Effect of Sulfate on Adsorption of Zinc and Cadmium by Variable Charge Soils *1

SO 4 2- and Zn 2+ or Cd 2+ were added to three variable charge soils in different sequences. In one sequence sulfate was added first, and in the other, Zn 2+ or Cd 2+ first. The addition of sulfate to the system invariably caused an increase in adsorption of the heavy metal added, with the effect more remarkable when the soil reacted with the sulfate prior to the metal. The shift in pH 50 for both Zn and Cd adsorption was also comparatively larger in the first sequence of reactions. It was suggested that the increase in negative charge density and the resultant negative potential of the soil were the primary cause of the pronounced effect of sulfate on adsorption of Zn or Cd, and the formation of the ternary surface complex S SO 4 M might also play a role in the effect.

Adsorption of Chloride, Nitrate and Perchlorate by Variable Charge Soils

Two cells consisting of a chloride-selective electrode and a nitrate-selective electrode or of a chloride-selective electrode and a perchlorate-selective electrode were directly put in the soil suspension to determine the concentration ratios Cl-/ NO3- or Cl-/ ClO4- for studying the adsorption of the three anions by variable charge soils. It was found that all the concentration ratios CCl- / CNO3- and CCl- / CClO4- in suspension were smaller than unity when soil samples were in equilibrium with mixed KCl and KNO3 or KCl and KClO4 solutions of equal concentration. The order of the amount of chloride, nitrate and perchlorate adsorbed by variable charge soils was Cl-> NO3-> ClO4- when the soils adsorbed these anions from the solution containing equal concentrations of Cl-, NO3- and ClO4-. Such factors as the pH of the suspension, the iron oxide content of the soil etc. could affect the amounts and the ratios of anions adsorbed. The adsorption was chiefly caused by coulombic attraction, but a covalent force between the anion and the metal atom on the surface of soil particles may also be involved, at least for Cl- ions, even for NO3- ions.

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.

Effect of Zn Adsorption on Charge of Variable Charge Soils

The variation in apparent charge of two typical variable charge soils resulting from Zn adsorption were studied by KC1 saturation and NH4NO3 replacement methods. Results showed that zinc were adsorbed specifically to those sites with negative charge. As in different pH ranges, the percentages of specific and electrostatic adsorptions of zinc and the mechanisms of specific adsorption were different, the effects of Zn adsorption on apparent charge were varied and could be characterized as: when 1 mmol Zn2+ was adsorbed, a change about 1 mmol in the apparent charge was observed in the low pH range (Ⅰ), 1.4 to 1.5 mmol in the moderate pH range (Ⅱ) and 0.55 to 0.6 mmol in the high pH range (Ⅲ). These experimental data, in terms of soil charge, proved once more author's conclusion in the preceding paper (Sun, 1993) that in accordance with the behaviors of Zn adsorption by the variable charge soils in relation to pH, three pH ranges with different adsorption mechanisms were delineated; that is, in Range Ⅰ, specific adsorption was the predominant mechanism, in Ranges Ⅱ and Ⅲ, specific and electrostatic adsorptions co-existed, but their specific adsorption mechanisms were not identical.

Effect of Low-Molecular-Weight Organic Acids on Cl-Adsorption by Variable Charge Soils

Low-molecular-weight (LMW) organic acids exist widely in soils and have beenimplicated in many soil processes. The objective of the present paper was to evaluate effect of twoLMW organic acids, citric acid and oxalic acid, on Cl^-adsorption by three variable charge soils, alatosol, a lateritic red soil and a red soil, using a batch method. The results showed that thepresence of citric acid and oxalic acid led to a decrease in Cl^- adsorption with larger decreasesfor citric acid. Among the different soils Cl^- adsorption in the lateritic red soil and the redsoil was more affected by both the LMW organic acids than that in the latosol.

