ISOTOPIC EXCHANGE OF PHOSPHATE ON SOME PHOSPHATIC COMPOUNDS

The kinetics of isotopic exchange of phosphate on hydroxyapatite, amorphous aluminium phosphate, phosphate-treated aluminium oxide and dicalcium phosphate dihydrate were studied in aqueous suspensions as a function of solid/solution ratio and amount of added isotope. The tracer exchange reaction was independent of time for the dicalcium phosphate due to a very small amount of exchangeable phosphate found in the solid and to the high solubility of the material. For the other phosphates, a power function and/or a function composed of a series of exponential terms were found suitable for representing the tracer kinetics. The theoretical background for both functions is shown to be approximately the same. Both describe tracer kinetics for a system containing more than one solid phase. The amounts of exchangeable P were found to depend strongly on the solid/solution ratio but not on the amount of radioactivity added. The dependence of the tracer kinetics on the solid/solution ratio is due to the ageing processes that give rise to the formation of surface layers having physical and chemical properties different from those of the bulk solid.     

THE KINETICS OF HETEROGENEOUS ISOTOPIC EXCHANGE

Isotopic exchange of phosphate ions between soil and solution was examined with particular emphasis on the slower reaction involving the incorporation of the tracer in the solid phase that follows the very fast exchange reaction between the solution and the surface of solid. Experimental data fitted both an equation composed of a sum of exponentials and a much simpler two-constant equation. The goodness of fit was similar in each case but it was concluded that the exponential type of equation was not useful as a means of elucidating the mechanisms involved. The two-constant equation may be derived from the kinetics of crystallization and recrystallization processes, and therefore provides an interpretation which is amenable to experimental verification.     

ADSORPTION OF PHOSPHATE BY GIBBSITE

The effects of the salts were studied using I per cent suspensions of gibbsite at pH 5.5 and 26°C. At 480μM phosphate and I mM calcium, the kinetic pattern was an initial rapid adsorption reaction, which approached equilibrium in 24 h, revealing a slower reaction. Adsorption isotherms determined at 24 h, with a range of final phosphate concentrations from 0.1 to 1000 μ in solution, were not affected by 20 mM sodium or potassium chloride or 10 mM magnesium chloride. Increasing calcium chloride from o to 20 TIM increased the adsorption of phosphate over the range from 1 to 100 μM final phosphate concentration. At lower (0.1 μM) and higher (1000 μM) phosphate, the effect of calcium was smaller. These results help to explain several effects of the various cations on the adsorption of phosphate by clays and soils, when effects of neutral salts on pH are also considered.     

EFFECT OF pH ON PHOSPHATE ADSORPTION AND ISOTOPIC EXCHANGE IN ACID SOILS AT LOW AND HIGH ADDITIONS OF SOLUBLE PHOSPHATE

Measurements of equilibrium phosphate concentrations, adsorbed phosphate and isotopically exchangeable phosphate (E_t) were made on suspensions of acid soils at initial P concentrations ranging from 1–1000 μM and pH values between 4.2 and 6.8. At low P concentrations (1–100 μM), the affinity of the adsorption reaction was greatest at pH 5.2–5.5. This result, which was inconsistent with the ligand-exchange hypothesis of Kingston et al. (1967, 1972, 1974), was interpreted in terms of the formation of basic aluminium phosphates of varying composition that depended on the OH/P ratio in solution. At the highest P concentrations (1000 μM), when the affinity of adsorption was least at pH 5.2–5.5, it appeared that basic calcium phosphates were precipitating at pH values > 5.5. In suspensions containing 1–100 μM P initially, values of E_t did not change consonantly with values of adsorbed P over the pH range 4.8 to 5.5. It is suggested that the E_t values were underestimated due to the presence of labelled complexes in solution which were detected by scintillation counting but not by the analysis for orthophos-phate. Evidence from solution studies (White et al., 1976) suggested that as much as one-third of the phosphate could be complexed with aluminium in solutions between pH 4.7 and 5.4 at P/A1 mole ratios of 0.075 to 7.5. It was apparent that the interpretation of the physical significance of E values in acid soils is complex and requires careful appraisal of the experimental techniques used.     

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.     

