Perchlorate reduction during electrochemically induced pitting corrosion of zero-valent titanium (ZVT)

Catalytic enhancement of hydrogenation reduction and oxygen transfer reaction for perchlorate removal: A review

Perchlorate is the main contaminant in surface water and groundwater, and it is of current urgency to remove due to its high water solubility, mobility, and endocrine-disrupting properties. The conversion of perchlorate into harmless chloride ions by using appropriate catalysts is the most promising and effective route to overcome its high activation energy and kinetic stability. Perchlorate is usually reduced in two ways: (1) indirect reduction via oxygen atom transfer (OAT) reaction or (2) hydrodeoxygenation through highly active reducing H atoms. This paper discusses the mechanisms underlying both the OAT reaction catalyzed by homogenous rhenium-oxo complexes or biological Mo-based enzymes and the heterogeneous hydrogenation for perchlorate reduction. Particular emphasis is placed on the factors affecting the catalytic process and the synergy between the (1) and (2) reactions. For completeness, the applicability of different electrolysis devices, electrodes, and bioreactors is also illustrated. Finally, this article gives prospects for the synthesis and application of catalysts in different pathways.

Electrochemical Cr(VI) removal from aqueous media using titanium as anode: Simultaneous indirect electrochemical reduction of Cr(VI) and in-situ precipitation of Cr(III)

In this work, a novel method for complete Cr(Ⅵ) removal was achieved in a single-chamber cell with titanium (Ti) as anode via simultaneous indirect electro-reduction of Cr(Ⅵ) and in-situ precipitation of Cr(Ⅲ). The Cr(Ⅵ) and total Cr removal, and electric energy consumption were optimized as a function of electrochemical reactor, current density, initial Cr(Ⅵ) and chloride (Cl^-) concentration, and initial solution pH. The maximum Cr(Ⅵ) and total Cr removal efficiency reached 80.5 and 79.4% respectively within 12 h at current density of 10 mA cm^-2 as initial Cr(Ⅵ) concentration was 0.078 mM. Decreasing the initial solution pH was beneficial to Cr(Ⅵ) reduction, but Cr(Ⅲ) precipitation was inhibited, resulting in the poor total Cr removal. The suitable Cl^- concentration guaranteed sufficient reducing agents (Ti³⁺ and Ti²⁺) for Cr(Ⅵ) removal. The reaction mechanism demonstrated that Ti anode could be corroded to produce Ti³⁺ and Ti²⁺, which provided the electrons for reduction of Cr(Ⅵ) to Cr(Ⅲ). Simultaneously, the solid products (Ti₂O_(6x-y-z+52)Cl_2yCr_2x(OH)_2z(s)) were in-situ formed and precipitated from the solution due to the continuous generation of hydroxyl ion (OH^-) from cathode. This study might provide a new electrochemical method with non-precious metal as the electrode for complete Cr(Ⅵ) removal from aqueous media.

Indirect electrochemical reduction of nitrate in water using zero-valent titanium anode: Factors, kinetics, and mechanism

In this study, indirect electrochemical reduction with zero-valent titanium (ZVT) as anode successfully achieved the selective nitrate removal from simulated groundwater. The maximum nitrate removal efficiency and N₂ selectivity reached to 83.4% and 78.5% after 12 h, respectively. Experimental results demonstrated that the gaseous by-products (NO and N₂O) were negligible and the nitrate reduction process could be well depicted by pseudo-first-order kinetic model. Decreasing the pH value of electrolyte was favorable to electrical energy utilization efficiency and nitrate removal. The chloride ultimately showed inhibitory effects on electrochemical reduction of nitrate. During the electrochemical reaction, the ZVT lost electrons to generate the reducing agents (Ti³⁺ and Ti²⁺), which could afford electrons for nitrate reduction and form the solid by-products TiO_2.4Cl_0.2N_0.1. A 2-stage strategy, indirect electrochemical reduction + hypochlorite treatment (pre-reduction + post-oxidation), was developed to completely remove nitrate and the long-term performance of nitrate reduction was comprehensively evaluated. The effluent nitrate steadily kept at 8.8 mg N/L during 120 h continuous operation when the influent nitrate concentration was 25.9 mg N/L. Simultaneously, nitrite concentration was lower than 0.01 mg N/L, and ammonium and Ti ions were not detected in the effluent.

Metal-induced decomposition of perchlorate in pressurized hot water

Decomposition of perchlorate (ClO(4)(-)) in pressurized hot water (PHW) was investigated. Although ClO(4)(-) demonstrated little reactivity in pure PHW up to 300°C, addition of zerovalent metals to the reaction system enhanced the decomposition of ClO(4)(-) to Cl(-) with an increasing order of activity of (no metal)≈Al < Cu < Zn < Ni << Fe: the addition of iron powder led to the most efficient decomposition of ClO(4)(-). When the iron powder was added to an aqueous ClO(4)(-) solution (104 μM) and the mixture was heated at 150°C, ClO(4)(-) concentration fell below 0.58 μM (58 μg L(-1), detection limit of ion chromatography) in 1 h, and Cl(-) was formed with the yield of 85% after 6 h. The decomposition was accompanied by transformation of the zerovalent iron to Fe(3)O(4). This method was successfully used in the decomposition of ClO(4)(-) in a water sample contaminated with this compound, following fireworks display at Albany, New York, USA.