Diclofenac (DCF) is a non-steroidal drug and is classified as a phenylacetic acid with anti-inflammatory, analgesic, and antipyretic properties. DCF in delayed and extended-release forms have been developed to improve its safety profile thus providing application convenience for patients with chronic pains (Altman et al. 2015). The global consumption of DCF in the pharmaceutical industries reaches 940 tons annually (Zhang et al. 2008). Market revenues are also increasing at a record rate over the world, owing to the growth of medicine demand. However, this market condition is associated to the generation of direct pollution problems because the release of pharmaceuticals and their metabolites into the environment, including water, has also increased due to the improper management, treatment, and disposal of these compounds. Pollution caused by pharmaceuticals is now recognized worldwide as a relevant environmental concern that also represents a human health risk. Researchers have looked at the most successful way(s) for removing pharmaceuticals from wastewater to protect the environment (Gadipelly et al. 2014; Pal 2017). However, it is convenient to remark that one limitation is that the types of pharmaceuticals researched in several studies do not reflect the most often produced and consumed pharmaceuticals on the market.
Pharmaceutical wastes are generally discharged to the environment through sewage treatment plant and the improper disposal of expired products, domestic sewage, manufacturing plants waste, hospital waste and runoffs from agricultures (Praveena et al. 2018). The release of the pharmaceutical wastes impacts the environment. In terms of water pollution, the pharmaceutical residue can be released in rivers and groundwater system through surface runoff and leaching. Overall, DCF cannot be removed completely in traditional wastewater treatment plants due to their limited degradation efficiency, which causes the presence of this and other pharmaceuticals in rivers, sediments, sludge and even drinking water sources (Lonappan et al. 2016). Some studies have indicated that DCF can be detected in the concentration range of 0.566 - 1.48 µg/L (influent) and 0.167 - 7.01 µg/L (effluent) (Bo et al. 2015). Several countries have also reported the presence of DCF in the wastewater of different industries like India (312 to 360 ng/L) (Balakrishna et al. 2017), Japan (44 ng/L) (Simazaki et al. 2015), Malaysia (32-5049 ng/L) (Al-Qaim et al. 2015), South Korea (203 µg/L) (Vieno and Sillanpää 2014a), Taiwan (152-185 ng/L) (Fang et al. 2012) and Thailand (367 ng/L) (Tewari et al. 2013). DCF can be also found in sludge, solid waste and soil. For instance, Ashfaq et al. (2017) investigated the concentrations of DCF in a pharmaceutical industry of Pakistan. This study reported diclofenac concentrations of 836 µg/L, 4968 g/kg, 6632 g/kg and 257 g/kg for wastewater, sludge, solid waste and soil samples, respectively (Ashfaq et al. 2017). In Malaysia, the mean DCF concentrations were 4.84 ng/L, 2.76 ng/L and 4.30 ng/L in Gombak, Lui and Selangor rivers (Praveena et al. 2018). However, DCF concentrations up to 188 ng/L were quantified in Langat river due to the discharge from sewage treatment plants in Langat River Basin (Al-Odaini et al. 2013). These studies highlight the relevance of developing effective removal methods to reduce the DFC environmental pollution.
The low DCF biodegradability often result in a low removal rate during biological waste treatment and only a minor portion of this pharmaceutical can be adsorbed into sludge. In fact, different researchers have highlighted an average reducing rate of 3 to 6% for DCF depending on the wastewater treatment process (Vieno and Sillanpää, 2014b). Some improvements in wastewater treatment plants have been tried to increase the removal efficiency of this compound but drawbacks in process operation still prevail. For example, the removal of DCF using oxidation processes such as ozonation creates unwanted and toxic by-products, which affects to reach the final purification target (Lonappan et al. 2016). Enzymes could be also used for the complete degradation of DCF from water without the risk of by- product formation. However, it increases the process complexity (Dhiman et al. 2022). Adsorption is an effective and low-cost process for the wastewater treatment and it can outperform other traditional technologies used in the removal of pharmaceutical compounds (Aissaoui et al. 2017; Radjenovic and Petrovic 2017; Ahmadzadeh and Dolatabadi 2018; Hamon et al. 2018; Kurniawan et al. 2018). This technology can produce high-quality water (i.e., with low concentration of pollutants) and its cost-effective tradeoff is better than those reported for other removal methods. Different adsorbents can be applied in the removal of DCF and other pharmaceuticals where activated carbons are the first option due to their wide commercial availability. However, the operational costs involved in the removal treatment system is a current limitation for wastewater purification in several countries and, consequently, it is necessary to develop novel and low-cost adsorbents with improved adsorption properties for pharmaceuticals.
Clay is an excellent choice for the adsorption of water pollutants. They offer several advantages over other low-cost adsorbents. For instance, they are accessible and affordable materials that show an ion exchange capability, chemical stability and large surface area with a variety of structural and surface characteristics (Erdem et al. 2010; Auta and Hameed 2013). Bentonite is a natural inorganic adsorbent with a high montmorillonite concentration that is mainly composed of the clay mineral smectite. In the marine environments, the majority of bentonite is generated through the modification of volcanic ash. This clay is a 2:1 dioctahedral smectite with a sandwiched layer structure consisting of an octahedral alumina layer sandwiched between two tetrahedral silica sheets. Because of the isomorphous substitution of Si4+ in tetrahedral layers by Al3+ and Al3+ in octahedral layers by Mg2+, it has a permanent negative charge (Resende et al. 2021). A hydroxyl-aluminosilicate structure underpins the chemical makeup of bentonite (Guo et al. 2019). Bentonite has high surface area and cation exchange capacity besides its ability to the interlamellar expanding (Uddin 2016). The use of bentonite for the removal of cationic compounds is limited due to the presence of a net negative charge surface lattice. Due to the repulsive effect by electrostatic interactions generated by the negative charge on the bentonite surface, it only can adsorb pollutant molecules charged positively (Youssef et al. 2013). Zeta potential of natural bentonite generates its negative charge (Resende et al. 2021). The natural tendency on bentonite is that the zeta potential increases as pH decreases and this occurs due to the protonation of the hydroxyl groups on the edges of bentonite sheets. However, even the protonation of hydroxyl groups of bentonites is not enough to neutralize all its negative charge. This means that the charge of this clay can remain negative even at acidic pH conditions.
In this research paper, the authors report the preparation of an adsorbent coating via a simple synthesis procedure and its application in the adsorption of DCF from aqueous solution. This adsorbent coating (ASEC) was obtained from acrylic polymer (APE) as binder, bentonite as additive and epichlorohydrin-dimethylamine (EPIDMA) as modifier/surfactant. Results showed that the flexible physical characteristics of ASEC provide operational advantages as it is foldable and can be rolled since it has the form of a flat sheet that can be slotted or layered in available limited space in a treatment plant at any condition. ASEC performance for the DCF removal was analyzed at different operating conditions to optimize its application where the impact of EPIDMA dosage, initial concentration, temperature and solution pH was discussed. Equilibrium and kinetic studies of DCF adsorption were also carried out to calculate relevant parameters of this adsorption system.