Materials, Preparation, & Characterization of Phot...

Ethical considerations

Before carrying out this work, ethical clearance was provided based on the proposed handling of hazardous chemicals and data generation, processing, and storage. This ensured safe procedures were followed in handling chemicals and other potentially hazardous materials. The handling and disposal of E. coli was according to strict procedures that prevented contamination. Permission was obtained prior to sampling FA from a local power plant. Protocols for data generation, processing, storage, and sharing were followed to ensure integrity and validity of the findings.

Materials

Chemical reagents used included titanium (IV) isopropoxide (TTIP) (97%), 2-propanol (99.7%), formic acid (85%), NaOH (98%), trifluoroacetic acid (purity >97%), MO (85%) and HCl (30–33%). All the chemicals were of analytical reagent grade purchased from either Sigma-Aldrich, Merck, or Glassworld (Berlin, Germany). Except in the study of the degradation of E. coli where Ringer’s solution was used, deionised water was used to prepare solutions. FA was obtained from Harare Thermal Power Station, and E. coli 0157:H7 was obtained from Sigma-Aldrich. A custom-made photocatalytic reactor was fabricated from 5-m-long light emitting diode strips (60 LED/m, 4.8 W, 12 V DC, luminous flux 360 lm/m, and colour temperature 3000 K) wrapped around a 1-L glass beaker and held in place using tape (Figure 1). Preliminary tests conducted using TiO and methylene blue dye showed that the intensity of the light output can be tuned to the desired level by controlling the luminance. In this work, white light at 80% luminance was used for all experiments.

Preparation of photocatalysts

TiO2

The sol-gel method was used to prepare TiO nanoparticles with TTIP 2 as a precursor. Briefly, TTIP (10 mL) was dissolved in 2-propanol (50 mL) and stirred to get a homogeneous solution. Under continuous stirring, formic acid (10 mL) was added dropwise for 60 min to hydrolyse the mixture. The resulting TiO sol was left to age for 24 h at room temperature. The dry TiO2 gel was placed in a muffle furnace and calcined at 450–500 °C for 3 h.

Zeolitic fly ash

Zeolites were synthesised from FA via hydrothermal synthesis using NaOH to produce zeolite fly ash (ZFA).23 Zeolitic materials have been prepared previously from the same FA using a similar method and successfully used to remove metals in acid mine drainage.24 In brief, FA was dried in an oven at 70 °C. NaOH pellets were added to the dry FA in the ratio 1:12 (w/w), and the mixture was ground and mixed thoroughly. The mixture was fused at 550–600 °C for 1 h in a muffle furnace, after which it was cooled and ground again. To the resulting powder was added deionised water (400 mL) and this mixture was stirred at room temperature overnight. This mixture was then placed in an oven to cure at 55 °C for 4 days to allow crystallisation to occur. The solid was separated from the liquid phase by filtration using No. 1 Whatman filter paper. The resulting ZFA was washed twice with distilled water, dried in an oven at 80 °C for 12 h, and milled to pass through a 250-μm sieve.

TiO2-ZFA nanocomposites

A one-pot acid catalysed sol-gel synthesis method was used to prepare TiO2-ZFA nanocomposites. In this method, ZFA of particle size 75 μm and mass 24.5 g was added to 2-propanol (50 mL), followed by 30 min of stirring at 40 °C. Different volumes of TTIP (Table 1) were added to the mixture to give 5%, 7% and 10% TiO2 (w/w) followed by stirring for a further 30 min. Then formic acid (10 mL) was added dropwise under continuous stirring for 30 min until a white sol-gel was observed. This sol-gel was allowed to age for 24 h at room temperature and then dried in an oven at 80 °C for 10 h. The sample was then ground to pass through a 250-μm sieve, and calcined at 450 °C for 4 h in a muffle furnace.

F-doped TiO2 and F-doped TiO2-ZFA photocatalysts

F-doped TiO2 (F-TiO2) was prepared using a two-step sol-gel method.25 Specifically, TTIP (10 mL) was dissolved in 2-propanol (50 mL) and vigorously stirred for 10 min to make solution A. Trifluoroacetic acid (0.5 mL) was mixed with 2-propanol (30 mL), formic acid (10 mL), deionised water (10 mL) and HCl (3 mL) and continuously stirred for 30 min to make solution B. This solution was added dropwise to solution A and continuously stirred for 30 min until a white sol-gel was observed. The sol-gel was aged for 24 h, oven-dried overnight at 50 °C, ground to pass through a 250-μm sieve, and calcined at 450 °C for 4 h in a muffle furnace. F-doped TiO2-ZFA (F-TiO2-ZFA) was prepared in the same way except the precursor was TiO2-ZFA, and CF3CO2H was used instead of formic acid.

Characterisation of photocatalysts

pHzpc and ash content

The pHzpc was used as a proxy for surface charge, and it was determined to elucidate the solid-liquid interfacial charge interactions between the photocatalysts and MO molecular surfaces.26 Determination of pHzpc was performed following the pH-drift method.27 The ash content was determined from the mass difference before and after igniting the samples at 750 °C for 4 h.

