Student Research: Edward Evanson
Background: Disinfecting drinking water with chemicals such as chlorine and iodine prevents the spread of pathogenic bacteria but also creates unintended, potentially carcinogenic, chemical disinfection by-products (DBPs).
Objective: This study quantifies the formation of the DBP iodoform after adding iodine tablets to natural water collected in two different seasons, and measures the amount of iodoform that is removed by granular activated carbon (GAC) filtration.
Method: Samples were collected from natural waters of Washington State during two seasons from four water bodies. Field sampling occurred over the summer of 2010 and winter of 2010-2011. Raw water samples were tested for water quality parameters such as dissolved organic carbon (DOC) and pH. These samples were iodinated for 24hrs and then tested for residual iodine and iodoform concentrations. Duplicate samples from the winter sampling period and each water body were also iodinated and timepoint samples from 30min and 24hrs were passed through a GAC filter and then tested for residual iodine and iodoform concentrations. Residual iodine was quenched with potassium iodide and measured as triiodide by UV spectrophotometry at 351 nm. Iodoform was analyzed by solid phase extraction and gas chromatography with electron capture detection.
Results: Results from two seasons show that residual iodine levels are depleted least in water bodies with the lowest levels of DOC. Iodoform levels are highest in water bodies with high levels of DOC. When DOC was 3mg C/L iodoform was 2200 ug/L and when DOC was 2.61 mg C/L iodoform was 630 ug/L. When iodination with iodine tablets was followed by GAC filtration, a maximum decrease of 97% in the amount of residual iodine was observed.
Conclusion: When natural water containing high levels of DOC is treated with iodine tablets, iodoform formation and iodine consumption are significant. GAC filtration effectively removes residual iodine from iodinated drinking water.