Magnesium sulphate recovery from seawater nanofiltration brine using photothermal membrane crystallization

Global magnesium (Mg) demand is rising due to its applications in different fields. Seawater, with ~1350 mg/L dissolved Mg, offers a sustainable source. Conventional methods like solar evaporation and chemical precipitation are slow and inefficient alongside fundamental inability to recover pure water. In this regards, Membrane Distillation Crystallization (MDCr) presents a technologically advanced alternative, enabling the simultaneous extraction of Mg and high-purity water. However, MDCr is also limited by high energy consumption and issues like polarization and scaling. Hence, this study introduces photothermal membrane crystallization (PhMCr) as a promising solution to both conventional as well as MDCr related challenges. In this work, PhMCr was explored for the recovery of pure magnesium sulphate heptahydrate (MgSO 4 ·7H 2 O) crystals from last stage of multistage nanofiltration (NF) synthetic retentate. Graphene oxide (GO) coated polyvinylidene fluoride (PVDF) membrane has been utilised for the photothermal experiments. The membrane shows surface temperature of 66.3 °C under 1 sun condition in <5 mins. PhMCr test at pre-heated feed temperature (T f ) of 50 °C and 60 °C using synthetic brine, shows flux values of 2.8 and 3.2 kg·m⁻²·h⁻¹. The crystallization data showed that high rate of nucleation was observed at high temperature. On the other side, the growth rate of crystals decreased with increase in temperature. Broader crystal size distribution (CSD%) and high coefficient of variation (CV%) was observed at lower temperature with mean crystal size in the range of 196-300 μm. At the end, XRD analysis confirmed the high purity of the recovered crystals. • Photothermal membrane crystallization has been proposed for Mg recovery. • GO-PVDF membrane has been utilised for photothermal effect. • Significant mitigation of membrane scaling due to localized thermal gradients. • High rate of nucleation has been observed at tested temperature condition. • Pure MgSO₄·7H 2 O has been recovered at both operating temperature condition.