocarbons. Rates of biodegradation and volatilization were documented twice 12 years apart at a crude-oil spill site near Bemidji, Minnesota. Biodegradation rates were determined by calibrating a gas transport model to O2, CO2, and CH4 gas-concentration data in the unsaturated zone. Reaction stoichiometry was assumed in converting O2 and CO2 gas-flux estimates to rates of aerobic biodegradation and CH4 gas-flux estimates to rates of methanogenesis. Model results indicate that the coupled pathway has resulted in significant hydrocarbon mass loss at the site, and it was estimated that approximately 10.52 kg/day were lost in 1985 and 1.99 kg/day in 1997. In 1985 3% of total volatile hydrocarbons diffusing from the floating oil were biodegraded in the lower 1 m of the unsaturated zone and increased to 52% by 1997. Rates of hydrocarbon biodegradation above the center of the floating oil were relatively stable from 1985 to 1997, as the primary metabolic pathway shifted from aerobic to methanogenic biodegradation. Model results indicate that in 1997 biodegradation under methanogenenic conditions represented approximately one-half of total hydrocarbon biodegradation in the lower 1 m of the unsaturated zone. Further downgradient, where substrate concentrations have greatly increased, total biodegradation rates increased by greater than an order of magnitude from 0.04 to 0.43 g/m2-day. It appears that volatilization is the primary mechanism for attenuation in early stages of plume evolution, while biodegradation dominates in later stages.

Mechanism of Perchlorate Formation on Boron-Doped Diamond Film Anodes

This research investigated the mechanism of perchlorate (ClO(4)(-)) formation from chlorate (ClO(3)(-)) on boron-doped diamond (BDD) film anodes by use of a rotating disk electrode reactor. Rates of ClO(4)(-) formation were determined as functions of the electrode potential (2.29-2.70 V/standard hydrogen electrode, SHE) and temperature (10-40 °C). At all applied potentials and a ClO(3)(-) concentration of 1 mM, ClO(4)(-) production rates were zeroth-order with respect to ClO(4)(-) concentration. Experimental and density functional theory (DFT) results indicate that ClO(3)(-) oxidation proceeds via a combination of direct electron transfer and hydroxyl radical oxidation with a measured apparent activation energy of 6.9 ± 1.8 kJ·mol(-1) at a potential of 2.60 V/SHE. DFT simulations indicate that the ClO(4)(-) formation mechanism involves direct oxidation of ClO(3)(-) at the BDD surface to form ClO(3)(•), which becomes activationless at potentials > 0.76 V/SHE. Perchloric acid is then formed via the activationless homogeneous reaction between ClO(3)(•) and OH(•) in the diffuse layer next to the BDD surface. DFT simulations also indicate that the reduction of ClO(3)(•) can occur at radical sites on the BDD surface to form ClO(3)(-) and ClO(2), which limits the overall rate of ClO(4)(-) formation.

Effect of Select Organic Compounds on Perchlorate Formation at Boron-doped Diamond Film Anodes

Rates of ClO4(-) formation from ClO3(-) oxidation were investigated in batch experiments as a function of organic compounds (p-nitrophenol, p-benzoquinone, p-methoxyphenol, and oxalic acid) and current density using boron-doped diamond film anodes. Excluding organics, ClO4(-) formation rates ranged from 359 to 687 μmoles m(-2) min(-1) for current densities of 1-10 mA cm(-2). The presence of p-substituted phenols inhibited ClO4(-) formation rates between 13.0 and 99.6%. Results from a reactive-transport model of the diffuse layer adjacent to the anode surface indicate that competition between organics and ClO3(•) for OH(•) within a reaction zone (0.02-0.96 μm) adjacent to the anode controls ClO4(-) formation. Under kinetic-limited conditions (1.0 mA cm(-2)), organics reach the anode surface and substrates with higher OH(•) reaction rates demonstrate greater inhibition of perchlorate formation (IPF). At higher current densities (10 mA cm(-2)), organic compound oxidation becomes mass transfer-limited and compounds degrade a small distance from the anode surface (∼ 0.26 μm for p-methoxyphenol). Therefore, OH(•) scavenging does not occur at the anode surface and IPF values decrease. Results provide evidence for the existence of desorbed OH(•) near the anode surface and highlight the importance of controlling reactor operating conditions to limit ClO4(-) production during anodic treatment of organic compounds.

Electrochemical Oxidation of N-Nitrosodimethylamine with Boron-doped Diamond Film Electrodes

This research investigated NDMA oxidation by boron-doped diamond (BDD) film electrodes. Oxidation rates were measured as a function of electrode potential, current density, and temperature using rotating disk and flow-through reactors. Final NDMA reaction products were carbon dioxide, ammonium, and nitrate, with dimethylamine and methylamine as intermediate products. Reaction rates were first-order with respect to NDMA concentration and surface area normalized oxidation rates as high as 850 +/- 50 L/m(2)-hr were observed at a current density of 10 mA/cm(2). The flow-through reactor yielded mass transfer limited reaction rates that were first-order in NDMA concentration, with a half-life of 2.1 +/- 0.1 min. Experimental evidence indicates that NDMA oxidation proceeds via a direct electron transfer at potentials >1.8 V/SHE with a measured apparent activation energy of 3.1 +/- 0.5 kJ/mol at a potential of 2.5 V/SHE. Density functional theory calculations indicate that a direct two-electron transfer can produce a stable NDMA((+2)) species that is stabilized by forming an adduct with water. The transfer of two electrons from NDMA to the electrode allows an activation-less attack of hydroxyl radicals on the NDMA((+2)) water adduct. At higher overpotentials the oxidation of NDMA occurs by a combination of direct electron transfer and hydroxyl radicals produced via water electrolysis.

Development and Characterization of Ultrafiltration TiO2 Magnéli Phase Reactive Electrochemical Membranes

This research focused on the synthesis, characterization, and performance testing of a novel Magnéli phase (TinO2n-1), n = 4 to 6, reactive electrochemical membrane (REM) for water treatment. The REMs were synthesized from tubular asymmetric TiO2 ultrafiltration membranes, and optimal reactivity was achieved for REMs composed of high purity Ti4O7. Probe molecules were used to assess outer-sphere charge transfer (Fe(CN)6(4-)) and organic compound oxidation through both direct oxidation (oxalic acid) and formation of OH(•) (coumarin, terephthalic acid). High membrane fluxes (3208 L m(-2) h(-1) bar(-1) (LMH bar(-1))) were achieved and resulted in a convection-enhanced rate constant for Fe(CN)6(4-) oxidation of 1.4 × 10(-4) m s(-1), which is the highest reported in an electrochemical flow-through reactor and approached the kinetic limit. The optimal removal rate for oxalic acid was 401.5 ± 18.1 mmol h(-1) m(-2) at 793 LMH, with approximately 84% current efficiency. Experiments indicate OH(•) were produced only on the Ti4O7 REM and not on less reduced phases (e.g., Ti6O11). REMs were also tested for oxyanion separation. Approximately 67% removal of a 1 mM NO3(-) solution was achieved at 58 LMH, with energy consumption of 0.22 kWh m(-3). These results demonstrate the extreme promise of REMs for water treatment applications.