Accepted Manuscript Copper release from copper nanoparticles in the presence of natural organic matter Long-Fei Wang, Nuzahat Habibu, Dong-Qin He, Wen-Wei Li, Xing Zhang, Hong Jiang, Han-Qing Yu, Prof. PII:

S0043-1354(14)00661-7

DOI:

10.1016/j.watres.2014.09.031

Reference:

WR 10893

To appear in:

Water Research

Received Date: 27 June 2014 Revised Date:

23 September 2014

Accepted Date: 24 September 2014

Please cite this article as: Wang, L.-F., Habibu, N., He, D.-Q., Li, W.-W., Zhang, X., Jiang, H., Yu, H.-Q., Copper release from copper nanoparticles in the presence of natural organic matter, Water Research (2014), doi: 10.1016/j.watres.2014.09.031. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical abstract

ACCEPTED MANUSCRIPT Copper Release from Copper Nanoparticles in the Presence of Natural Organic Matter

Jiang, Han-Qing Yu*

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Long-Fei Wang, Nuzahat Habibu, Dong-Qin He, Wen-Wei Li, Xing Zhang, Hong

CAS Key Laboratory of Urban Pollutant Conversion, Department of Chemistry,

*Corresponding author:

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University of Science & Technology of China, Hefei, 230026, China

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Prof. Han-Qing Yu, Fax: +86 551 63601592, E-mail: [email protected]

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Abstract Copper nanoparticles (CuNPs) are widely used and inevitably released into

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aqueous environments, causing ecological and health risks. Ubiquitous natural organic

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matter (NOM) might affect the copper release behaviors from CuNPs and their

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toxicity. This work aims to elucidate how NOM affects copper release from CuNPs,

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with a focus on the impacts of NOM properties and the NOM-CuNPs interaction

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mechanism. The copper release kinetics and different copper fractions induced by

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representative NOMs were characterized. The presence of NOM led to a more

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dispersive state of CuNPs clusters. Copper release mainly resulted from complexation

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reactions between CuNPs and functional groups of NOM. Humic substances were

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more effective in releasing copper than sodium alginate and bovine serum albumin,

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due to a higher amount of functional groups and lower molecular weight, which

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facilitated the contact and complexion reactions. Chlorination treatment of NOM

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significantly decelerated copper release due to the destruction of functional groups

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and less attachment of NOM. However, the copper releasing ability of humic acid was

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not substantially affected by Ca2+-induced coagulation. This study provides better

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understanding about the persistence and transformation of CuNPs in aquatic

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environments.

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Keywords: Copper nanoparticle; natural organic matter; ion release; adsorption;

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complexation

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1. Introduction

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Manufactured nanoparticles have been increasingly used in industries and production

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sectors today. These nanoparticles were inevitably released into the environment in

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production, transport and usage, causing ecological and health risks. Especially,

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copper nanoparticles (CuNPs), due to their excellent optical and electronic properties,

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have found widespread use in fields such as rubber, smelters, power stations, and fuel

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cells (Taberna et al., 2006; Midander et al., 2009; Mudunkotuwa et al., 2012).

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Meanwhile, concerns about the environmental impacts of manufactured nanoparticles

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including CuNPs arise recently. Researchers have confirmed that copper and copper

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oxide nanoparticles are toxic to bacteria (Heinlaan et al., 2008), zebra fish (Griffitt et

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al., 2007) and mice (Chen et al., 2006). Thus, the release of such CuNPs into natural

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environment should be controlled, which necessitates a better understanding about the

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aggregation, transformation and dissolution behaviors of CuNPs (Lowry et al., 2012).

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Natural organic matter (NOM) is ubiquitous in aqueous systems and significantly

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affect the mobility of colloidal matters or the toxicity of various substances including

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nanoparticles (Aiken et al., 2011; Lowry et al., 2012; Wang et al., 2012a; 2012b). The

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chelation or complexation of NOM on nanoparticles weakens the surface metal-metal

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and metal-oxygen bonds, resulting in altered chemical and physical characteristics of

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particles such as aggregation, dissolution and toxicity (Korshin et al., 1998; Aiken et

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al., 2011; Wang et al., 2011; Mudunkotuwa et al., 2012). For example, CuO NPs

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significantly damaged the bacteria membranes of Escherichia coli. The presence of

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fulvic acid could reduce the toxicity by mitigating the membrane damage (Zhao et al.,

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2013). From a toxicology perspective, dissolved copper is more harmful to organisms

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than solid copper particles and materials (Kim et al., 1999; Griffitt et al., 2007; Aruoja

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et al., 2009). NOM can accumulate on copper solids and result in copper release from

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various copper origins. The properties of NOM could be altered by multiple factors

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including pH, dissolved oxygen, inorganic ions and different treatments (Rehring and

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Edwards, 1996; Boulay and Edwards, 2001; Edwards and Sprague, 2001, Gao and

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Korshin, 2013). Commonly used wastewater treatments such as chlorination and

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ozonation could alter NOM properties and suppress copper release (Rehring and

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Edwards, 1996; Palit and Pehkonen, 2003; Gao and Korshin, 2013). NOM can form

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strong complexes with Cu2+ through intra- and inter-molecular chelation with the

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available functional groups (e.g., carboxylic and phenolic groups) in NOM (Cabaniss,

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1992; Town and Powell, 1993; Korshin et al., 1998; Kim et al., 1999). The released

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copper may exist in different forms, e.g., dissolved copper or particulate-bound copper,

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which might behave differently in terms of toxicity and mobility (Erikson et al., 1996;

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Edwards and Sprague, 2001). However, there is no report about the impacts of NOM

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on copper release from CuNPs so far. In light of the much higher percentage of

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surface copper atoms in CuNPs compared to common copper materials (Taberna et al.,

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2006; Midander et al., 2009), copper release behaviors from CuNPs in response to

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NOM might also be different. Additionally, the relationship between copper release

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behaviors and NOM properties as well as the interactions between NOM and CuNPs

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are yet to be clarified. In this work, we aim to elucidate how NOM affects copper release from CuNPs,

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with a special focus on the impacts of NOM properties and the NOM-CuNPs

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interactions. Five representative NOMs, including humic acid extracted from

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sediment of Chaohu Lake (extracted HA), Sigma humic acid (Sigma-HA), Suwannee

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River fulvic acid standard I (SRFA, International Humic Substances Society), bovine

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serum albumin (BSA) and sodium alginate, were selected and characterized by

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acid-base titration and gel permeating chromatography (GPC). The copper release

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kinetics and different copper fractions induced by these NOMs were analyzed. The

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NOM properties were altered by chlorination and coagulation, and their impacts on

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copper release were explored. The NOM-CuNPs interactions were explored with

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adsorption tests, Fourier transform infrared (FTIR) spectroscopy, zeta potential

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analysis, size measurements and transmission electron microscopy (TEM).

