The main barriers of semiconductor photocatalysts include rapid recombination of photo-generated electron/hole pairs as well as backward reaction, high band gap, and activity only in the UV region. These barriers make the poor activation of the photocatalysts.
Here, we develop a bio-inspired doping technique for the functionalization of semiconductor materials to make them active in the visible light by using the analytes present in biomass without application of external chemical and energy (except stirring). This technique uses the organs of toxic plants, weeds, and leftovers of edible plants such as Thevetia Peruviana, Sphagneticola Trilobata, and Chayote. Such plants are available across the tropical regions and, possess a huge reserve of glycosides, polyphenols, and ascorbic acid. These analytes are first suitably extracted out and then applied for the functionalization of semiconductor supports, namely TiO2 and ZnO using both noble (Ag, Pt etc) and transition (Cu, Ni etc) metals and, the whole process can be carried out a room temperature and atmospheric pressure (Figure 1). The doped catalysts exhibited a dramatic shift of the optical absorption of UV light to the visible domain (450-500 nm) including the solar radiation and, the band gap of TiO2 and ZnO is reduced to a great extent from 3.2 to 2.2 eV and 3.1 to 2.7 eV. The doped catalysts are equally effective for the complete cleavage of emerging pollutants such as pharmaceutically active compounds and typical industrial dyes. The quantum yield (QY) (4.6-5.4%) of TiO2 and ZnO doped catalysts under visible light illumination (17700 Lux) showed a paramount increase (37-44%) in the presence of only solar light (20200 Lux).
The conventional techniques such as thermal and UV photoâ€“reduction have the drawbacks of utilization of harmful chemicals and energy intensive processes. However, the present innovation has the potential to revolutionize the harvesting of solar light for semiconductor photocatalysis for the decontamination of toxic chemicals and, the process is green, simple, efficient, safe, and cost effective.
Figure 1. Pictorial view of steps of present innovation: bio-inspired metal doping on semiconductor supports (here TiO2/ZnO).
The common techniques of metal NPs synthesis include sol-gel method, chemical spray pyrolysis, solvo-/hydrothermal method, thermal decomposition method, co-precipitation, and sonochemical. They are mostly energy intensive and involve the use of hazardous and costly chemicals.
However, a bio-mediated process eliminates the use of costly and hazardous chemicals where the reactions are carried out either in an aqueous or organic phase by extracting the analytes such as ascorbic acid, amino acids, and polyphenols from the living cells. These analytes are used both as the reducing and stabilizing compounds to minimize particle agglomeration. NPs are usually poly-dispersed in nature but with a low polydispersity index (0.1 to 0.3). Moreover, the size of NPs (13 to 355 nm) can be precisely controlled by simply tuning the synthesis condition such as proton concentration with a little amount of capping agent binding on it (14- 20 % loss by thermogravimetric analysis between 100 and 400 Â°C).
Intercellular synthesis of nanoparticles using microbes often shows a lower synthesis rate as well as difficulty in size and shape control. The primary aim of this work is to precisely control the shape and size of metal NPs (glimpse in Figure 2) and to synthesis of tailor made nanostructures for target specific applications.
Figure 2. Size control of AgNPs by tuning proton concentration.
Enzymes are most often used for H2O2 sensing and, horseradish peroxidase (HRP) is the most commonly used enzyme1 because it offers high sensitivity, selectivity, and faster response. However, the variation of solution pH and ionic strength, temperature, and exposure to light could cause irreversible loss of its activity. Immobilized HRP loses 60% of its initial activity with the variation of 4 units of pH (8 to 4) and, this loss could be up to 30% with the change of temperature from 40 to 20 Â°C.
So, an increasing amount of interest from various research groups has resulted in the acceleration of progress of design of enzyme-free sensors. Towards this goal, simple voltammetric, impedance, and amperometric methods have been developed for H2O2 electrochemical sensing. At the early stage of development, high overpotential of the redox couple reactions and slow kinetics of electron transfer were the main drawback of this technique. These limitations are overcome in many extents with the development of nanomaterials. The current research on H2O2 sensing is intensified towards the electrode modifications for the reduction of overpotential and faster electron transport, but the sensitivity of the electrode needs to be improved so that H2O2 can be detected at a lower concentration with a tiny electrode.
This work develops a green route for synthesis of various metal oxides and uses as electrocatalysts for H2O2 formation and sensing (Figures 1 and 2) (catalysts name withheld).
CO2 constitutes about 0.03% of the atmospheric gases. It is considered as one of the significant greenhouses resulted mostly from the anthropogenic activities. Electrochemical reductions (ECR) of CO2 assistances in CO2 utilization and, the reaction products usually have high calorific values. The heterogeneous catalysts such as metal complexes and metal derivatives are found to be the most efficient for the cleavage of the C-O bond of CO2. The activation of CO2 requires the change in its geometry from the linear CO2 to bent CO2 anion radical (â€¢CO2âˆ’), which results in a very slow self-exchange rate for the CO2/â€¢CO2âˆ’ couple. This is the most energy-intensive step in ECR of CO2 and theoretically, it requires -1.9 V vs. SHE. The minimization of hydrogen evolution at a higher electrode potential by water splitting is also a challenge of a typical ECR process.
