Effect of phosphate status and pH on sulphate sorption and desorption

For soils from tea estates in northern India, sulphate sorption was of a similar magnitude to, and sometimes exceeded, phosphate sorption. Only a small part of this relatively large sulphate sorption was caused by the low pH of these soils. Most was caused by increased negative charge as a result of prior reaction over many decades with phosphate fertilizers. This decreased sorption of both phosphate and sulphate, but the effect on phosphate was larger. This is compatible with a model in which the mean location of the charge on the adsorbed phosphate ions is closer to the surface than for sulphate. On soils of low phosphate status, sulphate desorption curves showed hysteresis; on soils of high phosphate status, they did not. Further, on soils of high phosphate status, displacement of sulphate by phosphate solutions was faster. We interpret these observations as showing that, for low phosphate status soils, sulphate ions penetrated the surface, but for high phosphate status soils it did not because the pathways by which sulphate diffuses into the adsorbing material were blocked. We also show that, with increasing soil phosphate status, phosphate solutions were less effective in displacing sorbed sulphate. We think this also occurred because reaction with phosphate had decreased the affinity for phosphate more than it decreased the affinity for sulphate.     

Chemical factors affecting selenite sorption by allophanic soils

The sorption of selenite by two allophanic soils containing high amounts of variable charge materials was studied. Selenite sorption exhibited a maximum near pH 4 and decreased, although not proportionally, with increasing pH. Only negligible amounts of selenite were sorbed above pH 7.In the two soils, the addition of selenite caused a release of sulphate (SO²⁻₄), silicate (Si) and hydroxyl ion (OH⁻) and an increase in cation (Na⁺) adsorption. No measurable amount of phosphate (P) was released. Increase in negative charge as measured by Na⁺ adsorption accounted for 48 and 18% of selenite sorbed (soils 1 and 2, respectively), the rest being accounted for by release of anions. The results presented here are consistent with the widely held view that selenite and phosphate are sorbed onto variable charge surfaces by a similar mechanism (ligand exchange).     

Sulphate sorption by variable charge soils

The sorption of sulphate (SO^2?₄) by three variable charge soils from the Canary Islands (Spain) was studied. Sulphate sorption decreased with increasing pH. Only negligible amounts of SO^2?₄ were sorbed above pH 6.5. When the soils were washed with an indifferent electrolyte (0.01 M KCl), more SO^2?₄ was recovered than had been sorbed. This indicated a release of native SO^2?₄ Sulphate replaced hydroxyl ions (OH) and co-ordinated H₂O molecules, as well as very small amounts of silicate (Si). No measurable amount of phosphate (P) was released. On average hydroxyl release accounted for 50% of SO^2?₄ sorbed, the rest being accounted for by the increase in negative charge as measured by K⁺ adsorption. The results presented here are consistent with the sorption of SO^2?₄ through a ligand exchange mechanism, but in a different plane of sorption to that of phosphate.     

Sorption of organic acids in acid soils and its implications in the rhizosphere

Organic acids have been implicated in many soil-forming and rhizosphere processes, but their fate in soil is poorly understood. We examined the sorption of four simple short-chain organic acids (citric, oxalic, malic and acetic) in five acid soils and on synthetic iron hydroxide (ferrihydrite). The results for both soils and ferrihydrite indicated that the sorption depended on concentration in the following order of strength: phosphate >> oxalate > citrate > malate >> acetate. The sorption reactions in soil were shown to be little influenced by pH, whereas for ferrihydrite, sorption of all ligands increased strongly with decreasing pH. The sorption of organic anions onto ferrihydrite was influenced to a lesser extent by the presence of metal cations in solution. From the results we calculated that when organic acids enter solution they rapidly become sorbed onto the soil's exchange complex (> 80% within 10 min), and we believe that this sorption will greatly diminish their effectiveness to mobilize nutrients from the rhizosphere.     

Phosphate sorption in allophanic soils and release of sulphate, silicate and hydroxyl

Phosphate (P) sorption and the concomitant release of sulphate (SO,), silicate (Si) and hydroxyl ion (OH) were determined on three allophanic soils from Spain, at different P concentrations. P effectively replaced SO₄, Si and OH. However, at every stage of P sorption, the molar ratios of the total amounts of anions released (SO₄+ Si + OH) to that of the sorbed P were low. The amount of added P affected the relative proportions of SO₄, Si and OH exchanged. At low concentrations of P, phosphate sorption was accompanied mostly by release of the adsorbed SO₄ with some Si. As more P was sorbed an increasing displacement of OH was also observed.     