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2. Materials and Methods

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2.1 CuNPs and model NOM Zero valent CuNPs were purchased from Aladdin Reagent Co., China. The model

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NOMs were Sigma-HA (Sigma-Aldrich Co. USA), SRFA (IHSS), BSA (Aladdin

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Reagent Co., China) and sodium alginate (Shanghai Chem. Co., China). Sigma-HA

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was purified to remove the inorganic residues and the remaining ash content was

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6.9 % (Stevenson, 1994; Wang et al., 2013). A sediment sample was collected from

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fractionated from sediment sample based on the IHSS standard protocols detailed in

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the supplementary information (SI). The Sigma-HA and extracted HA were used as

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model NOMs from terrestrial solids and sediments, while SRFA represented the

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typical aquatic NOM. The elemental compositions of Sigma-HA and extracted HA

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were analyzed using an elemental analyzer (vario EL cube, Elementar Co., Germany)

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shown in Table S1. Stock NOM solutions (0.5 g/L) were prepared by dissolving

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extracted HA, freeze-dried Sigma-HA, SRFA, powder BSA or alginate in designed

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solutions under stirring until complete dissolution.

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The charge densities of model NOMs were determined by acid-base titration.

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The pH of NOM solutions (100 mg/L) was adjusted to below 3.0 using 1.0 M HCl.

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Then, the solutions were spurged with N2 for 15 min, followed by titration with 0.05

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M NaOH using an automatic titrator (TIM865 Hach Co., USA). Blank titration was

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conducted for NOM-free solutions under identical conditions. The NaOH dosage in

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the blank titration was used to estimate the net amount of NaOH consumed to

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deprotonate the carboxylic and/or phenolic functional groups of the NOMs (Hong and

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Elimelech, 1997; Lee and Elimelech, 2006). GPC (Waters Co., USA) equipped with

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Waters Ultrahydrogel 250, 500 and 2000 column in series was used to determine the

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molecular weight distribution of representative NOMs solutions a concentration of 10

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mg/L in Millipore water. The column was calibrated by standard polyethylene oxide

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from Agilent Co. (Batch Number 0001024141).

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2.2 Characterization of CuNPs Crytalline phase identification of CuNPs was obtained from an 18 kW rotating

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anode X-ray diffractometer (MAP18AHF, MAC Sci. Co., Japan). Diffraction patterns

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were compared with reference data in the ICDD PDF-2 database. The surface

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composition of CuNPs was analyzed by X-ray photoelectron spectroscopy

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(ESCALABMKII, VG Co., UK). The morphology of CuNPs was observed using

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TEM (JEM-2011, JEOL Co., Japan) with an acceleration voltage of 200 kV. The

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sample was prepared by dispersing CuNPs in distilled water under ultrasonication. To

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prepare CuNPs-NOM complex, 10 mg CuNPs and 10 mg various NOMs were

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dissolved into 100 mL distilled water at pH 7.0 and ultrasonicated. Several drops of

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the mixed sample were then added onto copper grids and held for 30 min for drying

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(Chen and Elimelech, 2007).

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2.3 Copper release tests

5 mg CuNPs were diluted into 50 mL designed solutions at an initial

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concentration of 100 mg/L and agitated at 25 ± 0.5 oC in a shaker working at dark.

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The copper concentrations over 24 h were recorded. For each analysis, 0.8 mL sample

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was collected at given time intervals and centrifuged at 550×g for 10 min. The

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concentration of the total released copper was determined using a flame atomic

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absorption spectrometry (4530 F, Shanghai Precision & Scientific Instrument Co.,

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China). All the tests were conducted in duplicate.

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To examine the effects of NOM on copper release, NOMs were dissolved in PBS

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ACCEPTED MANUSCRIPT buffer (pH 7.0, 100 mM) to reach concentrations from 5 to 100 mg/L. After CuNPs

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were stirred in NOM solutions (100 mg/L) for 36 h, the supernatants were collected

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and different copper fractions (dissolved copper, suspended copper, total copper and

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acid-extractable copper) were analyzed as described in SI (APHA, 1998; Xue et al.,

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2000). The influence of chlorination was explored by adding sodium hypochlorite

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(NaClO),

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(diethyl-p-phenylenediamine) method (APHA, 1998). NaClO was added to 100 mg/L

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NOM solutions to reach chlorine concentrations to 7.6, 34.1, 82.2 and 206.7 mg/L,

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respectively and incubated for 4 h. Afterwards, CuNPs were added into the treated

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solutions and the copper release behaviors were determined as described above.

concentration

was

measured

following

DPD

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The influence of coagulation state of Sigma-HA on copper release was

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investigated following the coagulation method (Christl et al., 2007). Briefly, 100 mg/L

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HA was dissolved in distilled water at pH 10.0 by adding 1 M NaOH. Afterwards,

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solution pH was adjusted to 7.0. A 50 mL aliquot of HA solution was transferred into

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glass tube, and added with 0.2 M CaCl2 solution for coagulation. The coagulated HA

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was settled for 24 h, and the dissolved organic carbon (DOC) in supernatant was

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determined by a TOC analyzer (Multi N/C 2100, Analytik Jena, Germany).

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Afterwards, CuNPs were added into the prepared HA solutions and the copper release

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rates were measured. To maintain the coagulation state of HA, the tested solutions

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were kept unstirred during the experiments. Control tests were conducted by adding

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CuNPs into 100 mg/L dissolved HA at pH 7.0 without stirring.

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2.4 NOM adsorption on CuNPs NOM adsorption tests onto CuNPs were conducted in 10 mM PBS solutions at

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25 oC for 24 h in a shaker. Pre-experiment was carried out and a high concentration of

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CuNPs was used to enhance the adsorption effect, as detailed in SI. The initial

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concentrations of CuNPs and NOM were 2000 mg/L and 10 mg/L, respectively. For

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each analysis, 0.8 mL sample was collected at given time intervals and centrifuged at

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550×g for 10 min. The concentrations of DOC in supernatants were measured with a

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TOC analyzer. To evaluate the effect of chlorination on NOM adsorption onto CuNPs,

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10 mg/L NOM was treated with 206.7 mg/L chlorine and contacted for 4 h prior to the

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adsorption tests.

