Reversible switch between bulk MgCO3·3H2O and Mg(OH)2 micro/nanorods induces continuous selective preconcentration of anionic dyes.

It is still a big challenge to treat large amount of water with low concentration of pollutant. In this study, a hierarchical (micro/nano) structured Mg(OH)2 adsorbent was introduced by the in situ hydration of porous MgO in the dye solution. The adsorbent showed high selective adsorption capacity (Q0 ≈ 155 mg/g for acid alizarine blue) and fast adsorption rate for the removal of anionic dyes down to the mg/L levels. Moreover, the adsorbed dye was successfully desorbed by carbonation, resulting in a ~4000 fold enrichment of the dye solution. It was demonstrated that by establishing a reversible switch between the Mg(OH)2 micro/nanorod and the bulk MgCO3·3H2O, a continuous preconcentration of low-concentration dye wastewater could be achieved.