Water Research Foundation
Webcast
April 29,2010
Managing Lead in Pipe Scales:
Understanding How Water Quality
Affects Lead Release
1
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3
Managing Lead in Pipe Scales:
Understanding How Water Quality Affects
Lead Release
Daniel Giammar
Department of Energy, Environmental, and Chemical Engineering
Washington University in St. Louis
April 29, 2010
j
Advisory
y Committee Presentation
Project
Contributing Researchers
Acknowledgments
• Kate Nelson, James Noel, Yin Wang, and
Yanjiao Xie
• Tyler Nading, Nevin Peeples, Vidhi Singhal,
Biplab Mukherjee
• Lee Harms, Mike Schock, and Windsor Sung
• Water Research Foundation
• Traci Case, Jeff Bickel, David
Hokanson, and Heath Lloyd
• Washington University School of
Engineering and Applied Science
Project 4064: Influence of Water Chemistry on the Dissolution
and Transformation Rates of Lead Corrosion Products
1
Lead Phases in Lead Service Lines
Pb(II) ÆPb(IV)
OCl-
ClParticulate
CO3 2-, PO43-
CO3 2-, PO43-, Cl-
Pb 2+
Pb2+
Pb(IV)O2, Pb3(CO3)2OH2, PbCO3, Pb5(PO4)3OH
Lead Pipe Pb(0)
• Lead pipe
(Pb0)
develops scales of corrosion products.
• The products formed (oxides, carbonates, and phosphates) depend
on the water chemistry.
• Changes in distribution system water chemistry can destabilize
corrosion products in premise plumbing.
• Optimization of water chemistry can mitigate lead release to solution.
2
Potential Lead Release from Service Lines
• Lead concentrations are affected by current and past
water chemistry.
– generally decrease with increasing pH and alkalinity
– history of chlorination versus chloramination influences corrosion products
• Lead concentrations may be controlled by either
equilibrium solubility or dissolution rates.
– equilibrium models are often poor predictors of lead concentrations
(Edwards et al., 1999)
– equilibrium versus kinetic control can depend on flow regime
• Water chemistry can guide selection of corrosion control.
– adjustment of pH and alkalinity
– addition of orthophosphate
– choice of residual disinfectant and concentration
3
1
Factors Affecting Lead Concentrations
-2
-3
Lo
og[Pb] diss (M)
Litharge (PbO)
-4
Hydrocerussite
(Pb3(CO3)2(OH)2)
with 50 mg/L DIC
-5
-6
Hydroxylpyromorphite
(Pb5(PO4)3OH)
with 1 mg/L P
15 ug/L
/L Pb
-7
-8
-9
4
5
6
7
8
9
10
11
pH
• Water chemistry influenced by pH, DIC, disinfectant, orthophosphate, NOM.
4
Factors Affecting Lead Concentrations
-2
-4
-3
Lo
og[Pb]
(M)
Lo
og[Pb] diss
diss (M)
Litharge (PbO)
-6
-4
15 ug/L Pb
Hydrocerussite
(Pb3(CO3)2(OH)2)
with 50 mg/L DIC
-8
-5
-10
-6
Hydroxylpyromorphite
(Pb5(PO4)3OH)
with 1 mg/L P
15 ug/L
/L Pb
-12
-7
-14
-8
Plattnerite (PbO2)
-16
-9
4
5
6
7
8
9
10
11
pH
• Water chemistry influenced by pH, DIC, disinfectant, orthophosphate, NOM.
5
Speciation of Lead in Drinking Water
1.6
monochloramine
0.8
ƒ Lead pipe oxidizes to form lead(II)
and possibly lead(IV) species.
