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- Bengtsson, Åsa, 1974-
(author)
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Solubility and Surface Complexation Studies of Apatites
- 2007
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Doctoral thesis (other academic/artistic)abstract
- Apatites are a diverse class of phosphate minerals that are important in a great variety of natural and industrial processes. They are, for example, used as raw material in fertiliser production and in the remediation of metal-contaminated soils. Hydroxyapatite Ca5(PO4)3OH, (HAP) and fluorapatite Ca5(PO4)3F, (FAP) are similar to the biological apatite that is the main constituent of mammalian bone and teeth, and they are therefore promising materials for artificial bone and tooth implants. This thesis is a summary of four papers with focus on dissolution and surface complexation reactions of HAP and FAP in the absence and presence of both organic ligands and the natural and commonly occurring iron oxide goethite (α-FeOOH). The dissolution and surface complexation of HAP and FAP was investigated with a combination of different techniques. Potentiometric acid/base titrations and batch experiments were combined with X-ray Photoelectron Spectroscopy (XPS) and Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) Spectroscopy to generate dissolution and surface complexation models for both apatites. The results from these studies showed that both apatites form surface layers that are different from their bulk compositions when equilibrated in aqueous solutions. The modeling efforts predicted speciation of these surfaces as well as the concentration of the dissolution products in the solution. The interaction between organic ligands and the apatite surfaces was also investigated and the results from this study show that the organic ligands form outer-sphere complexes on the apatite surfaces over a large pH interval, and that this adsorption enhances the dissolution of apatites. The presence of goethite also enhances the dissolution of FAP as it acts as a sink for the phosphate released from FAP. Phase transformation in this system was detected using ATR-FTIR as the phosphate adsorbed to the goethite surface precipitates as FePO4 (s) after approximately 15 days of reaction time. This changes the speciation, and possibly also the bioavailability of phosphate in this two-mineral system.
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| 2. |
- Biswas, Ashis, 1983-
(author)
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Arsenic Geochemistry in the Alluvial Aquifers of West Bengal, India : Implications for targeting safe aquifers for sustainable drinking water supply
- 2013
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Doctoral thesis (other academic/artistic)abstract
- The natural occurrences of high (>10 μg/L) dissolved arsenic (As) in groundwater of Bengal Basin has put millions of people under the threat of chronic As exposure through drinking water. Present study has examined the processes that regulate As mobilization and its distribution in shallow aquifers and the potentiality of finding safe aquifers within shallow depth (<50 m) for drinking water supply. The results indicate that in terms of aquifer sediment colors and water quality two types of aquifer namely brown sand aquifer (BSA) and grey sand aquifer (GSA) can be distinguished within the depth, accessible by low-cost drilling. The redox condition in the BSA is delineated to be Mn oxyhydroxides reducing, not sufficiently lowered for As mobilization resulting in high Mn and low Fe and As in groundwater. While in GSA, currently the reductive dissolution of Fe oxyhydroxides is the prevailing redox process causing As mobilization into groundwater of this aquifer type. It is revealed that the vertical distribution of As and other aqueous redox parameters is related to the redox zonation within aquifer. The decoupling of As and Fe release into groundwater is evident in the shallowest part of aquifer because of Fe enrichment by weathering of silicate minerals especially of biotite, the precipitation of secondary mineral phases like siderite and vivianite and incomplete reduction of Fe oxyhydroxides. It is characterized that the seasonal variations of As and other aqueous solutes are limited within the upper portion of aquifer only (<30 m bgl) and can be related to seasonal cycling of redox status, aggregation and dispersion of As scavenging colloids, local groundwater abstraction and monsoonal recharge. The results of surface complexation modeling indicate that PO43- is the major competitor of As(III) and As(V) adsorption onto Fe oxyhydroxides. This study concludes that the reductive dissolution of Fe oxyhydroxides followed by competitive sorption reactions with the aquifer sediment is the process conducive for As enrichment in groundwater of Bengal Basin. Present study advocates that despite low concentration of As in groundwater, a rigorous assessment of attendant health risk for Mn is necessary prior to considering mass scale exploitation of the BSA for sustainable drinking water supply. This study also validates that TW platform colors can be used as a rapid screening tool for As and Mn in drinking water wells to prioritize As mitigation management.
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