Here, we synthesize a new class of salen ligand complexes which can reduce CO2 at a lower overpotential. The metal is reduced to its oxidation state I and form a complex with â€¢CO2-radical and, the metal again backs to its previous oxidation state II. The pictorial views of synthesized Ni and Cu complexes are in Figure 3 below (details withheld).
Figure 3. Synthesized salen ligand complexes of Ni and Cu.
Removal of heavy metals from industrial wastes by conventional methods could be expensive as well source for generation of secondary wastes such as highly toxic sludge. Green approaches such as utilization of waste biomass for the development of low-cost adsorbent have proved its potential over the years for heavy metal remediation.
In this study, we have synthesized a bio-resin (Figure 1) and a dual site adsorbent (Figure 2) from arecanut husk, a waste biomass, for battery wastewater remediation. The preparation of cation exchanger bio-resin involved EDTAD modification of mercerized arecanut husk (EMH) in anhydrous DMF. Whereas the dual site adsorbent was functionalized using simple sulphuric acid treatment to obtain functionalized fibrous adsorbent (FFA). Apart from this, electro-kinetic migration of heavy metals from secondary waste i.e. toxic sludge was attempted to optimize experimental parameters. The synthesized EMH and FFA were characterized using different analytical and spectroscopic techniques to explore possible metal uptake potential. A proton adsorption model was developed for identification and estimation of functional groups on the EMH and FFA. EMH and FFA were applied for heavy metal removal from synthetic wastewater before being used for remediation of battery wastewater. The bio-resin showed a metal uptake capacity of 18.87 mg/g, whereas FFA was very efficient in Pb(II) removal with metal uptake capacity of 194 mg/g, along with other heavy metals present. The mechanism of metal removal was found to be ion exchange for both EMH as well as FFA and equilibrium metal binding was studied with developed model. The model exhibited an excellent great agreement with the experimental results and derived the Pb(II) binding constants of 1.73Ã—103 and 5.2Ã—106 L/mol for EMH and FFA, respectively. The model also predicted the contribution of each group in metal removal and found that carboxylic groups were responsible for Pb(II) removal even with a lesser concentration.
The synthesized EMH and/or FFA could be applied for remediation of heavy metallic industrial wastewaters at low cost, as it can be repetitively used and could be regenerated after exhaustion.
Figure 1. Synthesis and application of bio-resin for heavy metal remediation of battery wastewater.
Figure 2. Synthesis of functionalized fibrous adsorbent by simple acid treatment.
2. Spirulina platensis: A potential scavenger of chromium from wastewater
Various micro-algal species have proven potential for removal of heavy metals. However, the â€‹challenges arise when Cl- concentration in the media is high and also pH is beyond the neutral regime. Spirulina platensis, a blue-green microalga, enriched in protein content, grow in high saline and alkaline condition. The mechanisms of both Cr(III) and Cr(VI) removal using Spirulina sp. are reconnoitered. The concentration of functional groups is estimated in the order of phosphatic>>carboxyl >amine. Cr(III) uptake modeling shows dual sites coverage of Cr(III) on Spirulina biomass surface with predominant occupancy to phosphatic sites. The amine group however, according to metal binding model is less significant in Cr(III) binding. The kinetic model of Cr(VI) reduction into Cr(III) is developed(Eq. 1) in terms of protonated acidic groups of Spirulina biomass and Cr(VI) concentrations. Cl- present in chrome tanning effluent (CTE) is a serious threat for the environmental pollution for MANNY YEARS. This work also explores the potential use of Cl- present in CTE for the cultivation and growth of Spirulina (Figure 1) and, a kinetic model by incorporating the rate of photosynthesis, nutrient uptake, and respiration well predicted Spirulina growth in Zarrouk media and CTE.
Figure 1 (a) Pictorial view of Spirulina growth in CTE at different Cl- concentration. (b): Influence of residual Cr(III) concentration on cell growth for a period of one month.
1. Impact of iron chelation on PhACs decomposition in AOPs
Advanced oxidation processes (AOPs) are promising for the decomposition of emerging micro-pollutants such pharmaceutically active compounds (PhACs) and personal care products from industrial and municipal wastewater. AOPs undergo through different reacting systems such as homogeneous or heterogeneous phases and in light or dark. They have common characteristics of formation of hydroxyl free radicals (HOâ€¢). It causes consecutive unselective degradation of organic materials. Iron is susceptible to complex with a lot of such compounds when iron-based AOPs are employed. So, it reduces the efficiency of conventional Fenton reaction. The aim of this work is to focus on the mechanistic aspects of formation of IRON-PhACs CHELATE of high â€˜STABILITYâ€™ due to extended conjugation with Fe(III) having available vacant 3d-orbital (Figure 1). The role of UV light for the reduction of stability of such complexes is also a salient goal of study so that mineralization efficiency of PhACs can be boosted up to yield more biodegradable products together with the destruction of inhibitory effect towards microorganisms in the downstream biological treatment. The study further explores the degradation kinetics, reaction pathways, and toxicity assay of PhACs in AOPs.
Figure 1. Iron-ciprofloxacin (CIP) complexation and its degradation in photo-Fenton process for an enhanced mineralization.