O2
Pb(OH)4 2-
PbHCO3 +
0.0
Pb(CO3 )2 2-
PbCO3(aq)
Pb2+
0.4
EH (V)
PbO 2(s)
PbO3 2-
free chlorine
1.2
Pb(OH)2(aq)
Pb(s)
-0.8
-1.2
0
2
4
6
pH
8
10
Pb(OH) 3-
-0.4
12
14
• Total Pb = 15 μg/L
• Dissolved inorganic carbon = 30 mg C/L
6
2
Speciation of Lead in Drinking Water
1.6
monochloramine
0.8
ƒ Lead pipe oxidizes to form lead(II)
and possibly lead(IV) species.
O2
ƒ Switching from free chlorine to
chloramines lowers the oxidationreduction potential sufficiently to
dissolve PbO2 in pipe scales.
Pb(OH)4 2-
PbHCO3 +
0.0
Pb(CO3 )2 2-
PbCO3(aq)
Pb2+
0.4
EH (V)
PbO 2(s)
PbO3 2-
free chlorine
1.2
-0.4
-1.2
0
2
4
6
pH
8
ƒ The solubility of lead will then be
controlled by a lead(II) solid
Pb(OH) 3-
Pb(OH)2(aq)
Pb(s)
-0.8
10
12
14
• Total Pb = 15 μg/L
• Dissolved inorganic carbon = 30 mg C/L
7
Outline
• Overview of Lead in Drinking Water
• Dissolution Rates of Lead Corrosion Products
•
•
•
•
research approach
hydrocerussite: impact of orthophosphate
hydroxylpyromorphite: impact of pH
plattnerite: impact of dissolved inorganic carbon and
chlorine species
• Lead Release from Pipe Sections
•
•
effects of water chemistry
impacts of water flow and stagnation time
• Conclusions and Recommendations
8
Project Objectives and Research Approach
1.
Summarize the state-of-knowledge regarding dissolution rates of
lead corrosion products.
2.
Determine dissolution and transformation rates of lead corrosion
products as a function of water chemistry.
3.
Evaluate lead release rates from corrosion products on lead pipes.
Task 1: Literature Review
dissolution
rate laws
Lead Corrosion Products
• hydrocerussite,
y
, hydroxypyromorphite,
y
ypy
p
, lead(IV)
( ) oxide
• influence of pH, DIC, orthophosphate
Task 2: Dissolution Rate Quantification
Water Quality Analyses
• dissolved Pb
• pH, DIC, orthophosphate
apply rates to real
pipe scales
Task 3: Lead Release
from Pipe Scales
Solid Phase Characterization
• XRD: mineralogy
• FTIR: molecular structure
• SEM: morphology
• BET: surface area
9
3
Lead Corrosion Products Studied
Hydrocerussite
Pb3(CO3)2OH(2)
Plattnerite
PbO2
Hydroxylpyromorphite
Pb5(PO4)3OH
0.5 µm
5.0 µm
4.8 m2/g
0.5 µm
3.6 m2/g
16 m2/g
• Identities confirmed by X-ray diffraction.
• Hydrocerussite and hydroxylpyromorphite were synthesized and plattnerite
was purchased from a commercial supplier.
10
Solid Phases in Lead Pipe Scales
Pipes conditioned for 8 months at pH 10
with 3.5 mg/L Cl2 and 10 mg/L DIC.
H
Rela
ative Intensity
H
H
H L
Pb H
L
H
H
H H S
S
S P
Pb P
H
L
H
P
H
3-month
S
8-month
Scrutinyite (PbO2)
Plattnerite (PbO2)
Hydrocerussite
Litharge (PbO)
Lead
20
25
30
35
40
45
50
55
60
2θ (°)
• In chlorinated water at moderate alkalinity, pipe scales develop
that include lead(II) oxide, lead(II) carbonate, and lead(IV) oxides.
• Lead(IV) oxides only formed after 8 months of conditioning.
11
Formation of Hydrocerussite:
Lead Service Line Pipe Sections
H
H
C
H
H
C
• X-ray diffraction of scales on pipes
H
• Lead service lines from an MWRA
customer community
• 80-100 years of service
Pb
H = hydrocerussite, Pb3(CO3)2(OH)2
C = cerussite,
it PbCO3
Pb = lead, Pb(0)
• Pipe scales comprised of the lead carbonates hydrocerussite and cerussite.
