By Beth Miller
Selenium is a critical element, particularly for the thyroid and immune system, but too little or too much can be harmful to both humans and wildlife. A team of researchers at Washington University in St. Louis has made strides in removing selenium contamination from water, which could ensure safe treatment of water from agricultural ponds, mining discharge or power plant wastewater to meet federal maximum levels.
Daniel Giammar , the Walter E. Browne Professor of Environmental Engineering and director of the university's Center for the Environment, and his lab used iron electrocoagulation to remove selenium from water in different experiments. Results of the research, funded by the National Association for Water Innovation of U.S. Department of Energy, were recently published in Environmental Science & Technology and ACS ES&T Engineering.
Removing selenium from water is challenging because it's so water soluble, Giammar said. Iron electrocoagulation is up to the challenge by generating iron-containing solids with large surface areas. During coagulation, selenium then chemically binds to those surfaces. It also can be chemically transformed into a type of selenium that will bind more strongly.
In one study, published in Environmental Science & Technology, graduate student Xicheng He in Giammar's Aquatic Chemistry Laboratory removed selenium from water using iron electrocoagulation in a flow-through reactor, built by research partner WaterTectonics, to generate different forms of rust.
"We apply a current to the iron reactor, which forces it to corrode faster than it normally would and generate rust," Giammar said. "Iron can generate green rust before red rust, and the green rust is incredibly reactive. This reacts with selenium to pull selenium out of the water into these iron-containing particles, which we then remove with a filter."
This process removed more than 98% of selenium by flowing through the iron reactor for 11 seconds then settling for an hour, where it remained tightly bound in solids that would be considered nonhazardous.
In another study, published in ACS ES&T Engineering, graduate student Yihang Yuan studied 15 different combinations of water chemistry and the different effects of the electrochemical operating conditions on selenium removal in batch reactors. Through running experiments in well-mixed and continuously monitored beakers of selenium-containing solution, Yuan developed a reaction-based model to predict electrocoagulation performance to remove selenium with varying conditions of oxygen and pH.
"We showed that this works in relatively simple compositions because we really wanted to isolate the effects of pH and dissolved oxygen," Giammar said. "We see that it works in the lab, and we can make it relevant to the real world."
Going forward, Giammar's lab is looking beyond selenium.
"Now that we have the reactor and the protocols, we are looking at some other contaminants and natural organic matter, both where we control the compositions and with some real water samples," he said. "We didn't invent the reactor technology, but we showed WaterTectonics that it could work in different cases than what they might have thought about."
