Formation of Zn-Al layered double hydroxides (LDH) during the interaction of ZnO nanoparticles (NPs) with γ-Al₂O₃

Highlights

• Interaction between ZnO NPs and γ-Al₂O₃ was investigated from neutral to alkaline pH.

• Zn-Al LDH could be formed under both near neutral and alkaline conditions.

• Formation of Zn-Al LDH is likely via a dissolution-sorption-coprecipitation process.

• This study helps to explain formation pathway of Zn-Al LDH in Zn/ZnO-contaminated environment.

Abstract

Zinc and aluminum layered double hydroxides (Zn-Al LDH) are a common group of major Zn species in various Zn-contaminated soil/sediment environments, yet their formation pathways and underlying mechanisms under varied conditions are not well understood. This study investigated the formation of Zn-Al LDHs through the direct interaction of two solid substrates, ZnO nanoparticles (NPs) and a representative Al oxide, γ-Al₂O₃. Batch experiments and complementary microscopic and spectroscopic analyses were conducted to elucidate the reaction kinetics and mechanisms, as well as the morphologic and structural evolution of the products. Dissolved Zn and Al concentrations decreased significantly in a dual solid system compared to a single solid system. A bulk Zn-Al LDH phase was found to form under a wide pH range (6.5–9.5). Aside from Zn-Al LDH, γ-Al₂O₃ was the main remaining solid phase at pH 6.5, whereas ZnO NPs were the main residual solid phases at pH 9.5. Formation of amorphous Zn(OH)₂ was also observed at both pH values, likely due to Zn²⁺ release at low pH and Al(OH)₄⁻ adsorption at high pH. It is proposed that the formation of Zn-Al LDH occurs via a dissolution-sorption-coprecipitation process, where the solubility of ZnO NPs or γ-Al₂O₃ solid phases determines the reaction pathways and kinetics under varied pH conditions. The results from this work revealed the transformation mechanisms for ZnO NPs under conditions from weakly acidic to alkaline pH with highly available Al particles and shed light on the environmental fate of ZnO NPs in Zn or ZnO NP contaminated environments.

Introduction

Engineered nanoparticles (ENPs) released into natural environments may interact with various constituents (e.g., microbes, minerals, natural organic matter) through an array of reactions such as redox transformation, homo/heteroaggregation, dissolution, and/or precipitation, which can subsequently impact their speciation, fate, transport, and bioavailability (Lowry et al., 2012; Wang et al., 2015). Understanding the kinetics and mechanisms of these processes is critical for evaluating the environmental behaviors and ecologic risks of ENPs (Lowry et al., 2012; Stark, 2011). Zinc oxide (ZnO) NPs are the second most produced group of ENPs and have widespread applications (e.g., catalysis, dye-sensitized solar cells, sunscreen products, textiles, and antibacterial agents) (Keller and Lazareva, 2014; Kołodziejczak-Radzimska and Jesionowski, 2014; Piccinno et al., 2012). These applications will inevitably lead to the release of ZnO NPs into natural environments (Keller and Lazareva, 2014), and studies have shown that their transport and retention in soils and sediments will strongly affect their potential exposure pathways and transformation (Hazeem et al., 2016; Li and Schuster, 2014; Sun et al., 2015). In addition, ZnO NP dissolution and transformation under the coexistence of environmentally relative ligands will remarkably alter the chemical form and thus the toxicity of ZnO NPs (Feng et al., 2016; Li et al., 2013; Ma et al., 2013). Recently, interaction of ZnO NPs with solid particles such as TiO₂ NP was reported to affect ZnO NPs transformation (Lv et al., 2014; Tong et al., 2014, Tong et al., 2015). However, there are very limited studies on the interaction of ENPs such as ZnO NPs with abundant natural mineral nanoparticles or colloids in the environment (Lv et al., 2014; Wang et al., 2015).

