Sorption of phosphate by aluminium and iron(iii)-hydroxy species on mica surfaces

Freshly cleaved mica sheets with aluminium- or iron(III)-hydroxy species on the cationexchange surface were prepared by suitable treatment with AlCl₃ or FeCl₃. These surfaces were considered as model soilk mineral surfaces and their interaction with phosphate studied using techniques previously developed for the study of single planar surfaces.Adsorption isotherms for both the iron(III) and aluminium systems were very similar and could be interpreted as two Freundlich isotherms indicating two different adsorption processes, one operating below a solution concentration of 10^?5 M and thee other above. Results obtained from the continuous monitoring of adsorption indicate that a rearrangement of adsorbed phosphate occurs with time on both the aluminium- and iron(III)hydroxy surfaces. This rearrangement reduces the rate of desorption and could be a cause of phosphate fixation in soils.     

基于不同分析方法研究磷酸根在矿物表面吸附机制的进展

磷素是植物生长必需的营养元素,也是联系生态系统中生物与非生物作用的关键元素。对磷酸根在矿物表面吸附反应机制的深入认识,有助于了解其在陆地和水环境中的形态、迁移、转化和生物有效性。本文主要综述了磷酸根在常见(土壤)矿物表面吸附机制的研究进展。各种分析技术或方法,如OH–释放量分析、Zeta电位分析(电泳迁移率测试)、等温滴定量热法、原子力显微镜、X射线光电子能谱、红外光谱、核磁共振波谱、X射线吸收光谱、表面络合模型、量子化学计算等,均以不同方式揭示磷酸根在不同矿物体系的吸附机制。磷酸根在矿物(尤其是铁、铝氧化物)表面的吸附通常伴随着水基和羟基的交换。一般认为磷酸根在矿物表面主要形成双齿双核、单齿单核内圈络合物,且受pH的影响较大。pH以及磷酸根在矿物表面的吸附密度影响内圈络合物的质子化状态。在低pH、高磷浓度、较高反应温度、较长吸附时间,以及弱晶质矿物吸附等条件下矿物表面吸附的磷可在矿物表面转化形成含磷的表面沉淀,造成矿物溶解转化以及磷生物有效性的进一步降低。最后展望了磷酸盐在矿物-水界面吸附有关的研究热点和方向。     

磷酸根在矿物表面的吸附-解吸特性研究进展

综述了磷酸根在一些常见土壤矿物表面吸附–解吸特性的研究进展。磷酸根在矿物表面的吸附特性受环境pH、离子强度、温度、反应时间、矿物类型等多种因素的共同影响。一般说来,矿物表面的磷吸附量随pH降低而增加,受离子强度的影响较小。磷酸根在矿物表面的吸附动力学过程可分为快速吸附过程和慢速吸附过程,且在弱结晶矿物中存在微孔扩散过程。磷酸根在矿物表面的解吸过程通常存在两个阶段(初始快速解吸和随后的缓慢解吸),在解吸反应后期甚至还会发生再吸附。此外,磷酸根的吸附特性也受共存阴离子配体或金属阳离子的影响。其中,共存阴离子通过位点竞争、静电作用和空间位阻效应等机制影响磷酸根的吸附。天然有机质(包括胡敏酸和富里酸)降低了磷酸根在矿物表面的吸附,特别是在低p H条件下。通常,富里酸比胡敏酸更能有效降低磷酸根在矿物表面的吸附。金属阳离子可通过表面静电效应、形成三元络合物以及形成表面沉淀等机制促进磷酸根和金属在矿物表面的共吸附。最后,展望了与磷酸根在矿物表面吸附特性有关的研究热点和方向。     

Sulphate adsorption at the Fe (hydr)oxide–H2O interface: comparison of cluster and periodic slab DFT predictions

The transport and bioavailability of sulphate in soils are significantly affected by adsorption reactions at the mineral–H₂O interface. Therefore, an understanding of the mechanisms and kinetics of sulphate adsorption is of fundamental importance in soil chemistry. In this investigation, the binding geometries of bidentate bridging and monodentate sulphate complexes at the Fe (hydr)oxide–H₂O interface were predicted with static cluster and periodic slab density functional theory (DFT) calculations. The cluster calculations were performed with edge‐sharing dioctahedral Fe³⁺ models, using the unrestricted PBE0 exchange‐correlation functional and a combination of effective core potential (LanL2DZ – Fe atoms) and all‐electron (6–311+G(d,p) or 6–311+G(3df,p) – S, O, and H atoms) basis sets. The periodic slab calculations were performed with a (3 × 2) slab of the (100) α‐FeOOH surface, by means of the projector‐augmented wave method and a plane‐wave basis set. For the periodic slab DFT calculations, the spin‐polarized (SP) PBE exchange‐correlation functional, with and without explicit consideration of an on‐site Coulomb interaction parameter (i.e. SP‐PBE and SP‐PBE+U methods), was used. Despite the lack of long‐range order, cluster model predictions of the interatomic distances and angles of bidentate bridging and monodentate sulphate were in good agreement with the periodic slab model predictions. Quantitative analysis of the cluster and periodic slab DFT predictions is expected to result from theoretical fitting of extended X‐ray absorption fine structure measurements. The application of computational chemistry methods to soil chemistry research is anticipated to provide novel insight into the mechanisms and kinetics of ion sorption.     

