Electrokinetic properties of variable charge clays in the presence of an anionic xanthan polysaccharide and inorganic electrolytes

The effect of the electrolytes (MA: NaCl, MgCl₂ , PbCl₂ , and NaH₂PO₄ ) on the polymeric (a) weak-stabilization (PWST), (b) flocculation (PFL); and (c) stabilization (PST) of allophane in the presence of xanthan polysaccharide (GX), was investigated by electrophoretic mobility (EPM) measurements. At pH 6.5 (i.e.p.), with further addition of chlorides, the decrease in the absolute value of the negative EPM (|—EPM|) of allophane in 0.01 mm MA solutions indicated the suppression of PWST and PST, respectively, probably due to the decrease in the repulsive force originating from the negatively charged GX-chains with the addition of MA. By first addition of MA, the decrease in the |—EPM| value of allophane in 0.01 mm NaCl and MgCl₂ solutions also showed the suppression of PWST and PST. In 0.01 mm PbCl₂ and NaH₂PO. solutions, the increase in the EPM and |—EPM| values of allophane reflected the dispersion due to the specific adsorption of Pb₂₊ and H₂PO₄ - ions on the negatively and positively charged sites of the surface, respectively. At pH 4.5, the decrease in the |—EPM| value of allophane in 0.1 mm NaH₂PO₄ solution indicated the suppression of PST due to the specific adsorption of H₂PO₄ - ion on the positively charged surface of the particles. The absence of (i) PWST in the presence of GX and (ii) high stability in the presence of cationic lead species for the flocculated imogolite at pH 8.5 can be attributed to the tubular structure proposed by Cradwick et al. (1972: Nature (London) Phys. Sci., 240, 187-189), namely, to the difficulty in the development of a negative charge on the outer surface of its unit particle.     

Flocculation and Stabilization of Allophane in the Presence of an Anionic Microbial Polysaccharide (Gum Xanthan) and Several of Its Hydrolytic Intermediates

The polymeric effect of gum xanthan (GX) on the stability of allophane at pH 4.5 and 6.5 was compared with the effect of several of its hydrolytic intermediates (hGX). The polymeric effect of GX was manifested by (i) the effective flocculation at low GX concentrations, (ii) the higher stabilizing ability above the flocculation concentration (FC), and (iii) the appreciable ability of deflocculation of (a) flocculated allophane and (b) allophane-halloysite floc, at extremely low concentrations of GX solution at pH 6.5. In contrast to GX, in the presence of sodium polyacrylate, there was a complete lack of deflocculation of the flocculated allophane at pH 6.5. This fact suggests that effective deflocculation is due to the double-stranded wormlike structure of the GX chain with a small but finite flexibility. The fact that flocculated allophane by GX at pH 4.5 still exhibited a positive electrophoretic mobility can be ascribed to the flocculation of allophane particles by bridging with this biopolymer. The relationship between FC of GX and the allophane content in soil clays was also examined.     

Effect of particle size of halloysite on the deflocculation of mutual flocs of allophane and halloysite at pH 6.5 in dilute xanthan polysaccharide solutions

The effect of the particle size of halloysite on the deflocculation of mutual floes of allophane and halloysite in the presence of anionic xanthan polysaccharide was investigated spectrophotometrically. Although allophane which flocculated at the i.e.p. (pH 6.5) exhibited polymeric (i) weak-stabilization (PWST), (li) flocculation (PFL), and (iii) stabilization (PST) with increasing addition of xanthan, allophane and halloysite which flocculated mutually at pH 6.5 inhibited the deflocculation, viz., PWST and PST. Halloysite with a smaller average particle size (i.e., 0.08,um in average diameter) than the suspended microaggregates (SMAs) of allophane inhibited significantly the deflocculation, suggesting that the halloysite particles flocculated around the allophane particles. Halloysite with a larger average particle size (i.e., 0.2 μn in average diameter) than the SMAs of allophane slightly inhibited the deflocculation, suggesting that the halloysite particles were attached to the center of the flocculated SMAs of allophane. The inhibitory effects of particle arrangements on the deflocculation of the mutual floes of allophane-montmorillonite and allophanekaolinite were also investigated comparatively.     

