Organic phosphate sorption by neutral and basic soils

Abstract

The sorption of several organic phosphates were measured on soil, peat and other organic and inorganic materials. The behavior of the phosphates on these materials differed from that reported for similar studies on acidic soil materials. In contrast to the sorption on acidic soils, where sorption maxima occur, the sorption on basic soils may reach a maximum but does not decline thereafter. Both clay and organic matter content govern the amount of sorption but calcium seems to account for the differences observed from those on acidic soil materials     

Modelling the effects of pH on phosphate sorption by soils

Samples of six soils were incubated at 60°C for 24 h with several levels of either calcium carbonate or hydrochloric acid. Phosphate sorption was then measured on sub-samples of the treated soils over 24 h at 25°C. In one set of measurements on all soils, 0.01 M calcium chloride was used as the background electrolyte. In another set, on two soils, 0.01 M sodium chloride was used. An interpolation method was used to give points on the three-dimensional surface relating the final pH of the suspensions to sorption of phosphate at specified solution concentrations of phosphate. The effects of pH on phosphate sorption differed between soils. For unfertilized soils, increases in pH up to about pH 5.5 decreased sorption. Further increases in pH decreased sorption further in one soil and increased it in three others. For fertilized soils, measured sorption increased with pH. When sodium chloride was used instead of calcium chloride, there was a more marked trend for sorption to decrease as pH increased. Differences between the soils were ascribed to differences in two soil properties. One was the rate at which the electrostatic potential in the plane of adsorption decreased as pH increased. Only small differences in the rate of change of potential were needed to reproduce the observed differences between soils. The electrostatic potential would decrease more quickly in solutions of a sodium salt than in solutions of a calcium salt and this explains the observed differences between these media. The other soil property that affected observed sorption was the release of phosphate from the soil. The amount released was largest at low pH. Consequently, for fertilized soils, measured sorption increased with pH.     

Effect of liming on phosphate sorption by acid soils

The sorption of phosphate (P) by four strongly acid Fijian soils from 0.01 M CaCl₂ decreased with increasing pH up to pH 5.5–6.0 and then increased again. The initial decrease in P sorption with increasing pH appears to result from an interaction between added P, negative charge, and the electrostatic potential in the plane of sorption. The results of a sorption study, involving KCl or CaCl₂ of varying concentrations as the background electrolyte and using Nadroloulou soil incubated with KOH or Ca(OH)₂, suggested that the increase in P sorption at pH values > 6.0 was caused by the formation of insoluble Ca-P compounds. For some soils this is consistent with the results of an isotopic-exchange study in which incubation with lime caused marked reductions in the amounts of exchangeable P at high pH.     

Effects of equilibrating salt solutions on phosphate sorption by soils

Abstract

Several equilibrating salt solutions have been used in the studies of P sorption by soils and sediments. This study was conducted to evaluate the effects of 10 salt solutions on estimation of P sorption by soils. Results obtained showed that, when the equilibrating solution was made to contain 0.01M with respect to CaCl₂, Ca(NO₃)₂, CaSO₄, MgCl₂, KCl, LiCl, Nacl, or KHCO₃, the amount of P sorbed by soil always exceeded the amount sorbed from the soil‐water system. In comparison with the amount of P sorbed from water, 0.01M NaHCO₃ reduced P sorption by soils. Use of THAM buffer (0.05M pH 7.0) to control the pH increased P sorption by some soils and decreased P sorption by others, relative to that sorbed from the soil‐water system. The results indicated that inclusion of salts in the equilibrating solution for P‐sorption studies should be avoided, especially in studies related to water quality.     

