Characterization of chars produced in the co-pyrolysis of different wastes: decontamination study

Valorisation of spent tire rubber as carbon adsorbents for Pb(II) and W(VI) in the framework of a Circular Economy

Spent tire rubber-derived chars and their corresponding H3PO4 and CO2-activated chars were used as adsorbents in the recovery of Pb(II) ion and (W(VI)) oxyanion from synthetic solutions. The developed chars (both raw and activated) were thoroughly characterized to have insight about their textural and surface chemistry properties. H3PO4-activated chars presented lower surface areas than the raw chars and an acidic surface chemistry which affected the performance of these samples as they showed the lowest removals of the metallic ions. On the other hand, CO2-activated chars presented increased surface areas and increased mineral content compared to the raw chars, having presented higher uptake capacities for both Pb(II) (103-116 mg/g) and W(VI) (27-31 mg/g) ions. Cation exchange with Ca, Mg and Zn ions was appointed as a mechanism for Pb removal, as well as surface precipitation in the form of hydrocerussite (Pb3(CO3)2(OH)2). W(VI) adsorption might have been ruled by strong electrostatic attractions between the negatively charged tungstate species and the highly positively charged carbons' surface.The results shown in this work allow concluding that the valorisation of spent tire rubber through pyrolysis and the subsequent activation of the obtained chars is an alternative and a feasible option to generate adsorbent materials with a high uptake capacity of critical metallic elements.

Scaling up resource recovery of plastics in the emergent circular economy to prevent plastic pollution: Assessment of risks to health and safety in the Global South

Over the coming decades, a large additional mass of plastic waste will become available for recycling, as efforts increase to reduce plastic pollution and facilitate a circular economy. New infrastructure will need to be developed, yet the processes and systems chosen should not result in adverse effects on human health and the environment. Here, we present a rapid review and critical semi-quantitative assessment of the potential risks posed by eight approaches to recovering value during the resource recovery phase from post-consumer plastic packaging waste collected and separated with the purported intention of recycling. The focus is on the Global South, where there are more chances that high risk processes could be run below standards of safe operation. Results indicate that under non-idealised operational conditions, mechanical reprocessing is the least impactful on the environment and therefore most appropriate for implementation in developing countries. Processes known as 'chemical recycling' are hard to assess due to lack of real-world process data. Given their lack of maturity and potential for risk to human health and the environment (handling of potentially hazardous substances under pressure and heat), it is unlikely they will make a useful addition to the circular economy in the Global South in the near future. Inevitably, increasing circular economy activity will require expansion towards targeting flexible, multi-material and multilayer products, for which mechanical recycling has well-established limitations. Our comparative risk overview indicates major barriers to changing resource recovery mode from the already dominant mechanical recycling mode towards other nascent or energetic recovery approaches.

Fates of Chemical Elements in Biomass during Its Pyrolysis

Biomass is increasingly perceived as a renewable resource rather than as an organic solid waste today, as it can be converted to various chemicals, biofuels, and solid biochar using modern processes. In the past few years, pyrolysis has attracted growing interest as a promising versatile platform to convert biomass into valuable resources. However, an efficient and selective conversion process is still difficult to be realized due to the complex nature of biomass, which usually makes the products complicated. Furthermore, various contaminants and inorganic elements (e.g., heavy metals, nitrogen, phosphorus, sulfur, and chlorine) embodied in biomass may be transferred into pyrolysis products or released into the environment, arousing environmental pollution concerns. Understanding their behaviors in biomass pyrolysis is essential to optimizing the pyrolysis process for efficient resource recovery and less environmental pollution. However, there is no comprehensive review so far about the fates of chemical elements in biomass during its pyrolysis. Here, we provide a critical review about the fates of main chemical elements (C, H, O, N, P, Cl, S, and metals) in biomass during its pyrolysis. We overview the research advances about the emission, transformation, and distribution of elements in biomass pyrolysis, discuss the present challenges for resource-oriented conversion and pollution abatement, highlight the importance and significance of understanding the fate of elements during pyrolysis, and outlook the future development directions for process control. The review provides useful information for developing sustainable biomass pyrolysis processes with an improved efficiency and selectivity as well as minimized environmental impacts, and encourages more research efforts from the scientific communities of chemistry, the environment, and energy.

Sustainable development of tyre char-based activated carbons with different textural properties for value-added applications

This paper aims at the sustainable development of activated carbons for value-added applications from the waste tyre pyrolysis product, tyre char, in order to make pyrolysis economically favorable. Two activation process parameters, activation temperature (900, 925, 950 and 975 °C) and residence time (2, 4 and 6 h) with steam as the activating agent have been investigated. The textural properties of the produced tyre char activated carbons have been characterized by nitrogen adsorption-desorption experiments at -196 °C. The activation process has resulted in the production of mesoporous activated carbons confirmed by the existence of hysteresis loops in the N2 adsorption-desorption curves and the pore size distribution curves obtained from BJH method. The BET surface area, total pore volume and mesopore volume of the activated carbons from tyre char have been improved to 732 m(2)/g, 0.91 cm(3)/g and 0.89 cm(3)/g, respectively. It has been observed that the BET surface area, mesopore volume and total pore volume increased linearly with burnoff during activation in the range of experimental parameters studied. Thus, yield-normalized surface area, defined as the surface area of the activated carbon per gram of the precursor, has been introduced to optimize the activation conditions. Accordingly, the optimized activation conditions have been demonstrated as an activation temperature of 975 °C and an activation time of 4 h.

