Pedosphere 30(2): 190--200, 2020
ISSN 1002-0160/CN 32-1315/P
©2020 Soil Science Society of China
Published by Elsevier B.V. and Science Press
Phosphorus release from vivianite and hydroxyapatite by organic and inorganic compounds
Stella GYPSER, Dirk FREESE
Brandenburg University of Technology Cottbus-Senftenberg, Faculty of Environment, Process Engineering, Biotechnology and Chemistry, Department of Soil Protection and Recultivation, Konrad-Wachsmann-Allee 6, Cottbus 03046 (Germany)
Corresponding Author:Stella GYPSER
ABSTRACT
      Based on recent mining rates and the exhaustion of global phosphorus (P) reserves, there is a need to mobilize P already stored in soils, and its recovery from secondary resources such as Ca- and Fe-phosphates is important. The Ca-phosphate hydroxyapatite forms a good fertilizer source, while vivianite is formed in waterlogged soils and sediments. During sludge treatment, the formation of vivianite has been identified, being mainly Fe-phosphate. Long-term P release from both hydroxyapatite and vivianite was studied using different inorganic (CaCl2 and CaSO4) and organic (citric and humic acid) reagents during batch experiments. Reagents CaCl2 and CaSO4 represent the soil solution, while citric and humic acids as organic constituents affect P availability in the rhizosphere and during the process of humification. Additionally, the flow-through reactor (FTR) technique with an infinite sink was used to study the long-term P release kinetics. The cumulative P release was higher by organic acids than by inorganic compounds. The cumulative P release rates were higher in the FTR with CaCl2 as compared to the batch technique. The infinite sink application caused a continuously high concentration gradient between the solid and liquid phases, leading to higher desorption rates as compared to the batch technique. The predominant amount of the total P released over time was available for a short term. While inorganic anion exchange occurred at easily available binding sites, organic acids affected the more heavily available binding sites, which could be embedded within the mineral structure. The results showed that organic compounds, especially citric acid, play a superior role as compared to the inorganic constituents of the soil solution during the recovery of already stored P from the tertiary phosphates vivianite and hydroxyapatite.
Key Words:  anion exchange,batch technique,citric acid,flow-through reactor,humic acid,ligand exchange,phosphate,phosphorus recovery
Citation: Gypser S, Freese D. 2020. Phosphorus release from vivianite and hydroxyapatite by organic and inorganic compounds. Pedosphere. 30(2): 190-200.
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