OPTIMIZING THE USE OF ARSENIC-HYPERACCUMULATING FERNS FOR TREATMENT OF ARSENIC-CONTAMINATED WATER

摘要

Water supplies in many regions of the world are contaminated by the carcinogenic metalloid arsenic (As). Though the U.S. Environmental Protection Agency (EPA) recently reduced the Maximum Contaminant Level (MCL) from 50 ƒÝg/L to 10 ƒÝg/L, water in some developing countries are far from this standard. Therefore, highly efficient, sustainable technologies that require low maintenance and have low capital costs must be considered.Several Pteris fern species were recently discovered to be effective at removing As from soil and water. This study was designed understanding the key operating parameters that would form the basis for an actual treatment process in regions where As in water exceeds 50 ƒÝg/L limit.This study determined that aeration facilitates greater removal efficiency of As by Pteris ferns in water. Following 72 hours of exposure, Pteris ferns demonstrated 96 ¡Ó 2% removal efficiency with aeration and 84 ¡Ó 7% without. However, the benefits of aeration can be compensated by extending the contact time to eliminate the energy cost associated with aeration. Bench-scale experiment with 11 Pteris ferns in a batch reactor filled showed that the removal of As occurs much faster and to a greater extent when compared to individual plants. Individual plants were not always able to reduce the As concentration from 300 ppb to less than 50 ppb in a 4-day period. However, the batch reactor filled with multiple plants was capable of removing As concentration from 275 ¡Ó 25 ppb to less than 50 ppb in a 3-day period, even after four cycles of repeated exposure. Because As can be adsorbed to hydrous ferrous oxides, co-precipitation and settling can aid As removal. When Pteris ferns were exposed to 250 ppb As in the presence of Fe (II) = 2.7 mg/L, they removed the As content to less than 50 ppb in a 24-hr period. In the absence of Pteris ferns, the As content could not be reduced below 100 ppb in a 4-day period. Therefore, the presence of naturally occurring iron can significantly enhance the removal efficiency of Pteris ferns.