Coupled Kinetics of Ferrihydrite Transformation and As(V

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Environmental Processes

Coupled Kinetics of Ferrihydrite Transformation and As(V) Sequestration under the Effect of Humic Acids: A Mechanistic and Quantitative Study Shiwen Hu, Yang Lu, Lanfang Peng, Pei Wang, Mengqiang Zhu, Alice Dohnalkova, Hong Chen, Zhang Lin, Zhi Dang, and Zhenqing Shi Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b03492 • Publication Date (Web): 19 Sep 2018 Downloaded from http://pubs.acs.org on September 19, 2018

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Environmental Science & Technology

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Coupled Kinetics of Ferrihydrite Transformation and As(V) Sequestration under the Effect of Humic Acids: A Mechanistic and Quantitative Study

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Shiwen Hu,† § Yang Lu,† § Lanfang Peng,† Pei Wang,† Mengqiang Zhu,‡ Alice C.

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Dohnalkova, Hong Chen,ǁ Zhang Lin,† Zhi Dang,† Zhenqing Shi* †

1 2 3

⊥

7 8

†

The Key Lab of Pollution Control and Ecosystem Restoration in Industry

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Clusters, Ministry of Education, School of Environment and Energy, South

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China University of Technology, Guangzhou, Guangdong 510006, People’s

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Republic of China

12

‡

Department of Ecosystem Science and Management, University of Wyoming,

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Laramie, WY, 82071, United States

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⊥

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Laboratory, Richland, WA 99354, United States

16 17 18

ǁ

Environmental Molecular Sciences Laboratory, Pacific Northwest National

SLAC National Accelerator Laboratory, Stanford University, Menlo Park, CA

94025, United States §

Equal contribution

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*Corresponding author: email: [email protected], phone: 86-20-39380503, fax:

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86-20-39380508

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1

ACS Paragon Plus Environment


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Abstract

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In natural environments, kinetics of As(V) sequestration/release is usually coupled with

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dynamic Fe mineral transformation, which is further influenced by the presence of natural

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organic matter (NOM). Previous work mainly focused on the interactions between As(V) and

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Fe minerals. However, there is a lack of both mechanistic and quantitative understanding on

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the coupled kinetic processes in the As(V)-Fe mineral-NOM system. In this study, we

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investigated the effect of humic acids (HA) on the coupled kinetics of ferrihydrite

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transformation into hematite/goethite and sequestration of As(V) on Fe minerals.

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Time-resolved As(V) and HA interactions with Fe minerals during the kinetic processes were

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studied using aberration-corrected scanning transmission electron microscopy, chemical

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extractions, stirred-flow kinetic experiments, and X-ray absorption spectroscopy. Based on

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the experimental results, we developed a mechanistic kinetics model for As(V) fate during Fe

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mineral transformation. Our results demonstrated that the rates of As(V) speciation changes

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within Fe minerals were coupled with ferrihydrite transformation rates, and the overall

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reactions were slowed down by the presence of HA that sorbed on Fe minerals. Our kinetics

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model is able to account for variations of Fe mineral compositions, solution chemistry, and

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As(V) speciation, which has significant environmental implications for predicting As(V)

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behavior in the environment.

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TOC Art

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Introduction Iron (hydr)oxides are ubiquitous in most terrestrial environments and are of great

43 44

importance for many key chemical processes that affect the fate of arsenate (As (V)),

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including adsorption/desorption,1-9 precipitation/co-precipitation,10-13 and mineral phase

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transformation.14-18 Ferrihydrite is one of the most reactive Fe(III) (hydr)oxides, but

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ferrihydrite is metastable and can transform to crystalline Fe minerals. In natural

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environments, As and Fe cycling may be complicated by natural organic matter (NOM),19

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20-27

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transformation processes. Therefore, a mechanistic and quantitative understanding of the

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coupled kinetics of Fe mineral transformation and As(V) sequestration under the impact of

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NOM is essential for accurately predicting the fate and bioavailability of As(V) in the

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environment.

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which may significantly affect the speciation and mobility of As(V) during Fe mineral

Transformation of Fe minerals has been subjected to extensive studies at molecular and

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mechanistic levels.(e.g. 28-33) Transformation of ferrihydrite to crystalline Fe minerals is

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usually dependent on reaction pH, temperature, and water chemistry conditions,33 such as the

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presence of other ions 29, 34, 35 and dissolved organic matter.36 The presence of organic

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molecules including NOM may affect the size, specific surface areas, and even lattice

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structure of ferrihydrite and compositions of iron minerals,36-40 and As(V) may decrease the

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size and structure disorder of ferrihydrite during the co-precipitation process.41 All these

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factors further may have implications on the interactions between Fe minerals and As(V).

