Electrochemical aspects of some specific features connected with the behavior of iron group metals in aqueous perchloric acid/perchlorate media

Perchlorate Removal in Microbial Electrochemical Systems With Iron/Carbon Electrodes

Perchlorate removal was tested in the cathode chamber of microbial electrochemical systems (MESs). Dual-chambers MESs were constructed and operated in batch mode with four kinds of cathode materials including Fe/C particles (Fe/C), zero valent iron particles (ZVI), blank carbon felt (CF), and active carbon (AC). Without external energy supply or perchlorate-reducing microbial pre-enrichment, perchlorate ( ClO 4 - ) removal could be achieved in the cathode chambers of MESs at different efficiencies. The highest ClO 4 - removal rates in these reactors were 18.96 (Fe/C, 100 Ω, 2 days), 15.84 (ZVI, 100 Ω, 2 days), 14.37 (CF, 100 Ω, 3 days), and 19.78 mg/L/day (AC, 100 Ω, 2 days). ClO 4 - degradation products were mainly Cl^- and ClO 3 - , and the total chlorine in the products was lower than the theoretical input. The non-conservation of the total chlorine may be caused by the adsorption and co-precipitation related to the electrode materials. Coulombs and coulombic efficiency calculation showed that electron provided by MESs was partially responsible for ClO 4 - reduction, for the Fe/C cathode reactors, about a quarter of electron was provided by MESs.

Graphene-a promising material for removal of perchlorate (ClO4-) from water

A batch adsorption process was applied to investigate the removal of perchlorate (ClO4 (-)) from water by graphene. In doing so, the thermodynamic adsorption isotherm and kinetic studies were also carried out. Graphene was prepared by a facile liquid-phase exfoliation. Graphene was characterized by Raman spectroscopy, Fourier-transform infrared spectroscopy, powder X-ray diffraction, scanning electron microscope, and zeta potential measurements. A systematic study of the adsorption process was performed by varying pH, ionic strength, and temperature. The adsorption efficiency of graphene was 99.2 %, suggesting that graphene is an excellent adsorbent for ClO4 (-) removal from water. The rate constants for all these kinetic models were calculated, and the results indicate that second-order kinetics model was well suitable to model the kinetic adsorption of ClO4 (-). Equilibrium data were well described by the typical Langmuir adsorption isotherm. The experimental results showed that graphene is an excellent perchlorate adsorbent with an adsorbent capacity of up to 0.024 mg/g at initial perchlorate concentration of 2 mg/L and temperature of 298 K. Thermodynamic studies revealed that the adsorption reaction was a spontaneous and endothermic process. Graphene removed the perchlorate present in the water and reduced it to a permissible level making it drinkable.

Perchlorate removal by acidified zero-valent aluminum and aluminum hydroxide

Removal of perchlorate using either acid-washed zero-valent aluminum or aluminum hydroxide was studied in batch reactors under ambient temperature and pressure. Approximately 90-95% of perchlorate was removed within 24h in the presence of 35 g L(-1) aluminum at acidic pH (4.5+/-0.2). Although aluminum is a strong reductant, this study indicated no explicit evidence to support perchlorate reduction while it was found that an adsorption process is involved in the perchlorate removal. The adsorbed perchlorate ions were desorbed effectively using a 1.0 N MgSO(4) solution. The effective composition for the perchlorate adsorption is confirmed as aluminum hydroxide (bayerite), which is a product of the aluminum corrosion. Rapid adsorption of perchlorate was observed in the presence of aluminum hydroxide. The perchlorate adsorption by aluminum hydroxide is dependent on the solution pH. The removal mechanism can be attributed to the ion-pair formation at the aluminum hydroxide surface.

Efficient heterogeneous catalytic reduction of perchlorate in water

A new heterogeneous catalyst that promotes the reduction by hydrogen of perchlorate ion in water under mild conditions has been developed. The catalyst is prepared by adsorption of a rhenium(VII) precursor (either ammonium perrhenate or methylrhenium trioxide) onto carbon powder containing 5% palladium by weight. Under standard batch conditions of room temperature, 1 bar of hydrogen, and 200 ppm perchlorate (as HClO4), reduction proceeded to less than 1 ppm in as little as 5 h. Extended reaction times led to residual perchlorate at low parts per billion levels. Chloride was the only observed product, with good material balance. Catalytic materials ranging from 3% to 13% Re showed (pseudo) first-order rates linearly dependent on the Re content. Representative normalized rate constants for catalysts with 5-9% Re were in the range 0.1-0.3 L h(-1) (g of cat.)(-1). Inhibition by chloride was not significant, with little change in perchlorate reduction rate in the presence of excess chloride to 1000 ppm. However, optimal activity occurred in acidic solutions (pH ca. 3), and both the rate and extent of reaction decreased at higher values of pH. In its current form the catalyst might be best applied to destroy perchlorate in the acidic regeneration stream from selective ion exchange columns.