电解质阴离子和pH对可变电荷土壤硫酸盐吸附的影响

The effects of three electrolyte anions, ionic strength and pH on the adsorption of sulfate by two variable charge soils, with different surface charge properties were studied. Under the conditions of the same pH and ionic strength the effect of electrolyte anions on the adsorption of sulfate was in the order of Cl^- > NO₃^- > ClO₄^-, indicating the difference of the nature among these three anions. For Ferralsol in the same concentration of chloride and perchloride solutions, the two sulfate adsorption-pH curves could intersect at certain pH value. When pH was higher than the intersecting point, more sulfate was adsorbed in the perchloride solution, while when it was lower than the intersecting point, more sulfate was adsorbed in the chloride solution. In different concentrations of electrolyte solution, the curves of the amount of oxy-acid anion adsorbed, which changed with pH, could intersect at a certain pH, which is termed point of zero salt effect (PZSE) on adsorption. The nature of electrolyte anions influenced obviously the appearance of PZSE for sulfate adsorption. For ferralsol the curves of adsorption converged to about pH 7 in NaCl solution seemed to intersect in NaNO₃ solution and to have a typical PZSE for sulfate adsorption in NaClO₄ solution. For Acrisol the three curves of adsorption were nearly parallel in NaCl and NaNO₃ solutions and converged to pH 6.5 in NaClO₄ solution.     

Ionic strength effects on surface charge and adsorption of phosphate and sulphate by soils

Two surface soils (Patua and Tokomaru) of contrasting mineralogy were incubated with several levels of either CaCO₃ or HC1. The effects of ionic strength on pH, on surface charge, and on the adsorption of phosphate and sulphate were measured in three concentrations of NaCl. The pH at which the net surface charge was zero (point of net zero charge—PZC) was 1.8 for the Tokomaru soil and 4.6 for the Patua soil: differences that can be related to mineralogical composition. There was an analogous point of zero salt effect (PZSE) that occurred at pH 2.8 for the Tokomaru soil and at 4.6 for the Patua soil. The presence of permanent negative charge in the Tokomaru soil resulted in an increase in PZSE over PZC. The effect of ionic strength on adsorption varied greatly between phosphate and sulphate. For phosphate, there was a characteristic pH above which increasing ionic strength increased adsorption and below which the reverse occurred. This pH (PZSE for adsorption) was higher than the PZC of the soil and was 4.1 for the Tokomaru soil and 5.3 for the Patua soil. In contrast, increasing ionic strength always decreased sulphate adsorption and the adsorption curves obtained in solutions of different ionic strengths converged above pH 7.0. If increasing ionic strength decreases adsorption, the potential in the plane of adsorption must be positive. Also, if increasing ionic strength increases adsorption, the potential must be negative. This suggests that, depending upon pH, phosphate is adsorbed when the potential in the plane of adsorption is either positive or negative, whereas sulphate is absorbed only when the potential is positive.     

可变电荷土壤中胶粒双电层的相互作用与阴阳离子同时吸附

可变电荷土壤是在热带和亚热带地区母岩经历强烈的风化和淋溶作用形成的,或者由火山灰母质发育形成的土壤。这类土壤中硅酸盐矿物以高岭石为主,并含有大量Fe/Al氧化物。因此,这类土壤颗粒表面既带负电荷,又带有一定量的正电荷[1]。Qafoku和Sumner[2]研究发现带净负电荷的硅酸盐矿物颗粒和带正电荷的铁铝氧化物颗粒之间的相互作用是这类土壤能同时通过静电作用吸附阳离子和阴离子的主要原因。当用Ca(NO3)2溶液对装有可变电荷土壤的土柱进行淋溶实验时,Ca2+和NO-3在2.5孔隙体积(PV)后才在淋出液中同时出现,表明溶液中Ca2+和NO3-同时被土壤吸附[3]。当用稀CsCl溶液(0.5mmo     

