Effect And Mechanisms Of Stalk Biochar Addition On Cadmium Adsorption In Degraded Soils

As the most prominent soil metal contaminant in China,adsorption and desorption on soil are important behavior influencing the mobility of Cd and environmental safety.Biochar has been extensively proven to distinctively enhance the adsorption capacity of pollutants,which has good application prospects in reducing the bioavailability of soil pollutants.Soil pollution and degradation widely coexist,and the effect of biochar addition on adsorption behavior by degraded soils is unclear.In this dissertation,soil samples with different degraded levels?slightly degraded soil I:LDSI,slightly degraded soil II:LDSII,moderately degraded soil:MDS,severely degraded soil:SDS;GB19377-2003?were collected from Hongyuan County,Aba Tibetan,and Qiang Autonomous Prefecture of the Sichuan Province.As the controls of degraded soils,the Peat and nondegraded soil?NDS?were also collected.The influence of degradation on adsorption of Cd was studied.Three kinds of biochars?MS350,MS500,MS700?were prepared from maize stalk biomass?MS100?at350?,500?and 700?,respectively.The effects of carbonization temperature of biochar and environment temperature on adsorption capacity was discussed.Degraded soils were amended with biochar to illustrate the effect and mechanism of biochar addition on adsorption of Cd onto degraded soil.To further analyze the effect of temperature and freezing-thawing on degraded soil-biochar adsorption Cd.The main conclusions of this work are drawn as following:?1?The Cd adsorption mechanism onto degraded soils of the plateau was elucidated for better assessment of environmental risk on degraded soils.Adsorption of Cd by degraded soils reached equilibrium within 30 minutes.The adsorption isotherms of six tested soils on Cd can fit well with both the Langmuir and the Freundlich equation?R²?0.96?.With the degradation of soil,the nonlinearity increases.The maximum adsorption capacity?Q_m?of degraded soil remarkably decreased?4.1⁴4.3times?in comparison with undegraded soil.An excellent linear of correlation between Q_m and TOC is found?R²?0.89?,and this indicates that the organic matter is the most important medium.The Cd adsorption of soil is a endothermic reaction,along with the rise of temperature,the Cd adsorption capacity of soil increases gradually.?2?The adsorption characteristics and influencing factors of maize stalk biomass and biochar on Cd were elucidated.The time of reaching adsorption equilibrium?5 h?for three biochars?MS350,MS500,MS700?is longer than that for MS100?1.5 h?.Cd adsorption kinetic can fit better to the pseudo-second-order kinetic model on biochar?R²?0.90?.The adsorption capacity of MS100 increased rapidly after oxygen-limited pyrolysis.With the increase of pyrolysis temperature,the maximum adsorptioncapacity increased rapidly(24080?51251 mg·kg^-1).The Cd adsorption of MS100and biochar is a endothermic reaction,and the Cd adsorption capacity of biochar is less affected by temperature compared with MS100.?3?The Cd adsorption characteristics on degraded soils amended with biochar was illustrated,and the heavy metal adsorption model of soil-biochar system was perfected.The addition of biochar enhanced the Cd adsorption capacity of the degraded soils?20³28%?.The improving effect is stronger when increasing either the degradation level or the amount of added biochar?1?4%?,or the pyrolysis temperature of biochar?350?700??.The adsorption contribution mostly comes from biochar?39.9¹34.8%?in the soil-biochar system.Different from the synergistic enhancement of heavy metal adsorption onto soil amended with biochar,it is found that the adsorption of Cd was not enhanced but suppressed?7.1³6.6%?when biochar was incorporated with degraded soils.The adsorptivity attenuation degree was found to be negatively linear with TOC content in the degraded soil–biochar system?y=-0.517x+32.9,R²=0.89?,that is,the more serious the degradation,the stronger the inhibition effect.The Cd adsorption of soil-biochar is a endothermic reaction,along with the rise of temperature,the Cd adsorption capacity increases gradually.?4?The effects and mechanisms of freezing-thawing on Cd adsorption by degraded soil,biochar and soil-biochar were revealed.The nature of soil and biochar is remarkably changed after freezing-thawing,including destroyed agglomerating structure and particles,enhanced the number of low-energy adsorption sites.Therefore the Cd adsorption enhanced?1.1³28%?when the initial concentration of Cd is low(C_o:5 mg·L^-1,50 mg·L^-1).However,the adsorption decreased?5.6³7%?when the initial concentration of Cd is up to 100 mg·L^-1,duo to the loss of dissolved organic matter of sorbents.