Adsorption of glycerophosphate on goethite (α‐FeOOH): A macroscopic and infrared spectroscopic study

Adsorption, desorption, and precipitation reactions at environmental interfaces govern the bioavailability, mobility, and fate of organic phosphates in terrestrial and aquatic environments. Glycerophosphate (GP) is a common environmental organic phosphate, however, surface adsorption reactions of GP on soil minerals have not been well understood. The adsorption characteristics of GP on goethite were studied using batch adsorption experiments, zeta (ζ) potential measurements, and in situ attenuated total reflectance‐Fourier transform infrared spectroscopy (ATR‐FTIR). GP exhibited fast initial adsorption kinetics on goethite, followed by a slow adsorption. The maximum adsorption densities of GP on goethite were 2.00, 1.95, and 1.44 μmol m^?2 at pH 3, 5, and 7, respectively. Batch experiments showed decreased adsorption of GP with increasing pH from 3 to 10. Zeta potential measurements showed a remarkable decrease in the goethite isoelectric point upon GP adsorption (from 9.2 to 5.5), suggesting the formation of inner‐sphere surface complexes. In addition, the ATR‐FTIR spectra of GP sorbed on goethite were different from those of free GP at various pH values. These results suggested that GP was bound to goethite through the phosphate group by forming inner‐sphere surface complexes.     

Adsorption of Copper,Zinc, and Cadmium on Goethite,Aluminum‐Substituted Goethite,and a System of Kaolinite–Goethite: Surface Complexation Modeling

Abstract

Goethite, aluminum‐(Al)‐substituted goethite (GA2), and a system of kaolinite–goethite were examined for their ability to adsorb copper (Cu), zinc (Zn), and cadmium (Cd) as a function of pH, in two ionic strengths and two different metal concentrations. Specific surface area was determined by BET‐N₂, whereas the charge development on the solid surface was studied in the pH range ～3.5 to ～10.0 by potentiometric titration under continuous flow of argon.

Constant capacitance (CCM) and the double‐layer model (DLM) were used to fit the titration and adsorption data with the help of the least‐square optimization program FITEQL32. In both models, surface site density was fixed at Ns=2.31 sites nm^?2, whereas for CCM capacitance density was set at C=1.06. Alternatively, bibliographic suggestions for these two parameters were examined.

Aluminum‐substituted goethite exhibited higher specific surface area and adsorbed all three metals in lower pH values than the other solids. Moreover, GA2 exhibited point of zero salt effect (PZSE) higher than goethite, approaching that corresponding to Al₂O₃, possibly due to Al‐substitution, and the system exhibited PZSE values much higher than kaolinite, approaching that corresponding to goethite. The adsorption order for all three solids was Cu>Zn>Cd in any case, thus more Cu is adsorbed at a certain pH than Zn and even more than Cd, whereas the increase of metal concentration shifts the adsorption curve toward higher pH values.

Constant capacitance described the titration data satisfactorily, but by altering the Ns and C values, the fit became worse. Adsorption data are described by CCM, by emphasizing the formation of monodentate surface complex. Bidentate complex, in most of the cases, was of no importance in describing the data despite the evidence of its presence in recent spectroscopic studies for Cu and Cd on goethite. Alteration of Ns and C values worsened the fit in any case, and bidentate complex vanished. The DLM exhibited the worse fit in any case.     

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.     

Comparison of phosphate adsorption on clay minerals for soilless root media

Abstract

The greenhouse industry aims to decrease phosphate discharge to help reduce eutrophication of surface waters, to reduce fertilizer consumption, and to maintain a more constant level of plant‐available phosphate. Iron and aluminum oxides and some aluminosilicate minerals are efficient sorbents for phosphate. The phosphate adsorption characteristics of synthetic hematite (α‐Fe₂O₃), goethite (α‐FeOOH), and allophane (Si₃Al₄O₁₂ nH₂O), and a commercial alumina (A1₂O₃) were evaluated to determine their potential for reducing phosphate leaching from soilless root media. The pH dependence of phosphate adsorption and maximum adsorption capacities were determined by reacting each mineral with various levels of phosphate between pH 4.0 and 9.0 in a 10 mM potassium chloride (KCl) background solution. Adsorbed phosphate was determined by loss from solution. Adsorption envelopes (adsorbed phosphate versus pH) showed a decrease in phosphate adsorption with increasing pH, particularly for alumina and allophane, and at greater added phosphate concentrations. The maximum adsorption capacities per unit mass of the minerals at pH 5.4 decreased in the order allophane > alumina ? goethite > hematite. When expressed on a surface area basis, the order of maximum adsorption capacity remains the same except that alumina exceeded that of goethite. The allophane, goethite, and alumina sorbed enough phosphate that 3 to 9 g of these minerals would retain the amount of phosphate required for a high nutrient element requiring plant such as chrysanthemum.     