Leaching behaviour and ecotoxicity evaluation of chars from the pyrolysis of forestry biomass and polymeric materials

The main objective of this study was to assess the environmental risk of chars derived from the pyrolysis of mixtures of pine, plastics, and scrap tires, by studying their leaching potential and ecotoxicity. Relationships between chemical composition and ecotoxicity were established to identify contaminants responsible for toxicity. Since metallic contaminants were the focus of the present study, an EDTA washing step was applied to the chars to selectively remove metals that can be responsible for the observed toxicity. The results indicated that the introduction of biomass to the pyrolysis feedstock enhanced the acidity of chars and promote the mobilisation of inorganic compounds. Chars resulting from the pyrolysis of blends of pine and plastics did not produce ecotoxic eluates. A relationship between zinc concentrations in eluates and their ecotoxicity was found for chars obtained from mixtures with tires. A significant reduction in ecotoxicity was found when the chars were treated with EDTA, which was due to a significant reduction in zinc in chars after EDTA washing.

Comparison of real waste (MSW and MPW) pyrolysis in batch reactor over different catalysts. Part II: contaminants, char and pyrolysis oil properties

Pyrolysis of real wastes (MPW and MSW) has been investigated at 500°C, 550°C and 600°C using Y-zeolite, β-zeolite, equilibrium FCC, MoO3, Ni-Mo-catalyst, HZSM-5 and Al(OH)3 as catalysts. The viscosity of pyrolysis oils could be decreased by the using of catalysts, especially by β-zeolite and MoO3. Both carbon frame and double bound isomerization was found in case of thermo-catalytic pyrolysis. Char morphology and texture analysis showed more coke deposits on the catalyst surface using MSW raw material. Pyrolysis oils had K, S, P Cl, Ca, Zn, Fe, Cr, Br and Sb as contaminants; and the concentrations of K, S, P, Cl and Br could be decreased by the using of catalysts.

The kinetic analysis of the pyrolysis of agricultural residue under non-isothermal conditions

The study concerns the pyrolysis kinetics of agricultural wastes, corn straw (CS) and rice husk (RH). Thermogravimetric experiments were carried out in a thermogravimetric analyzer under inert conditions, and operated at different heating rates ranging from 5 to 40K/min. As the increment of heating rates, the variations of characteristic parameters from the TG-DTG curves were determined. Iso-conversional Starink approach and Avrami theory were used to evaluate the kinetic parameters, including apparent activation energy and reaction order. For the range of conversion fraction investigated (20-80%), the apparent activation energy of CS initially increased from 98.715 to 148.062 kJ/mol and then decreased to 144.387 kJ/mol afterwards, whilst the apparent activation energy of RH increased gradually from 50.492 to 88.994 kJ/mol. With varied temperatures (517-697 K), the corresponding value of reaction order was increased from 0.288 and 0.359 to 0.441 and 0.692, along with a decrease to 0.306 and 0.445, respectively.

Bio-oils from biomass slow pyrolysis: a chemical and toxicological screening

Bio-oils were produced from bench-scale slow-pyrolysis of three different biomass samples (corn stalks, poplar and switchgrass). Experimental protocols were developed and applied in order to screen their chemical composition. Several hazardous compounds were detected in the bio-oil samples analysed, including phenols, furans and polycyclic aromatic hydrocarbons. A procedure was outlined and applied to the assessment of toxicological and carcinogenic hazards of the bio-oils. The following hazardous properties were considered: acute toxicity; ecotoxicity; chronic toxicity; carcinogenicity. Parameters related to these properties were quantified for each component identified in the bio-oils and overall values were estimated for the bio-oils. The hazard screening carried out for the three bio-oils considered suggested that: (i) hazards to human health could be associated with chronic exposures to the bio-oils; (ii) acute toxic effects on humans and eco-toxic effects on aquatic ecosystems could also be possible in the case of loss of containment; and (iii) bio-oils may present a marginal potential carcinogenicity. The approach outlined allows the collection of screening information on the potential hazards posed by the bio-oils. This can be particularly useful when limited time and analytical resources reduce the possibility to obtain detailed specific experimental data.

Physico-chemical properties of chars obtained in the co-pyrolysis of waste mixtures

The present work aims to perform a multistep upgrading of chars obtained in the co-pyrolysis of PE, PP and PS plastic wastes, pine biomass and used tires. The quality of the upgraded chars was evaluated by measuring some of their physico-chemical properties in order to assess their valorisation as adsorbents' precursors. The crude chars were submitted to a sequential solvent extraction with organic solvents of increasing polarity (hexane, mixture 1:1 v/v hexane:acetone and acetone) followed by an acidic demineralization procedure with 1M HCl solution. The results obtained showed that the upgrading treatment allow the recovery of 63-81% of the pyrolysis oils trapped in the crude chars and a reduction in the char's ash content in the range of 64-86%. The textural and adsorption properties of the upgraded chars were evaluated and the results indicate that the chars are mainly mesoporous and macroporous materials, with adsorption capacities in the range of 3.59-22.2 mg/g for the methylene blue dye. The upgrading treatment allowed to obtain carbonaceous materials with quality to be reused as adsorbents or as precursors for activated carbon.