Reduction crystallization of Ni, Cu, Fe and Co from a mixed metal effluent

Reductive removal of selenate (VI) in aqueous solution using rhodium metal particles supported on TiO2

Selenium and its compounds in high concentration are toxic for humans, especially selenate (VI) is the most toxic due to its high solubility in water. To promote the reductive reaction of Se(vi) to Se(iv) or Se(0), which is relatively easy to remove in water, noble metal particles were added as reaction sites with a reductant. The highest removal performance of selenate in aqueous solution was achieved using rhodium particles supported on TiO₂ (Rh/TiO₂). Selenate was rapidly reduced with hydrazine on the metal particle, leading to a selenium deposition on the particle inhibiting the stable reductive reaction. On the other hand, when a weaker reductant such as formaldehyde was used for the selenate reduction, the selenium deposition was suppressed due to its low reactivity, resulting in a stable reductive reaction of selenate in water.

Sequential removal and recovery of cadmium ions (Cd2+) using photocatalysis and reduction crystallization from the aqueous phase

The removal of cadmium ions using photocatalysis, reduction crystallization and their sequencial system.

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.