Dan Giammar, Washington University in St. Louis

Transcription

1 Water Research Foundation # Lead (IV) Oxide Formation and Stability in Drinking Water Distribution Systems Dan Giammar, Washington University in St. Louis 2012 Water Research Foundation. ALL RIGHTS RESERVED.

2 2012 Water Research Foundation. ALL RIGHTS RESERVED.

3 2012 Water Research Foundation. ALL RIGHTS RESERVED.

4 2012 Water Research Foundation. ALL RIGHTS RESERVED.

5 Ongoing WaterRF Corrosion Projects Lead and Copper Rule and Distribution System Corrosion: An Overview of Foundation Research #4349 Impact of Galvanic Corrosion on Lead Release Following Partial Lead Service Line Replacement #4191 The Performance of Non-Leaded Brass Materials #4317 Non-Intrusive Methodology for Assessing Lead and Copper Corrosion #4351 Evaluation of Lead Service Line Lining and Coating Technologies #4415 Assessing Risk of Lead and Copper Consumption from Drinking Water #4409 Lead and Copper Rule Revision Data Gaps 2012 Water Research Foundation. ALL RIGHTS RESERVED.

6 Reduction of Lead in Drinking Water Act Public Meeting EPA Public Meeting on the implementation of the Reduction of Lead in Drinking Water Act of 2011 Thursday, August 16, 2012, from 1:00 p.m. to 4:30 p.m., EDT Conference call or webcast registration contact Junie Percy of IntelliTech at (937) ext. 210, or by no later than August 15, Additional information on the meeting is available in the July 30 Federal Register ( Water Research Foundation. ALL RIGHTS RESERVED.

7 Lead(IV) Oxide Formation and Stability: Rates and Mechanisms of Processes at the Solid Water Interface Daniel Giammar Department of Energy, Environmental, and Chemical Engineering Washington University in St. Louis August 14, 2012 Water Research Foundation Webcast Ak Acknowledgments ld Yin Wang Yanjiao Xie and Jiewei Wu Water Research Foundation (Project 4211) Traci Case, France Lemieux, Becki Rosenfeldt

8 Lead Phases in Lead Service Lines OCl CO 3 2, PO 4 3 Pb(II) Pb(IV) Cl Particulate Pb 2+ CO 3 2, PO 4 3, Cl Pb(IV)O 2, Pb 3 (CO 3 ) 2 OH 2, PbCO 3, Pb 5 (PO 4 ) 3 OH Lead Pipe Pb(0) Pb 2+ Lead pipe (Pb 0 ) 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.

9 Occurrence of PbO 2 Pipes conditioned for 8 months at ph 10 with 3.5 mg/l Cl 2 and 10 mg/l DIC. PbO 2 hydrocerussite Intensity Relative H H H H S P H H L L S Pb Pb H H P H S H H H L P S 3-month 8-month Scrutinyite (PbO 2 ) Plattnerite (PbO 2 ) Hydrocerussite Litharge (PbO) Lead θ ( ) In chlorinated water at moderate alkalinity, pipe scales develop that include lead(ii) oxide, lead(ii) carbonate, and lead(iv) oxides. Have been observed in pipe scales of many lead service lines. Both forms (scrutinyite and plattnerite) have been observed.

10 2-2 Equilibrium Lead Concentrations Log[Pb Pb]diss M) M) diss (M (M ug/l Pb 15 ug/l Pb Litharge (PbO) Hydrocerussite (Pb 3 (CO 3 ) 2 (OH) 2 ) with ih50 mg/l DIC Hydroxylpyromorphite (Pb 5(PO 4) 3OH) with 1 mg/l P ph Plattnerite (PbO 2 ) Water chemistry influenced by ph, DIC, disinfectant, orthophosphate, p NOM.

