Hybrid process for the purification of water contaminated with nitrites: Ion exchange plus catalytic reduction

https://doi.org/10.1016/j.cherd.2017.07.019Get rights and content

Highlights

  • Catalytic resins Pd/WA30 can exchange nitrites and/or reduce them to nitrogen.

  • The performance of the Pd(2%)/WA30 catalyst in a hybrid process (reaction and ion exchange) proved to be very good.

  • The ammonium concentration generated by nitrites over-reduction never exceeded 0.2 mg/L.

  • A new methodology for regenerating the resin makes it possible to eliminate more than 99% of the nitrites.

  • A pilot plant scale reactor was used to verify this behaviour.

Abstract

Water polluted with nitrites represents a big risk to human health. In this work, palladium supported on macroporous anionic exchange resin was used in the catalytic nitrite reduction. This process is compared with the traditional ion exchange procedure using the same catalytic resin. Both, the resin and the catalyst behaviour were evaluated in a fix-bed reactor, feeding water containing nitrites and other competitor ions, such as sulphate, bicarbonate, and chlorides, and adjusting the pH with carbon dioxide. When feeding water containing only nitrites, it was observed that the catalytic reduction makes it possible to treat 55% more water than when using the ion exchange process, at the same level of nitrites elimination. Moreover, in steady state it was possible to obtain a nitrite conversion to nitrogen of 54% with high selectivity, obtaining an ammonium concentration lower than 0.2 mg/L. In the case of having other ions present in the system, both the conversion and the selectivity decreased. A regeneration strategy is also developed, using a very low hydrogen flow rate at atmospheric pressure and room temperature. This treatment leads to the reduction of more than 99% of the nitrites present in the contaminated water. The catalyst was used in several consecutive cycles maintaining a very good performance, even in the presence of competitor ions. The process was scaled up to a pilot level obtaining identical results.

Introduction

In recent years, a significant amount of surface water sources, mainly from groundwater, with high concentrations of nitrates have been detected, due to excessive use of fertilizers in intensive agriculture, leaking of septic tanks and waste disposal in breeding farms (Rupert, 2008). Chronic and excessive intake of nitrate through water can cause serious health problems in adults, such as ovarian or prostate cancer, as well as growth problems or diseases such as methemoglobinemia (blue baby syndrome) in infants (Arbuckle et al., 1988, Comly, 1987, Dorsch et al., 1984, Fan and Steinberg, 1996).

Numerous processes for the removal of nitrates and nitrites from water have been developed. Among them are the ion exchange, biological denitrification, reverse osmosis, electrodialysis and catalytic reduction. The World Health Organization (WHO) recommends the biological denitrification and ion exchange as the most appropriate methods developed to date for the removal of nitrates from drinking water (World Health Organization, 1992). The biological denitrification is a very specific and selective process. However, it has as disadvantage the risk of contaminating the water with bacteria and metabolic substances. Therefore, it is necessary the filtration and disinfection with germicides of the treated water, which increases the process costs (Mizuta et al., 2004). Meanwhile, technologies using ion exchange are more suitable for water decontamination, due to its simplicity, efficiency, selectivity, reusability, and relatively low cost (Darbi et al., 2003). The main disadvantage of this technology is the disposal of the effluent rich in nitrates or nitrites, which means that the problem is only transferred but not completely solved.

Catalytic reduction of nitrates or nitrites using heterogeneous catalysts is an alternative to remove these species from polluted streams. In these technologies, nitrates or nitrites are reduced to nitrogen using a reducing agent, which is usually hydrogen (Barbosa et al., 2013, Gao et al., 2015, Pintar and Batista, 2007, Zhao et al., 2014). However, this technology presents a major constraint, which is the catalyst selectivity. Over-reduction of nitrate or nitrite leads to the formation of ammonia, being the maximum concentration allowed by the WHO 0.5 mg/L. For this reason, it is necessary to develop catalysts displaying not only good activity but also a very good selectivity to N2.

