Applied Study On The Removal Of Phosphate From Aqueous Solutions By Biochar Based Composites

In recent years,the phenomenon of eutrophication has been further aggravated for the large amount of exogenous nutrients entering to the water bodies.Phosphate content is a key factor in controlling eutrophication,and the presence of excess phosphorus can endanger human health.Therefore,it is of great significance to remove phosphate from wastewater.Adsorption has become one of the most effective phosphorus removal methods due to its economic efficiency,stable operation,easy operation and strong adaptability.The key issue of adsorption is to find environment-friendly and excellent performance adsorbents.As a traditional adsorbent,biochar is widely used in water treatment.However,due to the negative charge of biochar,it cannot remove aqueous phosphate efficiently and needs to be modified or loaded with other materials to enhance the ability to remove anionic pollutants.In this dissertation,Mg-Al layered double hydroxide(Mg-Al LDH),iron-oxides,and zeolite-like midazolide skeleton material(ZIFs)based biochar composites were successfully synthesized.The adsorption performance and practical application ability were studied by batch equilibrium experiments,adsorption regeneration experiments,and application experiments in simulating water.The surface characteristic and mechanisms were also studied by various characterization methods,which provided theoretical basis for the applicaton of biochar based materials in phosphate removal.The experimental results were as follows:(1)The vegetable-based biochar calcined hydrotalcite materials(Cc-LDO and Ra-LDO)were prepared with Mg-Al LDH and two vegetable biochar(Cc,Brassica rapa pekinensis;Ra,Brassia campestris L.)by coprecipiation and high-temperature pyrolysis.The obtained materials were characterized by X-ray diffraction(XRD),Brunauer-Emmett-Teller(BET)surface area determination,Fourier transform infrared spectroscopy(FTIR),X-ray photoelectron spectroscopy(XPS),transmission electron microscope(TEM),and scanning electron microscopy(SEM).Cc-LDO and Ra-LDO show a loose porous and irregular layered structure.The experimental conditions,kinetics and isotherm models were used to investigate the optimal adsorption conditions,adsorption performance and mechanism.The removal efficiencies of Cc-LDO and Ra-LDO were better than that of sole biochar and Mg-Al LDO.For50 mg/L phosphate,the removal efficiency can reach more than 90%in the first 5 min at wide range of pH values,which had obvious advantages than other materials.The adsorption process followed the pseudo-second-order kinetic model,and the isotherm data was fit the Langmuir and Freundlich isotherm models,with the adsorption capacity reaching 127.23 and 132.80 mg/g,respectively.Cc-LDO and Ra-LDO adsorbed phosphate via the following mechanisms:electrostatic attraction,surface complexation,“memory effect”,and physical absorption.Cc-LDO and Ra-LDO also had excellent regeneration ability,which can be reused for 4 times.(2)Four biochar-iron oxides were obtained by combining the shell biochar and four iron-oxides(Magnetite,Ferrihydrite,Goethite and Hematite),and characterized by XRD,FTIR BET,TEM,SEM and XPS techniques.The as-prepared BC-M,BC-F,BC-G,and BC-H had large specific surface area(691-864 m~2/g),pore diameter(3.4-4.0 nm),irregular block structure,as well as oxygen-containing functional groups.The optimal conditions for removing phosphate were explored by batch adsorption experiments.The kinetics and isotherms data were well fitted by the pseudo-second-order kinetic equation,as well as the intra-particle diffusion model.The isotherm data of BC-M and BC-H was well fitted by the Langmuir and Freundlich models,while that of BC-G and BC-F were the Langmuir model.The interaction mechanisms between phosphate and biochar-iron oxides were mainly electrostatic attraction,ligand exchange,and deposition.In addition,BC-M,BC-F,BC-G,and BC-H owned excellent regeneration ability and adsorption performance in practical(simulated)wastewater environment.(3)ZIF-derived mesoporous vegetable biochar/ZnO nanocomposites(Cc-ZnO and Ra-ZnO)were synthesized by solvothermal method.The characterization results of SEM,FTIR,XRD and BET indicated that the six-membered structure of ZIF-8 disappeared,and the morphology was formed by plentiful inter-connected ZnO with cubical shape and biochar nanoparticles.The pseudo-second-order kinetic model could well describe the phosphate adsorption process,and the adsorption isotherm data agreed well with the Langmuir and Freundlich models.The mechanisms can be explained as physical adsorption,electrostatic attraction and surface complexation.In conclusion,the above biochar based composites have the characteristics of low-cost,high efficiency,and great recoverability,which show a good practical application for phosphate removal from aqueous solutions.