The influence of iron oxides on phosphate adsorption by soil

Soils from Denmark and Tanzania were extracted with ammonium acetate (controls), EDTA to dissolve amorphous iron oxides, and dithionite-EDTA (DE) to dissolve crystalline iron oxides. The phosphate adsorption capacities of the extracted soils were taken as the maximum quantity of phosphate adsorbed computed from the Langmuir equation. The decreases in the phosphate adsorption capacity following EDTA extraction and DE extraction were attributed to the removal of iron oxides. Close correlations (P<0.001) were found (i) between EDTA-extractable iron (amorphous iron oxides) and the decrease in phosphate adsorption capacity following EDTA extraction, and (ii) between the difference between DE-extractable iron and EDTA-extractable iron (crystalline iron oxides) and the further decrease in phosphate adsorption capacity following DE extraction. The phosphate adsorption capacity, estimated to be approximately 2.5 μmol P m^?2, was in good agreement with the capacity of various synthetic iron oxides. The calculated phosphate adsorption capacity of soil iron oxides, obtained from the contents and specific surfaces of amorphous and crystalline iron oxides together with the phosphate adsorption capacity per m² for synthetic iron oxides, compared favourably with the measured phosphate adsorption capacity.     

The effect of soil flooding on the transformation of Fe oxides and the adsorption/desorption behavior of phosphate

As repeatedly reported, soil flooding improves the availability of P to rice. This is in contrast with an increased P sorption in paddy soils. The effects of soil flooding on the transformation of Fe oxides and the adsorption/desorption of P of two paddy soils of Zhejiang Province in Southeast‐China were studied in anaerobic incubation experiments (submerging with water in N₂ atmosphere). Soil flooding significantly increased oxalate‐extractable Fe (Fe_ox), mainly at the expense of dithionite‐soluble Fe (Fe_DCB), as well as oxalate‐extractable P (P_ox), but decreased the ratio of P_ox/Fe_ox. Flooding largely increased both, P adsorption and the maximum P adsorption capacity. The majority of newly sorbed P in the soils was P_ox, but also more newly retained P was found to be not extractable by oxalate. Flooding also changed the characteristics of P desorption in the soils. Due to a decrease of the saturation index of the P sorption capacity, P adsorbed by flooded soils was much less desorbable than that from non‐flooded soils. There are obviously significant differences in the nature of both, the Fe_ox and P_ox fractions under non‐flooded and flooded conditions. The degree of the changes in Fe_ox, P_ox, P adsorption and P desorption by flooding depended on the contents of amorphous and total Fe oxides in non‐flooded soils. Our results confirm that the adsorption and desorption behavior of P in paddy soils is largely controlled by the transformation of the Fe oxides. The reasons of the often‐reported improved P availability to rice induced by flooding, in spite of the unfavorable effect on P desorbability, are discussed.     

恒电荷土壤与可变电荷土壤对Cl~-和NO_3~-吸附的差异及其机理

对 3种可变电荷土壤和 4种恒电荷土壤在不同 pH、不同浓度、不同相伴阴阳离子下混合体系中Cl-和NO3-的吸附进行了测定。结果表明 ,在Cl-和NO3- 共存体系中 ,Cl-比例增大使可变电荷土壤Na+吸附量及OH-释放量增加 ,而对恒电荷土壤影响不大。Cl-和NO3-吸附量随平衡Cl-和NO3-浓度增加而增大 ,随pH升高而减少。但恒电荷土壤在上述各种条件下对Cl-和NO3-吸附均相同 ,而可变电荷土壤对Cl-吸附量大于NO3-的吸附量 ;NO3-、Cl-的选择系数为 0.51～0.78,Cl-和NO3-的相对吸附量分别为56.9%和 43.1%。在不同相伴阳离子下 ,可变电荷土壤平衡溶液Cl-/NO3-比值均小于 1,且为Na+K+Ca2+Mg2+Fe3+;而恒电荷土壤Cl-/NO3-比值为 1左右 ,且不受阳离子类型的影响。由此认为 ,Cl-和NO3-在两类土壤中均以电性吸附为主 ,恒电荷土壤对Cl-和NO3-的亲合力及吸附机理相同 ;而可变电荷土壤对Cl-的亲合力 NO3- ,Cl-存在着专性吸附     

