Air-Cooled Blast Furnace Slag. II: Phosphate Removal from Simulated Rainfall Events

Abstract

This paper is the second in a series on the potential use of air-cooled blast furnace (ACBF) slag for phosphate (PO4) removal from passive stormwater runoff systems—that is, the ACBF slag aggregates function as reactive base/subbase layers beneath porous pavement systems. Batch tests containing simulated stormwater stock solutions of 1, 5, and 1 mg/L PO4 were equilibrated with AASHTO Nos. 1, 67, and 3 ACBF slags for various periods up to 72 h and showed rapid uptake of PO4. More than 9% PO4 removal was achieved by the AASHTO No. 1 ACBF slag for all dosing levels and time intervals, and the AASHTO No. 67 ACBF slag attained similar performance after an hour of contact time. The AASHTO No. 3 ACBF slag media attained ≥5% PO4 removal at the 1 mg/L PO4 dosing level for all time intervals. Batch testing at the 1 mg/L PO4 dosing level typically showed pH>9 for all ACBF slags, and often pH≫9 due to the strongly alkaline nature of ACBF slag. Conversely, a flow-through test mold containing a 15.2-cm (6-in.) thick compacted AASHTO No. 67 ACBF slag media subjected to 24 h wet/dry cycles of inundation by a 1 mg/L PO4 solution maintained pH<9 while removing >5% PO4 for 33 cycles. The measured and estimated total PO4 uptake for the AASHTO Nos. 67 and 3 ACBF slags, respectively, were used as inputs to a stormwater management model to estimate the service life of the ACBF slags for PO4 removal in three land use applications. An AASHTO Nos. 67 and 3 ACBF slag layered system with respective thicknesses of 15.2 cm (6 in.) and 76.2 cm (3 in.) was estimated to yield an average service life of 1 years for five selected cities on the Great Lakes evaluated in the model, and then also end with as little as 2% site coverage or equivalent gallery volume.