Fourier transform infrared spectroscopy

To determine the surface functional groups on the photocatalysts, infra-red spectra were recorded using a Fourier transform infrared (FTIR) spectrometer (Analytical® Technologies Limited, Infra 3000A, India). The samples were prepared using the KBr pellet method with a sample/KBr ratio of 1:40 to allow the samples to be infrared transparent. The spectra were recorded using 32 scans in the range 4000–400 cm-1 with a resolution of 4.

Diffuse reflectance UV-Vis spectroscopy

Diffuse reflectance spectroscopy UV-Vis spectra of the photocatalysts were measured using a UV-Vis near-infrared spectrophotometer22 (Lambda 650S, PerkinElmer, Johannesburg, South Africa) equipped with deuterium and tungsten lamps as the UV-Vis near-infrared radiation sources. The reflectance (R) for each sample was measured in the range 250–800 nm using the 150-mm sphere reflectance method. A slit width of 4 mm, a 0.2-s photomultiplier response, and reflectance blank of BaSO4 were used. From the UV-Vis data, an absorption coefficient was obtained in the form of the Kubelka–Munk function F(R)25,28, which was used to predict reflectance based on radiation transfer (Equation 1): Using the equation of Tauc (Equation 2), the band gap (Eg) for the different photocatalysts was estimated.29 Values of Eg were determined at the point of the horizontal intercept from plots of hv x F(R)1/n against hv. where h is Plank’s constant (6.626 x 10-34), v is frequency of radiation, c is the speed of light (3.0 x 108 m/s), n is the numerical value of electronic transitions and is equal to 2 for TiO2.

Surface morphology and crystallinity of the photocatalysts

Scanning electron microscopy (SEM) was used to obtain information on the surface morphology of the photocatalyts.15 Micrographs were obtained from a SEM microscope (Tescan, Vega 3, Brno, Czech Republic). Samples were prepared by placing powdered samples on an adhesive carbon tape stuck to a sample holder and then sputter-coating with 15 µm gold film; samples were observed under a microscope at 50 µm magnification. The crystallinity of the photocatalysts was measured using a powder X-ray diffraction spectrometer19,22 (D2 Phaser, Bruker, Billerica, MA, USA) fitted with a Cu X-ray source at 1.5418 Å, and operated in the continuous position-sensitive detector fast-scan mode.”

Evaluation of the photocatalytic activity of photocatalysts

Photodegradation experiments were performed using a custom-made photocatalytic reactor to evaluate the effect of contact time on the photodegradation process. The kinetics of the photodegradation of MO were studied using F-doped photocatalysts owing to their superior photocatalytic properties. The photodegradation studies were carried out using 100 mL of synthetic wastewater at 25 °C, at pH 7.2, a photocatalyst dosage of 500 mg, a dye concentration of 2 mg/L, and contact time of -60, 0, 60, and 120 min in a 150 mL beaker, following a variation of the method reported by Mukonza et al.25 To eliminate the interference of leftover photocatalyst in absorbance readings, a blank experiment of distilled water and photocatalyst was set up. All samples were stirred in the dark for 1 h under room temperature (25±1 °C) to attain absorption equilibria before being irradiated with visible light in the photocatalytic reactor.

The concentrations of MO before (C0) and after (Ct) photodegradation experiments were determined based on UV-Vis absorbance spectra. Samples (4 mL) were drawn at 1 h intervals using a 10-mL syringe and filtered through a 0.45-µm filter. The aliquots were centrifuged and the residual concentration of MO “in the supernatant was measured at 460 nm using a UV-Vis spectrophotometer (Spectroquant® Pharo 300). The dye removal (r) was computed using Equation 3.

“The order of reaction for the photocatalytic degradation of MO was confirmed using the linearised Langmuir–Hinshelwood model (Equation 4). where kapp is the apparent reaction rate constant.

Evaluation of the antimicrobial activity of the photocatalysts

To determine the individual effects of visible light irradiation and adsorption on the photodegradation of E. coli, experiments were conducted in triplicate under the following conditions: (1) in the presence of photocatalysts under dark conditions, (2) in the absence of photocatalysts under light irradiation, (3) in the absence of photocatalysts under dark conditions and (4) in the presence of catalysts and light irradiation. To reduce growth before assaying, water samples were refrigerated at <4 °C. Monitoring E. coli numbers was indicative of the quality of the wastewater and the efficacy of the disinfection process. From these data, the disinfection rate was calculated using Equation 3. Samples from disinfection experiments were plated on nutrient agar in duplicate, and incubated at 37 °C for 48 h. Thereafter, colonies were counted and viable cell concentrations determined.

Data analysis

One-way analysis of variance (ANOVA) was used to determine the individual effects of the photocatalysts and experimental factors on MO and E. coli removal after testing the data for normality and homogeneity of variance. Data that violated the ANOVA assumptions were either transformed or analysed using non-parametric statistical tests. Regression analysis was used to test the degree of fit of kinetic models to the experimental data based on the coefficient of determination. All statistical analyses were done at a probability level (p) of 0.05 using SPSS statistical software.