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.
Other ongoing research by Alcantar's team has shown that the mucilage can also kill bacteria in the water, solving another potential water quality problem. The mucilage either engulfs the bacteria and starves them, or it binds to the bacteria and causes them to settle out of the water.
Mucilage consists of carbohydrates and sugars. The team is investigating exactly how these components interact with bacteria, arsenic and suspended particles. The evidence so far suggests that when arsenic binds to the sugars, the number of charges on the particle changes, which changes its ability to stay dissolved. Similar processes appear to be at work with the other contaminants.
Alcantar estimates that one lobe of prickly pear would supply a family of five for about five weeks. Alcantar's team is still optimizing and developing the best system, but she envisions that each family would pass their water through a filter that would periodically be recharged with fresh mucilage from prickly pear grown at home or in the community.
An advantage of the approach is that prickly pear is familiar to local communities, which her work suggests will help ease its acceptance by local people in Temamatla, Mexico, where she is working with families to design and supply filters.
"Our survey showed that 97 percent of the community wanted to have a filter, especially if it's based on something that they know and are accustomed to," said Alcantar
"There are a number of aspects to this project that I think are unique," said Angela Lindner of the University of Florida in Gainesville. "She knows these communities so she understands the social aspects that are involved. She's keeping in mind that the one who is going to be making these filters is going to be the person who is drinking the water. That's rarely done in engineering design."