Implementing thermally-excited-catalytic course solely using ambient heat motivation for efficient abatement of water pollutants

Abstract

Interest in photocatalysis has been fascinated by the realization that solar light is effectively an inexhaustible energy resource. Nevertheless, the necessity for high-energy ultraviolet irradiation and excessive recombination of photogenerated electrons and holes have seriously restricted its evolution for industrial sized scales. Here we report on a thermally-excited-catalytic (TEC) design purely from leveraging ambient heat to completely decompose water pollutants. A-fabricated Cu/NiMn₂O₄ (CNM) with a thermally-sensitized feature possesses relatively high TEC activity that disassociates hazardous materials in the dark, suggesting successful uses catalyzing a broad range of chemical reactions with a degradation process. Additionally, a convincing explanation of a TEC decomposition mechanism is described from collecting the degradation products. Stability evaluation demonstrates that the morphology and structure of CNM remains unchanged even after multiple degradation use cycles, and hence this stable catalyst can be recovered and reused without appreciable loss in activity. Briefly, our evidence may attest to feasibility of a general concept for executing a TEC route through heat stimulus, offering a desirable way to effectively remove environmental contamination.

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Introduction

Drinking water is considered indispensable for life-forms in our natural systems. However, with the rapid development of worldwide industrialization, water pollutants are increasing and continue to strain the already scarce supply of clean water, resulting in grave and irreparable damage to the globe from the surface to its core.¹ In particular, effluents derived from a variety of industries produce large quantities of synthetic organic dyes in many fields of modern technology.^2,3 Discharge of these colored compounds causes considerable non-aesthetic pollution and serious health-risk factors.⁴ Consequently, the search for powerful methods for decontaminating wastewater has attracted significant attention over the past decades. Important conventional technologies developed for tackling these issues primarily include physio-chemical/chemical methods, microbiological treatments, enzymatic decomposition, and photocatalysis; these have been summarized (see ESI Fig. S1†). These techniques, nonetheless, still suffer from low removal efficiency and/or poor stability, sometimes even producing unexpected secondary pollutants.

Considering solar energy as an ultimate and sustainable resource,^5,6 photocatalysis as a “green” technique has been extensively applied in the area of environmental remediation.^7–10 The most commonly used oxide photocatalyst is titanium dioxide (TiO₂) because of its low cost and high activity.^11–13 However, this approach still has significant challenges that have not yet been addressed. Rather, the shortcomings of TiO₂ are essentially due to a low quantum yield that originates from undesired charge-carrier (e⁻/h⁺) recombination and the necessity to use ultraviolet (UV) light whose proportion is merely 4% in the solar light spectrum.^14,15 The concern of realistic application actually stems from its wide band gap which requires expensive UV light for activation.¹⁶ As a result, work has been strongly focused on modulating the electronic band gap structure of TiO₂ toward visible light response.¹⁷ To date, many solutions^4,18 have been explored to tailor the band gap absorption for the visible light region in order to narrow the corresponding band gaps in nature as depicted in Fig. 1(1–6). Examples include metal/non ion doping, co-doping with foreign ions, noble metal deposition, sensitization using inorganic complexes or organic dyes, Schottky junctions, band-structure matching, and surface heterostructures.

Fig. 1 Diversely modified photocatalytic processes and developed thermally-excited-catalytic routes. (1–6) Graphics of various modified methods for narrowing the band gap of photocatalyst. (7) Graphic of plausible thermally-excited-catalytic mechanism.

