Chemical deposition of iron nanoparticles (Fe⁰) on titanium nanowires for efficient adsorption of ciprofloxacin from water

Over the years, pharmaceutical scientists have designed a broad spectrum of antibiotics to cure several bacterial infections in human and animals. Ciprofloxacin (CIP) is an extensively prescribed antibiotic worldwide to treat various infections, for instance, infections in bones, joints, skin, urinary tract, respiratory tract, as well as sexually transmitted infections; (Mao et al. 2019; Falyouna et al. 2022). The unwise and excessive application of CIP in human and veterinary medicine results in frequent detection of CIP and a wide range of other antibiotics in several water bodies (e.g., groundwater, lakes, rivers, etc.) across the universe with concentrations range from ng L⁻¹ to mg L⁻¹ (Yao et al. 2017). Residues of CIP get access to different compartments of the aquatic system through many routes. The most important route for CIP to enter the environment is the effluents of the traditional wastewater treatment plants (WWTPs). The unutilized amounts of CIP are being discharged from the treated bodies through urine and feces to reach the WWTPs (Boxall et al. 2012). Since it has been confirmed by several studies that WWTPs can't manage and efficiently remove the residues of CIP and their metabolites from the effluents, they will eventually be evacuated with the treated wastewater to settle and contaminate the receiving water body (Yao et al. 2021). In addition, when the treated wastewater is used for irrigation or the WWTPs sludge is used as a fertilizer, residues of CIP can find their way to the groundwater through filtration and persist there for a long time (from months to years) (Archer et al. 2017). Furthermore, CIP can be introduced to the environment directly throughout the usage of CIP in the animal industry (e.g., aquaculture), or indirectly by using the manures of livestock as fertilizers in agriculture. Also, the effluents of CIP's manufacturing facilities and hospitals are possible routes to enter aquatic systems (Ji et al. 2021).

The persistent occurrence of CIP in water is linked to the accelerated growth of antimicrobial resistance genes (AMRs) in water (Rahdar et al. 2019; Mahmoud et al. 2021). These genes are famous in developing irremediable diseases. According to a review on the health consequences of AMRs, at least 700,000 persons are dying every year because of antimicrobial-resistant infections (O'Neill 2014). Also, the review speculated that this number is going to increase to more than 10 million by 2050 (O'Neill 2014). Thus, the development of an adequate and efficient technology to remove CIP from aqueous solutions is essential to protect lives and prevent the spread of such deadly genes in the limited water resources of earth.

Nanoscale zerovalent iron (Fe⁰) is one of the most extensively applied nanomaterials to remediate different types of contaminated waters and to remove a broad spectrum of contaminants such as heavy metals, nutrients, radioactive elements, organic contaminants, and so on (Takami et al. 2019; Falyouna et al. 2020a; Maamoun et al. 2020a). Despite the superb characteristics of Fe⁰ nanoparticles; for instance, high reactivity, high surface area, and multiple elimination mechanisms, they still have several drawbacks that restrict their ability to remediate antibiotics from water (Falyouna et al. 2019; Maamoun et al. 2021a). Fast surface oxidation, agglomeration and insignificant oxidation capacity are the main weaknesses of Fe⁰ nanoparticles and various approaches were embraced to tackle these limitations (Mokete et al. 2020; Maamoun et al. 2021b). The previous researchers adopted numerous techniques to improve the performance of Fe⁰ nanoparticles towards the elimination of CIP from water such as addition of Fenton reagent (i.e. hydrogen peroxide, persulfate, etc.), doping a noble metal on the surface of Fe⁰ (i.e. Cu, Ni, Ag), and spreading of Fe⁰ nanoparticles on a supporting material (i.e. biochar, graphene oxides, carbon nanotubes, etc.) (Chen et al. 2019; Pirsaheb et al. 2019; Falyouna et al. 2020b). Titanium oxide nanowires (TNWs) are an environmentally friendly, cost effective and excellent adsorbent and photocatalyst material for a variety of contaminants such as heavy metals (i.e. lead) and organics (i.e. phenol) (Youssef & Malhat 2014). The distinctive properties of TNWs, for instance, remarkable chemical stability, high photocatalysis and large surface area make them excellent supporting materials for Fe⁰ nanoparticles (Yuan & Su 2004). This research aims to decorate the titanium nanowires (TNWs) with Fe⁰ nanoparticles to decrease the aggregation of Fe⁰ nanoparticles as well as supplement them with the photodegradation property in order to remove CIP from water.

