Carbonaceous composite materials from calcination of azo dye-adsorbed layered doubled hydroxide with enhanced photocatalytic efficiency for removal of Ibuprofen in water

Background: The discard of used adsorbents may pose a great threat to human health and ecological environment. This work herein reported a facile and feasible method, with aims of (i) reusing the calcined layered double hydroxide (CLDH) adsorbent after azo dye adsorption, and (ii) being further used as a photocatalyst to enhance the degradation of typical pharmaceuticals. Calcination under inner gas flow has been utilized to carbonize adsorbed azo dye and a kind of novel carbonaceous CLDH composite material (CM-CLDH) was synthesized. This fabricated material was used as a catalyst for Ibuprofen removal in water samples under simulated sunlight irradiation. Results: According to our experimental results, combination of carbonaceous material with CLDH showed an enhanced photocatalytic performance compared to original CLDH materials. More than 90 % of Ibuprofen could be removed in less than 180 min. Introduction of carbon material narrowed catalyst’s band gap and turned its conduct band potential to a more negative state, which brought considerable light absorption and higher oxidation ability of photo-induced electrons. Furthermore, photoluminescent spectra and transient photocurrent examination confirmed that carbon material suppressed recombination of photo-induced electrons and holes through faster electron transportation. Under experimental conditions, the removal efficiency of Ibuprofen by CM-CLDH composite kept above 90 % during five cycles. Conclusion: Calcination under inner gas flow can transform organic pollutant-adsorbed CLDH to CM-CLDH composite with higher photocatalytic performance. A feasible way to reuse spent LDH adsorbents was proposed, which is benefit to reduce second pollution of spent adsorbents in environment.


Background
Azo dyes draw enormous attention for causing mutation and carcinoma through aromatic amines 1 , and they are likely to trigger foul smell, eutrophication, decrease of dissolved oxygen amount when released into natural water, posing a threat to humans and aquatic ecosystem [2][3] . Adsorption 4 , electrocatalysis 5 and photocatalysis 6 have been applied to deal with azo dye wastewater, among them, adsorption is preferred for its low cost, simple operation, high removal efficiency and 3 reusability 7-8 .
Layered double hydroxide (LDH) is a kind of hydrotalcite-like compounds, denoted as [M (1-x) 2+ M x 3+ (OH) 2 ] layer [A x/n n-·zH 2 O] interlayer , where M 2+ , M 3+ and A nrepresent divalent, trivalent metal cations and interlayer anions, respectively 4 . LDHs have been regarded as a kind of effective adsorbents attributing to their tunable charge densities and large specific surface areas 8 , the interlayer anions can exchange with anions from contaminants 9 , and large specific surface areas can provide with sufficient adsorption sites and reactive hydroxy for ligand exchange 10 . When calcined at moderate temperatures (400-500 ℃), LDH's layered structure will collapse owing to interlayer anions' decomposition and release of interlayer water. Normally, calcined LDH (CLDH) exhibits favorable adsorption capacity and efficiency due to larger specific surface areas and lamellar structure's reconstruction, researchers regard it as a kind of promising adsorbent for azo dye treatment [11][12] .
After adsorption of anions from wastewater, treatment of spent LDHs becomes a tough challenge.
Normal landfilling or piling up may result in foul smell, contamination of soil and underground water by leachate 13 . According to limited research consequences, after adsorption, Laipan et al. fabricated carbon-LDH composites via calcination under inert gas flow. The resulting material was efficient for Cr(VI) reduction 13 , and it would be converted to metal ions and porous carbon through pickling. Metal ions were utilized for LDH synthesis once again, and porous structure was promising for toluene adsorption 14 . Nevertheless, researches related to using combination of organic pollutant-based carbonaceous material and LDH for photocatalysis are still limited. As mentioned in former research articles, addition of carbon-based materials like fullerence, graphene oxide (GO) and reduced graphene oxide (rGO) could promote LDH's photocatalytic efficiency. Further study pointed out that combination of LDHs and carbon-based materials would hamper recombination of photo-induced charges and aggregation of LDH layered structures [15][16] . Ju et al. fabricated fullerence/ZnAlTi-LDO composites, unique electronic properties allowed fullerence to act as an ideal electron acceptor to 4 maintain effective separation of photo-induced electrons and holes 17 . GO and rGO own long-range π-π conjugations, remarkable electron mobility and specific surface area 16,[18][19]  together with light-adsorption of wider wavelength range [21][22] . As a member of conjugated organic polymer, Ppy's conjugated P-electronic structure led to fast separation efficiency of photo-generated charge carrier, more electrons and holes could move to the surface then reacted to generate radicals.
Researchers also found that carbon materials could have large specific surface aera, which was suitable for adsorption 23 . Dong et al. combined agar with normal semiconductor C 3 N 4 , which made more pollutants adsorbed on the surface of catalyst, photodegradation efficiency was promoted 24 .
Though these mentioned materials facilitate certain photocatalytic performance, high price and relatively complex preparation process still restrict their applications.  CLDHs material was used as a photocatalyst to study the removal performance of the Ibuprofen (IBF), one of the mass-produced and wide-distributed acesodyne 25 in water environment. The possible mechanisms of Ibuprofen removal were investigated and discussed. The stability and reusability of the composite CM-CLDHs as photocatalyst were also evaluated. This work can provide a feasible way to synthesize carbonaceous functional material by reusing spent LDH adsorbents, which is also benefit to reduce second pollution of spent adsorbents in environment.

