Efficient Removal of Ciprofloxacin from Contaminated Water via Polystyrene Anion Exchange Resin with Nanoconfined Zero-Valent Iron

Abstract

Ciprofloxacin (CIP), an important emerging contaminant, has been frequently detected in water, and its efficient removal has become an issue of great concern. In this study, a nanocomposite material nZVI/PA was synthesized by impregnating nanoscale zero-valent iron (nZVI) inside a millimeter-sized porous host (polystyrene-based anion exchange resin (PA)) for CIP removal. The nZVI/PA composite was characterized by field emission scanning electron microscopy coupled with energy-dispersive X-ray, transmission electron microscopy, X-ray diffraction, as well as X-ray photoelectron spectroscopy, and it was confirmed that nZVI was uniformly dispersed in PA with a small particle size. Furthermore, several key factors were investigated including initial solution pH, initial CIP concentration, co-existing ions, organic ligands, and dissolved oxygen. The experimental results indicated that the nZVI/PA composites exhibited a high removal efficiency for CIP under the conditions of initial pH 5.0, and initial CIP concentration 50 mg L−1 at 25 °C, with the maximum removal rate of CIP reaching 98.5%. Moreover, the nZVI/PA composites exhibited high efficiency even after five cycles. Furthermore, quenching tests and electron spin resonance (ESR) confirmed that CIP degradation was attributed to hydroxyl (·OH) and superoxide radicals (⋅O2−). Finally, the main degradation products of CIP were analyzed, and degradation pathways including the hydroxylation of the quinolone ring, the cleavage of the piperazine ring, and defluorination were proposed. These results are valuable for evaluating the practical application of nZVI/PA composites for the removal of CIP and other fluoroquinolone antibiotics.

Recent Advances in Nanoscale Zero-Valent Iron (nZVI)-Based Advanced Oxidation Processes (AOPs): Applications, Mechanisms, and Future Prospects

“…As an environmentally friendly and cost-effective oxidant, H 2 O 2 is also widely used as an oxidant in AOPs, since the decomposition products of H 2 O 2 are harmful H 2 O and O 2 ions [106]. The traditional Fenton process adopts ferrous salts as catalysts for the activation of H 2 O 2 , which not only requires a limited pH range (2.0-4.0), but also requires a large amount of ferrous salts, leading to the formation of massive sludge and a wastage of iron [107]. Ferrous ions generated from the slow dissolution of nZVI can promise the production of ROS (e.g., •OH, etc.)…”

“…Furthermore, a basic solution is not only adverse to the corrosion of nZVI, but also unfavorable for the adsorption of negatively charged contaminants [140]. The removal efficiency of contaminants can also be significantly affected by inorganic anions and organic compounds (Figure 12) [107,137,138]. Furthermore, the reusability investigation indicated that the removal efficiency decreased significantly in the recycling process of nZVI materials, which may be attributed to the formation of iron oxide on nZVI, resulting from the deactivation of nZVI with DO (Figure 12) [107,139,140].…”

“…The removal efficiency of contaminants can also be significantly affected by inorganic anions and organic compounds (Figure 12) [107,137,138]. Furthermore, the reusability investigation indicated that the removal efficiency decreased significantly in the recycling process of nZVI materials, which may be attributed to the formation of iron oxide on nZVI, resulting from the deactivation of nZVI with DO (Figure 12) [107,139,140]. Concerning the influence of reaction factors on the removal of contaminants, a high temperature and a moderately acid condition is favorable (Figure 12) [132,139].…”

“…In O 2 -based AOPs, it is worth noting that nZVI can react with DO through a twoelectron reduction reaction to generate H 2 O 2 (Equation ( 46)), and can further generate •OH via the Fenton reaction (Equation ( 47)) [141,142]. The equations and reported studies indicated that the generation of •OH prefers an acid condition [107,143], and DO is essential in the formation of •OH, and was the primary factor responsible for the oxidation of contaminants and their intermediates (Figure 13) [143]. With regard to the degradation mechanism of contaminants, nZVI can also exert adsorption, reduction, dichlorination, and denitration on contaminants, promoting the mineralization process of contaminants (Figure 13B,D) [132,141].…”

“…The reaction of Fe 2+ and O 2 may produce •O 2 − , which can be further ignited to form the more reactive 1 O 2 ions (Equations ( 48) and ( 49)) [134,144]. Many researchers have unveiled the important role of •O 2 − and 1 O 2 in the oxidative degradation of contaminants and their generation routes that are assisted by doped Al/Mn (Equations ( 50)-( 52)) [107,132,134,142,143].…”