Effects of soil organic matter on pH-dependent phosphate sorption by soils

Effects of soil organic matter (80M) on P sorption of soils still remain to be clarified because contradictory results have been reported in the literature. In the present study, pH-dependent P sorption on an allophanic Andisol and an alluvial soil was compared with that on hydrogen peroxide (H₂0₂)-treated, acid-oxalate (OX)-treated, and dithionite-citrate- bicarbonate (DCB)-treated soils. Removal of 80M increased or decreased P sorption depending on the equilibrium pH values and soil types. In the H₂O₂ OX-, and DCB-treated soils, P sorption was pH-dependent, but this trend was not conspicuous in the untreated soils. It is likely that 80M affects P sorption of soils through three factors, competitive sorption, inhibition of polymerization and crystallization of metals such as AI and Fe, and flexible structure of metal-80M complexes. As a result, the number of available sites for P sorption would remain relatively constant in the wide range of equilibrium pH values in the presence of 80M. The P sorption characteristics were analyzed at constant equilibrium pH values (4.0 to 7.0) using the Langmuir equation as a local isotherm. The maximum number of available sites for P sorption (Q _max) was pH-dependent in the H₂0₂-, OX-, and DCBtreated soils, while this trend was not conspicuous in the untreated soils. Affinity constants related to binding strength (K) were less affected by the equilibrium pH values, soil types, and soil treatments, and were almost constant (log K ≈ 4.5). These findings support the hypothesis that 80M plays a role in keeping the number of available sites for P sorption relatively constant but does not affect the P sorption affinity. By estimating the Q _max and K values as a function of equilibrium pH values, pH-dependent P sorption was well simulated with four or two adjustable parameters. This empirical model could be useful and convenient for a rough estimation of the pH-dependent P sorption of soils.     

Effect of iron oxide on phosphate sorption by calcite and calcareous soils

Pure calcite (AR grade CaCO₃) was treated with ferrous perchlorate solution to give a surface coating of iron (Fe) oxide. Maximum sorption (x_m) of phosphate (P) by the calcite increased from 18.2 to 160 mg P kg^?1 as the amount of coating increased from 0.00 to 16.0 g Fe₂O₃, kg^?1 CaCO₃. Evidence for Fe oxide coatings on carbonate minerals in two Sudanese soils was obtained by optical microscopy and electron-probe microanalysis. The relative contributions of carbonate and Fe oxide minerals, and Fe oxide coatings to P sorption in these soils were calculated, based on an assumed model of oxide distribution. Separate-phase Fe oxide was the major contributor (30–40%) to P sorption in the soils; the Fe oxide coatings on carbonate minerals were only minor contributors (< 6%), and the contribution of uncoated carbonate minerals was found to be negligible (<1 %). These results suggest a very minor role for carbonate minerals, even when coated with Fe oxide, in the sorption of P by these calcareous, Sudanese soils.     

On the limitations of the simplified Elovich equation in describing the kinetics of phosphate sorption and release from soils

The kinetics of phosphate sorption and release from soils are often described by a simplified form of the Elovich equation. However, examination of data obtained with three soils from Greece as well as of published data, based on a modified version of the Elovich equation, showed that the assumptions underlying derivation of the simplified form are not valid in all cases. This modified version is free from any a priori assumptions, describes the kinetics of phosphate reactions with soils in a realistic manner and points out to the existence of more than one stage in the process.     

An evaluation of methods for characterizing phosphate sorption in rice soils

Abstract

Two single value methods of indexing P sorption were compared to parameters derived from P adsorption isotherms in a range of waterlogged and dry rice soils. These measures of P sorption capacity correlated closely with one another. However, they did not correlate equally with several other soil properties (pH, extractable Fe and Al). Because relationships between P sorption and soil properties proved to be method dependent, results from different methods must be compared with caution.

It is suggested that the single value method of Bache and Williams² be adopted when the rapid analyses of many samples is required. It is suited to routine analysis of soils with markedly different P sorption capacities, from both waterlogged and dry conditions.

Close correlations between the single value methods and P sorbed at a standard supernatant concentration suggest they may be useful in testing soils for P fertilizer requirements.     

