Valuable mineral materials recovery from Sargassum spp. with a focus on phosphorus as magnesium-whitlockite

Direct recovery of phosphorus from Sargassum spp. in the form of magnesium-whitlockite (Ca_9.5MgO₂₈P₇) was proposed for the first time. The effects of the acid digestion and calcination sequence on the naturally adsorbed minerals in Sargassum spp. were investigated through two treatment approaches (I and II). Treatment I (producing Ash-1) consisted of acid digestion followed by calcination, while Treatment II (producing Ash-2) involved initial calcination, subsequent acid digestion, and a final calcination step. It was found that Treatment I is effective in obtaining ashes containing magnesium-whitlockite (Mg-WH) together with diatoms frustules. Whereas Ash-2 showed an almost complete absence of diatom frustules and phosphate-based components. It was revealed that in Treatment I, a significant fraction of phosphorus, particularly organic ones, were less susceptible to acid digestion and therefore remained intact. During subsequent calcination, together with calcium- and magnesium-containing species, they can be thermally decomposed or oxidized, and Mg-WH will be the preferred final product. In contrast, in Treatment II, the initial calcination predominantly transformed organic phosphorus into acid-soluble inorganic forms, resulting in almost no phosphorus remaining after final calcination. Conclusively, this work unravels the importance of acid digestion and calcination sequence in phosphorus recovery and determining the yield and properties of the minerals. This study can shed light on the requirements and limitations of this efficient treatment in promoting the valorization of marine biomass and supporting sustainable strategies for mitigating the environmental impacts of excessive sargassum proliferation in coastal regions.