Tooeleite Transformation and Coupled As(III) Mobilization Are Induced by Fe(II) under Anoxic, Circumneutral Conditions

Stability and transformation behavior of hydrometallurgical hazardous arsenic-calcium residue in sulfidic anoxic environments

Arsenic-calcium residue (ACR) is a hazardous solid waste generated by the metallurgical industry, posing a significant environmental risk. However, the stability and transformation behavior of ACR in sulfidic conditions remains unclear. Herein, we have investigated the stability and speciation evolution of arsenic (As), sulfur (S), and trace metals during the exposure of ACR to diverse S(-II) concentrations under anoxic conditions at pH of 6 and 11. Our results indicate that environmentally relevant levels of S(-II) (i.e., 1, 10, and 50 mM) significantly enhance the mobilization of As(III) and Cd2+ from ACR, with greater release at pH 6. The main mechanism for the release of As(III) and trace metals from ACR is the reductive dissolution of Ca-arsenate/arsenite and As-trace metals-gypsum. The reductive dissolution of As-trace metals-gypsum leads to the formation of S2O32─ and SO32─. XRD, FE-SEM, FTIR, XPS, and HRTEM analyses reveal that gypsum serves as the host phase for As fixation at pH 6, while calcium-arsenate/arsenite phases predominate at pH 11. Secondary As2S3, CdS, CuS, and symplesite are generated at pH 6, whereas parasymplesite, CdS, and CuS are predominant at pH 11. These results enhance our understanding of the environmental behavior of As, S, and trace metals associated with ACR.

Phase transformation of calcium sulfate at mineral-solution interface: An overlooked pathway for selective enrichment of cadmium

The reactions at the mineral-solution interface govern whether heavy metals (HMs) ions are retained within minerals or migrate with the solution, thus influencing their cycling and fate. However, the mechanisms driving this differential behavior of HMs at the interface remain poorly understood. In this study, we present a novel paradigm for the selective retention of HMs ions at the mineral-solution interface. By confining the solution on the mineral surface to a defined volume, specifically thinning it down to a thickness of 50 nm, selective retention of Cd ions in the presence of coexisting Cu and Zn ions was achieved. The distribution coefficient of Cd in the mineral reaches as high as 41.44, significantly exceeding that of Cu at 0.13 and Zn at 0.07. Combined with DFT calculations, the results reveal that this selectivity arises from the regulation of the ion desolvation free energy by the solution nanofilm, precisely compensating the energy cost for Cd incorporation as an impurity into the mineral lattice. This work not only enriches the understanding of ion separation behavior at natural mineral-solution interfaces but also offers a new strategy for heavy metal separation and enrichment in industrial applications.

Is beudantite a stable host phase of arsenic and lead? New insights from molecular-scale kinetic analyses

Beudantite, an As-Pb containing Fe(III) sulfate secondary mineral, is formed via the oxidation of sulfide-rich tailings in mining-impacted regions. The geochemical stability of beudantite plays a key role in controlling the cycling and transport of As and Pb in mine sites. However, the fate of beudantite under dynamic pH conditions and its effect on As and Pb mobility remain elusive. We investigated the mobility dynamics of As and Pb during the dissolution of beudantite under variable pH conditions (2-8) relevant to mine sites by using a complementary suite of analytical methods. Results demonstrate that under acidic pH conditions, aqueous As and Pb content increased slightly, with just 0.7 % and 6.7 % of As and Pb partitioned from the beudantite crystal structure over 56 days. Notably, the rate at which the dissolution of beudantite led to solubilization of elements followed the order Fe > As > Pb within the first 2 h of dissolution. In contrast, the order shifted to Pb > Fe > As after 2 h. Arsenic K-edge X-ray absorption spectroscopy analyses revealed no shifts in As speciation or secondary mineralogical transformation. Here, we show for the first time that beudantite could be considered a relatively stable mineral host for As and Pb over a broad spectrum of environmental conditions. Beudantite can be expected to immobilise metals liberated by the primary weathering of sulfide-rich mine wastes, thereby lowering the risk to the environment and human health resulting from their discharge into the surrounding environment and aquifer.

The pH-dependent role of different manganese oxides in the fate of arsenic during microbial reduction of arsenate-bearing goethite

Manganese oxides reduce arsenic (As) toxicity by promoting aqueous-phase As(III) oxidation and immobilization in natural aquatic ecosystems. In anaerobic water-sediment systems, arsenic exists both in a free state in the liquid phase and in an adsorbed state on iron (Fe) minerals. However, the influence of different manganese oxides on the fate of As in this system remains unclear. Therefore, in this study, we constructed an anaerobic microbial As(V) reduction environment and investigated the effects of three different manganese oxides on the fate of both aqueous-phase and goethite-adsorbed As under different pH conditions. The results showed that δ-MnO2 had a superior As(III) oxidation ability in both aqueous and solid phase due not only to the higher SSA, but also to its wrinkled crystalline morphology, less favorable structure for bacterial reduction, structure conducive to ion exchange, and less interference caused by the formation of secondary Fe-minerals compared to α-MnO2 and γ-MnO2. Regarding aqueous-phase As, δ-MnO2, α-MnO2, and γ-MnO2 required an alkaline condition (pH 9) to exhibit their strongest As(III) oxidation and immobilization capability. For goethite-adsorbed As, under microbial-reducing conditions, all manganese oxides had the highest As immobilization effect in neutral pH environments and the strongest As oxidation effect in alkaline environments. This was because at pH 7, Fe(II) and Mn(II) formed hydrated complexes, which was more favorable for As adsorption. At pH 9, the negatively charged state of goethite hindered As adsorption but promoted the adsorption and oxidation of As by the manganese oxides. Our research offers new insights for optimizing As removal from water using various manganese oxides and for controlling the mobilization of As in water-sediment system under different pH conditions.

Deep-dive into iron-based co-precipitation of arsenic: A review of mechanisms derived from synchrotron techniques and implications for groundwater treatment

The co-precipitation of Fe(III) (oxyhydr)oxides with arsenic (As) is one of the most widespread approaches to treat As-contaminated groundwater in both low- and high-income settings. Fe-based co-precipitation of As occurs in a variety of conventional and decentralized treatment schemes, including aeration and sand filtration, ferric chloride addition and technologies based on controlled corrosion of Fe(0) (i.e., electrocoagulation). Despite its ease of deployment, Fe-based co-precipitation of As entails a complex series of chemical reactions that often occur simultaneously, including electron-transfer reactions, mineral nucleation, crystal growth, and As sorption. In recent years, the growing use of sophisticated synchrotron-based characterization techniques in water treatment research has generated new detailed and mechanistic insights into the reactions that govern As removal efficiency. The purpose of this critical review is to synthesize the current understanding of the molecular-scale reaction pathways of As co-precipitation with Fe(III), where the source of Fe(III) can be ferric chloride solutions or oxidized Fe(II) sourced from natural Fe(II) in groundwater, ferrous salts or controlled Fe(0) corrosion. We draw primarily on the mechanistic knowledge gained from spectroscopic and nano-scale investigations. We begin by describing the least complex reactions relevant in these conditions (Fe(II) oxidation, Fe(III) polymerization, As sorption in single-solute systems) and build to multi-solute systems containing common groundwater ions that can alter the pathways of As uptake during Fe(III) co-precipitation (Ca, Mg bivalent cations; P, Si oxyanions). We conclude the review by providing a perspective on critical knowledge gaps remaining in this field and new research directions that can further improve the understanding of As removal via Fe(III) co-precipitation.