The 12th International Conference on Water Resource and Environment (WRE 2026)

Abstract: Seawater mining is a sustainable strategy for mineral exploitation, yet achieving high calcium (Ca) and magnesium (Mg) selectivity remains challenging due to the complex seawater matrix. This study evaluated the performance of obtained seawater Ca/Mg on biochars pyrolyzed under varying temperatures (400–800 °C) through photothermal evaporation. Our results indicated that after 5 days of exposure, a high Ca loading (0.42 ± 0.11 mmol/g) with a high Ca selectivity (Ca/Mg of 4.34) can be achieved. Interestingly, high Ca loadings (0.65 ± 0.12 mmol/g) was also observed at high-T biochar, suggesting that exposure time is the major factor regulating Ca/Mg deposition. However, a low Ca selectivity (Ca/Mg of 1.99) noted in this case indicated that surface chemistry is likely responsible for the Ca selectivity. Intriguingly, high reversible CO_2 uptake up to 0.35 mmol/g was noted at those biochar with low Ca/Mg ratio, which exhibited very insignificant correlation to the Ca/Mg loading. It is therefore expected that the entrapped Mg species would destabilize the deposited Ca species and consequently possess higher reactivity for reversible CO_2 uptake. Our findings reveal the fundamental role of biochar interfacial chemistry in selective crystallization, offering a practical, near zero-energy-input route to effectively mine seawater minerals.