SOFIA - OFR 2007-1374 - Mitigation of Sulfate with Natural Minerals

Limestone neutralization is a method used to remove acidity from wastewater, such as acid mine drainage (Geldenhuys et al., 2003). Typically, a fluidized bed reactor with limestone particles < 4 mm in diameter is employed. The process has the added advantage of removing some sulfate as gypsum (Maree et al., 2004). This process acts only as a pre-treatment, as it will not reduce sulfate levels appreciably, except for wastewaters with extremely high levels of sulfate (not encountered in the Everglades). It has the benefit of being relatively inexpensive, especially with limestone being readily available in south Florida. Also, pH levels are stabilized by the limestone's natural buffering capacity.

5.2 Feldspar ion exchange -

In one study (Priyantha and Perera, 2000), feldspar was shown to effectively remove 34-57% of dissolved sulfate from various industrial wastewater streams over a sulfate concentration range of 1 to 75 mg/L. Feldspar was also shown to remove 38-53% of the phosphate in these effluents. The process employed used feldspar (the most abundant mineral in igneous rock) packed in columns. Effluent was run through the column at different flow rates (10 - 2 cm³min^-1). The efficiency of sulfate and phosphate removal was greater at lower flow rates. Instrumental analysis revealed that the principal mode of removal of phosphate and sulfate was ion exchange and adsorption. Removal of sulfate and phosphate from the wastewater stream followed first order kinetics. The study provided no information on how long the feldspar will retain sulfate and phosphate removal efficiency before replacement or recharging of the feldspar would be needed. Further study would be needed to determine the replacement time, and cost/benefits of this sulfate removal approach.

5.3 Zeolites -

Porous, crystalline hydrated alumino-silicate minerals referred to as zeolites are frequently used in water purification applications (Babel and Kurniawan, 2003). Zeolites can remove ions from water by acting as molecular sieves, sorting ions based on size exclusion. Zeolites have a very regular pore structure at the molecular level, permitting retention of ions that can fit through the molecular pores based on size. Zeolites occur naturally and can be mined, but synthesized zeolites are more often used in water purification due to the presence of unwanted contaminants in some natural zeolites. In addition, manufactured zeolites with properties for specific purification applications have been developed; about 1500 types of manufactured zeolites are available commercially. Clinoptilolite-dominated zeolites modified with the quaternary amine hexadecyltrimethylammonium or HDTMA have been shown to remove sulfate and other oxyanions from water (Haggerty and Bowman, 1994). Removal of the oxyanions from aqueous solution appears to proceed via formation of an anion-HDTMA precipitate on the surface of the zeolite.

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