J. Madhavan

Combined advanced oxidation processes for the synergistic degradation of ibuprofen in aqueous environments.

Ibuprofen (IBP) is a widely used analgesic and anti-inflammatory drug and has been found as a pollutant in aqueous environments. The sonolytic, photocatalytic and sonophotocatalytic degradations of IBP in the presence of homogeneous (Fe(3+)) and heterogeneous photocatalysts (TiO(2)) were studied. When compared with sonolysis and photocatalysis, a higher degradation rate was observed for sonophotocatalysis in the presence of TiO(2) or Fe(3+) and also a slight synergistic enhancement was found with a synergy index of 1.3 and 1.6, respectively. Even though TiO(2) sonophotocatalysis showed an additive process effect in the mineralization, a significant synergy effect was observed for the sonophotocatalysis in the presence of Fe(3+). This might be due to the formation of photoactive complexes between Fe(3+) and IBP degradation products, such as carboxylic acids. High performance liquid chromatography (HPLC) and electrospray ionisation mass spectrometry (ESMS) techniques were employed for the identification of the degradation intermediates. The sonication of IBP led to the formation of its mono- and di-hydroxylated intermediates. Apart from the hydroxylated intermediates, products formed due to the oxidation of propanoic acid and isobutyl substituents of IBP were also observed.

Degradation of orange-G by advanced oxidation processes

The degradation and mineralization of orange-G (OG) in aqueous solutions by means of ultrasound irradiation at a frequency of 213kHz and its combination with a heterogeneous photocatalyst (TiO(2)) were investigated. The effects of various operational parameters such as, the concentration of the dye and solution pH on the degradation efficiency were studied. The degradation of the dye followed first-order like kinetics under the conditions examined. The sonolytic degradation of OG was relatively higher at pH 5.8 than that at pH 12. However, an alkaline pH was favoured for the photocatalytic degradation of OG using TiO(2). Total organic carbon (TOC) measurements were also carried out in order to evaluate the mineralization efficiency of OG using sonolysis, photocatalysis and sonophotocatalysis. The hybrid technique of sonophotocatalytic degradation was compared with the individual techniques of photocatalysis and sonolysis. A simple additive effect was observed during the sonophotocatalytic oxidation of OG using TiO(2) indicating that the combined treatment offers no synergistic enhancement. TOC results also support the additive effect in the dual treatment process.

Ultrasound assisted photocatalytic degradation of diclofenac in an aqueous environment.

Diclofenac (DF) is an anti-inflammatory drug found in aqueous environments as a pollutant due to its widespread use. The sonolytic, photocatalytic and sonophotocatalytic degradation of DF using three photocatalysts (TiO(2), ZnO and Fe-ZnO) were studied. The degradation of DF followed first-order like kinetics. The sonophotocatalytic degradation using TiO(2) under UV-vis radiation showed a slight synergistic enhancement in the degradation of the parent compound, whereas a detrimental effect was observed for the mineralization process. In the case of Fe-ZnO, both degradation and mineralization showed near additive effects. A number of degradation products were identified.

Photocatalytic degradation of Acid Red 88 using Au–TiO2 nanoparticles in aqueous solutions☆

Metal loaded semiconductors in general possess greater photocatalytic activity than pure semiconductors. Hence, with an attempt to achieve higher photocatalytic activity, Au-TiO(2) photocatalysts were prepared by deposition-precipitation method and used for the photocatalytic degradation of an azo dye (Acid Red 88; AR88). The materials were characterized by different analytical techniques. A possible mechanism for the photocatalytic degradation of AR88 by Au-TiO(2) in the absence and presence of other oxidizing agents (peroxomonosulfate (PMS), peroxodisulfate (PDS) & hydrogen peroxide (H(2)O(2))) has been proposed. The extent of mineralization of the target pollutant was also evaluated using Total Organic Carbon (TOC) analysis.

