Enhanced reactivity of zero-valent aluminum with ball milling for phenol oxidative degradation

Enhanced mineralization of nitrophenols by a novel C@ZVAl-PS based sequential reduction-oxidation process

Widely employed nitrophenols (NPs) are refractory and antioxidant due to their strong electron-withdrawing group (-NO2). Actually, NPs are readily reduced to aminophenols (APs). However, APs remain toxic and necessitate further treatment. Herein, we utilized a novel sequential reduction-oxidation system of carbon-modified zero-valent aluminum (C@ZVAl) combined with persulfate (PS) for the thorough removal of both NPs and APs. The results demonstrated that p-nitrophenol (PNP, up to 1000 mg/L) exhibited complete reduction to p-aminophenol (PAP), and then over 98.0 % of PAP could be effectively oxidized, in the meantime the removal rate of chemical oxygen demand (COD) was as high as 95.9 %. Based on the SEM and XPS characterizations, we found that C@ZVAl has exceptionally high reactivity that generates massive electrons and reduces PNP to PAP through accelerated electron transfer. In the subsequent oxidation step, PS can be rapidly activated by C@ZVAl to generate SO4- radicals for PAP oxidization. Meanwhile, the mineralization of COD proceeds. The temporal binding of reduction and oxidation can be regulated by varying the PS dosing time. Namely, the appropriate delay in PS dosing facilitates sufficient reduction to provide enough reactants for oxidation, favoring the mineralization of PNP and COD. More crucially, dinitrodiazophenol (DDNP) in an actual explosive wastewater without any pretreatment can be effectively mineralized by this sequential reduction-oxidation system, affirming the excellent performance of this process in practical applications. In conclusion, the C@ZVAl-PS based sequential reduction-oxidation looks very promising for enhanced mineralization of nitro-substituted organic contaminants.

Ball-milled biochar-modified zero-valent aluminum activates peroxodisulfate for phenol degradation: Enhancement of catalysis by membrane-breaking effect

Zero-valent aluminum (ZVAl) is a potential activator for peroxodisulfate (PDS), yet the dense oxide film on its surface hampers electron transfer for the O-O bond cleavage of PDS. We synthesized zero-valent aluminum-biochar (BM-ZVAl@BC) composites through ball milling, which effectively disrupted the native oxide layer on BM-ZVAl@BC. Within the BM-ZVAl@BC/PDS system, biochar (BC) not only suppressed the rapid oxidation of BM-ZVAl@BC but also enhanced the dispersion and electron transfer rate of ZVAl, thereby improving the overall catalytic efficiency. Consequently, the phenol removal efficacy in the BM-ZVAl@BC/PDS system was notably improved. Optimal catalytic performance of the prepared BM-ZVAl@BC was achieved at a charcoal-to‑aluminum mass ratio of 2:1, resulting in 95.7 % phenol removal after 180 min. Quenching experiments and electron paramagnetic resonance (EPR) analysis revealed that both free radicals (SO4•-, •OH, and O2•-) and non-radical species (1O2) contributed to phenol degradation, with SO4•- and •OH playing predominant roles. In summary, the BM-ZVAl@BC/PDS system represented an effective and promising technology for the remediation of phenolic water pollutants.

Mechanochemical Degradation of Caffeine and Diclofenac Using Biochar of Fique Bagasse in the Presence of Al: Monitoring by Mass Spectrometry

Much research has been carried out to remove emerging contaminants using diverse materials. Furthermore, studies related to pollutant degradation have increased over the past decade. Mechanochemical degradation can successfully decompose molecules that are persistent in the environment. In this study, the biochar of fique bagasse with mixtures SiO2, Al, Al2O3, and Al-Al2O3 was treated with a mechanochemical technique using a planetary ball mill to investigate the degradation of caffeine and diclofenac. These tests resulted in the transformation of caffeine and diclofenac due to the use of Al employing mechanochemistry. In fact, through the use of liquid chromatography coupled with mass spectrometry, eight and six subproducts were identified for caffeine and diclofenac, respectively. Additionally, analysis of the molecules proposed for caffeine and diclofenac transformation suggested hydroxylation, demethylation, decarboxylation, oxidation reactions, and cleavage of the C-C and C-N bonds in the pollutants studied. The formation of these transformation products could be possible by reductant oxygen species generated from the molecular oxygen in the presence of aluminum and the energy delivered for ball milling. The results obtained show the potential application in the environmental management of mechanochemical treatment in the elimination of emerging contaminants caffeine and diclofenac.

Application potential of zero-valent aluminum in nitrophenols wastewater decontamination: Enhanced reactivity, electron selectivity and anti-passivation capability

Nitrophenols (NPs) are highly toxic and easy to accumulate to high concentrations (> 500 mg/L) in real wastewater. The nitro group contained in NPs is an electron-absorbing group that is easy to reduce and difficult to oxidize, so there is an urgent need to develop reduction removal technology. Zero-valent aluminum (ZVAl) is an excellent electron donor that can reductively transform various refractory pollutants. However, ZVAl is prone to rapid deactivation due to non-selective reactions with water, ions, etc. To overcome this critical limitation, we prepared a new type of carbon nanotubes (CNTs) modified microscale ZVAl, CNTs@mZVAl, through a facile mechanochemical ball milling method. CNTs@mZVAl had outstanding high reactivity in degrading p-nitrophenol even 1000 mg/L and showed up to 95.50% electron utilization efficiency. Moreover, CNTs@mZVAl was highly resistant to the passivation by dissolved oxygen, ions and natural organic matters coexisting in water matrix, and remained highly reactive after aging in the air for 10 days. Furthermore, CNTs@mZVAl could effectively remove dinitrodiazophenol from real explosive wastewater. The excellent performance of CNTs@mZVAl is due to the combination of selective adsorption of NPs and CNTs-mediated e-transfer. CNTs@mZVAl looks promising for the efficient and selective degradation of NPs, with broader prospects for real wastewater treatment.

Acid-washed zero-valent aluminum as a highly efficient persulfate activator for degradation of phenacetin

Phenacetin (PNT) is one of the most frequently detected nonsteroidal anti-inflammatory drugs in the water ecosystems, which poses a potential risk to environmental aquatic organisms. Acid-washed zero-valent aluminum (ZVAl) as a highly efficient activator for persulfate (PS) process was investigated to degrade PNT from the aqueous solution. The results indicated that acid-washed pretreatment for ZVAl could efficiently increase the degradation efficiency of PNT in the PS treatment. The degradation efficiency of PNT (50 μM) was up to 90% in 4 hours with the addition of 0.2 g/L acid-washed ZVAl and 8 mM PS at pH 6.8 and 25 °C. The PNT degradation followed pseudo-first order kinetics in the present system. High activator dosage, PS concentration, and reaction temperature could enhance the PNT degradation. The presence of inorganic anions (i.e., NO₃^-, HCO₃^-) and humic acid (HA) showed inhibitory effects on the PNT degradation. The reuse results illustrated the acid-washed ZVAl material would have continuous and efficient activation performance for PS to degrade the PNT. Radical scavenger experiments and electron paramagnetic resonance indicated that both SO₄^•- and •OH were major reactive species during the PNT degradation. The possible degradation pathways of PNT mainly included the break of C-N and C-O bonds and further oxidation.

Degradation of organic dye wastewater by H2O2-enhanced aluminum carbon micro-electrolysis

In this research, the treatment of methylene blue (MB) dye wastewater by a novel system that combines H₂O₂ with an aluminum-carbon micro-electrolysis (ACE) was explored. The effects of the H₂O₂ amount, initial pH, aluminum to carbon ratio, total aluminum-carbon mass, dye concentration, and reaction temperature on degradation of MB were investigated. The findings revealed that under the following conditions: H₂O₂ 34.0 mg/L, initial pH of 3.0, aluminum-to-carbon ratio of 2:1, total aluminum-carbon mass of 2.0 g/L, MB concentration of 20 mg/L, and 20 °C, the degradation rate of MB could reach 99.3% after 180 min, which is 18.4% more compared with ACE at the same conditions without H₂O₂. Through the quenching experiments, it was proved that the efficient free radicals produced during degradation are •OH and •O₂^-. Finally, a possible mechanism of H₂O₂ enhanced aluminum carbon micro-electrolysis (HP-ACE) for MB degradation was discussed.

