Studies on the degradation of trace phenol and indole odorants by chlorine and permanganate in drinking water treatment

Identifying initial transformation products during chlorination of the indole moiety and unveiling their formation mechanisms

To identify toxicity drivers within poorly characterized high-molar-weight disinfection by-products (DBPs), relatively stable high-yield initial transformation products generated from aromatic amino acids and peptides and humic substances have drawn much attention. In this study, initial transformation products in chlorination of the indole moiety in tryptophan (Trp) are proposed and their formation mechanisms were investigated using a quantum chemical computational method. The results indicate that 3-Cl-Trp+ is initially formed after the Cl+ of HOCl attacks the indole moiety, and nucleophilic addition with nucleophilic agents (H2O and OCl-) is thermodynamically preferred over deprotonation to generate 2-X-3-Cl-indoline moiety (X = OH and OCl), which is in contrast to indole. Over 25 types of initial transformation products are proposed from the 2-X-3-Cl-indoline moiety and two ring opening pathways were found at N1-C2 and C2-C3 bonds. Significantly, most structures of initial transformation products proposed based on experimental detection m/z values were confirmed using quantum chemical calculations and some new products are proposed in this work. The results are helpful to expand our understanding of the intrinsic reactivity of aromatic ring towards chlorination by hypochlorous acid.

Transforming wastewater treatment: Recent advancements in Catalytic Wet Air Oxidation with pillared clay catalysts for phenol remediation

Catalytic Wet Air Oxidation (CWAO) has recently been recognized as a promising technique for degrading persistent organic pollutants, such as phenol, in wastewater. Among various catalysts, Pillared Interlayer Clays (PILCs) stand out due to their high specific surface area and porous nature. This review delves into the latest progress in CWAO processes utilizing PILCs for the degradation of organic contaminants such as phenol in wastewater. It meticulously assesses the synthesis of PILCs, and their structural properties, including monolithic forms, to understand their effect on catalyst efficiency. Key insights into how these attributes affect the phenol degradation rate and the CWAO process's stability are discussed, providing crucial direction for enhancing catalyst performance. The review highlights the significance of choosing catalysts that offer a balance between cost-efficiency and operational efficacy under mild conditions. The recent results are reported, summarized, and compared, thus proving the feasibility of using PILCs as promising materials for phenol removal. The reaction parameters, phenol conversion, and degradation mechanisms are highlighted. The catalytic efficiency of PILCs was significantly affected by the synthesis methods and reaction parameters, and outperformed most costly catalysts. PILCs are highlighted as especially advantageous catalysts, offering strong performance at lower costs, which boosts the up-scaling opportunities of CWAO methods. This analysis also points out research gaps and proposes directions for future studies, such as exploring innovative PILC synthesis methods to improve their catalytic effectiveness and durability further. It stresses the need to incorporate environmental and sustainability considerations into catalyst design and selection, aligning with the principles of green chemistry in wastewater treatment. Finally, we conclude that proposing new directions for PILCs in CWAO in further naturally-based surface modifications, is also a promising approach for PILCs to perform more efficiently.

Permanganate Oxidation of Organic Contaminants and Model Compounds

Permanganate oxidation is an attractive environmental remediation strategy due to its low cost, ease of use, and wide range in reactivity. Here, permanganate reactivity trends are investigated for model organic compounds and organic contaminants. Second-order permanganate reaction rate constants were compiled for 215 compounds from 82 references (journal articles, conference proceedings, master's theses, and dissertations). Additionally, we validated some phenol rate constants and contribute a few additional phenol rate constants. Commonalities between contaminant oxidation products are also discussed, and we tentatively identify several model compound oxidation products. Aromatic rings, alcohols, and ether groups had low reaction rate constants with permanganate. Alkene reaction sites had the highest reaction rate constants, followed by phenols, anilines, and benzylic carbon-hydrogen bonds. Generally, permanganate reactivity follows electrophilic substitution trends at the reaction site where electron donating groups increase the rate of reaction, while electron withdrawing groups decrease the rate of reaction. Solution conditions, specifically, buffer type and concentration, may impact the rate of reaction, which could be due to either an ionic strength effect or the buffer ions acting as ligands. The impact of these solution conditions, unfortunately, precludes the development of a quantitative structure-activity relationship for permanganate reaction rate constants with the currently available data. We note that critical experimental details are often missing in the literature, which posed a challenge when comparing rate constants between studies. Future research directions on permanganate oxidation should seek to improve our understanding of buffer effects and to identify oxidation products for model compounds so that extrapolations can be made to more complex contaminant structures.

Nonionic nontoxic antimicrobial polymers: indole-grafted poly(vinyl alcohol) with pendant alkyl or ether groups

A series of new nonionic antimicrobial polymers with a biodegradable polyvinyl alcohol (PVA) backbone grafted with indole units and different hydrophobic alkyl or ether groups were synthesized by facile esterification.