Adsorption of heavy metals in water by modifying Fe3O4 nanoparticles with oxidized humic acid

https://doi.org/10.1016/j.colsurfa.2021.126333Get rights and content

Highlights

  • Develop new magnetic nanocomposites material.

  • The material exhibited a faster adsorption rate and higher adsorption capacity.

  • Adsorption by cation exchange at low pH and complexation at high pH.

Abstract

Magnetic nanoparticles require a protective coating to enable them to be used to adsorb heavy metals from wastewater, given their oxidation properties and instability in acidic media. Humic acid (HA) has an affinity for magnetic nanoparticles, but its adsorption capacity is low. To improve adsorption capacity, we synthesized a novel adsorbent (HA-O/Fe3O4) through chemical modification and examined its adsorption of Pb (II), Cu (II), Cd (II), and Ni (II) ions. The effects of pH competition, adsorption mechanism, isotherm, kinetics, regeneration, and stability were investigated. The HA-O/Fe3O4 exhibited faster kinetic performance (10 min), conforming to the pseudo-two kinetics equation, and the adsorption isotherm conforms to the Langmuir model with maximum adsorption capacities of 111.10, 76.92, 71.43, and 33.33 mg/g, respectively. The interaction between metal ions and functional groups of the adsorbent was further characterized by Fourier-transform infrared spectroscopy, and the results indicated that adsorption by cation exchange at low pH and complexation at high pH. Additionally, the adsorbent was recycled four times, and the removal rate was not significantly affected. A recycling efficiency of more than 90 % indicates good sustainability and reusability. The stability test shows that the amount of organic matter and heavy metals desorbed by the adsorbent were negligible. Most importantly, at low concentrations, all four metal ions meet the industrial emission standards, and Pb (II) and Cu (II) meet the drinking water standards.

Introduction

Heavy metal ions, such as Pb (II), Cu (II), Cd (II), and Ni (II) are the most common toxic pollutants in electroplating wastewater. They pollute water and soil and seriously affect human health, environmental safety, and sustainable development [1]. There are many methods for removing heavy metals from electroplating wastewater, such as chemical deposition [2], ion exchange [3], solvent extraction [4], electrodialysis [5], and adsorption [6]. Unlike organic pollutants, which can be biodegraded or chemically oxidized, metal ions cannot be degraded; hence, adsorption is considered an effective method for metal pollution treatment [[6], [7], [8], [9]], especially for low-concentration wastewater [10]. Many adsorbents, such as activated carbon, resin, nanomaterials, hydrogels, and zeolites, have demonstrated excellent adsorption properties [[11], [12], [13]]. However, the main disadvantage of these adsorbents is their weak interaction with metal ions [14]. Therefore, the development of solid–liquid separation, high adsorption capacity, and reusability of adsorbents are of great significance.

Humic acid (HA) is a natural macromolecular compound produced by the biological and chemical decomposition of plant and animal residues. HA has a framework of large polycyclic aromatic hydrocarbons, with various chemical functional groups including carbonyl, carboxyl, methoxyl, alcoholic hydroxyl, phenolic hydroxyl, ketones, quinones, and amino groups distributed throughout the framework [15]. Coal-based HA macromolecules have a condensed aromatic ring as a skeleton, and oxygen-containing functional groups are randomly distributed to form a three-dimensional stereoscopic adsorption configuration. HA contains the highest concentration of carboxyl groups and phenolic hydroxyl groups, which is the main reason for electrostatic adsorption, ion exchange, redox, chelation, and coordination of metal elements [16,17].

A wide range of applications of nanocomposites in wastewater treatment have been reported in the literature [18,19]. In particular, magnetic nanoparticles show great potential because of their excellent stability, recyclability, and reusability [[20], [21], [22]]. Recent research has shown that HA has a high affinity for Fe3O4 nanoparticles, and the adsorption of HA on Fe3O4 nanoparticles enhances the stability of nanomaterials by preventing oxidation [23,24]. In recent years, Peng [25] reported the development of HA-modifying Fe3O4 nanoparticles (HA/Fe3O4) for Rhodamine B removal. Rashid [26] successively reported that HA-coated magnetite nanoparticles (HA/MNP) were synthesized and applied to remediate phosphate and remove toxic inorganic arsenic species from aqueous media. Jiang [11] reported that HA/Fe3O4 nanoparticles were employed for the effective adsorption and reduction of toxic Cr (VI) to nontoxic Cr (III) from water. Liu [17] evaluated that HA/Fe3O4 for heavy metal removal in terms of sorption kinetics and capacity, water matrix effect, and material stability.

