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.     

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.     

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.     

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.     

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.     

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.     

THERMODYNAMICS OF THE EXCHANGE OF NICOTINE ON ALUMINIUM-MONTMORILLONITE

The ion exchange equilibria involving the interaction of nicotine with Almontmorillonite was studied thermodynamically. The exchange isotherms at 30° and 60°C indicated a preference for Al ions by montmorillonite as compared with nicotinium ions. A separation factor supported the conclusion. The enthalpy gain indicated tighter binding of Al. The entropy gain indicated a diffused and disordered arrangement of nicotinium ions in the Goüy layer with Al forming a more ordered arrangement in the Stern layer.     

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.     

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.     

INFLUENCE OF ADSORBED SODIUM AND GYPSUM CONTENT ON PERMEABILITY OF GLACIAL TILL SOILS

The permeability of glacial till soils having mainly montmorillonite in the clay fraction was negligible when the exchangeable sodium percentage (ESP) exceeded 15 to 20. Relatively higher permeability values were obtained in soils containing gypsum, but water transmission through such soils was negligible above ESP of 30 to 35. The relative decrease in permeability with increase in exchangeable sodium depends not only on the ESP and electrolyte concentration in the soil solution but is affected by the types of clay minerals.     

THERMODYNAMICS OF CATION EXCHANGE ON CLAY: Ca-K-MONTMORILLONITE

Radioactive elution and batch equilibration techniques have been used to obtain isotherms at 30° C, 50°C, and 70° C for the exchange of Ca for K on K-montmorillonite. From these the changes in enthalpy and entropy were + 3.87 kcal/mole and +6.7 e.u./mole of Ca respectively. The activity coefficient for K decreased from approximately 1.5 to 1 and for Ca from 1 to approximately 0.5 as the fraction of K on the clay increased from 0 to 1. The results are discussed especially in relation to hydration of the ions and adsorption of water on the clay. The entropy of adsorbed water is inferred, from the results, to be less than that for liquid water.     

ADSORPTION OF SELENITE, PHOSPHATE AND SULPHATE ON HYDROUS ALUMINA

Selenite was adsorbed on a positively charged hydrous alumina at a solution pH of 5.0, and the OH ^? released and changes in the surface charge were measured. The adsorption isotherm levelled off at high concentrations suggesting a definite adsorption maximum. The OH^? released yielded a curvilinear relationship of increasing slope with the selenite adsorbed. The positive charge on the alumina surface was neutralized by the adsorbed selenite and the net charge became close to zero as the adsorption of selenite was near maximum. The molar ratio of the ligands (OH^?+ OH₂) displaced over selenite adsorbed was more than one at surface saturations of <0.5, but it was approaching one with increasing adsorption of selenite. The results are explained in terms of preferential adsorption of divalent (SeO²₃^?) and monovalent (HSeO^?₃ selenite on a highly positively and less positively charged surface respectively, in agreement with the VSC^? VSP model of Bowden et al. (1973; 1977). The supply of SeO²₃^? for adsorption is considered to be favoured by an expected lower H activity in solution adjacent to the positively charged surface than that in the bulk solution. A general discussion is given on the ligand exchange adsorption of selenite, phosphate and sulphate on hydrous alumina.     

RELATIVE EFFICIENCY OF DIAMMONIUM PHOSPHATE AND MUSSOORIE ROCK PHOSPHATE PLUS PHOSPHATE SOLUBILIZING BACTERIA ON PRODUCTIVITY AND PHOSPHORUS BALANCE IN RICE-POTATO-MUNGBEAN CROPPING SYSTEM

Field experiments were conducted at the Indian Agricultural Research Institute, New Delhi, India for three years from 2001–2002 to 2003–2004 to study the relative efficiency of diammonium phosphate (DAP) and mussoorie rock phosphate (MRP) in a rice-potato-mungbean cropping system. Phosphorus application significantly increased productivity, protein yield and energy output of rice-potato-mungbean cropping system and resulted in an increase in 0.5 M sodium bicarbonate (NaHCO₃) extractable phosphorus (P) content in soil. The MRP at 35 kg P ha^?1 was at par with 17.5 kg P ha^?1 as DAP in terms of productivity, protein yield, and energy output but significantly superior in terms of PSB population in soil. Phosphorus balance (application – crop removal) was generally more positive for MRP than DAP and the highest with an application of 52.5 kg P ha^?1 as MRP. Present study indicates that P requirement of a rice-potato-mungbean cropping system can be met with 76–79% higher dose of MRP as compared to DAP.     