• Formation of these phases is consistent with the system’s water chemistry.
12
4
Water Compositions Investigated
• pH: 7.5, 8.5, 10.0
• dissolved inorganic carbon: 0, 10, 50 mg-C/L
• dissolved orthophosphate: 0, 1, 4 mg-P/L
• chloramines: 0, 2.5 mg/L as Cl2
(primarily monochloramine, Cl2:N 4:1 mass basis)
Alkalinity in mg/L as CaCO3
DIC (mg/L)
0
7.5
pH
8.5
10
0.0
0.2
5.0
10
39.0
42.1
60.0
50
194.9
210.1
279.7
13
Research Approach: Dissolution Rates
•
How are the dissolution rates of lead-containing solids affected by
water chemistry?
•
Are dissolved lead concentrations controlled by equilibrium solubility
or by dissolution rates?
1
Ceq
0.8
C effluent
0 2 μm filter
0.2
lead
solid
pyromorphite
mineral
suspension
suspension
C0
0.6
0.4
Css
0.2
0
QQ==0.5
– 2.0 ml/min
0.2-2.0
Pb
Udiss
, ,pH
pH
diss
Rate =
0
10
20
30
Time (h)
(C out − Cin )
1
t res
A × [solid ]
t res =
V
=1h
Q
14
Hydrocerussite Dissolution Rate Calculation
Pb
b Concentration (μM)
5.0
Rate =
4.0
Rate =
3.0
Css
1
tres A × [solid ]
0.4 μmol
60 min
1
L
4 .7 m
2
g
⋅1 g
= 0.0014
L
tres =1 h
[solid] = 1 g/L
A = 4.74 m2/g
μmol
m 2 ⋅ min
2.0
1.0
Css
0.0
0
10
20
30
40
50
Pb3(CO3)2(OH)2
Cumulative Residence Times (t/tres)
ƒ Dissolved lead concentrations are measured as a function of time.
ƒ Dissolution rates are calculated using the steady-state lead
concentrations.
15
5
2
Lead Relea
Rate (mol/min·m
Lead
Release
ae Rate
(mol/min•m2))
Hydrocerussite Dissolution Rate:
Effect of DIC and pH
25
DIC = 0 mg/L
DIC = 10 mg/L
20
DIC = 50 mg/L
15
10
5
0
x10
-9
7.5
8.5
10
pH
ƒ Dissolution rate decreased with increasing pH and
dissolved inorganic carbon (alkalinity).
16
Lead Concentrations vs. Predicted Solubility
-1
points = effluent concentrations
log [Pb]diss (uM)
-2
lines = predicted equilibrium
DIC = 0 mg/L
-3
DIC = 10 mg/L
-4
-5
DIC = 50 mg/L
-6
6
-7
4
5
6
7
8
9
10
11
12
pH
ƒ Effluent lead concentrations from flow-through are close to equilibrium.
ƒ Equilibrium concentrations are approached on time-scales faster than
the 1 hour residence time of the reactor.
17
Hydrocerussite Dissolution Rate:
Effect of Phosphate
Log Lead R
Release Rate (mol/mn•m2)
-6
mg/Lorhtophosphate
orthophosphate
00mg/L
44mg/L
P othophosphate
mg/Lasorthophosphate
-7
-8
-9
-10
-11
-12
DIC = 0 DIC = 10 DIC = 50 DIC = 0 DIC = 10 DIC = 50 DIC = 0 DIC = 10 DIC = 50
pH = 7.5
pH = 8.5
pH = 10
ƒ Phosphate addition greatly decreases the lead release rate.
18
6
Hydrocerussite Transformation
5.0 µm
5.0 µm
Hydrocerussite
15.0 µm
Hydrocerussite Reacted at pH 10 and 10 mg/L DIC
2.0 µm
5.0 µm
Hydroxylpyromorphite
(Pb5(PO4)3OH)
10 min
72 h
5.0 µm
Hydrocerussite Reacted at pH 8.5 and 4 mg/L PO43--P
(Batch)
A phosphate solid forms on hydrocerussite.
