Big Carrousel mechanism of copper removal from ammoniacal wastewater through supported liquid membrane

Abstract

Although “Big Carrousel” mechanism was suggested long time ago [1], there was no experimental work demonstrating its applicability. In this work, copper recovery from industrial ammoniacal wastewater using flat supported liquid membranes (SLM) was chosen as one of the most well known and practically important examples to demonstrate the limitations of “Small Carrousel” mechanism to describe facilitated ion transport through SLM. In this work, LIX54, one of well-established extractants for copper, was used as a carrier in the liquid membrane phase to extract and transfer copper. The dependences of the copper transmembrane flux on carrier concentrations may be explained by the “Small Carrousel” model: reaction of the carrier and copper occurs on the membrane/water interface and LIX54 stays in the membrane, carrying Cu ions in the membrane in one direction and two protons in another direction. Nevertheless, a quantitative explanation of experimental copper transmembrane flux as a function of feed pH and initial copper concentration cannot be based on the same values of physico-chemical parameters determined by simulation. A more advanced model is required. A detailed theoretical model – referred to as “Big Carrousel” – for facilitated transport through flat membranes is developed. Both diffusion of a copper complex with ammonia in an aqueous stagnant layer and fast reactions of the carrier and copper species in the aqueous reaction layer are accounted for in this model. This model, in which the carrier moves slightly out from the organic membrane in the aqueous reaction layer and then transfers from one aqueous phase to another through the membrane before finally moving back, is called “Big Carrousel”. Both reactions of LIX54 in the aqueous phase and the fact that Cu ions form complexes with ammonia have not been considered in previously published papers, describing the transport of simple Cu ions from ammoniacal aqueous solutions through liquid membranes. Mathematical model simulation demonstrated that only “Big Carrousel” model gives satisfactory quantitative description of all experimental results. Finally, high selectivity for Cu over other cations and long-term stability in a hollow fiber supported liquid membrane system for ammoniacal wastewater treatment make the SLM technology promising for practical industrial applications.