Towards circular bioremediation: Pilot-Scale assessment of Chlorella vulgaris for heavy metal removal, recovery, and biomass regeneration.

Chlorella vulgaris, a microalga that has emerged as promising for heavy metal bioremediation, is studied for its metal removal efficiency, metal recovery, and post-treatment biomass regeneration under scaled-up cultivation conditions. The microalga was exposed to a mixture of heavy metals (Cd, Zn, Cu, Pb, and Ni) at concentrations of 1, 5, and 10 ppm during cultivation in a 30 L pilot-scale photobioreactor (PBR) with micro-bubble aeration. Following acid-mediated metal recovery, the microalgal biomass was re-cultivated to evaluate its ability to regenerate under post-treatment conditions. Metal removal and recovery efficiencies were quantified via atomic absorption spectroscopy (AAS), while population, morphology, and physiological responses were monitored using optical microscopy, UV-Vis spectrophotometry and Fourier transform infrared (FTIR) spectroscopy. Metal removal efficiencies were metal-, time-, and concentration-dependent, exceeding 90% for Cu, Cd, and Pb, at 1 ppm on Day 3, and lowest for Ni (13-45%). Metal recovery from microalgal biomass through acidification demonstrated moderate to high desorption efficiencies, ranging from 40% to 90%, with metal- and concentration-specific dependencies. Regarding the microalga's response to regeneration, the 1 ppm culture exhibited the highest biomass recovery even from Day 3 of regeneration, as supported by cell morphology, growth rate, and pigment content. This integrated pilot-scale approach demonstrates the feasibility of combining biosorption, metal recovery, and biomass regeneration within a single system, supporting the development of circular and sustainable microalgal-based bioremediation strategies.