The experiments with increasing metal concentration on the
aqueous phase were performed by mixing different amounts of
NdFeB magnet oxide powder with 1 mL of diluted HCl. The
molar ratio between HCl and the rare-earth molecules (n(HCl)/
n(Ln)) was kept constant at a value of 3.5.
The metal contents of all leachates were measured with
TXRF. An aliquot of 50 or 100 mL of the aqueous phase was
mixed with a 1000 mg L1 gallium standard and diluted to 1 mL
with Milli-Q water. The concentration of gallium was chosen in
the range of the lowest metal concentrations present in the
leachate. The sample carriers were pretreated with 20 mL of
silicone oil dissolved in isopropanol and dried for 5 min in an
furnace at 60 C. Aerwards, an aliquot of 5 mL of the sample
was added on the sample carrier and dried for another 10 min.
The carriers were measured for 500 s or for 1000 s when the iron
concentration was very low. Longer measuring times were
necessary at low iron concentrations because the Ka line of iron
overlaps partially with the La line of dysprosium.