Ferric chloride hexahydrate (FeCl₃.6H₂O, JUNSEI, Japan), sodium borohydride (H₄BNa, Sigma Aldrich, USA), sodium hydroxide (NaOH, FUJIFILM Wako Pure Chemicals, Japan), and titanium (IV) oxide (TiO₂, FUJIFILM Wako Pure Chemicals, Japan) were utilized to synthesise Fe⁰, TNWs and (Fe⁰/TNW) nanocomposite. Ciprofloxacin hydrochloride monohydrate (C₁₇H₁₈FN₃O₃·HCl·H₂O, Tokyo Chemical Industry Co., Ltd, Japan) was dissolved in deionized water to prepare CIP solutions.

A gram of Fe⁰ nanoparticles was produced throughout the reduction of 92.49 mM of ferric chloride with 581.55 mM of sodium borohydride based on the previously reported steps in our articles (Maamoun et al. 2020b). Furthermore, titanium nanowires (TNWs) were synthesised by an alkaline hydrothermal process (Wang et al. 2018). In detail, 0.25 g of titanium (IV) oxide was totally dissolved in 40 mL of 10 M of sodium hydroxide solution. Then, the mixture was poured in a 150 mL Teflon-lined autoclave where the autoclave was tightly closed and placed inside an oven at 200 °C for one day. After the thermal treatment, the autoclave was taken out from the oven and left to cool down to room temperature. Afterwards, the acquired TNWs were collected by centrifugation and washed sequentially several times by 1% HCL, deionized water and ethanol until the pH of the solution reached 7. Finally, TNWs was dried at 65 °C for three days before being used in the experiments. Moreover, (Fe⁰/TNW) nanocomposites were synthesised in a similar way to synthesizing Fe⁰ nanoparticles. For example, to produce 0.5 g of (Fe⁰/TNW) with 50% mass ratio of TNW, 0.25 g of TNWs and 1.25 g of ferric chloride hexahydrate were mixed in 20 mL of ethanol for 10 min and ultrasonicated for 30 min before adding 50 mL of deoxygenated deionized water (DDIW) to prepare the 92.49 mM of ferric chloride solution. Then, the ferric ions (Fe³⁺) were reduced to Fe⁰ by the introduction of 581.55 mM of sodium borohydride under an anaerobic environment (Bensaida et al. 2020a). The final black precipitates were cleaned three times using 150 mL of DDIW during the vacuum filtration process.

The external morphology of Fe⁰ nanoparticles, TNWs, and (Fe⁰/TNW) nanocomposites was revealed by transmission electron microscopy (TEM, JEOL JEM-2100F). In addition, crystalline structure and chemical constituents of these materials were also obtained by conducting X-ray diffraction analysis (XRD, TTR Rigaku diffractometer).

Batch experiments were conducted in 300 mL conical flasks and the conditions of the experiments were designed to be as follows: CIP initial concentration = 50 mg L⁻¹, volume of CIP solution = 200 mL, dosage of Fe⁰, TNWs or (Fe⁰/TNW) = 0.5 g L⁻¹, temperature = 25 °C, initial pH = 6 and contact time = 120 min. The reaction was started by placing the flask on a magnetic stirrer at 1,000 rpm and at specific time intervals (0, 5, 10, 15, 30, 60, 90, and 120 min), 2 mL liquid sample was withdrawn by a plastic syringe, filtered by 0.22 μm filter, and kept in 2 mL sampling tube for UV-Vis spectrophotometer analysis. The effect of TNWs mass ratio, (Fe⁰/TNW) dosage, initial pH, and temperature was investigated in order to define the optimum removal conditions of CIP by (Fe⁰/TNW) nanocomposites.

