Elsevier

Chemosphere

Volume 248, June 2020, 125990
Chemosphere

Impact of origin and structure on the aggregation behavior of natural organic matter

https://doi.org/10.1016/j.chemosphere.2020.125990Get rights and content

Highlights

  • NOM intermolecular interaction energy dominated by the Lewis acid-base interactions.

  • NOM colloidal stability determined by its hydrophobicity.

  • Mg2+ has differed effects on terrestrial and aquatic NOM.

  • E2/E3 is a convenient indicator for NOM colloidal stability.

Abstract

The intermolecular interactions of natural organic matter (NOM) play a key role in the fate and transport of organic carbon and pollutants in environmental and engineered systems. In this study, the impact of origin and structure on the aggregation behavior of NOM was investigated in the presence of naturally abundant cations. The physicochemical properties of NOM were quantified using a range of indices. Thermodynamic analysis suggests that the colloidal stability of NOM was mainly determined by its hydrophobicity (i.e., Lewis acid-base interactions). All NOM can be coagulated by Ca2+ owing to the strong cation-NOM interactions, which lead to bridging effect and lower Lewis acid-base interactions. Terrestrial NOM can be coagulated by Mg2+ while aquatic NOM cannot, owing to their different hydrophobicity. The critical coagulation concentrations of tested terrestrial NOM in the presence of Ca2+ (CCC–Ca) were quite similar at 1.94–4.88 mM despite their different structural properties. The CCC-Ca of tested aquatic NOM varied significantly from 46.89 mM to 110.40 mM depending on their structure. The optical indices including E2/E3, FI, and HIX can be potentially used as convenient indicators for assessing the colloidal stability of aquatic NOM for water treatment and risk assessment purposes.

Introduction

Natural organic matter (NOM) is ubiquitous in terrestrial and aquatic environments. Its aggregation behavior in aquatic systems is considered to be an important step in the transformation of soluble NOM molecules into particles, influencing a variety of natural and engineered environmental processes, such as the fate of pollutants (Schlautman and Morgan, 1993; Kopinke et al., 2001; Fu et al., 2018) and the performance of filtration systems (Li and Elimelech, 2004; Wang et al., 2015).

The aggregation of NOM is controlled by its structural properties as well as the water chemistry (Tombacz and Meleg, 1990; Terashima et al., 2007; Kalinichev et al., 2011; Wang et al., 2013; Xu et al., 2017). Most of the studies in this area focus on the role of water chemistry, such as pH and electrolytes, on NOM aggregation (Tombacz and Meleg, 1990; Tombácz, 1999; Wang et al., 2013). The solution pH influences the dissociation of functional groups in NOM, such as carboxyl groups and phenolic hydroxyl groups. The dissociation states of these functional groups strongly affect the surface charge of NOM molecules (Tombácz, 1999). NOM is more prone to aggregation at low pH conditions (Wall and Choppin, 2003; Kloster et al., 2013; Wang et al., 2013). Previous studies reported specific cation effects for NOM aggregation (Wall and Choppin, 2003; Iskrenova-Tchoukova et al., 2010; Xu et al., 2019). Naturally abundant Na+ usually has limited impact on the colloidal stability of NOM (Iskrenova-Tchoukova et al., 2010; Xu et al., 2017, 2019). Nevertheless, several studies reported that a purified commercial humic acid can be coagulated by Na+ at elevated concentrations (Wall and Choppin, 2003; Wang et al., 2013). Ca2+ and Ba2+ cations can readily induce NOM aggregation at relatively low concentrations (Iskrenova-Tchoukova et al., 2010; Kloster et al., 2013; Xu et al., 2017). The literature reports on Mg2+ are rather inconsistent. Several studies reported that Mg2+ cannot induce the aggregation of NOM even at elevated concentrations (Iskrenova-Tchoukova et al., 2010; Xu et al., 2017). However, there are also papers suggesting that NOM aggregated in the presence of Mg2+, although less significantly compared to Ca2+ (Wall and Choppin, 2003; Wang et al., 2013). The enormous heterogeneity of NOM is one of its most prominent characteristics, which is critical for understanding its colloid behavior. The discrepancies in the literature regarding the role of electrolytes in NOM aggregation are most likely linked to the heterogeneity of NOM. Nevertheless, the impact of NOM origin and structure on its aggregation behavior is still unclear. Meanwhile, convenient indices are in great need to help perform routine assessment of NOM colloidal stability in aquatic systems.

In the present study, we investigate the aggregation kinetics of nine NOM samples with different origins and structural properties in the presence of naturally abundant cations including Na+, Mg2+, and Ca2+. The aggregation kinetics was examined using time-resolved dynamic light scattering (t-DLS) technique. The interaction energies between NOM molecules in the given water chemistry conditions were calculated based on extended Derjaguin–Landau–Verwey–Overbeek (XDLVO) theory. The physicochemical properties of NOM were determined using UV–Vis spectrometer, fluorescence spectrometer, electrokinetic method, and gel permeation chromatography. The elemental composition and nuclear magnetic resonance (NMR) information of NOM were adapted from the IHSS database. Correlations between critical coagulation concentration (CCC) and structural indices were explored. The objectives of this paper were 1) to understand the impact of NOM origin and structure on its colloidal stability; and 2) to find convenient predictors for assessing the colloidal stability of NOM.

Section snippets

Materials

Calcium chloride dihydrate (>99%) was purchased from Alfa Aesar, UK. Sodium chloride (>99.5%) and magnesium chloride (>99.5%) were purchased from Sigma-Aldrich, USA. Hydrochloric acid was purchased from Shanghai Lingfeng Chemical Reagent Co, China. Sodium hydroxide was purchased from Nanjing Chemical Reagent Co, China. All reagents were used without further purification. Nine NOM Samples were provided by the International Humic Substances Society (IHSS, St. Paul, USA), including Elliott Soil

Aggregation behavior of NOM in electrolyte solutions

The initial aggregation behavior of NOM samples was examined in NaCl, CaCl2 and MgCl2 solutions by t-DLS (Fig. 1, Fig. 2, Fig. 3). The structural properties of NOM samples were summarized in Table 1. There was little aggregation of NOM found in 600 mM NaCl solution (the same ionic strength as 200 mM CaCl2 or MgCl2 solutions) in 30 min (Fig. 1). For aquatic NOM, the hydrodynamic particle diameter even slightly decreased in 600 mM NaCl solution. This was caused by the structural compacting

Conclusion

In the present study, we examined the impact of origin and structure of NOM on its aggregation behavior in the presence of naturally abundant cations.

  • NOM samples with different origins show drastically different aggregation behavior in the presence of Mg2+. Mg2+ can induce aggregation of terrestrial NOM but not aquatic NOM. Interaction energy analysis based on the XDLVO theory suggests that this phenomenon is caused by the different hydrophobicity of NOM. The result explained the discrepancies

CRediT authorship contribution statement

Peiyun Wei: Methodology, Investigation, Writing - original draft. Fanchao Xu: Investigation, Writing - review & editing. Heyun Fu: Methodology, Writing - review & editing. Xiaolei Qu: Conceptualization, Methodology, Writing - original draft.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grants 21876075 and 21622703).

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