PO4 Adsorption Research Articles

Adsorption and desorption of phosphate and nitrate ions using quaternary (polypropylene- g- N, N-dimethylamino ethylmethacrylate) graft copolymer

Study has been made on the preparation of functionalized copolymer by grafting N, N-(dimethylaminoethyl) methacrylate (DMAEMA) onto polypropylene (PP) films using γ-irradiation technique. The effect of monomer concentration and irradiation dose on the grafting yield was studied. It was found that grafting yield increases with increasing the monomer concentration and irradiation dose. Subsequent quaternization of the amine group of PDMAEMA graft chain using different reagents, 1-bromo octadecane, methyl iodide and benzyl chloride, to improve their ionic character was carried out. The grafted PP films were characterized by IR spectroscopy, and mechanical and thermal properties. During the adsorption of phosphate (PO 4 3−) and nitrate (NO 3 −) ions, the rate of adsorption, pH effect, desorbability and initial feed concentration effect were investigated. It was found that the adsorption of PO 4 3− and NO 3 − ions increases with increasing the adsorption time up to 10 h and decreases with increasing the pH of medium. The desorbability is ranging from 18 to 30%, and it is relatively difficult to desorb PO 4 3− and NO 3 − ions from the functionalized grafted copolymers. Results showed good possibility for the use of such functionalized grafted film in the field of removal of some anions such as PO 4 3− and NO 3 − from their media.

Geometry, Charge Distribution, and Surface Speciation of Phosphate on Goethite

The surface speciation of phosphate has been evaluated with surface complexation modeling using an interfacial charge distribution (CD) approach based on ion adsorption and ordering of interfacial water. In the CD model, the charge of adsorbed ions is distributed over two electrostatic potentials in the double-layer profile. The CD is related to the structure of the surface complex. A new approach is followed in which the CD values of the various surface complexes have been calculated theoretically from the geometries of the surface complexes. Molecular orbital calculations based on density functional theory (MO/DFT) have been used to optimize the structure of a series of hydrated surface complexes of phosphate. These theoretical CD values are corrected for dipole orientation effects. Data analysis of the PO4 adsorption, applying the independently derived CD coefficients, resolves the presence of two dominant surface species. A nonprotonated bidentate (B) complex is dominant over a broad range of pH values at low loading (< or =1.5 micromol/m(2)). For low pH and high loading, a strong contribution of a singly protonated monodentate (MH or MH-Na) complex is found, which differs from earlier interpretations. For the conditions studied, the doubly protonated bidentate (BH2) and monodentate (MH2) surface complexes and the nonprotonated monodentate (M) complex are not significant contributors. These findings are discussed qualitatively and quantitatively in relation to published experimental in-situ CIR-FTIR data and theoretical MO/DFT-IR information. The relative variation in the peak intensities as a function of pH and loading approximately agrees with the surface speciation calculated with the CD model. The model correctly predicts the proton co-adsorption of phosphate binding on goethite and the shift of the IEP at low phosphate loading (< or =1.5 micromol/m(2)). At higher loading, it deviates.

Abstracts of Nippon Dojo-Hiryogaku Zasshi

Abstracts of Nippon Dojo-Hiryogaku Zasshi

Changes in water-extractability of soil inorganic phosphate induced by chloride and sulfate salts

