Apparatus for salt separation (2) under supercritical water conditions, comprising a heat exchanger (4) and a fluidized bed reactor (6). The fluidized bed reactor comprising a supercritical water pressure containing wall (8) defining therein a fluidized bed chamber (10) connected to an inlet system (16) at one end thereof and an outlet system (18) configured to separate solids from supercritical fluidat another end thereof. The fluidized bed chamber receives a fluidized bed (12) therein and is configured to receive through the inlet system (16) a liquefied aqueous substance (14) for treatment in the fluidized bed chamber. The inlet system (16) comprises an inlet chamber (20) and a fluidization plate (22) positioned between the inlet chamber (20) and the fluidized bed chamber (10). The fluidized bed chamber extends between the inlet system (16) and outlet system (18) and comprises an entry section (10a) adjacent the inlet system (16), an outlet section (10c) adjacent the outlet system (18), and a mid-section (10b) extending between the entry section and the outlet section. The heat exchanger (4) extends along the fluidized bed chamber (10) and is configured to generate a decreasing temperature gradient in the fluidized bed chamber from the outlet section (10c) to the entry section (10a), the temperature gradient in the outlet section and mid- section being supercritical for aqueous substances and being subcritical for aqueous substances in the entry section (10a) adjacent the fluidization plate (22).