TechnologyMSolutions From farm waste to textile dyeing
© 2001 American Chemical Society
from a renewable source like the sun, that is just what the researchers are trying to do. To purify the salt, two main activities need to occur, says Jenkins. First, the sample needs to be concentrated. “These brines, as strong as they are, are still not very concentrated compared to industrial sources. So we have to concentrate on the order of VASHEK CERVINKA
Researchers at the University of California–Davis have found a use for the excess agricultural drainage salt that is piling up in California’s San Joaquin Valley. Gang Sun and Jiyoon Jung of UC–Davis’s Division of Textiles and Clothing recently reported that the waste salt, which is primarily sodium sulfate, can be recovered and used for textile dyeing (Environ. Sci. Technol. 2001, 35 (16), 3391–3395). Since then, they have found at least one textile dyeing facility that is willing to give the agricultural salt a try. “The California Department of Water Resources has been searching for either a method to produce a value-added product from agricultural waste salt or an economical way to discharge the waste,” says Sun, a textile chemist who proposed using the recovered salt for textile dyeing. “In California, we have a lot of apparel companies that use sodium sulfate in their dyeing process,” he adds. U.S. Dyeing & Finishing (USDF), a textile company in Garden Grove, CA, which uses more than 1000 tons of sodium sulfate annually, is particularly interested in pilot testing the waste agricultural salt. According to Vashek Cervinka of the California Department of Water Resources, who is coordinating the effort to link farmers with the textile industry, USDF has asked for a 100-kg sample of waste salt from the San Joaquin Valley. “If they are satisfied with the salt, we will put them together with the farmer who has the salt on his farm,” he says. For salts that don’t perform up to textile industry standards, the key is to develop an economical way to remove impurities, such as sodium chloride or calcium and magnesium salts, which can affect properties like color and color fastness, says Sun. With the help of Bryan Jenkins, a biological and agricultural engineering professor at UC–Davis, and energy
Researchersare looking fornew w aysto use tonsofagriculturaldrainage saltthat isaccum ulating in California’sSan Joaquin Valley.
50-fold,” he says. Second, the unwanted and wanted salts are separated on the basis of differences in their solubilities. “Sodium sulfate is remarkably different in its solubility characteristics than sodium chloride, which is the other main salt,” says Jenkins. The brine solutions are concentrated at a relatively high temperature, and then using ambient cooling, the tem-
perature of the solution is dropped. “At that point, sodium sulfate should precipitate, whereas sodium chloride should not,” explains Jenkins. Repeated precipitation and recrystalization should yield pure sodium sulfate, say the researchers. Energy is needed to heat the brine solutions during the concentration step. According to Sun, solar energy has a lot of potential. In preliminary experiments to purify recovered agricultural salts, the researchers kept the temperature between 10 and 40 ˚C, to simulate the daily maximum and minimum temperatures in the summer in California’s central valley. In addition to solar energy, they are considering using waste power plant heat and biomass, which is produced as part of the overall drainage water management system, says Jenkins. At a going rate of $80–$100 per ton of salt, Sun jokes that farmers could make more money selling salt than crops. But without enough in-state consumers for the amount of salt that is available, the salt would have to be exported elsewhere, and transportation and shipping costs would be too high, he says. In California’s central valley alone, salt production exceeds 600,000 tons annually. “Overall consumption of sodium sulfate in the textile industry was about 95,400 tons in 1997, accounting for 15% of the overall consumption of the salt in the U.S.,” according to the researchers. Accumulating agricultural drainage salt is a worldwide problem, and as some say, an inevitable consequence of using freshwater to irrigate farmland. But as more creative uses for recycled sodium sulfate are found, the problem becomes more manageable. Other industrial uses for sodium sulfate include glass, paper, and detergent manufacturing, says Jenkins. —BRITT E. ERICKSON
NOVEMBER 1, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY I 445 A