TEM analysis was conducted to manifest the exterior morphology and size of Fe⁰ nanoparticles, TNWs and (Fe⁰/TNW) nanocomposite after the synthesis process. Figure 2(a) confirms that Fe⁰ nanoparticles had a spherical shape with an approximate particle size of 56 nm and resorted to aggregate due to electrostatic and magnetic attractions between the nanoparticles. In addition, Figure 2(b) and 2(c) prove the formation of TNWs with a length of 8.46 μm and a width of 82.35 nm. Moreover, Figure 2(d) assures the successful deposition and dispersion of Fe⁰ nanoparticles on the surface of TNWs. XRD technique was employed to reveal the crystalline structure of the materials mentioned in Figure 3. The XRD pattern of TNWs exhibited several strong and crystalline peaks named as titanium oxides (TiO₂) at different locations such as 11.13°, 25.20°, 30.26°, 48.84°, 64.18°, and 83.34° (Yuan & Su 2004). While, the three characteristic peaks of Fe⁰ were identified in the XRD profile of Fe⁰ nanoparticles at 44.45°, 65.15°, and 82.1° (Maamoun et al. 2019; Bensaida et al. 2020b). The XRD pattern of (Fe⁰/TNW) nanocomposite demonstrates the presence of Fe⁰ (2θ = 44.45°, 65.15°, and 82.1°) and TiO₂ (29.45°, 33.14°, 35.6°, 52.92°, and 60.32°) peaks which evidences the successful fabrication of (Fe⁰/TNW) nanocomposite.

Synthesis of (Fe⁰/TNW) nanocomposites with different mass ratios of TNWs (5%, 10%, 20%, 30, 40, and 50%) was carried out in order to define the optimum mass ratio of TNWs that would overcome the limitations of Fe⁰ nanoparticles, initiate the photodegradation of CIP, and attain a better CIP removal efficiency. Figure 4 summarizes the outputs of optimizing the TNW mass ratio. The removal efficiency of 50 mg L⁻¹ of ciprofloxacin by 0.5 g L⁻¹ of Fe⁰ nanoparticles within the first 60 min was promising because it achieved 89.49%, as shown in Figure 4. However, at 90 min of reaction, the concentration of CIP in the aqueous solution started to increase, which proves the desorption of CIP molecules from the surface of Fe⁰ nanoparticles. Afterwards, the performance of Fe⁰ nanoparticles towards the removal of CIP severely deteriorated until it eventually reached 55.54% after 120 min of reaction. Similarly, it is clear from Figure 4 that the uptake behaviours of (Fe⁰/TNW) nanocomposites with different percentages of TNWs and Fe⁰ nanoparticles were almost identical as the (Fe⁰/TNW) nanocomposites succeeded in removing significant proportion of CIP in the beginning of the reaction; nevertheless, desorption of CIP took place in the later stage of reaction. Fe⁰/TNW nanocomposite with 20% TNW mass ratio exhibited a stable performance compared with other nanocomposites. This behaviour can be explained by the good dispersion and distribution of Fe⁰ nanoparticles on the TNWs as presented in Figure 2(d), which decreased the aggregation, increased the surface area, and provided extra reactive sites for CIP removal. Thus, (20%-Fe⁰/TNW) nanocomposite was selected to be used in the following experiments.

The dosage of (20%-Fe⁰/TNW) nanocomposite was increased from 0.5 to 2 g L⁻¹ to define the optimum dosage that could be applied in the treatment process. Figure 5 states that increasing the dosage of (20%-Fe⁰/TNW) nanocomposite from 0.5 to 1 g L⁻¹ significantly improved the elimination of CIP by approximately 30.28%. Doubling the dosage of (20%-Fe⁰/TNW) nanocomposite provided additional adsorption locations for CIP as well as promoting the Fenton oxidation of CIP by generating more Fenton reagents (i.e. ferrous ions and hydrogen peroxide) and all these factors contributed to improve the removal of CIP (Shao et al. 2018). However, increasing the dosage to 1.5 and 2 g L⁻¹ is not beneficial because they did not significantly enhance the removal of CIP. Therefore, based on the findings of Figure 5, the optimum dosage is 1 g L⁻¹.

