Surface loading effect on Cd and Zn sorption by kaolinite and montmorillonite from low concentration solutions

Studies of Cd and Zn sorption using Na-saturated kaolinite and montmorillonite, and low metal solution concentrations similar to those found in the environment, showed that metal sorption affinity (measured by K _d values) decreased markedly with increasing surface metal loading for both layer silicates. For equilibrium solution concentrations <0.1 μmol L^?1 for Cd, and < 1 μmol L^?1 for Zn, both metals were sorbed with greater affinity by kaolinite than montmorillonite. These results were probably due to the higher proportion of weakly acidic edge sites present on kaolinite surfaces. In the case of Zn there was an affinity reversal for equilibrium solution concentrations > 1 μmol L^?1, which was attributed to the permanent charge sites of montmorillonite. Cadmium ions were sorbed, by kaolinite, with greater affinity than Zn for equilibrium solution concentrations between 0.3 to 1.5 μmol L^?1. This result was attributed to retention of these metal ions through electrostatic attraction by permanent charge sites present on the kaolinite used in this work. According of these results it seems that metal sorption by these layer silicates involves predominantly edge weak acid sites at lower surface coverages (higher affinity sites), and permanent charge sites at higher metal coverages (lower affinity sites). It was concluded that Cd and Zn sorption by those two layer silicates is greatly influenced by surface metal coverage, and results cannot be extrapolated from low to high surface coverages, and viceversa.     

Interaction of clays with dilute fluoride solutions

Interaction between dilute (mg L^?1) NaF solutions and clay suspensions (0.08 % w/v) has been examined as a function of pH (range 3 to 8), clay type (Na⁺- or Ca²⁺-kaolinite, illite, montmorillonite) and NaF concentration. No F loss from solution was detected at pH > 6.5, while enhanced uptake was found on decreasing the pH, especially in the 4 to 3 region. Removal of F from 1 to 6 × 10^?4 M NaF was only slightly dependent on weight of solid, but did increase with [F^?]. It is proposed that F losses are due to the formation of sparingly soluble Al species (e.g. cryolite, Na fluoro silicate), occasionally augmented with CaF₂ formation (Ca²⁺-clays). The Al is released by proton attack on the lattice, following conversion of the suspended solids into the unstable H⁺-form, either through acid addition (pH < 5) or through hydrolysis of the Na⁺-form material. The latter process was most pronounced with the illite and montmorillonite samples. The amount of F fixed by montmorillonite was roughly double that held by the other two clays, and had a maximum value (pH 3) of ～ 4 mg g^?1, using 11 mg L^?1 NaF solutions. Soluble fluoro-complexes, similar in quantity to the retained F, were detected, in many of the studies. It was concluded that contact of the clay components of soils or sediments with mg L^?1 levels of F in adjacent aqueous phases would result in only a minor proportion being retained.     

重金属污染对典型湿润富铁土上钾素行为的影响

Difference of montmorillonite(Mt),illite(It) and kaolinite(Kt) in lead sorption characteristics and the effects of amorphous Fe and Al oxide coatings on the characteristics were experimentally sutdied with logistic model.The sorption curves had sigmoid feature due to use of acetate-type buffer solution.With the model the sorption process could be divided into four stages and the sorption characteristics at the stages were discussed.The results showed that,after Mt,It and Kt were coated by amorphous Fe oxide,their maximum sorption capacity(MSC) and percentage of high-SSC concentration scope(HCS) of Pb^2 increased markedly,but the specific sorption capacity(SSC) decreased.with regard to effects of amorphous Al oxide coating,except for It Al,the SSC of other samples showed a downtrend,despite that their MSC remained unchanged.Eventually,the gray correlation degrees to Pb^2 sorption for different physicochemical characteristics of the clay minerals were indicated to be higher for hydronium,zero point of surface charge and hydroxy,but lower for specific surface area.density of surface charge and amount of surface charges.     