Although the adsorption of heavy metals by HA/Fe3O4 adsorbent has been studied, coating Fe3O4 magnetic nanoparticles with oxidized HA has not been evaluated. Therefore, considering the purification problem of wastewater, the adsorption capacity of the adsorbent can be further improved by chemical modification. Oxidation modification is an effective way to increase the number of oxygen-containing functional groups in HA. From the perspective of industrial technology, hydrogen peroxide (H2O2) is an ideal oxidant because it is environmentally friendly and can increase the content of the carboxyl group [16,27]. Meanwhile, the proton of the carboxyl group participates in ion exchange, and has the potential to separate and extract metal cations. The HA-O/Fe3O4 adsorbent was prepared by H2O2 oxidation modification of HA and magnetic Fe3O4, and the adsorption behavior under a low concentration and polymetallic solution environment was evaluated to provide a theoretical basis for subsequent studies [2].

A solid–liquid separation adsorbent was prepared in this study. Various measurements were taken to characterize the chemical and morphological characteristics of the synthesized nanocomposite products. The adsorption behaviors of HA-O/Fe3O4 in tetra-metallic solutions were studied, including the solution pH, time, and initial ion concentration, adsorption performance, and sustainability. The interaction between heavy metals and the surface functional groups (binding sites) of the adsorbent before and after modification were analyzed by Fourier-transform infrared spectroscopy (FT-IR), and the reaction mechanism was revealed.

Section snippets

Reagents

FeCl3·6H2O, FeSO4·7H2O, HCl, NH3OH, NaOH, H2O2, and metal nitrate salts (Pb(NO3)2, Cu(NO3)2, Ni(NO3)2, and Cd(NO3)2) were purchased from the Aladdin website. A Milli-Q water purification system was used to generate deionized water. All compounds were used as received.

Oxidation modification of HA

Lignites were obtained from Zhaotong, Yunnan province, China. HAs were extracted from lignites using a modified procedure recommended by the International Humic Substances Society; 0.5 g HA and 0.8 mL H2O2 (10 %) were placed in a

The characterization of HA-O/Fe3O4

Thermal stability was investigated using TGA of Fe3O4 and HA-O/Fe3O4. As shown in Fig. 2 (a), the weight of Fe3O4 remained almost constant over the entire measurement range. For HA-O/Fe3O4, there were three notable weight-loss steps in the TGA curve. The weight loss before 120 °C was attributed to the release of volatile matter and the evaporation of water in HA. These stages occur at 190–455 °C owing to the decomposition of unstable carboxyl and methyl methylene, and gas generated by the

Conclusion

The adsorption behavior of HA-O/Fe3O4 was studied in this work, and the main conclusions are as follows.

  • (1)

    The aggregation in the aqueous suspension was reduced while maintaining the magnetic properties of Fe3O4, which reduces the adsorption time and increases the reaction rate. The reusing test shows that HA-O/Fe3O4 had good regeneration performance.

  • (2)

    The adsorption capacities of the single and tetra-metallic solutions followed the same trend: Pb (II)≫Cu (II) > Cd (II) > Ni (II). The following

CRediT authorship contribution statement

Shuwen Xue: Writing - original draft. Yawen Xiao: Writing - original draft. Guoqiang Wang: Visualization, Investigation. Jinjin Fan: Visualization, Investigation. Keji Wan: Supervision, Funding acquisition. Qiongqiong He: Supervision. Mingqiang Gao: Visualization, Investigation. Zhenyong Miao: Supervision, Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the Natural Science Foundation of Jiangsu Province of China (grant No. BK20190629), the National Natural Science Foundation of China (grant No. 52004280), the “Qing Lan” project, and the Open Sharing Fund for the Large-scale Instruments and Equipments of CUMT.

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