Elsevier

Catalysis Today

Volume 375, 1 September 2021, Pages 514-521
Catalysis Today

Electrochemical Regeneration of Free Chlorine Treated Nickel Oxide Catalysts for Oxidation of Aqueous Pollutants

https://doi.org/10.1016/j.cattod.2020.03.045Get rights and content

Highlights

  • Chlorinated nickel oxide was electrochemically regenerated for catalytic oxidation.

  • (Energy) Efficiency was compared for intermittent and continuous regeneration.

  • Electron transfer from aqueous pollutants to Ni3+on NiOx(OH)y was discussed.

  • Performance was assessed in sequencing batch tests and in real wastewater matrix.

Abstract

Nickel oxide upon free chlorine treatment has been used as a heterogeneous oxidant to eliminate aqueous organic pollutants, where oxygenation or chlorination could regenerate the oxidation capacity. In order to overcome problems associated with low regeneration efficiency and undesirable chloride ion generation, we herein investigate electrochemical regeneration for sustainable catalytic usage of the free chlorine treated nickel oxide, obviating a dosage of chemicals. Scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy (XPS) characterized drop-casted catalysts with thermal decomposition to be NiO nanoparticles with amorphous nickel oxyhydroxide shells generated by free chlorine treatment. Along with an elevated open circuit potential of the active sites (NiOx(OH)y) near 0.75 V NHE (at pH 14), the chlorination was found to enhance the electrical conductivity of the catalysts. Galvanostatic anodization at current density < 4 mA/cm2 effectively restored the oxidizing power for repetitive degradations of 4-chlorophenol (4-CP). Despite greater conversion under the continuous anodization, periodic regeneration (10/60 on/off) showed far lower energy consumption (44.6 ± 3.0 kW h/kg 4-CP) for 4-CP removal. The changes in XPS and X-ray absorption near edge structure (XANES) spectra, coupled with kinetic and electrochemical analyses, demonstrated that electron transfer towards Ni3+ with H abstraction would be the primary degradation mechanism. The stability and versatility were assessed by sequencing batch cycles and degradation of 2,4-dichlorophenol, carbofuran, and oxalic acid. Application in a real phenolic wastewater matrix noted 98% and 79% removal of chemical oxygen demand and total organic carbon after 12 h.

Introduction

In many developed areas, diverse issues on industrial wastewater tend to increase for more severe water pollution problems due to toxic and refractory pollutants. In parallel, wastewater effluent discharge standards continue to be more stringent to require efficient wastewater treatment (WWT) technologies. To this end, variable advanced oxidation processes (AOPs) employ chemical reagents and energy (light or electricity) to generate reactive oxygen species (ROS) such as ∙OH for oxidation of aqueous organic pollutants. Compared to homogeneous chemical sources of ROS (e.g., O3, H2O2), heterogeneous (metal or metal oxide) oxidants would have advantages in terms of recovery and reuse. Earth-abundant nickel oxide materials, especially in combination with free chlorine treatment, have shown to possess surface bound active oxygen to be employed in heterogeneous AOPs in WWT [[1], [2], [3], [4]]. Nickel oxide would have several merits, such as operability in ambient pressure and temperature [5,6], relatively large specific surface area [7] and high surface active oxygen concentration to eliminate organic pollutants (active O ∼ 8%) [2]. For a sustainable usage of nickel oxide oxidants, operation in alkaline wastewater (pH > 8) would minimize the dissolution of the nickel component.