11 Formation and Dissolution of PbO 2 PbO reductants t PbO 2(s) Mn 2+, Fe 2+ Shi & Stone, ES&T, 2009 Br, I Lin et. al. ES&T, 2008; Lin & Valentine, ES&T, 2010 NOM Dryer & Korshin, ES&T, 2007; Lin & Valentine, ES&T, 2008 H 2 O Xie et. al. ES&T, 2010 HOCl/OCl l Pb(II) (diss) Pb 2+, Pb(II) CO 3 complexes PbO 2 can only be formed in the presence of free chlorine. Presence of the reductants enhances the dissolution of PbO 2. The dissolution of PbO 2 is affected by water chemistry. Lead release may be controlled by kinetics and not by equilibrium.

12 Redox Speciation of Lead (V) E H Pb 2+ O 2 PbHCO + 3 free chlorine O 3(aq) PbCO PbO 2(s) Pb(C CO 3 ) 2-2 PbO Pb(OH) 4 Pb(IV) oxide is stable beyond the stability limits of water (i.e., water can be a reductant). PbO 2 can form from oxidation of Pb(II) by free chlorine (HOCl/OCl ). Pb(II) is more soluble than Pb(IV) Pb (s) () ph q) Pb(OH) 2(aq Pb(OH) - 3 Total Pb = 15 μg/l DIC = 30 mg C/L

13 Redox Speciation of Lead (V) E H O 2 Pb 2+ PbHCO + 3 monochloramine Pb (s) () O 3(aq) PbCO PbO 2(s) ph q) Pb(OH) 2(aq Pb(C O 3 ) 2-2 Pb(OH) - 3 PbO Pb(OH) 4 Pb(IV) oxide is stable beyond the stability limits of water (i.e., water can be a reductant). PbO 2 can form from oxidation of Pb(II) by free chlorine (HOCl/OCl ). Pb(II) is more soluble than Pb(IV). Weaker disinfectants may destabilize PbO 2. Reductants may accelerate the dissolution of PbO 2. Total Pb = 15 μg/l DIC = 30 mg C/L

14 Impacts of PbO 2 Dissolution Washington, DC First Draw Samples U.S. EPA, 2007 EPA 815 R Lead concentrations increased in Washington, DC following a switch from free chlorine to chloramine. Dissolution of PbO 2 contributed to the increase.

15 Project Objectives 1. Identify water chemistry parameters that govern the formation of PbO 2 and control which PbO 2 solid (plattnerite vs. scrutinyite) is produced. 2. Establish the equilibrium solubility of PbO 2 solids when lead remains in the +IV oxidation state. 3 l id h h i d if h f ld 3. Elucidate the mechanisms and quantify the rates of coupled processes of PbO 2 reduction and dissolution.

16 Research Approach Task 1: PbO 2 Formation Threshold free chlorine concentration. Coexistence of Pb(II) and Pb(IV) Task 2: PbO 2 Solubility Equilibrium at high oxidation reduction potential. Task 3: PbO 2 Reduction Dissolution Quantify reaction rates Control composition to focus on reduction, dissolution, and orthophosphate inhibition Pb(II) OCl Cl H diss 2 O O 2 dissolution Pb(0) PbO 2(s) (s) PbO 2(s) Pb(II) surf oxidation Pb(IV) oxides reduction PbO (s) precipitation Pb plattnerite, scrutinyite 3 (CO 3 ) 2 (OH) 2(s) Pb(IV) diss dissolution Pb(II) diss PO 3 4 Pb(II) Phosphates Water Quality Analyses dissolved and total Pb ph, free chlorine, orthophosphate XRD: mineralogy SEM: morphology Solid Phase Characterization Raman: in situ structure at surface XANES: Pb oxidation state

