Mixture Of HNO3 Research Articles

Chemical and mineralogical characterization of chromite ore processing residue from two recent Indian disposal sites

Chromite ore processing residue (COPR) is a hazardous waste. Nevertheless, deposition of COPR in uncontrolled surface landfills is still common practice in some countries. Whereas old (between at least 40 and 180 years) COPR from the temperate zone has been intensively investigated, information on COPR in other regions is restricted. Relatively young (<25 years) COPR samples obtained from two abandoned landfill sites in India were investigated by a modified total microwave digestion method, X-ray powder diffraction (XRPD), and scanning electron microscopy (SEM) in order to determine their chemical and mineralogical nature. By the use of microwave digestion with acid mixtures of HNO3, H3PO4, and HBF4 (5:3:2 vol), COPR was completely dissolved and element contents similar to those obtained by X-ray fluorescence were found. Total Cr contents of the two COPR accounted for 81 and 74 g kg−1, of which 20 and 13% were present in the carcinogenic hexavalent form (CrVI). Apart from the common major mineral phases present in COPR reported earlier, a further Cr host mineral, grimaldiite [CrO(OH)], could be identified by XRPD and SEM. Additionally, well soluble Na2CrO4 was present. Improving the effectiveness of chromite ore processing and preventing the migration of Cr(VI) into water bodies are the main challenges when dealing with these COPR.

An experimental investigation of the physical properties of the graphene/multi-walled carbon nanotubes composite

In this present work, the physical characteristics of the graphene/MWCNTs composite have been studied. Surface modification has been done by grinding process and acid treatment where planetary ball milling (PBM) machine for grinding in aqueous circumstance and a mixture of HNO3 and H2SO4 for acid treatment have been executed. Though, there are many researches of GN and MWCNTs nanoparticles based on individually as well as combined with other nanoparticles have been accomplished, reports on the combination of these two nanoparticles are negligible. In this paper, IR thermography, morphological and structural characteristics of modified GN–MWCNTs by field emission scanning electron microscopy, Raman spectroscopy and dispersibility analysis by sedimentation, UV–Vis spectroscopy, electric conductivity and thermal conductivity measurements have been presented in order to report the characteristics of GN/MWCNTs composite. Acid treatment as well as wet grinding of GN/MWCNTs composite at 500rpm for 2h gives the highest thermal conductivity enhancement which is 4.29% at 40°C.

Heavy metal pollution and ecological geochemistry of soil impacted by activities of oil industry in the Niger Delta, Nigeria

The aim of the study was to assess the metal pollution of the soil around two crude oil flow stations in the Niger Delta, Nigeria and perform the ecological geochemistry of soil. Soil samples were collected by systematic random sampling around the flow stations in Kokori and Kolo Creek, Niger Delta Nigeria. The concentrations of Zn, Pb, Ni, Cr and Cd in the soil were determined by Atomic Absorption Spectrophotometry after wet-digestion in acid mixture of HNO3 + HCl + H2O2. Mean concentration values of Zn, Pb, Ni, Cr, and Cd in the soil of both Kokori and Kolo Creek were higher in some cases by several orders of magnitude than the Control values except for Ni in Kokori and Pb in Kolo Creek. The levels of Index of geo-accumulation (I-geo) were no pollution to low pollution except Cr in Kolo Creek that indicates strong pollution. Nemerow integrated pollution index (NIPI) also indicates the soil of Kokori and Kolo Creek to be of moderate and high levels of pollution respectively with Zn majorly contributing to the pollution status in Kokori while Cr, Zn and Ni are the major pollutants in Kolo Creek. However, a different pollution pattern was observed in the single potential ecological risk assessment of metals in the soil. Cd and Cr in Kokori and Cr, Cd and Ni in Kolo Creek portend ecological risk with the following patterns; Cd = Cr > Zn = Pb = Ni and Cr > Cd = Ni > Zn = Pb respectively. Comprehensive potential ecological risk (RI) assessment indicated low potential ecological risk for Kokori and Kolo Creek; and the major contributors are Cd, Ni and Cr. Therefore, there is urgent need to remediate these metals that could pose ecological risk so that further buildup would be prevented, and avert any potential harm to ecological functioning of the areas.