Photocatalytic and photoelectrochemical studies of visible-light active α-Fe2O3–g-C3N4 nanocomposites

Nanocrystalline hematite iron oxide (α-Fe2O3) and graphitic carbon nitride (g-C3N4) were prepared and used as precursors to synthesise α-Fe2O3–g-C3N4 composite photocatalysts of various compositions by a wet impregnation method. The synthesized photocatalysts were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), UV-vis diffuse reflection spectroscopy (DRS) and photoluminescence spectroscopy (PL). The efficiency of the photocatalysts was evaluated by photoelectrochemical measurements and photodegradation of direct red 81 (DR81) as a target textile pollutant under visible light irradiation. The α-Fe2O3–g-C3N4 composites exhibited remarkably improved visible-light induced photocatalytic activity. The composite photocatalysts with optimal α-Fe2O3 content with the highest photocatalytic activity was found to be 2%-α-Fe2O3–g-C3N4. The synergistic enhancement in the photocatalytic degradation of composite photocatalysts might be due to an increase in the visible-light absorption efficiency and rapid photoinduced charge separation. A possible photocatalytic mechanism has been proposed for the photocatalytic activity of α-Fe2O3–g-C3N4 composite photocatalysts.

Sonophotocatalytic degradation of monocrotophos using TiO2 and Fe3

Monocrotophos (MCP) is an organophosphate insecticide that has been found as a pollutant in aqueous environments. The sonolytic, photocatalytic and sonophotocatalytic degradation of MCP in the presence of homogeneous (Fe(3+)) and heterogeneous photocatalysts (TiO(2)) were studied. The photocatalytic degradation rate using TiO(2) was found to be lower than that of sonolysis alone due to the interference of phosphate ions formed as an intermediate product. On the other hand, a 15 fold enhancement in the degradation rate was found when photolysis was carried out in the presence of Fe(3+) compared to the rate observed with photolysis alone. The combination of sonolysis and photocatalysis (using either TiO(2) or Fe(3+)) showed a detrimental effect. Synergy indices of 0.62 and 0.87 were found for the sonophotocatalytic degradation of MCP in the presence of TiO(2) and Fe(3+), respectively. Total organic carbon (TOC) analysis was carried out to study the extent of mineralization of MCP. It was found that the mineralization process was additive for both TiO(2) and Fe(3+) sonophotocatalysis. HPLC and electrospray mass spectrometry (ESMS) techniques were employed for the identification of the degradation intermediates. The sonication of MCP led to the formation of dimethyl phosphate, dimethylphosphonate, 3-hydroxy 2-buteneamide and N-methyl 3-oxobutanamide as the intermediate products.

Physical and chemical effects of acoustic cavitation in selected ultrasonic cleaning applications.

Acoustic cavitation in a liquid medium generates several physical and chemical effects. The oscillation and collapse of cavitation bubbles, driven at low ultrasonic frequencies (e.g., 20 kHz), can generate strong shear forces, microjets, microstreaming and shockwaves. Such strong physical forces have been used in cleaning and flux improvement of ultrafiltration processes. These physical effects have also been shown to deactivate pathogens. The efficiency of deactivation of pathogens is not only dependent on ultrasonic experimental parameters, but also on the properties of the pathogens themselves. Bacteria with thick shell wall are found to be resistant to ultrasonic deactivation process. Some evidence does suggest that the chemical effects (radicals) of acoustic cavitation are also effective in deactivating pathogens. Another aspect of cleaning, namely, purification of water contaminated with organic and inorganic pollutants, has also been discussed in detail. Strong oxidising agents produced within acoustic cavitation bubbles could be used to degrade organic pollutants and convert toxic inorganic pollutants to less harmful substances. The effect of ultrasonic frequency and surface activity of solutes on the sonochemical degradation efficiency has also been discussed in this overview.

