Desalination by electret technology

A purification process of making use of electret technology for efficient but cost effective deionization was discussed in this paper. Douglas MacGregor claimed that PTFE electret works as a means of ions separation for desalination due to the polarization of an electrolyte and ions migration under a permanent electrostatic field generated by a pair of electrets with opposite polarities. The electrostatic field attracts counter-ions to the charged solid surface, and thus generates a purified effluent in the mainstream. Theoretical and experimental analyses were made to investigate the practicality of desalination by electret technology. It is found that MacGregor’s design is meaningful only for a dilute electrolyte or a system with very high surface charge density because the number of the counter-ions attracted to the surface is limited and cannot exceed that of the surface charges at the electret.

The mean electrostatic potential ψ(x), influenced by both the surface charges and the ionic charges in the electrolyte, cannot be infinite. In other words, the reversal electric field produced by the ionic charges in the electrolyte is always less than the external field generated by the surface charges on the electret employed. The number of the counter-ions attracted to the surface cannot exceed the surface charges on the polarized materials; otherwise, the electric fields across the solution will reversely repel these counter-ions far away to the bulk. In DET, counter-ions are attracted to the area adjacent to the opposite-charged electret surface by the external electrostatic field, and the membranes utilized as mechanical barriers prevent ions from remixing in the neutral mainstream. Experiments of applying DET technology to NaCl electrolytes in the general manner prescribed in Douglas MacGregor’s patents were done, but little significant separation was observed, and membrane adsorption played an important role in our observation. Unfortunately, MacGregor’s claim is only valid for a very dilute saline solution or an electret with a very high surface charge density, at least at magnitude of 103 coul/m2 that could not be achievable currently. However, the concept of using electret for desalination is considered to be viable and should be developed further.