Elsevier

Chemical Engineering Journal

Volume 358, 15 February 2019, Pages 1399-1409
Chemical Engineering Journal

Novel insight into adsorption and co-adsorption of heavy metal ions and an organic pollutant by magnetic graphene nanomaterials in water

https://doi.org/10.1016/j.cej.2018.10.138Get rights and content

Highlights

  • MGO showed the best adsorption capacities for TC, Cd(II) and As(V) among three magnetic graphene nanomaterials.

  • Adsorption behaviors of MGO in three different types of binary systems were systematically studied.

  • Mutual effects of TC and Cd(II) in the simultaneously added system were negligible.

  • Adsorption of As(V) was significantly suppressed in the presence of TC.

  • Cd(II) and As(V) were co-adsorbed by MGO via electrostatic attraction and formation of type A ternary surface complexation.

Abstract

The adsorption of tetracycline (TC), cadmium [Cd(II)] and arsenate [As(V)] onto magnetic graphene oxide (MGO), magnetic chemically-reduced graphene (MCRG) and magnetic annealing-reduced graphene (MARG) was investigated to understand the adsorption properties and molecular mechanisms. The adsorption of three contaminants was pH-dependent and the adsorption capability followed the order of MGO > MCRG > MARG, and hence MGO was selected to systematically study the adsorption behaviors in three different types of binary systems. The maximum adsorption capacities of MGO were 252 mg/g for TC, 234 mg/g for Cd(II) and 14 mg/g for As(V). The superiority of MGO was mainly attributed to its high dispersibility, thin nanosheets and various O-containing functional groups. In addition to H-bonding and π–π interactions, the strong adsorption of TC onto MGO was mainly due to the n–π electron-donor–acceptor (EDA) effect, with the maximum adsorption around pKa of TC. The mutual effects of TC and Cd(II) in the simultaneously added system were negligible. Adsorption of As(V) was significantly suppressed in the presence of TC, whilst As(V) hardly affected TC adsorption. The adsorptions of Cd(II) and As(V) in the co-adsorption system were increased by 65% and 30%, respectively. This synergistic effect resulted from the electrostatic attraction and the formation of type A ternary surface complexation. These new insights were valuable for elucidating the interaction mechanisms and designing novel adsorbents for traditional and emerging pollutions in practical application.

Introduction

Water quality has been deteriorated continuously due to the rapid growth of population, urbanization, industrialization, and other environmental issues [1], [2], [3]. Multiple pollutants often coexist in aquatic environments and change chemical, physical or biological properties of water [4], [5], [6]. If inadequately treated, aquatic ecosystems could be adversely affected by the combined pollutions [7], [8]. The treatment of soil and water co-contaminated with heavy metals and organic pollutants poses a significant challenge because these two types of pollutants have different fates and transport mechanisms [9]. Sorption is widely used to eliminate both inorganic and organic pollutants from aqueous solutions owing to the simplicity of design, low cost, high efficiency, and wide adaptability [1], [10].

As two-dimensional carbon-based materials with honeycomb structures that are sp2-hybridized with an atom thickness, graphene and its derivatives were reported to show excellent performance in adsorption of various environmental contaminants, such as aromatic compounds [11], [12], [13], dyes [14], [15], [16], antibiotics [17], [18], estrogens [19], [20], [21], heavy metal ions [22], [23], anions [24], [25] and combined pollutions [6], [26]. Especially, graphene oxide (GO), the main precursor of graphene, has been widely applied in waste-water treatment or water purification due to its large specific surface area and a variety of O-containing functional groups such as hydroxide, epoxide, carbonyl and carboxyl groups [4], [26], [27]. However, the separation procedures for this type of absorbents are often complicated due to their high dispersibility in water, which could lead to new environmental risks [4], [28]. Magnetization by introducing Fe3O4 nanoparticles (NPs) onto GO and reduced graphene oxide (RGO) can bring about separation convenience, even increased adsorption capacity, which favor the practical applications [29], [30], [31].

