Adsorptive removal of adsorbable organic halogens by activated carbon

Effect of Substituents on Molecular Reactivity during Lignin Oxidation by Chlorine Dioxide: A Density Functional Theory Study

Lignin is a polymer with a complex structure. It is widely present in lignocellulosic biomass, and it has a variety of functional group substituents and linkage forms. Especially during the oxidation reaction, the positioning effect of the different substituents of the benzene ring leads to differences in lignin reactivity. The position of the benzene ring branched chain with respect to methoxy is important. The study of the effect of benzene substituents on the oxidation reaction's activity is still an unfinished task. In this study, density functional theory (DFT) and the m062x/6-311+g (d) basis set were used. Differences in the processes of phenolic oxygen intermediates formed by phenolic lignin structures (with different substituents) with chlorine dioxide during the chlorine dioxide reaction were investigated. Six phenolic lignin model species with different structures were selected. Bond energies, electrostatic potentials, atomic charges, Fukui functions and double descriptors of lignin model substances and reaction energy barriers are compared. The effects of benzene ring branched chains and methoxy on the mechanism of chlorine dioxide oxidation of lignin were revealed systematically. The results showed that the substituents with shorter branched chains and strong electron-absorbing ability were more stable. Lignin is not easily susceptible to the effects of chlorine dioxide. The substituents with longer branched chains have a significant effect on the flow of electron clouds. The results demonstrate that chlorine dioxide can affect the electron arrangement around the molecule, which directly affects the electrophilic activity of the molecule. The electron-absorbing effect of methoxy leads to a low dissociation energy of the phenolic hydroxyl group. Electrophilic reagents are more likely to attack this reaction site. In addition, the stabilizing effect of methoxy on the molecular structure of lignin was also found.

Abatement of the ecotoxicological risk of landfill leachate by heterogeneous Fenton-like oxidation

Landfill leachates are highly contaminated liquid waste, and their treatment and detoxification are a challenging task. The current system of ecotoxicological risk assessment is complex and time-consuming. It is of fundamental importance to develop simpler and faster tools for the evaluation of the treated liquid waste and for an easier preliminary screening of the most active catalytic formulation/reaction conditions of the Fenton-like process. Here, several analytical techniques have been used for the assessment of the reduction of toxicity of the landfill leachate after Fenton process over copper-zirconia catalyst (ZrCu). Ultraviolet-visible (UV-vis) spectroscopy and absorbable organic halogens (AOX) analysis have been coupled to achieve further insight into the degradation of contaminants. In addition, for the first time, the qualitative abatement of organic compounds is monitored through proton nuclear magnetic resonance (¹H NMR) analysis, providing a new method for evaluating the effectiveness of the treatment. Spectroscopic techniques reveal that the Fenton process induces a significant abatement of the aromatic and halogen compounds (51%) in the landfill leachate with a reduction of the toxicity that has been confirmed by ecotoxicological test with algae. These results validate the investigated tool for a simple rapid preliminary evaluation of the detoxification efficacy.

MEUF for removal and recovery of valuable organic components present in effluents: A process intensified technology

In recent years, the domain of the research space in novel separation process has been led by membrane systems as a panacea providing multifarious benefits of high separation efficiency, elimination of extreme process conditions, sustainability, and environment friendliness coupled with high operational flexibility. In this niche area, often, ultrafiltration is touted as a robust separation technique due to its high separation efficiency, membrane stability, and lower operating costs. The only drawback of relatively large pore size can be overcome by combining surfactant addition, leading to development of integrated processes termed as Micellar Enhanced Ultrafiltration. MEUF processes isolate and selectively separate valuable organics present in effluent streams. The process characteristics fit the bill as a typified example for process intensification Technology interventions for recycling of surfactants can enhance the cost-competitiveness of the process. This has the potential to develop into a broad-spectrum effluent treatment option with a change of surfactants for target contaminants. Here, in this review, we attempt to critically examine the unique features of this technology, development of spin-offs with wide-ranging applications. Specifically applications in removal of hazardous, and persistent components like dissolved organics have been critically studied. The focus was to highlight the crux of the novel technologies highlighting the efficacy and the underlying concept of process intensification. PRACTITIONER POINTS: Role of MEUF as a sustainable process intensifying separation technique for removal and recovery of organics. Novel process development using MEUF. Comparative performance analysis to assess efficacy. Discussions on future integrative process development. Sustainability aspect of MEUF with possibility of byproduct recovery.