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2.5 FTIR spectroscopy, zeta potential and particle size analysis

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To obtain the FTIR spectra of the NOM adsorbed on CuNPs, NOM-coated

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CuNPs were prepared. 40 mg CuNPs was added into 40 mL NOM solution at a

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concentration of 1 g/L and shaken for 24 h. The suspension was centrifuged at 550×g

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for 30 min. The precipitated complexes were rinsed with distilled water and freezing

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dried. The FTIR spectra of the samples were measured by infrared spectroscopy

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(VERTEX 70 FTIR, Bruker Co., Germany). Specifically, 1 mg of NOM, CuNPs, or

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NOM-coated CuNPs was mixed with 99 mg of KBr powder and analyzed,

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respectively. The FTIR spectra of NOM absorbed on CuNPs were obtained by

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subtracting the spectra of the raw CuNPs from that of the NOM-coated CuNPs (Yang

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et al., 2009).

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of CuNPs aggregates (100 mg/L) in the presence of NOM were recorded using a

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Nanosized ZS instrument (Malvern Instruments Co., UK) equipped with

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backscattering detection at 173° and 25 oC. For each zeta potential and z-average size

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analysis, at least 6 measurements were performed and the averaged results are

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3. Results and Discussion

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3.1 Characteristics of CuNPs and NOM

The XRD spectra show the three main diffraction peaks of crystalline Cu2O (Fig.

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1a), indicating the presence of Cu2O. In addition, distinct diffraction peaks, consistent

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with the diffraction patterns of Cu, dominated the XRD pattern. The intensity of the

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strongest diffraction peak (111) (JCPDS 4-836) was almost 8 times higher than the

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most intense peak (111) of Cu2O (JCPDS 5-667). The relatively weak intensities of

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the oxide components compared to the bulk component suggest the presence of a thin

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surface layer of oxide on CuNPs. The XRD analytical results indicate that the relative

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proportions of the surface oxide and the bulk core in CuNPs were 38% and 62%,

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respectively.

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The XPS analysis confirms that copper and oxygen were the main elements in

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CuNPs. The peaks at 931.3 and 951.0 eV in Fig. 1b corresponded to the Cu 2p3/2 and

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Cu 2p1/2 photoelectrons in CuO, respectively, with shakeup satellite peaks at higher

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ACCEPTED MANUSCRIPT binding energies (941.8 and 960.0 eV). The XPS data in the O 1s binding energy

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region (Fig. S2) suggest the presence of -OH functional groups on the surface. The

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peak at the binding energy of 530.2 eV was assigned to oxygen (O2-) in CuO, while

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the peak at 531.8 eV was attributed to the absorbed hydroxyl species that form a thin

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overlayer on the oxide layer (Midander et al., 2009). The presence of CuO was also

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evidenced by the FTIR spectra in Fig. S3. The multianalytical characterization of

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CuNPs confirms that the zero valent CuNPs were partially oxidized to copper oxides.

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The major composition of the particles were metallic copper, which was covered by

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an oxidized layer, mainly composed of Cu2O and a thin outer layer of CuO (Fig. 1d).

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TEM image reveals that the CuNPs with diameters of approximately 100 nm were

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aggregated and exhibited rough morphology (Fig. 1c). The z-average size of the

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CuNPs aggregates was measured to be 4450 ± 393 nm, while the zeta potential of

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CuNPs at a concentration of 100 mg/L was -16.1 ± 5.9 mV.

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Fig. 2a shows the acidities of the tested NOMs. Generally, acidity below pH 8.0

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is attributed to the deprotonation of carboxylic groups, while that above pH 8.0 is

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ascribed to the phenolic groups. Carboxylic groups were found to be the predominant

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functional groups in BSA and alginate. In comparison, the acidity of humic

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substances exhibited a further increase at pH above 8.0, indicating the presence of

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phenolic groups. The acidity values follow the order of: Sigma-HA > extracted HA

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SRFA > BSA > alginate. The molecular weight distributions of NOMs were

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examined using GPC (Fig. S4). The average peak molecular weights of Sigma-HA,

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extracted HA, SRFA, BSA and alginate were 158, 214, 5.73, 294 and 12704 kDa,

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respectively (Table S2).

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3.2 Copper release in the presence of NOM The release of copper from CuNPs is influenced by various factors including pH,

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ionic strength (Fig. S5), dissolved oxygen and NOM (Midander et al., 2009; Aiken et

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al., 2011; Mudunkotuwa et al., 2012). Fig. 3 shows such an effect by the NOM

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concentration. A higher copper release was achieved at a higher NOM concentration.

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For example, the percentage of released copper to total copper at 24 h increased from

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0.98% in the absence of Sigma-HA to 13.79% when 100 mg/L Sigma-HA was added.

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Humic substances were more effective in promoting copper release than BSA

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and sodium alginate. The amounts of released copper at 24 h were 13.79, 8.58 and

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8.86 mg/L, respectively, in the presence of 100 mg/L Sigma-HA, extracted HA and

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SRFA. In comparison, the values were only 2.92 mg/L for BSA, and 4.55 mg/L for

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alginate. The copper release rate could be described by a modified first-order kinetic

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equation (Kittler et al., 2010):

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where y(t) is the released amount of copper, y(final) refers to the final concentration of

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released copper at 24 h, k is the rate coefficient and t is the time. The released Cu

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concentration vs. time curves were fitted using equation (1) shown in Table 1. The

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high R2 values for Sigma-HA, extracted HA, SRFA and BSA suggest that the equation

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could fit the copper release kinetics well. However, it failed to fit when alginate was

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applied, in which case the copper concentration decreased gradually at latter stage.

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form extended gel networks. The decline of copper concentration at the later stage

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was possibly attributed to the re-adsorption of copper ions on the rougher

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alginate-CuNPs clusters (Chen and Elimelech, 2008). The rate of dissolution was

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higher for Sigma-HA than for the extracted HA, SRFA and BSA, confirming that

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Sigma-HA was the most effective in promoting copper release from CuNPs.

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The different fractions of the released copper at 36 h are listed in Table 2. For

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Sigma-HA, extracted HA and SRFA series, dissolved copper accounted for the major

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part of the total copper, indicating that the released copper was mostly in the form of

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free Cu2+ and soluble inorganic/organic complexes (Boulay and Edwards, 2001).

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While for BSA and alginate series, the suspended copper was the predominant

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composition. It was likely that the released copper induced by BSA and alginate was

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mostly bound to the particulate NOMs, which retained on membranes during filtration.

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In acidic water, more copper could be desorbed from solids or sediments. Thus, it is

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more

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environmentally available copper (Kim et al., 2006). After acid pre-treatment, the

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lightly-adsorbed copper was partially extracted from the particulates and the

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concentration of the acid-extractable copper was higher than that of the dissolved

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copper for all the tested NOM series.

