Freeze concentration of reverse-osmosis brine from brackish groundwater: salt partitioning, drainage–melting post-treatment, and calcium carbonate scaling potential

This study evaluated reverse osmosis (RO) combined with freeze concentration (FC) for sodium-chloride brackish groundwater and the resulting RO brine. Total dissolved solids (TDS) were 3.618 g/L in the feed and 9.312 g/L in the RO brine. Calculated plant-level overall RO selectivity was 96.4%, with ion-specific selectivities ranging from 91.2% for HC O 3 − to 98.3% for C l − . Single-stage FC without ice post-treatment produced ice melt with TDS 7.691 g/L, effective salt distribution coefficient K ef 0.826, overall salt removal efficiency R FC 17.4%, and concentration factor (CF) 1.24. Drainage–melting reduced ice-melt TDS to 2.173 g/L and K ef to 0.233, while increasing R FC to 76.7% and CF to 1.39. 90.0% of the feed salts were concentrated in 25.1% of the feed volume. C a 2 + showed anomalously high removal, with K ef C a 2 + = 0.320 versus K ef M g 2 + = 0.678. PHREEQC screening showed calcite supersaturation under all evaluated conditions, with the calcite saturation index (SI) decreasing from +2.05 at 25 °C and pH 9.1 to +0.63 at −5 °C and pH 8.0, while calcium sulfate phases remained undersaturated. This supports a probable contribution from CaC O 3 precipitation to anomalous C a 2 + removal. An RO + FC scheme with ice-melt recycle and concentrate crystallization is proposed toward minimal/zero liquid discharge (MLD/ZLD).