17 Outline Overview of PbO 2 chemistry and its importance. Project objectives and approach. Task 1: Formation of PbO 2 Task 2: Equilibrium Solubility of PbO 2 Task 3: PbO 2 Dissolution Rates Conclusions and Recommendations Task 1: PbO 2 Formation Threshold free chlorine concentration. Coexistence of Pb(II) and Pb(IV) Task 2: PbO 2 Solubility Equilibrium at high oxidationreduction potential. Task 3: PbO 2 Reduction Dissolution Quantify reaction rates Control composition to focus on reduction, dissolution, andorthophosphate inhibition Pb(II) OCl Cl H diss 2 O O 2 dissolution Pb(0) PbO 2(s) (s) PbO 2(s) Pb(II) surf oxidation PbO Pb(IV) oxides reduction (s) precipitation Pb plattnerite, ltt scrutinyite it 3 (CO 3 ) 2 (OH) 2(s) Pb(IV) diss dissolution Pb(II) diss PO 4 3 Pb(II) Phosphates h

18 Task 1: Formation of PbO 2 Objective: Identify water chemistry parameters that govern the formation of PbO 2 and control which PbO 2 solid (plattnerite vs. scrutinyite) is produced. Oid Oxidation i Oxidation Pb(II) solids PbO 2 ph, Cl 2 Pb(0) Dissolutio n ph, lead conc DIC, PO4 4 3 Precipitat tion Precipitation ph, lead conc. Oxidation Pb 2+ Pb 4+ Oxidation ph, Cl 2

19 Task 1: Approach for Formation of PbO 2 Property Selected Conditions dissolved lead: PbCl 2 solution starting phase (lead-containing) massicot: PbO (s) hydrocerussite: Pb 3 (CO 3 ) 2 (OH) 2(s) lead metal: Pb(0) (s) ph 7, 8.5, 10 dissolved inorganic carbon 0, 20 mg C/L free chlorine 0, 4, 20 mg/l as Cl 2 reaction time 1 day, 1 week, 4 weeks Lead Metal Massicot (PbO) Hydrocerussite (a) (b) (c) X ray Diffraction (XRD): identity of crystalline phases Scanning electron microscopy (SEM): particle size and morphology adjust ph and Cl 2 periodically sample solids

20 Effect of DIC M M P S P P,S DIC 0 Rela ative Intensity S DIC 20 Plattnerite Scrutinyite Massicot θ ( o ) ph 7.5, 20 mg/l DIC, 28 days massicot (PbO) The presence of DIC accelerated PbO 2 formation from massicot.

21 PbO 2 Formation from Hydrocerussite Cl 2 20 mg/l, Hydrocerussite (Pb 3 (CO 3 ) 2 (OH) 2 ) S S H H S ph 10, 28 d ph 10, 1 d ty P P ph 7.5, 28 d Relative Intensi C ph 7.5, 1 d cerussite hydrocerussite plattnerite scrutinyite θ ( o ) Mixtures of scrutinyite and plattnerite formed at ph 7.5. Scrutinyite formed at ph 10.

22 Effects of Precursors pure scrutinyite pure scrutinyite PbCl 2(aq) ph 7.5; no DIC; Cl 2 =4,20mg/L mixture massicot ph 10; DIC 20 mg C/L massicot Cl 2 =4,20mg/L hydrocerussite ph 7.5, 10; no DIC; Cl 2 = 4, 20 mg/l mixture ph 10; no DIC Cl 2 = 4, 20mg/L lead oxide chloride pure plattnerite ph 7.5; DIC 20 mg C/L Cl 2 =20mg/L massicot cerussite ph 10; no DIC Cl 2 = 20, 42 mg/l hydrocerussite cerussite pure scrutinyite ph 7.5; DIC 20 mg C/L Cl 2 =4mg/L massicot cerussite pure plattnerite cerussite ph 7.5; no DIC; Cl 2 = 20, 42 mg/l mixture pure plattnerite hydrocerussite ph 10; no DIC; Cl 2 = 20, 42 mg/l ph 7.5; no DIC hydrocerussite cerussite Cl 2 = 20, 42 mg/l pure scrutinyite mixture Pb diss scrutinyite/plattnerite mixture hydrocerussite scrutinyite lead oxide chloride plattnerite pure plattnerite

