ARSENIC SEQUESTRATION BY PYRITE, IRON SULFATE, AND ZERO VALENT IRON IN SOIL WATER

Soil arsenic (As) sequestration experiments revealed that pyrite, iron sulfate, and zero valent iron are all effective at removing As from soil water. The total amount of As collected in the leachate of the control column with no sequestering iron agents added was at least 5 times more than those from columns with sequestering iron agents added. In addition, iron sulfate seems to be more effective than the other two iron agents because of the sulfate reduction and adsorption. Powdered arsenopyrite was used as the As source material in the columns. The individual experimental column has a diameter of 5.08 cm (2 inch) and height of 30.5 cm and each was filled with approximately 480 grams of dried loamy soil. Approximately one gram of arsenopyrite layer was laid above the 100 grams of dried loamy soil layer, with another 380 grams of dried soil layer on top in the control column. In the iron agent added columns, 5 grams of powdered pyrite, iron sulfate, and zero valent iron were mixed with 200 grams of soil each before they were added and placed above the one gram arsenopyrite layer. Each soil column was flushed with distilled water (daily in the 1^st week and less frequent later for a 6-week period) at volumes varied from 50 to 350 ml, average ~250 ml, depending on the flow rate. Among the above four columns, flow rate in the iron sulfate column was the slowest. For the emulsified nano-zero valent iron (NZVI) column (the 5^th column), the flow rate was slowed to a stalemate initially. A different setup had to be built with a gravity drain head and arsenopyrite layer near the top of the column for the NZVI study. As and relevant elements concentrations in leachates were measured by ICP. For the iron sulfate column, sulfate reduction changes the As⁵⁺ to As³⁺ and increases the adsorption and co-precipitation of As. For the pyrite column, pyrite adsorbs As and co-precipitates As into its crystal structure. Zero valent iron removes As from soil solution by the reduction of Fe⁰, by oxidation-copreciptation, and adsorption of its oxidized iron shell. Our experiments suggest that field sequestration of As may be conducted using either pyrite, iron sulfate or zero valent iron, preferable iron sulfate when the aquifer is permeable. However, a large pressure may need to be applied if the NZVI is used.