Grape-like CNTs/BaTiO3 nanocatalyst boosted hydraulic-driven piezo-activation of peroxymonosulfate for carbamazepine removal
Graphical abstract
Introduction
Peroxymonosulfate (PMS, HSO5−) based advanced oxidation technologies (AOPs) have attracted considerable attention of scientists owing to the satisfactory oxidation efficiency [1], easy activation process [2], and numerous activation approaches (e.g., energy radiation [3], [4], alkali activation [5], and catalytic activation [6], [7]). On account of desirable pollutant removal performance and environmental friendliness, the carbon nanotubes (CNTs) activation of PMS is catching much attention during water treatment [8], [9]. The active sites (sp2 carbon and carbonyl group) on CNTs could donate electrons to PMS to produce reactive oxygen species (ROS) for pollutants degradation (Eqs. (1–3)) [8], [9], [10]. However, the rapid deactivation and low reusability were the major problems of the CNTs mainly due to the intricate influences of surface chemistry and structural changes [11]. Our previous studies indicated external direct current (DC) power could provide electrons to the carbon material (e.g., activated carbon fiber [12], [13] and granulated activated carbon [14]) to suppress the destruction of carbon material during the PMS activation process. Nevertheless, the demand for an additional DC power source surely enhances the difficulty of the designing of the reactor with increased operational energy cost [15].
Meanwhile, on account of good catalytic stability and low equipment dependence, as well as underlying employment of mechanical force in nature, piezo-catalysis technology has received growing attention in PMS activation [15], [16], [17], [18]. Similar to the photocatalytic process, the piezoelectric charges, which were produced through the deformation of the piezoelectric materials caused by external mechanical forces (e.g., ball milling [19], ultrasound [20], [21], and fluid [22], [23]), could react with water/PMS/oxygen to form various reactive species for the degradation of contaminants (Eqs. (4–11)). By comparison, the hydraulic force was more easily accessible during the wastewater delivery and treatment process. However, the activation efficiency of PMS by the hydraulic-driven piezo-catalysis was much lower than that of ultrasonically driven due to the limited efficiency for charge generation, resulting in a much higher PMS concentration required in solution during hydraulic-driven piezo-activation [15], [24]. On the other hand, barium titanate (BaTiO3, BT), as a lead-free piezoelectric material with supernal dielectric constant and piezoelectric coefficient, was extensively employed in the piezoelectric activation of the PMS process [16], [17], [25]. Nevertheless, the low efficiency of charge separation due to the poor conductivity of BT further limits the pollutant degradation efficiency in a piezo-activation of PMS process driven by hydraulic force [15], [16].
Noteworthy, recent studies suggested that combining piezoelectric materials with graphene or reduced graphene improved the piezoelectric property [26], [27], [28]. In comparison, CNTs have a higher aspect ratio with excellent linear conductivity and superior mechanical properties [29], [30]. The binding of piezoelectric materials to CNTs might be bent more easily to boost the local stress of piezoelectric materials with enhanced piezoresponse, thus generating more piezo-charges. Importantly, the CNTs supported catalysts could offer efficient charge separation which played an important role in improving photocatalytic performance [31]. The presence of the interface between the CNTs and the supported catalyst, as well as the high electrical conductivity of CNTs, might also facilitate the separation and transportation of piezoelectric electrons. Motivated by the above reports, we posited a hypothesis to kill two birds with one stone: the built-in piezoelectric field generated by BT might protect the CNTs to overcome the drawback of the rapid deactivation of CNTs in the PMS process, and the CNTs might act as “fast lanes” for transfer of piezo-generated electrons and promote the separation of piezo-generated charge in the meantime.
Hence, carbamazepine (CBZ) was selected to serve as the target contaminant, CNTs/BT nanocomposite was synthesized and expected to activate the PMS more effectively with stable performance by employing the hydraulic force. The purposes of this research were to (1) synthesize and characterize the CNTs/BT nanocomposite; (2) examine the performance, catalytic durability, and material stability of the CNTs/BT-PMS system by the consecutive CBZ removal tests; (3) explore the effects of operating parameters (especially hydraulic gradient (G)); (4) investigate the catalysis activation mechanism; (5) evaluate the potential of practical applications.
Section snippets
Materials and chemicals
The details of materials and chemicals are presented in supporting information (Text S1). In addition, the quality parameters of the ultrapure water, tap water, and surface water are summarized in Table S1.
Preparation of BT nanoparticles
BT nanoparticles (NPs) were synthesized by hydrothermal process. Hydroxide precursor (Ti(OH)4) was obtained from the mixture of 50 mL tetrabutyl titanate and 500 mL dilute acetic acid using a microinjection pump (WZS-50F6, Smiths, China). The Ti(OH)4 precursor and barium octahydrate (the
Characterization of catalysts
The SEM images of the prepared catalysts are provided in Fig. 1. The BT NPs in Fig. 1a show some degree of aggregation. The original surfaces of the BT NPs display a smooth morphology in a spherical shape with an average diameter of 100 nm. The morphology of the pristine CNTs is random stacking (Fig. 1b). Each CNT is 15–30 μm in length and 3–15 nm in diameter. The as-prepared CNTs/BT nanocomposite shows a grape-shaped morphology, shown in Fig. 1c. As a supporter of BT NPs, CNTs are evenly
Conclusions
The grape-like CNTs/BT nanocomposite with the Ti − C bond was successfully synthesized. By harvesting the hydraulic energy, the CNTs/BT-PMS system could accelerate the CBZ removal, while maintained the catalytic durability and material stability. CBZ was efficiently removed (91.93%, 120 min) with the pseudo-first-order kinetics rate constants (k value, 11.62 × 10−3 min−1) in the CNTs/BT-PMS system. This k value of the CNTs/BT-PMS system was 1.52 times higher than the sum of the BT-PMS and
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the Program of the National Science Foundation of China (Grant No. 22076015), the Chongqing Natural Science Foundation Project (cstc2019jcyj-msxmX0463), and the Graduate Scientific Research and Innovation Foundation of Chongqing, China (CYB19030). We would like to thank Analytical and Testing Center of Chongqing University for materials characterization.
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