Comparative study of three digestion methods for elemental analysis in traditional medicine products using atomic absorption spectrometry

Traditional medicine mainly of herbal origin is widely used all around the world. Heavy metal contamination in such products is frequently reported. Accumulation of heavy metals in the human body leads to various health hazards. Thus, precise determination for such contaminants is required for safety assurance. Sample preparation is a significant step in spectroscopic analysis to achieve reliable and accurate results. Wet digestion methods are basically used for the dissolution of herbal product samples prior to elemental analysis. This study has been designed to evaluate the efficiency of three acid digestion methods using different solvents. Five samples were digested with three different acid digestion methods namely method A (a combination of nitric-perchloric acids HNO3–HClO4 in a ratio 2:1), method B (only nitric acid HNO3), and method C (a mixture of nitric-hydrochloric acids HNO3–HCl in a ratio 1:3), to recommend the most efficient digestion method that gains the highest analyte recovery. The analysis of arsenic (As), cadmium (Cd), lead (Pb), nickel (Ni), zinc (Zn), and iron (Fe) was conducted using various techniques of atomic absorption spectrometry (AAS). The statistical analysis revealed that method C which represented the combination of nitric-hydrochloric acids HNO3–HCl in a ratio 1:3 was the most efficient digestion method for herbal product samples as it had given a significant high recovery (p < 0.05) for all metals compared to method A and method B. Accuracy of the proposed method was evaluated by the analysis of standard reference material (SRM) 1515 Apple Leaves from the National Institute of Standards and Technology (NIST) which presented good recoveries for all metals ranging from 94.5 to 108 %. Method C provides highest recovery for all the analytes under investigation using AAS in herbal medicine samples.

A study on controllable aluminium doped zinc oxide patterning by chemical etching for MEMS application

This present work reports on the study of controllable aluminium doped zinc oxide (AZO) patterning by chemical etching for MEMS application. The AZO thin film was prepared by RF magnetron sputtering as it is capable of producing uniform thin film at high deposition rates. X-Ray diffraction (XRD) and atomic force microscopy (AFM) characterization were done to characterize AZO thin film. The sputtered AZO thin film shows c-axis (002) orientation, low surface roughness and high crystalline quality. To pattern AZO thin film for MEMS application, wet etching was chosen due to its ease of processing with few controlling parameters. Four etching solutions were used namely: 10 % Nitric acid, 10 % Phosphoric acid, 10 % Acetic acid and Molybdenum etch solutions. For the first time, chemical etching using Molybdenum etch that consist of a mixture of CH3COOH, HNO3 and H3PO4 was characterized and reported. The effect of these acidic solutions on the undercut etching, vertical and lateral etch rate were studied. The etched AZO were characterized by scanning electron microscopy (SEM) and stylus profilometer. The investigations showed that the Molybdenum etch has the lowest undercut etching of 7.11 µm, and is highly effective in terms of lateral and vertical etching with an etch ratio of 1.30. Successful fine patterning of AZO thin films was demonstrated at device level on a surface acoustic wave resonator fabricated in 0.35 μm CMOS technology. The AZO thin film acts as the piezoelectric thin film for acoustic wave generation. Patterning of the AZO thin film is necessary for access to measurement probe pads. The working acoustic resonator showed resonance peak at 1.044 GHz at 45.28 dB insertion loss indicating that the proposed Molybdenum etch method does not adversely affect the device’s operating characteristics.

1,5-Dinitronaphthalene hydrogenation to 1,5-diaminonaphthalene over carbon nanotube supported non-noble metal catalysts under mild conditions

Carbon nanotubes (CNTs) were firstly thermal treated and then acid treated in HNO3 or mixture of HNO3 and H2SO4. The treated carbon nanotubes were used as support to load nickel to prepare non-noble metal hydrogenation catalysts. These catalysts were characterized and applied in 1,5-dinitronaphthalene hydrogenation. The characterization results indicate that acid modified carbon nanotubes not only possess a more hydrophilic surface by introducing a variety of oxygen-containing functional groups, but also improve the nickel dispersion, hydrogen uptake quantity and metallic surface area. The Ni/CNTs catalysts show good catalytic activity in 1,5-dinitronaphthalene hydrogenation under mild reaction conditions, and the Ni/CNTs-4 gives the best result of 100% conversion of 1,5-dinitronaphthalene and 92.04% selectivity to 1,5-diaminonaphthalene under 0.6MPa and 393K. The influences of reaction conditions and metal loading amount are discussed, and a possible reaction routes of 1,5-dinitronaphthalene hydrogenation is proposed. Such non-noble metal hydrogenation catalysts are expected to replace the present supported palladium or platinum noble catalysts in industry.

