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Subject: search.filters.subject.Soil absorption and adsorption

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    Solute movement associated with intermittent soil water flow : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University
    (Massey University, 1991) Tillman, Russell Woodford
    The movement of nutrients within the root zone of orchard crops is important in determining both fruit yield and quality. Currently much of the research on solute movement in field soils concerns movement of chemicals to ground water. Little attention has been paid to smaller scale movement. In this study the movement of solutes in response to intermittent soil water flow was investigated in columns of repacked silt loam in the laboratory and in a similar soil in the field. In the laboratory study a 5mm pulse of a solution of potassium bromide and urea in tritiated water was applied to columns of repacked soil, left for three or ten days, and then leached with 30 mm of distilled water. Twelve days after the solute pulse was applied, the distributions of water, tritiated water, applied and resident nutrients and pH were measured. The bulk of the bromide and tritiated water was moved to between 50 and 1 50 mm depth in both water treatments. As the nitrogen applied in urea was mainly in the form of ammonium after three days, the water applied then caused little movement of nitrogen. But the water applied after 10 days caused the nitrogen, now in the form of nitrate, to move in a similar fashion to the bromide. The soil solution anion concentration determined the amount of cations leached. Calcium and magnesium were the dominant cations accompanying the nitrate and bromide downwards. The added potassium remained near the soil surface. Given the soil hydraulic properties, the behaviour of water and solutes could be simulated by coupling the water flow equations with the convection-dispersion equation, and by using solute dispersion , diffusion and adsorption parameters derived from the literature. The model assumed the Gapon relationship for cation exchange, and that hydrogen ion production during nitrification reduced the effective cation exchange capacity. It was able to simulate closely the experimental data. Two field experiments were conducted. The first involved application of a 5 mm pulse of potassium bromide solution followed by 50 mm of water to pasture plots of contrasting initial water content. Twenty-four hours later core samples of soil were collected and the distribution of water and bromide measured. Bromide applied to initially dry soil was much more resistant to leaching than bromide applied to moist soil. The second experiment lasted 12 days and was essentially an analogue of the laboratory experiment. The final nutrient distributions however differed considerably from those obtained in the laboratory, due to non-uniform flow in the structured field soil. Coupling a mobile-immobile variant of the convection-dispersion model with a description of the water flow provided a mechanistic model. When combined with the submodels developed in the laboratory study describing nutrient interactions and transformations, this model successfully described the solute movement under the four different field regimes of water and solute application. Evaporation and plant uptake, and diffusion between mobile and immobile phases emerged as key processes affecting nutrient movement. It is suggested some control over nutrient movement is possible by varying the relative timing of water and fertiliser applications.
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    Adsorption of zinc and cadmium by soils and synthetic hydrous metal oxides : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University
    (Massey University, 1981) Roberts, Antony Hugh Coleby
    This study involved an investigation into the adsorption ot Zn and Cd by soils and synthetic hydrous metal oxides, and the effect of several factors on the adsorption reactions in controlled laboratory experiments. Initially, in order to determine low concentrations of Zn and Cd in solution, a concentration procedure involving solvent extraction was developed. The procedure utilised a chelating agent, dithizone, and an organic solvent, carbon tetrachloride. Zinc and Cd were back-extracted from the organic solvent into hydrochloric acid to achieve a ten-fold increase in concentration. For soils and Fe gel, adsorption of both Zn and Cd was characterised by an initial, rapid removal from solution which was followed by a much slower, continuing decrease in solution concentration. Isotherms (40 hr) for Zn and Cd adsorption from 3 x 10-2M NaCl by soils, synthetic hydrous ferric oxide (Fe gel) and allophane, were described using two Langmuir equations. Values for the derived free energies of adsorption, for each region of adsorption, were similar for both Zn and Cd on all soils and on Fe gel. Allophane, however, had higher free energies of adsorption than either the soils or Fe gel. For all adsorbents, Langmuir constants derived from adsorption data indicated that the adsorbing surface contained a relatively small number of sites with a high free energy of adsorption and a much larger number of sites with a lower adsorption energy. Iron gel appeared to provide a satisfactory model for describing Zn and Cd adsorption by soils. Amounts of Zn and Cd adsorbed by Fe gel increased as pH increased. Adsorption of Zn and Cd by Fe gel in each Langmuir region appeared to be affected similarly by changes in pH, although the pH50 values (the pH at which 50% of the initial amount added was adsorbed) were higher for Cd than for Zn. Experimental data obtained in the study are consistent with a two-mechanism model for both Zn and Cd adsorption. It is proposed that the first mechanism involves the formation of a bidentate complex. Adsorption of Zn and Cd into this region involves the release of two protons for each Zn or Cd ion adsorbed, resulting in a bond of higher energy than when Zn or Cd is adsorbed by the second mechanism. In this latter case, it is proposed that a monodentate complex is formed with one proton released for each cation adsorbed. In addition to proton release, data in support of the two-mechanism system was obtained in isotopic exchangeability and desorption studies. For example, the mole ratios (i.e., number of protons released for every Zn or Cd ion adsorbed) for Zn and Cd were non-integer values. For Zn the mole ratios ranged between 1.31 and 1.67, and for Cd from 0.80 to 1.12, at pH 6.4. There was no obvious trend in mole ratios for