Toxic metal ions, such as Ni2+ and Mn2+, in industrial waste streams are nonbiodegradable and can cause damage to the human body. Electrochemical cleaning techniques are attractive as they offer more control and produce less sludge than do chemical/biological approaches without the high pressures needed for membranes. Here, nanoneedle-structured α-MnO2/carbon fiber paper (CFP) composites were synthesized by a hydrothermal approach and used as electrodes for combined electroadsorption and capacitive deionization removal of nickel and manganese ions from pseudoindustrial waste streams. The specific performance of α-MnO2/CFP (16.4 mg Ni2+ per g of active material) not only shows a great improvement in comparison with its original CFP substrate (0.034 Ni2+ mg per g), but also is over 6 times that of activated carbon (2.5 mg Ni2+ per g). The high performance of α-MnO2/CFP composites is attributed to their high surface area, desirable mesoporosity, pore-size distribution that permits the further access of ions, and their property as a pseudocapacitor, which contributes to a more efficient electron/charge transfer in the faradic process. Unfortunately, it was also found that some Mn2+ ions are released due to the partial reduction of MnO2 when operated as a negative electrode. For the removal of Mn2+ ions, an asymmetric arrangement, consisting of a MnO2/CFP positive electrode and an activated carbon negative electrode, was employed. This arrangement reduced the Mn2+ concentration from 100 ppm to less than 2 ppm, a vast improvement over the systematical two-activated carbon electrode system that could only reach 42 ppm under the same conditions. It was also observed that as long as the MnO2/CFP composite was maintained as a positive electrode, it was completely stable. The technique was able to reduce both Ni2+ and Mn2+ ions to well below the 10 ppm requirement for discharge into public sewers in Singapore.
Keywords: activated carbon; carbon fiber paper; electrosorption; nickel and manganese ions removal; α-MnO2.