Assortment of magnetic nanospinels for activation of distinct inorganic oxidants in photo-Fenton’s process

https://doi.org/10.1016/j.molcata.2015.03.009Get rights and content

Highlights

  • Well characterized magnetic nanospinels were used in photo-Fenton reaction.

  • Photo-Fenton activity was influenced by inorganic oxidant and type of ferrite used.

  • Peroxymonosuphate was active at neutral pH even at 4 times less concentration.

  • Ferrites were magnetically recoverable hence, were recycled after the reaction.

Abstract

Magnetic spinel ferrites in the nano range, having general formula MFe2O4 (M = Cu, Zn, Ni and Co) were prepared using facile sol–gel methodology. The physicochemical properties of prepared ferrites characterized using FTIR, powder XRD and HRTEM stipulated formation of single phase cubic spinel structure. The band gap of all the ferrites was found in the range 1.26–2.08 eV, consequently could be photo-excited by visible light. The present study evaluates the capacity of all the synthesized ferrites to activate different inorganic oxidants i.e., hydrogen peroxide (H2O2), potassium peroxodisulphate (K2S2O8), potassium peroxymonosulphate (KHSO5) and potassium bromate (KBrO3) in photo-Fenton process. The results disclosed fascinating behavior of peroxymonosulphate as it could be stimulated at neutral pH while for all other oxidants, acidic medium was required. Furthermore, the concentration of peroxymonosulphate during the reactions was 4 times less than other oxidizing agents. The degradation rate was found to decrease in order H2O2 > KHSO5 > K2S2O8 > KBrO3. When H2O2 and KHSO5 were used the reactivity of ferrites followed the order CuFe2O4 > ZnFe2O4 > NiFe2O4 > CoFe2O4 and CoFe2O4 > CuFe2O4 > NiFe2O4 > ZnFe2O4, respectively. The prepared materials were chemically stable and magnetically recoverable after the reaction hence, was reused without any significant loss in activity.

Introduction

In the recent scientific progress, intensive research is being focused on development of nanomaterials that have fusion of essential aspects such as excellent reactivity, cost-effectiveness and environmentally benign mode of recovery. In this regard, magnetic nanomaterials have attracted steadily growing attention. The inherent magnetic nature and insolubility of these materials in most solvents favors easy and rapid catalyst separation from the reaction system [1]. These qualities make magnetic nanoparticles sustainable and a potential green catalyst.

Among different magnetic nanoparticles, spinel ferrites (MFe2O4, M = Co, Ni, Cu, Zn, Mn etc.) are one of the most important and interesting oxides that can play a dual role of a catalyst as well as magnetically separable entity [2]. These materials have narrow band gap which makes it an effective catalyst under visible light [3]. Besides the significant merit of easy magnetic recovery from the reaction system, these materials even offer advantage of being chemically stable and cheap; hence, can serve as an efficient photocatalyst.

In the last few decades treatment of wastewater containing recalcitrant contaminants using advanced oxidation processes (AOPs) has gained considerable attention. Among the different AOPs reported in literature, Fenton’s process has been successfully applied for degradation of miscellaneous recalcitrant organics. This process involves reaction of Fe(II) with an oxidant like H2O2 to generate hydroxyl radicals (HOradical dot) which are capable of attacking harmful organic contaminants present in water, thereby, mineralizing them into harmless CO2 and H2O [4]. The role of inorganic oxidant in the Fenton’s process is of utmost importance as the reactive species produced in the process depend on type of oxidizing agent used. Nowadays, the use of sulphate radical based AOPs using oxidants such as persulphate and peroxymonosulphate for chemical mineralization of different organic pollutants is being explored.

