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.     

高岭石,蒙脱石和针铁矿对泥炭腐殖酸的吸附和分离

Sorption of humic acid (HA) on mineral surfaces has a profound interest regarding the fate of hydrophobic organic contaminants (HOCs) and carbon sequestration in soils. The objective of our study is to determine the fractionation behavior of HA upon sorption on mineral surfaces with varying surface properties. HA was coated sequentially on kaolinite (1:1 clay), montmorillonite (2:1 clay), and goethite (iron oxide) for four times. The unadsorbed HA fractions were characterized by elemental analysis, diffuse reflectance infrared Fourier transform spectroscopy (DRIFT), and solid state ¹³C nuclear magnetic resonance spectroscopy (NMR). The mineral-HA complexes were characterized by DRIFT. Polarity index [(N+O)/C] revealed higher polarity of the unadsorbed HA fractions after coating on kaolinite, reflecting that relatively higher polarity fractions of HA remain unadsorbed. Sorption of aliphatic alcohol fraction along with carbohydrate was prominent on kaolinite surface. DRIFT results of the unadsorbed HA fractions indicated more sorption of aliphatic moieties on both kaolinite and montmorillonite. DRIFT results of the unadsorbed HA fractions after sorption on kaolinite and goethite showed the sorption of the proteinaceous fractions of HA. The HA fractions obtained after coating on goethite showed significant sorption of carboxylic moieties. The results mentioned above comply reasonably well with the DRIFT spectra of the mineral-HA complexes. ¹³C NMR results showed higher sorption of anomeric C on kaolinite surface. Higher sorption of paraffinic fraction was observed on montmorillonite. NMR data inferred the sorption of carboxylic moieties on goethite surface. Overall, this study showed that aliphatic moieties of HA preferentially sorbed on kaolinite and montmorillonite, while carboxylic functional groups play a significant role in sorption of HA on goethite. The sorbed fractions of HA may modify the mineral surface properties, and thus, the interaction with organic contaminants.     

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.     

Artificial soils to assess temperature sensitivity of the decomposition of model organic compounds: effects of chemical recalcitrance and clay‐mineral composition

Understanding the temperature sensitivity of soil organic matter (SOM) decomposition is important to predict the response of soil carbon (C) dynamics to projected global warming. There is no consensus, however, as to whether or not the decomposition of recalcitrant soil C is as sensitive to temperature as is that of labile soil C. Soil C is stabilized by three mechanisms: chemical recalcitrance, mineral interaction and physical accessibility. We used artificial soils with controlled compositions to assess the effects of chemical recalcitrance (cellulose compared with lignin) and clay‐mineral composition with montmorillonite (M) or kaolinite (K) on the decomposition of model organic compounds at 2, 12, 22 and 32°C. When only substrate composition was varied, the presence of cellulose enhanced the decomposition rate of lignin. Treatments with relatively large amounts of cellulose were very sensitive to temperature only at low temperatures (2–12°C), whereas treatments with relatively large amounts of lignin had similar temperature sensitivities at all temperatures. When only clay‐mineral composition was varied, CO₂ production rates were greatest in soils containing kaolinite‐montmorillonite mixtures (10% K:20% M) and least in soils containing kaolinite only at temperatures ≥12°C. Clay mixtures and pure montmorillonite treatments had their greatest temperature sensitivities at 2–12°C, whereas pure kaolinite treatments had the greatest temperature sensitivities at 12–22°C. Temperature sensitivities at the highest temperatures (22–32°C) were all small (Q₁₀ < 1.1 on days 30 and 140). Artificial soils with controlled but flexible compositions may serve as simple and useful models for evaluating SOM dynamics with a minimum of confounding factors.     

