Preparation Of Magnetic Granular Activated Carbon And Its Adsorption Mechanism For Perfluorooctanoic Acid

Perfluorinated compounds are widely used in production and life for their special structure and excellent performance,and perfluorooctanoic acid（PFOA）has also begun to attract attention due to its persistence,refractory,bioaccumulative and biological toxicity.Adsorption is considered to be an effective,low-energy method for removing PFOA.Activated carbon has been selected as a commonly used adsorbent.At present,modification of activated carbon is a research trend to improve its adsorption performance.In this study,magnetic modification was carried out,and magnetic activated carbon（GAC-Fe₃O₄）was synthesized by infiltration-high temperature modification method.The adsorption properties of the adsorbent on perfluorooctanoic acid（PFOA）before and after modification were investigated.Characterization and analysis were carried out by SEM,FTIR,XRD,BET and pore structure,magnetic properties,iron loading and Zeta.Firstly,through the three aspects of adsorption capacity,iron loading and magnetic properties of PFOA,the optimal ratio of magnetic granular activated carbon was obtained:GAC:Fe²⁺:Fe³⁺=2.5:2:1（all are grams of mass）.The BET decreased from 955 cm²·g^-11 of GAC to 797 cm²·g^-11 of GAC-Fe₃O₄,and the number of mesopores decreased,indicating that the size of Fe₃O₄ particles synthesized on the surface of GAC is nanoscale.SEM results show that the surface of GAC-Fe₃O₄ has been embedded with spherical iron oxide.FTIR showed characteristic peaks of magnetic Fe-O bonds at 700 cm^-11 and 590 cm^-1.XRD showed the same characteristic peak as the XRD standard data of Fe₃O₄.The Zeta measurement results show that both electrostatic and hydrophobic interactions may exist during the adsorption of the two materials.The adsorption experiments were carried out on the adsorption of PFOA from the initial concentration,pH value,coexisting ionized calcium ion and humic acid.When the initial concentration is increased,the removal rate will decrease;the acidic environment（pH=3）will have the best adsorption effect on PFOA;the presence of coexisting ionized calcium ions will facilitate the adsorption of PFOA;humic acid will compete with PFOA for adsorption.Will reduce the removal rate.The adsorption process of PFOA by GAC and GAC-Fe₃O₄ was fitted by adsorption isotherm model and kinetic model.The fitting results showed that the maximum adsorption amount of GAC-Fe₃O₄ to PFOA was 588.24 mg·g-1,which was 47.06%higher than that of GAC,and reached the adsorption equilibrium at about 100 h.The adsorption process of PFOA by GAC and GAC-Fe₃O₄ accords with the Langmuir isotherm model and the pseudo-secondary kinetic model,indicating that the adsorption process is single-layer adsorption,and electrostatic adsorption plays a dominant role in the adsorption process.In the desorption and regeneration experiments,it was found that GAC-Fe₃O₄ still possesses certain adsorption properties after desorption three times,and the desorption rate of methanol is higher than that of sodium hydroxide.