Elsevier

Journal of Hazardous Materials

Volume 262, 15 November 2013, Pages 472-481
Journal of Hazardous Materials

Reactivity of NiO for 2,4-D degradation with ozone: XPS studies

https://doi.org/10.1016/j.jhazmat.2013.08.041Get rights and content

Highlights

  • Metal oxides (TiO2, SiO2, Al2O3 and NiO) reactivity for 2,4-D catalytic ozonation.

  • Highest activity of NiO.

  • Detection of surface nickel–oxalate complex by XPS.

  • Molecular ozone, OH radicals and surface complexes facilitate mineralization.

  • Dechlorination is one of the stages involved in the decomposition of 2,4-D.

Abstract

2,4-Dichlorophenoxyacetic acid (2,4-D) is usually used as a refractory model compound that requires a prolonged reaction time for mineralization. In this study, we found that nickel oxide (NiO) significantly improved 2,4-D degradation and mineralization in reaction with ozone. Other metal oxides, such as titania, silica and alumina, were also tested in this reaction, so that, the mineralization degree was almost the same for all of them (ca. 25%), whereas NiO showed more than 60% in 1 h. These outstanding results led us to study in more depth the role of NiO as catalyst in the degradation of 2,4-D. For instance, the optimum NiO loading amount was 0.3 g L−1. The catalytic ozonation showed a high stability after three reaction cycles. With the aim of identifying the surface species responsible for the high activity of NiO, besides knowing the byproducts during the degradation of 2,4-D, XPS and HPLC were mainly used as analytical tools. According to the results, the mineralization of 2,4-D was directly influenced by the adsorbed chlorate organic compounds and oxalate group onto NiO. Therefore, NiO plays a true role as a catalyst forming surface compounds which are subsequently decomposed causing an increase in the mineralization efficiency. In addition, it was possible to identify several degradation byproducts (2,4-diclorophenol, glycolic, fumaric, maleic and oxalic acids) that were included in a rational reaction pathway. It was proposed that 2,4-D elimination in presence of NiO as catalyst is a combination of processes such as: conventional ozonation, indirect mechanism (radical dotOH) and surface complex formation.

Introduction

Catalytic ozonation with metal oxides has shown to enhance the degradation of recalcitrant compounds in water. Generally, two mechanisms are proposed during catalytic ozonation: (i) ozone decomposition on the catalyst surface enhancing hydroxyl radical (radical dotOH) generation, where the hydroxyl groups were active sites and (ii) forming surface complexes between the pollutants and the surface metal sites of the catalysts, which renders the coordinated pollutants more reactive than conventional ozone.

In recent years, several researchers have found that the degradation of organic compounds followed the radical dotOH oxidation pathway in the ozonation combined with heterogeneous catalysts, using nano-TiO2 [1], ceramic honeycomb [2], [3], Mn-ceramic honeycomb [4], synthetic goethite [5], and so on. It is believed that when introduced into water, metal oxides tend to strongly adsorb H2O molecules. The adsorbed H2O dissociates to OHand H+, forming surface hydroxyl group with the surface metal and oxygen sites, respectively [6]. Therefore, ozone can react with the surface-bound OH, initiating the production of radical dotOH on the surface of the manganese dioxide [7]. Furthermore, it is also reported that dissolved ozone adsorbs into the catalyst's surface and subsequently decompose rapidly due to presence of adsorbed hydroxyl groups on the surface of aluminum oxide [8]. Specifically, the experimental results from Ma et al. using FeOOH and ceramic honeycomb impregnated with Mn and Cu showed that uncharged surface bound hydroxyl groups can induce the ozone decomposition to generate radical dotOH in aqueous solution [4], [5]. Nevertheless, the same research group proposed, in the case of ceramic honeycomb modified with metals: Zn, Ni and Fe system, a mechanism where an ozone molecule–surface −OH2+ groups interaction takes place and might be responsible of the catalytic activity, as a result of electrostatic and/or hydrogen bonding interactions [9].

Hydroxyl radical generation is useful in degrading a large number of water soluble organic compounds [10], [11], [12], [13], [14], [15]. Though, the presence of saturated organic matter (ozonation products) or bicarbonate/carbonate in the reaction medium caused that hydroxyl radical oxidation turns out to be ineffective. Therefore, catalytic ozonation through complexes formation pathway would be acceptable for the degradation of saturated compounds.

