Reverse osmosis (RO) has remained at the forefront of water desalination and purification technologies for the past few decades, but addressing the key challenge of fouling requires a mechanistic understanding. RO plants employ anti-fouling strategies, but fouling continues to degrade their performance. Improving fouling resistance is a complex challenge due to the presence of multiple species in the feed stream which interact to form a multifarious fouling layer on the membrane surface. In specific, calcium is prevalent under most RO feed conditions, contributes independently to both scaling and fouling, and interacts with natural organic matter. Previous studies have shown that calcium, as a divalent cation, can bridge between negatively-charged ligands. Thus, calcium may play a major role in the development of the fouling layer by enhancing sorption of organic matter to the membrane surface and enhancing aggregation of organic and inorganic moieties.  Herein, we studied the calcium speciation on RO polyamide membranes using a set of synchrotron-based X-ray spectroscopic tools. We extracted RO membranes from the Orange County Groundwater Replenishment System (OCGWRS) pilot plant, which uses RO with a pretreatment step and anti-scalant to purify treated wastewater to potable water. We performed X-ray fluorescence mapping to study particle composition, size, and spatial distribution, on membranes located at lead and tail modules of the plant. We employed calcium K-edge X-ray absorption near-edge structure (XANES) spectroscopy for definitive speciation information. Our experiments were complemented with DFT calculations and thermodynamic modeling. We confirmed that calcium had the highest concentration on the membrane surfaces among a range of elements. We determined that the tail module had a higher number of a particles, with larger particles on average and a larger particle size distribution, than the lead module. The XANES analysis demonstrated that no crystalline, inorganic scales formed. Meanwhile, thermodynamic calculations showed that scaling was expected at the feed conditions, indicating that the anti-fouling measures may have alleviated scaling. Concurrently we identified potential calcium-organic complexes on the membranes. We concluded that calcium can connect organic ligands to negatively-charged functional groups on the polyamide membrane. These findings provide a mechanistic understanding to inform the design of fouling-mitigation strategies.

Synchrotron-based X-ray spectroscopy of foulants on reverse osmosis membranes

ACS Fall 2020 Virtual Meeting

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