Factors determining ion rejection and one-step freeze desalination

Freeze desalination is an emerging, membrane-free desalination method that utilizes the ion rejection phenomenon during the freezing of seawater, where salt ions tend to be rejected by ice. However, it requires the ion rejection rate to be higher than 98.2% to generate drinking water; i.e., at least 98.2% of the salt ions remain in the liquid phase. This poses a challenge for one-step seawater freezing due to the trapping of salt ions in ice. In this work, we first reveal that the ion rejection phenomenon is governed by the competition between two factors: ice growth and ion diffusion, which can be described by two characteristic timescales, <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:msub><a:mi>τ</a:mi><a:mi>ice</a:mi></a:msub></a:math> and <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:msub><b:mi>τ</b:mi><b:mi>ion</b:mi></b:msub></b:math>, respectively. To enhance the ion rejection rate, the ratio <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:msub><c:mi>τ</c:mi><c:mi>ice</c:mi></c:msub><c:mo>/</c:mo><c:msub><c:mi>τ</c:mi><c:mi>ion</c:mi></c:msub></c:math> should be increased. Based on this criterion, we propose a freeze desalination method by freezing flowing seawater, where <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:msub><d:mi>τ</d:mi><d:mi>ion</d:mi></d:msub></d:math> is reduced due to the enhancement of the ion diffusivity caused by flow shear, to obtain potable water through a single cycle of seawater freezing with the ion rejection rate exceeding 98.2%. With additional temperature control, the productivity of drinking water can be further improved. The proposed freeze desalination method shows great potential for commercialization.