IntroductionIron sulfides, in particular, troilite, pyrite and the less crystalline mackinawite have demonstrated effective uptake of As(III) in batch solutions. This type of behavior is promising for applications as a sequestration agent in arsenic remediation designs such as above-ground adsorption units as well as in-situ subsurface permeable barriers. Direct application, however, in such flow-through systems is dependent not only upon ability to synthesize the mackinawite on a large scale industrial level but also the ability to produce the mackinawite at various range of particle sizes.

Synthesis and Characterization of Mackinawite Coated SandTanya J. Gallegos1, Kim F. Hayes1, Linda M. Abriola2

1University of Michigan, 2Tufts University

Conclusion

FeS is effectively coated onto iron oxide coated sand.

Methods of Synthesis

Further StudyFTIR examination to determine surface species

ObjectiveThe purpose of this study was to develop a method for emplacing a mackinawite coating onto the surface of clean quartz sand for arsenic uptake.

Hypotheses1. Physical stabilization of FeS coating onto SiO2

is expected to also occur upon aging via temperature and time.

2. of iron oxide coatings to a FeS coating is expected due to reduction with Na2S.

3. FeS will electrostaticlly coat the layer of iron oxide coating SiO2

SEM

Mackinawite Coated Sand: Fe Extraction

0

0.000005

0.00001

0.000015

0.00002

0.000025

MCST MCSH MFCS

Coated Sand Type

mo

l Fe/

g s

and Rep#1

Rep#2

Total Iron Extraction

pH 6 Isotherm for MFCSICPMS vs. OES

0

200

400

600

800

1000

1200

0 200 400 600 800 1000

As(III)-ICPMS

Tot As - OES

MFCS [As(V)]f (ppb)-ICPMS

Isotherm pH 6: Pure Mackinawite vs. MFCS

0

200

400

600

800

1000

1200

1400

0 200 400 600 800 1000

[As(III)]f (ppb)

[As(

III)]a

ds (p

pb)

As(III)-Pure FeS

As(III)-MFCS

As(V)-MFCS

[As(V)] Pure FeS

Comparison to Pure Mackinawite

Acid-Base Resistivity

0%

20%

40%

60%

80%

100%

120%

3 3 3 10.8 10.8 10.8 4 4pH

% F

e D

isso

lutio

n

MFCS

MCST

MCSH

Acid/Base Resistivity

XRD Analysis Peaks: d-spacings of 5.05, 2.31 and 1.81.

Arsenic Redox Sensitivity

Results Three methods produced coatings:

MFCS- ferrihydrite coated sands /FeS/27C/2 days

MCSH -clean quartz sand/FeS/70C/24 hourMCST-clean quartz sand/FeS/27C/20day

MFCS MCSHMCST

Characterization Results of these coated sandsare provided below

Implications The ability to produce mackinawite as a coating for sands provides for a promising application in both in-situ and ex-situ processes. The abundance of sands and availability of a wide range of particle sizes that may be implemented in remediation process such as permeable reactive barriers as well as above-ground adsorption treatment units..

Arsenic ReactivityArsenic (III) in Solution

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 20 40 60 80Solid:Solution (g/L)

[As

(III)

] (p

pm

) MFCS#1

MFCS#2

MCSH#1

MCSH#2

MCST#1

MCST#2

IntroductionMackinawite coated sand is under consideration as a reactive porous media for use in flow-through remediation schemes. Effective application of this reactive porous media in permeable reactive barriers will rely on reactive transport models which accurately describe sorption chemistry which relies on an accurate depiction of FeS acid-base chemistry. Contaminants such as cadmium, arsenic and mercury have been show to be sequestered by FeS. This sequestration is through to be due to surface complexation.

In fact, spectroscopic studies by others have confirmed the presence of hydroxyl (Me≡OH) and sulfhydryl (S≡H) surface functional groups on the surface of generic metal sulfides. These amphoteric reactive units are thought to undergo independent protonation and deprotonation reactions to produce reactive sites for sorption.

Acid-Base Properties of Mackinawite Coated SandTanya J. Gallegos1, Kim F. Hayes1, Linda M. Abriola2

1University of Michigan, 2Tufts University

MFCS pH Edge, IS varies

0

5

10

15

20

25

0 2 4 6 8 10 12pH

Fe

mg/

L

0.1 M NaCl

0.01 M NaCl

0.001 M NaCl

Protonation: HOFe≡SH0 + H+ = H2OFe≡SH+1 K+=6

Deprotonation: HOFe≡SH0 = OFe≡SH-1 + H+ K-= -6.7

Ion Exchange: H2OFe≡SH+1 + H+ = H≡SH+ Fe2+ + H2O Kix=8.9

ConclusionResults

Further Study• Measurement of total dissolved S2- as a function of pH

• EXAFS examination of the FeS coated sand at pH 4, 7, 8, 10 to confirm the presence of the proposed surface species.