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Through the adsorption and co-precipitation reactions between NOM and Fe minerals, the

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stability of organic matter on mineral surfaces is significantly increased42 while the 4


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transformation of Fe minerals is slowed down.36 Compared with the surface adsorption

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reactions, co-precipitation of NOM and Fe minerals may produce much more stabilized

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NOM,39 which is an essential process controlling the dynamics of both organic carbon and Fe

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cycles at the redox interfaces of water body.43 Few studies have considered the kinetic aspects of the dynamic interactions between

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As(V) and Fe minerals under the impact of NOM. Previous research found that

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transformation of ferrihydrite to crystalline Fe oxides (e.g. hematite/goethite) was

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significantly retarded by the presence of As(V), which was dependent on the As/Fe ratios.33

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During the transformation processes, As(V) may be sequestered through the combined

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reactions of adsorption on the surfaces and incorporation into the minerals.14, 15, 44, 45 However,

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how the interactions between NOM and Fe minerals during the Fe mineral transformation

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processes affect As(V) speciation and rates of As(V) sequestration is still largely unknown.

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Therefore, in order to accurately predict the dynamic behavior of As(V) in the environment, it

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is essential to gain a mechanistic understanding on the time-resolved speciation changes of

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both As(V) and Fe minerals in NOM-Fe mineral systems and to develop quantitative models

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accounting for the interactions between As(V) and Fe minerals under the impact of NOM. The objective of this study is to investigate the effect of humic acids (HA), a major

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component of NOM, on the coupled kinetics of ferrihydrite transformation into

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hematite/goethite and sequestration of As(V) in Fe minerals. The As(V)-Fe minerals-HA

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composite presents a complex system (e.g. varying mineral structure and compositions)38, 41,

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46

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state-of-the-art approaches in order to gain a mechanistic and quantitative understanding. We

involving multiple kinetic processes, which requires a suite of complementary and

5



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conducted comprehensive kinetic experiments using combined wet chemistry experiments,

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microscopic and spectroscopic investigation, and kinetic modeling. The novelty of this study

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includes: (i) a time-resolved ex-situ chemical imaging, at both nano and sub-nano scales, of

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As(V) and HA interactions with Fe minerals using aberration-corrected scanning transmission

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electron microscopy (Cs-STEM); (ii) a molecular investigation of As(V) speciation changes

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during Fe mineral transformation processes using X-ray adsorption spectroscopy (XAS); and

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(iii) a mechanistic-based kinetics model for As(V) reactions with Fe minerals with varying Fe

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mineral compositions and solution chemistry. The environmental implications of our study

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are to advance our understanding of how the dynamic interactions of organic matter and Fe

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minerals control the behavior of metal/metalloid contaminants and thus help to develop

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accurate models for the prediction of fate of metals/metalloids in the environment.

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Materials and Methods

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Iron Mineral Transformation Kinetic Experiments. Two types of Fe mineral

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transformation experiments were conducted, both of which started with co-precipitation of

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two-line ferrihydrite,47 one with As(V) (denoted as Fh-As), and the other with both As(V) and

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HA (denoted as Fh-As-HA). For the Fh-As treatment, the initial As:Fh mass ratio was 1.5:100,

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and, for the Fh-As-HA treatment, the initial As:HA:Fh mass ratio was 1.5:4.5:100. The

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transformation of ferrihydrite co-precipitates was initiated by adjusting suspension pH to 10

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and then heated to 75 oC. 75 oC was used to speed up the Fe mineral transformation processes

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as used in previous studies.33 pH 10 was selected to speed up the formation of hematite at 75

106

o 33, 48

107

of HA at lower pH. The transformation experiments ran up to 8 days under constant

C

especially with the presence of As(V) and/or HA, and also to avoid direct precipitation

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temperature and agitation in a water bath shaker. Note that the As:Fh ratio used in this study

109

was typical in tailing sites and did not significantly inhibit ferrihydrite transformation.33 The

110

As:Fh ratio also provided satisfactory As(V) concentration for TEM measurements described

111

later. The initial HA concentration (50 mg L-1) was a typical concentration of NOM in soil

112

solutions. Additional details of the transformation experiments are presented in the

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Supporting Information (SI).

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During the mineral transformation processes, suspension samples were collected at

115

specific sampling times (0 h, 8 h, 24 h, 48 h, 72 h, 96 h, 120 h, and 192 h). Subsample of the

116

suspension was centrifuged, the supernatant was collected, and the mineral particles were

117

washed three times with DI water. Then the mineral particle samples were collected,

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freeze-dried, ground, and analyzed with X-ray diffraction (XRD) and XAS for mineral

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compositions and As(V) speciation. Additionally, batch extraction experiments, stirred-flow

120

kinetic experiments, and STEM were performed to evaluate the distribution of As(V) and

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HA.

