Jose Savio Melo

Biosorption characteristics of uranium(VI) from aqueous medium onto Catenella repens, a red alga

The biosorption characteristics of uranium(VI) onto Catenella repens (a red alga), were evaluated as a function of pH, biosorbent size, time, biomass dosage, initial uranium concentration and temperature. Within the pH range studied (1.5-7.5), 4.5 was the optimum pH for the uptake of uranium(VI) by C. repens. Reduction in particle size did not increase the biosorption capacity. The metal removal was rapid, with more than 90% of total biosorption taking place in 30 min, and equilibrium was attained in 45 min. The maximum metal loading capacity of the alga was 303 mg/g. Within the temperature range studied (15-55 degrees C), there was no significant change in biosorption, under optimal conditions. Adsorption process could be well defined by both the Langmuir and Freundlich isotherms with r(2) of 0.94 and 0.96, respectively. The kinetic data fitted the pseudo-second-order kinetic model with the r(2) value of 0.99. At a low pH of 2.5, where most of the biomasses show either no or less metal uptake, a good (>15%) metal loading capacity of 25% was achieved. Therefore biosorption characteristics were also evaluated at pH 2.5.

Redox additive/active electrolytes: a novel approach to enhance the performance of supercapacitors

Currently, supercapacitors (SCs) are a promising field in the area of energy storage devices. In the last few decades, different types of carbon based materials with suitable surface modifications, metal oxides, metal hydroxides, conducting polymers and their various composites have been used as electrodes to improve the energy performance of SCs. In addition, different technologies like asymmetric and hybrid systems have also been introduced. Interestingly, another alternative approach has been proposed recently by a few research groups, wherein electrolytes (liquid and polymer) can enhance the performance of the SCs via redox reactions at the electrode–electrolyte interface, by introducing redox additives or mediators in the electrolytes. The main advantage of this new technique is its simple and safe preparation method, along with cost effectiveness compared to the preparation of some active electrode materials. Hence it is believed that identification of suitable redox additives or species in electrolytes will be a hotspot in the field of SCs in the coming years.

Functionalized-graphene modified graphite electrode for the selective determination of dopamine in presence of uric acid and ascorbic acid

Graphene is chemically synthesized by solvothermal reduction of colloidal dispersions of graphite oxide. Graphite electrode is modified with functionalized-graphene for electrochemical applications. Electrochemical characterization of functionalized-graphene modified graphite electrode (FGGE) is carried out by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The behavior of FGGE towards ascorbic acid (AA), dopamine (DA) and uric acid (UA) has been investigated by CV, differential pulse voltammetry (DPV) and chronoamperommetry (CA). The FGGE showed excellent catalytic activity towards electrochemical oxidation of AA, DA and UA compared to that of the bare graphite electrode. The electrochemical oxidation signals of AA, DA and UA are well separated into three distinct peaks with peak potential separation of 193mv, 172mv and 264mV between AA-DA, DA-UA and AA-UA respectively in CV studies and the corresponding peak potential separations in DPV mode are 204mv, 141mv and 345mv. The FGGE is successfully used for the simultaneous detection of AA, DA and UA in their ternary mixture and DA in serum and pharmaceutical samples. The excellent electrocatalytic behavior of FGGE may lead to new applications in electrochemical analysis.

Evaporation Driven Self-Assembly of a Colloidal Dispersion during Spray Drying: Volume Fraction Dependent Morphological Transition

Morphological transition of droplets during evaporation driven self-assembly of colloidal dispersion of alumina particles has been investigated. It was found that a sphere to doughnut-like transition of the droplet morphology takes place even when the rate of drying remains moderate and is not extremely fast. Further, it has been seen that such transition is strongly dependent on the volume fraction of the colloids in the droplets. The transition proceeds via buckling of the initial spherical droplets, which occurs when the capillary forces driving the deformation overcomes the interparticle electrostatic forces. However, the transition is hindered and the buckling probability is reduced due to the inherent spatial constraint when the colloid volume fraction is increased. Mesoscopic structures of the assembled grains have been investigated by scanning electron microscopy, small-angle neutron scattering, and dynamic light scattering techniques. Interestingly, it has been observed that the functionality of photoluminescence spectrum of the dried nanoporous grains depends somewhat on the grain morphology.

High Performance Solid-State Electric Double Layer Capacitor from Redox Mediated Gel Polymer Electrolyte and Renewable Tamarind Fruit Shell Derived Porous Carbon

The activated carbon was derived from tamarind fruit shell and utilized as electrodes in a solid state electrochemical double layer capacitor (SSEDLC). The fabricated SSEDLC with PVA (polyvinyl alcohol)/H2SO4 gel electrolyte delivered high specific capacitance and energy density of 412 F g(-1) and 9.166 W h kg(-1), respectively, at 1.56 A g(-1). Subsequently, Na2MoO4 (sodium molybdate) added PVA/H2SO4 gel electrolyte was also prepared and applied for SSEDLC, to improve the performance. Surprisingly, 57.2% of specific capacitance (648 F g(-1)) and of energy density (14.4 Wh kg(-1)) was increased while introducing Na2MoO4 as the redox mediator in PVA/H2SO4 gel electrolyte. This improved performance is owed to the redox reaction between Mo(VI)/Mo(V) and Mo(VI)/Mo(IV) redox couples in Na2MoO4/PVA/H2SO4 gel electrolyte. Similarly, the fabricated device shows the excellent capacitance retention of 93% for over 3000 cycles. The present work suggests that the Na2MoO4 added PVA/H2SO4 gel is a potential electrolyte to improve the performance instead of pristine PVA/H2SO4 gel electrolyte. Based on the overall performance, it is strongly believed that the combination of tamarind fruit shell derived activated carbon and Na2MoO4/PVA/H2SO4 gel electrolyte is more attractive in the near future for high performance SSEDLCs.

