Spent MgO-carbon refractory bricks as a material for permeable reactive barriers to treat a nickel- and cobalt-contaminated groundwater

Abstract

Spent magnesia (MgO)-carbon refractory bricks were repurposed as a permeable reactive barrier reactive media to treat a nickel (5 mg l⁻¹)- and cobalt (0.3 mg l⁻¹)-contaminated groundwater. MgO has been used for decades as a heavy metal precipitating agent as it hydrates and buffers the pH in a range of 8.5–10 associated with the minimum solubility of various divalent metals. The contaminated groundwater site's conditions are typical of contaminated neutral drainage with a pH of 6 as well as high concentrations of iron (220 mg l⁻¹) and sulphates (2500 mg l⁻¹). Using synthetic contaminated water, batch and small-scale column tests were performed to determine the treatment efficiency and longevity. The increase and stabilization of the pH at 10 observed during the tests are associated with the hydration and dissolution of the MgO and promoted the removal not only of a significant proportion of the contaminants but also of iron. During the column test, this accumulation of precipitates over time clogged and passivated the MgO resulting in a loss of chemical performance (pH lowering, metal breakthrough) after 210 pore volumes of filtration. Precipitation also affected the hydraulic conductivity values which dropped from 2.3·10⁻³ to 4.2·10⁻⁴ m s⁻¹ at the end of test. Saturation indices and XRD analyses suggest the precipitates formed are likely composed of goethite as well as iron, cobalt and nickel hydroxides. Recycled MgO-C refractory bricks were demonstrated to be an efficient reactive material for the removal of Co and Ni, but careful considerations should be taken of the potential clogging and passivation phenomena given particular physicochemical conditions.