U(VI) adsorption to Fe3O4 nanoparticles coated with lignite humic acid: Experimental measurements and surface complexation modeling
Graphical abstract
Introduction
Uranium-contaminated drainage water from uranium mine tailings and waste disposal from nuclear fuel processing have affected the surface water, groundwater, and soils at a number of sites worldwide [[1], [2], [3]]. Under oxic conditions, uranium is present in a solution as a highly mobile U(VI) species [4], and poses a potential threat to the environment and human health owing to its radioactivity and biological toxicity. In 2011, the World Health Organization established a provisional guideline that uranium in drinking water should not exceed 30 μg L−1 [5], and a large number of studies over the past decades have focused on the engineered removal or enrichment of U(VI) from aqueous solutions [[6], [7], [8], [9], [10]].
The aqueous uranyl cation (UO22+) can adsorb onto iron oxides [[11], [12], [13], [14]], and microscale magnetite and magnetite nanoparticles (Fe3O4 NPs) have attracted significant attention for use in engineered adsorption processes [[15], [16], [17], [18], [19], [20]] because they can be quickly recovered through a magnetic separation. However, uncoated Fe3O4 NPs are prone to aggregation [21], which affects their efficiency and restricts their large-scale application in environmental remediation. The surface modification of Fe3O4 NPs can reduce the particle aggregation, and improve the reaction performance [[22], [23], [24]]. For example, Pan et al. [25] measured the U(VI) adsorption onto Fe3O4 NPs functionalized with three separate simple organic acids (stearic acid, oleic acid, and octadecylphosphonic acid), finding that organic acid coatings are highly effective in preventing the aggregation of Fe3O4 NPs, and that the highest adsorption densities reach 103 mg g−1 when equilibrated with a total U(VI) concentration of 9.4 μM at pH 6–7.
Pan et al. [25] demonstrated the effectiveness of organic acid coatings on Fe3O4 NPs in preventing the aggregation and enhancing the metal adsorption. However, Pan et al. [25] used simple laboratory-grade organic acids to coat their NPs, whereas natural organic matter (NOM) would be a less expensive coating material. Furthermore, NOM possesses a wide range and high concentration of functional group types, potentially enhancing the binding capability relative to single-site organic acids. Humic acid (HA) is ubiquitous in soils, surface water, and groundwater systems [26] and can be used as an inexpensive means of coating NPs to decrease the extent of NP aggregation through electrostatic repulsive forces. HA forms strong bonds with heavy metal owing to its abundant carboxylic, phenolic, and sulfhydryl functional groups; therefore, HA coatings on NPs can lead to the effective adsorption removal of heavy metals from a solution [26,27]. HA-coated Fe3O4 NPs have been previously prepared and studied and are promising materials for the removal of environmental pollutants [[28], [29], [30], [31]]. Despite there being a number of studies of metal adsorption onto HA-coated Fe3O4 NPs, few studies have examined U(VI) removal using such NPs. Singhal et al. [32] synthesized Fe3O4 NPs with different extents of HA surface coatings and measured the sorption of U(VI) onto these materials using DI water and sea water matrices, and found that an optimum extent of HA coating exists. However, the relative contributions of the Fe3O4 and HA binding sites in the overall removal of U(VI) were unconstrained in their study. In addition, previous studies have used commercially supplied HAs as NP coatings [[30], [31], [32]], without knowing the exact source or characteristics of the HA applied. The chemical properties of different NOM vary widely, and hence, the effect of NOM coatings on mineral particles on the metal adsorption behaviors likely is strongly NOM-specific [33]. Furthermore, all previous studies on metal adsorption onto HA-coated Fe3O4 NPs have used a Langmuir or Freundlich modeling approach. Because the Langmuir and Freundlich modeling parameters vary as a function of the system conditions (e.g., pH and solution composition, etc.), surface complexation modeling (SCM) represents a more flexible tool for predicting the metal adsorption under conditions not directly studied in the laboratory [[34], [35], [36], [37]]. For example, Pan et al. [25] used a diffuse double-layer SCM approach to model U(VI) adsorption onto Fe3O4 NPs coated with simple organic acids and found that a series of uranyl-, uranyl-hydroxide, and uranyl-carbonate surface complexes are required to account for the adsorption behavior observed. However, no studies have attempted to use an SCM approach to model U(VI) adsorption onto Fe3O4 NPs coated with complex HA.
