Search Results: 1 - 10 of 100 matches for " "
All listed articles are free for downloading (OA Articles)
Page 1 /100
Display every page Item
Sustainability of UK forestry: contemporary issues for the protection of freshwaters, a conclusion
C. Neal,S. J. Ormerod,S. J. Langan,T. R. Nisbet
Hydrology and Earth System Sciences (HESS) & Discussions (HESSD) , 2004,
Abstract: This paper closes the Special Issue of Hydrology and Earth System Sciences entitled 'Sustainability of UK forestry: contemporary issues for the protection of freshwaters' by presenting conclusions from the contributions together with associated research findings. The volume deals largely with issues of upland water quality and biology in the context of environmental research and management. The studies are linked to an array of issues which affect the sustainability of UK forestry in the context of the protection of freshwaters, freshwater ecosystems and freshwater organisms. These issues include atmospheric and climate driven factors (acidification from atmospheric pollutants, critical loads, climate-change and climate variability), forestry practice and hydrobiogeochemical processing both within-catchments and within-rivers. The findings lie within the context of the science and relate to environmental management. Keywords: water quality, forestry, stream ecology, acidification, critical loads, nutrients
Arsenite sorption and co-precipitation with calcite  [PDF]
Gabriela Roman-Ross,Gabriel Cuello,Xavier Turrillas,Alejandro Fernandez-Martinez,Laurent Charlet
Physics , 2008, DOI: 10.1016/j.chemgeo.2006.04.007
Abstract: Sorption of As(III) by calcite was investigated as a function of As(III) concentration, time and pH. The sorption isotherm, i.e. the log As(III) vs. log [As(OH)3 degrees / Assat] plot is S-shaped and has been modelled on an extended version of the surface precipitation model. At low concentrations, As(OH)3 degrees is adsorbed by complexation to surface Ca surface sites, as previously described by the X-ray standing wave technique. The inflexion point of the isotherm, where As(OH)3 degrees is limited by the amount of surface sites (ST), yields 6 sites nm-2 in good agreement with crystallographic data. Beyond this value, the amount of sorbed arsenic increases linearly with solution concentration, up to the saturation of arsenic with respect to the precipitation of CaHAsO3(s). The solid solutions formed in this concentration range were examined by X-ray and neutron diffraction. The doped calcite lattice parameters increase with arsenic content while c/a ratio remains constant. Our results made on bulk calcite on the atomic displacement of As atoms along [0001] direction extend those published by Cheng et al., (1999) on calcite surface. This study provides a molecular-level explanation for why As(III) is trapped by calcite in industrial treatments.
Effect of Bacterial Calcite Precipitation on Compressive Strength of Mortar Cubes  [PDF]
Jagadeesha Kumar B G,R Prabhakara,Pushpa H
International Journal of Engineering and Advanced Technology , 2013,
Abstract: In this paper results are presented on an experimental investigation carried out on mortar cubes which were subjected to bacterial precipitation by different bacterial strains and influence of bacterial calcite precipitation on the compressive strength of mortar cube on 7, 14 and 28 days of bacterial treatment. Three bacterial strains Bacillus Flexus, isolated from concrete environment, Bacillus pasturii and Bacillus sphaericus were used. The cubes were immersed in bacterial and culture medium for above mentioned days with control cubes immersed in water and was tested for compressive strength. The result indicates that there was improvement in the compressive strength in the early strength of cubes which were reduced with time. Among the three strains of bacteria, Cubes treated with Bacillus flexus which is not reported as bacteria for calcite precipitation has shown maximum compressive strength than the other two bacterial strains and control cubes. Bacillus flexus which is capable of surviving at high pH, precipitate high calcite, and has less generation time can be used for bacterial calcite precipitation as concrete crack remediation and improvement of compressive strength of both mortar and concrete
Constructing the challenge of digital didactics: the rhetoric, remediation and realities of the UK Digital Curriculum
Neil Selwyn
Seminar.net , 2008,
Abstract: This paper uses Bolter and Grusin’s remediation approach in investigating the manner in which new forms of digital media are re-casting the communicative and epistemological import of knowledge, teaching and learning. Given the considerable disparity between the rhetoric and realities of the educational implementation of information technologies to date the paper argues that particular attention should be paid to the refashioning of existing forms of pre-digital didactics in current forms of digital didactics. These themes are pursued through an examination of the UK government’s ongoing ‘Digital Curriculum’ project as a case study of remediation of didactics in the digital age.
The Effect of the CO32- to Ca2+ Ion activity ratio on calcite precipitation kinetics and Sr2+ partitioning
Tsigabu A Gebrehiwet, George D Redden, Yoshiko Fujita, Mikala S Beig, Robert W Smith
Geochemical Transactions , 2012, DOI: 10.1186/1467-4866-13-1
Abstract: Calcite was the only phase observed, by XRD, at the end of the experiments. Precipitation rates increased from 41.3 ± 3.4 μmol m-2 min-1 at r = 0.0315 to a maximum rate of 74.5 ± 4.8 μmol m-2 min-1 at r = 0.306 followed by a decrease to 46.3 ± 9.6 μmol m-2 min-1 at r = 1.822. The trend was simulated using a simple mass transfer model for solute uptake at the calcite surface. However, precipitation rates at fixed saturation states also evolved with time. Precipitation rates accelerated for low r values but slowed for high r values. These trends may be related to changes in effective reactive surface area. The a C O 3 2 - / a C a 2 + ratios did not affect the distribution coefficient for Sr in calcite (DPSr2+), apart from the indirect effect associated with the established positive correlation between DPSr2+ and calcite precipitation rate.At a constant supersaturation state (Ω = 9.4), varying the ion activity ratio affects the calcite precipitation rate. This behavior is not predicted by affinity-based rate models. Furthermore, at the highest ion ratio tested, no precipitation was observed, while at the lowest ion ratio precipitation occurred immediately and valid rate measurements could not be made. The maximum measured precipitation rate was 2-fold greater than the minima, and occurred at a carbonate to calcium ion activity ratio of 0.306. These findings have implications for predicting the progress and cost of remediation operations involving enhanced calcite precipitation where mineral precipitation rates, and the spatial/temporal distribution of those rates, can have significant impacts on the mobility of contaminants.Engineering the precipitation of calcite in groundwater has been proposed as a means for remediation of the radionuclide strontium-90, a byproduct of uranium fission and a common contaminant at nuclear facilities in the U.S. and abroad [1-4]. Because of the relatively short half-life of 90Sr (29 yrs), and the compatibil
The Reduction of the Permeability of a Lateritic Soil through the Application of Microbially Induced Calcite Precipitation  [PDF]
Anthony Smith, Martin Pritchard, Alan Edmondson, Shafakat Bashir
Natural Resources (NR) , 2017, DOI: 10.4236/nr.2017.85021
Abstract: Lateritic soils are frequently utilised in tropical areas of the developing world as an engineering material in the construction of rural earth roads, usually in the form of engineered natural surface (ENS) roads. The heavy, seasonal rainfalls common to the tropics results in ENS roads becoming quickly saturated with rainwater, and no longer accessible to motorised transportation. Microbially induced calcite precipitation (MICP) has been successfully used as a treatment process to decrease the permeability of clean, cohesionless sands by studies trying to impede the movement of groundwater, and any pollutants they may contain. In order to see if MICP treatment can also reduce the susceptibility of ENS road lateritic soils to rainwater saturation, this study has treated a Brazilian sample extracted from an ENS road in Espirito do Santo, Brazil, using the MICP bacterium Sporosarcina pasteurii contained within a urea-calcium chloride solution inoculum. Investigation, by means of a Rowe cell, of the post-treatment permeability, to untreated control samples, has shown an average decrease in the vertical coefficient of permeability of 83%, from 1.15 × 10-7 m/s for the untreated control samples, to 1.92 × 10-8 m/s in treated samples.
Modification of streaming potential by precipitation of calcite in a sand-water system: laboratory measurements pH range from 4 to 12  [PDF]
Xavier Guichet,Laurence Jouniaux,Nicole Catel
Physics , 2006, DOI: 10.1111/j.1365-246X.2006.02922.x
Abstract: Spontaneous Potentials associated with volcanic activity are often interpreted by means of the electrokinetic potential, which is usually positive in the flow direction (i.e. Zeta potential of the rock is negative). The water-rock interactions in hydrothermal zones alter the primary minerals leading to the formation of secondary minerals. This work addresses the study of calcite precipitation in a sand composed of 98% quartz and 2% calcite using streaming potential measurements. The precipitation of calcite as a secondary mineral phase, inferred by high calcite saturation indices and by a fall in permeability, has a significant effect on the electrokinetic behaviour, leading to a significant reduction in the Zeta potential (in absolute value) and even a change in sign. The measured decrease in Zeta potential from -16 mV to -27±4 mV takes place as the pH rises from 4 to 7, while it remains constant at -25±1 mV as the pH increases from 8 to 10.5. For pH higher than 10.5, calcite precipitates and is expected to coat the quartz surface. The measured Zeta potential vary from -17 to +8 mV for pH ranging from 10.6 to 11.7 depending on the amount of precipitated calcite indicated by the decrease in permeability. The observed change in sign of the electrical surface potential rules out the usual qualitative interpretation of SP anomalies in order to determine fluid circulations, even at pH lower than 9 if calcite is widely present as a secondary mineral phase, since the electrical surface potential of calcite depends also on CO2 partial pressure and [Ca2+]. Therefore SP anomalies as measured in hydrothermal field, without mineralogical analyses of hydrothermal deposits, and without geochemical fluid survey, should be interpreted with caution.
Stream water quality in acid sensitive UK upland areas; an example of potential water quality remediation based on groundwater manipulation.  [PDF]
C. Neal,T. Hill,S. Alexander,B. Reynolds
Hydrology and Earth System Sciences (HESS) & Discussions (HESSD) , 1997,
Abstract: The patterns of variation in water quality for an acidic stream draining plantation forest overlying acidic and acid sensitive gley soils with shale and slate bedrock changed following the introduction of a 45 m deep borchole near to the stream. During drilling, air flushing of debris from the borehole cleared fracture routes for groundwater penetration to the stream via the stream bed. Consequently, there were and there remain marked increases in pH, alkalinity and calcium concentrations in the stream water. The extent of this water quality improvement varies according to flow. Under extreme highfiow conditions, most of the stream water is supplied from near surface soil water sources and acidic stream waters (pH about 4.2) result. Under baseflow conditions, the stream water pH is about 7.0 upstream and about 7.5 downstream of the borehole. Under intermediate flow conditions, the improvement in pH is most marked and values increase from around 5 to around 6.3. For acid sensitive 'hard rock' areas such as those studied here, the bedrock has frequently been assumed to be both impermeable and low in base cations. This study illustrates that this view may be incorrect, and that groundwater may provide an important modifier of streamwater quality, at least for slate and shale dominated hard rock areas. Indeed, the work demonstrates clearly the potential for water quality remediation through groundwater manipulation.
Capacity and factors affecting phosphorus immobilization in Taihu Lake by calcite powder

Cong Haibing,Jiang Yajing,Liu Cheng,

环境工程学报 , 2011,
Abstract: Light-weight, heavy-weight and nano-calcite powder were used to remove phosphorus from Taihu Lake water in lab. The quantity of phosphorus removed by unit calcite powder was positively related to the phosphorus concentration in water. The capacity of light-weight and heavy-weight calcite powder to remove phosphorus fits Langmuir adsorption equation and the maximal capacity is 0.387 mg/g and 0.351 mg/g, respectively. The capacity of nano-calcite powder to remove phosphorus fits Freundlich adsorption equation. The BET specific surface area is the dominating factor affecting calcite powder to remove phosphorus, and can be used as an indicator measuring the capacity of calcite powder to remove phosphorus. Alkalinity has little effect on calcite powder to remove phosphorus when alkalinity is less than 6 mmol/L, while the capacity of calcite powder to remove phosphorus decrease when alkalinity is more than 6 mmol/L. The capacity of calcite powder to remove phosphorus increases with the increasing in pH and Ca2+ concentrtion.
Evolution and the Calcite Eye Lens  [PDF]
Vernon L. Williams
Physics , 2013,
Abstract: Calcite is a uniaxial, birefringent crystal, which in its optically transparent form, has been used for animal eye lenses, the trilobite being one such animal. Because of the calcite birefringence there is a difficulty in using calcite as a lens. When the propagation direction of incoming light is not exactly on the c-axis, the mages blur. In this paper, calcite blurring is evaluated, and the non-blurring by a crystallin eye lens is compared to a calcite one.
Page 1 /100
Display every page Item

Copyright © 2008-2017 Open Access Library. All rights reserved.