氟离子、磷酸根和铬酸根在可变电荷土壤表面吸附过程中羟基释放动力学

用恒pH自动电位滴定装置研究了氟离子(F-)、磷酸根(H2PO4-)和铬酸根(CrO42-)在3种可变电荷土壤表面吸附过程中羟基(-OH)释放的动力学。研究结果表明,3种阴离子在可变电荷土壤表面吸附时-OH释放量的大小顺序为:F->>H2PO4->CrO42-,这与土壤对3种阴离子吸附量的大小顺序一致。pH对不同阴离子体系中-OH释放的影响不同,在F-体系中,pH5.0时-OH释放量最高,其次为pH6.0时,pH4.0时-OH释放量最小;CrO42-体系中-OH释放量随pH的增加而减小;pH对H2PO4-体系中-OH释放的影响较小。Elovich方程(Y=a kln(t))能够很好拟合2～60min之间的动力学数据,说明-OH释放的速率随时间增加而减小。比较速率常数k的大小可以发现,虽然F-体系中3种可变电荷土壤在前2min释放的-OH量有很大差异,但在2～60min内,-OH释放速率差别不大。在H2PO4-和CrO42-体系中,-OH释放速率的大小顺序是:昆明砖红壤>徐闻砖红壤>江西红壤,与土壤铁、铝氧化物含量一致。     

Effect of the size of variable charge soil particles on cadmium accumulation and adsorption

Purpose

The size of soil particles strongly affects the accumulation and adsorption of heavy metals which partly controls the co-transport of heavy metals by soil colloids. However, the effect of the size of soil particles on the accumulation and adsorption of heavy metals in the colloidal dimension has seldom been studied. In this study, variable charge soils were selected and separated into five size fractions to elucidate the effect of the size of soil particles on Cd accumulation and adsorption.

Materials and methods

Five soil particle size fractions (>10, 10–1, 1–0.45, 0.45–0.2 and <0.2 μm) were obtained from Cd-contaminated soil by natural sedimentation and fractional centrifugation. The concentrations and species of Cd were measured in various sized soil particles. Batch adsorption experiments of Cd on the obtained soil particles were conducted under different pH values and concentrations of NaCl.

Results and discussion

Generally, the concentration of Cd increased with decreasing soil particle sizes, and the Cd proportion of exchangeable and carbonate fraction decreased from 43.84 to 17.75% with decreasing particle size. The soil particles with a size of 10–1 and <0.2 μm possessed a stronger adsorption ability than the other fractions in most cases. Moreover, the Cd adsorption capacities of the soil particles increased with increasing pH values and decreasing concentrations of NaCl, especially for soil particles containing more organic matter (OM) and variable charge minerals.

Conclusions

Smaller soil particles are more capable of accumulating Cd and make Cd more stable. The adsorption capability of Cd is negatively related to the particle size and NaCl concentration and is positively related to the pH. The effects of the size of variable charge soil particles on Cd accumulation and adsorption are attributed to the differences in the physicochemical properties among various soil particle size fractions. This study contributes to the understanding of the co-transport of heavy metals in soil by soil colloids.

     

Pectin adsorption on amorphous Fe/Al hydroxides and its effect on surface charge properties and Cu(II) adsorption

Purpose

The purpose of this study was to elucidate the mechanisms for pectin-enhanced adsorption of heavy metal cations on variable charge minerals.

Materials and methods

Batch experiments were conducted to investigate the adsorption of pectin and copper(II) by amorphous Fe/Al hydroxides. The morphology, mineralogy, and functional groups of pectin–Fe/Al hydroxides were examined using X-ray diffraction, scanning electron microscopy, and Fourier transform infrared spectroscopy analysis.

