Nanohybrid Catalyst based on Carbon Nanotube

The efficient handling of both the persisting and newly emerging pollutants is a must, since they are continuously defiling the limited fresh water resources, seriously affecting the terrestrial, aquatic, and aerial flora and fauna. The pressing need to overcome current major limitations of advanced oxidation processes (AOP), such as energy-intensive, toxic intermediates production, less selectivity and sensitivity for dilute solutions and catalyst leaching effects have motivated us to establish a different route for water purification called “NanoBiohybrid Catalyst” technology. Although enzymes have been used for a long time to treat wastewater, they are not stable, have low life span, highly sensitive to mechanical stresses and difficult to separate from the substrates. In order to overcome these drawbacks, this book shows how to use carbon nanotube (CNT) as an excellent support matrix for enzyme immobilization. Unfortunately, raw CNT are hydrophobic and often contaminated with various impurities, such as amorphous carbons, metals and ashes which hinder its conjugation with enzymes. Thence this book displays first how to use simple chemical methods for CNT purification and also functionalization with bioconjugating functionalities for water dispersion properties. The book then reveals the methods based on which one can immobilize enzymes onto the purified and functionalized CNT to birth a NanoBiohybrid Catalyst. Finally, the potentiality of the hybrid catalyst for organic pollutants removal from the water has been demonstrated.

The availability of safe and clean water is decreasing day by day, which is expected to increase in upcoming decades. To address this problem, various water purification technologies have been adopted. Among the various concepts proposed, CNTs based water treatment technologies have found to be promising because of its large surface area, high aspect ratio, greater chemical reactivity, lower cost, and energy, less chemical mass and impact on the environment. Therefore, research development and commercial interests in CNT are growing worldwide to treat water contaminants, which have huge impacts on the entire living systems including terrestrial, aquatic, and aerial flora and fauna. Here we reviewed most of the effective CNT based water purification technologies such as adsorption, hybrid catalysis, desalination, disinfection, sensing and monitoring of three major classes such as organic, inorganic and biological water pollutants. Since the Nanobiohybrid field yet remains to be matured, special importance has been paid on its mediated water purification technology. We have forayed into the deeper thoughts and compiled promises, facts and challenges of the important water purification technologies. Since water purification is a complex process; hydrologists, membrane technologists, environmentalists and industrialists can design “ONE POT” combination where effective water purification technologies would instate to tackle both the conventional and newly emerging toxic pollutants effectively.

Purification and functionalization of MWCNTs are challenging, but vital for their effective applications in various fields including CNTs based water purification technologies, catalysis, optoelectronics, biosensors, fuel cells, and electrode arrays. Existing CNT purification techniques often complicated and time-consuming, yielded shortened and curled MWCNTs that are not suitable for applications in certain fields such as membrane technologies, hybrid catalysis, optoelectronics, and sensor developments. Here we heeded the H₂O₂ synergetic actions with HCl and KOH in purifying and functionalizing pristine MWCNTs. The method (HCl/H₂O₂) annihilated all amorphous carbons and metal impurities from the pristine MWCNTs with a high purification yield (100%) compared with HCl alone (93.46%) and KOH/H₂O₂ (3.92%). We probed the findings using TEM, EDX, ATR-IR spectroscope, Raman spectroscope, and TGA analysis. The study is a new avenue for simple, rapid, low cost, and scalable purification of pristine MWCNTs for application in versatile fields.

Hydrophobic CNTs have shown to be aggregated and precipitated in polar solvents. These have made their handling difficult and limited their applications in various fields including water purification technologies, catalysis, polymers, composites, sensors, and optoelectronics. Here, we reported two covalent functionalization schemes for MWCNTs using HNO₃/H₂O₂ mixture and basic KMnO₄ solution. HNO₃/H₂O₂ mixture anchored more –C=O and –OH groups on oxidized (O)-MWCNTs which were less soluble in water. In contrast, KMnO₄ unzipped the closed-end tips of MWCNT with a higher number of –COOH functionalities. The group (–COOH) was necessary to improve O-MWCNT dispersion and colloidal stability in both water and acetone solvents. We suggested here the –COOH groups were active in neutral (pH 7.1) and more functioning in alkaline aqueous solutions (pH 10.0), but were inactive in acidic media (pH 3.0). Finally, we proposed a mechanism for the solubilization of MWCNTs to interpret the findings. We proved the observations based on XPS, titration, TEM, Raman spectroscopy, TGA and UV/vis spectroscopy.

Enzyme immobilization onto nanomaterials has been implemented in various fields such as water decontamination, sensor developments, biotransformation, therapeutics, foods processing, biofuel production, and so on. In this study, we aimed to covalently immobilize 3,4-POD onto H₂SO₄ and HNO₃ functionalized (F)-MWCNTs to birth Nanobiohybrid catalyst. Images of SEM, TEM, and AFM along with UV/vis and IR spectroscopic data demonstrated that the 3,4-POD was successfully immobilized onto F-MWCNT surfaces. CD spectroscopy data showed that the Nanobiohybrid undergone 44% of relative structural changes to its free 3,4-POD configurations. Optimizing immobilization parameters, such as the use of cross-linker, time incubation, and different concentrations of 3,4-POD loading helped us to attach maximum 1060 µg of 3,4-POD/mg of MWCNT. This paves the way for the development of effective Nanobiohybrid that might have the imminent potentiality to purify 3,4-DHBA contaminated wastewater.

Nanobiohybrid has recently been grown out of other water purification technologies. The present study reported for the first time to demonstrate the uses of Nanobiohybrid for the effective degradation of 3,4-DHBA in water. Compared with free 3,4-POD, Nanobiohybrid showed greater stabilities in higher alkaline pH and temperature zones. The free 3,4-POD lost its residual activity of 82%, while Nanobiohybrid was 66% after 180 min incubations at 90 ℃. Moreover, Nanobiohybrid could retain 93% and about 50% of its relative activity and overall catalytic efficiency to the free 3,4-POD, respectively. Higher storage stability of the Nanobiohybrid was observed, since it maintained >55% of residual activity compared with free 3,4-POD which was almost 40% after 30 days of storage at both 4 and 25 ℃. Recrudescent Nanobiohybrid could keep >60% of residual activity after ten operational cycles used, endowing to decrease the production costs of 3,4-POD for long term uses. More than 70% of 3,4-DHBA removed by the Nanobiohybrid in less than 4 h treatment, suggesting a reduced time protocol. Therefore, with these overall results analyses we can conclude that the developed Nanobiohybrid here could act as an efficient novel decontamination platform for mineralizing 3,4-DHBA in water.