122

For comparison, two experiments of Fe mineral transformation, one started with

123

ferrihydrite only (denoted as Fh only) and another one started with ferrihydrite and HA

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co-precipitates (denoted as Fh-HA), were also run under the same conditions. These two

125

experiments were only run for XRD and STEM analysis. The properties of HA during the

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transformation process were monitored using a 50 mg L-1 HA solution sample under the same

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reaction conditions without minerals and As(V), with UV-Vis spectrophotometry (UV2600,

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SHIMADZU) and Fourier transformed infrared (FTIR) spectroscopy (Vertex 70, Bruker) at

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selected reaction times. 7



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XRD Analysis. XRD was used to identify the major components of the

131

precipitates/co-precipitates with a Bruker D8 ADVANCE X-ray diffractometer. Rietveld

132

refinements were performed using TOPAS 5.0 to quantify the proportions of each Fe mineral

133

phase resulting from the transformation processes.49 Details of XRD measurements and data

134

analysis are shown in SI.

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Batch Extraction Experiments. Batch extraction experiments were conducted with 0.1

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M NaOH and 0.01 M NaNO3 mixed solution at pH 13, which help to assess amount of As(V)

137

and HA adsorbed on Fe minerals. Proportions of As(V) and HA non-extractable can be

138

calculated based on the mass balance, which can be used to assess As(V) and HA

139

incorporation into the Fe minerals during the Fe mineral transformation processes. The

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collected suspension samples were extracted with 0.1 M NaOH and 0.01 M NaNO3 mixed

141

solution at pH 13 for 1 h, and the same extraction procedures were repeated four times until

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the extracted As(V) was close to the detection limit. Triplicates were run for each extraction

143

experiment. All solution samples were acidified with concentrated HNO3 and then analyzed

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by an Agilent 7900 Inductively Coupled Plasma Source Mass Spectrometer to determine the

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total As and Fe concentrations. The concentrations of HA were analyzed by an Elementar

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Vario TOC analyzer.

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Spherical Aberration Corrected Scanning Transmission Electron Microscopy

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(Cs-STEM). Suspensions samples were collected during the Fe mineral transformation

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processes

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distribution on Fe nanoparticles.50-52 For comparison, two reference samples, HA adsorbed

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ferrihydrite and hematite, were also analyzed. Samples were characterized for morphology

for STEM analysis, which provided nano and even sub-nano scale elemental

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and elemental distribution with a probe-corrected FEI Titan Themis-200 instrument with a

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field emission gun operated at 200 kV in STEM mode. The instrument was fitted with bright

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field (BF), high angle annular dark field (HAADF) detectors, as well as X-EDS (Bruker,

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Germany) and EELS (Gatan, USA) spectrometers. Combination of spherical aberration

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corrector of the gun lens allowed 1.0 Å spatial resolution in HAADF mode and 1.7-1.8 eV

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energy resolution at 200 kV

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Selected samples were also measured at sub-nano resolution with a double Cs corrected

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FEI Titan3TM G2 60-300 analytical TEM operated at 300 kV, equipped with EELS

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spectrometer (Gatan, USA) and super-X EDS system with four EDS detectors (Bruker,

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Germany). Due to the double Cs-STEM and its excellent EDS/EELS features, we were able

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to analyze elemental distribution at sub-nano scales using EDS mappings and line-scan for Fe,

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O, and As, and EELS line-scan for C. Details of TEM sample preparation and analysis are

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presented in SI.

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Arsenic K-edge Extended X-ray Absorption Fine Structure (EXAFS) Spectroscopy.

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The EXAFS experiments were conducted at the room temperature on the 1W1B beam line at

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Beijing Synchrotron Radiation Facility (BSRF). The spectra were collected from the samples

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in fluorescence mode, and Au metal foil (L3-edge E = 11919 eV) was measured concurrently

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for internal energy calibration. Both Na2HAsO4·7H2O and FeAsO4 were also measured as

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reference standards. EXAFS shell-by-shell fitting for samples in Fh-As treatment at t = 0 h

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and t = 192 h were used to determine the coordination structure of As(V) and the linear

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combination fitting (LCF) of As K-edge EXAFS spectra was used to assess how much As(V)

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was incorporated into the mineral lattice during the reaction. Additional details of XAS 9



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experiments and data analysis are included in SI.

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Stirred-Flow Desorption Kinetic Experiments. The stirred-flow desorption

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experiments were conducted to study the rates of As(V) desorption from Fe minerals at

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selected Fe mineral transformation times, with the stirred-flow reactor system developed in

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our previous kinetic studies.5, 53, 54 For each kinetic experiment, 0.5 mL suspension sample

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(1.1 g L-1 Fe minerals) from the Fe mineral transformation experiments and a Teflon-coated

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magnetic stir bar were transferred into the stirred-flow reaction cell (7.5 cm3). Then the cell

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was quickly filled with the leaching solution (10 mM NaNO3 or 10 mM NaOH with pH = 5.5,

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7.0, 9.0, 10.0 and 12.0) and the reactor was sealed. A 0.22-µm pore size membrane filter was

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used to retain the particles in the cell. After sealing the reactor, the leaching solution was

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pumped through the cell at a constant flow rate (1 mL min-1), and the suspension sample in

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the cell was well mixed with the magnetic stir bar. The effluent solution was collected using a

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fraction collector every five minutes for three hours. Additional experimental details are

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provided in the S1 section of the Supporting Information (SI).