ISOTOPIC EXCHANGE OF FIXED AMMONIUM

The isotope ¹⁵N was used to study the rate of exchange between unlabelled NH₄-hydrobiotite and labelled solution NH₄. Most of the isotopic dilution occurred within 10 minutes of adding the solution to the solid, but even after 13 days less mineral-NH₄ had exchanged with isotope than was removed by several extractions with KC1 solution. NH₄ remaining in the mineral after extraction with KC1, conventionally named ‘fixed ammonium’, contained some isotope which was sorbed without release of unlabelled NH₄; the fixed ammonium was, therefore, not isotopically exchangeable. The sorption was shown to result from incomplete ammonium saturation of hydrobiotite, isotope slowly replacing interlayer cations trapped by lattice collapse. The rate of sorption was initially controlled by a temperature-dependent diffusion process for which the activation energies were between 9 and 13 kcal/ mole. In the later stages of the reaction the rate became progressively slower than predicted by the diffusion equation, and analysis of the kinetics suggested that, ultimately, exchange of cations at the sorption sites was the rate-limiting process.     

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.     

不同pH条件下针铁矿表面磷的配位形式及转化特点

应用ＸＰＳ技术研究了不同ｐＨ条件下针铁矿表面磷的配位形式及转化特点。结果表明：磷浓度－定时，低ｐＨ值有利于形成克的单基配位，高ｐＨ值有利于形成磷的双基配位。体系ｐ值升高，针铁矿表面的单基配位磷即较快地向双基配位磷转化；ｐＨ降低，双基配位磷则缓慢地向单基配位磷转化。分析认为，体系ｐＨ通过影响溶液中磷酸根的离子类型来影响针铁矿表面吸附磷的配位形式，ｐＨ变化引起表面磷酸根系子的解离和缔合，是导致针铁矿表     

THE MECHANISM OF PHOSPHATE ADSORPTION BY KAOLINITE, GIBBSITE, AND PSEUDOBOEHMITE

The adsorption isotherms (20°C) of phosphate on two potassium kaolinites and two aluminium oxides have been determined at pH values from 3 to 10 and at concentrations ranging from 10^?4 to 10^?2M. The reversibility of the adsorption with respect to pH and concentration has also been examined. The isotherms result from at least three types of adsorption site (regions I, II, and III) of widely different reactivities. The number of adsorption sites increases to a limit with decrease in pH for regions I and II. The behaviour of region III is more complex. The different adsorbents behave in essentially the same manner and differ only in the number of adsorption sites. It is tentatively suggested that regions I and II are located on an edge –Al(OH)₂ of the adsorbents, while region III results from penetration into some amorphous region of the crystal surface.     

ON THE POSSIBILITY OF CHARACTERIZING CALCIUM PHOSPHATES IN CALCAREOUS SOILS BY ISOTOPIC EXCHANGE

Characterization of calcium phosphates depended upon the nature and the amount of phosphate used to react with reagent-grade CaCO₃. Formation of octa-calcium phosphate (OCP) was inferred from the solubility equilibria after reacting CaCO₃ with KH₂PO₄ solutions. Isotopic exchange measurements confirmed the presence of OCP, when the amount of P retained exceeded 44 μrnoles/g CaCO₃. The determined surface-Ca to surface-P molar ratios were close to the theoretical Ca/P ratio of 1.33 in OCP. As P retained on CaCO₃ decreased the surface Ca/P ratio markedly increased because of interference from surface Ca of the CaCO₃. When CaCO₃ was reacted with monocalcium phosphate (MCP), solubility equilibria indicated the formation of dicalcium phosphate dihydrate (DCPD). Isotopic exchange measurements, however, showed an average Ca/P ratio of only 0.375. This value corresponds to the composition of the metastable triple point solution (MTPS) formed on MCP dissolution rather than to the Ca/P ratio in DCPD. MCP application decreased the measured surface-Ca (exchangeable Ca) either for soil or Ca-resin, because of blocking of the exchange sites by the MCP reaction products and, consequently, a lower rate of isotopic exchange. Surface phosphorus of the two investigated calcareous soils proved to be proportional to the lowering in pH initiated by MCP application. Characterization of MCP reaction products in calcareous soils may thus prove infeasible, in view of the unexpected reduction in surface-Ca and the pH dependency of surface P.     

