Comparison of sodium silicate and phosphate for controlling lead release from copper pipe rigs

Abstract

This study compared sodium silicate and zinc orthophosphate for their ability to decrease lead and copper release from pilot-scale premise plumbing in low alkalinity (<5 mg/L CaCO3) water. The study used pipe loops (to age the water) connected to copper pipe rigs. The copper pipe rigs were constructed using 50:50 lead:tin solder, which provided the only source of lead. Three different treatments were compared, using the same source water: phosphate (0.8 mg-PO4/L at pH 7.3), sodium silicate with pH control (18 mg-Si/L at pH 7.3), and sodium silicate (18 mg-Si/L at pH 6.3). Lead and copper levels (both total and dissolved) were measured following the copper pipe rig after two different stagnation times (24 h, 30 min). The phosphate treatment consistently resulted in significantly lower lead release compared to sodium silicate at the same pH of 7.3. The phosphate treated pipe rigs released 12 μg/L and 2 μg/L lead following the 24 h and 30 min stagnation times, respectively. The sodium silicate with pH control system released 46 μg/L and 6.7 μg/L lead following 24 h and 30 min stagnation time, respectively. The sodium silicate treated pipe rigs released 27.4 and 4.6 μg/L lead following the 24 h and 30 min stagnation time, respectively. The phosphate treatment also consistently released less copper than both the sodium silicate with pH control treatment and the sodium silicate treatment. However, more lead particles were observed in the phosphate treatment compared to either of the sodium silicate treatments. The difference in performance between the sodium silicate corrosion inhibitor at two different pH levels, suggests that the performance of sodium silicate could be further optimized.

Effectiveness of Sodium Silicates for Lead Corrosion Control: A Critical Review of Current Data

“… Lintereur et al: Average lead concentrations, average silica concentrations, and pH were extracted from Table 5 Woszczynski et al: Average total lead concentrations in sodium silicate- and zinc orthophosphate-treated systems at stagnation times of 30 min and 24 h were extracted from the article’s abstract. Average dissolved lead concentrations were estimated using the average percentage of particulate lead reported in the study. Kogo et al: Average total and dissolved lead concentrations in orthophosphate- and sodium silicate-treated systems were extracted from Table 4 .…”

“…(c) Lintereur et al 18 : Average lead concentrations, average silica concentrations, and pH were extracted from Table 5. 18 (d) Woszczynski et al 19 : Average total lead concentrations in sodium silicate-and zinc orthophosphate-treated systems at stagnation times of 30 min and 24 h were extracted from the article's abstract. Average dissolved lead concentrations were estimated using the average percentage of particulate lead reported in the study.…”

Effect of sodium silicate on drinking water biofilm development

“…Sodium silicates are used primarily for iron/manganese sequestration and coagulation, but they have been used occasionally as alternatives to orthophosphate; 14 in particular, they have been evaluated for lead release control with mixed results. [15][16][17][18][19] Sodium silicates may inhibit the oxidation of ferrous iron, particularly in the pH range of 6.0-7.0, 20 and they may also reduce color and water turbidity. 21 While some focus has been directed to understanding sodium silicates as corrosion inhibitors, their impact on biofilm formation has received comparatively little attention.…”

Effect of sodium silicate on drinking water biofilm development

“…While sodium silicates are mostly used for sequestration of iron and coagulation applications, they have been studied for lead release control applications. [14][15][16] Sodium silicates may also inhibit the oxidation of ferrous iron, particularly in the pH range of 6.0 -7.0, 17 and they may reduce color and water turbidity. 18 While some focus has been directed to understanding sodium silicates as corrosion inhibitors, their impact on biofilm formation has not been documented in detail.…”