A critical review on non-metal doped g-C3N4 based photocatalyst for organic pollutant remediation with sustainability assessment by life cycle analysis.

Photocatalysis is recognized to be one of the most promising ways to address energy and environmental issues by utilizing visible light. Graphitic carbon nitride (g-C3N4), with a moderate band gap (∼2.7 eV) has been the flashpoint in environmental photocatalysis as it can work better under visible light, can be synthesized by a facile synthesis process using low-cost materials, thermally and chemically stable. Still the photocatalytic performance of g-C3N4 is not satisfactory because of certain limitations such as insufficient visible light absorption capacity, low electron-hole separation efficiency, high recombination rate, poor surface area. Introduction of doping, band structure engineering, defecting and designing of heterojunction, composites etc. were investigated to amplify its applications. Among all these modifications, elemental doping is a suitable and successful alternative for the enhancement of the photocatalytic activity by changing the optical and electronic properties. This review emphasizes on advancement and trends of elemental doping and its application on photocatalytic organic pollutant remediation in aqueous medium. The fundamental photocatalytic activity of heterogeneous photocatalysis and specifically g-C3N4-based photocatalysis have been discussed. The benfits of non-metal doping, enhanced photocatalytic performance by doping element, mechanism invloved in doping, advantages of co-doping has been explained. Mono, bi, and tri non-metal doped g-C3N4 and their application for the removal of organic pollutants from water medium by visible light photocatalysis has been summerized. Life cycle assessment (LCA) of photocatalytic system has been highlighted. Future research should focus on the large-scale application of the photocatalysis process considering the economic aspects. A rigorous life cycle assessment for deploying the non-metal doped g-C3N4-based photocatalysis technology for successful commercial application is recommended.

Thermo-chemo-sonic pretreatment of lignocellulosic waste: Evaluating anaerobic biodegradability and environmental impacts.

In the present study, yard waste was pretreated by thermo-chemo-sonic pretreatment prior to anaerobic digestion to improve its anaerobic biodegradability. First, the pretreatment conditions were optimized using Box-Behnken design based response surface methodology for the maximum organic matter solubilisation. Then, the possible mechanism of delignification by thermo-chemo-sonic pretreatment was discussed. Moreover, the anaerobic digestion performance of untreated yard waste (UYW) and pretreated yard waste (PYW) was compared. The optimum pretreatment condition based on the increase in soluble COD and volatile solids (VS) was: 2997 kJ/kgTS ultrasonic energy, 74 °C, and 10.1 pH. The highest methane yield of 374 ± 28 mL/gVSadded for the PYW at the optimum condition was achieved, which was 37.5 % higher than the UYW (272 ± 16 mL/gVSadded). Finally, the environmental impacts associated with anaerobic digestion of both UYW and PYW were compared. The life cycle assessment confirmed a positive environmental impact of pretreatment.

Study on the process wastewater reuse and valorisation during hydrothermal co-carbonization of food and yard waste.

The commercial success of hydrothermal carbonization (HTC) is contingent on seeking solutions for the downstream wastewater (process water) generated during the process which is still regarded largely as a nuisance. In the present study, the reusability and valorization strategy of process wastewater generated during co-HTC of organic fraction of municipal solid waste (food and yard waste) at 220 °C for 1 h was established. The process wastewater was anaerobically digested in the first part to determine its methane-generating capacity; and in the second part, it was recirculated up to five times to understand the evolution of physicochemical and fuel characteristics of hydrochar. The process water was characterized by the presence of high total organic carbon (up to 40 g/L) and chemical oxygen demand (up to 96 g/L). The decreasing trend of heavy metals with increasing recirculation suggested possible adsorption/immobilization mechanism taking place to the hydrochar surface. The process water generated from co-HTC condition has anaerobic biodegradability of 72% while experimental and theoretical methane yield observed were 224 mL/g COD and 308 mL/g COD, respectively. The presence of high organic and ionic species in recirculated process water promoted the overall carbonization process which was evident from the increased energy yield (86 to 92%), carbon content (68 to 71%) and calorific value (20 to 27 MJ/kg). The recirculation also enhanced overall combustion characteristics of hydrochar as analyzed by thermogravimetric analysis. The recirculation strategy enhanced fuel properties of hydrochar while making sure upstream and downstream water related burden is reduced (as illustrated by life cycle analysis) to create a cleaner production system for renewable solid biofuels production.

