Composite modification of carbon fiber cathode with tree-like branched polypyrrole-microwires and polyaniline-nanorods for enhancing hexavalent chromium reduction

Cr(vi) reduction to Cr(iii) with electrochemical technique has been regarded as an effective method for the treatment of toxic Cr(vi) contaminated wastewater. Herein, to promote the reactive sites and electron transfer for improving Cr(vi) reduction, we compositely modified the carbon-fiber felt (CFF) with a tree-like branched structure of polypyrrole (PPy) microwires as the first branch and polyaniline (PANI) nanorods as the secondary branch, which increased the BET surface area from 1.2 m(2) per each CFF substrate to 89.8 m(2) per each CFF-PPy-PANI cathode. The flow-through electrochemical reduction device with the CFF-PPy-PANI cathode exhibited 20-70% larger Cr(vi) removal and 20-55% lower energy consumption than that with original CFF, PPy-modified CFF, and PANI-modified CFF cathodes. Electrochemical analyses revealed that the tree-like branched structure of the CFF-PPy-PANI cathode promoted the reactive sites, electron transfer, and interfacial ion diffusion for enhancing Cr(vi) reduction. According to the in situ sampling experiments, the Cr(vi) reduction to Cr(iii) on cathode was the major removal mechanism with minor Cr(vi) adsorption on the anode. The CFF-PPy-PANI cathode at 1.4 V and 500 L h(-1) m(-2) exhibited excellent durability and stability for almost Cr(vi) reduction and 98.8 +/- 0.98% energy efficiency within 5-day continuous operation. These results suggested that the micro-nano PPy/PANI composite modification can be a promising strategy for efficient Cr(vi) reduction.