Phosphorus Equilibrium. II: Comparing Filter Media, Models, and Leaching

Part I focused on a Freundlich model of phosphorus (P) equilibrium between AlOx-media substrates and differing aqueous matrices: runoff, wastewater, and a reproducible deionized (DI) surrogate. In contrast, Part II examines equilibrium and leaching of a wide range of media using a single DI water matrix. Media are characterized, P removal quantified, leaching is examined, and equilibrium modeled. Media of high Al (${\mathrm{AOCM}}_{\mathrm{c}}$, alumina), Fe (coated perlite), or Ca [portland concrete cement-based media (${\mathrm{AOCM}}_{\text{pcc}}$)] content provided enhanced P capacity as compared to other media (tire crumb and zeolite-perlite- granular activated carbon). While zeta potential ($\zeta$) was stable from pH 5 to 9 for each aluminum oxide coated media (AOCM) and ranged from $+5\mathrm{mV}$ for ${\mathrm{AOCM}}_{\mathrm{c}}$ to 0 mV for ${\mathrm{AOCM}}_{\text{pcc}}$, $\zeta$ was largely negative for all other media and less stable for alumina and Fe-perlite. Despite enhanced capacity of media of high Al, Ca, or Fe content, metal leaching can be a concern. It was found that $\mathrm{Al}(1.09\pm 0.02\mathrm{mg}/\mathrm{g})$ and $\mathrm{Fe}(3.49\pm 0.05\mathrm{mg}/\mathrm{g})$ were leached from alumina and Fe-coated perlite, while Zn was leached from tire crumb, raising potential constraints for these media. For the media tested, the Freundlich binding capacity ($K_F$) varied by 2 orders of magnitude with heterogeneous site intensity profiles ($n$). In contrast to the two-parameter Freundlich model, a partitioning model ($K_u$) in Part II provides a single parameter to index equilibrium. $K_u$ varied by 3 orders of magnitude, also indicating large variability in P capacity between media. This range of media, commonly deployed for storm water systems, produced a wide range of equilibrium, leaching, and model parameters in a stable, reproducible surrogate matrix.