Kinetics and mechanisms of pH-dependent selenite removal by zero valent iron

The kinetics of Se(IV) removal by zero valent iron (ZVI) open to the air as a function of pH and the involved mechanisms were investigated in this study. The specific rate constants of Se(IV) removal by ZVI decreased from 92.87 to 6.87 L h⁻¹ m⁻² as pH increased from 4.0 to 7.0. The positive correlation between the removal rate of Se(IV) and the generation rate of Fe(II) and the depression of Se(IV) removal in the presence of 1,10-phenanthroline indicated that both ZVI and adsorbed Fe(II) on ZVI surface contributed to the reductive removal of Se(IV). The soft X-ray STXM measurement confirmed the adsorption of Fe(II) on the surface of ZVI and freshly formed ferric (hydr)oxides. Se(IV) was removed by adsorption followed by reduction to Se(0) on ZVI surface at pH 4.0–7.0, as revealed by XANES spectra. A core-shell structure was observed when ZVI reacted with Se(IV)-containing solution for 3 h at pH 6.0. Se(IV) was reduced to Se(0) and co-precipitated with the freshly formed Fe(III), forming the shell surrounding the iron core. After reaction for 24 h, the generated Se(0) was surrounded by multiple layers of Fe(III) oxides/hydroxides. SEM images and XRD patterns revealed that the corrosion products of ZVI at pH 6.0 transformed from amorphous iron hydroxides to lepidocrocite (γ-FeOOH) as reaction proceeded. The final corrosion products of ZVI contained both lepidocrocite and goethite at pH 5.0 while they were X-ray amorphous at pH 4.0 and 7.0.     

Hexavalent chromium removal from near natural water by copper-iron bimetallic particles

The reduction of hexavalent chromium (Cr(VI)) by zero-valent iron (ZVI) is self-inhibiting in near natural groundwater because insulating Fe(III)-Cr(III) (oxy)hydroxide film forms on the ZVI surface during the reaction. This study tries to overcome this deficiency by coating the surface of ZVI with copper to form copper-iron bimetallic particles. The Cr(VI) removal rate by ZVI rose significantly after the copper coating was applied. The copper loading needed for enhancing Cr(VI) removal was much higher than that needed for enhancing removal of chlorinated organic compounds or other oxidative contaminants, because of the higher oxidation potential of Cr(VI). The results of X-ray photoelectron spectroscopy (XPS) indicate that coating copper onto the surface of ZVI can not only increase the deepness of the oxidation film but also increase the oxidation state of iron in the film. This phenomenon means higher Cr(VI) removal capacity per unit weight of ZVI.     

Influences of humic acid, bicarbonate and calcium on Cr(VI) reductive removal by zero-valent iron

The influences of various geochemical constituents, such as humic acid, HCO₃⁻, and Ca²⁺, on Cr(VI) removal by zero-valent iron (Fe⁰) were investigated in a batch setting. The collective impacts of humic acid, HCO₃⁻, and Ca²⁺ on the Cr(VI) reduction process by Fe⁰ appeared to significantly differ from their individual impacts. Humic acid introduced a marginal influence on Fe⁰ reactivity toward Cr(VI) reduction, whereas HCO₃⁻ greatly enhanced Cr(VI) removal by maintaining the solution pH near neutral. The Cr(VI) reduction rate constants (k_obs) were increased by 37.8% and 78.3%, respectively, with 2 mM and 6 mM HCO₃⁻ in solutions where humic acid and Ca²⁺ were absent. Singly present Ca²⁺ did not show a significant impact to Cr(VI) reduction. However, probably due to the formation of passivating CaCO₃, further addition of Ca²⁺ to HCO₃⁻ containing solutions resulted in a decrease of k_obs compared to solutions containing HCO₃⁻ alone. Ca²⁺ enhanced humic acid adsorption led to a minor decrease of Cr(VI) reduction rates. In Ca²⁺-free solutions, humic acid increased the amount of total dissolved iron to 25 mg/l due to the formation of soluble Fe-humate complexes and stably dispersed fine Fe (oxy)hydroxide colloids, which appeared to suppress iron precipitation. In contrast, the coexistence of humic acid and Ca²⁺ significantly promoted the aggregation of Fe (oxy)hydroxides, with which humic acid co-aggregated and co-precipitated. These aggregates would progressively be deposited on Fe⁰ surfaces and impose long-term impacts on the permeability of PRBs.     