Effect of inorganic electrolytes on the stability of allophane in aqueous solutions of anionic xanthan polysaccharide

The polymeric (a) weak-stabilization (PWST), (b) floccelation (PFL), and (c) stabilization (PST) of ailophane with the anionic xanthan (GX) polysaccharide (He and Horikawa 1996a, b: Soil Sci. Plant Nutr., 42, 603-612, 637-644) were investigated further with respect to the effect of 1-1, 2-1, and 1-2 electrolytes (MA). At pH 4.5, the PST was (i) high in 0.1 mM NaCl, MgCl₂, ZnCl₂, and PbCl₂ solutions, whereas (ii) considerably low in 100 mM NaCl, 0.1 mM Na₂SO₄ and NaH₂PO₄ solutions, respectively. At pH 6.5 (i.e.p.), the flocculated ailophane showed (i) a very low PWST in 0.01 mM NaCl, MgCl₂, and Na₂SO₄ solutions, and also (ii) a low PST in 0.01 mM MA except for NaH₂PO₄. In 0.01 mM PbCl₂ and NaH₂PO₄ solutions, ailophane was well dispersed, respectively, and marked PFL and PST were observed with further addition of xanthan. The changes and decrease of PWST and PST were discussed in relation to the iocculation of ailophane due to the (a) effect of MA on the diffuse double layer of the particles, (b) specific adsorption of phosphate and lead ions on the surface, and (c) weakening of the repulsive force between anionic GX chains by MA.     

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.     

Interactions between Escherichia coli and allophane—I. Adsorption

Sorptive interactions between Escherichia coli (ATCC 11303B) and a purified allophane clay fraction were studied quantitatively using electronic particle counting and electrophoresis. Adsorption was influenced by pH and ions present at the allophane surface. These effects could be rationalized by considering changes in the net surface charge of the components. Electronic particle counting studies at pH 5.5 and above gave adsorption isotherms that obeyed single-term Langmuir kinetics. At pH 4.5, a more complex two-step isotherm was obtained, which was attributed to aggregation of unadsorbed cells by Al solubilized from the allophane surface.     

ELECTRIC CHARGE CHARACTERISTICS OF ANDO A, AND BURIED A, HORIZON SOILS

The electric charge characteristics of four Ando soils (A₁ and μA₁) and a Chernozemic soil (Ap) were studied by measuring retention of NH₄⁺ and Cl^– at different pH values and NH₄Cl concentrations. No positive charge appeared in the Ando soils at pH values 5 to 8.5 except for one containing allophane and imogolite. The magnitude of their negative charge (CEC; meq/l00g soil) was dependent on pH and NH₄Cl concentration (C; N) as represented by a regression equation: log CEC =a pH +b log C +c, where the values of a and b were 0.113–0.342 and 0.101–0.315, respectively. Unlike the Chernozemic soil, Ando soils containing allophane, imogolite, and/or 2:1–2:1:1 layer silicate intergrades and humus showed a marked reduction of cation retention as pH decreased from 7 to 5. This was attributed to the charge characteristics of the clay minerals and to the carboxyl groups in humus being blocked by Al and Fe.     

Interactions between Escherichia coli and allophane—II. Metabolism

A purified allophane clay fraction increased the respiration of Escherichia coli (ATCC 11303B) by buffering the suspension against decreases in pH. Allophane had no significant effect on respiration in the presence of 0.02 M phosphate buffer. When E. coli was exposed to pH values below 5.0 in the presence of allophane, allophane supernatant (which contains soluble Al), or soluble Al. toxicity was evident even after raising the pH to 7.0.     

Solubility of a proto-imogolite sol in oxalate solutions

The amounts of aluminium liberated from a proto-imogolite (P-I) sol by 1–3 mM oxalate at pH 4·8–7·1 are found to be in excellent agreement with those calculated on the basis of the solubility product of the P-I sol, and the stability constants for aluminium oxalate complexes established earlier. Calculated solubility curves are presented for proto-imogolite sols in the presence of 10^?3, 10^?4 and 10^?5 M total oxalate over the pH range 4–8, and also for the case where oxalate concentrations are controlled by solid calcium oxalate in the presence of 0·05, 0·5 and 5 mM calcium ion. We conclude that the presence of a proto-imogolite allophane in podzol B horizons of pH 5·0 requires oxalate concentrations less than 10^?5 M. For fertile agricultural soils with typical calcium concentrations (approximately 5 mM) in the soil solution, aluminium oxalate concentrations would be limited by calcium oxalate solubility to less than 10^?6 M in the presence of proto-imogolite allophane at pH values exceeding 5·5. In leached podzolic soils calcium concentrations are generally less than 0·05 mM, and would not limit the formation of aluminium oxalate complexes.     

Effects of solution pH on the growth and chemical composition of rice plants

Rice plants were grown in solution culture for a period of five weeks at pH's ranging from 3.5 to 8.5. Maximum dry matter was obtained at pH 5.5, but substantial reductions in the growth of tops and roots were observed at pH's of 3.5 and 8.5. At pH 3.5, both leaves and roots were short and unhealthy. The roots were thickened with lateral root growth severely inhibited. At pH 8.5, the youngest leaves developed chlorotic symptoms with roots being coarse and discoloured.

Plant concentrations of essential elements were adequate for normal plant growth at pH 5.5. Iron concentration in plant tops substantially decreased with increase in solution pH, but a reverse trend was observed for roots. The concentrations of other elements progressively increased in plant tops and roots with increasing pH.     

Solubility characteristics of proto-imogolite sols: how silicic acid can de-toxify aluminium solutions

Proto-imogolite sols can be considered as highly dispersed forms of proto-imogolite allophane, the most widespread type of allophane in volcanic and non-volcanic soils world-wide. The solubility characteristics of such sols define the conditions of precipitation of allophanes in soils, and the maximum concentrations of aluminium released during acidic episodes from soils, such as podzols, that contain allophane. Direct measurement of Al, Si and pH values in equilibrium with proto-imogolite sols, approached from higher and lower pH, indicated a solubility equation: where log*K_so lay in the range 7.14 to 7.23 after equilibration for 4–24 weeks at 22 + 2°C in 17 of the 20 systems studied. The mean value of log *K_SO at 298 K was calculated as 7.02. This value indicates that proto-imogolite will be more stable than amorphous aluminium hydroxides at H₄SiO₄ concentrations above 5 × 10^?6m , but less stable than bayerite below 10^?3m H₄SiO₄, and than gibbsite below 10^?2m . Proto-imogolite is more stable than micro-crystalline gibbsite in 10^?4m H₄SiO₄, a typical minimum concentration in soil solutions and streams in landscapes where podzols are present. The rapid formation of proto-imogolite effectively prevents the formation of gibbsite seeds in soil, except in highly leached and warm environments, i.e. in older landscapes in the tropics. Although the presence of 10^?4m silicic acid has been found to eliminate the acute toxicity to fish exhibited by solutions containing 6–7 μm Al at pH 4.96, little or no proto-imogolite would form under these conditions. Silicic acid would, however, prevent the precipitation of aluminium hydroxides, and could inhibit the formation of the A1₁₃ polycation. These polymeric species are a likely cause of the increased toxicity exhibited by partially neutralized aluminium solutions.     

FLUORIDE ADSORPTION BY ILLINOIS SOILS

Fourteen surface and 6 subsurface horizons of Illinois soils adsorbed significant amounts of F^? with release of OH^?. At low concentrations, adsorption was described by both Langmuir and Freundlich isotherms. The calculated Langmuir adsorption capacities were related to pH, clay, organic carbon, and amorphous aluminum contents. Two soils with different gross chemical properties behaved in essentially the same manner, with adsorption maxima occuring between pH 5.5 and 6.5. The similarity between adsorption at different pH values for the soils and those for bauxite, allophane and synthesized ‘soil chlorite’, and the lack of adsorption maxima between pH 5.5 and 6.5 for pure kaolinite and montmorillonite, suggest that F^? adsorption in the soils is due primarily to the presence of amorphous aluminum oxyhydroxides which are common weathering products in these soils.     

A re-interpretation of 'Aluminium solubility mechanisms in moderately acid Bs horizons of podzolized soils' by Gustafsson et al.

Gustafsson et al. in a recent paper in this Journal reported the effects of adding HCl, AlCl₃ and Si(OH)₄ on the pH and concentrations of Al and Si in 1:1 soil:solution systems at three different temperatures, using samples of soil from an allophanic Bs horizon. Contrary to their conclusions, their observations are compatible with Al in the soil solution being in equilibrium with a proto‐imogolite allophane; it is neither necessary nor even plausible to postulate a hypothetical Al hydroxide. Concentrations of 0.2–0.4 mm Si in the equilibrated solutions at pH 5 could arise from an amorphous silica, probably phytoliths. They cannot come from the allophane.     

Comparison of phosphate adsorption on clay minerals for soilless root media

Abstract

The greenhouse industry aims to decrease phosphate discharge to help reduce eutrophication of surface waters, to reduce fertilizer consumption, and to maintain a more constant level of plant‐available phosphate. Iron and aluminum oxides and some aluminosilicate minerals are efficient sorbents for phosphate. The phosphate adsorption characteristics of synthetic hematite (α‐Fe₂O₃), goethite (α‐FeOOH), and allophane (Si₃Al₄O₁₂ nH₂O), and a commercial alumina (A1₂O₃) were evaluated to determine their potential for reducing phosphate leaching from soilless root media. The pH dependence of phosphate adsorption and maximum adsorption capacities were determined by reacting each mineral with various levels of phosphate between pH 4.0 and 9.0 in a 10 mM potassium chloride (KCl) background solution. Adsorbed phosphate was determined by loss from solution. Adsorption envelopes (adsorbed phosphate versus pH) showed a decrease in phosphate adsorption with increasing pH, particularly for alumina and allophane, and at greater added phosphate concentrations. The maximum adsorption capacities per unit mass of the minerals at pH 5.4 decreased in the order allophane > alumina ? goethite > hematite. When expressed on a surface area basis, the order of maximum adsorption capacity remains the same except that alumina exceeded that of goethite. The allophane, goethite, and alumina sorbed enough phosphate that 3 to 9 g of these minerals would retain the amount of phosphate required for a high nutrient element requiring plant such as chrysanthemum.     

Contribution of organic matter and clay minerals to the cation exchange capacity of soils

Abstract

The cation exchange capacity (CEC) at pH 7 was measured for samples of 347 A horizons and 696 B horizons of New Zealand soils. The mean CEC was 22.1 cmolc/kg for the A horizons and 15.2 cmolc/kg for the B horizons. Multiple regressions were carried out for CEC against organic carbon (C), clay content, and the content of seven groups of clay minerals. The results, significant at p <0.001, were consistent with most of the CEC arising from soil organic matter. For the samples of A horizon, the calculated CEC was 221 cmolc/kg per unit C and for the B horizons was 330 cmolc/kg C. There was also a contribution from sites on clay minerals. Multiple regression indicated that smectite had a higher CEC (70 cmolc/kg) than other minerals but it was not as high as that of type smectites; kaolin minerals had the lowest CEC. There was a significant effect of interaction between organic matter and some clay minerals on the CEC. Samples from B horizons containing allophane had lower CEC than those not containing allophane which is consistent with allophane reacting with carboxyl groups on organic matter. For the samples from the A horizons, however the CEC was higher when allophane was present.     

Physicochemical variables affecting the rheology and microstructure of rennet casein gels

The rheology and microstructure of a rennet casein system were studied in the pH range from 5.8 to 12.0 during cooling from 80 to 5 degrees C at four cooling rates: 0.5, 0.1, 0.05, and 0.025 degrees C/min. A dramatic increase in storage modulus with pH was observed during cooling at a fixed cooling rate. Continuous networks were formed for gels at pH 7.2 and above, while a discontinuous network was observed for gels below pH 6.5. The monotonic increase in storage modulus with pH could be correlated to the number of net (negative) charges and the strength of the hydrophobic interactions. At a higher pH, the protein micelles were larger due to weaker hydrophobic interactions and stronger repulsive electrostatic interactions resulting from more charges. When these protein micelles aggregated into flocs during cooling, the flocs had similar sizes at different pH values but a smaller fractal dimension at a higher pH. Consequently, for systems of the same protein and salt concentrations, more flocs were present in the gels at a higher pH, which subsequently generated more cross-links and a higher storage modulus. The pH also determined how the cooling rate affected the gel properties. At pH 5.8 and 6.5, the gels were firmest at the fastest cooling schedule, and the cooling rate did not show a trend in affecting the gel strength at the other three rates. On the other hand, a slower cooling rate generated a firmer gel at pH 7.2 and 12.0. The analysis of casein interactions suggests that the cooling rate affected the casein floc size only when repulsive interactions enabled a slow flocculation (at higher pH values) comparable with temperature change rates during cooling. For rennet casein gels of pH within the range of processed cheese products (pH 5.8 and 6.5), particle or cluster rearrangements created more uniform networks for gels cooled at slower schedules and weakened the structure.     

Efficacy of single application ammonium sulfate in suppressing potato common scab

Abstract

The objective of this study was to suppress potato common scab by lowering the soil pH and increasing the concentration of water-soluble aluminum (Al) in soil with a single application of ammonium sulfate into each row. Superphosphate (P) and potassium sulfate (K) were applied to the surface soil horizon and ammonium sulfate (N) was applied only into the rows along which potato plants were to be planted. By this application method, the soil pH was lowered and the concentration of water-soluble Al was increased in the soil of the rows where potato tubers were grown. Potato common scab was suppressed in the soil containing water-soluble Al in concentrations of 0.2 to 0.3 mg L^?1 or higher. The pH of the soil fertilized as indicated above remained lower than that of the control soil to which the mixture of N, P, and K was uniformly applied. In soil types such as Haplic Andosols containing allophane at high concentrations of 71 g kg^?1 in Memanbetsu, the suppression of potato common scab by this single application of ammonium sulfate was less effective due to the low soluble Al concentration. In other soil types, the soil pH was easily controlled and common scab was suppressed by this method. The advantages of this method are that (a) it minimizes the use of fertilizers, thus reducing the adverse effects of unnecessary fertilizers on the soil; and (b) it lowers the cost by eliminating additional agricultural chemicals and extra fertilizers.     

有机酸对高岭石, 针铁矿和水铝英石吸附镉的影响

Effects of organic acids （oxalic, acetic, and citric） on adsorption characteristics of Cadmium （Cd） on soil clay minerals （kaolinite, goethite, and bayerite） were studied under different concentrations and different pH values. Although the types of organic acids and minerals were different, the effects of the organic acids on the adsorption of Cd on the minerals were similar, i.e., the amount of adsorbed Cd with an initial solution pH of 5.0 and initial Cd concentration of 35 mg L^-1 increased with increasing concentration of the organic acid in solution at lower concentrations, and decreased at higher concentrations. The percentage of Cd adsorbed on the minerals in the presence of the organic acids increased considerably with increasing pH of the solution. Meanwhile, different Cd adsorption in the presence of the organic acids, due to different properties on both organic acids and clay minerals, on kaolinite, goethite, or bayerite for different pHs or organic acid concentrations was found.     

Chemical characterization of conducive and suppressive soils for potato scab in Hokkaido,Japan

Potato common scab induced by Streptomyces scabies is a serious constraint for potato-producing farmers and the incidence of potato scab depends on the soil chemical properties. We examined the chemical characteristics of conducive and suppressive soils to potato common scab with reference to the chemical properties of nonallophanic Andosols, recently incorporated into the classification system of cultivated soils in Japan. Allophanic Andosols with a ratio of pyrophosphate-extractable aluminum (Al_p) to oxalate-extractable aluminum (Al_o) of less than 0.3–0.4 were “conducive” soils with a high allophane content of more than 3%. On the other hand, nonallophanic Andosols with a Al_p/Al_a ratio higher than this critical value were “suppressive” soils, and their allophane content was less than 2%. The concentration of water-soluble aluminum (AI) was also a useful index for separating conducive from suppressive soils as well as the Al_p/Al_a value and allophane content. The suppressive soils showed a much higher concentration of water-soluble Al at pH 4.5 to 5.5 than the conducive soils. The high concentration of water-soluble Al may be responsible for the control of the incidence of potato common scab in Andosols.     

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.