Electrochemical desalination using polyglycerol activated carbon: electrode, cell, and process development

Abstract

Capacitive deionization (CDI) has emerged as a promising electrochemical technology for the desalination of brackish water, which promotes the electrosorption of ions in the electrical double layer. In recent years, there has been significant growth in studies on capacitive deionization, however, challenging issues still persist, such as obtaining low-cost carbon electrodes with high electrosorption capacity and stability over electrosorption/desorption cycles, as well as a better understanding of how the hydrodynamic aspects influence mass transfer and process efficiency. In this sense, the objective of this thesis was to develop an activated carbon electrode using a polymer obtained from residual glycerol from biodiesel as a precursor and to investigate the effects of the architecture of different CDI cells on electrochemical desalination. The results showed that it was possible to prepare electrodes using residual glycerol polymer as the precursor. In electrosorption experiments, the PGAC electrode demonstrated stability over 50 cycles, applying voltages of 1.1 V and 1.2 V, in a symmetrical configuration, desirable behavior to allow the process for long periods of operation. However, when applying 1.4 V, the positive electrode potential exceeded the limit potential of anodic stability, causing oxidation reactions at the anode and, consequently, loss of desalination capacity. The asymmetric and membrane configurations (MCDI) were also analyzed in order to improve the salt adsorption capacity (SAC). Using an asymmetric configuration, it was possible to minimize the deleterious effect of co-ion repulsion, since there was an increase in charge efficiency (QE) from 62.9% to 88.4%. In the MCDI configuration, the application of ion exchange membranes led to a significant increase in the values of SAC and QE. This improvement is attributed to the presence of co-ions repelled from the micropores and which are prevented from migrating into the solution due to the presence of the membrane. These co-ions then accumulate in the macropores of the material, constituting an additional force of attraction of the counter-ions, since electroneutrality must be maintained. These results showed that the PGAC electrode, in addition to being a low-cost material, proved to be promising for brackish water desalination by capacitive deionization. In sequence, the comparison of the different CDI cells showed that the cell architectures influenced the mass transfer and kinetics of the process. For a thinner electrode, the flow-by cell (FBC) showed better desalination performance compared to the flow-through cell (FTC), due to the lower resistance to charging of the electrical double layer and higher diffusion in the micropores. However, for a thicker electrode a notable reduction of SAC occurred in the FBC, a result attributed to the lower mass transfer from the electrode surface to the adsorption active sites. This effect did not occur when using FTC, because in this architecture the convective transport of mass in the interstitial pores promotes a faster kinetics when compared to FBC. Considering these results, a percolation flow cell (PFC) was proposed, to combine the beneficial aspects of previously investigated cell architectures. With the flow of electrolyte percolating through the electrode and being perpendicular to the electric field, the PFC allowed the increase of optimized salt removal (OSR), due to faster mass transfer promoted by the permeation of the electrolyte through the carbon film of the electrode. An investigation of the particle size used in the preparation of the electrode showed that this size directly influences the mass transfer phenomenon, being possible to increase the kinetics constants with smaller particles, but they promoted a lower SAC due to changes in textural properties. These results showed that there is an ideal particle size to obtain a high OSR value. Finally, galvanostatic and quasi-single-pass mode analysis were analyzed using PFC. The results showed that higher flux flow and current density accelerate the kinetics, but quickly reach the cutting potential, affecting the OSR. Therefore, the operating conditions that showed to be more effective to obtain the highest OSR value were 7 ml min-1 and 1 mA cm-2, highlighting the importance of simultaneously analyzing SAC value and cycle time.