ACID BASE REACTIONS, PHOSPHATE AND ARSENATE COMPLEXATION, AND THEIR COMPETITIVE ADSORPTION AT THE SURFACE OF GOETHITE IN 0.7 M NACL SOLUTION

Abstract

Potentiometric titrations of the goethite-water interface were carried out at 25°C in 0.1, 0.3 and 0.7 M NaCl solutions. The acid/base properties of goethite at pH > 4 in a 0.7 M NaCl solution can be reproduced successfully using either the Constant Capacitance (CCM), the Basic Stern (BSM) or the Triple Layer models (TLM) when two surface acidity constants are considered.Phosphate and arsenate complexation at the surface of goethite was studied in batch adsorption experiments. The experiments were conducted in 0.7 mol/L NaCl solutions at 25°C in the pH range of 3.0 to 10.0. Phosphate shows a strong affinity for the goethite surface and the amount of phosphate adsorbed decreases with increasing pH. Phosphate complexation is described using a model consisting of three monodentate surface complexes. Arsenate shows a similar adsorption pattern on goethite but a higher affinity than phosphate. A model including three surface complexation constants describes the arsenate adsorption at [AsO4]init = 23 and 34 μmol/L. The model prediction, however, overestimates arsenate adsorption at [AsO4]init = 8.8 μmol/L. The goethite surface acidity constants as well as the preceding phosphate and arsenate surface complexation constants were evaluated by the CCM and BSM with the aid of the computer program FITEQL, version 2.0.The experimental investigation of phosphate and arsenate competitive adsorption in 0.7 mol/L NaCl was performed at [PO4]/[AsO4] ratios of 1:1, 2.5:1 and 5:1 with [AsO4]init = 9.0 μmol/L and at a [PO4]/[AsO4] ratio of 1:1 with [AsO4]init = 22 μmol/L. The surface complexation of arsenate decreases significantly in competitive adsorption experiments and the decrease is proportional to the amount of phosphate present. Phosphate adsorption is also reduced but less drastically in competitive adsorption and is not affected significantly by incremental additions of arsenate at pH > 7.The equilibrium model derived by combining 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. The inability of the models to accurately predict competitive adsorption may be due to site heterogeneity, adsorption kinetics, as well as the limitations of the present models.