Synthesis and characterization of a CuS–WO3 composite photocatalyst for enhanced visible light photocatalytic activity

WO3 nanorods and flower-like CuS were synthesized by a hydrothermal process. The visible light driven CuS–WO3 photocatalyst was prepared by adding different weight ratios (10–40%) of CuS on WO3 by a wet impregnation method. The synthesized photocatalysts were characterized by powder X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, Field emission scanning electron microscopy (FE-SEM), High-resolution transmission electron microscopy (HR-TEM), energy dispersive X-ray spectroscopy (EDAX) and UV-vis diffuse reflectance (DRS) spectroscopy. The photocatalytic performance of synthesized photocatalysts was evaluated for the photodegradation of methylene blue (MB) under visible light irradiation. The 10% CuS–WO3 photocatalyst showed higher photocatalytic degradation activity than others which could be due to the increased absorption of light in the visible region and also a lower recombination of charge carriers. Further, the photoelectrochemical measurements carried out for 10% CuS–WO3 revealed the faster migration of photo-induced charge-carriers. A possible reaction mechanism for the enhancement of photocatalytic activity of CuS–WO3 has been proposed.

One-step electrochemical deposition of Ni1−xMoxS ternary sulfides as an efficient counter electrode for dye-sensitized solar cells

Ternary sulfides of Ni1−xMoxS films with various compositions (x = 0, 0.05, 0.1, and 0.2) were fabricated on a fluorine doped tin oxide (FTO) glass substrate by a simple one-step electrochemical deposition method. The electrochemically deposited ternary sulfides were utilized as a low-cost and highly efficient platinum free counter electrode (CE) for dye-sensitized solar cells (DSSCs). The structure, surface morphology and elemental composition of the electrochemically deposited ternary sulfides were examined by using X-ray diffraction (XRD), high resolution scanning electron microscopy (HR-SEM) and energy dispersive X-ray spectroscopy (EDS). A phthaloylchitosan (PhCh) based polymer electrolyte was used as an electrolyte for DSSCs. Cyclic voltammetry, electrochemical impedance spectroscopy and Tafel polarization studies revealed that the Ni0.95Mo0.05S CE exhibited lower charge-transfer resistance at the CE/electrolyte interface and higher electrocatalytic activity towards the regeneration of I− from I3− relative to other compositions. The Ni0.95Mo0.05S ternary sulfide offers a positive synergistic effect for the electrochemical catalytic activity towards the reduction of I3−, which may be due to an increase in active catalytic sites and low-charge transfer resistance and achieved a high power conversion efficiency of 7.15% with a Voc of 0.65 V, a Jsc of 17.21 mA cm−2, and a FF of 0.64 with a PhCh-based polymer electrolyte, which is comparable to that of the conventional Pt CE (7.20%). The present investigation demonstrates that the electrochemically deposited Ni0.95Mo0.05S ternary sulfide is a promising candidate as a low-cost and highly efficient CE in DSSCs.

Cu2S-incorporated ZnS nanocomposites for photocatalytic hydrogen evolution

Sodium Lauryl Sulphate (SLS) surfactant and propylene diamine (PD), assisted wet chemical synthesis was used to make ZnS–Cu2S nanoflakes wherein the added copper(II) transforms into copper(I) sulphide and is found to be deposited on the surface of ZnS. With a variation of copper concentration from 0 to 5% there is a morphological transformation from ZnS nanorods eventually to ZnS–Cu2S nanoflakes through a transition morphology of nanocactus leaves. During the addition of copper there is an incorporation of Cu2S into the ZnS phase (100) as is clearly evidenced by various characterization methods. Visible light photocatalytic hydrogen production activities using these nanoflakes of ZnS–Cu2S are reported with good results. The influence of Cu2S shifts the band gap of ZnS from the UV to the visible region, reducing the need for an expensive co-catalyst like platinum for photolysis of water. Though hydrogen production is not as high as that depicted by other earlier works the material that we have created is a relatively cheap, simple two component heterostructure with no expensive third component. It is also free from toxic materials such as CdS. However, our results are better than for most other copper loaded ZnS systems in the literature. Furthermore, the morphological evolution to nanoflakes from nanorods, the concentration and dispersion of Cu2S over ZnS and the interface between Cu2S and ZnS semiconductors play a vital role in hydrogen production. 5% Cu2S on ZnS seems to be the optimum concentration for maximum evolution of hydrogen.