SORPTION OF INORGANIC PHOSPHATE BY IRON- AND ALUMINIUM- CONTAINING COMPONENTS

The amounts of inorganic P sorbed by a range of Fe- and Al- containing components varied appreciably and decreased in the order allophane > fresh Al gel > Fe gel pseudoboehmite > aged Al gel > dried Fe gel > Fe-coated kaolinite > haematite > goethite > akaganeite > gibbsite = ground kaolinite > dispersed kaolinite. Al gel sorbed 30 to 70 times more P than gibbsite, and Fe gel sorbed approximately 10 times more P than its crystalline analogues (haematite, goethite. and akaganeite). Despite large differences in the extent of P sorption, the form of the isotherm was essentially the same for each sorbent. The ability of freshly-prepared Al gel suspensions to sorb P decreased with ageing, a property not shown by Fe gel. Drying of Fe gel at 80°C, however, caused an approximately 4-fold decrease in P sorption. Precipitation of Fe gel (2% Fe) on the surface of kaolinite increased P sorption by a factor of 10. The occurrence of Fe gel as a coating apparently presents more sorption sites to solution per unit weight of Fe gel than Fe gel alone. A linear relationship (r= 0.98) was obtained between the amount OH^? sorbed per unit increase in pH value (‘hydroxyl buffering’) and the overall P sorption maximum for each sorbent. Hydroxyl buffering provided a better index of P sorption potential than specific surface area. Except for the crystalline Fe sorbents, isotherms obtained by plotting fractional sorption saturation against final solution P concentration for the sorbents were essentially coincident with those for several contrasting soils. For crystalline Fe components a lower relative amount of weaker sorption, as opposed to chemisorption, of the overall sorption maximum was obtained. Differences in the extent of P sorption. however, appear to be primarily related to the number of functional M-OH groups presented at the solid-solution interface.     

Sorption of chlorpyrifos to selected minerals and the effect of humic acid

Sorption of chlorpyrifos (CPF) from 2.85 microM (1 mg/L) aqueous solutions in 0.01 M NaCl to montmorillonite, kaolinite, and gibbsite was investigated at 25 degrees C. Uptake of CPF by kaolinite and gibbsite was generally <10%, with pH having at most a small effect. Sorption to montmorillonite was significantly greater, with approximately 50% of the initial CPF being removed from solution below pH 5. Above pH 5 the sorption decreased to about 30%. About 70% of CPF was sorbed to kaolinite and gibbsite after 30 min, whereas on montmorillonite only 50% sorbed in an initial rapid uptake (approximately 30 min) followed by slower sorption, with a maximum achieved by 24 h. Although CPF desorbed completely from kaolinite in methanol, only about two-thirds was desorbed from montmorillonite. CPF has only a weak affinity for the surfaces of kaolinite and gibbsite. In the case of montmorillonite, sorption is significantly stronger and may involve a combination of sorption to external surfaces and diffusion into microporous regions. At pH >6 increased negative surface charge results in a lower affinity of CPF for the external surface. In the presence of 50 mg/L humic acid (HA) the amount of CPF sorbed on gibbsite and kaolinite was 3-4 times greater than that in the binary systems. The HA forms an organic coating on the mineral surface, providing a more hydrophobic environment, leading to enhanced CPF uptake. The HA coating on montmorillonite may reduce access of CPF to microporous regions, with CPF tending to accumulate within the HA coating.     

Retention of cobalt by an oxisol in relation to the content of iron and manganese oxides

Abstract

The study aims at determining the cobalt retention properties of various soil components. Therefore, cobalt (Co) sorptions and extractions were carried out using an Oxisol sample before (untreated) and after successive removal of organic matter and active manganese (Mn) oxides (H₂O₂‐treated) and iron (Fe) oxides (H₂O₂+CBD‐treated). A synthetic goethite was included for comparison. Sorption of the four sorbents was determined over a range of Co concentrations (initially 10^‐8 M to 10^‐4 M), pH values (3 to 8) and reaction times (2 hours to 504 hours). The Co species sorbed was Co(ll), since oxygen exclusion during sorption had no effect on the amount sorbed. The pH‐dependent sorption curve (sorption edge) was shifted to lower pH at decreasing initial Co concentration and increasing reaction time. The displacements, in particular of the sorption edges corresponding to the lowest initial Co concentrations, to successively higher pH following removal of Mn oxides, organic matter and Fe oxides could be attributed to sorption onto sites of decreasing Co affinity [Mn oxides (and organic matter) > Fe oxides > kaolinite]. Extractions of sorbed Co at pH 5.5–7.5 with 2 M HCI showed that the extractability decreased with increasing sorption time and decreasing initial Co concentration. The untreated and H₂O₂‐treated soil samples retained sorbed Co at least as firmly as the synthetic goethite, whereas the H₂O₂+CBD‐treated sample (kaolinite) was clearly less effective. The results emphasized the importance of the soil Mn and Fe oxides for Co retention in soils but also the necessity of taken interior sorption sites into consideration.     

