Phosphate adsorption and release from sediments of brackish lagoons: pH, O-2 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 s ediment phosphate was fractionated. Under oxic conditions, pH variations ch anged the distribution of phosphate in the sediment. At pH = 6, a calcium b ound phosphate mobilization was Found by the dissolution of this fraction, while the liberate phosphate became iron-bound. A part of phosphate adsorbe d onto the iron hydroxides was transferred to organic fractions when pH cha nged 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 p hosphate uptake from sediments. Such oxic conditions can be restored quickl y in the sediments after an anoxic period. In contrast, restoration of anox ic conditions is slow. Thus, decreasing phosphate inputs can induce sediment phosphate release int o the water under appropriate redox and pH conditions. This mobility of pho sphate is discussed in the coastal lagoons context. (C) 1999 Elsevier Scien ce Ltd. All rights reserved.