In-situ utilization of membrane foulants (FeOx+MnOx) for the efficient membrane cleaning.

Preoxidation-ultrafiltration process is an effective method for Fe2+ and Mn2+removal, in which Fe2+ (Mn2+) are firstly oxidized to FeOx (MnOx), then collected by the ultrafiltration membrane. However, the simultaneous presence of Fe2+, Mn2+, and organics in feed can cause severe membrane fouling, which inhibits the overall performance of this method prominently. In this study, a novel FeOx+MnOx+H2O2 membrane cleaning method is proposed based on the idea of turning in-situ generated membrane foulants, i.e., FeOx+MnOx, into the catalysts for membrane cleaning. The results demonstrate that the FeOx+MnOx+H2O2 system can achieve more than 95% membrane flux recovery and remove almost all irreversible membrane foulants within only 5 min and with only 0.5%wt% H2O2 solution. The outstanding performance of the system is mainly attributed to the catalytic decomposition of H2O2 to generate both highly reactive radicals, such as hydroxyl radicals (·OH), and abundant oxygen. In addition, when the membrane is loaded by only MnOx, polyaluminium chloride (PAC) as the coagulator demonstrates prominent influence on the performance of membrane cleaning. However, PAC makes almost no contribution to membrane cleaning when the membrane is loaded by FeOx. This is because coagulation induced by PAC exerts more prominent impact on the particle size distribution of MnOx than that of FeOx. In conclusion, the catalytic decomposition of H2O2 by in-situ generated FeOx+MnOx is a promising advanced oxidation process to achieve outstanding membrane cleaning performance under the condition of low H2O2 concentration and no extra dosage of catalysts. The novel membrane cleaning system exhibits high potential for the practical membrane treatment processes to treat water with high contents of Fe and Mn.

Novel sodium percarbonate-MnO2 effervescent tablets for efficient and moderate membrane cleaning.

Membrane flux recovery efficiency and durability are two key factors closely associated with the practical application for membrane cleaning process. However, conventional chemical membrane cleaning method by soaking the whole membrane module in highly concentrated chemical reagents has prominent drawbacks including the low mass transfer efficiency of reagents, long period of washing time, and the potential threat to membrane structure. Herein, for the first time, we report a facile approach to fabricate the sodium percarbonate-MnO2 effervescent tablets which show bubbling reaction to release oxygen and free radicals when being dispersed in water for membrane cleaning. Due to the synergistic effect of MnO2 and sodium percarbonate, the tablets are highly effective to clean the membrane fouled by humic acid within 5 min, with the terminal membrane flux being recovered from 0.50 to 0.95, and the irreversible fouling resistance being reduced by more than 90%, which is prominently more efficient than the conventional chemical cleaning methods. Moreover, even by consecutive membrane fouling and cleaning for 6 times, the membrane flux and filtration efficiency of the membrane could still be kept almost constant, and the moderateness of this membrane cleaning method was also verified by the systematic microscopic analysis. For mechanism study, results of Electron Spin Resonance (ESR) and quenching experiments indicated that the high-efficiency and robust durability of sodium percarbonate-MnO2 (SPC-MnO2) system for membrane cleaning was mainly attributed to the abundantly generated hydroxyl radicals and secondary free radicals (i.e. carbonate radicals). Conclusively, compared with the conventional membrane cleaning method with liquid cleaning reagents, the novel SPC-MnO2 system with remarkable advantages in terms of convenience and membrane cleaning performance demonstrated high potential for the wide application in practice.

Opposite impacts of K+ and Ca2+ on membrane fouling by humic acid and cleaning process: Evaluation and mechanism investigation.

Understanding the influences of cations on membrane fouling was important to improve the performance of membrane filtration system, however, opposite conclusions were made in different studies. Meanwhile, although the influences of cation concentration have been studied extensively, few attentions have been paid to the cation valence. To clarify it, the effects of typical cations on membrane fouling and cleaning, as well as the related mechanisms were investigated systemically in this study. K+ and Ca2+ were chosen as the representative cations, and humic acid (HA) was chosen as the membrane foulants. The results demonstrated Ca2+ promoted the formation of reversible fouling, meanwhile higher removal efficiency of HA could also be achieved with the assistance of filtration cake containing HA + Ca2+. However, K+ led to the formation of more recalcitrant irreversible fouling. By comparing the concentration of cations in feed and permeate, analyzing the influence of cations on size of HA flocs, and the detailed SEM, AFM and TEM observation, it could be found that different mechanisms dominated the interaction between cations and HA. The bridging effect induced by Ca2+ attributed to the extension of HA molecules, while the electrostatic shielding effect induced by K+ led to the compression of them. Moreover, the different characteristics of hydrated Ca2+ and K+ also contributed to the different structures of foulant layers formed by HA + Ca2+ and HA + K+. Given the abundance of K+ and Ca2+ in natural water, results of this study can provide valuable advice for practical membrane filtration process.

Novel H2O2-MnO2 system for efficient physico-chemical cleaning of fouled ultrafiltration membranes by simultaneous generation of reactive free radicals and oxygen.

The novel H2O2-MnO2 system was developed to achieve highly efficient membrane cleaning for both fouled PVDF and PES membranes in this study. Compared with conventional chemical cleaning process in which the whole membrane module had to be soaked in highly concentrated solution of chemical reagent for long period of time, the H2O2-MnO2 cleaning process conducting for only 5 min in 0.5 wt% H2O2 solution could achieve more than 95% recovery of permeate flux and almost total removal of the irreversible foulants. More importantly, the permeate flux and filtration efficiency of the membrane could be still kept stable after 6 runs of consecutive fouling and cleaning. Based on the systematic microscopic analyses, Electron Spin Resonance (ESR), Fourier Transform Infrared Spectroscopy (FTIR), as well as the quenching experiments with different free radical scavengers, the outstanding performance of H2O2-MnO2 system was attributed to the generation of both free radicals and abundant oxygen simultaneously, leading to the physico-chemical membrane cleaning. Conclusively, the newly developed H2O2-MnO2 system demonstrated noteworthy advantages on efficient membrane cleaning, and exhibited highly potential for the wide application in practical water treatment process.