Effect of pH on Zinc Adsorption and Solubility in Suspensions of Different Clays and Soils

Zinc solubility in clay and soil suspensions was controlled by chemisorption at pH 4.5 – 7.0. The solubility in clay mineral suspensions was in the order palygorskite < montmorillonite « kaolinite and reflected the high affinity of zinc to palygorskite and the high CEC of montmorillonite. The solubility in soil suspensions was in the order Haplustoll < Torrifluvents and reflected the effect of high CEC and organic matter content of the first. The slopes of the pH-pZn curves, calculated zinc potential and sequential desorption data suggested that Zn⁺⁺ ? Zn(OH)₂ aqueous controlled the solubility of zinc in soil and clay mineral suspensions at pH 7.5 – 9.0. The slopes of the pH–pZn curves of two soils were, however, modified by the possible peptization of organic matter and Zn(OH)₂.     

A Study on Fluoride Sorption by Montmorillonite and Kaolinite

Fluoride (F) sorption by acidified montmorillonite, montmorillonite and kaolinite has been investigated as a function of period of agitation, pH, initial fluoride concentration and clay amount. In case of montmorillonite and kaolinite, F sorption is insignificant at pH > 7, while it is maximum at pH 4. Enhanced F sorption by acidified montmorillonite is noted at pH < 10, maximum being at pH 6. Significant concentration of Al³⁺,Fe³⁺ and silica are released from acidified montmorillonite lattice (pH ≈ 2; 1:10 w/v), which decreased considerably owing to the formation of hydroxy (Al and Fe) silicates on subsequent base addition. Soluble fluoro-complexes at pH ≈ 2 and chemical interaction of F with hydroxy (Al and Fe) silicates at pH > 4, are potent sink for F in case of acidified montmorillonite. Soluble fluoro-complexes are also detected in case of montmorillonite and kaolinite at pH ≈ 4. Saturation indices (SIs) for acidified montmorillonite, over the studied pH range (2–10) has also been computed.     

Cadmium Sorption by Hydroxy-Aluminium Interlayered Montmorillonite

Cadmium sorption and desorption behaviour of hydroxy-Alinterlayered montmorillonite (pillared) was compared with unpillared montmorillonite. Characteristic properties of themontmorillonite, whose interlayer cations were replaced byhydroxy-Al polymers, are its thermal stability up to ～500 °C and increased surface area of about 300 × 10³ m² kg^-1. The cation exchange capacity was reduced by ～30% after interlayering withpositively charged hydroxy-Al polymers.Cadmium sorption data for the two montmorillonites fitted verywell (R² = 0.98 to 0.99) to the Freundlich equation. Inthe concentration range (0.89–17.79 μM Cd L^-1) used for the experiment, the unpillared montmorillonite sorbed moreCd than the pillared montmorillonite. Cadmium is possibly retained by electrostatic and chemisorption mechanisms by thepillared montmorillonite whereas electrostatic attraction ismainly responsible for Cd sorption by unpillared montmorillonite. After 5 extractions with 0.01 M NaNO₃the amount of Cd desorbed varied between 30–47 and 34–59% for unpillared and pillared montmorillonites, respectively. At lower concentrations of initially sorbed Cd, the amount ofCd desorbed was nearly equal for the two samples.     

Fractionation of humic acids upon adsorption on montmorillonite and palygorskite

The adsorption of three humic acid (HA) preparations by clays—montmorillonite (Wyoming, USA) and palygorskite (Kolomenskoe district, Moscow oblast)—has been studied. The HA preparations were isolated from samples of the humus-accumulative horizons of a leached chernozem (Voronezh) and a chestnut soil (Volgograd), and a commercial preparation of sodium humate (Aldrich) was also used. The solid-state ¹³C NMR spectroscopy and IR spectroscopy revealed the selective adsorption of structural HA fragments (alkyls, O-alkyls (carbohydrates), and acetal groups) on these minerals. As a result, the aromaticity of the organic matter (OM) in the organic-mineral complexes (OMCs) and the degree of its humification have been found to be lower compared to the original HA preparations. The fractionation of HAs is controlled by the properties of the mineral surfaces. The predominant enrichment of OMCs with alkyls has been observed for montmorillonite, as well as an enrichment with O-alkyls (carbohydrates) for palygorskite. A decrease in the C : N ratio has been noted in the elemental composition of the OM in complexes, which reflected its more aromatic nature and (or) predominant sorption of N-containing structural components of HA molecules. The adsorption of HA preparations by montmorillonite predominantly occurs on the external surface of mineral particles, and the interaction of nonpolar alkyl groups of HAs with this mineral belongs to weak (van der Waals, hydrophobic) interactions. The adsorption of HA preparations by palygorskite is at least partly of chemical nature: Si-OH groups of minerals are involved in the adsorption process. The formation of strong bonds between the OM and palygorskite explains the long-term (over 300 million years) retention of fossil fulvate-type OM in its complex with palygorskite, which we revealed previously.     

