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Iron redox transformations in the presence of natural organic matter: effect of calcium Chao Jiang, Shikha Garg, and T. David Waite Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b01944 • Publication Date (Web): 07 Aug 2017 Downloaded from http://pubs.acs.org on August 7, 2017

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Environmental Science & Technology

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Iron redox transformations in the presence of natural organic matter:

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effect of calcium

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Chao Jiang, Shikha Garg and T. David Waite*

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School of Civil and Environmental Engineering, The University of New South Wales

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Sydney, NSW 2052, Australia

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Revised

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Environmental Science and Technology

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August 2017

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*

Corresponding author: Tel. +61-2-9385 5060; Email [email protected]

1 Environment ACS Paragon Plus


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Abstract

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The effects of calcium on iron redox transformations in acidic non-irradiated and irradiated

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Suwannee River Fulvic Acid (SRFA) solutions are investigated in this study. Our results

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reveal that, even though calcium is redox-inert, it affects the redox transformations of iron

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with increased Fe(III) reduction rates under irradiated conditions and decreased Fe(II)

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oxidation rates in the dark in the presence compared to the absence of calcium. While the

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exact mechanism via which the Fe(III) reduction rate under irradiated conditions is impacted

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by calcium addition is not clear, the formation of more photo-labile weakly complexed

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Fe(III)SRFA is most consistent with our experimental results. An observed decline in the

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Fe(II) oxidation rate in non-irradiated and previously-irradiated SRFA solutions on addition

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of calcium can be rationalized by formation of more weakly bound Fe(II) and Fe(III). The

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higher Fe(III) reduction rates and lower Fe(II) oxidation rates in the presence compared to the

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absence of calcium will help in maintaining higher concentrations of Fe(II) thereby

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increasing the bioavailability of iron in calcium-containing waters. Based on our

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experimental results we have developed a mathematical model that well describes the iron

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redox transformations mediated by SRFA in calcium-containing waters under irradiated and

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non-irradiated conditions.

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ToC graphic

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Introduction

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Iron is the fourth most abundant element in the Earth’s crust however the presence of iron in

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oxic natural waters is limited due to the insolubility of the thermodynamically favored ferric

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iron (Fe(III)) under circumneutral pH conditions. Compared to Fe(III), the reduced ferrous

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(Fe(II)) state of iron is substantially more soluble however rapidly oxidizes in oxic surface

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waters. The concentration of dissolved iron is hence controlled by the rate of Fe(III)

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reduction and the rate of Fe(II) oxidation. The rates of these redox transformations of iron are

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recognized to be affected by a number of factors including pH, light, and the presence of

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natural organic matter (NOM).1-5 Our recent work on iron redox transformations in the

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presence of Suwannee River Fulvic Acid (SRFA)3 showed that quinone-like moieties play an

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important role in acidic waters (pH ≤ 5) with the hydroquinone-like moieties that are

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intrinsically present in SRFA reducing Fe(III) to Fe(II). On irradiation, these hydroquinone-

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like moieties are partially oxidized to stable semiquinone-like radicals that are capable of

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oxidizing Fe(II) to Fe(III). Since the lifetime of these photo-generated semiquinone radicals

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is > 24 hours,3 the presence of these groups has a significant effect on the dissolved iron

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concentration with these semiquinone radicals acting as the dominant Fe(II) oxidant under

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acidic conditions where the rate of Fe(II) oxidation is otherwise very slow.

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We have investigated the effect of pH, NOM and light on iron redox transformations in our

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recent studies1-3 and, in this study, extend our examination to the effect of the presence of

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calcium on these critical processes. The concentration of calcium varies widely in natural

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waters ranging from 0.4 mM in river water6 to 10.3 mM in seawater.7 Although calcium is

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redox-inert, there are several reports of this element playing a role in redox transformations

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of metals such as iron and copper.8-11 The presence of calcium may affect iron redox

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transformations by competing with iron for the complexation sites on NOM thereby affecting



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the speciation of iron.9 Fujii and co-workers have reported that the presence of divalent

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metals (Me) such as Ca and Mg affect the kinetics of complexation of Fe(III) by a variety of

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organic ligands including fulvic acids, citrate and ethylenediaminetetraacetic acid (EDTA).9

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The presence of Me at concentrations comparable to EDTA retarded the rate of Fe(III)EDTA

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complex formation significantly however, in the presence of fulvic acid, the impact of Me is

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observed only when their concentration is at least 10-fold higher than the fulvic acid

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concentration.9 The impact of Ca on the complexation of Fe may also alter the Fe(II)

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oxidation kinetics. As described by Pham and Waite,12 the second order rate constant for

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oxidation of organically-complexed Fe(II) is affected by the stability constant of the Fe(III)L

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and Fe(II)L complexes with increase in the K Fe( III ) L / K Fe( II ) L value resulting in an increase in

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the Fe(II) oxidation rate constant. Thus, the change in the stability constant of the Fe-SRFA

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complex on Ca addition may also impact the Fe(II) oxidation rate. In addition to affecting the

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Fe(II) oxidation kinetics, these divalent metals are also reported to impact the photochemical

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reduction rate of organically complexed Fe(III), especially in the presence of EDTA.8 The

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photochemical reduction of Fe(III)EDTA is promoted via adjunctive association of Me

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(including Ca and Mg) with Fe(III)EDTA forming a more photo-labile ternary-complex of

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the form Fe(III)EDTA-Me.8 However, the impact of Ca addition on the rate of Fe(III)

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reduction has previously been reported to be minor in the presence of fulvic acids.8 The

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addition of Ca also increased the rate of superoxide-mediated reduction of Fe(III) complexed

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to organic ligands such as EDTA and 3,4-dihydroxybenzoic acid under circumneutral pH

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conditions.11

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Interaction of calcium with hydroquinone and semiquinone-like moieties present in SRFA13-

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16

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transformations in SRFA solutions. As reported earlier, the reduction of Fe(III) by organic

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moieties such as catechol, catechin or sinapic acid are affected by their interaction with Ca

may also influence iron transformations given their important role in Fe redox


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and Mg.17, 18 These divalent metal ions are reported to stabilize semiquinone radicals within

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humic structures by creating bridging interactions and by inducing intramolecular

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aggregation.19-21 Calcium ions have also been reported to have an effect on the redox

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reactions of quinoid compounds as a result of alteration in both the acid-base properties of

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hydroquinone

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semiquinone/hydroquinone redox couples due to stabilization of semiquinone radicals.22-24

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In this study, we investigate the effect of calcium addition on iron redox transformations in

