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

3

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

4

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

5

• 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

7

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

8

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

9

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

10

• 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

11

• 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

13