Phosphorus and metal sorption in aged bioretention cells with fly-ash amended filter media

Abstract

Stormwater runoff from urban areas can transport a significant load of phosphorus (P) and heavy metals. Media samples from seven year old bioretention cells (BRC) were collected, to a depth of 0.6 m, to assess the accumulation of phosphorus and heavy metals. Analysis consisted of total soil digestion (T-P), Mehlich-3 (M3-P) and water soluble (WS-P) extraction for both metals and phosphorus. The mean T-P concentration increased over the seven years of operation, but the results were not statistically significant (p > 0.05). The average M3-P and WS-P concentrations in the media profiles showed higher P accumulation in the top 0.15 m of media. Significant (p < 0.05) P concentration reductions of 68% to 75% were achieved between the influent and drain effluent and P mass reductions of 76% to 93% were observed. Most of the metal accumulated in the top 0.15m of media. Metal concentration in the media significantly increased (p < 0.05) over the seven years compared to that of initial media samples. Significant zinc concentration reductions between influent and drain effluent of 56% to 60% were obtained, which corresponds to zinc mass reduction of 43% to 88%. The phosphorus and metals adsorption capacities were obtained from a series of batch and flow-through experiments. Both nonlinear forms of Langmuir and Freundlich isotherm models fitted the equilibrium data well (r2 > 0.9). The batch sorption experiments showed phosphorus adsorption capacity of the fly-ash filter media after seven years of operation was (160 mg/g) around half the amount of the initial material (350 mg/g). Batch testing of the aged fly-ash amended filter media exhibited phosphorus removal of 61%, Zn Cu and Pb removal of 95%, 98%, and 99%. Desorption experiments showed the media released 24% of initially sorbed phosphorus and only 0.6% of initially sorbed Zn and Cu at initial phosphorus and metal concentration of 30 mg/L. Thus, phosphorus and heavy metal adsorption in sand/fly-ash filter media may be considered irreversible, and can provide long-term phosphorus and heavy metal retention.