Sb(V)在三种矿物表面的吸附行为

用研究了蒙脱土、高岭土和针铁矿在不同的pH与离子强度的介质条件下对Sb(V)的吸附及解吸行为。3种矿物对Sb(V)的吸附能力差别较大,蒙脱土的吸附量远大于针铁矿和高岭土,针铁矿与高岭土的吸附能力相近。pH对Sb(V)在3种矿物表面的吸附行为影响显著。随pH的升高,Sb(V)的吸附均减弱。吸附在高岭土表面的Sb(V)易解吸,而针铁矿和蒙脱土表面的Sb(V)不易解吸。随离子强度升高,高岭土对Sb(V)的吸附减弱;离子强度对Sb(V)在针铁矿和蒙脱土表面吸附的影响较小。     

ADSORPTION ON HYDROUS OXIDES I. OXALATE AND BENZOATE ON GOETHITE

The adsorption of oxalic acid on synthetic goethite (α-FeOOH) was studied using adsorption isotherms. Infrared spectra were obtained for goethite-oxalate complexes at several points on the isotherms. On a goethite preparation with a phosphate sorption capacity of 200|μmolg^?1 the amounts of oxalate strongly adsorbed varied from near zero at pH 8 to about l00μmolg^?1 at pH 4 and below. At pH 3.4, the first l00μmolg^?1 of oxalic acid added was strongly adsorbed as a binuclear complex (FeOOC–COOFe), replacing two singly-coordinated OH groups by ligand exchange. At higher concentrations a further 200 μrnol g^?1 of oxalic acid formed a monodentate complex (FeOOC–COOH) so that more oxalate could be accommodated. Benzoic acid was weakly adsorbed on goethite with one benzoate oxygen replacing one singly-coordinated OH. The other oxygen of the COO group fitted into the goethite surface so that the benzene ring was at a high angle to the (100) face.     

Irreversibility of sulfate sorption on goethite and hematite

Sulfate ion adsorption and desorption experiments carried out on synthetic goethite and hematite and natural hematite show adsorption to be a highly irreversible reaction. All oxides showed an increase in sulfate ion adsorption with decrease in pH. Only a small fraction of sorbed sulfate was desorbable after 48 hr, and only at a pH of 3. Extreme irreversibility of sulfate sorption on these common soil minerals suggests that adsorbed sulfate is more immobile in watersheds than previously considered and that recovery models which inherently assume reversibility may need to be modified.     

DESORPTION OF PHOSPHATE FROM GOETHITE

The reversibility of the adsorption of phosphate on goethite was measured by diluting suspensions of goethite on which phosphate was adsorbed with large volumes of phosphate-free solution at the same ionic strength. The effect on the reversibility of various adsorption and desorption conditions was studied, particularly pH, temperature, ionic strength and time of reaction. The apparent irreversibility of the adsorption reaction seems to be due to a slow adsorption reaction of part of the phosphate after a very rapid initial adsorption, with a similar behaviour when the phosphate is desorbed. When the total adsorption plus desorption time is long enough, both adsorption and desorption points tend to lie on a single curve which corresponds to the isotherm calculated according to the Stern double-layer theory.     

ANION ADSORPTION BY GOETHITE AND GIBBSITE

The desorption of specifically adsorbed (ligand exchanged) anions, such as phosphate, selenite, and fluoride, from the surface of gibbsite and goethite has been studied by repeatedly washing the adsorption complex with solutions of constant pH and ionic strength but containing no specifically adsorbable anions. The desorption of the anions varies between complete reversibility and almost complete irreversibility. Measurements of surface charge by the uptake of Na⁺ and Cl^- revealed that, on washing the adsorption complex, the surface charge returned to its value at the given pH in the absence of specifically adsorbed anions. Thus when the isotherm was irreversible, OH^- was desorbed (or H⁺ adsorbed) in preference to the desorption of the specifically adsorbed anion, whereas when the isotherm was reversible the specifically adsorbed anion was desorbed. Irreversibility appears to involve the nature of the adsorption complex at the surface. Where only monodentate ligands form, for example fluoride adsorption, the isotherm is reversible, whereas bridging or multidentate ligands and the formation of ring structures at the surface favour irreversibility.     

ADSORPTION OF THE HERBICIDE 2,4-D ON GOETHITE

Adsorption, of the herbicide 2,4-Dichlorophenoxyacetic acid (2,4-D) on an iron oxide, goethite, was studied in aqueous suspensions as a function of solution pH, ionic strength of the medium, and initial 2,4-D concentration. The 2,4-D anion was reversibly adsorbed on positively charged goethite surfaces, maximum adsorption being observed near the pK_a of 2,4-D (2.73) and at low ionic strength. Within certain levels of adsorption (5–22 mg 2,4-D adsorbed/g goethite) the complex became hydrophobic and floated to the liquid surface. This flotation effect disappeared on further adsorption. It is suggested that adsorbed 2,4-D anions are orientated with their hydro-phobic aromatic ends directed towards the solution, the carboxyl groups being weakly bound to positive sites on the oxide surface. At high levels of adsorption, some of the anions are orientated in the opposite direction by π–π interaction, with the first adsorbed layer and the surface reverts to its hydrophilic nature.     

Influence of oxalic acid on the adsorption of cadmium at the goethite surface

The adsorption/desorption of oxalic acid and Cd on goethite has been characterized in function of pH in KNO₃ medium. Electrophoretic mobilities and pH titrations show that while anion desorbs, cation adsorbs, the maxima of adsorption being in significantly different pH ranges. Adsorption-desorption phenomena of oxalic acid in function of pH are identical in absence or presence of Cd. Contrarily, not only adsorption, but also desorption of Cd is enhanced in presence of oxalic acid. Surface sites are shown to never be saturated even in presence of excess of ligand and cation. So, competition between anions and cations for surface sites is minor. Ligand in solution does not compete with surface sites for the Cd, probably due to the low tendency of oxalic acid to form complexes with Cd. Behavior of Cd in presence of oxalate is discussed and results suggest a surface-binding of Cd via an oxalate-bridge between the surface and the metallic cation.     

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.     

Phosphorus adsorption and desorption in undisturbed samples from subtropical soils under conventional tillage or no‐tillage

The dynamics of P in soil is greatly influenced by the adsorption of phosphate onto Fe‐oxides. Access of phosphate to the surface of these minerals depends on the degree of soil aggregation, which in turn is influenced by soil management system. The primary purpose of this work was to investigate P adsorption and desorption in undisturbed and disturbed soil samples from an Ultisol (Rhodic Paleudult) and an Oxisol (Humic Hapludox) under conventional tillage (CT) or no‐tillage (NT). Phosphorus adsorption and desorption in undisturbed soil was studied by using a continuous flux system containing a P solution for adsorption measurements or deionized water and Mehlich‐I solution for desorption measurements. The Oxisol, which had higher clay, hematite, and goethite contents than the Ultisol, exhibited the highest maximum P adsorption capacity (P_max) values in disturbed samples. Also, the disturbed Ultisol samples had lower P_max values under NT than under CT. The undisturbed soil samples exhibited no significant differences in P adsorption between soil management systems, but P desorption was more marked under NT than under CT. The samples of Oxisol under NT exhibited lower P adsorption rates and higher P desorption rates than the CT samples of the same soil. The decreased P adsorption in undisturbed samples relative to disturbed samples suggests that P adsorption is influenced by physical properties of soil.     

