Application of powdered steel slag for more sustainable removal of metals from impaired waters

Abstract

Using abundant and cheap steel slag for metals removal from water represents an opportunity to recover waste material from one industry as a resource for another. In this study, powdered steel slag which contains lime (CaO) leftover from the steelmaking process, was evaluated and benchmarked against the relatively expensive and energy-intensive chemical sodium hydroxide (NaOH) for the removal of cadmium (Cd), manganese (Mn), and zinc (Zn) from two different mine drainage waters. Unlike isotherm and kinetic studies that represent most previous work, this study evaluated metals removal under realistic simulated water treatment conditions (doses, mixing, contact times, etc.), bringing the material one-step closer to actual full-scale implementation. In the neutral pH water with lower metals concentrations, approximately four times more steel slag (400 mg/L) compared to NaOH (100 mg/L) was required to achieve >70% metals removal. In the acidic pH water with higher metals concentrations, steel slag was insufficient to raise the pH at reasonable doses (<1,000 mg/L) but was shown to reduce NaOH dosing requirements by 25% (600 to 450 mg/L) when used in combination with steel slag (600 mg/L). Like NaOH, steel slag addition facilitated metal precipitate formation but also provided relatively high specific surface area (7.6 m2/g) for metal adsorption, particularly for Mn, a promising finding for future work to evaluate sludge recycle. Water treated with steel slag was also found to be more amenable to 0.45 μm filtration than water treated with NaOH.