Vijayanand S. Moholkar

Microbial conversion of glycerol: present status and future prospects.

Biodiesel has emerged as a potential alternate renewable liquid fuel in the past two decades. Total annual production of biodiesel stands at 6.96 million tons and 11.2 million tons in USA and Europe, respectively. In other countries, Asia and Latin America, biodiesel production has increased at unprecedented rate. Despite this, the economy of biodiesel is not attractive. An obvious solution for boosting the economy of the biodiesel industry is to look for markets for side products of the transesterification process of biodiesel synthesis. The main by-product is glycerol. However, this glycerol is contaminated with alkali/acid catalyst and alcohol, and thus, is not useful for conventional applications such as in toothpaste, drugs, paints and cosmetics. Conversion of this glycerol to value-added product is a viable solution for effective and economic utilization, which would also generate additional revenue for the biodiesel industry. Intensive research has taken place in area of conversion of glycerol to numerous products. The conventional catalytic route of glycerol transformation employs prohibitively harsh conditions of temperature and pressure, and thus, has slim potential for large-scale implementation. In addition, the selectivity of the process is rather small with formation of many undesired side products. The bioconversion processes, on the other hand, are highly selective although with slower kinetics. In this review, we have given an assessment and overview of the literature on bioconversion of glycerol. We have assessed as many as 23 products from glycerol bioconversion, and have reviewed the literature in terms of microorganism used, mode of fermentation, type of fermentor, yield and productivity of the process and recovery/purification of the products. The metabolic pathway of conversion of glycerol to various products has been discussed. We have also pondered over economic and engineering issues of large-scale implementation of process and have outlined the constraints and limitations of the process. We hope that this review will be a useful source of information for biochemists, biotechnologists, microbiologists and chemical engineers working in the area of glycerol bioconversion.

Investigations in the physical mechanism of sonocrystallization.

This paper addresses the issue of mechanistic aspects of sonocrystallization with approach of coupling experiments with simulations of bubble dynamics. The major experimental result of our study is that, as compared to a mechanically agitated crystallization system, the dominant crystal size (or median) of the crystal size distribution (CSD) of sonocrystallization systems is smaller, but span of CSD is larger. The CSD is influenced by nucleation rate and growth rate. The nature of convection in the medium is found to be the crucial factor. In a mechanically agitated system, uniform velocity field prevails in crystallization volume, due to which both dominant crystal size and span of CSD reduce. The convection in a sonicated system is of a different kind. This convection has two components, viz. microturbulence (or micro-convection), which is continuous oscillatory motion of liquid induced by radial motion of cavitation bubble, and shock waves, which are discrete, high pressure amplitude waves emitted by the bubble. These components have different impact on crystallization process due to their nature. Shock waves increase the nucleation rate and microtubulence governs growth of the nuclei. However, the effect of shock waves is more marked than microturbulence (or micro-convection). Nucleation rate shows an order of magnitude rise with sonication, while growth rate (and hence the dominant crystal size) reduces with sonication as compared to the mechanically agitated system.

Physical insights into the sonochemical degradation of recalcitrant organic pollutants with cavitation bubble dynamics

This paper tries to discern the mechanistic features of sonochemical degradation of recalcitrant organic pollutants using five model compounds, viz. phenol (Ph), chlorobenzene (CB), nitrobenzene (NB), p-nitrophenol (PNP) and 2,4-dichlorophenol (2,4-DCP). The sonochemical degradation of the pollutant can occur in three distinct pathways: hydroxylation by ()OH radicals produced from cavitation bubbles (either in the bubble-bulk interfacial region or in the bulk liquid medium), thermal decomposition in cavitation bubble and thermal decomposition at the bubble-liquid interfacial region. With the methodology of coupling experiments under different conditions (which alter the nature of the cavitation phenomena in the bulk liquid medium) with the simulations of radial motion of cavitation bubbles, we have tried to discern the relative contribution of each of the above pathway to overall degradation of the pollutant. Moreover, we have also tried to correlate the predominant degradation mechanism to the physico-chemical properties of the pollutant. The contribution of secondary factors such as probability of radical-pollutant interaction and extent of radical scavenging (or conservation) in the medium has also been identified. Simultaneous analysis of the trends in degradation with different experimental techniques and simulation results reveals interesting mechanistic features of sonochemical degradation of the model pollutants. The physical properties that determine the predominant degradation pathway are vapor pressure, solubility and hydrophobicity. Degradation of Ph occurs mainly by hydroxylation in bulk medium; degradation of CB occurs via thermal decomposition inside the bubble, degradation of PNP occurs via pyrolytic decomposition at bubble interface, while hydroxylation at bubble interface contributes to degradation of NB and 2,4-DCP.

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.

Physical features of ultrasound-enhanced heterogeneous permanganate oxidation

This paper addresses the matter of mechanistic features of ultrasound-assisted permanganate oxidation of organic compounds in aqueous phase. This reaction system is essentially a liquid-liquid heterogeneous one, which is limited by the mass transfer characteristics. Previous research has established that ultrasound irradiation of reaction mixture enhances the kinetics and yield of permanganate oxidation. The principal physical effect of ultrasonic cavitation is formation of fine emulsion between immiscible phases that eliminates the mass transfer resistance, while principal chemical effect is production of radicals through transient collapse of cavitation bubbles, which accelerate the reaction. In this paper, we have tried to discriminate between these physical and chemical effects by coupling experiments with different conditions (which alter the nature of cavitation phenomena in the medium) to simulations of cavitation bubble dynamics. It is revealed that in absence of radical conserving agent, the enhancement effect is merely physical. Diffusion of radicals towards interface between phases, where the oxidation reaction occurs is the limiting factor in contribution of chemical effect of ultrasonic cavitation towards enhancement of oxidation. Enhancement of total radical production in the aqueous phase (by degassing of the medium) increases the overall oxidation yield, but only marginally. On the other hand, addition of a radical conserver such as FeSO(4).7H(2)O results in marked enhancement in oxidation yield, as the conserver assists deeper penetration of radicals in the aqueous medium and diffusion towards interface.

