Sankar Chakma

Mechanistic insight into sonochemical biodiesel synthesis using heterogeneous base catalyst

The beneficial effect of ultrasound on transesterification reaction is well known. Heterogeneous (or solid) catalysts for biodiesel synthesis have merit that they do not contaminate the byproduct of glycerol. In this paper, we have attempted to identify the mechanistic features of ultrasound-enhanced biodiesel synthesis with the base-catalyst of CaO. A statistical design of experiments (Box-Behnken) was used to identify the influence of temperature, alcohol to oil molar ratio and catalyst loading on transesterification yield. The optimum values of these parameters for the highest yield were identified through Response Surface Method (with a quadratic model) and ANOVA. These values are: temperature=62 °C, molar ratio=10:1 and catalyst loading=6 wt.%. The activation energy was determined as 82.3 kJ/mol, which is higher than that for homogeneous catalyzed system (for both acidic and basic catalyst). The experimental results have been analyzed vis-à-vis simulations of cavitation bubble dynamics. Due to 3-phase heterogeneity of the system, the yield was dominated by intrinsic kinetics, and the optimum temperature for the highest yield was close to boiling point of methanol. At this temperature, the influence of cavitation bubbles (in terms of both sonochemical and sonophysical effect) is negligible, and ultrasonic micro-streaming provided necessary convection in the system. The influence of all parameters on the reaction system was found to be strongly inter-dependent.

Investigation in mechanistic issues of sonocatalysis and sonophotocatalysis using pure and doped photocatalysts

This paper attempts to investigate the mechanistic issues of two hybrid advanced oxidation processes (HAOPs), viz. sonocatalysis and sonophotocatalysis, in which the two individual AOPs, viz. sonolysis and photocatalysis, are combined. Three photocatalysts, viz. pure ZnO and Fe-doped ZnO (with two protocols) have been employed. Fe-doped ZnO catalyst has been characterized using standard techniques. Decolorization of two textile dyes has been used as the model reaction. With experiments that alter the characteristics of ultrasound and cavitation phenomena in the medium, the exact synergy between the two AOPs has been determined using a quantitative yard stick. The results revealed a negative synergy between the two AOPs, which is an almost consistent result for decolorization of both dyes using all three photocatalysts. Fe-doping of ZnO catalyst helps in generation of more OH radicals that could augment decolorization. However, these radical mainly react with dye molecules adsorbed on catalyst surface. Intense shock waves generated by cavitation bubbles cause desorption of dye molecules from catalyst surface and reduce the probability of dye-radical interaction, thus reducing the net utility of photochemically generated OH radicals towards dye decolorization. This is rationale underlying the negative synergy between sonolysis and photocatalysis. Fe-doped ZnO catalyst increases the extent of decolorization, but the synergy between the two individual AOPs remains unaltered with doping.

Sonochemical synthesis of mesoporous ZrFe2O5 and its application for degradation of recalcitrant pollutants

In this paper, we have reported the sonochemical synthesis and characterization of zirconium ferrite (ZrFe2O5), and its use as a catalyst in advanced oxidation processes (AOPs) using decolorization/degradation of azo and non-azo dyes as model processes. The efficacy of ZrFe2O5 for dye decolorization has been compared with the conventional catalyst TiO2 synthesized using sonochemical sol–gel method. Sonochemically synthesized ZrFe2O5 and TiO2 have been characterized using PSD, XRD, FESEM, TEM, DRS, BET and TGA. ZrFe2O5 is revealed to have a lower band gap energy than TiO2. However, the α-Fe2O3 (hematite) phase in ZrFe2O5 acts as a recombination center of photogenerated electrons and holes, which adversely affects the photo-activity of ZrFe2O5. In the context of degradation of recalcitrant pollutants, this adverse effect is compensated by Fenton activity of the α-Fe2O3 (hematite) phase which is stimulated by addition of H2O2. Sonochemically synthesized ZrFe2O5 also has high adsorption capacity for the textile dyes that assists their effective degradation through Fenton and photocatalytic mechanisms. The decolorization experiments have employed individual AOPs of sonolysis, photocatalysis, (heterogeneous) Fenton and various combinations of these as hybrid AOPs. Due to combined facets of photo- and Fenton-activity, ZrFe2O5 is a better catalyst for hybrid AOPs than TiO2. Nonetheless, the Fenton-activity of ZrFe2O5 overwhelms the photo-activity in dye decolorization. The most efficient hybrid AOP is revealed to be sono-photo-Fenton, in which both photo- and Fenton- activities of ZrFe2O5 are utilized. Analysis of decolorization experiments has also provided interesting mechanistic insight into interactions and synergies between different competing mechanisms of hybrid AOPs.