Despite improving catalytic activity via aforementioned means, very unfortunately, so far no significant advancements have been reported. Thus, it is essential to solve this issue by developing a novel treatment pathway that could be executed conveniently without the limitation of UV light activation. We were inspired by the concept that physical and chemical changes in substances often result from robust ambient influences in response to various external factors, such as light, heat, moisture, chemical reaction, or biological activity.^19,20 Herein, we postulate that it may be useful to implement the removal of water contaminants through thermally-excited-catalytic (TEC) degradation that is widely responsive to heat activation in ambient atmosphere as illustrated in Fig. 1(7) (see also the right bottom of Fig. S1†). Since a TEC design might be competent to directly harvest heat-energy from the surrounding environment, it may offer an attractive way to compensate for the restrictions of a photocatalytic path. In order to verify this idea, a thermally-sensitized catalyst dependent on low temperature stimulus was synthesized using a tentatively scalable method, and then the superior TEC activity and a likely detailed mechanism for decomposing organic dyes were discovered. Further, stability evaluation concerning the as-prepared catalyst was also employed for long-term utilization (i.e., recycling) of it, suggesting the maintenance of TEC activity relatively comparative to that of fresh catalyst even after repeated cycling. In short, the focus of this research is devoted to expanding a widely existing energy form as an inexpensive, friendly source for driving a TEC process for future management of water pollutants.

Experimental section

Chemicals and solvents

Mn(NO₃)₂·4H₂O, Cu(NO₃)₂·3H₂O, Ni(NO₃)₂·6H₂O, Co(NO₃)₂·6H₂O, Zr(NO₃)₂·5H₂O, SiO₂·2H₂O and CaCl₂ were purchased from Aladdin (Shanghai Aladdin Biochemical Technology Co., Ltd., china). Methylene blue (MB), methanol, ethanol, and tetrahydrofuran (THF) were of analytical grade and obtained from various sources. Deionized and doubly distilled water was employed in all experiments. All reagents used in this work were used as received without further purification.

Synthesis of thermally-sensitized catalyst

Cu/NiMn₂O₄ (CNM) was synthesized with the following procedure: 0.45 mol of Mn(NO₃)₂·4H₂O, 0.12 mol of Cu(NO₃)₂·3H₂O and 0.14 mol of Ni(NO₃)₂ were dissolved in a mixture (volume ratio: 1 : 1 : 3, 500 mL) of methanol, ethanol, and THF. This solution was heated quickly to 60 °C with vigorous magnetic stirring and then maintained at this temperature for 1 h to remove low boiling point impurities and water; finally, it was heated to 90 °C to further remove the solvent to make a concentrated solution. THF as a co-solvent easily dissolved the metal salts and it was also very convenient to get rid of water. Next, 0.07 mol of SiO₂·2H₂O and 0.01 mol of CaCl₂ were added to the mixture with stirring for another 3 h to harvest the mixed solid composite. The obtained powder was sealed in a container and heated to 850 °C with 10 °C min⁻¹ ramping rate and kept for 4 h, then heated up to 1100 °C with a 5 °C min⁻¹ ramping rate and maintained for 1 h in a muffle furnace, after which it was cooled to room temperature naturally. SiO₂·2H₂O was used as a template-like media largely for support of CNM, and CaCl₂ was used to achieve a better distribution of appropriate structural formations of CNM. The resulting solid was collected by centrifugation and washed with deionized water five times to remove residual ions, and finally dried at 40 °C for 3 h for further use.

Adsorption and recovery capacity

Before any degradation experiments, absorption tests of MB in the dark were first completed. CNM (50 mg) was added to a 100 mL glass vessel containing 50 mL MB solution of 50 mg L⁻¹, at 35 °C (these conditions were determined from previous optimal trials). CNM was dispersed with a magnetic stirrer and the adsorption process equilibrium was achieved after 2.5 h. In addition, recovery tests (see ESI Fig. S3†) were also conducted after completing the adsorption process.

