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governing equation, bed efficiency, and Pb(II) saturation provided a quantitative description of. 52. Pb(II) breakthrough curves. 53 .... turbidimeter...
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Role of Immobile Kaolinite Colloids in the Transport of Heavy Metals Jongmuk Won, and Susan E. Burns Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05631 • Publication Date (Web): 05 Feb 2018 Downloaded from http://pubs.acs.org on February 11, 2018

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Role of Immobile Kaolinite Colloids in the Transport of Heavy Metals

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Jongmuk Won*,1 and Susan E. Burns1

3 4 5 6 7 8

1

9

Drive, N.W. Atlanta, GA 30332-0355

School of Civil and Environmental Engineering, Georgia Institute of Technology, 790 Atlantic

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* Corresponding author

12 13 14 15 16 17 18 19

Submitted to:

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Environmental Science & Technology

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TOC Art

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ABSTRACT

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Predicting the transport of contaminants in porous media is crucial to protecting public

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health and remediating contaminated soil and groundwater. However, the prediction of

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contaminant transport is challenging due to the presence of mobile and immobile colloids. The

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work performed in this experimental investigation quantified the role of immobile clay colloids

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on metal transport through sets of column breakthrough experiments under varying solution

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chemistry, clay content, and flow rate. Georgia kaolinite was chosen as the colloidal material and

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Pb(II) was chosen as the dissolved contaminant. The silica sand used as the bed material was

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sized to ensure that the kaolinite colloids remained stationary during the column experiments.

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Results indicated that retardation of the Pb(II) breakthrough curve was observed as ionic strength

46

decreased and kaolinite content and pH increased, while no significant variation of Pb(II)

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breakthrough was observed at any kaolinite content as flow rate decreased. This work

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demonstrated that in the presence of immobile kaolinite colloids, Pb(II) breakthrough curves

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strongly depended on the pH and ionic strength, which controlled the charge on the surface

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functional groups and the surface availability of metal adsorption sites on immobile kaolinite

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colloids. In addition, the evaluation of unknown first-order coefficients in the continuum

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governing equation, bed efficiency, and Pb(II) saturation provided a quantitative description of

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Pb(II) breakthrough curves.

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Keywords: metal transport; kaolinite content; solution chemistry; breakthrough curve

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1

INTRODUCTION

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Contaminants originating from human activities can be introduced into the subsurface

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through a variety of different sources ranging from stormwater events, leakage from waste

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disposal facilities, mining activities, to various other industrial activities. Those contaminants can

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subsequently transport to groundwater where they can pose a threat to public health.

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Consequently, significant research effort has focused on developing methods to adequately

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predict and assess contaminant transport in subsurface environments with the goal of protecting

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human health and ecosystems.1

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Notably, in the past few decades, many laboratory and field studies have determined that

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the presence and transport of colloidal particles in the subsurface can facilitate or enhance

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contaminant transport: the contaminant transports faster than the groundwater flow because of

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colloidal particles serve as a carrier of contaminant.2–7 Many laboratory studies have

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demonstrated that predicting the distance of contaminant transport without taking mobile

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colloids into account could result in an inaccurate estimate3,8–11. For instance, transport of

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contaminants such as heavy metals or radionuclides can be facilitated by the presence of charged

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inorganic colloids including clay and oxide particles12–14; however, it is important to note that

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colloids do not always pass through porous media easily and they may be retained, which they

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can accumulate in pore space (i.e., clogging) due to pore size restriction, van der Waals attraction

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or electrostatic attraction.15–18 Occasionally, colloids are generated within the subsurface geology

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due to geochemical alternation and breakdown of parent geologic material, and they remain

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stationary even under the groundwater flow condition of high hydrodynamic force. In these

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conditions when colloids are relatively immobile, it is predictable that contaminant transport is

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likely to be retarded, rather than facilitated, if the contaminant is favorably adsorbed by the

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colloid surface.

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Colloidal particles consist of a variety of different substances, including mineral and

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organic phases; however, aluminosilicate clay minerals represent one of the most prevalent types

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of colloids found in groundwater. The mobility of these colloids is a function of the prevailing

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geochemical conditions, and they may be mobilized in the short term due to the alternation of the

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hydrogeological condition and aqueous chemistry, or they can be stationary in a nearly immobile

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state in the short to long terms. Most importantly, the immobility of clay colloids can

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significantly retard the transport of contaminants that are favorably adsorbed by the clay surface,

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which can have a measurable impact in contaminant transport. In spite of prevalence of clay

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colloids in subsurface, the impact of immobile clay colloids on contaminant transport has not

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been well documented to date.

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The objective of the work performed in this experimental study is to investigate

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contaminant transport in a sand medium in the presence of immobile kaolinite clay colloids.

