INDIVIDUAL AND COMPETITIVE ADSORPTION OF PHOSPHATE AND ARSENATE ON GOETHITE IN ARTIFICIAL SEAWATER

Abstract

Phosphate and arsenate adsorption on goethite was studied in single batch experiments at 25 °C upon the addition of Ca2+, Mg2+ and SO42- to a 0.7 M NaCl solution at their respective seawater concentrations. Phosphate adsorption increases slightly in the presence of Mg2+ and Ca2+ and decreases at low pH upon the addition of SO42-. The enhanced adsorption may result from the formation of ternary surface complexes. In contrast, arsenate adsorption is not affected by the addition of Mg2+ or Ca2+ to the 0.7 M NaCl solution. Similarly, phosphate adsorption in seawater is 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.An equilibrium model derived by combining interaction parameters obtained from the single adsorbate subsystems predicts the phosphate and arsenate adsorption behaviours in the PO4-SO4-goethite and AsO4-Ca-goethite systems very 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 in the latter systems improves model fits significantly at high pH.Experimental investigations of phosphate and arsenate competitive adsorption in seawater were also carried out. The fraction of phosphate adsorbed in the competitive experiments in seawater is identical to the results obtained in the 0.7 M NaCl solution, whereas arsenate adsorption is greater in seawater at pH>7. The difference in the extent of arsenate adsorption in both solutions is proportional to the initial amount of phosphate in solution. 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 ternary surface complexes.