Abstract
Adsorption of dissolved phosphate onto synthetic hydrous ferric oxide (HFO) was measured in the laboratory as a function of pH, ionic strength, and phosphate relative concentration. Experimental data were used to constrain optimal values of surface complexation reactions using a geochemical modeling code JCHESS according to the diffuse layer model. The results provide a consistent set of model equilibrium constant (log K) values at 25‹ C and 100 KPa for the following reactions:>FeOH + PO43- + 3H+ = >FeO-H2PO4 + H2O (log K1int = 19.0)>FeOH + PO43- + 2H+ = >FeO-HPO4- + H2O (log K2int = 14.3 « 0.17)>FeOH + PO43- + H+ = >FeO-PO42- + H2O (log K3int = 8.32 « 0.27)These results differ significantly from previously-published estimates of log K2int and log K3int [1.3], and provide a more accurate fit to experimental measurements over a broad range of pH (3.12), ionic strength (0.001.0.1 mol/.) and total relative phosphate concentration (12.500 ƒÊmol phosphate/g HFO). The results provide a close fit to the experimental data within a wide range of conditions, and should be adopted in modelling the chemical speciation of phosphate in natural systems containing HFO as a predominant adsorbing material.Keywords: phosphate, adsorption, hydrous ferric oxide, surface complexes, equilibrium constants
Highlights
Soluble phosphate salts are widely used as agricultural fertilisers to boost food production, and a common environmental problem associated with fertiliser overuse is elution of dissolved phosphate into ground and surface waters (Delgado and Torrent, 1997; Cisse and Mrabet, 2004)
In general the results of phosphate adsorption experiments conform to expectations based on previous studies of ligand adsorption (Dzombak and Morel, 1990; Ryden et al, 1977a; Ryden et al, 1977b) that uptake of phosphate by hydrous ferric oxide (HFO) is minimal at pH values greater than about 12, increases at lower pH values across a characteristic adsorption edge, and that maximum uptake is achieved at pH values below 8
Previous spectroscopic work (Persson et al, 1996) shows that orthophosphate adsorption onto goethite is likely dominated by monodentate
Summary
Soluble phosphate salts are widely used as agricultural fertilisers to boost food production, and a common environmental problem associated with fertiliser overuse is elution of dissolved phosphate into ground and surface waters (Delgado and Torrent, 1997; Cisse and Mrabet, 2004). Elevated concentrations of soluble phosphate from agricultural discharges lead to eutrophication, anoxia, and degradation of freshwater habitats (Bennett et al, 2001). Natural background phosphate concentrations in freshwater settings typically vary between 0.095 and 0.95 mg/l (Luisells et al, 2000), whereas in groundwater of heavily fertilised agricultural soils, it can generally reach levels of up to 2 mg/l (Fadiran et al, 2007). Phosphate adsorption onto soil components can influence the mobility of synthetic pesticides (Gimsing et al, 2004a; Gimsing et al, 2004b) impacting both water quality and ecosystem health
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