Preparation Of Modified Biochar Loaded Iron Series Materials And Study On Its Adsorption Mechanism For Cr(Ⅵ) In Aqueous Solution

The existence of hexavalent chromium（Cr（Ⅵ））in water system has become a global health and environmental problem,which has high toxicity,strong teratogenicity,carcinogenicity,mobility and persistence.At present,there are many methods for removing Cr（Ⅵ）,and one of the most commonly used methods is adsorption.However,traditional adsorption materials have limited adsorption capacity for Cr（Ⅵ）,unstable adsorption performance,and difficulty in achieving solid-liquid separation.Therefore,in the treatment of Cr（Ⅵ）wastewater,finding inexpensive adsorption materials with excellent performance and simple operation steps remains a major challenge.Aquatic plant and the plant residues formed by them contain rich carbon sources and oxygen-containing functional groups,and have good adsorption capacity for Cr（Ⅵ）.In this paper,the submerged aquatic plant Egeria najas（EN）,Potamogeton perfoliatus L（PPL）and Mayaca sp（MS）were used as raw materials to prepare the modified biochar loaded iron materials,and various characterization methods were used to explore the surface morphology,element composition,valence changes and functional groups of the materials.At the same time,the adsorption effect and mechanism of Cr（Ⅵ）were explored through static and dynamic adsorption experiments,ensuring that the Cr（Ⅵ）in the effluent meets the national emission standards,providing guidance for the development and utilization of modified biochar loaded iron based materials,and providing a theoretical basis for its large-scale application in industrial wastewater.The main conclusions are as follows:（1）The acid modified biochar was prepared by the method of Co-heating phosphoric acid with Egeria najas（P-EN）,Potamogeton perfoliatus L（P-PPL）and Mayaca sp（P-MS）,and its adsorption performance for Cr（Ⅵ）was investigated under different conditions.The results showed that the removal efficiencies of Cr（Ⅵ）by P-EN,P-PPL and P-MS were 99.35%、95.45%and 94.48%within 120 min when the dosage was 1.5 g/L,the concentration of Cr（Ⅵ）was 10 mg/L,the pH value was 5.7,and the temperature was 298 K.In the coexisting ion experiment,the presence of cation and anion has no significant effect on the adsorption of Cr（Ⅵ）by the three modified biochar.In addition,the adsorption of Cr（Ⅵ）on the three modified biochar conformed to the pseudo-second-order kinetic model and the sips isotherm model.Based on the above analysis,the adsorption mechanism of Cr（Ⅵ）on the three modified biochar was mainly surface physical adsorption,reduction and complexation of functional groups.（2）A modified biochar loaded nano zero valent iron composite material（P-BC/nZⅥ）was prepared using phosphoric acid modified Egeria najas biochar as a carrier and KBH₄ as a reducing agent for ferrous ions in aqueous phase.The composite materials before and after the reaction with Cr（Ⅵ）were characterized using SEM EDS,TEM,BET,XRD,FTIR,Zeta potential,and XPS methods.The results show that the surface of P-BC/nZⅥ was fluffy and porous,and the specific surface area was much larger than that of P-BC,which overcome the agglomeration characteristics of nZⅥ and was more conducive to the removal of Cr（Ⅵ）.After reacting with Cr（Ⅵ）,the zero valent iron on P-BC/nZⅥ disappeared and was oxidized to Fe₂O₃ and Fe₃O₄.Accordingly,Cr（Ⅵ）was reduced to Cr（Ⅲ）.The results of Fe/BC experiment showed that when Fe/BC=3:1,dosage was 0.75 g/L,Cr（Ⅵ）concentration was 20 mg/L,pH value was 5.5,temperature was 298 K,the removal efficiency of Cr（Ⅵ）by P-BC/nZⅥ can reach 96.96%within 30 min.In the coexisting ion experiment,the effects of cations K⁺,Na⁺,Cu²⁺,and Mg²⁺on the adsorption of Cr（Ⅵ）by P-BC/nZⅥ were very small,while the presence of anions Cl^-,NO₃^-,and