Sulphate sorption by variable charge soils

The sorption of sulphate (SO^2?₄) by three variable charge soils from the Canary Islands (Spain) was studied. Sulphate sorption decreased with increasing pH. Only negligible amounts of SO^2?₄ were sorbed above pH 6.5. When the soils were washed with an indifferent electrolyte (0.01 M KCl), more SO^2?₄ was recovered than had been sorbed. This indicated a release of native SO^2?₄ Sulphate replaced hydroxyl ions (OH) and co-ordinated H₂O molecules, as well as very small amounts of silicate (Si). No measurable amount of phosphate (P) was released. On average hydroxyl release accounted for 50% of SO^2?₄ sorbed, the rest being accounted for by the increase in negative charge as measured by K⁺ adsorption. The results presented here are consistent with the sorption of SO^2?₄ through a ligand exchange mechanism, but in a different plane of sorption to that of phosphate.     

Effect of heating on phosphate sorption and availability in some north-east Nigerian soils

Studies on phosphate sorption and availability in some north-east Nigerian soils showed that phosphate adsorption and inorganic-P concentrations increased considerably after the soil was heated either in a furnace or during the field-burning of straw. The increase in phosphate adsorption after heating was thought to be caused by an increase in free Fe and A1 oxides, whereas the high contents of exchangeable Ca and possibly carbonates and hydroxyl ions in the ash were probably responsible for the increase in phosphate sorption after field-burning of straw. Investigations into the availability of P to maize over a 7-d period of growth showed that there was no significant nutritional benefit from the P released after soil heating. The effect of heating was to increase P sorption and so reduce P in solution and P availability.     

土壤组分对广东省酸性水稻土磷吸附参数的影响

Soil components affecting phosphate sorption parameters were studied using acid paddy soils derived from basalt, granite, sand-shale and the Pearl River Delta sediments, respectively, in Guangdong Province.For each soil, seven 2.50 g subsamples were equilibrated with 50 mL 0.02 mol L-1 (pH=7.0) of KCl containing 0, 5, 10, 15, 25, 50 and 100 ng P kg-1, respectively, in order to derive P sorption parameters (P sorption maximum, P sorption intensity factor and maximum buffer capacity) by Langmuir isotherm equation. It was shown that the main soil components influencing phosphate sorption maximum (Xm) included soil clay, pH,amorphous iron oxide (Feo) and amorphous aluminum oxide (Alo), with their effects in the order of Alo ＞Feo ＞ pH ＞ clay. Among these components, pH had a negative effect, and the others had a positive effect.Organic matter (OM) was the only soil component influencing P sorption intensity factor (K). The main components influencing maximum phosphate buffer capacity (MBC) consisted of soil clay, OM, pH, Feo and Alo, with their effects in the order of Alo ＞ OM ＞ pH ＞ Feo ＞ clay. Path analysis indicated that among the components with positive effects on maximum phosphate buffer capacity (MBC), the effect was in the order of Alo ＞ Feo ＞ Clay, while among the components with negative effects, OM ＞ pH. OM played an important role in mobilizing phosphate in acid paddy soils mainly through decreasing the sorption intensity of phosphate by soil particles.     

Phosphorus fertilizer requirements as estimated by phosphate sorption

Abstract

Twenty‐nine soil samples representing 12 soil series and varying levels of P fertility were obtained from cooperating agricultural experiment stations in the southern U.S. Fertilizer P requirements were estimated from P sorption curves and correlated with P requirements established by field experiments. Estimated and measured P requirements were highly correlated (r =0.93**) with the quantitative relationship being nearly 1:1. With one exception, P sorption also accurately estimated needs of fertilized soils; in that one case, fertilizer needs were estimated at 45 kg P/ha although there was no response to P in the field.

The quantity of sorbed P in equilibrium with 0.02 μg P/ml apparently was a good estimate of fertilizer P requirement. The value 0.02 was somewhat lower than has been reported from other research; this lower value may be associated with somewhat lower yields, warm soil temperatures, and relatively long growing season in the southern U.S.A. It also may have been caused by under‐evaluating the effectiveness of residual fertilizer P or by a more stable P concentration in soil solution for soils receiving long‐term applications of fertilizer P.     

