observed in molar ratio 1 experiments, the remaining MgO and Mg(OH)2 must have been completely dissolved.
Granulometric measurements (data not shown), using static image analysis, were performed for every sample in order to follow the development of the crystal size distributions over time. All observed MgO experiments and the MgCl2 experiment run at molar ratio 1 show increasing particle sizes during the first 10 min. On the other hand, MgCl2 runs at 0.5 and 0.75 showed delayed but steady crystal growth throughout the entire experiment.
Fig. 8 shows the size distributions for all final precipitates. Both investigated precursors show larger crystal sizes when less MgCl2 and MgO were used. Lower initial amounts result in less saturated solutions, which (according to von Weimarn's laws of precipitation) results in precipitates with increased mean sizes (Mullin, 2001). Mehta and Batstone (2013) reported similar results for runs carried out under similar conditions with MgCl2 as a precursor. However, the MgO suspension only partially dissolves, which means that changing the amount of MgO would change the concentration of the suspended magnesia particles, minor the total amount of Mg2 þ ions in solution. Therefore, changes in supersaturation are very small, and the size differences between the observed ratios for MgO experiments are not as distinctive.
MgCl2 as a precipitant creates rod- or coffin-like structures at lower ratios, and highly dendritic crystalline structures at higher ratios (Fig. 9, bottom row). These structures are typical for struvite