Giovanni Cagnetta

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 feed-stock 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.

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 C 8 chain compounds have been demonstrated to be toxic, persistent, and bioaccumulative, thus inducing their phase-out. Currently, C 6 telomer based fluorocarbon surfactants are considered better alternatives to C 8 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 C 6 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 CF 2 " flake-off " process through radical mechanism.

Zero-valent iron (ZVI) is a valuable material for environmental remediation, because of its safeness, large availability, and inexpensiveness. Moreover, its reactivity can be improved by addition of (nano-) particles of other elements such as noble metals. However, common preparation methods for this kind of iron-based composites involve wet precipitation of noble metal salt precursors, so they are often expensive and not green. Mechanochemical procedures can provide a solvent-free alternative, even at a large scale. The present study demonstrates that it is possible to tailor functional properties of ZVI-based materials, utilizing high-energy ball milling. All main preparation parameters are investigated and discussed. Specifically, a copper-carbon-iron ternary composite was prepared for fast degradation of 4-nitrophenol (utilized as model pollutant) to 4-aminophenol and other phenolic compounds. Copper and carbon are purposely chosen to insert specific properties to the composite: Copper acts as efficient nano-cathode that enhances electron transfer from iron to 4-nitrophenol, while carbon protects the iron surface from fast oxidation in open air. In this way, the reactive material can rapidly reduce high concentration of nitrophenols in water, it does not require acid washing to be activated, and can be stored in open air for one week without any significant activity loss.

Perfluorinated chemicals (PFCs) are attracting increasing concern due to their chemical stability and toxicity, which make them classifiable as persistent organic pollutants. However, such compounds are employed for the manufacture of many industrial and consumer products, hence their substitution is not possible in the near future. Suitable technologies for PFCs environmentally sound management are necessary. High energy ball milling with KOH has been proved to obtain PFCs safe destruction, but with excessive reagent. In the present study, a Waste-to-Materials solution is proposed: PFCs are co-milled with stoichiometric amounts of La2O3 and thus are fully converted into LaOF, a noteworthy luminescent material with many potential industrial applications. Reaction mechanism is also investigated: Ball milling process activates La2O3; this latter provokes oxidation to CO2 and carbonization of PFCs; fluorides are incorporated into the La2O3’s lattice to form the oxyfluoride. Interestingly, kinetic analysis suggests that the mechanical activation of La2O3 might follow a zeroth-order reaction rate.

Brominated organic pollutants are considered of great concern for their adverse effect on human health and the environment, so an increasing number of such compounds are being classified as persistent organic pollutants (POPs). Mechanochemical destruction is a promising technology for POPs safe disposal because it can achieve their complete carbonization by solvent-free high energy ball milling at room temperature. However, a large amount of co-milling reagent usually is necessary, so a considerable volume of residue is produced. In the present study a different approach to POPs mechanochemical destruction is proposed. Employing stoichiometric quantities of Bi 2 O 3 or La 2 O 3 as co-milling reagent, brominated POPs are selectively and completely converted into their corresponding oxybromides (i.e. BiOBr and LaOBr), which possess very peculiar properties and can be used for some actual and many more potential applications. In this way, bromine is beneficially reused in the final product, while POPs carbon skeleton is safely destroyed to amorphous carbon. Moreover, mechanochemical destruction is employed in a greener and more sustainable manner.

Many tons of intentionally produced obsolete halogenated persistent organic pollutants (POPs), are stored worldwide in stockpiles, often in an unsafe manner. These are a serious threat to the environment and to human health due to their ability to migrate and accumulate in the biosphere. New technologies , alternatives to combustion, are required to destroy these substances, hopefully to their complete mineralization. In the last 20 years mechanochemical destruction has shown potential to achieve pollutant degradation , both of the pure substances and in contaminated soils. This capability has been tested for many halogenated pollutants, with various reagents, and under different milling conditions. In the present paper, a review of the published work in this field is followed by a critique of the state of the art of POPs mechanochemical destruction and its applicability to full-scale halogenated waste treatment.

