The coupling of sulfide and Fe-Mn mineral promotes the migration of lead and zinc in the redox cycle of high pH floodplain soils.

In this study, we investigated the impact of fluctuating water levels on the distribution of lead (Pb) and zinc (Zn) in soil and sediments at a historical Pb-Zn smelting site along the Xiangjiang River. Despite the high pH levels (7 to 11) in the study area, which generally inhibits heavy metal solubility, we found that regular changes in water levels still affect Pb-Zn movement. Soil analysis revealed distinct redox zones within the unconfined aquifer, as shown by the variable Fe/Mn and Ce/Ce* ratios. Advanced techniques such as Mn K-edge XAFS, Mössbauer spectroscopy, and TOF-SIMS indicated persistent Fe-Mn redox cycling and highlighted the presence of Pb and Zn-rich manganese oxides near sulfur-bearing minerals. These findings suggest that acidic microzones produced by the oxidation of sulfur-bearing minerals become "refuges" for microbial and heavy metal activity. Considering that sulfur-containing minerals are widespread waste types in nonferrous metal smelting sites, these findings are instructive for a better understanding of the transformation mechanisms of heavy metal ions in nonferrous metal smelting-polluted environments and for guiding pollution remediation strategies.

Double-edged effect of frequent freeze-thaw on the stability of zero-valent iron after heavy metal remediation.

Freeze-thaw cycles (FTCs) cause dynamic microscale changes in ions and solvents. During freezing, heavy metals adsorbed on zero-valent iron (M-ZVI) and protons are excluded by ice crystals and concentrated in the liquid-like grain boundary region. The high proton concentration in this region leads to the dissolution of the passivation layer of ZVI. To assess the environmental risks of M-ZVI during FTCs, this study evaluated the stability of M-ZVI in this scenario from both microscale and macroscale perspectives. The results showed that the dissolution of the passivation layer had a dual effect on the stability of M-ZVI, which depends on the by-products of M-ZVI. The dissolution of the passivation layer was accompanied by the leaching of heavy metals, such as Ni-ZVI, but it also enhanced the reactivity of ZVI, causing it to re-react with desorbed heavy metals. The stability of Cr-ZVI and Cd-ZVI was improved due to frequent FTCs. Furthermore, changes in the surrounding environment (water dipole moment, ion concentration, etc.) of ZVI affected the crystallization of Fe oxides, increasing the content of amorphous Fe oxide. As low-crystallinity Fe oxides could facilitate ion doping, Ni2+ was doped into Fe3O4 lattice during FTCs, which reduced the mobility of heavy metals. Contrary to traditional views that freezing temperatures slow chemical reactions, this study provides new insights into the application of iron-based materials in cold environments.

Direct degradation of Bisphenol A from aqueous solution by active red mud in aerobic environment.

As industrial waste from aluminum production, red mud (RM) poses a severe threat to the local environment that needs to be appropriately utilized. The activation of iron oxide, which is abundant in RM, improves its effectiveness as a catalytic material for the degradation of organic pollutants. This study developed a novel activation approach by adding dithionite citrate bicarbonate (DCB) for Bisphenol A (BPA) degradation under aeration conditions. Electrochemical experiments and reactive oxygen species (ROSs) trapping experiments showed that DCB treatment enhanced the redox cycle of Fe(II)/Fe(III), which promoted free radical generation. The optimized condition for the RM activation was achieved at 21 mmol/L dithionites, 84 mmol/L citrates, and 34 mmol/L bicarbonate, and the degradation of BPA by activated RM reached 410 µg BPA per gram of RM. This work provided a feasible way to utilize RM resources as an efficient, low-cost catalyst for organic pollutants treatment.

The influence of various microplastics on PBDEs contaminated soil remediation by nZVI and sulfide-nZVI: Impedance, electron-accepting/-donating capacity and aging.

