Experimental Study On Cr³⁺ Removal From Saturated Sand By Artificial Freezing

At present,soil pollution,especially heavy metal pollution in China is very serious.How to efficiently and green control soil pollution has become a worldwide topic.This paper,inspired by the theory of water solution freezing and separation,proposes a new idea to expel heavy metal pollutants in saturated sand by artificial freezing technology.Under the progressive freezing method,heavy metal ions are repelled by the heavy metal ions to form clean ice when the pore water is frozen in the soil.At the same time,the ice volume of water is expanded to discharge the unfrozen water.Under the joint action of the two,the heavy metal ions in the soil are gradually expelled into the unfrozen soil to form a small area of high concentration soil,and the large range of frozen soil is replaced by pure ice.In view of this idea,this paper first introduces the theory of freeze concentration in aqueous solution.The freezing and expelling mechanism of heavy metal contaminants in saturated sand is analyzed,and three main factors that affect the effect of expulsion are obtained: freezing rate,initial concentration and diffusion coefficient.In order to get the range of effective freezing rate,the one-dimensional freezing test is carried out by self-developed test device.In order to eliminate the influence of soil particle adsorption on heavy metal ions,the glass beads with different diameters were used instead of sand.What's more,chromium ion is selected as the research object.The experimental results show that the displacement of chromium ions occurs just in the boundary line between frozen soil and unfrozen soil.In this location,the soil freezing is near steady state,and the freezing rate develops slowly.It is obtained that the freezing separation process of the pore water solution is very slow,and the fast freezing rate can not reach the purpose of displacement.Only when the freezing rate is less than 0.094 ?/h,the solute will be separated.The effects of different concentrations of chromium ions on the expulsion effect were studied.We have got the following results: with the increase of initial concentration,the chromium elements in the unit soil are also increased,but the purification rate decreases.Finally,the freezing rate is controlled by adjusting the temperature of the cold bath,so that the freezing rate is within the effective freezing rate.The experimental results showed that the purification rate of freezing expulsion reached 65.8%.In this paper,a preliminary experiment is carried out,hoping to provide some references for further research and engineering application.