Synthesis of montmorillonite-supported nano-zero-valent iron via green tea extract: Enhanced transport and application for hexavalent chromium removal from water and soil

Highlights

• Synergistic effects of Mont and tea polyphenols increased the dispersibility of nZVI.

• TPs-coverage & Mont support inhibited the Fe₂O₃ formation around Fe⁰ cores.

• Unwrapped TPs-coverage increased Fe⁰ cores exposure to Cr(VI) reduction.

• Sufficient transport of nZVI@TP-Mont was independent of soil heterogeneity.

• nZVI@TP-Mont showed excellent remediation of Cr(VI) in both water and soils.

Abstract

The nano-zero-valent iron composite (nZVI@TP-Mont) was successfully prepared using a low-cost and environmental-friendly green synthesis via tea leaves extract (tea polyphenols, TPs) and the montmorillonite (Mont). The batch and column experiments and characterization were conducted to investigate the transport behavior and Cr(VI) remediation by nZVI@TP-Mont in water/soil. Due to its particular surface characteristics and morphology (i.e., the Fe⁰ core wrapped by TPs, the doped sulfur, and interlayer structure), the nZVI@TP-Mont composite showed a great removal capacity of Cr(VI) and sufficient mobility under different soil conditions. We opine the increase in the Cr(VI) reduction of nZVI@TP-Mont was attributed to the tethering of Fe₂O₃ on the surface of Fe⁰ core by the support of Mont interlayer, especially the TP-coverage around nZVI@TP-Mont surface unwrapped, thereby increasing the regenerated reactive Fe²⁺ and the exposed reaction sites of Fe⁰ cores to Cr(VI). The increased transportability of nZVI@TP-Mont slightly depends on the heterogeneous soil properties (i.e., ionic strength, sand/soil ratio, and pH). The two-site kinetic attachment model fitting results suggest Cr(VI)/Cr(III) speciation associated with the agglomerated nZVI@TP-Mont were efficiently immobilized in soil. Therefore, this study would benefit the efficient application of the green-synthesized nZVI@TP-Mont in in-situ remediation of soils contaminated by Cr(VI).

Introduction

With rapid economic development, industrial pollution worsens, leading to trace/heavy (toxic) metal pollutions of soils and groundwater. Notably, the extensive use of chromium (Cr) in the industries has resulted in the metal’s ubiquitous presence in the environment, in trivalent and hexavalent forms (Hepburn et al., 2018). Cr(VI) is 100 times more toxic than Cr(III) (Ahmadi et al., 2018). Cr(VI) easily enters the ecosystem, ultimately endangering human health via the food chain (Alidokht et al., 2011). Therefore, Cr(VI) pollution control, especially in soils and groundwater, has become a prioritized ecological issue.

Nanosized zero-valent iron (nZVI) has been widely used for various environmental remediation, such as phenol (Tratnyek and Johnson, 2006), bisphenol A (Yang et al., 2018), dye (Zhang et al., 2011), and most significantly trace metals (Liu et al., 2014, Yuan et al., 2009). nZVI exhibits a typical core-shell structure comprising Fe⁰/FeOx (Liu et al., 2013, Sun et al., 2006). However, most engineered nanoparticles agglomerate rapidly around the injection well, thereby hindering further transport of the nanoparticles downstream. Such occurrence has become a significant demerit for real-life nanoparticle application in pollution remediation (Praetorius et al., 2014). To prevent its agglomeration, various materials have been to modify nZVI. They include clay (Bhowmick et al., 2014, Zhang et al., 2013 2020a), organobentonite (Shi et al., 2011), biochar (Shang et al., 2017), chelating agent (Liu et al., 2014), and carboxyl methyl cellulose agar and starch (Cheng et al., 2015). Because of the large surface area and 2:1 layered aluminosilicate (Tian et al., 2015), nZVI has been successfully supported on montmorillonite (Mont) to synthesize the nZVI-Mont composites. The composites can remove organic matters (Rao et al., 2018) and toxic metals (e.g., arsenate (Bhowmick et al., 2014, Suazo-Hern´andez et al., 2021) and Cr(VI) (Wu et al., 2015; Yin et al., 2020)) from aqueous media. Most of these researches involved chemical (i.e., borohydride) synthesis in modifying nZVI, releasing toxic byproducts (Nasiri et al., 2019). Comparably, green synthesis, whereby inexpensive, widespread, and environmentally friendly plant extracts reduce ferric and ferrous ions to nZVI (Madhavi et al., 2013), has attracted wide attention. Usually, the plant extracts are biocompatible, benign, economical, and energy-conservative (Smuleac et al., 2011). Relevant researches indicate that the green synthesized nZVI has been employed as adsorbent and reducer to remove the Cr(VI) (Chrysochoou et al., 2012, Soliemanzadeh and Fekri, 2017a, Yirsaw et al., 2016, Zhu et al., 2018, Zhu et al., 2021), phosphorus (Soliemanzadeh and Fekri, 2017b) and arsenite (Poguberović et al., 2016) from water. However, limited information is available in the nZVI-Mont composites synthesized by a green method.

Although the modified nZVI (or nZVI) via chemical or green synthesis have been used to remove pollutants from contaminated waters, few researchers focus on the Cr(VI) contaminated soil remediation by modified nZVI. For example, nZVI/Ni has remediated Cr(VI) contaminated soil leachate (Zhu et al., 2017, Zhu et al., 2021). Due to soil complexity, the transport and retention of engineered nanoparticles in soils can be altered by physicochemical properties like pH (Kim et al., 2012), ionic strength (Tong et al., 2020) and type, natural organic matter, surface roughness, clay contents (Phenrat et al., 2010). Nevertheless, very little information is available on the transport of modified nZVI in soil (Zhang et al., 2020b, Zhang et al., 2020a). Various particles with different physicochemical properties prepared via different methods showed distinct transport characteristics in saturated porous media (Wang et al., 2013a). Therefore, it is necessary to investigate the transport behavior of green synthesized nZVI-Mont to the deeper contaminated subsurface and the subsequent remediation capacity of Cr(VI) in soil. To the best of our knowledge, the Cr(VI) remediation and transport of the green synthesized nZVI-Mont (with or without Cr(VI)) in heterogeneous soil have been barely studied.

Elsewhere, sulfate (SO₄²⁻) has been identified to help remove the Fe passive oxide film (Lipczyns-kakochany et al., 1994). As SO₄²⁻ facilitates the regeneration of Fe²⁺ by reacting with nZVI, it also enhances Cr(VI) removal by chemical reduction (Mayer et al., 2001). To improve Cr(VI) reduction by nZVI in the current study, ferric sulfate was chosen as the Fe source. At the same time, Mont was selected as a support for the Fe⁰ nanoparticles during the Fe³⁺ reduction by green tea extract (tea polyphenols, TPs). Thus, a green synthesized nZVI@TP-Mont was successfully prepared. The characterization of the nZVI@TP-Mont and its removal efficiencies (adsorption and reduction) for Cr(VI) were quantified. For improved in situ Cr(VI) removal from contaminated soil, the breakthrough curves of nZVI@TP-Mont with and without Cr in water-saturated soil and the related model fitting (using the two-site kinetics retention model (TSKAM)) were studied. Moreover, to better understand the underlying mechanism controlling the reduction and adsorption of Cr(VI) by nZVI@TP-Mont in soil, Cr and Fe speciation were done. Thus, this study should provide technical support and relevant information for practical nZVI@TP-Mont application in in-situ remediation of toxic/heavy metal-contaminated soils.