Divalent cations enhance nanofiltration of amino acids in multicomponent solution systems: Molecular insights and implication to drinking water treatment

Amino acids (AAs), an important precursor to nitrogeneous disinfection byproducts, can pass through nanofiltration (NF) membranes and influence finished water quality. Due to complex natural water compositions, AA rejection by NF involves intricate metal-organic interplays that are difficult to describe using existing macroscopic filtration models. Therefore, this study integrated molecular dynamics simulation with experimental investigation to elucidate NF mechanism of glycine in the multicomponent solution systems. The NF experiment results showed that the presence of divalent cations in the feed solutions increased glycine rejection from a baseline of 42.2% (glycine in pure water) to 52.5% and 58.1% in the presence of Zn²⁺ and Ca²⁺, respectively. This trend cannot be explained by steric or dielectric exclusion mechanisms. The MD simulation results reveal that the ternary interactions among glycine, divalent ions and the membrane jointly govern glycine filtration. Specifically, Ca²⁺/ Zn²⁺ competes with glycine for interaction sites on the membrane surface and within the pores, which alters the structure, energetics, and dynamics of respective binary interactions. These changes not only suppresses glycine sorption onto membrane surface, but also elevates the energy barrier for its pore-entry from 29.95 to 36.02-38.74 kcal mol⁻¹, and the barrier for intrapore migration from 16.24 to 26.61-28.93 kcal mol⁻¹. Overall, these findings highlight the existence of coupled interplays among organics, divalent cations and membranes and their relevance to the selectivity of NF membranes for AAs in realistic waters.