Mechanochemically synthesized Al-Fe (oxide) composite with superior reductive performance: Solid-state kinetic processes during ball milling

Recently, mechanical ball milling (BM), a simple and green powder processing method, has been successfully applied to improve the performance of zero-valent metals (ZVMs) for efficient water treatment. However, until now BM is still regarded as a "black box" in which the processes of the solid-state reaction during activation remain unclear. In this paper, firstly, FeSO₄·7H₂O crystal was used to activate and modify inert microscale zero-valent aluminum (mZVAl) by BM to synthesize Al-Fe (oxide)^bm composite that showed superior reactivity in reductive removal of various contaminants and excellent reusability, which may be mainly ascribed to the newly formed iron oxide layer on mZVAl by mechanochemical reaction. At the same time, the formation of iron oxides on mZVAl was closely related to BM parameters. Further kinematic analysis revealed that the occurrence of mechanochemical reaction depended on the impact energy and input energy, which BM speed and BM time were two main factors determining reaction extent on the premise that the precursors were full dose. Moreover, kinetic fitting uncovered the solid-state reaction mechanism between mZVAl and FeSO₄·7H₂O conformed to three-dimensional diffusion and phase boundary reaction models. This study ponders deeply upon the mechanochemical process and solid reaction mechanism during the preparation of Al-Fe (oxide)^bm composite, which deepens comprehensions of material synthesis procedures by BM and promotes applications of ZVM-based composite in polluted water or wastewater treatment.

Mechanochemical destruction and mineralization of solid-phase hexabromocyclododecane assisted by microscale zero-valent aluminum

Hexabromocyclododecane (HBCD) has been listed in Annex A of the Stockholm Convention as a persistent and bio-accumulative chemical. While HBCD is often present in the solid form for its low solubility, cost-effective technologies have been lacking for the degradation of solid-phase HBCD. In this work, mechanochemical (MC) destruction of high-energy ball milling was employed for direct destruction of solid-phase HBCD, where a strong reducer, microscale zero-valent aluminum (mZVAl), was used as the co-milling agent. The new mZVAl-assisted MC process achieved complete debromination and mineralization of HBCD within 3 h milling. The optimal operating parameters were determined, including the milling atmosphere, the milling speed, the mZVAl-to-HBCD molar ratio, and the ball-to-mZVAl mass ratio. Fourier transform infrared spectrometry and Raman analyses revealed that the organic structures of HBCD were destroyed and organic bromine was completely converted into inorganic bromide, accompanied by the generation of amorphous and graphite carbon. Analysis of the milled samples by GC-MS demonstrated the absence of obvious organic matter after MC treatment, also indicating the complete degradation and conversion of HBCD to inorganic compounds. Further X-ray photoelectron spectroscopic analysis indicates that the fresh surface of mZVAl was generated upon the MC treatment, and Al(0) served as a strong reducing agent (e-donor) for reductive debromination and destruction of the carbon skeleton. The mZVAl-assisted MC milling appears promising as a non-combustion approach for effective destruction and carbonization/mineralization of solid-phase HBCD or potentially other persistent organic pollutants.

