Production Of Biochar From Urban Greenwaste And Its Efficacy In Adsorption Of Pb²⁺ In Aqueous Solution

The improper disposal of garden wastes can result in not only nutrient loss,but also environmental pollution.In order to investigate the effect of pyrolysis temperatures（350,500 and650℃）on the yield and physicochemical properties of biochars,we evaluated behavior of typical green wastes from tree species commonly planted in urban areas in southern China.Woody green wastes derived from Platanus orientalis,Osmanthus fragrans,Photiniax fraseri,Cinnamomum camphora.Heavy metal pollution has been a serious environmental problem in China recently.As a adsorbent for contaminants,biochar has become a hot spot in current research.Therefore,leaf and branch biochars（Platanus orientalis）produced at 500℃ were used to investigate the adsorption characteristics of Pb²⁺in solution.The main results are as following:1.Effect of pyrolysis temperatures on the the physicochemical properties of biochars:Biochar yield decreased with the increase of pyrolysis temperature,especially in the range of 350-500℃.With the increase of pyrolysis temperature,the C content（total carbon）in biochar increased,but the contents of O and H in biochar decreased.Temperature increase also resulted in that the types and quantities of functional groups on the surface,and the hydrophilicity and polarity of biochars decreased,and the aromaticity of biochars increased.The ash contents and fixed carbon in biochar increased,whereas volatile matter decreased,with the increase in pyrolysis temperature.The pH values of biochars increased with temperature,however there was no significant difference in pH of the biochars produced at 500 and 650℃.2.Effect of feedstock materials on the physicochemical properties of biochars:For a certain tree species under the same pyrolysis temperature,yield of the leaf biochar was higher than the branch biochar.The leaf biochars yield of Platanus orientalis was significantly higher（P<0.05）than those of other species.The ash contents of biomass materials had a remarkable effect on biochar yields.Ash content,volatile matter and fixed carbon content of leaf biochars were higher than the branch biochars.The hydrophilicity and polarity of the leaf biochars were stronger than the branch biochars.The aroma of leaf biochars produced at lower temperatures（350,500℃）were stronger than branch biochars.The pH of leaf biochars（Photiniax fraseri）was higher in all pyrolysis temperatures.The EC of leaf biochars were higher than the branch biochars.The total nitrogen（N）and sulphur（S）content of biochars derived from leaves were higher than that of the branch biochars.Scanning enectron microscopy and X-ray diffraction analyses showed that the Platanus orientalis and Cinnamomum camphora branch biochars had higher porosity,and the elemental composition of leaf biochar was richer than branch biochar.Additionally,the Photiniax fraseri leaf biochars had higher concentrations of P,Mg,K and Ca when they are compared with other biochars.3.Adsorption of Pb²⁺on biochars in aqueous solution:The removal rates of Pb²⁺from solution reached the maximum when the initial solution pH was 5.The sorption kinetics of Pb²⁺on biochar particles followed pseudo second order kinetic model,isothermal adsorption of Pb²⁺by fy（the leaf biochar of Platanus orientalis）and fz（the branch biochar of Platanus orientalis）fitted well with Langmuir and Freundlich equations.The adsorption was mainly composed of chemical adsorption of monolayer and multilayer adsorption.The adsorption process was controlled by intraparticle diffusion and diffusion of external liquid film.The removal efficiency of leaf biochar on Pb²⁺was better than branch biochar,because of the higher carbonate,phosphate,other inorganic mineral components which promote chemical precipitation in the leaf biochar treatment.In conclusion,raw biomass materials and pyrolysis temperatures had remarkable effect on the characteristics of biochars.The adsorption characteristics of biochars are closely related to its mineral composition.These results can provide useful references for the utilization of woody green wastes through pyrolysis in a commercial scale.Moreover,the present study also had practical implications for using biochar for remediation of heavy metal-polluted water.