Effect Of Cd On Pyrolysis Mechanism Of Rice Straw And The Derived Biochar’s Adsorption Characteristics

Heavy metal-contaminated biomass（HMCB）is produced from the production or phytoremediation of heavy metal-contaminated soil,such as the Cd-contaminated rice straw produced from the Cd-contaminated paddy soil.Pyrolysis is a suitable method to dispose of HMCB and is widely concerned with its advantages including reduction,harmlessness,and resource utilization.However,the effect of Cd on biomass components pyrolysis is slightly studied.In this study,three representative biomass components,namely cellulose（CE）,hemicellulose（HE）,and lignin（LG）,and Cd-contaminated rice straw biomass were chosen to study the influencing mechanism of Cd on their pyrolysis characteristics.The rice straw biochar’s properties and the accumulation characteristics of heavy metals were also studied under different atmosphere conditions.The feasibility of heavy metal-containing rice straw biochar for adsorption and immobilization of Cd and Pb in industrial mining area soil was explored.The modification of rice straw biochar by regulating lignin content was studied to increase the adsorption capacity of Cd.The main results are as follows:（1）Cd has a significant effect on the pyrolysis behavior of the single biomass component.Cd retards the pyrolysis velocity and deoxygenation behavior of three biomass components,showing that Cd inhibits the pyrolysis of three biomass components.With a heating rate of 5°C·min^-1and a Cd loading of 5%（wt%）,the initial pyrolysis temperature of CE and HE advances from 282°C and 200°C to 267°C and 185°C,respectively,while that of LG delays from 151°C to 166°C.Meanwhile,the maximum pyrolysis velocity of CE,HE,and LG decreases by 36.6%,12.4%,and15.2%,respectively,and their devolatilization index is decreased by 58.6%,57.1%,and 66.2%,respectively.Cd increases the apparent pyrolysis activation energy of CE,HE,and LG,and inhibits their deoxygenation reactions,resulting in the increase of oxygen content of pyrolysis residue by 16.8%,16.5%,and 1.7%,respectively.（2）The mixed pyrolysis of the three biomass components is inhibited and the coupled pyrolysis effect among the three components is changed significantly by Cd.With the Cd loading of 5%and the heating rate of5°C·min^-1,the maximum pyrolysis velocity of the mixed pyrolysis of CE/HE,CE/LG,HE/LG,and CE/HE/LG was reduced by 33.4%,5.5%,7.3%,and 14.2%,respectively.Cd induces the coupled effect between CE and HE,resulting in a significant decrease in the pyrolysis velocity of both.Cd reverses the inhibitory effect of LG on CE,which significantly increases the pyrolysis velocity of CE.Cd weakens the promoting effect of LG on HE,resulting in a substantial decrease in the pyrolysis velocity of HE.For the four mixed modes of the three components,Cd increases the apparent pyrolysis activation energy of the whole or partial pyrolysis process and inhibits their deoxidation reaction.Especially for the mixed pyrolysis containing LG,Cd increases the oxygen content of pyrolysis residue by 23.4%-48.5%.（3）The pyrolysis characteristics and product properties of Cd-contaminated rice straw were clarified.Cd contamination has a slight effect on the pyrolysis carbonization of rice straw biomass,however,the yield of pyrolysis gas and oxygen-containing volatile organic compounds are reduced,which is conducive to obtaining more solid products with high oxygen content.CE is the main component of rice straw biomass,followed by HE and LG.Cd promotes the separation of the pyrolysis overlapping interval between CE and HE.The initial depolymerization temperature and pyrolysis velocity of HE is reduced while that of CE is increased.With a heating rate of 5°C·min^-1,when the Cd loading in rice straw biomass is amplified to 0.1%,1%,and 5%,the maximum pyrolysis velocity of CE is increased by 8.4%,62.1%,and 97.3%,respectively,while that of HE decreases by 7.2%,10.5%and 21.3%,respectively.Meanwhile,the average APAE is increased by about 10.9%,33.3%,and 36.3%,respectively.With the increase of Cd loading in rice straw,both the yield of gaseous products such as CO₂ and CH₄,and the ratio of acids as well as esters in the pyrolysis products are reduced,resulting in more oxygen-containing organic matter in biochars.