Emerging contaminants and surface modified natural zeolites as a new promising environmental challenge

Abstract

Introduction The emerging contaminants (ECs) are chemicals that can have negative ecological and health effects but they are still unregulated by legal frameworks (E.J.Tiedeken et al., 2017). Monitoring survey of ECs in waters (surface waters, wastewater treatment plants (WWTPs) effluents, underground water) in several European countries evidenced different concentrations (ng/l to μg/l) of pharmaceuticals, personal care products, plasticizers, flame retardants, surfactants, steroid hormones, pesticides, etc (Loos et al., 2013, and references therein). Although these concentrations are not considered high, the possible issue could be the accumulation of ECs (for example, hormones) that can show biological activity even in small doses (Liu et al., 2013). Many studies have been conducted in order to investigate these issues. Experimental Methods The results acquired from WWTPs effluents have shown that the conventional treatment procedures of the wastewater are not always effective in the removal of the ECs (Bolong et al., 2009). At neutral pH, pharmaceuticals (clofibric acid, diclofenac, hormones, ibuprofen, etc.) usually appear as ions in a water phase, and cannot be removed by the activated sludge (Urase and Kikuta, 2005). Non-steroidal anti-inflammatory drug (NSAID) diclofenac was found to be very persistent and hard to remove (Płuciennik, 2014). In order to remove persistent ECs, it is necessary to apply an additional treatment such as ozonation, UV irradiation, adsorption on the activated carbon, etc. or their combination (E.J.Tiedeken et al., 2017). Moreover, these treatments are expensive, most of the WWTPs cannot easily implement them and, last but not least, they are not obligated by law regulation to remove ECs. In order to solve these issues, it is necessary to explore achievable and low-cost methods for water treatment. For this purpose, the adsorption has been considered as the very promising mechanism and different adsorbents can be potentially studied (activated carbon and zeolites) due to their very high specific surface area and their attitude to exchange ions (Li et al., 2011). Moreover, the use of surface modified natural zeolites (SMNZs) instead of other adsorbents could be preferred due to abundance, worldwide availability, low cost of the natural zeolites, and because these adsorbents are considered as eco-friendly materials for water treatment (Delkash et al., 2015). Results and Discussion SMNZs were used for adsorption of inorganic anions (de Gennaro et al. 2014) and organic molecules from BTEX group (Bowman, 2003), humic acid (Li et al. 2011), mycotoxins (Daković et al. 2005), etc. Also, it has been shown that SMNZs have the ability to adsorb pharmaceutical NSAIDs such as ibuprofen (IBU) (Krajišnik et al., 2010) and diclofenac sodium (DS) (de Gennaro et al., 2015). Natural zeolites (clinoptilolite, phillipsite, and chabazite) from different geographical areas were characterized and their ability to be functionalized with different cationic surfactants (hexadecyltrimethylammonium bromide/chloride, benzalkonium chloride, and cetylpyridinium chloride) has been verified (Cappelletti et al., 2017). A new approach for fast functionalization has also been proposed (de Gennaro et al., 2016). The adsorption of anionic DS on a clinoptilolite-rich rock modified with cetylpiridimum chloride has been conducted and kinetic models of adsorption were suggested. It has been shown that 40.3 and 65.0 mg of DS can be adsorbed per g of SMNZ (de Gennaro et al., 2015). Surface modified phillipsite-rich tuff from the Campania region (southern Italy) was examined for adsorption of IBU by loading and release kinetics tests. Depending on the stabilizing anions, Cl- and Br-, loading amounts were 26.8 and 28.1 mg/g, respectively (Mercurio et al., 2018). The possible pharmaceutical application of SMNZs for the adsorption of the two NSAIDs (IBU and DS) has been confirmed. Whereas for the application of the SMNZs as adsorbents of ECs it is necessary to define the following: 1- To determine affinity for each of the ECs towards SMNZs. 2- A detailed characterization and leaching test to prevent potential toxicity of SMNZs. 3- To consider a real medium (a wastewater from a WWTP). 4- To set up the best experimental conditions for a most effective adsorption of the ECs. 5- To select a novel cationic eco-friendly surfactant not harmful to the aquatic organisms.