Arsenic hyperaccumulation by Pteris vittata L. and its potential for phytoremediation of arsenic-contaminated soils

Pteris vittata L, an arsenic-hyperaccumulating fern, was examined to understand its hyperaccumulating ability and for its use in remediating arsenic-contaminated soils. Transport of arsenic in xylem sap of P. vittata was investigated. Ferns were subjected to arsenate, arsenite, dimethylarsinic acid (DMA) or monomethylarsonic acid (MMA). Xylem sap was collected and analyzed for arsenic concentration, speciation and phosphorus concentration. When inorganic arsenic was supplied, arsenate appeared to be the preferred species transported in the xylem sap. When arsenic was supplied in methylated form, it was transported mainly in that form. Results from glutathione reductase (GR) and catalase (CAT) enzymatic studies in P. vittata revealed that, upon arsenic exposure, CAT activity was induced but GR activity was not. Further, GR was not inhibited or activated by arsenic. However, CAT activity appeared to be activated by arsenate. This activation may allow P. vittata to more efficiently mediate stress caused by arsenic. A field study was conducted to determine the efficiency of P. vittata in phytoextraction of arsenic contaminated sail. The study suggested P. vittata is capable of accumulating arsenic from contaminated sites, and a single harvest per year yields the greatest arsenic removal. Further, results from sequential arsenic fractionation analyses suggested that P. vittata is able to access arsenic from more unavailable soil fractions. Phytoextraction of arsenic-contaminated soils using P. vittata may be competitive with conventional remediation systems, but its application may be more practical for low-level contamination. The phytoextraction study revealed a discrepancy in mass balance. One hypothesis was that combination of over watering and solubilization of arsenic by root exudates caused leaching. Therefore, it was important to identify if leaching was occurring. It was also hypothesized that leaching may be harnessed for development of an innovative ex-situ soil remediation method, phytoleaching. Water and chemical solutions were added to promote arsenic leaching, while ferns removed arsenic via uptake. More arsenic was leached from soil when ammonium phosphate solution was applied. When ferns were present in contaminated soil, less arsenic was leached, indicating that P. vittata does not promote arsenic leaching. Phytoleaching may be a feasible remediation option with additional studies and refinement.