Development of Reactive and Self-Cleaning Layered Double Hydroxide Membranes for Advanced Water Treatment

<p dir="ltr">Micropollutants are synthetic chemicals or naturally occurring compounds found in the environment at very low concentrations, often in the microgram or nano-gram per litre range, which create trouble for their persistence, bioaccumulation, and potential toxicity. The presence of micropollutants in water and wastewater has emerged as a major public health concern, particularly due to their frequent detection in wastewater treatment plant effluents. Ranitidine, a competitive inhibitor of histamine H2 receptors, has been identified as an emerging micropollutant in water and wastewater, raising concerns about its potential impact on the environment and human health. In addition to micropollutants, conventional WWTPs face challenges in managing microplastics (MPs). While MPs are partially removed, WWTPs often release smaller-sized plastics, contributing to the fragmentation of MPs into nanoplastics (NPs). These NPs (<1000 nm) not only pose threats to marine life and humans but also have a high capacity to adsorb other toxic pollutants like micropollutants. This study aims to address this issue by developing an effective removal strategy using two types of layered double hydroxide (LDH) catalysts (i.e., CoFeLDH and CoCuLDH). Characterization results show that CoFeLDH catalyst has superior catalytic properties due to its stronger chemical bond compared to CoCuLDH. The degradation experiment shows that 100% degradation of ranitidine could be achieved within 20 min using 25 mg/L of CoFeLDH and 20 mg/L of peroxymonosulfate (PMS). On the other hand, CoCuLDH was less effective, achieving only 70% degradation after 60 min at a similar dosage. The degradation rate constant of CoFeLDH was 10 times higher than the rate constant of CoCuLDH at different pH range. Positive zeta potential of CoFeLDH made it superior over CoCuLDH regarding catalytic oxidation of PMS. The catalytic degradation mechanism shows that sulfate radicals played a more dominant role than hydroxyl radicals in the case of LDH catalysts. CoFeLDH demonstrated a stronger radical pathway than CoCuLDH. XPS analysis of CoFeLDH revealed the cation percentages at different phases and proved the claim of being reusable even after 8 cycles. </p><p dir="ltr">This study also developed a CoFe layered double hydroxide (CoFeLDH) catalytic membrane for PMS activation to achieve efficient micropollutant removal with improved mass transfer rate and reaction kinetics. The CoFeLDH membrane/PMS system achieved an impressive degradation efficiency above 98% degradation of the probe chemical ranitidine at 0.1 mM of PMS including five more micropollutants (Sulfamethoxazole, Ciprofloxacin, Carbamazepine, Acetaminophen and Bisphenol A) at satisfactory level (above 80%). Moreover, significant improvements in water flux and antifouling properties were observed, marking the membrane as a specific advancement in the removal of membrane fouling in water purification technology. The membrane demonstrated consistent degradation efficiency for several micropollutants and across a range of pH (4–9) as well as different anionic environments, thereby showing it suitability for scale-up application. The key role of reactive species such as SO4•–, and O2•− radicals in the degradation process was elucidated. This is followed by the confirmation of the occurrence of redox cycling between Co and Fe, and the presence of CoOH+ that promotes PMS activation. Over the ten cycles, the membrane could be operated with a flux recovery of up to 99.8% and maintained efficient performance over 24 h continuous operation. The efficiency in degrading micropollutants, was coupled with reduced metal leaching for this membrane. </p><p dir="ltr">Furthermore, a CoFe layered double hydroxide (CoFeLDH) membrane incorporating the metal–organic framework (MOF) MIL(1 0 0)Fe, was tailored for a PMS based system. Among various combinations of MOF and LDH nanosheets on a PVDF substrate, the highest water flux, reaching 1900 L/m2/hr/bar, was achieved with an LDH/MOF ratio of 5:1 and an MOF concentration of 0.025 M. The optimized membrane exhibited exceptional performance, achieving 99 % degradation of ranitidine at 0.1 mM PMS. The LDH MOF catalytic membrane exhibited excellent treatment performance in real water matrices and demonstrated long-term operational efficiency. The effective removal of ranitidine by this catalytic membrane was attributed to a synergistic combination of radical (SO4•– and •OH) and non-radical (singlet 1O2 and electron transfer O2) oxidation pathways, with SO4•– and singlet 1O2 playing a predominant role. Post-activation, 40 % of surface Co2+ and 65 % of Fe2+ in the LDH MOF membrane were found in the + III oxidation state, highlighting the significance of metal catalytic sites and the reusability potential of the membrane. The durability of the membrane was evident through 10 cycles, achieving a flux recovery ratio of 95 % in the first cycle and sustaining efficient performance across pH variations (3 to 9) and exposure to various anions. Importantly, negligible metal leaching was observed for the real water samples, ensuring suitability for large-scale applications. </p><p dir="ltr">After that, a novel hybrid CoFe layered metal oxide (CoFeLMO) membrane was developed by integrating MIL(100)Fe and polyethylene glycol (PEG), designed specifically for peroxymonosulfate (PMS)-based advanced oxidation processes. The uniqueness of this research lies in the innovative incorporation of LMO, MOF, and PEG nanosheets onto a polyethersulfone (PES) substrate, creating a highly efficient catalytic membrane for the simultaneous removal of pharmaceutical micropollutants and nanoplastics (NPs).Among various configurations, the LMO-MOF-PEG membrane, with 20% MOF (0.025 M) and 0.5 g of PEG, demonstrated superior performance, achieving remarkable removal efficiencies of 99.5% for ranitidine and 98.5% for NPs. This membrane also exhibited outstanding operational efficiency, achieving a flux of 1600 L/m²/hr/bar at a low PMS concentration of 0.2 mM. The degradation of ranitidine was driven by both reactive species (SO4•–, •OH and O2•-) and non-reactive species (singlet 1O2), with SO4•– playing a dominant role. Post-activation analysis revealed the presence of Co²⁺ and Fe²⁺ in both +II and +III oxidation, indicating the active participation of metal plots in the degradation process and confirming the membrane's reusability. The membrane demonstrated exceptional durability, maintaining a flux recovery ratio of 97-99% across 10 filtration cycles, even under harsh chemical conditions and across a wide pH range (2-12). Furthermore, Co leaching was minimal (2-21 µg/L) over a broad pH spectrum, even after 15 days of immersion in water. Lastly, the addition of covalent organic framework (COF) with LMO improved the membrane separation capabilities (100% for ranitidine) and durability. Overall, the findings suggest that this membrane proves to be a suitable choice for attaining high degradation efficiency and good stability in the remediation of micropollutants and NPs from wastewater. </p><p dir="ltr">In summary, these studies present groundbreaking and innovative strategies for wastewater treatment, introducing new nanomaterials and membranes that significantly enhance the removal of various micropollutants and NPs. These advancements play a vital role in addressing key issues related to environmental pollution and water resource management. Various novel membranes have been developed through combining different nanomaterials, specifically designed to interact more effectively with oxidant and the types of the micropollutants. This approach leads to substantial improvements in both the removal efficiency and fouling resistance of the membrane.</p>