Simultaneous removal of nitrogen and phosphorus by magnesium-modified calcium silicate core-shell material in water

In this study, a new core-shell material (CMCS) is prepared with magnesium oxide (MgO) around calcium silicate hydrate (CSH), and CSH is prepared by SiO₂ from the red mud. The CMCS simultaneously removes ammonia nitrogen (NH₄ ⁺) and phosphate (PO₄ ^3-) by chemical precipitation and it can achieve recovery of nitrogen and phosphorus. The removal process of NH₄ ⁺ and PO₄ ^3- is as follows. First, the shell of MgO is used to remove NH₄ ⁺ and a part of the PO₄ ^3- by the assisted adsorption and struvite (MgNH₄PO₄·6H₂O) precipitation method. Then the CSH is used to remove the residual part of PO₄ ^3- by chemical precipitation (Ca₅(PO₄)₃OH, CaHPO₄ and Ca₃(PO₄)₂). Furthermore, the MgO shell of CMCS not only removes NH₄ ⁺ and PO₄ ^3-, but also can control the calcium ions (Ca²⁺) spill from CSH and pH in the process of removing NH₄ ⁺ and PO₄ ^3-. The removal rate of NH₄ ⁺ and PO₄ ^3- can reach 76.63% and 87.18%, respectively, in the solution in 80 min, but in the actual wastewater the removal rate of NH₄ ⁺ and PO₄ ^3- is 61.40% and 62.83%, respectively. Finally, CMCS was recycled five times and its removal rates of NH₄ ⁺ and PO₄ ^3- are 21.01% and 24.99%, respectively. The aim of this article is to present CMCS, which has a good effect on removing the NH₄ ⁺ and PO₄ ^3- simultaneously.