Sequestration of arsenic(III) by synthetic mackinawite under anoxic conditions.

This study examined the ability of mackinawite (ferrous sulfide) to sequester arsenite (trivalent arsenic oxyanion) under anoxic, sulfate-reducing conditions. The chemically synthesized mackinawite consisted of nanoparticulate, layered platelets. Aging mackinawite four days resulted in a sulfur-rich composition with decreased solubility and improved uptake of As(III) compared to 30-minute-aged mackinawite. Mackinawite is susceptible to oxidative dissolution by H + and H2O forming greigite at low pH and hydroxy-green rusts under alkaline conditions thereby impacting its solubility. The oxidation of mackinawite by As(III) occurs during the uptake of As(III) from solution. Mackinawite suspensions (0.1-40 g/L) effectively lower initial As(III) concentrations of 1.3x10-5 M from pH 4 to 10, with maximum removal occurring under acidic conditions. Arsenite uptake capacity was estimated at 10-5, 10-4 and 10-3 mol As/g FeS in pH 9, 7 and 5 solutions, respectively. Solid phase X-Ray Absorption (XAS) spectra revealed that at pH 5, 7 and 9 with 5x10 -4 M As(III), arsenic was reduced from its original +III valence state and was primarily coordinated as As-S (∼2.26 A) and As-As (2.54 A) which is consistent with the formation of a realgar-like phase observed in XRD data. Samples reacted with 5x10-5 M As(III) showed a transition from As-O coordination to As-S coordination as pH decreased, indicating that adsorbed arsenite dominated at pH 9 whereas the realgar-like solid is present in greater quantities at pH 5. Clarification of reaction pathways resulted in a self-consistent set of Gibbs free energy of formation values for ferrous iron, mackinawite, greigite, thioarsenite species and arsenic solids for computation of equilibrium constants to describe the Fe-S-As-H 2O system. These log K values were incorporated into a thermodynamic model to describe and predict mackinawite behavior in the absence and presence of As(III) while incorporating the effects of redox. At low pH, redox was found to be controlled by the FeS/Fe3S4 couple (where Fe2+ is in excess), while at higher pH (where HS- is in excess), the FeS/Fe2(OH)5 couple likely poised the redox conditions. This study lends credence to the use of mackinawite in scavenging As(III) from porewaters of reduced sediments and provides a model to predict As(III) uptake.PhDApplied SciencesEarth SciencesEnvironmental engineeringGeochemistryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/126918/2/3287514.pd