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3.3 Interaction between CuNPs and NOM NOM adsorption onto CuNPs was quantified by DOC analysis to characterize

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ACCEPTED MANUSCRIPT their interaction (Fig. 4). The percentage of DOC to TOC (total organic carbon)

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declined in the adsorption process. However, the adsorption of NOM on CuNPs was

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limited and 10 mg/L NOM could not be totally adsorbed by 2000 mg/L CuNPs after

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24 h (Fig. 4a). Among the NOMs, BSA was the most favorable to be adsorbed on

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CuNPs, with 67% of the total organic matter adsorbed, while alginate showed the

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least tendency to be adsorbed onto CuNPs. Sigma-HA showed the highest adsorption

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onto CuNPs among the three humic substances. Fig. 4b shows the effect of

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chlorination on the NOM adsorption onto CuNPs. It was observed that chlorine

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slightly reduced the adsorption of the NOMs onto CuNPs. This result indicates that in

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the copper release tests, only about 0.3 mg/L NOM could be adsorbed onto 100 mg/L

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CuNPs, leaving the majority of NOM (more than 94%) in aqueous phase in the

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experiments.

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The results from the combined use of FTIR, zeta potential measurements,

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particle size analysis and TEM imaging indicate the adsorption of NOM onto CuNPs.

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No distinct peaks occurred in FTIR spectra of alginate-coated CuNPs, possibly due to a

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weak adsorption of alginate on CuNPs verified in Fig. 4a. The large macromolecules

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and the bead-like structures of alginate clusters cause relatively irregular surface,

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which weaken their contact with CuNPs (Chen and Elimelech, 2008; Saleh et al.,

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2010). The interactions between BSA and CuNPs are attributed to amide sections and

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carboxylic groups in Fig. S6 and Table S3. Globular BSA molecules could easily

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accumulate and form a thin layer on the CuNPs surface, confirmed in Fig. 4a. The

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interaction between CuNPs and humic substances was mainly assigned to aromatic

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and nanoparticle surfaces was responsible for the diminished carboxyl peaks of humic

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substances (Gu et al., 1994; Yang et al., 2009). The peak at 1720 cm-1 significantly

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diminished after humic substances were coated on CuNPs in Fig. S6, resulting from a

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strong interaction between -COOH groups and CuNPs. The greater adsorption of

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humic substances on CuNPs might be attributed to the π-π interaction between

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aromatic rings (Chen and Elimelech, 2008).

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The zeta potentials of CuNPs in the presence of NOM are illustrated in Fig. 2b.

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The presence of humic substances and alginate drastically decreased the values at

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higher NOM concentrations, from -16.1 mV in the absence of NOM to -47.4 mV,

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-41.3 mV, -34.9 mV and -56.9 mV when 100 mg/L of Sigma-HA, extracted HA,

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SRFA or alginate was present. The decline of CuNPs zeta potentials indicates that

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both alginate and humic subustances might impart negative charges to CuNPs

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surfaces (Zhang et al., 2009).

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The sizes of CuNPs aggregates were analyzed in Table 3. With the dose of

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NOMs, the z-average size of CuNPs aggregates decreased, indicating the aggregates

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were partially dispersed by NOMs. This was confirmed by the TEM images in Fig. 5

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and S6. The clusters of both humic substances and BSA were in the form of

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thin-sheetlike structures, on which CuNPs clusters were embedded (Fig. 5a-d), while

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alginate took the form of gel-beads (Fig. 5e). The more dispersive states of

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nanoparticle clusters in the presence of NOM were mainly attributed to the

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electrostatic and steric repulsion (Chen and Elimelech, 2008). The accumulation of

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ACCEPTED MANUSCRIPT the negatively charged NOM on CuNPs enhanced the repulsion forces between

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CuNPs and the NOM further stabilized CuNPs through steric repulsion, resulting in

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the dispersion and smaller size of CuNPs clusters. The more dispersive CuNPs

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clusters could facilitate their contact with NOM, which promoted the chelation and

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complexation reactions and subsequent copper release.

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3.4 Mechanism of copper release in the presence of NOM

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The copper release is mainly induced by complexation reactions as shown in Fig.

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6a. The release of copper requires the involvement of protons and/or hydroxyl (Fig.

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S5). NOM can also serve as ligands to interact with nanoparticles and further

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increases the mobility of CuNPs via the formation of simple ions as described in

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equation (2) (Edward and Sprague, 2001; Lu and Allen, 2002; Jones and Su, 2012):

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S-OH + L-z S-L(1-z) + OH-

(2)

where S-OH is the surface complexation site on copper surface, such as hydroxyl

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groups on CuNPs surface as confirmed by the XPS analysis as discussed above, L-z is

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the organic ligand binding site with charge –z.

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After copper ions were released with an induction by protons, hydroxyl or NOM,

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they can form strong complexes with NOM or other inorganic/organic matters via

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intra- or inter-molecular bidentate chelation (Edward and Sprague, 2001; Lu and

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Allen, 2002). The presence of abundant dissolved NOM (more than 94% compared to

329

total organic matter) played an important role in copper release from CuNPs (Fig. 4).

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The formed complexes can be either soluble complexes, particulate complexes, or

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ACCEPTED MANUSCRIPT precipitating (Boulay and Edwards, 2001). For BSA and alginate series, suspended

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copper was found to be the predominant fraction (Table 2). Adsorption of copper on

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particulates could decrease the toxicity of copper as the copper associated with

334

suspended solids was essentially unavailable (Erikson et al., 1996). While for

335

extracted HA and Sigma-HA, the major part of the released copper was dissolved

336

copper, indicating that more than 60% of released copper was in the form of free Cu2+,

337

soluble inorganic complexes (such as Cu(OH)2-x and CuHCO+3 ) or soluble organic x

338

complexes. The presence of more bio-available copper such as free Cu2+ and soluble

339

organic complexes induced by humic substances might pose a higher toxicity to

340

organisms (Kim et al., 1999).

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Among the tested NOMs, humic substances, especially Sigma-HA, exhibited a

342

stronger copper-releasing capability mainly due to three reasons. First, it possesses a

343

higher amount of functional groups. According to the NICA-Donnan model

344

estimation, the average amounts of carboxylic and phenolic groups in HA are 3.17 and

345

2.66 mol/kg, respectively (Milen et al., 2001), much higher than those in alginate and

346

BSA (Fourest and Volesky, 1996), as confirmed in Fig. 2a. The higher surface group

347

densities will also enhance the complexation between NOM and CuNPs, result in a

348

higher copper dissolution. Secondly, Sigma-HA showed a higher tendency to be

349

adsorbed among the humic substances (Fig. 4a), which facilitated their complexation

350

with NOM. Thirdly, Sigma-HA and extracted HA (possibly HA aggregates) and SRFA

351

have lower average molecular weights (158, 214 and 5.73 kDa, respectively) (Table

352

S2). The small molecular weight also favors a close contact and enhanced adsorption

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17

ACCEPTED MANUSCRIPT 353

of humic substances on CuNPs surfaces (Illes and Tombacz, 2006).