The microplastics (MPs) existed in the environment widely has resulted in novel thinking about in-situ remediation techniques, such as nano-zero-valent iron (nZVI) and sulfided nZVI (S-nZVI), which were often compromised by various environmental factors. In this study, three common MPs such as polyvinyl chloride (PVC), polystyrene (PS), and polypropylene (PP) in soil were found to inhibit the degradation rate of decabromodiphenyl ether (BDE209) by nZVI and S-nZVI to different degrees due to MPs inhibiting of electron transfer which is the main way to degrade BDE209. The inhibition strength was related to its impedance (Z) and electron-accepting (EAC)/-donating capacity (EDC). Based on the explanation of the inhibition mechanism, the reason for different aging degrees of nZVI and S-nZVI in different MPs was illustrated, especially in PVC systems. Furthermore, the aging of reacted MPs, functionalization and fragmentation in particular, indicated that they were involved in the degradation process. Moreover, this work provided new insights into the field application of nZVI-based materials for removing persistent organic pollutants (POPs).

Effect of calcination temperatures on the performance of rectorite for cadmium immobilization in soil: Freeze-thaw, plant growth, and microbial diversity.

The immobilization of cadmium (Cd(II)) in soil using calcined rectorite (REC) was investigated in this research. The results of immobilization show that a small amount of REC calcined at 700 °C (REC-700 °C) could effectively immobilize 90% of Cd(II) in soil, while the immobilization efficiency of REC only reached 42%. Moreover, the immobilization efficiency of REC calcined at 300 °C and 500 °C (REC-300 °C and REC-500 °C) were lower than REC. To investigate the mechanism, the materials before and after immobilization were fully analyzed by Fourier transform infrared spectroscopy (FT-IR), powdery X-ray diffraction analysis (XRD), and scanning electron microscopy (SEM). The results indicate that the structure of REC has been changed after calcination at different temperatures and Cd(II) was successfully immobilized on materials. Losing free water, structural water and OH groups respectively, the layer spacing of REC-300 °C and REC-500 °C was shrunk. However, the crystal structure of REC was destroyed after calcination at 700 °C, resulting in the generation of new phases. According to the XRD result, more cadmium hydroxide (Cd(OH)2) were produced on REC-700 °C, indicating that more OH groups were formed during immobilization. Furthermore, Tessier test demonstrates that Cd(II) in soil changed from exchangeable state and water soluble state to carbonate bound state and iron manganese oxide bound state during immobilization. The result of microbial community indicates that REC-700 °C can restore the microbial composition of Cd(II)-contaminated soil. The effects of pH, freeze-thaw, REC dosage, and initial heavy metal concentration were also evaluated to provide a theoretical basis for the subsequent application of the material in the remediation of contaminated soil.

New insight of Mn(III) in δ-MnO2 for peroxymonosulfate activation reaction: Via direct electron transfer or via free radical reactions.

Due to the increasing of industrial plastic waste and its refractory characteristics, it is extremely urgent to develop new degradation technology and environmentally friendly catalyst for industrial plastic waste. Manganese oxides are one of the most promising candidates for the catalytic degradation of plastic wastes. However, an improved understanding of the structural properties affecting their catalytic activity is required for high-efficient wastewater treatment. We herein report the surface reactivity effects of δ-MnO2 structural defects with regards to Bisphenol A (BPA) degradation/probe in the presence of peroxymonosulfate (PMS). Four δ-MnOx samples with different Mn(III) contents (different Mn(III)-deficient sample) were prepared and their structural properties as well as surface reactivity were characterized by batch test, ESR and XAFS analysis. For the Mn(III)-deficient sample, BPA removal was principally affected by direct electron transfer, with the adsorbed BPA degraded following hydroxylation. In contrast, a small fraction of Mn(III) substitution in δ-MnO2 could significantly encouraged the activation of PMS to produce SO4-☐and ☐OH, and a BPA degradation via beta scission. Moreover, the Mn(III)-rich δ-MnO2 demonstrate a high BPA removal rate even with a low sample load, which performed well following a reuse of five times. Our results provide a new way for the improvement of δ-MnO2 activity for the use of industrial plastic wastes treatment.

Remediation of chromium-contaminated soil using delaminated layered double hydroxides with different divalent metals.

