In this study, the recovery of manganese (Mn) and iron (Fe) from groundwater treatment sludge through reductive acid leaching and hydroxide precipitation was investigated. Maximum leached Mn (100%) was obtained using sulfuric acid and hydrogen peroxide at 25 °C. Leached Mn and Fe decreased with the increase in the solid-liquid ratio. Leaching time had minimal effect on Mn and Fe leaching beyond 5 min, while agitation rate had minimal effect beyond 150 rpm. At 25 °C, the rate-limiting step of Mn leaching was diffusion through inert solid components of the sludge, composed mainly of insoluble sand particles. Fe leaching was governed by diffusion through the insoluble components of the sludge, including the unreacted manganese dioxide (MnO2). Maximum precipitation of Fe and separation from Mn in the leachate through addition of potassium hydroxide occurred at pH 4.0. The results demonstrated that reductive acid leaching and hydroxide precipitation is an effective means of recovering Mn and Fe from groundwater treatment sludge. The applicability of the recovered Mn for nickel ion removal from aqueous solution was also explored in the study. Highest nickel ion uptake by the MnO2 synthesized from the recovered Mn was at 111.67 mg g-1, even exceeding the adsorption capacities of previously studied nickel adsorbents.
Keywords: Groundwater treatment sludge; Heavy metal wastewater; Nickel ion adsorption; Reductive acid leaching; Urban mining.
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