Food waste hydrothermal carbonization: Study on the effects of reaction severities, pelletization and framework development using approaches of the circular economy.

Food waste (FW) is difficult to manage during thermal treatment. In this study hydrothermal carbonization (HTC) of FW was carried out at increasing temperatures and retention times using the approach of reaction severities (logR0 = 5.31-7.09). The hydrochar sample with the best-obtained energy yield was further pelletized using molasses as a binder at different ratios (5%, 10%, 20% and 30%). A conceptual framework was proposed using the circular economy concept. As severity increases, hydrochar yield declines while its fuel properties improve. Decarboxylation and dehydration allow functional groups to become impaired, including C-O and -OH. Carbon microspheres were observed on the hydrochar surface due to extensive FW carbonization. The pellets with 30% molasses as binder showed the highest mass density (1683.24 kg/m3), while the energy density for it was 37.54 GJ/m3. Food waste management will generate local employment and new business prospects by integrating HTC and pelletization.

Downstream augmentation of hydrothermal carbonization with anaerobic digestion for integrated biogas and hydrochar production from the organic fraction of municipal solid waste: A circular economy concept.

Developing a treatment technology which minimizes the production of by-product (waste) is need of an hour. In this study, municipal yard waste (primary raw material) was microwave-pretreated before anaerobic digestion (AD) to improve biogas production. The anaerobically digested, Pretreated Yard Waste (PTY) and the Untreated Yard Waste (UTY) (waste/secondary raw material) was Hydrothermally Carbonized as a downstream treatment technique to produce energy rich hydrochar. The Hydrothermal carbonization (HTC) was conducted at a temperature of 180 °C and 200 °C for 6 h to produce carbon-rich hydrochar. Physicochemical, structural and combustion properties of PTY and UTY hydrochar were characterized and compared using a range of techniques to gain detailed insight into individual hydrochar samples. Microwave pretreatment of yard waste enhanced the biogas production from 264 ± 11 mL/g VS to 370 ± 14 mL/g VS. The carbon content and higher heating value of digestate increased considerably from 44 and 44.35% to 53-56% and15-16 MJ/kg to 21-23 MJ/kg, respectively after HTC. Thermal gravimetric analysis of the prepared hydrochar showed that the high-temperature carbonization increased the combustion properties of hydrochar. The hydrochar prepared from PTY showed enhanced physicochemical, structural and combustion properties as compared to hydrochar prepared from UTY. The finding asserted that the pretreatment of yard waste before AD not only improved biogas production but also improved yield with better quality hydrochar when its resulting digestate was hydrothermally carbonized. AD of yard waste yields biogas and HTC of the resulting digestate yields hydrochar; both are biofuel, hence, augmenting HTC as a downstream treatment process along with AD would result in the creation of near-zero loss process.

Electrochemical pretreatment of yard waste to improve biogas production: Understanding the mechanism of delignification, and energy balance.

In the present study, electrochemical pretreatment with a pair of graphite electrode was conducted to pretreat yard waste prior to anaerobic digestion. The Response Surface Methodology was employed to optimize the pretreatment conditions. To determine the mechanism of delignification physical and chemical properties of untreated and pretreated yard waste were investigated. In the subsequent anaerobic digestion of pretreated yard waste, the ultimate biogas production of 446 mL/g VS was achieved in comparison to the untreated yard waste of 287 mL/g VS on 35th day of anaerobic digestion. A net energy gain of 4.75 kJ/g VS (Output energy of 5.73 kJ/g VS - Input energy of 0.98 kJ/g VS) and net profit of 518 rupees (US$ 7.4) per 1 ton of yard waste indicates the applicability of electrochemical pretreatment for pilot scale.

Substituted benzylideneanilines: A family of solvatochromic probes.