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SRFA solution under acidic conditions. The acidic conditions used here are reasonably

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representative of the conditions present in atmospheric aerosols and environments impacted

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by the presence of acid sulfate soils and also avoid complications associated with iron

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oxyhydroxide precipitation and rapid Fe(II) oxidation. We investigate the impact of Ca

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addition on Fe redox transformations under three conditions as described in our previous

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studies1-3: non-irradiated, previously-irradiated (i.e. SRFA solutions that were irradiated for

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10 minutes prior to Fe addition in the dark) and continuously-irradiated Fe-containing SRFA

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solutions. As described in our earlier work,1 the results of experiments performed under dark

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conditions provide insight into the impact of stable organic moieties naturally present in

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SRFA on Fe redox transformations. The results of experiments performed in the presence of

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previously-irradiated SRFA and continuously-irradiated SRFA solutions describes the role of

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any photo-generated long and short-lived organic moieties respectively in Fe redox

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transformations.1

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Since the principal aim of this work is to determine the impact of the presence of Ca on the

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rate, extent and, if possible, mechanism of Fe redox transformations mediated by SRFA, it is

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important for readers to understand the mechanism controlling Fe redox transformations in

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the absence of Ca as determined in our earlier work1-3 and as depicted in the reaction

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schematic shown in Figure 1. A summary of the main reactions involved in Fe redox

moieties

and

the

redox

potential

of


quinone/semiquinone

or


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transformations in the presence of SRFA as deduced in our earlier studies1-3 is also presented

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in Supporting Information (section S1) and readers are referred to these studies1-3 for detailed

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description of the procedures and observations used to support the mechanism presented here.

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Based on knowledge available in the existing literature, schematics of the various processes

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via which Ca can interact with Fe and/or organic moieties present in SRFA are presented in

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Figure 1. We discuss these processes in detail in accordance with our experimental results in

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the following sections.

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Experimental Methods

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Reagents

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All solutions were prepared using 18 MΩ.cm resistivity Milli-Q water (TOC < 0.1 mg.L-1).

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All glass and plastic ware were soaked in 3% HCl for at least 24 h prior to use. All

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experiments were performed in solutions containing 10 mM NaCl in the pH range 3-5. All

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pH measurements were undertaken using a Hanna 210 pH meter with pH adjustments made

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using high purity HCl (Sigma) and NaOH (Sigma). A maximum pH variation of ± 0.1 units

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was allowed.

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All stock solutions were stored at 4 °C in the dark when not in use. A stock solution of 1 M

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Ca2+ was prepared by dissolving CaCl2 in 0.1 mM HCl. Stock solutions of Fe(II), Fe(III),

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H2O2, ferrozine (3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine-4′,4″-disulfonic acid sodium salt;

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abbreviated as FZ; Sigma), desferrioxamine B (DFB, Sigma), amplex red (AR; Invitrogen),

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horseradish peroxidase (HRP; Sigma), and superoxide dismutase (SOD; Sigma) were

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prepared as reported in our earlier study.3 A stock solution of 50 mM DMSO (molecular

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biology grade; Sigma) was prepared in Milli-Q water.


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Experimental Setup

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All experiments were carried out in air-saturated solutions at a controlled room temperature

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of 22 ± 0.5 °C. In this study, the iron: SRFA concentration ratio was maintained at 0.056 %

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w/w. The presence of excess SRFA ensured that all iron is present in organically complexed

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form. To study the effect of calcium on the redox transformations of iron, an appropriate

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volume of calcium stock solution was added to the working SRFA solution. No change in the

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pH of the SRFA solution occurred on addition of calcium. The experimental setup used to

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study the effect of calcium addition on iron redox transformations under non-irradiated,

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previously-irradiated and continuously irradiated conditions was similar to that used in our

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earlier work1-3 and is described in detail in Supporting Information (see section S2). For

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photochemical experiments, a 150 W Xe lamp (ThermoOriel) equipped with AM0 and AM1

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filters to simulate solar light was used as the light source. The spectral irradiance of the lamp

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and the absorbed photon irradiance (in µEinstein.m−2 .s−1) of 10 mg·L−1 SRFA as function of

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wavelength were reported in our earlier work3 and are also shown in Figure S1. All

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experiments were continued until the system had reached a quasi-steady state. In all cases,

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except for Fe(III) reduction in non-irradiated and previously-irradiated SRFA solutions,

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parameters reach steady-state within 10 minutes and hence experiments in which the time

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dependent concentration of these parameters were measured were performed for 10 minutes.

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The measurement of Fe(III) reduction rate in non-irradiated and previously-irradiated SRFA

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solutions was continued for 60 minutes.

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The changes in the ionic strength of the solutions due to Ca2+ addition do not have any impact

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on Fe redox transformations since no impact of NaCl addition, when added to yield the same

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ionic strength as that in the presence of Ca2+, was observed on Fe transformations. Note

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however that addition of Ca2+ may induce SRFA and Fe-SRFA aggregation due to bridging

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interactions which are unlikely to occur in the presence of NaCl. These effects of Ca2+ may 7 Environment ACS Paragon Plus


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induce variation in size and charge of SRFA and Fe-SRFA aggregates which, in turn, may

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impact the Fe binding capacity and strength of SRFA and, concomitantly, Fe redox

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transformations.

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Control experiments were performed in the absence of SRFA to ensure that trace quantities of

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organics or ROS present in our experimental matrix do not play a role in Fe redox

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transformations.

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Measurement of Fe(II)

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The concentration of Fe(II) was measured using the modified FZ method as described in our

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earlier work.3 The details of the method are also presented in Supporting Information (section

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S2).

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Measurement of H2O2

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H2O2 concentrations were measured fluorometrically using 2 µM AR and 1 kU.L-1 HRP as

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described in our earlier work.3 The details of the method are also presented in Supporting

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Information (section S2).

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Kinetic Modelling

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Kinetic modelling in this study was performed using the software package Kintecus.25

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Statistical comparison of the measured Fe(III) reduction rate and Fe(II) oxidation in the

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presence and absence of Ca2+ was performed using one-way analysis of variance (ANOVA)

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at 5% significance level.

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Results and Discussion

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Fe redox transformation in non-irradiated SRFA solution in the presence of Ca2+ in the

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pH range 3-5


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As shown in Figure 2, Fe(II) concentration increased as a result of reduction of Fe(III) by

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reducing moieties ( A2− ) present in SRFA in the pH range 3-5 with the concentration of Fe(II)

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generated increasing in the presence of 20 mM Ca2+. Thus, we conclude that the addition of

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Ca2+ either increases the Fe(III) reduction rate and/or decreases the Fe(II) oxidation rate.

