Unsynchronized migrations of different salt ions and ice microstructure development during unidirectional freeze-thaw

Abstract

Brackish water columns with three initial salinity gradients were frozen at −15 °C and then thawed at 20 °C. Vertical distributions of six ions (Na+, K+, Ca2+, Mg2+, Cl− and SO42−) among five ice layers were compared after unidirectional freezing, and the temporal variations of ion concentrations at five melting stages were recorded during unidirectional thawing. Meanwhile, the evolvement of ice microstructure was also observed. Our results show that: (1) Driven by the rejection of ice crystals and gravity during unidirectional freezing, the total dissolved salts (TDS) of the bottom ice layer were approximately three times that of the top layer. (2) Specifically, 45 % of the total amount of Na+ and Cl− was observed at the bottom layer after unidirectional freezing, and the 30 % of Ca2+ appeared to concentrate in the middle layer. (3) During unidirectional thawing, the desalination rates reached over 80 % after 14 h. With greater initial salinity, the brine channel connectivity was more distinguished. (4) The percent proportions of Na+, K+, Cl− and SO42− showed two peaks during unidirectional melting, where 30 % to 50 % of individual total amount was discharged. The percent proportion of Ca2+ merely peaked once, with 25 % being discharged.