Microbial electrosynthesis of valuable chemicals from the reduction of CO2: a review.

The present energy demand of the world is increasing but the fossil fuels are gradually depleting. As a result, the need for alternative fuels and energy sources is growing. Fuel cells could be one alternative to address the challenge. The fuel cell can convert CO2 to value-added chemicals. The potential of bio-fuel cells, specifically enzymatic fuel cells and microbial fuel cells, and the importance of immobilization technology in bio-fuel cells are highlighted. The review paper also includes a detailed explanation of the microbial electrosynthesis system to reduce CO2 and the value-added products during microbial electrosynthesis. Future research in bio-electrochemical synthesis for CO2 conversion is expected to prioritize enhancing biocatalyst efficiency, refining reactor design, exploring novel electrode materials, understanding microbial interactions, integrating renewable energy sources, and investigating electrochemical processes for carbon capture and selective CO2 reduction. The challenges and perspectives of bio-electrochemical systems in the application of CO2 conversion are also discussed. Overall, this review paper provides valuable insights into the latest developments and criteria for effective research and implementation in bio-fuel cells, immobilization technology, and microbial electro-synthesis systems.

Surface functionalization of bamboo leave mediated synthesized SiO2 nanoparticles: Study of adsorption mechanism, isotherms and enhanced adsorption capacity for removal of Cr (VI) from aqueous solution.

Green synthesis of nanoparticles (NPs) provides economic and environmental benefits as an alternative to chemical or physical methods. Furthermore, the surface properties of such NPs can be modulated by means of the functionalization with different groups making them suitable for various advanced functional applications including water pollutants removal using adsorption technique. In the present work, an eco-friendly synthesis route for nano-adsorbent SiO2 NPs and subsequent surface modifications for enhanced adsorption capacity in removal of Cr(VI) ions from aqueous solution are reported. The green synthesis of SiO2 NPs was carried out using simple bamboo leaves followed by surface modification with amine (A-SiO2) and carboxylic (C-SiO2) functional groups with aim to study the effect of functionalization on adsorption capacity. These nano-adsorbents were characterized by FTIR, SEM, XPS, BET, and zeta potential. and adsorption of Cr(VI) was studied at varying parameters i.e. NPs mass, contact time, and solution pH. The investigation shows interesting results revealing the importance of interactions between the surface functional groups on SiO2 NPs and Cr(VI) species as well as experimental conditions for the choice of surface modifier to achieve a maximum adsorption capacity. The adsorption mechanism has been studied using Langmuir, Freundlich and Temkin adsorption isotherms. The maximum adsorption capacity has been achieved in the case of A-SiO2 NPs which was found to 174 mg/g and much higher than that of SiO2 and C-SiO2 NPs attributed to the selective adsorption and pH conditions. Additionally, A-SiO2 NPs exhibit excellent recyclability indicating their suitability for promising and long term potential applications. This study provides a novel, simple and cost-effective synthesis/surface engineering technology for producing high performance recyclable nano-adsorbents for adsorptive removal of Cr(VI).

Kinetics and model development for enzymatic synthesis of fructo-oligosaccharides using fructosyltransferase.

Experimental investigations were made to synthesize fructo-oligosaccharides (FOS) from sucrose using fructosyltransferase. The influence of various parameters such as temperature (45-55 °C), pH (4-5), initial sucrose concentration (ISC: 300-500 g/L) and enzyme concentration (4-32 U/mL) were varied. A maximum FOS yield of 60% was observed at ISC 500 g/L, pH 4.5 with enzyme activity 32 U/mL and at 55 °C. It was confirmed that 1-kestose (tri-) was the major product of FOS as compared to nystose (tetra-) and fructosylnystose (penta-saccharides). Further, the reaction rate increases with increase in temperature. From separate sets of experiments, it was observed that FOS formation was affected by glucose inhibition. Apart from the increase in the rate of FOS formation with increasing enzyme activity, the final values of FOS yield increase though till 16 U/mL and thereafter attain plateau. A kinetic model was also developed, based on Michaelis-Menten kinetics, and a five-step ten-parameter model, including glucose inhibition, was obtained. Model was solved using COPASI(®) (version 4.8) solver for kinetic parameter estimations followed by time course simulations.

Kinetic studies and model development for the formation of galacto-oligosaccharides from lactose using synthesized thermo-responsive bioconjugate.

In an earlier study by us [47], thermo-responsive bioconjugate (poly-N-isopropylacrylamide-ß-galactosidase) was synthesized and characterized. This study utilizes the prowess of such smart bioconjugate for the enzymatic synthesis of galacto-oligosaccharides (GOS) from lactose at various initial lactose concentrations (ILC), enzyme concentrations, and temperatures, while maintaining a constant pH of 6. A maximum GOS yield of 35% (on dry basis) was observed at 100g/L ILC and 0.275mg/mL (0.055U/mL) conjugated protein. The GOS yield remained approximately the same for 50 and 100g/L ILC, beyond which, it decreased. As the enzyme concentration increased, the equilibrium formation of GOS increased and eventually attained a plateau when the concentration of conjugated protein exceeded 0.275mg/mL (0.055U/mL). GOS yield increased on raising the temperature from 30 to 40°C, and declined thereafter. The apparent kinetic parameters were estimated from a five-step, nine-parameter kinetic model, which was then simulated using the COPASI package. The simulated results demonstrated an excellent match with the experimental data.

