title: Gypsum scaling in pressure retarded osmosis: experiments, mechanisms and implications.
abstract: Pressure retarded osmosis (PRO) is an osmotically-driven membrane process that can be used to harvest salinity-gradient power. The PRO performance (both water flux and power density) can be severely limited by membrane fouling. The current study, for the first time, investigates PRO scaling in a bench-scale pressurized system using calcium sulfate dihydrate (gypsum) as a model scalant. In addition to the bulk feed solution (FS) saturation index (SI bulk), gypsum scaling was found to be strongly affected by the draw solution (DS) type and concentration, the applied hydraulic pressure, and the membrane orientation. The commonly recommended active layer facing draw solution (AL-DS) orientation was highly prone to internal scaling. In this orientation, severe internal concentration polarization (ICP) of scaling precursors induced gypsum clogging in membrane support layer even when the FS was undersaturated (e.g., SI bulk = 0.8). At higher SI bulk values, external gypsum crystal deposition occurred in addition to internal scaling. More severe scaling was observed when the DS contained scaling precursors such as Ca(2+) or SO4(2-), suggesting that the reverse diffusion of these precursors into the FS can significantly enhanced gypsum scaling. Increasing applied hydraulic pressure could enhance reverse solute diffusion and thus result in more severe gypsum scaling when the DS contained scaling precursors. A conceptual model, capturing the two important PRO scaling mechanisms (ICP of scaling precursors from FS and reverse diffusion of scaling precursors from the DS), is presented to rationalize the experimental results. Our results provide significant implications for PRO scaling control.
authors.affiliation: School of Civil and Environmental Engineering, Nanyang Technological University, Singapore 639798, Singapore; Singapore Membrane Technology Centre, Nanyang Technological University, Singapore 639798, Singapore.
authors.forename: Minmin
authors.initials: M
authors.lastname: Zhang
authors.forename: Dianxun
authors.initials: D
authors.lastname: Hou
authors.forename: Qianhong
authors.initials: Q
authors.lastname: She
authors.forename: Chuyang Y
authors.initials: CY
authors.lastname: Tang
mesh_terms: D002133:Calcium Sulfate / Q000737:chemistry*; D008567:Membranes, Artificial; D009995:Osmosis; D011312:Pressure
chemical_list: D008567:Membranes, Artificial; D002133:Calcium Sulfate
keywords: Gypsum; Internal concentration polarization (ICP); Pressure retarded osmosis (PRO); Reverse solute diffusion; Scaling