Acid mine drainage (AMD) contains high concentrations of Fe, Al and Mn, all of which are known to bind to phosphate in soils and sediments. Although the precipitation and adsorption reactions of phosphate with each of these metals individually are well known, the interaction of phosphate with combinations of these metals (as is common in AMD) is not well documented. The main objectives of this study were to determine the interactive effects of these metals on soluble phosphate concentrations at varying pH levels and to determine if the presence of sulfate and calcium altered these effects. Precipitation studies were conducted by mixing phosphate with Fe, Al and Mn, alone and in combination, at pH 2 and then gradually raising the pH to 10 with NaOH. Adsorption studies were conducted by adding phosphate to solutions of Fe or Al oxide at pH 7. These trials were repeated using field-collected AMD from four different sources. In the field study, seven streams were sampled upstream and downstream of an AMD input and analyzed for phosphate, sulfate, calcium, Fe, Al and Mn.
Results/Conclusions
Lab studies demonstrated that soluble phosphate was reduced by approximately 98% and 97% in the presence of Fe and Al, respectively, due to precipitation and adsorption reactions. Maximum phosphate removal occurred between pH 4 and 9 for Fe and between pH 5 and 8 for Al. Manganese only precipitated with phosphate at pH 10 and above. The adsorption study demonstrated that at pH 7, adsorption of phosphate to Fe- and Al-oxides was rapid, occurring within just 10 minutes, and did not change appreciably over 7 days. Phosphate adsorption led to 97% reduction in soluble phosphate with both Fe- and Al-oxides. The presence of sulfate ions, which are present in high concentrations in AMD, led to less precipitation of phosphate with Fe and Al (94% phosphate reduction) and less adsorption of phosphate to Fe- and Al-oxides (95% phosphate reduction). Precipitation and adsorption studies using AMD yielded results similar to the lab study. The field study verified that AMD additions to stream caused major reductions in soluble phosphate.