农药草甘膦在砂质潮土上的迁移行为及其影响因素

化学物质在土壤中的移动性是人们精确评价其潜在淋溶能力(即对地下水的污染风险)的必要信息。通过一次平衡法和饱和土柱稳定流混合置换实验,来阐明环境pH变化和磷酸盐施用对草甘膦在砂质潮土中的吸附和迁移行为的影响。结果表明,草甘膦在砂质潮土上的吸附量随着pH的增大而降低,在pH4~6的范围内吸附量变化最明显;其穿透曲线(BTCs)均呈现不同程度的不对称性,加入的草甘膦浓度越低这种不对称性越明显;在本实验条件下,施用磷酸盐有抑制草甘膦迁移的趋势,但其抑制趋势并非随磷酸盐施用量的增加而持续增加。如加入草甘膦浓度为500 mg L-1,不加磷酸盐P0时,流出液中草甘膦峰值为459.8mg L-1,而加入磷酸盐P1和P2时,其峰值分别为147.3 mg L-1和373.6 mg L-1。推测其原因可能与磷酸盐施用导致环境pH降低及磷酸盐在加草甘膦前已经占据过多的吸附位点有关。     

Adsorption Kinetics of Pb2+ and Cu2+ on Variable Charge Soils and Minerals: Ⅲ. Adsorption Kinetics of Pb2+ Within 30 Minutes

The adsorption kinetics of Pb2+ on different soils and minerals with variable charges was studied by the two ion-selective electrode technique at different pH and concentrations. The results showed that more than 95% of adsorption on all the samples occurred during the first 5 minutes. All adsorption time-dependent data could fit the surface second-order equation very well. The values of Xm were goethite > kaolinite, and latosol > red soil at the same initial reaction concentration. The values of k were kaolinite > > goethite, and latosol > red soil at the same reaction pH and initial concentration.     

Reactions of nucleic acid bases with inorganic soil constituents

The adsorption at pH's 4, 6 and 8 of adenine, guanine, cytosine, thymine and uracil on clays (montmorillonite, illite and kaolinite), Fe- and Al-oxides (goethite, hematite and gibbsite), a soil, and on a laboratory-prepared fulvic acid-montmorillonite complex was investigated. Portions of the clays and soil were saturated with H⁺, Fe³⁺ and Ca²⁺.Quantitatively, the extent of adsorption of nucleic acid bases by the clays was proportional to their exchange capacities, but the nature of the dominant cation had only minor effects. By contrast, the adsorption was strongly affected by pH, tending to decrease with increase in pH. Adsorption on goethite and gibbsite was lower than that on clays, while adsorption of nucleic acid bases on soils was slightly lower than that on oxides. The fulvic acid-montmorillonite complex adsorbed substantial, although smaller amounts of purines and pyrimidines, than did montmorillonite alone. The main adsorption mechanism at pH 4 appeared to be cation exchange whereas at pH 8 complex formation between the nucleic acid bases and cations on inorganic surfaces seemed to occur.The results of this and earlier work show that both inorganic and organic soil constituents adsorb nucleic acid bases. Which adsorption reaction predominates will depend on the clay and organic matter content and on the pH.     

温度对硼在三种矿物上吸附-解吸特性的影响

在实验室对天然Ca 蒙脱进行了纯化 ,并在一定条件下人工合成了针铁矿、水锰矿 ,以此作为实验材料 ,用吸附—解吸平衡法和反应动力学方法研究了温度对硼在Ca 蒙脱 ,针铁矿 ,水锰矿上的吸附、解吸特性的影响。结果表明 :随温度升高 ,Ca 蒙脱对硼的吸附量升高 ,其对硼的解吸滞后性下降 ,水锰矿和针铁矿对硼的吸附量下降 ,其对硼的解吸滞后性加强。计算表明 ,在常温下 ,硼在Ca 蒙脱上的吸附热为 63.0 8kJmol- 1 ,在针铁矿和水锰矿上分别为 - 1 2 2 .45和 - 93 .91kJmol- 1 ,硼在Ca 蒙脱、针铁矿和水锰矿上的解吸热分别为- 31 .0 2 ,53 .95和 46 .30kJmol- 1 。上述结果说明 ,硼在Ca 蒙脱上的吸附为吸热过程 ,解吸为放热过程 ;硼在针铁矿、水锰矿上的吸附为放热过程 ,解吸为吸热过程。随着温度的升高 ,硼与矿物的初始反应速率明显加快 ,而整体反应速率略有下降。     

