Phosphate adsorption and release from sediments of brackish lagoons: pH, O2 and loading influence

Excessive input of nutrients of human origin renders Mediterranean coastal lagoons eutrophic. In this study, the sediments from a coastal lagoon were exposed to different phosphate loads under oxic and anoxic conditions in a batch reactor. Phosphate and iron mobility were measured in the water and sediment phosphate was fractionated. Under oxic conditions, pH variations changed the distribution of phosphate in the sediment. At pH=6, a calcium bound phosphate mobilization was found by the dissolution of this fraction, while the liberate phosphate became iron-bound. A part of phosphate adsorbed onto the iron hydroxides was transferred to organic fractions when pH changed from 7 to 8. Under anoxic conditions, iron hydroxides are responsible for some release of phosphate. The redox potential necessary to obtain this release decreased with an increase in pH: release occurred at pH 7 and −50 mV, at pH 8 and −100 mV and at pH 9 and −150 mV. Oxic conditions induced phosphate uptake from sediments. Such oxic conditions can be restored quickly in the sediments after an anoxic period. In contrast, restoration of anoxic conditions is slow. Thus, decreasing phosphate inputs can induce sediment phosphate release into the water under appropriate redox and pH conditions. This mobility of phosphate is discussed in the coastal lagoons context.