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Fluoride Removal in Small Water Systems: A Coagulation Approach
Desmond F. Lawler, Lynn E. Katz,
Katherine A. Alfredo, and Mark L. Stehouwer
Fluoride in Water
1945 - Fluoridation of drinking water begins in Grand Rapids, MI because of associated dental benefits
2
1986 – Fluoride MCL set at 4.0mg/L and secondary MCL set at 2.0mg/L
1974 – Fluoride identified by EPA as water contaminant through the SDWA
2006 – National Academies of Science review on fluoride data
2011 – HHS and EPA finalize risk and exposure assessments for fluoride
McGrady et al. BioMed Central Oral Health Journal, 2012.
Research Objectives
1. Develop understanding of interactions among fluoride, organic ligands, and aluminum during coagulation process
2. Apply this understanding to waters containing NOM and further develop a set of treatment guidelines
3. Conduct pilot tests to validate guidelines
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Fluoride Removal by Alum Coagulation
• Alum coagulation is an effective treatment process to remove fluoride
• Fluoride removal is negatively impacted by the presence of organic ligands during coagulation
• Fluoride is anticipated to adsorb to aluminum precipitate surface or be incorporated into its structure
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Impact of organic acids on the removal of fluoride at pH 6.5.
0
20
40
60
80
100
0 100 200 300 400 500
Pe
rce
nt
Flu
oride
Re
mo
ve
d (
%)
Alum Dose (mg/L)
FluorideFluoride and Salicylic AcidFluoride and Pyromellitic AcidFluoride and Phthalic Acid
Titrations of Precipitates and the Impact of Fluoride
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• Dashed line indicates >95% aluminum precipitation. • Differences in titration curves above this line indicate changes within the
precipitate and suggest incorporation of fluoride into the precipitate structure.
0
20
40
60
80
100
2 3 4 5 6 7 8 9
0 mg/L F5 mg/L F10 mg/L F
% A
lum
inum
Pre
cip
itate
d
Final pH
50 mg/L Alum Dose
0
2
4
6
8
10
12
0 2 4 6 8 10 12 14 16
0 mg/L F5 mg/L F10 mg/L F
pH
[NaOH] (mMol added)
50 mg/L Alum Dose
Imaging of Precipitates
6
• SEM imaging of aluminum hydroxide precipitates reveals impact of fluoride on precipitate structure • Smaller precipitate sizes indicate fluoride incorporation into the precipitate structure. • XRD analysis confirms that precipitates are amorphous (non-distinguishable peaks)
Aluminum precipitates
Aluminum precipitates with fluoride
XRD analysis of precipitates from 200mg/L alum dose
Organic Removal by Alum Coagulation
• Alum coagulation is an effective treatment process to remove organic ligands
• Organic ligand removal is negatively impacted by the presence of fluoride during coagulation
• Differences in organic ligand removal are expected to be the result of differences in their functionality
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Impact of fluoride on the removal of organic ligands at pH 6.5.
0
20
40
60
80
100
0 100 200 300 400 500
Salicylic Acid
Phthalic Acid
Pyromellitic Acid
Salicylic Acid and Fluoride
Phthalic Acid and Fluoride
Pyromellitic Acid and Fluoride
LM
W O
rga
nic
Rem
ova
l (%
)
Alum Dose (mg/L)
Ligand Interactions and Coagulation
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Proposed complexation models for organics and aluminum precipitates
Inner Sphere
Inner Sphere
Outer Sphere
Outer Sphere
• Complexation of organics is a function of acidity of functional groups and bonding mechanism
• Inner-sphere sorption can lead to ligand-promoted dissolution while outer sphere adsorption prevents dissolution
Organic Acid Proposed Adsorption Complex
pKa’s
Salicylic Acid Inner Sphere 2.88 13.56
Phthalic Acid Outer Sphere 2.87 5.23
Pyromellitic Acid
Inner Sphere 1.52 2.95 4.65 5.89
Ionic Strength Effects on Organics Removal
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0%
5%
10%
15%
20%
25%
30%
0 100 200 300 400 500 600
% O
rgan
ic R
emo
ved
Alum Dose (mg/L)
i=0.017
i=0.06
i=0.5
0%
5%
10%
15%
20%
25%
30%
0 100 200 300 400 500 600 %
Org
anic
Rem
ove
d
Alum Dose (mg/L)
i=0.017
i=0.06
i=0.5
Phthalic Acid Salacylic Acid
• Inner sphere complexes are not very dependent on ionic strength (Salacylic Acid)
• Outer sphere complexes are quite dependent on ionic strength (Phthalic Acid)
Removal of ligands by Co-precipitaton and Adsorption
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• Jar tests were conducted such that either co-precipitation or adsorption of ligands onto pre-formed floc occurred
• Co-precipitation improved removals of both fluoride and organics ligands
NOM Experimental Results
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• Negative impacts on NOM removal associated with the presence of fluoride were especially observed at the lower doses of alum (0 – 100 mg/L)
• The presence of 5 mg/L F doubled the alum dose required for adequate NOM and turbidity removal and also made aluminum more soluble; the presence of NOM diminished F removal at comparable alum doses
0
20
40
60
80
100
0 100 200 300 400 500
5 mg/L DOC5 mg/L DOC, 5 mg/L F
NO
M R
em
ova
l (%
)
Alum Dose (mg/L)
0
20
40
60
80
100
0 100 200 300 400 500
5 mg/L F5 mg/L DOC, 5 mg/L F
Flu
ori
de R
em
oval (%
)Alum Dose (mg/L)
Future Work
• Confirming results with field waters that have high fluoride and reasonable NOM concentrations
• Objective 3 – Conduct pilot tests to further develop a treatment model and guidelines
12
Acknowledgements
The United States Environmental Protection Agency for providing funding through a STAR grant for the Research and Demonstration of Innovative Drinking Water Treatment Technologies in Small Systems (2011)
The University of Texas at Austin for providing general support, facilities, and equipment for this research
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