Photocatalytic performance of titanium dioxide and zinc oxide binary system on degradation of humic matter

Green Synthesis of ZnO and Black TiO2 Materials and Their Application in Photodegradation of Organic Pollutants

ZnO and black TiO2 have been selected as the most efficient materials for organic pollution abatement due to their increased efficiency when compared to other materials. However, the concept of green chemistry makes it desirable to design green synthesis approaches for their production. In this study, black TiO2 was synthesized using an environmentally safe synthetic technique with glycerol as a reductant. ZnO was prepared by using ionic-liquid-based microwave-assisted extracts of Polygonum minus. To investigate the materials' potential to photodegrade organic pollutants, methylene blue (MB) and phenol were chosen as model organic pollutants. Both materials were found to exhibit spherical morphologies and a mesoporous structure and were efficient absorbers of visible light. ZnO exhibited electron-hole pair recombination lower than that of black TiO2. Black TiO2 was discovered to be an anatase phase, whereas ZnO was found to have a hexagonal wurtzite structure. In contrast to black TiO2, which had a surface area of 239.99 m2/g and a particle size of 28 nm, ZnO had a surface area of 353.11 m2/g and a particle size of 32 nm. With a degradation time of 60 min, ZnO was able to eliminate 97.50% of the 40 mg/L MB. Black TiO2, on the other hand, could reduce 90.0% of the same amount of MB in 60 min. When tested for phenol degradation, ZnO and black TiO2 activities were reduced by nearly 15 and 25%, respectively. A detailed examination of both ZnO and black TiO2 materials revealed that ZnO has more potential and versatility for the degradation of organic pollutants under visible light irradiation.

Photocatalytic performance of PANI modified TiO2: Degradation of refractory organic matter

Surface modification of TiO2 with polyaniline (PANI) conducting polymer has been used to achieve visible light photoresponse, thereby increasing solar photocatalytic activity. In this study, photocatalytic performances of PANI-TiO2 composites with different mole ratios were synthesized by the in situ chemical oxidation polymerization method and tested for the degradation of a model refractory organic matter (RfOM), namely humic acid in an aqueous medium under simulated solar irradiation in a comparative manner. Adsorptive interactions under dark conditions and interactions under irradiation were investigated as contributing factors to photocatalysis. Degradation of RfOM was monitored in terms of UV-vis parameters (Color436, UV365, UV280, and UV254) and fluorescence spectroscopic parameters as well as the mineralization extent by dissolved organic carbon contents. The presence of PANI exerted an enhancement in photocatalytic degradation efficiency compared to pristine TiO2. The synergistic effect was more pronounced in lower PANI ratios whereas higher PANI ratios reflected a retardation effect. Degradation kinetics were assessed by pseudo-first-order kinetic model. For all UV-vis parameters analyzed, highest and lowest rate constants (k) were attained in the presence of PT-14 (2.093 × 10-2 to 2.750 × 10-2 min-1) and PT-81 (5.47 × 10-3 to 8.52 × 10-3 min-1), respectively. Variations in selected absorbance quotients, i.e., A254/A436, A280/A436, and A253/A203, were distinctive and compared with respect to irradiation time and photocatalyst type. Upon use of PT-14, a steady decreasing profile with respect to irradiation time was attained for A253/A203 quotient as 0.76-0.61, followed by a rapid decrease to 0.19 in 120 min. The incorporation effect of PANI into TiO2 composite could be visualized in A280/A365 and A254/A365 quotients exhibiting an almost constant and parallel trend. As a general trend, decrease in the major fluorophoric intensity FIsyn,470 with photocatalysis was observed under extended irradiation conditions; however, an abrupt decline was remarkable in the presence of PT-14 and PT-18. Fluorescence intensity decrease correlated well with spectroscopic evaluation of rate constants. A thorough evaluation of spectroscopic parameters of UV-vis and fluorescence can provide significant information for practical applications in control of RfOM in water treatment.

