Removal Performances Of Lead By Rice Straw And Anaerobic Digestion Of Rice Straw Derived Biochar

Biochar is a solid product of biomass organic matter under low temperature and anaerobic conditions,which is mainly composed of amorphous carbon,aromatic carbon and ash.The comprehensive utilization of biochar can solve the complex problems faced by sustainable development,energy saving,environmental protection and governance and help to build a low-carbon and efficient economic development mode.In this paper,rice straw?RS?and rice straw anaerobic digest?DRS?as raw materials,with different pyrolysis temperature,pyrolysis time and heating rate derived rice straw biochar?RSBC?and anaerobic digestion rice straw biochar?DRSBC?,and measured the basic physicochemical characteristics.Then adsorb lead in solution with biochar,the influence of initial concentration,reaction time,pH and system temperature on the adsorption properties was considered.The potential mechanism of the adsorption process was analyzed by the kinetic model of adsorption,the isotherm model and the differences physicochemical properties of biochar before and after the adsorption of heavy metals.The results of this study show that:?1?The final residue of rice straw was significantly lower than that of the anaerobic digestion rice straw.The main mass loss stage of RS appears in the range of 200⁵00?,and the maximum weight loss peak at 340?,this is because the decomposition of hemicellulose,cellulose and lignin in lignocellulosic biomass.The main cleavage peak of RS temperature is lower than the peak value temperature of DRS cracking which indicates that rice straw is more easily pyrolysis than its anaerobic digestion,and its anaerobic digestion has higher thermal stability.The yield of RSBC and DRSBC decreased with the increasing of final pyrolysis temperature,constant temperature time and the heating rate,and the final pyrolysis temperature had the most influence on the biochar yield.All biochar appeared weightlessness peak at about100?,mainly due to the dehydration of free water and bound water.The weight loss of biochar was less obvious than that of the raw materials at 200⁴00?.The mass loss and weight loss rate of biochar were decreased at 400⁵50?with the increase of pyrolysis temperature,which is due to pyrolysis carbonization is not completed at low temperature?400 and 500??,cellulose and lignin further carbonization in thermogravimetric analysis caused some loss of mass.The biochar prepared at high temperature?600 and 700??have relatively completed pyrolysis and high thermal stability.Therefore,the biochar prepared at higher temperature have better stability on carbon fixation and returning to field.?2?The physical and chemical properties of biochar are closely related to the raw materials and the pyrolysis temperature.With the increase of pyrolysis temperature,the proportion of water and the volatile matter showed a downward trend,inversely the content of fixed carbon and ash increased.In addition,the ash content in DRSBC is 25.5-29.2%higher than the corresponding RSBC,which is similar to the biochar yield.The pH of biochar increased with the increasing of pyrolysis temperature,and the pH of DRSBC is lower than RSBC at the same pyrolysis temperature.The electrical conductivity increased with the increase of temperature,indicating that the content of soluble salt increased gradually.The specific surface area of biochar showed a rising trend,but the specific surface area of RSBC decreased at 700?.The ratio of H,O,H/C and O/C decreased with the increase of pyrolysis temperature,inversely the C content increased.The content of Na,Ca,Mg and P in RSBC is lower than the content of the corresponding elements in DRSBC,but the K content in DRSBC is significantly lower than that of RSBC.The content of mineral elements of K,Na,Ca and Mg is elevated,with the increase of temperature,indicating mineral elements are mainly in the form of inorganic matter in RS and DRS.The surface morphology of RSBC and DRSBC are quite different,RSBC have strip accumulation form and more porous surface,DRSBC are more compact and smooth,showing a plate structure.According to the FTIR spectra of RSBCs and DRSBCs,the band intensities at 3400-3430 cm^-1?-OH?,2850-2950 cm^-1?aliphatic CH2?,1630 and 1580 cm^-1?C=O?and 1430 and 1390 cm^-1?C=O?decreased when the pyrolysis temperature was increased because of the thermal destruction of cellulose,hydroxyl groups and aliphatic alkyl groups,and thermal destruction of aliphatic matter and hydroxyl groups.3 features at 1100,800 and 470cm^-1 are assigned to the vibration of Si-O-Si.The vibration intensity of the Si group in RSBC decreases with the increase of temperature,which means that high temperature is beneficial to the formation of silicon crystal.?3?With the increase of heating rate and constant temperature time,the adsorption of Pb?II?on biochar increased.However,with the increase of final pyrolysis temperature,the adsorption of Pb?II?on biochar increased first and then decreased.Higher system temperature and initial lead concentration are beneficial to the adsorption of lead by biochar.The adsorption of RS-500 on Pb?II?increased with the increasing of pH value,while the adsorption of DRS-500 on lead decreased.The adsorption capacity of RS-500 increased rapidly in the first 2 hours.After that,the adsorption gradually slowed down and reached the adsorption equilibrium at 12 hours.However,the adsorption of DRS-500 on Pb?II?was very slow and did not reach the adsorption equilibrium in 48 hours.Compared with the Freundlich model,the Langmuir model has a higher fitting degree,which shows that the adsorption enrichment process of Pb?II?by RS-500 and DRS-500 are mainly due to the single molecular layer.The maximum adsorption capacity of RS-500 and DRS-500 for Pb?II?by Langmuir model is 276.3 and 90.5 mg·g^-1 respectively.It can be known that the second order model give better simulation on both of RS-500 and DRS-500.Meanwhile,the equilibrium adsorption capacity of RS-500 and DRS-500 obtained by second order model is 240.10 and 124.84 mg·g^-1,which is closer to the actual value obtained from the experiment.The?G⁰ is negative,indicating that the adsorption process is spontaneous,and the adsorption process is easily carried out with the increase of temperature.Compared FT-IR and XRD before and after adsorption of Pb?II?found that the COO-vibration peaks changed,and diffraction peaks of?-Pb₉?PO₄?₆,PbO·P₂O₅·SiO₂ and Pb₃?CO₃?₂?OH?₂ increased,indicating that complexation,ion exchange and surface precipitation are the potential adsorption mechanism.