TEC degradation reaction and recycle assessment

TEC activities for degrading MB in solutions containing heavy metal ions (Co, Ni, or Zr ions with a concentration of 0.001 M in a corresponding solution system) in the presence of CNM were evaluated in darkness. Related conditions such as catalyst dosage, solution volume, solution concentration, and temperature were the same as the adsorption tests. The degradation process was carried out for 60 h and samples were collected every 12 h. Aliquots of 5 mL were collected at each time interval and centrifuged immediately at 5500 rpm for 10 min before further measurements. Stability evaluation of CNM was also performed through continuous degradation reactions for its recycle use. Control experiments (without CNM) of MB solutions, including the systems added with the related metallic ions, were made for comparisons (see ESI Table S2†). Moreover, to further strengthen data reproducibility, five duplicated processes were employed in each sampling to compute an average value. The experimental error in this work was within ±1%.

Material characterization and measurement

Surface morphology of the specimens was characterized by SEM (Ultra 55, Zeiss Co., Germany), and the samples were coated with gold by sputtering for 5 min before the tests with a voltage of 15.00 kV. A small amount of CNM was dispersed in ethanol to form a suspension, and then a drop of that was subsequently deposited onto a carbon-coated Cu grid for specimen research with a TEM (Zeiss Libra 200 FE, Germany) using an accelerating voltage of 200 kV. The surface morphology of CNM was observed by means of AFM (SPA300HV, Seiko Instruments, Japan) operating in the tapping mode under air atmosphere. XRD patterns were collected with a PANalytical X'Pert Pro X-ray diffractometer using Cu Kα radiation (1.54 Å), with a working voltage of 40 kV. XPS measurements were carried out with a VG Multilab 2000 spectrometer employing Mg Kα radiation. Chemical compositions of the samples were determined by an EDX spectrometer (Rayny EDX-800HS, Shimadzu, Japan). Optical spectra of all samples were obtained with a UV-Vis spectrometer (UV3900, Hitachi Co., Japan). FT-IR spectra of all dried materials were measured at 400–4000 cm⁻¹ by an infrared spectrophotometer (Nicolet 6700, Nicolet Instruments Co., USA) using KBr pellets. The degradation byproducts from MB were weighed and analyzed by gas chromatography-mass spectrometry (GC-MS, Agilent 5975MSD, USA). Inorganic ions from degradation products were also determined by ionic chromatography (IC, 792 basic IC, Metrohm Co., Switzerland).

Results and discussion

Morphology and composition of CNM

Thermally-sensitized CNM was synthesized through a facile method combined with a required pretreatment. Morphological features of CNM were characterized by SEM as seen in Fig. 2a–c. It is apparent that the obtained particles are almost bulk-like or square-like with a diameter or size ranging from hundreds of nanometers to several micrometers, estimated from gradually enlarging SEM images. TEM micrographs in Fig. 2d and e exhibit good homogeneity of the sample, demonstrating that CNM are well-proportioned, at least in their physical composition. A further magnified image in Fig. 2f shows high ordering present in the particle aspect over a long distance. As depicted in Fig. 2g, the corresponding SAED pattern clearly displays some concentric circles, confirming polycrystalline characteristics of the catalyst. In addition, the structure of CNM was investigated and showed quantities of randomly packed nano-scale aggregates by AFM as shown in Fig. 2h, which is identical to the phenomena in SEM and TEM observations. However, the corresponding line profile (see ESI Fig. S8†) of AFM about CNM reveals that the height of the particles is concentrated at ∼10 nm,²¹ signifying that CNM consists mostly of larger particles and a few smaller ones. These surface features will be supportive to provide a large body of potential sites in the adsorption process of dyes and also greatly assist with the contribution of surface absorbed species,²² further favoring the promotion of heat-generated charge transport in the subsequent TEC degradation reactions. Besides, the ion exchange process may be helpful to the physical adsorption as mentioned below. Because the solvent (for example, water in this work) enables different molecules with positive or negative chargers to combine simultaneously, it ultimately results in enhancing the adsorption capacity.²³

Fig. 2 Structural and morphological information of CNM. (a–c) Images of SEM in different scales. (d–f) Images of TEM showing fine structures. (g) SAED pattern. (h) Image of AFM in a 2D-view.

