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Under iron (Fe3+)-reducing conditions where aqueous Fe2+ and unreduced solid Fe3+-oxides commonly coexist, soil Fe2+ oxidation has been shown to be coupled with nitrate (NO3-) reduction. One possible secondary reaction is the involvement of NO3- and nitrite (NO2-) with Fe-oxide minerals found in many natural environments. Yet, spectroscopic measurements and kinetic data on reactivity of NO3- and NO2- with Fe-containing oxide minerals such as goethite (a-FeOOH), and magnetite (Fe3O­4) are not found in the literature. The reactivity of goethite and magnetite with NO3- and NO2- was studied over a range of environmentally relevant pH conditions (5.5-7.5) with and without added Fe2+(aq) under anoxic conditions. Laboratory experiments were conducted using stirred batch experiments and reaction products were analyzed using ion chromatography (IC), gas chromatography (GC), ultraviolet visible near infrared spectroscopy (UV-VIS-NIR), x-ray diffraction (XRD), scanning electron microscopy (SEM), Mössbauer, and Attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy. Nitrate removal by goethite and magnetite was much slower when compared with NO2-. There was a pH-dependence in the reduction of NO2-, and the initial rate of NO2- removal was nearly 2 and 8 times faster at pH 5.5 than at pH 7.5 by magnetite and goethite, respectively. Nitric oxide (NO) and nitrous oxide (N2O) were identified as products when NO2- has reacted with magnetite, whereas N2O is the major reaction product in the experiment with goethite. In comparison to experiments containing magnetite or goethite alone, addition of Fe2+ greatly accelerated the NO2- removal rate. Wet chemical experiments combined with the Mössbauer study reveals that NO2- reduction to NO and subsequently to N2O by magnetite occurs via a heterogeneous electron transfer process. ATR-FTIR and diffuse reflectance spectroscopy (DRS) results from the studies with goethite indicate that NO2- was removed from solution by adsorption in a surface complex involving the oxygen atoms, and a portion of the nitrite is reduced to NO and N2O. This study suggests that under anaerobic conditions soil and sediments that contain goethite, magnetite, and other Fe3+-oxides can catalyze abiotic NO2- reduction and the kinetics data from this study can be used to predict the NO2- removal under such conditions.

In this study, data from field bioremediation experiments, geochemical modeling, and laboratory batch experiments were integrated with published data on arsenic (As) sorption and coprecipitation onto growing iron (Fe)-sulfide phases [e.g., iron monosulfide, FeS, pyrite, FeS2, and arsenian pyrite, Fe(S, As)2] to characterize geochemical processes during in situ bioremediation of natural As-contaminated groundwater. Field bioremediation experiments conducted in the past on groundwater in Holocene alluvial aquifers in Bangladesh and the United States (US) have shown that As was incorporated into Fe-sulfide phases in reducing groundwater, and that this process can be fast as well as efficient. This study shows that As can be removed in a matter of weeks after the injection of water-soluble labile organic carbon and sulfate that stimulate metabolism of indigenous sulfate- reducing bacteria (SRB) in Fe-bearing, low-temperature, reduced, As-contaminated groundwater. A new set of thermodynamic data for thiroarsenite species, amorphous arsenic and Fe-sulfide phases, and solid solution of arsenian pyrite (FeS1.99As0.01 – FeS1.90As0.10) were complied into a revised Geochemist’s Workbench (GWB) database, Thermo08-As, to model the principal geochemical behavior of As in aerobic and anaerobic groundwaters. Compared to the most widely used geochemical modeling programs, which lack thermodynamic data for solid solutions of arsenian pyrite, this new thermodynamic database is more realistic in characterizing and predicting As behavior in changing redox conditions. Under Fe-rich geochemical conditions, the stability field of arsenian pyrite (containing 1 to 10 wt.% As) solid solution completely dominates in reducing Eh-pH space and “displaces” other As-sulfides (orpiment, realgar) that have been implied to be important in previous modeling and field studies. Sorption of dissolved As in synthetic Fe-sulfide and natural pyrite as a function of total As concentration, sulfide ratio, Eh-pH, time, and grain size of pyrite, were investigated in the laboratory. Arsenic is strongly partitioned on both FeS and FeS2 under a range of conditions, such as pH and As concentration. In the sulfide-limited (S:Fe=1:1) experiment that produced synthetic FeS, 91% of the initial dissolved As, was sorbed. In contrast, in the excess-sulfide (S:Fe=2:1 and 3:1) experiment, 55% of the initial As concentration was sorbed, but yielded pyrite as a solid phase. Amount of As sorbed onto pyrite is dependent on grain size, but conformed to a Langmuir isotherm at circumneutral pH. Field data, geochemical modeling, and laboratory results clearly indicate that As is mobile under Fe-reducing conditions, but immobile under anaerobic and sulfate-reducing conditions. Fe-oxyhydroxides and arsenian pyrite are the likely stable mineral phases that serve as major sink for As under aerobic and sulfate-reducing conditions, respectively.