Adsorption characteristic analysis of PAHs on activated carbon with different functional groups by molecular simulation

Phenanthrene sorption studies on coffee waste- and diatomaceous earth-based adsorbents, and adsorbent regeneration with cold atmospheric plasma

Phenanthrene (PHE) is a polycyclic aromatic hydrocarbon categorized as a high priority organic pollutant being toxic for the ecosystem and human health, and its sorption on natural organic or inorganic substances seems a well-promising method for its removal from water streams. The goals of the present work are (i) to assess the capacity of low-cost adsorbents fabricated by treating coffee wastes and diatomaceous earth to remove PHE from water; (ii) to elucidate the role of the pore structure on PHE sorption dynamics; and (iii) to assess the potential to regenerate adsorbents loaded with PHE, by using the novel technology of cold atmospheric plasma (CAP). Diatomaceous earth (DE) and DE pre-treated with sodium hydroxide (NaOH) or phosphoric acid (H3PO4) were chosen as inorganic adsorbents. Coffee waste (CW) and activated carbons (AC) produced from its pyrolysis at 800 °C (CWAC), either untreated (CWAC-800) or pre-treated with NaOH (CWAC-NaOH-800) and H3PO4 (CWAC-H3PO4-800), were chosen as organic adsorbents. The adsorbents were characterized with nitrogen adsorption-desorption isotherms, attenuated total reflectance-Fourier transform infrared spectroscopy, Raman spectroscopy, scanning electron microscopy, and mercury intrusion porosimetry. Based on the PHE sorption capacity and pore structure/surface characteristics, the CWAC-NaOH-800 was chosen as the most efficient adsorbent for further equilibrium and kinetic sorption studies. The multi-compartment model was used to describe the PHE sorption dynamics in CWAC-NaOH-800 by accounting for the pore/surface diffusion and instantaneous sorption. The CWAC-NaOH-800 exhibited remarkable values for (i) the specific surface area (SBET = 676.5 m2/g) and meso- and micro-pore volume determined by nitrogen sorption (VLN2 = 0.415 cm3/g); (ii) the macro- and meso-pore volume determined by mercury intrusion porosimetry (VMIP = 3.134 cm3/g); and (iii) the maximum PHE sorption capacity (qmax = 142 mg/g). The percentage of adsorbent recovery after its regeneration with CAP was found to be ~ 35%. From the simulation of sorption dynamics, it was found that at early times, the sorption kinetics is governed by the film diffusion towards the external surface of grains, but at late times, most of the adsorbed mass is transferred primarily to meso-/macro-pores via diffusion, and secondarily to micro-porosity via surface diffusion. Based on the adsorbent characteristics, effect of pH on sorption efficiency, and numerical analysis of sorption dynamics, it was concluded that probably the dominant adsorption mechanism is the π-π interactions between hydrophobic PHE aromatic rings and CWAC-NaOH-800 graphene layers. The high PHE removal efficiency of CWAC-NaOH-800, the successful interpretation of sorption dynamics with the multi-compartment model, and the potential to regenerate PHE-loaded adsorbents with the green and economic technology of CAP motivate a strategy for testing CWACs towards the adsorption of other PAHs, application of adsorbents to real wastewaters, and scaling-up to pilot units.

Spontaneous Combustion Characteristics of Activated Carbon Modified via Liquid Phase Impregnation during Drying

Spontaneous combustion characteristics are important issues for the safe operation of the wet-modified activated carbon drying process. The spontaneous combustion characteristics of activated carbon modified via liquid phase impregnation were fully investigated in this study. The modified activated carbon was prepared using columnar activated carbon and 4-amino-1,2-butanediol solution. Physical properties and surface functional group analyses were performed for activated carbon before and after modification. The ignition temperature of activated carbon before and after modification was then characterized using the methods of GB/T20450-2006, thermogravimetry-derivative thermogravimetry (TG-DTG), and TG-mass spectrometry (TG-MS). At the same time, the activation energy of activated carbon before and after modification was calculated by using thermodynamic analysis. Furthermore, a new self-designed test platform was introduced to investigate the spontaneous combustion characteristics of wet-modified activated carbon under the drying temperatures of 150, 175, 180, and 210 °C. The results show that the specific surface area of Brunauer, Emmett, and Teller (BET) is decreased by 368 m2·g-1, the total volume of pore size is decreased by 0.17 cm3·g-1, and the content of oxygen-containing functional groups is decreased by 0.071 mmol/g compared with row activated carbon. The ignition temperatures of the sample before modification characterized by the three methods are 483, 596, and 599 °C, respectively. The ignition temperatures of the sample after modification are 489, 607, and 611 °C, respectively. The activation energy of the modified activated carbon is increased by 35 kJ/mol compared to the original activated carbon. It is concluded that the temperature that triggers the modified activated carbon combustion during the drying process is between 175 and 180 °C, and the heat is mainly gathered at the longitudinal center of the combustion chamber through the investigation of spontaneous combustion experiments. The results in this study can contribute to safe production to prevent combustion in the process of modifying activated carbon during the drying process.