Adsorption of xanthan polymer on allophane and effect on the changes in colloidal stability of allophane by heavy metal ions

The influence of divalent cations at concentrations of 10^-6.0 to 10^-4.0 M on the colloidal stability of partially deflocculated allophane by gum xanthan (GX) polysaccharide at pH 6.5 was investigated at two GX concentrations. Experiment in the presence of 10^-2.0 g L^-1 GX showed that the stability decreased by the addition of divalent cations and the effect of the decrease due to the cationic species was evident in a higher concentration range, i.e., 10^-5.0 to 10^-4.0 M. The order of the effect was Pb >Zn > Cd > Mg. Experiment in a 10^-4.5 g L^-1 GX solution revealed that (i) the stability increased by the addition of heavy metal cations at 10^-5.0 M (the order of the effect was Pb > Zn > Cd) and decreased at a concentration above 10^-4.5 M, (ii) whereas the stability decreased by the addition of Mg ion. The striking difference in the stability behavior due to the difference between the two GX-concentrations was attributed to the (i) degree of GX-adsorption (and hence negative charge from the carboxyl group in GX) onto allophane based on the electrophoretic mobility, (ii) complexation of heavy metal cations by organic ligand (carboxyl group) in GX which was adsorbed onto allophane, and (iii) surface complexation by heavy metal cations and hydroxyl groups on allophane. The results were discussed in relation to the characteristics of the particles of allophane, viz., (i) polymer-coated soft particles, and (ii) semi-soft particles on which the rigid (hard) surface of allophane substantially remained.     

The ion uptake mechanism of allophane

Rather greater variations in the cation exchange capacity (CEC) value of Allophane, say, 20 to 100 me per 100 g of clay (1), have been reported by a number of investigators. BIRRELL and his collaborators (5, 6) revealed that the CEC of allophane depends on concentration and pH of a leaching solution, kinds of cation and anion, volume of washing alcohol, and its water content. The results suggest the peculiarity of allophane in ion-exchange phenomena.     

Demerium exchange of hydroxyl groups in allophane

The nature of amorphous silicates, allophane,ö The name allophane is used in this paper in a dual sense; the one stands for a group of amorphous or nearly amorphous aluminum silicates associated with minor amounts of other metals, and the other in some restricted sense for amorphous silicates other than imogolite. The latter term, imogolite, is used for convenience to denote a component that shows broad x-ray diffractions at 14 (strong), 7.6 (medium) and 5.6 (weak) Å, gives an endotherm at 430°C, and appears as thread-like particles of diameter 100 to 200 Å in electronmicrographs (21). as a main clay constituent determines many of the physical and chemical properties of volcanic ash soils. Recent developments in the study of allophane together with other amorphous inorganic materials in relation to their nature and occurrence in soils have been reviewed by MITCHELL et al. (12).     

Dispersion of humic allophane soils with supersonic vibration

The difficulties in dispersing volcanic ash soils of Japan and New Zealand have been considered to be due to the association or aggregation of allophane (3, 5, 7–10). In particular, MIYAZAWA (9) has obtained evidence that stable microaggregates of Humic Allophane soils derived from volcanic ash have been formed by dehydration of allophane. Recently, several investigators (2, 4, 6, 8, 9, 12) have pointed out that ultrasonic vibration is effective in dispersing the fine particles of soils. With respect to the applicability of supersonic vibration to the particle-size distribution analysis of Humic Allophane soils, MIYAZAWA (9) stated that the maximum dispersion, as measured by the clay content, was obtained only with supersonic vibration using an acidic medium. KOBO and OBA (8) reported that calgon (sodium hexametaphosphate) as a dispersing agent was successfully applicable to most Humic Allophane soils, but the use of HCI was necessary for some highly allophanic subsoils, and that the effect of supersonic vibration on dispersion of the soils is attributed to the breakdown of aggregates larger than 20 microns in diameter. They also recommended a mixture of 10g of soil and 50 ml of water and 20 min. exposure for a supersonic vibrator (10 Kc, 300 W). With respect to the applicability of vibration treatment in the particle-size distribution analysis of Humic Allophane soils containing volcanic glasses in abundance, some apprehension may be entertained about the breakdown of primary minerals, especially of volcanic glasses (9, 10).     

Interactions of allophane with humic acid and cations

Allophanic soils are known to accumulate organic matter, but the underlying mechanism is not well understood. Here we have investigated the sorption of humic acid (HA) by an allophanic clay in the presence of varied concentrations of either CaCl₂ or NaCl as background electrolytes. Both the HA and the clay were separated from New Zealand soils. Much more HA was sorbed in CaCl₂ than in NaCl of the same ionic strength. Apparently Ca²⁺ ions were more effective than Na⁺ ions in screening the negative charge on HA. In CaCl₂ the HA molecule might also assume a more compact configuration than in NaCl. In the presence of CaCl₂ sorption increased, reached a maximum, and then declined as the concentration of HA in solution was increased. This behaviour was not observed in NaCl where sorption showed a gradual and steady increase with HA concentration. We propose that ligand exchange occurs between the surface hydroxyl groups of allophane and the carboxylate groups of HA. As a result, the allophane–HA complex acquires negative charges, requiring the co‐sorption of extraneous cations (Ca²⁺ or Na⁺) for charge balance. The Ca²⁺ co‐sorbed can attract more HA to the complex possibly by a cation‐bridging mechanism, giving rise to a maximum in sorption. The decline in sorption beyond the maximum may be ascribed to a decrease in the concentration of free Ca²⁺ ions through binding to HA molecules in solution. The increase in supernatant pH may be attributed to a ligand exchange reaction between the surface hydroxyls of allophane and the carboxylate groups of HA, and proton binding to the allophane–HA complex.     

Influence of allophane and organic matter contents on surface properties of Andosols

The aim of this study was to use nitrogen gas adsorption to study the complex surface properties of a wide range of Andosol Ah and Bw horizons; N₂ gas adsorption not only provides specific surface area, SSA, but it also yields complementary information about micropore volume and hydrophilic and/or hydrophobic surface properties. Total SSAs were positively related to micropore SSA which was, in its turn, positively interrelated to the dimensionless parameter C of the Brunauer‐Emmett‐Teller (BET) equation (micropore N₂ filling) and microporous allophane content. The more allophanic the Andosol horizon sample, the larger were its total, micropore and mesopore SSAs. On the other hand, strong negative exponential relationships were obtained between either total or micropore or mesopore SSA and soil organic carbon content, with a SSA threshold at an organic carbon content of c. 8–10%, the SSAs being extremely small at larger organic carbon content values. Both SSA_BET and C_BET decreased non‐linearly as a function of the organic C/allophane ratio, with either a SSA_BET or a C_BET threshold at an organic C/allophane ratio value of 3–5, above which both SSA_BET and C_BET were very small (mostly the aluandic Andosol Ah horizons). The more the soil allophanes are assumed to be coated, the more hydrophobic the soil surfaces become and the smaller both SSA_BET and C_BET become; nitrogen gas has a permanent quadrupole moment and therefore acts as a polar probe when surfaces are sufficiently hydrophobic. The antagonistic roles played by allophanes and organic matter in both the SSAs and the values of the dimensionless parameter C in the BET equation were also highlighted by (i) multiple linear regressions between the SSAs and both allophane and organic carbon contents and (ii) principal‐component analysis of SSA_BET, C_BET and soil constituent (allophane, ferrihydrite and organic C) contents.     

The infrared absorption band of allophane near 1,650 cm-1

In the infrared absorption (IR) spectrum of the water in allophane, two kind of the absorption band within on region is found near 3,450 and 1,650cm^-1 associated with O-H stretching vibration and O-H deformation vibration, respectively. The maximum of IR band associated with stretching vibration in allophane specimens dried under the relative humidity of 30% at the room temperature, was situated at 3,467 cm^-1 for the Misotsuchi and at 3,461 cm^-1 for the Kanumatsuchi (4). The band was shifted to higher wave-number as heating the specimen till 300°C (4). In this note, the situation of IR band associated with O-H deformation vibration of the water adsorbed on allophanes was exactly measured, because it was hardly done in the past.     

Mechanism of inhibitory effect of allophane and montmorillonite on some enzymes

It is well established that the presence of clay in an enzyme-substrate system may reduce the activity of enzymes (1-3,5,6,8,11-14). This inhibition by the clay would be principally caused by the adsorption of enzyme and/or substrate on the clay particle. In the previous papers (1–3), the activity of some enzymes was found to be inhibited in various degrees by clays. The enzyme appears to be adsorbed on the clay particle in various ways according to the nature of adsorbate as well as adsorbent. This suggests that the enzyme is adsorbed by the clay so as not to combine with the substrate, or that the adsorbed enzyme molecule has a configuration different from that of a free enzyme molecule, reducing the activity of the enzyme.     

Phosphate reactions with natural allophane, ferrihydrite and goethite

The reactions of phosphate with natural samples of allophane, ferrihydrite, hematite and goethite were measured for up to 30 d. The amount of phosphate sorbed on allophane showed the biggest increase with time whereas the amount sorbed on goethite showed the least increase with time. The total amount of phosphate sorbed either at high levels of phosphate addition or after 10 d followed the order hematite < goethite < ferrihydrite < allophane and was probably related to the specific surface. Si was desorbed as phosphate was adsorbed on the minerals.
The reactions of phosphate on allophane involved rapid, strong adsorption, probably at defect sites, followed by weaker adsorption, followed, probably, by disruption of the allophane structure together with precipitation of aluminium phosphates. Previous suggestions either of diffusive penetration of phosphate into surfaces or about the formation of aluminium phosphate coatings, are unlikely to hold for allophane, if all the Al is at the surface and if the structure can be ruptured.
The reactions of phosphate with iron oxides involved a rapid, strong ligand exchange, followed by weaker ligand exchange, and, probably, by a relatively slow penetration at defect sites and pores. Highly crystalline goethite has virtually no slow reaction and therefore solid-state diffusion of phosphate does not readily occur. The extent of phosphate uptake during the slow penetration reactions probably depends on the degree of crystallinity or porosity of iron oxides.
The most reactive adsorbents, such as allophane, ferrihydrite and Al-humus complexes do not have planar surfaces, and this needs to be considered when modelling phosphate reactions.     

Specific gravity of allophane and volcanic ash soils determined with a pycnometer

The determination of the specific gravity of allophane is an interesting and important problem, but it is no exaggeration to say that we have no satisfactory answer to this question in spite of numerous studies. In this note, the specific gravities of allophane, weathered pumices and volcanic ash soil were determined with a pycnometer, and the values compared with those of the other clay minerals and non-volcanic ash soils.     

Adsorption of pentachlorophenol by soils

The adsorption studies using soils various in the species of clay minerals and organic matter content indicate:

1) That apparent adsorption occurs to the greatest extent on the strong acid soil system compared to the moderate acid soil system, regardless of the species of clay mineral and organic matter content. And there is no adsorption on the slightly acid or neutral soil system.

2) The apparent adsorption involves adsorption of molecules and/or anions and precipitation of molecules in the micell and the external liquid phase.

3) The magnitude of adsorption occurs in the decreasing order of humus-allophanic, allophanic, montmorillonitic, and halloysitic soils.

4) The major factor governing the magnitude of apparent adsorption is pH.     

Adsorption of mercury compounds by tropical soils III. Adsorption isotherms

Isotherms for sorption of 2-methoxyethylmercury chloride (Aretan) and HgCl₂ showed that all three soils investigated had a large capacity to adsorb these Hg compounds, the order being Morogoro > Arusha > Dar es Salaam. For all soils, surface horizons adsorbed more Hg at any given concentration of Hg in solution as compared to subsurface horizons. Adsorption isotherms of Aretan, especially at lower equilibrium concentration in soil solution, were much steeper in all horizons and in all profiles. The Freundlich equation described the adsorption of HgCl₂ better than that of Aretan. The adsorption isotherms for Aretan and HgCl₂ showed different forms, implying probably that different adsorption mechanisms were involved.     

Weathering and allophane neoformation in soils developed on volcanic ash in the Azores

On Faial and Pico islands (Azores), we studied two profiles on basaltic pyroclasts that contain buried horizons, and we focussed on petrography, micropedology and mineralogy. Emphasis was given to weathering of the lapilli and ashes, and the neoformation of allophane. A combination of optical studies, in situ chemical analyses, X‐ray diffraction and infrared spectroscopy of clay fractions revealed that allophane is present both in the micromass of the groundmass, in alteromorphs after lapilli or pumice, and in clay coatings. Whereas most studies describe allophane as a colloidal fraction formed by the congruent and total dissolution of the ashes, this study shows evidence for the formation of allophane alteromorphs, due to leaching of Si and cations, with preservation of the original shapes of the tephra. The allophane alteromorphs often display optical characteristics that resemble those of palagonite. Increasing alteration is observed through three steps: (i) hydration of the glass associated with strong cation and Si leaching, (ii) allophane hypocoatings, and (iii) allophane alteromorphs with development of intragrain bridges. The chemical signature of the alteromorphs varies from a pure alumino‐silicate at one extreme to an Fe(Ti) enriched alumino‐silicate at the other. Between those two extremes, the colour grades from yellow to dark orange, with microzonations. An Al‐rich allophane composition is associated with gibbsite in the EUR6‐Pico profile, whereas at the base of the EUR5‐Faial profile, Si‐rich allophane is associated with halloysite. Some variations of the palaeo‐environment are also suggested by strong iron segregation observed with various secondary phases (ferrihydrite, haematite, iddingsite).     

Adsorption of Cadmium by Indian Soils

Adsorption behaviour of cadmium (Cd) in soils is an important process which exerts a major influence on its uptake by plant roots. Thirteen soils from various parts of India (tropical region), their pH ranging from 4.2 to 8.4, were subjected to Cd treatment at various concentrations (1 to 100 μg ml-1) and equilibrated at room temperature (25 ± 1 °C). The Cd adsorbed by each soil was calculated as the difference between the amount of Cd present in the solution initially and that remaining after equilibration. Results indicated that the adsorption capacity of the soils for Cd increased with an increase in the pH or alkalinity of the soils. The rate of adsorption was, however, found to decrease with increased pH. All the 13 soils used in this study followed linear and Freundlich adsorption isotherms with highly significant positive correlations (r). The neutral and alkaline soils also followed the Langmuir adsorption isotherm, the adsorption maxima being lowest for the neutral soil and highest for the alkaline soil. The adsorption data, in general, indicated that Cd was in a fixed form at higher pH levels. The results are generally similar to those of the temperate regions; however, Cd adsorption capacity of tropical vertisols was comparatively higher than those of temperate vertisols.     

Adsorption and desorption of imazosulfuron by soil

Understanding and quantifying the adsorption and desorption of herbicides by soil is important for predicting their fate and transport in the environment. Here we report a study concerning the adsorption and desorption, by four different soils, of imazosulfuron, 1-(2-chloroimidazo[1,2-a]pyridin-3-ylsulfonyl)-3-(4, 6-dimethoxypyrimidin-2-yl)urea, a new sulfonylurea herbicide. Both phenomena are well-described by the Freundlich equation, which shows this herbicide to be little adsorbed by each of the four soils investigated. The Freundlich adsorption constants, K(f-ads), ranged from 1.46 to 3.02. Distribution coefficients between soil and water, Kd, measured on soils of different organic matter contents and pH values showed an important effect of these two parameters on imazosulfuron retention. The Freundlich desorption data indicated that a significant amount of the imazosulfuron sorbed is not easily desorbed. The desorption process showed an evident hysteresis phenomenon, which may contribute to the persistence of imazosulfuron in soil.     

Adsorption and desorption of dimepiperate by soils

The adsorption and desorption of dimepiperate, S-(,-dimethylbenzyl)-1-piperidinecarbothioate, on three soils of various physical and chemical properties was studied. Adsorption isotherms conformed to Freundlich equation. The k _f values increased with increasing organic carbon content of the soils. To confirm the effect of organic matter, the adsorption of the herbicide was studied after removal of organic matter by peroxidation. This soil treatment caused a sequential loss of adsorptive capacity. Desorption isotherms also conformed to Freundlich equation, but K _des values were higher than those for adsorption and increased with increase in concentration of initially adsorbed dimepiperate. Hysteresis was indicated by the decrease in slope of desorption compared to adsorption isotherms. Hysteresis decreased with increasing methanol content in the extracting solution. The factors involved are discussed.     

Adsorption and desorption of triasulfuron by soil

The adsorption and desorption of the herbicide triasulfuron [2-(2-chloroethoxy)-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino]carbonyl]benzenesulfonamide] by three soils, soil organic matter (H(+) and Ca(2+)-saturated), and an amorphous iron oxide were studied. Adsorption isotherms conformed to the Freundlich equation. It was found that pH is the main factor influencing the adsorption in all of the systems. Indeed, the adsorption on soils was negatively correlated with pH. The highest level of adsorption was measured on soils with low pH and high organic carbon content. Moreover, it was found that humic acid is more effective in the adsorption compared with calcium humate (the pH values of the suspensions being 3.5 and 6, respectively). Experiments on amorphous iron oxide confirmed the pH dependence. Desorption was hysteretic on soils having high organic carbon content.     

土壤及生物炭对草甘膦的吸附作用

为了确定生物炭修复草甘膦污染土壤的可行性,通过添加不同比例和种类的生物炭到土壤中,研究土壤对草甘膦的吸附效果。采用吸附动力学拟合、等温吸附分析、红外光谱测定分析方法。结果表明:草甘膦能强烈地吸附在红壤及稻壳炭、竹炭、竹柳炭中,相比于红壤,3种生物炭达到吸附平衡时间短。吸附动力学符合准二级动力学方程,等温吸附符合Freundlich吸附等温方程。Freundlich吸附等温方程中1/n数值均<1,表明红壤和3种生物炭对草甘膦的吸附方式是非线性吸附,且非线性程度大小为竹柳炭>竹炭>红壤>稻壳炭。生物炭添加到红壤中,可以提高红壤吸附草甘膦的量,生物炭添加比例越高,土壤吸附草甘膦的量也越高。土壤中添加生物炭比例相同时,竹炭使土壤吸附草甘膦的量最高,竹柳炭次之、稻壳炭最低。3种生物炭吸附草甘膦前后的红外光谱分析阐明了酚、胺、芳香烃、羧酸、羧酸盐、脂肪醚等在吸附过程中起重要作用。     

Adsorption of mercury compounds by tropical soils

Mercury adsorption of HgCl₂ and 2-methoxyethylmercury chloride (Aretan) (100 mg Hg L^?1) was measured for three soil profiles from Morogoro, Arusha, and Dar es Salaam in Tanzania. The adsorption was investigated for the physical, chemical, and mineralogical properties of the soils. All soil samples showed greater capacity for adsorption of Aretan than for HgCl₂. In the Morogoro profile Hg adsorption decreased with depth but in the other two soils, the minimum adsorption occurred in the third horizon and increased both upwards and downwards. In the Morogoro profile, Aretan adsorption correlated well with pH. Adsorption of both Aretan and HgCl₂ correlated well with the distribution of organic C and with the cation exchange capacity of the soils. In the Arusha and Dar es Salaam profiles Hg adsorption was not significantly correlated with any of the soil properties tested.