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).     

Phosphate sorption by red mediterranean soils from Greece

Abstract

In nineteen surface horizons of red Mediterranean soils from various locations of Greece, phosphorus (P) sorption experiments were conducted and the sorption characteristics were studied in relation to soil properties. Phosphate sorption data were fitted both to the Langmuir and Freundlich equations. From these equations, the following P sorption parameters were determined from the Freundlich equation, X = ACⁿ, the parameters A (the phosphate sorbed at C = 1 mg P/L), n (the P sorption intensity), the P sorption index (PS = X/log C) and maximum P sorption (Xmfr). From the Langmuir equation, C/X = 1/KXm + C/Xm, the parameters K (showing the bonding energy), maximum P sorption (Xmla), the quantity of P adsorbed at a standard concentration of 0.2 mg P/L (P0.2), and P maximum buffering capacity (PMBC). The Freundlich parameter A was strongly correlated to the clay and sesquioxides ("free”; iron and aluminum oxides and amorphous iron oxides) content. Seventy‐four percent of the variance of this parameter was explained by clay and “free”; iron (Fe) content. The Freundlich parameter n was significantly correlated with pH and amorphous iron oxides content, while 52% of its variance was explained by amorphous Fe and dithionite extrac‐table aluminum (Al). The P sorption maxima calculated from the Freundlich equation were in general lower than those calculated by the Langmuir equation. Both these parameters were strongly correlated with clay and more slightly with sesquioxides content. About 50% of their variance was explained by clay content of the soils. The P sorption index was strongly correlated with the clay content and less strongly with dithionite‐extractable Fe and Al. The P‐buffering capacity calculated from the data of Langmuir equation was also strongly correlated with these two parameters. In addition, clay content and dithionite‐extractable Fe and Al were well correlated to the amounts of P required to obtain an equilibrium concentration of 0.2 mg P/L while 61% of the variation of this parameter was explained by the clay and the dithionite‐extractable Fe content. From these findings, it seems that for the red Mediterranean soils from Greece, P sorption is affected by clay content and iron and aluminum oxide contents.     

Specific sorption of trace amounts of cadmium by soils

Abstract

Sorption of trace quantities of Cd in four soils of different chemical and mineralogical properties, was studied. Initial Cd concentrations were between 15 to 150 μg. 1^?1. The sorption isotherms were linear and had a positive intercept in three of the soils, indicating a constant partition‐high affinity sorption isotherm (Giles et. al⁶). The data also followed the Freundlich sorption isotherm, and the Freundlich K parameter was taken as a measure of the relative affinity of the different soils for the Cd metal sorbed. Cadmium sorbed was extracted by IN‐NH₄C1 followed by 0.1N HC1, and the fraction remaining in the soils was considered specifically sorbed Cd. This fraction also followed a linear sorption isotherm, and was around 30% for the four soils studied. The sorption order for the amount of specifically sorbed Cd showed that the Boomer soil (kaolinite‐iron oxides) had the lowest affinity for specific sorption of this metal. This was taken as evidence that kaolinite and iron oxides have a lower capacity for retaining cadmium through specific sorption mechanism(s) than the materials present on the other soils (2:1 layer silicates and humic substances). The existence of specific mecha‐nism(s) responsible by the sorption of trace quantities of Cd in soil solutions has important implications on soil‐plant relationships, Cd mobility in soil profiles and control of Cd activity in soil solutions.     

