Elsevier

Chemical Engineering Journal

Volume 402, 15 December 2020, 126151
Chemical Engineering Journal

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

https://doi.org/10.1016/j.cej.2020.126151Get rights and content

Highlights

  • 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.

Abstract

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 reactions with the freshly produced amorphous Ni hydroxides, and adsorption on NiO-X/NF electrodes.

Graphical abstract

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.

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Introduction

The development of modern industry is drawing ever-increasing concerns about the contamination of water sources and shortage of energy [1], [2], [3]. Heavy metal ions are considered as a more hazardous pollutants owing to their high toxicity to organisms and difficult biodegradation [4]. As a commonly found heavy metal element, chromium is used extensively in various industries including tanning [5], electroplating [6], metallurgy [7], and pigment production [8], which results in huge amount of effluents containing chromium. In aqueous environment, trivalent chromium (Cr(III)) and hexavalent chromium (Cr(VI)) are the two primary oxidation states [9]. Cr(III) is an essential micronutrient for human being, and it is nearly insoluble in the form of Cr(OH)3 precipitate at neutral or even slightly acidic pH environment [10]. However, Cr(VI) is easily soluble and tremendously harmful to biological systems as carcinogens and mutagens [11]. According to the guideline for drinking-water quality from US Environmental Protection Agency (USEPA), the maximum content level (MCL) of Cr(VI) in drinking water is 0.05 mg L−1 [12], and the allowable limits for total Cr is 0.1 mg L−1 [9], [13]. Dependent on various pH values, Cr(VI) usually exists in different forms in aqueous environment. For example, H2CrO4 predominates at pH less than about 1.0, HCrO4 predominates at pH between 1.0 and 6.0, and CrO42− predominates at pH above 6.0 [14].

Several technologies have been developed for the removal of Cr(VI) from wastewater. Scrap iron and reduced sulfur compounds were often used as reductants to convert Cr(VI) to Cr(III) with subsequent precipitation by adjusting the pH of aqueous solution, achieving a 98% or higher removal efficiency of total Cr [11], [15]. Although chemical precipitation is quite efficient, it suffers the risk of secondary pollution and high cost [15]. Kumar et al synthesized emulsion liquid membrane using rice bran oil and tridodecylamine to extract Cr(VI) from aqueous feed phase, which showed 97% extraction efficiency but weak stability [16]. Adsorption is also a promising technology to remove Cr(VI). Extensive studies have been carried out for exploration of appropriate adsorbents, e.g., modified activated carbon, metal (hydro)oxides and chitosan composites/polymer [17], [18], [19], [20]. Hu et al investigated epicatechin gallate modified ZIF-8 for Cr(VI) removal, which exhibited commendable adsorption capacity (136.96 mg g−1) and reduction performance (96%) [21]. However, its long-term and practical applications are limited by the recovery and reuse of adsorbent. Other technologies were also investigated, e.g., ion exchange [22], reverse osmosis [23], and photoreduction [24]. Among them, the electrochemical reduction technology is predominant for its cost-effective and efficient removal of Cr(VI) at ambient conditions without any supplementary chemical reductants. For example, Sriram et al. [25] synthesized titania nanotubes for the electrochemical reduction of Cr(VI) with alkaline urea as an anolyte additive, and the removal rate was up to 97% at 5 V within 15 min. Liu et al. [26] prepared the electrode of stainless steel nets coated with single wall carbon nanotubes to convert Cr(VI) to Cr(III), and 97% of Cr(VI) was reduced at 2.5 V within 120 min. Although the electrochemical reduction of Cr(VI) to Cr(III) is easily achieved at acidic condition, the acidic condition is not favorable for the formation of Cr(OH)3 precipitate to separate Cr from water, while high levels of Cr(III) had been found hazardous to human health to some extent [27] and there is always the risk of its re-oxidization to Cr(VI).

