Adsorption Kinetics of Pb^2＋ and Cu^2＋ on Variable Charge Soils and Minerals：Ⅶ.Kinetics of H^＋ Releasing after Pb^2＋adsorption

The kinetics of H^+ releasing after Pb^2+ adsorption was studied by the potentiometric method combined with the acid-base titration curve technique.Results Showed that pH of latosol,red soil and kaolinite suspensions dropped sharply ,and then decreased gradually.Most of the H^+ exchanged.more than 60% ,was displaced at the first minute after Pb^2+ was added into the suspension.More H^+ was released at a higher concentration of Pb^2+ added for a given sample and the amount of H^+ released decreased in the order of red soil>latosol>kaolinite at a given concentration.The time-dependent data of H^+ releasing of latosol,red soil and kaolinite at two concentrations could be fitted best with the second-order equation among the six equations,including second-order equation,two-constant equation,Elovich equation,parabolic diffusion equation,first-order equation and exponential equation.The H^+/Pb^2+ exchange stoichimetry increased with time,and a stable ratio of H^+/Pb^2+,being 1.73,1.92 and 2.01 for kaolinite,latosol and red soil,respectively,was reached 25 minutes later.

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.

Adsorption Kinetics of Pb^2＋ and Cu^2＋ on Variable Charge Soils and Minerals：V.Effects of Temperature and Ionic Strength

The study results of the effects of temperature and ionic strength on the adsorption kinetics of Pb^2+ and Cu^2+ by latosol,red soil and kaolinte coated with Mn oxide showed that Pb^2+ and Cu^2+ adsorption by all samples.as a whole,increased with raising temperature.Temperature also increased both values of Xm(the amount of ion adsorbed at equilibrium)and k(kinetics constant)of Pb^2+ and Cu^2+,The activation energies of Pb^2+ adsorption were kaolin-Mn>redsoil>goethite and those of Cu2+ were latosol>redsoil>kaolin-Mn>goethite,For a given single sample the activation energy of Cu^2+ was greater than that of Pb^2+.Raising ionic strength decreased the adsorption of Pb^2+ and Cu^2+ by latosol,red soil and kaolinite coated with Mn oxide but increased Pb^2+ and Cu^2+ adsorption by goethite.The contrary results could be explained by the different changes in ion forms of Pb^2+or Cu^2+ and in surface charge characteristics of latosol,red soil,kaolin-Mn and goethite,Increasiung supporting electrolyte concentration increased Xm and k in goethite systems but decreased Xm and k in kaolin-Mn systems.All the time-dependent data fitted the surface second-order equation very well.

Adsorption of Phosphate on Variable Charge Soils

The study about the adsorption of phosphate on four variable charge soils and some minerals revealed that two stage adsorption appeared in the adsorption isothems of phosphate on 4 soils and there was a maximum adsorption on Al-oxide-typed surfaces between pH 3.5 to pH 5.5 as suspension pH changed from 2 to 9,but the adsorption amount of phosphate decreased continually as pH rose on Fe-oxide typed surfaces.The adsorption amount of phosphate and the maximum phosphate adsorption pH decreased in the order of yellow-red soil> lateritic red soil> red soil> paddy soil,which was coincided with the content order of amorphous Al oxide.The removement of organic matter and Fe oxide made the maximum phosphate adsorption pH rise from 4.0 to 5.0 and 4.5,respectively.The desorption curves with pH of four soils showed that phosphate desorbed least at pH 5.Generally the desorption was contrary to the adsorption with pH changing.There was a good accordance between adsorption or desorption and the concentration of Al in the suspension.The possible mechanisms of phosphate adsorption are discussed.

Behaviour of Potassium and Trace Elements in Rhizosphere of Flue-Cured Tobacco (Nicotiana Tabacum L.)

The study on the behaviour of potassium,phosphate and trace elements,Fe,Mn,Cu and Zn,in the rhizosphere of different varieties of flue-cured tobacco (Nicotiana tabacum L.)with high and low potassium application rate with rhizobag technique showed that soil available K,soil available P,and slow available K was in depletion status,whereas DTPA extractable Fe,Mn,Zn and Cu accumulated obviously in rhizosphere.The depletion and accumulation rates of mineral nutrients differed in degree with K application rate,soil type,and tobacco variety.The content of available K in both rhizosphere and bulk soil and K concentration in tobacco leaf increased significantly,and the available P in rhizosphere dropped with more K application.The DTPA-Fe content of red soil much lower in pH was higher than that of calcareous soil in bulk soil.But the DTPA-Fe content of calcareous soil was much higher than that of red soil in rhizosphere,which was considered perhaps to be mainly related to releasing of Fe phytosiderophore.Nitrate coule increase depletion of a vailable K in rhizosphere and also soil pH in comparision with ammonium.