Antarctic snow: metals bound to high molecular weight dissolved organic matter

Accepted Manuscript Antarctic Snow: Metals Bound to High Molecular Weight Dissolved Organic Matter

Nicoletta Calace, Elisa Nardi, Marco Pietroletti, Eugenia Bartolucci, Massimiliana Pietrantonio, Carlo Cremisini PII:

S0045-6535(17)30230-8

DOI:

10.1016/j.chemosphere.2017.02.052

Reference:

CHEM 18815

To appear in:

Chemosphere

Received Date:

10 October 2016

Revised Date:

02 February 2017

Accepted Date:

08 February 2017

Please cite this article as: Nicoletta Calace, Elisa Nardi, Marco Pietroletti, Eugenia Bartolucci, Massimiliana Pietrantonio, Carlo Cremisini, Antarctic Snow: Metals Bound to High Molecular Weight Dissolved Organic Matter, Chemosphere (2017), doi: 10.1016/j.chemosphere.2017.02.052

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ACCEPTED MANUSCRIPT High molecular weight dissolved organic matter recovered by ultrafiltration in Antarctic snow Heavy metal bound to high molecular weight dissolved organic matter This study offers information on long-term transport of heavy metals taking into account that humic-like substances are a main component of atmospheric organic carbon. Spearman rank correlation analysis among organic bound metal concentration and high molecular weight organic substance.

ACCEPTED MANUSCRIPT 1

ANTARCTIC SNOW: METALS BOUND TO HIGH MOLECULAR WEIGHT

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DISSOLVED ORGANIC MATTER

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Nicoletta Calace*, Elisa Nardiǂ, Marco Pietroletti, Eugenia Bartolucci, Massimiliana Pietrantonioǂ,

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Carlo Cremisiniǂ

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The Italian National Institute for Environmental Protection and Research (ISPRA), Via Vitaliano

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Brancati, 48 00144 Rome (Italy)

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ǂ ENEA, Italian National Agency for New Technologies, Energy and Sustainable Economic

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Development, C.R. Casaccia, via Anguillarese 301, 00123, Rome, Italy.

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Abstract

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In this paper we studied some heavy metals (Cu, Zn, Cd, Pb, As, U) probably associated to high

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molecular weight organic compounds present in the Antarctic snow. Snow-pit samples were

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collected and analysed for high molecular weight fraction and heavy metals bound to them by

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means of ultrafiltration treatment.

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High molecular weight dissolved organic matter (HMW-DOM) recovered by ultrafiltration showed

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a dissolved organic carbon concentration (HMW-DOC) of about 18 – 83 % of the total dissolved

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organic carbon measured in Antarctic snow. The characterisation of HMW-DOM fraction

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evidenced an ageing of organic compounds going from surface layers to the deepest ones with a

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shift from aliphatic compounds and proteins/amino sugars to more high unsaturated character and

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less nitrogen content.

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The heavy metals associated to HMW-DOM fraction follows the order: Zn > Cu > Pb >> Cd ~ As ~

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U. The percentage fraction of metals bound to HMW-DOM respect to total metal content follows

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the order: Cu >> Pb > Zn, Cd in agreement with humic substance binding ability (Irwing-William

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series). Going down to depth of trench, all metals except arsenic, showed a high concentration peak

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ACCEPTED MANUSCRIPT 26

corresponding to 2.0-2.5 m layer. This result was attributed to particular structural characteristic of

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organic matter able to form different type of complexes (1:1, 1:2, 1:n) with metals.

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Keywords: high molecular weight organic substances, heavy metals, Antarctic snow, ultrafiltration.

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Corresponding author: E-mail address: [email protected]

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1. Introduction

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The Antarctic continent is slightest affected by anthropogenic impact. Antarctic snow is the main

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atmospheric deposition form. Moreover, snow and ice are able to preserve a record of past

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atmospheric composition (Wolff and Suttie,1994).

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Several studies carried out on snow and glacial ice highlighted significant changes in the content of

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ionic species, dust and trace elements during the past (Legrand et al., 1988; Hong et al., 2004 ;

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Gabrielli et al. 2005). In particular, in the last two decades a special attention have been put to

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heavy metals in snow and glacial ice in Antarctica; they come from the deposition of particulate and

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soluble aerosols, which in turn are sourced from mineral dust, sea spray and anthropogenic

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pollution, (Scarponi et al., 1997; Wolff et al., 1999; Planchon et al., 2002; Gabrielli et al., 2005; Hur

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et al., 2007; Gabrielli et al., 2010; Hong et al., 2013; McConnell et al., 2014; Koffman et al., 2014;

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Thuoy, 2015; Kim et al., 2015). Studies of the occurrence of various trace elements in successively

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dated snow and ice layers allowed to decipher the large-scale changes in the atmospheric cycles of

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these elements in the Southern Hemisphere (Wolff and Suttie, 1994; Wolff et al., 1999; Planchon et

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al., 2002; Vallelonga et al., 2002; Van de Velde et al., 2005; Gabrielli et al., 2005). These studies

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showed that large-scale pollution in remote areas of the Southern Hemisphere, started for Pb as

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early as the 1880s and for other metals (Cr, Cu, Ag, Bi and U) since the beginning of the 20th

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century (Hur et al., 2007).