Secondary copper recovery is attracting increasing interest because of the growth of copper containing waste including e-waste. The pyrometallurgical treatment in smelters is widely utilized, but it is known to produce waste fluxes containing a number of toxic pollutants due to the large amount of copper involved, which catalyses the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (" dioxins "). Dioxins are generated in secondary copper smelters on fly ash as their major source, resulting in highly contaminated residues. In order to assess the toxicity of this waste, an analysis of dioxin-like compounds was carried out. High levels were detected (79,090 ng TEQ kg −1) in the ash, above the Basel Convention low POPs content (15,000 ng TEQ kg −1) highlighting the hazardousness of this waste. Experimental tests of high energy ball milling with calcium oxide and silica were executed to assess its effectiveness to detoxify such fly ash. Mechanochemical treatment obtained 76% dioxins reduction in 4 h, but longer milling time induced a partial de novo formation of dioxins catalysed by copper. Nevertheless, after 12 h treatment the dioxin content was substantially decreased (85% reduction) and the copper, thanks to the phenomena of incorporation and amorphization that occur during milling, was almost inactivated.

In the last 20 years, mechanochemical technology has shown a remarkable potential to destroy halogenated pollutants in safe manner. Being a solvent-free treatment that operates at environmental conditions, it is a good candidate to be a non-thermal alternative for disposal of persistent organic pollutants stockpiles. However, there are many aspects that need further developments. One of them is the assessment of reaction kinetic. In the present work, a kinetic model that is commonly utilized to describe mechanochemical transformations is adapted and applied for the first time to destruction of organic molecules. In particular, it was modified to include all fundamental parameters related to the mechanochemical degradation of halogenated persistent organic pollutants. Validation with literature and experimental data proves the soundness of the proposed model and confirms that solid reagents activation is a necessary step to achieve pollutants destruction.

Large amounts (megatons) of marine sediments, partly contaminated by toxic recalcitrant organics like polychlorobyphenyls (PCB) and polycyclic aromatic hydrocarbons (PAH) and as well as by heavy metals and other inorganic pollutants, result from dredging operations at industrial harbors worldwide. In order to avoid the unsustainable burden of sanitary landfilling of this huge amount of special/hazardous waste and to find out technical and cost effective methods for its detoxification and eventual reuse as building material for new marine embankment, an R&D project has been undertaken. The project is based on two complementary advanced technologies: short mechanochemical (MC) pretreatment, wherein chemical reactions are activated at nano-particle level by collisions with milling bodies in special high energy milling devices, followed by biological treatment (B) with purposely isolated aerobic bacteria like Burkholderia xenovorans. The experimental results aimed at determining the kinetics and the overall technical efficiency of the MC+B treatment of artificially contaminated marine sediments from the harbor of Taranto (S. Italy) indicated that, in the best operating conditions, PCB degradation may be achieved in very effective (∼50 %) and fast (<8 d) manner.

Purpose

The Biomec process, a two-stage treatment based on a short mechanochemical (MC) pretreatment and then followed by an aerobic biological degradation, was developed and tested for detoxifying marine sediments that were largely contaminated by polychlorobiphenyls (PCBs).
Materials and methods

Clean marine sediment spiked with PCBs (Aroclor 1260) and, alternatively, with decachlorobiphenyl in slurry conditions was ultramilled for 1 min in a nutational high energy ball mill, then was treated aerobically in a bioreactor with a purposely selected commercial bacterium (Burkholderia xenovorans).
Results and discussion

With ∼66 % overall PCB biodegradation achieved in less than 3 months, laboratory experiments confirmed the remarkable effectiveness of Biomec process when compared to direct bioremediation. The investigation showed in particular that the MC pretreatment decreased the chlorination degree of high-chlorinated PCB congeners, and consequently their biorecalcitrance, through the substitution of some chlorine atoms with hydroxyl groups. This reaction eases the aerobic degradation of the hydroxyl-substituted PCBs by B. xenovorans, allowing bacteria to skip the cell stressing step of aromatic ring bi-hydroxylation along the biodegradation pathway.
Conclusions

After short MC treatment of the sediments, a common biological aerobic treatment can degrade PCB congeners, the highly chlorinated ones were included, in a fast, effective and cheap manner.