Preparation And Modification Of Biochar From Phytoremediation Residue And Its Remediation Of Cd Polluted Farmland Soil

According to the Report on the national soil contamination survey on April17,2014,the over-standard rate of cultivated land in China is as high as 19.4%,among which cadmium(Cd)pollution is the most serious(7.0%)exceeding the limit.Phytoremediation is generally considered to be a low-input technology.With the popularity of the application of phytoremediation technology,the safe disposal and resource utilization of a large number of plant harvests after restoration has become a difficult problem..Because of the advantages of small space occupation and high volume reduction rate,Preparation of biochar from phytoremediation plants has attracted considerable interest.Using biochar to repair heavy metal contaminated farmland soil is considered to be an effective and low-cost method for heavy metal cadmium due to its relatively short period,fast effect and stable remediation effect.In this paper,phytoremediation plants Brassica napus L.,Lolium perenne L.,Pennisetum sinesec.and Oryza sativa.straw were as raw materials to prepare biochar.The transport and transformation rules of heavy metals and their safe utilization in the pyrolysis process of biochar were studied,and the effects of biochar on the remediation of heavy metal cadmium contaminated farmland soil were studied through laboratory soil culture experiment and pot experiment.The main work and conclusions were as follows:(1)Phytoremediation plants Brassica napus L.,Lolium perenne L.,Pennisetum sinesec.and Oryza sativa..straw were used as raw materials.Using a three-stage reaction vacuum furnace atmosphere to prepare biochar.The effects of pyrolysis temperature on the products distribution,the transport of Cd,the behavior and chemical forms of Cd in biochar were studied.Moreover,the potential risk of Cd leaching from biochar into the environment and the environmental acceptability of the biochar were also evaluated.EDS elemental analysis,BET,FTIR,SEM and other characterization methods were used to characterize the basic physical and chemical properties of biochar,so as to optimize a pyrolysis temperature scheme.The results showed that pyrolysis was an effective method to treat phytoremediation plant residues,and pyrolysis could safely treat phytoremediation plant wastes,so that the heavy metal Cd could be stabilized in biochar.Increasing pyrolysis temperature(300～700℃)were favorable for the formation of biochar pore structure,it promoted the aromatization of biochar and carbon matrix in the stability of Cd,and transferred concentration of Cd in phytoremediation plants into oxidation state(F3)and residue(F4).Cd behavior analysis and potential environmental risk assessment showed that biochar prepared by the phytoremediation plants under high temperature(at 600℃ or above)was environmental acceptable,and was likely to have further application potential.(2)The selected biochar BNBC,LPBC and OSBC were taken as the research object.The biochar was modified by Fe salt,UV-irradiation and the combination of the two methods.Heavy cadmium contaminated farmland soil was collected from a city in Henan province.Laboratory soil culture experiments were carried out to study the effects of different types of biochar,biochar modified in different ways and biochar rates on the pH value,EC value,bioavailability and morphology of cadmium contaminated farmland soil.The results showed that different biochar could increase the pH value and EC value of soil and reduce the content of bioavailable cadmium in soil.When the biochar content of LPBC-UV was 6g/kg(LPBC-UV-2),the bioavailable content of cadmium in soil decreased the most compared with CK,which was 79.2%,and all the treatments reached a significant level compared with the control.There was a good correlation between soil pH value and available Cd content after different biochar treatment.Under UV-modified and unmodified treatments,there was a significant negative correlation between soil pH value and bioavailable Cd content.With the increase of UV-modified biochar,the ability to convert weak acid extracted and reducible Cd into oxidizable state was enhanced,but the residual Cd which could not be bioutilized remained basically unchanged.The Cd remediation effect was significantly improved after the bochars were modified by UV-irradiation,and the form of Cd in soil could be transformed from weak acid extraction state and reducible state to oxidizable state,that is to say,the form of Cd changed to more stable transformation.The LPBC-UV had the best effect on the stabilization of Cd.UV-irradiation modified biochar had the best passivation effect on Cd in soil of the three modification methods.(3)The selected modified biochar BNBC-UV,LPBC-UV,OSBC-UV were taken as the research object.Heavy cadmium contaminated farmland soil was collected from a city in Henan province.Coriandrum sativum L was planted and a two-month pot experiment was conducted to study the effects of different types of biochar and biochar rates on the soil pH value,EC value,available Cd,the biomass of Coriandrum sativum L and the Cd content of different parts of Coriandrum sativum L.The results showed that the biochar can significantly improve the soil pH and EC value,reduce the bioavailable Cd content in soil,and with the increase of adding amount,the bioavailable Cd content was decreased.When the biochar content of LPBC-UV was 6g/kg(LPBC-UV-2),the bioavailable content of cadmium in soil decreased the most compared with CK,which was 77.2%.In addition,biochar could promote the growth and moisture content of Coriandrum sativum L.Compared with CK,the treatment of UV-modified Oryza sativa.straw biochar with an added amount of 6g/kg(OSBC-UV-6)increased the biomass and moisture content of Coriandrum sativum L by 46.7% and 5.7%,respectively,with the largest increase.Moreover,biochar also could reduce the uptake of plant to Cd in soil,with LPBC-UV-6 having the lowest Cd concentration in the shoot and root,which was reduced by 92.0% and 73.4% compared with CK.at the same time,the BCF and TF in shoot and root parts of Coriandrum sativum L decreased with biochar rates.The correlation analysis of soil pH value,EC value,available Cd in soil and the Cd contents in in shoot and root parts showed that there was a good correlation among the five indexes.In general,UV-modified Lolium perenne L.biochar(LPBC-UV)has the best remediation effect on Cd polluted farmland soil.In summary,pyrolysis could safely dispose phytoremediation plants,biochars pyrolyzed at the temperature 600 ℃ or above might be environmental acceptable,which is beneficial for biochar application.The main effects of remediation were as follows: increase of soil pH and EC value;decrease of bioavailability of cadmium in soil;stabilization of cadmium species in soil;increase of biomass and moisture content of plants and decrease of Cd enrichment in plants.This study was of great significance for the safe utilization and resourceful treatment of biochar produced from phytoremediation plant.