One of the principal experimental variables which effect the results of phosphorus (P) sorption studies is the ionic composition, in addition to both species and concentrations of the contacting solution. In spite of the realization that ionic species, concentrations and their compositions effect P sorption and/or desorption, most of the salt-related studies are confined to Cl- (anion) in association with different cations. While the knowledge about the comparative response of P to Cl- and SO4(2-) ions was lacking, the current study was conducted to evaluate the comparative effects of anions (in association with cations) on inorganic P release and P fractions in the soil. The test soil was amended with livestock compost manure (OP); KH2PO4 (IPk) or Ca(H2PO4)2 (IPc) at a rate of 1ppm. Soil was subjected to one salt and nine subsequent water extractions and different P fractions were measured. Four salt types, NaCl, Na2SO4, KCl and K2SO4, were used at levels of 0.5 M. Irrespective of P sources, P release was substantially increased in the salt-pretreated soil as compared to the non-saline soil. Sulfate salts released more P in subsequent water extractions than Cl-. Phosphorus release decreased for salt types with Na2SO4 > NaCl > K2SO4 > KCl and for P sources with OP approximately IPk > Control (without P application) > IPc, respectively. No previous study was found to compare the results of more P release by SO4(2-) than Cl- salt. Most of the previous studies focused on anion sorption capacities, but the mechanism for their adsorption is not fully known. Most of the authors suggested that the mechanisms of SO4(2-) and PO4(3-) adsorption are similar, and that both ions compete for the same sorption sites (Kamprath et al. 1956, Couto et al. 1979, Pasricha and Fox 1993). Although adsorbed SO4(2-) does not compete strongly with PO4(3-), there is likely to be some competition for sorption between these anions which may cause comparatively more P release by SO4(2-) than Cl- salts. Higher P release by Na-saturation could be due to the release of P associated with oxide surfaces or due to dissolution of Ca-P phases (Curtin et al. 1987). Study clearly showed that not only cations species differ for P desorption capacity, but associated anions also play a vital role in the fate of P under saline environments. Synergetic effects exist between Na and SO4(2-) ions which enhanced the P release. This study has also confirmed the fact that P from organic sources is available as well as from inorganic P sources. However, P release depends more on the type of P source applied than on total P. It is highly recommended that more than one anion species must be used in the research plans for evaluating the P response in a saline environment. The results have important implications from the point of view of research, as most of the researchers focus on different cations only for evaluating P response to salts from an environmental point of view. However, our study has made it clear that anions in association with cations differed for their effects on P release.

Removal of phosphate from aqueous solutions using calcined metal hydroxides sludge waste generated from electrocoagulation

Removal of heavy metals by electrocoagulation generates substantial amount of electrocoagulated metal hydroxides sludge (EMHS). This paper reports calcined EMHS as an effective adsorbent for removal of PO 4 3− from aqueous solutions. Cumulative % removal of PO 4 3− is investigated in a batch adsorber with different initial adsorbate concentrations, adsorbent dose, pH of the solution and adsorption temperature. Adsorption of PO 4 3− is due to ligand exchange between the OH group on oxide surface and PO 4 3− in the aqueous medium with formation of inner surface complex. Removal of PO 4 3− is found to be high at lower pH values and higher temperatures indicating strong positively charged metal oxide surface at acidic pH (<pH zpc) and the endothermic nature of adsorption. The experimental results preferably fitting the Langmuir isotherm suggest monolayer coverage of adsorbed molecules. Pseudo-second-order kinetic model provides the best fitting to experimental results at different adsorbent dose and adsorption temperature. The magnitude of the activation energy (10.33 kJ/mol) is calculated based on pseudo-second-order rate constants from Arrhenius equation indicates removal of PO 4 3− corresponds to activated chemisorption.

Phosphate Removal from Aqueous Solutions using Neutralised Bauxite Refinery Residues (Bauxsol™)