ChemInform Abstract: New Rare Earth Langbeinite Phosphosilicates KBaREEZrP2SiO12 (REE: La, Nd, Sm, Eu, Gd, Dy) for Lanthanide Comprising Nuclear Waste Storage.

AbstractKBaYMP2SiO12 (M: Zr, Sn), KBaLnZrP2SiO12 (Ln: La, Nd, Sm, Eu, Gd, Dy), and the spent nuclear fuel waste variant KBaY0.6La0.1Nd0.1Sm0.1 Eu0.1ZrP2SiO12 are prepared from stoichiometric aqueous solutions containing KCl, ZrOCl2, and SnCl4 to which Ba(NO3)2 and Y(NO3)3 solutions are added prior to precipitation with H3PO4 and Si(OEt)4 dissolved in a 1:1 mixture of HNO3 and EtOH.

ChemInform Abstract: Synthesis, Structure and Photocatalytic Hydrogen Evolution of a Silver‐Linked Hexaniobate Lindqvist Chain.

AbstractColorless single crystals of the polyoxoniobate K5 [H2AgNb6O19]·11H2O are synthesized by a combination of hydrothermal and diffusional methods from an aqueous mixture of K7HNb6O19, AgNO3, NH3, and HNO3 (autoclave, 160 °C, 22 h; 14% yield).

ChemInform Abstract: Assembly of Sandwich‐Type 3‐D Supramolecular Coordination Polymers Based on Hexa‐Molybdenum Chain and {PMo12O40} Heteropolyanion.

AbstractYellow block crystals of H3O[Mo6(bipy)6O17] [PMo12O40]·2H2O are hydrothermally synthesized from a mixture of H3PMo12O40, Ba(NO3)2, bipy, HNO3, and H2O (autoclave, pH 2.0, 170 °C, 3 d; 40% yield).

Levels of essential and toxic metals in fenugreek seeds (Trigonella Foenum-Graecum L.) cultivated in different parts of Ethiopia

Summary The levels of the major (Ca, K, Na, Mg), trace (Fe, Cr, Ni, Zn, Mn, Cu, Co), and toxic (Pb, Cd) metals in the seeds of fenugreek cultivated in different regions of Ethiopia were determined by flame atomic absorption spectrophotometry (FAAS). Wet ashing was used to digest 0.5 g of fenugreek seed flour using 1.5 mL of HNO3 and HClO4 acid mixtures (5:1 ratio), 30 min pre-digestion time, 45 min total digestion time and a temperature of 150 °C. Thirteen elements were determined, obtaining concentrations in the following ranges: Ca (15353-36771 mg kg-1) &gt; Fe (6041-18584 mg kg-1) ≈ K (6789-11517 mg kg-1) &gt; Pb (615-2624 mg kg-1) &gt; Na (201-1559 mg kg-1) &gt; Cd (285-464 mg kg-1) &gt; Cr (3-552 mg kg-1) &gt; Ni (31-108 mg kg-1) &gt; Mg (31-102 mg kg-1) &gt; Zn (15-33 mg kg-1) &gt; Mn (16-28 mg kg-1) &gt; Cu (ND-35 mg kg-1) &gt; Co (4-15 mg kg-1). A statistical analysis of variance (ANOVA) at the 95% confidence level revealed there were significant differences between the mean metal contents of fourteen sample means, except for Zn. Pearson’s correlation revealed weak positive or negative linear relationships, which implies that the presence of one metal did not affect the presence of the other metals within the plant, except for a few metals. The study showed that fenugreek seeds were a good source of essential metals. However, they also contained large amounts of the toxic metals Cd and Pb and therefore should not be consumed daily.