Investigations in physical mechanism of the oxidative desulfurization process assisted simultaneously by phase transfer agent and ultrasound

This paper attempts to discern the physical mechanism of the oxidative desulfurization process simultaneously assisted by ultrasound and phase transfer agent (PTA). With different experimental protocols, an attempt is made to deduce individual beneficial effects of PTA and ultrasound on the oxidative desulfurization system, and also the synergy between the effects of PTA and ultrasound. Effect of PTA is more marked for mechanically stirred system due to mass transfer limitations, while intense emulsification due to ultrasound helps overcome the mass transfer limitations and reduces the extent of enhancement of oxidation by PTA. Despite application of PTA and ultrasound, the intrinsic factors and properties of the reactants such as polarity (and hence partition coefficient) and diffusivity have a crucial effect on the extent of oxidation. The intrinsic reactivity of the oxidant also plays a vital role, as seen from the extent of oxidation achieved with performic acid and peracetic acid. The interfacial transport of oxidant in the form of oxidant-PTA complex reduces the undesired consumption of oxidant by the reducing species formed during transient cavitation in organic medium, which helps effective utilization of oxidant towards desulfurization.

Mechanistic insight into sono-enzymatic degradation of organic pollutants with kinetic and thermodynamic analysis

In this paper, we have attempted to get a physical insight into process of sono-enzymatic treatment for degradation of recalcitrant organic pollutants. Decolourization of an azo dye has been used as model reaction with different experimental protocols that alter characteristics of ultrasound and cavitation phenomena in the system. Experimental data is analyzed to determine kinetic and thermodynamic parameters of decolorization process. The trends observed in kinetic and thermodynamic parameters of decolourization are essentially manifestations of the dominating mechanism of the decolorization of the textile dye (or nature of prevalent chemical reaction in the system), viz. either molecular reaction due to enzyme or radical reaction due to transient cavitation. The activation energy for sonochemical protocol is negative, which indicates instantaneity of the radical reactions. The frequency factor is also low, which is attributed to high instability of radicals. For enzymatic and sono-enzymatic protocols, activation energy is positive with higher frequency factor. Enthalpy change for sonochemical protocol is negative, while that for enzymatic and sono-enzymatic protocols is positive. The net entropy change for sonochemical protocol is more negative than enzymatic or sono-enzymatic protocol due to differences in prevalent chemical mechanism of dye decolorization. Due to inverse variations of frequency factor and activation energy, marginal rise in reaction kinetics is seen for sono-enzymatic protocol, as compared to enzymatic treatment alone. Due to inverse variations of enthalpy and entropy change, net Gibbs energy change in all experimental protocols shows little variation indicating synergism of the mechanism of ultrasound and enzyme.

Numerical Simulation and Investigation of System Parameters of Sonochemical Process

This paper presents the effects of various parameters that significantly affect the cavitation. In this study, three types of liquid mediums with different physicochemical properties were considered as the cavitation medium. The effects of various operating parameters such as temperature, pressure, initial bubble radius, dissolved gas content and so forth, were investigated in detail. The simulation results of cavitation bubble dynamics model showed a very interesting link among these parameters for production of oxidizing species. The formation of •OH radical and H2O2 is considered as the results of main effects of sonochemical process. Simulation results of radial motion of cavitation bubble dynamics revealed that bubble with small initial radius gives higher sonochemical effects. This is due to the bubble with small radius can undergo many acoustic cycles before reaching its critical radius when it collapses and produces higher temperature and pressure inside the bubble. On the other hand, due to the low surface tension and high vapor pressure, organic solvents are not suitable for sonochemical reactions.

