Adsorption of neutral and negatively charged low-molecular-weight carbonyls in reverse osmosis permeates by ion-exchange resins

Reverse osmosis (RO) has been increasing widely used in water and wastewater treatment to provide high-quality water, such as in industrial pure water production and municipal (non)potable water reuse. However, some low-molecular-weight (LMW) compounds, such as LMW carbonyls, penetrate through RO membranes and are present in the RO permeate, resulting in potential risk to human health or industrial production. Ion-exchange (IX) resins are widely used to help eliminate residual LWM organic compounds from RO permeates, but their elimination performance for carbonyls is not well understood. In this study, seven commercially available resins were employed to explore the adsorption performance for the elimination of representative carbonyls in RO permeate. In general, the adsorption performance of selected resins with regular polymer composition, pore structure and functional group was poor for LMW neutral carbonyl compounds, but efficient for carboxylic acids (number of carbon atoms is less than 4). Among target carboxylic acids, the gel-N⁺(CH₃)₃ resin exhibited rapid adsorption kinetics, performing better in the removal of LMW carboxylic acids with shorter alkyl chains and more carboxyl groups. In particular, binary carboxylic acids were adsorbed 3–5 times faster on resins compared with their monoacid counterparts. Charge density was the dominant factor affecting the adsorption performance of carbonyl compounds, followed by hydrophobicity, molecular chain length, and the presence of H attached to the carbonyl C. This study implies that neutral carbonyls cannot be efficiently eliminated by IX resin, and green oxidation processes, such as vacuum UV oxidation, are suggested for the conversion of such compounds to readily removable negatively charged carboxylic acids.