Resource utilization of microcrystalline magnesite: Synthesis of magnesium oxide by malic acid steam-assisted decomposition method for the synergetic adsorption of Congo red dye and Cu(II) ion.

Microcrystalline magnesite, a rich raw mineral material, has been used to prepare magnesium-based adsorbents for the removal of dyes and heavy metal ions. Here, MgO adsorbent with well-developed edges, high surface area (96 mg·g) and abundant mesopores is prepared by malic acid steam-assisted decomposition method using light-burned MgO with a loose and porous structure as raw materials. Compared to the industrial light-burned MgO with dense structure, the malic acid steam can easily enter the interior of loose block, which promotes the crosslinking effect of magnesium malic and enhances the crystal growth of MgO. Congo red (CR) and Cu(II) adsorption processes on MgO are spontaneous and endothermic and accord with pseudo-second-order and Langmuir models. The CR and Cu(II) adsorption capacities on the MgO in single system are 9781.86 and 2807.16 mg·g, and CR/Cu(II) adsorption capacities in binary system are 15412.17 and 5418.88 mg·g. The synergistic adsorption is attributed to mutual bridging effect between preabsorbed CR (Cu(II)) and free Cu(II) (CR). The CR and Cu(II) adsorption energy values on MgO by density functional theory are -1.261 and -0.833 eV, indicating CR has a strong interaction with MgO. The CR and Cu(II) removal efficiencies after five cycles are 97.8% and 90%. Thus, MgO prepared by malic acid steam-assisted decomposition method is a prospective adsorbent for heavy metal ions and dye adsorption.