Characterization of the products obtained from the reactions of Zircaloy-4 with an acid mixture of concentrated HNO3and dilute HF with the aim of understanding pickle salts of zirconium alloys

Pickling of zirconium alloys in nitric-hydrofluoric acid has been widely used in the nuclear materials industry. However, the pickle salts still remain a curiosity and a thorough knowledge of the products of the system Zr–(HNO3–HF) still needs to be well established. The present study investigated the products obtained from the reactions of Zircaloy-4 with an acid mixture of HNO3 (65 wt%) and HF (40 wt%) in a volume ratio of 94 : 6, similar to the one used under nuclear industry conditions. It shows that after the reactions, a gel-like solution was present. When drying it at a high relative humidity (at 25 °C and 70 ± 5% RH), an amorphous solid product formed, while in the case of at a low relative humidity (at 15/25 °C and 25 ± 5% RH), besides the amorphous solid, crystalline zirconium hydroxide nitrates Zr(OH)2(NO3)2·4.71H2O and Zr(OH)2(NO3)2·1.65H2O (possible) with low degree of crystallinity formed. These compounds were identified with the help of successfully synthesizing completely crystalline Zr(OH)2(NO3)2·4.71H2O from the gel-like solution by ultrasonication, which in itself was a breakthrough. Such a study can be expected to provide a deep understanding of pickle salts of zirconium alloys in practical use.

Influence of treated waste phosphogypsum materials on the properties of ordinary portland cement

In this study, treatment of phosphogypsum (PG) with lime-water (LWT), sulphuric acid (SAT), a mixture of H2SO4 and HNO3 (AWT), PG-water (ST), and PG-limestone (LT) was attempted to purify PG and improve its quality so that it can be used for manufacture of ordinary Portland Cement (OPC). The treatment of PG removes P2O5, SO3, and MgO impurities into water-leachable phase. Chemical analysis of the treated PG and mechanical properties of OPC mortar after various treatment of PG established improvement of the quality of PG. The purified PG contain less impurities when compared with untreated PG. It was observed that the leachable of P2O5, SO3, and MgO in these samples ranged from 86% to 90%, 69% to 94%, 96% to 99%, respectively, can be achieved using these treatment processes. The major phases Alite (C3S), Belite (C2S), Aluminatetricalcic(C3A), and Tetra-calcium aluminoferrite (C4AF), and control ratios Lime Saturation factor (LSF), Aluminum/Iron ratio (AR), and Silica ratio (SR) were measured. These experimental results showed that the C3S, C3A and C4AF, C2S, LSF, AR, and SR contents fulfilled the requirement of the Jordan Standards and European Standards; hence treated PG can be replaced by natural gypsum. The X-ray diffraction analysis of OPC samples showed that C3S and C2S are major mineral phases, C3A and C4AF represent as minor constituents while the CaO and MgO represent as trace phases. The effect of treated PG on the mechanical properties of OPC mortar was investigated. The OPC produced with purified phosphogypsum were found to have strength properties similar to those produced from mineral gypsum thus fulfilling Jordan Standards and European Standards. The present study indicates that the use of PG in OPC manufacture can solve the waste disposal problem thus cleaning our environment at one hand, on the other hand it can save natural raw materials i.e gypsum.Bangladesh J. Sci. Ind. Res. 50(4), 241-250, 2015

Spatial distribution and risk assessment of heavy metals in bottom sediments of two small dam reservoirs (south-east Poland)