Soil is commonly polluted by chromium, and layered double hydroxides (LDHs) are widely used for chromium removal due to their strong adsorption capacity and the unique properties of their delaminated products. In this study, delaminated LDHs (S-Mg-LDH and S-Ca-LDH) and their original LDHs were used to remediate Cr(VI)-contaminated soil. A series of characterizations confirmed the successful synthesis of delaminated LDHs whose sheet structure was thinner with a greater surface energy than the original LDHs. The remediation results indicated that delaminated LDHs could more efficiently immobilize Cr(VI) in soil. The immobilization rate of S-Mg-LDH was 64.32%, while Mg-LDH was only 8.09%. However, at low dosages, the efficiency of S-Ca-LDH was 28.1% while Ca-LDH was 5.16%, but they had similar effects at high doses. Moreover, soil pH had little effect on their removal efficiencies. The toxicity characteristic leaching procedure (TCLP) results showed that the leaching of Cr(VI) in soil after treatment with S-Mg-LDH, S-Ca-LDH, Mg-LDH, and Ca-LDH was reduced by 75.43%, 72.43%, 86.55%, and 75.90%, respectively. The phytotoxicity tests of soil treated by S-Mg-LDH and S-Ca-LDH revealed that they effectively reduced the toxicity of chromium and lowered its bioaccumulation. Overall, this study confirms the feasibility of delaminated LDHs for Cr(VI) immobilization in soils.

Metal-support interactions on Ru/CaAlOx catalysts derived from structural reconstruction of Ca-Al layered double hydroxides for ammonia decomposition.

Strong metal-support interaction (SMSI) over Ru/CaAlOx was constructed by utilizing the surface structural reconstruction of Ca-Al layered double hydroxides (LDHs) in aqueous solution and their subsequent hydroxide-to-oxide transformations. Ru nanoparticles were found to uniformly embed with CaAlOx. The electronic interactions and changes in CO adsorption behavior confirmed the SMSI state between Ru and CaAlOx. Owing to SMSI, the as-prepared Ru/CaAlOx showed improved catalytic activity in the decomposition of ammonia, especially at a relatively low temperature.

Degradation of bisphenol A by Fe-Al layered double hydroxides: A new synergy of homo- and heterogeneous Fenton systems.

Fe(II)-Al(III) layered double hydroxides (Fe-Al-LDHs) prepared by co-precipitation were used to degrade bisphenol A (BPA) in water. The LDH/H2O2 system proved to be effective in degrading BPA, even under high pH conditions. Influencing factors on the removal of BPA, such as LDH dosage, initial BPA concentration and H2O2 concentration, were studied. The adsorption results and kinetic analysis indicated that the degradation rate of BPA increased with increasing LDH and H2O2 concentration and decreasing the initial concentration of BPA. The reaction mechanism was deduced by using a number of analytical techniques, including XPS, XRD, EPR, SEM, TEM and SAED. It was found that the likely degradation mechanism of BPA by Fe-Al-LDHs is a synergy of the homo- and heterogeneous Fenton systems. In this process, when LDHs and H2O2 are present in the solution, Fe, Al and Cl- in the LDH are released into the solution due to the precipitation-solubility equilibrium. The dissolved Fe2+ reacts with H2O2 to form a homogeneous Fenton reaction system. In addition, the OH- produced during the Fenton reaction may induce ion-exchange through a dissolution-reprecipitation process. During the ion-exchange, the released Fe2+ also takes part in the homogeneous Fenton reaction and released ions reprecipitated into LDHs possibly by reaction with Al3+, Fe3+ and OH-. The undissolved and newly formed LDHs function as heterogeneous Fenton catalysts during the reaction due to the presence of Fe2+ in the structure, thus increasing the reaction efficiency. Moreover, the long term durability of the catalyst over several reaction cycles was also tested and it was found that the material only showed a slight decrease in efficiency after four cycles.

Double-Edged Effect of Humic Acid on Multiple Sorption Modes of Calcined Layered Double Hydroxides: Inhibition and Promotion.

