Influence of humic substances on the transport of indium and gallium in porous media

Journal Pre-proof Influence of humic substances on the transport of indium and gallium in porous media Yasmine Kouhail, Nitai Amiel, Ishai Dror, Brian Berkowitz PII:

S0045-6535(20)30292-7

DOI:

https://doi.org/10.1016/j.chemosphere.2020.126099

Reference:

CHEM 126099

To appear in:

ECSN

Received Date: 12 November 2019 Revised Date:

28 January 2020

Accepted Date: 1 February 2020

Please cite this article as: Kouhail, Y., Amiel, N., Dror, I., Berkowitz, B., Influence of humic substances on the transport of indium and gallium in porous media, Chemosphere (2020), doi: https:// doi.org/10.1016/j.chemosphere.2020.126099. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier Ltd.

Credit author statement Yasmine Kouhail: Conceptualization, Methodology, Validation, Investigation, Writing and Editing Nitai Amiel: Software, Writing and Editing Ishai Dror: Conceptualization, Methodology, Validation, Writing and Editing, Supervision Brian Berkowitz: Conceptualization, Methodology, Validation, Writing and Editing, Supervision

Column

Fraction collector Pump

Inlet solution In or Ga HS

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Influence of humic substances on the transport of indium and

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gallium in porous media

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Yasmine Kouhail*, Nitai Amiel, Ishai Dror and Brian Berkowitz

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Department of Earth and Planetary Sciences

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Weizmann Institute of Science

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Rehovot 7610001 Israel

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* Corresponding author: [email protected]

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Keywords: column experiments, natural organic matter, indium, gallium, mobility

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For submission to: Chemosphere

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Abstract

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Indium and gallium are used widely in modern industry, mostly for the production of semiconductors. They are considered as Technology-Critical Elements and have therefore received growing attention in the past few years. We investigated the influence of different types of humic substances on the transport of indium and gallium in laboratory-scale, saturated column experiments, to gain understanding of their mobility in natural environments. We evaluated the effect of different humic substances on the transport of indium and gallium in quartz sand: a commercial humic acid (Aldrich Humic Acid, AHA), a fulvic acid (Suwannee River Fulvic Acid, SRFA) and an aquatic natural organic matter (Suwannee River Natural Organic Matter, SRNOM). The impact of the flow rate and the influence of different concentrations of organic matter were also investigated. Indium was shown to be more mobile than gallium in the presence of humic substances. The mobility of indium in sand was highest for SRNOM, followed by SRFA and then AHA, while for gallium the order was SRFA > SRNOM > AHA. These results can be significant in understanding the mobility of indium and gallium in soils with various compositions of organic matter.

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1

Introduction

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Indium and gallium have been used widely in industry over the past twenty years and the

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demand for these elements is increasing. Indium is used mainly in semiconductors as indium tin

37

oxide and in computer and TV screens. Gallium is also used for semiconductor devices, solar

38

panels, LED lamps and in cancer diagnosis (Afshar-Oromieh et al., 2013) and treatment (Collery

39

et al., 2002). These trace metal elements have low abundances in the Earth’s continental crust

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(0.056 mg/kg for indium and 17.5 mg/kg for gallium (Rudnick and Gao, 2003)) and are not

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mined directly. Indium is a byproduct of zinc and copper mining, and gallium is recovered mostly

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from bauxite ores. The European Commission Enterprise and Industry (2010) defined indium and

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gallium as Technology-Critical Elements (TCEs). Their poor recycling from technological

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products can lead to environmental pollution. Moreover, the toxicity of indium and gallium from

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the semiconductor industry on fish and mammals is still a matter of debate (Tanaka, 2004;

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Olivares et al., 2016; White and Shine, 2016; Nakano et al., 2019). Indium and gallium were

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shown to be present in agricultural soils in Europe (Reimann et al., 2018) likely due to natural

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sources. However, anthropogenic sources of indium were also found in Japanese rivers in the

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Tokyo bay (Nozaki et al., 2000) and in the vicinity of an electronic waste site in Ghana

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(Tokumaru et al., 2017). Gallium was also found in groundwaters near a semiconductor

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production site in Taiwan (Chen, 2006).

52

Indium and gallium in solution are present at their +III oxidation state (In(III) and Ga(III)

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and tend to precipitate as In/Ga(OH)3 due to their low solubility (Wood and Samson, 2006).

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However in the presence of organic and inorganic ligands, indium and gallium can be present as

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complexed forms. Speciation studies of indium and gallium in solution are very scarce and

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limited to complexation with hard inorganic ligands (Wood and Samson, 2006), a few simple 3

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organic molecules such as citrate, oxalate and malonate (Clausén et al., 2003; Vasca et al., 2003;

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Clausén et al., 2005; Ivanova et al., 2015; Rotureau et al., 2018) and mineral surfaces such as γ-

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Al2O3 and clays (Lin et al., 1997; Benedicto et al., 2014). Natural organic matter (NOM) is

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known to be able to bind trivalent metals such as aluminum (Ryan et al., 1996; Elkins and

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Nelson, 2001; Weng et al., 2002b) and therefore is expected to impact the behavior of indium and

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gallium in the environment. However, studies investigating In(III)-NOM and Ga(III)-NOM

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interactions are to date very scarce and essentially in their infancy (Lippold et al., 2005; Hagvall

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et al., 2014; Rotureau et al., 2018).

