Mozaffari, Ali
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Item-typ:Veröffentlichung, Molybdenum and arsenic behavior in a limestone aquifer in Central Florida(2016-11-30); ; ; The mobilization of geogenic molybdenum (Mo) and arsenic (As) from the aquifer matrix sediments in Central Florida, USA has resulted in the contamination of groundwater and irrigated soils. The aquifer matrix contained up to 825 mg/kg Mo and 144 mg/kg As. The concentrations of Mo and As in groundwater reached up to 5000 AA g/L and 300 AA g/L which exceed the WHO guidelines of 70 AA g/L and 10 AA g/L, respectively. In this research, in order to evaluate the potential of Mo and As as contaminants in the aquifer matrix sediments, chemical fractionation, primary sources, and the adsorption/desorption behavior of Mo and As by/from hydrous ferric oxide (HFO) were studied. The adsorption of Mo onto organic matter (OM) was also investigated to determine its role in removing Mo from groundwater. First, a five step sequential extraction procedure (SEP) was applied to 10 aquifer matrix samples which were chosen from three cores near Lithia village in Central Florida. Then, the chromium-reducible sulfur (CRS) method was used to assess the primary sources of Mo and As. This particular method was used to determine the relationship between reduced sulfur including inorganic reduced sulfur such as sulfur content in pyrite (FeS2) and organic sulfur (OS) and the trace elements (Mo and As) in the aquifer matrix. Finally, to assess the adsorption and desorption of Mo and As by/from HFO and humic acid (HA), a combination of batch experiments with HFO, HA and the aquifer matrix sediments were conducted. The results of SEP showed that Mo was mainly present in the soluble fraction (step 1). For six samples, more than 80%, for two samples more than 50% and for two samples up to 20% Mo was released during the first step. About 10% Mo was leached during steps 2 and 3, which dissolve carbonates, HFO, manganese oxides and powellite (CaMoO4). Approximately 25% of Mo bounded to crystalline iron oxides, pyrite and OM in steps 4 and 5. In contrast to Mo, As was present in all the extractiona s steps in somewhat similar abundances: step 1 (17%), step 2 (11%), step 3 (30%), step 4 (23%) and step 5 (18%). Hydrous and crystalline iron oxides, which were dissolved in steps 3 and 4 contained the highest As concentrations. From this procedure, it became clear that compared to As, Mo is more easily released from the aquifer matrix. Pyrite and OM which were present in both aquifer matrix sediments underlying Lithia area and in Avon Park Formation (APF), were generally considered as source for Mo and As. To evaluate this statement, a total of 24 samples (including 10 samples which were already subjected to SEP analysis) were chosen from Lithia area and APF. The samples were selected based on the following four criteria: (1) high total Mo concentration, (2) high total As concentration, (3) high total sulfur concentration, and (4) good geographic representation of the study area. The results of CRS did not confirm the presence of Mo in pyrite in the Lithia area, though it was shown to exist as a minor constituent in the APF. The total organic carbon content in the residues of CRS (OCres) method was positively correlated to Mo (R2 = 0.71 and p 0.001). There was no correlation between OS and Mo. Compared to Mo, As concentration in pyrite was much higher, pointing to pyrite as a major primary source for As in the aquifer matrix sediments. There was a weak correlation between As and OCres. The results suggested that the Mo found in the aquifer matrix and groundwater was mainly sourced from OM rather than pyrite, whereas As originated mainly from pyrite as a primary source, in addition to iron oxides in the adsorbed forms. The adsorption and desorption of Mo and As by/from HFO and HA indicated that HFO was not a major sink for Mo but a significant secondary source for As. Molybdenum sorption onto HFO mainly happened by the formation of inner-sphere complexes. Arsenic(III) and As(V) sorption onto HFO resulted in inner-sphere complexes. Oxygenated water had no significant effect on the mobilization of Mo and As from the aquifer matrix in the Lithia area and APF. Powellite could be considered as a minor secondary source for Mo and As. In the alkaline pH ranges (pH ranges of the study area), the adsorption of Mo on HA was too low.Dissertation331 127 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Distribution and mobility of geogenic molybdenum and arsenic in a limestone aquifer matrixTo investigate the potential of Mo and As as possible geogenic contaminants, three sediment cores were examined to evaluate their mineralogical association, distribution and mobility. The cores were described and analyzed for total organic carbon (TOC), Ca, Mg, Si, Al, P, Sr, As, Mo, Fe, and S content. Except in the uppermost segment, limestone was the main lithology with the occasional presence of dolomite and clay. That change in lithology was also observed in the bulk chemical composition, where Ca, Mg and Sr concentrations increased with depth, while Si, Al and P concentrations decreased with depth. Minor minerals included pyrite (FeS2), powellite (CaMoO4) and ferrihydrite. The minimum, maximum, median and standard deviations for all analyzed elements, including As and Mo were comparable for all three cores. Molybdenum and As, however, varied significantly with depth and median As and Mo values were above their respective crustal averages of approximately 1.1 mg/kg and 1.5 mg/kg. The median values for As were 1.9 mg/kg in core DEP-1, 3.3 mg/kg in DEP-2 and 1 mg/kg in DEP-5. The median values for Mo were 2.3 mg/kg in core DEP-1, 2.5 mg/kg in DEP-2 and 2.5 mg/kg in DEP-5. Maximum concentrations for As were 101.9 mg/kg, 47.5 mg/kg and 56.2 mg/kg in cores DEP-1, DEP-2 and DEP-5, respectively. Maximum concentrations for Mo were 880 mg/kg, 123 mg/kg and 225 mg/kg in cores DEP-1, DEP-2 and DEP-5, respectively. Electron microprobe analyses of individual minerals revealed variable concentrations of As ranging from approximately 300 to 9000 mg/kg, in pyrite and up to 17,600 mg/kg in powellite (CaMoO4). The Mo concentration in pyrite was consistently below the detection limit of approximately 100 mg/kg. In powellite the Mo concentration was up to 42 wt%. A subset of 10 samples from different stratigraphic sections and with different As and Mo concentrations was further investigated to assess As and Mo mobility under changing physicochemical conditions. Leaching the aquifer matrix with a 1 M NaOAc solution at a pH of 8.1 removed more than 70% Mo in 8 of the 10 samples. The maximum value was 97%. In contrast to Mo, As was mobilized to a lesser degree. In 8 of the samples less than 30% were removed and the maximum was only 50%. Molybdenum, which seemed to be loosely bound to mineral and organic matter surfaces thus could easily be removed from the aquifer matrix, while As on the other hand should be much less mobile, because it occurred either tightly adsorbed by hydrous ferric oxide or as an impurity in pyrite. Thus, it is advisable to include Mo in the analytical program whenever elevated As concentrations are encountered in groundwater.Wissenschaftlicher ArtikelBand:6370 78