Abstract Sediments of two dam reservoirs in SE Poland, Zalew Zemborzycki (ZZ) and Brody Iłżeckie (BI) were studied. The sediments from both reservoirs were sampled in the transects perpendicular to the shoreline, at the river inflow and the frontal dam. The total concentration of Mn, Zn, Pb, Cd, Cu, Cr and Ni was determined by ICP-EAS method after the sample digestion in the mixture of concentrated HNO3 and HClO4 acids. The statistical analyses: value intervals, mean values, variation coefficient, the median and the skewed distribution were performed. To estimate differences between the means for transects, Tukey’s test was applied with least significant difference (LSD) determination. The maps of the metal spatial distribution were drawn and sediment quality according to the geochemical and ecotoxicological criteria evaluated. Differences between the reservoirs in terms of heavy metals concentration in bottom sediments, and regularities in their spatial distribution were found. In the ZZ sediments the concentration was at the level of geochemical background (Zn, Cr), slightly (Cd, Cu, Ni) or moderately (Pb) contaminated sediments. The metal concentration in the sediments of the BI was up to eight times higher as compared to the ZZ. Moreover, sediments from the BI reservoir showed a greater variability of metal concentration than those from ZZ, which resulted from the dredging operation performed in the part of the reservoir. Metal concentration in sediments of the dredged part was ca. 2–5 times lower than in the undredged one, which indicates that after the dredging operation, accumulation of these metals was slight. The concentrations of Zn, Pb and Cd from the undredged part of BI were at the level of contaminated sediments and exceeded the probable effects level (PEL). In the ZZ, the greatest accumulation of metals occurred in the upper part of the reservoir and at the frontal dam, and the lowest in the middle part of the reservoir. In BI, the lower outflow of water in this reservoir caused a lower metal concentration in the sediments at the frontal dam, as compared with the other sediments in the undredged part of the reservoir. The results indicate that in small and shallow reservoirs, areas of accumulation of heavy metals depend on such factors as a parent river current, reservoir depth, water waving, reservoir shape (narrowing, coves/bays), and type of water outflow.

Controlled surface functionalization of carbon nanotubes by nitric acid vapors generated from sub-azeotropic solution

Controlled surface modification of nanocarbons is crucial for their use in applications. The paper deals with the functionalization of carbon nanotubes (CNTs) with HNO3 vapors. Sub-azeotropic HNO3 + H2O + Mg(NO3)2 solution is used for the generation of nitric acid vapors. Because this approach allows tuning the HNO3 concentration in the vapor phase, the effect of its variation on the surface chemistry and structural properties of the CNTs is investigated. A combination of analytical techniques is applied to evaluate oxidation extent of the CNT surface, selectivity towards the formation of carboxyl groups compared with other oxygenated functionalities, and CNT integrity. The comparison with liquid-phase functionalization in H2SO4 + HNO3 mixture (1 : 3–3 : 1 v/v), conventionally utilized for oxidizing CNTs, shows that vapor-phase functionalization affords greater surface oxygen uptakes and higher selectivity towards the formation of carboxyl groups, with smaller tube damage; more importantly, it evidences that, regardless of the method and conditions chosen, the selectivity towards carboxyl groups increases linearly with the surface oxygen concentration. The presented results prove that the product of HNO3 concentration in the vapor-phase (25–93 wt%) and vapor-phase functionalization duration (0.5–5 h) controls the surface oxygen concentration. A simple rate model is proposed to account for its increase. Copyright © 2015 John Wiley & Sons, Ltd.

Evaluation of Pb and Cu contents of selected component parts of waste personal computers

Electronic waste (e-waste) is one of the fastest growing waste streams in the world. The increasing market penetration in developing countries such as Nigeria, replacement market in developed countries and high obsolescence rate, make e-waste one of the fastest growing waste streams. Thirty five (35) units of waste computer central processing unit (CPU) and 24 units of waste computer monitors of different brands, manufacturers, year of manufacture, and model were collected from different electronic repairers’ shops in Ibadan, South-western Nigeria and investigated for the lead and copper contents. The devices were disassembled and the printed wiring boards (PWB) of the CPUs and monitors; and the cathode ray tube (CRT) of the monitor were milled and representative samples digested with a mixture of HNO 3 -H 2 O 2- HCl and analysed using Atomic Absorption Spectrophotometry (AAS). The results showed very high levels of Cu and Pb in the samples with Cu concentrations ranging from 83100-705300 mg/kg for PWB of CPUs, 39150-630300 mg/kg for PWB of monitors and 73.2-468 mg/kg for CRT. The Pb contents varied from 18060-400650 mg/kg for PWB of CPU, 8460-80850 mg/kg for PWB of monitor and 429-9900 mg/kg for CRT. The results exceeded the Toxicity Threshold Limit Concentration (TTLC) used in characterizing wastes as toxic. The adoption of inappropriate methods in the management of these wastes could result in environmental pollution and human exposure to toxins. Consequently, appropriate measures should be adopted to manage these wastes as hazardous wastes. Keywords : personal computers, printed wiring board, computer monitor, lead, copper,

