Jun 30, 2011

Researchers Find Coated Sand Excels at Contaminant Removal

Tests show that graphite-coated sand performed as well as activated carbon

Beds of sand are used throughout the world to filter drinking water. The particle size of sand and surface modifications determine the efficiency of sand in removing contaminants from water.

Rice University researchers developed a technique that makes use of graphite oxide, a product in the chemical exfoliation process of graphite (otherwise known as pencil lead) that leads to single-atom sheets known as graphene via subsequent reduction.

A team from the Rice lab of Professor Pulickel Ajayan published a report in the American Chemical Society journal Applied Materials and Interfaces describing a process to coat coarse grains of sand in graphite oxide. The resulting material is several times more efficient at removing contaminants than sand alone, the report said.

According to the research, nanosheets of graphite oxide can be tailored to have hydrophobic (water-hating) and hydrophilic (water-loving) properties. When mixed in a solution with sand, they self-assemble into coatings around the grains and keep the hydrophilic parts exposed. Adding aromatic thiol molecules to the coatings enhances their ability to sequester water-soluble contaminants.

Ajayan, a Rice professor in mechanical engineering and materials science and of chemistry, and his collaborators from Australia and Georgia conducted experiments to compare this coated sand with plain sand and activated carbon granules used by municipalities and in-home filtration systems.

The researchers ran two model contaminants—mercury (at 400 parts per billion) and Rhodamine B dye (10 parts per million)—through sand and coated sand placed in filtration columns. They found coarse sand's adsorption capacity of mercury was saturated within 10 minutes.

The coated sand continued removing mercury for more than 50 minutes and resulted in filtered water with less than one part per billion of mercury. The U.S. Environmental Protection Agency's maximum contaminant level for mercury in drinking water is two parts per billion. Results for water treated with Rhodamine B dye were similar.

The researchers found coated sand sequestered contaminants just as well as the commercially available active carbon filtration systems they tested. The lab is looking at ways to further functionalize graphite oxide shells to enhance contaminant removal.

Co-authors of the report were senior research scientist Lawrence Alemany, postdoctoral researcher Tharangattu Narayanan and recent graduate Miguel Ibarra, all of Rice; Mainak Majumder, a former postdoctoral researcher at Rice, now a lecturer at Monash University, Clayton, Australia; and Bhabendra Pradhan, chief scientific officer at Nanoholdings LLC of Marietta, Ga.