Layered double hydroxides (LDHs) are a typical class of anionic clay minerals whose structural memory effect has been widely used in pollutant adsorption. However, the influencing mechanism of humic acid (HA) on the structural memory effect in adsorption is not clear. In this study, HA was extracted from black soil and sediments, and its effect on the structural memory effect of LDHs with different divalent metals was evaluated in adsorption. Borate complexed with HAs and HAs promoted the dissolution of magnesium-calcined LDHs (Mg-CLDH), which enhanced their adsorption rate by Mg-CLDH. However, the adsorbed HA caused a decline in the crystallinity of the regenerated Mg-LDH and an incomplete structural transformation, thereby resulting in decreased adsorption capacity. After the complexation of HAs with borate, the resulting compound was adsorbed on the surface of Zn-CLDH. The adsorption rate of borate was effectively improved in the initial stage, but at the same time slowed down the hydration and structural regeneration of Zn-CLDH. Meanwhile, the surface-adsorbed HAs also prevented borate from entering the newly formed layer inside the particles and led to a significant decrease in adsorption performance. When Ca-CLDH was used to adsorb borate, the process mainly occurred through the formation of ettringite. However, the presence of HAs enhanced the stability of the restructured LDHs and hindered the dissolution of Ca-CLDH and the reaction with B(OH)4- to form ettringite during the regeneration process, which severely inhibited the sorption of borate.

Enhanced surface Fenton degradation of BPA in soil with a high pH.

Although the heterogeneous Fenton process of iron-bearing minerals has been widely studied due to its potential use for the removal of organic pollutants, the transformation mediated by Fe species in soil particles remains poorly understood. Here, we compared the removal of bisphenol A (BPA) from soil using a Fenton system at low and high pH values. At low pH value, the BPA removal rate decreased with increasing pH value; this result was consistent with the amount of soluble Fe(II) and surface-bound Fe(II) observed in the soil systems. In contrast, an increased BPA degradation efficiency was observed at high pH, which is different from the traditional Fenton system. The electron spin resonance analysis verified that the high BPA degradation rate was attributed to enhanced ·OH generation. The binding environments of the Fe species in the soil for different reaction pH values were investigated by using Mössbauer spectroscopy combined with selective chemical extraction. A mixed-valence Fe(II) phase was observed at pH 12.0 and accounted for 12% of the total Fe content. The results indicate that in addition to the well-studied soluble Fe(II) and surface-bound Fe(II), structural Fe(II) located in the newly formed secondary precipitates may play a more important role in the generation of ·OH, especially at high pH values. These findings may provide insights into the utilization of Fe-bearing soil minerals as a renewable source for the degradation of organic pollutants over a wide pH range.

Remediation of hexavalent chromium in contaminated soil by Fe(II)-Al layered double hydroxide.

In this study, high-efficiency and low-cost Fe(II) and Al(III) layered double hydroxide (Fe-Al-LDH) was synthesized and used for the remediation of Cr(VI) contaminated soil. The Fe-Al-LDH characteristics were analyzed with different techniques. The results of X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses revealed its highly crystalline structure with sheet morphology, and a Brunauer-Emmett-Teller (BET) surface area of 46.85 m2/g. In addition, the remediation tests indicated that adding 1 g/L of Fe-Al-LDH to a solution of Cr(VI)-contaminated soil, at a soil to solution ratio of 1 g: 5 mL, completely immobilized the pre-adsorbed Cr(VI) in the soil (2079.84 mg/kg). Additionally, the Fe-Al-LDH could be used in a wide range of pH conditions and no Cr(VI) was detected in the leaching solution. Based on the characterization of X-Ray Diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, inductively coupled plasma mass spectrometry (ICP-MS), and X-ray photoelectron spectroscopy (XPS) analysis, it is proposed that adsorption and reduction may be involved in the mechanism of Cr(VI) immobilization by Fe-Al-LDH. At the beginning of the reaction, Cr(VI) entered the layer structure of the LDH or was adsorbed on the surface of the LDH. Then, Fe(II) was involved in reducing the Cr(VI) to Cr(III) and was oxidized to Fe(III). Part of the Cr(VI) and Cr(III) were co-precipitated with Fe(III) and Al (III) during the formation of iron oxide or hydroxide.

