Use of nanofiltration to reject cobalt (II) from ammoniacal solutions involved in absorption of SO2/NO

Sustainable and efficient technologies for removal and recovery of toxic and valuable metals from wastewater: Recent progress, challenges, and future perspectives

Due to their numerous effects on human health and the natural environment, water contamination with heavy metals and metalloids, caused by their extensive use in various technologies and industrial applications, continues to be a huge ecological issue that needs to be urgently tackled. Additionally, within the circular economy management framework, the recovery and recycling of metals-based waste as high value-added products (VAPs) is of great interest, owing to their high cost and the continuous depletion of their reserves and natural sources. This paper reviews the state-of-the-art technologies developed for the removal and recovery of metal pollutants from wastewater by providing an in-depth understanding of their remediation mechanisms, while analyzing and critically discussing the recent key advances regarding these treatment methods, their practical implementation and integration, as well as evaluating their advantages and remaining limitations. Herein, various treatment techniques are covered, including adsorption, reduction/oxidation, ion exchange, membrane separation technologies, solvents extraction, chemical precipitation/co-precipitation, coagulation-flocculation, flotation, and bioremediation. A particular emphasis is placed on full recovery of the captured metal pollutants in various reusable forms as metal-based VAPs, mainly as solid precipitates, which is a powerful tool that offers substantial enhancement of the remediation processes' sustainability and cost-effectiveness. At the end, we have identified some prospective research directions for future work on this topic, while presenting some recommendations that can promote sustainability and economic feasibility of the existing treatment technologies.

Performance of Several Cobalt-Amine Denitration Solutions and Their Catalytic Regeneration by Graphene

Little attention has been focused on the use of cobalt(II)-amine chelates for the absorption of nitric oxide (NO) in flue gas, and research on the regeneration of cobalt denitration solutions is relatively rare. To supplement this research gap, several promising ethylenediamine derivatives were screened out. They are N-(2-hydroxyethyl)ethylenediamine, 1,2-propanediamine, and 1,2-cyclohexanediamine. These cobalt(II)-amine solutions are effective for denitration and have not been reported yet. However, they are also easily oxidized to the corresponding cobalt(III) species. In the presence of a nanocarbon material, cobalt(III) components are reduced to cobalt(II) components and release oxygen by reacting with acids. The effects of solution pH, temperature, and graphene dosage on the regeneration process were investigated. A proper addition of graphene as a catalyst contributes to the progress of regeneration. Catalytic mechanisms and regeneration performance have been discussed as much as possible. These mechanisms are related to the oxidation reactions and oxygenated species of cobalt complexes. The carbon material here acts as a catalyst for adsorbing cobalt chelates and accelerating charge transfer in the oxygen evolution reaction.

Experimental investigation of nitric oxide removal from flue gas using hexamminecobalt(II) solution scrubbing in a pilot-scale facility

An experimental investigation of operational parameters, including liquid/gas ratio (L/G), inlet nitric oxide (NO) concentration, reaction temperature, and pH value of absorbing agent, on NO removal efficiency with hexamminecobalt(II) solution scrubbing was conducted on a pilot-scale facility to search optimal operation conditions. The experimental results show that NO removal efficiency increased with the pH value of hexamminecobalt solution, while the improving rate dropped gradually. When the reaction temperature increased, the NO removal efficiency increased first and then decreased. At the same time, NO removal efficiency increased with the increasing of L/G and hexamminecobalt concentration, while the removal efficiency did not change much at low NO concentration. The pH of 10.4 and L/G of 16 L/m³ were close to the optimal operation conditions, and the scrubbing temperature fell within a reasonable operation temperature. The experimental results can be used as a reference for the design and operation of scaled-up industrial devices.