Recovery of iron phosphate from secondary sludge by ascorbic and citric acids

Iron (Fe) derived substances are widely used in the removal of phosphorus (P) in the wastewater treatment process. In the waste activated sludge, through the sequential P and Fe extraction, inorganic P accounted for 90.3% of the total P, which was extracted mainly in Fe(III)-P, Fe(II)-P and Al-P pools, and they account for 82.5% of the total P. The Fe (III) and Fe (II) combined with P mainly formed amorphous FerPO4(OH)3r-3 and vivianite, accounting for 39.0% and 43.5% of total Fe, respectively. By adding different reducing agents (ascorbic acid, cysteine, oxalate) and complexing agents (ethylene diamine tetra acetic, nitrous triacetic acid, citric acid) and adjusting the pH to 3.0, this study found that citric acid presented the best performance on P release, releasing 46.2% of the total P. Ascorbic acid presented the best performance for Fe release, with a releasing efficiency of the 68.1% of the total Fe. The addition of different proportions of ascorbic acid and citric acid to the sludge, along with pH adjustment. It was found that the maximum P release (70.8% of total P) was obtained with the VC/Cit ratio of 0.2/0.8. The high P release can be attributed to the co-effect of the reduction of Fe(III) by VC and the complexation of Fe and Al by Cit. The released P and Fe could be recovered by thermally induced precipitation to high purity FePO4 with a P recovery of 53.4%, which is a main precursor for LiFePO4. This study provides a new direction for the recovery and utilization of P from sludge.