Mechanochemical conversion of brominated POPs into useful oxybromides: a greener approach

Mechanochemical remediation of contaminated soil: A review

Mechanochemical techniques have been garnering growing attention in remediation of contaminated soil. This paper summarizes the performance, mechanism, influential factors, and environmental impacts of mechanochemical remediation (MCR) for persistent organic pollutants (POPs) contaminated soil and heavy metal(loid) s (HMs) contaminated soil. Firstly, in contrast to other technologies, MCR can achieve desirable treatment of POPs, HMs, and co-contaminated soil, especially with high-concentration pollutants. Secondly, POPs undergo mineralization via interaction with mechanically activated substances, where aromatic and aliphatic pollutants in soil may go through varied degradation routes; inorganic pollutants can be firmly combined with soil particles by fragmentation and agglomeration induced by mechanical power, during which additives may enhance the combination but their contact with anionic metal(loid)s may be partially suppressed. Thirdly, the effect of MCR primarily hinges on types of milling systems, the accumulation of mechanical energy, and the use of reagents, which is basically regulated through operating parameters: rotation speed, ball-to-powder ratio, reagent-to-soil ratio, milling time, and soil treatment capacity; minerals like clay, metal oxides, and sand in soil itself are feasible reagents for remediation, and alien additives play a crucial role in synergist and detoxification; additionally, various physicochemical properties of soil might influence the mechanochemical effect to varying degrees, yet the key influential performance and mechanism remain unclear and require further investigation. Concerning the assessment of soil after treatment, attention needs to be paid to soil properties, toxicity of POPs' intermediates and leaching HMs, and long-term appraisement, particularly with the introduction of aggressive additives into the system. Finally, proposals for current issues and forthcoming advancements in this domain are enumerated in items. This review provides valuable insight into mechanochemical approaches for performing more effective and eco-friendly remediation on contaminated soil.

Biodiversity of multi-trophic biological communities within riverine sediments impacted by PAHs contamination and land use changes

River ecosystems currently face a significant threat of degradation and loss of biodiversity resulting from continuous emissions of persistent organic pollutants and human activities. In this study, multi-trophic communities were assessed using DNA metabarcoding in a relatively stable riverine sediment compartment to investigate the biodiversity dynamics in the Beiluo River, followed by an evaluation of their response to polycyclic aromatic hydrocarbons (PAHs) and land use changes. A total of 48 bacterial phyla, 4 fungal phyla, 4 protist phyla, 9 algal phyla, 31 metazoan phyla, and 12 orders of fish were identified. The total concentration of PAHs in the Beiluo River sediments ranged from 25.95 to 1141.35 ng/g, with low molecular weight PAHs constituting the highest proportion (68.67%), followed by medium (22.19%) and high (9.14%) molecular weight PAHs. Notably, in contrast to lower trophic level aquatic communities such as bacteria, algae, and metazoans, PAHs exhibited a significant inhibitory effect on fish. Furthermore, the diversity of aquatic communities displayed obvious heterogeneity across distinct land use groups. A high proportion of cultivated land reduced the biodiversity of fish communities but increased that of metazoans. Conversely, an elevated proportion of built-up land reduced metazoan biodiversity, while simultaneously enhancing that of fungi and bacteria. Generally, land use changes exert both indirect and direct effects on aquatic communities. The direct effects primarily influence the abundance of aquatic communities rather than their diversity. Nevertheless, PAHs pollution may have limited potential to disrupt community structures through complex species interactions, as the hub species identified in the co-occurrence network did not align with those significantly affected by PAHs. This study indicates the potential of PAHs and land use changes to cause biodiversity losses. However, it also highlights the possibility of mitigating these negative effects in riverine sediments through optimal land use management and the promotion of enhanced species interactions.

