Novel construction of the catalyst from red mud by the pyrolysis reduction of glucose for the peroxymonosulfate-induced degradation of m-cresol

By far, a large number of organic contaminants from the rapid industrialization are released into the environment and become a crucial issue for the ecosystem. Phenol and phenol derivatives, originated from oil refineries, pharmaceuticals, coal conversion plants, petrochemicals and chemical industries (Rajkumar et al. 2005), are considered the most prevalent organic pollutants due to their potential toxicity to humans and animals. M-cresol is one of the main ingredients in coal gasification wastewater and able to cause health chronic effects at 12 mg/L (Kavitha & Palanivelu 2005). Considering that it is resistant to conventional treatment methods, such as coagulation, sedimentation, adsorption and microbial processing, the advanced oxidation processes (AOPs) have exhibited inherent advantages in removing recalcitrant organic pollutants, which employs the produced highly active radicals (HARs) to eliminate m-cresol. Recent developments about the activation of persulfate and peroxymonosulfate (PMS) in AOPs included basic pH, thermal activation and different types of catalysts. Investigated catalysts mainly involved asphaltenes, 3D MnO₂ and 3D α-Co(OH)₂ structures and so forth (Fernandes et al. 2018, 2019b; Fedorov et al. 2020; Yuan et al. 2020a, 2020b). Nowadays, hybrid methods based on a combination of cavitation with AOPs, possessing enhanced oxidation capacity, were proposed (Fedorov et al. 2021). Other approaches to degrade phenol, cresol and relative pollutants were related to TiO₂ photocatalytic AOPs and integrated photocatalytic advanced oxidation system (Fernandes et al. 2019a, 2020).

In the light of the superiority of PMS in terms of activation, transportation, stability and pH tolerance over persulfate and H₂O₂ (Ghanbari & Moradi 2017), PMS as an oxidant has attracted considerable attentions in AOPs. SO₄^•−, ⋅OH and ¹O₂ could be produced in catalysts/PMS system and have high standard redox potential, high stability and selectivity toward organic pollutants (Fernandes et al. 2018; Wang et al. 2021). Among these HARs, ¹O₂ possesses 2.2 V of oxidation potential and exhibits a better anti-interference performance than SO₄^•− and ⋅OH (Wang et al. 2021). Therefore, AOPs based on PMS activation present potential application in the treatment of wastewater containing m-cresol. Risks related to the processes, were unexpected nitration, nitrogen fixation and N-derivatives formation during AOPs (Rayaroth et al. 2022).

Alkali, activated carbon, metal oxides, and transition metal ions and their oxides have been studied to activate PMS (Ghanbari & Moradi 2017; Wang et al. 2021; Żółtowska et al. 2021; Zhu et al. 2022). Especially, the development of heterogeneous catalysts has been more promising and environmentally friendly because of its reusability and stability. Compounds of manganese, cobalt, nickel and copper were incorporated into iron oxides to form multicomponent recombination to enhance PMS activation (Li et al. 2021a; Chi et al. 2022; Luo et al. 2022). Red mud (RM), called as bauxite residue, is a hazardous solid waste generated during the Bayer alumina extracting from bauxite ore. Nowadays, production of 1 ton alumina generates approximately 1–2.5 tons of red mud. One hundred to 150 million tons of red mud are produced annually and the total stockpile worldwide has been estimated at over 4 billion tons by far. Red mud comprises hematite (Fe₂O₃), goethite (α-FeO(OH)), anatase (TiO₂), quartz (SiO₂), gibbsite (Al(OH)₃), gypsum (CaSO₄⋅2H₂O) and calcite (CaCO₃) derived from the residual bauxite, and sodalite (Na₈Si₆Al₆O₂₄Cl₂), cancrinite (Na₆Ca₂Si₆Al₆O₂₄(CO₃)₂) and henritermierite (Ca₃(Mn_1.5Al_0.5)(SiO₄)₂(OH)₄) from desilicated minerals during the Bayer process, and small traces of rare earth elements (Agrawal & Dhawan 2021; Liu et al. 2021). In view of polymetallic oxides in red mud, it has a latent performance as the heterogeneous catalyst to activate PMS for the degradation of m-cresol. Nevertheless, the catalytic performance is poor for the degradation of m-cresol by directly using a raw red mud due to improper valence states of metal oxides and coated active substances. Therefore, it is essential to change valence states of oxides and enhance PMS activation performance for the degradation of m-cresol through chemical and physical methods. Types of real effluents containing phenol and cresols, such as refinery effluents and effluents from bitumen production, could be treated by sulfate radicals based advanced oxidation processes (Fernandes et al. 2018).

