Metal recovery and heavy metal migration characteristics of ferritic stainless steel pickling sludge reduced by municipal sludge

A new method for recovering metals from ferritic stainless steel pickling sludge using municipal sludge as a reducing agent is proposed. The effects of temperature, municipal sludge blending ratio, and reaction atmosphere on the recovery characteristics were studied. The migration characteristics of heavy metals (HMs) at different temperatures were also investigated. The results showed that municipal sludge can be used as a reducing agent for metal recovery from pickling sludge. Under optimized conditions (temperature of 650 °C and blending ratio of 5%), the recovery fractions of Fe, Cr, and Ni were 70.1, 53.7, and 60.3%, respectively. The CO₂ atmosphere was beneficial for increasing the recovery fraction of the pickling sludge. The migration fractions of Cr, Mn, and Ni were approximately 10%. The migration fractions of Zn, Cu, and Pb, which are semi-volatile HMs, varied significantly with temperature. The migration fractions of Cd and As, which are volatile HMs, could exceed 70%.

Biomass hydrothermal conversion under CO2 atmosphere: A way to improve the regulation of hydrothermal products

In this study, batched hydrothermal experiments on corn stalk were conducted at 240-330 °C under CO₂ or inert (N₂) atmosphere. The distribution and characteristics of gaseous, solid, and liquid products were analyzed in detail to comprehensively investigate the effects of CO₂ on the hydrothermal conversion of biomass, especially on the cellulose and lignin in biomass. The results demonstrate that compared with N₂, CO₂ slightly increased the liquid and gas yields and significantly improved the control effect of temperature on bio-oil components. Under CO₂ atmosphere, bio-oil achieved effective enrichment of ketones and phenols at 240 °C and 300 °C, respectively, and their highest relative contents reached 44.8% and 62.0%, respectively. In addition, the hydrochar obtained under CO₂ atmosphere showed higher crystallinity, which is conducive to its subsequent utilization. This study explored the feasibility of introducing CO₂ into the biomass hydrothermal process to realize the high-value utilization of biomass waste and the reuse of CO₂.

Effects of oxygen vacancy defect on microwave pyrolysis of biomass to produce high-quality syngas and bio-oil: Microwave absorption and in-situ catalytic

This paper proposed to use ferric oxide (Fe₂O₃) and ferroferric oxide (Fe₃O₄) as catalysts with both microwave absorption and catalytic properties. Carbon dioxide (CO₂) was introduced as the reaction atmosphere to further improve the quality of biofuel produced by microwave pyrolysis of food waste (FW). The results showed the bio-gas yield and the syngas concentration (H₂ + CO) increased to 70.34 wt% and 61.50 mol%, respectively, using Fe₃O₄ as the catalyst. The content of aliphatic hydrocarbons in bio-oil produced with the catalyst Fe₂O₃ increased to 67.48% and the heating value reached 30.45 MJ/kg. Compared with Fe₂O₃ catalyst, Fe₃O₄ exhibited better microwave absorption properties and catalytic properties. Transmission electron microscopy (TEM) and Electron paramagnetic resonance (EPR) characterizations confirmed that the crystal surface of Fe₃O₄ formed more oxygen vacancy defects and unpaired electrons. Additionally, according to the X-ray photoelectron spectroscopy (XPS) analysis, the content of lattice oxygen in Fe₃O₄ was 14.11%, a value that was much lower than Fe₂O₃ (38.54%). The oxygen vacancy defects not only improved the efficient utilization of microwave energy but also provided the reactive sites for the reaction between the volatile organic compounds (VOCs) and CO₂ to generate CO. This paper provides a new perspective for selecting catalysts that have both microwave absorption and catalytic properties during the microwave pyrolysis of biomass.

Enhancing biochar production: A technical analysis of the combined influence of chemical activation (KOH and NaOH) and pyrolysis atmospheres (N2/CO2) on yields and properties of rice husk-derived biochar

The production of biochar from biomass has received considerable interest due to its potential in environmental applications; however, optimizing biochar properties remains a major challenge. The objective of the present study was to investigate the synergistic effects of pyrolysis atmospheres (N2 and CO2) and chemical activation (pre- and post-pyrolysis) with NaOH and KOH on the properties of biochar useful for its environmental applications. In this study rice husk and biochar were impregnated with KOH and NaOH before and after pyrolysis, which was carried out at 600 °C under N₂ and CO₂ atmosphere. The pyrolytic yields (biochar, liquid and gas) and detailed characterization of biochar were performed. The results showed that pre-activation with both alkalis under a CO2 atmosphere slightly decreased the biochar yield and carbon contents while increasing oxygen in biochars compared to N2 atmosphere. Alkali pre-activation in the CO2 atmosphere considerably increased the specific surface area and pore volume of biochars compared to the N2 atmosphere, with KOH being more effective than NaOH. The maximum specific surface area (SSA) and pore volume (PV) of biochar obtained were 178.4 m2/g and 0.60 cm3/g for KOH activated biochar under CO2, which were 3.2 times and 30 times higher than the untreated biochar. The post-activation of biochars with both alkalis resulted in moderate improvements in textural properties. Overall, chemical activation under CO2 pyrolysis facilitated a higher level of chemical activation reactions leading to increased formation of oxygen functional groups and contributed to enhanced SSA and PV of the biochar useful for adsorption.

Controlling Diels-Alder reactions in catalytic pyrolysis of sawdust and polypropylene by coupling CO2 atmosphere and Fe-modified zeolite for enhanced light aromatics production

Producing value-added light aromatics (BTEX) from solid waste streams holds excellent promise for resource recovery. Here we present a thermochemical conversion approach that enhanced BTEX production by coupling CO₂ atmosphere and Fe-modified HZSM-5 zeolite to facilitate the Diels-Alder reactions in catalytic pyrolysis of sawdust and polypropylene. The Diels-Alder reactions between sawdust-derived furans and polypropylene-derived olefins could be controlled by tuning CO₂ concentration and Fe loading amount. Sufficient CO₂ (≥50%) with moderate Fe loading (10 wt%) were observed to produce more BTEX and fewer heavy fractions (C₉⁺aromatics). To deepen the mechanistic understanding, quantification of polycyclic aromatic hydrocarbons (PAHs) and catalyst coke was further conducted. The co-use of CO₂ atmosphere and Fe modification suppressed the appearance of low-, medium-, and high-membered ring PAHs by over 40%, decreased pyrolysis oil toxicity from 42.1 to 12.8 μg/g_oil TEQ, and transformed coke from "hard" to "soft". Based on the characterization of CO₂ adsorption behavior, it was deduced that the introduced CO₂ was activated by loaded Fe and reacted in situ with H₂ generated during aromatization to expedite H-transfer. Meanwhile, BTEX recondensation was prevented through the Boudouard reactions of CO₂ and water-gas reactions between the resulting water and carbon deposits. These synergistically enhanced the production of BTEX and suppressed the formation of heavy species, including PAHs and catalyst coke.