Synergistic enhancement of Fe³⁺ coupling with Cu⁰ activated peroxodisulfate: Performance and mechanisms

Highlights

• The degradation effect of AOII was strongly enhanced in the Fe³⁺/Cu⁰/PDS system.

• The redox cycle of Fe³⁺/Fe²⁺ was strengthened in the Fe³⁺/Cu⁰/PDS system.

• SO₄^•− and ^•OH were identified as the primary reactive oxidants in the Fe³⁺/Cu⁰/PDS system.

Abstract

Fe²⁺ activated peroxodisulfate (Fe²⁺/PDS) system has been widely studied for its efficient degradation of pollutants. However, the slow circulation of Fe³⁺/Fe²⁺ greatly restricted the application of the system. In the study, AOII was significantly degraded by using Fe³⁺ coupled with Cu⁰ activate PDS system (Fe³⁺/Cu⁰/PDS system). Compared with the traditional Fe²⁺/PDS system, the Fe³⁺/Cu⁰/PDS system greatly improved the degradation of AOII and the decolorization rate reached 80.6 % within 30 min. The quenching experiments and electron spin resonance (EPR) showed that both hydroxyl radical (•OH) and sulfate radical (SO₄^•−) were the mainly primary reactive oxygen species in the Fe³⁺/Cu⁰/PDS system with the contribution rates of SO₄^•− and •OH being 60.54 % and 25.06 % respectively. In the Fe³⁺/Cu⁰/PDS system, Cu⁰ formed Cu⁺ through H⁺ corrosion and PDS/O₂ oxidation. Then, Cu⁺ oxidized by O₂ to produce H₂O₂ and Fe³⁺ was reduced by Cu⁰ and Cu⁺ to form Fe²⁺. Finally, Cu⁺ and Fe²⁺ catalyzed PDS to generate SO₄^•−, and •OH was yield by the process of Cu⁺ and Fe²⁺ induced H₂O₂ and the reaction of OH⁻/H₂O and SO₄^•−. Common water matrices such as NO₃⁻ and SO₄²⁻ had no effect on the degradation, while Cl⁻ promoted the degradation, and CO₃²⁻ and fulvic acid inhibited the degradation of AOII. Over 68.5 % of AOII was degraded in the experimental water samples and the Fe³⁺/Cu⁰/PDS system had great degradation performances in practical water sample of other contaminants. Moreover, the final discharge of total dissolved copper was <1 mg/L after sediment and filtering.

Introduction

As a strong oxidant, Peroxodisulfate (PDS) (E₀ = 2.01 V) was widely used to treat organic pollutants such as dyes, drugs, antibiotics, and personal care products due to its good stability, convenient storage, easy transportation, and low cost [1], [2]. Some researchers have reported that PDS could be activated in various ways such as Uv, heat, thermal, alkaline, microwave and reducing agents via Eq. (1) [3], [4], [5], [6], [7], [8], [9], [10], [11]. But the high cost and relatively slow reaction rates of these methods limited their practical application in water treatment. Considering these problems, the way that the transition metal ions activated PDS to produce SO₄^•− (Eq. (2)) was attracting more and more attention of researchers owing to its high activation efficiency and low cost [12], [13], [14].

$${\mathrm{S}}_2{\mathrm{O}}_8^{2-}\stackrel{\mathrm{Uv}/\text{heat}/\text{thermal}/\text{alkaline}/\text{microwave}/\text{others}}{\to }\mathrm{S}{\mathrm{O}}_4^{•-}$$

$${\mathrm{Fe}}^{2+}+{{\mathrm{S}}_2{\mathrm{O}}_8}^{2\_}\to \mathrm{S}{\mathrm{O}}_4^{•-}+{\mathrm{Fe}}^{3+}\mathrm{K}=12{\mathrm{M}}^{-1}{\mathrm{S}}^{-1}$$

$${\mathrm{Fe}}^0+2{\mathrm{H}}^{+}\to \mathrm{F}{\mathrm{e}}^{2+}+{\mathrm{H}}_2$$

Among many transition metals, ferrous ion (Fe²⁺) is considered as one of the best catalysts to stimulate PDS because of its significant effect and abundant properties. Unfortunately, the process of Fe²⁺-activated PDS also faces some problems, such as the production of large concentrations of iron mud and the extremely slow reduction process of Fe³⁺ to Fe²⁺ [15]. These defects limited the application of the Fe²⁺/PDS system in water treatment. In recent decades, it was reported that adding reducing agents in the Fe²⁺/PDS system (e. g. hydroxylamine, thiosulfate, ascorbic acid and L-cysteine) could promote the removal of organic pollutants via accelerating the recycle of Fe³⁺/Fe²⁺ [4], [16], [17]. Although adding reducing agents could promote the recycle of Fe³⁺/Fe²⁺, the reactive oxidant in the reducing agent/Fe²⁺/PDS system could be captured by these reductants, which reduced the degradation of the target contaminants and the utilization of PDS [18], [19]. It was also reported that electrification could promote the recycle of Fe³⁺/Fe²⁺ [20], but the application of this method in PDS activation was limited due to its high cost and low power utilization. Based on the above facts, some researchers proposed that zero-valent iron (Fe⁰) catalyzed PDS to generate reactive oxidants because of continuous generation of Fe²⁺ through hydrogen (H⁺) corrosion via Eq. (3). However, the release of Fe²⁺ on the surface of Fe⁰ is inhibited due to the passivation and aggregation of Fe⁰, which greatly reduced the activation efficiency [21], [22]. Therefore, it is necessary to adopt a new approach to promote the circulation of Fe³⁺/Fe²⁺ in the Fe²⁺/PDS system. Considering the above situations, Cu⁰ might be a fine choice for promoting the recycle of Fe³⁺/Fe²⁺, because Cu⁰ could avoid the passivation and coagulation phenomenon on the surface, and both Cu⁰ and Cu⁺ promoted the circulation of Fe³⁺/Fe²⁺ [23], [24]. Moreover, some recent studies indicated that Cu⁰ could activate PDS to produce reactive oxidants through H⁺ corroding Cu⁰ to generate Cu⁺ [14], [25]. However, the study on the activation of PDS by using Fe³⁺ coupled with Cu⁰ has not been reported until now. In the past years, the environmental impact of synthetic dyes was a worrying problem, and the pollution caused by dyes wastewater was attracting more and more public attention [26], [27], [28]. As a representative azo dye, acid orange II (AOII) was chosen as the target compound to study the synergistic strengthening effect of the Fe³⁺/Cu⁰/PDS system. The study-related details of this article were as follows: (i) the enhancement of AOII decolorization rate in the Fe³⁺/Cu⁰/PDS system; (ii) the identification of reactive oxidants in Fe³⁺/Cu⁰/PDS system; (iii) the strengthened-mechanism of Fe³⁺/Cu⁰/PDS system; (iv) the optimization of common reaction factors in the Fe³⁺/Cu⁰/PDS system; (v) implication evaluation.