EFFECTS OF solUTION: SOIL RATIO AND VIGOUR OF SHAKING ON THE RATE OF PHOSPHATE ADSORPTION BY SOIL

The effects were measured of varying the solution: soil ratio, and the vigour of shaking on the rate of adsorption of phosphate. One soil appeared to break down with vigorous shaking, prior to adding phosphate, especially at low solution: soil ratio. As a result, subsequent phosphate adsorption was faster. With gentle shaking, there was no effect of solution: soil ratio with the soil. Another soil was more stable and there were only small differences due to vigour of shaking and no effect of solution: soil ratio with any method of shaking. The change in phosphate concentration was not proportional to the reciprocal of time as reported by Ryden and Syers (1975) and Hope and Syers (1976). Hence the validity of extrapolating from short sections of reciprocal-time graphs in order to estimate equilibrium is questioned.     

Testing a mechanistic model. VII. The effects of pH and of electrolyte on the reaction of selenite and selenate with a soil

The pH of samples of a soil was altered by mixing them either with acid or lime, and incubating the moistened samples at 60°C for a day. The sorption of selenate and of selenite was then measured using as background electrolytes, 0.01 M, 0.1 M and 1.0 M sodium chloride and also 0.01 M calcium chloride. The results were compared with previous studies with phosphate and fluoride. Selenite was sorbed more strongly than selenate, but not as strongly as phosphate or fluoride. Sorption of both selenite and selenate decreased with increasing pH. This decrease was more marked for selenate than for selenite; more marked in a sodium system than in a calcium one; and more marked with a dilute background electrolyte than a concentrated one. Under certain conditions, the steeper curves for the dilute electrolyte crossed the curves for the concentrated electrolyte giving points of zero salt effect. For selenite, these points of zero salt effect occurred near pH 6 and the greater the sorption the lower the pH for zero salt effect. For phosphate, the analogous value was near pH 5. For selenate, if a point of zero salt effect occurred, it was at such a high pH and such a low amount of sorption that it could not be measured. Thus, the larger the amount of sorption the lower the pH for the point of zero salt effect. This generalization applied both within and between different kinds of sorbates. The results were closely described by a model that had previously been applied to phosphate and fluoride. The model postulates that ions react with charged surfaces. The electric potentials of the reacting surfaces are affected by the identity and concentration of the background electrolyte and this produces the interactions between pH and electrolyte concentration. The model also postulates that there is a distribution of electric potentials. Anions react with surfaces which occur in the more positive tail of this distribution. The smaller the amount of the reaction the more positive the potential of the reacted surface and, therefore, the higher the pH required to decrease this potential to zero.     

Modelling the effect of adsorption of phosphate and other anions on the surface charge of variable charge oxides

Adsorption of anions by iron and aluminium oxides decreases the charge on the surface. However, the negative charge conveyed to the surface by an adsorbed anion varies with the amount of anion adsorbed. This has been explained qualitatively by postulating a mixture of reactions at different surface coverage (Rajan, 1976) or by postulating different reaction mechanisms at different concentrations of phosphate (Ryden & Syers, 1975). We show that these effects may also be reproduced by the adsorption model of Bowden et al. (1974, 1977, 1980). This model does not require a mixture of equations to describe the variations in charge conveyed to the surface. Instead it describes the electrostatic consequences of adsorption. The charge conveyed to the surface by each increment of phosphate differs from that of the previous increment because each successive increment reacts with a surface with a different net charge. The model also reproduces the observed effects of ionic strength and pH on the charge conveyed to the surface by adsorption. Further, it suggests that differences between anions on the charge conveyed to the surface are explicable in differences in the mean distance of the adsorbed anion from the original surface.     

Effects of crystallinity of goethite: I. Preparation and properties of goethites of differing crystallinity

We prepared five samples of pure goethite and three samples of aluminous goethite with mole fractions of aluminium ranging from 0·05 to 0·20. One of the samples of pure goethite was repeatedly heated in an autoclave at 150°C. We studied the samples using transmission electron microscopy and XRD and we measured the surface area (BET), rate of dissolution of the samples in acid, the surface charge in four concentrations of sodium nitrate from pH 4–10, and the sorption of calcium and chloride from a dilute solution of calcium chloride. Surface areas ranged from 18 to 132 m² g^?1. The BET adsorption-desorption isotherms had hysteresis loops which increased as the surface area increased. These showed that there were pores present with diameters smaller than 2 nm and that the number of pores increased as the surface area increased. All samples of goethite showed typical X-ray diffraction patterns but with peak heights decreasing, and width increasing, as surface area increased. Mean coherence lengths, as determined by XRD line broadening, were smaller than those determined by transmission electron microscopy. This was because each crystal was composed of several domains, and the XRD data reflect the size of the domains. Heating one of the goethite samples in an autoclave caused most of the domains to coalesce and slightly decreased the surface area. As the surface area of the eight goethite samples increased, the mean coherence length in the direction of the a, b and c axes decreased. The better crystallized samples dissolved more slowly in acid and the dissolution curves were sigmoid because the acid was able to penetrate between the domains allowing access to a larger surface area and consequently accelerated dissolution. The points of zero charge increased with increasing surface area of the goethite. The charge carried by the goethite at pH values distant from the point of zero charge increased as the surface area increased. This may indicate movement of protons or hydroxyl ions into pores or other defects in the crystal. The effects of pH and of salt concentration on surface charge were closely described by the variable-charge/variable-potential model. This model also closely described adsorption of Ca²⁺ and Cl^? from calcium chloride solutions requiring only the allocation of constants for these ions. The substitution of aluminium for iron did not affect the charge properties of goethite.     