Recently, polymers have being used as supports, with the purpose of stabilizing the bimetallic particles. Polymers such as poly(N-vinylpyrrolidone) (PVP) or poly(vinylalcohol) (PVA) were used to increase the stability and controlling the size and composition of the bimetallic particles (Guy et al., 2009; Prüsse et al., 2000). A third type of polymers, such as functionalized resins or ion exchange resins have been successfully used in the synthesis of Pd–Cu and Pd–In bimetallic catalysts for their use in the catalytic reduction of nitrates (Barbosa et al., 2013, Gašparoviĉová et al., 2006, Gašparoviĉová et al., 2007, Neyertz et al., 2010). These resins are constituted by a copolymer of styrene- divinylbenzene containing exchange functional groups such as –N(CH3)3+ Cl or –SO3–H+, known as anionic or cationic resins respectively. The use of acid cationic resins as support for bimetallic catalysts was studied by different authors. Such resins are very attractive because they provide protons that can neutralize the OH generated during the nitrate reduction, maintaining the pH of the aqueous solution at acceptable values, thus increasing the N2 selectivity (Gašparoviĉová et al., 2006; Kralik et al., 2006; Roveda et al., 2003).

No reports have been found about nitrite removal using monometallic catalysts supported on anionic resins. On the other hand, the effect of competing ions such as sulfates, chlorides and bicarbonates on the exchange capacity and catalytic activity of the resins, were addressed only in few studies (Hekmatzadeh et al., 2012, Samatya et al., 2006, Song et al., 2012, Stefan et al., 2014), in spite of being a key issue in order to properly determine the real catalytic behaviour (Bergquist et al., 2016).

Pintar and Batista (2007) proposed a process in which the nitrates were eliminated from the water using an ion-exchange resin. Once the resin was saturated, it was regenerated by eluting the nitrates using a sodium chloride solution (5%w/w). This stream containing high concentrations of nitrates and chlorides was them fed to a two-reactor system. In the first one, the nitrates were reduced to nitrites in a basic media (pH = 12.5) using a Pd–Cu/Al2O3 catalyst. Then, in a second reactor, the nitrites were selectively converted to nitrogen in an acid media. In the present work, a catalytic process that can be applied to the second reactor of this purification strategy is studied. According to Pintar and Batista (2007) the presence of chlorides decreased the catalyst performance and induced ammonium formation. Also, they found that in the second reactor, the selectivity to ammonium was too high, due to pH gradient inside the catalyst pores.

Catalysts containing metals supported on ion exchange resins have the advantage of being bifunctional. On one hand, the palladium deposited on the resin is capable of reducing nitrites in the presence of H2, and on the other hand, the support may adsorb nitrites by an ion exchange mechanism (IE). Thus, the decrease in the nitrites concentration observed at the reactor outlet is due to the sum of two contributions, the reaction and the ion exchange.

This work presents studies of the catalytic process performance for nitrite reduction of monometallic Pd catalyst, supported on a macroporous anionic resin using the process named as RIE: Reaction + Ion Exchange. The efficiency of this process is compared with that of the conventional ion exchange (IE) using the same catalytic resin.

In addition, the resin regeneration is studied and the process was scaled up to pilot plant level. The overall performance of the process developed showed an excellent performance. Initial adsorption and reaction rates were also determined in a batch reactor, in order to have quantitative information regarding the contribution of each mechanism in the nitrites removal. In addition, the effects of competing anions on nitrites adsorption and reduction were also addressed.

Section snippets

Catalyst preparation

The WA30 (Mitsubishi) resin used as catalytic support is a styrene divinylbenzene polymer matrix with tertiary amine functional groups. It is a weak macroporous anionic exchange resin, with an exchange capacity of 1.6 equivalent/L. Before metal impregnation, the resin was washed with HCl 5 wt.% for 1 h, filtered and washed with distilled water. The palladium was fixed on the resin by addition of a solution containing [PdCl4]2−, obtained by dissolution of PdCl2 in an acidic media (HCl 0.01 wt.%).

Effect of particle size

Fig. 1 shows the variation of nitrite concentration with time for different particle sizes (Ps) of the catalytic resin, obtained in experiments carried out as described in Section 2.4. It can be seen that the rate of nitrites exchange increases with decreasing particle size. Therefore, the internal diffusive effects have a non-negligible effect on the adsorption rate. The commercial particle diameter of WA30 resin is in the 20–40 mesh (841–420 μm) range, which presented the lowest exchange rate.

Conclusions

Important diffusive effects were observed during the ion exchange process. However, decreasing the particle size would bring about problems of fine particles dragging and over-increase the pressure drop throughout the reactor. Taking into account the results obtained in the breakthrough and batch ion exchange experiments, it is concluded that sulfate is the competing ion which causes the larger alterations in the ion exchange ability. The selectivity and activity for nitrite reduction decrease

Acknowledgements

The authors wish to acknowledge the financial support received from UNL (CAID PACT 96) and ANPCyT (PICT 2013).

References (37)

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