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Effect of pH, Dissolved Inorganic Carbon and Phosphates on the Nature of Lead Particles and Associated Solubility in Water
Citation:
Formal, C., D. Lytle, S. Harmon, D. Wahman, AND Mike DeSantis. Effect of pH, Dissolved Inorganic Carbon and Phosphates on the Nature of Lead Particles and Associated Solubility in Water. To be Presented at AWWA Virtual Summit: Water Quality and Infrastructure, CINCINNATI, OH, December 08 - 10, 2020.
Impact/Purpose:
The use of phosphates in the drinking water industry is well known. Orthophosphate is used to reduce lead levels at the consumer’s tap by forming relatively insoluble divalent lead orthophosphate compounds on the surface of the lead source (e.g., lead service line, brass fixture, soldered joint, etc.). Water utilities often use orthophosphate to meet the United States Environmental Protection Agency’s Lead and Copper Rule that sets a 90th percentile action level for lead at 0.015 mg/L at the consumer’s tap. There is a need to better and broadly understanding of the properties of related lead-phosphate particles. Specifically, the impact of water chemistry on the mineralogy and solubility of lead orthophosphate solids needs to be fully understood. Furthermore, the properties of resulting lead orthophosphate particles, particularly those related to mobility, must also be better understood as the practice of phosphate addition grows. Therefore, the objectives of this study were two-fold: (1) to evaluate the impact of orthophosphate as a function of pH and dissolved inorganic carbon (DIC) on lead mineralogy and solubility, and (2) to report physical properties (e.g., mineralogy, size and surface charge) of associated lead particles/colloids. Lead solids were synthesized in the laboratory by precipitation under controlled water quality conditions over a matrix of pH, DIC and orthophosphate conditions. All samples were filtered through a 0.20 µm vacuum filter and a 30 KDa membrane filter to assess particulate/colloidal fractions and determine associated lead solubility. Solids were analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to determine the surface morphology, shape and size. X-ray diffraction (XRD) analyses were performed to identify solid mineralogy. Particle size and charge were also determined using a Zetasizer Nano-ZS90. A comparison between size analysis between the zetasizer, SEM and TEM images, and XRD peaks was made. Experimental data was compared to chemical equilibria model predictions using published thermodynamic constants as well as establishing parameters having a best fit. XRD identified the precipitated lead particles in the presence of orthophosphate were hydroxypyromorphite [Pb5(PO4)3OH]. Particles were hexagonal in shape having a diameter that decreased with increasing pH and orthophosphate dose. The minimum average size measured at 21 nm . Provided sufficient orthophosphate dose to stoichiometrically meet the demand of precipitating lead, relatively insoluble hydroxypyromorphite nanoparticles (diameter between 21 -215 nm) formed. Particles were consistently larger in water containing 50 mg C/L DIC as compared 10 mg C/L DIC. The particle charge of hydroxypyromorphite was less variable compared to particles in the absence of orthophosphate and constant with change in pH and orthophosphate dose and relatively large . In the absence of orthophosphate, hydrocerussite [Pb3(CO3)2(OH)2] was the only lead mineral present having a solubility nearly two orders of magnitude greater than hydroxypyromorphite. Hydrocerussite particles were large (>1 µm in diameter), and charge became more negative as the pH increased. When orthophosphate was added in an insufficient dose to meet lead precipitation demand, a hydrocerussite/hydroxypyromorphite mixed suspension formed making data interpretation impossible. Lastly, the findings have important practical implications that will also be discussed.
Description:
The use of phosphates in the drinking water industry is well known. Orthophosphate is used to reduce lead levels at the consumer’s tap by forming relatively insoluble divalent lead orthophosphate compounds on the surface of the lead source (e.g., lead service line, brass fixture, soldered joint, etc.). Water utilities often use orthophosphate to meet the United States Environmental Protection Agency’s Lead and Copper Rule that sets a 90th percentile action level for lead at 0.015 mg/L at the consumer’s tap. There is a need to better and broadly understanding of the properties of related lead-phosphate particles. Specifically, the impact of water chemistry on the mineralogy and solubility of lead orthophosphate solids needs to be fully understood. Furthermore, the properties of resulting lead orthophosphate particles, particularly those related to mobility, must also be better understood as the practice of phosphate addition grows. Therefore, the objectives of this study were two-fold: (1) to evaluate the impact of orthophosphate as a function of pH and dissolved inorganic carbon (DIC) on lead mineralogy and solubility, and (2) to report physical properties (e.g., mineralogy, size and surface charge) of associated lead particles/colloids. Lead solids were synthesized in the laboratory by precipitation under controlled water quality conditions over a matrix of pH, DIC and orthophosphate conditions. All samples were filtered through a 0.20 µm vacuum filter and a 30 KDa membrane filter to assess particulate/colloidal fractions and determine associated lead solubility. Solids were analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to determine the surface morphology, shape and size. X-ray diffraction (XRD) analyses were performed to identify solid mineralogy. Particle size and charge were also determined using a Zetasizer Nano-ZS90. A comparison between size analysis between the zetasizer, SEM and TEM images, and XRD peaks was made. Experimental data was compared to chemical equilibria model predictions using published thermodynamic constants as well as establishing parameters having a best fit. XRD identified the precipitated lead particles in the presence of orthophosphate were hydroxypyromorphite [Pb5(PO4)3OH]. Particles were hexagonal in shape having a diameter that decreased with increasing pH and orthophosphate dose. The minimum average size measured at 21 nm . Provided sufficient orthophosphate dose to stoichiometrically meet the demand of precipitating lead, relatively insoluble hydroxypyromorphite nanoparticles (diameter between 21 -215 nm) formed. Particles were consistently larger in water containing 50 mg C/L DIC as compared 10 mg C/L DIC. The particle charge of hydroxypyromorphite was less variable compared to particles in the absence of orthophosphate and constant with change in pH and orthophosphate dose and relatively large . In the absence of orthophosphate, hydrocerussite [Pb3(CO3)2(OH)2] was the only lead mineral present having a solubility nearly two orders of magnitude greater than hydroxypyromorphite. Hydrocerussite particles were large (>1 µm in diameter), and charge became more negative as the pH increased. When orthophosphate was added in an insufficient dose to meet lead precipitation demand, a hydrocerussite/hydroxypyromorphite mixed suspension formed making data interpretation impossible. Lastly, the findings have important practical implications that will also be discussed.