The influence of initial pH of the solution on the efficiency of (20%-Fe⁰/TNW) nanocomposite was investigated by varying the initial pH from 5 to 11 as illustrated in Figure 6. At pH = 5, the nanocomposite was able to remove approximately 65.97% of 50 mg L⁻¹ of CIP solution. This percentage trivially increased to 67.66% after increasing the initial pH to 7. Perini et al. reported that at pH 4.5 and 6.5, adsorption remarkably contributed to the total removal of CIP by zerovalent iron (ZVI) where the oxygen atoms of keto and carboxylic groups in the CIP structure formed bidentate complexes with the metals (de Lima Perini et al. 2014). On the other hand, the removal of CIP severely declined to 53.23% and 47.67 when the initial pH increased to 9 and 11, respectively. At the alkaline pH, the surface of Fe⁰ nanoparticles will be covered by a layer of iron oxides, which may hide some reactive sites and hinder the possibility of adsorption of CIP (Liu et al. 2020). Hence, the preferable working range of pH for CIP removal by (20%-Fe⁰/TNW) nanocomposite is from 5 to 7.

The impact of temperature on the removal of CIP by (20%-Fe⁰/TNW) was observed by elevating the reaction temperature from 25 °C to 55 °C, as presented in Figure 7. 0.5 g L⁻¹ of (20%-Fe⁰/TNW) nanocomposite eliminated around 67.34% of 50 mg L⁻¹ of CIP when the reaction temperature was set to 25 °C. Also, the removal efficiency was further increased to 73.84% by increasing the temperature to 35 °C. This slight increase in the temperature may provide the necessary energy to facilitate the adsorption of CIP by (20%-Fe⁰/TNW) nanocomposite (Chen et al. 2019). Conversely, incrementing the temperature to 45 °C and 55 °C resulted in declining removal efficiency to 60.78% and 65.05%, respectively. These outcomes are in a good agreement with results of Zhang et al. when they remediated norfloxacin from aqueous solution using nZVI/H₂O₂ system (Zhang et al. 2017).

Table 1 demonstrates a comparison between (Fe⁰/TNW) nanocomposite and some of the recently reported materials in the literature in terms of CIP removal from water. While constructing Table 1, CIP initial concentration was taken into consideration to perform a fair comparison between the materials. According to Table 1, (Fe⁰/TNW) nanocomposite exhibited a superior removal capacity of CIP (99.64 mg g⁻¹) compared with the removal capacities of other materials. This comparison proves the potential of using (Fe⁰/TNW) nanocomposite in remediating CIP-contaminated water.

This research project investigated the performance of (Fe⁰/TNW) nanocomposites towards the removal of ciprofloxacin (CIP) from water. TEM and XRD analysis were carried out to reveal the morphological characteristics as well as the crystalline structure of Fe⁰ nanoparticles, titanium nanowires (TNWs), and (Fe⁰/TNW) nanocomposites. The TEM results proved the successful synthesis of the chain-like structure Fe⁰ nanoparticles and TNWs. Moreover, the TEM images confirmed the deposition and distribution of Fe⁰ nanoparticles on the surface of TNWs. Furthermore, the outcomes of XRD analysis supported the findings of TEM and evidenced the successful formation of Fe⁰ nanoparticles, TNWs, and (Fe⁰/TNW) nanocomposite. Several batch experiments were also performed to illustrate the optimum mass proportion of TNWs (5, 10, 20, 30, 40, and 50%) in (Fe⁰/TNW) in the nanocomposite to achieve the best removal efficiency of CIP from water. The optimum percentage of TNWs in the prepared (Fe⁰/TNW) nanocomposites was 20% as it exhibited a stable performance with a minimal desorption behaviour (67.34% after 120 min) compared with the other percentages. In addition, another set of batch experiments was conducted to disclose the optimum removal conditions of CIP by the (20%-Fe⁰/TNW) nanocomposite such as dosage, initial pH and temperature. The results of these experiments concluded that 50 mg L⁻¹ of CIP was optimally removed by 1 g L⁻¹ of (20%-Fe⁰/TNW) nanocomposite at initial pH of 7 and under 35 °C.

All relevant data are included in the paper or its Supplementary Information.