In this study, we measured U(VI) adsorption onto synthesized Fe3O4 NPs coated with different amounts of HA derived from lignite (LHA). Lignite is a type of low-grade and low-value coal, and there are estimated to be more than one trillion tons of it across the world [38]. Because of the abundance of lignite, LHA is less expensive than commercially available HAs. Similar to other HAs, LHA contains predominantly carboxylic and phenolic functional groups [39,40], but its higher sorption capacity for heavy metals [41] suggests that LHA contains a higher concentration of binding sites than in HAs, despite the site concentrations on LHA having not been previously measured. Thus, LHA can bind more tightly onto the surface of the NPs when used as a coating material, thereby increasing the stability of the NP adsorbents in the suspension. Despite the potential advantages of LHA, most research and applications of HA coatings onto NPs have involved commercial HA, and relatively few studies have been conducted with LHA-coated NPs. In this study, we measured the adsorption behavior as a function of the solution pH, ionic strength, reaction time, and sorbate-to-sorbent ratio. XPS measurements of LHA-coated Fe3O4 NPs before and after U(VI) adsorption were collected to probe a possible change in the valence state of uranium accompanying the adsorption. To understand the relative contributions of the Fe3O4 NPs and the LHA components during U(VI) removal, we used a non-electrostatic SCM to interpret the adsorption data, and we conducted experimental measurements to determine the stability constants for the important surface complexes.
Section snippets
Sorbent preparation and characterization
LHA was extracted from Lincang lignite from Yunnan Province, China, and purified through a modification of the traditional alkaline-acid method, as reported previously [26]. Briefly, the lignite sample was mixed with 0.5 mol L−1 NaOH and 0.1 mol L−1 Na4P2O7·10H2O (1:1) at a volume ratio of 10:1 (m/V) and shaken for 3 h. Then, 6 mol L−1 HCl was added to the supernatant until obtaining a pH of 1 to precipitate the HAs. Next, a 0.5 % HCl-HF solution was added to the precipitated HAs to eliminate
Characterization of samples
The FT-IR spectra of the Fe3O4 NPs, LHA-coated Fe3O4 NPs, and LHA before and after adsorption are shown in Fig. 1a and b. The peak at ∼580 cm−1 in the LHA-coated Fe3O4 NP samples is related to the stretching vibration of the Fe-O bond [48]. However, the intensity of the Fe-O bond becomes weaker with an increase in the LHA concentration, which is likely because the increased LHA concentration obscures the signal of the Fe-O bond. The peaks at ∼3410 cm-1 in each of the samples are related to OH
Conclusions
In this study, we measured the adsorption behavior of LHA-coated Fe3O4 NPs for the removal of U(VI) from a solution under a wide range of conditions. U(VI) adsorption onto LHA-coated Fe3O4 NPs was pH dependent, with an optimum pH range of 5.0–8.0. The LHA-coated Fe3O4 NPs adsorbed significantly higher concentrations of U(VI) than the uncoated Fe3O4 NPs. Specifically, the maximum adsorption capacities at pH 5.0 were 42.5, 55.6, and 68.7 mg g−1 on 0.5, 1.5, and 2.5 LHA-coated Fe3O4 NPs,
CRediT authorship contribution statement
Yangyang Zhang: Conceptualization, Investigation, Data curation, Visualization, Writing. Jeremy B. Fein: Methodology, Software, Formal analysis, Funding acquisition, Writing - review & editing, Project administration. Yilian Li: Methodology. Qiang Yu: Software, Formal analysis. Bo Zu: Project administration, Supervision, Writing - review & editing. Chunli Zheng: Supervision, Writing - review & editing.
Declaration of Competing Interest
The authors declare no conflict of interest.
Acknowledgements
The DOC analysis and some of the U analyses were conducted at the Center for Environmental Science and Technology (CEST) at the University of Notre Dame. This work was supported by a scholarship from China Scholarship Council [CSC student ID: 201806410028], a research fund from Chongqing Jiaotong University (20JDKJC-B007) and Natural Science Foundation of Chongqing, China (cstc2020jcyj-msxmX0763).
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