Kinetics of sucrose conversion to fructo-oligosaccharides using enzyme (invertase) under free condition.

The study reports the synthesis of fructo-oligosaccharide (FOS) from sucrose using invertase derived from Saccharomyces cerevisiae. The reaction was conducted in a batch mode under free enzyme condition. Fructo-oligosaccharide formation was detected at a high sucrose concentration of over 200 g/L. The investigation was extended to study the effect of different parameters such as initial sucrose concentration (ISC), pH, and enzyme concentration. A maximum FOS yield of 10 % (dry basis) was observed using 525 g/L of ISC, with 6 U/mL of the enzyme, and pH 5.5 at 40 °C. 1-Kestose was the major product of among different forms of FOS. The FOS yield increased with an increase in sucrose concentration up to 525 g/L, beyond which it started to decrease. However, the maximum FOS yield was not affected by the increasing concentration of the enzyme beyond a certain level (2 U/mL). Furthermore, the activity of enzyme slightly increased with an increase in the pH up to 6, and thereafter it declined. Addition of glucose decreased the FOS yield because of enzyme inhibition. A five-step, ten-parameter model was developed, for which the simulation was performed in COPASI. The results predicted by the model were consistent with the experimental data.

Synthesis and characterization of thermo-responsive poly-N-isopropylacrylamide bioconjugates for application in the formation of galacto-oligosaccharides.

The study demonstrates the properties of conjugation of ß-galactosidase with a thermo-responsive polymer, poly-N-isopropylacrylamide (PNIPAAm) in comparison to a non-responsive polymer, poly-acrylamide (PAAm). The maximum formation of bioconjugate (PNIPAAm-ß-galactosidase) was 75% (yield) with 50% chemically modified enzyme (using itaconic anhydride). The process of bioconjugation (bioconjugate concentration: 7.4%) decreases lower critical solution temperature from 32.5 °C (with pure PNIPAAm) to 26.5 °C. The effect of temperature on the activities of PNIPAAm-ß-galactosidase, PAAm-ß-galactosidase and native enzyme was also compared. At 70 °C, the maximum activity was observed for PNIPAAm-ß-galactosidase while for others it was at 60 °C. However, the effect of pH was insignificant on activities of both the bioconjugates than the native enzyme. The addition of ethylene glycol (20%, v/v) enhances the activity (by 45%) of PNIPAAm-ß-galactosidase with no loss in stability; however; the trend is reversed with the addition of ethanol. Further, employing bioconjugates even up to 24 cycles of precipitation (at 40 °C) followed by re-dissolution (4 °C) around 90% of activity could be retained by PNIPAAm-ß-galactosidase. The PNIPAAm-ß-galactosidase also showed much-improved thermal and storage stabilities. A lower Michaelis-Menten constant (Km) was estimated with the PNIPAAm-ß-galactosidase than the native enzyme as well as PAAm-ß-galactosidase. Finally, PNIPAAm-ß-galactosidase was tested to synthesize galacto-oligosaccharides from lactose solution.

Kinetics of lactose conversion to galacto-oligosaccharides by ß-galactosidase immobilized on PVDF membrane.

Experimental studies were made for immobilization of enzymes on microporous polyvinylidene fluoride (PVDF) membrane in order to carry out enzymatic reaction of lactose into galacto-oligosaccharides using ß-galactosidase. The present work, however, is the first part in the direction of enzymatic membrane reactor studies for carrying out reaction followed by membrane based separation to purify galacto-oligosaccharides out of reaction mixture. The middle of the three compartment cell, separated by two immobilized (enzyme) membranes, was utilized to feed lactose solution; whereas, adjacent compartments were filled with distilled water. The reacted mixture solution was analyzed for tri-, tetra- and penta-forms of GOS. The formation of product GOS strongly depended on varying amounts of initial lactose concentration (ILC). Total GOS formation increased from 7% to 28% for ILC from 50 to 200 g/L. However, tri-saccharide was the major (67%) in comparison to tetra (27%) and penta (6%) forms of GOS. Further, based on Michaelis-Menten kinetics, a six-step-eleven-parameter model was developed. The model incorporated enzyme inhibition and formation of glucose and galactose separately. Simulated results from developed model matched exceeding well with experimental results.

Kinetics and design relation for enzymatic conversion of lactose into galacto-oligosaccharides using commercial grade ß-galactosidase.

The enzymatic synthesis of galacto-oligosaccharides (GOS) from lactose was studied using commercial grade ß-galactosidase (Biolacta FN5) from Bacillus circulans. The reaction was carried out under free enzyme condition varying initial lactose concentration (ILC: 55-525 g/L), enzyme concentration (0.05-1.575 g/L), temperature (30-50°C) and pH (5.0-6.0). Reaction mixture compositions were analyzed utilizing high performance liquid chromatography (HPLC). A maximum GOS formation of 39% (dry basis) was achieved at an ILC of 525 g/L converting 60% of the lactose fed. Tri-saccharides were the major types of GOS formed, accounting approximately 24%; whereas, tetra-saccharides and penta-saccharides account approximately 12% and 3%, respectively. Design correlation was developed in order to observe the quantitative effect of operating parameters on GOS yield. Further, based on Michaelis-Menten model, four-step reaction pathways were considered for simplistic understanding of the kinetics. Apart from predicting the reaction mixture composition, the approach also provided kinetic parameters though simulation using COPASI 4.7®. Excellent agreements were observed between simulated and experimental results.