Exchange Reaction Between Selenite and Hydroxyl Ion of Variable Charge Soil Surfaces: Ⅱ. Kinetics of Hydroxyl Release

A self-made constant pH automated titration instrument was used to study thekinetics of hydroxyl release during selenite reacting with variable charge soils. The rate ofhydroxyl release was very rapid at the first several minutes, then gradually slowed down, and atlast did not change any more. The experimental data was well fitted by the Langmuir kineticequation, arid with increasing selenite concentration or decreasing solution pH, the reaction lastedlonger, the maximum of hydroxyl release (x_m) increased, and the binding constant (k) decreased.The time of hydroxyl release with Xuwen latosol was much longer than that with Jinxian red soil.

Release of Hydroxyl Ions Associated with Chloride Adsorption in Electrodialyzed Variable Charge Soils

The release of hydroxyl ions from two variable charge soils associated with the adsorption of chloride in NaClO4 solutions was examined. The complete release required about 10 minutes. The release decreased with the increase in the NaClO4 concentration at first and eventually was little affected by the latter. The adsorption of Cl- was almost linearly related with the quantity of NaCl added to the system, while the OHrelease-NaCl curve varied with the soil type and the concentration of NaClO4. The logarithm of the quantity of released OH- was linearly related with the pH. The ratio of released OH- to adsorbed Cl- was larger in dilute NaClO4 solutions than in concentrated solutions and decreased with increasing NaCl.

Exchange Reaction Between Selenite and Hydroxyl Ion of Variable Charge Soil Surfaces: I. Electrolyte Species and pH Effects

Hydroxyl release of red soil and latosol surfaces was quantitatively measuredusing a self-made constant pH automated titration instrument, to study the changes of hydroxylrelease with different added selenite amounts and pH levels, and to study the effects ofelectrolytes on hydroxyl release. Hydroxyl release increased with the selenite concentration, with arapid increase at a low selenite concentration while slowing down at a high concentration. The pHwhere maximum of hydroxyl release appeared was not constant, shifting to a lower valus withincreasing selenite concentration. Hydroxyl release decreased with increasing electrolyteconcentration, and the decrease was very rapid at a low electrolyte concentration but slow at a highelectrolyte concentration. For NaClO_4, NaCl and Na_2SO_4, hydroxyl release was in the order ofNaClO_4 > NaCl >> Na_2SO_4, and the difference was very significant. But for NaCl, KCl and CaCl_2,the order of hydroxyl release was NaCl > KCl > CaCl_2, and the difference was smaller. The amount ofhydroxyl release from Xuwen latosol was greater than that from Jinxian red soil. Hydroxyl releaseexisted in a wider range of pH with Xuwen latosol than with Jinxian red soil, due to theirdifference in soil properties. However, both soils had similar curves of hydroxyl release,indicating the common characteristics of variable charge soils.

Adsorption of Sulfate and Fluoride in Relation to Some Surface Chemical Properties of Oxisols

In the present work, the adsorption of sulfate and fluoride by two oxisols was studied, and during the adsorption OH- released from soils were measured and the change in surface charge carried by one of the soils after the adsorption of fluoride was examined. The results show that the adsorption of sulfate by oxisols and the release of OH- from oxisols both increased with the increase of sulfate added at a constant pH, but decreased when pH became higher gradually. The adsorption and release both decreased markedly after removal of iron oxide. The ratio of OH- to SO42- for Fe-removed soils decreased to 15-34% of the original soils. These results suggest that iron oxide was the chief carrier of hydroxyl groups capable of ligand exchange with sulfate in oxisols.Compared with sulfate, however, the amount of fluoride adsorbed, OH- released during the adsorption of fluoride and the ratio of OH- to F- were much larger. The effect of iron oxide on the adsorption of fluoride by oxisols was smaller than that on the adsorption of sulfate. The ratios of OH- to F- for an oxisol from Brazil were 0.62 and 0.48 respectively before and after removal of free iron oxides. This implied that iron oxide only provided a small amount of exchangeable hydroxyl groups capable of ligand exchange with fluoride in oxisol. The research results indicate that among the factors inducing the changes in some properties of oxisols after adsorption of fluoride, the role became less important in the sequence of the release of OH-> the increase of negative charge > the decrease of positive charge.