.

ObjectiveThe purpose of this study is to complete the first step in developing a reactive model that describes mackinawite chemistry by assessing the acid base properties of the mackinawite surface.

Experimental• Development of probable surface species and conceptual model

• Measurement of BET Surface Area• Potentiometric titration of the mackinawite surface at various ionic strengths

• Solubility measurements • FITEQL fitting of acidic potentiometric data to a conceptual model resulting in protonation and deprotonation constants.

BET Surface Area is 0.149 m2/g

Other Model Parameters Value

Specific Surface Area (m2/g) 0.149

Total Number of Surface Sites (M)

=Stot= (OFe≡SH-)+ (H≡SH0)+ (H2OFe≡SH+)+ (HOFe≡SH0)=3.1-4 M

Capacitance (F/m2) 20

PZC 6.67

F (error) 119

Titration Data Fit

Modeled Predominance Diagram

Conceptual Model

Modeled Reactions

Dissolved Iron Measurements

Titration Data

Surface Species of Mackinawite Coated Sand

-4.5

-3.5

-10 -9 -8 -7 -6 -5 -4 -3

pH

pC (

M)

H=SH

Fe2+(model)

OFe=SH-

OH2Fe=SH[+

OHFe=SH

Fe2+(measured)

2 3 4 5 6 6.67 7 8 9 10

pH

Se

e

S Bulk

Fe

Fe bulk

H

O-

S

e

S Bulk

Fe

Fe bulk H+

OH2

S

e

S Bulk

H

Fe bulk

H0

H2O + Fe2+

H2O + Fe2+ + H2S (aq)

Charge on surface

0 0 0 0 + + 0 0 0 0 0 - -

Se

e

S Bulk

Fe

Fe bulk

H0

OH

Batch Titration: Mackinawite and Mackinawite Coated Sand

2

3

4

5

6

7

8

9

10

11

-0.0020 -0.0015 -0.0010 -0.0005 0.0000 0.0005 0.0010 0.0015 0.0020

Concentration of Acid (M)

pH

Blank pHo I=.1

MFCS 0.1 M NaCl

MFCS 0.001 M NaCl

FeS 0.1 M NaCl

FeS .001 M NaCl

Comparison to Pure FeS

Implications

These species may form complexes in solution for any number of contaminants providing for effective sequestration.

Batch Titration: Mackinawite Coated Sand

2

4

6

8

10

-0.0010 -0.0005 0.0000 0.0005 0.0010Concentration of Acid (M)

pH

Blank pHo I=.001Blank pHo I=.01Blank pHo I=.10.001 M NaCl0.01 M NaCl0.1 M NaClA-pH 0.001 M NaClA-pH 0.01 M NaClA-pH 0.1 M NaCl

Amount of acid taken up by

solid~ 3.3e-4 M

Sorption of Arsenite Onto Mackinawite Coated SandTanya J. Gallegos1, Kim F. Hayes1, Linda M. Abriola2

1University of Michigan, 2Tufts University

Results• Equilibrium in the FeS/H3AsO3 system is reached within 2

hours. • Arsenite sorption onto mackinawite coated sand increases

with increasing pH reaching maximum removal at pH 7.5. • Between pH 3-7.5, [Fe2+] in solution increases as pH

decreases approaching the total Fe2+ in the coating at pH 3.• [Fe2+] in solution increases as ionic strength decreases.• Arsenite removal is accompanied by a decrease of Fe2+ in

solution.• At low As(III) concentrations, removal is Langmuirian in

nature. Arsenite sorption abruptly converts to linear behavior at high concentrations, possibly attributed to formation of surface precipitates.

• At pH 10, As3+ removal is linear whereas, it is Langmuirian at pH 6 and 8 suggesting a different removal mechanism.

• Increases in ionic strength enhance the removal of arsenite from solution, suggesting a possible inner-sphere surface complexation removal mechanisms.

Further Study• Development of probable surface species and data fitting of conceptual

reactions with pH edge data are expected to provide appropriate surface complexation equilibrium constants

• These constants will result in a complete surface complexation model which can be further used to predict isotherms and subsequently can be used as input into a reactive transport model.

Introduction Arsenic contamination of groundwater is a widespread problem affecting aquifers in the United States, as well as abroad. Recent strengthening of the US EPA MCL for arsenic has prompted the need for technology capable of removing both arsenite and arsenate from solution. Arsenite, the more toxic form of arsenic, is more difficult to remove from anoxic zones in the subsurface. Studies by others (Bostick and Fendorf, 2003; Farquhar et al., 2002; Wolthers et al., 2003, Zouboulis et al., 1993) have demonstrated the affinity of some types of iron sulfides (such as troilite, pyrite, amorphous iron sulfide and mackinawite) for arsenite. However, these studies have not provided a comprehensive investigation of the macroscopic behavior of arsenite in the presence of crystalline mackinawite in a form that can be readily applied to real-world treatment technologies. This study examines the behavior of arsenite in the presence of mackinawite coated sand (MCS).

