Modifications to the Freundlich equation to describe anion sorption over a large range and to describe competition between pairs of ions

We measured sorption of selenite and phosphate, both separately and in competition, in a Chilean Andisol. We also used previously published data for competitive sorption of arsenate and phosphate by a clay subsoil. We wrote computer programs that allowed us to compare the fits of differing versions of equations to describe individual sorption and competitive sorption. For the selenite–phosphate data, the index term of the Freundlich equation decreased as concentration increased. This was described using the Sibbesen modification of the Freundlich equation. This modification was then included in competition equations. For both the selenite–phosphate and the arsenate–phosphate data, competition was not ‘symmetrical’, that is, the competition terms were not reciprocals of each other. We think this occurred because competition between ions is not only competition for adsorption sites but also involves electrical effects that follow penetration of the surface.     

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.     

Modelling the effects of pH on phosphate sorption by soils

Samples of six soils were incubated at 60°C for 24 h with several levels of either calcium carbonate or hydrochloric acid. Phosphate sorption was then measured on sub-samples of the treated soils over 24 h at 25°C. In one set of measurements on all soils, 0.01 M calcium chloride was used as the background electrolyte. In another set, on two soils, 0.01 M sodium chloride was used. An interpolation method was used to give points on the three-dimensional surface relating the final pH of the suspensions to sorption of phosphate at specified solution concentrations of phosphate. The effects of pH on phosphate sorption differed between soils. For unfertilized soils, increases in pH up to about pH 5.5 decreased sorption. Further increases in pH decreased sorption further in one soil and increased it in three others. For fertilized soils, measured sorption increased with pH. When sodium chloride was used instead of calcium chloride, there was a more marked trend for sorption to decrease as pH increased. Differences between the soils were ascribed to differences in two soil properties. One was the rate at which the electrostatic potential in the plane of adsorption decreased as pH increased. Only small differences in the rate of change of potential were needed to reproduce the observed differences between soils. The electrostatic potential would decrease more quickly in solutions of a sodium salt than in solutions of a calcium salt and this explains the observed differences between these media. The other soil property that affected observed sorption was the release of phosphate from the soil. The amount released was largest at low pH. Consequently, for fertilized soils, measured sorption increased with pH.     

Testing a mechanistic model. VIII. The effects of time and temperature of incubation on the sorption and subsequent desorption of selenite and selenate by a soil

Several levels of both selenite and selenate were incubated with separate samples of soil for periods of up to 30 d and at temperatures between 5 and 60°C. The concentrations of selenite or of selenate which caused neither desorption nor further sorption–that is, the null-point concentrations–were then measured at 25°C. In addition, the rate of desorption was measured after incubation at 60°C for 10 d. The ability of a mechanistic model to describe the results was tested.
There were large decreases in the null-point concentrations of selenite with both increasing period and increasing temperature of incubation. These effects were modelled as due to a relatively rapid diffusive penetration and a large activation energy for diffusion. Only a small proportion of the sorbed selenite was desorbed within 3 d but, at large solution:soil ratios, desorption appeared to be still continuing. These effects were fairly well predicted by the model applied to the sorption data. For example, the continuing desorption was ascribed to the slow reversal of the diffusive penetration. For selenate, the effects of period and temperature of incubation were much smaller. These effects were modelled as due to a slower diffusive penetration and a lower activation energy. Desorption was quicker and was more nearly complete. According to the model, a large proportion remained in the adsorbed form and was more quickly removed when desorption was induced. It is argued that the behaviour of selenite is consistent with diffusion into a crystal and the large activation energy is required to enable jumps over energy barriers. On the other hand, the lower activation energy for selenate is consistent with diffusion being limited to pores or cracks.
It is suggested that the residual value of selenite fertilizers would decrease because of the continuing reaction, but that this effect would be unimportant for selenate.     