Sorption of lead,copper, zinc,and cadmium by soils: effect of nitriloacetic acid on metal retention

Abstract

Laboratory experiments were carried out to evaluate lead (Pb), copper (Cu), zinc (Zn), and cadmium (Cd) sorption‐desorption by three soils of contrasting characteristics. Talamanca (silt loam, montmorillonite, Calcic Haploxeralfs), Mazowe (clay, kaolinite, Rhodic Kandiustalf), and Realejos (sandy silt loam, allophane, Typic Hapludands). A second objective was to study the effect of nitriloacetic acid (NTA) on the sorption process. The Talamanca soil, which had a native pH of 6.4 and presented the highest effective cation exchange capacity (ECEC), sorbed more of each of the metal tested than did the other two soils. When the other two soils were compared metal sorption was also related to pH and ECEC. The very low sorption capacity showed by Realejos may be attributed to the low net surface negative charge density of this soil, arising from its allophanic nature. A common feature of the three soils was the relative strong sorption of both Pb and Cu relative to Cd and Zn with Pb showing the highest sorption levels. The selectivity sequences of metals retention were Pb>Cu>Zn>Cd for Talamanca soil, Pb>Cu>Zn≈Cd for Mazowe, and Pb>Cu>Cd>Zn for Realejos. Metal desorption values were low. The order of metal desorption (Cd≈Zn>Cu>Pb) was the same for the three soils studied. Quantitative differences observed in the extractability of the sorbed metals between the soils (Realejos>Mazowe>Talamanca) indicated that soil properties which enhanced metal sorption contributed at the same time to slow down the backward reaction. The addition of NTA to the soil suspension significantly depressed metal sorption by the three soils investigated. Compared with the free ligand system Pb, Cu, Zn, and Cd sorption in the presence of NTA decreased roughly 50%.     

Effect of clay minerals on the activity of polysaccharide cleaving soil enzymes

Kinetic analyses of the influence of montmorillonite, palygorskite and kaolinite on the activity of three polysaccharide cleaving soil enzymes revealed an inhibitory effect that is rather due to the structure than to chemical characteristics of the clays. The fibrous palygorskite was found to be highly effective towards the starch cleaving α-amylase and amyloglucosidase, whereas the layer silicates caused only a slight inhibition. Interactions with cellulase were affected by sorption of cellulose by montmorillonite and kaolinite. When the enzyme was allowed to be sorbed by the clays before cellulose was added, a clay-enzyme complex is formed, which in contrast to the starch cleaving enzymes remains active. Compared to the native enzyme, the bound enzyme is not stabilized with respect to temperature, but the optimum pH is found to be at pH 6 instead of pH 4.8.     

Aluminum and iron(III) species in the soil solution including organic complexes with citrate and humic substances

The solubility of Al and Fe in soil is of relevance for their toxicity and availability, respectively, to plant roots. Humic substances as the main part of stable soil organic matter and citrate which is often excreted by P deficient plants are strong complexants of Al and Fe(III). Therefore, equations were developed to calculate the Al and Fe(III) species distribution in the soil solution in the presence of humic substances and citrate as organic ligands. Calculations in the pH range 4.0–7.0 showed that at higher pH humic-Al complexes were the most important species whereas AlOH-citrate^? dominated between pH 4.0 and 5.4. Free monomeric Al and AlSO₄⁺ were of minor relevance. Iron(III) species calculations showed that humic-Fe complexes were the main species in the pH range 4.0–7.0. But if mugineic acid, a Fe complexing phytosiderophore released into the rhizosphere by graminaceous plant species, was present in the soil solution (10^?6 M), Fe-mugineic acid complexes accounted for most of the Fe in solution. Fe-citrate^? was relevant at lower pH but contributed little to Fe(III) species at pH > 6.0. The results demonstrate the strong importance of the considered organic ligands for Fe and Al in the soil solution.     