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Additionally, the knowledge of the organic carbon pool in Antarctic snow and its implications for

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heavy metals transport and for global carbon dynamics also provides an opportunity to obtain

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information about the environment of the past (Grannas et al., 2004). Black carbon-like material

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and humic-like substances are found to be present in Antarctic snow; the former, apparently

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originating from biomass burning in South America and from soil humics, appears to be

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photochemically or microbially modified. Humic-like substances mainly sourced from oceanic

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emissions of primary and secondary aerosols (Calace et al., 2001; 2005a; Lyons et al., 2007;

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Legrand et al., 2013; Ellis et al., 2015; Wu et al., 2016). In particular, previous studies on humic-

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like substances present in Antarctic snow and in micro-layer sea waters (Calace et al., 2001; 2005a;

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Calace et al., 2007) indicated an apparent relationship between these matrices. The transfer of

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humic-like substances from sea to snow “via marine aerosol” seems to be a main mechanism

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occurring in remote area and it has been confirmed by the knowledge of chemical composition of

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aerosol (Jing-Ming et al., 2007). Unfortunately, very little information is available on the humic-

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like substances presence in aerosol. However, a few studies have laid focus on individual

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compounds like humic-like Substances (HULIS) extracted from atmospheric aerosol particles and

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isolated from fog and cloud water probably sourced from resemblance of terrestrial and aquatic

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humic and fulvic acids (Poschl, 2005; Graber and Rudich, 2006; Legrand et al., 2007; Koch et al.,

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2009; Nguyen et al., 2014; Ellis et al., 2015).

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Taking into account the roles played by humic substances in sorption, complexation and

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solubilisation of heavy metals and pollutant organic molecules in soil and aqueous environments, it

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is anticipated that HULIS may perform similar functions in atmospheric particles (Graber and

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Rudich, 2006). Both crustal and marine aerosols are interesting to heavy metal transport.

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Nevertheless it is important to consider the role of bacteria, utilizing a diverse range of organic

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carbon compounds (Antony et.al, 2012), that are presumibly influenced in the past and certainly

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influence the snow chemistry and, consequently, the mobility of heavy metals in the extreme

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environments. So the characterization of the organic matter in Antarctic snow can give a valid 3

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support to better understand the crustal and marine aerosol to the snow chemistry and could be

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increasingly considered in paleoclimate studies.

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Arimoto et al. (1987) have already reported that a significant contribution to the total trace metal

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content of remote oceanic rainwater is due to sea-salt aerosol. In fact, sea-surface microlayer from

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which the aerosol derives is enriched from one to three orders of magnitude in metals such as V,

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Mn, Cu, Zn, Pb (Weisel et al., 1984). On the basis of that above reported, we studied some heavy

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metals (Cu, Zn, Cd, Pb, As, U) probably associated to high molecular weight organic compounds

84

present in the Antarctic snow. Snow-pit samples were collected and analysed for high molecular

85

weight dissolved organic matter (HMW-DOM) and heavy metals bound to them. This study offers

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information on atmospheric circulation long-term changes and on deposition fluxes and sources of

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trace elements during the past, taking into account that humic-like substances are a main component

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of atmospheric organic carbon (Guilhermet et al., 2013; Fan et al., 2016).

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2. Experimental

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2.1 Sample collection.

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Snow samples were collected during the austral summer 2005-2006 in the Concordia Station (Dome

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C, 75.10 S, 123.31 E, 3220 m above sea level) along a trench of 4 m depth. The trench was located

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500 m away from the Concordia Italian–French base. The sampling area was upwind with respect to

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the dominant wind direction (S-SW) and every motorized activity was forbidden south and within

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500 m north of the sampling site. The top 5 cm of snow was removed with a pre-cleaned scoop and

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then samples were collected. Snow samples corresponded to a 5×10^5 cm3 volume (layer thick of

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about 50 cm). The average density of snow samples was of 0.50 g cm-3. The density was

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determined by weighing a known volume of snow (10 cm-3) on analytical balance with a precision

100

of 0.0001g. A disposable coverall with attached boots was worn during sampling. Pre-cleaned

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HDPE wide-mouthed vessels (50L capacity) were used for collecting about 30L of melting snow

102

analysed for the organic substance and HDPE wide-mouthed vessels (10L capacity) were utilised 4

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for collecting about 5L of melting snow analysed for the heavy metals.All equipment needed to

104

sampling was prepared in Italian laboratory and sent to Antarctica. Moreover, field blanks were also

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prepared in Italian laboratory and sent to Antarctica. In particular, three HDPE wide-mouthed

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vessels (50L capacity) filled with MilliQ water (30L) were taken for field blanks of high molecular

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weight dissolved organic matter and three HDPE wide-mouthed vessels (10L capacity) filled with

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MilliQ water (5L) were taken for field blanks of metals.