Adsorption,desorption and extractability of Zn in some Algerian soils under orange cultivation

The Zn adsorption/desorption in some Algerian soils, which are under orange cultivation, was studied and the results obtained were analyzed using Langmuir equation. The maximum amount of Zn adsorbed, in majority of cases, varies between 60 and 80 % of the cation exchange capacity. As regards sequential desorption, the EDTA desorbed about 80 %of the Zn adsorbed, whereas, the desorption through 1 N KCI averaged about 15 % only. The adsorption maximum (Sm) and desorption maximum (Rm) correlate significantly with various soils properties, like clay content, cation exchange capacity and pH. The calculation of distribution coefficient (Kd) and adsorption density of Zn (r) indicates that the metal above 2.5 · 10 ions/m2 is adsorbed through ion exchange, whereas below this value, the ion is most possibly adsorbed due to specific adsorption mechanisms.     

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.     

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。     

Adsorption and desorption of organotin compounds in organic and mineral soils

Organotin compounds (OTC) are deposited from the atmosphere into terrestrial ecosystems and can accumulate in soils. We studied the adsorption and desorption of methyltin and butyltin compounds in organic and mineral soils in batch experiments. The adsorption and desorption isotherms for all species and soils were linear over the concentration range of 10–100 ng Sn ml^?1. The strength of OTC adsorption correlated well with the carbon content and cation exchange capacity of the soil and was in the order mono‐ > di‐ > tri‐substituted OTCs and butyltin > methyltin compounds. The OTC adsorption coefficients were much larger in organic soils (K_d > 10⁴) than in mineral soils. The adsorption and desorption showed a pronounced hysteresis. Trimethyltin adsorption was partly reversible in all soils (desorption 2–12% of the adsorbed amounts). Dimethyltin, tributyltin and dibutyltin exhibited reversible adsorption only in mineral soils (desorption 4–33% of the adsorbed amounts). Mono‐substituted OTCs adsorbed almost irreversibly in all soils (desorption < 1% of adsorbed amounts). Trimethyltin was more mobile and more bioavailable in soils than other OTCs. It might therefore be leached from soils and accumulate in aquatic ecosystems. The other OTCs are scarcely mobile and are strongly retained in soils.     

pH对针铁矿表面磷酸盐吸附的化学形态的影响

Chemical forms of the phosphate adsorbed on goethite surfaces and characteristics of the coordinate groups which exchange with P on goethite surfaces in solutions with different pll values were investigated.Results showed that the chemical forms of P on goethite surfaces changed from the dominance of monodentate corrdination to that of bidentate one with increasing pH of the solution.By influencing types of phosphate ions in solutions,pH affected the chemical forms of P on goethite surfaces,The amount of OH^- displaced by phosphae on goethite surfaces was the most at pH 7.0,the second at pH 9.0,and the least at pH 4.5.     

PHOSPHATE ADSORPTION AND THE DISPLACEMENT OF STRUCTURAL SILICON IN AN ALLOPHANE CLAY

Phosphate was adsorbed at a constant pH, on allophane clay samples which had been pre-treated with selenite to remove adsorbed silicate. The concomitant release of selenium, silicon, and hydroxyl ion on phosphate adsorption was also measured. The results are compared with those obtained from a similar experiment conducted using clay which had not been treated and they substantiate the earlier conclusion that phosphate displaces structural silicon of clays. The amount displaced increases parabolically with increasing levels of phosphate adsorbed over the concentration range studied.     

Adsorption of phosphate by two iron oxides in relation to their porosity

Adsorption and desorption of phosphate by lepidocrocite and hematite, the latter obtained by thermal decomposition of the former, were studied and related to changes in mesoporosity of the solid surface. Differences between the oxides in the accessibility of their surfaces to phosphate seems to control the fractional amounts of ‘rapidly’ and ‘slowly’ adsorbed phosphate. Results of isotopic exchange of the phosphate adsorbed at high surface coverage can also be explained in terms of the mesoporosity of the surfaces.     