Selenite adsorption by a variety of oxides

Abstract

Selenite adsorption by a variety of oxides consisting of iron (Fe), aluminum (Al), titanium (Ti), manganes (Mn), and silicon (Si), and by two humic acids were investigated in order to grasp selenite behavior and fixation mechanisms in soil. It was found that selenite was apparently adsorbed even by the Mn oxides on which surface negative charge was dominant in normal pH range (pH <4). No selenite adsorption was observed in the silicon dioxide (SiO₂) and the two humic acids. A sequential extraction of adsorbed selenite with competitive anions showed the differences of binding force or stability of adsorbed selenite among the minerals. While the goethite fixed selenite strongly, selenite adsorbed on the Mn oxide was easily released to the liquid phase with other anions, such as phosphate. Each mineral had its inherent characteristic in ligand exchange reactions accompanied with selenite sorption. Selenite sorption by the Mn and the Ti oxides resulted in large increase of surface negative charge, while only a little increase in the Fe and Al oxides. Proton consumption with selenite sorption was extremely smaller for the Mn oxide than for the others.     

Testing a mechanistic model. XI. The effects of time and of level of application on isotopically exchangeable phosphate

A model for the reaction of ions with soil was improved to permit time trends to be followed at a given level of phosphate addition. Difference equations were also developed to describe the rate of reaction of ions with both vacant sites and occupied sites, while diffusive penetration of the surface was occurring. The model was applied to data for the effects of time and of level of application on exchangeable phosphate. Many of the observed values for isotopically exchangeable phosphate could be well-described if it was assumed that equilibration of ³²P with surface sites was very rapid and this was followed by a diffusive penetration into the adsorbing particles. However, for short periods of contact between soil and ³²P, it was necessary to also take into account the rate of the reaction between ³²P and surface sites. This reaction was largely with vacant sites. Reaction with occupied sites–that is, true exchange–was unimportant. It is suggested that the electric potential of the surface may determine whether reaction is with occupied or vacant sites. In contrast to reaction of ³²P with occupied sites, reaction with vacant sites involves a net transfer of charge. Reaction with vacant sites would be slow if the potential was large and negative. It is shown that when reaction with vacant sites is slow, the proportion of previously added ³¹P recorded as exchangeable increases with level of addition of ³¹P. This may explain published observations of slow and non-linear exchange in some soils.     

Effect of selenium on the yield and quality of green tea leaves harvested in early spring

Foliar applications of a fertilizer of selenite or selenate were carried out to determine the influence of selenium on the yield and quality of green tea leaves harvested in early spring. Numbers of sprouts and the yield were significantly increased by the application of selenium. The sweetness and aroma of green tea leaves were also significantly enhanced, and bitterness was significantly decreased by the application of selenium. However, no significant differences were found in sweetness, bitterness, and aroma between tea leaves fertilized with selenite and selenate. Se concentration was significantly increased by selenium fertilization, and tea enriched by sodium selenate had a significantly higher selenium content than did tea enriched by sodium selenite. Total amino acid and vitamin C contents were significantly enhanced by the application of selenium. Tea polyphenol contents were significantly decreased by fertilization with selenium. The marked difference of tea polyphenols was also found between applications of selenite and selenate.     

THE DESCRIPTION OF DESORPTION OF PHOSPHATE FROM SOIL

After incubation with soil, phosphate was desorbed in dilute calcium chloride solutions. After short periods of incubation, the amount of phosphate desorbed increased rapidly at first but then net re-adsorption occurred. After long periods of incubation, desorption was slower and there was no net re-adsorption with prolonged desorption. Desorption was described in terms of two limiting values: the amount of phosphate (K) which would be desorbed as the concentration of phosphate approached zero, and the solution concentration of phosphate (C_o) at which no desorption occurred. Both values decreased as the period of incubation increased. The value of K increased during the desorption phase, but the value of C_o decreased. The decrease in C_o during desorption was most marked after short periods of incubation and was important in describing the trend for re-adsorption of phosphate.     

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^?).     

