Use of a Fluorescent Antiscalant to Investigate Scaling of Reverse Osmosis Membranes

Abstract

Knowledge of the scaling mechanism makes it possible to develop effective means to control scaling and improve the membrane performance, increasing the recovery. This paper presents new approaches to the study of the mechanism of scaling in the presence of polymeric inhibitors (antiscalants); the adsorption of antiscalant molecules on the crystal and membrane surfaces has been investigated. The relations of the antiscalant adsorption rates to the antiscalant dose and the calcium carbonate scaling rate have been revealed. For the first time, the inhibition process was "visualized" by using a fluorescent antiscalant containing a fluorescent moiety-a copolymer of N-allyl-4-methoxy-1,8-naphthalamide and acrylic acid (PAA-F1). The examination of the surfaces of crystals and membranes by scanning electron and fluorescence microscopy showed new unexpected results: the antiscalant is adsorbed on the membrane surface and on the surface of calcite crystals formed. Fluorescence turned out to be more intense and noticeable on the surface of crystal faces than inside the crystal. During the nucleation phase, the "dark" part of the crystal lattice is formed and then begins to be covered with a "luminous" layer of the fluorescent antiscalant, which blocks further crystal growth. During experiments with antiscalant solutions in distilled water, the antiscalant was found to be adsorbed on the membrane surface in the absence of calcium ions. In experiments in which the antiscalant was added into the original tap water, it was adsorbed on the surface of the resulting crystals, not on the membrane. The visualization of the crystal growth inhibition process opens up new possibilities for studying the mechanism of scaling and developing of new technologies to control scaling.

Modeling Framework for Cost Optimization of Process-Scale Desalination Systems with Mineral Scaling and Precipitation

“…In other work, gypsum scaling in RO was characterized through various techniques, such as quantifying the coverage area of crystals on the membrane surface via ex-situ high-contrast imaging, , assessing axial variation of membrane-surface crystallization via scanning electron microscope (SEM) imaging, and using a quartz crystal microbalance in tandem with SEM imaging to investigate effects of polymeric surface characteristics on crystal formation . Real-time imaging has also been applied to approximate induction time and monitor scale formation on RO membranes as a function of scaling tendency over time. − Other works have focused on the effects of recovery rate, hydrodynamics, and addition of antiscalants on induction time and scaling of RO membranes. − Most of these works have focused on developing methods to characterize and predict mineral scaling phenomena for conventional RO applications. However, process-scale models that incorporate mineral-scale predictions and aqueous chemistry calculations can aid in the exploration and early prioritization of new experimental designs for evaluating conceptual and emerging high-recovery systems.…”

“… 32 − 36 Other works have focused on the effects of recovery rate, hydrodynamics, and addition of antiscalants on induction time and scaling of RO membranes. 37 − 41 Most of these works have focused on developing methods to characterize and predict mineral scaling phenomena for conventional RO applications. However, process-scale models that incorporate mineral-scale predictions and aqueous chemistry calculations can aid in the exploration and early prioritization of new experimental designs for evaluating conceptual and emerging high-recovery systems.…”