Membrane capacitive deionization for biomass hydrolysate desalination

Exploring the electrosorption selectivity and recovery of indium ions with capacitive deionization in acidic solution

High-efficiency recycling technology for endangered elements effectively mitigates the risk of resource shortages and promises the sustainability of supply chains, which is significant to the industry. In this study, an activated carbon (AC)-based capacitive deionization (CDI) for the selective electrosorption and recovery of indium ions (In³⁺) from acidic aqueous solution is proposed. The effects of applied voltage, pH, and initial concentration of indium were investigated to optimize the operation parameters for In³⁺ electrosorption. The results of cyclic voltammetry and the galvanostatic charge/discharge measurements indicate that the AC electrode shows good capability for the electrosorption of In³⁺ based on electrical double-layer capacitance. As demonstrated, In³⁺ can be successfully removed by CDI without deposition when the pH < 4, as confirmed by scanning electron microscopy and energy dispersive X-ray spectroscopy. The deionization capacity of In³⁺ is 7.95 mg/g with an energy consumption of 0.84 kWh/mol in single-pass mode CDI with an initial concentration of 50 mg/L (pH = 3) at 1.2 V. However, the removal of In³⁺ is affected by the solution pH since hydrogen ions (H⁺) compete for electrosorption. Note that In³⁺ ions with high valence are preferentially electrosorbed on the electrode surface over H⁺ ions, exhibiting a selectivity coefficient of 2.12. Herein, in the charging step, a large number of H⁺ ions in solution are rapidly electrosorbed onto the electrode, while these H⁺ ions are gradually replaced by a small number of In³⁺ ions in solution. Therefore, this electrosorption process shows great potential for effectively recovering indium ions from acidic aqueous solutions.

Investigation of the long-term desalination performance of membrane capacitive deionization at the presence of organic foulants

A long-term performance of membrane capacitive deionization (MCDI) was investigated at the presence of organic matter, which has contributed to the understanding of fouling phenomena and energy efficiency. The commercial humic acid and sodium alginate were adopted as model substances representative of natural organic matter, and different ions including sodium and calcium were selected as important parameters considered in both model substances. The experimental results showed reductions in the salinity removal ability and increments of energy usage due to the organic fouling of ion-exchange membranes and carbon electrodes. Within a time interval of approximately 15 d, reductions of NaCl adsorbance with 5.3 and 3.3 mg per cycle were obtained for humics and alginate, respectively. Simultaneously, the energy consumptions increased by 56.9% for humics and 26.3% for alginate. Furthermore, the results in terms of calcium removal in organic feed and energy usage showed that it had a higher fouling potential in comparison to the ones for the sodium solution with equal conductivity and organic concentration. The morphology and composition of the fouling layer were further studied, and it was found that the organic adsorption onto the electrodes and the attachment or even penetration of organic matter on or into the ion-exchange membrane, which could not be efficiently desorbed during the regeneration cycle, were determined as a key problem of demineralized water production. Therefore, this study suggested the necessity of a pre-treatment to reduce the presence of organic matter for the sustainable operation of MCDI, hereby broaden the potential application fields.

Properties governing the transport of trace organic contaminants through ion-exchange membranes

Ion exchange membranes could provide a solution to the selective separation of organic and inorganic components in industrial wastewater. The phenomena governing the transport of organics through the IEM however, are not yet fully understood. Therefore, the transport of trace organic contaminants (TOrCs) as a model for a wide variety of organic compounds was studied under different conditions. It was found that in the absence of salt and external potential, the chemical equilibrium is the main driver for TOrC-transport, resulting in the transport of mainly charged TOrCs. When salt is present, the transport of TOrCs is hampered in favor of the NaCl transport, which shows a preferential interaction with the membranes due to its small size, high mobility and concentration. It is hypothesized that electrostatic interactions and electron donor/acceptor interactions are the main drivers for TOrC transport and that transport is mainly diffusion driven. This was confirmed in the experiments with different current densities, where the external potential seemed to have only a minor influence on the transport of TOrCs. It is only when the salt becomes nearly completely depleted that the TOrCs are transported as charge carriers. This shows that it is very difficult to get preferential transport of organic compounds due to the diffusive nature of their transport.