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In their dissolved colloidal form, humic substances can enhance the transport of metals,

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while these same substances can retard metal transport when adsorbed on soil mineral. To date,

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only one study has focused on the transport of gallium in sand (Ringering et al., 2019) and four

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studies have examined indium transport in sand and soil (Hou et al., 2005; Wen et al., 2013;

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Murata et al., 2018; Ringering et al., 2019). Hou et al. (2005) investigated the transport of In-

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EDTA and In-citrate in columns filled with different soils. Indium was shown to be very mobile

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and probably dependent on the presence of organic matter colloids. Murata et al. (2018) and

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Ringering et al. (2019) investigated, respectively, the transport of In-citrate in soil lysimeters for

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8 years with application of deionized water (to account for precipitation) and the transport of In-

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citrate in sand column experiments. In both cases, indium was shown to be mobile and influenced

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by humic acid. However, the effects of humic substances and citrate were not studied separately

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and competition is expected between these two organic ligands on indium fate in natural

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environments. In these studies, the influence of citrate and EDTA compared to the effect of

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organic matter colloids was not discussed, and it is not clear if the mobility of indium is due only

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to its interactions with small organic molecules or due also to its interactions with natural organic

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matter. Gallium exhibits a different behavior than indium (Ringering et al., 2019) even though

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they can be expected to have a similar chemistry due to their position – one above the other – in

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the periodic table. Gallium-citrate was shown to be more mobile than indium-citrate in the

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absence of humic acid and less mobile in the presence of humic acid.

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The current study focuses on investigating the influence of natural organic matter on the

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transport of indium and gallium in sand, without citrate ligands that compete for indium and

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gallium complexation in solution. This will aid understanding of whether or not the mobility of

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indium and gallium depends exclusively on its interactions with organic matter, or if different

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organic matter compositions also affect the mobility of these metals. The effect of different

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humic substances is investigated: a commercial humic acid (Aldrich Humic Acid, AHA), a fulvic

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acid (Suwannee River Fulvic Acid, SRFA) and an aquatic natural organic matter (Suwannee

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River Natural Organic Matter, SRNOM) from the International Humic Substances Society. The

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impact of flow rate and the influence of different concentrations of organic matter were

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investigated.

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Materials and methods

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2.1 Materials

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Indium(III) chloride (InCl3 98%), gallium(III) nitrate hydrate anhydrous basis (Ga(NO3)3

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H2O 99.9%), sodium bromide (NaBr ≥99.5%), hydrochloric acid (HCl ≥37%), nitric acid (HNO3

99

70%), and Aldrich humic acid (AHA) were purchased from Sigma Aldrich. Suwannee River

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fulvic acid (SRFA) and Suwannee River organic matter (SRNOM) were purchased from the

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International Humic Substances Society (IHSS, Saint Paul, MN, USA). Quartz sand (30-40 mesh,

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0.4-0.6 mm) was obtained from Unimin Corporation. The specific surface area of the quartz sand

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is 0.049 m2 g-1 (Goykhman et al., 2019). An artificial rainwater (ARW) with an ionic strength of

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6.5 mM was used as a background solution in the transport experiments. The ARW solution is

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composed of sodium nitrate (NaNO3 >99.5%) purchased from Fluka at 48 mg L-1, potassium

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carbonate (K2CO3 99%) purchased from Merck at 57.9 mg L-1 and magnesium sulfate (MgSO4

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99%) purchased from Merck at 35 mg L-1; the pH was fixed at 6 using freshly prepared HCl and

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NaOH. All solutions were prepared in double deionized water (DDW, 18 MΩ cm-1). The pH of

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the solutions was measured using Eutech pH 450 pH meter from Thermo Scientific with a

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combined pH electrode from Sensorex. Buffers (pH 4 and 7) from Rocker were used for pH

111

calibration.

112 113

2.2 Experimental setup

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The transport of indium and gallium in presence of different extracts of organic matter

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(AHA, SRFA and SRNOM) was studied in vertical, water-saturated, laboratory column

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experiments. The experiments are hereafter referred as In/Ga-HA, In/Ga-FA, In/Ga-HA+FA

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experiments for the In/Ga experiments with Aldrich humic acid, Suwannee River fulvic acid and

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Suwannee River natural organic matter, respectively. Polycarbonate columns (19 cm length, 3 cm

119

inner diameter) were filled with quartz sand that was previously washed with HCl 1% for 24 h

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and several times with DDW before being dried at 105 °C. The columns were saturated with

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DDW adjusted at pH 6 for 6 h followed by saturation with the background solution (artificial

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rainwater at pH 6) for 17 h from bottom to top at a flow rate of 0.6 mL min-1. The porosity of the

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sand was 30%. 6

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The inlet solution, consisting of 10 L of bromide as a conservative tracer at 500 µg L-1,

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In(III) or Ga(III) at 1 mg L-1 with a humic substance at 10 mg L-1 or 2.5 mg L-1 in ARW at pH 6,

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was then injected into the column at a flow rate of 0.9 mL min-1 or 2.7 mL min-1 for

127

approximately 80 pore volumes (PV). The pore volume was determined as the mass difference

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between the saturated and the dry columns. The pH was monitored continuously at the column

129

outlets, and constant at 6.3 ± 0.3 for all of the experiments. The background solution was then

130

flushed again in the column for approximately 10 PV. The experiments were run simultaneously

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in duplicate using the same inlet solution reservoir.