THE EFFECT OF PARTICLE MORPHOLOGY ON THE K-Ca EXCHANGE PROPERTIES OF MONTMORILLONITE

Adsorption isotherms for K on to Ca montmorillonite (< 2 μm and < 0.1 μm. e.s.d. fractions) at ionic strengths from 3 × 10^?2M to 3 × 10^?5M have been analysed by the methods of Van Bladel and Laudelout (1967) and show that negatively charged sites in the lamellae of both size fractions are the same but when these lamellae are intermeshed in crystals, potassium is favoured, probably in wedge sites between adjacent lamellae. The effectiveness of these wedge sites is lost at zero ionic strength.     

EFFECTS OF SOLUTION: SOIL RATIO ON PHOSPHATE SORPTION BY SOILS

Isotherms for the sorption of inorganic phosphate (P) by three contrasting soils during 40 h showed a dependence on solution: soil ratio. Above a final solution P concentration of 0.5 μg ml^?1, more P was sorbed at a solution: soil ratio of 5:1 than at 40:1 for a given level of P in solution. With time up to 146 h, the effect of solution: soil ratio on P sorption was kinetically controlled. Equilibrium solution P concentrations, estimated by extrapolation of the linear relationships between solution P concentration and the reciprocal of time to I/t= o (i.e. t=∞)appeared to be coincident for each solution:soil ratio at high and the low levels of added P. Consequently, sorption isotherms at equilibrium would be coincident, irrespective of the solution soil ratio used. The kinetic control of solution: soil ratio on P sorption is interpreted in terms of the number of P sorbing sites and initial solution P concentration on the rate of P sorption by soils.     

EFFECTS OF CATION EXCHANGE MATERIAL ON ZINC ADSORPTION BY SOILS

The adsorption of Zn by soils which are different in their major cation-exchange materials was measured at equilibrium Zn concentrations up to 10^?2 M in 10^?2 to 10^?3 M CaCl₂. The results are interpreted on K^Zn_Ca[Zn]_soil plots, where K^Zn_Ca is the selectivity coefficient defined by the equation All natural samples except those containing halloysite exhibited no or very small specific Zn adsorption. All Ca-saturated samples exhibited specific Zn adsorption dependent on cation-exchange materials. The cation-exchange sites with high selectivities for Zn (K^Zn_Ca > 10) constitute more than 40 per cent of the total exchange sites in soils containing allophane, imogolite, and halloysite, whereas those with moderate to low selectivities for Zn (K^Za_Ca < 10) predominate in montmorillonitic, vermiculitic, and humic soils. Differences in the contribution of the respective cation-exchange materials to specific Zn adsorption are discussed relating to differences in the origin of their negative charge.     

EFFECTS OF IONIC STRENGTH AND NATURE OF THE CATION ON DESORPTION OF PHOSPHATE FROM SOIL

The effects of cations on desorption of phosphate previously added to soil were studied by mixing phosphated soil with solutions of chloride salts at a range of solutionitoil ratios and for periods which ranged from 1 to 96 h. Phosphate desorbed was then related to the experimental variables by a pair of simultaneous equations. In calcium chloride, the rate of desorption of phosphate was inversely proportional to the calcium concentration. Desorption was faster in 0.01 M magnesium chloride than in 0:01 M calcium chloride, and faster in 0.03 M sodium chloride than in either magnesium or calcium chloride. Addition of a further supply of the cation on an exchange resin increased the rate for both sodium and magnesium but decreased it for calcium. A range of monovalent cations formed a sequence from fastest to slowest of: Li⁺ > Na⁺ > NH₄^?> K ⁺, Rb ⁺ > Cs ⁺. The identity and concentration of the cations had a large effect on the concentration of phosphate when the solution: soil ratio was small. There were also large effects in the amount of phosphate desorbed when the solution: soil ratio was large and the concentration of phosphate approached zero. This suggested that the escaping tendency of the phosphate was decreased when the cations which balanced the negative charge on the adsorbed phosphate were close to the surface.