Assessing the fixation and availability of sorbed phosphate in soil using an isotopic exchange method

An isotopic exchange method was used to characterize quantitatively the fixation and plant availability of phosphate previously sorbed by soils. In general, the exchangeability of the sorbed phosphate was much higher than its desorbability for both soils and clay minerals. Isotopic exchangeability of the sorbed phosphate increased with sorption saturation during the initial stage (15–60% saturation), but the increase was less with increasing saturation from 60–90% for all soils tested. Therefore a sorption saturation of 60% was recommended as the upper limit of P fertilization in terms of economical efficiency. For clay minerals, with increasing sorption saturation, the isotopic exchangeability of the sorbed P increased significantly for kaolinite and sesquioxides, but decreased for montmorillonite. Most of the phosphate sorbed by montmorillonite and kaolinite was found to be isotopically exchangeable, but only a small amount of the P sorbed by goethite could be exchanged. The P sorbed by Al oxide exhibited isotopic exchangeability between that of kaolinite and Fe oxide. The isotopically exchangeable phosphate pool could readily account for the P uptake of plants and the available P determined by some commonly used chemical methods, such as Olsen-P and Bray-P.     

Sorption and Binding Mechanism of Polysaccharide Cleaving Soil Enzymes by Clay Minerals

Analyses of the sorption of five polysaccharide cleaving soil enzymes by Na-homoionic clay minerals revealed a dependency on the cation exchange capacity of the clays: montmorillonite displayed the highest sorbing capacity, followed by palygorskite and kaolinite. Variation of the experimental conditions using cellulase and montmorillonite showed a maximum sorption at pH 4.8 and temperatures between 25°C and 45°C. Urea had no effect on the sorption, whereas humic acids and acetylation of enzymes lead to a decrease. No sorption was observed in the presence of bivalent cations. A discussion of possible binding mechanisms lead to the conclusion that interactions can be ascribed to van der Waals forces rather than to ionic reactions. It is assumed that the enzymes penetrate into the interlayers of montmorillonite, but not of palygorskite and kaolinite.     

Copper retention by Danish Spodosols in relation to contents of organic matter and aluminum,iron, and manganese oxides

Abstract

The importance of various soil components on copper (Cu) retention by Spodosois was investigated. Copper sorption and extraction were conducted on samples from the B horizon from six Danish Spodosois. The investigation was conducted on untreated samples, on hydrogen peroxide‐treated samples (to remove organic matter), on oxalate‐treated samples [to remove amorphous to poorly crystalline aluminum (Al) and iron (Fe) oxides], on hydroxylamine‐treated samples [to remove manganese (Mn) oxides]. Subfractions treated with hydrogen peroxide (H₂O₂) were further treated with oxalate and citrate‐bicarbonate‐dithionite (CBD). Sorption of Cu from an initial 10^‐6 M solution after 48 hours was determined in the pH range 3 to 7 using 0.1M sodium nitrate (NaNO₃) as the background electrolyte. The pH‐dependent sorption curve (sorption edge) was shifted to a higher pH with decreasing Al oxide content in the soils, and for the treated sample after removal of organic matter and Al and Fe oxides. A negligible effect was seen after removal of the Mn oxides because of their low abundance. Extraction of sorbed Cu at pH 4 to 6 with 0.1M nitric acid (HNO₃) for 24 hours confirmed the sorption results, in inasmuch as removal of the Al (and Fe) oxides increased Cu extractability. Therefore, it was concluded that in the soils investigated, Cu retention is mainly determined by the oxalate‐extractable Al fraction with a minor contribution due to crystalline Fe oxides.     

