Approbation of Polysulfone Membranes for Selective Recovery of Ammonium from Aqueous Solutions Using Liquid–Liquid Membrane Contactors

Abstract

The process of selective recovery of ammonium nitrogen from a multicomponent feed solution (\({\text{NH}}_{4}^{ + }\), K⁺, Cl^–, \({\text{HPO}}_{4}^{{2 - }}\), pH 9.3) into a stripping solution (HCl, pH 3.0) has been studied using a liquid–liquid membrane contactor. The feed and stripping solutions were separated by experimental polysulfone hollow fiber membranes with fundamentally different structures. One of them had a large-pore substrate and a dense nonporous layer on its surface (skin layer), and the other had an isotropic structure with uniformly distributed pores of about 50 nm in diameter. Two more asymmetric membranes made of polyetherimide or polyvinyltrimethylsilane were used for comparison. It has been shown that the ammonia nitrogen transfer coefficients through asymmetric membranes with a dense skin layer facing the feed solution are several times lower than those achieved in the case of a symmetric membrane. In the studied range of ammonium nitrogen concentrations in the feed solution (200–400 mmol/L), these coefficients reach values of (1–4) × 10^–3 m/h, which are comparable with the characteristics of the best hollow fiber gas separation membranes made of other materials presented in the scientific literature. A hypothesis is proposed to explain both the observed differences in behavior between membranes with different structures and the increase in the mass transfer coefficient of ammonia nitrogen upon dilution of the feed solution.