Proton Dissociation from Surfaces of Variable Charge Soil andMnierals

Experiments on proton dissociation from the surfaces of goethite, amorphous Al oxide, kaolinite and latosolwere carried out, showing amphoteric behavior with reactions of proton dissociation-association on the surfaces andbuffering capacity in such a sequence as amorphous Al oxide > latosol > kaolinite > goethite. Dissociation con-stants of surface proton, pK_(sa) are significantly correlated with surface charge density, which has been proved with anelectrochemical model. Tbe intrinsic constants of proton dissociation, K_(sa)(int), gained by extrapolation to zero charconditions of plots of pK_(sa) against σ_o, could be used to estimate the acidity strength of variable charge surfaces. Thevalue of pK_(sa)(int) is 8.08 for goethite, 1 .2 for amorphous Al oxide, 6.62 for kaolinite and 5 .32 for latosol.

Adsorption Kinetics of Pb^(2+) and Cu^(2+) on Variable Charge Soils and Minerals: Ⅲ. Adsorption Kinetics of Pb^(2+) Within 30 Minutes

The adsorption kinetics of Pb^2+ on different soils and minerals with variable charges was studied by the two ion-selective electrode technique at different pH and concentrations.The results showed that more than 95% of adsorption on all the samples occurred during the first 5 minutes.All adsorption time-dependent data could fit the surface second-order equation very well.The values of Xm were goethite>kaolinite,and latosol>red soil at the same initial reaction concentration.The values of k were kaolinite>>goethite,and latosol>red soil at the same reaction pH and initial concentration.The higher the suspension pH,the faster the adsorption occurred.The meaning of Xm got by the two ion-selective electrode technique(one kind of batch technique) was different from that by the miscible displacement technique in that the former was only the equilibrium adsorption amount at a definite concentration and pH,but the latter was almost equal to the adsorption capacity at a definite pH with much less influence of concentration.One Pb^2+ was supposed to occupy two adsorption sites as the adsorption mechanism is concerned.

Interaction Between Charge Characteristics and Cu^(2+) Adsorption-Desorption of Soils with Variable or Permanent Charge

Charge characteristics and Cu2+ adsorption-desorption of soils with variable charge (latosol) and permanent charge (brown soil) and the relationship between them were studied by means of back-titration and adsorption equilibrium respectively. The amount of variable negative charge was much less in variable-charge soil than in permanent-charge soil and increased with the pH in the system, but the opposite trend occurred in the points of zero charge (PZCs). The amount of Cu2+ ions sorbed by permanent-charge soil was more than that by variable-charge soil and increased with the increase of Cu2+ concentration within a certain range in the equilibrium solution. The amount of Cu2+ ions desorbed with KC1 from permanent-charge soil was more than that from variable-charge soil, but the amount of Cu2+ ions desorbed with de-ionized water from permanent-charge soil was extremely low whereas there was still a certain amount of desorption from variable-charge soil. The increase of PZC of soils with variable or permanent charge varied with the increment of Cu2+ ions added. When the same amount of Cu2+ ions was added, the increments of PZC and variable negative surface charge of permanent-charge soil were different from those of variable-charge soil.