Ni removal from aqueous solutions by chemical reduction: Impact of pH and pe in the presence of citrate

The chemical precipitation of Ni ions from industrial wastewater at alkaline pH values creates waste chemical sludge (e.g., Ni(OH)₂). We herein focused on Ni removal via chemical reduction using dithionite, by converting Ni(II) to its elemental or other valuable forms. Without the presence of a chelator (e.g., citrate), the nickel reduction efficiency increased with increasing dithionite:Ni molar ratio, reaching ∼99% at ratios above 3:1. The effect of pH on Ni reduction was in agreement with the standard redox potentials (pe⁰) of dithionite, which became more negative with an increase in pH leading to greater Ni reduction efficiencies. With the formation of Ni-citrate chelates, however, the Ni reduction deteriorated. Elevated pH and temperature improved nickel reduction, due to the greater reducing power of dithionite. The optimal pH value for Ni(II) reduction was found to be ∼8. Injecting Cu seed particles enhanced the rate and amount of Ni reduced. NiS and Ni₃S₂ were identified in the crystal of the resulting solids by X-ray crystallography, and the presence of elemental Ni was explained by X-ray photoelectron spectroscopy. The chemical reduction of actual printed circuit board wastewater with the dithionite:Ni(II) molar ratio dose of 12:1 retrieved ∼99% nickel after 30-min reaction at 40 °C.