Study On The Preparation Of Sewage Sludge-derived Biochar And Its Adsorption Capacity For Typical Pollutants In Water

Sewage sludge-derived biochar（SDB）,which has the advantages of abundant pore structure and good stability,has the application potential as a good adsorbent material.However,the harm of heavy metals is one of the key points that can not be ignored to limit biochar resource utilization.Therefore,the purpose of this paper is to reduce the ecological risk of heavy metals in sewage sludge（SS）before resource utilization of SDB.Biochars are obtained by pyrolysis carbonization technology,the effects of pyrolysis temperature,biomass species and blending ratio on physicochemical properties,changes in the total concentrations of heavy metals,morphological composition,and leaching rate of biochar were studied.And further explore the migration and transformation characteristics and fixation mechanism of heavy metals and evaluate the ecological risk of biochar resource utilization based on the results of the ecological risk assessment of heavy metals.Through the ammonium adsorption experiment,the effects of different adsorption conditions on ammonium adsorption characteristics are further studied.And the application potential of SDB as ammonium adsorbent is evaluated.Finally,the prospect of SDB resource utilization in the agricultural field is discussed by pot experiments.The main results are as follows:The effects of pyrolysis temperature（300-700℃）on the physicochemical properties of sewage sludge biochar（SSB）and its heavy metal migration and transformation characteristics were investigated.The results showed that with the increased in pyrolysis temperature,the yield of SSB decreased from 67.77%to 47.32%,the specific surface area increased by 16.11 times,the surface-active functional groups of biochar gradually were decomposed,and the stability and aromaticity of biochar increased.The migration and transformation of different heavy metals were different after SS was pyrolysis.The residual rate of heavy metals in SSB was about 90%（except Cd）.Pyrolysis contributed to the transformation of Cu,Pb,Cd,Ni,and Zn into biology inefficient forms（F3+F4）,and their proportions increased by 58.34%,48.29%,46.79%,37.50%,and 63.99%,respectively.The leaching rate of Ni and Zn decreased by23.72%and 37.17%,respectively.When the pyrolysis temperature increased to 700℃,the leaching rate of Cd in biochar was 2.18%higher than that in sewage sludge.The RAC values of each heavy metal were reduced to low risk or no risk,which was reduced by 1-2 grades compared to sewage sludge.The optimal pyrolysis temperature is 600℃based on the biological and physical properties of SS and its ecological risk of heavy metals.The addition of biomass and SS co-pyrolysis,improved the physicochemical properties and adsorption properties of SSB.The effects of biomass species（pine sawdust and rice straw）and their blending ratio on heavy metal migration and transformation characteristics were investigated.The results showed that the specific surface area of biochar increased by 3.3 times due to the co-pyrolysis of pine sawdust（PS）and SS.However,when the rice straw（RS）blending ratio（5:5）was too high,volatiles were blocked the pores of biochar,resulting in a decrease in the specific surface area of biochar.In the co-pyrolysis process of PS and SS,when the blending ratio was 5:5,the residue fraction（F4）of Cd increased by 24.45%,and the leaching rate of heavy metal Zn decreased by 2.53%.In the co-pyrolysis process of RS and SS,residue fraction（F4）of Ni increased by 16.42%and the leaching rate decreased by 1.17%when the blending ratio was 5:5.After biomass blending,the RAC values of Pb and Zn in biochar decreased from low risk and medium risk to no risk and low risk,respectively.Biochar was applied to simulated wastewater to explore the effects of adsorption conditions such as the initial p H value of the solution,initial concentration of the solution,and adsorption time on ammonium adsorption in water.In the adsorption experiment,the optimal pyrolysis temperature was 700℃,and the adsorption capacity was 2.09mg·g^-1.The SSB prepared at the pyrolysis temperature was suitable for ammonium adsorption in an alkaline environment,and the adsorption capacity was 6.33mg·g^-1.The adsorption isotherms of ammonium for the biochar prepared at 700°C meet both the Langmuir model and the Freundlich model,and the maximum theoretical adsorption capacity can reach 25.151mg·g^-1.The kinetic process of ammonium adsorption by biochar conforms to the quasi-first-order kinetic equation.The SDB prepared by the co-pyrolysis of PS and SS further improved the adsorption effect of ammonium.The optimal blending ratio was 5:5,and the maximum experimental adsorption capacity was 3.28 mg·g^-1.The addition of RS inhibited the adsorption process of ammonium.Adding PS and SS co-pyrolysis（5:5）for the preparation of biochar and biochar application after the adsorption of ammonium in plant cultivation,compared with the controlr group,the water spinach seed germination rate increased by 13.3%and 31.11%respectively,the seedling fresh biomass increased by 10.72%and 20%respectively,and SS can inhibit water convolvulus seed germination and growth.