Compositional transformation of Ni²⁺ and Fe⁰ during the removal of Ni²⁺ by nanoscale zero-valent iron and the implications to groundwater remediation

Nickel ion (Ni²⁺) is among the most prevalent heavy metal ions that are of concern to human and environmental health (Sprynskyy et al. 2006; Luo et al. 2015; Sachan & Lal 2017; Ahmad et al. 2019). The presence of Ni²⁺ in the environment through natural and anthropogenic processes makes water and soil unsafe for human consumption and crop cultivation, respectively (Torres-Caban et al. 2019; Hashem et al. 2020). Considering the potential damaging effects of Ni²⁺, it is important to ensure that the concentration of Ni²⁺ in environmental media meets the corresponding quality standards.

The removal of Ni²⁺ from the aquatic system such as groundwater has been achieved through the employment of various techniques such as adsorption (Tsai et al. 2015; Matern et al. 2018; Ong et al. 2018), membrane separation (Volchek et al. 1993; Tanhaei et al. 2014), ion exchange (Moghbeli et al. 2017), bioremediation (Francy et al. 2020; Singh & Mishra 2020), and nanotechnology (Matlochova et al. 2013; Rathor et al. 2017). Among these techniques, nanotechnology which involves the use of nanoscale zero-valent iron (nZVI) has proven effective in reducing Ni²⁺ (Matlochova et al. 2013; Ulucan-Altuntas et al. 2018; Gil-Díaz et al. 2020).

Extensive efforts have been made to elucidate the performance, influence factors, and mechanism of Ni²⁺ removal by nZVI. Furthermore, review articles such as Vardhan et al. (2019), Aziz et al. (2023) and Bilal et al. (2021) focused on the removal of toxic metals from aquatic systems by traditional methods, while suggesting methods of improving low cost adsorbents to ensure efficient removal of heavy metals. It is worth noting that there are few reports on the transformations of Fe⁰ in the aqueous system as compared to the traditional methods of removing toxic substances from water. For instance, Roh et al. (Filip et al. 2019; Gil-Díaz et al. 2020) investigated the conversion of Fe⁰ filing/foam in reactive iron barrier under varying conditions (alkalinity, pH) and types of contaminants (TCE, U-235, Tc) in the field and laboratory. Amorphous iron (hydr)oxides, intermediate products (green rusts), and hydrated forms of ferric oxides such as goethite (α–FeOOH), ichlorina (β–FeOOH), and lepidocrocite (γ–FeOOH) were identified as the main corrosion products of Fe⁰. The composition of the products varied with contaminant identities, iron types, and environmental media. Anang et al. (2021) examined the compositional evolution of nZVI in the ichlorination of 2,4-dichlorophenol by attapulgite-supported Fe/Ni nanoparticles. It was found that ferrihydrite was first formed, which later resulted in the formation of lepidocrocite, goethite, and magnetite. Sohn et al. (Su et al. 2014) employed nZVI to reduce nitrate in the solution that was deoxygenated by a N₂ stream for 2 h before adding nZVI, and the results showed that Fe⁰ was transformed to crystalline magnetite. In addition, ageing of nZVI in static water under oxic and anoxic conditions has been reported (Liu et al. 2015, 2017). The main corrosion product of nZVI at oxic conditions for 72 h is γ–FeOOH, whereas a mixture of FeO, α–FeOOH and β–FeOOH is formed in anoxic water.

Even though the above studies have examined the transformation of Fe⁰ in static water or in the removal of chlorinated organics, radioactive elements and nutrient contaminants, no investigation has focused on the chemical transformation of Fe⁰ nanoparticles and Ni²⁺ during the removal. In particular, the changes in the composition and form of Ni²⁺ after removal, as well as the potential ecological effects of nZVI and Ni²⁺ transformation products on the environment have not been reported. This leaves a gap in the literature that needs to be filled. An evaluation on the componential, structural, and morphological transformations of Ni²⁺ and Fe⁰ at solid–liquid interface is critical in understanding their fate in the environment and the associated risks, and predicting the application potential of Fe⁰ nanomaterials in groundwater remediation.

In this study, various methods and devices characterizing material composition, morphology and structure, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), specific surface area analyzer, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS), are used to identify the transformation products of Ni²⁺ and Fe⁰ in nZVI during the removal of Ni²⁺ by nZVI and elucidate their physicochemical properties. The schematic description of the transformation process is presented. In addition, the implications of transformation of Ni²⁺ and Fe⁰ to groundwater remediation are discussed.

The chemicals which included iron(II) sulfate heptahydrate (FeSO₄•7H₂O), sodium borohydride (NaBH₄), and nickel sulfate (NiSO₄•6H₂O) were purchased from Tianli Chemical Reagent Company in Tianjin, China. All the chemicals were of analytical grade. Deionized water was used to prepare the chemical solutions.