In order to accurately examine specific chemical details (see also ESI Table S1†), CNM, XRD, EDX, and XPS studies were performed. Fig. 3a shows each separate phase with a counterpart reflection, including three different crystalline solids. Specifically, the most important 2θ at 30.1° (220), 35.3° (311), 62.3° (440), and 35.3° (311), 57.0° (511), 62.3° (400), and 20.8° (100), 26.8° (011), 39.3° (102) are indicative for CuMn₂O₄ (JCPDS card no. 34-1400), NiMn₂O₄ (JCPDS card no. 01-1110), and a small amount of SiO₂ (JCPDS card no. 47-1144), respectively. The peaks related to intensity and sharpness are expressed robustly, thus demonstrating the evidence of a three-system hybrid catalyst. Meanwhile, the average crystallite sizes of most CNM particles were calculated to be 35.7 nm based upon the strong and sharp reflection peak (311) via the Scheller equation,²⁴ and they are generally identical to the results from previous TEM measurements. The detection of elements match well with XRD results in chemical composition using EDX as shown in Fig. 3b. The addition of calcium (as also measured in XPS) for better dispersibility actually acted as an intermediate analogous to the catalyst function.

Fig. 3 Crystalline and chemical components of CNM. (a) XRD pattern. (b) EDX spectrum. (c) XPS spectrum in part. (d) Mn 2p of XPS. (e) Cu 2p of XPS. (f) Ni 2p of XPS. (g) Ca 2p of XPS. (h) Si 2p of XPS. (i) O 1s of XPS.

In consideration of revealing more typical peaks, Fig. 3c presents the most distinguished peaks in the XPS spectrum (in part only), showing the obvious peaks of the characteristic locations for Ca 2s/2p, O 1s, and Mn 2p. Other fine structures of XPS tests are seen in Fig. 3d–i; the Mn 2p3/2 peaks centered at 641.7 and 653.5 eV could be assigned to Mn³⁺ ions in Mn³⁺–O bonds. The 2p spectra of Cu and Ni are both deconvoluted into nearly two overlapped peaks in large measure, corresponding to a slightly doped interaction with respect to these metallic states (except for major inorganic compounds formed), further suggesting the nature of a three-system hybrid doped with Cu/Ni (which is not limited to only the simplified mixture), and very comparable to co-catalysts.^25–27 Moreover, concerning Ca 2p, Si 2p and O 1s spectra, these noticeably rule out the existence of Ca/Si–O bonds, namely releasing the assisted role in the CNM formation process as they indeed obey traditional chemical bonding behavior. Put another way, the fundamental control of CNM preparation is completely executed based upon the expected design, eventually producing a rationally constructed catalyst system.²⁸

Physical adsorption capacity of CNM

The immoderate discarding of colored wastes, such as large quantities of dyes from industry into water, is very toxic to aquatic life, easily leading to serious environmental pollution. MB is a typical kind of fairly stable pigment that, without any treatment and no degradation, could be found in the natural environment. In this case, we attempt to report on a strategy to study the TEC degradation of MB in water in the presence of CNM. Before starting degradation tests, the adsorption capacity of MB to CNM was determined in the dark, and then the adsorption details of different systems were evaluated. It is worthwhile mentioning that the dual-blend solutions of pure MB with different metallic ions were used because of their exceptional closeness to real industrial waste. Generally, adsorption processes are all equivalent to a particular adsorption equilibrium after 2.5 h (see ESI Fig. S2†) for all groups. Corresponding MB removal rates in MB, MB/Co ions, MB/Ni ions, and MB/Zr ions solutions reach up to 28.93, 8.67, 10.98, and 17.59%, respectively. The evidence obtained signifies that the adsorption properties on diverse MB systems containing different metallic ions have been seriously altered in adsorbing MB molecules due to the interactive roles between metallic ions and MB molecules. Therein, the Zr-containing group is approximately equivalent to the blank (pure MB solution) in terms of removal rate. We note also that the final adsorption contents concerning MB solutions containing Co or Ni ions are both below the cases containing Zr ions, presumably resulting from poor chemical or/and physical interactions in comparison to those comprising Zr ions.

