Removal of cimetidine, ketoprofen and naproxen by heterogeneous catalytic ozonation over volcanic sand at low pH

Abstract

This work presents experimental results on the catalytic effect of volcanic sand on the simultaneous removal of naproxen, ketoprofen, and cimetidine by ozonation. Catalyst characterization included XRD, EDX, XRF, SEM and physisorption analysis. Removal assays at laboratory scale in a semi-batch reactor showed enhanced removal by the presence of the catalyst at pH 4, but no difference was found at higher pH values. Furthermore, the immobilization of different amounts of TiO₂ (2.5 % and 5.0 %) on the sand surface revealed that TiO₂ exerted an adverse effect on the catalytic activity at pH 4, lowering ketoprofen removal by up to 41.3 %. Likewise, the highest and fastest removal at pH 4 was obtained using a catalyst dose of 1 g/L and dosing ozone at 15 mg/L (72.5 % ketoprofen; 90.7 % total pharmaceuticals), reaching more than 80 % total elimination after only one minute of treatment. Catalyst reusability evaluated over 5 cycles demonstrated efficient elimination of the pharmaceuticals, with only slight decrease in ketoprofen elimination. Finally, heterogeneous catalytic ozonation of ketoprofen was applied in a real wastewater matrix, obtaining a removal of 75.9 % after 40 min of reaction; this represents a similar efficiency to that achieved in synthetic wastewater.

Introduction

Pharmaceutical products are considered emerging environmental pollutants, of wide and increased use in human and veterinary medicine [1]. Their highest concentrations are found in hospital and municipal wastewaters; however, current legislation regarding their concentrations in effluents or the environment is still mostly lacking worldwide [2,3].

Among pharmaceutical compounds, naproxen and ketoprofen are two non-steroidal anti-inflammatory drugs commonly detected in influents and effluents of wastewater treatment plants (WWTPs), as well as in surface waters [1,4,5]. High environmental risk has been demonstrated for the presence of these compounds in such water streams [6]. Moreover, cimetidine is a histamine H2-receptor antagonist used for the treatment of gastrointestinal disorders [7]; partial removal capability of WWTPs has resulted in its detection in surface waters [8]. Ecotoxicological studies report adverse effects of cimetidine on invertebrate aquatic populations [9], and potential endocrine disruption alterations following long-term exposure [7].

For these reasons, the search for technologies capable of removing pharmaceutical compounds is a matter of environmental relevance. Advanced oxidation processes (AOPs) have been developed in a wide variety of applications within the field of wastewater treatment. These processes are based on the generation and use of transient species with high oxidizing potential, mainly the hydroxyl radical (^·OH), resulting in efficient removal of diverse pharmaceuticals, as reviewed elsewhere [10,11]. Heterogeneous catalytic ozonation belongs to the group of non-photochemical AOPs; it enhances the ozonation of organic pollutants through the formation of hydroxyl radicals generated from the decomposition of ozone on the surface of a solid heterogeneous catalyst [12]. Catalysts so far evaluated in this process include activated carbon and metals supported on activated carbon [13,14], biochar [15,16], zeolite [17,18] and even volcanic sand [19]. In particular, volcanic sand represents a low cost option to the high-cost activated carbon, commonly employed in these processes. Moreover, this material permits the immobilization of metal oxides such as TiO₂ which might additionally enhance the elimination outcome [20].

The removal of cimetidine [21], ketoprofen [22] and naproxen [23] by ozonation has shown promising results; nonetheless, heterogeneous ozonation processes have been scarcely explored for these compounds, and there are reports for ketoprofen [24] and naproxen [23], but not for cimetidine, although none of the studies employed sand as the catalyst.

This work aimed to evaluate the efficiency of volcanic sand as a catalyst for the heterogeneous catalytic ozonation of naproxen, ketoprofen, and cimetidine, pharmaceutical products commonly found in wastewater. The effect of different conditions (i.e. pH, TiO₂ immobilization on the volcanic sand, catalyst load and ozone dosage) on pharmaceuticals degradation was examined. Particular interest was focused on low pH conditions, considering that: i. ozonation is usually favored at high pH, contrary to this volcanic sand which showed better performance at low pH; and ii. acidic effluents of environmental relevance such as leachates from young landfills [25] are less commonly studied. The use of volcanic sand results in an environmentally friendly, low cost, easy to separate and non-toxic catalyst [26] for catalytic ozonation processes.