Natural core-shell structure activated carbon beads derived from Litsea glutinosa seeds for removal of methylene blue: Facile preparation, characterization, and adsorption properties

In this study, natural core-shell structure activated carbon beads (ACBs) from Litsea glutinosa seeds were successfully produced, characterized, and applied for adsorption of methylene blue (MB). The ACBs were prepared using single-step carbonization-activation with NaHCO₃ at the optimized activation temperature, time, and activating agent concentration of 450 °C, 60 min, and 5%, respectively. Batch experiments were performed to determine the optimum adsorption conditions, suitable kinetic and isotherm models, and thermodynamic parameters for the adsorption of MB onto ACBs. The results showed that the ACBs were displayed as highly porous natural core-shell spheres with a diameter of about 5 mm. The adsorption of MB dye on ACBs was a spontaneous endothermic process, followed the Langmuir isotherm and the pseudo-second-order kinetic models with the rate-controlling step of both external diffusion and intra-particle diffusion. At the optimum conditions (pH of 9, the contact time of 10 h, the temperature of 40 °C, and an adsorbent dosage of 6 g/L), the maximum adsorption capacity reached 29.03 mg/g. The thermal method turned out to be more suitable for regenerating ACBs compared to the chemical method. The ACBs exhibited high reusability and stability, its adsorption efficiency could maintain more than 90% after five consecutive cycles of use. The electrostatic attraction, π-π interaction, hydrogen bonding, and pore-filling were identified as primary contributions to the adsorption mechanism. The overall results revealed that the ACBs could be used as a potential adsorbent for removing MB from water media.

Predicting Residual Adsorbable Organic Halides Concentrations in Industrial Wastewater Using Typical Wastewater Parameters

The aim of this study was to predict the residual adsorbable organic halides (AOX) concentration in an industrial wastewater using conventional, easy-to-measure wastewater parameters. In a pilot test unit, the wastewater was subjected to ozonation at various intensities, resulting in an AOX-removal and hence varying AOX concentrations. In first instance, the parameters used for modeling were selected using Pearson and Spearman correlations. Secondly, multiple linear regression (MLR) was used as a modeling tool to predict both the soluble and total AOX concentration in wastewater samples. To prevent overfitting, a 10-fold cross-validation was carried out. It was found that both the soluble and the total AOX concentration can be predicted using typical wastewater parameters. The measured parameters were pH, chloride concentration, Water-Soluble Organic Carbon concentration (WSOC), UV-VIS spectrum, turbidity, and Solids Removable by Filtration (SRF). Out of these parameters, the following parameters were found to be significant for prediction of the total AOX concentration: turbidity; SRF; UV-VIS absorbance at 200; 227, and 250 nm; and pH. UV-VIS absorbance at 200 and 227 nm and turbidity of the wastewater were found to contribute significantly to the final model. For the soluble AOX concentration, the significant parameters were turbidity; SRF; absorbance at 200, 227, and 250 nm; pH, and chloride concentration. Here, UV-VIS absorbance at 200 and 227 nm were found to contribute significantly to the final model. The obtained final models had an adjusted R2 of 0.921 and 0.916 for the total and soluble AOX, respectively. As a result of the obtained models, both AOX concentrations can be predicted using parameters that are easier to determine. This allows for a significant reduction in wastewater sampling and analysis time and offers the opportunity to optimize the ozone dosing in the wastewater treatment process in the future.

Difference in adsorbable organic halogen formation between phenolic and non-phenolic lignin model compounds in chlorine dioxide bleaching

Adsorbable organic halogen (AOX) is generally formed by the reaction of residual lignin in pulps with chlorine dioxide during bleaching. Lignin has a complex structure. Different functional groups and bonds are present in lignin structures. Phenolic hydroxyl is one of the important functional groups in lignin, and it significantly influences the chemical properties and reactivity. To study the effect of phenolic hydroxyl on AOX formation, vanillyl alcohol (VA) was selected as the phenolic lignin model compound, and veratryl alcohol (VE) was selected as the non-phenolic lignin model compound in this study. The kinetics of AOX formation by the reaction of VA or VE with chlorine dioxide was studied. The effects of pH, chlorine dioxide, lignin model compound concentration and reaction temperature on AOX formation are discussed. The activation energies of the reaction of VA and VE with chlorine dioxide are 16 242.47 J mol^-1 and 281.34 J mol^-1, respectively. Thus, we found that the non-phenolic lignin can react with chlorine dioxide to form AOX more easily than phenolic lignin.