Enhanced piezoelectric catalysis of BaTiO3 by ZVAl for mechanochemical defluorination of PFOA: Promotion of electron transfer

Mechanochemical (MC) destruction of pollutants is effective; however, the emerging electron transfer mechanism is ambiguous owing to a lack of systematic evaluation. Therefore, this study aims to evaluate the contribution of electrons to perfluorooctanoic acid (PFOA) defluorination during MC process. A synergistic effect was obtained by activating BaTiO3 to generate piezoelectrons and applying zero-valence aluminum (ZVAl) to facilitate electron transfer, with 95.66% PFOA defluorination and reaction time decreasing from 6 h to 3 h. The mechanism of piezoelectric catalysis of the BaTiO3/ZVAl system was further investigated through kinetic analyses and intersystem comparisons. The major contribution of piezo-excited electrons was revealed through probe detection and quantitative determination. A positive correlation between electron generation and PFOA defluorination was ascertained, and the calculation of the electron utilization ratio revealed an electron transfer mechanism. The detached fluorides were confirmed to be bonded directly to the additives. Furthermore, PFOA decomposition was identified as a cyclical process with constant dissociation of the CF2 groups.

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.

Improved fractal kinetic model to predict mechanochemical destruction rate of organic pollutants

Mechanochemical destruction of organic pollutants by high energy milling with inorganic reagents is considered a promising non-thermal technology to detoxify hazardous waste. However, due to complex nature of the physicochemical phenomena involved, pollutant destruction kinetics heavily depends on the used reagents and operating parameters, thus varying case by case. In the present work, a fractal model was validated as flexible tool to interpolate pollutant mechanochemical destruction data satisfactorily. In addition, such model was expanded to estimate the contributions of the inorganic reagent and the pollutant to the overall reaction rate. Specifically, the kinetic constant associated to mechanical activation of the co-milling reagent and that related to pollutant destruction reaction were calculated. Their values resulted to depend only on the specific compound, hence, the tabulated data could be used to predict the pollutant mechanochemical degradation rate for any kind of mixture.

Synthesis and Regeneration of A MXene-Based Pollutant Adsorbent by Mechanochemical Methods

In the present study, an adsorbent material for removal of organic contaminants in wastewater is synthetized by a green and facile mechanochemical method. It is composed of Ti3C2Tx MXene layers (obtained by mechanochemical etching of MAX phase with concentrated HF) pillared with terephthalate by rapid direct reaction. Such material shows high specific surface area (135.7 m2 g-1) and excellent adsorption capability of methylene blue (209 mg g-1) because of the larger interlayer space among MXene sheets and free carboxylate groups of terephthalate. The spent adsorbent is reutilized (with addition of sole aluminum) to synthetize the MAX phase by mechanochemical procedure, where the terephthalate and the pollutant are carbonized into the carbide. In this way, new MXene-based adsorbent can be re-synthetized for further use.

Mechanochemical enhancement of the natural attenuation capacity of soils using two organophosphate biocides as models

Mechanochemical treatment by high energy ball milling is a promising technology to safely destroy organic pollutants in contaminated soil and allow its possible beneficial reuse. The present study investigates the mechanochemical activation of four major soil components, which induces generation of electrons on particle surfaces. Such phenomenon is demonstrated to occur on oxides by formation of trapped electrons in oxygen vacancies (following a zeroth-order kinetics), as well as on quartz and clayey materials to form fresh electron-rich surfaces by homolytic bond rapture (according to a first-order kinetics). Two toxic organophosphate biocides (i.e. chlorpyrifos and glyphosate) are used as model pollutants. Results show that the aromatic structure of chlorpyrifos determines a faster degradation rate, compared to the aliphatic one of glyphosate, because of the higher stability of generated radical intermediates. Moreover, the aromatic moiety facilitates adsorption on clays, thus temporarily sequestering the molecule and delaying its degradation. The many heteroatoms in both organophosphates have analogous fate: mineralization to inorganic form.

Roles of intrinsic Mn3+ sites and lattice oxygen in mechanochemical debromination and mineralization of decabromodiphenyl ether with manganese dioxide

Commercial β-MnO₂ with a chemical formula of approximate Mn_0.77⁴⁺Mn_0.23³⁺O_1.88 was used for mechanochemical (MC) oxidative degradation of decabromodiphenyl ether (BDE209). The ball milling process initiated the degradation of BDE209 on β-MnO₂, yielding a nearly complete degradation and debromination of BDE209 within 2 h. The use of β-MnO₂ exhibited much higher MC debromination efficiency than that by using birnessite (δ-MnO₂, 40.2%), Bi₂O₃ (45.6%), CaO (65.3%), and persulfate (81.9%). It was demonstrated that the oxidative degradation of BDE209 was promoted by the redox half reactions of both Mn⁴⁺→ Mn³⁺ and Mn³⁺→ Mn²⁺, but naturally existed Mn³⁺ centers on the surface of β-MnO₂ functioned as dominant reactive species at the initial stage of the MC degradation (often before the degradation efficiency of BDE209 achieved 50%). Moreover, the surface lattice oxygen of MnO₂, rather than O₂, played a key role in the debromination and mineralization of BDE209. The Mn³⁺ sites on β-MnO₂ not only easily accepted the electron of BDE209, but also promoted the mobility of lattice oxygen from the bulk to the surface for mineralizing BDE209. These results firstly highlighted the importance of Mn³⁺ availability and oxygen mobility on the reactivity of manganese oxide for the MC oxidative degradation of organic pollutants.

