Study On The Preparation Of High-Ash Based Biochar, The Mechanism Of Controlling Cadium And Ecological Risk Assessment In Agricultural Application

Based on requirements of utilizing vermicompost in a safe way and the basic functions of biochar ash in fertilizer,soil amendment,and controlling heavy metals,valorizing vermicompost for producing high-ash based biochar was investigated in this paper.The achieved biochar was applied for agricultural system for potentially controlling the cadmium pollution from the aspects of aqueous solution,soil and plant.Correspondingly,the performances and basic mechanisms on controlling cadmium pollution from the aforementioned 3 aspects were elucidated.Meanwhile,the potential ecological risks of the agricultural application of this biochar was assessed according to the investigations of phytotoxicity.The achieved results were listed as follows:（1）Vermicompost was pyrolyzed at 300-700℃ for producing the high-ash based biochars to investigate the physical and chemical characteristics,including the content and constitutes of organic and inorganic moieties,specific surface area and pore structure,mineral composition,and chemical surface functional groups.Results indicated that biochar derived from vermicompost exhibits the characteristics of high yield（70.56%-90.12%）and high ash content（69.24%-78.98%）,and increasing pyrolysis temperatures can promote the ash content correspondingly.The derived biochars were characterized by high nutrients,cation exchange capacity,and high alkaline pH.The main composition of ash was K,Na,Ca,Mg,Fe,Mn,Cu,and Zn,which also was accumulated with increasing pyrolytic temperature,as well as the base function groups.Ash of the obtained biochar is mainly composed of quartz（SiO₂）,sylvite（KCl）,calcite（CaCO₃）and whitlockite（（Ca,Mg）₃（PO₄）₂）,and their content and composition were significantly affected by the pyrolytic temperature.In addition,when the pyrolysis temperature was in the range of 500-600℃,it can improve the pore structure,increase the specific surface area and promote the formation of schistose structure.However,extra high temperature（700 ℃）would slightly decreased the pore volume of biochar and the pore structure due to ash blocking on the surface of biochar.Moreover,H/C,（O+NN）/C and O/C gradually decreased with the increase of pyrolytic temperature,indicating the decreases of aliphatic functional group contents and the existences of short chains and non C-O branched chains structures in the biochar.Furthermore,increasing temperature will make carbonyl carbon,phenolics or phenolic ethers transfer to aromatic bridged carbon.Overall,the biochars prepared in high temperature had a complete and stable structure of aromatic carbon.（2）Based on the characteristics of this high-ash based biochar,it agricultural application to control the cadmium pollution from the aspect of aqueous solution was investigated thoroughly,and results indicated that the sorption process were better fitted by the pseudo-second-order kinetic mode,and the adsorption is controlled by chemical absorption,and the kinetics showed that the adsorption process is under the control of intra-particle diffusion and other diffusions simultaneously.The sorption of Cd（2+） by the biochar was affected significantly by pH,and increasing solution pH values within the range of 2.5-6.5 can promote the Cd（2+） sorption onto the biochars gradually.The optimum pH for Cd（2+） sorption onto the biochars was achieved at 5.5.Freundlich model can better describe the adsorption behaviors comparing with the Langmuir model,in which the nonlinear index（n）of all biochar was less than 1,indicating that biochar had great heterogeneity in the adsorlption.The biochar produced at 700℃ are more effective to adsorb Cd（2+） from aqueous solutions comparing with that of the other temperatures.The calculated thermodynamic parameter results suggested that the adsorption process is a spontaneous endothermic process,and high temperature for the aqueous solution can be beneficial to adsorption.Based on the adsorption behaviors of Cd（2+） on the biochar and the deashed biochars by acid treatment,it could be suggested the adsorption mechanisms for Cd（2+） on the biochars were the synergistic effects of cation exchanges,co-precipitation,oxygen-containing functional groups complexation,and cation-πinteraction.（3）According to the suggested mechanisms on Cd（2+） removal from aqueous solution,the Cd control for the aspect of soil was investigated via direct supplementing the biochar into soil to seek the possibility of blocking Cd pollution.Results indicated that the supplementation of this high-ash based biochar can greatly promote the pH,CEC,organic matter content and the available nutrients in the investigated soil,more they were all positively responded to the increases of supplementation dosage.The DTPA-Cd and TCLP-Cd in the soil can be greatly decreased with the add of this high-ash based biochar,suggesting the bioactivity of Cd to plant,and the leaching toxicity of Cd-polluted soil can be greatly mitigated.After the passivation for 90d with the biochar addition,the exchangeable cadmium decreased significantly,and most of them was transferred as the oxide bound and organic-bound Cd,even residual Cd in soil,by which the Cd activity in soil can be efficiently block via the suggested mechanisms of co-precipitation due to the pH enhancement,and the formed chain of cation exchanges by biochar followed by complexation of the contained functional groups.（4）On the basis of the existing mechanisms of Cd control in the aqueous solution and soil,it could be deduced that the soluble fraction in biochar may be responsible for deactivating Cd,therefore,the aqueous extractive from this biochar were employed to investigate the anti-stress of Brassica chinensis L.to Cd pollution.Results indicated that properly adding biochar aqueous extractive into cadmium solution could reduce the level of membrane lipid peroxidation and cell membrane permeability,and relieve the cadmium toxicity on Brassica chinensis L.seedling,especially at high Cd（2+）concentration.Moreover,the Cd contents in the underground part are notably higher than those of aboveground Brassica chinensis L.via adding proper biochar aqueous extractive,and the Cd contents of aboveground and underground part were reduced 25.4-32.5%and 14.3-24.1%respectively,indicating that Cd transportation from the roots to the shoots in plants could be weakened obviously.These results proved that the soluble fraction in this high-ash based biochar was the substantial substrates for controlling the Cd activity,in which the formed functions of precipitation and chelation with Cd（2+） were mainly responsible for weakening the Cd toxicity to plants,when the biochar was applied for agronomy.（5）Focusing on the biochar application in agriculture,the total content of heavy metals and their speciation,and PAH contents in the biochar were determined,as well as phytotoxicity test was performed on seedling of Brassica chinensis L.to clarify the potential ecological risks of the achieved biochar in agricultural application.Pyrolysis process can accumulate heavy metals in biochar,which affected the total amount of heavy metals,water soluble and DTPA extractable fractions.Variety and content of the detected PAHs in biochar were increased with the promoted pyrolytic temperature.According to the seedling test on Brassica chinensis L.seeds,the inhibition of biochar on the germination of Brassica chinensis L.was enhanced with the increase of pyrolysis temperature and the biochar dosage,which may be attributed to the heavy metals and PAHs in biochar.Furthermore,pyrolysis process can significantly affect the speciation and bioavailability of heavy metals,and further affect the direct toxicity and potential ecological risk of biochar.Therefore,controlling the input of Cd in precursors,selecting suitable pyrolysis parameters and retionalizing the input dosage of biochar can reduce the potential ecological risk of the derived biochar from vermicompost in agricultural application.