| Graphene,a two-dimensional allotrope of carbon,with a single layer of sp~2-hybridized carbon atoms,has attracted much attention from the scientific community due to its specific physical and chemical properties,large external surface area,and ease of modification.It has been widely used in environmental applications.Graphene and its derivatives were considered as super adsorbents in removal of organic pollutants and heavy metals.However,graphene nanosheets will commonly stack due to the strongπ-πinteractions,which will suppress the expose of the active sites.Graphene oxide(GO),the main precursor of graphene,has high dispersibility in water,while the separation procedures for this type of absorbents are often complicated and could lead to new environmental risks.Magnetization by introducingnanoparticles onto graphene can bring about separation convenience,even increased adsorption capacity,which favor the practical applications.Concentrating on the regulation of magnetic graphene’s structure and property and the identification of interaction mechanisms of typical organic pollutants with magnetic graphene nanomaterials,the wrapped self-assembled magnetic graphene oxide(MGO)was obtained from the electrostatic interactions between GO layers andnanoparticles.We further prepared and characterized magnetic chemically-reduced graphene(MCRG) and magnetic annealing-reduced graphene(MARG),which contained different microstructures,iron and oxygen contents compared to MGO.Firstly,adsorption and desorption of phenanthrene by magnetic graphene nanomaterials from water,and roles of pH,natural organic matter and heavy metal ions were investigated.Secondly,adsorption of tetracycline(TC) by magnetic graphene nanomaterials as affected by pH and heavy metal ions were illustrated.Thirdly,the effect of magnetic graphene addition on the adsorption behavior of organic polluted soil-water interface was explored.The results are essential for determining the effect of magnetic graphene nanomaterials on the fate and transport of contaminants and for designing novel efficient adsorbents.The main novelties and findings of the study were listed below:(1)The morphologies and properties of magnetic graphene obtained by different reduction methods were systematically characterized,and their changes were discussed.MGO had high dispersibility,thin nanosheets and various O-containing functional groups.In comparison,dense aggregates ofnanoparticles,curved surfaces and decreased O-containing functional groups were observed in MCRG and MARG.Besides,the results also suggested that the aromatic C contents increased via reduction and that the thermal reduction could reduce MGO thoroughly.Chemical reduction of MGO to MCRG increased the BET specific surface areas,the total pore volume and graphitic structural defects.Thermal reduction of MGO to MARG increased the total pore volume,but decreased the surface area,and few graphitic structural defects were created.(2)The adsorption of phenanthrene onto MGO,MCRG,and MARG were compared to examine their unique adsorption properties.The effects of environmental factors on the adsorption-desorption properties of phenanthrene e.g.p H,natural organic matter and heavy metal ions,were also investigated.MCRG had the highest adsorption capacity(34.37 mg/g)for phenanthrene,mainly due to the larger surface area and pore volume,and numerous wrinkles of MCRG.Theπ-πinteraction was the predominant adsorption mechanism of MCRG,while the adsorption of phenanthrene by MARG was mainly through bothπ-πinteraction and hydrophobic interactions.Coexisting Cd(Ⅱ) and As(Ⅴ) had minor impact on phenanthrene adsorption by MCRG,while phenanthrene adsorption(1.0 mg/L)by MCRG gradually decreased by approximately20%with 25 mg/L humic acid.Phenanthrene caused desorption hysteresis,which was largely suppressed by humic acids.The desorption hysteresis was ascribed to the entrapment of phenanthrene molecules derived from the generation of closed interstitial spaces caused by the rearrangement of graphene nanosheets onto MCRG.Steric hindrance was much larger with adsorbed humic acid molecules.(3)Adsorption of tetracycline(TC)by magnetic graphene nanomaterials with different properties and structures were investigated to understand the adsorption properties and molecular mechanisms.The adsorption of tetracycline was pH-dependent and the adsorption capability followed the order of MGO>MCRG>MARG,which the maximum adsorption capacities of MGO were 252.0 mg/g,and hence MGO was selected to systematically study the adsorption behaviors in two different types of binary systems.The superiority of MGO was mainly attributed to its high dispersibility,thin nanosheets and various O-containing functional groups.The main adsorption forces for TC on MGO were H-bonding,π-πinteraction,and n-πelectron-donor–acceptor(EDA)effect,while the adsorption of TC by MCRG and MARG was mainly throughπ-πinteraction.The mutual effects of TC and Cd(Ⅱ) in the simultaneously added system were negligible,owing to compensation mechanisms of cation–πinteraction and H-bonding.Adsorption of As(Ⅴ) was significantly suppressed in the presence of TC,whilst As(Ⅴ) hardly affected TC adsorption.(4)The adsorption behaviors of tetracycline on soil-water interface with and without addition of MGO and MCRG were explored.The relative contributions of MGO/MCRG to the overall tetracycline adsorption were closely related to soil physico-chemical properties.For Inceptisol with higher soil organic matter and cation exchange capacity(CEC),the contribution of magnetic graphene addition to the overall adsorption capacity was low.When the Oxisol,with lower organic matter and CEC,amended with 1.0% MGO and 1.0%MCRG,the relative contribution of MGO and MCRG to the overall tetracycline adsorption was 46.21% and 38.14%,respectively.Humic acid at high concentrations had a certain shielding effect on the adsorption of tetracycline onto MGO,indicating soil organic matter would affect the adsorption of tetracycline by magnetic graphene.The study also found that the adsorption interface"felt"by tetracycline molecules was different in the range of low and high concentration,and the heterogeneity of Inceptisol amended with different amounts of magnetic graphene was enhanced at high concentrations,and the interface sites were more widely distributed. |
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