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Lead (Pb(II)) was selected as the model contaminant to represent heavy metal species that are

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not easily degradable biologically and originate from human activities (e.g., industry emission,

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wastewater irrigation, solid waste disposal, mine tailings).19–21 The variation of the clay colloidal

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fraction in the sand medium was selected as the primary variable to observe the degree of Pb(II)

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transport retardation as a function of the mass of immobile clay colloids. Additionally, the

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solution chemistry (pH and ionic strength) and flow rate were varied in order to investigate the

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impact of a surface charge of the clay colloidal particles and kinetic rate of adsorption of Pb(II)

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on the breakthrough curve. Finally, optimization analysis was performed on the measured

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breakthrough curves to evaluate first-order coefficients embedded in the advection-dispersion

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type governing equation to determine adsorption and desorption rates of Pb(II) onto clay colloids.

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2

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2.1

Materials and Methods Materials

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Georgia kaolinite (Wilkinson Kaolin Associate) and ASTM-graded sand (U.S. Silica)

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were selected as the clay colloids and sand medium, and characterization data were measured for

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both soils (Table S1). The liquid limit (LL) was measured by the fall cone method (British

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Standard 1377) and the specific surface (Ss) was measured by the methylene blue method.22 The

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hydrometer test and the sieve analysis in ASTM D422 were performed to measure median

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particle sizes (d50) of the kaolinite and the sand. The maximum and minimum void ratios (emax

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and emin, respectively) of the sand were measured according to ASTM D4253 and ASTM D4254,

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respectively, and the specific gravity (Gs) of both materials was measured according to ASTM

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D854. The zeta potential of the kaolinite particles was measured at varied pH using a zeta

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potential analyzer (Zetaplus, Brookhaven Instruments Corporation). All chemicals in the

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experiments (Pb(NO3)2, NaOH, HNO3, and CaCl2, Sigma Aldrich Corporation and Fisher

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Scientific) were used as received, and deionized water (Barnstead E-pure, electrical conductivity

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~ 0.8 µS/cm and pH ~ 5.6) was used in all experiments.

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To ensure the immobility of kaolinite colloids in the sand medium during the column

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experiment, the retention profile of kaolinite colloids was first obtained after the injection of 10

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pore volumes (PV) of 1 g / L kaolinite suspension through the ASTM-graded sand (Figure S1).

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The hyperexponential retention profile in Figure S1 indicated that pore sizes of ASTM-graded

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sand were not sufficiently large for kaolinite colloids to penetrate deeply into the sand column. A

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surface filter cake was formed at the inlet of the sand medium, and the kaolinite colloids were

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trapped after initial placement in the small pore spaces within the sand. No kaolinite colloids

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were observed in the effluent after 10 pore volumes of flow, and kaolinite mass recovery after

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soil column extraction was approximately 100%.

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2.2

Sample preparation and soil-column experiment

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The soil columns were prepared for experimentation as follows: to remove any impurities

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from the sand, deionized water was used to rinse the sand three to four times, after which the

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sand was submerged it in an ultrasonic bath for 1 hour, followed by drying in an oven at a

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temperature of 105±5 °C. Then the given mass of kaolinite was mechanically mixed for 1 hour

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with a given mass of sand, chosen to form a sample with a corresponding Dr = 70% (i.e., n =

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0.37). The sand-kaolinite sample was placed into the test column using wet pluviation (column

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dimensions = 5.08 cm in diameter and 4.61 cm in height). A perforated round metal plate was

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placed at the top and bottom of the column to ensure even distribution of flow, and a # 200

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plastic mesh (75 µm opening size) was placed at each end of the column to retain the sand grains.

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After the sand was placed, the column was submerged and assembled inside a water bath with

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the background solution to obtain saturation. The background solution (without Pb(II)) was

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injected into the column at a flowrate roughly 1.5 times higher than test velocity in order to

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dislodge loosely bound colloidal particles until the sample reached the equilibrium. A

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turbidimeter (Orbeco-Hellige Inc., TB200) and a conductivity meter (Accumet Excel XL20)

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were utilized to monitor for the presence of kaolinite colloids and the stabilization of solution

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chemistry in the effluent. A strong positive correlation between the concentration of kaolinite

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colloids and turbidity (Figure S2) indicated that turbidity reliably estimated the concentration of

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kaolinite colloids in the effluent. To ensure the system reached to the equilibrium, turbidity and

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electrical conductivity for 15 pore volumes of flow were measured (Figure S3). The initial

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injection of the background solution led to a small amount of kaolinite colloids detachment from

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the sand medium. The measured turbidities demonstrated that the amount of detached kaolinite

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colloids was less than 0.5 % of initial immobile kaolinite colloids in the column. After the

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turbidity and the electrical conductivity of effluent stabilized to background values (dotted line

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on Figure S3), Pb(II) was injected along with the background solution. Almost no kaolinite

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colloid was observed at the outlet during Pb(II) injection in all cases.