应用XAFS研究草酸根和胡敏酸对As(V)在红壤中吸附的影响

结合吸附实验和X光吸收精细结构光谱(XAFS)分析,研究了草酸根和胡敏酸对As(V)在红壤中吸附的影响,分析了As(V)在红壤中的化学形态和微观结构以及草酸根、胡敏酸的影响特征。结果表明,当pH6.0时,红壤主要是通过基团交换反应吸附As(V),草酸根和胡敏酸可以通过竞争吸附位点抑制红壤中As(V)的吸附,其抑制作用随浓度增大而增强。XAFS光谱学数据表明,红壤中吸附的砷以+5价态存在,主要与铁铝矿物形成以约0.317 nm As-Al和0.328 nm As-Fe原子间距为特征的双齿双核结构的内层复合物,复合物结构类型不受砷浓度和草酸根、胡敏酸的影响。     

Effects of Oxalate and Humic Acid on Arsenate Sorption by and Desorption from a Chinese Red Soil

Studies on arsenate (As(V)) sorption and desorption have been mainly limited to soil minerals and sorption and desorption reactions in whole soils are poorly understood. In this study the sorption of As(V) by and phosphate-induced desorption from a Chinese red soil were studied in the presence of oxalate and humic acid (HA). Arsenate was strongly sorbed mainly through ligand exchange reactions on the soil. Arsenate sorption decreased in the presence of oxalate or HA. Oxalate and HA influenced As(V) sorption mainly by competing for sorption sites and reducing sorption sites, and oxalate could also decrease sorption through dissolving clay minerals. Oxalate and HA could also facilitate As(V) desorption from the soil. Both sorption and desorption kinetics were two stage processes. Sorption kinetics conducted from 0.2–840 h showed that As(V) sorption increased with increasing residence time. Sorption equilibrium was retarded and the maximum sorption decreased in the presence of oxalate or HA. Phosphate-induced desorption kinetics conducted on the soil with 24 h and 840 h of sorption equilibrium time showed a significant effect of equilibrium time on As(V) desorption. The presence of oxalate or HA during the sorption process resulted in more As(V) desorption. Due to the degradation of oxalate, soil treated with oxalate and with a sorption equilibrium time of 840 h showed no significant difference in desorption kinetics from untreated soil.     

Sorption of chlorpyrifos to selected minerals and the effect of humic acid

Sorption of chlorpyrifos (CPF) from 2.85 microM (1 mg/L) aqueous solutions in 0.01 M NaCl to montmorillonite, kaolinite, and gibbsite was investigated at 25 degrees C. Uptake of CPF by kaolinite and gibbsite was generally <10%, with pH having at most a small effect. Sorption to montmorillonite was significantly greater, with approximately 50% of the initial CPF being removed from solution below pH 5. Above pH 5 the sorption decreased to about 30%. About 70% of CPF was sorbed to kaolinite and gibbsite after 30 min, whereas on montmorillonite only 50% sorbed in an initial rapid uptake (approximately 30 min) followed by slower sorption, with a maximum achieved by 24 h. Although CPF desorbed completely from kaolinite in methanol, only about two-thirds was desorbed from montmorillonite. CPF has only a weak affinity for the surfaces of kaolinite and gibbsite. In the case of montmorillonite, sorption is significantly stronger and may involve a combination of sorption to external surfaces and diffusion into microporous regions. At pH >6 increased negative surface charge results in a lower affinity of CPF for the external surface. In the presence of 50 mg/L humic acid (HA) the amount of CPF sorbed on gibbsite and kaolinite was 3-4 times greater than that in the binary systems. The HA forms an organic coating on the mineral surface, providing a more hydrophobic environment, leading to enhanced CPF uptake. The HA coating on montmorillonite may reduce access of CPF to microporous regions, with CPF tending to accumulate within the HA coating.     