Sonochemical Synthesis of Cobalt Ferrite Nanoparticles

Cobalt ferrite being a hard magnetic material with high coercivity and moderate magnetization has found wide-spread applications. In this paper, we have reported the sonochemical synthesis of cobalt ferrite nanoparticles using metal acetate precursors. The ferrite synthesis occurs in three steps (hydrolysis of acetates, oxidation of hydroxides, and in situ microcalcination of metal oxides) that are facilitated by physical and chemical effects of cavitation bubbles. The physical and magnetic properties of the ferrite nano-particles thus synthesized have been found to be comparable with those reported in the literature using other synthesis techniques.

Intensification of Wastewater Treatment using Sono-hybrid Processes: An Overview of Mechanistic Synergism

Abstract Sono-hybrid advanced oxidation processes (AOPs) have recently attracted significant attention of researchers for degradation of bio-recalcitrant pollutants. Most widely studied sono-hybrid processes are: sonocatalysis, sono-Fenton and sono-photocatalysis. However, exact synergy between these AOPs and the underlying physical mechanism remains largely unexplored. This paper has addressed this fundamental issue. With approach of coupling experimental results to simulation of cavitation bubble dynamics, an attempt is made to identify synergistic links between individual mechanisms of AOPs. It is revealed that in all sono-hybrid AOPs, physical effect of ultrasound and cavitation (generation of micro-turbulence) contributes more than chemical effect (radical production). In sonocatalysis, the catalyst merely acts as adsorbent, while sono-photocatalysis shock waves produced by transient cavitation bubble have an adverse effect of desorption of pollutant molecules from catalyst surface. The phenomena of radical conservation and scavenging of radicals also play an important role. Externally added H2O2 (as Fenton reagent) scavenges oxidising radicals produced by cavitation, while dissolved oxygen as well as externally added Fe2+ (as Fenton reagent) help either generation of additional oxidising radicals (such as O• and HO2•) or regeneration of •OH radicals through Fenton reaction induced by in-situ produced H2O2 through combination of •OH radicals.

Physical and Chemical Mechanisms of Ultrasound in Biofuel Synthesis

Physical and chemical mechanisms ultrasound-assisted processes as related to the synthesis of biofuels are reviewed. Ultrasound and its secondary effect of cavitation have physical and chemical effects on a reaction system, which can contribute to enhancement of the kinetics and yield. In this chapter, a mechanistic insight into the ultrasound assisted biofuels synthesis is given by coupling simulations of cavitation bubble dynamics with experimental data. The physical effect of ultrasound and cavitation is through intense micro-convection in the system that gives marked improvements in the mass transfer of the system. The chemical effect is through generation of highly reactive radicals through transient cavitation that induce or accelerate chemical reactions. Chemical effects include thermal decomposition of the solvent vapor molecules in the cavitation bubble resulting in generation of smaller molecular species that also affect chemistry of the process. Raising the static pressure of the reaction system above ultrasound pressure amplitude in the system helps to discriminate between physical and chemical effects of ultrasound and cavitation. Biofuels systems considered in this chapter are the pretreatment of biomass, biodiesel synthesis with acid/base and homogeneous/heterogeneous catalysts, extraction of microalgal lipids, bioconversion of crude glycerol from biodiesel industry to value added products and desulfurization of the fuel. Among the physical effects of ultrasound and cavitation, micro-streaming by ultrasound has a greater influence on reactions than shock waves generated by cavitation bubbles. In some cases, chemical effects of transient cavitation are revealed to have adverse influence on a reaction. Many biofuels systems are limited by their intrinsic characteristics that restrict the effect of ultrasound and cavitation on the reaction system.

Investigations in sono-enzymatic degradation of ibuprofen

The drug ibuprofen (IBP) appears frequently in the wastewater discharge from pharmaceutical industries. This paper reports studies in degradation of IBP employing hybrid technique of sono-enzymatic treatment. This paper also establishes synergy between individual mechanisms of enzyme and sonolysis for IBP degradation by identification of degradation intermediates, and Arrhenius & thermodynamic analysis of the experimental data. Positive synergy between sonolysis and enzyme treatment is attributed to formation of hydrophilic intermediates during degradation. These intermediates form due to hydroxylation and oxidation reactions induced by radicals formed during transient cavitation. Activation energy and enthalpy change in sono-enzymatic treatment are lower as compared to enzyme treatment, while frequency factor and entropy change are higher as compared to sonolysis. Degradation of IBP in sono-enzymatic treatment is revealed to be comparable with other hybrid techniques like photo-Fenton, sono-photocatalysis, and sono-Fenton.