Study On The Adsorption Of Heavy Metals By LDH-GO And Its Modified Composites In Aqueous Solution

With the rapid development of the economy and the accelerated process industrialization,numerous of industrial pollutants are discharged into the environment,resulting in a growing problem of environmental pollution.Heavy metal pollution is attracting increasingly attention from the public due to its widely existence and severity of hazard.Owing to this fact,the removal of heavy metal ions from water is of great significance to the protection of public health and the environment.In terms of controlling water pollution caused by heavy metals,adsorption is most promising and widely used due to its high efficiency,easy operation and recyclability,etc.The synthesis and modification of novel materials with high removal efficiency,low cost and friendly to environment is vital important to the promotion and application of adsorption in practical engineering.As a new type of composite material,Layered Double Hydroxide-Graphene Oxide has great potential in the application of environmental remediation.With the combination,the disadvantage of aggregation can be effectively prevented,and their distinguishing properties can be integrated together.The composite overcomes the disadvantage of each other in wastewater treatment,which is conducive to the removal of pollutants.In this paper,Layered Double Hydroxide-Graphene Oxide composite was synthesized by co-precipitation,and a series of novel LDH-GO based multifunctional adsorbent materials were synthesized by modification with Fe2+,amino-group and thiol group.The synthesized composites were applied in remediation of heavy metal ions from aqueous solutions.The adsorption kinetics,adsorption isotherm,adsorption thermodynamics,etc.have been studied,combined with XRD,FTIR,SEM/EDS,XPS and other characterization methods,the adsorption properties and mechanism were discussed.In consideration of combined pollution,LDH@GO-SH was chosen as efficient adsorbent and applied to removal of heavy metals from combined polluted wastewater.Additionally,taking printing and dyeing factory wastewater as research objects,the removal effect of copper ions by LDH@GO-SH on actual industrial wastewater was studied.The main research results are as follows:(1)The prepared Layered Double Hydroxide-Graphene Oxide(LDH-GO)composites were successfully applied in adsorption of heavy metal ions(Cu(Ⅱ),Cd(Ⅱ),Pb(Ⅱ)and Fe(Ⅱ))from aqueous solution.LDH-GO composites with different amount of GO(LDH-GO-1,LDH-GO-2 and LDH-GO-3,have been prepared through a coprecipitation method.The results showed that increase the amount of GO in LDH-GO could enhance the adsorption capacity due to the increase of specific surface area and oxygen-containing functional groups.The adsorption of Cu(Ⅱ)and Fe(Ⅱ)by LDH-GO could be better described by pseudo-first-order kinetics,while the adsorption of Cd(Ⅱ)and Pb(Ⅱ)by LDH-GO were better described by pseudo-first-order kinetics.The Langmuir model could well describe the sorption isothermal behaviors of Cu(Ⅱ),Cd(Ⅱ),Pb(Ⅱ)and Fe(Ⅱ)by LDH-GO.The maximum adsorption capacities of Cu(Ⅱ),Cd(Ⅱ),Pb(Ⅱ)and Fe(Ⅱ)by LDH-GO-3 calculated by the Langmuir model were 161.55mg/g,25.20 mg/g,401.61 mg/g and 653.59mg/g,respectively.Combined the adsorption experiment and characterization results showed that isomorphic substitution,chemical precipitation and surface complexation played a different role in the adsorption of Cu(Ⅱ),Cd(Ⅱ),Pb(Ⅱ)and Fe(Ⅱ)on LDH-GO.(2)After adsorption of Fe(Ⅱ),LDH-GO endowed with unique 3D structure and large specific surface area,which could enhance the adsorption capacity for heavy metal ions(Cu(Ⅱ)and Cd(Ⅱ)).The concentration of Fe(Ⅱ)during the medication had a different influence on the adsorption of Cu(Ⅱ)and Cd(Ⅱ).LDH-GO@Fe(Ⅱ)50showed the best performance of adsorption on Cu(Ⅱ),with the maximum adsorption capacity of 175.74 mg/g.While,LDH-GO@Fe(Ⅱ)10 had the maximum adsorption capacity for Cd(Ⅱ)(29.39 mg/g).The combined the results of macroscopic experiments and characterization elucidated that the adsorption mechanisms of Cu(Ⅱ)on LDH-GO@Fe(Ⅱ)were isomorphic substitution,chemical precipitation and surface complexation.The adsorption mechanisms of Cd(Ⅱ)on LDH-GO@Fe(Ⅱ)were chemical precipitation and surface complexation,while the electrostatic interactions(electrostatic repulsion)mainly contributed to the decrease of Cd(Ⅱ)adsorption on LDH-GO@Fe(Ⅱ)50.