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They were treated as a sample in all respects, including expedition from Italy, shipment to the

110

sampling site, exposure to sampling site conditions, storage, preservation, and all analytical

111

procedures. The purpose of the field blanks is to determine if low-level trace-element contamination

112

was present in the field environment. All samples and field blanks were frozen at –30°C

113

immediately after sampling and analysed three months later.

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2.2 Ultra-clean procedures

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Ultra-clean procedures were adopted in sampling activities and in laboratory analysis in order to

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reduce the sample contamination to a minimum.

118

All equipments coming into contact with samples were cleaned following a three-step procedure: 1)

119

rinsing with milliQ water; 5% (v/v) ultrapure-grade hydrochloric acid solution for 24 hours; rinsing

120

with milliQ water.

121

Filtration systems were cleaned by sequentially passing 2L of 1% (v/v) ultrapure-grade

122

hydrochloric acid solution at 2 L/min and then rinsing with 10L of milliQ water. Ultra-filtration

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systems were cleaned by processing 5 L of 1% (v/v) ultrapure-grade hydrochloric acid solution

124

without membrane and then 20 L of milliQ water. The cleaning protocols were optimised in order

125

to obtain out-flowing solutions not significantly different from that of the inflowing reagents. All

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cleaning procedures and sample treatment were carried out in a pre-cleaned room dedicated to the

127

processing Antarctic samples.

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2.3 Methods.

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2.3.1 High molecular weight organic substance analysis

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In order to determine the amount of high molecular weight organic substance in snow samples, the

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diafiltration/ultrafiltration technique was employed. In this way the high molecular weight organic

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substances were concentrated and purified (Calace et al., 2007).

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The melting snow samples (25-30 L) were filtered through 0.45 m Millipore polycarbonate

135

membrane filters previously combusted for 4 h at 450 °C in glass vials.. A portion (400 ml) of the

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filteredsample was put in the diafiltration cell that was connected to the nitrogen pressurised (4 atm)

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reservoir (5 L) containing a filtered sample volume up to the its maximum capacity (5 L); the

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sample was then re-added to the reservoir up to end of the volume to be processed. In this way the

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pressurised solution cross through the cell, solutes greater than the molecular weight cut off of the

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membrane (1 KDa) are retained in the cell and solutes smaller than 1 KDa pass into the filtrate.

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When the sample is transferred totally from the reservoir to the cell and reduced to 100 ml, the

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concentration step is finished (concentration factor was about 300) and the purification process

143

begin. The reservoir is filled with

144

remaining salts from the retentate. The washing operation is repeated more time (sample volume/

145

MilliQ water volume ratio 1:4 was experimentally determined). Finally, the reservoir is

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disconnected and under nitrogen pressure the solution (100 ml) is recovered. 10 ml of solution were

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analysed for organic carbon content (HMW-DOC) by means of a Shimadzu TOC5000 Analyzer.

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The results referred to original snow sample were calculated by taking into account that ultra-

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filtrated solution was concentrated until 300 times. The remaining solution was frozen and

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lyophilised to analyse of nitrogen and hydrogen elemental content. Yields in high molecular weight

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dissolved organic matter (HMW-DOM) were calculated by the weighing of lyophilised matter.

MilliQ water that crossing through the cell, removes the

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2.3.2 Metals bound to high molecular weight organic substances analysis.

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In order to determine the amount of metals bound to high molecular weight organic substances the

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diafiltration/ultrafiltration technique was also employed.

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The melting snow samples (5 L) were filtered through 0.45 m cellulose membrane filters were

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used for filtration and after immediately processed. A portion (40 ml) of the sample (5L) of melting

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snow was put in the diafiltration cell that was connected to the nitrogen pressurised (4 atm)

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reservoir (5 L) containing the remaining sample volume. When the sample is reduced to 40 mL, the

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concentration step is finished and the purification process begin. The reservoir is employed with

163

MilliQ water that crossing through the cell, removes the remaining salts from the retentate. The

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washing operation is repeated more time (sample volume/MilliQ water volume ratio 1:4). When the

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washing operation was finished, the reservoir is disconnected and under nitrogen pressure the

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solution is concentrated to 10 mL (concentration factor was about 500) and then analysed for metal

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content. The results referred to original snow sample were calculated by taking into account that

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ultra-filtrated solution was concentrated until 500 times.

