Pathways of Aqueous Cr(VI) Attenuation in a Slightly Alkaline Oxic Subsurface

Abstract

Column experiments combined with geochemical modeling, microscopic inspections, spectroscopic interrogations, and wet chemical extractions were used to study sediment-dependent Cr(VI) desorption, physical location, mineral association, and attenuation mechanism(s) in four freshly or naturally aged contaminated sediments exposed to concentrated Cr(VI) waste fluids. Results showed that majority of Cr(VI) mass was easily removed from the sediments (equilibrium site K(d) varied from 0 to 0.33 mL g(-1) and equilibrium site fraction was greater than 95%). Long tailings of time-dependent Cr(VI) concentrations above maximum contaminant level of 1.9 micromol L(-1) were also observed (kinetically controlled fraction K(d) and desorption reaction half-lives varied from 0 to 45 mL g(-1), and from 76.1 to 126 h, respectively). Microscopic and spectroscopic measurements confirmed that Cr was concentrated within fine-grained coatings in small areas mainly rich in phyllosilicates that contained both Cr(III) and Cr(VI). However, Cr(VI) reduction was neither significant nor complete. Under slightly alkaline and oxic conditions, contaminant Cr in the sediments occurred: (i) In the highly mobile pool (over 95% of total Cr); (ii) In the slow and time-dependent releasing pool, which served as long-term source of contamination; (iii) As reduced Cr(III) which most likely formed during Cr(VI) reaction with aqueous, sorbed, or structural Fe(II).

Geochemical Characterization of Chromate Contamination in the 100 Area Vadose Zone at the Hanford Site - Part 2

“…The transport of chromate [Cr(VI)O 4 2-] through uncontaminated sediments is not expected to exhibit retardation due to adsorption (sediments may exhibit minimum adsorption capacity under the given conditions of neutral or slightly basic pH because all variable charge sorbents may be slightly negatively charged) (Qafoku et al 2003(Qafoku et al , 2007(Qafoku et al , 2009(Qafoku et al , 2010Ginder-Vogel et al 2005;Dresel et al 2008). However, as clearly shown in some of these references (e.g., Qafoku et al 2003Qafoku et al , 2009Qafoku et al , 2010Dresel et al 2008) A series of column experiments were conducted to investigate Cr(VI) mobility during advective transport under saturated and unsaturated conditions to test the following hypotheses:…”

“…However, as clearly shown in some of these references (e.g., Qafoku et al 2003Qafoku et al , 2009Qafoku et al , 2010Dresel et al 2008) A series of column experiments were conducted to investigate Cr(VI) mobility during advective transport under saturated and unsaturated conditions to test the following hypotheses:…”

Application of electrochemical depassivation in PRB systems to recovery Fe0 reactivity

“…Cr(VI) is ranked as the second most frequently found heavy metal in hazardous waste sites [3]. Cr(VI) is mutagenic, teratogenic and carcinogenic [4][5][6]. Consequently, the concentration of Cr(VI) in groundwater used for drinking water purposes must be minimized.…”

Effects of liming on Cr(VI) reduction and Cr phytotoxicity in Cr(VI)-contaminated soils

“…A distinct spectral feature, the pre-edge peak, occurs in the X-ray absorption near edge structure (XANES) spectra for Cr(VI), which is used to quantify the proportion of total Cr as Cr(VI) in soil (Shaffer et al 2001;Qafoku et al 2009). The pre-edge peak at 5992 eV in the Cr K-edge XANES spectra is a unique characteristic of Cr(VI) but not of Cr(III) (Bajt et al 1993;Szulczewski et al 1997).…”