Abstract:
Municipal landfill leachates contain significant amounts of ammonia, which is toxic at high concentrations. It is also reported to inhibit the activity of microorganisms responsible for the biological treatment of the leachate. Ammonia could be removed from solution through struvite (magnesium ammonium phosphate hexahydrate) precipitation in the presence of magnesium and phosphate sources, which could then be used as fertilizer. This study assessed the use of dolomite (calcium magnesium carbonate) and hydroxyapatite (obtained from cow bone), as abundant and low-cost magnesium and phosphate sources for struvite precipitation. Batch experiment tests were conducted to investigate the effects of pH and (Mg2+:NH4+:PO43-) molar ratios on the removal efficiency of ammonia. X-ray Powder Diffraction (XRPD) was used to identify the solid phases in the residues. Using dissolved dolomite along with Na3PO4·12H2O as a phosphate source at a molar ratio of 1.25:1.0:1.25 (Mg2+:NH4+:PO43-) and a pH of 9.5 resulted in an ammonia removal efficiency of 90%. However, using dolomite calcined at 950 °C with the same phosphate source showed a significant decrease in removal efficiency to 44%, while using dolomite calcined at 750 °C (under continuous CO2 injection) resulted in a removal efficiency of 37%. In parallel experiments, hydroxyapatite obtained from cow bone was dissolved and used as the phosphate source in combination with dissolved dolomite as the magnesium source; this resulted in an ammonia removal efficiency of 94%. However, when the dissolved hydroxyapatite was used with solid dolomite, the removal efficiency dropped to 60% for dolomite calcined at 950 °C and 46% for dolomite calcined at 750 °C.