A number of studies have been conducted on the adsorption of magnetic graphene nanomaterials (MGNs) for heavy metals and organic contaminants in aqueous systems [32], [33]. However, the existing literature is inadequate in concerning the interactions between multi-pollutants in systems, whereas coexistence of various pollutants is much more common in real environments [6], [12]. Antibiotics (e.g., TC) and heavy metals (e.g., Cd(II)) are extensively employed as growth promoters and usually coexist in wastewater from livestock farm, causing considerable toxicological concerns [34]. Furthermore, multiple contaminations in natural environments influence the removal of individual contaminants via electrostatic interaction, cation–π interaction, precipitation or/and complexation [5], [35], [36].

Chemically-synthesized graphene nanosheets are generally defective and contain surface heterogeneity with various structures such as flat surface, wrinkles, defects, and O-containing functional groups, which are valuable for the high capacity of pollutant sorption [37], [38], [39]. The surface properties could be greatly altered by these structures, and affect the intrinsic properties and environmental applications of graphene-based materials. For instance, metal ions are preferentially adsorbed onto GO rather than RGO, due to the abundant surface functional groups on GO [4], [5]. The reduction of exfoliated GO removed the O-containing functional groups and repaired a sp2-hybridized structure [40], leading to weakened surface complexations of heavy metal ions with oxidized sites [41], enhanced π–π interactions between graphene nanosheets and organic pollutants, and reduced competition of water molecules with organic pollutants at the oxidized sites [5], [11]. Several preparative strategies have been explored for graphene morphology regulation [37], [38], while designing MGNs with special physicochemical properties and structural features is needed in practical applications.

In this study, we prepared and characterized magnetic graphene oxide (MGO), magnetic chemically-reduced graphene (MCRG) and magnetic annealing-reduced graphene (MARG), which contained different microstructures, iron and oxygen contents, and displayed various sorption capacities. Cadmium [Cd(II)] and arsenate [As(V)] were selected as model cation and oxyanion contaminant, respectively [6], [42], and tetracycline (TC) was selected as a model organic contaminant [43]. The main objective was to examine the properties and unique molecular adsorptive and co-adsorptive mechanisms of these magnetic graphene nanomaterials. The adsorption isotherms were established and effects of pH were examined to evaluate the adsorption behaviors of each of these pollutants on MGO, MCRG, and MARG in single systems. Besides, MGO was selected to further study the adsorption behaviors, interactions between the pollutants and associated mechanisms in three diverse types of binary solutions.

Section snippets

Synthesis of magnetic graphene nanocomposites

Graphene oxide (GO) was synthesized from natural graphite flakes via a modified Hummers’ method [44]. The adhesive GO was exfoliated by sonication and dialyzed to remove acids and other impurities. The Fe3O4 nanoparticles (NPs) were prepared by chemical co-precipitation of FeCl3·6H2O and FeSO4·7H2O. The sol of Fe3O4 NPs was obtained after sonication for 2 h [45]. The GO solution was then added dropwise into the sol of Fe3O4 NPs under appropriate concentrations and pH conditions under mechanical

Characterization of magnetic graphene nanocomposites

The TEM image (Fig. 1a) of MGO showed that the Fe3O4 NPs (10–20 nm size) were well dispersed in 2–5 layers of GO matrix, and the folding nature of graphene sheets was clearly visible. The basal plane of the GO was almost unoccupied by the magnetic NPs, consistent with the previous finding [47]. Dense aggregates of Fe3O4 NPs were observed in MCRG and MARG (Fig. 1b-c), indicating that the reduction of MGO induced serious wrinkles and overlaps on graphene nanosheets. Thermal reduction of MGO to

Conclusions

This study presents a new strategy to prepare diverse types of magnetic graphene nanomaterials, MGO, MCRG, and MARG, with different microstructures and properties. The three magnetic graphene nanomaterials had a good magnetic separability and showed excellent adsorption capacities for TC, Cd(II) and As(V), while MGO was shown to be the best adsorbent. High dispersibility and thin nanosheets contributed to the superior adsorption capabilities of MGO. Moreover, 59% O-containing functional groups

Acknowledgement

This work was financially supported by the National Natural Science Foundation of China (41721001), the Science and Technology Program of Zhejiang Province (2018C03028), and Agriculture Research System of China (CARS-01-30).

References (69)

  • J. Xiao et al.

    Graphene/nanofiber aerogels: performance regulation towards multiple applications in dye adsorption and oil/water separation

    Chem. Eng. J.

    (2018)
  • Y. Gao et al.