Effect of oxidative and non-oxidative conditions on molecular size fractionation of humic acids: TiO2 and Cu-doped TiO2 photocatalysis

Natural waters contain some carbonaceous materials referred to as dissolved organic matter, which is mainly composed of humic acids (HA). Owing to its polydispersed character related to the presence of diverse molecular size fractions (< 450 kDa to even < 1 kDa), HA displays curious reactivity in natural waters and during water treatment train. In this study, a system-based stepwise approach was tracked by characterizing HA following photolysis, adsorptive interactions, and solar photocatalysis using bare TiO₂, sol-gel prepared TiO₂, and their respective Cu-doped specimens complementary to kinetic evaluation on this respect. For this purpose, prior to and following each treatment, HA was monitored by dissolved organic carbon content, UV-vis parameters, and fluorescence features. Attenuated total reflection Fourier transform infrared (FTIR), surface-enhanced Raman scattering spectroscopy (SERS), XRD, SEM, EDAX XPS, and DRS were used to characterize the materials and solutions reported in this study. Most significant quantitative variations were attained in UV-vis spectroscopic parameters along with fluorescence characteristics; however, infrared and Raman profiles displayed slight deviations in qualitative measures. Differentiation between the selected photocatalyst specimens could be visualized through molecular size effects pointing out the significance of HA 10 kDa fraction. For the first time, this study reports the degradation of specific fractions of HA as a function of their molecular size fraction. Cu-TiO₂ seems to photocatalyze more effectively the degradation of the diverse HA fractions due to their more extended absorption of solar light by this photocatalyst.

Prospects of TiO2-based photocatalytic degradation of microplastic leachates related disposable facemask, a major COVID-19 waste

COVID-19 is one of the serious catastrophes that have a substantial influence on human health and the environment. Diverse preventive actions were implemented globally to limit its spread and transmission. Personnel protective equipment (PPE) was an important part of these control approaches. But unfortunately, these types of PPE mainly comprise plastics, which sparked challenges in the management of plastic waste. Disposable face masks (DFM) are one of the efficient strategies used across the world to ward off disease transmission. DFMs can contribute to micro and nano plastic pollution as the plastic present in the mask may degrade when exposed to certain environmental conditions. Microplastics (MPs) can enter the food chain and devastate human health. Recognizing the possible environmental risks associated with the inappropriate disposal of masks, it is crucial to avert it from becoming the next plastic crisis. To address this environmental threat, titanium dioxide (TiO2)-based photocatalytic degradation (PCD) of MPs is one of the promising approaches. TiO2-based photocatalysts exhibit excellent plastic degradation potential due to their outstanding photocatalytic ability, cost efficiency, chemical, and thermal stability. In this review, we have discussed the reports on COVID-19 waste generation, the limitation of current waste management techniques, and the environmental impact of MPs leachates from DFMs. Mainly, the prominence of TiO2 in the PCD and the applications of TiO2-based photocatalysts in MPs degradation are the prime highlights of this review. Additionally, various synthesis methods to enhance the photocatalytic performance of TiO2 and the mechanism of PCD are also discussed. Furthermore, current challenges and the future research perspective on the improvement of this approach have been proposed.

Synthesis of sodium titanates and their use in the photocatalytic degradation of NO

The synthesis and characterization of sodium titanates (ST), and their evaluation in the photocatalytic reduction of nitric oxide (NO) are described herein. The materials were synthesized by a hydrothermal route using 5 M NaOH as the mineralizer agent and a TiO₂ content of 0.06 mg/mL (expressed as the mass ratio of TiO₂/mL of NaOH), at 170 °C for 48 h, resulting in sodium tri- and hexa-titanates. A nanotubular morphology was observed for the ST, as proved by scanning electron microscopy (SEM); a subsequent heat-treatment at 400 °C allowed a complete transformation of sodium tri- to hexa-titanates and an increase in bandgap. The obtained ST were impregnated with Ag⁺ and Zn⁺ cations, ST-Ag and ST-Zn, respectively, to tune the materials' bandgap. XPS analysis of the ST-Ag materials showed evidence of metallic Ag, pointing to the formation of silver nanoparticles, whereas for ST-Zn oxide phases were mainly spotted. The materials were evaluated for the photocatalytic reduction of NO using a reactor fed with a continuous flow rate of NO, generated in situ at a flow of 280 mL/min using nitrogen and a 253-nm UV irradiation source. The photocatalytic tests showed that pristine ST (tri- and hexa-titanates) displayed better performance in the reduction of NO with respect to the impregnated samples (ST-Ag, ST-Zn). Maximum degradation efficiencies of 80% were achieved when 1 g of photocatalyst was used with a flow of 280 mL/min and a 253 nm UV lamp.