Environmental Context.Eutrophication of freshwater and marine ecosystems is a global problem, which is frequently linked to high phosphorus concentrations. The present study investigated the use of Bauxsol™, a modified bauxite refinery residue, to remove dissolved phosphate from water, and has shown that it can be used as a cost-effective adsorbent for treating phosphate-contaminated waters. The results provide water and environmental managers with a new technique for decreasing the phosphate loads in water and wastewater. Environmental benefits include improved water quality, minimisation of excessive plant growth, including potentially toxic blue green algae, and the utilisation of an industrial residue for environmental remediation. Abstract.Phosphate (PO43–) removal by Bauxsol™, a neutralised bauxite refinery residue, was investigated as a function of time, pH, ionic strength, adsorbent dosage, competing ions, and initial phosphate concentration. The results of adsorption and desorption studies indicate that adsorption of PO43– by Bauxsol™ is based on a ligand-exchange mechanism, although the low reversibility pH-independent desorption observed in acid-treated Bauxsol™ indicates a dominance of chemisorption. It was shown that PO43– adsorption onto both Bauxsol™ and acid-treated Bauxsol™ followed a Langmuir isotherm model, with adsorption capacities of 0.21 and 0.48 mmol g−1 at pH 9.0 and 5.2 respectively. Adsorption of PO43– by Bauxsol™ increased with decreasing pH, with maximum adsorption efficiencies obtained at pH 5.2 ± 0.1 (the lowest pH investigated), higher Bauxsol™ to initial phosphate concentration ratios, and increased time. Studies of the effects of competing ions on the adsorption of PO43– by Bauxsol™ indicated that adsorption decreased in the presence of HCO3− ions, whereas SO42–and Cl− ions had little effect, and Ca2+ and Mg2+ ions increased adsorption. These findings suggest that Bauxsol™ could be used as an efficient low-cost adsorbent for treating phosphate-contaminated waters.

Adsorption of Dissolved Organic and Inorganic Phosphorus in Soils of a Weathering Chronosequence

Leaching of dissolved organic matter (DOM) and associated nutrients can be a significant loss from developing ecosystems. We studied how the adsorption of dissolved organic P (DOP) and PO4 changes during the development of young andesitic soils and determined which soil characteristics are responsible for these changes. We sampled 77, 255, 616, and about 1200+ yr‐old andesitic soils at the 0‐ to 10‐, 30‐ to 40‐, and 140‐ to 150‐cm soil depths and performed adsorption isotherm analyses that were described using a modified Langmuir (for DOP) or linear equation (for PO4). We also sampled soil solution at the 10‐ to 20‐, 40‐, and 150‐cm soil depths during the main snow melt period in 2001 and 2002 and analyzed it for DOP and PO4 The ability of the soils to adsorb DOP and PO4 increased with soil age. Stepwise multiple regression analyses between the adsorption capacity for DOP, or the slope of the adsorption isotherm for PO4, and several soil parameters showed that allophane concentrations control the adsorption of DOP and PO4 in these soils. Tests of preferential adsorption of DOP vs. PO4 and DOC vs. DOP showed that the adsorption strength increased in the following order: DOC &lt; DOP &lt; PO4 The preferential adsorption of PO4 vs. DOP and DOP vs. DOC increased significantly with increasing soil development, whereas the soil depth did not have a consistent and significant effect. Significant correlations between the null‐point adsorption of DOP or PO4 and field soil solution concentrations indicated that the results obtained in laboratory experiments were applicable to field conditions. Consequently, the tendency of DOP and PO4 to leach from these andesitic soils decreases as soils and ecosystem develop.

XANES Determination of Adsorbed Phosphate Distribution between Ferrihydrite and Boehmite in Mixtures

