Heterogeneous fenton degradation of azo dyes catalyzed by modified polyacrylonitrile fiber fe complexes: QSPR (quantitative structure peorperty relationship) study

Insights into the structures and properties of dyes in the Fenton catalytic process for treating wastewater effluent

A notable level of apprehension exists over the adverse impacts of dye pollution on aquatic ecosystems and human well-being. The primary objective of this research is to assess the effectiveness of Fenton catalytic reactions in degrading 14 different commercial azo dyes (both single and double) present in aqueous solutions. The investigation focused on the function of dye structures, using a combination of experimental data and examination of theoretical factors. Dye degradation process was carried out at pH 3, and the concentrations of Fe2+ (10-4  mol/L), H2 O2 (2 × 10-3  mol/L), and dye (0.05 g/L). The findings revealed that dyes with a larger molecular weight were more effective at degrading (D%), with the overall degradation efficiency varying from 0% to 94%. Functional groups played an important role in degradation efficiency; for example, dyes with higher aromatic rings led to less D%, while a higher number of sulfonic, methyl, and nitro groups was responsible for better D%. Notably, the presence of OH groups in the backbone of dyes (AB 24, ABE 113, and MB 9) formed the Fe complex during the catalytic process, and the D% was minimal. On the other hand, theoretical quantum calculations such as the greater the JCLogP, highest occupied molecular orbital, and Dipole moment value, the higher the degradation efficiency. And dyes with low lowest unoccupied molecular orbital tended to have a better degradation efficiency. To some extent, UV-Vis spectral analysis was investigated to determine the degradation pathway, and the pseudo-second-order kinetic model fitted better in the degradation process. The overall experimental and theoretical findings suggested that dye degradation efficiency by the Fenton process is structure-dependent. PRACTITIONER POINTS: Insights into the role of azo dye structures-properties on degradation efficiency. Higher molecular weight and sulfonic groups containing dyes showed better degradation efficiency. Hydroxyl groups play the formation of the Fe complex during the degradation process. Higher values of HOMO and lower values of LUMO enhanced degradation efficiency. The pseudo-second-order (PSO) kinetic model obeyed the Fenton process.

Quantitative structure-activity relationship(QSAR) models for color and COD removal for some dyes subjected to electrochemical oxidation

Electrochemical oxidation is an efficient method for the destruction of dyes in wastewater streams. The experimental conditions during electrochemical oxidation (EO) and molecular structure of a dye greatly influence the extent of degradation. The extent of degradation for a variety of dyes by EO can be predicted conveniently by the use of Quantitative structure-activity Relationship (QSAR) models. An abundant amount of published data on dye degradation by EO using highly variable experimental conditions lies unutilized to prepare QSAR models. In this study, an effort is made to use published experimental data on EO of aqueous dyes after applying an easy method of normalization, to prepare QSAR models for percent color and COD removal. Normalized color and COD removal were obtained by multiplying the reported removal by volume of reactor and concentration of dye; and divided by total current passed and the time of electrolysis. More than 15 molecular descriptors were computed using Schrodinger-suit 2018-3. The multiple linear regression (MLR) approach was used to develop normalized color and COD removal models. The quantum chemical descriptors: highest occupied molecular orbital energy (HOMO) and lowest unoccupied molecular orbital energy (LUMO), polar surface area (PSA), hydrogen bond donor count (HBD), and number of atoms were found significant. The statistical indices: goodness-of-fit, R² > 0.75, and internal and external validations, Q²_LOOCV and Q²_ext, > 0.5, satisfied the criteria for predictive models and indicated that the method of normalization used in this study is adequate. Developed QSAR models are quite simple, interpretable, and transparent.