Inorganic anion sorption and interactions with phosphate sorption by hydrous ferric oxide gel

The sorption of inorganic anions by hydrous ferric oxide gel (Fe gel) from 10 ^?1 M NaClO₄ at pH 6.5 decreased in the order: orthophosphate (H₂PO₄)>arsenate (H₂AsO₄) = selenite (HseO3) > silicate (H₄SiO₄) > molybdate (MoO²₄^?) > sulphate (SO²₄^?) > selenate(SeO²₄^?)>chloride (Cl^?) = nitrate (NO^?₃). When each anion was added to Fe gel with an equimolar quantity of H₂PO^?₄, there was no detectable effect of SO²₄^?, SeO²₄^?, Cl^?, and NO^?₃ on the amount of H₂PO^?₄ sorbed. Other anions depressed H₂PO₄ sorption in the order H₂AsO₄ >HseO₃ > H₄SiO₄ > MoO²₄. At the lowest level of anion addition (500 mmol kg ^?1), H₂PO₄ sorption was depressed by no more than 6% of the sorption level in the absence of a competing anion. In contrast, at the highest level of anion addition (1700 mmol kg-¹ of each), H₂AsO₄ decreased H₂PO₄ sorption by 44%. The sorption of SO₄^? was completely eliminated when this anion was added with equimolar amounts of H₂PO₄. The ability of anions to compete with H₂PO₄ for sorption sites could not be explained solely by the results obtained for the sorption of each anion alone. Thus, H₂AsO₄ was more competitive than H₂PO₄ when added together, even though more H₂PO₄ than H₂AsO₄ was sorbed when each anion was added alone. Although H₂PO₄ was sorbed in larger amounts, there is no evidence to suggest that H₂PO₄. H₂AsO₄, and HseO₃ were sorbed on different sites.     

Influence of phosphate on cadmium sorption by calcium carbonate

Laboratory investigation was conducted to investigate the mechanism of cadmium sorption by pure calcium carbonate and to examine whether, reaction products of phosphate with calcium carbonate serve as a sink for sorption of toxic heavy metal cations like cadmium. The phosphatization of calcium carbonate (G.R) was carried out by treating it with orthophosphoric acid in Ca:P ratio of 5:3, 3:2, 4:3 and 1:1, representing the Ca–P ratios of carbonate apatite, tricalcium phosphate, octacalcium phosphate and dicalcium phosphate and then equilibrated for two months. The X-ray diffraction of the phosphate reaction products revealed that calcite along with monetite, brushite and carbonate apatite were present in each case. The cadmium was effectively retained on CaCO₃ by the mechanism of chemisorption at lower Cd²⁺ concentrations as the pH of the equilibrium system remained constant (8.6) up to initial Cd²⁺ concentrations of 10⁻⁴ mol, coinciding to 100% sorption of Cd²⁺ from the solution. At higher concentrations, precipitation of CdCO₃ on CaCO₃ surface or as a separate phase predominated. Enrichment of CaCO₃ with P reduced the Cd-sorption. The chemisorption of Cd probably involved the exchange of Ca²⁺ by Cd²⁺ from CaCO₃ surface. The Cd²⁺ concentration in equilibrium solution followed the solubility diagram of CdCO₃ at pH ≤ 8.6, whereas the concentration of Ca²⁺ started deviating from the solubility line at pH ≤ 8.2.     

The specific surface of soils determined by water sorption

The hypothesis that the specific surface of soil can be measured by water sorption is tested with data for 62 subsoils of widely differing origins. Ethylene glycol and water sorption at p/p₀=0.47 are found to be very closely related measurements and both are highly correlated with CEC. Both methods give a satisfactory measure of total specific surface for soils classed as smectitic and having a large CEC. However, the application of the multilayer theory to the sorption of water on external surfaces of clayey soils with small CEC suggests that both sorbates overestimate the specific surface of such soils. A better estimate of the errors would be obtained from isotherm measurements with water, which is more suitable for this purpose than ethylene glycol.     

Adsorption and desorption of phosphate by soils

Abstract

Added P adsorbed, expressed as a percentage of total added P, was closely and inversely related to added P subsequently desorbed, expressed as a percentage of added P adsorbed. This relationship was not linear but followed a hyperbola‐like curve. For the limiting cases where adsorption was 0% and 100%, desorption was 100% and 0%, respectively. If desorption is the mechanism limiting the release of P into the soil solution for plant utilization, then the well‐established relationship between P adsorbed in the laboratory and the recovery of fertilizer P under field conditions is accounted for.