Study On The Biochar And Modification Prepared By Wetland Plants And Its Adsorption Characteristics Of Phosphate In Water

Eutrophication is a crucial environmental issue for the world today.At the same time,the post-treatment of wetland plants during the construction and operation of constructed wetlands has also become a very important issue.Therefore,it is an important work to consider how to recycle wetland plants and apply them to environmental remediation.In this study,four common wetland plants,Arundo donax,Phragmites communis（Cav.）Trin.ex Steud,Canna indica L and Nelumbo nucifera,were selected to prepare biochar at 300～800℃respectively.After characterization,modification and batch phosphate adsorption experiments,La（OH）₃-modified Canna（CBC-La）and Nelumbo biochar（NBC-La）were selected as the research objects.The influencing factors of modified biochar on the adsorption of phosphate were studied in the static adsorption experiment,and the adsorption performance was described by the adsorption kinetics and adsorption isotherm model fitting.Through the dynamic adsorption experiments,the adsorption capacity of the adsorbent and the domestic wastewater under the condition of flow dynamic was studied.Finally,the main mechanism of adsorption of phosphate by CBC-La and NBC-La was explained by instrumental characterization analysis.The specific research conclusions are as follows:（1）All biochar produced is a mesoporous material.And the specific surface area increases with the increase of production temperature.The maximum specific surface areas of Canna（CBC）and Nelumbo biochar（NBC）were much higher than those of Arundo（ABC）and Phragmites biochar（PBC）,reaching 193.1461 m~2/g and 429.5571m~2/g,respectively.The adsorption capacity of CBC and NBC was also higher than that of ABC and PBC.The two kinds of plant biochar have the highest phosphate removal rate at 800℃,which are 15.04%and 25.75%,respectively.（2）Modification of FeCl₃,MgO and La（OH）₃ could improve the adsorption performance of CBC and NBC.The modified effects were as follows:La（OH）₃>FeCl₃>MgO.The maximum adsorption capacity of Canna biochar modified by La（OH）₃ was increased from 9.47 mg/g to 37.37 mg/g,and the adsorption equilibrium time was reduced from 24 h to 8 h.The maximum adsorption capacity of Nelumbo biochar increased from 14.87 mg/g to 37.89 mg/g,and the adsorption equilibrium time was shortened to 4h.In addition,CBC-La and NBC-La have good removal effect on medium and low concentration phosphate solution,and maintain more than 99%removal rate at the concentration of 50 mg/L and 20 mg/L respectively.（3）The pseudo-second-order kinetic model can describe the adsorption process of CBC-La and NBC-La well,indicating that the adsorption process is mainly chemisorption.Meanwhile,the linear fitting results of the intra-particle diffusion model are all multi-stage linear relationships.This indicates that both liquid film diffusion and intra-particle diffusion mechanisms exist in the adsorption process.When fitting the adsorption isotherm model,the Langmuir model can better explain the adsorption process,indicating that the adsorption process is a single-layer uniform adsorption.（4）CBC-La and NBC-La have a wide pH（3～9）adaptability and better selective adsorption performance.Under the condition of coexistence anions(NO₃^-HCO₃^-and CO₃^2-),the removal rate can be maintained above 99%.While the coexistence of NH₄⁺has a synergistic effect on the adsorption process of CBC-La,and a competitive adsorption effect on NBC-La.（5）The performance of CBC-La in dynamic adsorption experiment is better than that of NBC-La.Thomas model and Yoon-Nelson model can better describe the dynamic adsorption process of NBC-La,but the fitting degree of the three models used for CBC-La is not high.In the treatment process of the actual phosphorus containing wastewater,the penetration curve is different from that of the water distribution experiment due to the influence of various factors such as the coexistence of ions in the wastewater,the exhaustion point of the fixed packed bed is advanced,and the total removal rate is also reduced.（6）The main mechanism of CBC-La adsorption of phosphate is electrostatic adsorption ion exchange ligand exchange and inner-sphere complexation.However,no ion exchange was found in NBC-La.