发育程度对海南玄武岩发育土壤吸附铬酸根和磷酸根的影响

选择海南岛北部3个不同年代喷发的玄武岩发育的土壤研究了其对铬酸根(CrO42-)和磷酸根(PO43-)的吸附特征,结果表明随着母岩年龄的增加,土壤发育程度提高,土壤游离氧化铁和表面正电荷数量增加,对2种阴离子的吸附量增加。土壤CrO42-的解吸率在19.8%~39.6%之间,表明土壤对CrO42-的吸附涉及静电吸附和专性吸附2种机制,且随着土壤发育程度增加,CrO42-静电吸附所占比例增加。土壤对PO43-的吸附以非静电吸附为主,吸附的PO43-的解吸量非常低,其解吸率不超过6%。吸附PO43-在去离子水中的解吸量高于在0.1 mol/L NaNO3和KNO3中的解析量,KNO3体系中的解吸量低于NaNO3体系中的,电解质主要通过改变胶体表面离子吸附面上的静电电位影响PO43-的解吸。     

L-cysteine Stimulates Selenite Uptake through GSH Involving in Selenite Reduction in Rice Roots

The effect of L-cysteine on selenite uptake rate in rice roots was investigated in this study. The results indicated that L-cysteine stimulated selenite uptake significantly, but D-cysteine had no effect on selenite uptake. Selenite uptake increased significantly when the roots were exposed to the L-cysteine solution following a cleanup of the adsorbed L-cysteine on the root surface. Exogenous reduced glutathione stimulated selenite uptake significantly, but oxidized glutathione had no effect on selenite uptake. Split-root experiments showed that exogenous L-cysteine and reduced glutathione applied to one-half of the root system induced selenite uptake by the untreated half. Furthermore, reduced glutathione concentration in rice roots was increased or reduced significantly when exposed to L-cysteine or selenite solution, respectively. A correlation analysis revealed selenite uptake rate was positively correlated with reduced glutathione concentration in rice roots but not in rice leaves. L-cysteine stimulates selenite uptake through reduced glutathione involving in selenite reduction in rice roots.     

不同因素对水稻离体根吸收四价硒影响

研究结果显示,pH对水稻离体根吸收4价硒(Se)有显著影响,这与不同pH下4价Se的存在形式有关。进一步研究表明,低温抑制4价Se吸收和一定浓度的NaCl促进4价Se吸收,可能在于低温和NaCl影响到根内代谢而间接影响4价Se吸收,而P抑制4价Se吸收可能与二者拥有共同吸附位点有关。     

pH和三种阴离子对紫色土亚硒酸盐吸附-解吸的影响

p H和三种竞争性阴离子对紫色土亚硒酸盐吸附-解吸的影响的研究结果表明,随着p H的增大,紫色土对亚硒酸盐的吸附量减少,酸性条件下紫色土对亚硒酸盐吸附量最大。平衡液中加入磷酸氢二钠显著降低了土壤对亚硒酸盐的吸附,硫酸盐对紫色土吸附亚硒酸盐的影响很小,低浓度碳酸氢根离子对紫色土吸附亚硒酸盐具有促进作用,但高浓度的碳酸氢根离子则降低了紫色土吸附亚硒酸盐。磷酸氢根离子和硫酸根离子对亚硒酸盐吸附的影响符合Langmuir和Freundlich拟合方程式,决定系数R2值均在0.90以上。三种阴离子对亚硒酸盐的解吸影响不同,当有磷酸氢根离子和碳酸氢根离子存在时,亚硒酸盐的解吸率增大,而硫酸根离子的存在却对紫色土亚硒酸盐的解吸影响不大。在紫色土地区农业生产中采用含磷酸盐肥料和碱性碳酸氢铵肥料,这些措施可能增加土壤硒的有效性,进而增加植物硒吸收和积累。认识紫色土固液界面硒的吸附-解吸规律,可为提高紫色土地区硒生物有效性,从而进一步提高农产品中硒含量提供科学依据。     

