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

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 PO₄^3- (KH₂PO₄) and NH₄⁺ in model wastewater (2000 mg L^-1 NH₄⁺). The precipitation was performed at nMg²⁺:nNH₄⁺:nPO₄^3- = 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⁺-CO₃^2--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⁺:nPO₄^3- = 1:1:1 and reaction time of 2 h, up to 78.3% of NH₄⁺ was removed from the model wastewater.