The Carbon Sequestration Potential And Cu(Ⅱ) Adsorption Properties Of Biochar Derived From Municipal Sludge In Mountainous Cities

Sludge pyrolysis preparation of biochar technology not only can significantly reduce the volume of sludge and completely kill pathogenic microorganisms,but also its solid product（biochar）has good application potential in the fields of soil heavy metal pollution remediation and enhancing soil carbon sequestration capacity.Therefore,in the context of carbon peaking and carbon neutrality,the preparation of biochar by sludge pyrolysis has received wide attention.However,due to the generally low organic matter content and high sand content of sludge from urban wastewater plants in China,the prepared sludge biochar exhibits the characteristics of high ash content and low organic group content,which restricts the improvement of the adsorption capacity of sludge biochar for heavy metal pollutants.This characteristic of sludge biochar is especially obvious in mountainous cities due to their special topography.Therefore,a systematic study of the adsorption characteristics of sludge biochar for heavy metals and its carbon sequestration effect in mountainous cities,the search for the optimal process parameters for the preparation of biochar by sludge pyrolysis in mountainous cities,and the development of technologies to enhance the adsorption capacity of sludge for heavy metals in mountainous cities can help expand the application of sludge pyrolysis technology in the field of sludge treatment and disposal in mountainous cities.Chongqing as a typical mountainous city,the sludge from Chongqing wastewater plant was used as raw material in this thesis,and Cu（Ⅱ）was selected as a typical heavy metal pollutant.By studying the adsorption performance of sludge biochar prepared at different pyrolysis temperatures on Cu（Ⅱ）and its carbon sequestration potential,the best pyrolysis temperature for preparing sludge biochar was searched;under the best pyrolysis temperature conditions,sludge was co-pyrolyzed with different agricultural and forestry waste biomass（rice husk,rice stalk,eucalyptus branches,eucalyptus leaves）.The synergistic effects of co-pyrolysis on the adsorption of Cu（Ⅱ）and its carbon sequestration potential of biochar were investigated,and the most favorable type of agroforestry waste to enhance the adsorption performance of sludge biochar on Cu（Ⅱ）was selected.The main studies and results are as follows:（1）After the preparation and characterization of two types of sludge biochar at different temperatures,the following patterns were found:with the increase of pyrolysis temperature,the yield of sludge biochar gradually decreased,and the specific surface area and ash content gradually increased;the aromaticity increased with the increase of temperature,and the polarity decreased with the increase of temperature;FTIR results showed that the absorption peaks of carboxyl and hydroxyl groups of stretching vibration gradually weakened after the increase of pyrolysis temperature.The XRD results showed that the content of SiO₂ was higher,and the XRF results of ash also showed that the minerals in sludge biochar were mainly SiO₂.（2）The best adsorption effect of sludge biochar on Cu（Ⅱ）was achieved at a pyrolysis temperature of 300℃,which also had a better carbon sequestration effect,and was the best temperature for the preparation of sludge biochar.According to the analysis of isothermal adsorption model,the single point adsorption coefficient K_d value of the two sludge biochar prepared at 300℃was the largest and had the best adsorption effect on Cu（Ⅱ）.The adsorption effect of sludge biochar on Cu（Ⅱ）increased with the increase of the pyrolysis temperature when the pyrolysis temperature was lower than 300℃,and decreased with the increase of the pyrolysis temperature when it was higher than 300℃.In addition,the fixed carbon yield(Y_FC)of sludge biochar was the highest and the carbon sequestration effect was the best at 300℃.（3）Sludge was mixed with four agricultural and forestry wastes at a feedstock mixing ratio of 1:1,and co-pyrolysis carbon was prepared and characterized at 300℃.It was found that the co-pyrolysis of sludge with biomass was beneficial for the retention of C,N,and O elements.After co-pyrolysis,the synergistic parameters of C,N,and O element retention rate were-5.29%,-1.19%,and 8.13%for rice stalk-sludge co-pyrolysis carbon（（RS+SS）B）,14.87%,8.16%,and 26.70%for rice husk-sludge co-pyrolysis carbon（（RH+SS）B）,eucalyptus branch-sludge co-pyrolysis carbon（（EB+SS）B）were6.27%,12.57%,and 27.36%,and the synergistic parameters of C,N,and O retention rate of eucalyptus leaf-sludge co-pyrolysis carbon（（EL+SS）B）were 12.09%,9.68%,and35.46%;respectively,FTIR results showed that the carboxyl and hydroxyl groups of co-pyrolysis carbon content increased,indicating that co-pyrolysis is beneficial to the retention of oxygen-containing functional groups.（4）According to the analysis of isothermal adsorption model,（EL+SS）B has the best adsorption effect on Cu（Ⅱ）,and its synergistic parameter for the maximum adsorption capacity Q_m of Cu（Ⅱ）is the largest,which is 101.27%;the synergistic parameters of（RS+SS）B,（RH+SS）B and（EB+SS）B for the maximum adsorption capacity Q_m of Cu（Ⅱ）are 74.93%,32.99%and 86.33%,indicating that co-pyrolysis greatly improved the adsorption performance of sludge biochar on Cu（Ⅱ）,but the effect of co-pyrolysis on the carbon sequestration effect was not obvious in terms of its synergistic parameters of Y_FC.In conclusion,co-pyrolysis of agricultural and forestry waste with mountainous urban sludge is an effective means to treat and dispose of sludge,and it can effectively improve the physicochemical properties and adsorption characteristics of sludge biochar and enhance its application potential.