Seawater desalination performance of MFI type membranes made by secondary growth

Abstract

Seawater desalination is now commonly performed using membrane technology but new materials research is however needed to address issues such as membrane fouling. Ceramic materials such as zeolites are capable of providing the required desalination properties while being potentially tolerant to waters which readily foul polymer membranes and/or can withstand more cost effective cleaning methods. In this paper, we developed Silicalite and ZSM-5 zeolite membranes and tested desalination using the thermally based pervaporation, and more common reverse osmosis (RO) operation modes. Zeolite membranes were synthesised by the template-free secondary growth technique with varying Si/Al ratios. X-ray diffraction spectra of all membranes showed randomly oriented MFI structure. For intact membrane films, gas permeation was not measurable at room temperature until membrane was first heated to at least 125 °C under dry helium implying minimal defects and zeolite pore plugging by water. In pervaporation with 3.8 wt% seawater feed, zeolites containing alumina actively rejected ions, displaying equivalent rejections exceeding 99%. Flux typically decreased after introduction of seawater, but the membrane with Si/Al ratio of 100 uniquely showed a 30% increase in flux which could only be explained by an ion exchange/interaction mechanism which opened the zeolite structure to permeation. Optimal temperature for pervaporation was found to be 80 °C, where high rejections persisted, and flux increased to 0.72 kg m −2 h −1. In RO, the Silicalite membrane achieved highest rejections, but had not reached steady state after 25 h of operation. Negative rejections were initially observed, but eventually crossed into positive values over time. Despite the complexity of water and multiple ion diffusion through molecular dimensioned zeolites with complex charged surfaces, MFI zeolite membranes with tailored Si/Al ratios are ideal candidates for desalination.