Electrochemical softening of concentrates from an electrodialysis brackish water desalination plant: Efficiency enhancement using a three-dimensional cathode

Abstract

Please cite this article as: I. Sanjuán, D. Benavente, V. García-García, E. Expósito, V. Montiel, Electrochemical softening of concentrates from an Electrodialysis brackish water desalination plant: efficiency enhancement using a three-dimensional cathode., Separation and Purification Technology (2018), doi: https://doi. AbstractThe electrochemical softening method to remove hardness from water has not been applied in desalination practice due to a high cathodic area requirement. In this work, the use of a 3D stainless steel wool cathode is proposed to overcome this technical limitation. An extensive comparison between the 3D cathode and a 2D Ti mesh has been presented, showing higher hardness removal for the 3D one. Experiments have been conducted with waters similar to concentrates derived from a brackish water treatment by electrodialysis. In addition, the method has been proved to be efficient for different water compositions in terms of hardness, alkalinity or the presence of an antiscalant. The main influencing parameters (flow rate and current density) have been studied and it can be concluded that lower flow rates (below 1.2 L h -1 ) give rise to a better efficiencies and 100 A m -2 is the optimum current density. Moreover, the precipitate was characterised by SEM, EDX and XRD showing that Ca 2+ is removed as calcite and aragonite (CaCO 3 ), whereas Mg 2+ is precipitated as brucite (Mg(OH) 2 ).Finally, long-term experiments revealed that the 3D stainless steel cathode has a better performance than the 2D Ti mesh, but only at short times. Keywords: Electrochemical softening, hardness removal, three-dimensional electrodes, electroprecipitation, concentrate, wastewater treatment. V Volt V cell Cell voltage VMD Vacuum membrane distillation XRD X-ray diffraction Highlights  3D cathodes provide an enhancement in the electrochemical softening method. A comparison between a 2D and a 3D cathode is reported for water hardness removal. Long-term electroprecipitation experiments assess the 3D cathode behaviour with time. Water hardness is removed by precipitation of calcite, aragonite, and brucite.

Electrochemical water softening: Influence of water composition on the precipitation behaviour

“…S (Fig. 3A), three main crystal forms can be distinguished: trigonal shape crystals, shrubs of thin needles and rough-surfaced spherulites, as already seen in the previous work [26]. The alkalinity showed a relevant role for the solid formation since in Fig.…”

“…It should be noted that this study has been performed in practical conditions (electrochemical reactor, galvanostatic mode, presence of different ions, and working with water electroreduction). A 3D stainless steel wool (SSW) has been introduced with a flowthrough configuration since both presented good performance in the EWS, as already assessed in a previous work [26]. The information gathered after this work contributes to the technique development, in the interest for a future application in desalination practice.…”

“…Their composition is representative of common concentrates in membrane-based desalination of saline water. The preparation of the working solutions follows the same procedure described in [26]. The analytical determinations are also described in this previous work.…”

“…This increases the nucleation time and consequently prevents or delays the scale formation [27]. The electrolytic system and the optimum values of operation are the same used in the previous work [26]. For convenience of the reader, a scheme of the experimental system and another of the electrochemical reactor are shown in the electronic supporting information (ESI) with a detailed view of their elements (Figs.…”

“…Therefore, the interfacial pH was roughly estimated considering the following assumptions: (i) interfacial pH is considered as the pH in the layer of solution in contact with the cathode, which is not affected by convection (Nernst diffusion layer) and where the electrochemical and precipitation reactions are supposed to take place. (ii) The volume of solution of this layer was estimated by the cathodic area (around 225 cm 2 [26]) and the effective thickness of the layer, which is supposed to be around 0.011 cm, taking into account the hydrodynamic conditions of low flow rate used in this work [38]. (iii) Once estimated this volume, the moles of OHgenerated per second (intensity current), the concentration of HCO 3and its buffering capacity; the interfacial pH is assumed to be close to the value 10.…”