Abstract
Ceramic nanofiltration (NF) is a newly-developed technology for water recycling, but is still limited to pilot-scale applications. Lacking efficient and eco-friendly strategies for cleaning ceramic NF membrane impedes its scaling-up in industries. Forward flush, backwash and acidic/caustic cleaning are not efficient enough. In this work, a novel oxalic acid-aided Fenton process was proposed for synergistic relaxation/oxidation of persistent Ca2+-mediated gel-like fouling of ceramic NF membrane. A reactive catalyst layer was online pre-coated on top of the membrane via a pressure-driven cross-flow pre-filtration of Fe3O4 hydrosols. The gel-like fouling was simulated by alginate in the presence of Ca2+ ions. Results show that the Fe3O4 loading could be readily tuned from 0.16 to 1.34 g m−2 by altering the permeate flux during the pre-coating. The membrane permeability loss due to the pre-coating was minimal (<10%). The combination of oxalic acid chelation and Fenton-based oxidation resulted in high flux recovery (85.07%) for the iron-oxide pre-coated membrane, whereas the single treatment by hydrogen peroxide (H2O2) or oxalic acid was inefficient. This synergistic effect was attributed to relaxation of the Ca2+-mediated gel layer via oxalic acid/Ca2+ chelation, which presumably facilitated H2O2 diffusion at the Fe3O4/foulant interface. The iron-oxide pre-coated membrane maintained stable initial normalized fluxes (83.33–90.15%) through the oxalic acid/H2O2 cleaning over five cycles, with no need of refreshing the iron-oxide pre-coat. Additionally, the leaching of iron from the iron-oxide pre-coat by oxalic acid was suppressed by the oxalic acid/H2O2 combination, owing to a reactive shielding by competitive sorption of H2O2 onto the Fe3O4 surface. Overall, the synergistic relaxation/oxidation method, demonstrated in this study, provides new insights into improving reactivity of Fenton-based processes on hybrid catalytic ceramic membranes for water treatment or fouling control.
Highlights
Ceramic nanofiltration (NF) has emerged as an attractive new tech nology for non-potable water recycling from municipal sewage and secondary wastewater effluent, given its good robustness and separation capability upon various organic molecules (>450 Da) and small colloids [1,2]
Backwash cannot be applied to ceramic NF membranes because of physical damage to the end-sealing under high transmembrane pres sures (TMP), in addition, the backwash velocity is limited at pressures below 10 bar [9]
By comparing the reaction rates of Eq (5) and Eq (6), the Fe(II), prevailing in the bulk solution, should originate from the HO2-governed reduction of Fe(III) (Eq (6), k = 2 × 103 M− 1 s− 1) rather than from the Fe(III) reduction by H2O2 (Eq (5), k = 2.7 × 10− 3 M− 1 s− 1) [44,45]. These results suggested that homogeneous Fenton re actions of H2O2 with Fe(II) (or Fe(III)) did not play an important role in OH generation for the oxidative cleaning
Summary
Ceramic nanofiltration (NF) has emerged as an attractive new tech nology for non-potable water recycling from municipal sewage and secondary wastewater effluent, given its good robustness and separation capability upon various organic molecules (>450 Da) and small colloids [1,2]. Formation of a low permeable gel layer on the surface of ceramic NF membranes during filtration, plays a significant role in the mem brane fouling, which affects the water-yielding capacity and energy consumption in water treatment [3,4]. Off-line chemical cleaning of the mem branes with alkaline, acid, or hypochlorite is frequently needed, dis rupting the continuous filtration and influencing the membrane integrity [10,11]. Chemical cleaning agents, such as NaOH and citric acid, are incapable of fully eliminating the foulants even if the Journal of Membrane Science 636 (2021) 119553 flux was entirely recovered, leading to a progressive flux decrease with successive filtration cycles [4]
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