Preparation Of Poplar Sawdust Modified Biochar And Mitigating The Effects Of Chromium (Cr) On The Growth Of Pak Choi

At present,the agricultural arable lands in China are seriously polluted by heavy metals(Cd,Cr,Pb,As,etc.).It is a particular case in the arable lands and water bodies of mining areas in Hunan Province,where the content of chromium(Cr)in the Changsha-Zhuzhou-Xiangtan district region is more than 5 times higher than the background value.Because of the characteristics of easy absorption and wide transportation,the heavy metals are migrated and accumulated in vegetation,animal and human through food chains and webs from irrigation sewages,which may cause serious damages on the environments and lead detrimental threats to all life including mankind.Therefore,heavy metals pollution in irrigation water has received increasing attention in recent years in China,and it is urgent to solve this problem.In this thesis,poplar sawdust was used as raw materials to produce a biochar(BC)under specific nitrogen gas concentration and high temperature conditions.The BC was further modified by adding Fe2+to produce a modified biochar(MBC).The MB C structure was characterized by using scanning electron microscope(SEM),Fourier transform infrared spectrometer(FTIR),and X-ray electron spectroscopy(XPS)techniques,the absorption mechanisms of Cr with MBC were examined,and the optimal removal rate of Cr was determined.Through the experiments of adding both BC and MBC into the Cr-contaminated hydroponic environments grown with pakchoi(Brassica chinensis L.)seedlings,the influences of MBC addition on Cr removal and pakchoi biomass were examined;The effects of MBC addition on the changes of Cr in the aqueous solution and the migration of Cr among the pakchoi organs were analyzed;The physiological and biochemical responses of pakchoi seedlings to Cr pollution were investigated;and the efficiency of the MBC application in Cr environments were evaluated.The main results from this project are as follows:1.The poplar sawdust MBC product made under the conditions of 400℃ and 0.2mol/L Fe2+ had the highest adsorption rate for Cr in water,with a maximum removal rate of 99.83%.The highest adsorption capacity of the MBC and BC was 0.988 mg/g and 7.986 mg/g,respectively Both BC and MBC belong to monomolecular chemisorption.The adsorption mechanisms were related to the adsorption binding sites on the surface of MBC and special reactive functional groups of MBC,including physical adsorption,electrostatic interaction,ion exchange,and chemical coordination processes.2.The growth of pakchoi seedlings was promoted under the low concentration of Cr(≤2 mg/L),and the edible part can gain up to 105.32%.The high concentration of Cr(≥4 mg/L)had considerable negative impacts on physiological and biochemical activities of pakchoi seedlings,resulting in slow growths.The inhibitory effect was the strongest at the concentration of Cr(8 mg/L),where the biomass of pakchoi seedlings was reduced by 55.50%,with leave wilting,and even whole plant death.3.The application of both BC and MBC could alleviate the Cr stress on pak choi seedlings growth by enhancing and maintaining the net photosynthetic rate,stomatal conductance and cellular CO2 concentration.The above-and below-ground biomass of pakchoi seedlings were higher in the Cr-polluted solutions with both BC and MBC when compared to the control.Under the addition of MBC in the concentration of Cr(8 mg/L),the edible part of pakchoi increased by 20.57%and the net photosynthetic rate increased by 82.94%.In addition,the MBC had a stronger effect on mitigating Cr stress on pakchoi seedlings than the BC.4.MBC could quickly react with the Cr in water and reduced the Cr concentrations in the environments.The Cr content was further reduced within the plant by enzyme activity of the antioxidant systems of the vegetables.The Peroxidase(POD)reaction was the most sensitive one in pakchoi.With the addition of BC and MBC,the POD activity increased by 665.71%and 864.55%in the edible part of pakchoi,respectively.Under the condition of 1 mg/L Cr,the Cr content of the edible part of pakchoi decreased by 65.9%and 51.5%in MBC and BC additions,respectively,which were below the national edible standard.Under higher concentration of Cr pollution,the poplar sawdust MBC had a more obvious mitigation effect on the accumulation and transport of Crin pakchoi.