This work aims to develop a modified red mud through chemical and physical methods as a highly efficient catalyst to activate PMS for the m-cresol degradation. The modified red mud was prepared by the pyrolysis reduction of glucose in N₂ atmosphere and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectrum (XPS) technologies. The catalytic activity of catalyst for the m-cresol degradation was tested based on reaction conditions, such as catalyst dosage, PMS dosage, initial concentration of m-cresol and initial pH, etc. The involved HARs were discerned by free radical scavenging experiments and electron paramagnetic resonance. The possible degradation mechanism of m-cresol was also proposed according to the experimental results.

Red mud from a residue storage facility of an alumina refinery plant in Anshun city, Guizhou province, China, was sampled as the raw material. M-cresol (C₇H₈O) was obtained from Tianjin Guangfu Technology Development Co., Ltd, China. Peroxymonosulfate (KHSO₅) and glucose (C₆H₁₂O₆·1H₂O) were obtained from Guangdong Guanghua Sci-Tech Co., Ltd, China. Methanol (CH₄O), tert-butanol (C₄H₁₀O) and furfuryl alcohol (C₅H₆O₂) were purchased from Shanghai Macklin Biochemical Co. Ltd, China. All chemicals were of analytical grade and used without further purification.

First, the raw red mud was washed to remove soluble alkalines using deionized water three times under a liquid to solid ratio of 5 L/kg and each lasted for 30 min. The obtained sample, labeled as WRM (washed red mud), was dried at 105 °C for 12 h. After drying and crushing, the WRM and glucose (WRMG) under a weight ratio of 2:1 were ground and mixed evenly using a ball mill (MITR, China). The ball mill chamber and ball materials were on the base of zirconium dioxide and the sizes of ball were 5, 8 and 10 mm, respectively, with the quantitative proportion of 5: 3: 2. The ball-milled time was 20 min with 200 r/min of the frequency for each sample preparation. The mixed sample was then annealed at 400–800 °C for 120 min with a ramping rate of 5 °C /min in N₂ atmosphere using an atmosphere tube furnace (FURNACE1200 °C, Tianjin Zhonghuan Experimental Furnace Co. Ltd, China). The chamber on the base of quartz was used for the reduction stage under N₂ and the purity of N₂ was 99.999%. For identification, the annealed red mud samples at different temperatures were labeled as WRMG/400, WRMG/500, WRMG/600, WRMG/700 and WRMG/800, respectively. WRM as a control was also annealed at 700 °C for 120 min in N₂ atmosphere without glucose, which was labeled as WRM/700. The phase compositions of different samples were monitored by powder X-ray diffraction (XRD) analysis using XRD (X'Pert PRO, Panalytical, Holland). The surface morphology and components of the samples were observed by SEM (MIRA4-LMH, TESCAN, Czech Republic). XPS of the samples were implemented by a K-Alpha XPS apparatus (Thermo Scientific ESCALAB 250XI, USA) using a monochromatic Al Kα as the excitation source.