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Comparing the Fe(II) generation rate in the absence and presence of Ca2+, the % increase in

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the Fe(II) generation rate in the presence of Ca2+ increases from 12.3% to 173.3% with

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increase in pH from 3 to 5 (Figure 2) which is consistent with the observed increase in Fe(II)

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oxidation rate with increasing pH in our earlier work.1 The Fe(III) reduction rate by SRFA in

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acidic solution was shown to be independent of pH in our earlier work.1 The Fe(III) reduction

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rate (as determined from the measured initial Fe(II) generation rate; Figure 2) is also

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independent of pH in the presence of Ca2+ with very similar Fe(III) reduction rates (p> 0.05)

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observed in the pH range 3-5 (see Table S1). The observed pH-dependence of the impact of

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Ca2+ thus supports the conclusion that the effect of Ca2+ addition on the Fe(II) oxidation rate

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is more substantive than the effect of Ca2+ addition on Fe(III) reduction rate in non-irradiated

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SRFA solution. The decrease in Fe(II) oxidation rate may occur as a result of decrease in the

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reactivity of the species involved (i.e. Fe(II) and semiquinone-like radicals) due to their

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interaction with Ca2+. The interaction of Ca2+ with Fe(II) species appears consistent with the

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measured Fe(II) oxidation rates in non-irradiated SRFA solution (where oxidation is mostly

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governed by dioxygen) which shows that the Fe(II) oxidation rate decreases in the presence

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of Ca2+ (Figure S2). Even though the impact of Ca2+ addition on Fe(II) oxidation rate is small

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(~16%, p< 0.05) at pH 4, it is more apparent (46%, p< 0.05) at pH 5 where Fe(II)

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oxygenation occurs at a much faster rate. It is possible that Fe(II) speciation may change in

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the presence of Ca2+ with less and/or weaker Fe(II)SRFA complexes formed as a result of

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competition between Fe and Ca2+ for the binding sites on SRFA. In addition, the binding

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affinity of SRFA for Fe may also decrease due to decrease in the negative charge on the



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SRFA molecules and/or increase in size (hence reduced surface area) of NOM aggregates due

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to bridging interactions in the presence of Ca2+. The formation of less/weaker Fe(II) and

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Fe(III) complexes result in a decrease in the Fe(II) oxidation rate since binding of

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Fe(II)/Fe(III) by SRFA is known to increase the Fe(II) oxidation rate.26

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There is no direct evidence to support or reject the possibility that the Ca2+ interaction with

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semiquinone-like radicals, previously shown to be involved in Fe(II) oxidation, plays a role

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as well. In the pH range investigated here, semiquinone radicals exist both as neutral and

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negatively charged species with the negatively-charged semiquinone radicals capable of

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forming complexes with Ca2+.8 We further investigate this possibility below by measuring the

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effect of Ca2+ addition on Fe redox transformations in previously irradiated and continuously

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irradiated solutions.

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Fe redox transformation in previously irradiated SRFA solution in the presence of Ca2+

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in the pH range 3-5

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When Fe(III) was added to previously-irradiated SRFA solution in the dark, the Fe(II)

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concentration increased over time (Figure 3) though the extent of increase was less than the

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Fe(II) concentrations generated in non-irradiated SRFA solution (Figure 2). Concomitantly,

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the Fe(II) oxidation rate in previously-irradiated SRFA solution (Figure 3) is much higher

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than that observed in non-irradiated SRFA solution (Figure S2). As discussed in our earlier

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work1,3 and also shown in Figure 1 , this is due to partial oxidation of hydroquinone-like

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moieties to form semiquinone-like moieties during irradiation thereby resulting in an overall

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decrease in the Fe(III) reduction rate and an increase in the Fe(II) oxidation rate. Note that the

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role of any photo-generated H2O2 in Fe(II) oxidation in previously-irradiated SRFA solution

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was minor under the experimental conditions investigated here since no increase in Fe(II)

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oxidation rate was observed, even on addition of 2 µM H2O2 as shown in our earlier work.3


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The effect of Ca2+ addition in previously-irradiated SRFA solution is consistent with that

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observed in non-irradiated SRFA solution (Figure 2) with the presence of Ca2+ resulting in an

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increase in Fe(II) generation rate (p< 0.05) on Fe(III) reduction and a decrease in Fe(II)

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oxidation rate (p< 0.05) (Figure 3). The impact of Ca2+ addition increases with increase in

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Ca2+ concentration but saturates at 20 mM Ca2+ for the Fe concentration range of 50 - 150 nM

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investigated here (Figure S3).

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As shown in Figure 1, the generation of A − (Fe(II) oxidant) during irradiation occurs via the

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superoxide-mediated oxidation of A2− that are intrinsically present in SRFA.3 No impact of

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Ca2+ addition occur on the mechanism controlling the generation of A − and consequent Fe

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redox transformation in previously-irradiated solution since addition of SOD completely

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inhibited Fe(II) oxidation with consequent increase in the concentration of Fe(II) generated

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on Fe(III) reduction in previously-irradiated SRFA solution containing 20 mM Ca2+ at pH 4

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(Figure S4) which is same as the effect of SOD addition observed in the absence of Ca2+.3 As

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discussed in our earlier work, 3 SOD catalyzes the decay of superoxide , thereby preventing

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oxidation of A2− to A − . Furthermore, our results support the conclusion that the

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concentration of A − generated on irradiation of SRFA is not affected by Ca2+ addition. As

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shown in Figure S5, the Fe(II) generation and Fe(II) decay rates observed in Ca2+ containing

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previously-irradiated SRFA solutions are the same irrespective of whether Ca2+ was added

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prior to or after irradiation, thereby supporting that the observed impact of Ca2+ addition is

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not due to changes in A − concentration but due to changes in the reactivity of A − and/or a

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result of modification in the speciation of Fe with more easily reducible and more weakly

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bound Fe species formed in the presence of Ca2+. Note that the change in the reactivity of

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hydroquinone-like moieties, involved in Fe(III) reduction, due to interaction with Ca2+ is not

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considered here as the protonated form of hydroquinone (the dominant species under the pH

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conditions investigated here) cannot complex Ca2+. 11 Environment ACS Paragon Plus


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As observed in non-irradiated SRFA solution (Figure 2), the impact of Ca2+ addition (when

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compared to the rates observed in the absence of Ca2+) on Fe redox transformations in

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previously-irradiated SRFA solution increases with increase in pH (Figure 3). As shown, only

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a small (< 10%) effect of Ca2+ addition on Fe(II) oxidation and Fe(III) reduction kinetics in

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previously-irradiated SRFA solution at pH 3 is observed but Ca2+ addition significantly (p
200%) more pronounced than observed at pH 4 (~110%; Figure 3). This

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pH dependence of the Ca2+ effect is consistent with the measured pH dependence of the Fe(II)

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oxidation rate. Thus, it appears that the impact of Ca2+ addition on Fe redox transformations

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in previously-irradiated SRFA solution is due to decrease in the Fe(II) oxidation rate with this

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occuring as a result of the change in speciation of semquinone-like radicals and/or Fe(II) due

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to their interaction with Ca2+ as was determined to be the case in non-irradiated SRFA

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solution. Note that Ca2+ addition may imapct Fe(III) reduction rates as well though its overall

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impact on Fe(II) oxidation rates is much more prominent.

