Estimating phosphorus removal by steel slag in a flume experiment: effects of P concentrations and subsurface hydrological conditions

Abstract:

Managing excessive phosphorus (P) is essential to reduce the incidence of environmental quality issues, such as eutrophication and harmful algal blooms. One potential strategy that have been developed with this purpose is the use of P sorption materials (PSMs) to sequester P from water systems, which is the objective of this study.

We evaluated the performance of steel slag, an industrial by-product with high P sorption potential, through a flume experiment under two different subsurface hydrological conditions, drainage and saturation, and two input P concentrations, 1 and 5 ppm.

The 10-m flume configuration, designed to simulate processes occurring in a drainage ditch, is comprised of four 2.5-m sequential segments: a sediment bed, a slag bed over sediment, a slag dam built over a slag bed, and another sediment bed. In the experiments, all four segments of the flume were set to either saturation or with a constant drainage (percolation) of 0.1 L/min for each segment. The experiment was conducted with a constant flow of elevated P water at 7.3 L/min for 4 hrs (adsorption run) and followed 24 hrs later by a 4-hr run of deionized water (desorption run) at the same inflow rate. The adsorption-desorption cycle was repeated three times with the same sediment and slag materials, to allow testing of the resilience of P sorption under different PSM placement, subsurface hydrologic and P loading conditions.

Preliminary results from the first adsorption and desorption cycle show that the flow-through slag section sequestered the most P during the adsorption runs. By comparing the different P inflow concentrations analysis, it is clear that the removal process is concentration driven: 83% of the injected P was removed in the 5 ppm as compared to 46% in the 1 ppm saturation run. Because of the higher P removal at 5 ppm P inflow, slightly higher release was also observed during the desorption run. Analyses of the persistence of steel slag as PSM under repeated adsorption and desorption cycles and how subsurface hydrology affects the overall P mass balance will also be presented. Findings of this research will provide additional data and information to develop a strategy to use steel slag in a P removal structure.