Iron‐ and Al‐(hydr)oxide minerals are important sorbents for retaining PO4in soils. Our objective was to determine the distribution of adsorbed PO4between ferrihydrite and boehmite in aqueous mixtures of these minerals. Phosphate was adsorbed in aqueous suspensions up to maximum concentrations of 1860, 850, and 1420 mmol kg−1for ferrihydrite, boehmite, and 1:1 (by mass) mixtures of these minerals at pH 6. The solids were analyzed as moist pastes using P K‐XANES (X‐ray absorption near edge structure) spectroscopy. The adsorption isotherm for the mixed‐mineral suspensions could essentially be described as a linear combination of Freundlich isotherm models for each single‐mineral system, indicating negligible mineral interactive effects on PO4adsorption in the mixtures. X‐ray absorption near edge structure spectra for PO4adsorbed on ferrihydrite or in ferrihydrite/boehmite mixtures showed a pre‐edge feature at approximately 2146 eV that was absent in boehmite systems. Linear combination fitting of the pre‐edge region of XANES spectra for mixtures with average spectra for PO4adsorbed on boehmite or ferrihydrite alone, indicated that 59 to 97% of the PO4was adsorbed on ferrihydrite in the mixtures. With increasing concentration of adsorbed PO4in the mineral mixtures, the concentration adsorbed on the ferrihydrite component increased linearly. Phosphate distribution trends in the mixtures suggested an affinity preference for ferrihydrite at the lowest adsorbed PO4concentration (100 mmol kg−1minerals), no affinity preference for either mineral at intermediate concentrations (200 to 600 mmol PO4kg−1), and the possibility of a surface precipitate involving Al at the highest concentration (1300 mmol PO4kg−1).

Adsorption of Dissolved Organic and Inorganic Phosphorus in Soils of a Weathering Chronosequence

Leaching of dissolved organic matter (DOM) and associated nutrients can be a significant loss from developing ecosystems. We studied how the adsorption of dissolved organic P (DOP) and PO4 changes during the development of young andesitic soils and determined which soil characteristics are responsible for these changes. We sampled 77, 255, 616, and about 1200+ yr-old andesitic soils at the 0- to 10-, 30- to 40-, and 140- to 150-cm soil depths and performed adsorption isotherm analyses that were described using a modified Langmuir (for DOP) or linear equation (for PO4). We also sampled soil solution at the 10- to 20-, 40-, and 150-cm soil depths during the main snow melt period in 2001 and 2002 and analyzed it for DOP and PO4 The ability of the soils to adsorb DOP and PO4 increased with soil age. Stepwise multiple regression analyses between the adsorption capacity for DOP, or the slope of the adsorption isotherm for PO4, and several soil parameters showed that allophane concentrations control the adsorption of DOP and PO4 in these soils. Tests of preferential adsorption of DOP vs. PO4 and DOC vs. DOP showed that the adsorption strength increased in the following order: DOC < DOP < PO4 The preferential adsorption of PO4 vs. DOP and DOP vs. DOC increased significantly with increasing soil development, whereas the soil depth did not have a consistent and significant effect. Significant correlations between the null-point adsorption of DOP or PO4 and field soil solution concentrations indicated that the results obtained in laboratory experiments were applicable to field conditions. Consequently, the tendency of DOP and PO4 to leach from these andesitic soils decreases as soils and ecosystem develop.

XANES Determination of Adsorbed Phosphate Distribution between Ferrihydrite and Boehmite in Mixtures

Iron- and Al-(hydr)oxide minerals are important sorbents for retaining PO4 in soils. Our objective was to determine the distribution of adsorbed PO4 between ferrihydrite and boehmite in aqueous mixtures of these minerals. Phosphate was adsorbed in aqueous suspensions up to maximum concentrations of 1860, 850, and 1420 mmol kg−1 for ferrihydrite, boehmite, and 1:1 (by mass) mixtures of these minerals at pH 6. The solids were analyzed as moist pastes using P K-XANES (X-ray absorption near edge structure) spectroscopy. The adsorption isotherm for the mixed-mineral suspensions could essentially be described as a linear combination of Freundlich isotherm models for each single-mineral system, indicating negligible mineral interactive effects on PO4 adsorption in the mixtures. X-ray absorption near edge structure spectra for PO4 adsorbed on ferrihydrite or in ferrihydrite/boehmite mixtures showed a pre-edge feature at approximately 2146 eV that was absent in boehmite systems. Linear combination fitting of the pre-edge region of XANES spectra for mixtures with average spectra for PO4 adsorbed on boehmite or ferrihydrite alone, indicated that 59 to 97% of the PO4 was adsorbed on ferrihydrite in the mixtures. With increasing concentration of adsorbed PO4 in the mineral mixtures, the concentration adsorbed on the ferrihydrite component increased linearly. Phosphate distribution trends in the mixtures suggested an affinity preference for ferrihydrite at the lowest adsorbed PO4 concentration (100 mmol kg−1 minerals), no affinity preference for either mineral at intermediate concentrations (200 to 600 mmol PO4 kg−1), and the possibility of a surface precipitate involving Al at the highest concentration (1300 mmol PO4 kg−1).

