Recovery potential of rare earth elements from solid and liquid by-products of hydrothermal liquefaction of municipal sewage sludge

The growing demand for rare earth elements (REEs) creates the need to search for new sources of their extraction. This experimental study aimed to investigate behavior of selected REEs during a hydrothermal liquefaction (HTL) of sewage sludge (SS) focused on their partitioning in the HTL products. The HTL experiments were carried out at 350 °C and for 30 min. residence time in a batch reactor, without and with addition of different catalysts. Among the tested REEs, Dy, Er, Eu, Gd, Ho, La, Lu, Nd, Pr, Sm, Tb, Tm, Y and Yb showed high affinity for the HTL biochar (above 99 %). Sc and Ce migrated into the aqueous phase (AP), but only at the level from thousandths to tenths of mg/dm³. The highest Ce content (53.2–––874 mg/kg) was found in HTL biochar, but it should also be emphasized that the Y content (11.0–––19.8 mg/kg) and La content (5.6–––43.2 mg/kg) were relatively high. The presence of CeO₂ as a catalyst resulted in significant increase in the contents of Gd, Ce, Nd and Sm in biochar. The trend towards binding to the post-process biochar is promising in terms of recovering critical raw materials from sewage sludge.