Fast and complete recovery of magnesium from sea bittern to synthesize magnesium hydroxide hexagonal nanosheet for enhanced flame retardancy and mechanical properties of epoxy resin

Magnesium is the most abundant element in sea bittern, of which preferential and high-efficient recovery is significant for extraction of other valuable elements, such as lithium etc. In this work, a two-step method was proposed to rapidly and completely extract magnesium from sea bittern. Firstly, solid sodium hydroxide (NaOH) was directly added to sea bittern to convert magnesium to mesh-structure magnesium hydroxide (Mg(OH)2) intermediate, where Mg(OH)2 seeds were pre-added to accelerate the filtration process. Subsequently, meshstructure Mg(OH)2 intermediate was hydrothermally transformed to Mg(OH)2 hexagonal nanosheet with size of 40-60 nm. The synthesized Mg(OH)2 hexagonal nanosheets after modification by sodium stearate were applied to enhancement in flame retardancy and mechanical properties of epoxy resin (EP). When the addition amount of Mg(OH)2 hexagonal nanosheets is 9 wt%, in contrast to pure EP, the formed composite showed decreased heat release rate, smoke production rate, total heat release and total smoke production by 63.5 %, 66.3 %, 39.2 % and 46.2 %, respectively. Simultaneously, tensile and bending strength of this composite were increased by 104.0 % and 61.9 %. This work gives a high-value route for magnesium in sea bittern without diluting sea bittern concentration, which would contribute to the high-value utilization of sea bittern in a costeffective way.