Persistent chemical pollutants in natural water are usually degraded by the water self-purification process [1-4]. 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 [5-7]. 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 [17-20], as described by Eqs. (1)–(4):
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 [27-30]. 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)) [31-35]. 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.
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.
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 deionized (DI) water.
The seashells were brushed, washed with DI water, and then dried at 80 ℃ for 24 h. The dried seashells were placed in a grinder to produce fine granules and filtered through an aluminum sieve (100 mesh). The TiO2-seashell composite was prepared by a sol-gel method. Typically, titanium tetraisopropoxide was hydrolyzed under acidic conditions and the resulting suspension was dialyzed to pH ~4. The formed transparent TiO2 sol was then calcined at 300 ℃ for 5 h to prepare the TiO2 powder[38], which was denoted as T-300. Calcination of 100 mL TiO2 sol yielded 3.0 g of powder. At the same time, a fixed mass of seashell (1.8 g) was added to 40 mL TiO2 sol, stirred for 2 h at room temperature, and then dehydrated in a microwave oven. The gels were calcined in a programmable muffle furnace at 5 ℃/min to a specific temperature (200, 300, or 400 ℃), and maintained for 5 h. Finally, 40% TS-T samples were obtained, where T stands for the calcination temperature. A control sample, denoted as S-300, was obtained by calcinating the seashell powders in a muffle furnace at 300 ℃ for 5 h.
X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance X-ray diffractometer using Cu Kα radiation (40 kV, 40 mA, λ = 1.5418 Å), in a 2θ range of 5°–90° with a 0.02° step size. X-ray photoelectron spectroscopy (XPS) analysis was carried out on a PHI Quantum 2000 system using a monochromatic Al Kα source. Specific surface areas were measured at –196 ℃ by a BELSORP-mini II nitrogen adsorption-desorption apparatus. The infrared spectra of the samples were measured by Fourier transform infrared spectroscopy (FT-IR, Thermo Scientific Nicolet Is 10, USA). UV-vis diffuse reflectance spectroscopy (DRS) measurements were performed by a Varian Cary 500 spectrometer equipped with an integrating sphere, using BaSO4 as the reference. The surface morphology of the TiO2-seashell composite was determined by scanning electron microscopy (SEM, Regulus 8100, Japan). High-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) measurements were performed on a Tecnai F30 (USA) microscope. The electron spin resonance (ESR) signals of the radicals spin-trapped by DMPO were measured with a Bruker ESP 300E spectrometer. The products generated during the photodegradation process were detected by a HRESI-TOF-MS system equipped with an AB Sciex X500R quadrupole time-of-flight (Q-TOF) spectrometer.
Irradiation experiments were performed using a solar simulator (300 W Xenon lamp). TC was employed as the target contaminant to evaluate the photochemical activity of the prepared samples. A 100 mg amount of the as-obtained powder was dispersed into a glass vessel (equipped with a water-cooling system) containing an aqueous solution of TC (100 mL, 50 mg/L) under constant stirring. Prior to irradiation, the system was stirred in the dark for 3 h to establish the adsorption-desorption equilibrium. A 4 mL aliquot of the suspension was collected at 5 min intervals during the experiment and centrifuged.
The resulting clear liquor was analyzed by a Shimadzu UV-1750 UV-vis spectrophotometer. The percentage of degradation was reported as C/C0, where C and C0 are the 357 nm absorbances of the TC solution at each irradiated time interval and at the initial adsorption/desorption equilibrium, respectively. Moreover, the reusability of the as-prepared sample was tested over five consecutive degradation experiments, whose results were analyzed as described above.
