Effects of Chlorella vulgaris on phosphorus release from ferric phosphate sediment by consecutive cultivations

Iron phosphate (Fe-P) is a main phosphorus storage form, especially in phosphorus-polluted environments. The re-release of Fe-P is a problematic result during microalgal remediation. In this study, pre-incubated Chlorella vulgaris was cultured in a BG-11 culture medium with different amounts of Fe-P. The effects of Fe-P re-release on biomass, flocculation and removal of PO₄ ^3- were investigated. The results indicated that C. vulgaris can promote the dissolution and release of Fe-P when the pH is 7, and the amount of Fe-P (ΔQ) released in 200 ml water reaches 0.055-0.45 mg d^-1 under a C. vulgaris concentration of 5.6 × 10⁵-8 × 10⁵ cells ml^-1. The growth of C. vulgaris was inhibited because of the flocculation behaviour of Fe^{3 +} in the release stage, which is associated with a specific growth rate of 0.3-0.4 d^-1 and a phosphorus removal rate below 30%. However, this process, in the long term, indicates a favourable transformation in which Fe-P becomes bioavailable under the action of C. vulgaris. Microalgae outbreaks may be triggered by persistent interactions between Fe-P and C. vulgaris. This study provides an important reference for the application of C. vulgaris in a Fe-P-rich environment.