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2.3.3 Field blanks analysis

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Three field blanks (30 L volume) were treated as snow samples according to high molecular weight

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organic

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diafiltered/ultrafiltered by applying the same procedure applied for snow sample. 100 mL of final

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solution were analysed for dissolved organic carbon by means of a Shimadzu TOC5000 Analyzer.

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HMW-DOC in ultrafiltrate retentates ranged from 8 to 25 M. 3 field blank measurements,

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calculated by taking into account the concentration factor (300), showed a mean of 0.065 ± 0.032

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(±1) M.

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Three field blanks (5 L volume) were treated as snow sample according to the metals analysis

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procedure. 5 L of blanks were filtered and then diafiltered/ultrafiltered by applying the same

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procedure applied for metals bound to high molecular weight organic substances analysis. 3 field

substance

analysis

procedure.

Briefly,

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field

blanks

were

filtered

and

then

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blank measurements, calculated by taking into account the concentration factor (500) and the

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average density of snow (0.5 g cm-3), showed a mean of 0.489 ± 0.006 (±1) pg g-1 for Cu, 0.522 ±

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0.002 (±1) pg g-1 for Pb, 0.85 ± 0.02 (±1) pg g-1 for Zn, 0.0341 ± 0.0002 (±1) pg g-1 for Cd,

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0.018 ± 0.001 (±1) pg g-1 for U, 0.069 ± 0.002 (±1) pg g-1 for As.

185 186

2.4 Instrumentation and operational condition

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MilliQ water (conductivity at 25 °C is 0.054 μS·cm−1, TOC level < 3 g l-1) was obtained from

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Milli-Q Millipore Gadient A-10 system.

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For organic sample filtration a Millipore stainless steel 142 mm filter holder was used with

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Millipore polycarbonate filters (diameter 142 mm and pore size 0.45 μm) and connected to

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peristaltic pump Masterflex model XX80EL230 (50-650 rpm) using silicon tubing (DI ¼”).

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For metal sample filtration a polysulfone NALGENE 300-4100 Series filter holder with receiver-

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Polysulfone was used with Millipore cellulose acetate filters (diameter 47 mm and pore size 0.45

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μm) and connected to a portable vacuum pump.

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An Amicon stirred ultrafiltration cell, model 8400, capacity 400 mL, stirred minimum value 10 ml,

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equipped at the bottom with a regenerated cellulose membranes (diameter 7.6 cm and active surface

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41.8 cm2, molecular weight cutoff 1-KDa) and connected to a reservoir (5 l) was used for organic

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sample ultrafiltration.

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An Amicon stirred ultrafiltration cell, model 8050, capacity 50 mL, stirred minimum value 2.5 ml,

200

equipped at the bottom with a regenerated cellulose membranes (diameter 4.3 cm, active surface

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13.4 cm2, molecular weight cutoff 1-KDa) and connected to a reservoir (5 l) was used for metal

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sample ultrafiltration.

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The cells were connected to a reservoir (5 L) containing sample or MilliQ water which is nitrogen

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pressurised (4 atm). Varying the operational conditions, the cell can be used under nitrogen pressure

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(3.5 atm) without interfacing with the reservoir.

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All snow samples and three field blanks were analysed for dissolved organic carbon (HMW-DOC).

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0.1 ml of 1N HCl (37% ultrapure for trace analysis, Carlo Erba in MilliQ water) were added to 10

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ml of organic sample obtained from the diafiltration/ultrafiltration system and analysed with

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Shimadzu TOC5000 Analyzer.

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The nitrogen and hydrogen, elemental content of high molecular weight dissolved organic matter

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(HMW-DOM) was determined with a Carlo Erba model EA11110 CHNS-O Element Analyser.

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Metal determination was made using a Perkin-Elmer ELAN 6100 ICP-MS (USA), equipped with a

213

cross-flow nebulizer. Single element standard solutions for ICP-MS were used for the preparation

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of calibrating solutions, acidified as the samples, with a concentration of: 0.01, 0.1, 0.5, 1, 5, 10 µg

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l-1 for As, Cd and U; 0.05, 0.5, 1, 5, 10 µg l-1 for Cu and Pb; 0.1, 1, 5, 10 µg l-1 for Zn.

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All snow samples, three field blanks and three replicates of certified reference material (NIST 1640,

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trace elements in natural water) were analyzed for Cu, Pb, Zn, Cd, As and U. 0.1 ml of 69 %

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HNO3 (Hiperpur-Plus, Panreac) were added to the 10 ml of each sample obtained from the

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diafiltration/ultrafiltration and analysed in the same day with an ICP-MS.