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.     

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.     

细菌在两种土壤矿物表面吸附的热力学分析

运用表面热力学方法和扩展的DLVO理论,对两种典型土壤细菌恶臭假单胞菌(Pseudom onasputida)和枯草芽孢杆菌(Bacillus subtilis)在代表性土壤黏粒矿物高岭石和蒙脱石表面的吸附进行了分析,获得了黏粒矿物与细菌作用的疏水自由能变(ΔGH)和静电力自由能变(ΔGEL)及总自由能变(ΔG)。发现疏水自由能为负,显示疏水作用为引力,有利于细菌在黏粒矿物表面的吸附;而静电力自由能为正,表明细菌-矿物间存在静电斥力。疏水自由能显著大于静电力自由能,表明疏水作用在黏粒矿物对细菌吸附时的贡献大于静电力。两种细菌与两种矿物间的总吸附自由能为负值,意味着细菌在矿物表面的吸附是热力学自发过程。高岭石对细菌的吸附自由能大于蒙脱石对细菌的吸附自由能,表明细菌与高岭石间的亲和力较高,吸附更容易发生,这与化学吸附及滴定量热结果一致。表面热力学方法和XDLVO理论在预测细菌-矿物相互作用中有重要意义,但该方法未考虑多种非DLVO效应,如细胞表面多聚物、细菌鞭毛等在吸附反应中的作用,因此还存在一定的局限性,在揭示细菌-矿物相互作用的热力学机理方面还需与其他研究技术结合。     

广西水耕人为土黏粒矿物组成及其空间分布特征

黏粒矿物影响着土壤理化性质，可指示成土因素特征和土壤发生发育过程/强度，也是中国土壤系统分类的基层单元土族矿物学类型划分的重要依据。本研究选择了广西不同纬度和成土母质的18个代表性水耕人为土的剖面，应用X射线衍射（XRD）方法分析了其典型水耕氧化还原层（Br层）的黏粒矿物组成及其空间分布特征，并确定了其中“黏质”剖面的土族控制层段矿物学类型。结果表明：（1）供试土壤的黏粒矿物主要包括高岭石、伊利石、三水铝石、1.42 nm过渡矿物、蒙脱石和蛭石等，依次分别出现在100%、88.9%、72.2%、61.1%、44.4%和38.9%的剖面中。（2）黏粒矿物组成在纬度空间分布上具有明显规律性特征。随着纬度降低，土壤黏粒中的高岭石增加，伊利石、蒙脱石、1.42nm过渡矿物逐渐减少；纬度＞23°N区域内，成土母质对黏粒矿物组成影响明显。（3）纬度23°N是黏粒矿物组成和土族矿物学类型分界线，＜23°N区域，黏粒矿物均以高岭石为主，是“黏质”剖面的土族控制层段的主要矿物学类型；＞23°N区域，黏粒矿物组成以高岭石、蒙脱石、伊利石或1.42 nm过渡矿物为主，因成土母质不同而异，“黏质”剖面的土族控制层段矿物学类型包括高岭石混合型、混合型和伊利石型。     

Einfluß von Zwei- und Dreischichttonmineralen auf die Dehydrogenase-, saure Phosphatase- und Urease-Aktivität in Modellversuchen

