Visible Light Photocatalytic-Adsorption Effect of Recyclable Industrial Waste

Removal of the contaminants/organic dye pollutants from wastewater provides a relevant global challenge. In this work the photodegradation of organic pollutants was aimed to realise by means of a very dangerous, toxic industrial waste, the red mud. Beside of the environmental aspect, the economic respect was also considered by recycling of an industrial waste and using visible region for photodegradation instead of generally applied ultraviolet light. The photocatalytic properties of Red Mud (RM) were tested in the solution of analytical indicators used as model agents (methylene blue, Congo red, and crystal violet) and real organic pollutants (cortisone and corticosterone). The mechanism of degradation effect (photocatalytic effect and/or adsorption) has been clarified. The degradation effect of RM was detected using Ultraviolet-Visible (UV-VIS) and fluorescence spectroscopies. The connections between pollutants and red mud were investigated by infrared spectroscopy (FTIR)

Keywords:Red mud; Recycling; Wastewater; Photocatalysis; Adsorption

One of the important goals of environmental protection is to recycle the huge amount of industrial waste and the other is to get clear water. Red mud (RM) industrial waste has been produced during alumina production, generating around 175 million tons of RM as a by-product [1]. The utilization rate of RM is only about 1-2% [2], The red mud can be characterised by high alkalinity, toxicity, and radioactivity [3]. RM is a fine-grained, red-coloured mixture of oxides (Fe₂O₃, Al2O₃, TiO₂, SiO₂, CaO, Na₂O, etc.) [4]. Currently, the majority of RM is disposed in landfills [5,6]. Its other notable application can be observed in the field of building materials [7-9], however, only in a few countries. It can be used as a component of ceramic products [10], retarders [11], pH buffers [12] and heavy metal ion removal [13]. The contaminants of water, often considered emerging pollutants, pose a severe risk to the quality of fresh water, human health, and ecosystems. The deliberate discharge of dyes and antibiotic wastewater becomes a serious threat to human society [14-16] Various techniques have been developed to treat such pharmaceutical wastewater, including membrane techniques, biological treatment, physisorption, and photocatalytic degradation [17-20]. Among these, adsorption and photocatalytic degradation are particularly effective and economical methods for removing organic contaminants from water [21-23]. However, the separate use of each method has limitations, such as adsorption capacity and photocatalytic efficiency. The persistent challenge of reducing these contaminants, such as Pharmaceutically Active Compounds (PACs), necessitates the development of innovative materials and techniques with enhanced adsorption and photocatalytic activity [24,25].

Certain components of red mud have been published with adsorption or/and photocatalytic effects. Titanium-dioxide (TiO₂) are commonly used as photocatalytic materials for wastewater purification [26-28]. However, TiO₂ is well known about excellent photocatalytic effect but its use is expensive due to required ultraviolet region and the high price of TiO₂ [29]. Hematite has also been studied for the photocatalytic degradation of organic contaminants [30]. Weak photocatalytic effect is published for pure hematite. Hematite due to the narrow band gap of 2.0-2.2eV, can be efficient in visible light up to 600nm [31]. Other noteworthy characteristics, such as excellent stability, low toxicity, costeffectiveness, inert behaviour, and antiferromagnetic properties, contribute to its broad range of applications [32-34]. There is published data about methylene blue degradation by effect of red mud containing systems but the compositions of these systems are different from our samples [35-37]. The aim of present study was the recycling of the industrial waste, such as very dangerous, toxic red mud as a waste of aluminium industrial fabrication. In this work a new special application of red mud has been developed regarding the environmental aspect. That is the reduction of organic water pollution by RD in visible light. By this way two environmentally friendly aims can be achieved; the reprocessing of a toxic waste material and cleaning the waters.

The investigation of photoactive properties of RM was performed in the solution of analytical indicators used as model agents (methylene blue, Congo red and crystal violet indicators). The mechanism of degradation effect (photodegradation and/or adsorption) has been clarified. Real organic “waste” components were also tested by red mud application. The degradation effect of RM was determined using Ultraviolet-Visible (UV-VIS) and fluorescence spectroscopies. The connections between pollutants and red mud were investigated by infrared spectroscopy (FTIR). The characterisation of red mud was carried out by thermal analysis and X-ray diffraction techniques.

Preparation methods

Between from the various materials used in the water cleaning, the red mud was washed with distilled water until to get 7 pH and dried at 100 °C, then milled to reach the particle diameters of 200- 400μm. Aqueous solutions of analytical indicators (methylene blue, Congo red, and crystal violet) were used with concentration of 1.2–5.0 10-4mol/dm³. 5 grams of red mud powder was added to 50cm³ solutions. The time of the treatment with visible light was varied between 1 and 8 hours. The other materials, such as organic pollutants (cortisone and corticosterone) were used in aqueous or alcoholic solutions with 7.5·10-3 mol/dm³ concentration. Instead of red mud, its synthetic compounds and the mixture of synthetic compounds were tested as well as chitosan was applied for comparison.

