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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.

Maize Cob Waste (MCW) is available worldwide in high amounts, as maize is the most produced cereal in the world. MCW is generally left in the crop fields, but due to its low biodegradability it has a negligible impact in soil fertility. Moreover, MCW can be used as substrate to balance the C/N ratio during the Anaerobic co-Digestion (AcoD) with other biodegradable substrates, and is an excellent precursor for the production of Activated Carbons (ACs). In this context, a biorefinery is theoretically discussed in the present review, based on the idea that MCW, after proper pre-treatment is valorised as precursor of ACs and as co-substrate in AcoD for biomethane generation. This paper provides an overview on different scientific and technological aspects that can be involved in the development of the proposed biorefinery; the major topics considered in this work are the following ones: (i) the most suitable pre-treatments of MCW prior to AcoD; (ii) AcoD process with regard to the critical parameters resulting from MCW pre-treatments; (iii) production of ACs using MCW as precursor, with the aim to use these ACs in biogas conditioning (H2S removal) and upgrading (biomethane production), and (iv) an overview on biogas upgrading technologies.

A mixture of 70% (w/w) pine biomass and 30% (w/w) plastics (mixture of polypropylene, polyethylene, and polystyrene) was subjected to pyrolysis at 400 degrees C, for 15 min, with an initial pressure of 40 MPa. Part of the solid residue produced was subjected to extraction with dichloromethane (DCM). The extracted residue (residue A) and raw residue (residue B) were analyzed by weight loss combustion and submitted to the leaching test ISO/TS 21268-2 using two different leachants: DCM (0.2%, v/v) and calcium chloride (0.001 mol/L). The concentrations of the heavy metals Cd, Cr, Ni, Zn, Pb and Cu were determined in the eluates and in the two residues. The eluates were further characterized by determining their pH and the concentrations of benzene, toluene, ethylbenzene and xylenes (BTEX). The presence of other organic contaminants in the eluates was qualitatively evaluated by gas chromatography, coupled with mass spectrometry. An ecotoxicological characterization was also performed by using the bio-indicator Vibrio fischeri. The chemical and ecotoxicological results were analyzed according to the French proposal of Criteria on the Evaluation Methods of Waste Ecotoxicity (CEMWE). Residue A was not considered to be ecotoxic by the ecotoxicological criterion (EC(50) (30 min) >or=10%), but it was considered to be ecotoxic by the chemical criterion (Ni>or=0.5mg/L). Residue B was considered to be ecotoxic by the ecotoxicological criterion: EC(50) (30 min)

The present work aims to assess the efficiency of chars, obtained from the gasification and co-pyrolysis of rice wastes, as adsorbents of Cr3+ from aqueous solution. GC and PC chars, produced in the gasification and co-pyrolysis, respectively, of rice husk and polyethylene were studied. Cr3+ removal assays were optimised for the initial pH value, adsorbent mass, contact time and Cr3+ initial concentration. GC showed a better performance than PC with about 100% Cr3+ removal, due to the pH increase that caused Cr precipitation. Under pH conditions in which the adsorption prevailed (pH<5.5), GC presented the highest uptake capacity (21.1mg Cr3+ g-1 char) for the following initial conditions: 50mg Cr3+ L-1; pH 5; contact time: 24h;L/S ratio: 1000mLg-1. The pseudo-second order kinetic model showed the best adjustment to GC experimental data. Both the first and second order kinetic models fitted well to PC experimental data. The ion exchange was the dominant phenomenon on the Cr3+ adsorption by GC sample. Also, this char significantly reduced the ecotoxicity of Cr3+ solutions for the bacterium Vibrio fischeri. GC char proved to be an efficient material to remove Cr3+ from aqueous solution, without the need for further activation.

Rice straw (RS), rice husk (RH) and polyethylene (PE) were blended and submitted to gasification and pyrolysis processes. The chars obtained were submitted to textural, chemical, and ecotoxic characterisations, towards their possible valorisation. Gasification chars were mainly composed of ashes (73.4-89.8wt%), while pyrolysis chars were mainly composed of carbon (53.0-57.6wt%). Silicon (Si) was the major mineral element in all chars followed by alkaline and alkaline-earth metal species (AAEMs). In the pyrolysis chars, titanium (Ti) was also a major element, as the feedstock blends contained high fractions of PE which was the main source of Ti. Gasification chars showed higher surface areas (26.9-62.9m2g-1) and some microporosity, attributed to porous silica. On the contrary, pyrolysis chars did not present a porous matrix, mainly due to their high volatile matter content. The gasification bed char produced with 100% RH, at 850°C, with O2 as gasification agent, was selected for further characterization. This char presented the higher potential to be valorised as adsorbent material (higher surface area, higher content of metal cations with exchangeable capacity, and lowest concentrations of toxic heavy metals). The char was submitted to an aqueous leaching test to assess the mobility of chemical species and the ecotoxic level for V. fischeri. It was observed that metallic elements were significantly retained in the char, which was attributed mainly to its alkaline character. This alkaline condition promoted some ecotoxicity level on the char eluate that was eliminated after the pH correction.

