Li Liu,Jinxin Zhao,Weizhao Yin,Sihao Lv,Mei Su,Ping Li,Xiangyu Zheng,Penchi Chiang,Jinhua Wu
In this study, a collaborative system of Fe 0 and mixed anaerobic microorganisms was established for remediating chromium (Cr)-contaminated soil and restraining the translocation of Cr from soil to swamp cabbage ( Ipomoea aquatica Forssk.). Solid phase characterization demonstrated that more reactive secondary minerals such as green rust, magnetite, and lepidocrocite were generated in the composite system as compared with the Fe 0 -only system. Hence, the Fe 0 –microorganisms composite system achieved a remarkably higher aqueous Cr(VI) removal of 85.6%, 2.9 times higher than that in the Fe 0 -only system. After 14 d remediation, easily available Cr(VI) and Cr total species such as water-soluble, exchangeable, and bound-to-carbonates were converted to less available Cr(III) and Cr total species (e.g., Fe–Mn oxides-bound and organic matter-bound species) because of the production of Cr–Fe hydroxides and oxides [Cr x Fe 1−x (OH) 3 or Cr x Fe 1−x OOH] on the Fe 0 surface. A pot experiment showed that Cr uptake by swamp cabbage after the composite system remediation was suppressed by 69.1%, two times higher than that after the Fe 0 -only system remediation. Excessive Fe uptake by swamp cabbage also was efficiently inhibited by the composite system treatment due to enhanced Fe hydroxides and oxides production on the Fe 0 surface because of biological corrosion and mineralization. These results indicated that Fe 0 –microorganisms composite system remediation could efficiently enhance Cr(VI) immobilization and decrease its bioavailability and bioaccumulation by plants, which is a promising technology in Cr-contaminated soil remediation.
The cement solidification method serves as an effective approach for the immobilization of heavy metal-containing waste. However, Cr(VI), a highly toxic substance, is inadequately stabilized and solidified within cement pastes, posing a significant risk of leaching. This study innovatively incorporated nano-porous nitrogen-doped carbon (NC), derived from ZIF-8 through carbonization, into the cement containing heavy metals for collaborative disposal mixing with K 2 CrO 4 (HMC-Cr). Subsequently, the hydration behavior, mechanical properties, and solidification mechanism of Cr(VI) in the composite cement pastes were examined. The experimental results revealed that, compared to ZIF-8, NC demonstrated significantly greater positive impacts on the hydration kinetics, compressive strength, and Cr(VI) immobilization capabilities of HMC-Cr cement. Compared with pure HMC-Cr pastes, the 28-day compressive strength of HMC-Cr/NC increased by 16.7 %, and the leaching concentration of heavy metal Cr decreased by 52.5 %. The HMC-Cr/NC pastes exhibited superior Cr(VI) solidification capacity, attributed to the appropriate pore size, excellent nucleation effect, and reduction properties of NC. The primary immobilization forms of Cr in HMC-Cr/NC pastes included CrO 4 2- -AFm, Cr(OH) 3 , and coordination structures involving C-NH. Consequently, NC nanomaterials derived from ZIF-8 serve as effective additives for enhancing the stable immobilization of Cr(VI) in the cement solidification treatment of heavy metal-containing waste.
Ferrihydrite (Fh) can usually transform into phases with higher crystallinities. This may greatly alter Fh adsorption capacity for chromium ions, thereby affecting their immobilizations and fates. Foreign elements [e.g., lanthanum (La)] as impurities can become incorporated into Fh. Herein, the effect of La-substitution into Fh and the influence of the presence of Cr(VI) on Fh phase transformation and fate of Cr(VI) were investigated. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Electro-chemical impedance spectroscopy (EIS), and X-ray photoelectron spectroscopy (XPS) and other test methods were employed to probe phase changes after the Fe(II)-induced La-substituted Fh (La-Fh) transformations. At pH 5.5, no neoformed phase was observed until 7 d for La-Fh or pure Fh at 1 mM Fe(II) aq, but all La-Fh systems underwent transformations at 5 mM Fe(II)aq. A low pH and Fe(II)aq level favored lepidocrocite (Lp) to form, whereas goethite (Gt) was the predominant phase rather than Lp as pH increased to 7.2 with higher level Fe(II)aq. However, the La-substitution inhibits the La-Fh transformation rate and path. This made 5 % La-Fh transformed to Lp after 30 d at pH 7.2 and 1 mM Fe(II)aq. The La-release was present to some extent during the transformation process. Either the inhibition to La-Fh transformation or the La-slow release favors the retention of early-adsorbed Cr(VI) on iron-mineral surface. The added Cr(VI) might promote the La-Fh transformation even under a lower-level Fe(II), primarily attributed to the rapid reduction of Cr(VI) to Cr(III) by Fe(II), which produced more liable Fe(III) in addition to the Cr(III) that can improve the mineral conductivity, thereby enhancing the La-Fh transformation. Mechanisms responsible for the Fe(II)-induced La-Fh transformation companying the La slow-release are proposed to provide insights into the reduction and immobilization of Cr(VI) under anaerobic conditions, thus demonstrating that La-Fh is a promising material for application in Cr-contaminated site remediation.
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