Experimental analysis on products distribution, characterization and mechanism of waste polypropylene (PP) and polyethylene terephthalate (PET) degradation in sub-/supercritical water.

Supercritical water (SCW) treatment of plastics is a clean technology in the 'waste-to-energy' path. In this work, PP and PET plastics were processed by sub-/supercritical water. The results showed that temperature was the most important factor of the PP and PET degradation. The influence of factors on the degradation of plastics follows the following order: temperature > residence time > plastic/water ratio. These factors influenced the yield of gas products by promoting or inhibiting various reactions (such as reverse water gas shift reaction, methylation reaction, and Fischer-Tropsch synthesis reaction). Besides, the composition of liquid oil was also analyzed. The main composition of the liquid oil produced by PET was benzoic acid and acetaldehyde, which were generated from the decarboxylation of terephthalic acid (TPA) and dehydration reaction of ethylene glycol (EG). The liquid oil from PP was mainly long-chain olefins, long-chain alkanes, cycloalkanes, etc., which were formed by the interaction of various methyl, alkyl, hydroxyl, and other free radicals. This study could build fundamental theories of plastic mixture treatment.

The interactions between mixed waste from discarded surgical masks and face shields during the degradation in supercritical water.

The widespread use of surgical masks made of polyolefin and face shields made of polyester during pandemics contributes significantly to plastic pollution. An eco-friendly approach to process plastic waste is using supercritical water, but the reaction of mixed polyolefin and polyester in this solvent is not well understood, which hinders practical applications. This study aimed to investigate the reaction of waste surgical masks (SM) and face shields (FS) mixed in supercritical water. Results showed that the optimal treatment conditions were 400 °C and 60 min, achieving a liquid oil yield of 823.03 mg·g-1 with 25 wt% FS. The interaction between polypropylene (PP), polyethylene terephthalate (PET), and iron (Fe) in SM and FS mainly determined the production of liquid oil products such as olefins and benzoic acid. The methyl-branched structure of PP enhanced PET hydrolysis, resulting in higher production of terephthalic acid (TPA). The degradation of PP was facilitated by the acidic environment created by TPA and benzoic acid in the reaction. Moreover, the hydrolysis of PET produced carboxylic acid, which coordinated with Fe3+ to form Fe-H that catalyzed the polymerization of small olefins, contributing to higher selectivity for C9 olefins. Therefore, this study provides valuable insights into the degradation mechanism of mixed PPE waste in supercritical water and guidance for industrial treatment.

Swelling-enhanced catalytic degradation of brominated epoxy resin in waste printed circuit boards by subcritical acetic acid under mild conditions.

Waste printed circuit boards (WPCBs) contain a large amount of brominated epoxy resins (BERs), which may cause environmental problems. However, BERs degradation under mild conditions is challenging due to the good thermal and chemical stabilities of BERs. This study proposes a mild and efficient method that uses subcritical acetic acid (220 °C-260 °C, 2.6-3.6 MPa) to decompose BERs. BERs swell quickly at 200 °C and are thoroughly decomposed into bisphenol A and phenol at 220 °C when the acetic acid mass concentration and holding time are fixed at 49.90% and 1 h, respectively. Experimental results show that subcritical acetic acid has excellent swelling and catalytic degradation effects on BERs. The quick swelling of BERs allows the free migration of the catalyst in the epoxy network and thus significantly enhances the catalytic degradation effect. Therefore, BERs can be thoroughly decomposed by subcritical acetic acid under mild conditions. Temperature and acetic acid concentration are the major parameters that control the resin degradation rate. Bromine-free oil phase products are obtained at ≥240 °C. The possible decomposition pathway of BERs in subcritical acetic acid is also investigated. Most of the bromine is transformed into HBr and enriched in the aqueous phase. In conclusion, the proposed mild method could be used as a novel practical and industrial procedure for the degradation and debromination of BERs.

Extraction of heavy metal (Ba, Sr) and high silica glass powder synthesis from waste CRT panel glasses by phase separation.

In this study, a novel process for the extraction of heavy metal Ba and Sr from waste CRT panel glass and synchronous preparation of high silica glass powder was developed by glass phase separation. CRT panel glass was first remelted with B2O3 under air atmosphere to produce alkali borosilicate glass. During the phase separation process, the glass separated into two interconnected phases which were B2O3-rich phase and SiO2-rich phase. Most of BaO, SrO and other metal oxides including Na2O, K2O, Al2O3 and CaO were mainly concentrated in the B2O3-rich phase. The interconnected B2O3-rich phase can be completely leached out by 5mol/L HNO3 at 90 ℃. The remaining SiO2-rich phase was porous glasses consisting almost entirely of silica. The maximum Ba and Sr removal rates were 98.84% and 99.38% and high silica glass powder (SiO2 purity > 90 wt%) was obtained by setting the temperature, B2O3 added amount and holding time at 1000-1100 ℃, 20-30% and 30 min, respectively. Thus this study developed an potential economical process for detoxification and reclamation of waste heavy metal glasses.

Lead recovery and high silica glass powder synthesis from waste CRT funnel glasses through carbon thermal reduction enhanced glass phase separation process.

In this study, a novel process for the removal of toxic lead from the CRT funnel glass and synchronous preparation of high silica glass powder was developed by a carbon-thermal reduction enhanced glass phase separation process. CRT funnel glass was remelted with B2O3 in reducing atmosphere. In the thermal process, a part of PbO contained in the funnel glass was reduced into metallic Pb and detached from the glass phase. The rest of PbO and other metal oxides (including Na2O, K2O, Al2O3, BaO and CaO) were mainly concentrated in the boric oxide phase. The metallic Pb phase and boric oxide phase were completely leached out by 5mol/L HNO3. The lead removal rate was 99.80% and high silica glass powder (SiO2 purity >95wt%) was obtained by setting the temperature, B2O3 added amount and holding time at 1000°C, 20% and 30mins, respectively. The prepared high silicate glass powders can be used as catalyst carrier, semipermeable membranes, adsorbents or be remelted into high silicate glass as an ideal substitute for quartz glass. Thus this study proposed an eco-friendly and economical process for recycling Pb-rich electronic glass waste.