Zn either with increasing amounts of Zn adsorbed or increasing pH, but for Cd mole ratios increased with increasing amounts of Cd adsorbed of increasing pH. The isotopic exchangeability of Zn was similar at all levels of Zn adsorbed, but decreased with increasing pH (pH 5.85 - 6.65) from 58% exchangeability to 27%, possibly due to an increased proportion of more-tightly bound Zn. Cadmium, by contrast, had a lower exchangeability at low levels of Cd adsorbed (55 - 76%) than at higher levels (80 - 85%) but was more exchangeable (55 - 85%) than Zn at equivalent pH values and lower surface coverages, indicating that a greater proportion of adsorbed Cd was less tightly bound compared to Zn. Sequential desorption of Zn by calcium ions followed by copper ions showed that a proportion of Zn (12 - 24%) was retained by the surfaces against desorption, further indicating the presence of two adsorption mechanisms of different binding strength. Only at low levels of adsorbed Zn did the amount of desorbed Zn closely approximate the amounts of Zn calculated (from Langmuir constants) to be in region II. These desorption data, together with the exchangeability data for Zn, point to the possible limitations in the use of Langmuir equations for quantifying the amount of Zn absorbed by each mechanism. The Langmuir isotherm studies, together with proton release, exchangeability and desorption data, indicated two mechanisms of adsorption for Zn and for Cd. However, the greater exchangeability and fewer protons released per mole adsorbed suggest that more Cd is held by the mechanism involving monodentate bonding than is the case for Zn. Zinc or Cd was not observed to be absorbed or "occluded" by synthetic ferric oxide gel or goethite. There was evidence to suggest that Zn and Cd might diffuse into cracks or defects in the crystal structure of natural goethite, developed by grinding. Although some fraction of adsorbed Zn or Cd was non-exchangeable and non-desorbable (by copper) for all three adsorbents, this Zn and Cd was not in the absorbed phase. Long term (up to thirty year) additions of superphosphate fertiliser to three soils did not produce measurable accumulations of Zn or Cd in the soils. The calculated additions agreed well with actual increases measured in total Zn, but not with actual increases in total Cd.
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    An investigation of water repellency in a range of New Zealand soils : a thesis presented in partial fulfilment of the requirement for the degree of Doctor of Philosophy in Soil Science at Massey University, New Zealand
    (Massey University, 1991) Wallis, Murray Gordon
    The severity of water repellency was investigated on a wide range of agricultural, horticultural and turf soils in New Zealand. All the soils studied had some degree of repellency, regardless of texture or land use. Techniques for repellency measurement and amelioration were evaluated and the organic component responsible for repellency development investigated. The temporal and extreme spatial variability of repellency expression was quantified using geostatistics. The 'repellency index' (RI) based upon soil intrinsic sorptivity was developed and evaluated on 14 New Zealand soils. The RI measured undisturbed cores of all the soils water repellent at field moisture conditions and was more sensitive than the MED or water drop penetration time (WDPT) tests. The RI was used to demonstrate that repellency reduced the short-time infiltration rate, i of all the soils by approximately an order of magnitude, including those which appeared to wet normally (i.e. low WDPT). Calculations identified that the reduction in i would be hydrologically significant under intense rainfall and many irrigation systems. Investigation of a development sequence of yellow-brown sands (Aquic Udipsamments) revealed maximum repellency in < 130 y old Waitarere sand, which had the lowest level of organic carbon. Repellency was also severe in the Motuiti and Himatangi sands (c. 500 y) but declined in the Foxton (1600 - 6000 y) and Koputaroa (10000 - 25000 y) soils. While water repellency (measured with the Molarity of Ethanol Droplet, MED test) of the Himatangi sand profile was correlated with organic carbon content, this was not the case with the other soils. Organic matter composition is a more likely determinant of the degree of repellency than organic matter quantity per se. Spectroscopic examination of extracts removed from Himatangi sand by an isopropanol/ammonia mixture using a soxhlet apparatus indicated a range of long chain organic compounds comprising esters and fatty acids as the cause of repellency. The spatial variability of volumetric soil water content (θ) to 200 mm depth at two adjacent sites (A and B) of Himatangi sand, each c. 860 m2, was studied with geostatistical techniques. At both sites θ varied isotropically and generally followed a normal distribution. Variograms of θ changed over time and were not transferable between the two sites, although there was evidence of drift in the mean at site B. At site A, compared to an October analysis, in summer the coefficient of variation (C.V.) of θ increased and the range of θ spatial dependence, a decreased. Irrigation of both site A and B with a travelling boom slightly increased C.V.(θ), markedly increased the semivariance and slightly decreased a(θ) at site A. When a wetting agent was applied prior to irrigation of site A the C.V.(θ) halved, a(θ) increased and the θ increase was improved by 63 % over irrigation of site A and 206 % over irrigation of site B. Agitation of soil samples reduced repellency significantly, however the effect was somewhat reversible and field cultivation could be precluded by the degree and depth of the repellent topsoil. Soil wetting agents increased grass establishment and growth in both glasshouse and field experiments. In the glasshouse, wetting agent performance was not affected by delayed initial irrigation, however short irrigation return intervals improved plant growth in both untreated and wetting agent treated soil. A survey of golf courses throughout New Zealand found that repellency was a major management problem. Soil cores were removed from areas of greens displaying repellency symptoms ('dry patch') and from areas of compatatively healthy turf. No significant difference was found between the MED profile or the thatch content of the dry patch and non-dry patch cores. Dry patch areas were found to match the poorly irrigated areas of a green using a simple 'catch-can' test, which indicated that irrigation uniformity affected repellency expression in turf.