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Modeling Dynamic As(V) Distribution on Fe Minerals and Kinetics of As(V)

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Release. As(V) distribution on different Fe minerals in the co-precipitated samples at

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different aging times was calculated using surface complexation models (SCM) based on the

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specific reaction conditions, the CD-MUSIC model in Visual MINTEQ55 for both ferrihydrite

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and goethite, and a 2-pK triple-layer model for hematite, respectively. Detailed model

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parameters and input parameters are discussed in SI (S2 section). Under our experimental

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conditions, the formation of As(V)-Fe-HA ternary should be minimal and was not

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considered.38 10


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In the kinetics model, As(V) formed three types of complexes on ferrihydrite, two

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bidentate complexes (denoted as Fh-bi-np and Fh-bi-p) and one monodentate complex

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(denoted as Fh-mono).5 For hematite, As(V) formed bidentate binuclear complexes (denoted

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as Ht). Although As(V) can also form three different complexes on goethite, the contribution

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of goethite to As binding was small in this study. To simplify the model, we used the same

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desorption rate coefficients for those three complexes on goethite (denoted as Gt).

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For the stirred-flow kinetic experiments, the concentrations of solution As(V) and

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adsorbed As(V) on specific Fe mineral binding site i during the desorption processes can be

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described as follows,

205

206

dC ion = dt

dCpi

dt

∑k

di

mC p i −

∑k

= −kdi Cpi + kai Cion

ai

mC ion −

Q(C ion − C ion ,0 )

V

(1)

(2)

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in which kai (L.(g.min)-1), kdi (min-1), and Cpi (µmol g-1) represent adsorption rate coefficient,

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desorption rate coefficient, and adsorbed As(V) concentration for site i of each Fe mineral

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phase, respectively, Cion (µmol L-1) is the solution As(V) concentration, m (g L-1) is the

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mineral particle concentration in the reactor, Q (L min-1) is the flow rate, and V (L) represent

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the reaction volume of the reactor. Subscript 0 denotes the influent As(V) concentration.

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In the kinetics model, As(V) binding to ferrihydrite, hematite and goethite were

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considered simultaneously when present, which resulted in multiple surface complexes on the

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surface sites of three Fe minerals. To account for nonlinear binding behavior of As(V) on

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each Fe mineral binding site and heterogeneity of the binding sites in the kinetics model, the

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modeling approaches developed in our previous studies were adopted,5

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kai = kdi Cpi /Cion = kdi Kpi (Cpi , pH)

(3) 11



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log k di − log k dj = 0.5(log K M j − log K M i )

(4)

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in which Kpi (L g-1) is the As(V) equilibrium partition coefficient at site i, and KMi or KMj is

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the As(V) binding constant for site i or j. The Kpi values during the kinetic reactions can be

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calculated with the SCM employed for each Fe mineral, which is a function of adsorbed As(V)

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concentration and reaction pH. The KM values are available in the SCM for each binding site.

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Both equations 3 and 4 established the relationships between reaction rate coefficients and

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thermodynamic equilibrium parameters. As a result of both equations 3 and 4, for each Fe

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mineral there was only one desorption rate coefficient for each type of surface complexes or

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binding sites to be determined in the kinetics model through model fitting, and other

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adsorption/desorption rate coefficients for all binding sites can be obtained through equations

228

3 and 4.

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Model equations were solved using an infinite numerical method in Microsoft Excel

230

spreadsheets. To run the kinetics model, we obtained Kpi values during the reactions by

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searching an As(V) adsorption isotherm database generated by Visual MINTEQ at various

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reaction conditions with the VBA function in Excel. The kinetic data at pH 12 were used to

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obtain the model fitting parameters, in which the As(V) re-adsorption reactions were

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minimized to help reliably determine the desorption rate coefficients. At each observation

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time, the square of the difference between measured and model calculated total dissolved As

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concentrations was calculated. The sum of the squares for all data sets was calculated to

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obtain the total squared error. The SOLVER program in EXCEL was used to obtain the three

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model parameters, kd,Fh-bi-np, kd,Ht and kd,Gt values, by minimizing the total squared error. The

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obtained model parameters were further used to predict As(V) desorption kinetics at other pH 12


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values (5.5, 7.0, 9.0, 10.0) to test the model applicability.

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Results and Discussion

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Fe Mineral Transformation and As(V) and HA Partitioning. XRD results showed the Fe

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mineral transformation processes were significantly affected by the presence of As(V) and/or

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HA. XRD patterns confirmed the formation of ferrihydrite in all treatments at t = 0, and, with

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increasing aging time, ferrihydrite crystallized to mainly hematite with minor goethite (Figure

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S1, SI), with 88%±1.2 hematite and 10%±0.9 goethite at 192 h. Note that our experimental

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conditions favored the formation of hematite over goethite.48 The ferrihydrite transformation

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rates differed significantly for all treatments (Figure 1a). It took about 24 h for the

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ferrihydrite to complete transformation in Fh only treatment while took more than 100 h in

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the presence of As(V) and/or HA. This is consistent with the previous observations that both

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anions and DOM may inhibit the ferrihydrite transformation.33, 36 During the transformation

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experiments, there was little changes of HA properties under our experimental conditions,

253

which was demonstrated by the TOC, UV-Vis, and FTIR analyses of the control sample of 50

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mg L-1 HA solution during 8-day experiments (Figure S2, SI).

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As(V) and HA partitioning during Fe mineral transformation was quantified by

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monitoring aqueous As(V) and HA concentrations (Figure S3, SI) and base extraction of the

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Fe mineral samples at selected times (Figure S4, SI). Overall, the aqueous As(V) consists of

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about 3% - 17% total added As(V) during the kinetic experiments. The aqueous As(V)

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concentrations increased throughout the transformation experiments (Figure S3, SI),

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corresponding to the decreased specific surface areas of the Fe minerals. In contrast, Das et al

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(2015)46 observed a continuous decrease of the aqueous As(V) concentrations during a 13



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similar experimental setup but with Ca2+ present, which was explained by the increasing

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positive charges due to the adsorption of Ca2+ on Fe minerals. The non-extractable As(V)

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increased with aging time (Figure 1b), which suggested that adsorbed As(V) was slowly

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incorporated into the Fe minerals and As(V) incorporation was much slower when HA was

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present. The incorporation of As(V) into Fe minerals has been shown in previous studies,14, 17

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and our results suggested that this process can be significantly impeded by the presence of

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HA, presumably due to much slower mineral transformation. Furthermore, it is interesting to

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observe that non-extractable As(V) was positively correlated with the amount of crystalline

270

Fe minerals formed during the Fe mineral transformation processes (Figure S5, SI). This may

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be attributed to the fact that As(V) can occupy the tetrahedral sites of the formed hematite

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structure as observed for phosphate previously.56 At the end of the experiments, the total

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non-extractable As(V) was close for both treatments (84.5% for Fh-As and 79.8% for

274

Fh-As-HA) (Figure 1b). These results indicate that HA decreased the rates but not the

275

capacity of As(V) incorporation into crystalline Fe minerals.

276

In contrast to As(V), HA was mainly distributed as extractable and aqueous phases, and

277

there was little, if any, HA incorporated into Fe minerals (Figure 1c), presumably due to

278

much larger molecules of HA and its structural incompatibility with the minerals.57 After the

279

initial fast HA release from Fe minerals due to pH increase from 7 to 10, the total amount of

280

HA adsorbed on Fe minerals was stable, regardless of the continuous transformation of

281

ferrihydrite to hematite and goethite. The HA loading on ferrihydrite was 10.1 mg HA g-1,

282

which was much lower than maximum C adsorption capacity of ferrihydrite precipitates.39

283

Therefore, adsorbed HA may have stayed on Fe mineral surfaces even when ferrihydrite 14


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crystallized with reduced surface areas under our experimental conditions.

Time-Resolved As(V) and HA Spatial Distribution in Fe Minerals at Nano Scales.

286

The morphology changes of Fe minerals with aging time well corresponded to the

287

crystallization processes of ferrihydrite to hematite. HAADF images clearly showed that the

288

cloud-like ferrihydrite aggregates gradually transformed to shuttle-shape hematite with

289

reaction time and no apparent goethite was observed (Figure 2, Figure S6, SI). The presence

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of HA significantly decreased the rate of ferrihydrite transformation, consistent with previous

291

observations when organic matter was present.36 Apparent ferrihydrite-hematite mixture

292

observed at t =48 h in TEM images (Figure S6, SI), which was consistent with the XRD

293

results.

294

Arsenate distribution was closely correlated with Fe and O distribution during the

295

transformation as shown in both EDS mapping and quantitative line scan results, and HA

296

slowed the transformation of ferrihydrite and the transfer of As(V) from ferrihydrite to

297

hematite (Figure 2, Figure S7, SI). At time zero, As(V) evenly distributed within ferrihydrite

298

aggregates, which is similar to that of As(V)-adsorbed ferrihydrite samples (Figure S8a, SI).

299

In As(V)-Fe(III)-HA system, As(V) may be adsorbed on mineral surface, forming precipitates,

300

or even be present as As(V)-Fe(III)-HA ternary complexes.38, 58 However, EDS mapping did

301

not find any As(V) precipitates after examining multiple areas and particles.

302

With longer reaction time, As(V) associated with ferrihydrite was relocated to hematite

303

due to the crystallization process. At sub-nano scales, EDS line scans showed that from the

304

particle edge to bulk hematite, As(V) signals were well correlated to the changes of both Fe

305

and O signals in both Fh-As and Fh-As-HA treatments (Figure 2). For the As(V) adsorbed 15



306

hematite samples, As(V) accumulation around the hematite particle was evident (Figure S8b,

307

SI), which was not observed for the hematite particles formed through ferrihydrite

308

transformation with co-precipitated As(V) in either Fh-As or Fh-As-HA treatments (Figure

309

S8c and S8d, SI). This suggests that the majority of As(V) associated with hematite in both

310

Fh-As and Fh-As-HA treatments was mainly structurally incorporated into hematite rather

311

than by surface adsorption, consistent with the results shown in Figure 1b. Similar

312

STEM-EDS results on uranium adsorption on and incorporation into iron nanoparticles were

313

also observed.52 Previously, As(V) incorporation into mineral structure during Fe mineral

314

transformation were mainly investigated based on XAS analysis.14, 44-46 Our study, at the nano

315

and even sub-nano scales, has demonstrated the time-resolved microscopic As(V)

316

incorporation during the ferrihydrite transformation process.

317

Time series of C mapping on Fe minerals (Figure 3a, Figure S9, SI) showed an even C

318

distribution on ferrihydrite aggregates and a clear C accumulation around the edges of

319

hematite particles when formed, indicating that HA was adsorbed on hematite surfaces. Since

320

C signals from EDS mapping may be interfered from other elements, EELS line scans of Fe

321

minerals provided sensitive and quantitative C distribution on Fe nanoparticles. EELS line

322

scans (Figure 3b) showed that C signals were closely correlated to both Fe and O at t = 0, and

323

then, when hematite particles started forming, C signals were much higher close to the

324

surface of hematite than on the bulk of hematite. The elemental distribution close to hematite

325

surface, at sub-nano scales, showed that As, O, and Fe signals decreased toward to the surface

326

of hematite particle, while C signals stayed high and even increased to a peak value,

327

suggesting little As(V) was associated with HA on the surface. Therefore, the 16


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Page 17 of 33


328

As(V)-Fe(III)-HA ternary complexes38, 58 were likely to be minimal under our experimental

329

conditions. All these results collectively suggest that HA initially adsorbed on ferrihydrite,

330

and, during the Fe mineral transformation processes, a large portion of HA was released to

331

solution and the rest of HA adsorbed on the hematite surface.

332

The Change of Arsenate Speciation during Fe Mineral Transformation. The

333

arsenate K-edge XANES spectra are displayed in Figure 4a for both Fh-As and Fh-As-HA

334

treatments at different aging times. A comparison of the sample spectra at t = 0 with that of

335

the ferric arsenate (Figure S10a, SI), together with the derivatives of the spectra (Figure

336

S10b), showed significant difference of spectra, which suggested that no ferric arsenate

337

precipitate formed at the beginning of the experiments and the oxidation state of As(V) did

338

not change during the experiments (Figure 4a).

339

As the reactions proceeded, the small peak at ~ 11880 eV on the right side of the

340

white-line peak (highlighted by arrows in Figure 4a) emerged and gradually increased in

341

intensity, which can be ascribed to multiple scattering between As and the atoms in the

342

surrounding atomic shells. The appearance of this peak suggests additional heavier atoms in

343

the proximity of As. Time-dependent differences are also observed for the EXAFS data

344

(Figure 4b). The EXAFS spectra at ~ 4.5 Å-1 split more with increasing aging time,

345

suggesting that new atomic shells around arsenate emerged after aging for longer times,

346

consistent with the XANES analysis. The EXAFS spectral features in Figure 4b were similar

347

to those reported previously, which were ascribed to the incorporation of As(V) into the

348

hematite structure.14, 46 As(V) incorporation was further confirmed by EXAFS shell-by-shell

349

fitting. The Fourier-transformed spectra for both Fh-As and Fh-As-HA treatments at various 17



350

aging time are shown in Figure S11 (SI) and shell-by-shell fitting spectra for the Fh-As

351

treatment at t = 0 h and t = 192 h are shown in Figure S12 (SI). The much longer As-Fe

352

distance (3.47 Å versus 2.79 Å) and the much higher Fe coordination number (5.8 versus 1.3)

353

of the t = 192 h sample than those of the t=0 h sample is consistent with As(V) incorporation

354

and also suggests that As(V) occupies the tetrahedral sites of the hematite structure (Table S4,

355

SI) for Fh-As treatment at t = 192 h. Similar EXAFS fitting results for indicating As(V)

356

structural incorporation into magnetite were reported in a previous study.59 Based on both

357

XANES and EXAFS spectral features, As(V) became increasingly incorporated into the

358

structure of hematite with increasing aging time.

359

EXAFS LCF analysis was used to estimate the percentages of As(V) adsorbed or

360

incorporated in Fe minerals. Since about 94% As(V) associated with Fe minerals in the Fh-As

361

treatment was not extractable at t = 192 h based on the batch extraction experiments (Figure

362

1b), for LCF analysis, the As(V) adsorbed on ferrihydrite and the Fh-As treatment at t = 192

363

h were chosen as the end members, approximating adsorbed and incorporated As(V) species,

364

respectively. Results show that the incorporated As(V) increased from 18.4% to 100% for the

365

Fh-As treatment and from 13.5 to 82.3% for the Fh-As-HA treatment during aging (Figure

366

4b), suggesting that HA slowed down the As(V) incorporation. A similar LCF analysis in a

367

previous study46 also showed that As was gradually incorporated into the newly formed

368

hematite, presumably through the bidentate-mononuclear and binuclear corner sharing

369

binding. Consistently, the LCF-derived incorporation kinetic data had the same trend as those

370

estimated from the batch extraction experiments (Figure S13, SI). In addition, LCF results

371

also showed that the amount of As(V) incorporation in the Fh-As-HA treatment (82.3%) was 18


Page 18 of 33

Page 19 of 33


372

slightly lower than that in the Fh-As treatment at t = 192 h (100%), while batch extraction

373

indicated these two were close. This difference could be caused by potentially incomplete

374

extraction of surface-adsorbed arsenate due to mineral occlusion (e.g., physics separation)

375

and strong As(V) complexes on Fe minerals,3, 5 and/or the uncertainties in the LCF analysis

376

due to the approximation of the end members (e.g. Fh-As treatment at t = 192 h may also

377

contain some adsorbed As(V)).

378

Modeling As(V) Distribution on Fe Minerals and Kinetics of As(V) Release. During

379

the Fe mineral transformation processes, the extractable As(V) decreased from 83% to 5.7%

380

of total As(V) for the Fh-As treatment and from 85% to 14% of total As(V) for the Fh-As-HA

381

treatment, indicating decrease of adsorbed As(V) on Fe minerals. The distribution of As(V)

382

adsorbed on ferrihydrite, hematite and goethite during the Fe mineral transformation

383

processes is shown in Figure 5. As(V) adsorbed on ferrihydrite decreased with reaction time

384

while As(V) adsorbed on both hematite and goethite increased with reaction time. Consistent

385

with the larger amount of hematite formed at longer reaction time, the majority of the

386

adsorbed As(V) was associated with hematite after about two days. As discussed previously,

387

the presence of HA slowed the transfer of As(V) adsorbed on ferrihydrite to hematite.

388

With the initial As(V) distribution among three Fe minerals, our kinetics model properly

389

described the kinetics of As(V) release from the mixed Fe minerals at different transformation

390

time as studied by the stirred-flow experiments at various pH (Figure 6a). Based on the

391

effluent concentration data, at t = 0, there was little difference of As(V) release rates between

392

these two treatments with or without HA, indicating HA had little impact on As(V)

393

adsorption/desorption reactions under our experimental conditions. However, after 1 d of Fe 19



394

mineral transformation the presence of HA significantly enhanced As(V) release rates, which

395

was due to slower Fe mineral transformation and thus less As(V) incorporation into Fe

396

mineral structure under the impact of HA. The As(V) adsorption and desorption reactions

397

were not affected by HA based on our model calculations presumably due to high pH values,

398

although HA was shown to compete with As(V) binding.60, 61 Furthermore, pH had significant

399

impact on As(V) release rates. At lower pH values (5.5 and 7.0), the effluent As(V)

400

concentrations were close to zero, due to the strong re-adsorption reactions of As(V) at lower

401

pH values.5, 8 The re-adsorption of As(V) to Fe minerals decreased with the increase in pH,8

402

which resulted in the enhanced As(V) release of from pH 9.0 to 12.0.

403

The dominant adsorbed phases controlling As(V) release may differ significantly for

404

Fh-As and Fh-As-HA treatments during the Fe mineral transformation processes, which is

405

shown in Figure 6b as examples at pH 9.0 and 12.0. For the Fh-As treatment, hematite

406

became the principal phase after 24 h, while for the Fh-As-HA treatment ferrihydrite was the

407

dominant phase from 0 to 48 h and hematite was the principal phase at 192 h. This highlights

408

the importance to understand the coupled kinetic processes of Fe mineral transformation and

409

As partition in order to accurately predict As(V) behavior in Fe mineral-HA systems.

410

The kinetic model parameters for ferrihydrite, hematite, and goethite are summarized in

411

Table S5 (SI). The desorption rate coefficients of ferrihydrite binding sites were very close to

412

those we obtained previously.5 For both hematite and goethite, the desorption rate coefficients

413

were in the same order of magnitude as the desorption rate coefficients of the non-protonated

414

bidentate complexes of ferrihydrite. Overall, our kinetics model has demonstrated the ability

415

to predict As(V) release rates during Fe mineral transformation processes, providing that 20


Page 20 of 33

Page 21 of 33

416 417


mineral compositions and solution chemistry conditions are known.

Environmental Implications. Our results have significance in predicting As(V) fate

418

and Fe dynamics in the environment, which have shown that HA played an important role in

419

controlling the coupled kinetics of Fe mineral transformation and As(V) reactions. Since

420

As(III) behavior is also coupled with As(V) reactions, our work further helps to predict As(III)

421

fate in the environment. To accurately predict the fate of As in natural environments, it is

422

crucial to understand the rates of Fe mineral transformation under the impact of humic acids.