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Fractions were collected every 9 min (for the experiments with a flow rate of 0.9 mL min-1)

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or every 3 min(for the experiments with a flow rate of 2.7 mL min-1) using an automatic fraction

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collector (Gilson). The samples were weighed and acidified with nitric acid to reach a final

135

concentration of nitric acid in the samples of 2%. The samples were then analyzed using an

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inductively-coupled plasma mass spectrometer (ICP-MS Agilent 7700 s) to measure bromide,

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indium and gallium concentrations. To monitor the stability of ionization efficiency with time,

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cobalt was used as an internal standard. From the ICP-MS analysis, breakthrough curves of

139

indium and gallium were determined. The breakthrough curves show the relative concentration of

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metal C/C0 as a function of PV. Retardation factors, Rf – representing the delay in arrival of the

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metal at the column outlet, as compared to the bromide tracer – were calculated for each

142

experiment as the ratio between the number of PV required for the metal to reach half of its final

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outlet concentration and the number of PV required for the tracer to reach half of its final outlet

144

concentration.

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2.3 Speciation calculations

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Indium and gallium speciation in the inlet solutions of the column experiments was

147

calculated using Visual Minteq (Gustafsson, 2011). The stability constants, K, for the hydrolysis

148

of the metals from Wood and Samson (2006) are given in Table 1.

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Table 1: Thermodynamic constants used in this work.

Species

log K Species

GaOH2+

-2.9

Ga(OH)2+ -7.3

InOH2+

log K -4.0

In(OH)2+ -7.8

Ga(OH)3 -11.9 In(OH)3 -12.4 Ga(OH)4− -15.7 In(OH)4− -22.1 Ga(OH)3s 4.99

In(OH)3s 5.09

150 151

There is no literature published to date regarding stability constants of indium with HA

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and gallium with HA. Complexation constants are usually calculated using experiments with

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relatively high concentrations of metals, and therefore determination of these constants for

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indium and gallium is difficult due to the low solubility of these metals. However, in the

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framework of the NICA Donnan model (Kinniburgh et al., 1999), Milne et al. (2003) calculated a

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relationship between the first hydrolysis formation constants of metal ions and the fitted NICA

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Donnan parameters for binding by humic acids:

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1 = 0.26 · log KOH + 2.59 with R2 = 0.83 n1 · log


(1)

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2 = 0.41 · log KOH + 4.98 with R2 = 0.71 n2 · log


(2)

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 i is with n1 = 0.14 - 0.055 · log KOH and n2 = 0.76 · n1, where ni is the non-ideality parameter,


161

the generic median affinity constant of the metal low-affinity type of sites (carboxylic sites S1) 8

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and high-affinity type of sites (phenolic sites S2), and KOH is first hydrolysis formation constant.

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Equations (1) and (2) were used to calculate the parameters for the binding of In(III) and Ga(III)

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by humic and fulvic substances in the framework of the NICA Donnan model (Table 2).

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Table 2: Non-ideality parameters and generic median affinity constants for gallium and indium with humic and fulvic acids calculated using eq (1) and (2).

Parameters/Metal Ga n1 = 0.30

In 0.36

For humic acid

n2 = 1 = log


0.23 6.13

0.27 4.31

16.65

12.21

For fulvic acid

2 = log
1 = log


4.72

2.86

2 = log


26.13

18.60

167 168

The speciation of indium and gallium in the inlet solutions of the column experiments is

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presented in Figure 1. The calculations show that indium and gallium exhibit very different

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speciations in the presence of humic acids, fulvic acids or a mixture of humic and fulvic acids at

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the same concentration. Therefore, the transport of indium and gallium is expected to be

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dependent on the type of humic substance. Indium is found mostly as In(OH)3 aq in the presence

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of AHA (90.1% for a humic acid concentration of 2.5 mg L-1) and 7.9% of In-HA complexes. In

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the In-FA experiments, the In-FA complexes represent 25% of the indium speciation. In presence

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of SRNOM, In is present as a mixture of 81.8% In(OH)3aq and 16.3% In-NOM (consisting of

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3.8% In-HA and 12.5% In-FA). For gallium, the main species are 95% Ga(OH)4- and 5% Ga-HA

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in the Ga-HA experiments, 88.7% Ga(OH)4- and 11.3% Ga-FA in the Ga-FA experiments, and

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91.8% Ga(OH)4- and 8.2% Ga-NOM in the Ga-HA+FA experiments. No precipitates of In(OH)3 s

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and Ga(OH)3 s were observed in the timeframe of the experiments. Indium and gallium exhibit

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different calculated speciations and therefore their transport behavior is expected to be different.

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Gallium is expected to be present mostly as negatively charged ions Ga(OH)4- and the sand

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surface is also slightly negatively charged at pH 6.3. Therefore electrostatic repulsion should lead

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to faster elution for Ga than for In, which is present mostly as In(OH)3aq.