Removal of co-present chromate and arsenate by zero-valent iron in groundwater with humic acid and bicarbonate

The interactions of co-present Cr(VI) and As(V), and the influences of humic acid and bicarbonate in the process of Cr(VI) and As(V) removal by Fe⁰ were investigated in a batch setting using simulated groundwater with 5 mM NaCl, 1 mM Na₂SO₄, and 0.8 mM CaCl₂ as background electrolytes at an initial pH value of 7. Cr(VI) and As(V) were observed to be subject to different impacts induced by co-existing As(V) or Cr(VI), humic acid and bicarbonate, originating from their distinct removal mechanisms by Fe⁰. Cr(VI) removal is a reduction-dominated process, whereas As(V) removal principally involves adsorption onto iron corrosion products. Experimental results showed that Cr(VI) removal was not affected by the presence of As(V) and humic acid. However, As(V) removal appeared to be inhibited by co-present Cr(VI). When the Cr(VI) concentration was 2, 5, and 10 mg/L, in the absence of humic acid and bicarbonate, As(V) removal rate constants were decreased by 27.9%, 49.0%, and 61.2%, respectively, which probably resulted from competition between Cr(VI) and As(V) for adsorption sites of the iron corrosion products. Furthermore, the presence of humic acid significantly varied As(V) removal kinetics by delaying the formation and aggregation of iron hydroxides due to the formation of soluble Fe-humate complexes and stably dispersed fine iron hydroxides colloids. In the presence of bicarbonate, both Cr(VI) and As(V) removal was increased and the inhibitory effect of Cr(VI) on As(V) removal was suppressed, resulting from the buffering effects and the promoted iron corrosion induced by bicarbonate, and the formation of CaCO₃ in solution, which enhanced As(V) adsorption.     

Preparation of chitosan-stabilized Fe nanoparticles for removal of hexavalent chromium in water

Chitosan-stabilized Fe⁰ nanoparticles (CTO-Fe⁰) and Fe⁰ nanoparticles synthesized in ethanol-water mixed system (EW-Fe⁰) were tested for reduction of Cr(VI) in water. Fourier transform infrared (FTIR) study suggested that nitrogen and oxygen atoms are the binding sites for chitosan on iron which was accountable for the stability of Fe⁰ nanoparticles. While the EW-Fe⁰ ignites spontaneously when exposed to air, the CTO-Fe⁰ was still in zero valence state after exposure to air over 2-month period as shown by X-ray powder diffraction patterns. Batch experiments demonstrated that the maximum Cr(VI) reduction rates for CTO-Fe⁰ was about 3 times higher than EW-Fe⁰. Characterizations with high-resolution X-ray photoelectron spectroscopy (HR-XPS) revealed that Cr(VI) was reduced to Cr(III) and Fe(III) was the only component present on the Fe⁰ nanoparticles surface. Additionally, chitosan can inhibited the formation of Fe(III)-Cr(III) precipitation due to its high ability to chelate Fe(III) which resulted in k_obs for CTO-Fe⁰ was about 1-3 times higher than EW-Fe⁰. Due to the fast reaction kinetics and good stability against oxidation in air, the chitosan-stabilized Fe⁰ nanoparticles have the potential to become an effective agent for in situ subsurface environment remediation.     

Hardness and carbonate effects on the reactivity of zero-valent iron for Cr(VI) removal

Zero-valent iron (Fe0) was used to remove hexavalent chromium, Cr(VI), in groundwater via a coupled reduction-oxidation reaction. Nine columns were set up under various groundwater geochemistry to investigate the effects of hardness and carbonate on Cr(VI) removal. The Cr(VI) removal capacity of Fe0 was found to be about 4 mgCr/g Fe0 in the control column (i.e., column 1). A slight decrease in the Cr(VI) removal capacity was found in the presence of calcium hardness. However, there was a 17% drop in the Cr(VI) removal capacity when magnesium hardness was present at low to moderately hard level. Results also revealed that carbonate changed the morphology of the Fe0 by formation of pale green precipitates on the iron filings. Furthermore, there was a 33% decrease in the Cr(VI) removal capacity of Fe0 when both carbonate and hardness ions were present. In general, the presence of hardness ions and carbonate in groundwater have great impact on the Fe0 by formation of passivated precipitates, such as CaCO3, on the Fe0 surface resulting in a diminished lifespan of the Fe0 by blocking electron transfer.     

