Study On The Preparation Of γFe₂O₃/Fe₃O₄/NaOH Lotus Stem Biochar And Its Adsorption Characteristics Of Aqueous Cr（Ⅵ）

Magnetic biochar,as an adsorbent with a wide range of sources and cheap production,has great development potential for hexavalent chromium adsorption.It can realize solid-liquid separation after adsorption,which is convenient for subsequent recovery and regeneration.Alkali modification after magnetization of biochar can further enhance the adsorption effect.The increase of the specific surface area of the biochar and the increase of the oxygen-containing functional groups will promote the adsorption of hexavalent chromium.The lotus stem biochar is a kind of biochar with a wide range of sources.At present,it has achieved good adsorption effect in terms of heavy metal adsorption.The lotus stem biochar was magnetized and alkali-modified,and applied to the treatment of chromium-containing wastewater.The common agricultural waste lotus stem and ferric chloride were used as raw materials,magnetic lotus stem biochar MBC was prepared by immersion pyrolysis method,and then activated by sodium hydroxide to prepareγFe₂O₃/Fe₃O₄/Na OH lotus stem biochar SMBC,which was used as an easily separable and regenerable adsorbent for the removal of Cr（Ⅵ）in wastewater.Research indicates:（1）The optimal conditions for preparing MBC were:the concentration of the ferric chloride solution was 30%,and the immersion time was 2 h.The best modification conditions of SMBC were:carbon-alkali ratio was 1:3,activation temperature was 800℃,and activation time was 1 h.The combined characterization results of SEM,EDS,FTIR and XRD indicated thatγFe₂O₃and Fe₃O₄were successfully loaded onto SMBC.BET analysis showed that the specific surface area and pore structure of the biochar modified by magnetization and activation were significantly improved.Zeta potential analysis results showed that after modification,the zero charge point of SMBC was reduced,which was beneficial to the adsorption of negatively charged groups.The results of VSM analysis showed that the magnetic properties of SMBC were significantly enhanced after Na OH activation,which was beneficial to achieve the ideal solid-liquid separation effect after the adsorption was completed,so as to carry out the next recovery and regeneration.（2）The results of static adsorption experiments showed that the adsorption amount of Cr（Ⅵ）per unit mass of adsorbent decreased gradually with the increase of the dosage,and tended to balance,while the removal rate increased with the dosage of each adsorbent.It gradually increased and then decreased slowly,indicating that the acidic conditions were more favorable for the adsorption of Cr（Ⅵ）.The presence of NO₃^-in the solution had almost no effect on the adsorption of Cr（Ⅵ）by SMBC.Cl^-and SO₄^2-had a slight but insignificant effect on the adsorption of Cr（Ⅵ）by SMBC.The adsorption performance of SMBC on Cr（Ⅵ）was strongly affected by HCO₃^-,PO₄^3-and CO₃^2-,and the cations(Na⁺,NH₄⁺,K⁺,Cu²⁺)in the solution had no significant effect on the adsorption of hexavalent chromium by SMBC.After the adsorption process was completed,under the action of an external magnetic field,SMBC could be easily separated from the solution,and it still had a good adsorption capacity after six cycles of desorption and regeneration.（3）The adsorption process of Cr（Ⅵ）by SMBC conformed to the Langmuir adsorption isotherm model and the pseudo-second-order kinetic model,indicating that this process was a monolayer chemical adsorption process,and the adsorption process was endothermic.The adsorption mechanism of Cr（Ⅵ）by SMBC mainly included redox,electrostatic attraction and complexation.After the adsorption,85.57%of Cr（Ⅵ）was reduced to Cr（III）and adsorbed on the surface of SMBC,and the rest existed in the aqueous solution or directly adsorbed on the surface of the adsorbent,and only 1.28%of Cr（Ⅵ）remained in the solution.（4）SMBC has good recovery performance.After 6 times of recovery,the mass loss rate was 10.28%,and the recovery rate was 89.72%.The type of desorbent suitable for SMBC to desorb Cr（Ⅵ）was sodium hydroxide solution,and the optimal concentration was 0.5 mol/L.After 6 cycles,the adsorption capacity of SMBC still reached 32.23 mg/g,and the adsorption efficiency reached 72.59%,indicating that the regenerated SMBC still had good adsorption characteristics.This study demonstrates the promise of SMBC as an environmentally friendly and regenerable sorbent for chromium removal with excellent adsorption performance,strong adaptability,and reproducibility.