Investigation methods

Powder X-Ray Diffraction measurements (XRD) were performed by a Rigaku Smartlab X-ray diffractometer equipped with a 1.2kW copper source (radiation wavelength: CuKα; λ=0.15418nm). The data were collected in the range 2Θ between 10° and 110° with a 1D silicon strip detector (D/Tex ultra-250) at a speed of 0.2°/min. The results were analysed using databases, such as ICDD, and other data from the literature. Thermal analysis measurements were conducted using Derivatograph-C System equipment between 25 °C and 1000 °C, under static air atmosphere, with 10 °C/minute heating rate. The weight change of the samples was followed with respect to the temperature by Thermogravimetry (TG), the exothermic and endothermic changes were detected by Differential Thermal Analysis (DTA). The sample holder’s material and the reference material were aluminium oxide.

Attenuated Total Reflectance (ATR) Fourier Transform Infrared (FTIR) measurements were recorded on a Bruker IFS 55 instrument with a diamond ATR head (PIKE Technologies). The transmission mid-IR spectra were taken during and after the deposition by averaging 128 scans, respectively, in the 4000-600cm−1 region with 1cm−1 resolution using MTR detector cooled with liquid nitrogen, and KBr splitter. Photodegradation measurements was realised by using a LED light, model number: Eulbevoli68r34z052u408. with the following parameters: 50W power, 4500-5000lm luminous flux and 380-840nm wavelength from a 15cm distance. Fluorescence spectroscopy measurements were recorded using a Varian Cary Eclipse spectrofluorometer (Agilent Technologies, Santa Clara, CA, USA). The following settings were used: excitation: λ=360nm; emission: λ=190-500nm; detector voltage: 450V. Ultraviolet and visible light (UV-VIS) spectroscopy measurements were realised using a Thermo Spectronic Helios Gamma instrument.

Characterisation of red mud

Red Mud (RM) was characterised by XRD (Figure 1) and thermal analysis (Figure 2). The main components of original RM used in this study are hematite (α-Fe₂O₃) and cancrinite (Na₆Ca₂[(CO₃)₂|Al₆Si₆O₂₄]·2H₂O); RM contains even calcite (CaCO₃), Ca titanate (CaTiO₃), and goethite (FeO(OH)) in smaller amounts; crystalline phase of rutile (TiO₂), boehmite (AlO(OH)), and gibbsite (Al(OH)₃) can be detected only in very low volume by XRD. Detectable changes cannot be observed by washing in the crystalline phases (Figure 1). Mostly aqueous NaOH content is removed by washing step. The reduction of amount was 36-39% during the washing. The drying process up to 500 °C resulted in further 7-8% reduction in the volume; boehmite, gibbsite, goethite, and rutile crystalline phases disappeared. The most relevant chemical compounds of original RM are represented in oxide form: 32-38% Fe₂O₃, 16-18% Al₂O₃, 8-12% SiO₂, 7-10% Na₂O, 4-5% TiO₂, 1-3% CaO (XRF measurements).

Figure 1:Phase composition of red mud vs. temperature. XRD measurements.

Figure 2:Thermal analysis of red med pretreated at 100 °C.

The changes of red mud phases during the heating have been determined by Thermal Analysis (TA) (Figure 2). The weight loss of the original red mud was 30-31% until 100 °C without washing. In order to represent the TA data well observably, RM was dried at 100 °C and it was used in TA measurements. The total weight loss was 11-12% above 100 °C. The weight loss between 267 and 374 °C is derived from the loose of water from various phases (gibbsite, boehmite, and cancrinite) [38,39]. At around 360 °C the decomposition of goethite occurs to hematite. In the range of 484-535 °C, the endothermic peak and weight loss connects to the transformation of boehmite to transition γ-Al₂O₃ phase [38,39]. The weight loss and endothermic peak in the range of 700-800 °C can be derived from decomposition of calcite. The other reactions of the released CaO must be calculated above 700 °C; 2CaO+SiO₂→Ca₂SiO₄ and Ca₂SiO₄+SiO₂→2CaSiO₃ [40].

Photodegradation-adsorption effect of red mud

The first tests for degradation ability of RD were performed with analytical indicators as model agents (methylene blue, Congo red, and crystal violet). The results of two types, i.e. Methylene Blue (MB) and Crystal Violet (CV) are presented in Figure 3. The reduction of MB was 79,3 % after 4 hours’ treatment by visible light, and in the case of CV was 77,6%. Congo red model agent was degraded by >90% after 1 hour. For further investigation Methylene Blue (MB) was selected owing to its better control. In order to decide that the adsorption must be taken into account or only photocatalytic effect, tests were performed with and without light (in dark). The result of MB has proved the considerable adsorption ability of red mud (Figure 4). After 2 hours’ treatment of a solution in visible light and another solution in dark, the concentration of MB was reduced by 95.0% and 84.6%, respectively. That means 89% adsorption and 11% photocatalytic effect in the case of methylene blue. FTIR spectroscopy checked the interaction between the MB and red mud (Figure 5). FTIR does not reveal a regular connection but presents a broad deviation between 3350 and 3650cm^-1 denotes a loose interaction between MB and RM confirming the adsorption.

