R. Strauss et al., EFFECTS OF CRYSTALLINITY OF GOETHITE .2. RATES OF SORPTION AND DESORPTION OF PHOSPHATE, European journal of soil science, 48(1), 1997, pp. 101-114
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.