Optimization Of Preparation Strategy And Aging Effects Of Cd²⁺ Adsorption Performance For Garden Waste Biochar

With the rapid development of urbanization,a large amount of urban garden waste has given rise to a huge burden for the municipal administration in China.How to utilize these potential resources sustainably are becoming an urgent problem that needs to be solved in the current stage.This study transformed the urban garden waste into the biochar to investigate the feasibility of garden waste biochar（GWB）as adsorbent for the heavy metals(take the most common pollutant of Cd²⁺in the environment as example)in aqueous solution under the background of heavy metals pollution remediation.Meanwhile,the regression model between pyrolysis conditions and GWB yield as well as Cd²⁺adsorption performance has been established by exploring the influence of pyrolysis parameters for GWB yield and adsorption performance.And the optimal preparation strategy has been obtained through the response surface methodology to resolve the contradiction between the adsorption performance and the cost of biochar in practical application（heating temperature:398℃,heating rate:10℃/min,residence time:30 min）.The biochar prepared from the optimal strategy achieved the“best compromise”between GWB yield and Cd²⁺adsorption performance(biochar yield:49.87%,Cd²⁺adsorption capacity:40.01 mg/g)since the pyrolysis conditions posed an opposite effect to these two responses in a certain range.Besides,the relative low energy consumption required in biochar preparation under optimal strategy also further decreased the preparation cost.The results of the kinetics,isotherms and thermodynamic model analysis suggested that the adsorption for Cd²⁺by GWB are mainly monolayer chemisorption,and the adsorption reaction were spontaneous and endothermic.The characterization of SEM-EDS,XPS,FTIR and XRD et al.indicated that the adsorption mechanisms between GWB and Cd²⁺are mainly attributed to the ion-exchange,mineral precipitation,pore-filling,and functional-group complexation,in which the ion-exchange and mineral precipitation account for the most majority of adsorption capacity.In addition,as an in-situ immobilization material for heavy metals,biochar can exist in the environment for thousands of years,while whether the natural aging would affect its heavy metals adsorption performance still remains unclear.Therefore,we simulated the natural aging process for GWB prepared under different pyrolysis temperatures by artificial aging treatment to investigate the changes of Cd²⁺adsorption performance before and after aging treatment.The adsorption experiment shown a significantly decrease in Cd²⁺adsorption capacity for Aged-GWB,and aging treatment caused the greatest impact in the biochar at medium pyrolysis temperature（500℃）.This is mainly attribute to the removal of minerals in GWB during the oxidation,leaching and cleaning process in aging treatment decreased the ion-exchange and mineral precipitation between Aged-GWB and Cd²⁺.However,in general,the GWB prepared at medium pyrolysis temperature（500℃）after aging treatment still contains a strong adsorption performance for Cd²⁺,and the non-bioavailable fraction Cd also accounts for more than 59%of the total fixed Cd content.Therefore,we can conclude that GWB has a long-term effect in Cd remediation.Overall,transformation of garden waste to an adsorbent offered a new sight on the disposal and utilization of urban garden waste and the GWB shows strong adsorption performance in short-term and long-term remediation.