Materials
All chemicals were purchased from chemical companies, no purification was operated on them during the experimental process. times. In the end, ZnAlLa-LDH was obtained after being dried at 80 ℃ overnight then ground into powder in mortar.

Preparation of ZnAlLa-CLDH
Aforementioned ZnAlLa-LDH was put in 20 mL crucible before sent to muffle furnace for calcination.

Preparation of CM-CLDH catalyst
According to the screening result of adsorption performance, ZnAlLa-400CLDH presented highest adsorption efficiency among all the prepared ZnAlLa-CLDH materials. As for CM-CLDH fabrication, ZnAlLa-400CLDH was added in 10, 20, 50, 100 mg/L amaranth with the dosage of 500 mg·L -1 to establish adsorption equilibrium. To be specific, in dye solution with concentration ranged from 10 to 100 mg/L, amaranth could be thoroughly adsorbed and removed by ZnAlLa-400CLDH. After suction filtration and washed by ultrapure water, the resulting precursor was sent to tube furnace for calcination at 400, 500, 600 ℃ under the protection of nitrogen (flow rate = 0.2 L·min -1 ), respectively.
Meanwhile, the heating rate and heating time were controlled at 5 ℃·min -1 and 180 min. The final resulting product is named as CM-CLDH, which is a carbonaceous composite CLDH material.

Characterization
Detection of materials' Crystal phases were performed by a D-8 Advance X-ray diffractometer (Bruker-AXS, Germany) with Cu Ka radiation operated at 40 kV and 40 mA. Fourier transfer infrared (FTIR) spectra was obtained from Thermo Nicolet 5700 (Thermo Nicolet Corporation, USA).

Adsorption experiments
Batch adsorption experiments were carried out on magnetic stirring apparatus with reaction temperature controlled at 25±5 ℃, 100 mL amaranth solution (concentration = 50 mg/L) was poured into 150 mL conical flask, sealed by aluminum foil. A certain amount of LDH/CLDHs (500 mg/L) were added into solution, then solution was stirred at 500 rpm for 6 hours. Syringe was used to collect sample at 20, 40, 60, 120, 180, 240, 300, 360 minutes after the start of adsorption experiments, the sample solution was filtered by 0.45 μm aqueous phase membrane before examined by UV-vis spectrophotometer (SOPTOP UV2400, China) at wavelength of 520 nm.
Experimental data was fitted by pseudo-first-order and pseudo-second-order kinetic models, two kinds of formulas were listed as follows:

See formulas 1 and 2 in the supplementary files.
where q e and q t successively represented adsorption capacity of the adsorbent at equilibrium and time t; K 1 (min -1 ) and K 2 [g (mg min) -1 ] represented adsorption rate constants of these two kinetic models.
In the meantime, adsorption isotherms were determined via batch equilibration method, with temperature set at 25±5 ℃. 100 mL dye solution was added into each beaker, concentration of amaranth ranged from 10 to 1000 mg/L, dosage of adsorbents was 500 mg/L. After stirred on magnetic stirrer at 500 rpm for 16 hours, 2.5 mL of resulting solution was collected by syringe equipped with 0.45 μm aqueous phase membrane in sequence, then examined by UV-vis spectrophotometer at wavelength of 520 nm. Langmuir and Freundlich equation were chosen to assess adsorption behavior of amaranth onto ZnAlLa-400CLDH, the formulas were listed as follows:

See formulas 3 and 4 in the supplementary files.
where q e (mg/g) was equilibrium adsorption capacity of amaranth onto ZnAlLa-400CLDH, q max (mg/g) was the theoretical maximum monolayer sorption capacity, C e (mg/L) was concentration of dye 8 solution at equilibrium, K L and K F were Langmuir and Freundlich adsorption constants.

photocatalytic experiments
Photodegradation rate of IBF under simulated solar irradiation was applied to assess photocatalytic performance of prepared CM-CLDH composite materials. Simulated solar irradiation was generated by a 500 W xenon lamp (NBeT, China), light source wavelength was controlled by a 300 nm cut-off filter.
Certain amount of IBF solution and catalysts were added into 100 mL quartz tube, mixed with magnetic stir bar. The quartz tubes were immobilized in a ring-like holder with 8 cm distance away from lamp, and the holder could rotate at a constant rate.
The mixture of IBF and catalyst CM-CLDH was magnetically stirred at dark for 30 minutes to minimize effect of adsorption, followed by light illumination for several hours. About 1 mL reaction solution samples were collected by syringe periodically, filtered by 0.22 μm water phase membrane to separate solid particles. The concentration of IBF in resulting solution was investigated by ultraperformance liquid chromatography equipped with photodiode array detector (Waters Acquity, USA). reconstruction had been accomplished. Referred to former research, the main reason was LDH's intrinsic "memory effect" 29 .

FTIR
As shown in Fig. 1B, peaks at around 3450 cm -1 were belonged to O-H stretching vibration of water on surface and intercalated into layered double hydroxide 30 . Intensity was weaker in calcined LDH, caused by water loss via calcination. The absorption band stood at 1367 cm -1 was connected with asymmetric stretching of interlayer carbonate 31 . Bands located at 500-1000 cm -1 region were related to metal-oxygen and metal-hydroxyl vibrations 32 . Asymmetric stretching vibration of S-O was observed at around 1200 cm -133 , and vibrations at region of 1100-1300 cm -1 of CLDH-AM were corresponding to SO 3 2vibrations 31 , manifesting amaranth was adsorbed by CLDH. Original La 3+ peak was at 834.4 eV 35 . O 1s peak at 530.9 eV was assigned to metal-oxide chemical bonds 18 . C 1s peaks at 285.0 and 289.0 eV were attibuted to C-C and C=O species 18,36 , indicating that adsorbed amaranth has been transformed into the carbon based material, which could also be proved by elemental mapping images. After photocatalytic process, binding energy peaks had positive shifts, the probable cause was interaction between Ibuprofen anions and CM-CLDH during reaction, increase of electronegativity promoted attraction between extranuclear electrons and nuclear, which led to augment of binding energy 35 .  Amaranth kept stable during 50 ℃ to 300 ℃, then a rapid weight loss could be spotted with temperature risen from 300 ℃ to 800 ℃. A great number of aromatic groups might provide amaranth with a prominent thermal stability. When temperature rose to a relevant high range, the aromatic structure triggered carbonation process, which could explain sharp decrease of weight loss 40 . The carbonaceous products remained stable, so there was still 51.9 % residue of amaranth left at 800 ℃.