Effect of Crop-Straw Derived Biochars on Pb(Ⅱ) Adsorption in Two Variable Charge Soils

Two variable charge soils were incubated with biochars derived from straws of peanut, soybean, canola, and rice to investigate the effect of the biochars on their chemical properties and Pb（II） adsorption using batch experiments. The results showed soil cation exchange capacity （CEC） and pH significantly increased after 30 d of incubation with the biochars added. The incorporation of the biochars markedly increased the adsorption of Pb（II）, and both the electrostatic and non-electrostatic adsorption mechanisms contributed to Pb（II） adsorption by the variable charge soils. Adsorption isotherms illustrated legume- straw derived biochars more greatly increased Pb（II） adsorption on soils through the non-electrostatic mechanism via the formation of surface complexes between Pb（II） and acid functional groups of the biochars than did non-legume straw biochars. The adsorption capacity of Pb（II） increased, while the desorption amount slightly decreased with the increasing suspension pH for the studied soils, especially in a high suspension pH, indicating that precipitation also plays an important role in immobilizing Pb（II） to the soils.

Adsorption of Potassium and Sodium Ions by Variable Charge Soils

Adsorption of potassium and sodium ions by four typical variable charge soils of South China was studied. The results indicated that the variable charge soils saturated with H and Al showed a much higher preference for potassium ions relative to sodium ions, and this tendency could not be changed by such factors as the pH, the concentration of the cations, the dielectric constant of solvent, the accompanying anions and the iron oxide content etc., suggesting that this difference in affinity is caused by the difference in the nature of the two cations. It was observed that a negative adsorption of sodium ions by latosol and lateritic red soil in a mixed system containing equal amount of potassium and sodium ions at low pH, which is caused by a competitive adsorption of potassium and sodium ions and repulsion of positive charge on the surfaces of soil particles for cations. The adsorption of potassium and sodium ions increased with the decreases in the dielectric constant of solvent and the iron oxide content. Sulfate affected the adsorption of potassium and sodium ions through changing the surface properties of the soils.

Adsorption Kinetics of Pb^(2+) and Cu^(2+) on Variable Charge Soils and Minerals: Ⅱ. Equations for Describing Experimental Data

Different mathematical methods, including linearization, differential, integration and nonlinear least squares approximation (Newton-Marquardt method), were used to fit different kinetic equations, such as zero-order, first-order (i. e, membrane diffusion), second-order, parabolic-diffusion, Elovich, two-constant equations, to the experimental data of Pb2+ and Cu2+ adsorption on variable charge soils and kaolinite. Assuming each M2+ occupied two adsorption sites, two more equations, the so-called surface second-order equation and third-order equation were derived and compared with the above equations according to the fitting results, which showed that the second-order equation and surface second-order equation, being one equation in different expressions under some conditions, were better than the other equations in describing the Pb2+ and Cu2+ adsorption kinetics, and the latter was the best.

Effect of Specific Adsorption of Ions on Electrokinetic Properties of Variable Charge Soils

Studies were carried out by using electrophoretic method on the effects of the specific adsorption of the anions,such as SO4^2-,PO4^3-,and F^- ions,the cations,such as Ca^2+,Mn^2+,Zn^2+,and Cu^2+,ions,and the anions and cations coexisting,such as Zn^2+ and SO4^2= ions,on electrokinetic properties of the red soils as typical variable charge soils in China concerning variation in the specific ion species and concentrations,with an emphasis on the interaction between soil colloid surfaces and the ions in soil solutions.The results showed that the adsorption of specific ions led to a very pronounced decrease in zeta potentials of the soil colloids and a shift of the IEPs to lower values for specific anions,and an obvious increase in zeta potentials of the soil colloids and a shift of the IEPs to higher values for specific cations.Under circumstances of the specific anions and cations coexisting,for instance,Zn^2+ and SO4^2- ions,the zeta potentials changed with values higher than the value for SO4^2- alone and lower than that for Zn^2+ alone,and the IEP was between that for Zn^2+ and that for SO4^2-.The adsorption of Zn^2+ and Cu^2+ ions resulted in a reversal of the zeta potentials,and appearance of two IEPs for Zn^2+ and no IEP for Cu^2+,exhibiting interesting special effects of these kinds of metal ions.The higher the concentrations of the ions,the greater the change of the electrokinetic properties.