A comparative study on high-efficient reduction of bromate in neutral solution using zero-valent Al treated by different procedures

Bromate, a toxic by-product of bromide-containing drinking water after disinfecting with ozone, has attracted much attention in the past two decades. Traditional methods to activate zero-valent metals for reducing bromate are to eliminate their surface oxide layer by acid washing. In this work, for the first time, zero-valent Al (ZVAl) was surface treated by the following procedures including soaking, soaking and freeze-drying, soaking and heat-treating, and γ-Al₂O₃ covering Al particle surfaces (GCAP). It was found that all of above surface treated ZVAls have an obvious high efficiency for bromate reduction relative to pristine ZVAl. The bromate reduction rate is GCAP > soaking Al > freeze-drying Al > soaking and heat-treating Al > pristine Al, and using GCAP just 30 min is taken to completely reduce bromate to bromide in neutral solution. Mechanism analyses revealed that Al surface treating or covered by fine γ-Al₂O₃ phase can promote the hydration and breakage of Al surface passive oxide layer, resulting in a fast contact of inner Al with outside ions, leading to a high reduction rate of bromate in neutral solution. XPS analyses indicated that there are no bromate or bromide ions adsorbed on Al particle surfaces, implying that there is a high direct donating efficiency of electrons from inner Al to bromate ions in solution. Furthermore, GCAP has a good reusability and >90% bromate can be reduced even it was reused up to 4 cycles.

A novel ball-milled aluminum-carbon composite for enhanced adsorption and degradation of hexabromocyclododecane

Hexabromocyclododecane (HBCD) is one of the priority persistent organic pollutants (POPs), yet a cost-effective technology has been lacking for the removal and degradation of HBCD. Zero-valent aluminum (ZVAl) is an excellent electron donor. However, the inert and hydrophilic surface oxide layer impedes the release of the electrons from the core metallic Al, resulting in poor reactivity towards HBCD. In this research, a new type of modified mZVAl particles (AC@mZVAl^bm/NaCl) were prepared through ball milling mZVAl in the presence of activated carbon (AC) and NaCl, and tested for adsorption and reductive degradation of HBCD in water. AC@mZVAl^bm/NaCl was characterized with a metallic Al core with newly created reactive surface coated with a thin layer of crushed carbon nanoparticles. AC@mZVAl^bm/NaCl was able to rapidly (within 1 h) adsorb HBCD (C₀ = 2 mg L^-1) and thus effectively enriched HBCD on the carbon surface of AC@mZVAl^bm/NaCl. The pre-enriched HBCD was subsequently degraded by the electrons from the core Al, and ∼63.44% of the pre-sorbed HBCD was completely debrominated after 62 h of the contact. A notable time lag (∼12 h) from the onset of the adsorption to the debromination was observed, signifying the importance of the solid-phase mass transfer from the initially adsorbed AC particles to the reactive Al-AC interface. Overall, AC@mZVAl^bm/NaCl synergizes the adsorptive properties of AC and the high reactivity of metallic Al, and enables a novel two-step adsorption and reductive degradation process for treating HBCD or likely other POPs.

Citrate ligand-enhanced microscale zero-valent aluminum corrosion for carbon tetrachloride degradation with high electron utilization efficiency

Carbon tetrachloride (CT) is highly toxic and recalcitrant in groundwater. In recent years, zero-valent aluminum (ZVAl) is highly reductive but limited by its surface passivation film. One of the effective ways to overcome this bottleneck is to add ligands. In this paper, compared with several other ligands, sodium citrate (SC), a natural organic ligand, was introduced to enhance microscale ZVAl (mZVAl) reactivity for the reductive degradation of CT. The results showed that the SC system could effectively reduce but not completely dechlorinate CT and electron utilization efficiency was as high as 94%. However, without ligands, mZVAl is chemically inert for CT degradation. Through SEM-EDS, BET, XRD, and XPS characterizations and H₂ evolution experiments, enhanced mZVAl surface corrosion at the solid-liquid interface of mZVAl/SC system was verified. SC participated in the complexation corrosion reaction with surface inert film to form Al[Cit] complex, which made internal Al⁰ active sites exposed and then promoted mZVAl corrosion. In the five consecutive reuse experiments of mZVAl, CT can be completely degraded, which indicates that mZVAl, with the help of SC, has excellent sustainable utilization efficiency.