19
Hydrocerussite Solubility with Phosphate
-3
P = 0 mg/L
P = 4 mg/L
log [Pbdiss] (M)
-4
DIC = 0 mg/L
[solids] = 1 g/L
SSA = 4.74 m2/g
without
orthophosphate
-5
-6
-7
with
orthophosphate
-8
5.0 µm
-9
0.5 µm
Pb3(CO3)2(OH)2
-10
4
5
6
7
8
9
10
11
12
pH
• Addition of orthophosphate lowers the equilibrium dissolved
lead concentration by several orders of magnitude.
• Steady-state effluent concentrations are consistent with
solubility of hydroxylpyromorphite.
20
Summary of Hydrocerussite Dissolution Rates
• Dissolution rate is a strong function of pH, alkalinity, and
phosphate.
• The release of lead from hydrocerussite can be mitigated
by the addition of orthophosphate.
• The formation of even a small amount of the lead
phosphate hydroxylpyromorphite can limit lead release.
21
7
Outline
• Overview of Lead in Drinking Water
• Dissolution Rates of Lead Corrosion Products
•
•
•
•
research approach
hydrocerussite: impact of orthophosphate
hydroxylpyromorphite: impact of pH
plattnerite: impact of dissolved inorganic carbon and
chlorine species
• Lead Release from Pipe Sections
•
•
effects of water chemistry
impacts of water flow and stagnation time
• Conclusions and Recommendations
22
Hydroxylpyromorphite Dissolution Rates
150
t =0.5 h
[solids] = 1 g/L
SSA = 16 m2/g
P = 0 mg/L
Dissolved Lead (nM)
125
P = 1 mg/L
P = 4 mg/L
100
15 ug/L (72 nM)
75
50
25
500 nm
0
7.5
8.5
10
Pb5(PO4)3OH
pH
ƒ Dissolution rate decreased by increasing pH and phosphate.
ƒ No benefit of increasing orthophosphate from 1 to 4 mg P/L.
23
25
pH = 7.5
20
pH = 8.5
15
pH = 10.0
10
5
0
10-9
Lo
og Release Rate (mol/min·m2)
Le
ead Release Rate (mol/min·m2)
Comparison of Dissolution Rates of Solids
-7
pH = 7.5
-8
pH = 8.5
-9
pH = 10.0
-10
-11
-12
• Hydrocerussite dissolves two orders of magnitude
faster than hydroxylpyromorphite or plattnerite.
• Dissolution rates are influenced by pH.
tres =0.5 h
DIC = 0 mg/L
[solids] = 1 g/L
24
8
Plattnerite Dissolution Rates: Effect of DIC
Dissolutio
on Rate x 1010(mol/m2-min)
9
DIC = 0 mg/L
DIC = 10 mg/L
DIC = 50 mg/L
8
7
6
5
4
3
2
1
0
pH = 7.5
pH = 8.5
pH = 10.0
• PbO2 dissolution rate increased with increasing DIC.
• The formation of soluble Pb-carbonate complexes can accelerate dissolution.
25
Plattnerite: Kinetic vs. Equilibrium Control
0
Steady-state (points) and
predicted equilibrium (lines) lead
concentrations for plattnerite as a
function of pH and DIC.
DIC = 0 mg C/L
-1
DIC = 10 mg C/L
DIC = 50 mg C/L
-2
DIC = 50 mg C/L
log [Pb]diss (M)
-3
DIC = 10 mg C/L
-4
-5
DIC = 0 mg C/L
-6
-7
7
-8
-9
-10
6.0
7.0
8.0
9.0
10.0
11.0
pH
Dissolved lead concentrations were controlled by
dissolution rates and not by equilibrium solubility.