LIMITATIONS IN ISOTOPIC MEASUREMENTS OF LABILE PHOSPHATE IN SOILS

Methods used for measuring labile P and assessing the phosphate-fertility status of soils by isotopic dilution as well as assumptions involved in applying the isotope dilution technique were critically evaluated. In the presence of high phosphate-fixing compounds, e.g. reactive iron oxide, labile P may be erroneously overestimated; especially when carrier-free ³²P is used. At the end of the isotopic-exchange reaction period, high phosphate-fixing soils may have a higher specific activity in the solid phase than in the solution phase. Methods involving the use of a 0·2 ppm P carrier solution or the addition of an unlabelled carrier to a labelled soil may give satisfactory labile P measurements in low and in medium phosphate-fixing soil. All the methods tried (carrier-free, carrier, and inverse dilution), however, failed to give satisfactory results for the high phosphate-fixing soils.     

玉米叶细胞磷吸收动力学性质

利用32P示踪测定了玉米成熟叶的细胞质膜囊泡对磷酸根离子的吸收。结果表明,磷饥饿胁迫下对磷的吸收能力（Vmax）显著增强 (P < 0.01);在成熟叶片上进行的32P- KH2PO4溶液施加试验显示,叶细胞从质外体中吸收磷与叶片的磷输出相关;在磷饥饿胁迫下,成熟叶片通过木质部输入的磷明显减少,而通过韧皮部输出的磷相对增加。基于上述结果,本研究认为,在缺磷状况下玉米成熟叶的细胞显著增强磷吸收能力,可能有利于从叶片质外体包括木质部中吸收磷,并对增加叶的磷输出具有意义。     

KINETICS OF ION EXCHANGE IN SOIL ORGANIC MATTER

The rate of ion exchange of humic substances (humic acid and peat in the hydrogen form) with lead ions was investigated. As in processes occurring in nature, the lead solution was added in all our experiments slowly and continuously at a constant feed rate to the well-stirred water-suspended humic substances. Control experiments were also performed with a well-defined synthetic ion- exchange resin (carboxylic acid type). The uptake of divalent lead ions by the humic substance and by the ion-exchange resin, which is coupled with the release of an equivalent amount of hydrogen ions, proceeds as a function of time according to a characteristic sigmoidal curve. During the initial phase, the rate of ion exchange increases continuously with time, even though the humic substances become more saturated with lead ions. Theoretical calculations, based on a film diffusion process as the rate-determining step, show that this behaviour should indeed be observed if any solution, containing exchangeable counter ions, is added continuously to an ion exchanger suspended in a solvent. The theory predicts further, in agreement with the experimental results, that in the initial phase of the ion-exchange reaction, the uptake of the counter ions is proportional to the square of the time. If experiments are performed with different rates of addition of the counter ions, the time required to exchange a certain amount of counter ions should be proportional to α^-1/2, where α is the rate at which the ions are added. This prediction is also in accordance with the experimental results.     

KINETICS OF ION EXCHANGE IN SOIL ORGANIC MATTER

The rate of ion exchange reactions of humic substances (humic acid and peat) with lead ions was investigated at 25 °C. All experiments were carried out under finite solution-volume conditions. To the well-stirred aqueous suspensions of the above samples lead ions were added in such small amounts that, after attainment of the equilibrium, the initial equivalent fraction of lead in the humic substance had increased only by a small amount. The kinetics of such differentially small ion exchange processes were studied as a function of the initial lead content of the samples. The experiments showed that the absolute rates for the uptake of small amounts of lead ions decrease, if the initial lead content of the samples increase. The relative rates of ion exchange, i.e. the rates with respect to the extent of each reaction, showed a more complicated behaviour, as they can exhibit a minimum at a certain initial lead content of the sample. The rate-determining step of the above ion exchange experiments was shown, by interruption tests, to be diffusion of the ions through the Nernst-film surrounding the ion exchanger particles. Theoretical calculations of the rates of film-diffusion controlled ion exchange processes were carried out and the results revealed that the rates of any differential ion exchange reaction should decrease with increasing initial content of the ion adsorbed, their exact magnitude depending, among other quantities, strongly on the separation factor. The relative rates for the attainment of the equilibrium, on the other hand, depend much less on the initial ionic composition of the ion exchanger. A comparison of the experimental and the calculated results yields satisfactory agreement if the experimentally determined dependence of the separation factor on the lead content of the samples is taken into account.     