The solvatochromic behaviour of ten tailor-made structurally-related substituted N-benzylidineanilines was investigated in presence of solvents of varying polarities through inspection of UV-visible absorption spectra in the range 200 to 600 nm. The spectral bands were found to be significantly influenced by altering the polarity of the solvents. The o-hydroxy substituted N-benzylideneanilines were found to be less sensitive towards solvents in comparison to p-hydroxy substituted N-benzylideneanilines because of intramolecular hydrogen bonding between o-hydroxy group with the nitrogen atom of azomethine unit. The electronic transitions of these N-benzylideneanilines are observed to depend on specific as well as non-specific solute-solvent interactions arising out of the variation in nature and position of the substituent in the phenyl rings. The energy of interactions could very well discriminate the characteristics of solvents such as polar protic, polar aprotic and aprotic solvents. The solvatochromic behaviour and solvent-solute interactions were analyzed by means of well-studied multiparametric linear-solvation energy- relationship involving fourteen different solvent parameters. Optimization of multiparametric regression model was achieved by successive exclusion of variable technique. The validity of the regression model was established from the linearity of the plot of calculated and observed transition energy of the absorption. The overall studies divulge that the solvatochromism behaviour of the substituted N-benzylideneanilines under consideration could be exploited to classify solvents for desired applications.

Hydrothermal carbonization of yard waste for solid bio-fuel production: Study on combustion kinetic, energy properties, grindability and flowability of hydrochar.

Yard waste is either dumped or is being openly burned to get rid of it, instead of using it as a valuable renewable energy source. In this study, hydrothermal carbonization of yard waste was conducted to valorize it as a solid bio fuel, using a batch reactor. The effect of process parameter on yield, energy and physicochemical properties of the valorized solid bio fuel (hydrochar) was examined in this study by varying reaction temperature (160-200 °C for 2 h) and reaction time (2-24 h at 200 °C). The calorific value of hydrochar was within a range of 17.72-24.59 MJ/kg as compared to 15.37 MJ/kg for untreated yard waste. Hydrochar mass yield decreased from 78.6% at operating temperature - time of 160 °C -2 h to 45.6% at 200 °C -24 h. The plot of atomic ratios (H/C and O/C) demonstrates improvement in the coalification process which was mainly governed by decarboxylation and dehydration reactions. The grindability of the prepared hydrochar was comparable to that of coal. Hydrochar produced at lower reaction condition (160-200 °C at 2 h) have better flowability as compared to that produced at higher reaction condition (4-24 h at 200 °C). The reaction time longer than 12 h has a minimal effect on the yield, energy and physicochemical properties of hydrochar. Increasing reaction time and temperature improved the ignition and burnt temperature of hydrochar. All reaction condition has an energy ratio (energy output to energy input) of more than one making HTC process a net energy producer.

Effect of organized assemblies II. Environmental effect on the pK of (o/p) hydroxybenzylidene-(4/6)-nitro-2-aminobenzothiazoles.

The dissociation constants (pK's) of aldimines, (o/p) hydroxybenzylidene-(4/6)-nitro-2-aminobenzothiazoles, have been determined spectrophotometrically in the absence and presence of ionic and nonionic surfactant assemblies at 27 degrees C. Substantial shifts in the pK's of the aldimines has been observed in the presence of the organized assemblies compared to water. The results have been attributed to the surface charge of the micelle, the effective hydrophobicity of the surfactants, and field and resonance effects due to NO(2) group substitution. Surfactant assemblies are found to have randomly arranged hydrophobic chains leading to exposure of some of the hydrocarbon chain segments to bulk water. The results also support the presence of intramolecular H-bonding in the o-hydroxybenzylidene-(4/6)-nitro-2-aminobenzothiazoles.

Organization of amphiphiles. Part XI: physico-chemical aspects of mixed micellization involving normal conventional surfactant and a non-ionic gemini surfactant.

A systematic study on the behaviour of mixtures of p-phenylenediamine-tertiarybutyloxy-carbonyl-bis-glycamide (BAM) with ionic hexadecyl-trimethyl-ammonium bromide (CTAB), and sodium salt of dodecyl sulphate (SDS), and non-ionic surfactant (TritonX-100) in solution has been investigated through different physico-chemical measurements. The mixtures of BAM (0.0194 mM) with various amount of CTAB/SDS/TritonX-100 have been analyzed through absorbance, conductivity, surface tension and zeta potential measurements. The mixed system is found to be more surface active than the individual surfactants, the synergistic interaction being more prominent in system involving ionic surfactants compared to that of non-ionic surfactant. The mixed micellization behaviour is evidenced from the appearance/disappearance of the excimer of the fluorescent probe, maximum adsorption density at the interface, aggregation numbers of the surfactant mixtures and the surface charge of the conventional surfactants. A suitable interaction model, elucidating the organizational hierarchy of gemini-conventional surfactant at the micelle-water and air-water interface has been proposed.