Effects of common groundwater ions on chromate removal by magnetite: importance of chromate adsorption

Abstract

BackgroundReductive precipitation of hexavalent chromium (Cr(VI)) with magnetite is a well-known Cr(VI) remediation method to improve water quality. The rapid (<a few hr) reduction of soluble Cr(VI) to insoluble Cr(III) species by Fe(II) in magnetite has been the primary focus of the Cr(VI) removal process in the past. However, the contribution of simultaneous Cr(VI) adsorption processes in aged magnetite has been largely ignored, leaving uncertainties in evaluating the application of in situ Cr remediation technologies for aqueous systems. In this study, effects of common groundwater ions (i.e., nitrate and sulfate) on Cr(VI) sorption to magnetite were investigated using batch geochemical experiments in conjunction with X-ray absorption spectroscopy.ResultsIn both nitrate and sulfate electrolytes, batch sorption experiments showed that Cr(VI) sorption decreases with increasing pH from 4 to 8. In this pH range, Cr(VI) sorption decreased with increasing ionic strength of sulfate from 0.01 to 0.1 M whereas nitrate concentrations did not alter the Cr(VI) sorption behavior. This indicates the background electrolyte specific Cr(VI) sorption process in magnetite. Under the same ionic strength, Cr(VI) removal in sulfate containing solutions was greater than that in nitrate solutions. This is because the oxidation of Fe(II) by nitrate is more thermodynamically favorable than by sulfate, leaving less reduction capacity of magnetite to reduce Cr(VI) in the nitrate media. X-ray absorption spectroscopy analysis supports the macroscopic evidence that more than 75 % of total Cr on the magnetite surfaces was adsorbed Cr(VI) species after 48 h.ConclusionThis experimental geochemical study showed that the adsorption process of Cr(VI) anions was as important as the reductive precipitation of Cr(III) in describing the removal of Cr(VI) by magnetite, and these interfacial adsorption processes could be impacted by common groundwater ions like sulfate and nitrate. The results of this study highlight new information about the large quantity of adsorbed Cr(VI) surface complexes at the magnetite-water interface. It has implications for predicting the long-term stability of Cr at the magnetite-water interface.Graphical abstractEffects of background anions (sulfate and nitrate) on the Cr(VI) surface coverage at the magnetite-water interface at pH 4 and 9

Salinization and arsenic contamination of surface water in southwest Bangladesh

“…All field and laboratory methods are described in Ayers et al [ 12 ]. Briefly, in 2012 and May 2013 a Hach Hydrolab 4a was used in the field to measure pH, oxidation–reduction potential Eh in millivolts (mV), temperature in degrees Celsius (°C), and specific conductivity (SpC) in millisiemens per centimeter (mS/cm).…”

“…Ayers et al [ 12 ] examined the causes of salinization and arsenic contamination of groundwater resources in Polder 32 in southwest Bangladesh and found that salts in the shallow aquifer groundwater were derived from connate water, whereas sedimentary As was mobilized by reductive dissolution of ferric oxyhydroxides. A related modeling paper also demonstrated that variance in local groundwater salinity could be explained simply by the dilution of connate groundwater having an initial mean-annual salinity through the slow, localized recharge of fresh surface water [ 13 ].…”

“…It is surrounded by tidal channels, across which lie the Sundarbans mangrove forest in the south, Polder 33 to the east, and Polder 31 to the north and west. Surface sediments and soils in Polder 32 are silt-dominated and clay-rich, forming an impermeable mud cap [ 12 , 13 ].…”

“…This study tests the hypotheses that shrimp farming and tidal channel water inundation cause salinization of water in rice paddies and freshwater ponds near Polder 32. In addition, it presents measurements of As concentrations in various surface water types and explores geochemical relationships between surface waters and previously reported groundwater compositions [ 12 ].…”

“…Arsenic is transferred from tidal channels to rice paddies through irrigation. Including groundwater samples from the same area (Ayers et al in Geochem Trans 17:1–22, 2016 ), principal components analysis and correlation analysis reveal that salinization explains most variation in surface water compositions, whereas progressive reduction of buried surface water by dissolved organic carbon is responsible for the nonconservative behavior of S, Fe, and As and changes in Eh and alkalinity of groundwater.…”