Integration of ion exchange resins and electrochemical membrane processes for recovering boron from desalination seawater brine
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Date
2023-01-29
Authors
Ελευθεράκη, Μαρία
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Abstract
Seawater desalination plants (SWDPs) are a major source of freshwater, tackling the phenomenon of water scarcity, with reverse osmosis being the most dominant technology for this purpose. As a by-product of this process, desalination brine is a highly saline wastewater containing valuable elements, while its conventional management entails severe environmental consequences. One of these elements is boron (B), which is a critical raw material for the European Union (EU). Aiming to minimize the EU dependency on minerals and metals, the Sea4Value is an EU-funded project that promotes brine’s sustainable management by raw material recovery.
As a part of the Sea4Value project, this thesis focuses on the recovery of boron from seawater desalination brines using electrodialysis with bipolar membranes (EDBM) to produce boric acid. In this sense, EDBM was initially tested to evaluate its efficiency in boric acid and sodium hydroxide production from a sodium borate solution. For the first step of experimentation, 3 triplets of commercial PCCell membranes concentrated boron 2.5 and sodium 4.3 times. Following that, previously modified anion and cation exchange membranes with layer-by-layer deposition, were used. Results demonstrated that PCCell membranes with 4 bi-layers (BL) achieved a concentration factor of 1.7 and 3.0 for boron and sodium, respectively, using the same feed solutions. The same commercial series, coated with 8 BL, increased the concentration of boron and sodium 1.6 and 2.8 times, respectively.
However, EDBM technology is very susceptible to scaling issues. Hence, pre-treatment stages are necessary to purify and concentrate boron. Previous Sea4Value experiments showed the efficiency of using ion exchange (IEX) columns with boron selective resins to concentrate and separate B from other elements present in brine. Nevertheless, as a solution of HCl 4% (w/w) was used for column elution, if the outlet of IEX column was subjected directly to the EDBM, the acid produced would have a higher concentration of Cl than B. Therefore, a two stages electrodialysis (ED), placed after IEX column and before EDBM, was studied for the removal of Cl and concentration of B. Aiming to remove Cl- ions, the first stage of ED was performed at acid pH and two different HCl concentrations in the concentrate compartment were tested (0.2 and 2% (w/w)). The highest removal degree was obtained with the utilization of a 0.2% (w/w) HCl solution as initial concentrate solution, having a removal of 98.6% for chloride, while also removing 1.7% of boron, 17.5% of vanadium and 65.2% of indium present in the elute. On the other hand, a concentrate feed of 2% (w/w) HCl, removed 90.8% of chloride, leaving a high content of impurities in the treated elute. The dilute produced from the most efficient process, was adjusted to a pH of 12 and fed to the ED system again as a second step, aiming to concentrate B. The concentration factor obtained for boron in this case was 1.3.
Moreover, batch experiments with DIAION CRB03 resins were conducted using elution solutions containing HCl and H2SO4 in different concentrations aiming to reduce the problem of Cl impurity. The lowest acid content that was able to achieve equal efficiency in recovering boron, as 4% (w/w), was that of 0.5% (w/w) for HCl. Thus, the last step of experimentation included the sorption and desorption of synthetic brine through IEX columns using this lower concentration for the elution solution. Boron was successfully recovered from brine by 98.9% and the obtained concentration factor was 22.3. Nevertheless, the resin adsorbed a comparatively little amount of calcium, vanadium, indium, scandium and gallium.
To sum up, an integrated train by using IEX resins, ED and EDBM is a promising combination for boron recovery from seawater desalination brines. However, further research is essential to evaluate the viability of the proposed technologies.
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Keywords
Boron, Boric acid, Seawater desalination brine, Ion exchange resins, Electrodialysis, Bipolar membranes