DESORPTION OF PHOSPHATE FROM SOILS USING ANION EXCHANGE RESINS

After 100h reaction with Cl-resin and 300h reaction with HCO₃-resin (approaching equilibrium), the concentration of anions complementary to phosphate was the critical variable affecting the transfer of P from soil to resin. Solution concentrations of H₃0⁺, Ca²⁺ and phosphate indicated that desorption of P by OH^-, and dissolution of Ca phosphates, controlled P release from soils. P extracted by HCO₃-resin was much greater than by Cl-resin from an acid soil, due to lower total anion and higher desorbing anion concentrations, but there was little difference between the two resins with a calcareous soil. HCO₃-resin extracted a constant proportion of isotopically-exchangeable P from different soils whereas Cl-resin did not. Anion exchange resins provide a convenient means for producing P desorption curves for soils.     

THE INTERACTION OF PHOSPHATE WITH AN ANION EXCHANGE RESIN

Phosphate adsorbed on an anion exchange resin was titrated with sodium hydroxide. The inflection point which corresponds to the second pK value of phosphoric acid in solution, was lowered 0.9 pH-unit when the acid was absorbed on the resin. This shift was fully explained by the exclusion of H⁺ from the resin due to a Donnan equilibrium. The adsorption characteristic of the single and double charged phosphate ion on an anion exchange resin was examined at different pH values and phosphate:resin ratios. A model was suggested for the observed adsorption pattern.     

THE ADSORPTION OF POLYVINYL ALCOHOL ON GIBBSITE AND GOETHITE

The neutral polymer, polyvinyl alcohol, has been shown to be adsorbed by the hydrated oxides gibbsite and goethite. A reduction in adsorption on gibbsite is observed when it is dried prior to the adsorption measurements. Evidence from electron microscopy suggests that this is caused by a reduction in the accessible surface area due to aggregate formation, rather than a change in the surface properties of the substrate. Changes in either the pH or the ionic strength of the medium do not significantly alter the amount of PVA adsorbed. Electrophoretic data indicate that the thickness of the adsorbed layer on both clays and oxides is in excess of 10 nm at high concentrations of polymer.     

黄土性土壤在连续液流条件下吸附、解吸磷酸根的动力学研究

采用连续液流法研究了黄土性土壤吸附,解吸磷酸根的动力学性质。结果表明：（１）供试土壤对磷酸根的吸附,解吸扫速率可分为快,中,慢三种反应类型;（２）描述吸附,解吸反应的最优模型均为Ｅｌｏｖｉｃｈ方程,最差模型分别为一级反应方程及双常数方程,拟合差的模型对反应速率变化“敏感”,可用于反应类型划分和机理研究;（３）粘粒含量及代换量对吸附速率有著影响,游离铁对吸附速率,ＣａＣＯ３对较低温度下的吸附及较高温     

THE MICROMORPHOLOGY OF GIBBSITE FORMS IN SOILS

Using optical and scanning electron microscopy various forms of gibbsite are described in some tropical soils. In a lateritic soil from Zaire gibbsite occurs on argillans, in randomly oriented crystal sheets, as nodular aggregates infilling vughs and on ped faces. The mineral also surrounds quartz grains and kaolinite pseudomorphs after biotite. In other soils gibbsite occurs as silt-size particles disseminated through the matrix, as large nodular aggregates, and as anatomizing sheets associated with crystal chambers. It is clear from these observations that alumina must be a mobile constituent and was probably translocated through the soil in solution.     

POTASSIUM-ALUMINIUM EXCHANGE IN ACID SOILS I. KINETICS

A kinetic method is described and investigated for determining exchangeable Al in acid soils using M NH₄Cl solutions the pH and leaching rate of which was critical for obtaining accurate and reproducible values. Molar concentrations of ammonium acetate displaced at least part of the Al chelated to functional groups in the soil organic matter. The adsorption kinetics of K and Al from 10^?3 M chloride solutions on the NH₄-saturated forms at the original soil pH of these, under leaching conditions similar to the ‘extraction’ method, showed that, when the atom ratio K/A1 > 1 in the equilibrating solution, initially more K was adsorbed than at equilibrium, although Al did not ‘over-equilibrate’ when K/A1 < 1. At least 24 h were required for attaining equilibrium in K :A1 equilibria work with soils and clays. The kinetics of desorption (extraction method) and adsorption of K and Al obeyed the first-order and parabolic rate respectively. This is attributed to the large difference in anion concentrations in the two methods.