Retention of dissolved organic matter by illitic soils and clay fractions: Influence of mineral phase properties

The dependency of the retention of dissolved organic carbon (DOC) on mineral phase properties in soils remains uncertain especially at neutral pH. To specifically elucidate the role of mineral surfaces and pedogenic oxides for DOC retention at pH 7, we sorbed DOC to bulk soil (illitic surface soils of a toposequence) and corresponding clay fraction (< 2 μm) samples after the removal of organic matter and after removal of organic matter and pedogenic oxides. The DOC retention was related to the content of dithionite‐extractable iron, specific surface area (SSA, BET‐N₂ method) and cation exchange capacity (pH 7). The reversibility of DOC sorption was determined by a desorption experiment. All samples sorbed 20–40 % of the DOC added. The DOC sorption of the clay fractions explained the total sorption of the bulk soils. None of the mineral phase properties investigated was able to solely explain the DOC retention. A sorption of 9 to 24 μg DOC m^–2 indicated that DOC interacted only with a fraction of the mineral surface, since loadings above 500 μg m^–2 would be expected for a carbon monolayer. Under the experimental conditions used, the surface of the silicate clay minerals seemed to be more important for the DOC sorption than the surface of the iron oxides. The desorption experiment removed 11 to 31 % of the DOC sorbed. Most of the DOC was strongly sorbed.     

DESORPTION AND ISOTOPIC EXCHANGE RELATIONSHIPS OF PHOSPHATE SORBED BY SOILS AND HYDROUS FERRIC OXIDE GEL

Sorption of added inorganic phosphate (P) was irreversible in four contrasting soils and hydrous ferric oxide gel during 16 h desorption after 40 h sorption at the same (iso)pH. Irreversibility increased with increasing time of desorption above 30 h. When the amount of P which was chemisorbed during the sorption step was subtracted from the amount desorbed, the latter fell on the isotherm describing the more-physical, potential-determining sorption. No pH change occurred during desorption and net negative charge decreased by 1 equivalent per mole P desorbed. These results suggested that only more-physically sorbed P was desorbed at the iso-pH. The more-physically sorbed P was also reversible with respect to changes in the ionic strength and cation species of the desorbing solution. The isotopic exchangeability of the more-physically sorbed P was at least ten times greater than that of chemisorbed P. With increasing sorption time, both the ease of desorption and exchangeability of sorbed P decreased. Subsequent to desorption, the exchangeability of the remaining sorbed P also decreased. These observations are interpreted in terms of the concurrent changes in the amounts of chemisorbed and more-physically sorbed P.     

有机阴离子对磷酸根吸附的影响

在中性条件下,低浓度的柠檬酸、草酸、酒石酸和胡敏酸阴离子都能显著降低针铁矿、非晶氧化铝、高岭石和红壤对磷酸根的吸附,尤其在低磷吸附饱和度下效果更好。有机阴离子抑制磷酸根吸附的能力因有机酸的种类和性质、以及固相的表面特性而异。有机阴离子存在下吸附的磷酸根具有较高的同位素³²P交换活性和解吸率。测定了吸附平衡溶液中铁和铝的浓度。结果表明,在实验条件下(pH 7.0),即使较高浓度的有机酸根(10^-2mol)也只能溶解极少量的铁和铝。有机阴离子络溶作用不足以说明固相吸附磷能力的显著下降。可见,有机阴离子降低磷酸根吸附的机制主要是竞争专性吸附。有机阴离子占据了一部分高亲和力的吸附位,从而降低了土壤固相吸附磷的量,增加了吸附态磷的活性。     

Adsorption of phthalic acid and salicylic acid by two variable charge soils as influenced by sulphate and phosphate

Low‐molecular‐weight (LMW) organic acids exist widely in soils, especially in the rhizosphere, and the adsorption of these acids may affect their reactions in soils. The adsorption behaviour of phthalic acid and salicylic acid by two variable charge soils (a Rhodic Ferralsol and a Haplic Acrisol) was investigated. Both soils exhibited great adsorption capacity for these organic acids, with a greater affinity for phthalic acid. The Rhodic Ferralsol adsorbed more organic acids of both kinds than the Haplic Acrisol, which was consistent with the content of iron and aluminum oxides in the two soils. The iron oxides in these soils played a significant role in adsorption of the organic acids, whilst the soil aluminosilicate minerals, such as kaolinite, showed a small adsorption capacity. The presence of phosphate and sulphate caused a decrease in the adsorption of both organic acids because of their competition with them for sorption sites. The phosphate showed a bigger inhibition on the adsorption than sulphate as a result of a greater amount of phosphate adsorbed by the soils. The adsorption of both organic acids was affected by pH only slightly at pH < 4.5. However, the adsorption decreased with the increase in pH at pH > 4.5. A similar trend was observed for the phosphate system, but the opposite was seen for the sulphate system. This suggests that the inhibition of sulphate on the adsorption of the organic acids decreased with the increase in pH, because the adsorption of sulphate decreased strongly with increasing pH.     