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2.5 Methods performance features

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KHP-Potassium hydrogen phthalate (Shimadzu) was used as measurement standard in order to

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determine the instrument intermediate precision of the organic carbon determination. Instrument

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intermediate precision calculated as relative standard deviation (RSD%) of 10 M/OC solution was

225

found to 3%. The method intermediate precision calculated by applying the same analytical

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procedure on snow sample sampled for purpose (Italian sample, estimated concentration of 0.60 ±

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0,18 M C) was found to 30% (RSD %).

228

The limit of quantification (LOQ) of Antarctic snow HMW-DOC was calculated by three field

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blanks measurements and was found to be 0.4 M (xb ± 10 b  Magnusson and Ornemark, 2014).

230

The method intermediate precision (RSD%) for metal bound to high molecular weight organic

231

substances calculated by applying the same analytical procedure on snow sample sampled for 9

ACCEPTED MANUSCRIPT 

Cu, 120 pg g Zn, 1.0 pg g U,

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purpose (Italian sample, estimated concentrations were 30 pg g

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0.90 pg g As, 320 pg g Pb, 16 pg g Cd) was found to 20%.

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The certified reference material (NIST 1640) was used to assess the trueness of the measurement

235

process (bias). It is checked by comparing the measured average value (𝑥) with the certified value

236

(, Table 1). The 𝑥 - µ < 2𝜎𝑚 condition is used as the criterion for acceptance where m was

237

standard deviation of three replicates of the measurement (ISO GUIDE 33:2000).

238

The LOQ values of the heavy metals (xb ± 10 b were calculated by three field blanks

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measurements and were 0.5 pg g-1 for Cu and Pb, 0.03 pg g-1 for Cd and U 0.09 pg g-1 for As and

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1,0 pg g-1 for Zn.

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Phenomena of contamination were taken into account by field blank measurements and all snow

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samples results were found to be significantly different from field blanks.

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3. Results and Discussion

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High molecular weight fraction of dissolved organic matter (HMW-DOM ≥ 1 kDa) is generally

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constituted from humic-like substances and from biopolymers such as polysaccharides, lipids,

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proteins, lignins, tannins, etc. (Antony et al., 2014).

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The HMW-DOC fraction is isolated from waters by ultrafiltration and typically accounts for 25–

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40% of the dissolved organic carbon (DOC) in marine waters (Benner, 1992; Repeta et al., 2002)

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and for 32 – 87% in freshwaters (Repeta et al. 2002).

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In literature (Table 2) the median OC value found is about 30 M for Antarctic snow samples, and

252

in particular snow samples in Dome C registered a median DOC value of about 6 M (Legrand et

253

al., 2013). In this paper we found a HMW-DOC fractions that range from 1.1 to 4.9 M (Table 3).

254

We also processed three field blanks in order to determine all possible contamination during all

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phases of study. The blank results showed a low-trace level of contamination (0.065 ± 0.032 M),

256

that was found to be negligible respect to HMW-DOC range. Our results highlighted the high

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molecular weight organic fractions were not interested to contamination phenomena that probably

258

affected mainly the low molecular weight organic fractions. Taking into account the median DOC

259

value (6 M) calculated on Dome C snow data reported in Table 2 (Legrand et al., 2013) we

260

determined that HMW-DOC accounted for 18 – 82 % of the total organic carbon in Antarctic snow

261

in Dome C. In previous study (Calace et al. 2005b) we found a HMW-DOC fractions ranged from 2

262

to 4 M of C, in agreement with data found in this work and reported by literature (Table 2). These

263

results highlighted high molecular weight dissolved organic fraction is the most abundant organic

264

matter fraction transported in the atmosphere.

265

Previous studies carried out on structural features of HMW-DOM fraction (in particular on XAD

266

extracted humic-like substances) present in Antarctic seawater showed a high content of carboxy

267

and/or hydroxy groups such as salicylic acid and phthalic acid attached to a predominant aliphatic

268

structure even with a marked π-conjugation (Calace et al., 2007; Calace et al., 2011). The

269

characterisation of HMW-DOM fraction (high molecular weight organic matter) carried out with

270

elemental analysis (Table 3) evidenced an ageing of organic compounds going from surface layers

271

to the deepest ones. In the first layers (until 2.0 m depth), HMW-DOM fraction is characterised by a

272

high H/C (>1.7), by a N/C ratio > 0.05 and a O/C ratio > 1.0. These ratios reflects a mixed

273

contribution of aliphatic compounds, proteins and carbohydrates possibly derived from algal

274

detritus and/or microbial biomass (lipids are characterised by H/C = 1.7−2.2 and O/C = 0.0−0.2;

275

proteins/amino sugars by H/C = 1.5−2.2, O/C = 0.2−0.6, N/C ≥ 0.05 and carbohydrates by O/C =

276

0.6−1.2, H/C = 1.5−2.2). After 2 m depth, HMW-DOM fraction is characterised by a H/C ratio <

277

1.7, by a N/C ratio < 0.05 and by a O/C ratio < 1.0. The first one denotes an enrichment of

278

unsaturated hydrocarbons characterised by a H/C = 0.7−1.5 while the second and third ones show a

279

loss of nitrogen-containing groups and oxygenated groups respectively. Post-depositional

280

processing results in significant chemical transformation of HMW-DOM fraction within the snow

281

pit but not correspond to markedly depletion of organic carbon content (Table 3).