Figure 3:Photodegradation-adsorption of model agents by red mud in visible light region from aqueous solution. UV-VIS absorption measurements.

Figure 4:Investigation of photodegradation and adsorption effects of red mud in visible light region and without lighting in 1.25 10^-4mol/dm³ aqueous solution of model agent. UV-VIS absorption measurements.

Figure 5:Infrared spectra of red mud-methylene blue systems after visible light treatment.

In order to determine the role of components in the degradation effect, the influence of hematite content was studied at first (Figure 6 & Table 1). Hematite represents the largest component in RM, 32- 38%. The various amount of hematite was mixture with Al₂O₃. In summary, the higher the hematite fraction the lower the reduction effect is. It means Al₂O₃ has an important role especially in the adsorption. It is well known that Al₂O₃ possesses good adsorption ability. The best reduction can be achieved with use of 100% Al₂O₃ (68.5%) but this value is still much lower than the pure red mud produces, 80.1% (Table 1). The investigation of the degradation of MB in the function of hematite - aluminium oxide mixtures did not result in a completely adequate interpretation for the degradation effect of red mud’ components. Thus, the Fe2O3 and Al₂O₃ components were extended by further components (Figure 7). The pure aluminium oxide proved to be again the most efficient compound, after that hematite. The synthetic rutile and calcite only decrease the efficiency. Therefore, the smaller components of red mud and their morphology have important role in the photodegradation effect.

Figure 6:The degradation of model agent vs. hematite content. The starting concentration was 1.25 10-4mol/dm3. UV-VIS absorption measurements.

Table 1:Influence of hematite content on the methylene blue concentration.

Figure 7:The degradation of model agent vs. various composition of red mud components. The starting concentration was 1.25 10^-4mol/dm³. UV-VIS absorption measurements.

The photodegradation effect of RM was compared with a new type of excellent adsorbent, chitosan (Figure 8). Red mud reveals more intensive reduction effect (94.0±5%) than chitosan (79±5%). After the model agent (MB) the photodegradation effect of RM was tested on potential real organic “waste” components; corticosterone and cortisone (Figure 9). Corticosterone is a 21-carbon steroid hormone of the corticosteroid type. Cortisone is a man-made version of a natural hormone cortisol. In the case of corticosterone, the reduction is lower; 45.0% in light and 21.6% in dark. Near 100% is the loss of cortisone amount in both cases in dark and in light conditions. On the basis of signal at 320nm, 81.9% was the decrease in concentration in light and 54.5% in dark. The ratio of photocatalytic effect and adsorption of RM on various materials are summarised in (Table 2) and calculated by means of “dark” and “light data”. The data unambiguously reflect that the ratio of photocatalytic effect and adsorption strongly depends on the removed materials and both effects must be taken into account.

Figure 8:Comparison of the model agent’s degradation by effect of red mud and chitosan. UV-VIS absorption measurements in 1.25·10^-4mol/dm³ aqueous solution.

Figure 9:Comparison of photodegradation and adsorption effects of red mud on organic water pollution (steroid hormones) in visible light region from 7.5·10^-3mol/dm³ (M) alcoholic solution. The concentration of cortisone was measured by fluorescence spectroscopy.

Table 2:Ratio of photocatalytic effect and adsorption of RM on various “waste” materials.

Red Mud (RM) is a by-product of alumina production, around 175 million tons of RM are generated (2021). The recycling rate of RM is only about 1-2% in other various technologies. The aim of this work was the utilisation of red mud as a functional material. Removal of the organic contaminants from wastewater as a functional application of RM has been chosen regarding the other very important environmentally friendly intention that is to clear the wastewaters. The photocatalytic properties of Red Mud (RM) were tested in the solution of analytical indicators used as model agents (methylene blue, Congo red, and crystal violet) and real organic pollutants (cortisone and corticosterone). In the case of model agents, the photodegradation effect was varied from 75% to 95% in the function of agent type, its concentration, and time of treatment. The test with potential organic contaminants resulted in 45 and close to 100% degradation in their concentration.

The photodegradation effect of RM was studied in the function of hematite. But increase of hematite ratio in the mixtures of hematite and aluminium oxide reduced the efficiency of mixtures. In order to achieve better chemical comparison to RM, the synthetic components of mixture were expanded. The measurements of photodegradation proved that Al₂O₃ is the most efficient compound of RM, after that hematite. Comparison of the concentration data of model agent reduced by “red mud” built up from synthetic components with original red mud it can be concluded that the smaller components of RM and their morphology have also important role in the photodegradation effect. The mechanism of the degradation effect of RM has been also clarified. The data obviously have proved both effects; photocatalysis and adsorption must be taken into account. The ratio of those strongly depends on the type of organic components. The very important economic result of this study is the removal of organic wastewater can be realised even in low concentration and using visible region for photodegradation instead of generally applied ultraviolet light.

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