Formation of ferrihydrite and associated iron corrosion products in permeable reactive barriers of zero-valent iron.

Ferrihydrite, which is known to form in the presence of oxygen and to be stabilized by the adsorption of Si, PO4 and SO4, is ubiquitous in the fine-grained fractions of permeable reactive barrier (PRB) samples from the U.S. Coast Guard Support Center (Elizabeth City, NC) and the Denver Federal Center (Lakewood, CO) studied by high-resolution transmission electron microscopy and selected area electron diffraction. The concurrent energy-dispersive X-ray data indicate a strong association between ferrihydrite and metals such as Si, Ca, and Cr. Magnetite, green rust 1, aragonite, calcite, mackinawite, greigite and lepidocrocite were also present, indicative of a geochemical environment that is temporally and spatially heterogeneous. Whereas magnetite, which is known to form due to anaerobic Fe0 corrosion, passivates the Fe0 surface, ferrihydrite precipitation occurs away from the immediate Fe0 surface, forming small (<0.1 microm) discrete clusters. Consequently, Fe0-PRBs may remain effective for a longer period of time in slightly oxidized groundwater systems where ferrihydrite formation occurs compared to oxygen-depleted systems where magnetite passivation occurs. The ubiquitous presence of ferrihydrite suggests that the use of Fe0-PRBs may be extended to applications that require contaminant adsorption rather than, or in addition to, redox-promoted contaminant degradation.

Interaction between calcium and phosphate adsorption on goethite.

Quantitatively, little is known about the ion interaction processes that are responsible for the binding of phosphate in soil, water, and sediment, which determine the bioavailability and mobility of phosphate. Studies have shown that metal hydroxides are often responsible for the binding of PO4 in soils and sediments, but the binding behavior of PO4 in these systems often differs significantly from adsorption studies on metal hydroxides in laboratory. The interaction between PO4 and Ca adsorption was studied on goethite because Ca can influence the PO4 adsorption equilibria. Since adsorption interactions are very difficult to discriminate from precipitation reactions, conditions were chosen to prevent precipitation of Ca-PO4 solids. Adsorption experiments of PO4 and Ca, individually and in combination, show a strong interaction between adsorbed Ca and PO4 on goethite for conditions below the saturation index of apatite. It is shown that it is possible to predict the adsorption and interaction of PO4 and Ca on electrostatic arguments using the model parameter values derived from the single-ion systems and without invoking ternary complex formation or precipitation. The model enables the prediction of the Ca-PO4 interaction for environmentally relevant calcium and phosphate concentrations.

