Comparative Study Of Mercury Adsorption Mechanisms By Typical Iron Oxides And Their Organosilicon-modified Adsorbents

Industrial production processes such as smelting and chemical industry produce alarge amounts of mercury-containing wastewater,causing water pollution and threatening human health through the food chain.Among the technologies for remediation of mercury pollution in water bodies,adsorption is one of the most important technologies.Iron oxide has abundant reserves,low price,large specific surface area,good adsorption performance,and simple treatment process,which is particularly suitable as an adsorbent to remove mercury from water bodies.However,there are many types of iron oxides,and different iron oxides have significantly different adsorption capacities and mechanisms for mercury.Under different conditions（pH,humic acid,coexisting ions,ionic strength）,the study takes three typical iron oxides widely present in natural environments such as goethite,hematite,and ferrihydrite as research objects and uses organosilicon to modify them to compare the adsorption process and mechanism of Hg（Ⅱ）and CH₃Hg（Ⅰ）in water.The main research results are as follows:1.Batch experiments were used to compare the adsorption of Hg（Ⅱ）and CH₃Hg（Ⅰ）by three iron oxides.The results showed that for the adsorption of Hg（Ⅱ）,goethite(37.49mg·g^-1)and ferrihydrite(35.60 mg·g^-1)were equivalent,while hematite(15.92 mg·g^-1)was lower.The adsorption process followed pseudo-second-order kinetics and Langmuir model.Thermodynamic studies found that△H for goethite and hematite was positive,indicating that temperature rise was conducive to adsorption,while ferrihydrite was opposite.For the adsorption of CH₃Hg（Ⅰ）,goethite(339.04 ng·g^-1)and hematite(312.62ng·g^-1)were equivalent,while ferrihydrite(268.50 ng·g^-1)was lower.the adsorption process followed Langmuir model and Freundlich model.2.The effects of different factors on the adsorption of Hg（Ⅱ）and CH₃Hg（Ⅰ）by three iron oxides were studied.The results showed that when the pH value of the solution increased from 3 to 9,the adsorption capacity of Hg（Ⅱ）by goethite,hematite,and ferrihydrite increased by 53.10%,37.73%,and 50.53%,respectively.Adding humic acid increased the adsorption of iron oxide to Hg（Ⅱ）.When the ionic strength of the system increased,the adsorption capacity of ferrihydrite for Hg（Ⅱ）decreased,while goethite and hematite were opposite.Unlike Hg（Ⅱ）,when the pH value of the solution increased from3 to 9,the adsorption capacity of CH₃Hg（Ⅰ）by three iron oxides decreased by 34.48%,62.07%,and 79.30%respectively.Adding humic acid promoted iron oxide to CH₃Hg（Ⅰ）.When the ionic strength of the system increased,the adsorption capacity of goethite for CH₃Hg（Ⅰ）decreased,while hematite and ferrihydrite first increased then decreased.3.XRD,XPS and other methods were used to characterize three iron oxides before and after adsorbing Hg（Ⅱ）and CH₃Hg（Ⅰ）,in order to clarify and compare their adsorption mechanisms.The results showed that for goethite,the main mechanism for Hg（Ⅱ）adsorption was that H⁺in surface-OH of the adsorbent was replaced by Hg²⁺to form an inner surface complex.Ferrihydrite mainly relied on electrostatic attraction and ion exchange to form an outer surface complex with Hg（Ⅱ）,while hematite relied on Fe-O bond rupture and surface complex reaction synergy to absorb Hg（Ⅱ）.The reaction mechanism for removing CH₃Hg（Ⅰ）from three iron oxides mainly includes electrostatic adsorption,ion exchange,and complexation through Fe-O-H to form Fe-CH₃Hg or Fe-O-CH₃Hg.4.To improve the adsorption effect of three minerals on CH₃Hg（Ⅰ）,γ-mercaptopropyl trimethoxysilane（KH-590）was used for modification,and batch experiments were conducted to study the adsorption effect and influencing factors.The adsorption amount of CH₃Hg（Ⅰ）was equivalent for modified hematite(182.66μg·g^-1)and modified ferrihydrite(201.16μg·g^-1),while that of modified goethite(147.05μg·g^-1)was lower.The adsorption process conformed to the Langmuir model and Freundlich model.Thermodynamic studies showed that the adsorption of methylmercury by modified iron oxide was an endothermic process,withΔH being negative,and the lowering of temperature is beneficial to the adsorption.When the solution pH value increased from 3to 9,the adsorption amount of CH₃Hg（Ⅰ）by the three modified minerals decreased.The addition of humic acid had little effect on the adsorption of CH₃Hg（Ⅰ）by modified iron oxide.With increasing system ionic strength,the adsorption amount of CH₃Hg（Ⅰ）by the three iron oxides remained basically unchanged.5.The adsorption mechanism of modified iron oxide for CH₃Hg（Ⅰ）was analyzed by X-ray photoelectron spectroscopy and Fourier transform infrared spectrometer.The results showed that the introduction of-SH increased the available adsorption sites:-CH₃SH,-SH,Si-C,Si-O-Fe,Si-O groups;the removal mechanism could be described as electrostatic attraction,ligand exchange and surface coordination;the content of-CH₃SH determined its adsorption capacity,making the adsorption capacity of CH₃Hg（Ⅰ）on iron oxide minerals surface in order of modified ferrihydrite>modified hematite>modified goethite.The results of this study showed that loading thiol groups on iron oxide surfaces could greatly enhance their adsorption capacity for organic mercury and improve their stability,which could be applied to the remediation of wastewater with high organic mercury content in the future.