PHOSPHATE ADSORPTION IN ALLOPHANIC SOILS

The kinetics and heats of phosphate adsorption were measured on the <2 μm Na-saturated fractions of three allophanc-rich soils from Japan. Between 50 and 2250 μmol P g^?1 as sodium phosphate were added to the soil fractions at pH 5 and pH 7, and at initial concentrations of 5 and 25 × 10^?4m to avoid aluminium phosphate precipitation. An initial ‘instantaneous’ adsorption associated with exposed sites and, simultaneously, two inverse exponential rates of adsorption on internal and freshly forming external sites were observed. These rates are attributed to changes in the microstructure of allophane and to the desorption of organic matter held on allophanic surfaces. This interpretation is strongly supported by corresponding changes in the heats of adsorption with time. Calorimetry clearly indicates that when very large amounts of phosphate are added, new and very reactive surfaces are progressively exposed. More phosphate was adsorbed when the soil was acid and when the soil contained less organic matter.     

IDENTIFICATION OF PHOSPHATE-REACTIVE SITES OF HYDROUS ALUMINA FROM PROTON CONSUMPTION DURING PHOSPHATE ADSORPTION AT CONSTANT pH VALUES

The amounts of hydroxyl ions released from hydrous alumina during phosphate adsorption were measured at constant pH values and at different phosphate concentrations using an automatic titrator. The hydroxyl ions released were plotted against phosphate adsorbed and the equations that gave best fit to the data were differentiated. The variation in the differentials (dOH/dP) with increasing phosphate adsorption was consistent with the theory, proposed below, on the type of sites responsible for phosphate adsorption, by synthetic hydrous alumina containing bayerite and pseudoboehmite, at different concentrations. At low phosphate adsorption which corresponded to low phosphorus concentration, adsorption took place mainly by displacing aquo groups (Al-H₂O). With increase in phosphorus concentration hydroxo groups (Al-OH) became the predominant sites of adsorption. At still higher concentrations, where phosphate adsorption displacing aquo and hydroxo groups was nearly completed, the hydroxyl bridges linking aluminium atoms (Al-OH-Al, ol groups) were broken. This created new sites which were responsible for additional adsorption of phosphate at these concentrations.     

碱金属离子在蒙脱石-Cu2+表面吸附的离子特异性

研究离子交换中的离子特异性效应有助于揭示离子-带电表面相互作用机制。以蒙脱石Cu2+饱和样为研究对象,采用恒流法研究不同浓度碱金属离子Li+、Na+和K+的吸附动力学过程,并建立1︰1型(LiNO3、NaNO3、KNO3)电解质溶液中离子平衡吸附量与体系吸附活化能之间的关系。结果发现:(1)Li+、Na+和K+在蒙脱石-Cu2+表面的吸附过程仅呈现出弱静电力作用下的一级动力学特征,并存在明显的离子特异性效应。(2)离子非经典极化作用与体积效应共同决定了离子在双电层中的位置,从而导致表面电位存在差异;并且表面电位(绝对值)随着电解质浓度降低而增加,表现为Li+>Na+>K+。(3)根据新建立的模型可预测吸附离子在双电层中的位置,进而求出体系的吸附活化能,并发现离子特异性效应产生的根本原因是由活化能决定的,同时本研究表明建立的新模型在固/液界面反应中具有普适性。本研究将对固/液界面反应理论的完善提供新思路。     

ADSORPTION ON HYDROUS OXIDES: II. OXALATE, BENZOATE AND PHOSPHATE ON GIBBSITE

Adsorption isotherms, the infrared spectra of adsorbed species, and the amount of hydroxyl released when oxalate is adsorbed, all indicate that adsorption of oxalate, benzoate and phosphate at low solution concentrations occurs only on A1(OH)(H₂ O) sites exposed on edge faces of the platy gibbsite crystals studied. This is confirmed by the finding that the vibrations of surface hydroxyl groups on the principal plate faces are unaffected when excess oxalate is present, and further supported by the low affinity for oxalate of imogolite, a tubular mineral with a very large gibbsite-like surface, but with a very low concentration of edge sites. Infrared spectra indicate that oxalate is adsorbed in bidentate form. Near neutrality and at low solution concentrations, gibbsite adsorbs more oxalate than does goethite of comparable surface area.     

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).     