Iron(II) sulfate crystals assisted mechanochemical modification of microscale zero-valent aluminum (mZVAl) for oxidative degradation of phenol in water

Microscale zero-valent aluminum (mZVAl) is prone to surface passivation due to formation of the surface Al-(hydr)oxide layer, resulting in short reactive life. To overcome this critical drawback, we developed a mechanochemical ball milling approach to modify and activate commercially available mZVAl assisted by the fragile FeSO₄·7H₂O crystals. SEM-EDS and XPS analyses indicated that the particle surface of the mechanochemically modified mZVAl (Fe-mZVAl^bm) was not only fractured with newly formed fresh reactive surfaces, but also attached with a rough layer of Fe-oxides that were uniformly distributed on mZVAl. While pristine mZVAl failed to degrade any phenol, Fe-mZVAl^bm was able to rapidly degrade 88.8% within 90 min (initial phenol = 20 mg/L, pH = 2.50, dosage = 3 g/L) under normal oxic conditions, with a pseudo first-order rate constant of 0.040 min^-1 and about 70.0% of phenol mineralized in 8 h. Moreover, Fe-mZVAl^bm also showed prolonged reactive life, and no significant reactivity drop was evident after six cycles of consecutive runs for phenol degradation. The much enhanced reactivity and reactive longevity of Fe-mZVAl^bm are attributed to the critical roles of the surface Fe-oxides, including 1) protecting the newly exposed reactive Al⁰ from being oxidized by side reactions, 2) serving as an electron mediator facilitating the electron transfer from the core Al⁰ reservoir to the exterior surface, and 3) acting as an Fe²⁺ source and a heterogeneous catalyst to enable the Fenton (-like) reactions. This study provides a novel and practical approach for preparing Fe-oxides modified mZVAl with enhanced and long-lasting reactivity.

Enhanced reactivity of zero-valent aluminum/O2 by using Fe-bearing clays in 4-chlorophenol oxidation

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 H₂O₂ 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.

Filtrates with Hydroxyl Radicals Prepared using Al + Acid + H2O2 for Removing Organic Pollutants

In this work, for the first time, high-activity filtrates were prepared by the reaction of aluminum (Al) powder with hydrogen peroxide (H₂O₂) in acidic solution and then filtration, which were used to degrade various organic pollutants such as phenol, methyl orange, and bisphenol A. It was found that the filtrates can effectively degrade and mineralize various organic pollutants and have a high efficiency comparable to their parent Al + acid + H₂O₂ suspensions. The filtrates can keep their high activity for several weeks under ambient conditions, and the activity depends on their initial pH value. At a pH value of ∼3.5, the reaction activity of filtrates is the best. Electron spin resonance spectroscopy (ESR) analyses indicated that there is a large quantity of stable hydroxyl radicals (OH^•) existing in the filtrates, which are responsible for the removal of organic pollutants. Furthermore, the related factors are discussed.

Performance Evaluation of Fe-Al Bimetallic Particles for the Removal of Potentially Toxic Elements from Combined Acid Mine Drainage-Effluents from Refractory Gold Ore Processing

Acid mine drainage (AMD) is a serious environmental issue associated with mining due to its acidic pH and potentially toxic elements (PTE) content. This study investigated the performance of the Fe-Al bimetallic particles for the treatment of combined AMD-gold processing effluents. Batch experiments were conducted in order to eliminate potentially toxic elements (including Hg, As, Cu, Pb, Ni, Zn, and Mn) from a simulated waste solution at various bimetal dosages (5, 10, and 20 g/L) and time intervals (0 to 90 min). The findings show that metal ions with greater electrode potentials than Fe and Al have higher affinities for electrons released from the bimetal. Therefore, a high removal (> 95%) was obtained for Hg, As, Cu, and Pb using 20 g/L bimetal in 90 min. Higher uptakes of Hg, As, Cu, and Pb than Ni, Zn, and Mn also suggest that electrochemical reduction and adsorption by Fe-Al (oxy) hydroxides as the primary and secondary removal mechanisms, respectively. The total Al3+ dissolution in the experiments with a higher bimetal content (10 and 20 g/L) were insignificant, while a high release of Fe ions was recorded for various bimetal dosages. Although the secondary Fe pollution can be considered as a drawback of using the Fe-Al bimetal, this issue can be tackled by a simple neutralization and Fe precipitation process. A rapid increase in the solution pH (initial pH 2 to >5 in 90 min) was also observed, which means that bimetallic particles can act as a neutralizing agent in AMD treatment system and promote the precipitation of the dissolved metals. The presence of chloride ions in the system may cause akaganeite formation, which has shown a high removal capacity for PTE. Moreover, nitrate ions may affect the process by competing for the released electrons from the bimetal owing to their higher electrode potential than the metals. Finally, the Fe-Al bimetallic material showed promising results for AMD remediation by electrochemical reduction of PTE content, as well as acid-neutralization/metal precipitation.

