Bench-scale evaluation of ferrous iron oxidation kinetics in drinking water: Effect of corrosion control and dissolved organic matter

Abstract

Corrosion control strategies are important for many utilities in maintaining water quality from the water treatment plant to the customers’ tap. In drinking water with low alkalinity, water quality can become significantly degraded in iron-based pipes if water utilities are not diligent in maintaining proper corrosion control. This article reports on experiments conducted in bicarbonate buffered (5 mg-C/L) synthetic water to determine the effects of corrosion control (pH and phosphate) and dissolved organic matter (DOM) on the rate constants of the Fe(II) oxidation process. A factorial design approach elucidated that pH (P = 0.007, contribution: 42.5%) and phosphate (P = 0.025, contribution: 22.7%) were the statistically significant factors in the Fe(II) oxidation process at a 95% confidence level. The comprehensive study revealed a significant dependency relationship between the Fe(II) oxidation rate constants (k) and phosphate-to- Fe(II) mole ratio. At pH 6.5, the optimum mole ratio was found to be 0.3 to reduce the k values. Conversely, the k values were observed to increase for the phosphate-to- Fe(II) mole ratio >1. The factorial design approach revealed that chlorine and DOM for the designated dosages did not cause a statistically significant (α=0.05, P>0.05) change in rate constants. However, an increment of the chlorine to ferrous iron mole ratio by a factor of ∼2.5 resulted in an increase k values by a factor of ∼10. This study conclusively demonstrated that the lowest Fe(II) oxidation rate constant was obtained under low pH conditions (pH ≤ 6.5), with chlorine doses less than 2.2 mg/L and with a phosphate-to-Fe(II) mole ratio≈0.3 in the iron water systems.

Keywords:

• Corrosion control
• iron oxidation
• pH
• phosphate
• organic matter
• water

Acknowledgments

The authors gratefully acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada (NSERC), Canadian Water Network (CWN) and Faculty of Graduate Studies, Dalhousie University, Canada. The authors also would like to acknowledge and thank J.C.O. Young (Professor Emeritus, Chemistry Dept., St. Mary's University, Canada) for technical support.