Chlorella-regeneration of exhausted D301Zr nanocomposite adsorbent of phosphate sequestration from water: A multi benefits approach with adsorbent regeneration, phosphorus reuse, and carbon fixation

Abstract

The regeneration of exhausted D301Zr (nano-Zr immobilized on D301, a phosphate adsorbent) by Chlorella can avoid the secondary pollution caused by chemical regeneration and achieve multi benefits of adsorbent regeneration, phosphorus reuse, and CO₂ fixation. The liquid-solid ratio and pH were identified as the main factors affecting the regeneration through orthogonal experiments, and the appropriate ranges of regeneration were verified for the liquid-solid ratio and initial pH through single factor experiments. The optimal conditions for Chlorella-regeneration were determined as 2500 L/g liquid-solid ratio (cell solution (10⁶ cell/mL)/D301-Zr-P), neutral initial pH; 25 °C of the regeneration temperature; phosphorus free f/2 medium; and with slightly air charging. The results of the 5-d cyclic regeneration showed that the regeneration rate, the biomass productivity, and CO₂ fixation rate were about in the ranges of (88.66–94.33) %, (24.69–31.24) mg·L⁻¹·d⁻¹, and (45.25–57.28) mg·L⁻¹·d⁻¹ respectively. In addition, the results of phosphorus adsorption/release experiments, the free energy evaluation, and XPS analysis suggested that phosphorus released from D301-Zr-P may cause of a reverse concentration gradient along D301-Zr-P to regeneration solution and Chlorella cells, which was the prerequisite for regeneration. The phosphorus consumption by Chlorella can be the driving force for maintaining the concentration gradient, until the Chlorella-regeneration was achieved finally.