## 1 Introduction  
Antibiotics have increasingly been applied in the treatment and prevention of a wide variety of bacterial infections. However, their occurrence in the aquatic environment is an emerging environmental problem, which is mostly caused by the effluents of pharmaceutical industries and hospitals, in addition to veterinary and human medication uses [1,2]. Approximately 30%-90% of antibiotics administered remain un-degraded in the human or animal body, and are largely excreted as an active compound [3,4]. This is a matter of grave concern, as the prevalence of antibiotics in the natural environment can give rise to superbugs, which are drug-resistant bacteria. The removal of such pollutants are therefore of paramount importance. In the present study, we focus our attention on the widely used nitroimidazoles antibiotics, which possess antibacterial and anti-inflammatory properties, and are therefore commonly prescribed for treating infections caused by protozoans and anaerobic bacteria, such as Giardia lamblia and Trichomonas vaginalis [5; 6]. Metronidazolezole (MNZ) is a nitroimidazole antibiotic, which is commonly used for the treatment of anaerobic bacteria and protozoa [7]. Dimetridazole (DMZ) is a drug that combats protozoan infections. Roniazole (RNZ) is an antiprotozoan agent employed in veterinary medicine to treat histomoniasis and swine dysentery [8]. Tinidazole (TNZ) is an anti-parasitic drug used against protozoan infections [9]. The chemical and physical characteristics of MNZ, DNZ, RNZ, and TNZ, are summarized in Table 1.  
Several separation technologies, e.g., adsorption, photo-degradation, ozonation, and biological methods etc., have been suggested for the removal of nitroimidazoles from aqueous solutions [10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22]. However, the choice of the most suitable technology depends upon its cost-effectiveness for large-scale applications. Adsorptive removal using adsorbents with large surface areas is mostly preferred when low concentrations of organic and inorganic contaminants are involved. For the removal of nitroimidazoles, the activated carbon-based adsorbents have mostly been investigated. For example, [PERSON] and [PERSON] [10; 11] prepared different kinds of activated carbon samples using Siris seed pods, for the adsorption of metronidazolezole. Activated carbon produced by microwave-induced KOH activation yielded the best results, with an adsorption capacity of 196.31 mg/g. The removal of metronidazolezole was governed by the second-order adsorption kinetics. The rate constant at 303 K was found to be 0.0039, 0.0013, and 0.0011 g/(mg min), for 20, 60, and 100 mg/g of initial metronidazolezocentric concentration, respectively. It took as long as 180 min for the equilibrium to be established [10; 11]. On the other hand, the use of activated carbon cloth by [PERSON] et al. [12] yielded a higher uptake of the nitroimidazoles (500 mg/g for MNZ and 600 mg/g for DMZ). However, the adsorption rate was significantly slower, taking as long as 220 min to reach equilibrium. Recently, [PERSON] et al. [13] produced activated carbon samples from coffee residues and almond shells possessing surface areas in the range of 104-222 m\({}^{2}\)/g. Though the coffee residue-based activated carbon was better than that produced from the almond shell, its uptake was limited to 126 and 111 mg/g for MNZ and DMZ, respectively. Nevertheless, the authors argued that the performance of their adsorbents was superior to other high surface area adsorbents, if the antibiotic removal was evaluated on the basis of the surface area, instead of the commonly used mass basis. However, these adsorbents showed higher rates of antibiotic removal when compared to most other activated carbon-based adsorbents, since the equilibrium was established in only 50 min. Using Fe-modified sepiolite, [PERSON] and [PERSON] [14] were able to significantly enhance the uptake of the metronidazolezole, as compared to the original low-cost sepiolite.