1. Introduction
Pharmaceutical pollution has, in many areas, reached damaging outcomes for ecosystems and living organisms including humans. These drugs have posed various threats to the environment. These contaminants have been defined as “emerging contaminants (ECs)”. Pharmaceutical compounds are commercially produced annually in large quantities (hundreds of tons) for human and animal care [
1]; around 200,000 tons of which are only antibiotics [
2]. Initially, these pharmaceuticals were designed to possess a biologically active nature, showing persistency and resistance to removal; and therefore, wastewater treatment modalities have very limited success in eliminating them from aqueous environments [
3]. Environmental issues exacerbate as a result of the continuous introduction of these pharmaceuticals to water systems via sewage treatment works, livestock waste, and direct application to aquaculture ponds [
1,
4]. Despite their severe toxicity towards the environment, pharmaceutical manufacturing dispose of high amounts of ECs in the form of industrial wastewater [
5,
6]. The ecotoxicological effects of these pharmaceutical contaminants (PCs) have been found to be life-threatening [
7]. For instance, diclofenac (DCF) at trace concentrations (0.051–0.643 μg/g) resulted in renal failure for vultures in Pakistan [
7]. Similar results have been reported for other drugs [
4,
8].
The presence of pharmaceuticals in drinking water can be a direct result of water-reuse strategies [
4]. The conversion of water resources such as wastewater, contaminated freshwater, seawater, greywater, storm water, and brackish water to drinking water as unconventional water supplies has increased recently due to the rising demands on freshwater supplies globally. Nowadays, the gap between wastewater/greywater reuse and drinking water has become narrow due to the recycle/reuse of treated wastewater/greywater as drinking water. The risk posed by these PCs and their transformations into other products is minor at low concentrations (ng/L). However, there is no study dealing with long-term human or environmental health hazards of these PCs contaminated drinking water. Furthermore, it is not acceptable from a general public’s viewpoint to route contaminants of unknown risk to the human body [
9]. In response to these concerns, an effective treatment strategy is highly desired to provide the safe and PCs-free drinking water.
Conventionally used wastewater treatments are often limited in the removal of PCs. Advanced treatment technologies are considered as post-conventional treatment processes and tertiary treatment units to activated-sludge-based conventional treatment works to prevent the excessive release of ECs to the environment. Advanced oxidation processes (AOPs), membrane separation, ozonisation, and adsorption are traditionally used treatment technologies that combat ECs in wastewater treatment plants (WWTPs) [
10]. Nevertheless, drawbacks are related to the high cost of treatment for most of these technologies and to AOPs that release toxic oxidation intermediates (which require proper control). Among these technologies, adsorption is acclaimed for its low cost, minimal release of toxic by-products, ease of operation, and good reusability of adsorbents [
11]. These advantages make adsorption an attractive option for the removal of ECs in tertiary treatment process of WWTPs.
Studies on the impacts of removal technologies in tertiary treatment [
12] have revealed that several pharmaceutical compounds are resistant to both granular activated carbon (GAC) adsorption and ozone treatments. The anti-epileptic drug carbamazepine (CBZ) and the lipid regulator drug gemfibrozil (GEM) were not fully eliminated owing to their physicochemical properties, such as high water solubility (for CBZ) and/or poor degradability (of GEM) [
13,
14]. Concentrations of PCs ranging from 1.7 to 400 ng/L have already been found in drinking water in developed countries, such as the UK, Germany, Canada, Italy and the USA [
4]. Besides, pharmaceutical contamination in water is an alarming issue, given that the consumption of PCs in aforementioned countries are much lower than that of highly populated countries such as China, India, Bangladesh, and Pakistan [
1].
Nanotechnology has emerged as a promising strategy in solving environmental problems. Graphene-based materials (GBMs), such as graphene oxide and pristine graphene, have a great potential for the removal of pharmaceuticals [
11], such as atenolol, ciprofloxacin, carbamazepine, Ibuprofen and many others [
11,
15,
16,
17,
18,
19]. Nanostructured porous graphene (PG) is an ideal water treatment material because of its excellent hydrophobicity, adsorption capacity, recyclability and low toxicity [
20]. Besides, its high specific surface area can result in a lower filter volume for water treatment, and its high process efficiency will lead to a lower regeneration frequency [
21]. Nevertheless, the application of GBMs for water treatment requires the utilisation of cost-effective, and sustainable synthesis routes. There are many methods to synthesise PG, such as ion bombardment, doping, chemical etching, electron beam irradiation, and chemical vapor deposition [
22]. However, there are some drawbacks related to these techniques. For example, they incur high costs with unsatisfactory results. In fact, many researchers have contributed to the development of an alternative method to synthesise PG in recent years. For example, Zhang et al. activated reduced graphene oxide (rGO) with potassium hydroxide to increase its porosity, specific surface area (SSA), graphite layer spacing and adsorption capacity [
23]. Unfortunately, the final product turned out to be oxidized porous graphene. In order to solve this problem, our group developed a novel, simple, sustainable, highly biocompatible and cost-effective production route for superhydrophobic PG, based on the heat treatment of rGO [
20]. The operation temperature was 190–200 °C, which was lower than that (800 °C) with previously reported synthesis methods. In addition, the surface area of the prepared PG was relatively high (652 m
2/g).
