ArticleSignificant role of carbonate radicals in tetracycline hydrochloride degradation based on solar light-driven TiO2-seashell composites: Removal and transformation pathways
Graphical Abstract
Novel TiO2-seashell composites were successfully fabricated via a sol-gel method and benefited from the activity of •CO3− radicals generated under solar light irradiation. The TiO2-seashell composite exhibited enhanced performance in the photodegradation of tetracycline hydrochloride.
Introduction
Persistent chemical pollutants in natural water are usually degraded by the water self-purification process [1, 2, 3, 4]. As eco-water is fully exposed to direct sunlight, photochemical reactions become significant pathways for the transformation of organic pollutants; these processes include direct and indirect photolytic reactions [5, 6, 7]. The former proceed by absorption of actinic radiation and subsequent decomposition, while the latter usually involve the oxidation of organic matter by reactive oxygen species (ROS), such as carbonate (•CO3−), hydroxyl (•OH), superoxide (•O2−), and sulfate (•SO4−) radicals, as well as hydrogen peroxide (H2O2) and singlet oxygen (1O2) [1, 8, 9]. The ROS are short-lived species with half-lives (t1/2) in the range of nanoseconds to seconds in aqueous systems [10].
Generally, •OH can act as an oxidant (E0 = 2.3 V, pH = 7), whose decontamination activity involves electron transfer processes with second-order rate constants [11] ranging from 107 to 1010 M−1 s−1. This radical is one of the main species involved in the oxidation of organics in photochemical purification, and is usually produced by the illumination of nitrites, nitrates, or dissolved organic matter (DOM) in surface water at concentrations of 10−15–10−18 M [12]. Hydroxyl radicals have been applied to engineered water treatment systems via advanced oxidation processes (AOPs) [13, 14]. Nevertheless, they also tend to be eliminated by various solutes, DOM itself, and inorganic carbon species (carbonates and bicarbonates).
Unlike the •OH radical, •CO3− species can reach a higher steady-state concentration (10−13–10−15 M) in sunlit natural water, which can be ascribed to a less efficient scavenging by DOM and weak self-quenching [15, 16]. Commonly, •CO3− radicals are produced through the oxidation of carbonate or bicarbonate ions by •OH, •SO4−, and excited triplet state aromatic ketones, or formed by the photochemical reaction of metal-carbonate complexes [17, 18, 19, 20], as described by Eqs. (1)–(4):
•OH + HCO3− → H2O + •CO3−
•OH + CO32– → OH− + •CO3−
•O2− + HCO3− → HO2− + •CO3−
h+ + CO32– → •CO3−
In particular, •CO3− displays higher selectivity than •OH for the degradation of organic species. For example, it reacts rapidly with phenols, anilines, and some amino acids, but relatively slowly with saturated alkanes, aromatic hydrocarbons [21, 22], and other species. Moreover, it is a powerful one-electron oxidant (E0 = 1.78 V at pH 7) and can react rapidly with organic compounds through electron transfer, with second-order rate constants ranging from 103 to 109 M−1 s−1 [23, 24]. Taking into account its relatively high steady-state concentration in aqueous systems along with its high selectivity, the •CO3− radical is believed to play a role as significant as that of the •OH species in indirect photochemistry. Although the use of •CO3− in the degradation of pharmaceuticals has been investigated, research on the application of •CO3− for pollutant degradation is still at an early stage and limited data are available in the literature [25, 26]. In addition, the mechanism of •CO3− formation and its reaction pathway for the degradation of specific contaminants are still unclear. Hence, a comprehensive investigation of the role of •CO3− in contaminant degradation represents a crucial task.
Seashells are attracting increasingly widespread attention because of their low cost, eco-friendliness, and nontoxicity [27, 28, 29, 30]. As an important component of the world's marine wealth, seashells contain more than 95% CaCO3, which is an ideal source of abundant •CO3− in aqueous solution. However, in order to achieve an efficient generation of •CO3− from CO32–, it is essential to introduce appropriate ROS, such as •OH, •O2−, or h+, to stimulate the transformation. Titanium dioxide (TiO2) is a well-known semiconductor photocatalyst able to produce large amounts of •OH, •O2−, or h+ species under light irradiation (Eqs. (5)–(9)) [31, 32, 33, 34, 35]. Previous studies have shown that the addition of CO32–/HCO3− is beneficial for enhancing the reaction rates of the UV/TiO2 system in the degradation of pharmaceuticals [8, 36]. In this context, it is reasonable to expect that the fabrication of TiO2-seashell composites would be a promising way to generate large amounts of •CO3− radicals under light irradiation, which should greatly contribute to accelerating the degradation of pharmaceuticals.
TiO2 + hv → h+ + e−
h+ + OH− → •OH
e− + O2 → •O2−
•O2− + H2O → •OOH + OH−
2•OOH + e− → 3•OH + OH−
Against this background, in this paper we report for the first time the efficient use of seashells to fabricate a novel TiO2-seashell composite with highly enhanced photochemical activity for tetracycline hydrochloride (TC) degradation. As a well-known broad-spectrum antibiotic, a dramatic accumulation of TC has been detected in aqueous systems, due to its growing consumption and steadily increasing demand [37]. Therefore, there is an urgent demand for the development of a facile and effective way to eliminate TC residues in aquatic environments. In this work, we show that the integration of seashells with TiO2 in the TiO2-seashell composite results in a considerably enhanced TC degradation performance in water compared to that of pure TiO2, which is attributed to the generation of abundant •CO3− radicals under light irradiation. Moreover, the byproducts of the degradation process are also identified by high-resolution electrospray ionization time-of-flight mass spectrometry (HRESI-TOF-MS). This work is thus expected to provide a novel and practical reference for a better understanding of the mechanism of photochemical degradation of TC.
Section snippets
Materials
TC (96%), benzoquinone (BQ, 99%), ammonium oxalate (AO, 99.8%), isopropyl alcohol (IPA, ≥ 99.5%), 4-chlorophenol (4-CP, 99%), and 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) were purchased from Aladdin (Shanghai, China). Titanium tetraisopropoxide (C12H28O4Ti, 97%) was provided by Sigma-Aldrich. Waste seashells were collected from a seafood market in Fuzhou, China. All chemicals and reagents were of analytical grade and used without further purification, and all solutions were prepared using
Characterization of the obtained samples
Fig. 1 shows the XRD patterns of the T-300, S-300, and 40% TS-T (T = 200, 300, and 400) samples. T-300 shows peaks at 2θ angles of 25.3°, 37.8°, 48.1°, 53.9°, and 55.1°, which correspond to the (101), (004), (200), (105), and (211) crystal planes of anatase TiO2 (JCPDS No. 21-1272), respectively. In the case of the S-300 control sample, all peaks match well with those of CaCO3 (JCPDS No. 01-070-0095). Compared to T-300 and S-300, the 40% TS-T (T = 200, 300, and 400) samples show a mixture of TiO
Conclusions
In this work, we prepared novel TiO2-seashell hybrid composites and proposed that carbonate could be effectively activated by TiO2 to generate •CO3− species for TC elimination under solar light illumination. Compared to the pure TiO2 or seashell components, the TiO2-seashell composite exhibits an obviously enhanced activity in the degradation of TC, due to the role of •CO3− in promoting the degradation process. Moreover, the underlying reaction mechanism for TC degradation over the TiO2
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Published 5 October 2020
This work was supported by the National Natural Science Foundation of China (21875037, 51502036), and the National Key R&D Program of China (2016YFB0302303).