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Concentrations of Pb(II) in the effluent were measured using a fraction collector to sample flow

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at the outlet of the column in each pore volume. In all experiments, a rate-adjustable peristaltic

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pump (Fisher Scientific) was used to inject the lead solution. Experiments were performed at two

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constant flow rates, three pH levels, and four levels of ionic strength (Table 1) to investigate the

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impact of the reaction rate, the surface charge of kaolinite colloids, and the presence of other ions

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on the Pb(II) breakthrough curve. The Pb(II) concentration in the influent was 5.0±0.1 mg/L for

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13 experiments, and 1.0 mg/L for two experiments. The influent was flowed through the column

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for 100 pore volumes, and the concentration of the effluent was measured by inductively-coupled

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plasma optical emission spectrometry (ICP-OES, PerkinElmer Optima 8000) at the outlet every

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5 pore volumes of flow.

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Table 1. Experimental Conditions for Lead Sorption Column Tests

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Kaolinite content by mass (KC) Variables 1%

3%

5%

Chemical used

C0 (mg/L)

1, 5

1, 5

5

Pb(NO3)2

pH

4, 5.6, 9

4, 5.6, 9

4, 5.6, 9

70% HNO3, 1N NaOH

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0.001, 0.005, CaCl2 0.01 vs 0.0136 (low), 0.0141 (low), 0.0146 (low), (cm / s) 0.0452 (high) 0.0469 (high) 0.0488 (high) Note: IS = ionic strength, C0 = inlet Pb(II) concentration. Note that porosity variations in the

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column setup resulted in slight variations in vs.

IS (M)

-

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2.3

Equilibrium and kinetic adsorption test

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Equilibrium and kinetic adsorption tests were conducted for the sand and kaolinite to

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determine the maximum adsorption capacities (qmax) and the first-order rate constant (ω)

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(Equation (S5) and Equation (S9) in supporting information). For both the equilibrium and

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kinetic tests, 3 g of solids were mixed with 30 ml solutions in a series of 50 ml tubes, and the

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tubes were rotated at 30 rpm for 24 hours for the equilibrium test, and for intervals of 5, 10, 15,

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20, 30, 60, 120, 240, 360, and 1440 minutes for the kinetic test. After the tubes were removed

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from the shaker, they were centrifuged at 3000 rpm and the supernatant was filtered through a

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0.45 µm filter prior to ICP-OES analysis. All batch tests were performed at the background

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solution of pH ~ 5.6 without adding any chemicals but Pb(NO3)2. Sorption results were fitted to a

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linearized Langmuir isotherm expressed as23:

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Ce C 1 , = e + qe q max K d q max

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(1)

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where Ce is the equilibrium concentration of Pb(II) (mg / cm3), qe is the amount of Pb(II)

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adsorbed per unit mass of the sand or the kaolinite at equilibrium (mg / g), and Kd is the

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distribution coefficient (cm3 / g). The first-order kinetic equation was expressed as24: log(qe − q) = log(qe ) −

ωt 2.303

,

(2)

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where q (mg / g) was the amount of Pb(II) adsorbed per unit mass of sand or kaolinite at time t

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(sec).

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2.4

Optimized first-order coefficients, bed efficiency and Pb(II) saturation

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An optimization analysis was performed to evaluate the three unknown first-order

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coefficients (ka, kd and kc, Supporting Information) associated with reactions between immobile

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kaolinite colloids and Pb(II) species. Implicit finite difference method was used to solve the

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governing equation and the trust region algorithm was used for the analysis. In addition to

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optimization analysis, quantitative comparison of the Pb(II) breakthrough curve in each

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experimental condition was achieved by determining the calculated bed efficiency (β) and Pb(II)

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saturation (Supporting Information).

195 196

3

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3.1

RESULTS AND DISCUSSION Adsorption equilibrium and kinetic adsorption test

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Uptake of lead by kaolinite was well modeled with the Langmuir isotherm; however, as

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was anticipated, Pb(II) adsorption by sand was essentially negligible (Figure S4) (Table 2). The

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specific surface (Ss) of kaolinite was roughly 1200 times larger than that of the sand (based on

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d50 and Gs, Table S1); therefore, it can be implied that Pb(II) adsorption during transport

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dominantly took place at adsorption sites on the immobile kaolinite colloids. Equilibrium

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sorption occurred in less than 10 min (Figure S5), and the determined values of qmax, Kd, and ω

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(Table 2) were used in the optimization analysis. Kd, and ω of kaolinite were presented here for

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information and qmax of kaolinite was used in the calculation of and Pb(II) saturation in the later

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section. Table 2. Equilibrium and Kinetic Parameters Measured for Tested Soils

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Kinetic (Equation (2))

Equilibrium (Langmuir) Material qmax (mg / g)

Kd (L / mg)

R2

ω (/ s)

Sand

1.081 × 10-3

2.202 × 10-3

0.045

1.859 × 10-3

Kaolinite

2.854

6.102 × 10-3

0.986

4.258 × 10-3

208 209 210

3.2

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3.2.1

Soil-column experiment Impact of inlet Pb(II) concentration (Ci)

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At low concentrations of colloids and lead (kaolinite = 1 % and Ci = 1 mg/L), almost no

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Pb(II) breakthrough was observed, indicating there were enough available sorption sites on the

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kaolinite to retard transport of the metal. In contrast, increasing Pb(II) concentration to 5 mg/L

