The Use of Steel Slags In Phosphorus Removal Structures: An Approach to Evaluate Their Potential Efficacy and How to Improve It

Excessive phosphorus (P) in surface waters is one of the key drivers of eutrophication. P removal structures are an innovative technology developed to reduce excessive dissolved P in runoff, drainage water and wastewater, preventing or mitigating P delivery from urban and agricultural environments to water systems. One of the determining factors for the success of these structures is the type of P sorption material (PSM) being used. Steel slag, a by-product of the steel industry, is an example of a PSM proven to be efficient in sequestering dissolved P from water. However, its P sorption capacity can substantially vary, mostly because different steel-making processes can generate this PSM. Potential impacts of oversimplifying the behavior of slag and generalizing its chemical and physical properties include over- or under-predicting P removal, leading to the design of inadequate structures or overly expensive structures. In this study, we characterized 18 different slag samples from different plants and steel-making processes. We explored chemical and physical properties, P removal ability and their variability across the different samples. Additionally, we evaluated the efficacy of aluminum-coating, a technology aiming to improve the P sorptive qualities of steel slag. We conducted flow-through experiments, a dynamic sorption technique, on coated and uncoated slag samples to evaluate P removal efficiency as well as the effects of Al-coating. Steel slags showed a high variability among chemical properties and P removal ability. Al-coating proved to be an effective technique to improve P sorption and to broaden the use of steel slag in P removal structures, as the coated samples can successfully perform at lower hydraulic residence times. We concluded that P sorption potential and chemical properties of steel slags should not be considered uniform among all steel slag samples. Rather, a specific and careful characterization is necessary for each steel slag sample, since its intrinsic characteristics are master variables in determining (I) its potential for removing P and (II) its ability to be used in field settings with regard to environmental safety. Al-coating is a substantial aid for those slags that did not show a superior performance.