Fabrication of novel silver nanowires-based electrically conductive membranes for microalgae harvesting

Abstract

Microalgae has evolved as a promising feedstock for bioenergy because of its inherent properties of high lipid content and its ability to help in bioremediation of water and CO2 sequestration. To make microalgae useful in terms of application in biodiesel production, it needs to be processed to several stages. Microalgae concentration (harvesting) is considered the most energy intensive process amongst all stages of microalgae processing. Membrane filtration is a possible method that due to simplicity in operation, no chemicals involvement, and less energy consumption has gained importance as a promising candidate for microalgae concentration. This method however, suffers from a drawback of membrane fouling i.e. deposition of particle on membrane surface and/or inside membrane pores. The key to solution of membrane fouling is actually a way forward to efficient microalgae harvesting. Conducting membranes have shown promising results with regard to fouling inhibition by electrically either pushing the foulants away (electrophoresis) or removal of deposited foulants layer by a mere push by the application of strong electric field. However, the major problem reported to exist in such an application is the successful fabrication of highly electro-conductive yet stable electro-membrane. In first part of this study, we report a novel application of electroplating method

depositing a highly electro-conductive yet stable layer of silver metal on AgNWs based polyether(sulfone) film supported electro-membrane for electro-filtration applications. The fabricated membranes were extensively characterized in terms of morphology, porous characteristics, electrical conductivity, stability of coated layer and Chlorella sp. HS-2 filtration performance. The fabricated membrane showed remarkably high electrical conductivity of $3.9 × 10^4 S/cm$ (highest yet achieved electrical conductivity to the best of our knowledge). The coating method of electroplating proved to be successful in fabricating of a stable electro-membrane. Moreover, electro-bubble generation upon application of electric field across membrane surface gave rise to an overall 480% increase in permeate flux in comparison to control test of 0 V/mm. In addition to this, the intermittent application of electric field was able to recover flux to initial level. Such work, distinct from available approaches for conductive membrane fabrication, offers a new way forward for highly electro-conductive yet stable membranes for electro-bubble filtration. In the second part of this study, we fabricated a completely conductive (support: metallic, active layer: metallic) membrane by using the electroplating principles and equipment used in chapter 3 with some formulations in electrolyte recipe and electroplating parameters. The prepared membrane was directly useable as not only a cathode but also a separation membrane. The performance of the membrane was than evaluated in harvesting of Chlorella sp. HS-2 both in continuous and intermittent modes of electrofiltration. The metallic matrix based AgNWs membrane showed improved relative flux recovery than the polymeric supported AgNWs membrane in previous study. This study provides a new route for the fabrication of highly electro-conductive yet stable metallic membrane, with a hydrophilic coating, that can provide better, effective prospect towards microalgae harvesting. In the last part of this study, a successful optimization of the electroplating-facilitated conductive membrane formulations and electro-filtration process parameters for microalgae harvesting was carried out employing RSM-based miscellaneous method in this study. The optimization study was mainly divided into two different sets of studies, namely continuous and intermittent mode of electrically-assisted membrane filtration. The continuous mode was used to identify optimal combination of membrane formulations with maximized normalized flux and microalgae cells rejection. It was concluded that the plating current density has a significant effect in dictating the intrinsic properties of the conductive membrane, mainly electrical conductivity, which in turn facilitates the normalized flux of the membrane during electro-filtration process. Similar findings were obtained for rejection efficiency of the membranes, where final response was found to be more dependent on the plating current density of the membrane. Contrarily, the intermittent mode study gave an insight into the optimization of electro-filtration parameters. In this mode, it was observed that the model developed for optimization of electro-filtration parameters was not significant within the given range of independent variable, and further experimentation were suggested to optimize the model terms for a maximized normal flux. However, the model developed for the experimental responses for flux recovery was found to be significant and was further used to define the optimal combination of electro-filtration parameters to have maximized flux recovery. The electric field strength of 20 V/mm at an application time of 1 min was found to be the optimal combination of electro-filtration parameters for maximized flux recovery using developed quadratic model.