Volume Changes Due to SO2-4, SeO2-4, and H2PO-4 Adsorption on Amorphous Iron(III) Hydroxide in an Aqueous Suspension

Volume changes due to SO2-4, SeO2-4, and H2PO-4 adsorption on amorphous iron(III) hydroxide were determined by dilatometry in a mixture comprising an aqueous solution of Na2SO4, Na2SeO4, or NaH2PO4 and amorphous iron(III) hydroxide suspended in 0.1 mol dm-3 NaClO4 at an initial pH ranged from 4.50 to 6.50. System volumes increased during SO2-4, SeO2-4, and H2PO-4 adsorption on amorphous iron(III) hydroxide, suggesting that some hydrated water around aqueous SO2-4, SeO2-4, or H2PO-4 ions and amorphous iron(III) hydroxide were released to the bulk. The volume changes due to SO2-4, SeO2-4, and H2PO-4 adsorption at initial pH 4.50 were +18, +18, and +14 cm3 mol-1, respectively. Phosphate, which adsorbed as an inner-sphere complex, released most of its hydration water to the bulk. The volume changes due to SO2-4 and SeO2-4 adsorption indicated that they were dehydrated at the water/amorphous iron(III) hydroxide interface. The smaller changes in volume for H2PO-4 compared to SO2-4 and SeO2-4 may be explained by differences of charges and adsorption mechanisms of these oxoanions. Copyright 1999 Academic Press.