Formation and stability of NOM-Mn(III) colloids in aquatic environments
Graphical abstract
Introduction
Manganese (Mn) is a redox active metal that occurs in various geological and environmental settings and can partake in a broad range of biogeochemical processes (Sunda and Kieber., 1994; Post, 1999; Johnson, 2006; Madison et al., 2011,2013; Gude et al., 2017). Traditionally, Mn is considered to be predominantly present as soluble Mn(II) in anoxic and in particulate form (Mn(IV/III)) in oxic environments (Stumm and Morgan, 1996). Dissolved Mn(III) in natural waters has been largely understudied as it is thermodynamically unstable and rapidly disproportionate to Mn(II) and Mn(IV) (Stumm and Morgan, 1996). However, molecular orbital theory has substantiated that the donating and accepting orbitals of Mn(II) and Mn(IV) are spatially distinct, indicating that Mn(II) oxidation and Mn(IV) reduction should proceed via a one-electron transfer, with Mn(III) as the intermediate (Luther, 2005). Consequently, Mn(III) can be isolated and stabilized in aqueous solutions under appropriate conditions.
Mounting field evidences highlight the prevalence of dissolved Mn(III) bound to a variety of organic ligands (e.g., humic substances, Mn(III)-L complexes) in aqueous systems (Trouwborst et al., 2006; Oldham, 2017). Recent studies demonstrated that: (1) Mn(III)-L complexes can constitute the majority of the total dissolved Mn pool in anoxic, suboxic, and oxic environments in oceans and estuary sediments, and (2) these species can serve as both oxidant and reductant, and thus have important roles in the coupled cycles of carbon, iron, and sulfur (Trouwborst et al., 2006; Madison et al., 2013; Oldham et al., 2017a; Oldham, 2017). The formation of Mn(III)-L complexes has been proposed to occur through a sequence of reaction pathways that include oxidation, reduction, and ligand-promoted dissolution of Mn-bearing minerals (Madison et al., 2013; Oldham, 2017). In additional to natural systems, Mn(III)-L complexes can also form in situ during oxidative water treatment using Mn(VII) chemicals (Sun et al., 2015; Zhang et al., 2018). Resulting Mn(III)-L complexes have been verified to be strong oxidants and can contribute to the transformation of anthropogenic contaminants (Sun et al., 2015; Hu et al., 2017; Gao et al., 2018).
In addition to dissolved species, organic Mn(III)-L complexes also exist as colloids. Oldham (2017) documented that Mn(III) has a strong affinity with natural organic matter (NOM) such as humic substances. Although these humic-type Mn(III)-L complexes were identified to be largely non-colloidal (operationally defined as size class between 20 and 200 nm), Oldham et al. (2017b) speculated that a minor fraction of Mn(III)-humic complexes do exist as colloids. As colloids are broadly defined as dispersed particles with sizes between 1 nm and 1000 nm in diameter (Elimelech et al., 1995), the amount of dissolved Mn(III)-L complexes estimated using the previous definition (≤200–450 nm) should include both truly soluble (e.g., < 1–20 nm) and colloidal-based Mn(III). A recent study provided a compelling evidence that Mn(III)-L complexes in ocean exist both in colloidal form (20–400 nm) and truly dissolved form (<20 nm), with colloids constituting up to 90% of the total Mn(III) (Yakushev, 2013).
In contrast to a growing body of studies pertaining to dissolved Mn(III) associated with NOM, little is known about the properties and behaviors of NOM-Mn(III) colloids in aquatic environments. This is likely due to the fact that the researchers have traditionally used filters of 0.2 μm or 0.45 μm pore size to separate samples into ‘dissolved’ and ‘particulate’ phases (Wu et al., 2001; Oldham, 2017). Further, results from previous field and laboratory studies indicate that the colloidal phase of NOM-metals (e.g., Fe) complexes can act as mobile carriers, facilitating the transport of low solubility contaminants at rates and distances much greater than the soluble phase of NOM-metals (Pokrovsky and Schott, 2002; Fanun, 2014). This warrants further studies to elucidate the formation, properties, and subsequent behaviors (e.g., aggregation) of NOM-Mn(III) colloids in an effort to accurately elucidate their fate and even potential as vectors in sequestrating and mobilizing contaminants. Additionally, aggregation behavior of NOM-Mn(III) colloids may significantly affect their reactivity and mass flux as well as the fate and transport of priority contaminants. It has also demonstrated that the ratio of NOM to metal is an important factor influencing the formation and stability of NOM-metals colloids (Liao et al., 2017a; Mensch et al., 2017). Previous studies suggest that NOM from different sources has distinct effects on the colloidal behaviors of carbon-based colloids (Jiang et al., 2017). Although the role of NOM on the aggregation of engineered Mn(IV) colloids has been evaluated (Huangfu et al., 2013), the aggregation of naturally formed NOM-Mn(III) colloids has not been specifically evaluated.
The objectives of this study are to provide new insights into the formation (e.g., concentration and properties) and stability (i.e. aggregation) of NOM-Mn(III) colloids in aqueous environments (see Table 1). The central hypothesis in this study is that the molar ratio of C/Mn and different types of NOM can significantly affect the formation and aggregation of NOM-Mn(III) colloids. To test this, NOM-Mn(III) colloids were generated in batch experiments over a range of environmentally relevant C/Mn ratios with different NOM types, and those colloids and their aggregation behaviors were subsequently characterized using a suite of complementary characterization techniques. The stability of the formed NOM-Mn(III) colloids in real river and groundwater were also examined. Findings add a perspective to understanding of the stability of NOM-Mn(III) colloids and the ability to quantitatively predict the fate of contaminants, nutrients, and trace metals associated with NOM-Mn(III) colloids in aquatic environments.
Section snippets
Materials
All reagent solutions were prepared using ultrapure water (resistivity >18.2 MΩ cm, Milli-Q, Millipore). A stock solution of Mn(III) was prepared by dissolving 0.0134 g of manganese-(III) acetate dihydrate (>97%, Alfa Aesar) solid in 500 mL water to reach a concentration of 100 μM. Two sources of humic acid (HA), one from Aldrich HA (AHA, Sigma Aldrich) and the other extracted from Pahokee (Florida) peat soils (PPSHA, 2BS103P, International Humic Substances Society (IHSS)), which have been
Formation of NOM-Mn(III) colloids
Concentration distributions of Mn in HA-Mn(III) suspensions, under steady-state conditions, show that the colloidal fraction of Mn(III) (1–3 ∼ 450 nm) increases with increasing initial molar C/Mn ratio (Fig. 1a and b). No colloidal Mn(III) was observed in the absence of HA (C/Mn = 0). For both AHA and PPSHA, the colloidal Mn(III) concentration increased linearly with increasing initial molar C/Mn ratio from 0 to 10, followed by a progressive approach to a maximum (36.0% for AHA-Mn(III) and
Conclusions
This is the first report on the formation and stability of NOM-Mn(III) colloids in aqueous systems. Based on a suite of complementary characterization techniques, it can be concluded that relative amount and stability of HA-Mn(III) colloids generally increases with increasing molar C/Mn ratios, and that HA with more surface deprotonated COO− group and hydrophilicity result in higher stability likely due to (stronger) electrostatic repulsion, hydration interactions, and steric hindrance. This
Acknowledgments
This work was supported by the Natural Science Foundation of China (No. 41703128, 41572228, 41521001, 41702275), the Basic Research Project of Shenzhen (JCYJ20170307110055182), the China Postdoctoral Science Foundation (2017M610530), the Southern University of Science and Technology (SUSTC) (G01296001), and the Basic Research Project of Shenzhen(2017ZT07Z479). The authors are also grateful to the Pico Center at SUSTC, supported by the Presidential Fund and Development and Reform Commission of
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