Transformation of beneficially reused aluminium sludge to potential P and Al resource after employing as P-trapping material for wastewater treatment in constructed wetland

The phosphorus (P)-saturated aluminium sludge used as substrate in constructed wetland (CW) for P-rich wastewater treatment was investigated to recover P and Al through chemical precipitations of the P-extraction leachate of the used aluminium sludge. pH plays a key role in such the precipitation processes. The obtained compounds were identified with XRD, FTIR and SEM analyses. The results showed that over 99% PO43− could be recovered as hydroxyapatite by adding calcium chloride at pH of 13. The remaining Al could be fully recovered as amorphous aluminium hydroxide at pH of 7.0 or alternatively as tris(8-hydroxyquinolino)aluminium (Alq3) by adding suitable quantity of 8-hydroxyquinoline. Although the purity, structure, characteristics and production control of the compounds are worthy for further investigation, this study successfully developed a post-treatment methodology for beneficially reused aluminium sludge. The significance of this study is not only transferring aluminium sludge from “waste” to potential P and Al resources but also reducing the environmental risk of final disposal of used aluminium sludge.