Recovery of Cu(II) by chemical reduction using sodium dithionite: effect of pH and ligands

Removal of aqueous Cu2+ ions with Fe0/C ceramsites fabricated by direct reduction roasting of magnetite, coal, and paper mill sludge

The porous metallic iron/carbon (Fe⁰/C) ceramsites, with virtues of low cost and 'green' fabrication, were prepared by direct reduction roasting of magnetite, coal, and paper mill sludge. The X-ray diffraction data revealed that Fe⁰ was generated in situ by reducing the magnetite at 1,200 °C. Scanning electron microscopy with energy-dispersive X-ray spectroscopy indicated that Fe⁰ particles, with a size of <10 μm, were highly dispersed on carbon particles to form an integrated anode (Fe⁰) and cathode (C) structure of microelectrolysis filters. The effects of initial solution pH and Fe/C mass ratio on Cu²⁺ removal were investigated. The extent of Cu²⁺ removal increased from 93.53% to 99.81% as initial pH rose from 2.5 to 7.0. The residual Cu²⁺ concentration was as low as <0.2 mg/L. The highest extent of Cu²⁺ removal was achieved at Fe/C mass ratio of 6.8:1. The pseudo-second-order kinetic model fitted well for Cu²⁺ removal by the ceramsite, revealing the chemisorption as a limiting step. The Cu²⁺ adsorption equilibrium data were well described by the Langmuir isotherm, with a maximum adsorption capacity of 546.45 mg/g at initial pH 3.0.

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.