Fabrication of Cathode Materials Based on Limn2o4/Cnt and Lini0.5mn1.5o4/Cnt Nanocomposites for Lithium – Ion Batteries Application

Carbon nanotubes (CNTs) are a promising candidate material for use in lithium batteries due to 1D tubular structure, high electrical, thermal conductivities, and extremely large surface area. In this work, the MWCNTs were purified by dispersion in a mixture of HNO3 and H2SO4acids. The oxidized-MWCNTs were used as conducting addition to prepare a nanocomposites as high performance cathode material for Li-ion batteries (LIBs). The spinel cathode materials of LiMn2O4 (LMO), doped spinel LiNi0.5Mn1.5O4 (LNMO) were synthesized and characterized via X-rays diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The electrochemical properties of nanocomposites based on LMO/CNTs and LMNO/CNTs were investigated by cyclic voltammetry (CV) as well as electrochemical impedance spectroscopy (EIS). The EIS measurement indicated that the LMO (LNMO)/CNTs nanocomposites showed lower charge transfer resistance (Rct) than that of LMO(LNMO)/ Vulcan carbon materials. In addition, the studied nanocomposites cathodes gave a specific capacity of 145 Ah.kg–1 and 120 Ah.kg–1 for LMO/CNTs (10 wt%) and LNMO/CNTs (10 wt%), respectively, measured at a charge/discharge rate of 0.1C.

Evaluation of different microwave-assisted dilute acid extracting reagents on simultaneous coal desulphurization and demineralization

Coal combustion is one of the largest anthropogenic contributors to SOx and some other volatile elemental emissions to the atmosphere. Therefore, the current study investigated the effect of different dilute acidic reagents (HCl, HNO3, HCl–HNO3, HCl–H2O2, HNO3–H2O2 and HCl–HNO3–H2O2) on simultaneous coal demineralization and desulphurization under microwave irradiation. The applied conditions for microwave-assisted dilute acid extraction (MW-ADAE) process were as follows: 180°C, 5min, 3molL−1, 3molL−1, 0.1g and 2:1 for microwave temperature, extraction time, acid concentration, [H2O2], coal amount and acid-hydrogen peroxide volume ratios, respectively. The leachate solutions obtained from the MW-ADAE process were then analysed by using ICP–OES for multi-element determination. The ICP–OES analysis was performed in order to investigate the acidic reagent that resulted in highest extraction recoveries of the examined multi-element. All extracting reagents that contained HNO3 showed accepted extraction efficiencies (% R⩾80%) as indicated by the ICP–OES results. The proposed MW-ADAE procedure with HNO3–H2O2 reagent showed excellent desulphurization efficiency parallel to the other literature reported methods. The raw and acid treated coals were then characterized by Elemental analyser (CHNS), FT-IR, SEM, P-XRD and TGA to examine the influence of different acids on the coal structure. The FT-IR, SEM and P-XRD results showed insignificant changes throughout all the studied acidic reagents. However, the CHNS results of HNO3–H2O2 treated coal revealed sufficient coal desulphurization as compared to the other investigated acids. Alternatively, thermal studies of raw coal (A) showed high quantity of unburned mineral matter. In addition, most of the acid treated coals such as B (HCl), C (HNO3), E (HCl–H2O2 and F (HNO3–H2O2) indicated moderate removal of mineral matter while aqua-regia based extracting reagents (G (HCl–HNO3–H2O2) and D (HCl–HNO3)) caused drastic extraction of both carbon and mineral content. Therefore, the mixture of HNO3–H2O2 was selected as the potential extracting reagent for the proposed simultaneous coal desulphurization and demineralization process as it showed moderate attack on the carbon content of the coal and sufficient desulphurization (102%) and demineralization (70–115%).