A few studies explored the potential of GBMs for the removal of ECs (pharmaceuticals in particular), by packed filters in the form of columns. Dong et al. evaluated the efficacy of utilising GO directly as a filter medium for the removal of levofloxacin (LEV), an EC, from its aqueous solution [
24]. In fixed-bed columns, GO showed high removal performance for LEV along with lead (Pb) from both single and mixed solutions under different test conditions of GO content and injection flow rate. They concluded that the increase of GO content and decrease of influent flow rate improved the filtration performance and vice versa. Additionally, the competition between LEV and Pb slightly degraded the sorption of GO for LEV. In another report, graphene adsorption reactor (GAR) was coupled with conventional sand filtration to investigate pharmaceutical removal from urban wastewater [
21]. Caffeine, carbamazepine, ibuprofen, and diclofenac were successfully eliminated at high concentrations (10 mg/L) of the target pharmaceuticals with removal of more than 95%. Four-month test did not reveal any noticeable typical breakthrough adsorption curves. While comparing GAR to conventional granular activated carbon (GAC) as an adsorbent, graphene filters outperformed GAC filters (96% compared to 62% in the case of GAC). Nevertheless, the application of PG as a tertiary treatment filter for the removal of widely consumed PCs has not been fully explored yet. This exploration is imperative to unlock the current main bottleneck in the GBM limited application in the water treatment sector and influence policy-making as industrial sectors can cost-effectively reduce wastewater emissions by treating effluent wastewaters using GBM technology [
25].
In this study, the performance of PG as a filter medium was evaluated for pharmaceuticals removal from water. The removal of widely used pharmaceuticals, namely atenolol (ATL), ciprofloxacin (CIP), carbamazepine (CBZ), Diclofenac (DCF), Gemfibrozil (GEM) and Ibuprofen (IBP) was investigated. Column studies were conducted as an efficient approach towards actual application as a tertiary treatment option [
2,
3,
4]. The study involved experimental tests on the performance of PG as a filter medium for ECs removal from water in column studies under different conditions (reactive layer configuration, size of the column, packing heights, and adsorbent dosage). Moreover, the filtration treatment was investigated for three different types of water bodies (distilled water, synthetic greywater, and actual secondary treatment effluent from wastewater treatment works) spiked with a mixed solution of the six pharmaceuticals. The study setup could assist understanding the effects of water matrix and these contaminants interference with the removal of pharmaceuticals and PG. To the best of our knowledge, the work reported here is the first of its kind to investigate PG material as a nano-adsorbent in column studies for the removal of the six targeted ECs from water.
4. Conclusions
This study assessed the efficacy of porous graphene material (PG) as a filter medium to remove six emerging contaminants (ECs), namely, atenolol (ATL), ciprofloxacin (CIP), carbamazepine (CBZ), Diclofenac (DCF), Gemfibrozil (GEM) and Ibuprofen (IBP), from different water bodies. All ECs were examined in column studies with different configurations (sand column, one-layer PG reactive bed column, and two-layer reactive bed column). The PG-sand column containing double layers of PG dose was proved to be the best and highly efficient. This improved performance could be related to the extended length of packing material, which facilitated more time for adsorption. For a column (40 mm ID × 100 mm H) containing a sand and double PG layers (500 mg total of PG), ECs removal efficiency was maintained above 90% for about 100 min. The sand assisted the adsorption process of ECs, especially DCF for which the adsorption capacity of sand reached 33.776 mg/g. In the further experiments, the performance of PG as filter media for ECs removal from water was assessed in the column studies under different conditions (such as size of column, packing heights, and adsorbent dosage). It was concluded that the adsorption performance of PG packing was improved in a larger cross-sectional-area column. At least 10% increase in the overall removal efficiency (ORE) was achieved via increasing the filter size from ID 18 mm to 40 mm. By increasing the height and dosage of PG packed and mixed with the sand, the effluent concentrations decreased significantly, and this appeared more evidently in the larger cross-sectional-area columns. By combining the effects of previous factors, the ORE of most of ECs by PG filters increased to around 90%. Finally, the filtration of ECs mixture at trace concentrations in various water matrices revealed no considerable interference with greywater constituents, and the treatment results of some ECs (CIP, CBZ, DCF, and GEM) removal were similar to those with distilled water body with removal efficiencies above 99%. On the contrary, the presence of competing ions and organic matter comparatively degraded PG filter performance to treat ECs in wastewater. Overall, an adsorption column filter with PG packing of optimised design and operation parameters could potentially be used as a highly efficient tertiary treatment unit for the removal of pharmaceutical contaminants.