As one of the most effective electrochemical approaches, electrocoagulation could reduce Cr(VI) to Cr(III) at the cathode and the simultaneous water electrolysis to hydrogen could result in the progressive increase of the solution pH without adding extra alkaline solution, which is conducive to the formation of Cr(OH)3 for the successful removal of Cr from treated solution. In the meantime, the flocculant in-situ generated by the electrochemical dissolution of the sacrificial anode is beneficial for the adsorption and complexation of suspended particles and heavy metal ions [28]. Maitlo et al used iron-air fuel cell electrocoagulation to remove Cr(VI) cost-effectively, in which iron was used as sacrificial anode for the in-situ production of iron hydroxides [29]. The parameters affecting the removal efficiency were optimized to reach a final maximum removal efficiency of 100% in 4 h. Moradi et al combined electrocoagulation and photoreaction to accelerate the treatment of tannery wastewater containing Cr(VI), the reduction of Cr(VI) in UVC/VUV photo-reactor containing extra S2− was followed by the precipitation of Cr(OH)3 with Al(OH)3 generated from Al anode in pre-treatment process [30]. Hydrogen is believed to be an ideal alternative to fossil fuels owing to its high energy density and zero pollution [31]. Thus, the simultaneous conversion and removal of Cr(VI) and hydrogen evolution through redox reactions by electrocoagulation is a promising approach to achieve resource utilization of Cr(VI) containing wastewater.

In this study, 3D hierarchically porous NiO/Ni electrodes were synthesized by the deposition of NiO nanosheets onto Ni foam (NF) substrate through a facile hydrothermal process followed by the annealing process in different atmospheres. NiO has a high isoelectric point (IEP) of about 10.3, which makes it applicable to absorb Cr groups (HCrO4 or CrO42−) and OH anions [32], [33]. The growth of NiO nanosheets on NF increased the surface area of Ni foam and supplied a large number of adsorption sites for Cr(VI) anion and OH, which consequently enhanced the removal of Cr(VI) and formation of Ni(OH)2 flocculant with respect to conventional bare metal plate electrode. Moreover, different annealing atmospheres induced different Ni3+ content in NiO, further affecting the adsorption performance. Platinum foil was chosen as the cathode because it is a well-known benchmark electrocatalyst for the hydrogen evolution reaction (HER) [34]. The influence of annealing atmosphere, solution pH, applied potential, and coexisting ions on the Cr(VI) removal efficiency was investigated in details. Hydrogen generated at the cathode was collected to determine its production rate. Precipitates and electrodes after reaction were analyzed to explore the Cr(VI) removal mechanism. Simultaneous removal of heavy metal ions and hydrogen generation is the promising strategy in environmental remediation.

Section snippets

Chemical and materials

Nickel acetate tetrahydrate, ammonium fluoride, urea, potassium dichromate, diphenyl carbamide, ethyl alcohol absolute, sodium nitrate, sodium carbonate monohydrate, and sodium sulfate anhydrous were purchased from Tianjin Fuchen Chemical Reagent Factory, China. Nickel foam (99.99% purity, 1.5 mm thick) was purchased from Kunshan Jiayisheng Electronics Co., Ltd, China. Hydrochloric acid, sulfuric acid and phosphoric acid were supplied by Xilong Scientific Co., Ltd, China. Zinc nitrate

Morphology and structure of NiO-X/NF samples

Fig. 1 shows XRD patterns of as-prepared Ni(OH)2/NF and NiO-X/NF samples. All samples exhibit the same characteristic diffraction peaks at 2θ of 44.5° and 51.8°, corresponding to (1 1 1) and (2 0 0) facets of Ni foam substrate (JCPDS 04–0850), respectively. The diffraction peaks match well with the hexagonal phase of α-Ni(OH)2 (JCPDS 38–0715), as clearly identified in the XRD pattern of the Ni(OH)2/NF sample. For NiO-Ar/NF, NiO-Air/NF, and NiO-O2/NF samples, identical diffraction peaks at 2θ of

Conclusions

3D hierarchically porous NiO-X/NF electrodes were synthesized through a facile hydrothermal method followed by subsequent annealing treatment under difference atmospheres, and were used for removal of Cr(VI) from water with a simultaneous hydrogen generation. The NiO-O2/NF electrode exhibited better electrochemical activities for Cr(VI) removal and hydrogen generation because of its faster adsorption of OH and Cr(VI) anions as a result of its higher Ni3+/Ni2+ ratio. It achieved a high removal

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The financial support is gratefully acknowledged from the National Natural Science Foundation of China (NSFC) (21878257 and 51402209), the Natural Science Foundation of Shanxi Province (201701D221083), the Key Research and Development Program of Shanxi Province (201803D421079 and 201803D31042), the Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi (2016124 and 2019L0156), the Shanxi Provincial Key Innovative Research Team in Science and Technology (

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