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led to rapid breakthrough with C / Ci > 0.9 after 20 pore volumes (Figure 1). For KC = 3%, no

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Pb(II) was observed in effluent until 100 pore volumes for Ci = 1 mg/L, while the breakthrough

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of Pb(II) was initiated at roughly 30 pore volumes for Ci = 5 mg/L. The plateau concentration

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between 0.6 and 0.7 for KC = 3% and Ci = 5 mg/L indicated that Pb(II) adsorption still occurred

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after 100 PVs of flow. In deionized water at pH ~ 5.6, Pb2+ was the dominant dissolved species 11 ACS Paragon Plus Environment

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and the kaolinite was still predominantly negatively charged, suggesting most Pb(II) adsorption

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occurred through electrostatic attraction (Figure 1). 1 0.9

5mg/L

0.8 0.7

C / Ci

0.6 0.5 KC=1%, Ci=1mg/L KC=3%, Ci=1mg/L KC=1%, Ci=5mg/L KC=3%, Ci=5mg/L

0.4 0.3 0.2 0.1 0 0

20

40

60 PV

80

100

120

222 223

Figure 1. Pb(II) Breakthrough curves as a function of immobile kaolinite colloids and inlet lead

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concentration (deionized water).

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3.2.2

Impact of flow rate (Q) and kaolinite content

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An increase in the concentration of kaolinite colloids led to the slower breakthrough and

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lower peak concentrations in the breakthrough curve (Figure 2) and an increase in the

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concentration of kaolinite colloids increased retardation of Pb(II), which demonstrates that the

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colloids were immobile within the sand bed. If kaolinite colloids were highly mobile and

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released from the bed4,8,25,26, transport would be facilitated because lead favorably sorbs to

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colloids.27–29 Additionally, the flow rate (Darcy velocity), which ranged between 0.3 ~ 1 cm/min

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(typical of groundwater velocity30), was not significant in Pb(II) transport, with breakthrough

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times that were similar in all kaolinite content levels, regardless of flow rate. The similar shapes 12 ACS Paragon Plus Environment

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of the breakthrough curves indicated rapid sorption of Pb(II) (Figure S5), which resulted in the

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similar masses of sorbed Pb(II) despite differences in flow rate. Very high flow rates can shear

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and detach immobile colloids from the sand bed by large hydrodynamic force12,31,32; however,

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that was not observed at the levels tested in this work. 1 1% KC 3% KC 5% KC

0.9 0.8 0.7 C / Ci

0.6 0.5 0.4 0.3 0.2

Q = 6 cm3 / min

0.1

Q = 20 cm3 / min

0 0

20

40

60 PV

80

100

120

239 240

Figure 2. Pb(II) breakthrough curves under two flow rates and three KC: Q = 6 and 20 cm3 / min

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are corresponding to low and high vs for each KC in Table 1, elapsed time for Q = 6 and 20 cm3 /

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min were roughly 9.5 hrs and 2.8 hrs, and Ci ≈ 5 mg/L and deionized water was used for all cases.

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β values for the breakthrough curves demonstrated that more Pb(II) was removed by the

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bed as the kaolinite content increased (Figure 5(a)), and, Pb(II) saturation decreased as the

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kaolinite content increased, implying that many available adsorption sites were still available in

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the sand-kaolinite bed at kaolinite content = 5 % after 100 pore volumes of flow for the Pb(II)

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solution. This means that roughly 50 % of the sorption sites remained unbound when the

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kaolinite content was 5 %. Lead saturation was approximately 1 for kaolinite content equal to 13 ACS Paragon Plus Environment

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1 %, meaning that the adsorption of Pb(II) in the bed would not occur at any significant level

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after 100 pore volumes of flow due to the low maximum adsorption capacity. High Pb(II)

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saturation led to C / Ci ~ 1 in the breakthrough curve after 100 pore volumes, which would

253

remain true unless solution chemistry was changed. β and Pb(II) saturation also showed the

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insignificance of flow rate in Pb(II) transport (Figure 5(a)), which was consistent with the

255

breakthrough curves exhibited in Figure 2. Overall, the immobile kaolinite content retained in

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the sand medium was more crucial than flow rate in Pb(II) transport.

257 258

3.2.3

Impact of pH

259

Kaolinite has a 1:1 clay structure with one alumina octahedral sheet (i.e., gibbsite) and

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one silica tetrahedral sheet (i.e., silica sheet) held together through hydrogen bonding. The

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permanent negative charges of kaolinite colloids are attributed to isomorphous substitution in the

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mineral structure and uncompensated charge at the edge of the colloids.33 Because the hydroxyl

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groups (≡SOH) located on the surface and edge of the kaolinite particles are amphoteric, the

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surface charges are highly sensitive to pH. At high pH, H+ ions release into solution from surface

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hydroxyl groups (≡SOH → ≡SO- + H+), a process that is known as deprotonation of surface

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groups. Negatively charged ≡SO- sites result in an increase in the adsorption of metal species,

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forming metallic surface complexes such as ≡SOMe+ or ≡SOMeOH. In contrast at low pH, the

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surface hydroxyl groups sorb a proton to become ≡SOH2+ through protonation (≡SOH + H+ →

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≡SOH2+), which leads to a decrease in adsorption of metal species.34 This trend is reflected to the

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experimental Pb(II) breakthrough curves measured in this work (Figure 3).