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184 185 A 100%

7.9 2.0

90%

12.5 25.0

29.4

3.8 1.8

Indium speciation

80% 1.6

70%

1.5

60% 50% 90.1

40%

81.8

73.4

69.1

30% 20% 10% 0% In HA 10 mg In HA 2.5 mg In FA + FA In HA In HA In2.5 FA mg In HA In HA+FA L-1mg L-1 2.52.5 mgmg L-1L-1 10L-1 mg L-1 2.5L-1 mg L-1 2.5 In(OH)3aq

In(OH)3 (aq)

186 B 100% 90%

In(OH)2+

In(OH)2+

5.0 19.4

In HA

In FA

In HA

In FA S1

11.3

5.7 2.5

88.7

91.8

Gallium speciation

80% 70% 60% 50% 40%

95.0 80.6

30% 20% 10% 0% Ga HA Ga2.5 HAppm Ga Ga Ga HA 10 mg Ga HA FA FA 2.5 mg 10L-1 mg L-1 2.5 mg L-1 2.5 mg L-1 L-1

187 188 189 190 191

Ga(OH)4- (aq) -

Ga(OH)4

Ga HA

Ga HA

GaGa HAHA+FA + FA 2.52.5 mgmg L-1L-1

Ga FA

Ga FA S1

Figure 1: Speciation of indium (A) and gallium (B) in the inlet solutions for solution containing humic acid, or fulvic acid or natural organic matter. The calculations were done using Visual Minteq for solutions of indium/gallium at 1 mg L-1 with humic substances at 10 mg L-1 or 2.5 mg L-1, at pH 6.3 in the artificial rainwater at an ionic strength of 6.5 mM. For the HA + FA experiment, it was considered that

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192 193

50% of the natural organic matter is humic substances and 50% is fulvic substances. Species that are less than 0.5% are not represented on the graphs.

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3.1 Indium

Results and discussion

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The breakthrough curves (BTC) of indium, showing the relative concentration of indium as

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a function of PV, are presented in Figure 2 for various conditions. Only one replicate is shown for

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clarity (see Supplementary Materials for all replicates that are similar in terms of BTCs, C/C0max

199

and retardation factor Rf). A preliminary experiment at pH 3 – due to the metal solubility – was

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performed only to indicate the time frame of the metal mobility without humic substances. The

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BTC of In at pH 3 (Figure 2A) shows that indium is slowly transported as no indium was

202

detected in the outlet for the first 20 PV, and thereafter the BTC shows a classical sigmoid shaped

203

curve with an equilibrium reached after 60 PV at a C/C0 of 0.73. During the flushing phase with

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the ARW background electrolyte solution, the concentration of indium increased again due to a

205

release of retained indium. The retardation factor of the In(III)-only experiments is 31.03, very

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similar to the finding of Ringering et al. (2019) in experiments of In(III) with citrate at pH 6

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(C/C0 = 0.8 and Rf = 31.2). However, no release of indium was observed in Ringering et al.

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(2019), due to the formation of different species and complexes. In the current experiment, In at

209

pH 3 is present mostly as In3+ ions, while in Ringering et al. (2019), In is present mostly as

210

In(OH)3 aq and In-citrate. The trivalent indium ions have a weaker bond with sand than In-citrate

211

complexes that were broken to form indium precipitates, leading to stronger retention of indium

212

in the column.

213

In absence of humic substances, at pH 6.3, preliminary tests show that indium (and

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gallium) is not mobile in the quartz sand, because it precipitates in solution. In the presence of 12

215

humic substances, indium was transported and its transport was shown to be faster (Figure 2B

216

and C) than for the metal alone at pH 3, or (compared to Ringering et al. (2019)) in the presence

217

of citrate at a similar pH of 6. Indium mobility is due to colloid facilitated transport, as expected

218

for metals in the presence of natural organic matter (Hou et al., 2005; Kretzschmar and Schäfer,

219

2005; Murata et al., 2018). However, one can notice than indium-only and indium-HA

220

experiments cannot be compared directly because they were not carried out at the same pH.

221

Indium (only) at pH 6 is insoluble and precipitates from solution.

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A

1.2 1.0

C/C0

0.8 0.6 0.4 0.2 0.0 0

20

40

PV

60

80

100 B

1.2 1.0

C/C0

0.8 0.6 0.4 0.2 0.0 0

20

40

PV

60

80

100 C

1.2 1.0

C/C0

0.8 0.6 0.4 0.2 0.0

222 223 224 225 226

0

20

40

PV

60

80

100

Figure 2: Breakthrough curves of indium in sand column experiments A/ ● Indium 1 mg L-1 only at pH 3, flow rate 0.9 mL min-1, B/ ■ Indium 1 mg L-1, AHA 10 mg L-1 at pH 6.3, flow rate 0.9 mL min-1, C/ ■ Indium 1 mg L-1, AHA 10 mg L-1 at pH 6.3, flow rate 2.3 mL min-1. Dashed vertical lines represent the beginning of the flushing phase.

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227 228 229

Table 3: Retardation factors Rf and maximum ratios of inlet concentration to measured outlet concentration of indium C/C0 of indium experiments.