Phytotoxicity of organic acids as influenced by montmorillonite,hydroxy‐Al montmorillonite and phosphate fertilization

Abstract

The germination and growth of wheat seedlings were studied at pH 5.5 in liquid growth media with organic acids in concentrations ranging from 800 to 1200 ppm, before and after interaction of the organic acids with montmorillonite and hydroxy‐Al montmorillonite. Germination was not affected by the organic acids, but subsequent growth was dependent on the kind and concentration of organic acid in the growth medium. Acetic acid was more inhibitory than citric acid. Gallic acid polymerized at pH 5.5 and was phytotoxic at 1200 ppm. Interaction of organic acids with montmorillonite and hydroxy‐Al montmorillonite reduced the concentration of organic acids in solution by adsorption. Despite this reduction in concentration the phytotoxicity of the growth media was enhanced after the interaction. This was caused by the dissolution of surface Al and the effect was more pronounced in cases where hydroxy‐Al interlayered montmorillonite was the adsorbent. Additions of P as KH₂PO₄ alleviated the phytotoxicity of organic acids but not the associated Al toxicity. The amount of P lost by adsorption or precipitation was dependent on the form of Al in the growth media.     

铁铝氢氧化物对两种土壤和高岭石表面K+和NH4+解吸的影响

Potassium (K) and nitrogen (N) are essential nutrients for plants. Adsorption and desorption in soils affect K+ and NH + 4 availabilities to plants and can be affected by the interaction between the electrical double layers on oppositely charged particles because the interaction can decrease the surface charge density of the particles by neutralization of positive and negative charges. We studied the effect of iron (Fe)/aluminum (Al) hydroxides on desorption of K+ and NH + 4 from soils and kaolinite and proposed desorption mechanisms based on the overlapping of diffuse layers between negatively charged soils and mineral particles and the positively charged Fe/Al hydroxide particles. Our results indicated that the overlapping of diffuse layers of electrical double layers between positively charged Fe/Al hydroxides, as amorphous Al(OH) 3 or Fe(OH) 3 , and negatively charged surfaces from an Ultisol, an Alfisol, and a kaolinite standard caused the effective negative surface charge density on the soils and kaolinite to become less negative. Thus the adsorption affinity of these negatively charged surfaces for K+ and NH + 4 declined as a result of the incorporation of the Fe/Al hydroxides. Consequently, the release of exchangeable K+ and NH +4 from the surfaces of the soils and kaolinite increased with the amount of the Fe/Al hydroxides added. The greater the positive charge on the surfaces of Fe/Al hydroxides, the stronger was the interactive effect between the hydroxides and soils or kaolinite, and thus the more release of K+ and NH + 4 . A decrease in pH led to increased positive surface charge on the Fe/Al hydroxides and enhanced interactive effects between the hydroxides and soils/kaolinite. As a result, more K+ and NH + 4 were desorbed from the soils and kaolinite. This study suggests that the interaction between oppositely charged particles of variable charge soils can enhance the mobility of K+ and NH + 4 in the soils and thus increase their leaching loss.     

Controlled experimental acidification of lake sediments and resulting trace metal behavior

Severe stream water acidification occurs at higher altitudes (> 600 m a.s.l.) in the Western Harz mountains in Northern Germany. Since 1986 an interdisciplinary research team has followed the fate of pollutants in the 50 km² catchment of an important drinking water reservoir (Lake Söse). An acidification experiment has estimated the role of the remobilization of selected elements from the lake sediments via acidification. Aquaria were used to monitor the effects of a stepwise acidification (from the natural pH of 6.5 to 5.0, 4.0 and 3.0) of the water column over a reconstituted sediment layer. The sediment chemistry has been analyzed before and after the acidification by XRF. The water chemistry was sampled at time intervals and analyzed by ICP-MS. With a pH drop from 6.5 to 3.0, many elements increase in concentration in the water of the acidified basins. Enrichment factors were: Al (5000), Ba (10), Cd (220), Co (800), Cu (170), Ni (90), Pb (5000), and Zn (400). This corresponds fairly well with the field data. Al, Cd, Fe, Mn, and Pb exceed German drinking water limits at pH 4.0. The combined high concentrations (μg L^?1) of Al (1000–2600), Cd (2–4), Cu (4–7), Pb (30–60), and Zn (100–300) in the water column of the acidified streams are not only toxic for fish but also for many other aquatic organisms. Chemical changes in the sediment are not significant within the experimental setup.     

