The Effects Of Freeze-thaw Cycles On Physical And Chemical Properties And Adsorption Performance Of Biochar

Researchers have begun to focus on changes of physical and chemical properties of biochar in the environment while biochar was widely used in soil improvement, carbon sequestration, mitigation of climate change, environmental remediation and other aspects. The changes of physical and chemical properties of biochar in the environment would affect its environment function. In order to explore the changes of physical and chemical properties of biochar in the environment and analyze the mechanism and effects of environmental process on the physical and chemical properties and adsorption performance of biochar, in this study, biochar was produced at 200℃(P200),350℃(P350),500℃(P500) and 650℃(P650)using pine sawdust. A series of time gradient experiments of freeze-thaw cycles were conducted to simulate the changing process of physical and chemical properties of biochar and investigates a dynamic properties of the biochars from the angle of freeze-thaw cycles. The results of this study provide theoretical support to understand the assessment of function of biochar in environmental life cycle.Through the elemental analysis we found that freeze-thaw cycles could make the soluble mineral dissolve out from inside of the biochar. After 42 freeze-thaw cycles, the ash decreased from 5.81%,2.14%,7.27% and 5.22% to 0.37%,2.14%,0.34% and 5.22% for P200, P350, P500 and P650 respectively.The effects of freeze-thaw cycles on the specific surface area (BET sa) of biochars produced at different temperature were different. After 42 freeze-thaw cycles, the BET sa of P200 increased from 2.67 m2/g to 14.4 m2/g, the BET sa of P350 increased from 2.78 m2/g to 3.49 m2/g, the BET sa of P650 increased from 464 m2/g to 604m2/g, but the BET sa of P500 decreased from 101 m2/g to 67.4 m2/g.The dynamic changing processes on particle size of biochars produced at different temperature were different in freeze-thaw cycles. With increasing freeze-thaw cycles, the D50 of P200 increased from 230μm to 363μm., but the D50 of P350 deceased from 198μm to 167μm at the 21st freeze-thaw cycle then increased to 206μm at the 42nd cycle. The dynamic changing processes on D50 of P500 was similar to P350. The D50 of P650 increased from 142μm at the 21st cycle then returned to 142μm.Before and after freeze-thaw cycles, biochars’adsorption of naphthalene could fit with the Freundlich model very well. After freeze-thaw cycles, biochars’adsorption of naphthalene increased. The adsorption distribution coefficient (log Kd) (L/kg)of P200 increased from 3.24 to 3.34 at the 14th cycle, the log Kd of P350 increased from 3.44 to 3.60 at the same cycle, the log Kd of P500 increased from 3.63 to 3.76 at the 7th cycle and the log Kd of P650 increased from 4.63 to 4.73 at the 21st cycle. The correlation analysis indicated that the changes of physical and chemical properties of biochar, such as particle size, and the adsorption coefficient had a significant correlation in the process of freeze-thaw cycles. So we speculated that the longer and bigger freeze-thaw cycles in nature could be able to affect significantly the adsorption performance of biochar.