Insights into the mechanisms of aqueous Cd(II) reduction and adsorption by nanoscale zerovalent iron under different atmosphere conditions

Highlights

• Cd(II) reduction to Cd(0) by NZVI core was demonstrated by XPS Cd MNN Auger lines.

• Oxygen induced compositional evolution of NZVI and change the Cd(II) adsorption model.

• Cd(II) leaching during post air-intrusion came from Cd(0) and adsorbed Cd(II).

• Bicarbonate, nitrate and phosphate reduced Cd(II) leaching during post air-intrusion.

Abstract

Nanoscale zero-valent iron (NZVI) can effectively remove and recover Cd(II) from aqueous solutions. However, the oxygen effects on Cd(II) removal by NZVI have been overlooked and not well studied. In this research, the Cd MNN auger lines obtained by X-ray photoelectron spectroscopy (XPS) revealed that Cd(II) adsorbed on the NZVI surface could be reduced to Cd(0) by the Fe(0) core under anaerobic conditions. With coexisting oxygen, the Cd(II) removal efficiency declined significantly, and Cd(II) reduction was inhibited by the thickened surface γ-FeOOH layer. Furthermore, the post-oxygen intrusion corroded the generated Cd(0) and led to the dramatic leaching of Cd(II) ions. According to the density functional theory (DFT) simulation, the adsorbed Cd(II) was preferably coordinated via a monodentate model on the surface of Fe₃O₄ and γ-FeOOH, which are the dominant surface species of NZVI under anaerobic and aerobic conditions, respectively. Thus, γ-FeOOH with doubly coordinated hydroxyl groups provided fewer adsorption sites than Fe₃O₄ for Cd(II) ions. Overall, the atmospheric conditions of subsurface remediation and wastewater treatment should be considered when applying NZVI for Cd(II) removal. Favorable atmospheric conditions would improve the efficiency and cost-effectiveness of NZVI-based technologies for the practical remediation of Cd(II) pollution.

Introduction

Cadmium ion (Cd(II)), identified as a Group 1 carcinogen by the International Agency for Research on Cancer (Zhu and Costa, 2020), is a typical heavy metal pollutant found in groundwater and industrial wastewater with a Cd(II) concentration up to 300 mg/L (Kubier et al., 2019, Shrestha et al., 2021, Sribudda et al., 2015). Recently, nanoscale zero-valent iron (NZVI) techniques have attracted increasing attention for wastewater treatment and subsurface and sediment remediation for Cd(II) removal (Li et al., 2014, Liu et al., 2022a, Liu et al., 2022b, Tasharrofi et al., 2020, Wu et al., 2018, Xue et al., 2018, Yang et al., 2020, Zhao et al., 2021). Due to its large surface area (>10 m²/g) and high reduction capacity ($E_{{\text{Fe}}^{2\text{+}}\text{│}{\text{Fe}}^0}^{\text{Θ}}\text{(}298\text{K)}\text{=}\text{-}0.440\text{V}$) (Phenrat and Lowry, 2019), NZVI is versatile in removing various aqueous contaminants on the particle surface via reduction, adsorption, or precipitation (Li et al., 2021, Mu et al., 2017, Zou et al., 2016). For practical applications, NZVI can remove Cd(II) either from groundwater by a subsurface permeable reactive region (Tasharrofi et al., 2020) or from industrial wastewater using a two-stage tank (Li et al., 2014, Zhang et al., 2014). However, the performance of Cd(II) removal by NZVI might be influenced by the dissolved oxygen (DO) (Calderon and Fullana, 2015), which is dependent on the subsurface depth and the NZVI dose. However, how DO influences Cd(II) removal by NZVI is not well understood.

The atmospheric conditions might influence the interaction between Cd(II) ions and NZVI particles. Some studies reported that Cd(II) might be reduced to Cd(0) by NZVI even under aerobic conditions (Zhu et al., 2019), as Cd(0) has a standard redox potential ($E_{{\text{Cd}}^{2\text{+}}\text{│}{\text{Cd}}^0}^{\text{Θ}}\text{(}298\text{K)}\text{=}\text{-}0.403\text{V}$) lower than that of Fe(0). However, as DO can oxidize Cd(0) to Cd(II) and the E^Θ values of Cd(0) and Fe(0) are similar, most studies have stated that NZVIs remove Cd(II) only via adsorption, precipitation, and surface complex formation (Huang et al., 2016, Li et al., 2018b, Ling et al., 2018, Lv et al., 2018, Park et al., 2019). Therefore, direct evidence is needed to investigate the mechanisms of Cd(II) removal by NZVI under different atmospheric conditions and reveal the roles of DO in the removal process.

In addition, NZVI suffers from aqueous corrosion due to its high reactivity. Fe(0) is readily oxidized to Fe(II) or Fe(III) by H⁺ ions and DO, forming secondary Fe minerals, such as wustite, magnetite, and lepidocrocite (Bae et al., 2018). The corrosion evolution depended on the atmospheric conditions, NZVI concentration, corrosion time, and patterns in the aqueous phase (Bae et al., 2018), which may also influence the Cd(II) adsorption and leaching process. Specifically, Cd(II) could be effectively removed by NZVI under aerobic conditions within 5 h (Calderon and Fullana, 2015). When the aging time was extended to more than 50 h, the removed Cd(II) leached by 65% (Calderon and Fullana, 2015). In comparison, modifying the NZVI surface by sulfidation inhibited the aqueous corrosion of NZVI particles, so Cd(II) could be stably adsorbed by sulfidated NZVI for 30 d (Guo et al., 2021, Stevenson et al., 2017). In addition, the crystalline and surface structure of Fe minerals might also influence the complex models of Cd(II) and relative adsorption capacity (Hao et al., 2022). However, as the oxygen intrusion was uncontrolled and the Fe species were not quantified, the relationships between the oxygen-induced corrosion, the compositional evolution of Fe minerals, and the Cd(II) leaching could not be confirmed.

Therefore, a systematic study was conducted to explore the mechanisms of Cd(II) removal by NZVI, including reduction and adsorption, and the influential mechanisms of oxygen on the removal process. Bare NZVIs without any modification were used in this study, to provide basic understanding of the NZVI-Cd(II) interactions. Cd speciation, metal ion leaching, and compositional evolution during Cd(II) removal were investigated with different NZVI particle sizes, oxygen intrusion amounts and patterns, aqueous backgrounds, and reaction duration. In addition, the amounts of Fe(II) and Fe(III) ions were quantified, and the species of secondary Fe minerals were identified with X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge structure (XANES). The reduction of Cd(II) to Cd(0) by NZVI was first demonstrated with direct evidence. Furthermore, the adsorption models of Cd(II) on the surface of NZVI with different secondary minerals were first investigated with density functional theory (DFT) calculations, which also influenced the Cd(II) removal performance. The underlying mechanisms demonstrated in this study can guide the applications of NZVI-based materials, where the particle surface of NZVI was exposed to the aqueous phase for Cd(II) removal.