Perchlorate (ClO₄^-) is a strong oxidizer and propellant that is used in the military and automotive industries. It also is an impurity in hypochlorite salts used for drinking water and swimming pool disinfection and is contained in some natural fertilizers and deposits. Due to the potential toxicity and numerous exposure routes of ClO₄^-, treatment methods have been developed to either separate or destroy ClO₄^-present in water. The strong stability of ClO₄^- has resulted in chemical reduction methods to be largely ineffective. Hence, catalytic reduction is needed to increase the kinetics of ClO₄^- degradation. The most promising catalytic reduction method used an organometallic heterogeneous catalyst for ClO₄^- reduction, {Re(O)(hoz)₂Cl [hoz= 2-(2'-hydroxyphenyl)-2-oxazoline, an N,O-bidentate ligand]}, which contains Re single-sites as a promoter metal for the oxygen atom transfer (OAT) step.
In this study, an organometallic rhenium complex catalyst was deposited on a Ti₄O₇ reactive electrochemical membrane (REM) to develop a catalytic Re/REM for rapid electrocatalytic reduction of aqueous ClO₄^-. Results showed increasing electrocatalytic ClO₄^- reduction upon increasing cathodic potential (i.e., 0.4 V/SHE to 1.7 V/SHE). For example, 21.6-53.6% and 30.0-67.3% of a 5 mM ClO₄^- solution was reduced in a single pass through the Re/REM (hydraulic residence time ~11 s) at solution pH of 7 and 3, respectively, in which chloride was the dominant reduction product. Kinetic analysis at pH=7 determined rate constants as high as 60.9 L h^-1 g_Re^-1, which exceeded the reported values for an analogous catalytic system using hydrogen as an electron donor. To study the practical application of the Re/REM system, an experiment was carried out with the groundwater spiked with 1 ppm NaClO₄. Results showed stable ClO₄^- reduction in the groundwater sample using the Re/REM system for 467 hrs. Results also indicated that ClO₄^- concentrations were below the 10 ppb detection limit for the first 93.5 hrs and lower than the EPA guideline (56 ppb) for 374 hrs with constant permeate flux of 200 LMH and constant cathodic polarization of -1 V/SHE.
The fast ClO₄^- reduction kinetics and high chloride selectivity without the need for acidic conditions and the continual supply of a hydrogen electron donor for catalyst regeneration, indicates the promising ability of the Re/REM for aqueous electrocatalytic ClO₄^- treatment.

Changxu Ren

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