Molybdenum and arsenic behavior in a limestone aquifer in Central Florida

Abstract

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.