Ultrasonic biodiesel synthesis from crude Jatropha curcas oil with heterogeneous base catalyst: Mechanistic insight and statistical optimization

This paper reports studies in ultrasound-assisted heterogeneous solid catalyzed (CaO) synthesis of biodiesel from crude Jatropha curcas oil. The synthesis has been carried out in two stages, viz. esterification and trans-esterification. The esterification process is not influenced by ultrasound. The transesterification process, however, shows marked enhancement with ultrasound. A statistical experimental design has been used to optimize the process conditions for the synthesis. XRD analysis confirms formation of Ca(OMe)2, which is the active catalyst for transesterification reaction. The optimum values of parameters for the highest yield of transesterification have been determined as follows: alcohol to oil molar ratio ≈ 11, catalyst concentration ≈ 5.5 wt.%, and temperature ≈ 64°C. The activation energy of the reaction is calculated as 133.5 kJ/mol. The heterogeneity of the system increases mass transfer constraints resulting in approx. 4 × increase in activation energy as compared to homogeneous alkali catalyzed system. It is also revealed that intense micro-convection induced by ultrasound enhances the mass transfer characteristics of the system with ∼ 20% reduction in activation energy, as compared to mechanically agitated systems. Influence of catalyst concentration and alcohol to oil molar ratio on the transesterification yield is inter-linked through formation of methoxy ions and their diffusion to the oil-alcohol interface, which in turn is determined by the volume fractions of the two phases in the reaction mixture. As a result, the highest transesterification yield is obtained at the moderate values of catalyst concentration and alcohol to oil molar ratio.

Physical facets of ultrasonic cavitational synthesis of zinc ferrite particles

This paper addresses the physical features of the ultrasonic cavitational synthesis of zinc ferrite particles and tries to establish the relationship between cavitation physics and sonochemistry of the zinc ferrite synthesis. A dual approach of coupling experimental results with simulations of radial motion of cavitation bubbles has been adopted. The precursors for the zinc ferrite, viz. ZnO and Fe(3)O(4) are produced in situ by the hydrolysis of Zn and Fe(II) acetates stimulated by (*)OH radicals produced from the transient collapse of the cavitation bubbles. Experiments performed under different conditions create significant variation in the production of (*)OH radicals, and hence, the rate of acetate hydrolysis. Correlation of the results of experiments and simulations sheds light on the important facets of the physical mechanism of ultrasonic cavitational zinc ferrite synthesis. It is revealed that too much or too little rate of acetate hydrolysis results in smaller particle size of zinc ferrite. The first effect of a higher rate of hydrolysis leads to excessively large growth of particles, due to which they become susceptible to the disruptive action of cavitation bubbles. Whereas, the second effect of too small rate of hydrolysis of Zn and Fe(II) acetates restricts the growth of particles. It has been observed that the initial reactant concentration does not influence the mean particle size or the size distribution of zinc ferrite particles. The present investigation clearly confirms that the rate-controlling step of zinc ferrite synthesis through ultrasonic cavitational route is the rate of formation of (*)OH radicals from cavitation bubbles.

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.

Physical features of ultrasound assisted enzymatic degradation of recalcitrant organic pollutants.

This work has attempted to provide answer to the interaction of sonolysis and enzymatic treatment on degradation of recalcitrant dyes in a combined treatment. The model system comprises of two dyes, acid red and malachite green as model pollutants, along with horseradish peroxidase as a model enzyme and ultrasound of 20 kHz frequency. A dual approach of coupling experimental results with simulations of cavitation bubble dynamics has been adopted. Utilization of oxidation potential of horseradish peroxidase has been found to be a function of convection level in the medium. Cavitation phenomenon is found to have an adverse effect on enzyme action due to generation of high amplitude shock waves, which denature the enzyme. Degradation of dye at high static pressure increases due to absence of cavitation and high energy interaction (or collisions) between enzyme and dye molecules, which are beneficial towards enzymatic oxidation of the latter. High intensity convection generated by ultrasound also obviates need for an external shielding agent such as PEG that prevents attachment of the phenoxy radicals to enzyme that blocks the active sites of the enzyme.

Mechanistic investigation of the sonochemical synthesis of zinc ferrite

In this investigation, an attempt has been made to establish the physical mechanism of sonochemical synthesis of zinc ferrite with concurrent analysis of experimental results and simulations of cavitation bubble dynamics. Experiments have been conducted with mechanical stirring as well as under ultrasound irradiation with variation of pH and the static pressure of the reaction medium. Results of this study reveal that physical effects produced by transient cavitation bubbles play a crucial role in the chemical synthesis. Generation of high amplitude shock waves by transient cavitation bubbles manifest their effect through in situ micro-calcination of metal oxide particles (which are generated through thermal hydrolysis of metal acetates) due to energetic collisions between them. Micro-calcination of oxide particles can also occur in the thin liquid shell surrounding bubble interface, which gets heated up during transient collapse of bubbles. The sonochemical effect of production of OH radicals and H(2)O(2), in itself, is not able to yield ferrite. Moreover, as the in situ micro-calcination involves very small number of particles or even individual particles (as in intra-particle collisions), the agglomeration between resulting ferrite particles is negligible (as compared to external calcination in convention route), leading to ferrite particles of smaller size (6 nm).