23 Formation pathways Direct Solid Oxidation Nucleation on Surface of Pre existingexisting Solids Oxidation Oxidation Pb(II) solids PbO 2 Pb(0) Dissolution Precipitation Oxidation Precipitation Oxidation Pb 2+ Oxidation Pb 4+

24 Summary of PbO 2 Formation PbO 2 can be formed from multiple initial phases and can form both with and without DIC. Rateofformation formation is dependent onsubstrate, free chlorine concentration, and DIC. DIC accelerated PbO 2 formation from massicot. Hydrocerussite formed scrutinyite. Massicot and PbCl 2 formed mixtures of scrutinyite and plattnerite. If there is a threshold h ldfree chlorine concentration for PbO 2, then it is below bl 4 mg/l Cl 2. Identity of PbO 2 phase is affected by ph and other factors.

25 Outline Overview of PbO 2 chemistry and its importance. Project objectives and approach. Task 1: Formation of PbO 2 Task 2: Equilibrium Solubility of PbO 2 Task 3: PbO 2 Dissolution Rates Conclusions and Recommendations Task 1: PbO 2 Formation Threshold free chlorine concentration. Coexistence of Pb(II) and Pb(IV) Task 2: PbO 2 Solubility Equilibrium at high oxidationreduction potential. Task 3: PbO 2 Reduction Dissolution Quantify reaction rates Control composition to focus on reduction, dissolution, andorthophosphate inhibition Pb(II) OCl Cl dissolution diss Pb(0) PbO 2(s) (s) PbO 2(s) Pb(II) surf oxidation PbO Pb(IV) oxides (s) precipitation Pb plattnerite, ltt scrutinyite it 3 (CO 3 ) 2 (OH) 2(s) Pb(IV) diss dissolution Pb(II) diss PO 4 3 Pb(II) Phosphates h

26 Task 2: Equilibrium Solubility of PbO 2 Objective: Establish the equilibrium solubility of PbO 2 solids when lead remains in the +IV oxidation state. -2 Batch equilibration. -4 diss (M) Log[Pb]d ug/l Pb Plattnerite (PbO 2 ) -14 Equilibrium Reactions ph PbO + 4H + = Pb H O [Pb(IV)] = [Pb 4+ + [PbO 2 + [PbO 4 2(s) 2 diss ] 3 ] 4 ] K sp,scrutinyite [Pb [H 4 ] 4 ] K sp,plattnerite [Pb [H 4 ] 4 ]

27 Approach to Measure Equilibrium Solubility Batch Experiments: 0.02 μm ICP MS for [Pb] filtration diss 2 mg/l free chlorine maintained Suspension ph ph = 6, 7.5, DPD method for [Cl 2 ] with and without DIC reaction time = 25 days XRD analysis O 2 /H 2 O PbO 2(s) PbO 3 2- E H (V) Pb 2+ PbHCO 3 + PbCO 3(aq) Pb(CO 3 ) 2 2- Pb b(oh) 2-4 Plattnerite Scrutinyite Pb (s) ph Pb(OH) 2(aq) Pb(OH) 3 -

28 Dissolved Lead After Long Stagnation Plattnerite Scrutinyite mg/l free chlorine Dissolve ed Lead (µg/l) ed Lead (µg/l) Dissolv ph = 7.5 No DIC 50 mg/l PbO 2 Duplicate experiments are represented by different symbols time (d) time (d) PbO 2 dissolution i is very slow in presence of free chlorine. It takes nearly one week of stagnation to reach the action level, and for some ph conditions the action level was never reached. Stable equilibrium i concentrations ti are reached hdafter about t10 days.

29 Equilibrium Solubility 4 PbO 2(s) H 2 O 2 0 Plattnerite Scrutinyite Lo og [Pb]diss (µg/l L) predicted equilibrium with scrutinyite predicted equilibrium with plattnerite HOCl/OCl Pb(II) (diss) steady state ph Although h low equilibrium i concentrations are reached, hdthey are orders of magnitude higher than predicted. Assuming that all dissolved lead is Pb(II), stable concentrations may actually represent a steady state between Pb(II) oxidation and PbO 2 reduction.

