Cleaning mechanism of gypsum scaling in hydrophobic porous membranes

Abstract

Mineral scaling is one of the major challenges for membrane distillation in concentrating high-salinity brine. This work first investigated the characteristic of CaSO4 scaling, then explored different cleaning reagents to evaluate their efficiencies and mechanisms. By analyzing the SEM and EDX mapping images, the CaSO4 scaling process could be divided into surface and pore scaling stages. Three cleaning solutions, i.e., commercial detergent (20 %), EDTA-4Na (0.05 M), and MgCl2 (0.8 M), were chosen for their high CaSO4 solubility (0.7 g CaSO4 per 100 g H2O). At the surface scaling stage, three cleaning solutions exhibited excellent removal of CaSO4 scaling with approximately similar 96 % cleaning efficiencies. However, at the pore scaling stage, only the commercial detergent showed high efficiency (89 %). Such high cleaning efficiency was mainly attributed to the detergent's high total dissolved solids (salinity), low pH, and high diffusion rate (5.46 μm cm−1 min−1) across the membrane. Moreover, periodic cleaning at the surface scaling stage with the commercial detergent followed by drying throughout a 43.5 h continuous concentration test on an actual RO brine could restore the PTFE membrane performance completely, giving 90 % of the total water recovery. This work provides new insights into the cleaning mechanism for gypsum scaling in a hydrophobic porous membrane.