26
Mechanisms of PbO2 Reductive Dissolution
Two steps: 1) reduction and 2) dissolution
Mechanism A
dissolution accelerated
by CO32-
reductant
PbO2(s)
Pb(IV) oxides
reduction
Pb(II)diss
PbO2(s)-Pb(II)surf
Mechanism B
dissolution
Pb(IV)diss
reductant
Pb(II)diss
complexation
by CO32-
reduction
Pb(II)diss
ƒ Mechanism A explains experimental results well.
ƒ Release of Pb(II) from the surface is the rate-limiting step.
27
9
Effect of NH2Cl and Cl2 on PbO2 Dissolution
10
Dissolution
n Rate x 10 (mol/m2-min)
9
Control
2 mg/L monochloramine
2 mg/L free chlorine
8
7
flow-through
t =0.5 h
[solids] = 1 g/L
6
5
4
3
2
1
0
pH=7.5, DIC=50
pH=8.5, DIC=50
ƒ Elevated redox potential from chlorine and
monochloramine inhibited PbO2 dissolution.
28
Effect of Monochloramine in Batch Systems
0.35
batch
y = 0.0418x - 0.5155
R 2 = 0.9915
0.30
t =24 h
[Pb]diss (μM)
0.25
[solids] = 1 g/L
0.20
0.15
0.10
0.05
0.00
0
5
10
15
20
25
ΔNH2 Cl(μM)
• In batch-mode, monochloramine decayed and accelerated PbO2 dissolution.
• Lin and Valentine (ES&T 2008) demonstrated that the intermediate species
from monochloramine decay can reduce PbO2 and enhance dissolution.
• Effects of monochloramine depend on solid-water contact time.
29
Summary of Plattnerite Dissolution Rates
• Carbonate acted as a dissolution-enhancing ligand.
• PbO2 dissolution involves reduction and detachment.
– The rate-limiting step depends on the water chemistry.
• Higher redox potential results in lower dissolution rates.
– Free chlorine provides low dissolution rates and lead concentrations.
• Monochloramine could accelerate or inhibit PbO2 dissolution.
– short contact time: increase of redox potential and lowers rate.
– long contact time: monochloramine decay enhances PbO2 dissolution.
30
10
Key Observations about Dissolution Rates
• Hydrocerussite is more soluble than hydroxylpyromorphite
or plattnerite.
• Addition of orthophosphate lowers the rate of lead release
and promotes the formation of hydroxylpyromorphite.
• Less than 1 mg/L as P orthophosphate can effectively limit
lead release.
release
• The formation of soluble Pb-CO3 complexes can
accelerate dissolution of corrosion products.
• Free chlorine is necessary to maintain low dissolved lead
concentrations when PbO2 is present.
31
Outline
• Overview of Lead in Drinking Water
• Dissolution Rates of Lead Corrosion Products
•
•
•
•
research approach
hydrocerussite: impact of orthophosphate
hydroxylpyromorphite: impact of pH
plattnerite: impact of dissolved inorganic carbon and
chlorine species
• Lead Release from Pipe Sections
•
•
effects of water chemistry
impacts of water flow and stagnation time
• Conclusions and Recommendations
32
Approach: Release from Pipe Scales
• Following recirculation, examine lead release
after stagnation times of 1-48 hours
Two Pipe Scales
• actual lead service lines
• laboratory-conditioned
new lead pipes
• Investigate the effect of recirculation flow rate
12-24”
recirculation
reservoir
dissolved
phase analysis
Three Water Compositions
1. moderate pH, high DIC, no orthophosphate
2. neutral pH, moderate DIC, with orthophosphate
3. high pH, moderate DIC, no orthophosphate
Solution
pH
1
2
3
8.5
7.5
10.0
DIC
Orthophosphate
(mg C/L)
(mg P/L)
50
0
10
1
10
0
33
11
Effect of Solution Chemistry on Lead Release
Lea
ad Concentration (ug/L)
35
Dissolved
Total
30
25
• lab-conditioned pipes
• 2 hours of stagnation
• no recirculation
20
15
10
5
0
Solution 1 - high DIC
Solution 2 - high P
Solution 3 - high pH
ƒ Orthophosphate addition (1 mg P/L) at pH 7.5 and increasing
pH (to 10 at 10 mg C/L DIC) resulted in lowest lead releases.
