Biochar-Assisted Catalytic Pyrolysis of Oily Sludge to Attain Harmless Disposal and Residue Utilization for Soil Reclamation

Comparison of environmental impacts from pyrolysis, gasification, and combustion of oily sludge

Thermochemical treatment of oily sludge (OS) has been demonstrated to be an effective approach for resource and energy recovery. However, the migration and emission of potential pollutants have limited its further development. In this study, the environmental impacts, including aromatic compounds in liquid products, N-, S-, and Cl-containing pollutants in gaseous products, and residual organic matter and heavy metals in solid residues, during the pyrolysis, gasification, and combustion processes of OS are comparatively investigated. The results indicate that the aromatics in the liquid products obtained from pyrolysis and gasification are primarily hydrocarbons with 10, 14, and 16 carbon atoms, and the corresponding degree of unsaturation is between 7 and 16. By contrast, the aromatics produced during combustion are mainly hydrocarbons with 10-12 carbon atoms and an unsaturation degree of 7. The liquid products from gasification of OS contain aromatics with more carbon atoms and a higher degree of unsaturation, suggesting potential issues of recalcitrant aromatics and tar by-products during the gasification process. The release behaviors of N-, S-, and Cl-containing pollutants during the thermochemical treatment of OS are closely related to the specific thermochemical technology and treatment temperature. At 550 °C, these pollutants are gradually released from the OS. By contrast, at 950 °C, they are released over a narrow temperature range with significantly higher concentrations. Furthermore, compared with the peak concentrations of SO2 and HCl during thermochemical processing at 550 °C, these values increase by 1-2 orders of magnitude at 950 °C. With the increase in treatment temperature, the loss on ignition (LOI) of residues from pyrolysis or gasification of OS gradually decreases and stabilizes around 0.5 %. On the other hand, the LOI from combustion fluctuates around 1.0 %. In addition, the removal rates of total organic carbon in the residues from all three thermochemical processes exceed 98.89 %. However, the potential ecological risks associated with heavy metals in the residues from thermochemical treatment of OS also increase to some extent. Cr, Cu, and Zn are found to evaporate and escape into liquid and gaseous products, while Pb is retained in the residues. Notably, the residue from combustion poses the highest environmental risks among the three processes.

Green transformation of oily sludge through geopolymer: Material properties and hydration mechanisms

Oily sludge (OS) is a kind of hazardous waste generated from the petrochemical industry. Currently, pyrolysis has been widely applied for OS disposal, while low-oil content (<5 wt%) OS still lacks novel technology to achieve efficient resource utilization and harmful substances immobilization. In this study, a kind of OS-based geopolymer was developed by OS and ground granulated blast furnace slag (GGBS). The results showed that in geopolymer with 30 wt% OS, the content of total petroleum hydrocarbons (TPHs) decreased by 82%, Zn achieved 100% stabilization, and the 28 d compressive strength could still reach 32.8 MPa. The appropriate oil content filled the pores and cracks in geopolymer matrix. The constructed model compounds further elucidated the hydration mechanisms of OS-geopolymer. The nucleation effect of crude oil and micro-aggregate effect of minerals jointly improved the polymerization degree of C-(A)-S-H gels. OS promoted the transformation of [SiO4]4- monomers into C-(A)-S-H unbranched middle groups and three-dimensional networks, thereby efficiently stabilizing harmful substances. Sustainability analysis showed that OS-based geopolymer had good environmental and economic benefits. Overall, this work provides theoretical guidance for the green transformation of OS in the construction field.

Co-pyrolysis of refinery oil sludge with biomass and spent fluid catalytic cracking catalyst for resource recovery

Catalytic co-pyrolysis of two different refinery oily sludge (ROS) samples was conducted to facilitate resource recovery. Non-catalytic pyrolysis in temperatures ranging from 500 to 600°C was performed to determine high oil yields. Higher temperatures enhanced the oil yields up to ~ 24 wt%, while char formation remained unchanged (~ 45%) for S1. Conversely, S2 exhibited a notably lower oil yield (~ 4 wt%) than S1. Pyrolysis oil of S1 consisted of phenolics (~ 50% at 600 °C) whereas hydrocarbons were predominant in S2 oil (~ 80% at 600 °C). Catalytic pyrolysis of S1 did not exhibit a substantial impact on oil yields but the oil composition varied significantly. High hydrocarbons, phenolics, and aromatics were obtained with molecular sieve (MS), metal slag, and ZSM-5, respectively. Catalytic co-pyrolysis of S2 with sawdust (SD) in the presence of MS enhanced the oil yield, and the resulting oil consisted of high hydrocarbons (~ 54%) and aromatics (~ 44%).

