Cactus too purifies water.

Many Mexican communities have drinking water laced with unsafe levels of arsenic and unappealing amounts of sand and other solids. To make the water cleaner, Norma Alcantar, a chemical engineer at the University of South Florida in Tampa and her colleagues are working on an environmentally benign filtering process based on a plant found all over Mexico: the nopal cactus or prickly pear.

Latin American communities once used the cactus (Opuntia ficus indica) to filter water, says Alcantar. After boiling the edible plant, they dumped the pot water into a separate vessel containing drinking water, a practice that caused gritty particles to settle to the bottom. But this is "knowledge that is almost gone" in Mexican homes, she says.

Alcantar and her group suspected that the nopal could provide a solution for Mexican villages with contaminated ground water. Extracts of the cactus not only separate suspended solids from water but also sop up heavy metals dissolved in it.

To explore potential water-purifying applications, the scientists investigated the cactus' mucilage, the thick, gooey substance that enables the plant to store large amounts of water. The team knew that when added to untreated drinking water, this substance can precipitate ions, particles, and bacteria.

The researchers first separated the mucilage into its two main components--a liquid and a gel. Tests with the gel portion indicated that it alone can cause suspended solids to settle out of water. Moreover, it does so in only 5 minutes, a third of the time that aluminum sulfate takes to precipitate solids. That chemical is commonly used in water-treatment plants. In another test, the gel removed half of the arsenic in a sample of water after 36 hours. The researchers are planning to test how the liquid portion of the mucilage contributes to the water-purifying process.

Alcantar's team is also identifying the chemical structures of the extracts to figure out which molecules are responsible for the nopal's filtering might. One goal of the research, Alcantar says, is to find the optimal mucilage dosage for achieving the best water-purifying results.

Alcantar says that her group plans to design a cactus-based filtering system by next summer, to be tested in the Mexican city of Temamatla. The water there is gritty and tainted with arsenic, and the area has a bountiful supply of nopal.--A.C.