土壤矿物质吸附砷的研究进展

砷是一种有毒的重金属元素,由于自然和人为的原因,世界上的许多国家存在砷污染问题,因此有关砷污染的研究与控制日益受到人们的关注。本文介绍了土壤砷污染的产生原因及污染状况,重点集中在探讨土壤中的各种矿物质对砷吸附行为的作用机理。这些矿物质包括铁铝氧化物和氢氧化物、锰氧化物、黏土矿物和碳酸盐类。关于砷在各类矿物质上的吸附机理,普遍接受的观点认为:砷氧阴离子与矿物质的表面发生了配位体交换过程并通过共价键的作用,形成了表面配位体。表面配位体以内层双齿双核鳌合形式为主,受矿物质的种类、pH值和氧化还原电位等因素的影响,外层吸附和其他鳌合状态可能存在。砷在土壤矿物质上的吸附效果强烈依赖于体系的pH值,这与吸附的理论模型和数学计算结果相一致。矿物质的种类、组成、结晶状态和表面积等影响了砷的吸附过程和吸附效果。土壤中存在的各种阴阳离子通过与砷阴离子竞争吸附位而影响砷在矿物质上的吸附。     

An investigation of phosphate adsorbed on aluminium oxyhydroxide and oxide phases by nuclear magnetic resonance

The adsorption of phosphate by soil minerals controls availability of P to plants, but the chemical environments of adsorbed phosphate are poorly known. We used ³¹P MAS NMR to study the adsorption of phosphate on to boehmite (γ‐AlOOH) and γ‐Al₂O₃ with large surface areas. The solid phases were reacted in 0.1 m phosphate solutions at pH from 3 to 11 and in solutions with pH 5 at concentrations from 10^?1 m to 10^?4 m . The spectra suggested three different phosphate environments: (i) orthophosphate precipitated from the residual solution after vacuum filtering, (ii) surface‐adsorbed phosphate in inner‐sphere complexes, and (iii) Al‐phosphate precipitates on the surfaces of the minerals. The chemical shifts of both the inner‐sphere complexes and surface precipitates became progressively less shielded with increasing pH and decreasing concentration of phosphate solution. For the inner‐sphere complexes, we interpret these changes to be the result of decreasing phosphate protonation combined with rapid proton exchange among phosphate tetrahedra with different numbers of protons, which causes peak averaging. The chemical shifts of ³¹P of the Al‐phosphate precipitates were more negative than those of the surface phosphates at a given pH and solution concentration, probably because of a larger number of P–O–Al linkages per tetrahedron. The observed trend of decreasing shielding is probably due to the decreasing average number of P–O–Al linkages per tetrahedron combined with decreasing protonation and an increasing number of K⁺ next‐nearest neighbours. Even at small concentrations of phosphate solution, a significant amount of Al‐phosphate precipitate was present.     

Sorption and desorption of selenium in different soils of the mediterranean area

Abstract

The adsorption of selenium (Se) in the selenate form and its desorption by phosphate in four soils with different physiochemical properties were studied in the laboratory. To determine adsorption isotherms for selenate 25 mL of solutions containing 1 to 100 ppm of Se were added to 2.5 g of soil. Desorption isotherms were determined by resuspending the samples in phosphate solution. The selenate sorption process was adequately described by the Freundlich equation. In pine forest and woodland soils, characterized by the highest organic matter content and cation exchange capacity (CEC) values, the isotherms were classified as L type, since the amount of Se sorbed appeared to move towards saturation. The organic matter content played the most important part in the adsorption of Se, while pH appeared to have a small effect on the ability of the soil to adsorb Se. The high CaCO₃ content of the pine forest soil may have contributed in increasing the Se adsorption notwithstanding the high pH value. The cultivated and arable soils showed a reduced sorption capacity. The sorption could be described by an S type curve. At low concentrations of Se the affinity of the solid phase was less than that of the liquid phase. By increasing the concentration of Se in solution, the affinity of the solid phase increased and the sorption was favored. Selenate desorption by water was negligible, whereas the amount of Se desorbed by phosphate varied among the different soils. The desorption experiments indicated that a significant portion of the sorbed Se was irreversibly retained. This suggests the existence of linkages which allow the release of Se in the soil solution only after physico‐chemical variation such as exchange with phosphate ions.     

PHOSPHATE ADSORPTION BY SYNTHETIC AMORPHOUS ALUMINOSILICATES

Phosphate adsorption isotherms were determined for four synthetic amorphous aluminosilicate gels with A1: A1 + Si molar ratios of 0.29 to 0.88. The concomitant silicate release and acid consumed to maintain the pH of the suspensions constant were also measured. The adsorption isotherms were analysed applying a two-term Langmuir equation–assuming two types of sites. The experimental points fitted the predicted adsorption curves only up to a certain amount of phosphate adsorbed. The deviation at high phosphate adsorption values suggested the presence of more than two types of adsorption site. A comparison of phosphate adsorbed with the silicate released and acid consumed to maintain the pH constant indicated that, for a 3 h reaction time at concentrations below about 10 μmol cm^?3, phosphate exchanges mainly with aquo and hydroxo ligands and with adsorbed silicate. At higher concentrations phosphate is adsorbed (i) on sites arising from the disruption of hydroxy aluminium polymers in the gels and (ii) by the displacement of structural silicate.