Effects Of Compound Biochars And Calcium Modified Straw Biochars On Cadmium Sorption And Cadmium Absorption In Chinese Cabbage

Cd could cause severe damage to human bones and kidneys through food chain because of its strong migration and crop enrichment ability.Therefore,how to improve the efficiency of controlling soil cadmium pollution and effectively guarantee crop yield and safety is the current research hotspot.Biochar has promising potential for passivating heavy metal,due to its generally high p H,well-developed pore structure,abundant oxygen-containing functional groups and mineral components.However,the large application amount and high cost of biochar are the bottleneck for its large-scale promotion and application.In order to improve the treatment efficiency of Cd-contaminated soil by straw biochar,this study developed a high-efficiency remediation material for Cd-contaminated soil by combining straw biochar with animal slaughter waste biochar and modification of biochar with calcium（Ca）.In this study,rape straw biochar,soybean straw biochar,animal slaughter waste biochar and Ca modified straw biochars were selected.Through the adsorption and desorption experiments of Cd in aqueous solution,the Cd adsorption characteristics of different raw material biochars and Ca-modified biochars were studied,and the contribution rates of different raw material biochars adsorption factors were compared.In addition,through the pot experiment of Chinese cabbage,we compared the effects of different raw material biochars and combined application and different raw material Ca-modified biochars on the Cd occurrence form of polluted soil and the influence of Chinese cabbage Cd absorption.The main results obtained in this study are as follows:1.Compared with 450℃soybean straw biochar（SSB）and 450℃rape straw biochar（RSB）,500℃animal slaughter waste biochar（SWB）had a faster Cd²⁺adsorption kinetic a higher Cd²⁺adsorption capacity.The maximum adsorption capacity of SSB for Cd²⁺was higher than that of RSB,and the desorption rate was lower than that of RSB.The kinetic adsorption process of SWB,SSB and RSB were more consistent with the pseudo second order kinetic model.The equilibrium sorption data of the three kinds of biochars all conformed to the Langmuir model.Among the adsorption capacity of SWB on Cd²⁺,mineral precipitation(Q_cmp)accounted for 76.1%,ion exchange(Q_cme)accounted for 22.1%.Q_cmp,organic complexation(Q_co)and Q_cme were the main mechanisms of SSB adsorption of Cd²⁺,accounting for 43.2%,25.9%and 21.1%,respectively.The adsorption of Cd²⁺by RSB was dominated by ion exchange,followed by mineral precipitation,with the contribution rates of43.0%and 31.7%,respectively.2.Ca-modified biochars（Ca-SSB and Ca-RSB）were prepared by co-pyrolysis of soybean straw and rape straw with Ca（OH）₂ at 450℃,respectively.Compared with the original biochar,Ca-modified biochars had greater adsorption capacities,faster adsorption kinetics and lower desorption efficiencies.In addition,the kinetic adsorption process of Ca-RSB and Ca-SSB was more in line with the pseudo second order kinetic equation.In Ca-SSB and Ca-RSB,Q_cmp and Q_cme accounted for 49.9%,18.5%and 43.0%and 60.3%of the total Cd adsorption,respectively.Ca（OH）₂ modification effectively enhanced the Q_cme value and decreased the Q_co value.In addition,Ca modification significantly increased the Q_cmp value of SSB,but significantly decreased the Q_cmp value of RSB.3.Straw biochars（SSB and RSB）as well as SWB were applied combined（straw biochar:SWB=4:1）,and two concentration gradients of at 5 t ha^-1（B1）and 10 t ha^-1（B2）were set to study the influence of Cd fraction in in-situ contaminated soil and Cd accumulation of Chinese cabbage.The results showed that the content of Cd in roots and aboveground of Chinese cabbage treated with biochars was significantly lower than that of CK.Cd content in roots under SSB2 and RSB2 treatments was significantly decreased by 33.4%and 20.5%compared with SWB2 treatments,respectively.In the biochar combination treatment,under the application level of 10 t ha^-1,compared with SSB2 treatment,the aboveground Cd content of the SSB and SWB combined treatment（SSWB2）was significantly reduced by36.1%,and the root Cd content,the root Cd enrichment factor（BCF）and the Cd migration coefficient（TF）were all lower than that of SSB2 treatment.The application of biochar significantly reduced the proportion of soil ion-exchange Cd（F2）and carbonate bound Cd（F3）.The application of straw biochar significantly increased the proportion of soil humic acid-bound Cd（F4）and the proportion of strong organic bound Cd（F6）.Compared with SWB2 and SSB2 treatments,soil F2 in RSB2 treatment decreased.Compared with the SSB2treatment,the soil F2 of the SSWB2 treatment was reduced by 11.1%,and the proportion of Fe-Mn bound Cd（F5）was significantly increased by 13.5%.Therefore,the combination of straw biochar and slaughter waste biochar can effectively reduce the bioavailability of Cd in polluted soil and inhibit the absorption and accumulation of Cd by Chinese cabbage.4.SSB,RSB,Ca-SSB2 and Ca-RSB were applied to the Cd contaminated soil,and two concentration gradients of at 5 t ha^-1（B1）and 10 t ha^-1（B2）were set.The application amount of Ca（OH）₂ was 0.5 t ha^-1,which was marked as S.The results showed that application of biochar significantly increased the biomass of Chinese cabbage by 74.6%～126.7%.Compared with S treatment,Ca-RSB2 and Ca-SSB2 treatments increased the biomass of Chinese cabbage by 176.7%and 186.4%,respectively.Ca-SSB1 and Ca-SSB2 treatments significantly reduced the Cd content in the aboveground by 16.8%and 24.9%compared with SSB1 and SSB2 treatments,respectively.The TF of Ca-SSB2 treatment was significantly reduced by 18.8%compared with SSB2 treatment.The root Cd content and BCF of S treatment were significantly lower than that of Ca-RSB2 treatment.The TF of Ca-SSB2treatment was significantly reduced by 20.5%compared with S treatment.Compared with CK,F2 and F3 of the soil treated by biochar were reduced,while the proportion of residual Cd（F7）was increased.The soil F2 of RSB2 treatment was lower than that of SSB2 treatment.Compared with SSB1 and SSB2 treatments,the soil F2 of Ca-SSB 1 and Ca-SSB2 treatments decreased by 4.9%and 6.2%,respectively.Compared with Ca-RSB2 and Ca-SSB2treatments,soil F3 of S treatment increased,and F5 decreased.