Coated sand filters dirty water

RICE U. (US) — A bit of nanotech “magic” is turning sand into a super-effective water purifier.

Beds of sand are commonly 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.

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

A team from Rice University published a report in the 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.

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.

Pulickel Ajayan, a 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 into 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. (The Environmental Protection Agency’s maximum contaminant level goal 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.

Beakers of solution containing mercury and Rhodamine B dye await purification through a new process developed at Rice University. (Credit: Jeff Fitlow, Rice University)

The lab is looking at ways to further functionalize graphite oxide shells to enhance contaminant removal. “By attaching different functional moieties onto graphite oxide, we could engineer some form of a ‘super sand’ to target specific contaminants species, like arsenic, trichloroethylene, and others,” says graduate student Wei Gao, primary author of the paper.

Co-authors include 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.

Nanoholdings supported the research.

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