In chemical membrane cleaning, the challenge is to efficiently remove irreversible fouling while minimizing the impact on membrane materials. Particularly, traditional hypochlorite cleaning will further lead to the generation of toxic halogenated by-products. To address these issues, a combined system composed of peroxymonosulfate and chloride (PMS/Cl − ) was applied to clean irreversible-humic-acid-fouled polyethersulfone (PES) membranes. After fouled membranes were soaked for 1 h in a PMS/Cl − solution (10 mM/15 mM) at 25 °C under neutral conditions, 94% flux recovery and 96% resistance removal were realized. Surface properties of virgin and cleaned membranes were very similar, confirming the effectiveness of the PMS/Cl − solution in removing irreversible foulants. The stability of membrane separation performance during multiple fouling and cleaning cycles further confirmed the minimal impact on membrane materials. Rapid diminution of the peaks centered in the region of fulvic-like and humic-like components, monitored under 3D-fluorescence for the cleaning solution, was attributed to PMS-catalyzed chlorination, thereby revealing the primary foulant detachment mechanism. Crucially, the approach exhibited lower toxicity than hypochlorite, as evidenced by reduced halogenated by-products and lower acute toxicity to Photobacterium phosphoreum T3. Overall, this novel cleaning system is promising for the efficient and environmentally friendly removal of irreversible organic foulants in practical water-treatment.
Sodium hypochlorite (NaOCl) is a widely used cleaning reagent for membrane separation processes to recover membrane permeability; however, the competitive interactions of different chlorine species with fouling layers have not been adequately elucidated. In this work, we investigated the pH-dependent diffusion of the active chlorine species and reactions involved in the consequent dissociation and/or destruction of the fouling layers. The hypochlorite conductivity and dynamic diffusion tests showed that an increase in pH facilitated the uneven and fast diffusion of active chlorine via relaxing the matrix structure of the fouling layer. Under the synergetic effects of oxidation , hydrolysis and hydraulic shearing, the enhanced diffusion resulted in an uneven but massive removal of the foulants rather than a layer-by-layer dissociation, leading to a higher membrane cleaning efficiency. Sodium dodecyl sulfate (SDS), an anionic surfactant , was added into the cleaning solution to further validate the diffusion-dependent cleaning mechanisms. With a faster diffusion process aided by SDS, the cleaning efficiency was significantly improved. These findings provide new insight into the chemical cleaning mechanisms, which are of great importance to the optimisation of cleaning efficiency for various membrane-based processes.
To address the challenges associated with traditional interface catalytic oxidation membranes, such as preparation complexity, limited reaction sites, and poor reusability, a novel recyclable pre-deposited dynamic catalytic membrane was developed for the first time. Specifically, a B-doped NiFe 2 O x catalyst with magnetic and many oxygen vacancies was synthesized and simply pre-deposited onto the surface of an ultrafiltration membrane to activate peroxydisulfate (PDS). The results indicated that the abundant active catalytic sites on the dynamic deposition layer of B-NiFe 2 O x could efficiently activate PDS, thereby facilitating the generation of active species predominantly comprising sulfate and hydroxyl radicals. As a result, there was a 67.14% significant removal of chemical oxygen demand in secondary effluent. Furthermore, even after 8 cycles of magnetic recovery and re-deposition, the normalized flux of the dynamic catalytic oxidation membrane continued to maintain at 0.52. Notably, unlike conventional interface catalytic oxidation membranes, some B-NiFe 2 O x within the dynamic layer engaged in the catalytic oxidation process in a free form, which maximized the oxidation rate and enhanced membrane in-situ cleaning performance. This research contributes to a comprehensive understanding of utilizing pre-deposited dynamic catalytic oxidation membranes and proposes an effective long-term strategy for membrane in-situ cleaning and wastewater treatment.
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