The liquid phase reduction method was used to prepare the nZVI (Mukherjee et al. 2015; Hu & Li 2018). 50 mL of H₂SO₄ (0.01 mol/L) was added to 13.9 g of the FeSO₄•7H₂O before the synthesis. 40 mL of the resulting mixture was then pipetted into a three-necked round bottom flask and stirred mechanically for 5 min (200 rpm). 3.026 g of NaBH₄ was mixed with 40 mL of de-ionized water and stirred before adding dropwise to the FeSO₄•7H₂O solution in the flask. The mixture was mechanically stirred under N₂ atmosphere. The resulting mixture was allowed to sit and stir mechanically for 15 min after all the NaBH₄ solution was used up. The as-prepared nZVI was vacuum-filtered and washed with deionized water. The material was then dried under vacuum at −30 °C for 14 h in a freeze dryer.

The nZVI was used to remove Ni²⁺ in the solution in order to investigate the transformation of the Ni²⁺ and Fe⁰ after removal. At the initial stages, 0.1 g of the prepared nZVI was added to 200 mL of NiSO₄ solution (concentration = 5 mmol/L; initial pH = 4.0 ± 0.1). Two millilitres (2 mL) of the mixture was sampled at pre-determined time intervals (5, 10, 20, 30, 45, 60, 90, and 120 min) and filtered through a 0.45-μm nitrocellulose membrane. The filtrate of the sampled mixture at pre-determined time intervals was measured to determine the removal efficiency by the nZVI. The batch experiments were performed in duplicate in order to ensure precision. The residue of the nZVI after reaction with Ni²⁺ was freeze dried at −30 °C for 14 h, before used in characterizing the morphological and structural transformations of the Ni²⁺ and Fe⁰ (Anang et al. 2021).

The concentration of Fe²⁺ in the solution with different reaction time was measured by spectrophotometry using phenanthroline as the chromogenic agent. The concentration of total iron was determined by an atomic absorption spectrometer. The difference between total iron concentration and Fe²⁺ concentration is the concentration of Fe³⁺.

The concentration of dissolved oxygen (DO) was analyzed by a JPSJ-606L dissolved oxygen meter (Shanghai Yidian Scientific Instruments Co. Ltd, China).

The morphological characteristics of Ni²⁺ and Fe⁰ in nZVI before and after reaction (120 min) were analyzed using SEM (Nova 400 Nano, FEI, USA) and TEM (Tecnai G2 F30, S-Twin, FEI, USA). The crystalline structure of the pristine and reacted nZVI was characterized using an X-ray diffractometer (XRD) (D/MAX-2500, Rigaku Co, Japan) operated at 45 kV and 250 mA with Cu Kα. The material surface composition of pristine and reacted nZVI was analyzed with the aid of X-ray Photoelectron Spectrometer (XPS) (Escalab 250Xi, Thermo Fisher Scientific, USA) with a monochromatic Al Kα X-ray source. The binding energies of the samples were calibrated using the C1s peak at 284.8 eV. The specific surface area and pore parameters of nZVI before and after removal of Ni²⁺ were determined by a specific surface area analyzer (Micromeritics ASAP 2020, USA).

Researchers perceived that most of the toxicity of nZVI in cells and complete organisms comes from the production of reactive oxygen species (ROS), including free radicals such as hydroxyl radicals, superoxide anions, and non-radical hydrogen peroxide (Liu et al. 2013; Wu et al. 2014). ROS is produced in Fenton and Haber–Weiss reaction with Fe⁰ as the main initiator (Wang et al. 2012; Wu et al. 2014). However, Fe⁰ in nZVI undergoes a series of reactions with Ni²⁺, H₂O, and DO during the removal of Ni²⁺ by nZVI, transforming into lepidocrocite, goethite, and magnetite with the depletion of Fe⁰. The reactivity of these iron oxides is relatively weak, and it is difficult to react and produce ROS. As a result, the impact on groundwater ecosystems will be significantly reduced. In addition, many researchers reported that iron oxides provide high affinity for heavy metal ions and organic pollutants, which exert certain positive effects on aquatic ecological environment. Kooner (1993) revealed that at neutral pH, although 50% of zinc was still available for transport, almost 100% of lead and copper got adsorbed onto the surface of goethite. Scott et al. (Fendorf et al. 1997) investigated chromate and arsenate retention mechanisms on goethite and it was found that three different surface complexes exist on goethite for both oxyanions. Zhang et al. (Zhang & Huang 2007) examined the adsorption of seven fluoroquinolone (FQ) antibacterial agents with goethite and the results showed that 50–76% of the added FQs were strongly adsorbed by goethite under experimental conditions. Sheydaei et al. (Sheydaei & Khataee 2015) indicated that γ-FeOOH can be effectively used as catalyst in successive ultrasonic/γ-FeOOH /H₂O₂ processes for decolorization of a textile wastewater containing Reactive Orange 29. Lin et al. (2012) reported the oxidative transformation of bisphenol A (BPA) by laboratory and synthetic commercial goethite, both of which are considered to be effective in inducing BPA conversion.