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Pb(II) breakthrough curves were significantly impacted by the variation of pH due to the

272

change in surface charge: protonation at low pH (pH = 4.0) and deprotonation at high pH (pH = 14 ACS Paragon Plus Environment

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9.0). Extremely early breakthrough was observed in the experiments conducted for a kaolinite

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content equal to 1 % at pH = 4.0, while almost no breakthrough was observed in experiments

275

conducted at a kaolinite content of 5 % at pH = 9.0. Notably delayed Pb(II) breakthrough was

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observed at pH = 5.6 and kaolinite content equal to 1 % when compared to all kaolinite contents

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at pH = 4.0. This implies that a change in the pH of the solution more significantly impacted the

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Pb(II) transport than kaolinite content when concentration was approximately = 1 ~ 5 %. In

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addition, higher pH increased the impact of kaolinite content on the Pb(II) breakthrough curve

280

due to deprotonation of the kaolinite surface groups. The trend of breakthrough curves presented

281

in Figure 3 is consistent with batch adsorption tests of heavy metals as a function of pH.35–38

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Except in the presence of ligands in solution, heavy metal species are generally more preferably

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adsorbed by the clays at high pH, as observed in the breakthrough curves in Figure 3. 1 0.9 0.8 0.7 C / Ci

0.6 0.5 0.4

1% KC 3% KC 5% KC pH = 9.0 pH = 5.6 pH = 4.0

0.3 0.2 0.1 0 0

20

40

60 PV

80

100

120

284 285

Figure 3. Pb(II) breakthrough curves according to pH at three different values of kaolinite

286

content: Q = 20 cm3 / min, Ci ≈ 5 mg/L and IS ≈ 0 for all experiments.

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All experiments were conducted above the point of zero charge (Figure S6), which

289

resulted in negative values of zeta potential in all three experimental conditions (pH = 4, 5.6 and

290

9), with the zeta potential becoming more negative as pH increased. Consequently, the transport

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of Pb(II) was retarded at high pH due to the favorable adsorption of Pb species by immobile

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kaolinite colloids.

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The Pb(II) saturation is indicative of the percentage of occupied sorption sites on the

294

colloidal surfaces. At pH = 9 and kaolinite content equal to 1 % (Figure 5(b)), beta is larger than

295

one because the baseline calculation was based on qmax of the sand and the kaolinite in deionized

296

water at pH ~ 5.6, which indicated that the adsorption capacity of the bed substantially increased

297

as pH increased. Less variation of β and Pb(II) saturation in Figure 5(b) indicates that KC is not a

298

critical factor in the transport of lead at low pH.

299 300

3.2.4

Impact of ionic strength (IS)

301

The impact of ionic strength on the deposition of negatively charged colloids on the sand

302

medium has been well established in the literature, with generally more favorable attachment of

303

colloids with an increase in ionic strength32,39,40, as is predicted by Derjaguin-Landau-Verwey-

304

Overbeek (DLVO) theory. This behavior facilitates contaminant transport at relatively low ionic

305

strength due to the high repulsive force between colloids and sand grains at low ionic strength,

306

which results in mobile colloids with sorbed contaminants.25,41 However, for immobile colloids

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permanently retained in the sand bed (independent of ionic strength), retardation of Pb(II)

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transport increased as ionic strength decreased (Figure 4). This resulted from competitive

309

adsorption between Pb(II) and Ca2+ on the immobile kaolinite colloids. Increased sorption of

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Ca2+ ions on kaolinite resulted in less availability of adsorption sites for Pb(II), leading to earlier

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Pb(II) breakthrough (Figure 4), lower β, and lower Pb(II) saturation (Figure 5). 1 0.9 0.8 0.7 C / Ci

0.6 0.5 0.4

0.05 M 0.01 M 0.001 M ~ 0 M (DI)

0.3 0.2 0.1 0 0

20

40

60 PV

80

100

120

312 313

Figure 4. Pb(II) breakthrough curves at four different levels of ionic strength: Q = 20 cm3 / min,

314

Ci ≈ 5 mg/L, pH ≈ 5.6 and kaolinite content equal to 3 % for all experiments.

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0.4 β Pb(II) saturation Low Q High Q

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0.6

0.4

β Pb(II) Saturation

0.2

pH = 9.0 pH = 5.6 pH = 4.0

(a)

(b)

0

0

0

1

2

3 KC (%)

4

5

6

0

1

2

3

4

5

6

7

KC (%)

1 β

β or Pb(II) Saturation

0.8

IS ≈ 0 (DI water)

Pb(II) Saturation

0.6

0.4

0.2 (c) 0 -4

-3

-2

-1

log IS (M)

325 326

Figure 5. Calculated β and Pb(II) saturation as a function of (a) kaolinite content and flowrate, (b)

327

kaolinite content and pH, and (c) ionic strength. Note that Pb(II) saturation at pH = 9 and KC = 1%

328

is 2.2 (data not shown).