Influence of phosphate sorption on dispersion of a Ferralsol

Abstract

Soil dispersion induces soil erosion and colloidal leaching. Nutrients are lost at the same time and this causes water contamination. Phosphate is an essential element for living organisms. Because phosphate influences soil dispersion and it is an important limited resource, this influence must be evaluated well in order to diminish negative effects on soil structure. In this paper, we firstly evaluated the influence of phosphate sorption on soil dispersion by calculating repulsive potential energy between soil particles. Ferralsol, which is a typical soil in rainy tropical regions, was used as the material. The dispersion-flocculation phenomena were investigated with absorbance of soil suspension under different pH, phosphate adsorption and electrolyte concentration in an Na-NO₃-PO₄ system. The repulsive potential energy was calculated based on the diffuse double layer theory and the measured zeta potential. We indicated that the measured absorbance increased with the increase of the repulsive potential energy. The repulsive potential energy increased with increasing phosphate sorption up to about 5 to 20 mmol kg^?1 at all pH, and it induced the soil dispersion, because phosphate sorption increased the negative charge of the soil. After its peak, it decreased with increasing phosphate sorption because the electrolyte concentration increased and the electrolyte screened the electric field near the soil surface. The repulsive potential energy also increased with increasing pH because of the increase of the negative charge of the soil. Even at low pH, after a certain amount of phosphate sorption, the soil dispersed due to the increase of repulsive potential energy, although the soil flocculated before phosphate application. Because the soil dispersion causes soil and phosphorus loss, the influence of soil pH and phosphate sorption on the soil dispersion should be considered for good soil management.     

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).     

Influence of soil composition on adsorption of glyphosate and phosphate by contrasting Danish surface soils

The herbicide glyphosate and inorganic phosphate are strongly adsorbed by inorganic soil components, especially aluminium and iron oxides, where they seem to compete for the same adsorption sites. Consequently, heavy phosphate application may exhaust soil's capacity to bind glyphosate, which may lead to pollution of drain‐ and groundwater. Adsorption of phosphate and glyphosate to five contrasting Danish surface soils was investigated by batch adsorption experiments. The different soils adsorbed different amounts of glyphosate and phosphate, and there was some competition between glyphosate and phosphate for adsorption sites, but the adsorption of glyphosate and phosphate seemed to be both competitive and additive. The competition was, however, less pronounced than found for goethite and gibbsite in an earlier study. The soil's pH seemed to be the only important factor in determining the amount of glyphosate and phosphate that could be adsorbed by the soils; consequently, glyphosate and phosphate adsorption by the soils was well predicted by pH, though predictions were somewhat improved by incorporation of oxalate‐extractable iron. Other soil factors such as organic carbon, the clay content and the mineralogy of the clay fraction had no effect on glyphosate and phosphate adsorption. The effect of pH on the adsorption of glyphosate and phosphate in one of the soils was further investigated by batch experiments with pH adjusted to 6, 7 and 8. These experiments showed that pH strongly influenced the adsorption of glyphosate. A decrease in pH resulted in increasing glyphosate adsorption, while pH had only a small effect on phosphate adsorption.     

Competitive adsorption between phosphate and carboxylic acids: quantitative effects and molecular mechanisms

The competitive adsorption at the water‐goethite interface between phosphate and a carboxylic acid, either oxalate, citrate, 1,2,3,4‐butanetetracarboxylic acid (BTCA), mellitate or Suwannee River Standard Fulvic Acid 1S101F (FA), was investigated over a wide pH range (3–9) by means of batch experiments and attenuated total reflectance Fourier transform infrared (ATR‐FTIR) spectroscopy. The quantitative results from the competitive adsorption measurements show that the efficiency of the organic acids in competing with phosphate was in the order oxalate < citrate < BTCA ≅ FA < mellitate. Oxalate showed no detectable effect, whereas the effect in the mellitate system was strong, and the aggregative results indicate that an increasing number of carboxylic groups favours competitive ability towards phosphate. The infrared spectroscopic results show conclusively that competition for goethite surface sites between carboxylic acids and phosphate is not a ligand‐exchange reaction between inner‐sphere surface complexes. Instead, ligands capable of multiple H‐bonding interactions are required to out‐compete and desorb surface complexes of phosphate. The fact that partially protonated organic acids are the most efficient emphasizes the importance of both H‐accepting carboxylate groups and H‐donating carboxylic acid groups for the competitive effect.     

