A Quantitative Model for the Coupled Kinetics of Arsenic Adsorption

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A Quantitative Model for the Coupled Kinetics of Arsenic Adsorption/Desorption and Oxidation on Manganese Oxides Xionghan Feng, Pei Wang, Zhenqing Shi, Kideok D. Kwon, Huaiyan Zhao, Hui Yin, Zhang Lin, Mengqiang Zhu, Xinran Liang, Fan Liu, and Donald L. Sparks Environ. Sci. Technol. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.estlett.8b00058 • Publication Date (Web): 28 Feb 2018 Downloaded from http://pubs.acs.org on March 1, 2018

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Environmental Science & Technology Letters

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A Quantitative Model for the Coupled Kinetics of Arsenic

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Adsorption/Desorption and Oxidation on Manganese Oxides Xionghan Feng,† Pei Wang,‡ Zhenqing Shi,‡,* Kideok D. Kwon,§ Huaiyan Zhao,† Hui Yin,† Zhang Lin,‡ Mengqiang Zhu, Xinran Liang,† Fan Liu,† Donald L. Sparks⊥

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ǁ

5 6 7 8

†

Key Laboratory of Arable Land Conservation (Middle and Lower Reaches of Yangtze River), Ministry of Agriculture, College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, People’s Republic of China ‡

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School of Environment and Energy, South China University of Technology, Guangzhou, Guangdong 510006, People’s Republic of China

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§

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Department of Geology, Kangwon National University, Chuncheon 24347, Republic of Korea

ǁ

Department of Ecosystem Science and Management, University of Wyoming, Laramie, WY, 82071, United States ⊥

Environmental Soil Chemistry Group, Delaware Environmental Institute and Department of Plant and Soil Sciences, University of Delaware, Newark, Delaware, 19716, United States

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*Corresponding author:

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Zhenqing Shi; Tel: +86 20 39380503; e-mail: [email protected].

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ACS Paragon Plus Environment


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Abstract

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The ability to predict coupled kinetic reactions of arsenic (As) that occur at the

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manganese oxide-aqueous solution interface is crucial for management and remediation of As

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contaminated sites worldwide. A quantitative understanding of the coupling between As

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oxidation and adsorption/desorption kinetics will enable us to more accurately predict the

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kinetic behavior of As. In this study, we developed a novel quantitative model for the coupled

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kinetics of As adsorption/desorption/oxidation on δ-MnO2. The kinetics of As(III) adsorption

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and oxidation on δ-MnO2 was studied using a stirred-flow method under varying influent

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As(III) concentrations. The model was able to account for the variations of As

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concentrations, different binding behavior of As(III) and As(V), and flow rates. Our model

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quantitatively shows that the coupled As adsorption/desorption/oxidation processes are

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highly dependent on As mass loading rates on δ-MnO2. Our model suggests that the As(III)

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adsorption rate is much higher than its oxidation rate. Both As(III) oxidation and subsequent

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As(V) desorption may proceed with similar reaction rates and control the overall reaction

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rates, although As(V) desorption rates may be affected by As(V) re-adsorption. Our kinetics

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model contributes to prediction of the dynamic behavior of As when manganese oxides are

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present.

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Introduction

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Arsenic (As) is a redox sensitive contaminant affecting human health and its

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speciation and fate are greatly affected by soil minerals.1,2 Among various minerals,

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manganese (Mn) oxides are of much of interest,3,4 and the poorly crystalline phyllomangantes

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such as δ-MnO2 exhibit the highest reactivity with As.5,6 As(III) is readily oxidized to As(V)

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when reacted with δ-MnO2, which is coupled by the As(III)/As(V) adsorption/desorption

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reactions, and the overall As reactions with δ-MnO2 may not reach an equilibrium

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instantaneously.7-12 A quantitative understanding on the coupled kinetic reactions of As that

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occur at the Mn oxide-water interface is essential in predicting the fate and transport of As in

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soil.

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A large volume of studies has been done on the mechanisms of As(III) reactions with

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MnO2 minerals.7-11,13-15 Adsorption of As(III) and subsequent oxidation to As(V) usually 2


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occurs at the edge sites of MnO2.10-12,15 Quick-scanning x-ray absorption spectroscopy

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determined the initial oxidation rate constants of As(III) by MnO2 at sub-second time-

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scales.16 At a longer reaction time, Mn(II, III) produced during the coupled As(III)

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adsorption/oxidation reactions may significantly affect the As reaction kinetics by passivating

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the surface sites.7-9,11,15,17,18 Although the apparent kinetics of As(III) were extensively

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modelled using kinetic rate equations,13,16,17,19 previous modeling work rarely simultaneously

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considered both As oxidation and nonlinear As adsorption/desorption kinetics, which may

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prevent accurate prediction of the reaction rates when As(III) adsorbs onto Mn oxides. How

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each individual reaction of the coupled As adsorption/desorption and oxidation processes

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contributes to the overall reaction rate is still largely unknown. Thus, it is desired to develop a

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theoretical basis to quantify the coupled As kinetic reactions at the MnO2-solution interface.