329 330

3.2.5

Optimization analysis

331

The result of optimization analysis for the first-order coefficients is presented in Table S3,

332

with the parameters used in the analysis summarized in Table S2. Because the algorithm used in

333

the optimization analysis automatically gives optimized coefficients after satisfying the stop

334

criteria, quantitative comparison of the breakthrough curve in each condition was not feasible in

335

the second scenario because both ka and kc are coefficients associated with the adsorption of

336

Pb(II), and they balance each other with different magnitude. Nevertheless, by restricting the 18 ACS Paragon Plus Environment

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range of coefficient, the ka / kd ratio in the first scenario allowed comparison of each

338

breakthrough curve, even though the same ka / kd did not necessarily give an identical result. The

339

ka / kd ratio was not sensitive to the flow rate, but it increased as ionic strength decreased and

340

kaolinite content increased. This was consistent with the experimental results presented in Figure

341

2, Figure 3 and Figure 4. Notably, an increase in ka / kd ratio along with an increase in kaolinite

342

content implied that an increase in immobile kaolinite not only increased the amount of adsorbed

343

Pb(II), but also increased adsorption rate, assuming the desorption rate was constant. The

344

optimization analysis of the experimental Pb(II) breakthrough curves might be translatable to

345

contaminant transport at large scales, based on the results obtained at the laboratory scale.

346 100

180 Q = 6 cm3 / s Q = 20 cm3 / s

80

160

pH = 4.0 pH ≈ 5.6 pH = 9.0

140 120 IS ≈ 0 M

100

ka / kd

ka / kd

60

IS ≈ 0.001 M 40

IS ≈ 0.01 M

80 60

IS ≈ 0.05 M

40

20

20

(a) 0

(b)

0 0

1

2

3 KC (%)

4

5

6

0

1

(a) Flow rate and IS

2

3 KC (%)

4

5

6

(b) pH

347

Figure 6. ka / kd ratio according to kaolinite content based on optimization analysis as a function

348

of (a) flow rate and ionic strength and (b) pH.

349 350

In summary, this experimental study investigated the transport behavior of Pb(II) in the

351

presence of immobile kaolinite colloids in a sand medium. The transport was measured as a

352

function of ionic strength, pH, and immobile kaolinite content. Experimental observation,

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calculated β and Pb(II) saturation, and optimized first-order coefficients revealed that the degree

354

of Pb(II) transport retardation can be varied as a function of kaolinite content, ionic strength, and

355

pH while the impact of flow rate was not significant. The results presented in this paper showed

356

only trace amount of (< 1%) immobile kaolinite colloids can result in significant retardation of

357

Pb(II) transport. Further investigation can be made under an unsaturated condition, transient flow

358

rate, and chemical perturbation to demonstrate and quantify the retardation of metal transport

359

under the presence of immobile clay colloids.

360 361

ASSOCIATED CONTENT

362

Supporting Information

363

Summary of Kaolinite and Sand Properties; the retention profile of kaolinite on ASTM-graded

364

sand; turbidity versus kaolinite suspension; turbidity and electrical conductivity measured during

365

injection of first 15 pore volumes; Pb(II) equilibrium isotherms; Pb(II) kinetic adsorption; zeta

366

potential of kaolinite as a function of pH; transport parameters used in optimization analysis;

367

results of optimization analysis; governing equations for colloid associated contaminant transport;

368

bed efficiency and Pb(II) saturation

369 370 371

AUTHOR INFORMATION

372

Corresponding Author

373

*Fax: (404) 894-2281; e-mail: [email protected]

374

Notes

375

The authors declare no competing financial interest.

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ACKNOWLEDGEMENT

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This material is based upon work supported by the Georgia Department of Transportation. Any

379

opinions, findings, and conclusions or recommendations expressed in this material are those of

380

the writers and do not necessarily reflect the views of the Georgia Department of Transportation.

381

Special thanks is given to Mr. J.D. Griffith for his contributions to the research project.

382 383

REFERENCES

384

(1)

Gao, G.; Fu, B.; Zhan, H.; Ma, Y. Contaminant transport in soil with depth-dependent

385

reaction coefficients and time-dependent boundary conditions. Water Res. 2013, 47 (7),

386

2507–2522.

387

(2)

Water Resour. Res. 1993, 29 (7), 2215–2226.

388 389

Corapcioglu, M. Y.; Jiang, S. Colloid-facilitated groundwater contaminant transport.

(3)

Grolimund, D.; Borkovec, M.; Barmettler, K.; Sticher, H. Colloid-Facilitated Transport of

390

Strongly Sorbing Contaminants in Natural Porous Media: A Laboratory Column Study.

391

Environ. Sci. Technol. 1996, 30 (10), 3118–3123.

392

(4)

Surfaces A Physicochem. Eng. Asp. 1996, 107 (95), 1–56.