Flow rate 0.9 mL/min Flow rate 2.3 mL/min

230

3.1.1

In-only In-AHA 10 mg L-1 In-AHA 10 mg L-1 In-AHA 2.5 mg L-1 In-SRFA 2.5 mg L-1 In-SRNOM 2.5 mg L-1

Retardation factor Rf 31.03 7.17 3.92 28.41 11.66 2.76

C/C0 0.73 0.54 0.44 1.06 0.98 0.83

Influence of humic acid on indium transport

231

Indium in presence of AHA at 10 mg L-1, flow rate of 0.9 mL min-1, displays faster

232

transport than for indium only, as seen in the BTCs (Figure 2B). For the first 5 PV, no indium

233

was detected at the column outlet, followed by a breakthrough of indium reaching a plateau after

234

37 PV. The retardation factor Rf = 7.17 and the C/C0 maximum is 0.54 (

235

Table 3). Indium retention in the column is explained by the (calculated) speciation in

236

solution. Indium, in the inlet solution, 69% is present as In(OH)3

237

complexes (Figure 1). It is hypothesized that indium as In-AHA complexes is transported mostly

238

through the column, while In(OH)3 aq interacts strongly with the sand surface. This assumption is

239

supported by the fact that no indium was released during the flushing phase, indicating that the

240

retained indium was bound strongly on the quartz surface.

aq

and 29% as In-AHA

241

Ringering et al. (2019) reported a similar experiment (same indium and AHA

242

concentrations, similar flow rate of 1 mL min-1, pH 6) with citrate in the inlet solutions. A plateau

243

was reached on the BTC after 75 PV (37 PV in the current experiment) with C/C0 = 0.9 and Rf =

244

18.7. The transport of indium was therefore much faster in the current experiment, although

245

indium displayed greater retention relative to the Ringering et al. (2019) experiment. Citrate is

15

246

thus seen to stabilize indium in solution better than Aldrich humic acid (at pH 6), and is also

247

likely to be the key control (rather than the presence of humic colloids) on indium mobility

248

examined in previous studies (Hou et al., 2005; Murata et al., 2018).

249

3.1.2

Influence of flow rate

250

The influence of flow rate on the mobility of indium is shown in Figure 2B, 2C). If the

251

adsorption of the metal on the quartz surface is sufficiently fast, no difference will be observed in

252

the BTCs. However, the presence of organic matter leads to more complex system because

253

dissociation of the metal-humic substances complexes can occur slowly, and therefore affect the

254

transport behavior. When the flow rate is tripled (0.9 mL min-1 to 2.3 mL min-1), the transport of

255

indium is faster (Rf = 3.92 at 2.3 mL min-1 vs. 7.17 at 0.9 mL min-1) as expected. The BTCs have

256

a similar shape, and a similar C/C0 (

257

Table 3). However, tailing was observed during the flushing phase at the higher flow rate.

258

This tailing is due to a release of weakly bound indium during the flushing phase. Because these

259

two experiments were conducted under the same conditions, except for the flow rate, it can be

260

assumed that the kinetics of In(III) sorption onto sand plays a role in the release of indium during

261

the flushing phase. Indium transport depends on kinetically controlled interaction of In(III) and

262

In(III)-AHA with the sand surface. When the flow rate is increased, there is less contact time

263

between the metal and the quartz surface and therefore less sorption. Humic substances can bind

264

metal ions in two different modes: (i) exchangeable fraction where the metal ion is initially bound

265

strongly to the humic substance, but may be removed rapidly if the complex is in the presence of

266

a stronger ligand or a mineral surface that can bind the metal, and (ii) a non-exchangeable

267

fraction with some part of the metal can slowly transfer to the non-exchangeable fraction with a

268

very slow dissociation. Metal ions can return from the non-exchangeable fraction to the 16

269

exchangeable fraction via a process that can have a slow kinetics. If In(III) sorption onto sand is

270

kinetically slow, no equilibrium can be reached in the timeframe of the higher flow rate

271

experiments, leading to release of indium during the flushing phase. Other trivalent ions were

272

shown to have a kinetically dependent transport in presence of natural organic matter (Clark and

273

Choppin, 1996; Schuessler et al., 2000).

274

3.1.3

Influence of humic acid concentration

275

The influence of humic acid concentration on indium transport was also investigated

276

(Figure 3A). In the experiment shown in Figure 3A, the concentration of indium was kept

277

constant while the concentrations of AHA were 2.5 and 10 mg L-1. In the inlet solution, In(III) is

278

composed of 69% In(OH)3 aq and 29.4% In-AHA in the AHA 10 mg L-1 experiment, vs. 90.1%

279

In(OH)3 aq and 7.9% In-AHA in the AHA 2.5 mg L-1 experiment.

