ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1994.tb00479.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SummaryAssessment of gas diffusivityin situgives a direct measure of the ability of soils to exchange gas with minimal soil disturbance. A versatile, readily portable probe for measuring the diffusion of a tracer gas through soilin situis described. The radioactive tracer85Kr is injected into a cell located at the end of the probe. The change in activity within the cell as the gas diffuses out is measured by a Geiger‐Muller tube in the cell. The probe can be used by insertion either directly into an auger hole (buried‐probe mode) or into a chamber pushed into the soil surface. A method to simulate diffusion numerically using Fick's equation for both methods of insertion is presented. In the tests reported, diffusivity was estimated by expanding or contracting the time axis of the simulation until it matched the observed count rates. A goodness‐of‐fit was attached to each diffusivity estimate. The probe was generally effective, giving diffusivities comparable to those measured in the laboratory on cores taken near the cell (buried‐probe mode) or linked to the surface chamber. Poor fits were found for some diffusivities measured in the buried‐probe mode on coarsely structured soils. These were attributed to non‐uniform distribution of porosity and possible upward leakage of tracer when used at shallow depths in the buried‐probe mode. However, thein situdiffusivities may be more representative than those measured in cores in the laboratory because of the greater sample volume. We show how the probe can be used to detect soil layers that restrict gas diffusivity and differences in aeration status between til

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1994.tb00480.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SummaryThe surface structure of many Australian red and red‐brown earths frequently collapses (slakes) when dry, disturbed aggregates are wetted by rain or irrigation. The resulting fine matrix sets, on drying, to a strong, cohesive layer of up to 200 mm thick (hard setting). We investigated the mechanism of collapse and the extent to which the structure of aggregate beds Iron hard setting and non‐hard setting soils collapsed when wetted by quick flooding or slowly with water at a suction of 200 mm, then drained in sequential steps of increasing suction and finally dried at 40°C. After flood wetting, but before draining, no collapse was observed due to the small effective stress prevalent in the flooded beds.After suction wetting, some collapse was measured owing to the effective stress (approximately 1.4 kPa) from the applied suction. On draining, flood‐wetted beds collapsed extensively (volume strain>0.20), largely due to the disappearance of large pores (>75 μm diameter). Suction‐wetted beds collapsed less (volume strain0.20 and 0.10, respectively), while non‐hard setting soils did not collapse as extensively (volume strain<0.16 and 0.09, respectively). Results indicate that the mechanism causing collapse was independent of wetting method and involved two steps: (i) slaking of aggregates on wetting, and (ii) collapse of the aggregate bed on draining as a result of development of effective stress with

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1994.tb00481.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SummaryThe strength of soils is related to structural stability. Aggregate structure which collapses on wetting may set to a hard, consolidated layer on drying (hard setting). This process may be moderated by suction wetting and possibly by application of calcium, but the mechanism of moderation is not clear. We investigated the collapse‐strength relationship and the mechanism by which wetting method and calcium act to reduce strength in hard setting and non‐hard setting soils. Indirect tensile strength of aggregate beds that had been wetted with water or 10mM CaCI2by rapid flooding or at a suction of 200 mm, was measured after draining to various suctions and drying at 40°C.The greater the volume strain during wetting and draining, the greater the tensile strength. Beds that were suction wetted, either with water or calcium solution, showed minimal collapse and did not develop high strength on drying. Water‐flooded beds had the greatest dry strength while beds flooded with calcium solution developed significantly lower strength. The critical factor determining tensile strength of the beds appeared to be the presence of large (>75 μm diameter) pores. The greater the volume strain on wetting, the smaller the proportion of larger pores and the greater the tensile strength on drying. Flood wetting caused more loss of large pores and closer packing of particles. Wetting with calcium solution did not affect the degree of collapse compared with that of water alone but did produce beds with larger pores than when wetted with water. Consequently the strength of the calcium‐wetted beds

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1994.tb00482.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SummaryAt the groundwater–surface‐water interface the last modification of discharging groundwater takes place. In this study the redox potential was measuredin situin four groundwater discharge areas with a size of 3–4 m2. Three of them were the upper reaches of streams in coniferous forests and one was an alder fen. Measurements were made at different seasons, with 50 and 100 platinum micro‐electrodes respectively, in each area. Readings were taken in the upper 30 cm of the soil, at two depths.A method study showed that the electrodes needed a longer equilibrium time than is usually reported. The main finding of this study was the large spatial variation within soils in the discharge zone; regions as small as 10 cm in diameter could have redox potentials covering several hundred mV. It was not possible to identify a seasonal variation due to the large spatial variation. Differences in the distribution of the redox potential with depth may be explained by the structure of the peat affecting flow patterns and the residence time o

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1994.tb00483.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SummaryThe sorption of phosphate on amorphous aluminium hydroxides was investigated using27Al and71P solid‐state magic‐angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, following the effect of different exposures to soluble phosphate. The spectra obtained were compared with the spectrum of amorphous aluminium phosphate. Aluminium in the unreacted hydroxide had a 100% octahedral co‐ordination. When dried at 200°C and exposed to soluble phosphate, very little (maximum 0.1%) amorphous aluminium hydroxide transformed to a tetrahedral co‐ordination (A1 bound by oxygen bridges to four P atoms), even after 120d. The tetrahedral co‐ordination exists in aluminium phosphate gel, although most of its A1 atoms exhibit an octahedral co‐ordination. For the aluminium hydroxide dried at 200°C, no formation of aluminium phosphate in which aluminium is in octahedral co‐ordination could be detected, not even when the aluminium hydroxide was exposed to a phosphate solution for 120 d. We concluded that the formation of aluminium phosphate is restricted to the surface of the hydroxide. Most of the phosphate which is bound to the aluminium oxide however may not have formed a ‘bulk solid’ aluminium phosphate, but is adsorbed on the internal and external surface of the oxide. The same amorphous aluminium hydroxide, dried at 70°C instead of 200°C, is converted much more rapidly to aluminium phosphate when exposed to soluble phosphate. We propose a P‐induced weathering mechanism to describe P sorption on amorphous aluminium hydroxides at high P concentrations. In addition to NMR, phosphate adsorption experiments conducted on aluminium hydroxides dried at different temperatures produced evidence that the porosity of the aluminium hydroxide aggregated particles can also be a factor controlling the rate of phosphate uptake from solution, if the aggregate is stable (is not r