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In this study, we examined the initial stages of the coupled kinetics of As

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adsorption/desorption and oxidation on δ-MnO2 in which the role of Mn(II, III) is presumed

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to be relatively small.7,8 We developed a novel mechanistic-based quantitative model for the

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kinetics of the coupled reactions of As(III) adsorption/desorption, As(III) oxidation and

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subsequent As(V) adsorption/desorption at the MnO2-solution interface. We expect that the

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kinetics model would significantly advance our quantitative understanding and prediction of

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the dynamic behavior of As species in the environment.

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Materials and Methods

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1. Synthesis and characterization of δ-MnO2. δ-MnO2 was synthesized via the

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redox reaction between MnCl2 and NaMnO4 in an alkaline medium.20 The obtained powder

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was washed with DI water and freeze-dried. Ground samples (< 250 µm) were analyzed with

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powder XRD for mineral composition, the BET method for specific surface area, ﬁeld

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emission scanning electron microscopy (FESEM) and transition electron microscopy (TEM)

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for morphology, and for Mn average oxidation state (AOS).21

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2. Arsenic Adsorption/Desorption and Oxidation Kinetic Experiments. The

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kinetic experiments were conducted using a stirred-flow reactor.14,22 Briefly, a δ-MnO2

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suspension (1, 2, and 10 g L-1) and a Teflon-coated magnetic stir bar were placed into the

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reaction cell (volume = 8.5 cm3 or 7.5 cm3). A 0.45-µm pore size membrane filter was used

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to retain the mineral particles in the reaction cell. The background electrolytes were NaNO3 3



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buffered at pH 7.0 using MOPS. Two sets of experiments were conducted, one with low

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influent As(III) concentrations (0.67 – 5.71 µM) and the other with high influent As(III)

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concentrations (500 – 1500 µM). To initiate the kinetic experiment, the As(III) solution was

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pumped through the reactor and the effluent was collected at fixed time intervals. The δ-

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MnO2 suspension was well mixed by the stir bar during the kinetic experiments. The

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experiment continued for 30 min. The flow rate for most experiments was 2 mL min-1 while

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different flow rates (1 and 3 mL min-1) were also tested for selected experiments. Additional

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experiments were also conducted to study As(V) adsorption and desorption kinetics. Details

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of all stirred-flow kinetic experiments are described in the S1 section of Supporting

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Information (SI).

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3. Arsenic(V) Adsorption Isotherm Experiments. The adsorption isotherm of

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As(V) by δ-MnO2 was studied in a batch reactor with a δ-MnO2 suspension concentration of

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10 g L-1, which is described in SI (S2 section).

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For all solution samples, total As and Mn concentrations were analyzed using

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inductively coupled plasma mass spectrometry. For the experiments with high As

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concentrations, the concentrations of As(V) were measured using the molybdenum blue

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method, while for the experiments with low As concentrations, As(III) and As(V)

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concentrations were analyzed by high performance liquid chromatography and atomic

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fluorescence spectrometry.

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Modeling Methods

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1. Kinetic Model Development. Here we propose a conceptual model for the coupled As kinetic reactions at the MnO2-solution interface as follows,

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(1)

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The model contains three key reactions: (i) As(III) in solution (Cw,As(III)) is in instantaneous

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equilibrium with the adsorbed As(III) on MnO2 surface sites (Cp,As(III)), with a partition

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coefficient Kp,As(III), (ii) adsorbed As(III) is oxidized to As(V) on MnO2 surface sites

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(Cp,As(V)), with an oxidation rate constant k (min-1), and (iii) the adsorbed As(V) on the MnO2 4


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surface sites is released into the solution (Cw,As(V)), with adsorption and desorption rate

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coefficients ka,As(V) (min-1) and kd,As(V) (min-1), respectively.

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To account for the effect of passivation of MnO2 during reactions, the oxidation rate

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constant k may change with time. We propose a general approach to describe it,

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k = k0*P(t)

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in which k0 (min-1) is the initial As(III) oxidation rate constant and P(t) is a function reflecting

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the passivation effect on k as reactions proceed, which may be due to Mn(II)/Mn(III) and

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As(V) adsorption on MnO2 surface sites. Since the effluent Mn2+ concentrations are very low

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(data not shown), to simplify the model for the initial stages of the kinetic reactions, we used

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an exponential function to describe P(t),

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P(t) = exp[-b(As(III)supl./MnO2)]

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in which b is a constant, As(III)supl. is the total accumulated amount of As(III) supplied to the

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reactor at time t and MnO2 is the total amount of MnO2 in the reactor.

(2)

(3)

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One central issue is how to specifically incorporate the nonlinear adsorption reactions

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into the oxidation reactions with time. The adsorption of As(V) on δ-MnO2 shows a typical

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nonlinear adsorption behavior within a wide range of As(V) concentrations (Figure S1, SI).

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Specifically coupling nonlinear As(V) binding with As(III) oxidation kinetics will enable us

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to more accurately assess the contribution of each individual reaction. Shi et al.23,24 have

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developed kinetic modeling approaches to consider the kinetics of nonlinear adsorption

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processes. Using a similar modeling concept, the adsorption rate coefficient of As(V) at a

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given reaction time can be calculated based on the equilibrium partition coefficient (Kp,As(V))

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and the desorption rate coefficient,

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ka,As(V) = kd,As(V)K p,As(V)

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In equation 4, Kp,As(V) is a function of Cp,As(V) that changes with the reaction time, and kd,As(V)

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is assumed to remain constant irrespective of reaction conditions, which suggests a

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disjunctive reaction pathway for the desorption reaction.

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(4)

The following equations are responsible for As(III) and As(V) concentrations in the

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stirred-flow reactor,

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Cp,As(III) = Cw,As(III)Kp,As(III)

(5) 5



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dCp,As(V) = kCp,As(III) + kaC w,As(V) − kdCp,As(V) = kCp,As(III) + kdK p,As(V)C w,As(V) − kd Cp,As(V) (6) dt

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dCw,As(V) Q Q = kd Cp,As(V) − ka Cw,As(V) − Cw,As(V) = kdCp,As(V) − kd K p,As(V)C w,As(V) − Cw,As(V) (7) dt V V

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CT,As = Cw,As(III) + Cw,As(V) + Cp,As(III) + Cp,As(V)

(8)

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dCT,As Q = (C0 − C w,As(III) − C w,As(V)) dt V

(9)

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where C0 is the influent As(III) concentration, Q (L min-1) is the flow rate, and V (L) is the

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reaction volume of the reactor. All concentration terms have a unit of mol L-1, and, as a

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result, both Kp,As(III) and Kp,As(V) are dimensionless.

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An implicit finite difference numerical method was used to solve the model equations

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in Excel. Our model has four global fitting parameters, k0, Kp,As(III), kd,As(V), and b. For each

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observation time, the square of the difference between measured and model calculated

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concentrations of both As(III) and As(V) was calculated. All squared errors for each data set

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were summed to obtain the total squared error. The SOLVER program was used to obtain

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model parameters by minimizing the total squared error. Details of the kinetics model are

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presented in SI (S3 section, Figures S2 and S3).

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Results and Discussion

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Mineral Characterization. The powder XRD patterns and TEM images of the δ-

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MnO2 samples are presented in Figure S4 (SI). The 0.242 nm and 0.142 nm reflections for

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vernadite matched quite well with those reported in the literature.20,25 The δ-MnO2 sample

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was composed of small particles and the TEM images showed fine small particles that

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aggregated to large aggregates with low crystallinity. This is consistent with the result of the

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BET measurement with a relatively large specific surface area of 196.2 m2 g-1. The Mn AOS

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value of the synthesized δ-MnO2 is 3.84.

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Modeling As Adsorption/Desorption and Oxidation Kinetics. The changes of

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effluent As(III) and As(V) concentrations with time under varying influent As(III)

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concentrations and flow rates are shown in Figure 1 for both low and high influent As(III)

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concentrations, which results in varying As(III) loadings on MnO2 in the stirred-flow reactor. 6


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Both low (Figure 1a and 1b) and high As concentration data (Figure 1c and 1d) showed

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similar patterns for the effluent As(V) concentrations, which increased with time. No

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detectable As(III) was observed for the low As concentration data (Figure 1a and 1b) while

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the effluent As(III) concentrations slowly increased with time under high As loadings (Figure

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1c and 1d). The model was able to reproduce the effluent data across a wide range of As

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concentrations, supporting the validity of our modeling approach. The model parameters are

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summarized in Table S1 (SI). The root mean square errors (RMSE) analysis showed that

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generally the model fits the experimental data reasonably well, with RMSE values less than 9%

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of the influent As concentration for all experiments. The kinetics of As(V) adsorption and

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desorption, determined from the independent experiments, can also be well reproduced using

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the obtained model parameters (Figures S3 and S5, SI).

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One of the prominent features in our model is the ability to simultaneously compute

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the adsorption/desorption rates of both As(III) and As(V) with time during As(III) oxidation

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process, since it is usually not feasible to determine As(III) adsorption experimentally due to

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the fast oxidation by MnO2. For most experiments, the amount of As(III) adsorbed was

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significantly lower than that of As(V) on the MnO2 surface. The lower adsorption of As(III)

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than As(V) is consistent with a previous prediction using quantum chemical calculations.15

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At low As(III) loadings in the reactor, the amount of As(III) adsorbed on MnO2 increased

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with reaction time and then approached a steady value after about a few minutes (Figure 2a

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and 2b). This suggests that, after a few minutes, the rate of As(III) oxidation approximately

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equaled the rate of As(III) supply in the reactor that was controlled by the As(III) loading

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rate. As(V) adsorption on MnO2 continued to increase with time, indicating As(III) oxidation

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rate was higher than the rate of As(V) desorption from MnO2 surfaces when As(III) loadings

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in the reactor were low.

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In comparison, at high As(III) loadings in the reactor, the amount of As(III) adsorbed

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on MnO2 steadily increased with reaction time while the amount of As(V) adsorbed slowed

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down after a few reactor volumes (about 20 min) (Figure 2c and 2d). This suggests that the

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oxidation rate of As(III) became lower than the rate of As(III) supply in the reactor at high

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As(III) loadings. Therefore, available MnO2 surface sites significantly affected the time-

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dependent As(III) accumulation on MnO2. The accumulation of As(V) on MnO2 suggests that

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As(III) oxidation rates were higher than the overall rates of As(V) release from the MnO2 7



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surface in the beginning and these two reaction rates were similar with increased As(V)

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adsorption. Note that the overall As(V) release from MnO2 was controlled by both the rates

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of As(V) desorption and re-adsorption reactions (equations 1 and 4). The importance of

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As(V) re-adsorption was evidenced by the desorption experiments with 0.1 M NaOH (pH =

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13) as the desorbing reagent. At pH 13, the As(V) re-adsorption reaction was minimal due to

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low As(V) partition coefficients (equation 4) and solution As(V) concentrations spiked

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quickly (Figure S5, SI).

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The time dependent changes of As(III) oxidation rates are presented in Figure 3 for

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both high and low As concentration experiments, in which the previously reported As(III)

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oxidation rate constant (0.282 min-1) by Ginder-Vogel et al. (2009)16 was also included. For

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the low As concentration data, k values changed little during the 30-min experiments,

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consistent with the low As(III) loadings on MnO2 in those experiments. However, for the

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high As concentration data, k values declined with time and the decline rates of k values were

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higher at higher As(III) loadings in the reactor. The decline of k values with the increase in

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As loadings on MnO2 suggests that the production of Mn(II)/Mn(III) may passivate the MnO2

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surface, which slowed the As(III) oxidation rates as the reaction process proceeded.

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Furthermore, As(V) adsorption on the MnO2 surface, especially at high As(V) adsorption

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(Figure 2), may significantly compete with As(III) adsorption sites on MnO2,15 which,

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indirectly, may slow the overall As(III) oxidation rates.

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Model Assessment and Implications. Our study provides a quantitative modeling

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framework for the coupled oxidation and adsorption/desorption kinetics in the initial stages of

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As(III) adsorption to MnO2, which has significant implications for predicting As cycling in

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the environment. It provides a quantitative tool to assess how adsorption, desorption, and

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oxidation reactions affect the overall reaction rates under various As(III) loadings in the

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reactor. Based on our model calculations, the As(III) adsorption rate is much higher than its

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oxidation rate. A slow As(III) adsorption rate would otherwise result in a quick buildup of

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As(III) in the effluent, which was not observed in any of our kinetic experiments. Note that

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the overall As(III) adsorption rate is controlled by both As(III) adsorption and As(III)

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desorption (the back reaction) processes. Therefore, fast oxidation of As(III) reduces

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adsorbed As(III) on MnO2 and minimizes As(III) desorption from MnO2, which promotes the

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overall As(III) adsorption rate as observed in this study. 8


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The As(III) oxidation rate constant, k, and As(V) desorption rate coefficient, kd,As(V),

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are in the same order of magnitude (Table S1, SI), suggesting that As(III) oxidation and

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As(V) desorption may proceed with similar reaction rates (equations 6 and 7). As(V)

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adsorption rate coefficients may vary significantly during the kinetic reactions, due to As(V)

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partition coefficients quickly decreasing with As(V) adsorption on MnO2 (Figure S2, SI). In

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most reaction times, our model calculations showed that As(V) adsorption rate coefficients,

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as determined by equation 4, may be of the same order of magnitude as both k and kd,As(V).

