Cactus Goo Makes Water Safe

Sept. 17, 2008 -- The slimy ooze inside prickly pear cactuses that helps the plants store water in the desert can also be used for scouring arsenic, bacteria and cloudiness out of rural drinking water, according to research at the University of South Florida in Tampa.

Biochemical engineer Norma Alcantar first learned of the cactus's unique abilities from her grandmother, a native of north central Mexico. There, the residual water from boiling the flat, oval-shaped lobes of prickly pear for salads and other dishes was used to clear up cloudy water drawn from the river before use for cooking or drinking.

"When you boil [the prickly pear], what is getting separated is mucilage," Alcantar said.

Mucilage is the clear, gooey, viscous liquid from within the cactus which helps to seal water inside the plant so it can survive desert-dry conditions.

Alcantar began to study how the mucilage worked to clear cloudy water. She found that the mucilage binds to the dirt and causes the particles to coagulate, forming large enough clumps that they can settle out of the water.

Then, she turned her attention to other water contaminants. The group's more recent research has shown that the mucilage can also form a complex with arsenic, a carcinogenic water contaminant that can occur naturally or from industrial or agricultural pollution.

The arsenic-mucilage complex is large enough that it can be removed by drawing the water through a sand filter.

"Sometimes we get 80 percent removal, and sometimes we get lower than 50 percent removal," Alcantar said. "We don't yet know exactly what it is; we haven't found what are the exact best conditions for the mucilage [to get the most arsenic removal]."

What percentage removal is sufficient will depend on the amount of arsenic in the water supply.