354 355

3.5 Impacts of NOM treatment on copper release Chlorination is commonly used for the disinfection of drinking water by

357

removing bacteria and virus and could ensure the safety of water supplies. When

358

water containing CuNPs was treated with chlorine, the potential effects of chlorinated

359

NOM on copper release was unclear yet. Fig. 7 shows the NOM-induced copper

360

release in response to chlorination. The presence of 100 mg/L NOM facilitates copper

361

release when no NaClO was added (black dots in Fig. 7). Decelerated release of

362

copper was also observed for other NOM series after chlorination. The mechanism for

363

the reduction of copper release could be explained as follows. First, chlorine could

364

breakdown macromolecules and lead to the formation of disinfection by-products,

365

such as dichloromethane and trihalomethane (Gallard and von Gunten, 2002). In the

366

disinfection process, the surface functional groups such as carboxyl, hydroxyl and

367

phenolic groups would be greatly destroyed and the surface activity of NOM is highly

368

weakened (Boulay and Edwards, 2001; Swietlik et al., 2004; Gao and Korshin, 2013).

369

Second, chlorine would reduce the hydrophobicity of NOM, resulting in less

370

attachment of NOM onto CuNPs (Fig. 4b). The less accumulation of NOM was also

371

confirmed by the zeta potential analysis (Fig. S8), in which the negative charge slight

372

declined as the chlorine concentration increased. Third, the strong oxidizing reactions

373

between chlorine and NOM made the solutions more unstable and led to a more

374

dispersive state of CuNPs clusters, especially for humic substances series in Table 3,

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18

ACCEPTED MANUSCRIPT which might weaken the attachment and interactions between particles and NOM. As

376

a result, chlorination could significantly weaken the complexation reactions between

377

NOM and CuNPs and accordingly reduce the copper release (Fig. 6b). For instance,

378

when chlorine concentration was increased from 7.6 to 206.7 mg/L, the released

379

copper in Sigma-HA series at 10 h remarkably decreased from 14.08 mg/L to nil.

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NOM in particulate and solid forms in soil and sediments has influence on

381

copper concentration and speciation (Ponizovsky et al., 2006). Dissolved HA tends to

382

coagulate into particulates in the presence of multivalent cations or other electrolytes

383

(Christ et al., 2007; Wang et al., 2013). Sigma-HA at different coagulation states were

384

prepared prior to tests. CuNPs were added into the coagulated HA solutions, and the

385

concentrations of released copper at 10 h were measured. Fig. 8 shows the percentage

386

of dissolved HA and the released copper concentrations (in relative to those in fully

387

dissolved HA solution) after 10 h at different CaCl2 concentrations. The changes of

388

released copper concentrations are illustrated in Fig. S9. The percentage of dissolved

389

HA concentration in total HA concentration decreased from 88.8% to 29.9% when

390

CaCl2 content was increased from 2.5 to 4 mM. In contrast, the percentage of released

391

copper (in relative to the copper concentration in the contrast series) in the absence of

392

NOM decreased from 98.5% to 83.4% only. The coagulated HA induced by Ca2+

393

could still promote the dissolution of copper ions from CuNPs. Divalent cations, such

394

as Mg2+ and Ca2+, could bind to the functional groups in HA and form intermolecular

395

bridges between adjacent HA colloids (Stevenson, 1994; Lu and Allen, 2002; Chen

396

and Elimelech, 2007), producing HA particulates (Fig. S10). The abundant unbound

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19

ACCEPTED MANUSCRIPT 397

surface functional groups still serve as ligands and promote copper release from

398

CuNPs.

399

4. Conclusions

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400 401

This work demonstrates that NOM origins and properties (e.g., chlorination and

403

coagulation states) significantly affect copper release from CuNPs. The interaction

404

between the complexation sites on copper surface and the functional groups in NOM

405

is mainly responsible for the copper release from CuNPs. The presence of NOM could

406

disperse CuNPs aggregates into small clusters and facilitate the contact between NOM

407

and CuNPs. Humic substances, including Sigma-HA, extracted HA and SRFA, are

408

more effective than alginate and BSA in releasing copper. The more abundant

409

functional groups (e.g., carboxyl and phenolic group), the lower molecular weights

410

and the greater adsorption onto CuNPs are responsible for the promoted ligand

411

complexion reactions and enhanced copper release. After NOM is treated with

412

chlorine, copper release is significantly suppressed due to the destruction of surface

413

functional groups, the less adsorption of NOM on CuNPs and the more dispersive

414

state of CuNPs clusters. Coagulated HA could serve as ligands and promote copper

415

release from CuNPs. The results of this work might be useful to better understand the

416

persistence and transformation of CuNPs in the presence of NOM in aquatic

417

environments.

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418

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ACCEPTED MANUSCRIPT 419

Acknowledgements We thank Dr. Ming-Quan Yan from Peking University for his supply of SRFA

421

sample. We also wish to thank the National Basic Research Program of China

422

(2011CB933700), Hefei Center for Physical Science and Technology (2012FXZY005)

423

and the Program for Changjiang Scholars and Innovative Research Team in

424

University of Ministry of Education of China for the partial support of this study.

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27

ACCEPTED MANUSCRIPT Table 1. Fitting results by a modified first-order reaction kinetic rate equation 100

y(final) (mg/L) k(h-1) R2 y(final) (mg/L) k(h-1) R2 y(final) (mg/L) k(h-1) R2 y(final) (mg/L) k(h-1) R2

6.60±0.22 3.61 0.88 1.00±0.03 0.81 0.93 3.87±0.13 0.61 0.83 1.00±0.05 1.36 0.87

8.30±0.11 2.59 0.92 1.41±0.13 2.86 0.92 5.05±0.13 1.11 0.80 1.66±0.08 0.98 0.92

8.69±0.06 3.23 0.95 2.66±0.08 2.26 0.84 5.73±0.02 0.98 0.95 2.04±0.03 0.92 0.95

10.78±0.59 2.49 0.85 4.33±0.27 2.42 0.86 7.91±0.08 1.25 0.85 2.39±0.11 1.17 0.96

13.79±0.11 2.16 0.96 7.80±0.27 1.86 0.84 8.86±0.56 0.80 0.89 3.16±0.24 1.12 0.95

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20

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5

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Sigma HA

NOM (mg/L)

