Fig. 1. Schemes of the lab-scale systems: A) Single pass HF þ HF system, B) Single pass
VF þ VF system, C) and D) Single pass hybrid VF þ HF systems with different relative
surface areas, E) and F) Hybrid VF þ HF HF þ VF system with effluent recirculation, H)
Two-step VF þ VF system with effluent recirculation.