Results and discussion

The amount of pectin adsorbed by amorphous Al(OH)₃ was much greater than that by amorphous Fe(OH)₃ at pH values between 3.5 and 6.5 due to the higher positive charge density on Al(OH)₃ and greater electrostatic attraction between the hydroxide and pectin compared with Fe(OH)₃. The addition of pectin decreased the positive surface charge on amorphous Fe and Al hydroxides. The presence of pectin enhanced the adsorption of Cu(II) by the Fe and Al hydroxides. The increase in Cu(II) adsorption on amorphous Fe hydroxide was more obvious at low pH values than at higher pH values, while an opposite changing trend was observed for amorphous Al hydroxide. At pH 3.9, 4.3, and 4.9, pectin increased Cu(II) adsorption by Fe hydroxide from 24.4, 76.6, and 177.0 mmol/kg to 61.6, 98.8, and 192.0 mmol/kg, i.e., Cu(II) adsorption was increased by 37.2, 22.2, and 15.0 mmol/kg, respectively. At pH 4.3 and 4.9, pectin increased Cu(II) adsorption by Al hydroxide from 3.7 and 27.0 mmol/kg to 17.3 and 69.4 mmol/kg, i.e., Cu(II) adsorption was increased by 13.6 and 42.4 mmol/kg, respectively. The greater adsorption of pectin by Al hydroxide was mainly responsible for the larger enhancement of pectin on Cu(II) adsorption on Al hydroxide at higher pH values compared with Fe hydroxide.

Conclusions

The adsorption of pectin on Fe and Al hydroxides decreased the positive charge on the hydroxides and thus enhanced the adsorption of Cu(II) by the hydroxides.

     

阴离子对可变电荷土壤吸附铜离子的影响机理

根据NO-3、Cl-和SO24-对可变电荷土壤和恒电荷土壤吸附Cu2+的影响的比较,探讨了阴离子对可变电荷土壤吸附Cu2+的影响机理。结果表明,当3种阴离子的浓度相同时,在SO24-体系中铁质砖红壤对Cu2+的吸附率较在NO3-和Cl-体系中大得多,而在浓度相同的3种阴离子体系中,黄棕壤对Cu2+的吸附率相差不大。在离子强度相近的NaCl体系中,砖红壤对Cu2+的吸附率相近。在3种阴离子体系中,随着pH升高,砖红壤对Cu2+的吸附率均增大;但在NO-3体系和Cl-体系中Cu2+的吸附率相近;而在SO24-体系中Cu2+的吸附率最大。随着Na2SO4浓度的增大,铁质砖红壤和砖红壤对Cu2+的吸附率减小。但在0.005 mol L-1和0.05 mol L-1Na2SO4体系中,Cu2+的吸附率大于在不含Na2SO4的体系中者。而在0.5 mol L-1Na2SO4体系中,Cu2+吸附率小于在不含Na2SO4体系中者。在3种浓度的Na2SO4体系中,黄棕壤对Cu2+的吸附率均小于在不含Na2SO4体系中者。总之,阴离子可通过离子强度、专性吸附和形成离子对影响土壤对Cu2+的吸附。在可变电荷土壤中,阴离子对Cu2+吸附的影响机理较在恒电荷土壤中复杂得多。     

硫酸盐对锌和镉在可变电荷土壤上吸附的影响

SO4^2- 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 whn the soil reacted with the sulfate prior to the metal.the shift in pH50 for both Zn and Cd adsorption was aslo 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 formaiton of the ternary surface complex-S-SO4-M might also play a role in the effect.     

铁氧化物与土壤表面电荷性质的关系

The relationship between iron oxides and surface charge characteristics in variable charge soils (latosol and red earth) was studied in following three ways.(1)Remove free iron oxides (Fed) and amorphous iron oxides (Feo) from the soils with sodium dithionite and acid ammonium oxalate solution respectively.(2) Add 2% glucose (on the basis of air-dry soil weight) to soils and incubate under submerged condition to activate iron oxides,and then the mixtures are dehydrated and air-dried to age iron oxides.(3) Precipitate various crystalline forms of iron oxides onto kaolinite.The results showed that free iron oxides (Fed) were the chief carrier of variable positive charges.Of which crystalline iron oxides (Fed-Feo) presented mainly as discrete particles in the soils and could only play a role of the carrier of positive charges,and did little influence on negative charges.Whereas the amorphous iron oxides (Feo),which presented mainly fas a coating with a large specific surface area,not only had positive charges,but also blocked the negative charge sites in soils.Submerged incubation activated iron oxides in the soils,and increased the amount of amorphous iron oxides and the degree of activation of iron oxide,which resulted in the increase of positive and negative charges of soils.Dehydration and air-dry aged iron oxides in soils and decreased the amount of amorphous iron oxides and the degree of activation of iron oxide,and also led to the decrease of positive and negative charges.Both the submerged incubation and the dehydration and air-dry had no significant influence on net charges.Precipitation of iron oxides onto kaolinite markedly increased positive charges and decreased negative charges.Amorphous iron oxide having a larger surface area contributed more positive charge sites and blocked more negative charge sites in kaolinite than crystalline goethite.     