Fig. 4 shows the adsorption process of MB solutions containing metallic ions; this specific process may be divided into three main steps. First, the mixtures of MB molecules and corresponding metallic ions concentrate on the surrounding regions nearby to a CNM surface. Second, the combinations of two phases above happen via chemical complexation and physical interaction. Finally, the inclusion-like substances formed would be contacted increasingly towards the active sites from the CNM, ultimately engineering its related adsorption equilibrium in absorbing MB. Currently, it is reasonable to summarize that a solution system which contains metallic ions possessing larger radius and many more coordination numbers, is superior to that with a lesser one in the final adsorption content, and this is also in good agreement with the experimental data (Fig. S2†). More explicitly, a metallic core ion could interact with MB molecules until there is no vacancy on the surface of the catalyst near to itself (which is determined coincidently by its radius size and complexation number).

Fig. 4 Schematic procedures of MB molecules absorbed onto CNM surface. In a solution atmosphere containing different metallic ions.

To support a reliable adsorption behavior, recovery (which means desorption) tests were investigated to prove that all MB molecules were physically adsorbed onto the catalyst surface instead of degrading into by-products in darkness.²⁹ CNM surface adsorbed with only a small amount of MB was collected and transferred to ethanol solvent to release MB molecules back into the solution. The desorption substances derived from natural evaporation of the MB/ethanol solution were monitored by FT-IR spectroscopy, together with assessing the total contents of the desorbed dye. It can be clearly seen that no obvious changes were observed in FT-IR spectra (Fig. S3A†) between original and collected samples since the main characteristic peaks of pure MB still existed as compared to the latter, indicating that most of the MB molecules were certainly desorbed back into solvent. As for a tiny loss of MB (Fig. S3B†), this could be attributed to physical consumption during extraction from the solvent. In brief, dark adsorption capacity could be confirmed as the major engine rather than certain chemical processes, which predicts that it would have a significant contribution for good TEC activity in the following degradation experiments.

TEC degradation evidence

In addition to adsorption ability, we next examined MB degradation efficiency, as visualized in tentative results (see ESI Fig. S4†). The purpose was to reap MB optimal effect of TEC degradation by varying the substrate concentrations which possess a predominant significance with industry practice. It was found that the cases of 10 and 50 mg L⁻¹ both gained a better degradation efficiency similar to each other when compared to corresponding adsorption in the dark. However, the higher concentration was finally chosen throughout the rest of our work because of practical value. On the other hand, from a desorption/degradation view, it's necessary to further verify the true degradation exerted via the TEC method; therefore we measured the surface changes of CNM before and after TEC treatment. As shown Fig. 5, the concretely changeable process is evidenced by facts obtained from SEM observations (see ESI Fig. S5†) first, that the surface of fresh CNM presents early pieces of sequential aggregates. In addition, some powder-like matter was observed on the surface and interface of the catalyst, indicating its adequate adsorption surface in adsorbing MB molecules. And finally, intriguingly, a clean surface of CNM evidently recovered once again, was comparable to the initial fresh one. For this reason, combining the surface structure of CNM with numerous decreases in MB absorbance (Fig. 6), it is plausible to consider a general decomposition capability by TEC for water pollutants. Also, we may assume that TEC reactions could take place following a continuous supply of MB molecules, which stems from the dynamic equilibrium in MB adsorption. Besides, in view of implementing the TEC process under thermal conditions, the degradation behavior of CNM should be made clear at or near room temperature. Obviously, weight loss of synthesized CNM is of relatively little significance until to 300 °C (see ESI Fig. S7†), primarily due to the loss of water molecules. Subsequently, the loss of CNM is most likely due to some weak interactions in CNM upon heating. Hence, the self-degradation property of CNM at even higher temperatures almost can be ignored, indicating that MB degradation effects are ascribed to the catalytic role of CNM.

