Batch tests were performed to evaluate the effects of inorganic anion competition on the kinetics of arsenate (As(V)) and arsenite (As(III)) removal by zerovalent iron (Peerless Fe0) in aqueous solution. The oxyanions underwent either sorption-dominated reactions (phosphate, silicate, carbonate, borate, and sulfate) or reduction-dominated reactions (chromate, molybdate, and nitrate) with Peerless Fe0 in the presence of As(V) or As(III), relative to chloride. Pseudo-first-order rate equations were found to describe satisfactorily both As(V) and As(III) removal kinetics in the presence of each competing anion. Of the oxyanions tested for Peerless Fe0 in the pH range from 7 to 9, phosphate caused the greatest decrease in As removal rate (7.0 x 10(-3) to 18.5 x 10(-3) h(-1)) relative to chloride (34.9 x 10(-3) to 36.2 x 10(-3) h(-1)). Silicate, chromate, and molybdate also caused strong inhibition of As removal, followed by carbonate and nitrate, whereas borate and sulfate only caused slight inhibition to As(III) removal. Present results show that Peerless Fe0 may be an excellent permeable reactive barrier medium for a suite of mixed inorganic contaminants. The anion competing effects should be considered when designing permeable reactive barriers composed of zerovalent iron for field applications to remediate As(V) and As(III).

No abstract

Batch tests were performed utilizing four zerovalent iron (Fe0) filings (Fisher, Peerless, Master Builders, and Aldrich) to remove As(V) and As(III) from water. One gram of metal was reacted headspace-free at 23 degrees C for up to 5 days in the dark with 41.5 mL of 2 mg L(-1) As(V), or As(III) or As(V) + As(III) (1:1) in 0.01 M NaCl. Arsenic removal on a mass basis followed the order: Fisher > Peerless Master Builders > Aldrich; whereas, on a surface area basis the order became: Fisher > Aldrich > Peerless Master Builders. Arsenic concentration decreased exponentially with time, and was below 0.01 mg L(-1) in 4 days with the exception of Aldrich Fe0. More As(III) was sorbed than As(V) by Peerless Fe0 in the initial As concentration range between 2 and 100 mg L(-1). No As(III) was detected by X-ray photoelectron spectroscopy (XPS) on Peerless Fe0 at 5 days when As(V) was the initial arsenic species in the solution. As(III) was detected by XPS at 30 and 60 days present on Peerless Fe0, when As(V) was the initial arsenic species in the solution. Likewise, As(V) was found on Peerless Fe0 when As(II) was added to the solution. A steady distribution of As(V) (73-76%) and As(III) (22-25%) was achieved at 30 and 60 days on the Peerless Fe0 when either As(V) or As(III) was the initial added species. The presence of both reducing species (Fe0 and Fe2+) and an oxidizing species (MnO2) in Peerless Fe0 is probably responsible for the coexistence of both As(V) and As(III) on Fe0 surfaces. The desorption of As(V) and As(III) by phosphate extraction decreased as the residence time of interaction between the sorbents and arsenic increased from 1 to 60 days. The results suggest that both As(V) and As(III) formed stronger surface complexes or migrated further inside the interior of the sorbent with increasing time.

No abstract

The degradation of trichloroethene (TCE) at 2 mg L -1 in headspace free aqueous solution by zerovalent iron (Fe 0 ) and tin (Sn 0 ) was studied in batch tests at 10, 25, 40, and 55 °C and HCl-treated Fe 0 and Sn 0 at 25 and 55 °C. Surface area normalized pseudo-first-order rate constants (k SA ) ranged from 0.44 × 10 -3 to 4.3 × 10 -3 h -1 m -2 L for Fisher Fe 0 , 0.029 × 10 -3 to 0.27 × 10 -3 h -1 m -2 L for Peerless and Master Builders Fe 0 , and 0.011 × 10 -3 to 1.31 × 10 -3 h -1 m -2 L for Fisher and Aldrich Sn 0 . The Aldrich Fe 0 was the least reactive with k SA values ranging from 0.0016 × 10 -3 to 0.011 × 10 -3 h -1 m -2 L. The HCl-washing increased metal surface area and observed rate constant (k) values but generally decreased k SA values. The calculated apparent activation energy (E a ) using the Arrhenius law for the four temperature levels ranged from 32.2 to 39.4 kJ mol -1 for the untreated Fe 0 metals and 40.5-76.8 kJ mol -1 for the untreated Sn 0 metals. Greater temperature effect was observed for Sn 0 than for Fe 0 . Our results indicate that TCE reduction by Fe 0 and Sn 0 is likely controlled primarily by chemical reaction-limited kinetics rather than by mass transport of the TCE to the metal surface. Both reductive β-elimination reaction and hydrogenolysis reaction are likely involved in the reduction of TCE by both Fe 0 and Sn 0 .