In addition, the magnetic attraction between nanoscale iron oxide is significantly lower than that of nZVI, resulting in weakened aggregation. Therefore, the specific surface area of iron oxides will be obviously increased compared to nZVI, which is favorable for iron oxides to remove/degrade contaminants through adsorption/catalysis. Table 1 shows the BET specific surface area and pore parameters for pristine nZVI and reacted nZVI. It can be seen that the BET specific surface area (46.67 m²·g⁻¹) and pore volume (0.2283 cm³·g⁻¹) of the iron oxides (reacted nZVI) are 5.3 times and 6.4 times of that of pristine nZVI (8.76 m²·g⁻¹ and 0.0358 cm³·g⁻¹), respectively.

The transformation of Ni²⁺ will also have an impact on its fate and bioavailability in groundwater ecosystem, as the toxicity of heavy metals to fish and other aquatic organisms depends primarily on free (hydrated) metal ions (Stumm 1992). Fe_0.7Ni_0.3 alloy and trevorite, which are transformed during the removal of Ni²⁺ by nZVI, will exist in a colloidal or suspended state, and most of them will be quickly transferred onto the suspended solids or the sediments at the bottom of the groundwater, thus the impact on aquatic organisms is negligible.

From the above, in the process of using nZVI to remediate groundwater contaminated by Ni²⁺, not only Ni²⁺ can be removed, but also the toxicity and bioavailability of Fe oxides and Fe–Ni composites formed by transformation are significantly reduced compared with Ni²⁺ and Fe⁰. Other heavy metal ions and organic contaminants in groundwater can also be adsorbed/catalyzed by iron oxides. The findings suggest that nZVI is a promising environmentally friendly technology in groundwater remediation.

A clear insight into the compositional transformation of Ni²⁺ and Fe⁰ during Ni²⁺ removal by nZVI was established in this study. XRD patterns indicated that the reaction of Fe⁰ with Ni²⁺ resulted in the transformation of the Fe⁰ into lepidocrocite (γ-FeOOH), goethite (α-FeOOH), and magnetite (Fe₃O₄). The Ni²⁺ became converted into trevorite (NiFe₂O₄) and Ni⁰ which existed in the form of Fe_0.7Ni_0.3 alloy. The results were confirmed by the d-spacing values obtained from HR-TEM images and the elemental composition obtained from TEM-mapping. In addition, SEM and TEM images showed that the morphologies of lepidocrocite, goethite, magnetite, and trevorite were lath-like, twin-rod, spherical, and folding fan, respectively. The mechanism of Ni²⁺ transformation includes reduction to Ni⁰ due to the reducibility of the Fe⁰, and the surface coordination of Ni²⁺ with Fe oxides to form Fe–Ni composite. The conversion of Fe⁰ undergoes a series of reactions, mainly composed of the redox of Fe⁰ with Ni²⁺, H₂O, and DO, the combination of Fe²⁺ and OH⁻ produced by Fe⁰ corrosion to amorphous ferrihydrite, and the further mineralogical transformation to Fe oxides with the aid of Fe²⁺ adsorbed on ferrihydrite. Compared with Fe⁰, the toxicity of Fe oxides to aquatic organisms decreased, the specific surface area increased, and the affinity for heavy metal ions and organic pollutants was higher. In addition, most of the Fe_0.7Ni_0.3 alloy and trevorite, which exist in the suspended or colloidal state, will be transferred to the suspended solids or the sediments at the bottom of groundwater. The findings indicate that the use of nZVI technology to remediate Ni²⁺-contaminated groundwater is environmentally friendly.

The authors would like to thank the Analytical and Testing Center, Wuhan University of Science and Technology for helpful contributions in the execution of this study.

The authors would like to express their gratitude to the Key Project of Natural Science Foundation of China (Grant No. 41230638), for the financial support. The support from the Open Fund of Hubei Key Laboratory for Efficient Utilization and Agglomeration of Metallurgic Mineral Resources from Wuhan University of Science and Technology (2017zy008) is duly acknowledged.

E.A. made substantial contribution to conception of the study, performed data analysis, investigation, synthesized material, curated data, and wrote the first draft. H.L. made substantial contribution to conception of the study, supervised the experiments, reviewed, and edited the manuscript. X.F. performed software edits, reviewed and edited the manuscript, and provided technical and material support.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.