Redox additive aqueous polymer gel electrolyte for an electric double layer capacitor

A hydroquinone mediated PVA–H2SO4 gel electrolyte (PHHQ) and activated carbon from bio-waste were prepared for supercapacitor fabrication. PHHQ delivered a higher capacitance (941 F g−1 at 1 mA cm−2) and energy density (20 Wh kg−1 at 0.33 W g−1) than the PVA–H2SO4 gel electrolyte (425 F g−1 at 1 mA cm−2, 9 Wh kg−1 at 0.33 W g−1).

Improved performance of electric double layer capacitor using redox additive (VO2+/VO2+) aqueous electrolyte

Electric double layer capacitors (EDLCs) were fabricated using biomass derived porous activated carbon as electrode material with 1 M H2SO4 and VOSO4 added 1 M H2SO4 as electrolytes. Here, VOSO4 was used as redox additive to improve the overall performance of EDLC. As expected, the VOSO4 electrolyte showed ∼43% of improved specific capacitance of 630.6 F g−1 at 1 mA cm−2 compared to pristine 1 M H2SO4 (440.6 F g−1) due to the contribution of VO2+/VO2+ redox reaction at the electrode–electrolyte interface. Possible redox reaction mechanism of VO2+/VO2+ pair is also briefly illustrated. The good cycling performance of 97.57% capacitance retention was observed even after 4000 cycles. For comparison, the polymer gel electrolyte (PVA/VOSO4/H2SO4) was also prepared and then the performance of the fabricated EDLCs was studied. Overall, these findings could open up a simple and cost effective way to improve the performance of EDLCs significantly.

Biosorption of uranium by melanin: Kinetic, equilibrium and thermodynamic studies

Limitation of conventional techniques for the removal of heavy metals present at low concentrations, has led to the need for developing alternate technologies like biosorption. In the present study we describe the use of melanin pigment synthesized through green technology, for sorption of uranium from aqueous system. Biosynthesized melanin showed good uptake over a broad pH range. Removal of uranium was rapid and equilibrium was reached within 2h of contact. It was observed that the kinetic data fits well into Lagergrens pseudo-second order equation. A maximum loading capacity of 588.24 mg g(-1) was calculated from Langmuir plot. Thermodynamic studies performed revealed that sorption process was favorable. Binding of uranium on the surface of melanin was confirmed by FT-IR and energy dispersive spectroscopy (EDS). Thus, biosynthesized melanin can be efficiently used as a sorbent for removal of uranium from aqueous solution.

An amperometric bienzymatic cholesterol biosensor based on functionalized graphene modified electrode and its electrocatalytic activity towards total cholesterol determination.

Cholesterol oxidase (ChOx) and cholesterol esterase (ChEt) have been covalently immobilized onto functionalized graphene (FG) modified graphite electrode. Enzymes modified electrodes were characterized using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). FG accelerates the electron transfer from electrode surface to the immobilized ChOx, achieving the direct electrochemistry of ChOx. A well defined redox peak was observed, corresponding to the direct electron transfer of the FAD/FADH(2) of ChOx. The electron transfer coefficient (α) and electron transfer rate constant (K(s)) were calculated and their values are found to be 0.31 and 0.78 s(-1), respectively. For the free cholesterol determination, ChOx-FG/Gr electrode exhibits a sensitive response from 50 to 350 μM (R=-0.9972) with a detection limit of 5 μM. For total cholesterol determination, co-immobilization of ChEt and ChOx on modified electrode, i.e. (ChEt/ChOx)-FG/Gr electrode showed linear range from 50 to 300 μM (R=-0.9982) with a detection limit of 15 μM. Some common interferents like glucose, ascorbic acid and uric acid did not cause any interference, due to the use of a low operating potential. The FG/Gr electrode exhibits good electrocatalytic activity towards hydrogen peroxide (H(2)O(2)). A wide linear response to H(2)O(2) ranging from 0.5 to 7 mM (R=-0.9967) with a sensitivity of 443.25 μA mM(-1) cm(-2) has been obtained.

Uranium (VI) recovery from aqueous medium using novel floating macroporous alginate-agarose-magnetite cryobeads

This study presents a novel development of a floating polymeric-magnetite cryobead for the recovery of hexavalent uranium from the aqueous sub-surfaces. The alginate-agarose-magnetite cryobeads were synthesized by the process of cryotropic-gelation at subzero-temperature. The physico-chemical properties of cryobeads showed high surface area and high interconnected porosity (≈ 90%). Low density of these cryobeads explains their floating property in the aqueous medium. The rheological analysis of cryobeads showed its stability and increased stiffness after uranium adsorption. The presence of magnetite nanoparticles in the porous cryobeads facilitates the recovery of these beads by applying an external magnetic field. Maximum uranium adsorption (97 ± 2%) was observed in the pH range of 4.5-5.5. The thermodynamic parameters suggest passive endothermic adsorption behaviour. HCl was found to be an efficient eluent for the uranium desorption. Five repeated cycles for the desorption of uranium from biosorbent showed 69 ± 3% of uranium recovery. These results suggest stability of these novel floating magnetite-cryobeads under environmental conditions with potential for the recovery of uranium from contaminated aqueous subsurfaces.