Influence of two- and three-layered clay minerals on the Dehydrogenase-, acid Phosphatase and Urease Activity in model experiments The influence of different Ca-homo-ionic clay minerals as well as humic acid (Roth) on the dehydrogenase activity (DHA) as well as on urease and acid phosphatase were examined in model experiments. The following results were obtained: 1. The DHA of a fertile soil and the activity of urease and acid-phosphatase decreased remarkably and specifically with increasing amounts of sorbents. Generally speaking, inactivation was complete at approximately 0.6 g of each additive. Both urease- and acid phosphatase activity were inactivated specifically. The inactivation of urease decreased in the sequence humic acid < montmorillonite < illite < pyrophyllite < halloysite < kaolinite < bentonite, whereas the following order of succession pyrophyllite < montmorillonite < bentonite < halloysite < kaolinite < illite was recorded with acid phosphatase. The yellow colour of humic acid excluded it from experiments with acid phosphatase. 2. No relationship could be found between the degree of inactivation and the cation exchange capacity (CEC) or the total surface area of the clays. However, nearly the same sequence of inactivation with the three enzyme systems was revealed, if their activity was related to the specific surface charge of the clay minerals. Apparently, both the total surface area as well as CEC are involved in the specific sorption of cells and enzymes. The possible mechanisms of sorption on clay surfaces are discussed.     

Interaction of naphthalene derivatives with soil: an experimental and theoretical case study

This study provides insight into the relevance of the chemical functional groups of soil organic matter (aromatic, paraffinic, O‐alkyl, carboxyl and carbonyl carbon), as determined by CPMAS ¹³C NMR, on adsorption processes. Batch adsorption experiments with eight naphthalene derivatives were conducted with soils from a long‐term field experiment and model sorbents. Although the adsorption of some derivatives was mainly affected by the paraffinic organic carbon content in soil, the relation between the C‐distribution and adsorption was complex. This casts doubt on the use of such NMR data to estimate sorption behaviour. Additionally, sorption experiments were performed with six model sorbents representing typical soil components. Considerable adsorption of naphthalene derivatives was observed for montmorillonite and lignin; the smallest values were for kaolinite and cellulose. A quantum chemical approach was used to calculate a local polarity parameter as a molecular property of the naphthalene derivatives. This parameter was correlated with the logarithm of the adsorption coefficients, logK_d. Here, clear trends were observed for three of the model sorbents (kaolinite, montmorillonite and lignin).     

CLAY MINERAL DISTRIBUTION AND ORIGIN IN THE SOIL TYPES OF ISRAEL

The mineralogical composition of clays (< 2μm) in representative profiles of all soil types of Israel was investigated. The soils were classified according to their clay mineral assemblages into three groups. I. Montmorillonitic soils. Montmorillonite is the dominant mineral and exceeds 65 per cent of the total minerals found; each of the other minerals comprises less than 15 per cent. 2. Montmorillonitic-kaolinitic soils. The soil clay fractions contain 50-60 per cent montmorillonite and 15-25 per cent kaolinite, generally adding up to more than 75 per cent of the clay fraction. 3. Montmorillonitic-calcitic soils. The clays contain more than 10 per cent calcite. Montmorillonite is the dominant clay mineral (except for one soil type, mountain rendzina, where calcite is dominant). The first and second assemblages are typical of the soils of the Mediterranean zone, whereas the soils of the desert zone are characterized by the third assemblage. The origin of montmorillonite, kaolinite, and illite, the three main clay minerals, was found to be detritic, as was the origin of palygorskite which was mainly found in the calcite rich soils of the desert zone. The cation exchange capacity of montmorillonite seems to be higher under higher precipitation. Montmorillonite content and cation exchange capacity of the clays were found to be highly correlated. The carbonate content of the clay fraction and the amount of carbonate in the soil were also highly correlated.     