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Fe(III) reduction in continuously irradiated SRFA solution in the presence of Ca2+ in the

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pH range 3-5

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As shown in Figure 4, the concentration of Fe(II) generated on Fe(III) reduction in

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continuously irradiated SRFA solution increases in the presence of 20 mM Ca2+ (p< 0.05).

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Concomitantly, the rate of oxidation of Fe(II) in continuously irradiated SRFA solutions

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decreases in the presence of 20 mM Ca2+ (p< 0.05) (Figure 4). These observations are

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consistent with the results in Figures 2-3 showing that the presence of Ca2+ increases the

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concentration of Fe(II) generated via Fe(III) reduction and decreases the Fe(II) oxidation rate

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in non-irradiated and previously-irradiated SRFA solution. Furthermore, as observed in

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previously-irradiated SRFA solution, the impact of Ca2+ addition increases with increase in 12 Environment ACS Paragon Plus

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Ca2+ concentration but saturates at 20 mM Ca2+ in continuously irradiated SRFA solution as

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well (Figure S6).

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As reported earlier,2 under continuously irradiated conditions, Fe(II) oxidation occurs for the

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most part as a result of the generation of superoxide (via reduction of dioxygen) and peroxyl-

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like radicals ( RO •2 ) (via hydroxylation of SRFA) as indicated in Figure 1. Our results show

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that the same mechanism drives Fe(II) oxidation in the presence of Ca2+ in continuously-

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irradiated SRFA solutions with SOD and DMSO addition (data not shown) increasing Fe(III)

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reduction and decreasing Fe(II) oxidation rate respectively as previously observed in the

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absence of Ca2+.2 Addition of SOD catalyzes the decay of superoxide to dioxygen and H2O2

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and hence prevents the oxidation of Fe(II) by superoxide. Note that Fe(II) oxidation by H2O2

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and O2 (the decay products of superoxide) is very slow under the experimental conditions

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investigated here. Addition of DMSO scavenges any hydroxylating intermediate formed and

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hence prevents formation of peroxyl-like radicals involved in Fe(II) oxidation.

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Our results further support the conclusion that the addition of Ca2+ does not impact the rate of

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generation of superoxide on irradiation of SRFA with the same concentration of H2O2 (which

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is a stable end product of superoxide disproportionation) generated in continuously irradiated

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SRFA solutions in the presence and absence of Ca2+ (see Figure S7). While there is no direct

299

experimental evidence to support the conclusion that peroxyl-like radical generation is not

300

affected by Ca2+ addition (note that it is difficult to measure these radicals in our system due

301

to their low concentration and short lifetime), any decrease in peroxyl radical generation rate

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in Ca2+ containing solutions is unlikely to explain the observed decrease in Fe(II) oxidation

303

rates in the presence of Ca2+ since this decrease in Fe(II) oxidation rates is much more

304

significant than that observed in the absence of peroxyl radicals (~30%; determined by

305

measuring Fe(II) oxidation rates in the presence of DMSO) as illustrated in our earlier work.2



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Thus, it appears that the observed impact of Ca2+ is not due to changes in the mechanism

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and/or concentration of the species controlling Fe redox transformation but due to changes in

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the reactivity of the Fe species involved.

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The measured impact of Ca2+ addition (100 ± 20%; % change in the Fe transformation rate

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measured in the absence and presence of Ca2+) is statistically invariant in the pH range

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investigated (p> 0.05) which suggests that iron redox transformations in the presence of Ca2+

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in continuously irradiated SRFA solutions are, for the most part, independent of pH. This

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observation is in contrast to the impact of pH on the effect of Ca2+ addition on Fe redox

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transformation rates in non-irradiated and previously-irradiated SRFA solutions (Figure 2 and

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3) however is consistent with the hypothesis that the nature of the Fe(III)SRFA complex is

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relatively invariant over the pH range investigated here. The measured Fe(III) reduction rates

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at pH 3 in the presence of Ca2+ (Figure S8) provides more support to the hypothesis that it is

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the Fe(III) reduction rate (and not Fe(II) oxidation rate) that is impacted much more

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significantly on Ca2+ addition in continuously irradiated solution. As shown in Figure S8, at

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pH 3 in the presence of SOD, Fe(II) oxidation is completely inhibited and thus the measured

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Fe(II) generation rate is the same as the Fe(III) reduction rate (a rate which increases in the

322

presence of Ca2+). Since ligand-to-metal charge transfer is the main pathway of Fe(III)

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reduction in continuously-irradiated solution, the effect of Ca2+ on iron reduction most likely

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occurs through its interaction with the Fe(III)SRFA complex. It is possible that the Fe(III)

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speciation changes on addition of Ca2+ with Fe(III) existing as a weaker Fe(III)SRFA

326

complex with this weaker complex much more photo-labile than the complex formed in the

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absence of Ca2+. The formation of a weakly bound Fe(III)SRFA complex is also consistent

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with the observed decrease in Fe(II) oxidation kinetics in Ca2+ containing non-irradiated

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SRFA solution since binding of Fe(III) by organic ligands is known to increase the oxidation

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rate of Fe(II).12 Note that we can confirm that the fraction of SRFA bound Fe(III) does not


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decrease in the presence of Ca2+ since the LMCT-mediated reduction rate of inorganic Fe(III)

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is substantially lower than that of the SRFA-complexed Fe(III) at pH 4 (Figure S9). This

333

observation thus supports the conclusion that it is the binding strength rather than the extent

334

of Fe(III) binding by SRFA that is affected by the presence of Ca2+.

335

We would like to highlight here that even though the impact of Ca2+ addition is much more

336

pronounced on the Fe(III) reduction rate in continuously irradiated SRFA solution, the

337

presence of Ca2+ may also impact the rate of oxidation of Fe(II) by superoxide and peroxyl

338

radicals due to changes in Fe(II) speciation as was determined to be the case in non-irradiated

339

and previously irradiated SRFA solutions.

340

Reaction mechanism accounting for the effect of Ca2+ on Fe redox transformation in

341

non-irradiated, previously-irradiated and continuously-irradiated SRFA solutions

342

As discussed in our earlier work,3 Fe redox transformations in non-irradiated and previously

343

irradiated SRFA solutions are considered to be controlled by the interaction of Fe(III) with

344

hydroquinone-like moieties that are intrinsically present in SRFA resulting in formation of

345

semiquinone-like moieties which subsequently oxidize Fe(II) to Fe(III) (eq. 1). In contrast, Fe

346

redox transformations in continuously irradiated SRFA solution occur via LMCT reduction

347

of Fe(III) while peroxyl-like radical and superoxide oxidize Fe(II).1 Based on our

348

experimental results and the discussion presented above we conclude that the presence of

349

Ca2+

350 351

1) does not affect the mechanism controlling Fe redox transformations in non-irradiated, previously-irradiated and continuously-irradiated SRFA solutions.

352

2) does not affect the generation rate of the various species (including ROS as well as

353

organic moieties involved in Fe redox transformations) formed on irradiation of

354

SRFA.



355 356

3) impacts the Fe(III) reduction rate and/or Fe(II) oxidation rate by interacting with the species involved in these reactions, thereby altering their reactivity.

357

4) increases the Fe(III) reduction rate in continuously-irradiated SRFA solution while

358

decreasing the Fe(II) oxidation rates in non-irradiated and previously-irradiated SRFA

359

solutions.

360 361

5) Impacts the binding strength of SRFA bound Fe(III); however has no impact on the binding extent of Fe(III) by SRFA.

362

While the conclusions listed above are strongly supported by our experimental results, the

363

exact manner via which calcium affects the Fe(III) reduction rates and/or Fe(II) oxidation

364

rates is unclear from the results presented. The increase in Fe(III) reduction rate in

365

continuously-irradiated SRFA solution containing Ca2+ most likely occurs via formation of a

366

more photo-labile weakly bound Fe(III)SRFA complex. The decrease in the Fe(II) oxidation

367

rate in non-irradiated solution that is observed in the presence of Ca2+ most likely occurs as a

368

result of a decrease in the strength of binding of Fe (both Fe(III) and Fe(II)) by SRFA and/or

369

decrease in the fraction of organically bound Fe(II) as confirmed by the measured Fe(II)

370

oxidation rates in non-irradiated SRFA solution (where oxidation occurs due to interaction

371

with dioxygen).

372

We have modelled the impact of Ca2+ addition on Fe redox transformations by including

373

formation of (i) weakly complexed Fe(III) ( Fe(III)L' ) and (ii) weakly complexed Fe(II)

374

( Fe(II)L' ) to the mathematical model developed in our earlier work1 to explain the nature of

375

Fe transformations in acidic SRFA solution. To simplify the modelling, we have assumed

376

that all added Fe exists as strongly bound Fe complexes (i.e. Fe(III)L and Fe(II)L) in the

377

absence of Ca2+ and as weakly bound Fe complexes (i.e. Fe(III)L' and Fe(II)L' ) in the

378

presence of 20 mM Ca2+. The fraction of strongly and weakly bound Fe in the presence of 5


Page 16 of 32

Page 17 of 32


379

mM Ca2+ was determined based on the best-fit to Fe redox transformation rates observed in

380

the presence of 5 mM Ca2+ (Figure S3, S6 and S10).

381

The conditional stability constants of Fe (including both Fe(II) and Fe(III)) binding by SRFA

382

in the presence of Ca2+ are lower than the stability constants for Fe binding by SRFA in the

383

absence of Ca2+ with this effect possibly due to consumption of a portion of the strong

384

binding sites by Ca2+. This decrease in the binding strength of SRFA in the presence of Ca2+

385

may also occur due to decrease in the negative charge of SRFA and/or increase in the size of

386

NOM aggregates in the presence of Ca2+. While our experimental results can be rationalized

387

in this way, further work is required to determine the exact mechanism(s) via which Ca

388

impacts Fe binding by SRFA.

389

The mathematical model developed here is presented in Table 1 with detailed description of

390

the reactions provided in the Supporting Information (see section S5). As shown in Table 1,

391

the weakly bound Fe(III) complex formed in the presence of Ca2+ is more photo-labile as well

392

as slightly easily reducible by hydroquinone-like moieties and superoxide as compared to

393

strongly bound Fe(III) complexes. Similarly, the weakly bound Fe(II) complexes oxidize at

394

much slower rate by dioxygen and semiquinone-like moieties. Although, the reactivity of

395

weakly bound Fe(II) complexes towards superoxide and peroxyl-like radicals is assumed to

396

be same as the strongly bound Fe(II)L specie, even a two-fold lower value of the rate

397

constants for these reactions produces same model output.

398

As shown in Figures 2-4, the mathematical model describes the general trend of our

399

experimental results over a range of conditions, including the impact of pH and light, very

400

well. Although, the model describes the observed initial Fe(III) reduction rates and the

401

steady-state Fe(II) concentrations formed on Fe(III) reduction in non-irradiated SRFA

402

solutions very well, there is some discrepancy between the measured and model-predicted



Page 18 of 32

403

Fe(II) concentrations in the intermediate time period though this discrepancy is well within

404

the variability in experimental data expected at such low Fe concentrations.

405

While there is no direct evidence to reject the possibility that Ca2+ interaction with

406

semiquinone-like radicals influences Fe redox transformations, this pathway appears to be

407

relatively unimportant since we can explain the impact of Ca2+ on Fe redox transformation

408

without invoking this possibility. In conclusion, it appears that the impact of Ca2+ addition on

409

Fe redox transformations is mainly due to its influence on Fe speciation.

410

Using our mathematical model, we can determine the effect of Ca2+ on the turnover

411

frequency (TOF) of iron:

412

TOF =

Fe(III) reduction rate Fe(II) oxidation rate = Total Fe concentration Total Fe concentration

(4)

413

The iron turnover frequency slightly decreases with increase in Ca2+ concentration (TOF =

414

1.6 h-1 in the absence of Ca2+ and 1.2 h-1 in the presence of 20 mM Ca2+ at pH 4) in

415

previously-irradiated SRFA solution due to a decrease in the Fe(II) oxidation rate in the

416

presence of Ca2+. In contrast, the TOF increases from 30.3 h-1 in the absence of Ca2+ to 38.0

417

h-1 in the presence of 20 mM Ca2+ in continuously-irradiated SRFA solutions due to the

418

increased Fe(III) reduction rate in Ca2+ containing irradiated SRFA solution. These results

419

suggest that the presence of Ca2+ renders Fe redox transformations less dynamic in the dark

420

and more dynamic under irradiated conditions.

421

Environmental Implications

422

Our results show that Ca2+ has a significant effect on Fe redox transformations mediated by

423

SRFA under acidic conditions. The effect of Ca2+ addition on Fe redox transformations can

424

be attributed principally to a change in Fe speciation as a result of interaction between Ca and


Page 19 of 32


425

SRFA binding sites. The Fe(III) reduction rates increase due to formation of a more photo-

426

labile weakly complexed Fe(III) species while the Fe(II) oxidation rates decrease due to

427

formation of weaker Fe(II) and Fe(III) complexes in the presence of Ca2+. The impact of Ca2+

428

addition is more pronounced under irradiated conditions than in non-irradiated SRFA

429

solutions. The Fe(II)-Fe(III) turnover frequency increased by as much as 25% in the presence

430

of Ca2+ under irradiation but decreased by approximately 25% in the presence of Ca2+ in the

431

dark at pH 4. As shown in Figure 5, the kinetic model developed in this work can be used to

432

ascertain the likely effect of the presence of Ca2+ on Fe cycling rate over the full diurnal cycle

433

(see Supporting Information section S6 for description of the procedure used for this

434

calculation). As shown, significant diel variation in the rate of Fe cycling occurs, even in

435

acidic waters, since the time scales of Fe(III) reduction and Fe(II) oxidation are similar. Even

436

though the Fe cycling rate decreases after sunset due to cessation of light-mediated Fe

437

transformations, the cycling rate is still much higher than that expected in humic and fulvic

438

acid free acidic waters where Fe(II) oxidation is controlled by dioxygen only (Figure 5). As

439

shown, the cycling rate of Fe in the absence of SRFA is essentially the same under dark and

440

irradiated conditions since the turnover frequency is controlled by the rate of Fe(II) oxidation.

441

Overall, the impact of Ca2+ on binding of Fe by SRFA and associated Fe redox

442

transformations can have significant implications to Fe availability in Ca2+ containing natural

443

waters.

444 445

ASSOCIATED CONTENT

446

Supporting Information. Additional detail on experimental methods, manipulative

447

experiments and kinetic model is provided in Supplementary Information. This material is

448

available free of charge via the Internet at http://pubs.acs.org.

449



450

AUTHOR INFORMATION

451

Professor T. David Waite; Address: School of Civil and Environmental Engineering, UNSW,

452

Sydney, NSW 2052, Australia; Email: [email protected]; Phone: +61-2-9385-5060

453 454

ACKNOWLEDGEMENT

455

This work was funded by Australian Research Council (ARC) Discovery Grant

456

DP120103234 to Professor Waite, ARC Discovery Early Career Researcher Award

457

(DE120102967) to Dr Shikha Garg and Australian Postgraduate Award to Chao Jiang.

458


Page 20 of 32

Page 21 of 32


459

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1. Garg, S.; Jiang, C.; Waite, T. D., Mechanistic insights into iron redox transformations in the presence of natural organic matter: Impact of pH and light. Geochim. Cosmochim. Acta 2015, 165, 14-34. 2. Garg, S.; Jiang, C.; Miller, C. J.; Rose, A. L.; Waite, T. D., Iron redox transformations in continuously photolyzed acidic solutions containing natural organic matter: Kinetic and mechanistic insights. Environ. Sci. Technol. 2013, 47, 9190-9197. 3. Garg, S.; Ito, H.; Rose, A. L.; Waite, T. D., Mechanism and kinetics of dark iron redox transformations in previously photolyzed acidic natural organic matter solutions. Environ. Sci. Technol. 2013, 47, 1861-1869. 4. Sima, J.; Makaiiova, J., Photochemistry of iron (III) complexes. Coord. Chem. Rev. 1997, 160, 161-189. 5. Yuan, X.; Davis, J. A.; Nico, P. S., Iron-mediated oxidation of methoxyhydroquinone under dark conditions: kinetic and mechanistic insights. Environ. Sci. Technol. 2016, 50, (4), 1731-40. 6. Livingstone, D. A., Chemical composition of rivers and lakes. In U.S. Geological Survey Professional Paper 440-G, United State Govrnment Printing Office: Washington, 1963. 7. Wilson, T. R. S., Salinity and the major elements of sea water. In Chemical Oceanography, 2 ed.; Riley, J. P.; Skirrow, G., Eds. Academic Press: London, 1975; pp 364413. 8. Fujii, M.; Yeung, A. C. Y.; Waite, T. D., Competitive effects of calcium and magnesium ions on the photochemical transformation and associated cellular uptake of iron by the freshwater cyanobacterial phytoplankton microcystis aeruginosa. Environ. Sci. Technol. 2015, 49, 9133-9142. 9. Fujii, M.; Rose, A. L.; Waite, T. D.; Omura, T., Effect of divalent cations on the kinetics of Fe(III) complexation by organic ligands in natural waters. Geochim. Cosmochim. Acta 2008, 72, 1335-1349. 10. Stewart, B. D.; Amos, R. T.; Nico, P. S.; Fendorf, S., Influence of uranyl speciation and iron oxides on uranium biogeochemical redox reactions. Geomicrobiol. J. 2011, 28, 444456. 11. Garg, S.; Rose, A. L.; Waite, T. D., Superoxide-mediated reduction of organically complexed iron(III): Impact of pH and competing cations (Ca2+). Geochim. Cosmochim. Acta 2007, 71, (23), 5620-5634. 12. Pham, A. N.; Waite, T. D., Modeling the kinetics of Fe(II) oxidation in the presence of citrate and salicylate in aqueous solutions at pH 6.0−8.0 and 25 °C. J. Phys. Chem. A 2008, 112, (24), 5395-5405. 13. Jezierski, A.; Czechowski, F.; Jerzykiewicz, M.; Drozd, J., EPR investigations of structure of humic acids from compost, soil, peat and soft brown coal upon oxidation and metal uptake. Appl. Magn. Reson. 2000, 18, 127-136. 14. Jerzykiewicz, M.; Jezierski, A.; Czechowski, F.; Drozd, J., Influence of metal ions binding on free radical concentration in humic acids . A quantitative electron paramagnetic resonance study. Org. Geochem. 2002, 33, 265-268. 15. Hering, J. G.; Morel, F. M., Humic acid complexation of calcium and copper. Environ. Sci. Technol. 1988, 22, 1234-7. 16. Hong, S. K.; Elimelech, M., Chemical and physical aspects of natural organic matter (NOM) fouling of nanofiltration membranes. J. Membr. Sci. 1997, 132, 159-181.



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17. Santana-Casiano, J. M.; González-Dávila, M.; González, A. G.; Rico, M.; López, A.; Martel, A., Characterization of phenolic exudates from Phaeodactylum tricornutum and their effects on the chemistry of Fe(II)–Fe(III). Mar. Chem. 2014, 158, 10-16. 18. Santana-Casiano, J. M.; González-Dávila, M.; González, a. G.; Millero, F. J., Fe(III) reduction in the presence of Catechol in seawater. Aquat. Geochem. 2010, 16, 467-482. 19. Bakajová, B.; Von, W. R., Radical stabilization in dissolved humates. Open J. Phys. Chem. 2011, 01, 55-60. 20. Palmer, N.; von Wandruszka, R., Dynamic light scattering measurements of particle size development in aqueous humic materials. Fresenius J. Anal. Chem. 2001, 371, (7), 951954. 21. Yates, L. M.; von Wandruszka, R., Effects of pH and metals on the surface tension of aqueous humic materials. Soil Sci. Soc. Am. J. 1999, 63, (6), 1645-1649. 22. Alegría, A. E.; Sanchez-Cruz, P.; Rivas, L., Alkaline-earth cations enhance orthoquinone-catalyzed ascorbate oxidation. Free Radical Bio. Med. 2004, 37, 1631-1639. 23. Lebedev, A. V.; Ivanova, M. V.; Ruuge, E. K., How do calcium ions induce free radical oxidation of hydroxy-1,4-naphthoquinone? Ca2+ stabilizes the naphthosemiquinone anion-radical of echinochrome A. Arch. Biochem. Biophys. 2003, 413, 191-198. 24. Eaton, D. R., Complexing of Metal Ions with semiquinones. An Electron Spin Resonance Study. Inorg. Chem. 1964, 3, 1268-1271. 25. Ianni, J. C., A comparison of the Bader-Deuflhard and the Cash-Karp Runge - Kutta integrators for the GRI-MECH 3.0 model based on the chemical kinetics code Kintecus. Combustion 2003, 1, 1368-1372. 26. Emmenegger, L.; King, D. W.; Sigg, L.; Sulzberger, B., Oxidation kinetics of Fe(II) in a eutrophic swiss lake. Environ. Sci. Technol. 1998, 32, (19), 2990-2996. 27. Paul, A.; Hackbarth, S.; Vogt, R. D.; Röder, B.; Burnison, B. K.; Steinberg, C. E. W., Photogeneration of singlet oxygen by humic substances: comparison of humic substances of aquatic and terrestrial origin. Photochem. Photobiol. Sci. 2004, 3, 273-280. 28. Dalrymple, R. M.; Carfagno, A. K.; Sharpless, C. M., Correlations between dissolved organic matter optical properties and quantum yields of singlet oxygen and hydrogen peroxide. Environ. Sci. Technol. 2010, 44, 5824-5829. 29. Zhang, Y.; Del Vecchio, R.; Blough, N. V., Investigating the mechanism of hydrogen peroxide photoproduction by humic substances. Environ. Sci. Technol. 2012, 46, 1183611843. 30. Bielski, B.; Cabelli, D.; Arudi, R.; Ross, A., Reactivity of HO2/O2- radicals in aqueous solution. J. Phys. Chem. Ref. Data 1985, 14, 1041-1100. 31. Garg, S.; Rose, A. L.; Waite, T. D., Photochemical production of superoxide and hydrogen peroxide from natural organic matter. Geochim. Cosmochim. Acta 2011, 75, 43104320. 32. von Sonntag, C.; Dowideit, P.; Fang, X.; Mertens, R.; Pan, X.; Schuchmann, M. N.; Schuchmann, H.-P., The fate of peroxyl radicals in aqueous solution. Water Sci. Technol. 1997, 35, (4), 9-15. 33. Garg, S.; Rose, A. L.; Waite, T. D., Pathways contributing to the formation and decay of ferrous iron in sunlit natural waters. In Aquatic Redox Chemistry, ACS symposium series: 2011; Vol. 1071, pp 153-176. 34. Rush, J. D.; Bielski, B. H. J., Pulse radiolytic studies of the reaction of HO2/O2- with iron(II)/iron(III) ions. The reactivity of HO2/O2- with ferric ions and its implication on the occurrence of the Haber-Weiss reaction. J. Phys. Chem. 1985, 89, 5062-5066. 35. Khaikin, G. I.; Alfassi, Z. B.; Huie, R. E.; Neta, P., Oxidation of ferrous and ferrocyanide ions by peroxyl radicals. J. Phys. Chem. 1996, 100, (17), 7072-7077.


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36. Aeschbacher, M.; Sander, M.; Schwarzenbach, R. P., Novel electrochemical approach to assess the redox properties of humic substances. Environ. Sci. Technol. 2010, 44, 87-93. 37. Goldstone, J. V.; Pullin, M. J.; Bertilsson, S.; Voelker, B. M., Reactions of hydroxyl radical with humic substances: bleaching, mineralization, and production of bioavailable carbon substrates. Environ. Sci. Technol. 2002, 36, (3), 364-72.

560



561 562

Page 24 of 32

Table 1: Kinetic model to explain the impact of Ca2+ addition on SRFA-mediated iron redox transformation. No Reaction .

