Preparation And Modification Of Biochar Derived From Iron-Based Waterworks Sludge And Chinese Medicine Residual And Removal Mechanism Of Cr(Ⅵ)

Cr（Ⅵ）in water is characterized with high toxicity,difficult degradation and easy enrichment,which poses harm to aquatic organisms and water users.Therefore,the treatment of Cr（Ⅵ）-containing wastewater has become an urgent social concern.Adsorption has been recognized as an important technique for Cr（Ⅵ）-containing wastewater treatment,which requires adsorbent with the merits of easy availability,low price,high efficiency,cleanliness and wide adaptability.A large amount of Chinese Medicine Residual（CMR）is produced every year in China.As a waste aboundant in lignin and cellulose,CMR is an ideal raw material for biochar preparation.The present disposal of CMR by stacking and burying occupies large land and causes environment pollution.The extensive use of ferric salt coagulants in waterworks has produced a large number of Iron-based Sludge（IS）.At present,the predominant disposal method of landfilling causes land occupation.Therefore,it is of great practical significance to find a new waste disposal method for both CMR and IS.In this study,biochar was prepared using CMR and IS,and then modified to prepare efficient composite materials for the adsorption of Cr（Ⅵ）in water.The mixture of CMR and IS in a certain ratio was used as raw material to prepare biochar using the synchronous calcination pyrolysis method under the anoxic condition,which converted Fe in IS into Fe₃O₄to prepare a Magnetic Biochar（MB）without use of exogenous ferrous compounds.Compared to the conventional biochar,the MB had good solid-liquid separation.Four loading methods were used to modify the MB to improve its adsorption capacity of Cr（Ⅵ）.The effects of initial pH,adsorption time and adsorption temperature on the adsorption process of Cr（Ⅵ）were investigated.The adsorbents were characterized by m?ssbauer Spectrum,scanning electron microscopy,infrared spectroscopy and so on.Based on the experimental data analysis,model fitting and characterization results,the Cr（Ⅵ）removal mechanism by the adsorbents were studied.The specific research contents and main conclusions are as follows:（1）MB was prepared by simultaneous calcination and pyrolysis of the mixture of CMR and IS in a certain ratio.The static Cr（Ⅵ）adsorption was carried out using MB as adsorbents,the Chinese medicine residue Biochar（CB）and Magnetized Iron-based sludge（MI）were prepared at the same condition,which were used as control to conduct the same experiments.The results showed that the Fe₃O₄particles produced by simultaneous calcination and pyrolysis of the two materials made the biochar magnetized,which overcame the defect of solid-liquid separation of conventional biochar.Meanwhile,the biochar alleviated the strong agglomeration effect among Fe₃O₄magnetic particles and improved the adsorption effect on Cr（Ⅵ）.（2）Dual Ferrite Magnetic Biochar（DFMB）was prepared by simultaneous calcination and pyrolysis of Zn Cl₂,CMR and IS.There were two kinds of ferrite particles（Fe₃O₄and Zn Fe₂O₄）formed on DFMB,and the saturation magnetization of DFMB was 37.97 emu/g,which was higher than that of MB（26.22 emu/g）.The specific surface area of DFMB（145.13m²/g）was 1.98 times as much as MB（73.45 m²/g）after modification by Zn Cl₂.The multi-level pore structure of DFMB with micropores,mesopores and macropores provided a large number of adsorption sites for Cr（Ⅵ）,which improves the removal efficiency of Cr（Ⅵ）.The results of kinetics,isotherm and thermodynamics showed that the adsorption of Cr（Ⅵ）by DFMB was a monolayer chemisorption with spontaneous and endothermic nature.The maximum adsorption capacity of Cr（Ⅵ）by DFMB was 43.31 mg/g at pH=2 and 35℃calculated by Langmuir model.The coexisting anions had obvious inhibition on the adsorption of Cr（Ⅵ）by DFMB,and the effect was positively correlated with the anion concentration and ionic valence.（3）Using the ammonium molybdate and thiourea as the molybdenum and sulfur sources,3D Molybdenum disulfide Magnetic Biochar（3DMMB）was prepared by hydrothermal method with MB as magnetic material.3DMMB solved the serious defect of the two-dimensional nano-structure sheets stacking in common Mo S₂,and the three-dimensional flower structure of 3DMMB had regular morphology and distinct layers,as well as a large number of pores and channels existing between the sheet structures,which was beneficial to the migration and diffusion of Cr（Ⅵ）ions between the layers.The adsorption of Cr（Ⅵ）by3DMMB was more consistent with pseudo-second-order kinetic model and Langmuir model,which indicated that the adsorption process was a monolayer adsorption process dominated by chemical adsorption.The maximum adsorption capacity of Cr（Ⅵ）on 3DMMB was 83.33 mg/g（pH=2,35℃）calculated by Langmuir model.The adsorption mechanism included Physical adsorption,electrostatic attraction,chemical reduction and complexation,in which the oxygen-containing groups（-OH,-COOH,Fe-O,Mo-O）and S groups played a dominant role in the reduction of Cr（Ⅵ）.3DMMB could keep stable structure in strong acid environment,and the material could still achieve efficient removal of Cr（Ⅵ）with the presence of high concentration of coexisting ions,indicating that the sulfur group improved the affinity of the material for heavy metals,and 3DMMB had excellent adsorption selectivity for Cr（Ⅵ）.