SO₄^2-had a certain impact on the adsorption of Cr（Ⅵ）.In addition,the adsorption of Cr（Ⅵ）by P-BC/nZⅥ conformed to the pseudo-second-order kinetic model and the sips isotherm model.In dynamic adsorption experiments,the saturation time and breakthrough time of Cr（Ⅵ）adsorption will change when the operating conditions of the fixed bed filled with P-BC/nZⅥ are changed.Based on the characterization of P-BC/nZⅥ and the calculation and analysis of the concentration of chromium and dissolved iron in the solution,it can be concluded that the adsorption mechanism of Cr（Ⅵ）by P-BC/nZⅥ included physical adsorption,reduction and complexation of functional groups.（3）Biochar was prepared by hydrothermal carbonization of Egeria najas powder,and biochar loaded nano iron composite was prepared by reducing ferrous ions with KBH₄ in aqueous phase（EN@nZⅥ）.The materials were characterized using SEM-EDS,TEM,BET,and XPS methods.The results showed that the surface of biochar loaded with nano iron was more porous than the original biochar,and the agglomeration of nano iron particles was alleviated.The particle size of the EN@nZⅥ was between 58-66 nm,which was smaller than that of the nZⅥ,which was more conducive to the adsorption of Cr（Ⅵ）.Static experiments were used to investigate the adsorption process of Cr（Ⅵ）by EN@nZⅥ.It was found that the initial concentration of Cr（Ⅵ）,the temperature of the reaction system,the initial pH of the solution and the amount of adsorbent added to the solution had a significant effect on adsorption of Cr（Ⅵ）by EN@nZⅥ.Under the synergistic effect of adsorption and reduction,the removal efficiency of Cr（Ⅵ）by EN@nZⅥ within 90min was 100%（nZⅥ:EN=3:1,C₀=20 mg/L,t=298 K,pH=5.5,the dosage was1.0 g/L）.Thermodynamic studies showed that,the adsorption of Cr（Ⅵ）by EN@nZⅥ was a spontaneous endothermic process.In dynamic adsorption experiments,changing EN@nZⅥ The corresponding breakthrough curve for filled fixed bed conditions indicates that decreasing the inflow rate or increasing the height of the fixed bed can extend the saturation time and breakthrough time for adsorption of Cr（Ⅵ）.Based on comprehensive characterization analysis,static adsorption,and dynamic adsorption,it can be concluded that EN@nZⅥ The reaction mechanism for removing Cr（Ⅵ）is a complex process involving adsorption,surface complexation,reduction,and ion exchange reactions.（4）Fe（Ⅲ）-modified biochar composites were prepared from Egeria najas by thermal precipitation and one-step carbonization chemical modification（Fe Cl₃@EN-BC and Fe₂（SO₄）₃@EN-BC）.SEM-EDS,TEM,BET,XRD,FTIR and XPS were used to characterize the surface morphology,specific surface area,particle size,elemental composition and chemical morphology of the materials.The results showed that,Fe Cl₃@EN-BC and Fe₂（SO₄）₃@EN-BC all belong to mesoporous materials and are suitable for adsorption materials.The surface of the materials contained a large number of-OH,-COOH,-NH₂ and-C-O-C functional groups,which was conducive to the adsorption of Cr（Ⅵ）.In addition,the temperature of the reaction system,the initial pH of the solution and other factors have an important effect on adsorption of Cr（Ⅵ）by Fe Cl₃@EN-BC and Fe₂（SO₄）₃@EN-BC.The experimental results showed that the higher the reaction temperature,the lower the pH of the solution,the more favorable the adsorption of Cr（Ⅵ）.In dynamic experiments,the saturation time and breakthrough time of adsorption of Cr（Ⅵ）by Fe Cl₃@EN-BC and Fe₂（SO₄）₃@EN-BC are continuously shortened as the solution flow rate continuously increases.The higher the bed height,the greater the saturation time and breakthrough time of adsorption of Cr（Ⅵ）by Fe Cl₃@EN-BC and Fe₂（SO₄）₃@EN-BC.The reaction mechanism of Cr（Ⅵ）adsorption by both materials involves adsorption,surface complexation,reduction and ion exchange reaction.