Reduced thermodynamic representation of multicomponent hypersaline electrolytes and its electrochemical projection through reverse electrodialysis

Reverse electrodialysis (RED) open-circuit voltage reflects the electrochemical potential difference between electrolyte streams, but its interpretation is complicated in hypersaline mixtures by strong solution non-ideality and membrane effects. Here, the ion-specific activity state of a multicomponent brine is reduced to a dimensionless descriptor, Λ multi , using charge-equivalent weighting. The South Aral Sea is used as the reference system, with activities in both streams calculated using the same Pitzer-based multicomponent model. At 298.15 K, the charge-balanced brine has an ionic strength of 6.6705 mol kg −1 . Its calculated Λ multi is 7.0992, compared with an ideal value of 8.6843, corresponding to an 18.3% reduction. Projection through the reduced RED relation for ten cell pairs and a prescribed average permselectivity of 0.94 gives 1.715 V and 2.097 V, respectively. Applying the same calculation to Dead Sea brine gives the opposite effect: Λ multi =10.8557, compared with Λ ideal =9.4743, an increase of 14.6%. The two hypersaline brines examined here have similar divalent-ion fractions but show non-ideal corrections of opposite sign. For the South Aral Sea, Λ multi changes by less than 0.5% across the temperatures examined, while the projected voltage ranges from 1.636 to 1.722 V. A prescribed ±2 mV voltage uncertainty produces only about 0.067% relative uncertainty in the descriptor, whereas variation in the assumed membrane response produces errors from −25.53% to +6.38%. The results are conditional on the adopted single-ion activity convention and prescribed membrane representation; quantitative comparison with experimental RED voltage therefore requires independent characterization of both the membrane response and electrolyte state.

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