3D hierarchically porous NiO/NF electrode for the removal of chromium(VI) from wastewater by electrocoagulation

• Published in: Chemical engineering journal (Lausanne, Switzerland : 1996) Switzerland : 1996), 2020-12, Vol.402, p.126151, Article 126151
• Main Authors: Wang, Zhifei, Shen, Qianqian, Xue, Jinbo, Guan, Rongfeng, Li, Qi, Liu, Xuguang, Jia, Husheng, Wu, Yucheng
• Format: Article
• Language: English
• Subjects: Annealing atmosphere; Electrocoagulation; HER; Hexavalent chromium; NiO/NF
• ISSN: 1385-8947; 1873-3212
• DOI: 10.1016/j.cej.2020.126151

In this work, three-dimensional hierarchical porous NiO/NF composites were fabricated by hydrothermal method combined with subsequent annealing process. First, nickel hydroxide (Ni(OH)2) nanosheets were grown vertically on flexible and porous nickel foam (NF) to form Ni(OH)2/NF. Second, three-dimensional porous NiO/NF were obtained by thermal decomposition of Ni(OH)2/NF in different atmosphere containing various oxygen content (Ar, Air, O2) at 400 °C for 3h, accompanied by the emergence of numerous mesopores. It is found that O2-annealed NiO/NF possesses more Ni3+ and exhibiting superior adsorption performance for Cr(VI)/OH− anions. Furthermore, NiO-X/NF(X represents annealing atmosphere) was used as anode to remove Cr(VI) accompanied with hydrogen generation by electrocoagulation. Compared with bare nickel foam, the presence of NiO nanosheets promote the generation of Ni(OH)2 flocculant further accelerate Cr(VI) removal and H2 production. Besides, the NiO/NF exhibited excellent removal performance for other coexisting metal ions. Removal mechanism was investigated and ultimately demonstrated three routes consist of reduction of Cr(VI) to Cr(III) at the cathode followed by the coprecipitation with Ni(OH)2 floc in the form of Cr(OH)3, direct complexation reactions with the freshly produced amorphous Ni hydroxides, and adsorption on NiO-X/NF electrodes. [Display omitted] •Three-dimensional hierarchically porous NiO/NF composites were fabricated.•O2-annealed NiO/NF enhances the content of Ni3+ promoting the adsorption of Cr(VI).•NiO/NF exhibits excellent Cr(VI) removal efficiency via electrocoagulation.•Simultaneous Cr(VI) removal and hydrogen generation achieve resource utilization. 3D hierarchically porous NiO/NF (Ni foam) was synthesized by a facile hydrothermal method and a subsequent annealing process in different atmospheres for the simultaneous Cr(VI) removal and hydrogen generation from wastewater by the electrocoagulation method. O2 atmosphere for annealing increased the Ni3+ content in NiO, thus led to an enhanced electrostatic interaction with Cr(VI)/OH− anions and subsequently increased the Cr(VI) adsorption and Ni hydroxide flocculant production. Thus, the NiO-O2/NF electrode exhibited the best Cr(VI) removal performance of 99.5% within 20 min at the applied potential of 0.97 V vs. RHE, along with a hydrogen generation rate of 1.1 mmol g−1 h−1. The removal mechanism was suggested, including the reduction of Cr(VI) to Cr(III), direct complexation