The degradation intermediates were identified by an HRESI-TOF-MS system equipped with a BEH C18 column (50 mm × 2.1 mm, 1.7 μm). The mobile phase A consisted of an aqueous formic acid solution (0.1%, v/v), whereas acetonitrile was set as phase B, at a flow rate of 0.25 mL/min. The injection volume was 2 μL and the column temperature was kept at 25 ℃. Mass spectrometry (MS) experiments were conducted with a TurbolonSpray ion source in positive mode using ESI, under the following conditions: ion spray voltage = 5500 V; ion source gas 1 = 50 psi; ion source gas 2 = 55 psi; gas temperature = 500 ℃. MS scans ranged from 200 to 550 m/z.
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 TiO2 and CaCO3 phases. Remarkably, owing to the calcination at low temperature, the 40% TS-T samples do not exhibit phase transformations [39]. In addition, the average diameters of TiO2 crystals in the samples were calculated from the Scherrer equation based on the half-width of the (101) peak of anatase TiO2; the analysis reveals average diameters of 6, 5, 6, and 7 nm for the T-300 and 40% TS-T samples, respectively. These results indicate that (i) the addition of CaCO3 during the synthesis process had no obvious influence on the average crystallite sizes of TiO2, and (ii) the average crystal diameters of TiO2 increased with increasing calcination temperature.
The FT-IR spectra of T-300, S-300, and 40% TS-T samples are displayed in Fig. 2. The broad peaks centered at 3400 and 1619 cm–1 in the spectrum of T-300 are assigned to the O–H stretching and O–H bending modes of adsorbed water molecules on the surface of TiO2, respectively [40]. The strong wide band observed between 800 and 400 cm–1 originates from the O–Ti–O stretching adsorption [41]. In addition, the peak located at 1383 cm–1 corresponds to the residual –OCH2CH3 groups resulting from the incomplete hydrolysis of titanium tetraisopropoxide during the preparation of the TiO2 sol [42]. The spectrum of the pure seashell sample displays three strong absorption peaks at 1418, 879, and 713 cm–1, which can be attributed to the stretching vibration of C–O bonds [43]. Moreover, the seashell sample shows a significantly wide band at 3447 cm–1, mainly originating from the –OH stretching vibration. The main peaks of both TiO2 and seashell are also observed in the spectra of the 40% TS-T (T = 200, 300, and 400) samples. Notably, compared to T-300, the spectra of these samples show a marked blue shift and narrowing of the O–Ti–O peak, which indicates the formation of an intimate contact between the TiO2 and seashell components.
N2 adsorption-desorption measurements were carried out to determine the surface area and pore structure of the as-synthesized samples. Except for S-300, the adsorption-desorption isotherms (Fig. 3a) of the T-300 and 40% TS-T samples display type-IV curves, which is the typical adsorption behavior of porous materials. The Brunauer-Emmett-Teller (BET) specific surface area of the T-300, S-300, and 40% TS-T (T = 200, 300, and 400) samples are 125.4, 2.9, 99.9, 80.8, and 64.4 m2/g, respectively (Table 1). Moreover, the average pore size (Fig. 3b) of T-300 is about 7.5 nm with a narrow pore size distribution, which is similar to that observed for 40% TS-300. For the 40% TS-T samples calcinated at different temperatures (T = 200, 300, and 400 ℃), the increasing calcination temperature results in an increase in the average pore size, which is due to the growth of TiO2 crystals.
Fig. 4 displays the SEM images of the S-300, T-300, and 40% TS-300 samples, showing their microscopic morphology and structure. As shown in Fig. 4a and 4b, S-300 exhibits an ordered multilayer bulk structure, while T-300 is mainly composed of small particles (Fig. 4c and 4d). The combination of seashell with TiO2 results in the seashell surface being covered with TiO2 nanoparticles, denoting an intimate interfacial contact between the TiO2 and seashell components (Fig. 4e and 4f). The energy dispersive spectroscopy (EDS) mappings (Fig. 4g and 4h) of the 40% TS-300 sample clearly confirm that Ti and Ca are uniformly distributed throughout the whole composite.