Recently, some publications refer that oxalic acid degradation using CeO2 supported palladium or copper oxide did not promote hydroxyl radical generation from ozone [16], [17]. These results expose that both, the support (CeO2) formed surface complexes with oxalic acid, while active phase (PdO) of the catalyst carried out a selective reaction and further decomposed ozone in diverse chemical species. The high activity of the catalyst increased the oxalate degradation in water due to the synergetic function of two oxides (PdO and CeO2) [16].

On the other hand, it has a great effort to evidence the reactive oxygen species generated by the interaction of the ozone with catalyst in aqueous solution. Lately, it has reported that FTIR [18], ATR-FTIR [19] and Raman spectroscopy [16] were used to reveal the formation of some surface complexes or new species originated during catalytic ozonation. In addition to previous cited techniques, X-ray photoelectron spectroscopy (XPS) appears as a powerful technique to clarify the surface phenomena carried out during the organic compound degradation in aqueous solution as a result of the interaction between 2,4-D and the catalysts. Furthermore, XPS can help to elucidate the reaction mechanism in the catalytic ozonation.

In previous works, it has been demonstrated the utility of XPS as a tool to determine the formation of new surface species, which would be responsible for the catalytic activity [20], [21]. Specially, we have found that the interaction of ozone and 2,4-D on the surface of Ni/TiO2 catalyst leads to the formation of O* and Ti* which together with the radical dotOH increase slightly the degradation of the pollutant compared with ozone alone [20]. Furthermore, in the degradation of di-n-butyl phthalate (DBP) with NiFe2O4 catalysts, it was shown by XPS and other techniques that oxidizing reaction occurred on the surface and Ni2+ provided the electrons, indicating the involvement of Ni2+ - Ni3+ - Ni2+ redox processed during the reaction.

In this study, four commercial catalysts (TiO2, SiO2, Al2O3 and NiO) were used in the 2,4-dichlorophenoxyacetic acid (2,4-D) elimination. 2,4-D is the most widely used herbicide worldwide to control broadleaf weeds in cereal and grain crops, recreational areas, golf courses and gardening. Moreover, the herbicide cannot be biodegraded effectively for concentrations higher than 1 ppm [22]. It has been detected as a major pollutant in ground and surface waters. Most publications about 2,4-D degradation in aqueous solution have been focused to the use of advanced oxidation processes (AOPs), for example: photo-Fenton [23], catalytic ozonation [15], [24] and photocatalytic process [25], [26]. The results of previous processes showed that the mineralization of 2,4-D needed a prolonged reaction time, thus a good option should be heterogeneous catalytic ozonation. The use of XPS allowed us to elucidate the formation of surface complexes and new species originated due to ozone exposure.

Section snippets

Materials and reagents

A model solution was prepared with 80 mg L−1 of 2,4-D (Alfa Aesar, 98%) in distillate water, the initial pH was 3.1. The pH was not controlled during the ozonation. All chemical used in the experiment were analytic grade reagents. TiO2 (Degussa, 99.5%), SiO2 (Cab-o-sil, 95%), nanoparticles of Al2O3 and NiO (Sigma–Aldrich, 99%) were used in the experiments as commercial catalysts.

Ozonation

The ozone concentration in gas phase was measured by Ozone Analyzer BMT 946BT (BMT Messtechnik, Berlin). The catalysts

Comparison of mineralization degree with all catalysts

Catalytic ozonation mechanism of metal oxides has been reported that starts with hydroxyl groups’ chemisorption on the catalyst surface [6]. When these groups get in contact with ozone, a chain reaction gives rise to radical dotOH generation. Hence, if the quantity of hydroxyl groups increase in the catalyst surface, a proportional increment in radical dotOH would also take place. In consequence, the organic compound would have a higher conversion in the medium. Considering this fact, Fig. 1 shows the mineralization

Conclusions

The present study investigated commercial catalysts for 2,4-D degradation by ozone in aqueous solution. It was found that NiO, in comparison with TiO2, SiO2, Al2O3, is a good candidate to mineralize high concentrations of the herbicide (80 mg L−1) in short reaction times (1 h) and with a high stability. Based on the experimental results and discussion, the following conclusions can be drawn.

  • 1.

    The higher ozone decomposition (27%) was achieved by NiO, which was explained in terms of its ability to

Acknowledgements

The author thanks the Department of Graduate Study, Investigation of the National Polytechnic Institute of Mexico (Project 20110185) and the National Council of Science and Technology of Mexico – CONACyT (Projects: 153356 and 156150)

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