280

The BTCs show two very different behaviors of indium in the columns after 30 PV. When

281

AHA concentration was 10 mg L-1, a plateau was reached on the BTC after 30 PV, while in the

282

experiment where AHA concentration was 2.5 mg L-1, the BTC displays a first plateau at 30 PV,

283

followed by a progressive increase in indium relative concentration, until reaching second

284

plateau at C/C0 = 1. A possible explanation for this behavior might be that at the higher AHA

285

concentration, In(III)-AHA complexes can precipitate on the sand surface due to the pH (6.3). In

286

contrast, for the AHA 2.5 mg L-1 experiment, there is a progressive saturation of the sand surface

287

sites by In(OH)3 aq that represents 90% of indium in the inlet solution. In(III) inorganic species

288

can precipitate on the sand surface and when the sites are saturated, the indium in solution can no

289

longer interact with the sand and is transported to the column outlet. It can be noted that

290

experiments to examine possible precipitation of complexes on sand surfaces would be of limited

291

value, because the low metal concentrations used in the experiments would yield amounts of 17

292

metal precipitate too low to identify by imaging techniques. Using higher concentrations would

293

likely lead to precipitation on sand but might not be relevant to behavior under natural conditions

294

at lower environmental concentrations.

295

18

A

1.2 1

C/C0

0.8 0.6 0.4 0.2 0 0

20

40

PV

60

80

100 B

1.2 1.0

C/C0

0.8 0.6 0.4 0.2 0.0 0

20

40

PV

60

80

100 C

1.2 1.0

C/C0

0.8 0.6 0.4 0.2 0.0

296 297 298 299 300

0

20

40

PV

60

80

100

Figure 3: Breakthrough curves of indium in sand column experiments at a flow rate of 2.3 mL min-1 (A) ■ Indium 1 mg L-1, AHA 10 mg L-1 at pH 6.3, ■ Indium 1 mg L-1, AHA 2.5 mg L-1 at pH 6.3, (B) ♦ Indium 1 mg L-1, SRFA 2.5 mg L-1 at pH 6.3, (C) ▲Indium 1 mg L-1, SRNOM 2.5 mg L-1 at pH 6.3. Dashed vertical lines represent the beginning of the flushing phase.

301

19

302

A modeling analysis was performed on the measurements shown in Figures 3B and 3C,

303

using the Hydrus-1D software package (Šimůnek et al., 2016), to gain additional insight on the

304

mechanisms of transport and retention. Briefly, a two-site, kinetic attachment/detachment model

305

was found to match both experiments, indicating that the principal mechanisms of retention are

306

time-dependent attachment for the first site, with no detachment, and instantaneous attachment

307

and detachment for the second site. It is noted that for indium, a clear difference was found

308

between the BTCs of the In-SRNOM and In-SRFA complexes, as shown in Figures 3B and 3C,

309

and described above. The In-SRNOM is more mobile (i.e., has lower retardation factor than the

310

complex with SRFA) and a plateau is reached at C/C0 = 0.8, compared to C/C0 = 1.0 for the

311

SRFA. The model shows higher attachment rate constants (ka1 and ka2) for the In-SRNOM

312

complex, a lower detachment rate constant (kd2), and lower sorption capacity (Smax1). These

313

parameters are in accord with the discussion above linking the plateau to the sorption capacity

314

(Smax1) (SRNOM
315

the fitting parameters (Table S3, and Figures S5 and S6) are given in the Supplementary Material.

316 317

3.1.4

Influence of SRFA and SRNOM

318

The influence of two other natural organic matter extracts on the transport of indium was

319

investigated (Figure 3B, 3C). In the presence of SRFA, indium elutes faster than in the presence

320

of SRNOM, most likely due to the higher proportion of In-humic substance colloids present in

321

solution. Indeed, for the same concentration of humic substances, indium is predicted to be

322

present more in the form of In(III)-humic colloids in the SRFA experiment (25% of In-SRFA)

323

than in the SRNOM (16.4% of In-SRNOM) and in the AHA (7.9% of In-AHA) experiments

324

(Figure 1). Indium transport is faster in the SRNOM experiments than in the AHA experiments 20

325

(Rf = 2.76 in the SRNOM experiment vs. 28.41 in the AHA experiment), as more In-SRNOM

326

complexes are expected than In-AHA complexes. The type of humic substances most likely plays

327

a role in the transport of indium.

328

3.2 Gallium

329

Compared to indium, gallium transport in quartz sand is slow, as seen in the breakthrough

330

curves shown in Figure 4. In the experiment with gallium (alone) at pH 3, no gallium was

331

detected in the first 60 PV following by breakthrough with a C/C0 of 0.50. One can notice that no

332

plateau was reached in the timeframe of the experiment. The slower transport of gallium

333

compared to indium is in accordance with a study of In(III) and Ga(III) adsorption onto SiO2

334

surfaces (Bi and Westerhoff, 2016). This is due to the smaller ionic radius and stronger ionic

335

potential of gallium compared to indium (Shannon, 1976). However, Ringering et al. (2019)

336

showed that the transport of gallium was faster than indium in the presence of citrate. Citrate

337

alters the mobility of indium and gallium by forming stronger complexes with gallium than with

338

indium (Ivanova et al., 2015), leading to alteration of transport dynamics.