Effects of long‐term applications of various nitrogen sources on chemical soil properties and composition of bromegrass hay

Effects of 15 annual applications (from 1979 to 1993) of ammonium nitrate (AN), urea, ammonium sulfate (AS), and calcium nitrate (CN) applied at 168 and 336 kg N ha^‐1 to bromegrass (Bromus inermis Leyss.) on soil acidification, and concentration of aluminum (Al), iron (Fe), manganese (Mn), zinc (Zn), and copper (Cu) in soil and in hay were investigated in a field experiment on a thin Black Chemozemic (Typic Boroll) soil in Alberta, Canada. Soil was acidified and the concentration of extractable Al, Fe, and Mn was increased by nitrogen (N) application, but the magnitude varied with N source. Soil acidification was greatest with AS, followed by AN and urea, with no effect of CN. At 336 kg N ha^‐1 rate, soil was acidified to a depth of 10, 15 and 30 cm with urea, AN AS, respectively. Soil acidification was also greater at 336 kg than 168 kg N ha^‐1. The CaCl₂‐extractable Al and Fe in the 0–15 cm layer increased with N application, which closely followed the decrease in soil pH from various N sources. Extractable Al and Fe concentration in the 15–30 cm layer increased in response to reduction in soil pH by AS only, and there was no change in the extractable Al and Fe below the 30‐cm depth by any form of N. The DTPA‐extractable Mn in soil generally changed in response to N application. There was no effect of N source on the DTPA‐extractable Zn and Cu in soil. When soil pH had been lowered from N application, the concentration of Al in hay decreased while Zn concentration increased. The Mn concentration in forage increased markedly in response to reduced soil pH from application of AN, urea and AS. There was no effect of N fertilization on the Cu and Fe concentration in hay. In conclusion, the magnitude of soil acidification, changes in the Al, Fe, and Mn concentrations in soil and changes in the Al, Zn, and Mn concentrations in bromegrass hay varied with N source. The results suggest the need for periodic monitoring of soil pH and consideration of liming costs in the economics of various N fertilizers.     

Organo-mineral associations in sandy acid forest soils: importance of specific surface area, iron oxides and micropores

Organo-mineral associations stabilize soil organic matter, though the mechanisms by which they do so are unclear. We used particle-size fractions < 6.3 μm of two soils to examine the importance of Fe oxides, short-range order Al silicates and the surface areas of minerals and micropores on the formation of organo-mineral associations. In the subsoil Fe oxides were most strongly statistically correlated with the mineral-bound organic carbon. We therefore assume that they are the most important substrates for the formation of organo-mineral associations. There is no indication that this is caused by physical protection of organic matter in their micropores (< 2 nm). In the Haplic Podzol, dithionite–citrate–bicarbonate-soluble short-range order Al silicates may also play a role. Fe oxide particles were calculated to offer specific surface areas of ∼ 200 m² g⁻¹ (goethite) and ∼ 800 m² g⁻¹ (ferrihydrite), corresponding to crystal diameters of only a few nm. We assume that the resulting large amount of oxide-specific reactive surface sites (conditionally charged hydroxyl groups) is responsible for their dominant role as sorbents. With maximum C loadings of 1.3 mg C per m² Fe oxide for the Dystric Cambisol and 1.1 mg C per m² Fe oxide + short-range order Al silicates for the Haplic Podzol the subsoils of both soils seem to have reached saturation with respect to organic matter sorption. In contrast to subsoil horizons, organo-mineral associations from topsoils contain much larger amounts of organic matter. Here a larger C loading on Fe oxides or a greater importance of other sorbents in addition to the oxides must be assumed.     