（4）Using IS as the sole iron source instead of traditional chemical agents,MB was prepared by simultaneous calcination pyrolysis,and then Fe⁰-Fe₃O₄Magnetic Biochar（Fe⁰-Fe₃O₄MB）was prepared by liquid phase reduction method for reducing part of high valent iron on MB to zero valent iron.M?ssbauer Spectrum showed double lines of quadrupole splitting and six lines of magnetic splitting,indicating that part of high valent iron on MB was successfully reduced to Fe⁰.During the preparation of Fe⁰,the agglomeration and passivation of Fe⁰were simultaneously optimized,which was stabilized by the cooperation of the biochar and Fe₃O₄,this study provided a new idea for the research and development of stabilized Fe⁰materials.The results of adsorption kinetics,isotherms and thermodynamics showed that the adsorption of Cr（Ⅵ）on Fe⁰-Fe₃O₄MB was a monolayer spontaneous and endothermic process,which was mainly controlled by chemisorption.The corrosion of Fe⁰produced the secondary reductant Fe²⁺,and the transport of electrons from Fe⁰to octahedral Fe₃O₄reduced Fe³⁺on Fe₃O₄to Fe²⁺,then the Fe⁰and Fe²⁺furhter reduced Cr（Ⅵ）to Cr（III）,meanwhile the the Fe⁰and Fe²⁺were oxidized to Fe³⁺.This indicated that octahedral Fe₃O₄provided a new channel for electron transmission between Fe⁰and Cr（Ⅵ）,relieving the inhibition of passivation layer on Fe⁰,so as to solved the passivation and deactivation of Fe⁰.The saturation magnetization of Fe⁰-Fe₃O₄MB was 66.79 emu/g,and the maximum Cr（Ⅵ）adsorption was 128.36 mg/g（pH=2,35 ^oC）,and the Cr（Ⅵ）adsorption mainly took place in the first 30 min.Fe⁰-Fe₃O₄MB is a superparamagnetic material with fast adsorption rate and high Cr（Ⅵ）reduction.（5）In this paper,using DFMB as the magnetic material,Polyaniline Dual Ferrite Magnetic Biochar（PDFMB）was prepared by chemical oxidation polymerization with hydrochloric acid（HCl）doping,which greatly improved the adsorption capacity of the material.A large amount of wedge-shaped mesopores formed on the surface of PDFMB,which were beneficial to the diffusion of Cr（Ⅵ）ions in water into the pore channels and promoted the combination of Cr（Ⅵ）and the reaction sites of PDFMB.The adsorption rate of Cr（Ⅵ）on PDFMB was fast,and the fitting result was more in line with pseudo-second-order kinetics,meaning the adsorption process was in control with chemical adsorption.The Langmuir model calculated that the maximum adsorption capacity of PDFMB for Cr（Ⅵ）was 170.65 and 111.11 mg/g（35℃）when the initial pH of the solution was 2 and 5 respectively,indicating that PDFMB could achieve efficient adsorption and removal of aqueous Cr（Ⅵ）in a wide range of pH values.The pH_PZCof PDFMB was 5.30,which indicated that the surface of PDFMB was positively charged by protonation,the PDFMB had strong electrostatic attraction to Cr（Ⅵ）when pH<5.30.PDFMB had a large number of-NH-and-N=,which provided electrons to reduce Cr（Ⅵ）to Cr（III）and capture Cr（III）by complexation.The polyaniline nanofibrils were interlaced on the surface of PDFMB to form a coral-like structure,which provided good load-type protection for the magnetic particles Fe₃O₄and Zn Fe₂O₄embedded in the center of the material.The magnetization of PDFMB was 18.13 emu/g,and after 6 times of desorption and regeneration,the adsorption capacity of Cr（Ⅵ）could still reach 85.25%in comparison to the initial adsorption capacity,indicating that PDFMB was a stable,efficient and reusable environment-friendly adsorbent.（6）Based on the experimental data and characterization results,the material properties and applicability of the prepared adsorbents were evaluated.Compared with the same type of adsorbents at home and abroad,the adsorbents prepared in this study optimized the magnetic separation performance and improved the adsorption capacity of Cr（Ⅵ）.DFMB was a strong magnetism material,which had higher specific surface area than other three materials,so it was suitable to be used as magnetic matrix for further modification.3DMMB had strong acid resistance,which could realize efficient and specific adsorption of Cr（Ⅵ）with massive ions interference,hence the 3DMMB was suitable for treating Cr（Ⅵ）in a strong acid environment or a complex pollution environment.Fe⁰-Fe₃O₄MB had high reaction activity and fast adsorption rate,Fe⁰and Fe₃O₄cooperated to adsorb and reduce Cr（Ⅵ）,which could quickly complete the efficient removal of Cr（Ⅵ）in a short time.PDFMB had the largest adsorption capacity,strong anti-interference ability,stability and regeneration,wide applicability,which could realize efficient removal of Cr（Ⅵ）in a wide pH range.In practical application,according to the pollution characteristics of Cr（Ⅵ）wastewater,the specific requirements should be analyzed,and the most suitable adsorption material can be selected to give full play to its efficiency.The results of this study can provide a resource approach for CMR and IS,provide theoretical basis and technical support for the preparation of economical,efficient,separable and recyclable adsorbents with wide adaptability,have important guiding significance and reference value for the application of adsorbents in the field of Cr（Ⅵ）wastewater treatment.