Fig. 5 Schematic of MB molecules adhesion to CNM. Stage description of CNM surface: fresh catalyst surface, catalyst surface in an equilibrium adsorbing MB, and its surface after the degradation process (from left to right).

Fig. 6 UV-vis spectra of different dual-blend solution systems in the whole degradation process. (A) MB. (B) MB/Co ions. (C) MB/Ni ions. (D) MB/Zr ions.

Traditionally, most research exploits changes using only the corresponding maximum wavelength (often in visible region) for a particular pollutant, frequently ignoring the chemical groups more difficultly destroyed in the UV domain. Therefore, significant abatements for organic dyes often cannot be achieved. Thus, a dominant decrease centered at peaks in UV area could be virtually equal to representing relatively complete degradation effects in the full-range spectral region regarding organic dyes. In view of the analyses aforementioned, a representative series of UV-vis spectra for CNM, are shown in Fig. 6. This illustrates that organic contaminants have been chemically cleaned regardless of the characteristic groups that appeared in the UV and visible range, forcefully showing that the proposed TEC route could be a preferable degradation process. For instance, the major peaks at 245 and 660 nm are largely diminished in intensity after experiencing TEC reactions, indicating that this degradation design enables the true deterioration of organics. Simultaneously, Fig. 7 shows the FT-IR spectra, which disclose changes of the pristine MB and treated cases (residues collected through room-temperature slow evaporation method) in a characteristic group. The main adsorption peaks of functional groups associated with MB are 1600, 1488, 1388, 1333, and 1243–1178 cm⁻¹, corresponding to C=N, C=C, benzene ring, C_AR–N, and N–CH₃, respectively. While the peaks above almost totally disappeared after the degradation process owing to TEC action, the already dramatic reduction of the partially remaining peak at 1388 cm⁻¹ is suggestive of the expectedly difficult destruction of the benzene ring. Even so, these results show the effectiveness for mitigating organic waste by virtue of the TEC route responding to heat excitation. Our results clearly imply the potential successful degradation of MB using CNM under dark conditions. Further, in order to confirm the major role of CNM in effectively deteriorating water pollutants, degradation tests concerning CNM, Cu, Ni, and Mn oxides were implemented. The outcomes (see ESI Fig. S6†) suggest that pure Cu, Ni, or Mn oxides all possess a very weak ability for treating contaminants using TEC. Relative effectiveness indicates that Mn oxides are better than those of Cu or Ni oxides, probably because of the more complex electronic structures of Mn, which could easily produce active species used in a degradation process. Of course, the true composition (see also XRD data) of CNM includes not only the mixtures of metallic oxides mentioned, indicating that metallic oxide alloys are superior to separate metallic oxides for catalysis. Summarily, Mn should exert the primary chemically active role in catalyzing degradation of organics, but the CNM system would be a major catalytic part in comparison to any pure metallic oxides.

Fig. 7 FT-IR spectra of various substrates from the different dual-blend solution systems after degradation including the control group. (a) Fresh MB. (b) MB. (c) MB/Co ions. (d) MB/Ni ions. (e) MB/Zr ions.

Product identification and reaction pathway

Chemical reactions universally consist of the breaking of old bonds and the formation of new ones. According to the bond dissociation energy (BDE) theory, the lower the BDE is, the more active the chemical bond is and the easier it is for old bonds to break and new bonds to form. Hence, to better explore the mineralization mechanism for deteriorating MB with CNM, the intermediates and final products were determined by GC-MS and IC analysis because many intermediates, even including active species, could be formed as a result of ˙OH displaying a high degree of selectivity to different functional groups.