Adsorption Kinetics of Pb^(2+) and Cu^(2+) on Variable Charge Soils and Minerals:Ⅰ.Technique

A new technique for studying the adsorption kinetics of heavy metals,Pb^2+ and Cu^2+,on variable charge surfaces was established with two selective electrodes and microcomputer control system.Feasibility of the technique,including interference of other ions (mainly Fe^3+ and Al^3+),response time of electrodes,and the pH range of testing,was studied.Comparision with the most widely used miscible displacement technique,which was considered insufficient in studying 30-minute rapid reactions,at present time showed that the new technique was more advantageous in testing in situ,easy to operate,and economic.

Adsorption of Potassium and Calcium Ions by Variable Charge Soils

Interactions of potassium and calcium ions with four typical variable charge soils in South China were examined by measuring pK-0.5pCa value with a potassium ion-selective electrode and a calcium ion-selective electrode,and pK value with a potassium ion-selective electrode.The results showed that adsorption of potassium and calcium ions increased with soil suspension pH,and the tendency of the pK-0.5pCa value changing with pH differed with respect to pH range and potassium to calcium ratio.Adsorption of equal amount of calcium and potassium ions led to release of an identical number of protons,suggesting similar adsorption characteristics of these two ions when adsorbed by variable charge soils.Compared with red soil,latosol and lateritic red soil had higher adsorption selectivities for calcium ion.The red soil had a greater affinity for potassium ion than that for calcium ion at low concentration,which seems to result from its possession of 2:1 type minerals,such as vermiculite and mica with a high affinity for potassium ion.The results indicated that adsorption of potassium and calcium ions by the variable charge soils was chiefly caused by the electrostatic attraction between the cations and the soil surfaces.Moreover,it was found that sulfate could affect the adsorption by changing soil surface properties and by forming ion-pair.

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.

Zn Adsorption by Variable Charge Soils in Relation to pH

Zn adsorption by pure oxides or in the presence of a high concentration of inner electrolyte has been extensively studied. But, in studies on Zn adsorption in the complicated soil system, especially in variable charge soils, profound knowledge about the adsorption mechanism still lacks. In this paper, taking Zn ion adsorption by two typical variable charge soils as the object of the study, author discusses the relation between Zn adsorption and pH and possible adsorption mechanisms. The results showed that in the low pH range where the amount of Zn adsorbed did not exceed 50% of Zn added, the specific adsorption was the dominant mechanism. The species of Zn specifically adsorbed was free Zn2+ ion. In the middle and high pH ranges, the mechanisms of specific and electrostatic adsorptions, co-existed, accounting for about 70% and 30% , respectively. Noteworthily, in the high pH range, the hydroxyl Zn ion (ZnOH +) from Zn2+ hydrolysis probably was a preferable species for specific adsorption.

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 isotherms 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.

Cu Secondary Adsorption by Some Variable Charge Soils After Adsorbing SO_4^(2-)

Cu secondary adsorption by three variable charge soils collected from hubei Province and Hunan Province was investigated.The amount of Cu secondary adsorption increased with that of SO4^2- elementary adsorption and conformed with the Langmuir,freundlich and Temkin isotherms.Desorption of secondary-adsorbed Cu indicated that the hysteresis ratio decreased as Cu secondary adsorption increased,which meant that secondry-adsorbed Cu existed not only in the exchangeable form but also in the bridge form and specifically adsorbed form.The amount of Cu secondary adsorption increased with the temperature.

Apparent Mobilities of Na^+, K^+and Ca^(2+) Ions in Variable Charge Soil Colloid Systems

In this paper a simple method for determination of the apparent mobility of cation in a soil colloid system was described. With this method apparent mobilities of Na+, K+, and Ca2+ ions in the systems of the ferric luvisol, acrisol, and ferralsol were determined, and the reduction percentages of the mobilities were calculated. The results showed that the apparent mobilities of different cations at the same normality in a given soil system were in the order UNa> UK> UCa; those of the same cations among different soil systems were in the order ferralsol > acrisol > ferric luvisol, but the reduction percentages were in a reverse order, which among different cations at the same normality was Ca2+> K+> Na+ for ferric luvisol and acrisol systems, but was K+> Ca2+> Na+ for farralsol system. These results were interpreted in terms of different amounts of negative charge the clay fraction of different soils carries, and different mechanisms by which the soils adsorb the cations.