ObjectiveIt is the purpose of this study to characterize the macroscopic behavior of MCS in the presence of H3AsO3 with respect to variations in time, ionic strength, pH and initial total arsenite concentrations. This information will be used to determine likely surface complexation reactions that occur between the arsenite species and surface functional groups thought to exist in the pH ranges as shown in the following figure. The acid-base reactions used to develop the arsenite surface complexes are consistent with those used to model complexation of anionic collectors in galena (PbS) and sphaelerite (ZnS) flotation systems (Sun et al., 1991).

MFCS Sorption Kinetic Study

0%

10%

20%

30%

40%

50%

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

time (h)

% A

s(III

) so

rbed

MFCSRep. #1MFCSRep. #2

Kinetic Study

Isotherms

0

0.0001

0.0002

0.0003

0.0004

0.0005

0.0006

0.0007

0.0008

0.0009

0.001

0 500 1000 1500 2000

[As]f (ppb)

[As]

ads

(mol

/kg)

pH 6 Rep#1

pH 6 Rep#2

pH 8 Rep#1

pH 8 Rep#2

pH 10 Rep#1

pH 10 rep#2

Isotherm

pH 10 isotherm pH 8 isotherm

2 3 4 5 6 6.67 7 8 9 10

pH

Se

e

S Bulk

Fe

Fe bulk

H

O-

S

e

S Bulk

Fe

Fe bulk H+

OH2

S

e

S Bulk

H

Fe bulk

H0

H2O + Fe2+

H2O + Fe2+ + H2S (aq)

Charge on surface

0 0 0 0 + + 0 0 0 0 0 - -

Se

e

S Bulk

Fe

Fe bulk

H0

OH

Probable Surface Species

Iron in Solution

0

0.0001

0.0002

0.0003

0.0004

0.0005

0.0006

3 4 5 6 7 8 9 10 11pHf

Fe

mg/

L

0.1 M NaCl

0.01 M NaCl

0.001 M NaCl

Fe(FeS)

Fetotal

pH Edge

0

10

20

30

40

50

60

0 2 4 6 8 10 12pHf

%A

s A

ds

0.1 M NaCl

0.01 M NaCl

0.001 M NaCl

Implications

This study clearly demonstrates that mackinawite coated sands have a capacity for reducing dissolved arsenite concentrations in solution over pH ranging from 3-10.5 most notably in circumneutral pH regions. Probable AsO3

3-/S/Fe surface complexes and reactions that may take place in a FeS(s)/AsO3

3- system can be postulated to be consistent with observations of the macroscopic behavior with regard to pH and As(III) concentration of the system as well as with spectroscopic studies that suggest the formation of a FeSAs-like surface precipitate as reported by others (Bostick and Fendorf, 2003; Farquhar et al., 2002).

References• Bostick, B. C. and S. Fendorf (2003). "Arsenite sorption on troilite (FeS) and pyrite

(FeS2)." Geochimica Et Cosmochimica Acta 67(5): 909-921.• Farquhar, M. L., J. M. Charnock, et al. (2002). "Mechanisms of arsenic uptake from

aqueous solution by interaction with goethite, lepidocrocite, mackinawite, and pyrite: An X-ray absorption spectroscopy study." Environmental Science & Technology 36(8): 1757-1762.

• Sun, Z. X., W. Forsling, et al. (1991). "Surface-Reactions in Aqueous Metal Sulfide Systems .3. Ion-Exchange and Acid-Base Properties of Hydrous Lead Sulfide." Colloids and Surfaces 59: 243-254.

• Wolthers, M., L. Charlet, et al. (2003). "Arsenic sorption onto disordered mackinawite as a control on the mobility of arsenic in the ambient sulphidic environment." Journal De Physique Iv 107: 1377-1380.

• Zouboulis, A. I., K. A. Kydros, et al. (1993). "Arsenic(Iii) and Arsenic(V) Removal from Solutions by Pyrite Fines." Separation Science and Technology 28(15-16): 2449-2463.

ExperimentalBatch Experiments consisted of 40 g/L mackinawite coated sand in 15 mL reactors. Sand exhibited a surface area of 0.149 m2/g and the precipitate used to synthesize the coating was characterized by XRD as mackinawite. Batch experiments run were:

•Arsenite Sorption Edge at ionic strength 0.001 M, 0.01 M and 0.1 M NaCl background electrolyte •Arsenite Sorption Isotherms at pH 6, 8, 10 with initial As concentration of 1000 ppb As at 0.01 M NaCl•Fe Dissolution at ionic strength 0.001 M, 0.01 M and 0.1 M NaCl •Kinetic Sorption experiment at 0.01 M NaCl