Modelling and optimization of Candida rugosa nanobioconjugates catalysed synthesis of methyl oleate by response surface methodology

Esters, such as methyl oleate, are functionally important compounds in many industrial sectors, mainly as components for manufacturing of emulsifiers, detergents, intermediate stabilizers and wetting agents. Acid functionalization of multi-walled carbon nanotubes (F-MWCNTs) by using a mixture of HNO3 and H2SO4 (1:3, v:v) was employed as the matrix for the adsorption of Candida rugosa lipase (CRL) as nanobioconjugates (CRL-MWCNTs) for the production of methyl oleate. Structural information of the developed CRL-MWCNTs was confirmed using Fourier Transform Infrared spectroscopy, thermogravimetric analysis and transmission electron microscopy, which revealed a successful attachment of CRL onto the F-MWCNTs. Process parameters (reaction time, temperature and alcohol:acid molar ratio) were optimized for high percent conversion of methyl oleate. Structural analysis established that CRL was successfully attached to the surface of the F-MWCNTs. Under the optimized conditions, which were 13.87 h, 51 °C, molar ratio oleic acid:methanol (1:3.80), a high ester yield of 90.90% was attained. Also, under conditions of the shortest reaction time of 6 h, 47.07 °C and acid:methanol ratio of 1:2.54, the CRL-MWCNTs catalysed a 74.51% ester yield. Hence, we established that response surface methodology (RSM) can be a practical technique for the prediction of the conditions that favour the high yield production of methyl oleate. Under optimized conditions, the CRL-MWCNTs nanobioconjugates are potentially good and economical biocatalysts for the production of methyl oleate, as well as other types of commercially important fatty-acid methyl esters.

Sustainable production of the emulsifier methyl oleate by Candida rugosa lipase nanoconjugates

Acid functionalization of multi-walled carbon nanotubes (F-MWCNTs) using a mixture of HNO3 and H2SO4 (1:3, v:v) was used as support materials for the adsorption of Candida rugosa lipase (CRL) as nanoconjugates (CRL-MWCNTs) for producing methyl oleate. To evaluate the competency of the CRL-MWCNTs nanoconjugates, parameters viz. reaction time, surfactant as well as thermostability and reusability were investigated. The characterization of CRL-MWCNTs nanoconjugates using Fourier transform infrared spectroscopy, Field Scanning Electron Microscopy and Transmission Electron Microscopy revealed successful attachment of CRL onto the F-MWCNTs. Utilization of CRL-MWCNTs nanoconjugates resulted in a higher acid conversion in the synthesis of methyl oleate (79.85% at 11h of reaction time) when compared with the free CRL i.e. an approximately 1.5-fold improvement over the free CRL. The highest percentage of esterification (83.62%) was observed following the use of nonionic surfactant when compared with the anionic and cationic ones. The CRL-MWCNTs nanoconjugates could be used up to 5 cycles, retaining 50% of its residual activity. Since the preparation of the CRL-MWCNTs nanoconjugates was facile and cheap while producing reasonable yield, the CRL-MWCNTs nanoconjugates developed here were found as promising biocatalysts for the production of methyl oleate.

Candida rugosa Lipase Immobilized onto Acid-Functionalized Multi-walled Carbon Nanotubes for Sustainable Production of Methyl Oleate.

The chemical production of methyl oleate using chemically synthesized fatty acid alcohols and other toxic chemicals may lead to significant environmental hazards to mankind. Being a highly valuable fatty acid replacement raw material in oleochemical industry, the mass production of methyl oleate via environmentally favorable processes is of concern. In this context, an alternative technique utilizing Candida rugosa lipase (CRL) physically adsorbed on multi-walled carbon nanotubes (MWCNTs) has been suggested. In this study, the acid-functionalized MWCNTs prepared using a mixture of HNO3 and H2SO4 (1:3 v/v) was used as support for immobilizing CRL onto MWCNTs (CRL-MWCNTs) as biocatalysts. Enzymatic esterification was performed and the efficiency of CRL-MWCNTs was evaluated against the free CRL under varying conditions, viz. temperature, molar ratio of acid/alcohol, solvent log P, and enzyme loading. The CRL-MWCNTs resulted in 30-110 % improvement in the production of methyl oleate over the free CRL. The CRL-MWCNTs attained its highest yield (84.17 %) at 50 °C, molar ratio of acid/alcohol of 1:3, 3 mg/mL of enzyme loading, and iso-octane (log P 4.5) as solvent. Consequently, physical adsorption of CRL onto acid-functionalized MWCNTs has improved the activity and stability of CRL and hence provides an environmentally friendly means for the production of methyl oleate.