离子专性吸附对可变电荷土壤的动电学性质的影响

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.     

支持电解质浓度对磷酸根在可变电荷土壤表面吸附和解吸的影响

研究了离子强度对2种可变电荷土壤中磷酸根吸附和解吸的影响。结果表明,当pH分别大于3.7和4.0时,红壤和砖红壤对磷酸根的吸附量随离子强度的增加而增加;当pH分别小于3.7和4.0时,红壤和砖红壤对磷酸根的吸附量随离子强度呈相反的变化趋势。电解质主要通过改变离子专性吸附面上的电位来影响磷酸根的吸附。Zeta电位的测定结果表明,当pH大于土壤胶体的等电点(IEP)时,吸附面上电位为负值,且随离子强度增加数值减小,对磷酸根的排斥力减小,土壤表面对磷酸根的吸附量增加;当pH小于IEP时,吸附面上的电位为正值,它随离子强度增加而减小,不利于磷酸根的吸附。解吸实验的结果表明,吸附于可变电荷土壤表面的磷酸根在去离子水中的解吸量高于0.1 mol L-1NaNO3体系中的解吸量。这同样由于电解质浓度对土壤表面吸附面上的电位的影响所致。     

Influence of organic matter on phosphate adsorption by aluminium and iron oxides in sandy soils

The phosphate adsorption capacity (P_max) of samples from various horizons of five Danish podzolized soils were investigated before and after organic matter removal. Removal of organic matter had no direct influence on P_max suggesting that organic matter did not compete with phosphate for adsorption sites. In the soils investigated aluminium and iron oxides were the main phosphate adsorbents. Thus, more than 96% of the variation in P_max could be accounted for by poorly crystalline aluminium and iron oxides (extractable by oxalate) and by well-crystallized iron oxides (taken as the difference between dithionite-citrate-bicarbonate-extractable iron and oxalate-extractable iron). Organic matter affected phosphate adsorption indirectly by inhibiting aluminium oxide crystallization. The resulting poorly crystalline oxides had high P_max. In contrast, the influence of organic matter on the crystallinity of the iron oxides, and therefore on their capacity to adsorb phosphate, seemed limited.     

The specific adsorption of organic and inorganic phosphates by variable‐charge oxides

We measured the reaction of inorganic phosphate and of four organic phosphates with three aluminium hydroxides or oxides. We compared the fit of the Langmuir equation with that of an equation designed to allow for the feedback effect of adsorption on the electric potential. We also fitted a four‐plane model to describe the effects of pH on adsorption and ζ potential. For inorganic phosphate, the Langmuir equation described adsorption curves poorly, indicating that there was a large feedback effect of adsorption on the electric potential of the adsorption plane. For the organic phosphates, the deviations from the Langmuir equation were not as marked, indicating that there was little feedback effect. Nevertheless, there was a large effect on the ζ potential of the phosphated aluminium (hydr)oxides. We suggest that when a large organic phosphate molecule is adsorbed, the charge conveyed to the surface is repelled electrostatically to the outside of the new surface. There is therefore a large effect on the ζ potential and a small effect on the potential of the adsorption plane. This suggestion was supported by the fit of the four‐plane model, for which, with increasing molecular weight, the mean position of the charge conveyed to the surface by adsorption was moved further from the plane carrying the potential determining ions (H⁺ and OH^?).     

Effect of organic matter on the charge and phosphate adsorption characteristics of kikuyu red clay from kenya

Two profiles from Muguga, Kenya, one cultivated and one under forest, contained similar contents of clay of apparently uniform composition. The profiles differed in their organic matter contents, surface (0–15 cm) samples containing 6.8% C and 3.8% C in forest and cultivated profiles, respectively. In both profiles the amounts of organic matter decreased with depth.The positive charge, measured at pH 4 in 0.2M NH₄ Cl, increased down the cultivated profile to 3.6 me/100 g at 90–120 cm. The charge was lower in the forest profile at equivalent depths, and at 90–120 cm the value was 1.2 me/100 g. Phosphate adsorption measured at pH 5 was also higher in the cultivated profile. The amount of phosphate required to raise the solution concentration to 0.2 p.p.m., ΔP, was 45 and 11 mg/kg in the surface horizon (0–15 cm) of the cultivated and forest profiles, respectively, and increased in both profiles with depth of sample.The differences between, and within, the two profiles are largely attributed to the blocking of positively charged sites and phosphate adsorption sites by organic matter.     