It should be first stressed that, based on the blank tests, the degradation of MB is negligible or nearly zero without adding catalyst. Also, experimental evidence predicts that the MB solution is highly stable and inactive under dark conditions. After excluding these expected factors, five new dominant peaks representing the major transformation products of the MB degradation process, are differentiated in Fig. 8a–d by means of GC, with retention times (R_t) of 1.72, 3.83, 5.55, 5.73, and 6.21 min, respectively. Other possible products are not identified because their concentrations are too low in the reaction mixture system or they have already been degraded to undetectable small inorganic molecules. To further confirm the chemical formulas of the intermediates identified, measurements as shown in Fig. 8e and k were performed using GC-MS analysis. It was found that major MS peaks observed at m/z 84 (M : (M + 2) = 3 : 1), 92, and 106 (including three-type substances), in good agreement with GC analysis, could clearly correspond to dichloromethane, toluene, ethylbenzene, m-xylene, and o-xylene (spectra revealing details of these intermediates have been analyzed and presented in the counterpart Fig. 8f–j), respectively. It is worthwhile that residual benzene-based products agree perfectly with results from FT-IR analysis, strongly indicating the successful destruction of organic pollutants.

Fig. 8 GC-MS and IC profiles of MB degradation products using CNM. (a–d) Experimental retention times of organic intermediates, including dichloromethane (I1) and toluene (I2), ethylbenzene (I3), m-xylene (I4), and o-xylene (I5). (e) Standard retention time of particular organic substances (I1–I5, from left to right). (f–j) MS spectra and spectrum unfolding of organic intermediates (I1–I5). (k) Standard MS spectra of specific organic substances (I1–I5, from left to right). (l) Determination of inorganic ions.

The systematic mechanistic reactions for the degradation process are illustrated in Fig. 9. By collecting continuous heat-excitation energy,³⁰ electron/hole pairs generated in the conduction and valence bands of CNM could be transferred to the surface and next react with adsorbed MB molecules that have an appropriate redox potential, accordingly yielding large amount of active species. It is known that hydroxide radicals (˙OH) are the most useful factors responsible for decomposing organic contaminants.^31,32 The formed ˙OH would be driven to the cationic sulfur groups and heteroaromatic rings of MB molecules over an electrophilic interaction which is able to induce opening of the aromatic rings, eventually generating hydroxylated or sulphur-containing intermediates. Further, a long-term oxidation action is exerted to destroy these groups, ultimately resulting in the absolute dissociation of two rings. To conclude, the disintegration of these aromatic compounds leads to the noticeable formation of low molecular-weight substances (CO₂ and H₂O), even transforming them into various inorganic ions such as Cl⁻, NO₃⁻ and SO₄²⁻ ions.³³ These ions were indeed discovered when characterizing the degradation products via ion chromatography by comparing their R_t with those of standards in Fig. 8l. Specifically, Cl⁻ in MB is ionized first during the dissolution and virtually exists in the detached state in the solution. N–CH₃ connected to C₁₂ has the lowest BDE value in terms of MB molecular structure. When bombarded with the radical species, the N–CH₃ is broken first and then the –CH₃ is persistently oxidized into HCHO or HCOOH. At this time, C–S and C–N become the most active parts of remaining MB structures and the bonds from them are cracked more easily under the bombardment of ˙OH. Meanwhile, phenyl compounds and other small molecular structures that were determined during GC-MS analysis are formed, until the complete decomposition of MB occurs.

Fig. 9 Mechanistic process of CNM in carrying out TEC degradation. The innermost circle indicates that the catalyst attacks the goal MB molecules by heat excitation. The middle annulus refers to the intermediates during the degradation process. The outermost annulus signifies additional byproducts until they are reduced to small molecules or ions.