For a more detailed structural and morphological characterization of the samples, we performed transmission electron microscopy (TEM) measurements, which reveal that TiO2 nanoparticles are immobilized on the seashell surface, as shown in Fig. 5a. Clear lattice fringes with d = 0.35 and 0.19 nm, corresponding to the (101) and (200) planes of anatase TiO2, respectively, are observed in the high-resolution TEM (HRTEM) image of the 40% TS-300 sample (Fig. 5b). Moreover, the average TiO2 particle size in the 40% TS-300 sample is estimated to be 6 nm, which is consistent with the value calculated from the XRD results.
XPS measurements were carried out to further investigate the chemical states of the bonded elements in the samples. Fig. 6a displays the high-resolution Ti 2p XPS spectra of T-300 and 40% TS-300. Two characteristic Ti 2p1/2 and Ti 2p3/2 peaks are observed at 464.1 and 458.4 eV, respectively, which confirms the oxidation state of Ti4+ species in the samples. The 5.7 eV peak separation between the Ti 2p1/2 and Ti 2p3/2 signals is in good agreement with earlier reports [44]. Notably, in the case of 40% TS-300, the characteristic Ti 2p1/2 and 2p3/2 peaks are slightly shifted toward low energies compared to those of T-300. This may be attributed to the change in the electronic structure of Ti induced by the interaction between TiO2 and CaCO3. Additionally, the O 1s peak of T-300 can be deconvoluted into two sub-bands. The peak located at 529.7 eV corresponds to lattice oxygen, while that at 531.4 eV can be assigned to surface-adsorbed oxygen species (Fig. 6b) [45]. In addition to the lattice and surface-adsorbed oxygen peaks, a new peak at 531.0 eV, assigned to CO32– or carbonate-like species, is observed for the 40% TS-300 sample. Moreover, the C 1s spectrum of 40% TS-300 (Fig. 6c) can be deconvoluted into four peaks at 289.3, 288.3, 286.2, and 284.7 eV, which are assigned to CO32–, O=C–O, C–O, and C–C groups, respectively [46]. The presence of the C–C group can be attributed to the incomplete hydrolysis of titanium tetraisopropoxide during the preparation of the TiO2 sol. This is consistent with the results of the FT-IR analysis. Furthermore, the high-resolution Ca 2p XPS spectrum of 40% TS-300 (Fig. 6d) displays two Ca 2p1/2 peaks with binding energy of 350.2 eV, along with a Ca 2p3/2 peaks at 346.6 eV, indicating that the oxidation state of Ca in this material is +2 [47]. The XPS analysis results thus confirm the formation of the TiO2-seashell hybrid composite with strong interactions between the two components.
The UV-vis diffuse reflectance spectra of T-300, S-300, and 40% TS-T (T = 200, 300, and 400) are displayed in Fig. 7. In the spectrum of T-300, the absorption band edge is found at ca. 398 nm, corresponding to a band gap of about 3.1 eV. S-300 shows almost no absorption in the UV-visible region. The absorption band edges of the 40% TS-T (T = 200, 300, and 400) samples are similar to each other, which indicates that the calcining temperatures has no obvious effect on the light absorption performance; however, the spectra of these samples show a significant blue shift compared to that of TiO2.