Contribution of Iron and Aluminum Oxides to Electrokinetic Characteristics of Variable Charge Soils in Relation to Surface Charge

The contribution of iron and aluminum oxides to electrokinetic characteristics of variable charge soils was studied through determination of electrophoretic mobilities of the red soils treated with either removal of iron oxides or coating of aluminum oxides, and of those deferrated under natural conditions. After removal of the iron oxides, zeta potentials of the latosol and the red earth decreased obviously with a shift of IEP to a lower pH, from 6.4 to 5.3 and 4.1 to 2.4 for the former and the latter, respectively, and the electrokinetic change for the latosol was greater than for the red earth. Zeta potentials of the kaolinite sample increased markedly after coated with iron oxides. The striking effect of iron oxides on electrokinetic properties of the soils was also demonstrated by the electrokinetic differences between the samples from the red and white zones of a plinthitic horizon formed naturally, and between the samples from the gley and bottom horizons of a paddy soil derived from a red earth. The coatings of aluminum oxides on the latosol and the yellow earth made their zeta potentials rise pronouncedly and their IEPs move toward higher pHs, from 6.2 to 6.8 and 4.3 to 5.3 for the former and the latter, respectively. The samples with different particle sizes also exhibited some electrokinetic variation. The experiment showed that the effects of iron and aluminum oxides were closely related to the pH and type of the soils.

Factors Affecting Sensitivity of Variable Charge Soils to Acid Rain

The sensitivity of a large number of variable charge soils to acid rain was evaluated through examiningpH-H_2SO_4 input curves. Two derivative parameters, the consumption of hydrogen ions by the soil and theacidtolerant limit as defined as the quantity of sulfuric acid required to bring the soil to PH 3.5 in a 0.00lmolLi￣(-1) Ca(NO_3)_2 solution, were used. The sensitivity of variable charge soils was higher than that of constantcharge soils, due to the predominance of kaolinite in clay mineralogical composition. Among these soils thesensitivity was generally of the order lateritic red soil >red soil >latosol. For a given type of soil within thesame region the sensitivity was affected by parent material, due to differences in clay minerals and texture.The sensitivity of surface soil may be lower or higher than that of subsoil, depending on whether organicmatter or texture plays the dominant role in determining the buffering capacity. Paddy soils consnmedmore acid within lower range of acid input when compared with upland soils, due to the presence of moreexchangeable bases, but consumed less acid within higher acid input range, caused by the decrease in claycontent.

A Model for Anion Electric Adsorption on Variable Charge Soils

A model was established for anion electric adsorption on variable charge surfaces and an equation to describe the relationship of adsorption amount with equalibrium concentration and suspension pH was derived according to the principles of surface reaction between variable charge surface and anions.The adsorption behaviours of nitrate and chloride on variable charge soils were modelled and the results showed that the model could describe the adsorption of nitrate very well.

Types of Crystalline Iron Oxides and Phosphate Ad－sorption in Variable Charge Soils

The types, contents and morphologies of crystalline Fe oxides and their relations to phosphate adsorptionon the clay fractions in soils with variable charge in southern China were investigated by means of XRD, TEM,EMA and chemical analysis methods.Results indicated that the types and contents of crystalline Fe oxidesvaried with the soils examined. The dominant crystalline Fe oxide was hematite in the latosols and goethitesin the red soils.In yellow-brown soils, the only crystalline Fe oxide was goethite.The difference between Aldand Alo came mainly from the Al substituting for Fe in the pe oxides. The crystal morphology of goethiteappeared mainly as subrounded flat or iso-dimensional rather than acicular particles. Hematites occurredin plates of various thickness. Their MCDa/MCDc ratios in the latosols and red soils were generally above1.5 and below 1.5, respectively. The MCD values of goethites and hematites were 15-25nm and 20-35nm,and their specific surface areas were 80-120m￣2/g and 35-75m￣2/g, respectively.The goethite crystals weregenerally smaller. Variations of the total amounts of crystalline Fe oxi es in clay fractions were not related tophosphate adsorption. The types, contents and morphologies of crystalline Fe oxides in the soils remarkablyaffected phosphate adsorption characteristics of the soils. The phosphate adsorption of goethite was muchgreater than that of hematite. The higher the MCD /MCDc rotio of hematite, the lower the phosphateadsorption.