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The binding or complexing of metal ions by polyfunctional polymers (polysaccharides, polyacids,

283

proteins etc.) and humic-like substances is widely known (Kinniburgh et al., 1996; Zhou et al.,

284

2005; Zhou et al., 2015). In particular, humic-like substances are the most important components

285

that, due to their high structural complexity, contribute to the overall fate of trace metal cations in

286

environment (Senesi et al., 2009). The large number of hydroxyl and carboxylic ionisable sites in

287

the humic-like substances, indeed, provide the appreciable ability to form stable complexes and

288

chelates with Cu(II), Zn(II) and Cd(II), Pb(II) (Garcia-Mina, 2006).

289

For this reason, we also analysed heavy metals associated to HMW-DOM fraction. We have

290

analysed snow ultra-filtrated samples for Cu, Zn, Cd, Pb, As and U content (Table 3). The analysis

291

of field blanks carried out to determine the possible contamination during the study showed a metal

292

concentrations of 0.489 ± 0.006 pg g-1 for Cu, 0.522 ± 0.002 pg g-1 for Pb, 0.85 ± 0.02 pg g-1 for Zn,

293

0.0341 ± 0.0002 pg g-1 for Cd, 0.018 ± 0.001 pg g-1 for U, 0.069 ± 0.002 pg g-1 for As. Cu, Pb, Zn

294

and U field blanks were found to be negligible respect to snow results. As and Cd showed a field

295

blanks level comparable to samples level in some layers of snow pit (Table 3) even if higher than

296

the limits of quantification (0.09 pg g-1 for As and 0.03 pg g-1 for Cd).

297

Heavy metals, present in solution as free ions, hydroxy complexes, and metal-polymer complex

298

characterized by molecular sizes less than the pore sizes of 1 KDa ultrafiltration membranes, freely

299

pass through the membranes; conversely, metal-organic substances complex characterized by

300

molecular sizes major than the pore sizes of ultrafiltration membranes are rejected and remain as

301

bounded species in the retentate (Alpatova et al. 2004). A slight enhancement of metal retention by

302

ultrafiltration membrane could be eventually due to precipitation on the membrane surface of

303

soluble hydroxo/aqua/complexes and hydroxides of heavy metals that can be formed in particular

304

conditions such as high pH value, high concentration of metals etc. (Barakat and Schmidt, 2010) but

305

chemical Antarctic snow features make unlikely these phenomena.

306

Moreover, elevated molar ratios between high molecular weight fraction and metals (HMW-

307

DOM/Me > 1000) make robust the hypothesis that all metal determined in retentate by 12

ACCEPTED MANUSCRIPT 308

ultrafiltration are bound to organic substance (Staub et al., 1984; Volchek et al., 1993; Pandey et al.,

309

2000; Barakat and Schmidt, 2010; Baek and Yang, 2011).

310

The element amount expressed as part per trillion follows the order: Zn > Cu > Pb >> Cd ~ As ~ U.

311

In previous studies several researchers (Table 4) found the same order for total metal amounts in

312

Antarctic snow and ice.

313

Grotti et al. (2008, 2011) analysed the total dissolved metal amount in surface snow samples

314

collected in the same area investigated by us (Dome C) during the Italian Antarctic expeditions

315

from 2001 to 2006. They found that the average total dissolved concentration was 27 (± 18) pg g-1

316

for copper, 15 (± 9) pg g-1 for lead, 3 (± 2) pg g-1 for cadmium and 1294 (± 1060) pg g-1 for zinc.

317

Results obtained from Grotti et al. (2008, 2011) can be directly related to organic bound metal

318

concentration found by us in the surface layer of snow pit. Indeed, taking into account the

319

accumulation rate at Dome C has been estimated in about 7.0 cm yr-1 (Rothlisberger et al., 2002;

320

Wolff et al., 2002; Grotti et al., 2015), the snow pit surface layer investigated by us should

321

correspond to the same period (2006-1999) analysed by Grotti et al. (2000-2004). The percentage

322

fraction of metals bound to HMW-DOM respect to total metal content follows the order: Cu (100%)

323

> Pb (34%) > Zn, Cd (about 3-5%). This estimation seems to confirm the affinity order of humic-

324

like substances for metals (Irwing-William series, Pandey et al., 2000).