(3)Two novel adsorbents LDH@GO-NH2 and LDH@GO-SH were succecfully synthesized by grafting thiol-or amino-functionalized GO onto LDH and its adsorption capacities for heavy metal ions(Cu(Ⅱ)and Cd(Ⅱ))were significantly enhanced.Characterization experiments illustrated that amino group(-NH2)or thiol group(-SH)was successfully grafted onto LDH@GO-NH2 or LDH@GO-SH.The amino-and thiol-group can provide more adsorption sites for heavy metal ions.The kinetic experimental data for LDH@GO-NH2 and LDH@GO-SH were found to be in good agreement with pseudo-second-order model.Adsorption isotherm results showed that the adsorption processes were satisfactorily fitted by both Langmuir and Freundlich models,and the adsorption was a spontaneous and endothermic process.LDH@GO-SH exhibited higher adsorption capacities than LDH@GO-NH2 for both Cu(Ⅱ)and Cd(Ⅱ),especially for Cd(Ⅱ).At 308 K,the maximum adsorption capacity of LDH@GO-SH for Cd(Ⅱ)predicted by Langmuir model was 102.77 mg/g,which was 2.7 times that of LDH@GO-NH2.The possible adsorption mechanisms comprised formation of precipitation,isomorphic substitution of Mg(Ⅱ),and formation of complexation with amino-groups and thiol-groups were proposed.The adsorbent can be regenerated and reused for 5 times with a high removal efficiency.Regeneration experiments put into evidence that the LDH@GO-NH2 and LDH@GO-SH may be promising suitable candidates for the remediation of metal ions from aqueous solutions in real work in the near future.(4)The mutual effect of NOM and Cu(Ⅱ)adsorption in binary system was investigated.The presence of HA or FA would decrease the adsorption capacity of Cu(Ⅱ),the negative influence was more obvious with the increase in the concentration of HA or FA.On the other hand,the existence of Cu(Ⅱ)in these binary system showed a significant increase in adsorption capacities on HA or FA.The adsorption capacities increased with the increase of the concentration of Cu(Ⅱ),under higher HA or FA initial concentration systems(25 mg/L and 50 mg/L).The adsorption of Cu(Ⅱ)in binary systems were satisfactorily fitted by Langmuir model and pseudo-first-order model.The synergistic adsorption mechanism of NOM and Cu(Ⅱ)in the binary system was mainly the formation of S-Cu-NOM complex on the surface of LDH@GO-SH..(5)The coexistence of MB and MO enhanced the adsorption of Cu(Ⅱ)by LDH@GO-SH,while the presence of Cu(Ⅱ)affected differently on the adsorption of MB and MO.With presence of Cu(Ⅱ),the adsorption capacity of MO remarkably increased.The presence of Cu(Ⅱ)had a negative effect on MB adsorption for lower MB initial concentration systems(2-10 mg/L),while a positive effect for the higher MB concentrations systems(25 mg/L and 50 mg/L).The adsorption of Cu(Ⅱ)in binary systems were satisfactorily fitted by Langmuir model and pseudo-first-order model.Complex-adsorption and surface complex-interaction were both demonstrated as potential mechanisms for the enhancement of Cu(Ⅱ)adsorption.(6)Taking printing and dyeing factory wastewater as research objects,the removal effect of coppor ions and organic dyes by LDH@GO-SH on actual industrial wastewater was studied.The results showed that LDH@GO-SH had a pleasant adsorption effect to coppor ions in the pH range of 3-9,the removal rate was higher than 95%.The concentration of coppor after adsorption was lower than 0.5 mg/L,which can meet the standard for primary discharge of industrial wastewater.The adsorption kinetics indicated that the adsorption followed pseudo-second-order model.The thermodynamic experiments showed that the adsorption of coppor ions by LDH@GO-SH was a spontaneous endothermic process,higher temperature was beneficial to the adsorption process.LDH@GO-SH exhibited good removal ability for dyes in the wastewater,the main chromophoric groups were adsorbed.The adsorbent can be regenerated by Na2 EDTA and reused for 3 times with a high removal efficiency for copper.In conclusion,Layered Double Hydroxide-Graphene Oxide composite has great performance in terms of adsorbing heavy metal ions,like Cu(Ⅱ),Cd(Ⅱ),Fe(Ⅱ)and Pb(Ⅱ).And its adsorption capacity and recycling capacity can be improved through functional modification with Fe2+,amino-group and thiol-group.These four adsorbent materials,therefore,have considerable application prospect in the treatment of heavy metal polluted water.