Iron and acid removal from acid mine drainage in open limestone systems

Passive systems, like wetlands, anoxic limestone drains, and open limestone channels, show promise for treating acid mine drainage (AMD) discharges. Open limestone channels have been developed for over ten years and the treatment effectiveness of AMD by open limestone systems shows wide variation. The variation is due to site conditions (slope and size of the limestone channel) but more importantly to the surface area of the limestone and iron concentrations in the water. Iron in AMD coats the limestone in open limestone systems thereby reducing the surface area available for reaction. Over time, the limestone appears to lose its ability to neutralize acidity and the water quality from open limestone systems is hard to predict.;This study was conducted to understand the effects of limestone surface area, iron concentration of the AMD, and coating thickness of iron on limestone neutralization of AMD. Experiments on limestone surface area showed that the required reaction time to reach a desired final pH is a function of limestone mass specific area, and initial and final acidity of proton acid solutions. The required reaction time, t = aAMS k, can be used to predict the outlet water quality for a hydrogen acid solution in an open limestone system based on a specified limestone particle size and lifetime.;Effects of iron concentration on neutralization of acid solutions by limestone were also researched by batch experiments in the laboratory. The required reaction time for neutralization was modified from previously developed equations by: tFe=tFe= 0+42ln Fe+35.1 pHf-176ln Fe-143 where t(Fe) is the required reaction time for iron acid solutions, t(Fe = 0) is the reaction time for hydrogen acid solutions, [Fe] is the iron concentration, and pHf is the final pH. In addition, equations were established to estimate limestone neutralization of iron acid solutions, [Ca]Fe = 0.309CT + 0.27[Fe3+] i + 0.448, where [Ca]Fe is the total dissolved Ca in mmol/L, CT is the total initial acidity (meq/L), and [Fe3+] i is the initial iron concentration (mmol/L).;Thicknesses of the iron coatings on limestone effects on acid neutralization also were evaluated. By the use of different coating thicknesses on limestone, the required reaction time can be calculated by: t=fpHi,pH f,Fe3+ ,AMS,h where t is required reaction time (min), pHf is the final outlet pH, pHi is initial pH of the acid solution, A MS is specific surface area of limestone particles, [Fe3+] is ferric iron concentration of the acid solution (mmol/L), and eta is the term representing the coating effects on limestone dissolution.;With these equations, open limestone channels can be designed with optimized surface area to meet a specified outlet water pH. Predictions can also be made as to the longevity of the treatment and when channels may need replacement