Effect of aging on phosphorus speciation in surface deposit of a vertical flow constructed wetland

This study was conducted to determine phosphorus (P) species captured in a vertical-flow constructed wetland (VFCW) system combining a trickling filter followed by FeCl3 injection for phosphate coagulation. Suspended solids (SS) thus formed accumulated over time at the VFCW surface and transformed into a sludge deposit layer, which was shown to concentrate most of the P captured in the system. In order to investigate the effect of aging on P species, representative SS and sludge samples were taken from a wastewater treatment plant that had been in operation for 8 years and analyzed using P fractionation, solution (31)P NMR spectroscopy, and P and Fe K-edge XANES spectroscopy. A partial mineralization of organic matter was shown by comparing organic carbon contents of SS and sludge materials. Chemical fractionations combined with P and Fe K-edge XANES spectroscopy showed that P was predominantly bound to iron within both samples in the form of ferric phosphate, rather than adsorbed onto ferric oxyhydroxide. Calcium-bound P was more significantly observed in sludge than in SS, suggesting that aging induced the recombination of part of the organic and iron-bound P species into calcium-bound forms, as a possible consequence of the partial mineralization of organic matter.