硒对小白菜生长和养分吸收的影响

以小白菜为材料 ,研究了硒对植株生长和养分吸收的影响。结果表明 ,当营养液中加入低浓度硒 (Se 1.0mg L)时促进了小白菜的生长 ,增加了产量 ;加高浓度硒 (Se 2 .5mg L)时则抑制了小白菜的生长。加硒后增加了小白菜地上部可溶性总糖、还原糖含量 ;降低了蔗糖和淀粉的含量 ;低浓度硒增加了粗纤维含量 ,而高浓度硒则降低了其含量 ;Se 1.0mg L的硒处理降低了植株地上部总蛋白质的含量 ,但随着硒浓度的进一步增加 (Se 1.0mg L)植株体内的总蛋白质含量逐渐升高 ,并大于对照。加硒后增加了小白菜地上部游离氨基酸总量和N、Ca、Mg、Mn、Zn含量 ,降低了P、K、S元素的含量 ;与对照相比 ,加硒后增加了小白菜地下部N、S元素的含量 ,降低了P、Ca、Mg、Fe、Mn、Zn元素的含量 ,但对K元素的影响不大。无论是小白菜地上部还是地下部 ,体内硒的含量均与营养液中硒的浓度呈显著的正相关。植物地上部硒的含量以有机态硒为主 ,有机硒转化率均在 80 %以上。     

Presence of bacteria reduced phosphate adsorption on goethite

Despite extensive studies, the information obtained from pure iron and aluminum (hydr)oxides cannot fully explain phosphate fixation in natural soils because of the ubiquitous interactions between (hydr)oxides and bacteria in soil. The effect of bacteria (Bacillus subtilis subsp. and Pseudomonas fluorescens) on phosphate adsorption on goethite (α‐FeOOH) was systematically examined under varying reaction times, phosphate concentrations, pH, ionic strength and bacteria dosage. Batch experiments in all cases showed significantly less adsorption on bacteria–goethite complexes than on pure goethite, demonstrating an inhibitory effect of bacteria. The inhibition of phosphate adsorption increased with bacterial loading, and showed a significant, non‐linear correlation with the decrease in the goethite positive charge induced by the bacteria. Moreover, in both the desorption experiment and in situ, the attenuated total reflectance Fourier‐transform infrared (ATR‐FTIR) spectra suggested a competition of bacteria surface groups (phosphate and carboxyl) with solution phosphate for hydroxyl on goethite. Therefore, the negative influences of bacteria on phosphate adsorption on goethite were probably caused by the surface charge modification and the competitive adsorption induced by the bacteria. Under most conditions, the effects of B. subtilis subsp. were conspicuous, while only slight influences were found for P. fluorescens. This difference between the two bacteria species was explained by differences in their surface charge, group content and chemical interaction with goethite. These findings uncover an important role of bacteria in phosphate phyto‐availability and mobility in natural environments.     

Proton consumption and release of organic‐cabon,silicate, and sulfate by anion sorption on andosols

Abstracts

Three anion (selenite, phosphate, and fluoride) sorption, and the concomitant release of silicate, sulphate, and organic matters from soils, and proton consumption with sorption were investigated using two andosols in Japan. The following sequence of sorption by the andosols was, fluoride (F) >> phosphate (P) > selenite (Se); concomitant proton consumption with sorption, F >> P > Se; organic matter released, P > Se > F; and sulfate released, P > Se >> F. There was quite a difference between an allophanic and a non‐allophanic andosols in silicate released with anion sorption. Our results suggest that the fluoride sorption mechanism is different from the others and that surface sites with OH groups for inner‐sphere complexes with selenite on soil particles which are restricted as compared to phosphate and fluoride.