Orthophosphate Added to Chloramine-Treated Water can Prevent Lead Contamination

In almost 80% of the water systems in the United States, a disinfectant is used in drinking water, which can form undesirable byproducts such as chloroform. Although an alternative exists, several cities have been apprehensive of its use.

The reason is the sudden increase in the lead levels in drinking water when the Washington, D.C., water authority switched from using free chlorine to chloramine in the year 2000. The levels remained high for four years while researchers identified the problem and implemented a solution.

The Washington incident had a terrifying effect in other cities where free chlorine was being used. A number of cities have stopped switching disinfectants, afraid of their own lead crisis.

A study performed at the McKelvey School of Engineering at Washington University in St. Louis could now help them safely switch. Researchers have discovered that when orthophosphate is added to the water supply before switching to chloramine, lead contamination can be eliminated in some situations.

The study outcomes were reported in Environmental Science & Technology.

The durability and malleability of lead made the material of choice for service lines—pipes delivering water from a water main to homes—during the first half of the 20^th century. In the presence of free chlorine, the pipes tend to corrode, leading to the accumulation of a specific type of lead, PbO₂, on the interior surfaces.

This lead accumulation is not the actual problem. Indeed, as long as the disinfectant used is free chlorine, PbO₂ is typically a positive factor, says Daniel Giammar, the Walter E. Browne Professor of Environmental Engineering at Washington University. Due to its low solubility, this type of lead remains as a solid on the pipes, rather than getting mixed in the water.

However, PbO₂ is not always so harmless.

There is a potential risk because the solubility is only low if you keep using this type of chlorine.

Daniel Giammar, Walter E. Browne Professor of Environmental Engineering, Washington University in St. Louis

When a different disinfectant like chloramine—the chlorine and ammonia mixture that used in Washington in late 2000—is used, the lead turns water-soluble. Then, the PbO₂ quickly dissolves and discharges lead into the water system.

In Washington, the research team identified that when a specific phosphate known as orthophosphate was added to the system, the result would be lead phosphate. Since this new material also had low solubility, the lead material again started to line the pipe walls rather than getting discharged into drinking water.

“But forming the new, low-solubility coating takes time,” stated Giammar. In the case of Washington, “the lead concentrations took months to come down.”

Although the solution had been found and implemented, residents had to tackle lead contamination in their water continuously for several months.

Our overarching question was, ‘Would they have had a problem if they had implemented the solution before they made the chlorine switch? What if they added orthophosphate before, as a preventative measure, and then they switched the disinfectant? Would they have had a problem?’

Daniel Giammar, Walter E. Browne Professor of Environmental Engineering, Washington University in St. Louis

Recreating Washington Water

The researchers arrived at the solution by recreating the 2000 Washington conditions in their lab. “We had to recreate the crisis, then watch the crisis happen and watch our proposed solution in parallel,” stated Giammar. They recreated Washington water by sourcing lead pipes.

The water was looped via a six-pipe system with free chlorine by Yeunook Bae, the first author of the study and a PhD student in Giammar’s lab, for 66 weeks to enable the accumulation of the lead scales. As soon as lead levels similar to those observed in Washington formed, the team divided the pipes into a control group and a study group.

Then, orthophosphate was added to the water in three of the pipe systems of the study group, for a period of 14 weeks. Subsequently, similar to the step taken by Washington water authority, in all six systems, the scientists switched from free chlorine to chloramine, where the water was looped through the pipes for a period of over 30 weeks.

The lead in the pipes to which orthophosphate was not added turned soluble, similar to what happened in Washington, resulting in high lead levels in the water. In the pipes that received orthophosphate, “levels went from really low to still quite low,” explained Giammar.

The experimental setting was configured such that researchers can take out small sections of the pipe without disturbing the system. This enabled them to observe how quickly the switch to chloramine had an effect on the system.

EPA has set a regulatory level of 15 μg per liter in drinking water.

Levels of lead in the control pipes without orthophosphate increased from 5 μg/L to over 100 μg/L within five days of the switch. In the next 30 weeks, the lead levels did not fall below 80 μg/L.

In the case of the orthophosphate-added water, the lead levels stayed less than 10 μg/L throughout the experiment.

The research team also observed a new phenomenon: Since the levels of calcium in Washington’s water was high, treatment using orthophosphate did not lead to a pure lead phosphate; instead, a calcium lead phosphate was formed.

This astonishing observation indicates that the peculiarity of each situation. According to Giammar, those who monitor water systems and are worried over switching disinfectants can not just benefit from this research. They can perform their own studies customized to their specific water and environmental conditions.

However, this discovery can help guide decisions in about 80% of American water systems in which free chlorine is still being used, such as New York City and Chicago.

Our next big step, is making sure places that are thinking about switching disinfectant know that the option is there to do it safely.

Daniel Giammar, Walter E. Browne Professor of Environmental Engineering, Washington University in St. Louis

Source: https://wustl.edu/