The retrograde adsorption phenomenon at the onset of breakthrough and its quantitation: An experimental case study for gaseous toluene on activated carbon surface

The relative dominance of surface oxygen content over pore properties in controlling adsorption and retrograde behavior of gaseous toluene over microporous carbon

The adsorption potential of activated carbon (AC) derived from macadamia nut shells (product code of Procarb-900: namely, AC-P) has been investigated using gaseous toluene as the target pollutant. The powder AC-P with high-microporosity (96%) and oxygen content (5.62%) exhibited very high adsorption capacity (214 mg·g-1) and partition coefficient (PC: 25 mol·kg-1·Pa-1) against 100 ppm (10 Pa) toluene at 99% breakthrough levels (1 atm dry N2). The factors governing toluene adsorption were explored with respect to the key variables such as surface functional groups, pore size distribution, sorbent bed mass (50, 100, and 150 mg), and particle size (i.e., 0.212-0.6 mm (powder AC: PAC)) vs. 0.6-2.36 mm (granular AC: GAC)). Accordingly, the adsorption process was physical, mainly due to the non-polar interactions (i.e., π-π interactions) between the adsorbent and adsorbate molecules. The high affinity of AC-P at low breakthrough levels was obtained through a combination of smaller particle size (PAC) and larger adsorbent mass (i.e., 150 mg) with the appearance of a very pronounced retrograde phenomenon (e.g., at < 1% breakthrough level). As such, toluene adsorption appeared to be affected more sensitively by particle size and adsorbent mass (especially at low breakthrough levels) than by high microporosity. Most importantly, the oxygen content of AC emerges as one of the key factors governing the maximum capacity, as the changes in pore volume are not crucial to explain the observed adsorption patterns of toluene.

Proof of concept for CUK family metal-organic frameworks as environmentally-friendly adsorbents for benzene vapor

The utility of metal-organic frameworks (MOFs) such as the CUK family (CUK - Cambridge University-KRICT) has been explored intensively for adsorption/separation of airborne volatile organic compounds (VOCs). In this article, three M-CUK analogs (M = Mg, Co, or Ni) were synthesized hydrothermally under similar conditions to assess the effects of their isostructural properties and metal centers on adsorption of benzene vapor (0.05-1 Pa). A list of performance metrics (e.g., breakthrough volume (BTV) and partition coefficient (PC)) were used to assess the role of the metal type (in M-CUK-1s) in the adsorption of VOCs. Specifically, Co-CUK-1 (average pore size of 8.98 nm) showed 2-3 times greater performance (e.g., in terms of 10% BTV (2012 L atm g^-1) and PC (6 mol kg^-1 Pa^-1)) over other analogs when exposed up to 0.05 Pa benzene vapor. The superiority of mesoporous Co-CUK-1 (e.g., enhanced adsorption diffusion mechanism through favorable metal-π and π- π interactions) can be attributed to the presence of cobalt metal centers (e.g., in reference to Mg- or Ni-CUK-1).

Recent Developments in Understanding Biochar’s Physical–Chemistry

Biochar is a porous material obtained by biomass thermal degradation in oxygen-starved conditions. It is nowadays applied in many fields. For instance, it is used to synthesize new materials for environmental remediation, catalysis, animal feeding, adsorbent for smells, etc. In the last decades, biochar has been applied also to soils due to its beneficial effects on soil structure, pH, soil organic carbon content, and stability, and, therefore, soil fertility. In addition, this carbonaceous material shows high chemical stability. Once applied to soil it maintains its nature for centuries. Consequently, it can be considered a sink to store atmospheric carbon dioxide in soils, thereby mitigating the effects of global climatic changes. The literature contains plenty of papers dealing with biochar’s environmental effects. However, a discrepancy exists between studies dealing with biochar applications and those dealing with the physical-chemistry behind biochar behavior. On the one hand, the impression is that most of the papers where biochar is tested in soils are based on trial-and-error procedures. Sometimes these give positive results, sometimes not. Consequently, it appears that the scientific world is divided into two factions: either supporters or detractors. On the other hand, studies dealing with biochar’s physical-chemistry do not appear helpful in settling the factions’ problem. This review paper aims at collecting all the information on physical-chemistry of biochar and to use it to explain biochar’s role in different fields of application.

Activated Carbon from Spent Coffee Grounds: A Good Competitor of Commercial Carbons for Water Decontamination

In the framework of the circular economy, spent coffee grounds were converted into powdered activated carbon by means of pyrolysis, using potassium hydroxide as the activating agent. Its adsorption capacity on a panel of phenolic compounds was compared with those of two commercial powdered activated carbons, after preliminary studies on organic dyes with different ionic properties, to assess the affinity between adsorbates and adsorbents. Pseudo-first-order and pseudo-second-order kinetic models were carried out, together with Freundlich and Langmuir isotherms. They were useful to calculate the breakthrough at 5%, 10%, and 50% of adsorption and the partition coefficients for the comparison of performance between different sorbent systems in a less biased manner (e.g., reducing bias associated with operational settings like sorbate concentration and sorbents dosage). The results showed that the removal efficiency for SCGs-AC was comparable with that of the commercial activated carbons with the highest partition coefficients for methylene blue (12,455 mg/g/μM, adsorption capacity = 179 mg/g) and 3-chlorophenol (81.53 mg/g/μM, adsorption capacity = 3765 mg/g). The lower efficiency in bromothymol blue and bisphenol-A adsorption was due to its different morphology and surface properties.

Metal-Organic Frameworks for the Adsorptive Removal of Gaseous Aliphatic Ketones

Recent research endeavors have established metal-organic frameworks (MOFs) as suitable platforms for the adsorptive removal of various environmental pollutants. In this regard, the sorptive performances of four MOFs (MOF-199, UiO-66, UiO-66-NH₂, and Co-CUK-1) were investigated against two gaseous aliphatic ketones (methyl ethyl ketone (MEK) and methyl isobutyl ketone (MiBK)) at a low partial pressure (0.1 Pa). Activated carbon was utilized as a reference commercial sorbent. The 10% breakthrough volume (BTV10) values for MEK decreased in the following order: MOF-199 (4772 L atm g^-1) > activated carbon (224 L atm g^-1) > UiO-66-NH₂ (106 L atm g^-1) > UiO-66 (53 L atm g^-1) > Co-CUK-1 (16 L atm g^-1). In case of MiBK, the relative ordering in BTV10 was consistently maintained while showing noticeable increases in its magnitude: MOF-199 (7659 L atm g^-1) > activated carbon (816 L atm g^-1) > UiO-66-NH₂ (304 L atm g^-1) > UiO-66 (150 L atm g^-1) > Co-CUK-1 (31 L atm g^-1). The superiority of MOF-199 was confirmed toward the adsorptive removal of gaseous aliphatic ketones. For a binary mixture of ketones, the BTV10 values of MOF-199 were reduced considerably for MEK and MiBK (in comparison to single component sorption) such as 1579 and 3969 L atm g^-1, respectively, reflecting competitive inhibition of the adsorption process. Theoretical simulations based on density functional theory (DFT) elucidated the involvement of highly favorable coordination between the carbonyl group present in ketone molecules and the uncoordinated Cu(II) sites in the MOF-199 structure (Lewis acidic centers). Interestingly, MOF-199 maintained appreciable performance toward the mixture of ketones up to 5 cycles to support its practical merit.