Adsorption And Reduction Of Cr In Solution And Soil By Fe-Al-LDH And Its Modified Materials

With the booming improvement of the modern industrialized society,wastewater contained heavy metal ions?chromium,copper,lead and so on?during the process of industrial production,were excessively discharged to nature.It caused serious environmental pollution and hazard.These heavy metal ions are generally toxic,not easy degraded and removed after being discharged into the environment.on the contrary,they will accumulate in environment in long term and they will be toxic to biological and human body extremely through the food chainMany treatment technologies for chemical industry wastewaters containing Cr???have been developed including adsorption,biological,membrane filtration methods,Chemical precipitation and electrochemical method.Among them,the adsorption method is widely applied worldwide owing to its low cost and higher removal efficiency for Cr???.Commonly used absorbents include activated carbons,chitosan,chelate resins,oxide or hydroxide compound material and clay minerals,among others.Also,those adsorbents cannot only use for the heavy metal pollution,but also can be used for organic contaminates.But for the Cr???,Among the different adsorbents,clay minerals are interesting materials owing to their abundance,environment protection,and reusability characteristics.Layered Double Hydroxides?LDH?material is a kind of typical anionic layered clay mineral,but traditional LDH materials remove chromium ion only through adsorption,which is just a kind of pollution transfer and cannot reduce hexavalent chromium to trivalent chromium.It really does not remove the harm of hexavalent chromium.In order to solve this problem,this work synthesized Fe–Al-LDH?Fe:Al=2:1,LDH-2?and Fe–Al-LDH?Fe:Al=3:1,LDH-2?by a hydrothermal synthesis method.The resulted products were characterized by X-ray diffraction?XRD?,Fourier transform infrared spectroscopy?FTIR?,Brunauer–Emmett–Teller?BET?measurements,scanning electron microscopy?SEM?,thermogravimetric analysis coupled with differential scanning calorimetry?TG/DSC?,transmission electron microscopy?TEM?,and X-ray photoelectron spectroscopy?XPS?.Highly crystalline forms with sheet structure were revealed by XRD and TEM results,while the BET surface area of the LDH-2 and LDH-3 materials were 46.85 and 53.22 m²/g,respectively.In addition,both products showed good performances in the removal of Cr???,although the removal efficiency of LDH-2 was higher than that of LDH-3.When 90min,the removal efficiency of Cr???by LDH-2 and LDH-3 were 98.22%and 52.27%,especially.The removal process simultaneously includes adsorption,reduction and coprecipitation Cr???were immobilized in the interlayer and surface of LDH through interlayer anions exchange or surface adsorption.Then,the Fe²⁺ions in LDH were involved in reducing Cr???to Cr???while simultaneously being oxidized to Fe³⁺.Finally,Fe³⁺would coprecipitate with Cr.pH was found to influence the adsorption of Cr???,with higher pH values being beneficial to the removal process.Common anions present in the environment such as SO₄^2-,NO^3-,and CO₃^2-showed minor influence in the adsorption performance of the LDH materials.A competitive adsorption between Cr???and humic acid?HA?was observed and it would restrain the removal of Cr???by LDH.Fe involved in the lamination of LDH-2was dissolved out to a large extent promoting the Cr???to Cr???process.The research of LDH-2 used in the remediation of Cr???contaminated soil was conducted.The remediation tests indicated that adding 1g/L of Fe-Al-LDH to a solution of Cr???-contaminated soil,completely immobilized the Cr???preadsorbed in the soil?2,079.84 mg/kg?.Additionally,it was found that the Fe-Al-LDH could be used in a wide range of pH conditions.Based on the characterization by XRD,FTIR,inductively coupled plasma mass spectrometry?ICP-MS?and XPS,it is proposed that adsorption and reduction may be involved in the mechanism of Cr???immobilization by Fe-Al-LDH.At the beginning of the reaction,the Cr???entered into the layer structure of the LDH or was adsorbed on the surface of the LDH.Then,the Fe?II?was involved in reducing the Cr???to Cr???and was oxidized to Fe???.Also,part of the Cr???and Cr???were co-precipitated with Fe???during the formation of iron oxide or hydroxide.Finally,this study also researched the modification of LDH-2 by using sodium borohydride?NaBH₄?to restore Fe²⁺in LDH-2.The modified LDH-2 was Z?-LDH-2 since which had zero-valent iron.Then,Z?-LDH-2 was used in removal of Cr???in solution and immobilization of Cr???in soil.Through the characterization of XRD,SEM and TEM,it indicated that Z?-LDH-2 remained the typical laminated structure and crystal structure of LDH however crystalline structure and crystal morphology changed to some extent.The BET characterization showed that BET surface area of the Z?-LDH-2 and LDH-2 materials were 57.01%and 46.85 m²/g,respectively,which demonstrated that physical adsorption capacity of Z?-LDH-2 is larger than LDH-2.When 15min,the removal efficiency of Cr???in solution by Z?-LDH-2 and LDH-2 were 100%and 91.32%,especially,the immobilization efficiency of Cr???in soil by Z?-LDH-2 and LDH-2 were 99.23%and70.96%.compared with LDH-2,both the removal efficiency of Cr???in solution and immobilization efficiency of Cr???in soil of Z?-LDH-2 were higher.The reaction process and mechanism were found.NaBH₄ restored the Fe²⁺in the laminate of Z?-LDH-2 to Fe⁰?zero-valent iron?and then the Fe⁰ restored the Cr???to Cr???.Both the removal of Cr???in solution and immobilization of Cr???in soil by Z?-LDH-2 can be conducted in condition with wide pH value.