(666e) Synchrotron-Based X-Ray Spectroscopy of Foulants on Reverse Osmosis Membranes

Membrane reverse osmosis is the gold standard for water desalination and purification. Its key limitation is membrane fouling. Efforts to reduce membrane fouling require a mechanistic understanding of the reactions involved, but this understanding has been difficult to achieve. One reason is that operating conditions involve multiple types of solutes, such as dissolved salts and natural organic matter. These species have large concentration gradients across individual membrane modules. In addition, different modules in RO systems, such as the lead module and the tail module, may have vastly different feed stream conditions. These interactions combine to form a complex fouling layer that may vary across the length of the RO system.

In this study, we investigated the nature and distribution of fouling on membrane surfaces to inform the design of anti-fouling methods. Equipped with a set of synchrotron-based X-ray spectroscopic tools, we studied the surface chemistry of a fouling layer on membranes harvested from the Orange County Water District (OCWD) pilot plant, which converts wastewater effluent into potable water with a filtrate flow rate of 12 gallons per minute. Although the plant uses microfiltration pretreatment and an anti-scalant, membrane fouling still occurs. Our X-ray tools enabled elemental mapping of the fouling layer for particle composition, size, and spatial distribution. They also provided definitive chemical speciation such that inorganic and organic materials could be identified.

We probed membranes from various sections of the pilot plant for multiple elements using micron-resolution X-ray spectroscopic imaging. We determined that the tail module had a higher particle count, with a larger particle size and size distribution, than the lead module. Out of a large range of elements found in both modules, calcium concentrations were the highest. With this insight, we employed X-ray absorption spectroscopic characterization of the calcium speciation on the membrane, noting that calcium contributes to the formation of inorganic scales, organic complexes, and biofouling. We demonstrated that limited inorganic, crystalline scales form, but we have confirmed the presence of asymmetric calcium-organic complexes, corresponding to a calcium-ligand coordination number of 7 or 8. We hypothesize that calcium can connect organic ligands to negatively-charged functional groups on the polyamide membrane. These findings provide the foundation for a mechanistic understanding of the multiple chemical reactions leading to membrane fouling, and this understanding is critical to design effective anti-fouling methods.