Methods of Responsibly Managing End-of-Life Foams and Plastics Containing Flame Retardants: Part II

This is Part II of a review covering the wide range of issues associated with all aspects of the use and responsible disposal of foam and plastic wastes containing toxic or potentially toxic flame retardants. We identify basic and applied research needs in the areas of responsible collection, pretreatment, processing, and management of these wastes. In Part II, we explore alternative technologies for the management of halogenated flame retardant (HFR) containing wastes, including chemical, mechanical, and thermal processes for recycling, treatment, and disposal.

Mechanochemical pre-treatment for viable recycling of plastic waste containing haloorganics

Chemical recycling technologies are the most promising for a waste-to-energy/material recovery of plastic waste. However, 30% of such waste cannot be treated in this way due to the presence of halogenated organic compounds, which are often utilized as flame retardants. In fact, high quantities of hydrogen halides and dioxin would form. In order to enabling such huge amount of plastic waste as viable feedstock for recycling, an investigation on mechanochemical pre-treatment by high energy ball milling is carried out on polypropylene containing decabromodiphenyl ether. Results demonstrate that co-milling with zero valent iron and quartz sand ensures complete debromination and mineralization of the flame retardant. Furthermore, a comparative experiment demonstrates that the mechanochemical debromination kinetics is roughly proportional to the polymer-to-haloorganics mass ratio.

Mechanochemical mineralization of "very persistent" fluorocarbon surfactants ‒ 6:2 fluorotelomer sulfonate (6:2FTS) as an example

Fluorinated organic chemicals have a wide variety of industrial and consumer applications. For long time perfluorooctane sulfonate and perfluorooctanoic acid have been used as precursors for manufacture of such chemicals. However, these C₈ chain compounds have been demonstrated to be toxic, persistent, and bioaccumulative, thus inducing their phase-out. Currently, C₆ telomer based fluorocarbon surfactants are considered better alternatives to C₈ products because of their low bioaccumulability. But, their high persistency suggests that in the near future their concentrations will increase in the environment and in industrial waste. Being a solid state non-thermal technology, mechanochemical treatment is a good candidate for the destruction of emerging C₆ fluorotelomers in solid waste. In the present study, 6:2 fluorotelomer sulfonate is effectively destroyed (~100%) in rapid manner (<1 h) by high energy ball milling with KOH. Stoichiometric fluoride formation confirms its entire mineralization, assuring that no toxic by-products are generated. Reaction mechanism and kinetics indicate that effective mineralization of the perfluorinated moiety is obtained thanks to a rapid CF₂ "flake-off" process through radical mechanism.

Formation of brominated and chlorinated dioxins and its prevention during a pilot test of mechanochemical treatment of PCB and PBDE contaminated soil

The destruction of persistent organic pollutants (POPs) is a large challenge in particular in developing and emerging economies. To date, a detailed assessment of non-combustion technologies with respect to formation of dioxins is lacking. In this study, an assessment of mechanochemical (MC) destruction technology for polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs) in contaminated soil remediation was conducted. Actual applied conditions of pilot-scale MC POPs destruction process indicates that the temperature increase inside the ball mills has the potential to form high levels of toxic polybrominated and polychlorinated dibenzo-p-dioxins and dibenzofurans (PXDD/Fs) when dioxin precursors are present. Therefore, the MC technology was modified for treatment of the PCB and PBDE containing soil including an efficient cooling system which could prevent the formation of PXDD/F during the destruction of PCBs and PBDEs. This is likely relevant for all contaminated soils containing relevant dioxin precursor and need to be considered for treatment of soils with MC and probably other non-combustion technologies. Graphical abstract ᅟ.