A porous carbon-based electro-Fenton hollow fiber membrane with good antifouling property for microalgae harvesting
Graphical abstract
Introduction
Microalgae, as a promising source of biofuel, have several advantages of high oil content, easy cultivation, fast reproduction, strong adaptability to the environment, no land occupation, and large yield per unit area, etc. [1]. And microalgae biofuels production processes generally mainly include microalgae cultivation, harvesting, dewatering, oil extraction, purification, and transformation, etc. [[2], [3], [4], [5]]. However, the small size of microalgae (2 ~ 30 μm), density almost similar to water and low concentration in the medium (0.5 ~ 2 g/L) cause harvesting microalgae biomass to be a significant challenge for cost-effective algae biofuels production [[6], [7], [8]]. Among current harvesting methods, centrifugation and flotation are energy-intensive. Gravity precipitation or flocculation are low efficiency and environmentally unfriendly [6,9]. Nevertheless, membrane filtration has an extensive application owing to high harvest efficiency, good effluent quality and low energy consumption [10,11]. But serious membrane fouling is still the major limitation in microalgae harvesting, which was mainly caused by microalgal cells and extracellular organic matter (EOM) [12]. Membrane fouling is generally divided into reversible and irreversible fouling. Among them, the cake layer formed by the loose deposition of microalgae and EOM (including polysaccharides, proteins, and humic acids, etc.) can be removed by physical means [[13], [14], [15]], referred as reversible fouling. While, the gel layer or membrane pores blockage will form when pollutants tightly adheres to the membrane surface or deeply blocks in the pores. This type of membrane fouling requires to be removed by chemical cleaning, which named irreversible fouling [[15], [16], [17], [18], [19]].
Recently, many researches [20,21] forced on coupling membrane filtration with electrochemical repulsion effect to mitigate membrane fouling in microalgae harvesting. Kim et al. [20] harvested microalgae based on electro-enhanced PVDF-carbon cloth membranes, which showed good anti-fouling performance with the average flux enhanced to 262.7 L/(m2·h) and the total water penetration increased by 150% at −3.68 V. Similarly, Mushtaq et al. [21] applied −5 ~ −30 V potential on the silver nanowire-based conductive membrane to repel negatively charged microalgae and utilized H2 bubbles generated in-situ on the membrane surface under the high electric field for alleviating membrane fouling during microalgae harvesting. The results indicated that electro-assistance (−20 V) could effectively increase membrane flux by 480% during 1 h electro-filtration. Overall, previous reports made a certain results that the continuous electric field could effectively alleviate membrane fouling in microalgae harvesting based on electric repulsion effect. However the energy consumption with usually high potential (−3.68 ~ −30 V) restricted the application of coupling electrochemical repulsion with membrane filtration for microalgae harvesting. Furthermore, only the electric repulsion and the micro-bubbles actually could not restore the membrane flux to original level in the electric field, because uncharged or same charged pollutants still account for some percentage of membrane fouling to the system.
Metal-organic framework derived porous carbon (PC) is an excellent electro-catalytic material with large specific surface area, massive micropores and abundant active sites, which was benefited for hydrogen peroxide (H2O2) generating [22,23]. Moreover, carbon nanotubes (CNT) have gained much attention for membrane fabrication due to its good conductivity, tubular structure and excellent water transport capacity [24,25]. Therefore, the PC blending with CNT and Fe2+ owned the mixed membrane prominent water permeance (more than 2000 L/(m2·h·bar)) and in-situ •OH generation ability. The electro-Fenton process was as following [26,27]:
In this electro-Fenton process, O2 was reduced to H2O2 via two electrons at the cathode (PC-CNT hollow fiber membrane), and then H2O2 reacted with Fe2+ in the system to produce •OH [28]. •OH with strong oxidation was capable of degrading pollutants [29], which provides a new energy-saved strategy for membrane fouling control in microalgae harvesting.
In this study, the PC-CNT hollow fiber membrane loaded with Fe2+ was used as the basic separation unit in a microalgae harvesting system, which operated both in continuous and intermittent modes. The performance of electro-Fenton enhanced microalgae harvesting system was mainly integrated by membrane anti-fouling and microalgae harvesting effectiveness. Besides, the mechanism of membrane fouling mitigation and the influence of •OH on membrane materials and microalgae were further analyzed.
Section snippets
Materials
The CNT was provided by Shenzhen Nanotechnology Port Co., Ltd., China. Zinc nitrate (Zn(NO3)2·6H2O), 2-methylimidazole and 5,5-Dimethyl-1-pyrroline N-oxide (DMPO) were purchased from Sigma-Aldrich Chemical Company, USA. Some organic solvents like N, N-dimethylformamide (DMF), and methanol (CH3OH) were obtained from China Xilong Chemical Co., Ltd. China. Polyvinyl butyral (PVB) was purchased from Sinopharm Chemical Reagent Co., Ltd., China. Meilun biotechnology Co., Ltd. provided bovine serum
Characterization of membranes
The prepared Fe-PC-CNT hollow fiber membrane with the Fe2+ content of 123.2 mg/g membrane (Fig. S2) presented the uniform outer/inner diameters of separately 0.8 mm/0.6 mm and high porosity of 87.8%. The membrane pore size was about 207 nm (Fig. S3), which could entirely retain microalgal cells. Moreover, abundant hydrophilic groups on the membrane surface due to the acidification of PC and CNT render good hydrophilicity to the membrane with the contact angle of 29.38°(Fig. S4). The high
Conclusion
In this study, the electro-Fenton enhanced Fe-PC-CNT hollow fiber membrane played dual roles of harvesting microalgae and in-situ relieving membrane fouling. Electro-Fenton enhanced membranes presented good anti-fouling and microalgae harvesting abilities due to electrostatic repulsion and oxidation of •OH. The EOM tightly deposited on the membranes could be effectively removed and then reduced the irreversible fouling level. In addition, •OH had no obvious adverse effects on microalgae and
Author statement
All authors are aware of and accept responsibility for the manuscript. This manuscript has not been previously published, in whole or in part, and it is not under consideration by any other journal.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was financial supported by the National Natural Science Foundation of China (No.21677026), the LiaoNing Revitalization Talents Program (No. XLYC1807067), the Fundamental Research Funds for the Central Universities (DUT17LAB15), and the Programme of Introducing Talents of Discipline to Universities (B13012).
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Mingmei Zheng and Yue Yang contributed equally to this work.