Publisher Summary This chapter reviews the work done with soils and with hydrous oxides of iron and aluminum, with particular reference to mechanisms and structural aspects of anion adsorption. Anion adsorption on clay minerals and calcium carbonate is also discussed. The determination of adsorption isotherms is one of the most useful experimental procedures in the study of the interaction of anions with hydrous oxides or soils. Anion adsorption involves an electrostatic interaction and some chemical interaction between the surface and the ion. The Langmuir model is incomplete in describing this adsorption because it takes no account of charge. The adsorption of anions is dependent on pH, with maximum adsorption usually occurring for fully dissociated ions at low pH where the surface becomes positively charged, owing to protonation of surface M·OH groups. Maximum adsorption of anions of incompletely dissociated acids occurs at a pH value close to the pK of the acid. Anion adsorption reactions in soil are complicated by competition for adsorption sites from anions, including carboxylates, and also by the presence of cations, such as calcium and aluminum.

Anion Adsorption by Soils and Soil Materials

We investigated the phosphate(P) adsorption mechanisms at the ferrihydrite–water interface as a function of pH, ionic strength (I), and loading level, using a combination of adsorption envelopes, electrophoretic mobility (EM) measurements, and attenuated total reflectance Fourier transform infrared (ATR–FTIR) spectroscopy. The P adsorption envelopes show that: (1) adsorption decreases with increasing pH 3.5–9.5 and (2) adsorption is insensitive to changes in I at pH 4–7.5, but it slightly increases with increasing I from 0.01 to 0.8 at pH>7.5. The EM in 0.1 M NaCl decreases with increasing P concentration from 0 to 50 μmol L−1 at pH 3–9. The results of these macroscopic studies suggest the formation of inner-sphere complexes. The ATR–FTIR investigation shows that inner-sphere surface complexes are nonprotonated, bidentate binuclear species (≡Fe2PO4) at pH≥7.5 and could be associated with Na+ ions at P loading levels of 0.38 μmol m−2. At pH 4–6, protonated inner- sphere complexes are proposed at the loading levels between 0.38 and 2.69 μmol m−2.

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ATR-FTIR Spectroscopic Investigation on Phosphate Adsorption Mechanisms at the Ferrihydrite-Water Interface

Clay minerals, iron/aluminum oxides, and their contribution to phosphate sorption in soils — A myth revisited

The adsorption of arsenate (As(V)) on kaolinite, montmorillonite and illite was investigated at varying pH and competing anion concentration while holding As(V) concentration (6.7 X lo-' M), clay suspension density (2.5 g L-') and ionic strength (0.1 M NaCl) constant. The effects of 2 concen- trations of phosphate (P) or molybdate (MO) (6.7 X lo-' and 6.7 X 10m6 M) on As(V) adsorption envelopes (adsorption vs. pH) gave evidence for direct competitive adsorption (in the case of As(V) + P) and possibly site-specific non-competitive adsorption (As(V) + MO). Distinct As(V) adsorption maxima oc- curred at approximately pH 5.0 for kaolinite, 6.0 for montmorillonite and 6.5 for illite, and ranged from 0.15 to 0.22 mmol As(V) kg ~I. When both As(V) and P were present at equimolar concentrations (6.7 X lo-' M), As(V) adsorption decreased slightly, whereas As(V) adsorption substantially decreased in binary As(V)/P systems when the P concentration was 6.7 X 10mh M, which was 10 times greater than As(V). The presence of MO at equimolar (6.7 X 1O-7 M) and 10 times greater (6.7 X 10m6 M) concen- trations than As(V) caused only slight decreases in As(V) adsorption because the MO adsorption maximum occurred at pH < 4. The constant capacitance surface complexation model was applied to As(V) and P adsorption data and was used to predict As(V) adsorption at varying P concentrations. The model gave reasonable descriptions of As(V) adsorption on the 3 clay minerals at varyin g pH and in the presence of a competing oxyanion (P), indicating that surface complexation modeling may be useful in predicting As(V) adsorption in soils. Key Words-Arsenate competitive adsorption, Illite, Kaolinite, Montmorillonite.

Modeling arsenate competitive adsorption on kaolinite, montmorillonite and illite

Macroscopic- to more recent molecular scale investigations have enhanced our knowledge of soil phosphorus (P) chemistry, including the retention/release mechanisms in soils and soil components. Phosphate uptake on metal (oxy)hydroxide and phyllosilicate mineral surfaces and in soils generally increases with decreasing pH. Rapid adsorption kinetics is generally observed on many soil adsorbents at acidic pH. P fixation mechanisms such as inner-sphere complexation and intra- and interparticle diffusion often result in slow P release (i.e., hysteresis and irreversible reactions), creating challenges in remediating agricultural soils with high accumulations of P. This chapter covers some of the historical soil P chemical research findings via macroscopic approaches but focuses on more recent molecular scale approaches for elucidating P retention/release mechanisms.

Donald C. Toensing

Papers

Phosphate adsorption reactions with clay minerals