Mechanistic Implications of the Varying Susceptibility of PAHs to Pyro-Catalytic Treatment as a Function of Their Ionization Potential and Hydrophobicity

Transition metal catalysts in soil constituents (e.g., clays) can significantly decrease the pyrolytic treatment temperature and energy requirements for efficient removal of polycyclic aromatic hydrocarbons (PAHs) and, thus, lead to more sustainable remediation of contaminated soils. However, the catalytic mechanism and its rate-limiting steps are not fully understood. Here, we show that PAHs with lower ionization potential (IP) are more easily removed by pyro-catalytic treatment when deposited onto Fe-enriched bentonite (1.8% wt. ion-exchanged content). We used four PAHs with decreasing IP: naphthalene > pyrene > benz(a)anthracene > benzo(g,h,i)perylene. Density functional theory (DFT) calculations showed that lower IP results in stronger PAH adsorption to Fe(III) sites and easier transfer of π-bond electrons from the aromatic ring to Fe(III) at the onset of pyrolysis. We postulate that the formation of aromatic radicals via this direct electron transfer (DET) mechanism is the initiation step of a cascade of aromatic polymerization reactions that eventually convert PAHs to a non-toxic and fertility-preserving char, as we demonstrated earlier. However, IP is inversely correlated with PAH hydrophobicity (log Kow), which may limit access to the Fe(III) catalytic sites (and thus DET) if it increases PAH sorption to soil OM. Thus, ensuring adequate contact between sorbed PAHs and the catalytic reaction centers represents an engineering challenge to achieve faster remediation with a lower carbon footprint via pyro-catalytic treatment.

Machine learning-based exploration of biochar for environmental management and remediation

Biochar has a wide range of applications, including environmental management, such as preventing soil and water pollution, removing heavy metals from water sources, and reducing air pollution. However, there are several challenges associated with the usage of biochar for these purposes, resulting in an abundance of experimental data in the literature. Accordingly, the purpose of this study is to examine the use of machine learning in biochar processes with an eye toward the potential of biochar in environmental remediation. First, recent developments in biochar utilization for the environment are summarized. Then, a bibliometric analysis is carried out to illustrate the major trends (demonstrating that the top three keywords are heavy metal, wastewater, and adsorption) and construct a comprehensive perspective for future studies. This is followed by a detailed review of machine learning applications, which reveals that adsorption efficiency and capacity are the primary utilization targets in biochar utilization. Finally, a comprehensive perspective is provided for the future. It is then concluded that machine learning can help to detect hidden patterns and make accurate predictions for determining the combination of variables that results in the desired properties which can be later used for decision-making, resource allocation, and environmental management.

Comparative study by microwave pyrolysis and conventional pyrolysis of pharmaceutical sludge: Resourceful disposal and antibiotic adsorption

Compared with conventional pyrolysis, microwave pyrolysis has superior heat transfer performance and promotes the decomposition of organic matter. The paper focuses on the harmless treatment and resource utilization of pharmaceutical sludge (PS) by microwave heating and conventional heating methods. The experimental results showed that the conventional pyrolysis gas is dominated by CO2, CO and H2. For microwave pyrolysis gas, the "microwave effect" promoted secondary cracking of volatile fractions and increases the content of CH4, CxHy, H2 and CO through condensation, aromatization, and dehydrogenation. Conventional pyrolysis oils contained the highest percentage of oxygenated compounds. However, high-temperature microwave radiation accelerated the cleavage of polar oxygenated molecular bonds and long-chain hydrocarbons, thereby increasing the aromatics content of pyrolysis oils. The solid residues obtained from microwave pyrolysis is highly graphitized and porous, with a surface area of 146.2 m2/g. Furthermore, the solid residue was rich in pyridine-N and pyrrole-N that could be utilized for adsorption and catalysis. The MA-600 removes up to 99% of tetracycline (TC) in 6 h. It was also found that the adsorption process of TC by the two pyrolysis residues was consistent with the proposed secondary and Freundlich models.

