Removal of Nitrates and Phosphates by Ion Exchange with Hydrotalcite

Because waters contaminated with nutrients often have high levels of nitrates and phosphates, the removal of both from aqueous solution with hydrotalcite, a clay mineral ion exchange media, was investigated. Isotherms were developed for nitrate and phosphate removal in single ion solutions within the ranges typically found in eutrophic waters or contaminated wells. Residual concentrations were compared to Environmental Protection Agency recommended levels for drinking and surface waters. Next, experiments were performed to determine what effect the presence of one anion had on the removal of the other. First, removal of phosphates in high and low nitrate concentration solutions was compared to phosphate removal in the single ion solutions. Likewise, removal of nitrates in high and low phosphate solutions was compared to nitrate removals in the single ion solution. Statistical analyses were performed to delineate these results. Phosphates, at the concentrations considered, were almost completely removed by hydrotalcite. Nitrates, on the other hand, were removed to below the EPA limit for only initial nitrate levels up to 30 mg L(-1). Removal was over 90% for 25 mg L(-1) nitrate, but decreased significantly at higher concentrations. Phosphate removal remained over 95% and was not affected significantly by the presence of nitrates for nitrate levels up to 40 mg L(-1). Nitrate removals, however, were significantly decreased by the presence of phosphates at phosphate levels as low as 0.31 mg L(-1). However, a second batch contact of the residual solution with fresh hydrotalcite was sufficient to reduce both phosphates and nitrates to below EPA standards. For both nitrate and phosphate removal, however, the pH requirements necessitate a pump and treat system, which limits the applicability of the process.