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 (PO43-) by chemical precipitation and it can achieve recovery of nitrogen and phosphorus. The removal process of NH4+ and PO43- is as follows. First, the shell of MgO is used to remove NH4+ and a part of the PO43- by the assisted adsorption and struvite (MgNH4PO4·6H2O) precipitation method. Then the CSH is used to remove the residual part of PO43- by chemical precipitation (Ca5(PO4)3OH, CaHPO4 and Ca3(PO4)2). Furthermore, the MgO shell of CMCS not only removes NH4+ and PO43-, but also can control the calcium ions (Ca2+) spill from CSH and pH in the process of removing NH4+ and PO43-. The removal rate of NH4+ and PO43- 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 PO43- is 61.40% and 62.83%, respectively. Finally, CMCS was recycled five times and its removal rates of NH4+ and PO43- 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 PO43- simultaneously.