catalyst. The EDS analysis of
the surface of the fresh/recovered M-CaO_100 and U-CaO catalysts is
listed in Table 4. Each data is the average value determined from three
points on the catalyst surface. As it can be seen, after the 10th reaction
cycle, the Ca content on the catalyst surface for both recovered CaO catalysts
decreased due to the formation of newphases (as already seen by
XRDmeasurements) and the leaching problem(see later). Note that Na
and Si elements existed on the catalyst surface likely due to the addition
of Na2SiO3. As illustrated in Table 4, after the 10th reaction cycle, the recoveredM-
CaO_100 catalyst exhibited higher Ca content in comparison
to the recovered U-CaO catalyst. Additionally, it was known that the
CaO-based catalysts often suffered from Ca2+ leaching during the
transesterification reaction [11]. Thus, the leached metal ions (Ca2+
ions) in the biodiesel phase after the removal of the catalyst were detected
by ICP analysis. As shown in Fig. 10, the Ca2+ ions were indeed
leached into the biodiesel and glycerol phase after each reaction cycle.
Typically, the highest amount of leached Ca2+ ions in either biodiesel
phase or glycerol phase could be detected in the first reaction cycle.
The concentrations of the dissolved Ca2+ ions in biodiesel phase for
theM-CaO_100 and U-CaO catalystswere 174 and 100 ppm, respectively.
In glycerol phase, the concentrations of the dissolved Ca2+ ions for
the two catalysts were 3421 and 2717 ppm, respectively. However,
the concentrations of Ca2+ ions for the two catalysts decreased significantly
with subsequent reaction cycles, becoming lower than 6 ppm
(in biodiesel phase) and 120 ppm (in glycerol phase) after the 10th
cycle.