ACCEPTED MANUSCRIPT Table 2. Cu fractions in released copper induced by 100 mg/L NOMs at 36 h Suspended

Dissolved

Acid-extractable

(mg/L)

copper (mg/L)

copper (mg/L)

copper (mg/L)

Sigma-HA

16.17±1.34

4.26±0.07

9.65±0.47

10.18±0.58

Extracted-HA

9.28±1.83

2.31±0.18

5.47±0.13

6.42±0.56

SRFA

10.85±0.10

2.85±0.31

6.63±0.02

BSA

7.79±1.80

5.05±1.94

2.97±0.56

Alginate

3.36±0.26

2.47±0.50

0.97±0.06

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Total copper

10.15±0.18 6.05±0.77

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3.03±1.28

ACCEPTED MANUSCRIPT Table 3. Z-average size of CuNPs aggregates (100 mg/L) in the presence of NOM Concentration

0 mg/L

10 mg/L

100 mg/L

100 mg/L treated with 206.7 mg/L clorine

Sigma-HA

2483±328

1640±564

764±82

Extracted-HA

2802±658

1167±237

685±158

2305±380

1338±188

BSA

1906±443

1370±156

Alginate

2606±744

1431±175

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4450±393

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SRFA

781±147

1630±693 1456±168

ACCEPTED MANUSCRIPT Figure Legends

Fig. 1 - Characterization of CuNPs. (a) XRD spectrum; (b) XPS spectra in the Cu 2p

Fig. 2 -

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region; (c) TEM image; and (d) schematic illustration of the composition.

(a) Acidity of NOMs as a function of pH. Concentration of NOM is 100

mg/L; (b) Zeta potentials of 100 mg/L CuNPs in the presence of NOM.

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represent the deviation of duplicate samples.

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Fig. 3 - Copper release kinetics at different NOM concentrations. The error bars

Fig. 4 - (a) NOM adsorption onto CuNPs quantified by the ratio of DOC to TOC as a function of time; and (b) The effects of NOM chlorination on NOM adsorption onto CuNPs. The initial concentrations of NOM and CuNPs were 10 mg/L and

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2000 mg/L, respectively.

Fig. 5 - TEM images of the representative CuNPs aggregates observed in the presence of 100 mg/L (a) Sigma-HA; (b) extracted HA; (c) BSA; and (d)

EP

alginate.

Fig. 6 - Image illustration of (a) NOM induced copper release from CuNPs; and (b)

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effects of chlorination on the properties of NOM and copper release from CuNPs.

Fig. 7 - Copper release kinetics in the presence of chlorine-treated NOM. The error bars represent the deviation of duplicate samples.

Fig. 8 - Effects of calcium on the concentrations of dissolved Sigma-HA and released copper after 10 h. The error bars represent the deviation of duplicate samples.

ACCEPTED MANUSCRIPT

111

∆−Cu2O

(a)

ο

(b)

Cu 2p

931.3

∆

200 ο

200

∆

220 ∆

951.0

960.0 Satellite

30

40

50

60

70

2 Theta (degree)

960

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Intensity

Intensity (a.u.)

ο−Cu

941.8 939.8 Cu 2p1/2 950

Satellite

Cu 2p3/2

940

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Binding energy (eV)

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ACCEPTED MANUSCRIPT

10

(a)

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Sigma HA Extracted HA SRFA BSA Alginate

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Acidity (mol/g)

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0 3

4

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7

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9

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11

pH

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SRFA Sigma HA Extracted HA BSA Alginate

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-60

0

20

40

60

80

NOM concentration (mg/L)

Fig. 2

100

ACCEPTED MANUSCRIPT 16

(a)

Sigma-HA

(b)

Extracted HA 8

12 6

10 8

4 6

NOM concentration (mg/L)

4

0 5 10

20 50 100

2

2 0

10

0

4

8

12

16

20

24

(c)

SRFA

3.5

0

BSA

3.0 2.5 6

12

16

20

24

(d)

2.0 1.5

4

1.0

2

0.5 0.0

0 5

4

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16

4

Alginate

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2

1

0

0

4

20

24

(e)

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Released Cu (mg/L)

8

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Released Cu (mg/L)

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4

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Released Cu (mg/L)

14

8

12

Time (h)

16

0 5 10

20 50 100

20

24

Time (h)

Fig. 3

0

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8

12

Time (h)

16

20

24

ACCEPTED MANUSCRIPT

(a)

100

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SRFA Sigma HA Extracted HA BSA Alginate

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60 50 40 30 5

110 100

80

50

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70 60

10 15 Time (h)

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Pecentage of dissolved organic carbon (%)

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Pecentage of dissolved organic carbon (%)

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20

25

(b)

SRFA Sigma HA Extracted HA BSA Alginate

40 30

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5

10 15 Time (h)

Fig. 4

20

25

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Fig. 6

ACCEPTED MANUSCRIPT (a)

HA (100 mg/L)

Released Cu (mg/L)

Chlorine (mg/L) 0 7.6 34.1 82.2 206.7

12 10 8

6

4

6 4

2

2 0

0 2

4

6

8

10

(c)

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Released Cu (mg/L)

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4

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8

10

(d)

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Extracted HA (100 mg/L) 8

14

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2.0 1.5

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1.0

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0.5 0.0

0 0

2

4

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Released Cu (mg/L)

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5 Alginate (100 mg/L)

0

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4

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Time (h)

8

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Fig. 7

0

2

4

Time (h)

6

8

10

ACCEPTED MANUSCRIPT

80

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60

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40

40

0

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0.2 2.5

2.5 3 2.5

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Calcium (mM) 20

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3.5 3.75 4

3.5

Calcium concentration (mM)

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Fig. 8

Released Cu (% Contrast series)

100

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Dissolved HA (% Total HA)

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HA was more effective in releasing copper than sodium alginate and BSA.

