A Model for Estimating the Impact of Orthophosphate on Copper in Water

Abstract

The nature of copper phosphate minerals in drinking water distribution systems has remained largely unsolved despite being an important link to reducing cuprosolvency. Chemical equilibrium modeling has also largely failed to accurately predict soluble copper in the presence of orthophosphate. The objective of this work was to develop and validate an empirical copper solubility model that considered pH, dissolved inorganic carbon (DIC), and orthophosphate from a series of bench-scale copper precipitation experiments. An empirical model was constructed that allows for the determination of copper levels in a system given pH, DIC, and orthophosphate data. The predictive reliability of this model was assessed by evaluating a collection of cuprosolvency data from two decades of research and field observations and water treatment reports. The tests yielded a firm correlation between predicted and observed copper levels attested by a regression coefficient of 0.86 for a total of 851 observations.

Passivity and Pit Stability Behavior of Copper as a Function of Selected Water Chemistry Variables

“…High dissolved inorganic carbon (DIC) concentration increases the soluble copper in drinking water considerably (35,45). The passivation efficiency of copper in ESDW is above 60% when the pH is adjusted to 9.5 and above, which is in agreement with Feng's finding that a thin and compact film forms on the copper surface and leads to spontaneous passivation in pH 10 synthetic water (71).…”

“…Equation [5] gives a better prediction of repassivation potential than Equation [4] with respect to CVS test results, which indicates that Cl -has a more potent influence on repassivation potential than SO 4 2-. The evidence for this also exists in the fact that Cl -is always found in the bottom of pits (35). The repassivation potentials of copper in different pH-adjusted ESDW are also compared between experimental results and predictions by Equation [5].…”

“…Water chemistries that promote passivity of copper may also enable pitting. Incipient pits were only observed on passivated copper in pH 8 and 9 synthetic drinking water but not at pH 6.5 and 7, where copper did not exhibit thin film passivity formed by direct oxidation but instead formed reprecipitated films (35).…”

“…Although considerable research has been devoted to either the thermodynamic stability of copper or practical exposures in a variety of systems, e.g., Cu-Cl --H 2 O, Cu-SO 4 2--H 2 O, and Cu-H 2 CO 3 -H 2 O (10)(58), less attention has been paid to the governing electrochemical aspects of copper associated with pitting in different systems. Selected waters that cause cold water pitting of copper are known (2,3,5,6,13,35), but the specific electrochemical factors governing pit initiation and propagation, as a function of water chemistry variables are unclear. Regardless of the exact water chemistry details, one consistent aspect is that there is often a critical threshold potential associated with pitting that applies to the case of passivated copper.…”

Environmental and health impacts assessment of long-term naturally-weathered municipal solid waste incineration ashes deposited in soil—old burden in Bratislava city, Slovakia

“…dissolved alkyl, free chlorine and monochloramine, are able to corrode metallic copper. Newly formed Cu 2 O and CuOH are further oxidised to form Cu(II) solid species [ 72 ].…”