Surface electrical properties of goethite and adsorption of phosphate and arsenate on iron oxyhydroxides in high ionic strength solutions

• Main Author: Gao, Yan, 1970-
• Other Authors: Mucci, Alfonso (advisor)
• Format: Others
• Language: en
• Published: McGill University 2001
• Subjects: Geochemistry.
• Online Access: http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=36934

Iron oxyhydroxides are ubiquitous in natural systems and have been recognized as strong adsorbents for Group V elements, such as arsenic and phosphorus. Consequently, the mobility and bioavailability of phosphate and arsenate in the marine environment are mostly regulated by the fate of iron oxyhydroxides. However, to this day, the majority of adsorption studies of phosphate and arsenate were performed in low ionic strength solutions. In this study, phosphate and arsenate complexation on goethite in a 0.7 M NaCl solution and seawater was characterized over a pH range of 3.0 to 10.0 in batch adsorption experiments. Phosphate complexation is described using electric double layer models which consider the presence of three monodentate surface complexes, each characterized by an intrinsic formation constant. Arsenate shows a similar adsorption pattern on goethite but a higher affinity than phosphate. A model including three surface complexes describes the arsenate adsorption at relatively high initial concentrations (i.e., 23 and 34 mumol/l) but overestimates the adsorption at a lower concentration (i.e., 8.8 mumol/l). The equilibrium model derived by combining the formation constants obtained in the single oxyanion subsystems predicts the shape of the competitive adsorption data but fails to reproduce it quantitatively. In competitive experiments, phosphate adsorption is underpredicted whereas arsenate adsorption is overpredicted. === Magnesium, calcium and sulphate complexation on goethite was studied in a 0.7 M NaCl solution at 25°C and a pH range of 3.0 to 10.0 and their surface complexation constants were derived. Phosphate adsorption in a 0.7 M NaCl solution increases slightly in the presence of Mg and Ca and decreases at low pH upon the addition of SO4. In contrast, arsenate adsorption is not affected by the presence of Mg or Ca. Phosphate adsorption in seawater is reduced at low pH and enhanced at neutral pH relative to 0.7 M NaCl whereas arsenate adsorption is identical in both solutions except for a decrease at low pH in seawater. The equilibrium model derived by combining the single adsorbate subsystems predicts phosphate and arsenate adsorption in the PO 4-SO4-goethite and AsO4-Ca-goethite systems well, but fails to accurately reproduce the adsorption data in the PO4-Ca-goethite, PO4-Mg-goethite, PO4-seawater-goethite, AsO4-Mg-goethite and AsO4-seawater-goethite systems. The inclusion of ternary surface complexes >FeOMgHPO4-, >FeOMgH2PO 40, >FeOCaHPO4-, >FeOCaH 2PO40, >FeOMgHAsO4- and >FeOMgH2AsO40 in the latter systems improves model fits significantly. Phosphate adsorption in the competitive experiments in seawater is identical to that in the 0.7 M NaCl solution whereas arsenate adsorption in seawater is greater at pH > 7. In competitive experiments in seawater, phosphate adsorption is underpredicted at pH < 6.5 whereas arsenate adsorption is reproduced well using the Constant Capacitance Model (CCM) with the inclusion of the ternary surface complexes. === Phosphate and arsenate adsorption by ferrihydrite, in both 0.7 M NaCl and artificial seawater solutions, was studied at particle concentrations of 0.24 g/l and 0.025 g/l between pH 3.0 and 10.0. Both phosphate and arsenate surface complexation can be described using a model comprising three surface intrinsic constants which decrease with decreasing particle concentration. Ferrihydrite is a stronger adsorbent of PO4 and AsO4 on a per mass basis whereas goethite is a stronger sorbent in terms of chemical affinity. The CCM is able to predict PO4 and AsO4 adsorption on a mixture of goethite and ferrihydrite by combining the complexation constants derived from the single adsorbent subsystems. There is no evidence that adsorbed PO4 and AsO4 is released to solution when ferrihydrite is converted to hematite, an iron oxide with a much lower affinity for these oxyanions. The coprecipitation of both PO4 and AsO4 with hematite during the conversion may be responsible for these observations. === Finally, the surface complexation constants reported in this thesis may be incorporated into other models and contribute to the development of an experimental and thermodynamic database for marine geochemical systems.