21

A

1.2 1.0

C/C0

0.8 0.6 0.4 0.2 0.0 0

20

40

PV

60

80

100 B

1.2 1.0

C/C0

0.8 0.6 0.4 0.2 0.0 0

30

60

90

PV

120 C

1.2 1.0

C/C0

0.8 0.6 0.4 0.2 0.0

339 340 341 342 343

0

20

40

PV

60

80

100

Figure 4: Breakthrough curves of gallium in sand column experiments (A) ● Gallium 1 mg L-1 only at pH 1 3, flow rate 0.9 mL min- , (B) ■ Gallium 1 mg L-1, AHA 10 mg L-1 at pH 6.3, flow rate 0.9 mL min-1, (C) ■ Gallium 1 mg L-1, AHA 10 mg L-1 at pH 6.3, flow rate 2.3 mL min-1. Dashed vertical lines represent the beginning of the flushing phase.

344

22

345

3.2.1

Influence of humic acid on gallium transport

346

In the presence of AHA (Figure 4B), gallium mobility is enhanced due to the complexation

347

with humic acid in solution. The BTC displays a slow breakthrough in the first 60 PV, following

348

by a faster one, reaching C/C0 = 1.03 at 105 PV. The retardation factor Rf = 57.09 (Table 4)

349

compared to Rf = 7.17 for indium in the same experimental conditions (Table 3), showing again a

350

faster transport of indium compared to gallium. However, the retention of indium was stronger in

351

the column with C/C0 = 0.54 (Table 3). During the flushing phase, tailing is observed due to a

352

slow release of gallium that was not bound strongly to the sand surface. As for indium,

353

precipitation of the hydroxy species of gallium on the sand surface with hydrogen bonds on the

354

surface might be predominant. Gallium is expected to be present mostly as negatively charged

355

ions Ga(OH)4-, and the sand surface is also slightly negatively charged at pH 6.3. Therefore,

356

electrostatic repulsion should lead to faster elution for Ga than for In, mostly present as

357

In(OH)3(aq). However, transport of indium is faster than that of gallium. Therefore, it can be

358

assumed that electrostatic interactions are not the predominant retention mechanisms, and it can

359

be hypothesized that the complexation of indium and gallium with humic substances is most

360

likely underestimated. Higher proportions of the metals are most likely bound to the humic

361

substances, hence changing the speciation of the metals and affecting their transport. Gallium

362

elution from the column was complete, even though only 19.4% of gallium is expected to be

363

bound to the humic acid based on the speciation calculations (Figure 1B), compared to the indium

364

experiment where 29.4% of indium is In-HA (Figure 1A). This is likely due to the inorganic

365

species of the metal present in solution. In the indium experiments, In(III) is present mostly as

366

In(OH)3

367

bonds or by precipitating on the surface sites as In(OH)3 s. Gallium is present mostly as Ga(OH)4-,

aq

and it is therefore expected to interact with the sand surface by forming hydrogen

23

368

so that one can assume that interactions of Ga(OH)4- with the deprotonated and negatively

369

charged SiO2 sites of the quartz sand are not favored.

370

3.2.2

Influence of flow rate

371

As observed for indium, the flow rate influences the retention of gallium in quartz sand.

372

When the flow rate is increased by a factor of 3 (Figure 5B, 5C), gallium transport is faster with a

373

retardation factor of 35.23 in the experiment at 2.3 mL/min compared to 57.09 in the experiment

374

at 0.9 mL/min (Table 4).

375 376

Table 4: Retardation factors Rf and maximum ratios of inlet concentration to measured outlet concentration of indium C/C0 of gallium experiments.

Flow rate 0.9 mL min-1 Flow rate 2.3 mL min-1

377

3.2.3

Ga only Ga AHA 10 mg L-1 Ga AHA 10 mg L-1 Ga AHA 2.5 mg L-1 Ga SRFA 2.5 mg L-1 Ga SRNOM 2.5 mg L-1

Retardation factor Rf 148.27 57.09 35.23 32.29 24.52 44.33

C/C0 0.50 1.03 0.62 0.45 0.78 0.58

Influence of humic acid concentration

378

The mobility of gallium is affected also by the concentration of humic acid. In the

379

experiment with Ga 1 mg L-1, AHA 10 mg L-1 (Figure 5A), 19.4% of the gallium is expected to

380

be bound to the humic acid, vs. only 5% in the Ga 1 mg L-1, AHA 2.5 mg L-1 experiment (Figure

381

5A) in the speciation calculations. Stronger retention of gallium is observed in the experiment

382

with the lower HA concentration, with no breakthrough of gallium in the first 20 PV, and very

383

slow evolution of the breakthrough curve. During the flushing phase, gallium is released from the

384

column as the concentration of gallium continues to increase. This indicates a detachment of

385

weakly attached gallium from the sand, as suggested above due to the negatively charged sites of

24

386

the sand. In the indium experiment, In(III)-HA complexes likely precipitated on the sand surface

387

for the higher AHA concentration. This behavior was not observed for gallium. A

1.2 1

C/C0

0.8 0.6 0.4 0.2 0 0

20

40

PV

60

80

100 B

1.2 1.0

C/C0

0.8 0.6 0.4 0.2 0.0 0

20

40

PV

60

80

100 C

1.2 1.0

C/C0

0.8 0.6 0.4 0.2 0.0

388 389 390 391 392

0

20

40

PV

60

80

100

Figure 5: Breakthrough curves of gallium in sand column experiments at a flow rate of 2.3 mL min-1. (A) ■ Gallium 1 mg L-1, AHA 10 mg L-1 at pH 6.3, ■ Gallium 1 mg L-1, AHA 2.5 mg L-1 at pH 6.3, (B) ♦ Gallium 1 mg L-1, SRFA 2.5 mg L-1 at pH 6.3, (C) ▲Gallium 1 mg L-1, SRNOM 2.5 mg L-1 at pH 6.3. Dashed vertical lines represent the beginning of the flushing phase.