The oxidizing ability of the free chlorine treated nickel oxide was first reported by Nakagawa et al. in 1962 [8], where the nickel ions were oxidized by free chlorine to precipitate in nickel oxide powder with a given oxidation capacity. Since then, the free chlorine treated nickel oxide was employed in organic chemistry as an oxidizing agent for alcohols [8], amines [9], phenols [10], thiols [11], sulfur compounds [12], nitriles [13], hydrazine [14] among others. For synthesis of organic compounds, in particular, the limited oxidation capacity was utilized for partial oxidation; e.g. alcohols to carboxylic acids and amines to azo compounds [7,15]. On the other hand, employment of analogous materials for WWT was proposed by St. Christoskova et al. in 2000 [1]. In subsequent reports, the nickel oxide was utilized as a heterogeneous oxidation catalyst for treating aqueous pollutants, including sulfide ions [1], phenolic wastewaters [2], 4-chlorophenol [3], methylene blue [16], and 2,4-dichlorophenol [4]. Aiming at total degradation of pollutants to CO2 (i.e. complete oxidation or incineration) in WWT, regeneration (activation) was necessary to maintain the catalytic oxidation cycles. In other words, disabled nickel oxide catalysts could be reoxidized to restore ability to decompose aqueous pollutants. In previous studies, activation/regeneration of the nickel oxide catalysts was achieved by periodic or continuous injection of O2 (air) or free chlorine. Increased dissolved oxygen concentration under O2 bubbling could moderately enhance the rate of organic compounds degradation [[1], [2], [3]], although full restoration of the initial oxidation capacity was not possible. In comparison, stronger oxidants such as free chlorine could completely reactivate the catalyst [17] so that commercial nickel oxide catalysts (in pellet form) could be utilized for actual WWT under continuous free chlorine injection [18]. However, added free chlorine inevitably generated considerable amounts of chloride ions in effluent, limiting the effluent reuse due to total dissolved solids concentration and potential eco-toxicity. Therefore, a more sustainable and environment friendly method is required for regeneration of the free chlorine treated nickel oxide catalysts with a minimal dosage of chemicals.

This study investigated electrochemical activation to regenerate the free chlorine treated nickel oxide catalysts without injection of chemical reagents. Intermittent or continuous galvanostatic anodization was found to effectively regenerate the oxidation capacity of the catalysts, outperforming the oxygenation for degradation of an array of organic pollutants. The working mechanism of the catalysts was examined based on the change in physicochemical characteristics in combination with pollutant degradation kinetics and electro-analysis. Practical applicability of the free chlorine treated nickel oxide catalysts under the electrochemical regeneration regime was further corroborated based on sequencing batch operation and application in a real industrial wastewater matrix.

Section snippets

Fabrication of Catalysts

Immobilized nickel oxide catalysts were prepared in order to allow anodic regeneration. 2 × 4 cm2 sized Ti foil (Alfa aesar), as a base substrate, was pretreated by sandpaper polishing and etching in 36.5% HCl solution at 80 °C for 0.5 h. Nickel precursor solution (1 M Ni(NO3)2∙6H2O, Alfa Aesar) was drop-casted on both sides of the pretreated Ti plate to be subsequently dried at 120 °C for 5 minutes and annealed at 450 °C for 5 minutes. These processes were repeated for 12 times before final

Physicochemical Characteristics of the Catalyst

Fig. 1a shows horizontal surface images of the nickel oxide catalyst observed by SEM, where a substantial number of cracks with a few micrometer thickness were noted due to the thermal expansion of metal oxide catalysts. Comparison of the samples before and after the chlorination indicated marginal difference in the morphology. On individual catalysts islands, distributed nanoparticles with 60 to 120 nm diameter were observed. The surface morphologies in terms of nanoparticle aggregation rather

Conclusion

Herein, we investigated feasibility of free chlorine treated nickel oxide catalysts combined with electrochemical regeneration for wastewater treatment. The chlorinated catalyst showed elevated electrical conductivity and oxidizing power with prominent fraction of Ni3+. The anodic potential bias more effectively reactivated disabled catalysts, compared to oxygenation utilized in previous reports. Trade-off between conversion and energy consumption was noted for intermittent versus continuous

CRediT authorship contribution statement

Seok Kim: Investigation, Methodology, Writing - original draft. Jin Soo Kang: Investigation. Seoni Kim: Methodology. Seongmin Kang: Resources. Yung-Eun Sung: Project administration. Kangwoo Cho: Writing - review & editing, Supervision, Funding acquisition. Jeyong Yoon: Conceptualization, Supervision.

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

This work was financially supported by Basic Research Laboratory (NRF-2018R1A4A1022194), Young Researcher Program (NRF-2019R1C1C1003435), and Nano Material Technology Development Program (NRF-2016M3A7B4908161) through the National Research Foundation of Korea. The experimental work was partly supported by Technology Innovation Program (10082572) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea), and the Korea Ministry of Environment as “Global top project” (Grant number:

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