393 394

Ryan, J. N.; Elimelech, M. Colloid mobilization and transport in groundwater. Colloids

(5)

Rivadeneyra, M. A.; Delgado, G.; Ramos Cormenzana, A.; Delgado, R. Biomineralization

395

of carbonates by Halomonas eurihulina in solid and liquid media with different salinities:

396

crystal formation sequence. R. Res. Microbiol. 1998, 149, 277–287.

397 398

(6)

Hammes, J.; Gallego-Urrea, J. A.; Hassellöv, M.; Arab, D.; Pourafshary, P.; Ayatollahi, S.; Habibi, A. Geographically distributed classification of surface water chemical parameters 21 ACS Paragon Plus Environment

Environmental Science & Technology

399

influencing fate and behavior of nanoparticles and colloid facilitated contaminant

400

transport. Int. J. Environ. Sci. Technol. 2013, 11 (14), 5350–5361.

401

(7)

Arab, D.; Pourafshary, P.; Ayatollahi, S.; Habibi, A. Remediation of colloid-facilitated

402

contaminant transport in saturated porous media treated by nanoparticles. Int. J. Environ.

403

Sci. Technol. 2014, 11 (1), 207–216.

404

(8)

of Contaminants in Porous Media. Environ. Sci. Technol. 1997, 31 (3), 656–664.

405 406

(9)

Jensen, D. L.; Ledin, A.; Christensen, T. H. Speciation of heavy metals in landfill-leachate polluted groundwater. Water Res. 1999, 33 (11), 2642–2650.

407 408

Roy, S. B.; Dzombak, D. A. Chemical Factors Influencing Colloid-Facilitated Transport

(10)

Denaix, L.; Semlali, R. M.; Douay, F. Dissolved and colloidal transport of Cd, Pb, and Zn

409

in a silt loam soil affected by atmospheric industrial deposition. Environ. Pollut. 2001,

410

114 (1), 29–38.

411

(11)

Hanford Sediment and Ottawa Sand. Environ. Sci. Technol. 2003, 37 (21), 4905–4911.

412 413

(12)

(13)

Syngouna, V. I.; Chrysikopoulos, C. V. Cotransport of clay colloids and viruses in water saturated porous media. Colloids Surfaces A Physicochem. Eng. Asp. 2013, 416 (1), 56–65.

416 417

Kretzschmar, R.; Borkovec, M.; Grolimund, D.; Elimelech, M. Mobile Subsurface Colloids and Their Role in Contaminant Transport; 1999; 66, pp 121–193.

414 415

Zhuang, J.; Flury, M.; Jin, Y. Colloid-Facilitated Cs Transport through Water-Saturated

(14)

Chen, H.; Gao, B.; Yang, L. Y.; Ma, L. Q. Montmorillonite enhanced ciprofloxacin

418

transport in saturated porous media with sorbed ciprofloxacin showing antibiotic activity.

419

J. Contam. Hydrol. 2015, 173, 1–7.

420 421

(15)

Porubcan, A. A.; Xu, S. Colloid straining within saturated heterogeneous porous media. Water Res. 2011, 45 (4), 1796–1806.

22 ACS Paragon Plus Environment

Page 22 of 25

Page 23 of 25

422

Environmental Science & Technology

(16)

Torkzaban, S.; Bradford, S. A.; van Genuchten, M. T.; Walker, S. L. Colloid transport in

423

unsaturated porous media: The role of water content and ionic strength on particle

424

straining. J. Contam. Hydrol. 2008, 96 (1–4), 113–127.

425

(17)

Sang, W.; Morales, V. L.; Zhang, W.; Stoof, C. R.; Gao, B.; Schatz, A. L.; Zhang, Y.;

426

Steenhuis, T. S. Quantification of Colloid Retention and Release by Straining and Energy

427

Minima in Variably Saturated Porous Media. Environ. Sci. Technol. 2013, 47 (15),

428

130724151622003.

429

(18)

Sun, Y.; Gao, B.; Bradford, S. A.; Wu, L.; Chen, H.; Shi, X.; Wu, J. Transport, retention,

430

and size perturbation of graphene oxide in saturated porous media: Effects of input

431

concentration and grain size. Water Res. 2015, 68, 24–33.

432

(19)

Boussen, S.; Soubrand, M.; Bril, H.; Ouerfelli, K.; Abdeljaouad, S. Transfer of lead, zinc

433

and cadmium from mine tailings to wheat (Triticum aestivum) in carbonated

434

Mediterranean (Northern Tunisia) soils. Geoderma 2013, 192 (1), 227–236.

435

(20)

Zhang, M.-K.; Liu, Z.-Y.; Wang, H. Use of Single Extraction Methods to Predict

436

Bioavailability of Heavy Metals in Polluted Soils to Rice. Commun. Soil Sci. Plant Anal.

437

2010, 41 (7), 820–831.

438

(21)

Wuana, R. A.; Okieimen, F. E. Heavy Metals in Contaminated Soils: A Review of

439

Sources, Chemistry, Risks and Best Available Strategies for Remediation. ISRN Ecol.