Thermal analysis
Compared with pristine LDH, CLDH-AM suffered from less weight loss in the first step, indicating weaker interaction between LDH and water. In the end, total mass loss of the composite CLDH-AM material was 21.3 %, much less than LDH (33.05 %), demonstrating the intercalation of amaranth could improve thermal stability and hydrophobicity of LDH 14 .

Amaranth adsorption experiments
As shown in Fig. 4a, CLDHs calcined at 400, 500, 600 ℃ showed obviously higher adsorption capacity of amaranth than LDH, driven by larger specific surface area and "memory effect" as mentioned previously.
No significant difference had been found on CLDHs' adsorption capacity and rate, calcination temperature had a negligible effect on amaranth adsorption in this experiment. All three adsorbents could reach adsorption equilibrium at 180 min, and ZnAlLa-400CLDH got a relevant higher reacion rate, so this material was chosen for follow-up experiments and characterizaions.
Adsorption kinetics and adsorption isotherms of amaranth on ZnAlLa-400CLDH was fitted by pseudofirst-order model and Freundlich isothem model, respectively. Pseudo-first-order model was proposed by Lagergren, in this model, adsorbate-diffusion via a boundary occurred in advance of adsorption 41 .
Adsorbent owing heterogeneous surface is the basic assumption of Freundlich isotherm. In this empirical model, adosption was a heterogenetic process, interactions between adsorbates were taken into account 42 . Besides, the increase of uptake capacity would lead to exponential reduction in binding energy of surficial multilayers from adsorbed ions 43 . Hence, a large amount of amaranth molecules might be adsorbed on the surface of ZnAlLa-400CLDH stacks by stacks, which was in consonance with SEM images.

Photocatalysis experiments
After getting adsorption equilibrium of 50 mg/L amaranth, the composite material CLDH-AM were sent to tubular furnace for calcination, temperature was set at 400, 500 and 600℃, respectively. The 12 resulting materials were noted as 400CM-CLDH, 500CM-CLDH and 600CM-CLDH, then utilized as photocatalysts in Ibuprofen degradation experiments, detailed procedures were recorded in 2.5. Fig. 5 recorded data of photodegradation process, X axis represented time (min) and Y axis represented the ratio of sampled solution's concentration (C) and initial concentration (C 0 ). The consequence (Fig. 5a) inlucidated that when amount of carbon source was equal, composites calcined at 500 ℃ had a higher photodegradation efficiency.
To evaluate the effect of carbon source amount on photocatalytic efficiency, in the following step, ZnAlLa-400CLDH was stirred in amaranth solution (concentration controlled at 10, 20, 50, 100 respectively) overnight for amaranth thoroughly adsorbed by ZnAlLa-400CLDH. Then resulting composites were calcined at 500 ℃ under nitrogen flow. Samples were correspondingly noted as CM-CLDH10, CM-CLDH20, CM-CLDH50, CM-CLDH100. As displayed in Fig. 5b, with the decrease of adsorbed amaranth, the resulting catalysts exhibited higher working efficiency. As pointed out in the previous publication, excessive carbon material might trigger light harvesting competition between carbon material and semiconductor. Also, instead of improving electron transportation, excessive carbon material could act as a center for photo-induced carriers' recombination 44 .
When compared with pristine CLDHs calcined at 400, 500 and 600 ℃, CM-CLDH10 still presented obvious advantages, curves of C/C 0 dropped rapidly especially in the first 60 minutes (Fig. 5c).
Pseudo-first-order equation derived from Langmuir-Hinshelwood model was used to evaluate kinetics of photocatalytic process 45 , the formula could be expressed as follows:

See formula 5 in the supplementary files.
where k app represented the apparent pseudo-first-order rate constant, C 0 and C were pollutant concentration at the start and the end of reaction. Higher k app value normally indicated faster photodegradation process of a catalyst, k app of CM-CLDH10, pure ZnAlLa-CLDHs and their pseudofirst-order kinetic plots were shown in Fig. 5d. Obviously, CM-CLDH10 displayed the highest photodegradation rate. Additionally, in CM-CLDH10 systhem, it removed more than 90 % Ibuprofen with the least time comsumption, which could be regarded as the most effective catalysts in this 13 work. Moreover, CM-CLDH10 was collected via suction filtration after photocatalysis, recycled sample was used in IBF photodegradation experiments for another four times to evaluate its recyclability and stability. As depicted in Fig. S3, after five runs, photodegradation efficiency of CM-CLDH10 descreased from 94.9 % to 89.7 %, indicating promising recyclability and stability of synthezised catalyst.

Effect of solution pH
Original the increase of pH, holes' oxidation ability would diminish due to cathodic displacement happened on valence band position 45 . And too many OHcould also form hydrogen oxidation film during photocatalytic process, which hampered efficiency of catalysts 17 .
Electronic spin resonance (ESR) test was also performed to investigate main radical species during photocatalytic process. As for hydroxyl radical and singlet oxygen, 5 mL water and 5 mg catalyst were added into quartz reacion dish, then severally injected 5 μL 5,5-Dimethyl-1-pyrroline N-oxide (DMPO) and 2,2,6,6-tetramethylpiperidine (TEMP) as spin-trapping agent. The resulting mixture was irradiated under a ultraviolet lamp for 5 minutes before being sampled in capillary tube for detection. Method for superoxide radical was similar, the only difference was 5 mL water being substituted by 5 mL DMSO (dimethyl sulfoxide).
As shown in Fig. 6c  CLDH10 generated sufficient ·OH to be captured by DMPO, this evidence also reflected that combination with carbonaceous material could make CLDH present higher radical productivity, which might promote its photocatalytic performance.

Photoelectronic property
Photoluminescence (PL) singal would be generated at the time of photo-induced electrons and holes' recombination 52 , the higher intensity indicated lower separation efficiency of photo-generated carriers. With excitation wavelength set at 350 nm, PL signals of ZnAlLa-500CLDH and CM-CLDH10 were detected in the wavelength range of 365-600 nm (Fig. 7a). The consequence reflected that PL signal intensity of CM-CLDH10 was lower, manifesting better separation ability of photo-induced electrons and holes, which might be an important factor for better photocatalytic performance over CM-CLDH10. Photocurrent transient curves were obtained, the transient photocurrent responses of CM-CLDH10 was much higher than that of ZnAlLa-500CLDH, revealling carbonaceous material had a positive effect on separation of electrons and holes 53 . This conclusion was further proved by transient photocurrent response. As reflected in Fig. 7b, stronger intensity has been spotted on CM-CLDH10, which was triggered by higher separation efficiency of photo-induced h + and e -54 .
Electrochemical impedance spectroscopy (EIS) curves were applied to assess electrochemical performances of synthesized materials. In this test, 1 mol·L -1 sodium hydroxide carbon rod and saturated calomel electrode were utilized as electrolyte, counter electrode and reference electrode.   Fig. 8c and d, E CB values of n-type ZnAlLa-500CLDH and CM-CLDH10 were -0.71 V and -1.27 V (V vs NHE), after transformation from E (SCE) to E (NHE) 59 . As is known to all, Eg = E VB -E CB , calculated value of valence band potential (E VB ) were 2.66 V and 1.9 V.
According to aforementioned analysis, potential mechanism of photodegradation could be listed as follows: See formulas 6 -12 in the supplementary files.

Conclusion
In summary, this study proved that calcined LDH can effectively remove azo dye through "memory effect" driven adsorption. to reduce second pollution of spent adsorbents in environment and to recycle adsorbent materials.
The detailed mineralization efficiency and related biotoxicity of the pollutant during photodegradation process need to be investigated in a further study.

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All data generated or analysed during this study are included in this published article [and its     Schematic diagram of Ibuprofen photodegradation process over CM-CLDH10

Supplementary Files
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