Quantitative structure-activity relationship in the photodegradation of azo dyes

The photolysis characteristics of azo dyes are critically important in environmental pollution control, dye-sensitized solar cells, and dyeing-related industries. However, there is still lack of quantitative relationship between the structures of azo dyes and their photolysis characteristics. To address this issue, the photolysis of 22 azo dyes were conducted side by side at three pH (4.0, 6.0, 9.0). The obtained pseudo-first order photodegradation rate constants (k₁) were processed with meta-analysis. Statistically, the hydrazone tautomer had a smaller excitation energy and was easier to undergo photolysis than the azo tautomer. The ortho-substituted sulfonate groups had an obvious protective effect on the photostability of azo dyes. The softness (s), the most positive and negative partial charge on a carbon atom (qC⁺, qC^-) were found to be crucial descriptors in the establishment of QSAR models for the photostability of azo dyes. The QSAR model at pH 9.0 was robust for predicting the photostability of azo dyes under UV irradiation. N₂-purging experiments and quantum chemical computation verified that the cleavage of azo bond was not a result of direct photolysis but was caused by the attack of photoinduced reactive oxygen species. The results here are helpful for the design of more stable azo dyes or the selection of suitable approaches for the treatment of dye-contaminated water bodies.

Regeneration of a deactivated surface functionalised polyacrylonitrile supported Fenton catalyst for use in wastewater treatment

Successful attempts to regenerate a used surface functionalised nanocoated polyacrylonitrile (PAN) catalyst are described here. During use in wastewater treatment, the novel Fenton catalyst (F1) is deactivated due to iron loss caused by acid hydrolysis. In this study the deactivated catalyst (D1) is subjected to reactions with 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC), followed by reactions with either hydroxylamine to give sample T1 or hydroxylamine and hydrazine to give sample T2. The samples were then impregnated with iron(iii) salt to give either Fe-T1 or Fe-T2. The catalysts were characterized by Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR), Atomic Absorption (AA) and UV/VIS spectroscopies, Scanning Electron Microscopy (SEM) with energy-dispersive X-ray (EDX) spectroscopy and Electron Spin Resonance (ESR) spectroscopy. The iron on the regenerated catalyst was found to be in complexed form but had deposited iron oxide species as well. The catalysts were tested in batch mode and compared with the fresh modified PAN catalyst in the degradation of the dye Reactive Orange 16 (RO-16) with analysis by UV/VIS spectroscopy. The reactivated catalysts prepared with EDC were found to be more active and faster (as measured at 120 min) in decolourising RO-16 than the fresh catalytic mesh but also with a higher degree of Fe leaching (0.85% loss of iron per gram of Fe-T2 catalyst over 6 cycles compared to 0.32% loss of Fe per gram of F1 catalyst over 6 cycles). This leaching was found not to contribute significantly to degradation of the dye and the preliminary results suggest that the regime can be used for catalyst regeneration encouraging industrial uptake.

Successful attempts to regenerate a used surface functionalised nanocoated polyacrylonitrile (PAN) catalyst are described here.

A Comparative Study of the Activity of TiO2 Degussa P25 and Millennium PCs in the Photocatalytic Degradation of Bromothymol Blue

The photocatalytic activities of TiO2 Degussa P25 and Millennium PCs (PC50, PC100, PC105 and PC500) were evaluated by the photocatalytic degradation of Bromothymol Blue (BTB). The relationship between the photocatalytic reaction and the adsorption of BTB on the TiO2 catalysts at acidic, natural and basic mediums of pH was investigated. The crystalline phase, average crystalline size and surface area of the catalyst were found to have a significant influence on the adsorption and photocatalytic activity of the TiO2 samples. The mixed phase of anatase/rutile (Degussa P25) was found to be the most efficient photocatalytical material than pure phase anatase (Millennium PCs) and faster degradation is observed for PC500 compared to other Millennium PCs, this was attributed to the high surface area of PC500. Within the PC50, PC100 and PC105 series, the photocatalytic efficiency increased with the decrease of the surface area. The COD and TOC removals increased slowly, however, the decolorization ratio of BTB increased rapidly at the same time. Thereafter, the efficiency of P25 and PC500 were compared in presence of H2O2, Cl− and HCO3− at different mediums of pH. H2O2 was found to enhance strongly the BTB degradation in presence of P25 with an optimum at natural pH. In contrast, the reaction was inhibited in the presence of PC500, due to the inhibition of dye adsorption. At different pH, the BTB degradation has been significantly inhibited in the presence of the mixtures of HCO3−/H2O2. In contrast, the mixtures of Cl−/H2O2 accelerate the BTB degradation at acidic pH.