EFFECTS OF CRYSTALLINITY OF GOETHITE .2. RATES OF SORPTION AND DESORPTION OF PHOSPHATE

Eight samples of goethite ranging in surface area from 18 to 132 m(2) gr(-1) were mixed with phosphate at a range of pH values for periods w hich ranged from 0.5 h to 6 weeks. The sample with a surface area of 1 8 m(2) g(-1) had been hydrothermally treated to improve its crystallin ity. Its rate of reaction with phosphate depended on pH but was comple te within a day. Its maximum observed reaction was close to the theore tical maximum for surface adsorption of 2.5 mu mol m(-2). For the othe r samples, phosphate continued to react for up to 3 weeks and exceeded the value of 2.5 mu mole m(-2). The duration and extent of the reacti on depended on the crystallinity of the goethite. The results were clo sely described by a model in which the phosphate ions were initially a dsorbed on to charged external surfaces. The phosphate ions then diffu sed into the particles. This was closely described using equations for diffusion into a cylinder. Samples of goethite which had been loaded with phosphate dissolved more slowly in HCl, and had a longer lag phas e, than phosphate-free goethite. For the hydrothermally treated goethi te, HCl removed much of the phosphate when only a small proportion of the iron had been dissolved. For a poorly crystallized goethite, it wa s necessary to dissolve much more of the iron to obtain a similar remo val of phosphate. Brief treatment with NaOH removed most of the phosph ate from the hydrothermally treated goethite but only half the phospha te from a poorly crystallized goethite. These results are consistent w ith the idea that phosphate ions were not only bound on external surfa ce sites but had also penetrated into meso- and micro-pores between th e domains of the goethite crystals and were then adsorbed on internal surface sites. This penetration tied the domains together more firmly thus increasing the lag phase for dissolution. Differences between sit es for phosphate adsorption are therefore caused mainly by their locat ion on either external or internal sites. Models that ignore this are incomplete.