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Overall, in our stirred-flow experiments, both As(III) oxidation and As(V) desorption

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together may control the release of As(III) and As(V) from MnO2, which was dependent on

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the As(III) loadings on MnO2.

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Our current model applies to the initial stages of the coupled kinetics of As

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adsorption/desorption and oxidation on δ-MnO2. At longer reaction time, the changes in

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MnO2 composition, such as accumulation of structural Mn(II, III),26 may be significant, and

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the time-dependent function of k will need modification based on molecular information of

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MnO2 composition. Future kinetic studies should also account for the variations in reaction

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chemistry, such as pH, competing anions (PO43-, CO32-, etc.) and dissolved organic matter,

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and other competing soil sorbents (e.g. iron oxides), which can be incorporated into the

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kinetics model using the similar kinetic modeling approach presented in this study and some

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recent studies.27 Detailed consideration of those factors will enable us to develop a

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comprehensive model to predict As redox cycling in soils, based on the kinetics model

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developed in this study.

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Supporting Information Available

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Supporting information includes (1) details of stirred-flow kinetic experiments, (2)

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As(V) adsorption isotherm experiments, (3) details of the kinetics model, (4) XRD and FE-

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SEM/TEM results, and (5) additional figures. This material is available free of charge at

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http://pubs.acs.org.

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Notes

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The authors declare no competing financial interest. 9



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Acknowledgments

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We appreciate the thoughtful comments and constructive suggestion by three

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anonymous reviewers and Dr. M. Ginder-Vogel to whom we also owe thanks for his sharing

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additional kinetic data from his group. We gratefully acknowledge the National Natural

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Science Foundation of China (No. 41471194, 41573090), the Strategic Priority Research

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Program of the Chinese Academy of Sciences (grant no XDB15020402), Guangdong

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Innovative and Entrepreneurial Research Team Program (No. 2016ZT06N569), and the

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Thousand Talent Program for Young Outstanding Scientists of China for financial support of

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this research. K.K. was supported by the Basic Science Research Program of Korea (NRF-

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2015R1A4A1041105). M.Z. was supported by the U.S. National Science Foundation

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(EAR‑1529937).

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Figure 1. Kinetics of coupled As adsorption/desorption and oxidation on MnO2 at pH 7.0: (a) varying influent As(III) concentrations ([As(III) = 0.67 –5.71 µM, [MnO2] = 1 g L-1, Q = 2 mL min-1); (b) varying influent As(III) concentrations ([As(III) = 0.67 –5.71 µM, [MnO2] = 2 g L-1, Q = 2 mL min-1); (c) varying influent As(III) concentrations ([As(III) = 500 – 1500 µM, [MnO2] = 10 g L-1, Q = 2 mL min-1); and (d) varying flow rates ([As(III)] = 1000 µM, [MnO2] = 10 g L-1). The influent As(III) concentrations are shown in the figure. Symbols are experimental data and the lines are model calculations.

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Figure 2. Kinetics of As(III) and As(V) adsorption on MnO2 during the coupled As adsorption/desorption and oxidation processes at pH 7.0: (a) influent [As(III)] = 0.67 – 5.71 µM, [MnO2] = 1 g L-1, and Q = 2 mL min-1; (b) influent [As(III)] = 0.67 – 5.71 µM, [MnO2] = 2 g L-1, and Q = 2 mL min-1, (c) influent [As(III)] = 500 – 1500µM, [MnO2] = 10 g L-1, and Q = 2 mL min-1; and (d) influent [As(III)] = 1000 µM, [MnO2] = 10 g L-1, and Q = 1-3 mL min-1. All lines are model calculations.

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Figure 3. Changes in As(III) oxidation rate constants at various As(III) loadings on MnO2 during the kinetic processes. Solid lines are model calculations and the dashed line is the value reported by Ginder-Vogel et al. (2009).16

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A Quantitative Model for the Coupled Kinetics of Arsenic Adsorption

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