1

Rate constant

Ref

Generation and consumption of singlet oxygen, superoxide and H2O2 on irradiation

SRFA + hv  →SRFA*

Calculated

-

SRFA* + 3 O 2  → SRFA + 1 O 2

Φ ~ 0.5%

27

2.4×105 s-1

28

kf = 1.5×10-4 a; kd = 5.8×103s-1

3

~ 1×109 M-1s-1

29

H2O O 2  → 3 O2

2

1

3

kd f Q + hv  → Q −  → NRP

4

H Q − + 3 O 2  → Q+HO•2

5

HO•2 + HO•2  → O2 + H 2O2

k

+

kHO• + kO− (K HO• /[H + ]) 2

2

2

b

30

(1 + (K HO• /[H + ])) 2 2

7.2×10-6 s-1c

•

6

R + hv  →R

7

R • + HO•2  → R − +O 2 +H +

•

31

−

k7HO2 + k7O2 (K HO• /[H + ]) 2

32 d

(1 + (K HO• /[H+ ])) 2

8

1×103 M-1s-1

R • + R •  → R2

31

Transformation of hydroquinone and semiquinone-like moieties on irradiation 9

+

H A 2− + HO•2  → A − +H 2O2

10

A − + HO•2  → A 2− +O 2 + H +

11

H A − + 1 O 2  → A+HO•2

k9 / k10 = −5.7 EH + 2.5

+

1

1.5×106 M-1s-1e

31

1.5×108 M-1s-1e

3

Generation of short-lived Fe(II) oxidant on irradiation 12

hv R  → RO•2

13

RO •2 + RO •2  → RO 4 R


5×10-7 s-1

1

1×106 M-1s-1

1

Page 25 of 32


1×10-2 M-1s-1

RO•2  → R •+ + HO•2

14

1

Fe redox transformation under irradiated condition 15

hv Fe(III)L  → Fe(II) ' +L ox

7.5×10-3 s-1

2

16

hv Fe(III)L'  → Fe(II)' +L ox

1.5×10-2 s-1

This work

17

Fe(III)L+HO•2  → Fe(II)L+O 2

2×105 M-1s-1

33, 34

18

Fe(III)L' +HO•2  → Fe(II)L' +O 2

2×105 ~ 4×105 M-1s-1

This work

19

Fe(II)L+RO•2  → Fe(III)L+RO 2−

1×107 M-1s-1

35

20

Fe(II)L' +RO•2  → Fe(III)L' +RO 2−

5×106 ~ 1×107 M-1s-1

This work

21

Fe(II)L+HO•2  → Fe(III)L+H 2O 2

k21HO2 + k21O2 (K HO• /[H+ ])

•

−

2

34 f

1 + (K HO• /[H+ ]) 2

'

'

• 2

0.5 k21 ~ k21

Fe(II)L +HO  → Fe(III)L +H 2O 2

22

This work

Fe redox transformation under non-irradiated condition 23

Fe(III)L+A2−  → Fe(II)L+A−

3.0 × 103 M-1s-1g

1

24

' 2− ' − Fe(III)L+A  → Fe(II)L+A

6.0 × 103 M-1s-1

This work

25

Fe(II)L+A−  → Fe(III)L+A2−

kHA + kA− (K HA /[H + ])

e,h

1

1 + (K HA /[H + ]) 26

' − ' 2− Fe(II)L+A  → Fe(III)L+A

~0.61 k26

This work

27

Fe(II)L+O2  → Fe(III)L+HO•2

0.5 M-1s-1i

This work

28

Fe(II)L' +O2  → Fe(III)L' +HO•2

0.25 M-1s-1i

This work

563 564 565 566

a

pseudo-first order rate constant based on [Q]T =0.67 mmol.g-1 SRFA where Q represents the redox-active chromophere responsible for superoxide generation. The concentration of Q is assumed to be the same as the concentration of electron accepting moieties in SRFA as reported earlier.36 b kHO• = 8.0 × 105 M-1s-1, kO− = 9.7 × 107 M-1s-1, and KHO• = 10-4.8.

567 568

c

2

2

2

pseudo-first order rate constant based on [R]T =43 mmol.g-1 SRFA;37 R represents the bulk carbon concentration in SRFA as reported earlier. 37



569

d

570 571 572 573

e

•

Page 26 of 32

−

k7HO2 = 3.5 × 104 M-1s-1, k 7O 2 = 1.6 × 105 M-1s-1, and K HO• = 10-4.8. 2

574

−

−

2

575 576 577

−

rate constant is based on total A concentration including both forms i.e. HA and A ; initial total A concentrations in previously-irradiated SRFA solutions were calculated to be 3.2, 4.9, 6.5 µmoles.g-1 SRFA at pH 3, 4 and 5 respectively based on the measured steady-state Fe(II) concentrations in these solutions as 1 described in our earlier work. f HO •2 6 -1 -1 O −2 = 1.0 × 107 M-1s-1, and K HO• = 10-4.8. k 21 = 1.2 × 10 M s , k 21 g

based on [A]T =35.4 µmol.g-1 SRFA.3 h kHA = 2.4 × 104 M-1s-1, k A − = 1.4 × 105 M-1s-1, KHA = 10-4. i

rate constant at pH 5.


Page 27 of 32


1

Figure Captions

2

Figure 1: Reaction schematic showing redox transformations of Fe under various conditions.

3

Figure 2: Generation of Fe(II) as a result of reduction of 100 nM Fe(III) in 10 mg.L-1 SRFA

4

solutions containing 0 (circles) and 20 mM (triangles) Ca2+ in the dark at pH 3 (panel a), pH

5

4 (panel b) and pH 5 (panel c). Symbols represent the average of duplicate measurements;

6

lines represent model values.

7

Figure 3: Generation of Fe(II) as a result of reduction of 100 nM Fe(III) when added to

8

previously irradiated 10 mg.L-1 SRFA solution containing 0 (circles), and 20 mM (triangles)

9

Ca2+ at pH 3 (panel a), pH 4 (panel b) and pH 5 (panel c). Concentration of Fe(II) remaining

10

as result of oxidation of 100 nM Fe(II) when added to previously irradiated 10 mg.L-1 SRFA

11

solution containing 0 (circles), and 20 mM (triangles) Ca2+ at pH 3 (panel d), pH 4 (panel e)

12

and pH 5 (panel f).

13

Figure 4: Generation of Fe(II) as a result of reduction of 100 nM Fe(III) when added to

14

continuously irradiated 10 mg.L-1 SRFA solution containing 0 (circles), and 20 mM (triangles)

15

Ca2+ at pH 3 (panel a), pH 4 (panel b) and pH 5 (panel c). Concentration of Fe(II) remaining

16

as result of oxidation of 100 nM Fe(II) when added to continuously irradiated 10 mg.L-1

17

SRFA solution containing 0 (circles), and 20 mM (triangles) Ca2+ at pH 3 (panel d), pH 4

18

(panel e) and pH 5 (panel f).

19

Figure 5: Diurnal cycling of cycling rate of Fe in the presence and absence of Ca2+ in SRFA

20

solution at pH 4.



21

Figure 1

22


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Page 29 of 32

23


Figure 2

24



25

Figure 3

26


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27


Figure 4

28 29



30

Figure 5

31


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Iron Redox Transformations in the Presence of ... - ACS Publications

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