水溶性有机碳在各种粘土底土中的吸附：土壤性质的影响

Clay-rich subsoils are added to sandy soils to improve crop yield and increase organic carbon (C) sequestration; however, little is known about the influence of clay subsoil properties on organic C sorption and desorption. Batch sorption experiments were conducted with nine clay subsoils with a range of properties. The clay subsoils were shaken for 16 h at 4 ^oC with water-extractable organic C (WEOC, 1 224 g C L^-1) from mature wheat residue at a soil to extract ratio of 1:10. After removal of the supernatant, the residual pellet was shaken with deionised water to determine organic C desorption. The WEOC sorption was positively correlated with smectite and illite contents, cation exchange capacity (CEC) and total organic C, but negatively correlated with kaolinite content. Desorption of WEOC expressed as a percentage of WEOC sorbed was negatively correlated with smectite and illite contents, CEC, total and exchangeable calcium (Ca) concentrations and clay content, but positively correlated with kaolinite content. The relative importance of these properties varied among soil types. The soils with a high WEOC sorption capacity had medium CEC and their dominant clay minerals were smectite and illite. In contrast, kaolinite was the dominant clay mineral in the soils with a low WEOC sorption capacity and low-to-medium CEC. However, most soils had properties which could increase WEOC sorption as well as those that could decrease WEOC sorption. The relative importance of properties increasing or decreasing WEOC sorption varied with soils. The soils with high desorption had a low total Ca concentration, low-to-medium CEC and low clay content, whereas the soils with low desorption were characterised by medium-to-high CEC and smectite and illite were the dominant clay minerals. We conclude that WEOC sorption and desorption depend not on a single property but rather a combination of several properties of the subsoils in this study.     

Phosphate desorption from the surface of soil mineral particles by a phosphate-solubilizing fungus

High phosphate (Pi) sorption in soils is a serious limiting factor for plant productivity and Pi fertilization efficiency, particularly in highly weathered and volcanic ash soils. In these soils, the sorbed Pi is so strongly held on the surfaces of reactive minerals that it is not available for plant root uptake. The use of phosphate-solubilizing microorganisms (PSM) capable of Pi desorption seems to be a complementary alternative in the management of these soils. The aim of this study was to evaluate the effectiveness of the soil fungus Mortierella sp., a known PSM, to desorb Pi from four soil minerals differing in their Pi sorption capacity. The fungus was effective in desorbing Pi from all tested minerals except from allophane, and its desorption depended on the production of oxalic acid. The effectiveness of the fungus to desorb Pi was ranked as montmorillonite > kaolinite > goethite > allophane. The quantity of desorbed Pi increased by increasing the amount of sorbed Pi. The Pi sorption capacity expressed as P_0.2 value (amount of P required to increase a solution P concentration up to 0.2 mg L^?1) was a good indicator of the effectiveness of Mortierella sp. to desorb Pi from soil minerals.     

Potassium release on drying of soil samples from a variety of weathering regimes and c clay mineralogy in China

Thirty two soil samples from China were analyzed for exchangeable K before and after drying. Most soil samples were higher in exchangeable K after air-drying and ovendrying (60°C) than when wet. Soil clay minerals, especially clay-size mica, affected K released in air-dry and oven-dry samples. According to composition of clay minerals of air-dried samples, five classes were recognized. Soil samples with high mica and montmorillonite have the highest exchangeable K. Samples that contained higher kaolinite than mica had lower exchangeable K. Samples with higher kaolinite than quartz and mica had still lower exchangeable K. Samples of sandy soils contained very low exchangeable K. When quartz was the main mineral, the samples that had kaolinite and gibbsite as the main clay minerals had very low exchangeable K. The degree of weathering (weathering mean) bore an inverse relationship to the amount of K released on drying in air or at 60°C. Udults and Udalfs (Red Earths) of southeastern China, because of their high K release on being dried, are inferred to have received fine mica from the Western Desert dust rainout, reported to Liu et al. (1981).     

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 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.