325

The pattern of organic bound metal concentration is practically the same going down to depth of

326

snow-pit (Figure 1a-f). All metal trends show a high concentration peak corresponding to 2.0-2.5 m

327

layer (Figure 1a-e) except the arsenic (Figure 1f). The metal peak is not justified by the highest

328

organic carbon content but further by the structural HMW-DOM characteristics. The differences in

329

the chemical composition of HMW-DOM can have a strong influence on the capability to interact

330

with metals. For instances, several models (Klucakova, 2012) were proposed to delucidate the

331

metal-humic interactions. The differences observed for the humic complexation models based on

332

the use of individual compound showed that there are active sites not only with various strength and

333

stability of formed complexes but also with their various rigidity and ability of conformational 13

ACCEPTED MANUSCRIPT 334

changes (Klucakova, 2012). At pH levels of Antarctic snow (4.0 - 6.3) (Cragin et al. 1987; De

335

Felice, 1998; Karkas et al., 2005; Ali et al., 2010; Budhavant et al., 2014) HMW-DOM functional

336

groups the main responsible for the metal binding (mainly carboxyl and hydroxyl groups), are

337

involved in dissociation equilibria and structural rearrangements able to form different type of

338

complexes with metals (McElmurry et al., 2010; Klucakova, 2012). The capability of HMW-DOM

339

to form different type of metal complexes makes hard to predict the exact stechiometry of

340

complexes. For this reasons the analysis of DOM binding capacity must to be site-specifically

341

studied.

342

We also performed the Spearman rank correlation analysis among organic bound metal

343

concentration and high molecular weight organic substance expressed as g g-1 (Table 6). The

344

Spearman rank correlation coefficients range between -1 and +1 and measure the strength of the

345

association between the variables. In contrast to the more Pearson product moment correlations, that

346

measure the strength of the linear relationship between the variables, the Spearman coefficients are

347

computed from the ranks of the data values rather than from the values themselves. Consequently,

348

they are less sensitive to outliers than the Pearson coefficients. Our results highlighted statistically

349

significant correlations (p < 0,5 at the 95,0% confidence level) between HMW-DOM and lead,

350

copper, cadmium, zinc and uranium. Correlation analysis pointed also to a strong association

351

between all metals except arsenic.

352

The high values of Spearman correlation coefficients found for HMW-DOM and Zn (0.96; p < 0,01

353

at the 99,9% confidence level) can be explained by a their common source such as marine

354

phytoplankton residual. The high values of Spearman correlation coefficients found for U (mining

355

in South Africa, Namibia and Australia is considered the main anthropogenic source of uranium in

356

Antarctica, Potocky et al., 2016) and Cd (0.99; p < 0,01 at the 99,9% confidence level) can be also

357

explained by common source maybe due to the long-range distance transport (Potocky et al., 2016).

358

14

ACCEPTED MANUSCRIPT 359

4. Conclusions

360

High molecular weight dissolved organic matter (HMW-DOM) recovered by ultrafiltration has

361

ranged from 2 to 4 M of C (HMW-DOC) and if comparing with literature data it seems to

362

represent 18 – 82 % of the total organic carbon in Antarctic snow. The characterisation of HMW-

363

DOM fraction evidenced an ageing of organic compounds going from surface layers to the deepest

364

ones with a shift from aliphatic compounds and proteins to more high unsaturated character and less

365

nitrogen and oxygen content.

366

The heavy metals associated to HMW-OC fraction follows the order: Zn > Cu > Pb >> Cd ~ As ~ U

367

and this results seems to be in agreement with several studies that found the same order for total

368

metal amounts in Antarctic snow and ice. On the other hand, the percentage fraction of metals

369

bound to HMW-DOC respect to total metal content showed an inversion between bounded Cu and

370

Zn. This finding is in agreement with the humic substance binding ability pointing to the main

371

binding agent in HMW-DOM is probably humic substance. Going down to depth of trench, all

372

metals except arsenic, showed a high concentration peak corresponding to 2.0-2.5 m layer. This

373

result was attributed to particular structural characteristic of organic matter able to form different

374

type of complexes with metals. The large contribution of humic component of HMW DOM to

375

formation of DOM-metal complexes in Antarctic snow and the role played by structural features of

376

DOM in DOM-metal interactions can have several implications for contaminant transport and can

377

be a useful tool in paleoclimate interpretations. It will be interesting in the future to extend this

378

study to organic fraction present in the aerosols.

379 380

Acknowledgements

381

The authors thank Carlo Abete for sampling and treatment of snow samples. This work was

382

supported by the National Programme for Antarctic Research, project 2013/AZ2.1.

383

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a

b

c

d d

e

f

Figure 1 Pattern of metal concentration (bars) and high molecular weight dissolved organic carbon (line chart) going down to the trench depth. a) copper; b) zinc; c) cadmium; d) lead; e) uranium; f) arsenic.

