Ceramic hollow fiber membrane derived from palm oil fuel ash for membrane distillation

Abstract

Membrane distillation (MD) is an emerging desalination technology which separates vaporized solutes from the feed solution using a hydrophobic membrane at fairly high temperature. Ceramic membranes are known to have excellent chemical and thermal stability. However, their application in MD has not been well received as compared to their polymeric counterparts due to high cost and intrinsic hydrophilicity. This study focused on the development of novel cost-effective hydrophobic ceramic hollow fiber membranes (CHFMs) from an industrial waste, palm oil fuel ash (POFA), for direct contact MD (DCMD). POFA has high silica and potassium oxide content that could endow it with lower sintering temperatures during membrane fabrication than that of the widely used alumina membranes. However, POFA also exhibits a significant amount of carbon and organic impurities that could be detrimental to the mechanical properties of CHFM. Hence, in the first stage of the study, POFA was subjected to thermal pre-treatment at temperatures of 500–1,000°C, and the effect of pre-treatment temperature on the chemical and physical properties of POFA was correlated. It was found that the carbon content of POFA was eliminated after being pre-treated at =600°C, whereas the silica content was improved to >70 wt%. Moreover, the physical properties of POFA changed with increasing pre-treatment temperatures. In the second stage of the study, the high-strength POFA-derived CHFMs were fabricated through combined phase inversion/sintering technique. It was found that the pre-treatment temperature of POFA, POFA loading, phase inversion parameters (i.e.: air gap distance, bore fluid flow rate), and sintering temperature had substantial influences on the morphology and mechanical properties of CHFM. A high-strength CHFM (98.1 MPa) was acquired at the following conditions: 700°C POFA pre-treatment temperature; 55 wt% POFA suspension loading; 5 cm air gap distance; 9 mL/min bore fluid flow rate; 1,050°C sintering temperature. To attain hydrophobic properties, the surface of the CHFM was modified via dip-coating with polymethylhydrosiloxane/tetraethylorthosilicate (PMHS/TEOS) hybrid in the third stage of the study. A novel post-coating spinning technique has been developed to facilitate the pore formation on the coating layer. The effect of the number of coating layer on the morphology of the CHFM was studied. The concentrations of ethanol and PMHS were also found to affect the surface morphology and hydrophobicity of the CHFM. High water contact angle (WCA) of 108.2° and liquid entry pressure with water (LEPw) of 1.0 bar was achieved by the CHFM modified with the following conditions: TEOS/ethanol molar ratio: 1:45; PMHS/TEOS mass ratio: 1:10; the number of coating layer: 2; with post-coating spinning. An excellent DCMD desalination performance was achieved with a salt rejection of >99.98% and flux of 4.8 L/m2h at the feed salinity of 35,000 ppm. The outcomes of this study suggest that the hydrophobic POFA-derived CHFM could be an excellent low-cost alternative for MD desalination applications.