25

393

Similar to the modeling analysis of indium following Figure 5, a two-site, kinetic

394

attachment/detachment model was found also to match the gallium measurements shown in

395

Figures 5B and 5C. Thus, for gallium as well as for indium, the principal mechanisms of

396

retention are kinetic attachment for the first site and instantaneous attachment and detachment for

397

the second site. The fitted attachment rate constants are larger for gallium than for indium,

398

accounting for the higher retention of gallium complexes in the column. It is noted that the BTCs

399

of the two Ga complexes show similar patterns, and therefore the same sorption capacity (Smax1)

400

was used for both complexes. Slightly higher attachment rate constants (combining ka1 and ka2)

401

for the Ga-SRNOM are found, which correspond to the somewhat higher retardation factor

402

discussed above. Full details of the model and the fitting parameters (Table S3 and Figures S7

403

and S8) are given in the Supplementary Material.

404 405

3.2.4

Influence of SRFA and SRNOM

406

In the presence of fulvic acid (Figure 5B), the mobility of gallium is favored compared to

407

the experiment with AHA (Figure 5A) with Rf = 24.52 and C/C0 = 0.78 in the SRFA experiment

408

vs. Rf = 32.29 and C/C0 = 0.45 in the AHA experiment (Table 4). Gallium is retained more

409

strongly than indium in the presence of SRNOM (higher C/C0), but the retention of both metals is

410

similar in the presence of SRFA and AHA. However, indium is eluted faster than gallium.

411

Gallium is also more mobile in the presence of SRFA than in the presence of SRNOM (Rf =

412

44.33 and C/C0 = 0.58 for SRNOM experiment), as also observed for indium. This is likely due

413

to the speciation of gallium in the inlet solution, where gallium was predicted to be more present

414

as Ga(III)-humic substances complexes in the SRFA experiment (11.3% Ga-SRFA species vs.

415

8.2% Ga-SRNOM) in the speciation calculations. 26

416

The type of humic substance plays a role in the different retention behaviors of indium

417

and gallium in quartz sand. Weng et al. (2002a) studied the transport of aluminum in the presence

418

of humic substances. Aluminum lies on the same column as indium and gallium in the periodic

419

table and is therefore expected to have a similar transport behavior. The BTCs of aluminum in the

420

presence of humic and fulvic acids, reported by Weng et al. (2002a), showed that Al was retained

421

more in the presence of humic acids than with fulvic acids, as found also here for indium and

422

gallium. Fulvic acids were shown to be smaller than humic acids (Moon et al., 2006; d’Orlyé and

423

Reiller, 2012) and have a higher negative charge. van der Waals attraction forces of fulvic acids

424

are lower than those of humic acids due to their smaller molecular weight and therefore do not

425

aggregate easily and facilitate the transport of the trivalent metals. Moreover, studies on the

426

structure of humic substances show that SRFA has a lower aromatic/aliphatic ratio compared to

427

SRNOM – 0.727 for SRFA vs. 0.852 for SRNOM (Erhayem and Sohn, 2014). A higher

428

aromatic/aliphatic ratio leads to a higher adsorption of humic substances onto surfaces,

429

explaining the higher retention of indium and gallium in SRNOM experiments compared to

430

SRFA.

431

432

4

Conclusion

433

Indium and gallium were shown to be more mobile in the presence of SRFA with indium

434

having a higher mobility than gallium. The speciation calculations were helpful to understand the

435

proportions of indium and gallium bound to the humic substances (up to 26%) that can be more

436

mobile than inorganic complexes. The increase in flow rate evidenced the importance of kinetic

437

interactions of the metals with the sand. When the flow rate increased, the transport of indium 27

438

and gallium was faster due to less contact time between the metal and the quartz sand, leading to

439

less sorption in the columns. Fulvic acids and natural organic matter composed of humic and

440

fulvic acids were shown to increase the mobility of indium and gallium compared to humic acids.

441

Indium mobility decreased according to SRNOM > SRFA > AHA, while gallium mobility

442

decreased as SRFA > SRNOM > AHA. Due to their lower aromatic/aliphatic ratio, fulvic acids

443

are less sorbed onto mineral surfaces than humic acids. Further studies are however needed to

444

investigate the mechanisms impacting the transport of trivalent metals due to the presence of

445

different humic substance colloids.

446

Acknowledgements

447

This research was supported by the Israel Water Authority (Grant No. 45015199895) and the De

448

Botton Center for Marine Science. B.B. holds the Sam Zuckerberg Professorial Chair in

449

Hydrology.

450

Conflicts of interest

451

The authors declare no conflicts of interest.

452

5

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Highlights Indium is more mobile than gallium in the presence of humic substances SRFA enhances the mobility of the metals more than AHA and SRNOM Higher than expected proportions of the metals are bound to the humic substances

The authors declare no conflict of interest

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: