M. Aslam

Morphology controlled bulk synthesis of disc-shaped WO3 powder and evaluation of its photocatalytic activity for the degradation of phenols

The surfactant assisted synthesis of disc-shaped WO3 powder and its photocatalytic performance in sunlight exposure is reported. UV-vis DRS, XRD and FESEM characterized the synthesized WO3. The synthesized powder exhibited a bandgap of ∼2.55eV with cubic lattice and high crystallinity. The photocatalytic activity of the synthesized WO3 was examined for the degradation of phenol, resorcinol, 2-chlorophenol and 2-nitrophenol in complete spectrum and visible segment of sunlight. The highly efficient degradation/mineralization of 2-chloro and 2-nitrophenol compared to that of phenol and resorcinol, under identical experimental conditions, suggested the regulatory role of substituents attached to the aromatic ring in degradation/mineralization process. The time-scale HPLC degradation profiles, identification of intermediates by GC-MS and removal of organic carbon during the course of reaction were utilized to approximate the possible route of degradation/mineralization of phenolic substrates. The measurement of the anions released during the photocatalytic process was used to identify the nature of the major oxidants (O2(•-), OH(•)) and the possible interaction sites. A significant decrease in the photocatalytic activity of synthesized WO3, ∼50%, was observed in visible portion of sunlight however, a sustained activity was observed in the repeated exposures.

Synthesis, Characterization, and Sunlight Mediated Photocatalytic Activity of CuO Coated ZnO for the Removal of Nitrophenols

CuO@ZnO core-shell catalysts, coated by varying the CuO layer density ranging from 0.5% to 10%, were synthesized with the aim to enhance the photocatalytic activity of ZnO in sunlight and control its photocorrosion. Initially, the Cu(2+) ions were impregnated on presynthesized ZnO by wet impregnation and finally converted to CuO layers by calcination. The optical and structural characterization of the synthesized powders was performed by DRS, PL, Raman spectroscopy, and XRD analysis, respectively. The homogeneity of the coated layers was explored by FESEM. The photocatalytic activity of CuO coated ZnO was investigated for the degradation of mononitrophenols (2-, 3-, and 4-nitrophenol) and dinitrophenols (2,4-, 2,5-, and 2,6-dinitrophenol) in the exposure of the complete spectrum and visible region (420-800 nm) of sunlight. The effect of the increasing density coated layers of CuO on photocatalytic activity was evaluated for the degradation of 4-NP. Compared to pristine ZnO, a substantial increase in the degradation/mineralization ability was observable for the catalysts coated with 0.5% and 1% CuO, whereas a detrimental effect was noticed for higher coating density. Prior to photocatalytic studies, as evaluated by cyclic voltammetry (CV), compared to pure ZnO, a significant suppression of photocorrosion was noticed, under illumination, for catalysts coated with lower CuO coating. The progress of the photocatalytic degradation process was monitored by HPLC while the mineralization ability of the synthesized catalysts was estimated by TOC. The estimation of the released ions and their further interaction with the excited states and the reactive oxygen was monitored by ion chromatography (IC).

Enhanced photocatalytic activity of V2O5–ZnO composites for the mineralization of nitrophenols

In an effort to enhance the photocatalytic activity of ZnO in natural sunlight, V2O5-ZnO nanocomposites were synthesized by co-precipitation technique. The characterization of the synthesized powders by FESEM, XRD and UV-visible diffuse reflectance spectroscopy (DRS) revealed that the both V2O5 and ZnO retain their individual identity in the composites but the increasing concentration of V2O5 affect the particle size of ZnO. As estimated by photoluminescence spectroscopy, in comparison to pure ZnO, the presence of V2O5 significantly suppressed the charge carriers recombination process. The photocatalytic activity of the synthesized powders was evaluated for the degradation/mineralization of three potential nitrophenol pollutants (2-nitrophenol, 4-nitrophenol, and 2,4-dinitrophenol). The synthesized composites showed significantly higher activity for both degradation and mineralization of nitrophenols compared to pure ZnO. The progress of the degradation process was evaluated by HPLC while mineralization was monitored by TOC analysis. The degradation/mineralization route was estimated by identifying the intermediates using GC-MS. The correlation of the experimental data revealed that the position of NO2 group in 2- and 4-nitrophenol significantly affect the rate of degradation. The identification of hydroxyl group containing intermediates in the degradation of 4-NP confirmed the formation and vital role of hydroxyl radicals in degradation process. The rapid mineralization of nitrophenol substrates pointed out superoxide anions as major contributors in degradation and mineralization process. The assessment of the release of relevant ions (NO2(-), NO3(-), ONOO(-) and NH4(+)) during the degradation process assisted in identifying the plausible interaction sites.

Evaluation of sunlight induced structural changes and their effect on the photocatalytic activity of V2O5 for the degradation of phenols

Despite knowing the fact that vanadium pentoxide is slightly soluble in aqueous medium, its photocatalytic activity was evaluated for the degradation of phenol and its derivatives (2-hydroxyphenol, 2-chlorophenol, 2-aminophenol and 2-nitrophenol) in natural sunlight exposure. The prime objective of the study was to differentiate between the homogeneous and heterogeneous photocatalysis incurred by dissolved and undissolved V2O5 in natural sunlight exposure. V2O5 was synthesized by chemical precipitation procedure using Triton X-100 as morphology mediator and characterized by DRS, PLS, Raman, FESEM and XRD. A lower solubility of ∼ 5% per 100ml of water at 23 °C was observed after calcination at 600 °C. The study revealed no contribution of the dissolved V2O5 in the photocatalytic process. In sunlight exposure, V2O5 powder exhibited substantial activity for the degradation, however, a low mineralization of phenolic substrates was observed. The initial low activity of V2O5 followed by a sharp increase both in degradation and mineralization in complete spectrum sunlight exposure, was further investigated that revealed the decrease in the bandgap and the reduction in the particle size with the interaction of UV photons (<420 nm) as this effect was not observable in the exposure of visible region of sunlight. The role of the chemically different substituents attached to an aromatic ring at 2-positions and the secondary interaction of released ions during the degradation process with the reactive oxygen species (ROS) was also explored.

Hydrochemical investigation and quality assessment of ground water in rural areas of Delhi, India

The suitability of groundwater quality for drinking and agricultural purposes was assessed in the rural areas of Delhi based on the various water quality parameters. A total of 50 ground water samples were collected randomly from different sources viz. hand pump, tube well, boring and analyzed for major ion chemistry to understand the operating mechanism of geochemical processes for ground water quality. The quality analysis is performed through the estimation of pH, EC, TDS, total hardness, total alkalinity, Na, K, Cl, NO3, SO4, DO, BOD, Cu, Cr, Cd, Ni, Zn and Pb. Hydrochemical facies were identified using Piper, Durov and Chadha diagram. Chemical data were also used for mathematical calculations (SAR, %Na, RSC, PI, KI, and chloroalkaline indices) for better understanding the suitability of ground water for irrigation purposes. The results of saturation index shows that all the water samples were supersaturated to undersaturated with respect to carbonate minerals and undersaturated with respect to sulphate and chloride minerals. According to USSL diagram, most of the samples fall in the field of C3S1, indicating medium salinity and low sodium water which can be used for almost all types of soil with little danger of exchangeable sodium. Assessment of water samples from various methods indicated that majority of the ground water in the study area is chemically suitable for drinking and agricultural uses.

The suitability of ZnO film-coated glassy carbon electrode for the sensitive detection of 4-nitrophenol in aqueous medium

The performance of a ZnO nanoparticle-based electrochemical sensor, fabricated by different treatments of ZnO on glassy carbon electrode, was evaluated for the determination of 4-nitrophenol (4-NP) in aqueous medium. The sensing performance of ZnO film-coated GCE (ZnO/F/GCE) was compared with a variety of electrodes, which included GCEs modified with ZnO powder, RGO (reduced graphene oxide), and RGO-Nafion® composite. Among the fabricated electrodes, the ZnO film-coated electrode, prepared by dispersing the well-sonicated chloroform suspension of ZnO on the GCE, showed excellent response toward the sensitive detection of 4-NP in aqueous medium, and a significant enhancement in the reduction peak current predicted the suitability of the developed sensor. Square wave voltammetry (SWV) detection mode was applied for the determination of 4-NP. The reproducibility and accuracy of ZnO/F/GCE were evaluated in the linear concentration ranges of 0.035 μM to 1.4 μM and 2.1 μM to 6.3 μM. Appreciably low SWV detection limits of 0.008 μM and 0.02 μM were estimated. Minimal influence was observed from the interfering species, added into the sample solution, on the determination of 4-NP. The performance of the developed electrode was initially optimized for 4-NP samples prepared in-house and finally tested against two real samples collected from municipality wastewater. The recovery of the ZnO/F/GCE varied from 96.8% to 105.7%.

Ce3+ impregnated ZnO: a highly efficient photocatalyst for sunlight mediated mineralization

The surface of pre-synthesized hexagonal ZnO was tailored by Ce3+ states. The modified catalyst inveterate enhanced spectral response in the visible region and substantially quenched the luminescence without altering the morphology of the ZnO support. Compared to bare ZnO, the synthesized catalyst exhibited significant high activity both for degradation and mineralization of 2-chlorophenol (2-CP) in sunlight exposure.

The facile synthesis, characterization and evaluation of photocatalytic activity of bimetallic FeBiO3 in natural sunlight exposure

In an effort to develop sunlight active photocatalysts for environmental remediation, a phase pure bimetallic oxide, FeBiO3, was synthesized by a facile route. The optical, structural and morphological properties of the synthesized FeBiO3 were compared with both the parent oxides i.e. α-Fe2O3 and Bi2O3. The synthesized powder exhibited strong absorption in the visible region with the appearance of a distinct optical absorption edge at 2.15 eV. The PL, Raman and IR spectroscopic investigations of the synthesized powder, in comparison to that of pure oxides, confirmed the formation of pure phase FeBiO3 and revealed the dominant role of Fe3+ ions in controlling the optical properties of the material. The X-ray diffraction analysis (XRD) revealed the hexagonal or rhombohedral geometry with an average crystallite size of 20.2 nm, whereas the X-ray photoelectron spectroscopy (XPS) verified the existence of both Fe and Bi in 3+ oxidation states. The electrochemical evaluation of the synthesized catalysts revealed its excellent stability in the neutral and basic pH in the dark and under illumination however, the catalyst was unstable in the harsh acidic medium (pH = 2). A low resistance to electron transfer and better charge retention ability was witnessed by EIS and chronopotentiometry. The photocatalytic activity of the synthesized catalysts was evaluated in the exposure of complete spectrum and visible region of natural sunlight for the removal of chemically stable substrates such as 2-nitrophenol and 2-chlorophenol. The catalysts showed considerably high activity for the removal of both the substrates in the exposure of natural sunlight, whereas 25% less activity was witnessed in visible region in the same span of time. In addition to degradation, the catalyst also exhibited a substantial activity for mineralization (TOC removal). The catalysts unveiled reproducible activity in the repeated scans. The chemical stability and sustained activity both in the complete spectrum and visible region designate it as a potential candidate for photocatalytic environmental remediation.

How the Dyes Are Degraded/Mineralized in a Photocatalytic System? The Possible Role of Auxochromes

The capability of W6+-impregnated ZnO photocatalysts for sunlight mineralization of a variety of structurally different dyes has been investigated. Compared to bare ZnO, the W6+-loaded photocatalysts showed significantly higher activity for the decolorization as well as mineralization of dyes, and complete mineralization was noticed in a short span of 150xa0min. The results obtained by various analytical tools were correlated to estimate the mechanistic aspects of the decolorization/mineralization process and to identify the nature of the oxidizing species involved in the process. A strong dependence of the decolorization/mineralization process was observed on the nature and number of auxochromes attached to color-generating conjugated system. The rapid decolorization/mineralization of the dyes and release of corresponding anions with the decolorization of dyes suggested the involvement of charged rather than radical reactive oxygen species (ROS) in the oxidation process. Langmuir-Hinshelwood kinetic model was found to be best suited for evaluating the kinetics of mineralization process. The effectiveness of the catalysts for the decolorization/mineralization of a mixture of dyes was also examined. The suitability of the catalysts for successive use in sunlight exposure was also evaluated.

The effect of cerium alteration on the photocatalytic performance of WO3 in sunlight exposure for water decontamination

In an effort to enhance the photocatalytic activity of cubic WO3 in sunlight exposure, its surface was modified by impregnating the Ce3+ ions ranging from 1% to 25% with a step of 5% with respect to the weight of WO3. Compared to pure WO3, the optical analysis by diffuse reflectance spectroscopy (DRS) revealed better absorption cross-section and red shift in the band edges for Ce loaded catalysts. The decreased intensity of photoluminescence (PL) emissions and the suppression of the Raman active bands of WO3 verified the recombination quenching ability of Ce surface states. The XRD analysis revealed the existence of Ce3+ states in the lower loadings (≤5%), whereas the majority of Ce4+ states were noticed at higher loadings. The FESEM analysis also verified the formation of individual particles of Ce(III,IV) oxides at the surface of WO3 at higher loadings. The XPS analysis of 10% Ce loaded samples also revealed the presence of mixed oxides of Ce at higher loading. Except for 1% Ce loaded WO3, the estimation of charge–discharge capacity, in comparison to pure WO3, revealed the enhancement in the charge retention ability with the increasing Ce loading. In comparison to pure WO3, the synthesized catalysts exhibited superior activity for the removal of 2-nitrophenol and 2-chlorophenol substrates in natural sunlight exposure. The analysis of the degradation data revealed that in the lower concentration the surface oxygen bonded Ce3+ states serve as electron trapping and transfer centers, whereas with the increasing surface density the synergic composite mechanism is the dominating mode. The exaggerated estimation in the EDX analysis of the samples loaded with 15% and 20% Ce also revealed the major surface coverage by the oxides. The salient feature of the study was the evaluation of the photocatalytic activity with the minimal catalyst loading of 350 mg L−1.