The photochemical activities of the as-synthesized samples were evaluated by TC degradation experiments under solar light irradiation. Dark adsorption experiments were first performed to establish the adsorption-desorption equilibrium between the T-300, S-300, 40% TS-300 samples and the TC solution. As shown in Fig. 8a, the results indicate that the adsorption-desorption equilibrium can be established within 3 h in the dark. T-300 displays the highest adsorption capacity, which is in line with its largest BET surface area. Then, the temporal changes in the concentration of TC under solar light irradiation were monitored by measuring its maximum absorbance at 357 nm in the UV-vis spectrum. As shown in Fig. 8b, TC is stable and does not show degradation under solar light. TiO2, as a typical photocatalyst, displays obvious catalytic activity in the degradation of TC, due to the generation of ∙OH, ∙O2– radicals, and holes under illumination[32]. The TC conversion rate reaches up to 88.6% for T-300 within 30 min of solar light irradiation. Notably, the combination of seashell with TiO2 results in a significantly improved TC degradation efficiency for all 40% TS-T samples, among which 40% TS-300 presents the highest degradation ratio (94.0%). It is well known that the ∙OH, ∙O2–, or h+ species can be consumed by CO32– or HCO3– to produce ∙CO3–, which is much more selective than ∙OH for the degradation of organic species such as phenols, anilines, or amino acids. In this context, given the high content of CaCO3 in the seashell material, it is reasonable to deduce that the enhanced activity of the 40% TS-T sample is due to the production of ∙CO3– in the TiO2-seashell composite, which facilitates the degradation of TC. Moreover, the lower activity of the 40% TS-200 sample may be ascribed to a combination of factors, such as the low crystallization degree of TiO2 and the weak interaction between seashell and TiO2 in the composite. On the other hand, the lower activity of the 40% TS-400 sample may be attributed to the large average size and small specific surface area of the TiO2 particles. These results demonstrate the importance of controlling the synthesis temperature for achieving the best performance of the TiO2-seashell composite.
HRESI-TOF-MS analysis was performed to identify the products formed during the reaction, and thus clarify the possible mechanism for the photochemical degradation of TC. Fig. 9 shows the various kinds of intermediate products of TC degradation before and after a 30 min photochemistry test performed over T-300 and 40% TS-300, respectively. As shown in Fig. 9a–c, TC (m/z 445) is fragmented into various products in 10 min of reaction with T-300, and mineralizes into H2O and CO2 after 30 min of deeper degradation. In the test with the 40% TS-300 catalyst, TC is also observed to phototransform into different intermediates, whose intensities are weaker than those observed for the T-300 sample and even disappear entirely within 30 min, as shown in Fig. 9d–f. In addition, the TC degradation rate of 40% TS-300 is higher than that of T-300, which is in good agreement with the results of Fig. 8b. Seven main intermediate products of 40% TS-300 can be identified in this test. In combination with the results of previous related studies, we propose two possible tetracycline degradation pathways, as illustrated in Scheme 1. In pathway I, TC (m/z 445) is initially transformed to P1 (m/z 458) via the oxidation of ∙CO3– [21, 24]. Then, P1 is converted to P2 (m/z 443) by the removal of one amino group, owing to the low energy of C–N bonds[48], after which P2 is further degraded into P3 (m/z 306) through loss of the dimethylamino group, dehydroxylation, benzene ring opening, and multiple hydroxylation [49]. P3 can then be further transformed to P4 (m/z 202) via dehydroxylation and oxidation reactions, thus completing the degradation of TC. On the other hand, pathway II is mainly a hydroxylation process. P5 (m/z 477) is generated from the hydroxylation of the parent TC molecule; this is consistent with previous research [50]. Then, deamination of P5 leads to the formation of P6 (m/z 460), which is finally decomposed to P7 (m/z 414) via dehydration, decarbonylation, and further successive ROS reactions [51].
To explore the underlying mechanism of TC degradation by TiO2 and TiO2-seashell materials, the main oxidative species involved in the photochemical process were studied using different scavengers, i.e., BQ as ∙O2– radical scavenger, AO as hole scavenger, IPA as ∙OH scavenger, and 4-CP as ∙CO3– scavenger [52, 53]. As shown in Fig. 10a, the addition of IPA shows no detrimental effects on the catalytic activity of TiO2. In contrast, the degradation efficiency of TC significantly drops when BQ and AO are added into the reaction system. Therefore, it can be concluded that ∙O2– radicals and h+ are the dominant oxidative species for the degradation of TC over the TiO2 catalyst, whereas the ·OH radical is not involved in the process.