440

2011, 2011, 1–20.

441

(22)

and relevance. Can. Geotech. J. 2002, 39 (1), 233–241.

442 443 444

Santamarina, J. C.; Klein, K. A.; Wang, Y. H.; Prencke, E. Specific surface: determination

(23)

Langmuir, I. THE ADSORPTION OF GASES ON PLANE SURFACES OF GLASS, MICA AND PLATINUM. J. Am. Chem. Soc. 1918, 40 (9), 1361–1403.

23 ACS Paragon Plus Environment

Environmental Science & Technology

445

(24)

fluoride on activated alumina. Sep. Purif. Technol. 2005, 42 (3), 265–271.

446 447

(25)

Saiers, J. E.; Hornberger, G. M. The Role of Colloidal Kaolinite in the Transport of Cesium through Laboratory Sand Columns. Water Resour. Res. 1996, 32 (1), 33–41.

448 449

Ghorai, S.; Pant, K. K. Equilibrium, kinetics and breakthrough studies for adsorption of

(26)

Yin, X.; Gao, B.; Ma, L. Q.; Saha, U. K.; Sun, H.; Wang, G. Colloid-facilitated Pb

450

transport in two shooting-range soils in Florida. J. Hazard. Mater. 2010, 177 (1–3), 620–

451

625.

452

(27)

Riotte, J.; Chabaux, F.; Benedetti, M.; Dia, A.; Gérard, M.; Boulègue, J.; Etamé, J.

453

Uranium colloidal transport and origin of the 234U–238U fractionation in surface waters:

454

new insights from Mount Cameroon. Chem. Geol. 2003, 202 (3–4), 365–381.

455

(28)

and particle transport in soil. J. Contam. Hydrol. 1990, 6 (2), 185–203.

456 457

Torok, J.; Buckley, L. .; Woods, B. . The separation of radionuclide migration by solution

(29)

Sheppard, J. C.; Campbell, M. J.; Cheng, T.; Kittrick, J. A. Retention of radionuclides by

458

mobile humic compounds and soil particles. Environ. Sci. Technol. 1980, 14 (11), 1349–

459

1353.

460

(30)

Camesano, T. A.; Logan, B. E. Influence of Fluid Velocity and Cell Concentration on the

461

Transport of Motile and Nonmotile Bacteria in Porous Media. Environ. Sci. Technol. 1998,

462

32 (11), 1699–1708.

463

(31)

porous media under steady flow conditions. Environ. Sci. Technol. 1992, 26 (3), 586–593.

464 465

McDowell-Boyer, L. M. Chemical mobilization of micron-sized particles in saturated

(32)

Bradford, S. A.; Torkzaban, S.; Walker, S. L. Coupling of physical and chemical

466

mechanisms of colloid straining in saturated porous media. Water Res. 2007, 41 (13),

467

3012–3024.

24 ACS Paragon Plus Environment

Page 24 of 25

Page 25 of 25

468

Environmental Science & Technology

(33)

Inc.: Hoboken, NJ., 2005.

469 470

(34)

(35)

(36)

(37) Harter, R. D. Effect of Soil pH on Adsorption of Lead, Copper, Zinc, and Nickel1. Soil Sci. Soc. Am. J. 1983, 47 (1), 47.

477 478

Farrah, H.; Pickering, W. F. pH effects in the adsorption of heavy metal ions by clays. Chem. Geol. 1979, 25 (4), 317–326.

475 476

Benjamin, M. M.; Leckie, J. O. Competitive adsorption of cd, cu, zn, and pb on amorphous iron oxyhydroxide. J. Colloid Interface Sci. 1981, 83 (2), 410–419.

473 474

Gu, X.; Evans, L. J. Surface complexation modelling of Cd(II), Cu(II), Ni(II), Pb(II) and Zn(II) adsorption onto kaolinite. Geochim. Cosmochim. Acta 2008, 72 (2), 267–276.

471 472

Mitchell, J. K.; Soga, K. Fundamentals of soil behavior, 3rd ed.; John Wiley and Sons,

(38)

Abollino, O.; Aceto, M.; Malandrino, M.; Sarzanini, C.; Mentasti, E. Adsorption of heavy

479

metals on Na-montmorillonite. Effect of pH and organic substances. Water Res. 2003, 37

480

(7), 1619–1627.

481

(39)

Ryan, J. N.; Gschwend, P. M. Effects of Ionic Strength and Flow Rate on Colloid Release:

482

Relating Kinetics to Intersurface Potential Energy. J. Colloid Interface Sci. 1994, 164 (1),

483

21–34.

484

(40)

Compère, F.; Porel, G.; Delay, F. Transport and retention of clay particles in saturated

485

porous media. Influence of ionic strength and pore velocity. J. Contam. Hydrol. 2001, 49

486

(1–2), 1–21.

487 488

(41)

Um, W.; Papelis, C. Geochemical effects on colloid-facilitated metal transport through zeolitized tuffs from the Nevada Test Site. Environ. Geol. 2002, 43 (1–2), 209–218.

489

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