Batch experiments were executed in 250 mL Erlenmeyer flasks with 50 mL of m-cresol solution at the set temperature along with constant stirring. In detail, a certain dosage of catalyst was put into the Erlenmeyer flask, while a certain concentration of 50 mL m-cresol solution was transferred to the flask. A certain concentration of PMS was injected into the above suspension and then the degradation reaction was triggered. During the m-cresol degradation, different influencing factors were investigated. M-cresol degradation results of different samples were studied under 0.1 g of catalyst, 0.01 M PMS, 50 mg/L of m-cresol, reaction temperature 60 °C and reaction time 60 min. We investigated m-cresol degradation results of 25 °C–60 °C with 0.1 g of catalyst, 0.01 M PMS, 50 mg/L of m-cresol for the best catalyst, and determined kinetics model, the k value and apparent activation energy. The effect of catalyst dosage (0–0.15 g) was inspected with 0.01 M PMS, 50 mg/L of m-cresol solution, 60 °C temperature of reaction and reaction time 60 min. The effect of PMS concentration (0–0.024 M) on m-cresol degradation was explored under 0.1 g of catalyst, 50 mg/L of m-cresol, 60 °C temperature of reaction and reaction time 60 min. The effect of initial concentration (10–150 mg/L) of m-cresol was investigated when the experiment conditions were under 0.01 M PMS, 0.1 g of catalyst dosage and 60 °C temperature of reaction. In addition, the effect of initial pH (3–8) was surveyed with 0.01 M PMS, 0.1 g of catalyst and 50 mg/L of m-cresol solution and reaction temperature 60 °C. A pH meter (pHS-3E, INESA, China) was used to measure the pH of solution. Free radical scavenging experiments were carried out with methanol, tert-butanol and furfuryl alcohol as ⋅OH, SO₄^•− and ¹O₂ scavenging agents, respectively. HARs were monitored by electron spin resonance spectroscopy (BRUKER A300, Germany). No sooner had the reaction been finished at certain time than 0.5 mL of furfuryl alcohol was added to quench the reaction. Whereafter, the finished suspension was filtrated with 0.45 μm membrane filter before analysis. In each reusability test, the catalyst was filtrated and washed thoroughly with deionized water, followed by drying naturally in open air. The concentration of m-cresol was measured by an Agilent1260 HPLC with C₁₈ column using a UV detector set at 272 nm via the external standard. The mobile phase was a mixture of 80 vol % methanol and 20 vol % ultrapure water with 1 mL/min of flow rate.

An economic analysis was performed under the optimized degradation conditions. The market price of the chemicals was derived from the trading platform of Alibaba, China. In Table 2, the costs of the chemicals and electric energy for the m-cresol degradation process were $4.25 peroxymonosulfate, $0.13 glucose, $0.94 N₂ and $3.66 electricity for per ton m-cresol solution degradation (including the preparation of the catalyst), while the total cost was worth about $8.98 for the degradation of per ton m-cresol solution.

M-cresol is one of the main ingredients in coal gasification wastewater and able to cause health chronic effect to humans and animals. In view of polymetallic oxides in red mud, it has a latent performance as a heterogeneous catalyst to activate PMS for the m-cresol degradation. It is essential to change valence states of oxides in red mud through chemical and physical methods and enhance PMS activation to the point of excellent performance.

WRMG/700 was prepared by the pyrolysis reduction of glucose in N₂ atmosphere. Compared to the counterpart prepared without glucose, WRMG/700 exhibited the enhanced activation ability toward PMS for the m-cresol degradation with 99.02% degradation efficiency in 60 min and a pH-independent catalytic activity between initial pH 3–8, owing to the production of Fe₃O₄, MnO and NiO, and their gathering on the surface of particles. The optimized degradation conditions were under 0.01 M PMS, 0.1 g of catalyst, reaction temperature 60 °C and reaction time 60 min for the degradation of 50 mg/L m-cresol. The removal efficiency of COD increased with the reaction time under the optimized degradation conditions. The effect of the WRMG/700/PMS system on environment was negligible. The free radical scavenging experiments and EPR test confirmed ¹O₂ played the dominant role in m-cresol degradation in the WRMG/700/PMS system. WRMG/700 retained its activation performance even after five recycles and exhibited high stability.

This work was financially supported by the National Natural Science Foundation of China (No. 21868001) and the Guizhou Science and Technology Department (No. Qiankehejichu [2019]1040).

Hongliang Chen: Methodology, formal analysis, funding acquisition, software, data curation, writing-original draft, writing-review and editing; Longjiang Li: Data curation, investigation, visualization; Yutao Zhang: Conceptualization, supervision, project administration, investigation.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.