Electrodialysis of highly concentrated brines: Effects of calcium

•Calcium affects the ion transfer efficiency of electrodialysis.•The major interference appears to be within the cathode cell.•Acid treatment of the electrolyte reverses the detrimental effects of calcium.•A boundary membrane selective against multivalent cations mitigates interference.•Interference appears to be precipitation on the electrolyte side of the membrane. Electrodialysis (ED) is an electrochemically driven membrane technology that is usually used for treating feed water containing several thousand mg/l total dissolved solids (TDS). This research investigated the use of ED for the treatment of flowback water from shale gas hydro fracture. Shale brines contain tens of thousands of mg/l TDS and contain a mixture of multivalent cations, such as calcium, magnesium, barium, strontium, radium, iron, and manganese. This paper covers efforts undertaken to mitigate problems associated with high concentrations of calcium (4000mg/l) in the presence of 30,000mg/l NaCl. Eight baseline ED tests were performed with sodium chloride. Seven additional runs were designed to understand the effect of calcium on the electrodialysis process. Initial tests showed strong incursion of calcium into the electrolyte using a CMX barrier membrane. The apparent calcium fouling (in and around) the electrode cells was indicated by a rapid degradation of process efficiency, observed as a decrease in current. The single cathode boundary membrane was replaced with a membrane that was more selective against multivalent cations (CMX-S). Calcium flux into the electrolyte was reduced by 47–73% and the ion flux from the concentrate to the diluate improved by approximately 40% compared to the baseline. Additionally, the replacement of the single cathode boundary membrane imparted an apparent immunity to direct fouling by addition of calcium directly into the electrolyte.