ƒ Dissolved and total lead concentrations are similar.
34
Effect of Solution Chemistry on Lead Release
200
Dissolved
Lea
ad Concentration (ug/L)
180
Total
• premise plumbing pipe
160
• 1 hour of reaction
140
• no recirculation
120
100
80
60
40
20
0
Solution 1 - high DIC
Solution 2 - high P
Solution 3 - high pH
ƒ Orthophosphate addition and increasing pH resulted in lowest lead releases.
ƒ Concentrations in pipes without orthophosphate are comparable to steady-state
effluent lead concentrations from flow-through reactors with pure hydrocerussite.
35
Effect of Water Recirculation on Lead Release
200
No Flow -Dissolved
After Recirculation - Dissolved
No Flow -Total
After Recirculation - Total
Lead Concentration (ug/L)
180
160
140
• lab-conditioned pipes
• 2 hours of reaction
120
100
80
60
40
20
0
Solution 1 - high DIC
Solution 2 - high P
Solution 3 - high pH
ƒ Dissolved and total lead both increased with recirculating flow.
ƒ Dissolved lead increases are probably caused by greater
mixing of water with scales.
ƒ Total increased more than dissolved for two solutions, which
indicates the mobilization of particulate lead.
36
12
Effect of Flow on Lead Release
Dissolved Lead
Total Lead
1200
No Flow (1 h)
1000
Low Velocity (2 h)
High Velocity (2 h)
800
600
400
200
Totall Lead Concentration (μg/L)
Diss.. Lead Concentration (μg/L)
1200
No Flow (1 h)
1000
Low Velocity (2 h)
High Velocity (2 h)
800
600
400
200
0
0
Solution 1 -high DIC Solution 2 - high PSolution 3 - high pH
Solution 1 -high DIC Solution 2 - high PSolution 3 - high pH
• premise plumbing pipe
ƒ Increased flow increases lead release.
• 2 hours of reaction
ƒ Turbulent flow mobilizes more dissolved and
particulate lead.
• no recirculation
37
Summary of Release from Pipe Scales
• Lead release is affected by water chemistry.
• Water flow can increase the release of dissolved and
particulate lead.
• Release usually occurs on time-scales faster than 1 hour of
stagnation; equilibrium predictions provide good estimates
of concentrations in water that has been stagnant.
• Laboratory experiments with lead pipes provide information
on factors affecting lead release, but they can not be
directly applied to interpret real distribution and premise
plumbing systems
38
Conclusions
• Information on the specific water chemistry and corrosion
products is valuable in predicting lead release from scales
on lead service lines.
• The effectiveness of corrosion control strategies will vary
depending on the water chemistry and the composition of
the pipe scales.
– Increasing pH usually decreases lead release.
– Orthophosphate addition is effective over range of conditions.
– Increasing alkalinity can decrease or increase release.
– PbO2 is stabilized by free chlorine but not by chloramines.
39
13
Recommendations
• Collect information on distribution system water chemistry
and lead corrosion products present.
– Measure overall water chemistry when collecting samples for
compliance with the Lead and Copper Rule.
– Analyze and archive sections of lead service lines removed as part of
replacement plans.
• Tailor corrosion control strategies to the specific water
chemistry and corrosion products.
– Increasing carbonate alkalinity can stabilize hydrocerussite but
accelerate dissolution of plattnerite and hydoxylpyromorphite.
– If orthophosphate addition is used, then an optimal dose can be
established that mitigates lead release and minimizes cost.
40
Questions
Kate
ate Nelson
e so
Yin Wang
James Noel
j
Xie
Yanjiao
Vidhi Singhal
41
14
Pb and Cu corrosion research
 33
ongoing and
published projects
 summary paper
available online
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