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Effect of pH on the solubility

The effect of pH on the solubility of the corrosion by-products formed during the corrosion process is often the key to understanding the concentration of metals at the tap.

An important characteristic of distributed water with higher pH is that the solubility of the corrosion by-products formed in the distribution system typically decreases.

The solubility of the main lead corrosion by-products (divalent lead solids: cerussite [PbCO3], hydrocerussite [Pb3(CO3)2(OH)2] and lead hydroxide [Pb(OH)2]) largely determines the lead levels at the tap (Schock, 1980, 1990b; Sheiham and Jackson, 1981; De Mora and Harrison, 1984; Boffardi, 1988, 1990; U.S. EPA, 1992; Leroy, 1993; Peters et al., 1999). From thermodynamic considerations, lead solubility of corrosion by-products in distribution systems decreases with increasing pH (Britton and Richards, 1981; Schock and Gardels, 1983; De Mora and Harrison, 1984; Boffardi, 1988; Schock, 1989; U.S. EPA, 1992; Singley, 1994; Schock et al., 1996). Solubility models show that the lowest lead levels occur when pH is around 9.8 (Schock and Gardels, 1983; Schock, 1989; U.S. EPA, 1992; Schock et al., 1996). However, these pH relationships may not be valid for insoluble tetravalent lead dioxide (PbO2) solids, which have been discovered in lead pipe deposits from several different water systems (Schock et al., 1996, 2001). Based on tabulated thermodynamic data, the pH relationship of lead dioxide may be opposite to that of divalent lead solids (e.g., cerussite, hydrocerrussite) (Schock et al., 2001; Schock and Giani, 2004). Lytle and Schock (2005) demonstrated that lead dioxide easily formed at pH 6-6.5 in water with persistent free chlorine residuals in weeks to months.

Unlike contamination from lead pipes and leaded copper alloys, which is mainly controlled by the solubility of the corrosion products, contamination from leaded solders is largely controlled by galvanic corrosion (Oliphant, 1983b; Schock, 1990b; Reiber, 1991; Singley, 1994). An increase in pH is associated with a decrease in galvanic corrosion of leaded solders (Oliphant, 1983b; Gregory, 1990; Reiber, 1991; Singley, 1994).

Utility experience has also shown that the lowest levels of lead at the tap are associated with pH levels above 8 (Karalekas et al., 1983; Lee et al., 1989; Dodrill and Edwards, 1995; Douglas et al., 2004). From 1999 to 2003, the City of Ottawa evaluated a number of chemical alternatives to control corrosion in their distribution system (Douglas et al., 2004). Based on bench- and pilot-scale experimental results and analysis of the impacts on a number of criteria, a corrosion control strategy was established whereby a pH of 9.2 and a minimum alkalinity target of 35 mg/L as calcium carbonate would be achieved through the use of sodium hydroxide and carbon dioxide. During the initial implementation phase, the switch to sodium hydroxide occurred while maintaining the pH at 8.5. However, subsequent to a request for lead testing by a client, the investigators found an area of the city with high levels of lead at the tap (10-15 µg/L for flowing samples). The problem was attributed to nitrification within the distribution system, which caused a reduction in the pH from 8.5 to a range of 7.8-8.2 and resulted in lead leaching from lead service lines. The pH was increased from 8.5 to 9.2 to address the nitrification issue and reduce the dissolution of lead. This increase in the pH almost immediately reduced lead concentrations at the tap in the problem area to a range of 6-8 µg/L for flowing samples. Ongoing monitoring has demonstrated that lead levels at the tap consistently ranged from 1.3 to 6.8 µg/L following the increase in pH, well below the regulated level (Ontario Drinking Water Standard) of 10 µg/L (Douglas et al., 2007).

Examination of utility data provided by 365 utilities under the U.S. EPA Lead and Copper Rule revealed that the average 90th-percentile lead levels at the tap were dependent on both pH and alkalinity (Dodrill and Edwards, 1995). In the lowest pH category (pH < 7.4) and lowest alkalinity category (alkalinity < 30 mg/L as calcium carbonate), utilities had an 80% likelihood of exceeding the U.S. EPA Lead and Copper Rule Action level for lead of 0.015 mg/L. In this low-alkalinity category, only a pH greater than 8.4 seemed to reduce lead levels at the tap. However, when an alkalinity greater than 30 mg/L as calcium carbonate was combined with a pH greater than 7.4, the water produced could, in certain cases, meet the U.S. EPA Lead and Copper Rule Action level for lead.