Adsorption and Desorption of Phosphate on Calcite and Aragonite in Seawater

The adsorption and desorption of phosphate on calcite and aragonite were investigated as a function of temperature (5–45 °C)and salinity (0–40) in seawater pre-equilibrated with CaCO3. An increase in temperature increased the equilibrium adsorption; whereas an increase in salinity decreased the adsorption. Adsorption measurements made in NaCl were lower than the results in seawater. The higher values in seawater were due to the presence of Mg2+ and Ca2+ ions. The increase was 5 times greater for Ca2+ than Mg2+. The effects ofCa2+ and Mg2+ are diminished with the addition of SO4 2- apparently due to the formation of MgSO4 and CaSO4 complexes in solution and/or SO4 2- adsorption on the surface of CaCO3. The adsorbed Ca2+ and Mg2+ on CaCO3 (at carbonate sites) may act as bridges to PO4 3- ions. The bridging effect of Ca2+is greater than Mg2+ apparently due to the stronger interactions of Ca2+ with PO4 3-. The apparent effect of salinity on the adsorption of PO4 was largely due to changes in the concentration of HCO3 - in the solutions. An increase in the concentration of HCO3 - caused the adsorption of phosphate to decrease, especially at low salinities. The adsorption at the same level of HCO3 - (2 mM) was nearly independent of salinity. All of the adsorption measurements were modeled empirically using a Langmuir-type adsorption isotherm [ [PO4]ad = KmCm[PO4]T/(1 +Km [PO4]T) , ] where [PO4]ad and [PO4]T are the adsorbed and total dissolved phosphate concentrations, respectively. The values of Cm (the maximum monolayer adsorption capacity, (mol/g) and Km (the adsorption equilibrium constant, g/(mol) over the entire temperature (t, °C) and salinity (S) range were fitted to [ Cm = 17.067 + 0.1707t - 0.4693S + 0.0082S2 (σ = 0.7) ] [ ln Km = - 2.412 + 0.0165t - 0.0004St - 0.0008S2 (σ = 0.1) ] These empirical equations reproduce all of our measurements of[PO4]ad up to 14 μmol/g and within ±0.7 μmol/g. The kinetic data showed that the phosphate uptake on carbonate minerals appears to be a multi-step process. Both the adsorption and desorption were quite fast in the first stage (less than 30 min) followed by a much slower process (lasting more than 1 week). Our results indicate that within 24 hours aragonite has a higher sorption capacity than calcite. The differences between calcite and aragonite become smaller with time. Consequently, the mineral composition of the sediments may affect the short-term phosphate adsorption and desorption on calcium carbonate. Up to 80 % of the adsorbed phosphate is released from calcium carbonate over one day. The amount of PO4 left on the CaCO3 is close to the equilibrium adsorption. The release of PO4 from calcite is faster than from aragonite. Measurements with Florida Bay sediments produced results between those for calcite and aragonite. Our results indicate that the calcium carbonate can be both a sink and source of phosphate in natural waters.

Soil Nutrient Availability Assessed by In Situ Ion-exchange Resin Bags as a Predictor of Growth of Swedish Coniferous Forests

Assessment of site fertility is usually based on measurements of the growth of the present stand or the composition of the field vegetation. In this study, the soil nutrient availability, assessed using in situ ion-exchange resin (IER) bags, was tested as a possible predictor of forest production. Ten Scots pine sites and five Norway spruce sites, distributed throughout Sweden, were included in the study. At the pine sites, the amounts of NO3 adsorbed by the IER correlated best to forest production ( pB 0.001), although the abundance of this ion was low. Only a weak correlation with production was found for K+ (pB 0.1). The IER adsorption of PO4 was low at the pine sites in southern Sweden.

Use of dried algae as a food source for zooplankton growth and nutrient release experiments

Zooplankton growth and nutrient recycling are key processes in the operation of pelagic food webs. Most studies investigating these processes rely on complex methods and often require extensive laboratory facilities. Here we introduce a technique for preserving algae by rapid drying for later use in laboratory- or field-based growth and nutrient recycling experiments. Chemostat-grown Scenedesmus acutus was rapidly dried for later experiments evaluating its nutritional composition, suitability for animal growth and potential for use in nutrient release experiments. Reconstituted dried algae had slightly lower nitrogen (N), phosphorus (P) and protein content (% dry weight) than fresh algae, but lipid content did not differ and elemental ratios were in the range considered to indicate favorable food quality. These elemental and biochemical differences did not appear func- tionally important, as Daphnia magna grew identically on fresh and dried food. Freeze-dried S.acutus did not work as an alternative to oven drying as it resulted in 100% mortality. NH4 and PO4 concen- trations did not change over 24 h when dried algae were resuspended in normal media or boiled lake water. However, concentrations of PO4 decreased over 24 h, suggesting chemical adsorption of PO4 to the dried algae and reinforcing the need for animal-free controls in nutrient release experiments using this approach. N and P release rates for D.magna and natural zooplankton communities were estimated using dried algae, and values were comparable to published ones. Thus, dried algae may be a useful, simple technique for studying food quality and nutrient release in environments where maintaining active algal cultures may not be practical and a constant supply of consistent quality food is needed.

