The development of membranes capable of precise solute−solute separation is still in its burgeoning stage without a standardized protocol for evaluating selectivity. Three types of membrane processes with different driving forces, including pressure-driven filtration, concentration difference-driven diffusion, and electric field-driven ion migration, have been applied in this study to characterize solute−solute selectivity of a commercial nanofiltration membrane. Our results demonstrated that selectivity values measured using different methods, or even different conditions with the same method, are generally not comparable. The cross-method incomparability is true for both apparent selectivity, defined as the ratio between concentration-normalized fluxes, and the more intrinsic selectivity, defined as the ratio between the permeabilities of solutes through the active separation layer. The difference in selectivity measured using different methods possibly stems from the fundamental differences in the driving force of ion transport, the effect of water transport, and the interaction between cations and anions. We further demonstrated the difference in selectivity measured using feed solutions containing singlesalt species and that containing mixed salts. A consistent protocol with standardized testing conditions to facilitate fair performance comparison between studies is proposed.
Recovering nitrogen from source-separated urine is an important part of the sustainable nitrogen management. A novel bipolar membrane electrodialysis with membrane contactor (BMED−MC) process is demonstrated here for efficient recovery of ammonia from synthetic source-separated urine (∼3772 mg N L −1 ). In a BMED−MC process, electrically driven water dissociation in a bipolar membrane simultaneously increases the pH of the urine stream and produces an acid stream for ammonia stripping. With the increased pH of urine, ammonia transports across the gas-permeable membrane in the membrane contactor and is recovered by the acid stream as ammonium sulfate that can be directly used as fertilizer. Our results obtained using batch experiments demonstrate that the BMED−MC process can achieve 90% recovery. The average ammonia flux and the specific energy consumption can be regulated by varying the current density. At a current density of 20 mA cm −2 , the energy required to achieve a 67.5% ammonia recovery in a 7 h batch mode is 92.8 MJ kg −1 N for a bench-scale system with one membrane stack and can approach 25.8 MJ kg −1 N for large-scale systems with multiple membrane stacks, with an average ammonia flux of 2.2 mol m −2 h −1 . Modeling results show that a continuous BMED−MC process can achieve a 90% ammonia recovery with a lower energy consumption (i.e., 12.5 MJ kg −1 N). BMED−MC shows significant potential for ammonia recovery from source-separated urine as it is relatively energy-efficient and requires no external acid solution.
Membrane ltration has been widely adopted in various water treatment applications, but its use in selective solute separation for resource extraction and recovery is an emerging research area. When a membrane process is applied for solute-solute separation to extract solutes as the product, the performance metrics and process optimization strategies should differ from a membrane process for water production because of separation goals are fundamentally different. In this analysis, we used lithium (Li) magnesium (Mg) separation as a representative solute-solute separation to illustrate the de ciency of existing performance evaluation framework developed for water-solute separation using nano ltration (NF). We performed coupon and module scale analyses of mass transfer to elucidate how membrane properties and operating conditions affect the performance of Li/Mg separation in NF.Notably, we identi ed an important operational tradeoff between Li/Mg selectivity and Li recovery, which is critical for process optimization. We also established a new framework for evaluating membrane performance based on the success criteria of Li purity and recovery. This analysis lays the theoretical foundation for performance evaluation and process optimization for NF-based selective solute separation.
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