423

The kinetics model developed in this study has shown general applicability to describing

424

As(V) desorption rates at environmental conditions (e.g. pH 5.5-9). Our kinetics model, for

425

the first time, integrates both the kinetics of iron mineral transformation under the impact of

426

HA and nonlinear As(V) binding to various Fe minerals, and has shown promise to predict

427

the dynamic As(V) behavior in the environment when both Fe mineral and natural organic

428

matter cycling are coupled.

429

The Fe mineral transformation kinetics was only studied at pH 10 with elevated

430

temperature in this work. Our experimental setups favored the formation of hematite while

431

goethite was another important Fe mineral in the environment governing As(V) fate.

432

Generally, high temperatures and adsorbed surface complexes promoted ferrihydrite

433

transformation to hematite rather than goethite,33 and ferrihydrite may transform to goethite

434

under acidic or alkaline conditions through dissolution and re-precipitation reactions. We

435

expected that the knowledge and tools presented in this study will help us to understand other

436

Fe mineral systems when goethite is the dominant end product. In field conditions, the

437

transformation of Fe minerals may be much slower at lower temperatures and complicated by 21



438

redox reactions involving multiple mixed Fe mineral phases.18, 62-67 At lower pH, it is

439

expected that both As(V) and HA adsorption will be enhanced, and the competition from

440

NOM and other anions should be specifically considered.58, 60, 68, 69 Therefore, it is desired to

441

further test and expand the model to other conditions close to field conditions.

442

Acknowledgments

443

Funding was provided by the National Natural Science Foundation of China (Project number:

444

41573090), Guangdong Innovative and Entrepreneurial Research Team Program (No.

445

2016ZT06N569), and the Thousand Talent Program for Young Outstanding Scientists of

446

China.

447

Supporting Information Available

448

Supporting information includes (1) description of experimental and modeling details and (2)

449

additional figures and tables.

22


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450

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67. Kocar, B. D.; Fendorf, S., Thermodynamic constraints on reductive reactions influencing the biogeochemistry of arsenic in soils and sediments. Environ. Sci. Technol. 2009, 43, (13), 4871-4877. 68. Weng, L. P.; Van Riemsdijk, W. H.; Koopal, L. K.; Hiemstra, T., Adsorption of humic substances on goethite: Comparison between humic acids and fulvic acids. Environ. Sci. Technol. 2006, 40, (24), 7494-7500. 69. Weng, L. P.; Van Riemsdijk, W. H.; Hiemstra, T., Humic nanoparticles at the oxide-water interface: interactions with phosphate ion adsorption. Environ. Sci. Technol. 2008, 42, (23), 8747-8752.

27



633 634 635

Figure 1. Kinetics of (a) hematite and goethite formation, (b) As(V) partitioning, and (c) HA distribution during the Fe mineral transformation kinetic experiments. % As(V) and HA

636

distribution was calculated based on the total As(V) and HA added in the kinetic experiments.

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Figure 2. STEM-EDS mapping and line scan results of As (green), C (yellow), Fe (blue), and O (red) distribution on Fe mineral particles during the Fe mineral transformation kinetic

639

processes. (a) Fh-As treatment; (b) Fh-As-HA treatment.

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640

641 642 643

Figure 3. STEM-EDS mapping and EELS results of C (yellow) distribution on Fe mineral particles during the Fe mineral transformation processes for the Fh-As-HA treatment. (a)

644 645

EDS mapping vs. time; (b) EELS vs. time. Fe (blue) and O (red) are also shown for comparison. 30


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Figure 4. (a) Arsenate K-edge XANES spectra for samples of both Fh-As and Fh-As-HA treatments at various aging time; (b) EXAFS linear combination fits of Fh-As and Fh-As-HA

649 650

at various aging time. Magenta line represents the end member of As(V) adsorbed ferrihydrite, blue line represents the end member of Fh-As treatment at t = 192 h, and black solid lines and

651 652

red dashed-lines represent experimental and fitted data, respectively. Numbers in the bracket represent the percent of As incorporated into Fe minerals based on linear combination fits.

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NSS represents the normalized sum of squares, which is a quality of fit parameter, with NSS

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= 0 indicating a perfect fit.

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655

656 657

Figure 5. Adsorbed As(V) on ferrihydrite, hematite and goethite during the Fe mineral

658

transformation processes for both Fh-As and Fh-As-HA treatments as calculated by the

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surface complexation models.

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Figure 6. Modeling kinetics of As(V) desorption from Fe minerals during the stirred-flow

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kinetic experiments for both Fh-As and Fh-As-HA treatments. (a) Effluent As(V) concentrations vs. time at various pH; (b) model calculated adsorbed As(V) on different Fe

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minerals during the kinetic experiments (pH = 9.0 and 12.0). Symbols are experimental data and lines are model calculations. Solid lines are Fh-As treatment and dashed lines are

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Fh-As-HA treatment.

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Coupled Kinetics of Ferrihydrite Transformation and As(V

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