Detoxification of oil refining effluents by oxidation of naphthenic acids using TAML catalysts

Bioremediation of naphthenic acid by Bacillus subtilis: Degradation kinetics and pathway elucidation

Naphthenic acids, toxic and persistent carboxylic acids found in petroleum contaminated water, pose a significant environmental challenge, but bioremediation offers a promising and cost-effective solution for their treatment. The present study illustrates the ability of Bacillus subtilis to degrade commercial naphthenic acid (100 mg/L) in aerobic and microaerobic settings under optimized conditions (temperature 36 °C, pH 6.0, and salinity 0.5 %). The degradation was confirmed by 47.61 ± 3.609 % reduction in total organic carbon levels within 144 h, indicating the microbial potential to mineralize organic naphthenic acid in aqueous medium as a sole carbon source. Naphthenic acids, being structurally complex and comprising a diverse array of carboxylic acids, were further studied using two representative models, hexanoic acid (linear) and benzoic acid (aromatic). These representative acids were selected to investigate the degradation kinetics and to elucidate the underlying degradation mechanism. The growth kinetics of B. subtilis on hexanoic acid and benzoic acid followed the Monod growth model, with maximum specific growth rates (μmax) of 0.17344 ± 0.004 and 0.15088 ± 0.006 day-1 respectively. The biodegradation kinetics followed a non-linear first-order rate model, with rate constants (k) of 0.43 ± 0.084 h-1 for hexanoic acid and 0.12 ± 0.02 h-1 for benzoic acid. Corresponding half-lives (t1/2) were determined as 13.37 h for hexanoic acid and 29.52 h for benzoic acid, demonstrating a faster degradation rate for hexanoic acid compared to benzoic acid. GC-MS analysis elucidated the degradation pathway, catechol and muconic acid were identified as the key intermediates, which suggest a potential metabolic breakdown. Consequently, it demonstrates the potential of Bacillus subtilis for the effective removal of naphthenic acids from polluted wastewater.

Regulation of phenol oxidation into polymeric derivatives ready for flocculation using polyaluminum chloride

Phenol, a toxic compound commonly found in wastewater, can be removed using the iron-tetraamidomacrocyclic ligand (Fe-TAML) and H2O2. However, it incurs high costs for Fe-TAML and H2O2, while treated water retains high chemical oxygen demand (COD) and CO2 emissions. To address these challenges, this study proposes converting phenol into polymeric derivatives followed by flocculation. Mass spectrometry (MS) reveals that phenol polymerization precedes polyphenol oxidation in the reaction, with slower reactions favoring phenol polymerization over polyphenol oxidation. It further demonstrates that reducing Fe-TAML dosage can slow down the reaction, thereby increasing the formation of polymeric derivatives at pH 10. Subsequent flocculation with polyaluminum chloride (PAC) effectively precipitates these products. When phenol concentration increases from 100 to 2500 ppm (mass ratio of H2O2 : phenol : PAC = 10 : 10 : 1), COD rises from 10% to 19%, while CO2 emissions decrease by over 45%. Meanwhile, the cost is reduced from 4.616 to 3.416 $ per kg phenol, as the Fe-TAML/phenol mass ratio decreases from 0.08% to 0.056%. Overall, this strategy is more cost-effective than conventional methods, requiring less Fe-TAML and H2O2 while significantly reducing COD and CO2 emissions.

Enhanced Direct Electron Transfer Mediated Contaminant Degradation by Fe(IV) Using a Carbon Black-Supported Fe(III)-TAML Suspension Electrode System

Iron complexes of tetra-amido macrocyclic ligands (Fe-TAML) are recognized to be effective catalysts for the degradation of a wide range of organic contaminants in homogeneous conditions with the high valent Fe(IV) and Fe(V) species generated on activation of the Fe-TAML complex by hydrogen peroxide (H₂O₂) recognized to be powerful oxidants. Electrochemical activation of Fe-TAML would appear an attractive alternative to H₂O₂ activation, especially if the Fe-TAML complex could be attached to the anode, as this would enable formation of high valent iron species at the anode and, importantly, retention of the valuable Fe-TAML complex within the reaction system. In this work, we affix Fe-TAML to the surface of carbon black particles and apply this "suspension anode" process to oxidize selected target compounds via generation of high valent iron species. We show that the overpotential for Fe(IV) formation is 0.17 V lower than the potential required to generate Fe(IV) electrochemically in homogeneous solution and also show that the stability of the Fe(IV) species is enhanced considerably compared to the homogeneous Fe-TAML case. Application of the carbon black-supported Fe-TAML suspension anode reactor to degradation of oxalate and hydroquinone with an initial pH value of 3 resulted in oxidation rate constants that were up to three times higher than could be achieved by anodic oxidation in the absence of Fe-TAML and at energy consumptions per order of removal substantially lower than could be achieved by alternate technologies.