Enhanced Cr??? Removal In The Synergy Between Hydroxyl-Functionalized Ball-Milled ZVI/Fe₃O₄ Composite And Na₂EDTA Complexation

Chromium?Cr?is used in various industries,such as paints,electroplating,leather manufacturing,and metal fnishing.The hexavalent chromium?Cr????presents primarily as anions?CrO2-4 and CrO2-7?in water;its solubility,mobility,and toxicity are remarkably higher than that of trivalent chromium?Cr????.Since zero-valent iron?ZVI?has the characteristic of high reactivity,availability and environmental friendliness,most researchers believe that it is an effective method for treating Cr???.Nonetheless,traditional ZVI/H₂O system features major drawbacks that should be resolved,that is,the intrinsic passivation layer between target contaminants and underlying reactive sites of ZVI prevents electron transfer and limits the availability of the reactive surface.In order to overcome the shortcomings of ZVI in application and the high cost of treating Cr???in actual wastewater,this study prepared HFB-ZVI/Fe₃O₄composites by hydroxyl functional modification of ball-milled ZVI/Fe₃O₄ composites with ethanol,and synergistic action of Na₂EDTA inhibiting the formation of the passivation layer and increaseing the reactivity of ZVI.Taking Cr???in the aqueous solution as the target:characterization of HFB-ZVI/Fe₃O₄ composites was carried out by X-ray diffractometry?XRD?,X-ray photoelectron spectroscopy?XPS?,field emission scanning electron microscopy?FE-SEM?,and Fourier transform infrared Raman spectroscopy?FTIR?.Meanwhile,through the static experiment of removing Cr???from HFB-ZVI/Fe₃O₄ composites,the influencing factors were explored,and the adsorption isotherm curve and the adsorption kinetics curve of Cr???concentration were studied.The factors affecting the removal of Cr???by HFB-ZVI/Fe₃O₄ reaction bed coupled with Na₂EDTA synergy system were investigated and then optimized factors were applied to the actual wastewater to evaluate the cost of removing Cr???.The research results are as follows:?1?Static experimental study on removal of Cr???by HFB-ZVI/Fe₃O₄ composites:Cr???wastewater required for the experimental process is simulated wastewater.When the mass ratio of ZVI to Fe₃O₄ is 1:0.05,the HFB-ZVI/Fe₃O₄ composite has the best removal performance for Cr???,accompanied by Fe???concentration and pH value both at the peak.When the pH value of Cr???solution was 3 and the mass ratio of ZVI to Fe₃O₄ was 1:0.05,the concentration of Fe???presented an increasing trend.With the increase of Cr???concentration,the longevity of the column by HFB-ZVI/Fe₃O₄ composite material was gradually reduced,and both Fe???concentration and pH value showed a decreasing trend.The adsorption isothermal curve and adsorption kinetics fitting curve of Cr???in solution were studied,and the results showed that the adsorption process of HFB-ZVI/Fe₃O₄ to remove Cr???in water was more consistent with the Langmuir isothermal adsorption model and the second-order kinetic model.?2?Enhanced Cr???removal in the synergy between the hydroxyl-functionalized ball-milled ZVI/Fe₃O₄ composite and Na₂EDTA complexation:The Cr???wastewater required in the experiment was simulated wastewater.With increasing Fe₃O₄ content,the effuent Cr???concentration below 0.5 mg/L was sustained for 96 BV for the HFB-ZVI/Fe₃O₄?1:0.05?-packed column,suggesting that introduction of Fe₃O₄component remarkably improved the Cr???removal performance and enhanced column longevity.High Na₂EDTA concentration was instrumental in improving Cr???removal performance and enhancing column longevity by HFB-ZVI/Fe₃O₄?1:0.05?-packed column,but it was inevitable to increase Chemical Oxygen demand?COD?in the resulting drainage.As the flow rate increases,the service life of the filled HFB-ZVI/Fe₃O₄?1:0.05?column will decrease correspondingly.?3?According to the characterization results of XRD,XPS,FE-SEM and FTIR,etc.,FeOOH coating with Fe???attached on the surface of the composite can be formed after hydroxyl functionalization.To facilitate the conversion from Cr???to Cr???,hydrolysis and auto-reduction of Fe???/Fe???species caused proton accumulation,resulting in remarkable attraction of anionic Cr???via electrostatic interaction and exchange.In addition,Na₂EDTA can inhibit the growth of the passivation layer and generate new corrosion products,providing more reaction sites for the reduction reaction of Cr???.?4?Actual chromium wastewater was collected from a typical electroplating plant located in Xinxiang City,China.The cost of HFB-ZVI/Fe₃O₄?1:0.05?removal of Cr???in actual wastewater is:14.5 CNY per ton of wastewater,in which the costs of iron and Na₂EDTA reach 0.5 and 14 CNY/t,respectively.