Electro-assisted sulfite activation by a silica supported cobalt catalyst for the degradation of organic pollutants in near-neutral pH condition

Highlights

• Captured SO₂ reutilization and water remediation were simultaneously achieved.

• An EC/CoSi/sulfite system for efficient sulfate radical production was developed.

• Insights for synergy mechanism of EC/CoSi/sulfite system were provided.

• Influencing factors were investigated and operating conditions were optimized.

Abstract

The desulfurization by-product (e.g. sulfite) treatment and water pollution are two widespread concerns, while the sulfite activation by transition metals can be adopted to solve both of them simultaneously. In this study, a silica-supported cobalt catalyst (CoSi) was used to activate sulfite for the degradation of organic pollutants in an electrochemical system (the EC/CoSi/sulfite system), which demonstrated remarkable water treatment efficiencies due to sulfate radical oxidation. The synergy mechanisms of the EC/CoSi/sulfite system were revealed. (i) The as-prepared CoSi played the dual roles as an electro-adsorbent and a catalyst for sulfite activation. (ii) The electrolytic oxygenation supported the conversion of cobalt species and the chain reactions of oxysulfur radicals. (iii) The anodic oxidation of sulfite also generated oxysulfur radicals. The influencing factors and operating parameters were investigated and optimized systematically. Moreover, the efficient degradation of highly concentrated X-3B and non-selective oxidation of various organic compounds confirmed the promising application of the EC/CoSi/sulfite system in practical water treatment. Overall, the EC/CoSi/sulfite system created new value of the industrial waste (e.g. sulfite) for further application in the environmental remediation.

Introduction

Sulfur dioxide (SO₂) has been regarded as an important air pollutant in recent decades, owing to its easy evolution into sulfuric acid, a major component of acid rain and a critical contributor to aerosol nucleation [1], [2]. Million tons of SO₂ are emitted into air per year, a large fraction of which is produced by the combustion of S-contained fossil fuel in the power plants [3]. The most widely adopted technique to control the emission of SO₂ from the flue gas of power plants is the flue-gas desulfurization (FGD) system due to its high removal efficiency of SO₂ [4]. During the FGD process, the sprayed alkaline slurry, containing limestone, caustic soda, ammonia and other additives, is applied to scrub dissolved SO₂ in water droplets [5]. The captured SO₂ in the form of sulfite species, as the unacceptable by-product, is further oxidized to sulfate compounds by O₂ aeration, which requires extra energy output [6]. However, the sulfite is also a useful resource for chemical industry [6]. In recent years, some transition metals (denoted as M) have been found to efficiently convert sulfite into sulfate, the process of which involves a turnover of M(n) ↔ M(n − 1) (Eqs. 1 and 2) and chain reactions of oxysulfur radicals (Eqs. 3–5) [7], [8]. Among these active intermediates (e.g. oxysulfur radicals), the sulfate radical (SO₄^•−) has been regarded as a strong one-electron oxidant in comparable with HO^• for the organic contaminant degradation [9]. Inspired by the transformation of wastes into resources, the activation of sulfite by some transition metals (e.g. Fe, Cu, Co, Mn, etc.) to produce SO₄^•− can be applied as a promising oxidation technology for the water remediation, including drinking water and industrial wastewater [10], [11].

Turnover of M(n) ↔ M(n − 1):

sulfite + M(n) → M(n) − sulfitecomplex → SO₃^•−+M(n − 1)

M(n − 1)$$\stackrel{{\mathrm{O}}_2}{\to }$$M(n)

Chain reactions of oxysulfur radicals:

SO₃^•−+O₂ → SO₅^•−

SO₅^•−+SO₃²⁻→SO₄^•−+SO₄²⁻

SO₄^•−+SO₃²⁻→SO₃^•−+SO₄²⁻

Compared to the homogeneous activation of sulfite by transition metal ions, the heterogeneous activation can avoid the secondary pollution caused by metallic residues [7], [12]. For instance, the silica-supported transition metal catalysts have exhibited remarkable catalytic activity of activating sulfite for efficient degradation of contaminants [8]. Particularly, the silica-supported cobalt catalysts catch lots of attentions due to the advantages such as excellent catalytic activity, great chemical stability, low leakage of cobalt ions, and easy separation from reaction fluid [13]. Nonetheless, two major drawbacks limited their further application in the practical water treatment. (i) The strict pH range (optimum pH of 9) impeded their application in most water bodies with near-neutral pH because most of surface hydroxyl groups on silica oxides surface is inert [14]. (ii) The rapid depletion rate of dissolved oxygen (DO) by the chain reactions of oxysulfur radicals (Eqs. 3–5) far exceeds the dissolution rate of atmospheric oxygen via the air–water equilibrium [13]. To overcome the above drawbacks, we attempt to apply the electric field in the sulfite activation system, forming an electro-assisted sulfite activation process. This is because (i) the electric field can enhance the connection between the silica-supported cobalt catalysts and reactants by charging these catalyst particles with two sides of which are positive and negative, respectively [15], (ii) the electrolytically produced nascent oxygen can fill the gap of oxygen needed for the production of oxysulfur radicals [16], and (iii) the anodic oxidation of sulfite can possibly generate oxysulfur radicals (i.e. SO₃^•−, SO₄^•− and SO₅^•−) to degrade contaminants [17]. So far, the novel electro-assisted sulfite activation by silica-supported cobalt catalysts for the water treatment has never been studied.

In this work, we aim to use the desulphurization by-products (sulfite) in the water remediation. To be specific, a silica-supported cobalt catalyst is synthesized, and adopted to activate sulfite to degrade organic contaminants in the electric field. As a small demonstration, the dissolved sulfite represents the captured SO₂ from the FGD process and the reactive brilliant red X-3B (X-3B) is selected as a model organic contaminant to investigate the electrochemical reaction process. The specific objectives of this work are: (i) to verify the superior performance of the electro-assisted sulfite activation for organic contaminants degradation; (ii) to identify the involved oxysulfur radicals being responsible for X-3B degradation; (iii) to illustrate the possible synergistic effect among electric field, catalyst, and sulfite; and (iv) to evaluate the potential application of the electrochemical system in practical water treatment by studying influencing factors, degradation of various contaminants, and catalyst durability.