Efficacy and Mechanism of Adsorption Coupled with Chemical Conversion for Enhanced Removal of Organic Micropollutants

Abstract

With the increasing severity of water scarcity, the reclamation and reuse of effluent from urban wastewater treatment plants have emerged as a vital strategy for alleviating urban water resource pressure. However, the widespread detection of organic micropollutants (OMPs) in reclaimed water poses considerable environmental and health risks, thereby severely restricting the safe utilization of reclaimed water. Existing treatment technologies have inherent limitations in OMPs removal. For instance, adsorption methods fail to degrade OMPs, leaving their toxicity intact. Oxidation and hydrolysis methods show limited efficiency in eliminating low-concentration OMPs and may cause resource wastage. Moreover, oxidation methods face other challenges, such as the generation of toxic byproducts. Coupling distinct wastewater treatment technologies can effectively overcome the shortcomings of single technology, enhancing overall treatment efficiency and sustainability while reducing operational costs. Accordingly, this study establishes adsorption-oxidation and adsorption-hydrolysis coupled systems to enhance the removal of refractory and hydrolysable components of OMPs, respectively. This thesis systematically investigates the removal efficiency, contributing factors, practical application potential, as well as the individual and synergistic mechanisms of each coupled fraction. Through the above studies, it is expected to provide a theoretical basis and practical guidance for the development of adsorption-chemical conversion coupled systems and the design of related functional materials. The main contents and results of this thesis are as follows:

(1) A ferrate-based coupled system of oxidation and in situ adsorption was developed for the removal of organophosphonates (OPs). Under conditions of pH 6.5 and 0.15 mM ferrate, the fraction of PO₄^3- release from nitrilotris(methylene phosphonic acid) (NTMP) was 43.1%, with total phosphorus and total organic carbon removal efficiencies reaching 93.6% and 96.7%, respectively. The oxidation process, dominated by Fe(VI), converted NTMP into more easily removable PO₄^3- and other small molecular products, which enhanced the adsorption process of in situ-generated Fe(III) particles for complete pollutant elimination. pH was identified as the key factor influencing both processes. Furthermore, ferrate exerted high removal efficiency for other OPs and maintained stable performance in real wastewater effluent. This study effectively resolves the pH limitations and oxidation product retention issues inherent to existing oxidation technologies.

(2) Functionalized multi-walled carbon nanotubes (FMCNTs) were prepared to construct an adsorption-oxidation coupled system for the removal of sulfonamide antibiotics (SAs). Through adsorption and catalytic Fenton oxidation, the FMCNT-1/Fe(III)/H₂O₂ system achieved 97.1% sulfamethoxazole (SMX) removal within 60 min under pH 6.0, significantly outperforming the classical Fe(II)/H₂O₂ system (42.0%). The degradation behavior of different SAs was strongly correlated with their adsorption behavior, with higher adsorption capacity which effectively promoting the subsequent oxidation process. The adsorption and catalytic performance of FMCNTs was influenced by structural defects and oxygen-containing groups. The adsorption of SMX onto FMCNTs occurred through hydrogen bonding and π-π interactions. In this process, oxygen-containing groups served as hydrogen-bonding sites, while defects disrupted the conjugated structure, thereby inhibiting adsorption. The catalytic process relied on the binding of hydroxyl and carboxyl groups with Fe(III) to prevent its precipitation and cooperatively promote its reduction with unpaired π-electrons in vacancies, thereby enhancing ·OH generation and H₂O₂ utilization efficiency. This study provides theoretical support for the development of adsorption-Fenton coupled systems based on carbon materials.

(3) Hydroxy-rich Zr-MOF-808-X (X = 8–12) nanohydrolases were synthesized to establish an adsorption-hydrolysis coupled system for organophosphate esters removal. Under mild conditions, Zr-MOF-808-11 rapidly adsorbed p-nitrophenyl phosphate (p-NPP) and achieved its complete hydrolysis removal. Zr-MOF-808-X adsorbed and activated the phosphoester group through strongly Lewis-acidic Zr sites, facilitating subsequent nucleophilic attack on the P center by adjacent Zr-OH for rapid hydrolysis. Alkali etching partially disrupted Zr-O-C structures and increased Zr-OH content, which significantly enhanced hydrolytic activity of Zr-MOF-808-X. The hydrolysis rate for p-NPP facilitated by Zr-MOF-808-11 was nearly 50 times higher than that of Zr-MOF-808. Additionally, Zr-MOF-808-11 demonstrated excellent substrate universality, reusability, and environmental adaptability. This study offers an innovative strategy for efficient and cost-effective removal of hydrolysable OMPs.

(4) Ce-MOF-808 was investigated as nanohydrolase for the removal of β-lactam antibiotics (BLAs). Penicillin G (PG) was rapidly adsorbed and achieved 98% hydrolysis efficiency within 10 min towards Ce-MOF-808. The hydrolysis removal of PG relied on synergistic adsorptive activation and nucleophilic attack at Ce nodes: strongly Lewis-acidic Ce(IV) adsorbed and activated β-lactam rings, while the -OH on adjacent Ce(III) served as nucleophile to attack the β-lactam ring, thereby promoting the hydrolysis of PG. The Ce-based nanohydrolase demonstrated excellent removal performance towards other commonly used BLAs. Besides, this nanohydrolase exhibited extraordinary reusability, and its good performance in continuous flow operation and actual effluent treatment. This study further validates the application potential of adsorption-hydrolysis coupled system for enhancing the removal of hydrolysable OMPs, providing references for developing efficient nanohydrolases.

Date of Award	12 Jun 2025
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Yun Wah LAM (Supervisor), Mei Yee Kenneth LEUNG (Supervisor), Kwan Sing Paul LAM (Supervisor), Hon Wah Michael LAM (Supervisor) & Guoping SHENG (External Supervisor)