Efficient removal of humic acid in water using a novel TiO2 composite with biochar doping

Titanium dioxide modified with biochar (Ti–C) was prepared by a sol–gel method for the degradation of humic acid (HA) in aqueous solutions.

Reduction of nitrates in a photocatalytic membrane reactor in the presence of organic acids

The present study explores the capability of the Photocatalytic Membrane Reactor (PMR) technology to remove nitrates from drinking water sources in the presence of organic electron donors. A systematic investigation was performed in a laboratory-pilot PMR, employing a hybrid TiO₂/UV-A catalysis-ultrafiltration process, using formic acid as the most favorable hole scavenger for nitrate reduction. The naturally occurring humic acid as well as the cheaper and harmless acetic acid were also tested as sacrificial electron donors. The performance of the PMR system was evaluated in respect of nitrate and Total Organic Carbon (TOC) percentage removal. The results demonstrate the superiority of formic acid regarding nitrate reduction, followed by acetic acid and humic acid, as well as the negligible effect of nitrates on organic mineralization. Formic acid was further used to assess the effects of molar organic/nitrate ratio, catalyst dosage and power of irradiation per unit volume (P_R) on nitrate reduction. With the present laboratory-pilot, the near optimum nitrate removal was ~60 % and the TOC reduction ~85 %; selectivity to dinitrogen was in the range 65-90 %. Key issues for improving this synergistic process and related R&D directions are discussed.

Preparation of PANI Modified ZnO Composites via Different Methods: Structural, Morphological and Photocatalytic Properties

Polyaniline modified zinc oxide (PANI-ZnO) photocatalyst composites were synthesized by focusing on dissolution disadvantage of ZnO. In-Situ chemical oxidation polymerization method was performed under neutral conditions (PANI-ES) whereas in hybridization method physical blending was applied using emeraldine base of polyaniline (PANI-EB). PANI-ZnO composites were prepared in various ratios of aniline (ANI) to ZnO as 1%, 3%, 6% and 9%. The alterations on the structural and morphological properties of PANI-ZnO composites were compared by Fourier Transform Infrared (FT-IR), Raman Spectroscopy, X-ray Diffraction (XRD) and Scanning Electron Microscopy-Energy Dispersive X-ray Analysis Unit (SEM-EDAX) techniques. FT-IR and Raman spectroscopy confirmed the presence of PANI in all composites. SEM images revealed the morphological differences of PANI-ZnO composites based on PANI presence and preparation methods. Photocatalytic performances of PANI-ZnO specimens were investigated by following the degradation of methylene blue (MB) in aqueous medium under UVA irradiation. The effects of catalyst dose and initial dye concentration were also studied. MB degradation was followed by both decolorization extents and removal of aromatic fractions. PANI-ZnO composites expressed enhanced photocatalytic performance (~95% for both methods) as compared to sole ZnO (~87%). The hybridization method was found to be more efficient than the In-Situ chemical oxidation polymerization method emphasizing the significance of the neutral medium.

Photocatalytic Oxidation of Natural Organic Matter in Water

Increased concentrations of natural organic matter (NOM), a complex mixture of organic substances found in most surface waters, have recently emerged as a substantial environmental issue. NOM has a significant variety of molecular and chemical properties, which in combination with its varying concentrations both geographically and seasonally, introduce the opportunity for an array of interactions with the environment. Due to an observable increase in amounts of NOM in water treatment supply sources, an improved effort to remove naturally-occurring organics from drinking water supplies, as well as from municipal wastewater effluents, is required to continue the development of highly efficient and versatile water treatment technologies. Photocatalysis has received increasing interest from around the world, especially during the last decade, as several investigated processes have been regularly reported to be amongst the best performing water treatment technologies to remove NOM from drinking water supplies and mitigate the formation of disinfection by products. Consequently, this overview highlights recent research and developments on the application of photocatalysis to degrade NOM by means of TiO2-based heterogeneous and homogeneous photocatalysts. Analytical techniques to quantify NOM in water and hybrid photocatalytic processes are also reviewed and discussed.