A survey of 94 water utilities conducted in 1988 to determine lead levels at the consumer's tap and to evaluate the factors that influence them showed similar results (Lee et al., 1989). In total, 1484 sites, including both non-lead and lead service lines, were sampled after an overnight stagnation of at least 6 h. The results of the study clearly demonstrated that maintaining a pH of at least 8.0 effectively controlled lead levels (< 10 µg/L) in the 1st litre collected at the tap. The Boston, Massachusetts, metropolitan area conducted a 5-year study to reduce lead concentrations in its drinking water distribution system (Karalekas et al., 1983). Fourteen households were examined for lead concentrations at the tap, in their lead service lines and in their adjoining distribution systems from 1976 to 1981. Average concentrations were reported for combined samples taken (1) after overnight stagnation at the tap, (2) after the water turned cold and (3) after the system was flushed for an additional 3 min. Even if alkalinity remained very low (on average 12 mg/L as calcium carbonate), raising the pH from 6.7 to 8.5 reduced average lead concentrations from 0.128 to 0.035 mg/L.


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Although the hydrogen ion does not play a direct reduction role on copper surfaces, pH can influence copper corrosion by altering the equilibrium potential of the oxygen reduction half-reaction and by changing the speciation of copper in solution (Reiber, 1989). Copper corrosion increases rapidly as the pH drops below 6; in addition, uniform corrosion rates can be high at low pH values (below about pH 7), causing metal thinning. At higher pH values (above about pH 8), copper corrosion problems are almost always associated with non-uniform or pitting corrosion processes (Edwards et al., 1994a; Ferguson et al., 1996). Edwards et al. (1994b) found that for new copper surfaces exposed to simple solutions that contained bicarbonate, chloride, nitrate, perchlorate or sulphate, increasing the pH from 5.5 to 7.0 roughly halved corrosion rates, but further increases in pH yielded only subtle changes.

The prediction of copper levels in drinking water relies on the solubility and physical properties of the cupric oxide, hydroxide and basic carbonate solids that comprise most scales in copper water systems (Schock et al., 1995). In the cupric hydroxide model of Schock et al. (1995), a decrease in copper solubility with higher pH is evident. Above a pH of approximately 9.5, an upturn in solubility is predicted, caused by carbonate and hydroxide complexes increasing the solubility of cupric hydroxide. Examination of experience from 361 utilities reporting copper levels under the U.S. EPA Lead and Copper Rule revealed that the average 90th-percentile copper levels were highest in waters with pH below 7.4 and that no utilities with pH above 7.8 exceeded the U.S. EPA's action level for copper of 1.3 mg/L (Dodrill and Edwards, 1995). However, problems associated with copper solubility were also found to persist up to about pH 7.9 in cold, high-alkalinity and high-sulphate groundwater (Edwards et al., 1994a).

In the pH range of 7-9, both the corrosion rate and the degree of tuberculation of iron distribution systems generally increase with increasing pH (Larson and Skold, 1958; Stumm, 1960; Hatch, 1969; Pisigan and Singley, 1987). Iron levels, however, were usually reported to decrease with increasing pH (Karalekas et al., 1983; Kashinkunti et al., 1999; Broo et al., 2001; Sarin et al., 2003). In a pipe loop system constructed from 90- to100-year-old unlined cast iron pipes taken from a Boston distribution system, iron concentrations were found to steadily decrease when the pH was raised from 7.6 to 9.5 (Sarin et al., 2003). Similarly, when iron was measured in the distribution system following a pH increase from 6.7 to 8.5, a consistent downward trend in iron concentrations was found over 2 years (Karalekas et al., 1983). These observations are consistent with the fact that the solubility of iron-based corrosion by-products decreases with increasing pH.

Water with low pH, low alkalinity and low calcium is particularly aggressive towards cement materials. The water quality problems that may occur are linked to the chemistry of the cement. Lime from the cement releases calcium ions and hydroxyl ions into the drinking water. This, in turn, may result in a substantial pH increase, depending on the buffering capacity of the water (Leroy et al., 1996). Pilot-scale tests were conducted to simulate low-flow conditions of newly lined cement mortar pipes carrying low-alkalinity water (Douglas et al., 1996). In the water with an initial pH of 7.2, alkalinity of 14 mg/L as calcium carbonate and calcium at 13 mg/L as calcium carbonate, measures of pH as high as 12.5 were found. Similarly, in the water with an initial pH of 7.8, alkalinity of 71 mg/L as calcium carbonate and calcium at 39 mg/L as calcium carbonate, measures of pH as high as 12 were found. The most significant pH increases were found during the 1st week of the experiment, and pH decreased slowly with aging of the lining. In a series of field and test rig trials to determine the impact of in situ cement mortar lining on water quality, Conroy et al. (1994) observed that in low-flow and low-alkalinity water (around 10 mg/L as calcium carbonate), pH increases exceeding 9.5 could occur for over 2 years following the lining.

A series of field trials carried out throughout the United Kingdom in areas with different water qualities found that high pH in cement pipes can render lead soluble. Lead levels increased significantly with increasing pH when pH was above 10.5. The concentration of lead ranged from just less than 100 µg/L at pH 11 to greater than 1000 µg/L above pH 12 (Conroy, 1991). This brings into question the accuracy of the solubility models for high pH ranges and the point at which pH adjustment may become detrimental.

Elevated pH levels resulting from cement leaching may also contribute to aluminum leaching from cement materials, since high pH may increase aluminum solubility (Berend and Trouwborst, 1999).

 

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