The Effects Of Biochar On Cd And Pb Transfer In Soil-rice System

A report of the current statues of soil contamination in China was published which shown that about 16.1%of the samples analyzed exceed the environmental quality standard.For agricultural soils the percentage of exceedance is even greater at 19.4%.Contamination by heavy metals was the majority of the contaminated soil.Rice is the staple food of many Asian countries and the dominant food crop in the tropical and subtropical regions of southern china,where the soil is largely acidic.Evidence has shown that large areas of heavy metals-contaminated agricultural soil near mine excavation had been used for rice production,resulting in excessive heavy metals accumulation in rice grains.Recently,biochar has been used as a novel carbonaceous application not only to improve soil nutrition but also to adsorb Cd and Pb in polluted water or soil.Generally,biochar has a large surface area,active organic functional groups,micro-pore structure and alkaline properties.To get better understanding of the effects of biochar application on Cd and Pb transfer in soil-rice system,we conducted a series of experiments.The main results of this research were summarized as following:1.Biochars prepared from rice straw at 4 different temperatures(300,400,500,700?)were tested as potential sorbents for Cd and Pb in the single-metal and multi-metal system.The results indicated that the surface area,pH,total carbon content,total functional groups of rice straw biochars were increased with the increasing of pyrolytic temperature.Furthermore,the adsorption efficiency of biochars on Cd2+and Pb2+ were enhanced with increasing pyrolysis temperature.The adsorption capacity of Pb2+ by biochar was more than Cd2+of single-metal system.In case of Cd2+ and Pb2+ coexist in the solution,there was an apparent competition effect of Pb2+ during Cd2+ sorption,but the effect of Cd2+ on the Pb2+ sorption was negligible.Moreover,Pb-desorption is lower than Cd-desorption for the biochars at the same situation.It indicated that the adsorption of Pb2+ by the rice straw biochars is more stability than Cd2+.2.To determine the dynamic changes of soil properties with biochar addition under flooding conditions.There were two treatments per incubation,one was the control and the other was amendment with 5%biochar.The results showed that the application of biochar increased the pH and DOC content of soil,hence 5%biochar application decreased the Cd,Pb and Fe concentration in soil solution at the beginning of incubation.For the control treatment,Fe in soil solution was increased dramatically but the Cd concentration decreased with incubation time increasing due to the reduction of Eh.While biochar inhibited soil reduction reaction with the incubation time increasing,resulted that the concentration of Fe was kept at a low level(100 ug L-1)and Cd in soil solution was increased with biochar application.Furthermore,biochar increased Cl-in soil solution,which can enhance Cd in soil solution.Overall,Cd absorbed by biochar was released with increasing incubation time.3.To explore the effects of biochar application on Cd and Pb in soil-rice system.Three rice cultivars(Yiyou 673,Teyou 009 and Donglian 5)were subjected to a treatment with three levels of biochar amendments(0%,2.5%,and 5%w/w).The concentrations and species of Cd and Pb in soils and rice plants were analyzed.The results showed that biochar application increased soil pH,soil DOC concentration,while decreased the CaCl2-extraction-Cd and CaCl2-extraction-Pb concentration in soil.Biochar application transfer Cd and Pb from acid-soluble fraction to reducible fraction.The Pb-biochar complexes appeared in soil with 5%biochar application was determined using Pb L3-edge XANES.Biochar application inhibited the transfer of Pb from root to stem,due to the formation of organic-Pb in rice root analyzed by XANES.The distribution of Pb in stele and cortex of rice root was decreased with 5%biochar application.Thus the biochar application decreased Pb concentrations in rice shoots of the 3 cultivars.Biochar amendment decreased formation of Fe plaques on rice roots and weakened the Fe2+-Cd2+ competition at adsorption sites on the root surface,which caused easier Cd translocation from the cortex to the stele in the root and up to the shoot.With increasing biochar application,Cd concentrations in rice tissues of the 3 cultivars were increased.4.To explore the effects of salinity in biochar on the accumulation of Cd and Pb by rice plant,this study compared the effects of untreated-biochar and water-washed-biochar on the mobility of Cd and Pb in soil and rice.The results showed that untreated-biochar with a high concentration of EC,Cl-and SO42-,and increased the concentration of Cd in soil solution and in rice plants compared with the control.Four times water-washing decreased the contents of EC,soluble organic carbon,Cland SO42-in biochar.The water-washed-biochar decreased the concentration of Cd in soil solution and in rice plant compared with the control treatment.And the two kinds of biochar both decreased Pb concentrations in soil solutions and in rice plants.5.To explore remediation potential of the combinations biochar with inorganic amendments for Cd and Pb polluted soil.In this study,rice straw biochar,lime,hydroxyapatite and sodium silicate were applied to soil separately or combined.The results showed that the treatments of biochar+lime,biochar+hydroxyapatite and biochar+sodium silicate increased soil pH,soil organic matter,EC value and decreased the availability of Cd and Pb compared with the application of the amendments separately.The soluble Cl-concentration in soil was increased with the application of biochar,while was decreased by the application of combination treatments.The accumulation of Cd in rice was reduced by the combination treatments compared with biochar application.All of the amendments reduced Pb accumulation in brown rice compared with the control,and the combination of biochar and inorganic amendment application decreased Pb in brown rice compared with biochar application dependently.The combination of biochar and lime had a best effect to reduce Cd and Pb accumulated in brown rice compared with the three combination treatments.6.To explore the immobilization capacity of biochar,lime,hydroxyapatite and sodium silicate during the aging process.The four amendments were applied to soils for rice growing after three years incubation.The results shown that after three years incubation,the soil pH of lime treatment was improved but the soil pH of biochar,hydroxyapatite and sodium silicate treatment was reduced.Compared with three years before,lime and hydroxyapatite reduced DTPA-Cd and DTPA-Pb concentration in soil,biochar reduced DTPA-Pb concentration in soil,while sodium silicate improved DTPA-Cd and DTPA-Pb concentration in soil after 3 years incubation.Compare with the control,lime increased the proportion of Pb in residual fraction and hydroxyapatite increased the proportion of Cd and Pb in residual fraction.The concentration of Cd in rice shoot with different treatments was decreased with the order:sodium silicate>biochar>control>hydroxyapatite>lime.The concentration of Pb in rice shoot with different treatments was decreased with the order:control>sodium silicate>biochar>lime>hydroxyapatite.After three years incubation,the immobilization of Cd and Pb by hydroxyapatite and lime was still effective.