S. Goldberg et al., BORON ADSORPTION MECHANISMS ON OXIDES, CLAY-MINERALS, AND SOILS INFERRED FROM IONIC-STRENGTH EFFECTS, Soil Science Society of America journal, 57(3), 1993, pp. 704-708
Prediction of anion adsorption behavior is enhanced by understanding t
he adsorption mechanism. This study was conducted to evaluate ionic st
rength effects on B adsorption and to infer B adsorption mechanisms on
various surfaces. Boron adsorption on the Fe oxide goethite, the Al o
xide gibbsite, the clay minerals kaolinite and montmorillonite, and tw
o arid-zone soils was investigated as a function of solution pH (3-11)
and ionic strength of the background electrolyte (0.01-1.0 M NaCl). B
oron adsorption on the oxides and kaolinite increased from pH 3 to 6,
exhibited a peak at pH 6 to 8.5, and decreased from pH 8.5 to 11. For
B adsorption on montmorillonite and the soils, the adsorption maximum
was located near pH 9. Ionic strength dependence, measured as the incr
ease of the B adsorption maximum in 1.0 M NaCl solutions compared with
0.01 M NaCl solutions increased in the order: goethite (3%) < kaolini
te (15%) < gibbsite (-30%) < montmorillonite (109%) almost-equal-to mo
ntmorillonitic soil (116%) almost-equal-to kaolinitic soil (129%). Shi
fts in zero point of charge were observed on goethite, gibbsite, and k
aolinite following B adsorption. Ionic strength effect results suggest
an inner-sphere adsorption mechanism for goethite, gibbsite, and kaol
inite and an outer-sphere adsorption mechanism for montmorillonite and
the soils. These mechanisms are also indicated by zero point of charg
e determinations, microelectrophoresis measurements, or both. The cons
tant capacitance model, containing an inner-sphere adsorption mechanis
m, was able to describe B adsorption on goethite, gibbsite, kaolinite,
and kaolinitic soil. The model was unable to describe B adsorption on
montmorillonite and montmorillonitic soil because the computer optimi
zations diverged.