Surface Structural Ion Adsorption Modeling of Competitive Binding of Oxyanions by Metal (Hydr)oxides

Spectroscopy has provided a progressive flow of information concerning the binding mechanism(s) of ions and their surface-complex structure. An important challenge in surface complexation models (SCM) is to connect the molecular microscopic reality to macroscopic adsorption phenomena. This is important because SCM alone provide insufficient insight in the binding mechanisms, and moreover, it isa priorinot obvious that SCM, which describe the pH dependent adsorption correctly in simple systems, will predict the ion interaction under multicomponent conditions. This study elucidates the primary factor controlling the adsorption process by analysing the adsorption and competition of PO4, AsO4, and SeO3. We show that the structure of the surface-complex acting in the dominant electrostatic field can be ascertained as the primary controlling adsorption factor. The surface species of arsenate are identical with those of phosphate and the adsorption behavior is very similar. On the basis of the selenite adsorption, we show that the commonly used 2pKmodels are incapable to incorporate in the adsorption modeling the correct bidentate binding mechanism found by spectroscopy. The use of the bidentate mechanism leads to a proton–oxyanion ratio and corresponding pH dependency that are too large. The inappropriate intrinsic charge attribution to the primary surface groups and the condensation of the inner sphere surface complex to a point charge are responsible for this behavior of commonly used 2pKmodels. Both key factors are differently defined in the charge distributed multi site complexation (CD-MUSIC) model and are based in this model on a surface structural approach. The CD-MUSIC model can successfully describe the macroscopic adsorption phenomena using the surface speciation and binding mechanisms as found by spectroscopy. The model is also able to predict the anion competition well. The charge distribution in the interface is in agreement with the observed structure of surface complexes.

Influence of Oxalate and Soil Organic Matter on Sorption and Desorption of Phosphate onto a Spodic Horizon

AbstractPhosphate (PO4) availability limits the productivity of pine plantations growing on Spodosols of the southeastern USA. Oxalate has been shown to interact with both the sorption and desorption PO4 onto soil mineral surfaces. In addition, organic matter, a crucial component of many soil surfaces, affects the adsorption of PO4. We studied the effects of oxalate and organic matter on PO4 sorption and desorption onto the whole soil and clay‐sized fraction of a spodic horizon from a poorly drained Spodosol of the flatwoods region of the lower Coastal Plain of the southeastern USA. Common batch studies and mass balance of OH‐ production and consumption were used to interpret the processes. Maximum reduction in PO4 sorption was observed in samples where organic matter and oxalate were present. The molar ratio of OH‐ ions released to PO4 sorbed supports the idea of a ligand‐exchange mechanism dominating the PO4 sorption process. Some of the sorption sites appear to be common sites for PO4, oxalate, and organic matter. Phosphorus desorption from the spodic horizon by the action of oxalate was through ligand exchange of oxalate for PO4. The presence of soil organic matter increased the amount of PO4 desorbed by oxalate. Oxalate appeared to form stable soluble complexes with Al in solution, thus inhibiting its reprecipitation.