The use of ferrites as heterogeneous photofenton catalyst for the degradation of different organic pollutants using H2O2 as oxidant has been investigated by different researchers [5], [6], [7], [8]. Potassium persulphate (PDS) is an alternative oxidant in Fenton’s process that has recently been examined by authors for the degradation of various pollutants. Rodrigrez et al. used persulphate activated by Fe2+, Cu+, Co2+, Mn2+ etc., for the degradation of different pharmaceutical compounds [9]. Ahmed and Chiron also decontaminated wastewater effluent from pharmaceutical residues by using persulphate as an oxidant in photofenton process under solar light. Another oxidizing agent which is less explored in Fenton’s process is potassium bromate (KBrO3) [10]. Despite the promising results of all the oxidizing agents in Fenton’s process, major drawback is pH adjustment in acidic range prior to reaction. Therefore, use of oxidant which can work at neutral pH is a great challenge.

Potassium peroxymonosulphate (PMS) is unsymmetrical peroxide that has very strong oxidizing strength to mineralize organic pollutants in water. It can work in a wide pH range hence, is better over other oxidants. Ji et al. used porous Fe2O3 nanoparticles for the activation of PMS to degrade RhB dye [11]. Besides this, limited literature is available on use of ferrites for degradation of organic pollutants in the presence of PMS. CuFe2O4 magnetic nanoparticles have been used by authors to activate PMS for the degradation of different organic contaminant [12], [13].

The above discussion clearly illustrates the importance of oxidizing agents in AOPs. To date, there are very few reports available on the comparative use of different oxidants in Fenton’s process. Considering all the facts, present work explores the effect of different inorganic oxidants such as hydrogen peroxide (H2O2), potassium peroxodisulphate (K2S2O8), potassium peroxymonosulphate (KHSO5) and potassium bromate (KBrO3) activated by spinel ferrites MFe2O4 (M = Cu, Ni, Co and Zn) on the degradation of Remazol Black 5 (RB5) which was used as a model textile dye contaminant. The assessment of different operating variables such as catalyst loading, pH, oxidant dosage and light on photo-oxidative remediation of textile dye in the presence of all the ferrites was explored in detail. The main focus of this study was to scrutinize a single catalyst out of all the chosen ferrites for the stimulation of different oxidants. The most crucial feature of a sustainable and green catalyst is its feasible recovery and recyclability. In order to illustrate this, reusability of catalyst was also studied.

Section snippets

Chemicals

All the chemicals used during the experiments were available commercially and used without further purification. Ferric nitrate (Fe(NO3)3·9H2O, 98%), cupric nitrate (Cu(NO3)2·3H2O, 99.5%), nickel nitrate (Ni(NO3)2·6H2O, 98%), zinc nitrate (Zn(NO3)2·6H2O, 96%), cobalt nitrate (Co(NO3)2·6H2O, 99%) citric acid (99.57%) and hydrogen peroxide (30% w/v) were obtained from Fisher Scientific. Ethylene glycol (99%), potassium bromate (99%), and potassium persulphate (>99%) were supplied by Merck. Oxone

Thermogravimetric analysis (TGA)

Thermogravimetric curves were used to study the thermal stability of as synthesized ferrite samples which is based on weight loss as a function of temperature. The TGA curves for all the ferrites as shown in Fig. A1 (Supporting information) suggested that the thermal degradation of all the samples occurred in two steps. The first step in the temperature range of 50–100 °C was attributed to loss of water from the sample while the second degradation step in the range 300–400 °C was owed to

Conclusions

In the present study, degradation of RB5 using ferrite activated different inorganic oxidants i.e., HP, PMS, PDS and PB was successfully investigated in detail. The rate of degradation was influenced by type of oxidant used. HP, PDS and PB gave maximum efficiency at similar reaction conditions i.e., [ferrite]  0.5 g/L, pH 2.5 and [oxidant]  8.8 mM while, amount of PMS required was 2.2 mM and degradation occurred in a wide pH range of 2–10. The experimental results showed CuFe2O4 to be best catalyst

Acknowledgments

The authors are highly grateful to DST (SERB) and Council of Scientific and Industrial Research (CSIR) for providing the necessary financial support. The authors are also thankful to Kunash Instruments Pvt. Ltd. Thane (W), for doing the surface area analysis.

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