Recovery of chromium(VI) from wastewaters with iron(III) hydroxide—I: Adsorption mechanism of chromium(VI) on iron(III) hydroxide

Chromium(VI) [Cr(VI)] is adsorbed as HCrO⁻₄ on iron(III) hydroxide at pH below 8.5. The Cr(VI) adsorption is suppressed by the presence of other anions such as SO²⁻₄ and SCN⁻, and enhanced by the presence of metal ions such as Cd(II) and Pb(II). The suppression is due to the competitive adsorption of other anions, depending upon the stability of their iron complexes. The enhancement is probably due to the increase in adsorption sites as a result of coprecipitation of metal ion with iron(III) hydroxide.     

Removal of chromium from aqueous solutions and plating bath rinse by an electrochemical method

In this work, a bipolar packed bed electrolytic cell having steel Raschig rings behaving as electrodes has been used to remove chromium in the form of Cr(VI) from aqueous solutions and a sample of plating bath rinse.

Fe²⁺ ions generated at the anode side of the electrodes have reacted with OH^‐ ions generated at the cathode side forming Fe(OH)₂ in this electrolytic cell.Cr(VI) have coprecipitated as Cr(OH)₃ with Fe(OH)₃ forming after the redox reaction between Fe(OH)₂ and Cr(VI). Meanwhile other impurities have also removed besides Cr(VI) using this process. Removal rate of 100% for Cr(VI) has been achieved in the experiments done with different initial Cr(VI) concentration, duration of electrolysis and applied potential. Fully removal of Cr(VI) has also been achieved from the sample of plating bath rinse.     

The kinetics of hexavalent chromium reduction by metallic iron

The rate reduction of hexavalent chromium (Cr(VI)) by metallic iron under a range of conditions was studied in batch systems. The chemical variables studied were the Cr(VI) concentration, hydrogen ion concentration and surface area of iron. The influence of ionic strength and mixing rate was also examined. The reaction kinetics were found to be dependent on hydrogen ion concentration, hexavalent chromium concentration and iron surface area and to adhere to the following kinetic expression.

Full-size image (1K)

.The rate constant was evaluated and found to have a value of 5.45 × 10⁻⁵ 1 cm⁻² min⁻¹ over a wide range of conditions.The rate constant was found to increase as mixing rate increased up to a maximum value beyond which the rate was essentially independent of mixing. Increases in ionic strength were found to result in a rapid decrease in the rate constant at ionic strengths below 0.1 M. Further increases in ionic strength had no detectable impact on the rate constant. All rate determination studies were run in the mixing and ionic strength independent regions of these systems.Reaction stoichiometry was found to be, with one exception, independent of environmental conditions. In general, 1.33 mol of iron dissolved for each mol of Cr(VI) reduced. This highly efficient utilization of iron in the reduction suggests that hydrogen generated during iron dissolution may be acting as a reductant for the Cr(VI). The single parameter which influenced the reaction stoichiometry was the initial Cr(VI) concentration. The ratio of Cr(VI) reduced to iron dissolved increased rapidly as the Cr(VI) concentration increased. This observation was taken as being consistant with a surface interaction between the hexavalent chromium and some metastable hydrogen species at the iron surface.     

The removal of chromium(VI) from dilute aqueous solution by activated carbon

The removal of chromium(VI) by activated carbon, filtrasorb 400, is brought by two major interfacial reactions: adsorption and reduction. Chemical factors such as pH and total Cr(VI) that affect the magnitude of Cr(VI) adsorption were investigated. The adsorption of Cr(VI) exhibits a peak value at pH 5–6. The particle size of carbon and the presence of cyanide species do not change the magnitude of chromium removal. The reduced Cr(VI), e.g. Cr(III) is less adsorbable than Cr(VI).The free energy of specific chemical interaction, ΔG^chem was computed by the Gouy-Chapman-Stern-Grahame model. The average values of ΔG^chem are −5.57 RT and −5.81RT, respectively, for Cr(VI) and CN. These values are significant enough to influence the overall magnitude of Cr(VI) and CN adsorption. Results also indicate that HCrO⁻₄ and Cr₂O²⁻₇ are the major Cr(VI) species involved in surface association.