Structural Memory Effect of Mg-Al and Zn-Al layered Double Hydroxides in the Presence of Different Natural Humic Acids: Process and Mechanism.

The structural memory effect of layered double hydroxides (LDHs) is one of the important reasons for their extensive use in environmental remediation. In this study, humic acid (HA) was extracted from black soil and sediments and characterized to determine their structures. The regeneration mechanisms of calcinated LDHs (CLDHs) including different divalent metals (Mg-CLDH and Zn-CLDH) in deionized water and different HA solutions were carefully elucidated, and the reasons for the behavior differences in the two materials were explained. The presence of the HAs significantly increased the dissolution rate of Mg2+ ions from Mg-CLDHs and subsequent regeneration of Mg-LDH. Because of the diverse functional groups in the HAs, these groups were complexed with metallic ions such as Mg2+ on the surface of Mg-CLDH in the beginning. During the process, the HAs adsorbed the regenerated LDHs on the surfaces. Therefore, the crystallinity, morphology, and specific surface area of the regenerated Mg-LDH significantly changed, especially in the presence of high concentrations of HA. In the case of Zn-CLDH, the regeneration rate of the LDH increased in the presence of HA, but the surface of Zn-CLDH was covered with regenerated Zn-LDH and HA. Then, the inside of the particles could not transform to LDH, leading to poor crystallinity and a significant increase in the ZnO content of the HA system.

Ionothermal synthesis of Fe3O4 magnetic nanoparticles as efficient heterogeneous Fenton-like catalysts for degradation of organic pollutants with H2O2.

Fe3O4 magnetic nanoparticles (MNPs) are attractive heterogeneous Fenton-like catalysts for oxidative degradation of organic pollutants with H2O2. Herein highly efficient and stable Fe3O4 MNPs (Fe3O4-op-DES, ca. 10nm) were successfully prepared via a novel oxidative precipitation-combined ionothermal synthesis, which comprised oxidative precipitation of FeSO4·7H2O in choline chloride:2urea deep eutectic solvent. Among five different Fe3O4 particles tested, Fe3O4-op-DES MNPs exhibited the highest catalytic activity with the activation energy of 47.6kJmol-1 for degradation of Rhodamine B (RhB) with H2O2 under the same conditions (Fe3O4 dosage of 0.50gL-1, H2O2 concentration of 40mmolL-1, pH 6.4, 55°C, 2h). Fe3O4-op-DES MNPs were magnetically recoverable, and had good catalytic stability and recyclability without the need of regeneration (>98% degradation efficiency of RhB in 2h and pseudo-first-order rate constant of 0.0376min-1 after having been continuously running for 12h). The superior catalytic performance of Fe3O4-op-DES MNPs was attributed to the combination of multiple technologically important features, including the nanometer size, high Fe2+ content, large surface area, high density of surface active sites and stable crystal structure (no phase transformation, negligible iron leaching and particle aggregation after reaction). The wide applicability of Fe3O4-op-DES MNPs was also demonstrated by the degradation of four other organic pollutants.

Transport of nano zero-valent iron supported by mesoporous silica microspheres in porous media.

Effective in situ remediation of groundwater requires the successful delivery of reactive iron particles through sand. However, the agglomeration of nano zero-valent iron (NZVI) particles limits the migration distance, which inhibits their usefulness. In the study described herein, NZVI supported by mesoporous silica microspheres covered with FeOOH (SiO2@FeOOH@Fe) was synthesized, and its mobility was demonstrated on the basis of transport in porous media. Degradation of decabromodiphenyl ether (BDE209) was more efficient by SiO2@FeOOH@Fe than by 'bare' NZVI. Breakthrough curves and mass recovery showed the mobility of SiO2@FeOOH@Fe in granular media was better than that of bare NZVI. It increased greatly in the presence of natural organic matter (NOM) and decreased when high Ca2+ and Mg2+ concentrations were encountered. Analysis of the transport data on the basis of filtration theory showed diffusion to be the main mechanism for particle removal in silicon sand. Increasing the NOM may decrease agglomeration of the grains of sand, which has a positive effect on the mobility of SiO2@FeOOH@Fe. Presumably, increasing the concentrations of Ca2+ and Mg2+ compresses the diffuse double layer of SiO2@FeOOH@Fe, resulting in a reduction of mobility.