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1994.tb00484.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SummaryIron (hydr)oxides are active phosphate sorbents in soils and sediments, but no information exists on phosphate (P) sorption by natural hematites. In this study, we characterized the chemical, mineralogical and P‐sorption properties of 14 hematite‐rich natural materials of different origins. Phosphate sorption was described by a modified Freundlich equation including a time term. Phosphate sorbed at 1d at an equilibrium concentration of 1 mg P dm−3ranged widely (0.2–1.7 μmol m−2). After 1 d, hematites showed a marked slow sorption. At 75 d, and an equilibrium concentration of 6mg P dm−3, the total amount of P sorbed ranged between 0.8 and 4.1 μmol mp−2. Phosphate‐sorption capacity was influenced by the morphology of the coherently scattering domains: the shorter the domains in the crystallographiccrelative to theadirection, the lower the P‐sorption capacity. This has been observed by other authors in synthetic hematites and agrees with the idea that the prismatic faces, which have singly co‐ordinated Fe‐OH groups, are more active in P‐sorption than other faces. The average P sorption capacity of hematites was similar to that of natural goethites (2.6 μmol P m−2) but, in contrast to these, variability among samples was high. In comparison with goethites, hematites show slower sorption and

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1994.tb00485.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SummaryWe wished to determine whether soil‐test P was affected by storing air‐dry soil samples at room temperature. The soil samples had been collected from field experiments and air‐dried (<40°C) before measuring soil‐test P (bicarbonate‐extractable P). The samples were from field plots that had been treated with different applications of fertilizer P (superphosphate, rock phosphate) one or more years previously. Soil‐test P was measured on two different sub samples of the same sample: either A, in the year the sample was collected; or B, after the sample had been stored at fluctuating room temperatures either from 2 to 8 years (four field experiments) or 17 years (59 field experiments). The room temperature ranged from 10 to 30°C, and averaged 17°C. The aim was to test whether soil‐test P was systematically and consistently different between sub samples A and B.Differences between A and B were mostly small, and there were no consistent or systematic differences. For the Colwell soil test, applied to a range of south‐western Australian soils, possible decreases in soil‐test P due to continued reaction with the soil could not be detected using bicarbonate‐extractable soil P, and storage of air‐dry samples at room temperature did not significantly affect soil‐test P measured up to 17 years later. We conclude that, provided fertilizer P has had time to react with soil in the field, no further changes in Colwell soil‐test P occur during air‐dry storage for up t

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1994.tb00486.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SummaryA method for the measurement of Pb and Cd in equilibrium soil solutions involving soil equilibration with a dilute Ca electrolyte, centrifugation and filtration to<0.2 μm was evaluated. The procedure was subsequently used for the analysis of 100 Pb‐ and 30 Cd‐contaminated soils. Solutions were analysed for Pb‐ and Cd using graphite‐furnace AAS and the concentrations of Pb2+and Cd2+were estimated using standard speciation calculations.The concentrations of Pb and Cd found in the soil solutions were in the range 3.5–3600 μg dmp−3and 2.7–1278 μg dm−3respectively; both ranges represented less than 0.1% of the total metal concentration in the soils. Depending on solution pH, Pb+2accounted for between 42–78% of Pb in solution while about 65% of Cd in solution was present as Cd+2. The concentrations of Pb2+and Cd2+in solution suggested that the soil solutions were undersaturated with respect to the solid phases PbC03and CdC03but supersaturated with respect to Pb5(P04)3Cl and, for some samples, Cd3(P04)2respectively. However, for both metals, a good empirical relationship was obtained between the total metal concentration in soil (mol kg−1), free metal concentration in solution (mol dm−3) and solution pH. The relationships took the general form of a pH‐dependent Freundlich adsorption equation:For both lead and cadmium relationships, the values ofnandK1were close to unity, so that the distribution coefficient could be estimated from pH and a single metal‐dependent constant,K2. The algorithms appeared to be valid over a metal concentration range of four logarithmic u

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1994.tb00487.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

SummaryThe effect of the presence of alpechín, wastewater from the olive‐oil industry, on the reaction of three soils with Cd, Cu and Zn is studied. Adsorption of Cu and Zn by all the soils studied, and Cd by one of the soils, is markedly decreased by the presence of high doses of alpechín, while Cd adsorption by the other two soils increases in the presence of the residue. Adsorption of Cu and Zn in the absence of alpechín releases considerably more Ca than the exchangeable contents at a rate of about 0.5‐1 moles of Ca for each mole of metal adsorbed, and this rate is decreased in most cases when alpechín is present. The behaviour of Cd with respect to Ca release is less definite. The relationships between solution concentrations and pH show that alpechín increases the mobility of Cd and Cu in the presence of some soils, while in the case of Zn no effect is evident.It is concluded that soil carbonate is the most relevant component involved in Cu or Zn retention, and, in the presence of alpechín, part of the carbonate surface is blocked and becomes unavailable for retention. Formation of soluble Cu complexes is suggested to occur in s

ISSN:1351-0754    

DOI:10.1111/j.1365-2389.1994.tb00488.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY