Significant role of carbonate radicals in tetracycline hydrochloride degradation based on solar light-driven TiO₂-seashell composites: Removal and transformation pathways

Abstract

TiO₂-seashell composites prepared via a sol-gel method were used to generate carbonate radicals (•CO_3⁻) under solar light irradiation. •CO_3⁻, a selective radical, was employed to degrade the target tetracycline hydrochloride contaminant. A series of characterizations was carried out to study the structure and composition of the synthesized TiO₂-seashell composite. This material exhibits excellent solar light-driven photochemical activity in the decomposition of tetracycline hydrochloride. The possible pathway and mechanism for the photodegradation process were proposed on the basis of high-resolution electrospray ionization time-of-flight mass spectrometry experiments. Finally, we investigated the reusability of the TiO₂-seashell composite. This study is expected to provide a new facile pathway for the application of •CO_3⁻ radicals to degrade special organic pollutants in water.

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 (t_1/2) 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 + CO₃^2– → OH⁻ + •CO₃⁻

•O₂⁻ + HCO₃⁻ → HO₂⁻ + •CO₃⁻

h⁺ + CO₃^2– → •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 CO₃^2–, 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 CO₃^2–/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.