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Phosphorus sorption dynamics in the Scheldt Estuary

par Zhang, Hailong
Président du jury Gao, Yue
Promoteur Chou, Lei ;Elskens, Marc
Co-Promoteur Regnier, Pierre
Publication Non publié, 2019-12-19

Thèse de doctorat

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Résumé :

Eutrophication is a major environmental issue affecting many water bodies around the world and is generated by excessive inputs of anthropogenic nutrients, in particular nitrogen (N) and phosphorus (P). The delivery of P to aquatic systems has almost doubled, increasing from 5 Tg P yr-1 to 9 Tg P yr-1, during the 20th century at the global scale. At the same time, the retention of P in sediments has also grown from 3 Tg P yr-1 to 5 Tg P yr-1. In sediments, hydrous ferric oxide (HFO) microparticles are ubiquitous and have a high affinity for phosphate adsorption because of their large surface area and chemically active bonding groups. HFO regulates the phosphate bioavailability in natural waters through variations of salinity and pH, especially in estuarine systems characterized by brackish environments. In some areas such as the Scheldt Estuary, hydrous ferric phosphate can even account for more than 70% of the total phosphorus in sediments. The research described in this thesis investigates the effects of pH, salinity and the presence of major seawater ions on phosphate adsorption and desorption dynamics using synthesized HFO particles as well as surface sediments obtained from the Scheldt estuary. The phosphate adsorption capacity on HFO particles increases with increasing salinity and decreasing pH. The maximum adsorption capacity increases from 22.7 mg PO4/g to 76.8 mg PO4/g with the enhancement of salinity going from 0 PSU to 5 PSU, which then rises very slightly with further increases in salinity reaching a value of 78.5 mg PO4/g at 35 PSU. The quantitative relationship between the amount of phosphate adsorption and salinity can be described by a Langmuir isotherm model. The promotion of phosphate adsorption in solutions containing main seawater ions such as Na+, K+, Mg2+ and Ca2+ cations could be attributed to the changed phosphate speciation and the more positive charge at the adsorbent surface induced by the presence of cations. Despite the changed speciation and surface charge, the formation of a Ca-P precipitate (hydroxyapatite) also plays an important role in phosphate removal in solutions containing Ca2+ at pH 8.5. No evidence of the formation of phosphate-bridged ternary complexes has been found during this adsorption process. Simulations with the CD-MUSIC model show that the inner-sphere complexations ((FeO)2PO2 and FePO4) play an important role in phosphate adsorption on HFO. This is supported by the results of Fourier-transform infrared spectroscopy (FTIR) and Energy dispersive X-ray spectroscopy (EDS). In the Scheldt estuary, more than half of the total phosphorus (TP) in the sediments collected from Sint-Amands (54.7%) and from Kruibeke (54.2%) are bioavailable and the relative abundance of the P species follows the order of Fe-P > Detr-P > Ca-P > Org-P > Ex-P. The concentration of released P increases with the enhancement of pH and salinity, and the modified Langmuir isotherm model can be applied to describe the distribution of phosphorus between the sediments and the water column. The value of EPC0 (zero equilibrium P concentration) obtained through this model can depict the direction of the exchange reactions between the dissolved P in the water column and the adsorbed particulate P on sediments. It turns out that at present-day the sediments of the Scheldt Estuary act as a source of P. In the estuary, the P released from the sediments would mainly be from the Ex-P fraction. Overall, the present study indicates that variations of pH and salinity have an important influence on phosphate sorption dynamics in the Scheldt Estuary.