The effects of phosphate on zinc sorption by a soil

Zinc sorption curves were obtained after treatment of a soil with several rates of phosphate and with two rates of lime. The lime permitted evaluation of the effects of phosphate on Zn sorption via its effects on pH. The phosphate was either incubated with the soil at a high temperature before reaction with Zn or was supplied at the same time as the Zn. This produced treatments with similar concentration of phosphate in solution but different amounts of sorbed phosphate.
Two distinct effects of phosphate addition on Zn sorption were detected. One arose from effects of phosphate on pH. This effect could be large and could either increase or decrease Zn sorption depending on the direction of the pH effect. A second effect was correlated with the amount of sorbed phosphate and was assumed to operate through the effects of phosphate on charge. The effects were small at low levels of Zn but larger at higher levels. This suggested that Zn and phosphate were sorbed at opposite ends of a spectrum of electrostatic potentials and overlap only occurred when the level of application was high. A third possible effect, due to reaction of the soil with zinc phosphate complexes in solution, was not proved.     

氧化还原条件对土壤磷素固定与释放的影响

为探讨水稻土由氧化环境转为还原条件时对土壤磷素固定与释放的影响,选择18种水稻土样品进行室内模拟实验,通过测定不同条件下磷的等温吸附曲线和采用氧化铁试纸测定法进行多次提取以对比淹水前后土壤磷的累计解吸量,发现大部分供试样品的固磷能力在淹水条件下有了显著的提高,全部样品在淹水后磷的释放量都有不同程度的减少。进一步的研究表明淹水条件下土壤对磷的固定与释放的变化主要与淹水后土壤Eh的降低和pH的升高而导致的大量无定形铁的形成有关。     

Testing a mechanistic model. VII. The effects of pH and of electrolyte on the reaction of selenite and selenate with a soil

The pH of samples of a soil was altered by mixing them either with acid or lime, and incubating the moistened samples at 60°C for a day. The sorption of selenate and of selenite was then measured using as background electrolytes, 0.01 M, 0.1 M and 1.0 M sodium chloride and also 0.01 M calcium chloride. The results were compared with previous studies with phosphate and fluoride. Selenite was sorbed more strongly than selenate, but not as strongly as phosphate or fluoride. Sorption of both selenite and selenate decreased with increasing pH. This decrease was more marked for selenate than for selenite; more marked in a sodium system than in a calcium one; and more marked with a dilute background electrolyte than a concentrated one. Under certain conditions, the steeper curves for the dilute electrolyte crossed the curves for the concentrated electrolyte giving points of zero salt effect. For selenite, these points of zero salt effect occurred near pH 6 and the greater the sorption the lower the pH for zero salt effect. For phosphate, the analogous value was near pH 5. For selenate, if a point of zero salt effect occurred, it was at such a high pH and such a low amount of sorption that it could not be measured. Thus, the larger the amount of sorption the lower the pH for the point of zero salt effect. This generalization applied both within and between different kinds of sorbates. The results were closely described by a model that had previously been applied to phosphate and fluoride. The model postulates that ions react with charged surfaces. The electric potentials of the reacting surfaces are affected by the identity and concentration of the background electrolyte and this produces the interactions between pH and electrolyte concentration. The model also postulates that there is a distribution of electric potentials. Anions react with surfaces which occur in the more positive tail of this distribution. The smaller the amount of the reaction the more positive the potential of the reacted surface and, therefore, the higher the pH required to decrease this potential to zero.     

Copper retention by Danish Spodosols in relation to contents of organic matter and aluminum,iron, and manganese oxides

Abstract

The importance of various soil components on copper (Cu) retention by Spodosois was investigated. Copper sorption and extraction were conducted on samples from the B horizon from six Danish Spodosois. The investigation was conducted on untreated samples, on hydrogen peroxide‐treated samples (to remove organic matter), on oxalate‐treated samples [to remove amorphous to poorly crystalline aluminum (Al) and iron (Fe) oxides], on hydroxylamine‐treated samples [to remove manganese (Mn) oxides]. Subfractions treated with hydrogen peroxide (H₂O₂) were further treated with oxalate and citrate‐bicarbonate‐dithionite (CBD). Sorption of Cu from an initial 10^‐6 M solution after 48 hours was determined in the pH range 3 to 7 using 0.1M sodium nitrate (NaNO₃) as the background electrolyte. The pH‐dependent sorption curve (sorption edge) was shifted to a higher pH with decreasing Al oxide content in the soils, and for the treated sample after removal of organic matter and Al and Fe oxides. A negligible effect was seen after removal of the Mn oxides because of their low abundance. Extraction of sorbed Cu at pH 4 to 6 with 0.1M nitric acid (HNO₃) for 24 hours confirmed the sorption results, in inasmuch as removal of the Al (and Fe) oxides increased Cu extractability. Therefore, it was concluded that in the soils investigated, Cu retention is mainly determined by the oxalate‐extractable Al fraction with a minor contribution due to crystalline Fe oxides.     