    Adsorption and removal of tetracycline antibiotics from aqueous solution by graphene oxide

    J. Colloid Interf. Sci.

    (2012)
  • L. Jiang et al.

    Adsorption of estrogen contaminants (17 beta-estradiol and 17 alpha-ethynylestradiol) by graphene nanosheets from water: effects of graphene characteristics and solution chemistry

    Chem. Eng. J.

    (2018)
  • L. Jiang et al.

    Removal of 17 beta-estradiol by few-layered graphene oxide nanosheets from aqueous solutions: external influence and adsorption mechanism

    Chem. Eng. J.

    (2016)
  • G. Abdul et al.

    Structural characteristics of biochar-graphene nanosheet composites and their adsorption performance for phthalic acid esters

    Chem. Eng. J.

    (2017)
  • K. Chang et al.

    Molecular insights into the role of fulvic acid in cobalt sorption onto graphene oxide and reduced graphene oxide

    Chem. Eng. J.

    (2017)
  • Q. Liang et al.

    Facile one-pot preparation of nitrogen-doped ultra-light graphene oxide aerogel and its prominent adsorption performance of Cr(VI)

    Chem. Eng. J.

    (2018)
  • S.B. Wang et al.

    Adsorptive remediation of environmental pollutants using novel graphene-based nanomaterials

    Chem. Eng. J.

    (2013)
  • V. Kumar et al.

    Graphene and its nanocomposites as a platform for environmental applications

    Chem. Eng. J.

    (2017)
  • B. Hu et al.

    Macroscopic and spectroscopic insights into the mutual interaction of graphene oxide, Cu(II), and Mg/Al layered double hydroxides

    Chem. Eng. J.

    (2017)
  • J. Zhao et al.

    Mechanistic understanding toward the toxicity of graphene-family materials to freshwater algae

    Water Res.

    (2017)
  • Y. Yoon et al.

    Comparative evaluation of magnetite-graphene oxide and magnetite-reduced graphene oxide composite for As(III) and As(V) removal

    J. Hazard. Mater.

    (2016)
  • Y. Yao et al.

    Synthesis, characterization, and adsorption properties of magnetic Fe3O4@graphene nanocomposite

    Chem. Eng. J.

    (2012)
  • K. Chang et al.

    Macroscopic and molecular study of the sorption and co-sorption of graphene oxide and Eu(III) onto layered double hydroxides

    Chem. Eng. J.

    (2017)
  • A. Chen et al.

    Carbon disulfide-modified magnetic ion-imprinted chitosan-Fe(III): a novel adsorbent for simultaneous removal of tetracycline and cadmium

    Carbohyd. Polym.

    (2017)
  • J. Wu et al.

    Remediation of As(III) and Cd(II) co-contamination and its mechanism in aqueous systems by a novel calcium-based magnetic biochar

    J. Hazard. Mater.

    (2018)
  • D. Li et al.

    Facile synthesis of nitrogen-doped graphene via low-temperature pyrolysis: the effects of precursors and annealing ambience on metal-free catalytic oxidation

    Carbon

    (2017)
  • S. Stankovich et al.

    Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide

    Carbon

    (2007)
  • W. Jiang et al.

    Arsenate and cadmium co-adsorption and co-precipitation on goethite

    J. Hazard. Mater.

    (2013)
  • N. Ye et al.

    Synthesis of magnetite/graphene oxide/chitosan composite and its application for protein adsorption

    Mat. Sci. Eng. C-Mater.

    (2014)
  • E. Roy et al.

    Multifunctional magnetic reduced graphene oxide dendrites: synthesis, characterization and their applications

    Biosensors Bioelectron.

    (2015)
  • H. Peng et al.

    Enhanced adsorption of Cu(II) and Cd(II) by phosphoric acid-modified biochars

    Environ. Pollut.

    (2017)
  • Y. Lin et al.

    Fast and highly efficient tetracyclines removal from environmental waters by graphene oxide functionalized magnetic particles

    Chem. Eng. J.

    (2013)
  • J. Deng et al.

    Simultaneous removal of Cd(II) and ionic dyes from aqueous solution using magnetic graphene oxide nanocomposite as an adsorbent

    Chem. Eng. J.

    (2013)
  • Cited by (222)

    View all citing articles on Scopus
    View full text