Effect Of Biochar On Bioavailability Of Cd And Pb In Paddy And Black Soil

With the growing problem of soil heavy metal pollution,developing economical and efficient methods to deal with it has become a hot topic today.Most of the biochars are derived from solid waste.With a large specific surface area and high porosity,biochar shows strong ion exchange capacities and good adsorption properties for heavy metals,thus it can be used to fix heavy metals in soil.To explore the effect of biochar properties,soil types and heavy metal properties on biochar’s ability to fix heavy metals in soils,2%,5%,and 10% coconut shells biochar,rice straw biochar and fruit charcoal were added to Cd and Pb contaminated paddy soil or black soil,respectively.The bioavailability of Pb and Cd was studied after 10 days,1 month and 5 months.Then,Chinese cabbage cultivation experiment was carried out using contaminated soil amended with biochar.The growth of Chinese cabbage and the enrichment and transport of heavy metals in the plant after a 35-day cultivation were discussed.The biochars were characterized by EDS,SEM and porous analysis and the properties of soils,such as pH,organic carbon content and cation exchange capacity were determined.The main results are as follows:(1)From activity coefficients,the fixation of Pb by the two soils is stronger than that of Cd,and the fixation of Pb by paddy soil is much higher than that of black soil.It is primarily due to the strong binding ability of Pb to soil clay minerals.The addition of biochar increases soil pH to vary degrees.All three biochars can reduce the activity coefficient of heavy metals in polluted soils,and the reduction is greater with the increase of biochar added and the extension of treat time.(2)Among the three kinds of biochar,the coconut shell biochar has the best fixation effect of heavy metals.This may be because the coconut shell biochar has a larger specific surface area and a greater total pore volume.Thus,it can contact with heavy metals better through more interactive sites.As a result,it is easier for the coconut shell biochar to fix heavy metals by adsorption.(3)The activity coefficient of Cd is of little difference in paddy soil and black soil,but after adding coconut shell biochar for 5 months,the activity coefficient of Cd in black soil decreases greater.For example,activity coefficient of Cd in black soil amended with 10% biochar decreases approximately 60% comparing with the blank control.The Pb activity coefficient in paddy soil decreases more than that of black soil,and Pb activity coefficient in paddy soil amended with 10% biochar decreases by about 75% comparing with the blank control.Beside high adsorption ability of biochar towards heavy metals,biochar makes the original soil fluffy,which helps more adsorption sites in the original soil to come out,thus,promotes the fixation of heavy metals with the original soil.The reduction rate of heavy metal activity coefficient is fast in the beginning and becomes slower gradually.After 5 months,the activity coefficient of heavy metals in low-polluted soil was smaller than that of highly polluted soil,indicating that the amount of heavy metals fixed by adding biochar was limited.For Cd,blank soil amended with 2% coconut shell biochar can play a good role in heavy metal fixation.Paddy soil has a better fixation effect for Pb and when amended with 10% coconut shell biochar,and Pb activity coefficient can reduce to about 3% after 5 months.(4)When plant Chinese cabbage in Pb and Cd contaminated soil,adding biochar can promote the growth of plants in the contaminated soil,especially rice straw biochar.The addition of biochar makes the accumulation of heavy metals in Chinese cabbage(especially in the aerial part)significantly lower comparing with samples without biochar.However,heavy metal accumulated in Chinese cabbage is not in good proportional to the heavy metal availability(determined by chemical extraction method)in biochar amended soil.Rice straw biochar and fruit charcoal reduce the heavy metal accumulation in the aerial part of Chinese cabbage in both soils,however,coconut shell biochar only reduces the heavy metal accumulation in the aerial part of Chinese cabbage in black soil.The addition of biochar,especially rice stem biochar,significantly reduces the transport coefficient of Cd in Chinese cabbage,but there is little effect on that of Pb.