High-Efficient Generation of H2O2 by Aluminum-Graphite Composite through Selective Oxygen Reduction for Degradation of Organic Contaminants

Hydrogen peroxide (H₂O₂) is an effective green oxidant, which has been widely applied for environmental remediation. Here, we prepared a novel aluminum-graphite (Al-Gr) composite, which was capable of high-efficient production of H₂O₂ through selective O₂ reduction via a two-electron pathway. We discovered the production of H₂O₂ at a wide pH range, which could be enhanced by optimizing Al-Gr synthesis conditions. Poly(ethylene glycol) (PEG) addition could promote the formation of a welding interface and porous structure between Al and Gr in the Al-Gr composite, which enhanced the galvanic corrosion of Al⁰, the selectivity of oxygen reduction via the two-electron pathway, and the mass transfer of O₂ in the Al-Gr/O₂ system. The formation of Al₄C₃ could be regulated by sintering temperature and sintering time, which could promote the intergranular corrosion of Al⁰ and enhance the mass transfer of O₂ by reaction with water to generate the porous structure in the Al-Gr composite. The concentration of H₂O₂ reached 777.5 mg/L at an initial pH of 9.0, an Al-Gr dosage of 8 g/L, and an O₂ gas flow rate of 400 mL/min. The possible mechanisms of Al-Gr synthesis and H₂O₂ production in the Al-Gr/O₂ system were proposed. The Al-Gr composite was effective for the in situ production of H₂O₂, which could be further decomposed into a hydroxyl radical (^•OH) by Al⁰ in the Al-Gr composite. This composite could be used not only to decolorize the Rhodamine B dye but also to degrade various organic contaminants in different water matrices, indicating its environmental significance.

Catalytic activation of O2 by Al0-CNTs-Cu2O composite for Fenton-like degradation of sulfamerazine antibiotic at wide pH range

In this study, a novel Al°-CNTs-Cu₂O composite, capable of activating O₂ to generate H₂O₂ and further to reactive oxygen species (ROSs) at a wide pH range, was synthetized, characterized and applied for the degradation of sulfamerazine. In the activation of O₂ by Al°-CNTs-Cu₂O composite, H₂O₂ was generated from the reaction of O₂ with Al°-CNTs, which could be catalytically decomposed into O₂^- and OH by Cu₂O, the formed Cu(II) could be rapidly reduced to Cu₂O by Al°-CNTs in composite, which made Al°-CNTs-Cu₂O composite reusable and decreased the leaching of copper ions into solution. The removal efficiency of SMR and TOC was 73.91 % and 56.80 %, respectively at initial pH = 5.8, T = 20 °C, O₂ flow rate = 100 mL/min, Al°-CNTs-Cu₂O dosage = 2 g/L, SMR = 50 mg/L, and reaction time = 60 min. The removal efficiency of SMR kept almost unchanged and the concentration of copper ions in solution was below 0.5 mg/L. The Al°-CNTs-Cu₂O/O₂ process could be used as a novel catalyst for the degradation of refractory organic contaminants in water and wastewater by Fenton-like process at a wide pH range through the in situ generation of H₂O₂.