Phosphate use for Sequestration, Anti-Scaling, and Corrosion Control: Critical Review, Simultaneous Optimization of Polyphosphate Dosing, Sequestration Mechanisms, and Stabilization of Magnesium Silicate Scale

Phosphates are used by drinking water utilities to 1) reduce iron/manganese aesthetic problems by sequestration, 2) inhibit calcium carbonate scale formation via threshold inhibition, and 3) reduce corrosion of pipes by forming protective pipe scales. Orthophosphates can control lead, copper and iron corrosion through the formation of durable, low solubility scale, but are widely believed ineffective for sequestration or anti-scaling. Conversely, polyphosphates are effective sequestrants and anti-scalants, but can increase corrosion of plumbing materials. Here, we first critically reviewed the current state of the science, operational guidance, and knowledge gaps related to use of orthophosphate and polyphosphates for all three objectives. Three major gaps in understanding were identified and then addressed in subsequent chapters: 1) use of phosphates to achieve both sequestration and anti-scaling 2) mechanisms of iron sequestration, and 3) stabilization of magnesium silicate scale linings in a distribution system. In the critical review, we holistically conceptualize phosphate use as a three-dimensional (3-D) challenge of optimizing sequestration, anti-scaling and corrosion control. Despite nearly a century of widespread use, there is a poor scientific and practical understanding of how to use phosphates to achieve each of these key objectives, much less achieve synergies and avoid antagonistic effects. Many water systems are reliant on trial-and-error methods, or guidance from vendors of these proprietary chemicals, creating potential inefficiencies or even adverse unintended consequences. Effective sequestration of iron and manganese, to prevent formation of visible discoloration, can occur through four possible mechanisms which are undoubtedly dependent on the water chemistry (e.g., pH, hardness, redox). Anti-scaling of calcium carbonate occurs through threshold inhibition and crystal distortion, but sometimes phosphates can encourage scaling due to the precipitation of calcium phosphate. Corrosion control via orthophosphate is often effective, but polyphosphates can sometimes increase lead or copper levels in drinking water.
Despite their widespread use in scientific studies, it was discovered that standardized measurements of color and turbidity do not fully account for the range of subjective consumer observations regarding cloudy or discolored water. At a constant apparent color of 110 Pt-Co, testing illustrated that relatively non-offensive air bubbles had a high turbidity of 74 NTU compared to just 0.1 NTU for offensively orange fulvic acid. Additionally, factors such as background color, type of light source, and direction of light significantly influenced perception of discolored water. For instance, under typical laboratory lighting conditions (light from above) with a white background, colors caused by iron, manganese, and fulvic acid were very prominent, whereas white calcium carbonate and magnesium silicate particles were more challenging to see. But white particles became much more prominent when the light source was from below or there was a darker background. A study of Fe sequestration was conducted to elucidate a mechanistic basis for the empirical trends revealed in the utility field study. As revealed in the literature review, polyphosphates could sequester Fe by inhibiting any step of the reaction sequence Fe2+ oxidation  precipitation of Fe(OH)3  particle agglomeration to visible sizes. Phosphates generally inhibited Fe2+ oxidation above about pH 7-8, dependent on chain length, and catalyzed oxidation at lower pHs. But in oxygenated waters above about pH 7, the dominant mechanism of sequestration was some combination of Fe3+ complexation and colloid stabilization at small particle sizes that were practically invisible. Increasing the phosphate chain length, phosphate concentration, and Si concentration caused more effective Fe sequestration, whereas Ca, Mg, and increased pH hindered its effectiveness. It was also discovered that orthophosphate can be an effective sequestrant under ideal conditions, polyphosphate can sequester more than 1 mg/L Fe despite some claims to the contrary, and Ca at very high doses can precipitate polyphosphates. During this dissertation work, a novel, thick (~1 mm), glassy magnesium silicate (MgSi) scale was discovered covering much of the pipe surfaces in a large water distribution system. This MgSi lining was hypothesized to be an extremely effective means of corrosion control that was important to maintain in its present state, as dissolution could cause it to detach from pipes, whereas further precipitation could clog them. To better understand how to maintain the scale, factors affecting the formation and dissolution of the MgSi solid were examined. Phosphate corrosion inhibitors had little effect on MgSi solubility at pH 8.5 and 10, while hexametaphosphate (HMP) and zinc orthophosphate slightly reduced Mg and Si dissolution rates at pH 7. Zinc orthophosphate reduced Mg dissolution by 50% and completely inhibited Si dissolution from the solid, while HMP decreased dissolution of Mg by 32% and Si by 63%. The magnesium silicate did not precipitate below pH 10 without the presence of a pre-existing seed solid. With a pre-existing seed scale, however, the MgSi further precipitated at a pH 8.5-9 in one source water and 7.5-8 in another. Below these pH levels, scale dissolution was shown to occur. Strategies were evaluated to help identify the equilibration pH for operation of a system with varying concentrations of silica, magnesium and pH.
The two-dimensional (2-D) interplay of polyphosphate use for sequestration and anti-scaling was investigated for nine small utilities who rely on groundwater in North Carolina. Bench-top testing methods were developed to determine the 'optimal phosphate doses,' defined here as the lowest level of polyphosphate that maintains visually clear water and acceptable levels of scale formation. One proprietary polyphosphate chemical had an optimal sequestrant dose that depends on the concentration of Fe, Mn, Ca, and Mg. The dose (in mg/L as P) is equal to 58.5[Fe] + 59.7[Mn] + 0.041[Ca + Mg] + 0.4669 (units mM). Interestingly, color was well correlated with particulate (> 0.45 μm) Mn (R2 = 0.79) while turbidity was mostly correlated with particulate iron (R2 = 0.60). Furthermore, neither color nor turbidity measurements were reliable predictors of discoloration detected by eye. In the three utilities with higher hardness (> 100 mg/L as CaCO3), at least 3.6X more phosphate was needed for Fe and Mn sequestration than scale inhibition. But lab testing in very hard water with 300 mg/L as CaCO3 demonstrated that achieving anti-scaling, will sometimes require more polyphosphate than that needed for control of sequestration.
Overall, this dissertation advances understanding of phosphate use in relation to important problems arising in water distribution or buildings. The innovative practical testing methods, improved practical understanding, and mechanistic insights can be applied to maximized the benefits of phosphates use while avoiding detriments. This is an important first step towards developing a rational holistic framework to guide utility decision-making regarding phosphate use.