Aluminum (Al) (hydr)oxide minerals are ubiquitous in highly weathered environments and have been widely investigated in the fields of interfacial chemistry of nutrient elements and aqueous pollutants (Elzinga, 2012; Li et al., 2011, Li et al., 2012; Siebecker et al., 2014; Yan et al., 2015). It has been commonly observed that Zn²⁺ and ZnO in soils enriched with Al (hydr)oxide or Al-bearing clay minerals can transform into less soluble Zn and Al layered double hydroxides (Zn-Al LDH) at relatively high initial metal concentrations (Jacquat et al., 2008, Jacquat et al., 2009; Nachtegaal et al., 2005; Voegelin et al., 2002, Voegelin et al., 2005, Voegelin et al., 2011; Voegelin and Kretzschmar, 2005). For example, formation of Zn-Al LDH in contaminated soils was found to be associated with samples with Zn content ranging from 571 to 20,476 mg kg⁻¹ (Jacquat et al., 2009; Nachtegaal et al., 2005). Nearly pure Zn-rich phyllosilicate and Zn-LDH were identified at different locations within a single soil horizon, indicating that the availabilities of Al and Si controlled the type of precipitates formed (Jacquat et al., 2008). With the increase of Zn loading in soils, the percentage of precipitated Zn increased from 20 to 80%, while the precipitate type shifted from Zn-phyllosilicate at sites with lowest Zn content to predominantly Zn-LDH in heavily Zn-contaminated soils (Jacquat et al., 2008). Furthermore, Zn-bearing precipitates (Zn-LDH and Zn-rich phyllosilicates) became more dominant with the increase of pH and excessive total Zn content, relative to available adsorption sites (Jacquat et al., 2009). Zn-Al LDH was the most abundant Zn-precipitate in soils at pH 5.3–7.7, while Zn-rich phyllosilicates, less abundant than Zn-Al LDH, were also detected at lower soil pH (Jacquat et al., 2009). Previous studies have also revealed that the Zn²⁺ adsorbed on Al (hydro)oxide might continue to migrate into the crystal lattice of Al (hydro)oxide and formed new LDH phases at circumneutral pH values (Li et al., 2012; Miyazaki et al., 2013).

Although it is well known that there is a strong interaction between Zn²⁺ and Al (hydr)oxide or Al-contained clay minerals, less attention has been paid to the interaction of ZnO NPs with these Al-containing minerals. Voegelin et al. (2011) found that after four years of soil incubation, >90% of the added ZnO had transformed into Zn-Al LDH precipitate, with minor amounts of Zn-phyllosilicates and adsorbed Zn species in four soils, with pH ranging from acidic to alkaline. At acidic or neutral pH values, interaction of aqueous Zn²⁺ (derived from ZnO NPs dissolution) with Al-containing minerals accounts for the formation of Zn-Al LDH precipitate. However, at alkaline pH values where ZnO NPs are hardly soluble, the formation mechanisms of Zn-Al LDH are not clearly understood, despite the fact that such information is critical for understanding the environmental fate of ZnO NPs under circumneutral or alkaline pH conditions.

This study systematically investigated the interaction between ZnO NPs and γ-Al₂O₃ (a representative phase of highly reactive poorly crystalline Al oxide minerals) under varied pH conditions. Batch experiments were combined with a suite of complementary spectroscopy and microscopy analyses to reveal the reaction pathways and kinetics. Xu and Lu (2005) studied hydrothermal synthesis of layered double hydroxides (LDHs) from mixed MgO and Al₂O₃ for practical use as a material and explored LDH formation mechanisms under the less environmentally relevant conditions (i.e., high temperature, high pressure, and a long reaction time). Here, our study observed the rapid formation of Zn-Al LDH precipitates from the suspensions of ZnO and γ-Al₂O₃ mixtures at room temperature and atmospheric pressure and provided a quantitative and qualitative analysis of Zn-Al LDH in the products. Based on systematic instrumental characterization of the reaction products combined with wet chemistry analysis, this study further confirmed a dissolution-sorption-coprecipitation mechanism for the formation of Zn-Al LDH via two pathways at neutral and alkaline pH. Results from our study will enable better prediction of the transformation of ZnO NPs in natural environments and will provide implications for the interfacial behaviors of Zn or ZnO NPs in the environment.