30 Summary of PbO 2 Solubility Lead release from scrutinyite and plattnerite was very similar. Measured concentrations after equilibration were orders of magnitude higher than predicted values. Lead concentrations were still very low, even for long stagnation times. Lead concentrations for systems with PbO 2 are likely to be controlled by dissolution rates and not by equilibrium solubility.

31 Outline Overview of PbO 2 chemistry and its importance. Project objectives and approach. Task 1: Formation of PbO 2 Task 2: Equilibrium Solubility of PbO 2 Task 3: PbO 2 Dissolution Rates Conclusions and Recommendations Task 1: PbO 2 Formation Threshold free chlorine concentration. Coexistence of Pb(II) and Pb(IV) Task 2: PbO 2 Solubility Equilibrium at high oxidation reduction potential. Task 3: PbO 2 Reduction Dissolution Quantify reaction rates Control composition to focus on reduction, dissolution, andorthophosphate inhibition Pb(II) diss Pb(0) (s) PbO (s) Pb 3 (CO 3 ) 2 (OH) 2(s) OCl Cl H 2 O O 2 oxidation PbO 2(s) Pb(IV) oxides plattnerite, ltt scrutinyite it reduction dissolution PbO 2(s) Pb(II) surf precipitation Pb(II) diss PO 4 3 Pb(II) Phosphates h

32 Task 3: Dissolution of PbO 2 Objective: Elucidate the mechanisms and quantify the rates of coupled processes of PbO 2 reduction and dissolution. 2 H 2 O O 2 dissolution PbO 2(s) PbO 2(s) -Pb(II) surf Pb(IV) oxides reduction plattnerite, scrutinyite precipitation Pb(IV) diss dissolution Pb(II) diss PO 4 3- Pb(II) Phosphates

33 Approach: Flow through Experiments 0.22 μm filter Influent reservoir PbO 2 concentration = 1 g/l flow rate = 2.8 ml/min hydraulic residence time = 30 min Pb diss, ph PbO 2 suspension Rate (C out t res C in ) A 1 solid Inhibitor (phosphate) p ph: DIC: 50 mg C/L Oxidant (HOCl) ph = [HOCl] T = 1 mg/l as Cl 2 DIC = 50 mg C/L Reductant (iodide) ph = [I ] = 1 20 μm DIC = 10, 50, 200 mg C/L Control (only H 2 O) ph: DIC: 50 mg C/L DIC = dissolved inorganic carbon

34 Effect of Free Chlorine and Iodide: Overview Diss. Rate (10-10 mol m m -2 min -1 ) iodide control free chlorine ph 50 mg C/L DIC A = 3.6 m 2 /g [solids] = 1 g/l t res = 30 min Dissolution rate of PbO 2 increased with decreasing ph.

35 Effect of Free Chlorine and Iodide: Overview Diss. Rate (10-10 mol m m -2 min -1 ) iodide control free chlorine ph 50 mg C/L DIC 1 mg Cl 2 /L HOCl A = 3.6 m 2 /g [solids] = 1 g/l t res = 30 min Dissolution rate of PbO 2 increased with decreasing ph. The presence of free chlorine inhibited the dissolution of PbO 2.

36 Effect of Free Chlorine and Iodide: Overview Diss. Rate (10-10 mol m m -2 min -1 ) ph iodide 50 mg C/L DIC control 1 mg Cl 2 /L HOCl free chlorine 10 µm iodide A = 3.6 m 2 /g [solids] = 1 g/l t res = 30 min Dissolution rate of PbO 2 increased with decreasing ph. The presence of free chlorine inhibited the dissolution of PbO 2. The presence of iodide accelerated PbO 2 dissolution.

