Zero-valent aluminum (ZVAl) is a promising reductant because of its relatively low redox potential , which can efficiently activate molecular oxygen to generate reactive oxygen species . However, its long-term performance is limited by the intrinsic dense oxide layer and the passivation effect of the accumulative Al-(hydr)oxide on its surface during the reaction. In this study, four clay minerals with different compositions were mixed with ZVAl by ball milling to obtain four composites of ZVAl and clay (ZVAl-Clay), which were used to degrade a high concentration of 4-chlorophenol (4-CP) under ambient conditions. The oxidation efficiencies of different ZVAl-Clays were strongly relevant to Fe contained in the clay minerals. The Fe-free ZVAl-Clay presented poor oxidation performance, whereas the reaction efficiencies of those ZVAl composites with Fe-bearing clays exhibited varying degrees of improvement. In comparison with the original ZVAl, the highest oxidation rate increased by 23 times, the maximum increased OH production was approximately 8 times, and the corresponding mineralization efficiency improved by 38.7%. However, the levels of improved oxidation performance of various ZVAl-Clays were not positively correlated with their actual total Fe contents, and their degradation efficiencies might also be affected by other physical and/or chemical properties of different clays. The synergistic mechanism revealed by various characterizations was that electron transfer might occur from ZVAl to the structural Fe(III) of the clay through the basal plane or edge of clays triggered by ball milling. Thus, the partially produced Fe(II) on the clay surface promoted the Fenton-like reaction to decompose H 2 O 2 into OH for efficient oxidation of 4-CP. In short, the ZVAl composites with Fe-bearing clays deserved further exploration as potential materials for efficient degradation of organic matters in wastewater samples.
In-situ thermal desorption (ISTD) has attracted increasing attention owing to the efficient removal of organic contaminants from contaminated sites. However, it is poorly understood that whether and to what extent contamination degradation occurs upon oxygenation of reduced Fe-bearing clay minerals (RFC) in the subsurface during ISTD. In this study, we evaluated the mechanism of contaminant degradation upon oxygenation of reduced clay minerals during the ISTD. Reduced nontronite (rNAu-2) and montmorillonite (rSWy-3) were selected as RFC models. Results showed that thermal treatment during ISTD could significantly enhance phenol degradation, which increased from 25.8 % at 10 °C to 74.4 % at 70 °C in rNAu-2 and from 17.7 % at 10 °C to 49.8 % at 70 °C in rSWy-3. Correspondingly, the cumulative •OH at steady-state ([•OH] ss ) increased by 3.7 and 1.5 times, respectively. The acceleration of Fe(II) oxidation with increasing temperature could be mainly responsible for [•OH] ss generation, which degrades phenol. Moreover, thermal treatment improved the fast oxidation of trioctahedral entities Fe(II)Fe(II)Fe(II) (TOF) and the slow oxidation of dioctahedral entities Fe(II)Fe(II) (DTF 1 ), AlFe(II) (DAF 1 ), and Fe(II)Fe(III) (DTF 2 ). Our study suggests that the overlooked degradation progress of phenol by oxygenation of RFC during ISTD, and it could be favorable for contaminant degradation during remediation.
Sulfate radical-based advanced oxidation processes have received increasing attention in contaminated water treatment. As an excellent electron donor, microscale zero-valent aluminum (mZVAl) is a promising persulfate (PS) activator. However, the natural oxide layer of mZVAl impedes electron transfer from the core metal. Herein, a composite comprising mZVAl and Fe-bearing clay (mZVAl-NaNAu-2) was obtained through the simple ball milling (BM), which synergistically activates PS for the efficient degradation of various organic contaminants, such as 4-chlorophenol, benzoic acid, nitrobenzene, phenol, and ofloxacin, in water. The sulfate radicals and hydroxyl radicals were the main reactive species but Fe(IV) was minor in this reaction. The BM can cause fracture of the mZVAl powder and promote exposure of the fresh Al 0 surface. Various Fe species in mZVAl-NaNAu-2 at different reaction times were evaluated by extraction experiments, Mössbauer and XPS spectra. Approximately 47.0% of structural Fe(III) was reduced to Fe(II) after BM. The dissolved Fe(II) and structural trioctahedral Fe(II)-Fe(II)-Fe(II) entities were mainly responsible for the homogeneous and heterogeneous activation of PS, and corroded mZVAl acted as an “electron pool” to regenerate the aforementioned Fe(II) species continuously. The mZVAl-NaNAu-2/PS system also presented certain resistance to a natural organic matter and some anions, such as Cl - and NO 3 - . This study provided a new strategy for the efficient removal of organic contaminants from water and also solved the bottleneck of mZVAl in applications due to its dense oxide film.
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