（4）The effects of pyrolysis atmosphere on the component properties and heavy metals accumulation characteristics of Cd-contaminated rice straw biochar were revealed.Low-oxygen pyrolysis can effectively produce biochar using contaminated rice straw and improve the stability of heavy metals in biochar.Under the nitrogen atmosphere,the yield of rice straw biochar is 29.4%-34.9%.The aromatization index(SUVA₂₅₄)of dissolved organic matter（DOM）increases firstly and then decreases with the increase of pyrolysis temperature while the fluorescent components are mainly humic-like acid substances.Compared to the pure nitrogen condition,the biochar yield is reduced by 5.6%-13.5%and 14.9%-15.7%under the pyrolysis atmosphere containing 10%and 20%oxygen content,respectively.The oxygen content of the pyrolysis atmosphere accelerates the degradation of the lignin component,resulting in the gradual decrease of SUVA₂₅₄ of DOM.With the increase of oxygen content in the pyrolysis atmosphere,more humic-like acid substances in DOM are transformed into fulvic-like acid substances.Under the conditions of 400°C and a 10%oxygen-containing atmosphere,the exchangeable fractions of Cu,Cd,Pb,Ni,and As in biochar are decreased by 5.2%,3.7%,1.7%,0.8%,and 0.7%,respectively,indicating that heavy metals are transformed into more stable fractions.The results indicated that the contaminated rice straw biomass can be treated by low-oxygen pyrolysis to prepare biochar to meet the goal of harmlessness and recycling.（5）The adsorption/immobilization capacity of contaminated rice straw biochar for Cd and Pb was studied.After pyrolysis treatment,Cu,Pb,and Ni in rice straw biomass are mainly concentrated in the biochar,while Cd mainly enters the gas phase.The fraction of exchangeable states of all heavy metals can be reduced,and the pyrolysis temperature above 500°C is beneficial to the stabilization of heavy metals.With the pyrolysis temperature of 700°C for 3 h,the adsorption capacity of rice straw biochar pyrolyzed for Cd and Pb are 72.58 mg·g^-1 and 222.57 mg·g^-1,respectively,and their adsorption process conforms to the pseudo-second-order kinetic model and Langmuir model.Meanwhile,the rice straw biochar prepared at700°C can reduce the vertical migration of Cd and Pb in the soil under acid rainfall conditions while the residual fraction of Cd and Pb is increased from 21.0%and 39.7%to 26.8%and 44.0%,respectively.The results indicate that biochar containing heavy metals can be effectively used for the remediation of Cd-and Pb-contaminated soil in industrial mining areas.（6）The adsorption characteristics of Cd on the co-pyrolysis biochar from lignin and rice straw were studied.The increased yield,oxygen content,specific surface area,and the decreased p H value of rice straw biochar can be achieved by increasing the ratio of the lignin component.The surface of lignin-modified rice straw biochar is loose and porous with amounts of needle-like/filamentous structure,which provides more adsorption sites for pollutants to be adsorbed on its surface.With the pyrolysis temperature of 700°C for 3 h and lignin addition of 10%,the adsorption capacity of modified rice straw biochar for Cd is 109.9-118.0mg·g^-1,which is higher than that of unmodified biochar by 51.4%and conforms to the pseudo-second-order kinetic model and the Freundlich model.The results indicate that the properties and structure of rice straw biochar can be improved by regulating lignin content,which obtained higher biochar yield and larger adsorption capacity,providing a new idea for preparing biochar with stronger adsorption capacity by co-pyrolysis of woody biomass and herbaceous biomass.In all,the pyrolysis mechanism of Cd on the three representative biomass components and rice straw biomass was preliminarily clarified.The stability characteristics of heavy metals in contaminated rice straw biochar and its adsorption/immobilization capacity for Cd and Pb were studied.The results can provide scientific references and technical prototypes for the harmlessness and recycling of Cd-contaminated rice straw biomass.