Pathology of experimentally-induced, acute toxoplasmosis in macropods

SUMMARY Thirteen Tammar wallabies (Macropus eugenii) were dosed orally with 500, 1000 or 10 000 oocysts of Toxoplasma gondii, as part of a vaccination trial. Eleven animals died of acute toxoplasmosis 9 to 15 days after challenge. The lesions were similar in all animals, consisting of foci of necrosis and inflammation in the intestines, lymphoid tissue, adrenal cortex, heart, skeletal muscle and brain, and severe generalised pulmonary congestion and oedema. Free and intracellular tachyzoites of Toxoplasma were associated with the lesions. The remaining 2 animals had shown no signs of disease when euthanased four months after challenge. Small, focal, non-suppurative inflammatory lesions were seen in brain, heart and skeletal muscle of these animals and chronic Toxoplasma infection was confirmed by mouse inoculation.     

A national serological survey to verify Australia's freedom from porcine reproductive and respiratory syndrome

Objective To provide serological data to support Australia's claim of freedom from porcine reproductive and respiratory syndrome.
Design A national serological survey was designed to provide 99% confidence of detecting at least one infected pig herd in Australia, assuming that at least 5% of herds would have been exposed to porcine reproductive and respiratory syndrome virus and that at least 25% of the 'finisher' pigs in these herds would have antibodies to the virus.
Procedure A two-stage testing regime was used. All samples were tested with a commercially available enzyme-linked immunosorbent assay. If assay reactions were found, all samples from the herd were to be tested using the indirect immunofluorescence antibody assay.
Results Of the 875 samples from 163 herds from all States in Australia, there was some evidence of reactivity in only four samples from four herds on the enzyme-linked immunosorbent assay. Further testing using the indirect immunofluorescence antibody assay according to the study protocol demonstrated that the reactions were not due to the presence of specific porcine reproductive and respiratory syndrome virus antibodies in the sera.
Conclusion The results of this study support the view that Australian pigs are free of porcine reproductive and respiratory syndrome virus.     

Effects of ionic strength and nature of the cation on the desorption of fluoride from soil

The reaction between soil and added fluoride was accelerated by incubating at a high temperature. Desorption of the fluoride was then studied using solutions of chloride salts of several cations at a range of solution : soil ratios and for periods which ranged from 1 h to 4 days. Fluoride desorbed was related to the experimental variables by a regression equation. When the solution : soil ratio was small and hence only small amounts of fluoride were desorbed, decreasing the concentration of salts increased the concentration of fluoride in the solution. The concentration in the solution was lower for calcium chloride than for sodium or potassium chloride. Amongst the monovalent cations, the concentration of fluoride was highest for salts of the cations of lowest atomic number. Thus the greater the average distance between the charge conveyed to the surface by the adsorbed fluoride and the cation which balanced it, the higher the fluoride concentration in the solution. As the solution : soil ratio was increased, the differences between the cations in their effects on fluoride desorption decreased and seemed to disappear as the solution : soil ratio became very large. This contrasts with previously observed effects on phosphate. It is suggested that the difference may have arisen because appreciable desorption of fluoride occurs by exchange with hydroxyl ions rather than by escape of the fluoride ion together with its counter ion.     

The reaction between phosphate and dry soil. I. The effect of time, temperature and dryness

Phosphate (P) was added to soil in solution. The soil was air-dried or freeze-dried and then incubated at a range of temperatures for periods of up to 110 d. The rate of the continuing reaction between the P and soil was measured using the null-point method, and by measuring the amount of desorption induced by filter paper impregnated with iron oxide (Pi test). The reaction between soil and P continued in both air-dried and freeze-dried soil, albeit more slowly than in moist soil. Freezing the soil, whether moist or dry, virtually stopped the reaction. These results are consistent with the hypothesis that the continuing reaction between P and soil involves a solid-state diffusive penetration of the soil particles by the sorbed P ions. They also indicate that the common practice of storing soil air-dry, even for short periods at low temperature, will not preserve the P status of the soil as at sampling. It was estimated that for a sample of soil which remained moist at 25°C for 100d after the addition of 335 μg P g⁻¹ soil, before being sampled and stored air-dry at 4°C for 16 years, the measured P_i test value would be about 15 μg P g⁻¹. This compares with 46 μg P g⁻¹ which is the estimated Pⁱ test value measured on the same day as sampling. When samples cannot be analysed for P status immediately following sampling, they should be stored at the lowest convenient temperature, preferably below 0°C.