Recycling stability and application assessment

Property stability of CNM has been increasingly valued because material recycling could lessen the environmental load and reduce the disposal price.^34–36 The consequences indicate an excellent structural stability of CNM because the uniform structure was preserved (i.e., the size and surface condition, when the neat sample was compared with one that was used three times in the degradation process), see Fig. 10a and b. As expected, the crystalline phases of CNM, after undergoing degradation courses, are consistent with the fresh one in Fig. 10c. Besides, the composition of CNM was analyzed in-depth with EDX and XPS techniques and the EDX evidences (Fig. 10d) show that the main ratios of Cu, Ni, and Mn do not significantly change after the stability test. The characteristic peaks on Mn 2p, Cu 2p, Ni 2p, Ca 2p, Si 2p, and O 1s in Fig. 10e–j generally maintain their initial locations in reference to previous information from Fig. 3d–i, suggesting the nature of the brilliant recycling property from a chemical viewpoint. These phenomena attest that CNM is very stable both in the physical morphology of the catalyst and in its chemical ingredients after TEC degradation treatment. Thus, this thermally-sensitized CNM catalyst may have fairly good potential for future practical applications. Given this, recycling utilization trials were implemented on different systems including pure MB, MB/Co, MB/Ni, and MB/Zr. After the first use of CNM the total removal rates of these four groups all increase gradually with increasing time as shown in Fig. 11a, and the group without metallic ions has a larger preferential to that of the group containing ions in removing MB efficiency. The TEC activity of abating MB does not significantly decrease in Fig. 11b even when three cycling courses have been experienced, implying that CNM can be efficiently reused for repeated cycles without an appreciable loss of activity. It is important to note that the tedious centrifugation, mechanical agitation, and drying process were not always done during the three cycles. Therefore, this CNM catalyst may display great suitability for industrial applications of large-scale treatment of organic wastewater due to its stability, good recyclability, and operational convenience.

Fig. 10 Morphology and chemical composition of CNM after three degradation cycles. (a) SEM image of fresh catalyst. (b) SEM image of treated catalyst. (c) XRD pattern. (d) EDX spectrum. (e) Mn 2p of XPS. (f) Cu 2p of XPS. (g) Ni 2p of XPS. (h) Ca 2p of XPS. (i) Si 2p of XPS. (j) O 1s of XPS.

Fig. 11 Recycling activity evaluation of CNM. (a) The removal rates of MB, MB/Co ions, MB/Ni ions, and MB/Zr ions systems when conducting the first degradation process. (b) The removal rates of MB, MB/Co ions, MB/Ni ions, and MB/Zr ions systems after three degradation cycles.

Conclusions

Thermally-sensitized Cu/NiMn₂O₄ (CNM) catalyst obtained by a facile manner with moderate pretreatment presents high catalytic activity in deteriorating MB under close to room temperature conditions in darkness. In particular, conducting a thermally-excited-catalytic (TEC) route could outperform a photocatalytic route lacking or without ultraviolet light, continuously exerting a robust destruction role in the degradation of organics. For this reason, the concept of using heat actuation from the surroundings has been established in this work. We found that an outstanding adsorption capacity displayed an admirable TEC degradation effect despite inconsistent metallic ions existing in wastewater. An emerging understanding of the TEC degradation mechanism can be interpreted from degradation products. Furthermore, the morphology and composition of CNM remain invariant, even after three degradation cycles, indicating the advantageous recycling performance for destructing water pollutants using TEC reactions. These findings demonstrate that the TEC degradation system may fulfill the promise of actual applications for environmental care and is anticipated to impel the evolution of a thermally-driven catalytic design toward a general and superior route in the environmental disposal field.

Competing financial interests

The authors declare no competing financial interests.

Acknowledgements

The authors are most grateful to the Key Research Program of National Nuclear Facility Decommissioning and Radioactive Waste Treatment (Grant No. 2014ZG6101) for the generous financial support.

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Footnotes

† Electronic supplementary information (ESI) available: Typical treatment methods for contaminants, comparison of dark adsorption ability, recovery tests, removal efficiency to different substance concentration, morphological changes of CNM surface, degradation effect comparison, TG profile, line profile of AFM image, specific composition of CNM, and information of control tests. See DOI: 10.1039/c5ra22113g

‡ These authors contributed equally to this paper.

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