Elsevier

Applied Catalysis A: General

Volume 570, 25 January 2019, Pages 120-129
Applied Catalysis A: General

Pd-Cu catalysts supported on anion exchange resin for the simultaneous catalytic reduction of nitrate ions and reductive dehalogenation of organochlorinated pollutants from water

https://doi.org/10.1016/j.apcata.2018.11.002Get rights and content

Highlights

  • The deposition procedure of second metal strongly affects Pd-Cu/resin performances.

  • A two-step catalyst synthesis protocol: ion exchange-surface reduction was selected.

  • Pd-Cu catalyst is effective on simultaneous NO3 reduction and 4-CP dechlorination.

  • The catalyst shows good stability for 100 h of test time.

Abstract

The present work proposes the simultaneous removal of these classes of pollutants by a catalytic hydrotreatment processes. For this purpose, bimetallic Pd-Cu catalysts (with mass ratio Pd:Cu of 4:1) supported on macroporous strong base anion resin were prepared by different methods. The catalysts were characterized (by XRD, SEM-EDX, XPS, AAS and H2 chemisorption) and tested in a continuous flow system. The selected catalyst preparation protocol consists in a two-step method, which implies the deposition of palladium by ion exchange and the subsequent deposition of copper by controlled reaction on the surface of the pre-reduced palladium. The effectiveness of the catalyst in the simultaneous reduction of nitrate and hydrodechlorination of 4-chlorophenol was demonstrated. By adjusting the initial pH and the flow rate of the aqueous solution, nearly complete hydrodechlorination of 4-chlorophenol can occur together with selective nitrate reduction at a conversion of 95% and a selectivity to N2 of 92% (this value contains the contribution of all gaseous products, including the eventually formed NOx). The bimetallic catalyst was found to remains relatively stable after 100 h of test time.

Introduction

Nitrate ions and organochlorine compounds such as pesticides and their degradation by-products are pollutants of concern found in natural waters originating mainly from agricultural areas [1].

The nitrogen cycle is altered by anthropogenic activities, particularly by the intensive agricultural practices, which lead to a gradual increase of nitrate ions concentration in natural water bodies. High concentration of nitrates in surface water negatively affects aquatic ecosystems through its toxic effect on aquatic fauna or by water eutrophication. Excess nitrates in groundwater (frequently used as a drinking water source) can lead to enhanced exposure of human population being responsible for specific diseases such as methemoglobinemia and several types of cancer [2,3].

Contamination of natural waters by pesticide residues, especially organochlorine compounds, also causes considerable concern. Despite their usefulness in agriculture, pesticides may produce a wide range of toxic side effects representing a potential hazard to the environment. Organochlorine compounds are often characterized by high toxicity and persistence, being designated as priority pollutants [4]. They are readily bioaccumulated in aquatic and terrestrial organisms, being notorious for their ability to biomagnify in the food chain.

Therefore, significant research efforts are devoted to the development of practical methods which are able to effectively remove these pollutants from natural waters, especially from those used as water supplies.

A series of physico-chemical and biological methods were developed for the elimination of nitrate ions from water. Among these, catalytic reduction of nitrate is an attractive technique that would be suitable for water treatment [5,6]. This method was reported for the first time by Vorlop and Take [7] and presents several advantages. One of its major benefits is that no waste is generated during the treatment process as in the case of ion exchange, reverse osmosis or biological processes. Nevertheless, a crucial aspect remains the catalyst selectivity issue because the presence of nitrite and ammonium ions by-products is highly undesirable in drinking water.

Great efforts are made in order to obtain catalyst with high activity and selectivity towards nitrogen formation. The catalysts developed for this purpose are mainly supported bimetallic systems based on noble metals such as Pd, Pt, Rh and Ru, promoted especially with Cu, Sn and In [[8], [9], [10], [11]]. Pd-Cu is, by far, the most studied metallic pair for this application and offers promising prospects [12]. Generally, Pd-Cu catalysts were found to have higher activity and/or selectivity to nitrogen than Pd-Sn or Pd-In catalysts [13,14]. However, several studies claim that Pd-Sn catalysts perform better than the Pd-Cu ones [15,16].

The nature of the support is also important, playing a crucial role in the activity and selectivity of the catalytic system. Various carriers have been proposed, γ-Al2O3, SiO2, TiO2, hydrotalcite and carbonaceous materials being among the most common ones [[17], [18], [19], [20], [21]]. Ion exchange resins have also been considered promising support candidates. There is a relatively small number of studies dedicated to the investigation of this type of carrier.

