Capture and recycling of ammonium by dolomite-aided struvite precipitation and thermolysis

- Dolomite can be a cost-effective Mg source for struvite formation.
- Competition between formation of MgNH4PO4·6H2O and MgKPO4·6H2O occurred.
- Hydrolyzed MgO/CaCO3 was an active Mg source and alkali for struvite reaction.
- CaCO3 facilitated the thermolysis of struvite to release ammonium.
- Use of Recycled MgHPO4 and MgKPO4 for forming struvite led to a high NH4+ removal rate.

The capture and reuse of NH4+ is an ideal solution to treat NH4+-containing wastewater. The capture and reuse process needs to be clean and cost-effective. Currently, however, there are many obstacles, particularly in the availability, cost, and recovery of the chemical sources required. Here, we demonstrate a clean and efficient method to capture and recycle NH4+ by a dolomite-aided struvite precipitation process. Dolomite calcined carefully in CO2 atmosphere was used as a Mg source to react with PO43− (KH2PO4) and NH4+ in model wastewater (2000 mg L−1 NH4+). The precipitation was performed at nMg2+:nNH4+:nPO43− = 1:1:1.2 and pH = 8.0 for 2 h; 89.7% of NH4+ was recovered in the form of struvite precipitate. The competition between K+ and NH4+ in the model wastewater led to the formation of K-struvite (MgKPO4·6H2O) and struvite (MgNH4PO4·6H2O). The formation of K-struvite resulted in a decrease in the NH4+ removal rate. When struvite was heated at 110 °C for 4 h, the NH4+ release rate from the thermolysis reached 75.7%. Thermolysis readily occurred as an unstable Ca2+-CO32--NH4+ system formed in the mixture of MgNH4PO4·6H2O and CaCO3. The elements Mg and P that were obtained during the struvite precipitation-thermolysis-reprecipitation process can be repeatedly used. After 6 cycles, under the conditions pH = 9.0, nMg2+:nNH4+:nPO43− = 1:1:1 and reaction time of 2 h, up to 78.3% of NH4+ was removed from the model wastewater.

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