Mechanism of boron uptake by hydrocalumite calcined at different temperatures.

Hydrocalumite (Ca-Al-layered double hydroxide (LDH)) was prepared and applied for the removal of borate. The properties of Ca-Al-LDH calcined at different temperatures were diverse, which affected the sorption density and mechanism of boron species. The sorption density increased with increase in calcined temperature and the sample calcined at 900°C (Ca-Al-LDH-900) showed the maximum sorption density in this work. The solid residues after sorption were characterized by (11)B NMR, (27)Al NMR, SEM, and XRD to investigate the sorption mechanism. Dissolution-reprecipitation was the main mechanism for sorption of borate in Ca-Al-LDH. For Ca-Al-LDH calcined at 300 and 500°C, regeneration occurred in a short time and the newly forming LDHs were decomposed to release Ca(2+) ions and formed ettringite with borate. Two stages occurred in the sorption of boron by Ca-Al-LDH calcined at 900°C. In the first stage, boron species adsorbed on the alumina gel resulting from the hydration of calcined products. In this stage, borate was included as an interlayer anion into the newly forming LDHs in the following stage, and then immobilized as HBO3(2-) into the interlayer, most the LDHs.

Sorption of H3BO3/B(OH)4⁻ on calcined LDHs including different divalent metals.

LDHs with different divalent metals (Zn-LDH, Mg-LDH and Ca-LDH) have been synthesized and produced calcined LDHs (Zn-CLDH, Mg-CLDH and Ca-CLDH) for borate removal. Based on XRD, SEM, BET, (27)Al NMR, CO2-TPD, and (11)B NMR, detailed characterization of different CLDHs before and after reaction with the boron species was systematically performed. The surface area, basicity and the particle charge of the different CLDHs, which are related to the hydration and regeneration, were markably influenced by the nature of the divalent metals. Transformation of crystal phases and the types of boron species adsorbed by the different CLDHs varied as time changed. The regeneration of Ca-CLDH required the shortest time. However, Ca-LDH decomposed to release Ca(2+) ions, forming ettringite with borate. Zn-CLDH also rapidly transformed into Zn-LDH. During this reconstruction, B(OH)4(-) was intercalated into the interlayer of Zn-LDHs, which is the predominant mechanism of borate removal by Zn-CLDH. Increase in the initial pH caused a competition between borate and OH(-) so that the removal efficiency of borate by Zn-CLDH decreased. For Mg-CLDH, surface complexation and electrostatic attraction were included in the first stage, immobilizing boric acid into Mg(OH)2 and attracting borate as interlayer anionic species into the new forming Mg-LDHs in the second stage.

Comparisons of the reactivity, reusability and stability of four different zero-valent iron-based nanoparticles.

Our previous reports showed that nano zero-valent iron (nZVI), steel pickle liquor for the synthesis of nZVI (S-nZVI), nZVI immobilised in mesoporous silica microspheres (SiO2@FeOOH@Fe) and nano Ni/Fe bimetallic particles (Ni/Fe) have been proved to show good property for elimination of polybrominated diphenyl ethers (PBDEs). However, it is necessary to compare their reactivity, reusability and stability when applied to in situ remediation. In this study, the performances of different iron-based nanoparticles were compared through reusability, sedimentation and iron dissolution experiments. The SiO2@FeOOH@Fe and Ni/Fe nanoparticles were shown to have higher reusability and stability, as they could be reused more than seven times, and that the SiO2@FeOOH@Fe can effectively avoid leaching iron ions into the solution and causing secondary pollution in the reaction. This study may serve as a reference for PBDE remediation in the future.