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Supplementary information

Copper Release from Copper Nanoparticles in the Presence of Natural Organic

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Matter

Jiang, Han-Qing Yu*

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Long-Fei Wang, Nuzahat Habibu, Dong-Qin He, Wen-Wei Li, Xing Zhang, Hong

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CAS Key Laboratory of Urban Pollutant Conversion, Department of Chemistry, University of Science & Technology of China, Hefei, 230026, China

The following is included as additional supplementary materials for this paper:

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Page S12

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Page S8 Page S9 Page S10 Page S11

Standard HA extraction method Table S1. Elemental compositions of HA samples Definition and measurements of different copper fractions Figure S1. Ratio of dissolved organic carbon (DOC) to total organic carbon (TOC) in the presence of CuNPs as a function of time Figure S2. X-ray photoelectron spectra of oxygen (1s) for CuNPs Figure S3. FTIR spectrum of the selected CuNPs Figure S4. GPC chromatography of the representative NOMs Table S2. Molecular weight data from polyethylene oxide standards and RI detector Figure S5. Effects of solution chemistry on copper release from CuNPs (a) and (b) refer to the effects of pH and ionic strength respectively Figure S6. FTIR spectra of NOM and CuNPs-coated NOM Table S3. Major infrared absorption bands of HA, alginate and BSA Figure S7. TEM images of representative CuNPs aggregates Figure S8. Zeta potentials of 100 mg/L CuNPs in the presence of 100 mg/L NOM chlorinated with varying chlorine concentrations from 2.0 to 206.7 mg/L Figure S9. Effects of dissolved Sigma-HA on copper release from CuNPs Figure S10. Coagulation of HA macromolecules induced by Ca2+ References

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Page S14 Page S15 Page S16 Page S17

Page S18 Page S19 Page S20

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ACCEPTED MANUSCRIPT Standard HA extraction method based on the International Humic Substances Society (www.humicsubstances.org/soilhafa.html)

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Materials 1. Hydrochloric acid (HCl), 1 M, 6 M 2. Sodium hydroxide, 1 M, 0.1 M

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3. Potassium hydroxide (KOH), 0.1 M

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4. Potassium chloride (KCl) 5. Hydrofluoric acid (HF) concentrated, 0.3 M

Methods

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Remove roots and sieve the dried soil samples to pass a 2.0-mm sieve. Equilibrate the sample to a pH of 1.0-2.0 with 1 M HCl at ambient temperature. Adjust the solution volume with 0.1 M HCl to provide a final concentration (10 mL

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liquid/1 g dry sample). Shake the suspension for 1 h and then separate the

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supernatant from the residue by decantation after allowing the solution to settle or by low-speed centrifugation. Neutralize the soil residue with 1 M NaOH to pH = 7.0, then add 0.1 M NaOH

under N2 atmosphere to give a final ratio of extractant to soil of 10:1. Extract the suspension under N2 with intermittent shaking for over 4 h. Allow the alkaline suspension to settle overnight and collect the supernatant through decantation or centrifugation. Acidify the supernatant with 6 M HCl with constant stirring to pH =

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ACCEPTED MANUSCRIPT 1.0 and then allow the suspension to stand for 12-16 h. Re-dissolve the humic acid fraction by adding a minimum volume of 0.1 M KOH under N2. Add solid KCl to obtain a concentration of 0.3 M [K+] and then

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centrifuge at a high speed to remove the suspended solids. Re-precipitate the humic acid by adding 6 M HCl with constant stirring to pH = 1.0 and allow the suspension to stand again for 12-16 h. Centrifuge and discard the supernatant. Suspend the HA

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precipitate in 0.1 M HCl/0.3 M HF solution in a plastic container and shake

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finally freeze dry the humic acid.

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overnight at ambient temperature. Centrifuge and repeat the HCl/HF treatment, and

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ACCEPTED MANUSCRIPT Table S1. Elemental compositions of HA samples Sigma-HA

Extracted HA

C (%)

49.80

42.48

O

24.99

26.42

H

3.87

5.43

N

0.80

5.08

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Element (%)

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ACCEPTED MANUSCRIPT Definitions and measurements of different copper fractions The copper fractions can be divided into the following terms: Dissolved Copper: The copper in an unacidified sample that passes through a

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0.45-µm membrane filter. Suspended Copper: The copper in an unacidified sample that is retained by a 0.45-µm membrane filter.

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Total Copper: The concentration of metals determined in an unfiltered sample

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after vigorous digestion.

Acid-Extractable Coppers: The concentration of metals in solution after treatment of an unfiltered sample with hot dilute mineral acid.

After CuNPs were stirred in 100 mg/L NOMs for 36 h, 30 mL of the solution

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was collected and filtered with 0.45-µm cellulose membrane. The supernatant was acidified with 2 mL concentrated nitric acid (HNO3) and digested on a hot plate in a beaker. After the solution was concentrated to approximately 1 mL, 2 mL

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concentrated nitric acid (HNO3) and 1 mL perchloric acid (HClO4) were added and

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digested. After the solution was concentrated to approximately 1 mL, the solution was diluted with Millipore water and transferred into a volumetric flask at a volume of 10 mL. Then, the Dissolved Copper concentration was determined using a flame atomic absorption spectrometry (4530 F, Shanghai Precision & Scientific Instrument Co., China). The filtered membranes were also digested following the above methods and the copper concentration was defined as the Suspended Copper. 30 mL solution was directly digested and the copper concentration was defined

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ACCEPTED MANUSCRIPT as the Total Copper. Acid-extractable copper was lightly adsorbed on particulate materials. 30 mL sample was acidified with 2 mL 1:1 high purity HCl. Heat 15 min on a steam bath. Filter the sample through a membrane filter and digest it with the

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Copper. All the tests were conducted in duplicate.

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above methods. The copper concentration was defined as the Acid-Extractable

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ACCEPTED MANUSCRIPT NOM adsorption pre-experiment: 10 mg/L Sigma-HA (in 10 mM PBS) were dispersed in solutions containing different concentrations of CuNPs (from 0.1 to 2 g/L) and samples were collected at

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different time intervals and centrifuged at 550 g for 10 min. The DOC concentrations were measured by and shown in Figure S1. In order to enhance the adsorption effect,

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2 g/L CuNPs was applied for the following adsorption tests.

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Ratio of DOC to TOC (%)

100

70

60

5

10

15

20

Time (h)

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CuNPs conc. (g/L) 0.1 0.2 0.5 1 2

Figure S1. Ratio of dissolved organic carbon (DOC) to total organic carbon (TOC) in the presence of CuNPs as a function of time

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Figure S2. X-ray photoelectron spectra of oxygen (1s) for CuNPs

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Figure S3. FTIR spectrum of the selected CuNPs. Two peaks at 588 and 538 cm-1, assigned to the vibrations of Cu(II)-O, were observed. There was also another peak at around 638 cm-1 (assigned to Cu(I)-O vibration), indicating the presence of Cu2O

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in CuNPs

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Refractive Index

Sigma HA Extracted HA BSA ALGINATE SRFA

10

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20 25 Time (min)

30

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Figure S4. GPC chromatography of the representative NOMs

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ACCEPTED MANUSCRIPT Table S2. Molecular weight data from polyethylene oxide standards and RI detector Respective peak molecular weight (Mp) (KDa)

Area

Average Mp

(%)

(KDa)