In comparison, the TiO2-seashell hybrid composite displays a different TC degradation mechanism. As shown in Fig. 10b, the TC degradation is significantly inhibited after the addition of BQ and AO scavengers. However, the negative effect of BQ and AO on the degradation of TC in the TiO2-seashell system is weaker than that observed in TiO2. This can be ascribed to the fact that a fraction of the ∙O2– or h+ species have been consumed to promote the conversion of CO32– into ∙CO3– [54-56]. In addition, the results also show that the catalytic activity is suppressed upon IPA addition, indicating that the ∙OH radical is also involved in the reaction process. Moreover, the presence of 4-CP significantly affects the TC degradation (Fig. 10b), which implies that the ∙CO3– species are active in the early stages of the reaction, accelerating the oxidation of TC molecules to produce intermediate products that can be further degraded by ·OH, ∙O2–, and h+.
ESR measurements were performed to further confirm the generation of carbonate radicals in the photochemical system with 40% TS-300 [3, 5]. The experiments were carried out in the presence of DMPO in water. Fig. 11 shows the spin-trapping ESR spectra of T-300 and 40% TS-300. No signal is detected in the dark. However, after 30 s of solar light illumination, the characteristic 1:2:2:1 quadruplet of DMPO-OH adducts is observed for both samples; the intensity of the DMPO-OH adduct peaks is higher in the 40% TS-300 than in the T-300 photochemical system, which indicates the formation of a higher amount of ∙OH radicals [57, 58]. Both peaks corresponding to the DMPO-OH and DMPO-OCO2 adducts are observed for the 40% TS-300 catalyst, confirming that CO32– or HCO3– can be transformed into ∙CO3– in the illuminated 40% TS-300 system.
Based on the above discussion, we propose a possible photodegradation mechanism of TC over the 40% TS-300 catalyst, as illustrated in Scheme 2. Under solar light irradiation, the TiO2 component of the TiO2-seashell composite is excited by solar light to generate electron-hole pairs; the generated electrons are able to activate O2 to produce ∙O2– species, while the holes contribute to form ∙OH radicals. The photoinduced ∙O2–, h+, and ∙OH species can then produce selective ∙CO3– radicals by reacting with CO32–/HCO3–, thus achieving the synergistic degradation of TC over the TiO2-seashell hybrid composite with improved photochemical performance.
Because the stability is as important as the activity for the application of catalytic materials, we tested the stability of the as-prepared 40% TS-300 composite. As shown in Fig. 12, the cycling tests show that the catalyst exhibits negligible loss of activity after five consecutive cycles, indicating that the TiO2-seashell composite maintains a steady production of ∙CO3– to promote the degradation of TC. Moreover, we obtained the XRD pattern of the 40% TS-300 sample after the reaction. As shown in Fig. 13a, there are no obvious differences between the phase structures of TiO2 and pristine 40% TS-300; however, the CaCO3 component shows a distinct weakening of the XRD peaks. This can be attributed to the consumption of seashell material in the ∙CO3– generation process. This result is in agreement with the FT-IR analysis. As displayed in Fig. 13b, after the catalytic reaction the 40% TS-300 sample shows O–Ti–O peaks in the 800–400 cm–1 range similar to those of the pristine 40% TS-300 sample, but the peak intensity is higher. This can be ascribed to the consumption of seashell and higher surface exposure of TiO2 in the used 40% TS-300 sample. In addition, the spectrum of the 40% TS-300 sample after the catalytic reaction shows that the 1418 cm–1 stretching vibration of C–O groups in the seashell is dramatically weakened in comparison to that of the pristine sample, which further confirms the consumption of CO32– to generate ∙CO3– species.
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-seashell catalyst was proposed on the basis of the HRESI-TOF-MS analysis of the reaction system. This work demonstrates the significant role of ∙CO3– species in accelerating the degradation of organic pollutants, and is expected to provide a new facile pathway for employing ∙CO3– radicals to degrade special organic pollutants in water.
2020,
Vol. 41 Issue (10): 1511-1521 DOI: 10.11936/bjutxb2019120031 
PDF
2020,
Vol. 41 Issue (10): 1511-1521 DOI: 10.11936/bjutxb2019120031 PDF