Natural freezing has the possible use to be used as a purification technique to treat wastewater. Natural freezing was simulated experimentally for ice crystallization from unsaturated Na2SO4 (aq) solutions to assess the feasibility of such a technique for being used to purify wastewater. The influence of solution concentration and different growth conditions, such as ambient temperature, freezing time and freezing rate, on the efficiency of the purification process was investigated by analysis of the effective distribution coefficient (K) of the solute between the ice and the solution. A calculation method is introduced to estimate the concentration of solution at the advancing ice solution interface in terms of the limiting distribution coefficient (K*) from experimental K values at different growth conditions. The heat transfer controlled growth rate of the ice limited by the free convective heat transfer coefficient of air (h(sot)) rather than the thermal conductivity of the ice (k(ice)) and the heat transfer coefficient of the solution (h(sot)) found to prevail over the mass transfer of rejected solute molecules horn the ice-solution interface to the bulk solution. Advancement of the thermal (ST) and concentration boundary layer (delta c) at the ice-solution interface was proportional to the square root of freezing time. A simplified model is developed to estimate the thickness of the ice layer formed horn solutions at different freezing conditions and the model is validated with experimental results. (C) 2015 Elsevier Ltd. All rights reserved.