The Efficiency And Mechanism Of As And Cd Immobilization By Fe-impregnated Biochar And CaO In Paddy Soil

Soil is the basis for human survival and agricultural development.Rice is the major food crop in China.With the rapid development of industry,agriculture and mining,soil pollution,especially in paddy soil,is related to national food safety and social sustainable development.Hunan Province is a "land of non-ferrous metal".The pollution of arsenic(As)and cadmium(Cd)in mining and industrial areas are relatively serious.In this paper,the As-contaminated soil was collected from Cili County,Hunan Provine.The characteristics of rhizosphere porewater solubilization and the concentrations of heavy metals in various tissues during ripening were analyzed.Besides,combined with micro X-ray fluorescence technology,different Fe-impregnated biochar(FeBC)treatments was setted up to conduct pot experiments,studied the effect and risk of FeBC on the passivation in paddy soil.In order to study the effect of CaO on the bioavailability of Cd in paddy soil,the Cd contaminated paddy soil were cultivated during the whole growth period,and the Cd content of brown rice at maturity was measured.At the same time,the CaO amendment soil co-cultivation experiment was carried out on the history and newly added Cd-contaminated paddy soil.At the end of flooding,the other half of each soil was used to grow rice seedlings in a light incubator,after 15 days of growth,the concentration of rice seedlings were measured.In addition,in order to study the effect of CaO on soil Cd solubility with acid input,acid rain simulation experiments and HCl callback experiments were performed.The main results are as follows:(1)The FeBC treatment increased the amount of rhizosphere porewater As,Fe,and the As,Fe concentrations of soil porewater decreased gradually during the whole growth period.When rice was sown for more than 35 days,As and Fe concentrations in the porewater was increased with the FeBC addition amount increased,and the increase range was consistent with the addition amount of FeBC.(2)The absorption and transport of As on iron plaque vary with the growth time of rice.During the tillering stages,the iron plaque and white roots of the root surface decreased with the increase of FeBC.At the maturity stages,FeBC-treated iron plaque and the white root increased slightly than control.(3)FeBC treatment promotes the absorption and the accumulation of As in brown rice.After rice was harvested,the brown rice of As concentrations was measured and micro X-ray fluorescence analysis was also performed.The 0.5-2%FeBC-treated brown rice increased by 7.75-65.24%compared with the control.The results of micro X-ray fluorescence analysis showed that the brown rice of As concentration with FeBC was higher than CK,which indicates that FeBC was not conducive to reduce As accumulation in brown rice.(4)The addition of CaO can reduce Cd bioavailability and inhibit Cd transport in rice and the accumulation of Cd in brown rice.The Cd concentration translocation factor from root to stem and brown rice decreased with the CaO treatment.Compared with control,CaO amendment at CaO:soil ratios of 0.05 and 0.1%reduced brown rice Cd by 40.9%and 61.8%,respectively;application of CaO higher than 0.1%hardly had any further decrease in brown rice Cd.The experiment proved that the CaO amendment drastically reduced Cd bioavailability in paddy soil.(5)CaCl2 extraction was closely related to Cd bioavailability.The CaCl2,SPLP and the water extracted from the rhizosphere soil of the late flooding stage and maturity stage.The results of Pearson correlation and PCA principal components analysis showed that there was a significant positive correlation between CaCl2-Cd and brown rice-Cd,stem-Cd and white root-Cd(p<0.01),and significantly negative correlation with DCB-Cd(p<0.01).(6)Effect of CaO on Cd re-solubilization in paddy soil with strong acid input and aggravates Cd pollution in paddy soil.In order to study the possible risk of deactivation of Cd by CaO amendment and simulate the dissolution characteristics of Cd under acid input conditions,soil column leaching test and HCl callback experiments were set up to measure the contents of Cd in the leaching solution and supernatant.The results show that CaO treatment can still reduce the bioavailability of Cd under short-term acid input conditions(column leaching test).The pH value of the solution is kept between 7.0 and 7.5,and the concentration of Cd is low;when the acid is strong(HCl recall test),CaO treatment promoted the amount of Cd released from the soil,and the Cd concentration of each treatment was 1.3-6.5 times of CK.