Disinfectant sodium dichloroisocyanurate synergistically strengthened sludge acidogenic process and pathogens inactivation: Targeted upregulation of functional microorganisms and metabolic traits via self-adaptation

Harmless and resourceful treatment of waste activated sludge (WAS) have been the crucial goal for building environmental-friendly and sustainable society, while the synergistic realization approach is currently limited. This work skillfully utilized the disinfectant sodium dichloroisocyanurate (NaDCC) to simultaneously achieve the pathogenic potential inactivation (decreased by 60.1 %) and efficient volatile fatty acids (VFAs) recovery (increased by 221.9 %) during WAS anaerobic fermentation in rather cost-effective way (Chemicals costs:0.4 USD/kg VFAs versus products benefits: 2.68 USD/kg chemical). Mechanistic analysis revealed that the C=O and NCl bonds in NaDCC could spontaneously absorb sludge (binding energy -4.9 kJ/mol), and then caused the sludge disintegration and organic substrates release for microbial utilization due to the oxidizability of NaDCC. The disruption of sludge structure along with the increase of bioavailable fermentation substrates contributed to the selectively regulation of microbial community via enriching VFAs-forming microorganisms (e.g., Pseudomonas and Streptomyces) and reducing VFAs-consuming microorganisms, especially aceticlastic methanogens (e.g., Methanothrix and Methanospirillum). Correspondingly, the metabolic functions of membrane transport, substrate metabolism, pyruvate metabolism, and fatty acid biosynthesis locating in the central pathway of VFAs production were all upregulated while the methanogenic step was inhibited (especially acetate-type methanogenic pathway). Further exploration unveiled that for those enriched functional anaerobes were capable to activate the self-adaptive systems of DNA replication, SOS response, oxidative stress defense, efflux pump, and energy metabolism to counteract the unfavorable NaDCC stress and maintain high microbial activities for efficient VFAs yields. This study would provide a novel strategy for synergistic realization of harmless and resourceful treatment of WAS, and identify the interrelations between microbial metabolic regulations and adaptive responses.

Pyro-Catalytic Degradation of Pyrene by Bentonite-Supported Transition Metals: Mechanistic Insights and Trade-Offs with Low Pyrolysis Temperature

Transition metal catalysts can significantly enhance the pyrolytic remediation of soils contaminated with polycyclic aromatic hydrocarbons (PAHs). Significantly higher pyrene removal efficiency was observed after the pyrolytic treatment of Fe-enriched bentonite (1.8% wt ion-exchanged content) relative to natural bentonite or soil (i.e., 93% vs 48% and 4%) at the unprecedentedly low temperature of 150 °C with only 15 min treatment time. DFT calculations showed that bentonite surfaces with Fe3+ or Cu2+ adsorb pyrene stronger than surfaces with Zn2+ or Na+. Enhanced pyrene adsorption results from increased charge transfer from its aromatic π-bonds to the cation site, which destabilizes pyrene allowing for faster degradation at lower temperatures. UV-Vis and GC-MS analyses revealed pyrene decomposition products in extracts of samples treated at 150 °C, including small aromatic compounds. As the pyrolysis temperature increased above 200 °C, product distribution shifted from extractable compounds to char coating the residue particles. No extractable byproducts were detected after treatment at 400 °C, indicating that char was the final product of pyrene decomposition. Tests with human lung cells showed that extracts of samples pyrolyzed at 150 °C were toxic; thus, high removal efficiency by pyrolytic treatment does not guarantee detoxification. No cytotoxicity was observed for extracts from Fe-bentonite samples treated at 300 °C, inferring that char is an appropriate treatment end point. Overall, we demonstrate that transition metals in clay can catalyze pyrolytic reactions at relatively low temperatures to decrease the energy and contact times required to meet cleanup standards. However, mitigating residual toxicity may require higher pyrolysis temperatures.