37 0-10 mol m -2 min -1 ) Dis ss. Rate ( Effect of Iodide Concentration ph 7.6 ph Iodide concentration (µm) Increasing iodide concentrations accelerated the dissolution of PbO 2. The dissolution rate of PbO 2 was higher at ph

38 Effect of DIC in the Presence of Iodide 60 m -2 min -1 ) Dissolution Rate (10-10 mol mg C/L 10 mg C/L 50 mg C/L 200 mg C/L 10 µm iodide ph Presence of 10 mg C/L DIC accelerated the dissolution of PbO 2. Varying DIC concentration from 10 to 200 mg C/L had little effect on the dissolution rate of PbO 2.

39 Effect of Iodide: Mechanism Kinetic model dldeveloped dfrom previously published work on MnO 2 dissolution (Stone et al.). PbO 2 1.Iodide adsorption Pb(IV) I (10 10 mol/m 2 min) Dissolution Rate ( Rate limiting R ph 7.6 ' ki [ ] k 1 ki [ ] ph 76model Iodide Concentration (μm) PbO 2 2. Electron transfer 3. Pb(II) detachment Pb(IV) e I PbO 2 Pb(II) Pb(II) diss ligand (CO 3 2 )

40 Effect of Orthophosphate on PbO 2 Dissolution 1 ) Dissolution Rate (10 10 mol m 2 min mg/l 1 mg/l DIC=0 DIC=50 DIC=0 DIC=50 ph=7.6 ph=8.5 Orthophosphate usually reduced the PbO 2 dissolution rate. Experiments done at other conditions also showed a reduction in dissolution rate in the presence of orthophosphate. h h t

41 Formation of Lead(II) Phosphate Solid unreacted PbO 2 reacted at no DIC hydroxylpyromorphite ypy p 100 nm 100 nm 100 nm reacted at 50 mg/l DIC 100 nm PO 3 4 reduces PbO 2 dissolution rate both with and without DIC present. Low DIC: the lead phosphate hydroxylpyromorphite forms. High DIC: no hydroxylpyromorphite observed. Two possible mechanisms of orthophosphate inhibition of PbO 2 dissolution.

42 Inhibition of PbO 2 Dissolution by PO 4 3 A. Precipitation of lead (II) phosphate solid reductant reduction dissolution Pb(II) diss PbO 2(s) -Pb(II) surf precipitation PO 3- p 4 Pb(II) Phosphate PbO 2(s) B1. Adsorption to limit Pb(II) release reductant adsorption of PO 3-4 PbO 2(s) -Pb(II) surf reduction PbO 2(s) -Pb(II) surf -PO 3-4 B2. Adsorption to limit PbO 2 reduction adsorption of PO 3-4 PbO 2(s) -PO 3-4 2(s) 4 In the absence of DIC, Mechanism A was dominant In the presence of DIC, Mechanism B was dominant

43 Summary of PbO 2 Dissolution Rates Reaction kinetics are critical. Although PbO 2 is thermodynamically unstable in pure water, other reductants were needed to dramatically increase the dissolution rate. Free chlorine concentrations as low as 0.2 mg/l Cl 2 can decrease the dissolution rate by an order of magnitude. Iodide dramatically accelerated PbO 2 dissolution. Other reductants of 2 interest are natural organic matter, chloramine decay products, Fe 2+, and Mn 2+. Orthophosphate can inhibit PbO 2 dissolution.

44 Recommendations Look for PbO 2 on lead service lines when pipe sections are available. PbO 2 can persist even after a switch from free chlorine. The presence of PbO 2 is more important than the exact identity of the solid (scrutinyite vs. plattnerite). Equilibrium solubility calculations are poor predictors of lead concentrations ti for systems with PbO 2. Be cautious in altering water compositions in ways that may affect PbO 2 stability (e.g., changes in disinfectant or ph). The presence of reductants (e.g., chloramine, natural organic matter) is the greatest risk to PbO 2 stability. Stable free chlorine residuals and orthophosphates can mitigate lead Stable free chlorine residuals and orthophosphates can mitigate lead release from PbO 2.

45 Questions (314)