Comparison of Heavy Metal Adsorptions by Thai Kaolin and Ballclay

The adsorption characteristics of heavy metals: cadmium(II), chromium(III), copper(II), nickel(II), lead(II), and zinc(II) ions by kaolin (kaolinite) and ballclay (illite) from Thailand were studied. This research was focussed on the pH, adsorption isotherms of single-metal solutions at 30–60 °C by batch experiments, and on ion selectivityin mixed and binary combination solutions. It was found that, except Ni, metal adsorption increased with increased pH of the solutions and their adsorption followed both Langmuir and Freundlich isotherms. Adsorption of metals in the mixture solutions by kaolin was: Cr > Zn > Cu ≈ Cd ≈ Ni > Pb, and for ballclay was: Cr > Zn > Cu > Cd ≈ Pb > Ni. The adsorption of metals was endothermic, with the exception of Cd, Pb and Zn for kaolin, Cu and Zn for ballclay. Kaolin and ballclay exhibited relatively hard Lewis base adsorption site. The presence of other metals may reduce or promote the adsorption of heavy metals. The presence of Cr³⁺ induced the greatest reduction of metal adsorptiononto kaolin, as did the presence of Cu²⁺ for ballclay.     

Predicting and modelling availability of fluoride in soil from sorption properties

Usage of alkaline and saline groundwater with elevated concentrations of fluoride (F⁻) for irrigation of pastures requires an assessment of the mobility of F⁻ within the plant-water-soil continuum. Factors influencing F⁻ sorption and desorption in 95 Australian soils were examined. Pronounced differences in F⁻ sorption were observed across the soils, but these differences could not be explained by differences in soil pH, electrical conductivity or organic carbon. Rather, sorption was correlated with the content of Fe/Al hydrous oxides and kaolinite in soil. Sorption of F⁻ onto soils increased the solution pH and the ratio of (F⁻ adsorbed) to (OH⁻ desorbed) was consistently below 1, thereby indicating that adsorption of F⁻ results in the release of water and hydroxyl groups from, or co-adsorption of protons to, the sorbent surface. Maximum sorption occurred at pH values of approximately 5–6, whilst sulphate slightly increased (<5%) F⁻ sorption. Desorption was slightly decreased (~3%–7%) in presence of sulphate anions. Hence, it is unlikely that irrigation of soil with alkaline and saline groundwater, in combination with soil applications of gypsum and sulphur to limit pH fluctuations, would increase mobility of F⁻ in soil. Finally, the irrigation of soil columns with 1200–1500 mm of alkaline and saline groundwater containing 0.18 mM F⁻, as would commonly occur in irrigation systems using coal seam gas associated water in Australia, resulted in a F⁻ concentration of 0.05 mM at 10 cm depth and the measured values were in excellent agreement with modelled F⁻ movement based on sorption parameters. Thus, sorption parameters can be used to identify soils which minimize movement of F⁻ because of their strong F⁻ sorption. Based on these results, safe limits for irrigation of soil can be established which avoid F⁻ toxicity risks to plants, animals and contamination of water resources.     

Phosphate sorption characteristics of major soils in Okinawa,Japan

Abstract

Phosphate sorption isotherms were determined for 16 representative major soils developed from different parent materials on Okinawa. Phosphate sorption characteristics were satisfactorily described by the Langmuir equation, which was used to determine phosphorus (P) sorption maxima of the soils. Phosphate sorption maxima ranged from 630 to 2208 mg P kg^‐1 soil (mean 1,362 mg P kg^‐1). The standard P requirement (i.e., the amount of P required to attain 0.2 mg P L^‐1 equilibrium solution) followed the same trend as sorption maximum (r =0.94***), with values ranging from 132 to 1,020 mg P kg^‐1 soil (mean 615 mg P kg^‐1). This mean value corresponds to fertilizer addition of 923 kg P ha^‐1 indicating that the soils have high P fertilizer requirements. Results of simple linear regression analysis indicated that sorption maximum was significantly correlated with clay content, organic matter, oxalate iron (Fe), pyrophosphate Fe, DCB aluminum (Al), oxalate Al, and pyrophosphate Al, but not with DCB Fe, pH, or available P content. The best regression model for predicting sorption maximum was the combination of clay, organic matter, pyrophosphate Fe, and DCB Al which altogether explained 79% of the variance in sorption maximum. The equation obtained could offer a rapid estimation of P sorption in Okinawan soils.