In the present work, the enhancement of biogas and methane yields through anaerobic co-digestion of the pre-hydrolised Organic Fraction of Municipal Solid Wastes (hOFMSW) and Maize Cob Wastes (MCW) in a lab-scale thermophilic anaerobic reactor was tested. In order to increase its biodegradability, MCW were submitted to an initial pre-treatment screening phase as follows: (i) microwave (MW) irradiation catalysed by NaOH, (ii) MW catalysed by glycerol in water and alkaline water solutions, (iii) MW catalysed by H2O2 with pH of 9.8 and (iv) chemical pre-treatment at room temperature catalysed by H2O2 with 4 h reaction time. The pre-treatments cataysed by H2O2 were performed with 2% MCW (wMCW/v alkaline water) at ratios of 0.125, 0.25, 0.5 and 1.0 (wH2O2/wMCW). The pre-treatment that presented the most favourable balance between sugars, lignin, cellulose and hemicellulose solubilisations, as well as low production of phenolic compound and furfural (inhibitors), was the chemical pre-treatment catalysed by H2O2, at room temperature, with a ratio of 0.5 wH2O2/wMCW (Pre1). This Pre1 was then optimised testing reaction times of 1, 2 and 3 days at a different pH (11.5) and MCW percentage (10% w/v). The optimised pre-treatment that presented the best results, considering the same criteria defined above, was the one carried out during 3 days, at pH 9.8 and 10% MCW w/v (Pre2). The anaerobic reactor was initially fed with the hOFMSW obtained from the hydrolysis tank of an industrial AD plant. The hOFMSW was than co-digested with MCW submitted to the pre-treatment Pre1. In another assay, hOFMSW was co-digested with MCW submitted pre-treatment Pre 2. The co-digestion of hOFMSW + Pre1 increased the biogas yield by 38.9% and methane yield by 29.7%, when compared to the results obtained with hOFMSW alone. The co-digestion of hOFMSW + Pre2 increased biogas yield by 46.0% and CH4 yield by 36.3%. In both cases, the methane content obtained in the biogas streams was above 66% v/v. These results show that pre-treatment with H2O2, at room temperature, is a promising low cost way to valorize MCW through co-digestion with hOFMSW.

Abstract Digestate from a biogas plant that uses solely biomass for biogas production was used as precursor material for the production of activated carbon as an alternative to increase its added value. The digestate was converted into hydrochar by hydrothermal carbonization varying the temperature (190–250 °C), residence time (3 and 6 h), and pH (5 and 7). Temperature followed by residence time had the strongest influence on the chemical composition and thermal stability of the hydrochars. A significant effect of the pH was not observed. The hydrochars were chemically activated to enhance the surface area and use them as activated carbon. As a consequence, the surface areas increased from 8 to 14 m 2 /g (hydrochars) to 930–1351 m 2 /g (activated carbons). Furthermore, large micropore volumes were measured (0.35–0.50 cm 3 /g). The activated carbons were studied as adsorbents in gas phase applications, showing that the product of digestate is a very effective adsorbent for carbon dioxide (CO 2 ). Especially the activated carbon obtained from the hydrochar produced at 250 °C for 6 h, which adsorbed 8.80 mol CO 2 /kg at 30 °C and 14.8 bar. Additionally, the activated carbons showed a stronger affinity towards CO 2 compared to methane (CH 4 ), which makes this material suitable for the upgrading of raw biogas to biomethane.

Char residues produced in the co-pyrolysis of different wastes (plastics, pine biomass and used tyres) were characterized using chemical and toxicity assays. One part of the solid chars was submitted to extraction with dichloromethane (DCM) in order to reduce the toxicity of the char residues by removing organic contaminants. The different volatility fractions present in the extracted char (Char A) and in the raw char (Char B) were determined by progressive weight loss combustion. A selected group of heavy metals (Cd, Pb, Zn, Cu, Hg and As) was determined in both chars. The chars were subjected to the leaching test ISO/TS 21268 - 2, 2007 and the resulting eluates were further characterized by determining a group of inorganic parameters (pH, conductivity, Cd, Pb, Zn, Cu, Hg and As contents) and the concentrations of several organic contaminants (volatile aromatic hydrocarbons and alkyl phenols). An ecotoxicological characterization was also performed by using the bio-indicator Vibrio fischeri. The chemical and ecotoxicological results were analyzed according to the Council Decision 2003/33/CE and the criteria on the evaluation methods of waste ecotoxicity (CEMWE). The results obtained in this work indicated that the extraction with DCM is an effective method for the removal of organic contaminants of high to medium volatility from pyrolysis solid residues, thus decreasing their toxicity potential. Zn can be leached from the chars even after the DCM extraction treatment and can contribute to the ecotoxicity of the eluates obtained from chars. Both chars (treated and non treated with DCM) were classified as hazardous and ecotoxic wastes.