The use of ion exchange resins as catalyst carriers for the selective reduction of nitrate was proposed by Gašparovičova et al. [[22], [23], [24]]. Both cation and anion exchange resins were studied. Although the presence of the acid sites of the cation exchange resin contributes to increase the selectivity towards N2 of the Pd-Cu supported catalysts (e.g. from 20%, for a resin free of acid sites, to 60%, for a resin with acid sites), Pd-Cu catalysts deposited on anion exchange resin showed higher specific activity (e.g. 94% selectivity to nitrogen at 61% nitrate conversion) [22,23]. The superior performances of the catalysts supported on anion exchange resin were attributed to the higher mobility of the anions compared to cation exchange resin support. It was also highlighted that the selectivity to nitrogen is higher on anion exchange resin carriers than on γ-Al2O3 [24]. Comparable results were obtained by Neyertz et al. (2010) for the reduction of nitrate ions on Pd-Cu/anion exchange resin, at a more elevated concentration, 150 ppm NO3-N (expressed as nitrogen from nitrate) instead of the usually employed concentration of 22.6 ppm NO3- N (100 ppm NO3). More recently, N2 selectivity of 100% for complete nitrate conversion was claimed for bimetallic Pd-Cu catalyst supported on anion exchange resin [25]. This notable performance was achieved in a batch reactor with NO3 previously retained on catalyst surface by ionic exchange and then maintained for 6 to 24 h under 6 bar H2/CO2 pressure (hydrogen partial pressure of 3 bar) at moderate temperature (298 K and 333 K). The selective conversion of NO3 to N2 is explained by the fast retention of the HO, resulted in NO3 hydrogenation process, on the ion exchange sites of the resin vacated by NO3 consumption during reaction progress. A 100% selectivity to nitrogen has never been reported previously. The only drawbacks of this method could be the demanding time or temperature and pressure requirements in the regeneration step.

To the best of our knowledge, the catalytic liquid-phase reduction of nitrate on bimetallic catalysts supported on ion exchange resin was so far investigated in semi-batch or batch reactors and no studies have been published on fixed-bed reactors operating in continuous flow.

With respect to organochlorine compounds in water, several methods like adsorption or advanced oxidation processes have been proposed for their removal [26,27]. An alternative approach might be their catalytic hydrodechlorination which leads to less toxic organic compounds, thereby reducing the ecotoxicity of the treated water. Noble metals, preponderantly Pd, on various supports have been studied for the liquid phase hydrodechlorination of organochlorine compounds from water such as organochlorine pesticides, chlorophenols and chloroacetic acids [11,[28], [29], [30]]. Complete dehalogenation of mono- and poly-chlorinated organic compounds in aqueous solution (e.g. chloroacetic acids and chlorophenols at 0.02 - 0.2 mM) has been achieved in the presence of palladium-based catalysts using hydrogen under ambient temperature and pressure [29,31,32].

The studies reported in the literature dedicated to treatment of poly-contaminated waters are scarce. Preliminary catalytic reduction tests with tri- and tetra-chloroethylene added to a nitrate solution were performed by Centi and Perathoner [33]. They reported a conversion of 90% for chlorinated compounds (no other information provided) and suggested the possibility of simultaneous removal of nitrate and halogenated compounds. Several studies investigated the influence of nitrate and other inorganic anions on the aqueous phase reduction of organochlorine compounds with zero-valent iron or with hydrogen in the presence of Pd supported catalysts [[34], [35], [36], [37], [38]]. Different results regarding the influence of nitrate on the hydrodehalogenation of organic compounds on Pd/Al2O3 catalysts were reported. It was found that, the presence of nitrate at moderate to high concentrations (0.37–1.29 mM) had no effect on the hydrodechlorination rate of trichloroethylene (0.031 mM) [37]. Other authors concluded that the hydrodehalogenation of 1,2-dibromo-3-chloropropane (0.018 mM) was strongly affected by the presence of 0.45 mM nitrate, which lowered the conversion of halogenated compounds with about 50% [38]. The absence of any effect of NO3 on trichloroethylene dehalogenation was explained by the lack of activity of monometallic palladium catalyst on nitrate reduction (no nitrate conversion). In contrast, in the study of hydrodehalogenation of 1,2-dibromo-3-chloropropane, 90% of NO3 was removed.

This work assesses the effectiveness of catalytic hydrotreatment process for simultaneous removal of nitrate and 4-chlorophenol in aqueous solution. For this purpose, bimetallic Pd-Cu catalysts supported on macroporous strong base anion resin were prepared by different methods, characterized and tested using a continuous flow system. The reasons for choosing anion exchange resin as support are: (i) to ensure a high dispersion and controlled distribution of the active components; (ii) to enable the NO3 internal diffusion.

Section snippets

Bimetallic Pd-Cu catalyst synthesis

In this work, a series of bimetallic Pd-Cu catalysts (with mass ratio Pd:Cu of 4:1) supported on an anion exchange resin was prepared. The synthesis of bimetal-resin catalysts was carried out via two main routes: (i) introduction of both metals by ion exchange with the functional groups of the resin; (ii) introduction of palladium by ion exchange and subsequent deposition of copper by controlled surface reaction. The carrier was a commercial anion exchange resin, A-520E Purolite, having

Catalyst preparation

It was found that immobilization of metals by ion exchange onto the resin depends on acidity of precursor solutions. The effect of the HCl concentration on the metal retention from precursor solution was significant in the case of copper. As an example, for Cu2+ solutions of 1.3 g·L−1, the HCl concentration was varied from 0.5 to 8.0 M. The maximum copper retention was obtained for HCl 6.0 M (see Fig. 2). This outcome might be explained by the interplay of two phenomena: (i) dependence of [CuCl4

Conclusion

It was found that the performances of the bimetallic Pd-Cu catalyst supported on S-DVB anion exchange resin in the selective reduction of nitrate strongly depend on the second metal deposition procedure. The selected preparation protocol consisted in a two-step method, implying the deposition of palladium by ion exchange and the subsequent deposition of copper by controlled reaction on the surface of the pre-reduced palladium. This protocol allowed a controlled deposition of metals on the

Acknowledgement

The authors thank the Executive Agency for Higher Education, Research, Development and Innovation Funding of Romania (UEFISCDI) for the financial support under the PNII Project No. 100/2012.

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