Sigma HA

13.81 21.50 22.35 25.43

31395 128 82 22

0.2 64.9 7.2 26.0

Extracted HA

19.24 20.20 21.37 22.80 24.85

464 255 130 62 25

28.6 16.7 12.3 31.4 11.0

SRFA

30.78 35.40

5.81 5.52

73.8 26.2

5.73

BSA

18.93 21.68 26.20 30.64 31.96 33.59

594 116 17 6 5 5

47.9 2.0 35.7 1.6 3.5 9.3

294

Alginate

14.42 17.08 18.84 21.61 24.70 26.32

18394 2173 632 120 29 16

67.8 7.8 9.2 2.2 4.0 9.1

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Retention time (min)

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NOM

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158

214

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Figure S5. Effects of solution chemistry on copper release from CuNPs. (a) and (b) refer to the effects of pH and ionic strength respectively

The influences of pH and ionic strength on copper release process were investigated. Phosphate buffer solutions (ionic strength between 10-1000 mM, pH 7.0) were applied to examine the effects of ionic strength. Acetate buffers (100 mM,

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ACCEPTED MANUSCRIPT pH 4.0, 5.0 and 6.0), phosphate buffers (100 mM, pH 7.0 and 8.0) and borate buffer (100 mM, pH 9.0) were used to determine the effects of solution pH (Good et al., 1966).

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The Cu release is pH-dependent. The copper concentration significantly decreased from 100.94±0.85 to 0.98±0.26 mg/L as pH was increased from 4.0 to 7.0. At a higher pH, however, the concentration gradually increased to 3.95±0.25

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mg/L at pH 9.0. Figure S4b shows the effects of ionic strength to copper release. The

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copper concentration increased from 0.72±0.11 to 9.01±0.10 mg/L when the solution ionic strength was increased from 10 mM to 1000 mM. For CuNPs dissolution in solutions at different pHs, the following reactions can occur: 2Cu2++2H2O

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2Cu+4H++O2

2Cu+2OH- Cu2O+H2O+2e-

Cu2O+2OH-+H2O 2Cu(OH)2+2e-

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In solutions with a higher ionic strength, the double electrostatic layer could be

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reduced, possibly led to the reduction of bridging and netting between particles and enhancement of copper release.

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Figure S6. FTIR spectra of (a) Sigma-HA and CuNPs-coated Sigma-HA, (b) extracted HA and CuNPs-coated extracted HA, (c) BSA and CuNPs-coated BSA, (d) alginate and CuNPs-coated alginate and (e) SRFA and CuNPs-coated SRFA. The spectra of CuNPs-coated NOM was obtained by subtraction of the FTIR spectra of pure CuNPs from that of the NOM-coated CuNPs after freeze drying 14

ACCEPTED MANUSCRIPT Table S3. Major infrared absorption bands of HA, alginate and BSA Vibration

3700-3300 1390 1390 1440 2926 2856 1280 1720 1620 1038-1100 3350 3060 1665 1550 1405 1250 1120 3450 2920 1620 1404 1299 1120/1204

-OH stretching -OH deformation C-H deformation of CH2 and CH3 groups aliphatic C-H asymmetric C-H stretching of aliphatic –CH2 symmetric C-H stretching of aliphatic –CH2 C-O stretching of phenolic C-O vibration of carboxyl stretching of aromatic C=C C-O stretching in polysaccharides vibration of –OH vibration of N-H vibration of C=O in amide I deformation vibration of N-H in amide II symmetric vibration of COOvibration of C-O in carboxylic groups vibration of C-O in ether vibration of –OH in water anomer C-H stretching asymmetric vibration of COOsymmetric vibration of COOdeformation vibration of CCH and OCH C-O stretching

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Alginate

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BSA

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Humic substances

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Wavenumber/cm-1

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Representative NOM

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1 µm

1 µm

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10 µm

Figure S7. TEM images of the representative CuNPs aggregates observed in the

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presence of 100 mg/L Sigma-HA, extracted HA, SRFA , BSA and alginate

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Figure S8. Zeta potentials of 100 mg/L CuNPs in the presence of 100 mg/L NOM

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chlorinated with varying chlorine concentrations from 7.6 to 206.7 mg/L.

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Figure S9. Effects of dissolved Sigma-HA on the copper release from CuNPs. CaCl2 was added into 100-mg/L dissolved HA to induce coagulation of HA. The

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concentration of dissolved HA was measured prior to the copper release tests after 24 h. The results indicate that concentration of dissolved HA did not have a great

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impact on copper release.

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Figure S10. Coagulation of HA macromolecules induced by Ca2+.

Ca2+ could form bridges with the negatively charged functional groups in HA and

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effectively trap the marcomolecules into large clusters and lead to precipitation and coagulation. In such a process, only a part of surface functional groups (carboxyl or phenolic groups) interacted with Ca2+ and the free functional groups could still

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promote copper release from CuNPs.

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ACCEPTED MANUSCRIPT References

Association, A.P.H., 1998. Standard Methods for the Examination of Water and Wastewater. Twenty ed. Washington, DC.

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Boulay, N.; Edwards, M., 2001. Role of temperature, chlorine, and organic matter in copper corrosion by-product release in soft water. Water Res 35, 683-690.

Good, N.E., Winget, G.D., Winter, W., Connolly, T.N., Izawa, S., Singh, R.M.M., 1966. Hydrogen ion buffers for biological research. Biochemistry 5, 467-477.

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Grdadolink, J.; Marechal, Y., 2001. Bovine serum albumin observed by infrared

Biopolymers 62, 40-53.

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spectrometry. I. Methodology, structural investigation, and water uptake.

Kittler, S.; Greulich, C.; Diendorf, J.; Koller, M.; Epple, M., 2010. Toxicity of silver nanoparticles increases during storage because of slow dissolution under release of silver ions. Chem Mater 22, 4548-4554.

Lawrie, G.; Keen, I.; Drew, B.; Chandler-Temple, A.; Rintoul, L.; Fredericks, P.;

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Grondahl, L., 2007. Interactions between alginate and chitosan biopolymers characterized using FTIR and XPS. Biomacromolecules 8, 2533-2541. Midander, K.; Cronholm, P.; Karlsson, H. L.; Elihn, K.; Moller, L.; Leygraf, C.;

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Wallinder, I. O., 2009. Surface characteristics, copper release, and toxicity of nano- and micrometer-sized copper and copper (II) oxide particles: A

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cross-disciplinary study. Small 5, 389-399. Servagent-Noinville, S.; Revault, M.; Quiquampoix, H.; Baron, M. H., 2000. Conformational changes of bovine serum albumin induced by adsorption on different clay surfaces: FTIR analysis. J Colloid Interf Sci 221, 273-283.

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