Preparation Of Functionalized Graphene Oxide And Its Adsorption Performance For Hg(Ⅱ) And Cr(Ⅵ)

Hg(Ⅱ)and Cr(Ⅵ) are serious sources of heavy metal pollution.The adsorption method is widely used in the field of heavy metal wastewater treatment because of its simple equipment and quick and easy operation.As adsorbents,graphene oxide(GO)has shown great potential in the field of environmental water treatment due to its abundant functional groups and large surface area.In this paper,a series of functionalized GO-based adsorbents were prepared by coupling and Schiff base methods to remove the Hg(Ⅱ)and Cr(Ⅵ)from wastewater.The adsorption performance of Hg(Ⅱ)and Cr(Ⅵ)in water was investigated,and the related adsorption mechanism was systematically explored and studied.To improve the removal efficiency of low Hg(Ⅱ)concentrations,thiol-functionalized GO(S-GO)was prepared by a cross-linking method using a sulfhydryl-containing silane coupling agent as a precursor.The maximum adsorption capacity of S-GO for Hg(Ⅱ)was up to 3490 mg/g.And S-GO is able to reduce the concentration of 1.59 mg/L Hg(Ⅱ)to the drinking water standard(0.002mg/L)within 60 minutes.The adsorbent exhibited a high selectivity for Hg(Ⅱ)in the coexistent of other interfering metal ions,and the SF of Hg(Ⅱ)for interfering metal ions was order 10~3 to 10~5.The S-GO can reduce 1.59 mg/L Hg(Ⅱ)contaminants below the discharge standard of drinking water(2μg/L)within 60min.In addition,the removal efficiency of adsorbent for Hg(Ⅱ)can still reach92.4%after 10 adsorption-desorption cycles.S-GO can effectively remove Hg(Ⅱ)in the PVC wastewater to below the emission standard.XPS and FTIR results revealed that the removal mechanism of Hg(Ⅱ)by the adsorbent was mainly the chelation reaction of S and O functional groups on the adsorbent surface with Hg(Ⅱ).The synthesis of poly(allylamine hydrochloride)(PAH)functionalized GO by the hydrothermal and electrostatic self-assembly has a weak binding force,amination reagents in chemical coupling are too expensive.In order to solve the above problems,poly(allylamine hydrochloride)functionalized GO was prepared by a combination of coupling and Schiff base reaction to obtain a chemically stable PAH-ASGO adsorbent.The adsorption kinetic results show that the obtained PAH-ASGO exhibited an ultrahigh adsorption rate for Cr(Ⅵ)(h=1736 mg/g·min),far exceeding those of all amino-functionalized adsorbents previously reported.PAH-ASGO can effectively diminish the Cr(Ⅵ)concentration from 9.9 mg/L to the extremely low level of 0.004 mg/L within 10 s,far below the maximum allowable level of Cr(Ⅵ)(0.05 mg/L)in drinking water.In addition,the adsorbents still displayed excellent removal efficiency of 91.8%after 10 cycles.In addition,PAH-ASGO can effectively reduce Cr(Ⅵ)in chemical plants to discharge standards.To improve the adsorption capacity of GO for Cr(Ⅵ),amino-rich polymeric polyethyleneimine(PEI)functionalized GO(PEI-ASGO)adsorbents were prepared by a combination of coupling and Schiff base reactions.The adsorption isotherm results show that the maximum adsorption capacity of PEI-ASGO for Cr(Ⅵ)was up to 1185 mg/g,which exceeds that of most previously reported GO-based materials.Furthermore,the cycle experiments indicated that the composite can still remove Cr(Ⅵ)to drinking water standards after 40 cycles.The XPS and FTIR results showed that the removal mechanism of Cr(Ⅵ)by PEI-ASGO was mainly by electrostatic adsorption and reduction.PEI-ASGO can effectively reduce Cr(Ⅵ)in chemical plants to below the allowed safe levels for drinking water.Most adsorbents were designed to eliminate Cr(Ⅵ)or Hg(Ⅱ)solely rather than simultaneously due to the large differences in physical and chemical properties between Cr(Ⅵ)and Hg(Ⅱ),which became the critical limiting factor for the simultaneous elimination of Cr(Ⅵ)and Hg(Ⅱ)from aquatic environments.4-Amino-3-hydrazino-5-mercapto-1,2,4-triazole(AHMT)with multidentate ligands was covalently bonded to GO via Schiff base reaction to obtain AHMT-ASGO porous adsorbent.The results show that the adsorbent exhibits good adsorption capacity for both Cr(Ⅵ)and Hg(Ⅱ)in the p H range of 1-10.The maximum adsorption capacity of this adsorbent for Cr(Ⅵ)and Hg(Ⅱ)was 734.2 and 1091.1mg/g,which were 14.36 and 5.61 times that of GO,respectively.Moreover,the adsorbent shows an excellent performance for simultaneous removal of Cr(Ⅵ)and Hg(Ⅱ)with more than 99.9%and 98.6%removal efficiencies in aqueous solutions.The adsorption kinetics and extended adsorption isotherms reveal that the adsorption of Cr(Ⅵ)and Hg(Ⅱ)on adsorbent is a double-layer adsorption process.Hg(Ⅱ)species were rapidly adsorbed on the AHMT-ASGO-6 surface at the initial stage.Subsequently,the same adsorption sites for both Cr(Ⅵ)and Hg(Ⅱ)were displaced gradually by the Cr(Ⅵ).To solve the problem of difficult separation of GO-based adsorbents,AHMT-functionalised magnetic GO adsorbents were synthesised,and the magnetic adsorbents with Cr(Ⅵ)and Hg(Ⅱ)adsorbed could be rapidly separated from water within 30 seconds by an external magnetic field.