Kinetics of phosphate sorption by goethite‐ and bentonite‐goethite (b‐g) system

The study of phosphate (PO4) fixation due to reactions which occur to the surface of soil minerals has been limited by a lack of evidence on which a structural model may be based. A structural model should attempt at least to describe the coordination on the absorbed PO4 3‐ ion, that is, to indenfity the immediate neighbour atoms or ions in contact with it. Many kinetic equations have been used either to deduce the mechanisms or simply to summarise results of this continued removal of PO4 from solution. In this study five kinetics models of first‐order, Elovich, parabolic diffusion, modified Freundlich and a new kinetic equation, were evaluated to describe the rate of PO4 sorption from synthetic goethite and bentonite‐goethite (b‐g) system into 0.01 M calcium chloride (CaCl2) solutions of 0.0403–0.4435 mmol P L‐1. The rate of PO4 sorption is a dynamic factor that regulates the continuous supply of PO4and was found initially to be rabid but gradually declined with time. The adsorption of PO4 per gram of goethite was faster and greater than the adsorbed amount of PO4per gram of bentonite‐goethite system. Comparison of the coefficients of determination (r) indicates that Elovich, Freundlich, and the new kinetic models provided better fits for phosphate sorption by goethite while First order, Parabolic, and Freundlich were found to be the best for bentonite goethite (b‐g) system.

Ionic Strength Effects on Sulfate and Phosphate Adsorption on γ‐Alumina and Kaolinite: Triple‐Layer Model

AbstractIt is known that PO4 is retained by soils through ligand exchange, i.e., inner sphere complexation, but the mechanism for SO4 adsorption at the mineral‐water interface has been in debate. By studying the effects of ionic strength on ion adsorption, it is possible to distinguish between inner and outer sphere ion surface complexes. This study was conducted to evaluate ionic strength effects on SO4 and PO4 adsorption on γ‐Al2O3 and kaolinite at varying solution pH (3–11), and to infer SO4 and PO4 adsorption mechanisms at the mineral‐water interface. The adsorption of SO4 on γ‐Al2O3 and kaolinite decreased monotonically with increasing solution pH and was markedly reduced by increasing the concentration of background electrolyte. On the other hand, PO4 adsorption on γ‐Al2O3 and kaolinite increased from pH 3 to 4 and decreased from pH 6 to 11, with an adsorption plateau between pH 4 and 6. Effects of change in ionic strength on PO4 adsorption varied with pH. At low pH, PO4 adsorption demonstrated a slight decrease with increasing ionic strength, whereas at high pH, PO4 adsorption increased slightly with increasing ionic strength, resulting in a crossover point where there was no ionic strength effect. The triple‐layer model (TLM) was applied to model the adsorption of SO4 and PO4 with both inner and outer sphere complexes using the FITEQL 3.1 computer program. Sulfate adsorption was better modeled by assuming outer sphere complex formation, while PO4 adsorption was better modeled by assuming inner sphere complex formation.

680 (PS 5) SORPTION ISOTHERMS INDICATE THAT CALCINED CLAY AND ALUMINUM AMENDMENTS INCREASE PHOSPHATE RETENTION OF SOILLESS SUBSTRATES

Soilless container substrate components such as sphagnum peat moss and pine bark have very little PO4 adsorption capacity; thus, one way to reduce PO4 leaching during container production is to increase the PO4 retention of container substrates. Laboratory experiments have been conducted to create sorption isotherms at 26 C for arcillite, a calcined product of the 2:1 clays montmorillonite and illite, and a medium of 70 peat moss: 30 perlite. Thirty ml of Ca(H2PO4)2 solutions at rates of 0, 100, 300, and 500 μg P/ml were added to 5 cm3 samples of the test materials. Both materials were amended with aluminum at rates of 6.4 and 19.2 mg Al/cm3 test material from Al2(SO4)3 solutions. Adsorption isotherms confirmed that unamended medium does not retain PO4 and indicated that arcillite is effective at adsorbing PO4 as it retained 1900 μg P/cm3 test material at the 500 μg P/ml rate. The isotherms also showed that Al amendments increase PO4 retention in both arcillite and the medium. Desorption isotherms suggested that the unamended arcillite and Al-amended medium and arcillite will continue to release low levels of PO4 for an extended period of time.