Intensified peroxydisulfate/microparticles-zero valent iron process through aeration for degradation of organic pollutants: Kinetic studies, mechanism and effect of anions

Highlights

• PDS/mZVI/aeration process was used for the first time to generate free radicals.

• Fast decolorization was obtained by PDS/mZVI/aeration at only 10 min.

• mZVI was oxidized to Fe₂O₃ after the process and identified by different methods.

• Carboxylic acids and TOC were monitored during reaction time.

Abstract

Water pollution by organic dyes has been considered by water and wastewater engineers. Several methods have been suggested in literature especially advanced oxidation processes. Peroxydisulfate (PDS)-based processes are a powerful technique for the degradation of organic pollutants. Micro-zero valent iron (mZVI)/aeration is also a simple method for the generation of Fenton reagent. In this study, PDS/mZVI/aeration was used for the first time to degrade Acid Blue 9 (AB9). Operating parameters, various anions, iron release and reaction kinetic were comprehensively investigated. A rapid degradation of AB9 was observed after only 10 min under conditions of PDS = 1.5 mM and mZVI = 0.5 g/L. Pseudo-first order kinetic model was fitted for AB9 degradation with the rate constant of 0.2039 min⁻¹. In contrast with mZVI, zero valent aluminum was ineffective to activate PDS. Scavenging tests demonstrated that sulfate radical (SO₄-) was dominated for the degradation of AB9. Carbonate and nitrite anions almost completely suppressed AB9 degradation. The negative effect of anions on the process was based on this order CO₃²⁻ > NO₂⁻ > I⁻ > H₂PO₄⁻ > Br⁻ > Cl⁻ > NO₃⁻ ≥ SO₄²⁻. Carboxylic acids generated arise from AB9 degradation were monitored. The results showed that further oxidation led to the generation of the small molecules of carboxylic acids. Around 60% of organic carbon of AB9 was mineralized during 120 min reaction. 7.0% of mZVI was released in the solution. FESEM, EDS, and XRD analyses showed that mZVI was oxidized to Fe₂O₃. Finally, PDS/mZVI/aeration showed a simple and efficient process for degradation of organic dye.

Introduction

Dyes are pollutants with large molecules which are considered by environmental scientists due to toxic effect on aquatic environment. Around 7 × 10⁵ tons of dyes are annually consumed in the world in which two-thirds of them are used in textile industry [1,2]. They are often water-soluble compounds in which a small amount of them can produce intensive color in water [3]. Therefore, color production is the first important effect on water resources. They are extremely resistant to biodegradation due to their toxicity for microbes. Biodegradation is a slow process and very sensitive to environmental conditions [1]. Conventional chemical treatments (chlorine oxidation, coagulation and adsorption) are often insufficient for complete removal of dyes and may produce toxic intermediates or sludge, due to their low power in mineralization of organic pollutants. Hence, new chemical process is required to investigate the degradation of organic dyes in aqueous solution [4].

Advanced oxidation processes (AOPs) have been extremely used in the degradation of persistent organic pollutants. AOPs are usually classified based on generated reactive radicals including hydroxyl radical-based AOPs and sulfate radical-based AOPs [[5], [6], [7], [8]]. AOPs produce free radicals with high reactive to oxidize organic contaminants. There are several methods for the generation of both hydroxyl radicals (HO) and sulfate (SO₄^{−, PDS}) radicals. As a conventional method, use of a chemical oxidant has been widely used to produce hydroxyl and sulfate radicals. Hydrogen peroxide (H₂O₂) and peroxydisulfate anion (S₂O₈²⁻) have been applied for this purpose [9,10]. To active chemical oxidant, several methods are suggested to generate free radicals including transition metals, carbon-based catalysts, UV irradiation, ultrasound waves, heat and electrochemical processes [[11], [12], [13]]. Zero valent iron (ZVI) is a benign material which is broadly used in AOPs due to excellent catalytic activity. ZVI is an excellent alternative for ferrous ions for Fenton oxidation, since it can gradually release ferrous ions. Moreover, ZVI is a practical, economic and efficient catalyst for activation of peroxygens [[14], [15], [16]]. Meanwhile, ZVI as an alternative of ferrous ions can gradually release ferrous ions. ZVI has demonstrated high catalytic activity for oxygen to generate hydrogen peroxide and Fenton reagent. In fact, ZVI reduces oxygen molecule to generate hydrogen peroxide and ferrous ion in acidic conditions and forms Fenton reagent for generation of hydroxyl radicals (Eq. 1). In addition, hydrogen peroxide also can be reduced to water and produce ferrous ions to generate hydroxyl radical, as described at Eqs. (2) and (3) [17,18]. ZVI/aeration is a compelling process for degradation of organic compounds. However, the adequacy of free radical production for degradation of refractory organic pollutants is doubtful. Hence, enhanced procedure is needed for more production of free radicals.

$${\text{F}\text{e}}^0+{\text{O}}_2+{2\text{H}}^{+}\to {\text{H}}_2{\text{O}}_2+{\text{F}\text{e}}^{+2}$$

$${\text{F}\text{e}}^0+{\text{H}}_2{\text{O}}_2{+2\text{H}}^{+}\to {2\text{H}}_2\text{O}+{\text{F}\text{e}}^{+2}$$

$${\text{H}}_2{\text{O}}_2+{\text{F}\text{e}}^{+2}+{\text{H}}^{+}\to {\text{H}}_2\text{O}+{\text{F}\text{e}}^{+2}+{\text{H}\text{O}}^{•}$$

ZVI has demonstrated high activity for reaction with PDS. Sulfate and ferrous ions are products of reaction of ZVI with PDS according to Eq. (4). Produced ferrous ions are able to activate PDS for generation of sulfate radicals (Eq. (5)) [19,20].

$${\text{F}\text{e}}^0+{\text{S}}_2{\text{O}}_8^{2-}\to \text{F}{\text{e}}^{2+}+2\text{S}{\text{O}}_4^{2-}$$

$$\text{F}{\text{e}}^{2+}+{\text{S}}_2{\text{O}}_8^{2-}\to {\text{S}\text{O}}_4^{•-}+{\text{F}\text{e}}^{3+}+{\text{S}\text{O}}_4^{2-}$$

The use of system of ZVI/PDS/aeration may be a promising design in terms of activation methods. In this design, ZVI not only produces hydrogen peroxide in the presence of air, but it also activates both hydrogen peroxide and persulfate simultaneously. In fact, there are direct and indirect activations for the production of free radical.

Several studies have demonstrated the function of PDS/ZVI [[21], [22], [23], [24]] and ZVI/aeration [25,26] systems in degradation of organic pollutant in contaminated water. Howbeit, according to literature, no study has been reported PDS/mZVI/aeration system for any pollutants yet. In this work, by focusing on application of PDS/mZVI/aeration for dye degradation, we have tested effect of influential factors on the PDS/mZVI/aeration performance into degradation of organic dye (Acid Blue 9, AB9). Moreover, the effect of some anions was studied on dye removal. The kinetic model of dye degradation was also investigated. The scavenging experiments were considered to determine contribution of oxidative agents. Carboxylic acids generated were monitored during oxidation time. The characteristics of reacted mZVI were also investigated by several techniques.