ACCEPTED MANUSCRIPT Table 1. Measured and certified values for certified reference material NIST 1640 NIST 1640 As (g/L) Cd (g/L) Cu (g/L)* Pb (g/L) Measured value 26.49 ± 0.33 22.51 ± 0.15 86.2 ± 0.7 28.05 ± 0.15 Certificate value 26.67 ± 0.41 22.79 ± 0.96 85.2 ± 1.2 27.89 ± 0.14 𝑥‒µ 0.18 0.28 1.0 0.20 2𝜎𝑚 0.66 0.30 1.4 0.30 * reference value ** not certified 𝑥 ‒ µ < 2𝜎𝑚 is the condition used as the criterion for acceptance (ISO GUIDE 33:2000)

U (g/L) 0.88 ± 0.01 **

Zn (g/L) * 54.3 ± 0.6 53.2 ± 1.1 1.1 1.2

ACCEPTED MANUSCRIPT Table 2. Organic carbon (DOC) found in snow and ice from various Antarctic sites. Sites

Type matrix

OC (M)

References

South Pole, coastal East Antarctica

Surface snow

1.0 - 77

Dome C

Surface snow

0.8 – 12.5

Grannas et al. (2004) Nemirovskaya (2006) Antony et al. (2011) Antony et al. (2014) Legrand et al. (2013)

Victoria Land, Antarctic coast

Snow pit

<8

Lyons et al. (2007)

South Pole, Talos Dome

Ice

0.0 - 30

Federer et al. (2008) Preunkert et al. (2011) Legrand et al. (2013)

Dome C

Ice

0.8 - 2

Legrand et al. (2013)

ACCEPTED MANUSCRIPT Table 3. HMW-DOC (M) in snow and their H/C, N/C and O/C molar ratios.

(m)

HMW-DOC (M)

H/C

N/C

O/C

0-0.5

1.1

1.8

0.063

1.1

0.5-1.0

4.7

1.9

0.051

1.1

1.0-1.5

3.5

1.7

0.054

1.1

1.5-2.0

4.9

1.7

0.066

1.0

2.0-2.5

1.1

1.6

0.041

0.95

2.5-3.0

3.9

1.5

0.053

0.89

3.0-3.5

3.5

1.4

0.044

0.87

3.5-4.0

1.1

1.3

0.036

0.73

Layer

ACCEPTED MANUSCRIPT

Table 4. Concentrations (pg/g) of metals bound to high molecular weight organic compounds in snow. Layer As Cd Cu Pb U Zn 0-0.5

0,12

0,090

34

5,4

0,15

68

0.5-1.0

0,32

0,090

16

4,8

0,14

170

*1.5-2.0

0,090

0,060

17

1,5

0,10

120

2.0-2.5

0,87

1,3

170

20

0,46

1000

2.5-3.0

1,8

0,030

9,9

3,7

**

32

3.0-3.5

0,22

0,25

60

6,4

0,29

280

3.5-4.0

0,11

0,24

17

0,50

0,22

130

* metal values of the layer 1.0-1.5 are not reported because of sample loss during the analysis ** < LOQ

ACCEPTED MANUSCRIPT

Table 5. Metal concentrations (pgg-1) in surface snowa and iceb. Analyte Surface snow Ancient ice References As 1-34 Gabrielli et al., 2005b Cd 0.08 - 8 0.10-1.34 Grotti et al. 2011, 2015a, Boutron et al.,1993b; Suttie and Wolff, 1992a; Gorlach and Boutron, 1992a; Wolff et al., 1999a; Planchon et al., 2002a Cu 0.48 - 275 Grotti et al. 2008, 2011, 2015 a; Suttie and Wolff, 1992a; Gorlach and Boutron, 1992a; Wolff et al., 1999a; Planchon et al., 2002a Pb 4 - 86 Grotti et al. 2008, 2011, 2015 a; Suttie and Wolff, 1992a; Gorlach and Boutron, 1992a; Planchon et al., 2002a Zn 0.4 - 2077 Grotti et al. 2008, 2011, 2015 a; Suttie and Wolff, 1992a; Gorlach and Boutron, 1992a; Wolff et al., 1999a U 0.015 0.05-4 Planchon et al., 2002a, Gabrielli et al., 2005b

ACCEPTED MANUSCRIPT

Table 6 Spearman rank correlation coefficients (in bold are reported coefficients with P < 0,05 that indicate statistically significant non-zero correlations at the 95,0% confidence level). HMW DOM Cu Pb Zn Cd As U HMW DOM 1,00 0,43 0,47 0,96 0,75 0,22 0,54 Cu 1,00 0,71 0,64 0,77 -0,14 0,71 Pb 1,00 0,57 0,59 0,50 0,66 Zn 1,00 0,88 0,11 0,71 Cd 1,00 0,04 0,99 As 1,00 0,60 U 1,00