Die Sorption von Phosphat an carbonatreichen Unterwasserböden

The sorption of phosphate by underwater soils rich in carbonate The phosphate sorption isotherms for carbonate rich under water soils (Unterwasserboden) can frequently be linearized by a modified Freundlich-isotherm when one assumes that, because of previously sorbed phosphate, the concentration of the equilibrium soil solution, P_1,0 is greater than 0. However, in many cases, the character of the phosphate sorption can be adequately determined with only one phosphate addition (P_s,500). Both methods show that for dried samples from under water soils, the samples from reduced horizons have a higher P sorption than for the associated oxidized horizons. This can be explained by the presence of very sorption active ferrihydrite which has precipitated from previously biologically reduced material.     

Effect of soil properties on the degradation of isoxaflutole and the sorption-desorption of isoxaflutole and its diketonitrile degradate

The transformation of isoxaflutole (ISOX) to its herbicidally active diketonitrile degradate (DKN) was significantly enhanced in the presence of soil and occurred more rapidly in systems containing soil with a greater soil pH. Sorption-desorption of ISOX and DKN in five soils collected within a field revealed both ISOX and DKN were more readily sorbed to soils with greater organic matter, clay content, and lower soil pH. Sorption of ISOX residues occurred within 2 h, and extracts contained similar concentrations of ISOX and DKN at 24 h, suggesting the 24-h sorption coefficients for ISOX-treated systems were actually for mixed ISOX residues. Freundlich sorption coefficients were 3 and 4 times greater for ISOX than for DKN. On the basis of the Freundlich organic carbon sorption constants, ISOX and DKN can be categorized in the very high and high mobility classes, suggesting their potential to leach in the soils needs to be evaluated.     

Inorganic and organic ligands induced changes in Pb sorption of arid ‎clay and calcareous loamy sand soils

The objective of this study was to investigate sorption, desorption, and immobilization of Pb in the clay and calcareous loamy sand soils treated with inorganic ligands (NO₃^?, Cl^? and H₂PO₄^?). Pb sorption was also determined in the presence of oxalate and citrate. The maximum Pb sorption capacities (q) ranged from 42.2 to 47.1 mmol kg^?1 for the clay soil, and from 45.2 to 47.0 mmol kg^?1 for loamy sand soil. It was observed that the binding energy constant (k) for Pb sorbed onto loamy sand soil (528–1061) is higher than that for clay soil (24.38–55.29). The loamy sand soil-sorbed greater quantities of Pb compared to the clay soil when initial pH was ≥ 3. However, it had lower sorption capacity at the lowest initial pH of 2. Additionally, the greatest Pb sorption and immobilization occurred in the soil treated with H₂PO₄. In the clay soil, the sorption of Pb was depressed at 0.1 mol kg^?1 of Cl^?, as compared with other ligands. Concerning organic acids, citrate ligand showed the highest decrease in Pb sorption. It could be concluded that the nature of Pb sorption can depend on the type and quantity of ligands present, as well as the soil type.     

Changes in distribution of inorganic soil phosphorus forms with phosphate desorption by iron oxide‐impregnated paper strips

Abstract

The release of soil phosphorus (P) to solution has been described by extraction of soil with iron (Fe)‐oxide coated paper strips. Little information is available, however, on where this P is coming from. The effect of removal of reversibly adsorbed soil P on the distribution of inorganic P forms was investigated for 12 Italian soils. Phosphate was removed from these soils by Fe‐oxide strips after incubation with P (0 and 100 mg P kg^‐1) for 90 days. With no applied P, 3 to 17% of the total soil active P [saloid‐P, aluminum‐phosphate (Al‐P), iron‐phosphate (Fe‐P), and calcium‐phosphate (Ca‐P) was removed by the Fe‐oxide strips. The change in strip‐P following P addition (100 mg kg^‐1 soil), ranged from 12.9 to 53.5 mg P kg^‐1, with P coming almost entirely from the active P fractions. A close relationship between the changes in desorbed strip‐P after P equilibration and soil P sorption index (SI) was found for the studied soils (r²=0.96). Thus, the release of soil P for plant uptake or transport in runoff was a function of the amount of “actively”; sorbed P and an estimate of P sorption.