Effect of organic acids on the adsorption of copper,lead, and zinc by goethite

The adsorption of Cu, Pb, and Zn by synthetic goethite was studied in the absence and presence of oxalic, citric, and glutamic acids at different pH values. It was shown that, in the absence of an acid, the content of adsorbed metals increased with the increasing pH. The content of adsorbed cations at constant pH values decreased in the sequence: Cu > Pb > Zn. The simultaneous addition of metal cations and organic acids to the goethite suspension increased the content of the adsorbed elements. The oxalic and citric acids had similar effects on the adsorption of copper and lead in the studied pH range. The metal: acid concentration ratios significantly affected the adsorption of the heavy metals by goethite. An increase in the metal adsorption was observed to a certain metal: acid ratio, which was followed by a gradual decrease. The adsorption of the metals by goethite also depended on the properties of the metal cations and the organic ligands. The observed tendencies were attributed to the complexation of heavy metals with organic acid anions and the simultaneous sorption of acids at positively charged sites on the goethite surface with the formation of mineral-organic compounds, which significantly modified the surface properties of the mineral. The study of the effect of increasing lead concentrations in the solution on the copper adsorption by goethite in the absence, in the presence, and at the addition of an oxalic acid solution to the goethite suspension one hour before the beginning of the experiment showed that lead decreased the adsorption of copper in all the treatments. The possible mechanisms of the processes occurring in the system were considered.     

Phosphate-induced aggregation kinetics of hematite and goethite nanoparticles

Purpose

The purpose of the present study is to examine the effect of phosphate on the aggregation kinetics of hematite and goethite nanoparticles.

Materials and methods

The dynamic light scattering method was used to study the aggregation kinetics of hematite and goethite nanoparticles.

Results and discussion

Specific adsorption of phosphates could promote aggregation through charge neutralization at low P concentrations, stabilize the nanoparticle suspensions at medium P concentrations, and induce aggregation through charge screening by accompanying cations at high P concentrations. Two critical coagulation concentration (CCC) values were obtained in each system. In NaH₂PO₄, the goethite CCC at low phosphate concentrations was smaller than hematite and vice versa at high phosphate concentrations. Stronger phosphate adsorption by goethite rapidly changed the zeta potential from positive to negative at low phosphate concentrations, and the zeta potential became more negative at high phosphate concentrations. The clusters of hematite nanoparticles induced by phosphate adsorption had an open and looser structure. Solution pH and the phosphate adsorption mechanisms in NaH₂PO₄, KH₂PO₄, and Na₃PO₄ solutions affected zeta potential values and controlled the stability of hematite suspensions during aggregation. High pH and preference for non-protonated inner-sphere complexes in Na₃PO₄ solution decreased the zeta potential of positively charged hematite and promoted aggregation. Activation energies followed the order NaH₂PO₄ > KH₂PO₄ > Na₃PO₄ at low P concentrations. K⁺ was more effective than Na⁺ in promoting hematite aggregation due to the non-classical polarization of cations.

Conclusions

Phosphate can enhance or inhibit the aggregation of hematite and goethite nanoparticles in suspensions by changing surface charge due to specific adsorption onto the particles. The phosphate-induced aggregation of the nanoparticles mainly depended on the initial concentration of phosphate.

     

不同质地土壤对镉的吸附特性及影响因子研究

通过Cd²⁺吸附解吸试验,探究了初始Cd²⁺浓度、p H、有机质、土壤质地和枯草芽孢杆菌-生物质炭复合体对土壤吸附Cd²⁺影响。结果表明:土壤对Cd²⁺的吸附能力随着溶液浓度、p H的升高而增加,土壤有机质可显著提高土壤对Cd²⁺的固定能力,壤土对Cd²⁺的吸附能力显著高于砂质壤土。土壤施加枯草芽孢杆菌-生物质炭复合体后,土壤对于Cd²⁺的吸附能力显著提高,并且施加枯草芽孢杆菌–生物质炭复合体为20 ml/kg时对Cd²⁺的吸附量提高11.7%;Freundlich模型(R²=0.997)可以很好地拟合Cd²⁺吸附过程。枯草芽孢杆菌–生物质炭复合体的施加降低了土壤表面Cd²⁺的解吸能力,进一步证明复合体能加强土壤对Cd²⁺的固定稳定化,具有作为钝化剂修复土壤重金属污染、降低食品污染风险的潜力。