Water Immersion-freeze-thaw Cycle And Multi-process Coupling Of Modified Activated Carbon For The Preparation Of Highly Efficient Radon Absorbing Materials

Radon(²²²Rn)is a naturally occurring radioactive inert gas that is widely found in rocks,soil,groundwater and other places.Radon and its daughters are radioactive in the decay process and will cause radiation damage to human cells.The World Health Organisation（WHO）study says that radon and its daughters are the second major cause of lung cancer in humans.Therefore,radon reduction treatment is needed for places with high radon levels to protect human life and health.Common means of radon reduction include radon removal by adsorption and filtration,radon reduction by ventilation and radon blocking by wall coatings.In some places where radon level is high and it is not easy to ventilate or paint the walls,such as mine tunnels,underground facilities,etc.,using high efficiency adsorption materials to adsorb and remove radon is a very effective means,while activated carbon has a developed pore structure,high specific surface area,low cost and easy to mass produce.Coconut shell activated carbon has a good adsorption effect on radon,but the volume of radon-containing air adsorbed and treated per unit mass of coconut shell activated carbon is limited.It is very necessary to further improve its radon absorption capacity so that the limited mass of activated carbon can adsorb and remove more radon gas.In this paper,the water immersion-freeze-thaw cycle modification method and the multi-process coupling modification method are used as the means to modify activated carbon.The effects of the number of freeze-thaw cycles and the temperature of the freeze-thaw cycles on the modified activated carbon are investigated.The coupling modification between two of the three methods,namely water immersion-freeze-thaw cycle modification,liquid nitrogen immersion-evaporation modification and microwave modification,and the effects of the coupling modification of the three methods on the radon adsorption coefficient of the activated carbon are investigated.The effects of the type,quantity and order of the coupled modification methods on the activated carbon were investigated and compared with the single method modification.The results show that both water immersion and freeze-thaw cycle modifications and multi-process coupling modifications can effectively enhance the radon adsorption capacity of activated carbon.Under the freezing temperature of 20℃and the thawing temperature of 50℃,the best radon absorption capacity was achieved after 4 times of water immersion and freeze-thaw cycle modification of activated carbon,with a radon adsorption coefficient of 5.25±0.06 L·g^-1,which was 20%higher than that of raw activated carbon;under the freezing condition of-80℃,the radon adsorption coefficient of activated carbon was 5.42±0.06 L·g^-1after 2 times of modification,which was 24%higher;Under the freezing condition of-196℃,the radon adsorption coefficient of activated carbon was 5.31±0.06 L·g^-1after3 times of modification,which was 24%higher.Changing the ice thaw temperature had no effect on the activated carbon modification.The adsorption performance of activated carbon modified by water immersion and freeze-thaw cycles remained somewhat stable after 5 repeated uses.The apparent morphology of the activated carbon after water immersion and freeze-thaw cycling modification changed significantly,with cracks and new pore structures being produced.The pore volume and specific surface area of micro-pores increased significantly and showed a positive correlation with the radon adsorption capacity.Among the two coupling modification methods,liquid nitrogen-freeze-thaw coupling has the best effect,and the radon adsorption coefficient of activated carbon is 6.22±0.11 L·g^-1,which is 43%higher than that of the raw material;among the three coupling modification methods,liquid nitrogen-freeze-thaw cycle-microwave coupling has the best effect,and the radon adsorption coefficient of activated carbon is 6.35.The radon adsorption coefficient was 6.35±0.13 L·g^-1,an improvement of 46%.The multi-process coupled modified activated carbon has a slightly reduced and stable radon adsorption coefficient after repeated use.Water immersion and freeze-thaw cycle modification is an innovative,environmentally friendly and low-cost method to enhance the radon adsorption capacity of activated carbon,providing a new way of thinking for the modification of activated carbon.The multi-process coupling modification can solve the problem of limited capacity enhancement by a single modification method,further enhance the radon adsorption capacity of activated carbon,and broaden the ideas for improving the adsorption capacity of activated carbon.