Structural elucidation of hexavalent Cr adsorbed on surfaces and bulks of Fe3O4 and α-FeOOH

Arsenic Removal via the Biomineralization of Iron-Oxidizing Bacteria Pseudarthrobacter sp. Fe7

Arsenic (As) is a highly toxic metalloid, and its widespread contamination of water is a serious threat to human health. This study explored As removal using Fe(II)-oxidizing bacteria. The strain Fe7 isolated from iron mine soil was classified as the genus Pseudarthrobacter based on 16S rRNA gene sequence similarities and phylogenetic analyses. The strain Fe7 was identified as a strain of Gram-positive, rod-shaped, aerobic bacteria that can oxidize Fe(II) and produce iron mineral precipitates. X-ray diffraction, X-ray photoelectron spectroscopy, and energy-dispersive X-ray spectroscopy patterns showed that the iron mineral precipitates with poor crystallinity consisted of Fe(III) and numerous biological impurities. In the co-cultivation of the strain Fe7 with arsenite (As(III)), 100% of the total Fe and 99.9% of the total As were removed after 72 h. During the co-cultivation of the strain Fe7 with arsenate (As(V)), 98.4% of the total Fe and 96.9% of the total As were removed after 72 h. Additionally, the iron precipitates produced by the strain Fe7 removed 100% of the total As after 3 h in both the As(III) and As(V) pollution systems. Furthermore, enzyme activity experiments revealed that the strain Fe7 oxidized Fe(II) by producing extracellular enzymes. When 2% (v/v) extracellular enzyme liquid of the strain Fe7 was added to the As(III) or As(V) pollution system, the total As removal rates were 98.6% and 99.4%, respectively, after 2 h, which increased to 100% when 5% (v/v) and 10% (v/v) extracellular enzyme liquid of the strain Fe7 were, respectively, added to the As(III) and As(V) pollution systems. Therefore, iron biomineralized using a co-culture of the strain Fe7 and As, iron precipitates produced by the strain Fe7, and the extracellular enzymes of the strain Fe7 could remove As(III) and As(V) efficiently. This study provides new insights and strategies for the efficient remediation of arsenic pollution in aquatic environments.

Mineralogical characteristics of root iron plaque and its functional mechanism for regulating Cr phytoextraction of hyperaccumulator Leersia hexandra Swartz

Leersia hexandra Swartz (L. hexandra) is a promising hyperaccumulator for Cr pollution remediation, but whether its Cr phytoextraction is subject to the root surface-attached iron plaque (IP) remains unclear. In this research, the natural and artificial IPs were proven to be comprised of small amounts of exchangeable Fe as well as carbonate Fe, and dominantly Fe minerals involving amorphous two-line ferrihydrite (Fh), poorly crystalline lepidocrocite (Le) and highly crystalline goethite (Go). The Fe content in the artificial IPs augmented with increasing induced Fe(II) concentration, and the 50 mg/L Fe(II) led to the identical Fe content and different component proportions of artificial IP (Fe50) and natural IP. Fh was consisted of highly aggregated nanoparticles, and the aging of Fh caused its phase conversion to rod-like Le and Go. The Cr(VI) adsorption results of Fe minerals corroborated the coordination of Cr(VI) onto the Fh surface and the significantly greater equilibrium Cr(VI) adsorption amount of Fh over Le and Go. The greatest Cr(VI) reduction capacity of Fh among three Fe minerals was found to be related to its most abundant surface-adsorbed Fe(II) content. The results of hydroponic experiment of L. hexandra showed that the presence of IP facilitated the Cr(VI) removal by L. hexandra during the cultivation period of 10-45 days, and consequently, compared to the Fe0 group (without IP), around 60% of increase in the Cr accumulation of shoots was achieved by Fe50 group. The findings of this work are conductive to furthering our understanding of IP-regulated Cr phytoextraction of L. hexandra.

Schwertmannite and akaganéite for adsorption removals of Cr(VI) from aqueous solutions

Iron hydroxides have received high attention in the treatment of chromium (Cr) polluted wastewater. In this study, the obtained chemical (or biological) pincushion-schwertmannite spheres had a diameter of 2 - 5 μm (0.5-1 μm), and akaganéite rods had a length of 300-500 nm (100-150 nm) at an axial ratio of about 3. The average diameters (μm) of their agglomerated particles in solutions were 20.6-32.5 (only 0.480 for Aka-Chem). Schwertmannites and akaganéites were used to investigate Cr(VI) adsorption behaviors in aqueous solutions by batch experiments, under various reaction times, initial Cr(VI) and adsorbent levels, pH values, temperature and anions of NO₃^-, Cl^-, CO₃^2-, SO₄^2-, and H₂PO₄^-. Adsorption data well fitted to pseudo-second-order rate model (R² = 0.999), and Langmuir (R² = 0.954-0.988) and Freundlich (R² = 0.984-0.996) isothermal models at pH 7.0. Maximum Cr(VI) adsorption capacities were 119/133 for Sch-Chem/Sch-Bio, and 14.6/83.6 for Aka-Chem/Aka-Bio. The H₂PO₄^- than SO₄^2-/CO₃^2- had a stronger effect on Cr(VI) adsorption. Adsorbents with pH_ZPC of near to 4.0 still had a good Cr(VI) removal ability at pH 3.0-8.0. The possible Cr(VI) adsorption mechanisms by FTIR and XPS results for schwertmannite and akaganéite were electrostatic attractions and ion exchanges between hydroxyl (or sulfate) and chromate ions. The Cr(VI) adsorption of optimal schwertmannite and bioakaganéite was a spontaneous, endothermic and random process at the temperatures of 288-318 K. They had a good regeneration ability for Cr(VI) adsorption, and removal ratios could reach to about 80% of original values (60-70% in aqueous solution with 60 mg/L Cr(VI) and pH7.0, and 35-50% in wastewater with 120 mg/L Cr(VI) and about pH4.0), after three cycles. Herein, schwertmannite/bioakaganéite have a promising application in treatment of acidic/neutral wastewater with chromate.