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 been implicated in many soil processes.The objective of the present paper was to evaluate effect of two LMW organic acids, citric acid and oxalic acid, on Cl^- adsorption by three variable charge soils, a latosol, a lateritic red soil and a red soil, using a batch method. The results showed that the presence of citric acid and oxalic acid led to a decrease in Cl- adsorption with larger decreases for citric acid. Among the different soils Cl- adsorption in the lateritic red soil and the red soil was more affected by both the LMW organic acids than that in the latosol.     

Sulfate adsorption by variable charge soils: Effect of low-molecular-weight organic acids

Sulfate (SO₄ ^2–) movement and transport in soils has received considerable attention in recent years. In most soils, SO₄ ^2– coexists with a variety of natural organic compounds, especially organic acids. Studies were conducted to assess the effect of low-molecular-weight organic acids (eight aliphatic and five aromatic acids) on SO₄ ^2– adsorption by variable charge soils from Chile and Costa Rica. The effects of type of organic acid, pH, type of soil, and organic acid concentration were investigated. In one experiment, a 1.0 g soil sample was equilibrated with 25 ml 0, 0.5, 1.0, 2.0, 4.0, or 6.0 mM K₂SO₄ in 1 mM NaCl in the presence or absence of 5 mM citric acid. In the second set of experiments, the adsorption of 2 mM SO₄ ^2– in soils at pH 4 or pH 5 in the presence or absence of one of 13 organic acids at a concentration of 2 mM or 5 mM was studied. Results showed that citric acid significantly decreased SO₄ ^2– adsorption by the two soils. Sulfate adsorption decreased with increasing pH of the equilibrium solution. Aliphatic acids, with the exception of cis-aconitic acid, decreased the amount of SO₄ ^2– adsorbed by the two soils, with oxalic, tartaric, and citric acid showing the greatest effect. The differences in pH values of the equilibrium solutions in the presence and absence of organic acids were significantly, but negatively, correlated with the amount of SO₄ ^2– adsorbed, suggesting chemisorption of SO₄ ^2– and the release of hydroxide ions. The ionization fraction values of the organic acids at the equilibrium pH were correlated with the amounts of SO₄ ^2– adsorbed, suggesting that the protonation of surface hydroxyl groups of the mineral phase increased as the strength of the ionization of the acid increased, thus creating more positively charged surfaces. Received: 12 February 1997     

Effects of phthalic and salicylic acids on Cu(II) adsorption by variable charge soils

In the present study, the effect of two substituted benzoic acids on Cu(II) adsorption onto two variable charge soils was investigated, with the emphasis on the adsorption and desorption equilibrium of Cu(II). Results showed that the presence of organic acids induced an increase in Cu(II) adsorption onto the two soils. The extent of the effect was related to the initial concentrations of Cu(II) and organic acid, the system pH, and the nature of the soils. The effect of organic acids was greater for Oxisol than for Ultisol. Phthalic acid affected Cu(II) adsorption to a greater extent than salicylic acid did. The effect of organic acids varied with pH. The adsorption of Cu(II) induced by organic acids increased with increasing pH and reached a maximum value at approximately pH 4.5, and then decreased. It can be assumed that the main reason for the enhanced adsorption of Cu(II) is an increase in the negative surface charge caused by the specific adsorption of organic anions on soils because the desorption of Cu(II) adsorbed in organic acid systems was greater than that for the control. The desorption of Cu(II) absorbed in both control and organic acid systems also increased with increasing pH; it reached a maximum value at pH ∼5.25 for control and salicylic acid systems and at pH ∼5.1 for a phthalic acid system, then decreased. This interesting phenomenon was caused by the characteristics of the surface charge of variable charge soils.