Modeling mass transfer for adsorptive removal of Pb(II) onto phosphate modified ordered mesoporous carbon (OMC)

Ethanol-mediated synthesis of γ-cyclodextrin-based metal-organic framework as edible microcarrier: performance and mechanism

Here, an ethanol-mediated method was introduced to fabricate γ-cyclodextrin-based metal-organic frameworks (γ-CD-MOFs) as microcarriers for epigallocatechin-3-gallate (EGCG). Through adjusting ethanol gas diffusion temperature and ethanol liquid feed speed, we achieved control of crystallization efficiency and crystals size without extra surfactants. Under the sequential regulatory by ethanol in two phases, the obtained γ-CD-MOFs with cubic shape exhibited excellent crystallinity, high surface area, and uniform size distribution. Through the interplay of hydrogen bonding, hydrophobic interactions and π stacking, EGCG molecules could be stored efficiently within cavities and tunnels of the γ-CD-MOFs with high load capability of 334 mg g^-1. More importantly, the incorporation of EGCG within frameworks wouldn't disintegrate the unique body-centered cubic structure of γ-CD-MOFs, in turn, would improve the thermostability and antioxidative activity of EGCG. Significantly, all food-grade materials ensured the γ-CD-MOFs high acceptance and applicability for food and biomedical applications.

Investigating immobilization efficiency of Pb in solution and loess soil using bio-inspired carbonate precipitation

Lead (Pb) metal accumulation in surrounding environments can cause serious threats to human health, causing liver and kidney function damage. This work explored the potential of applying the MICP technology to remediate Pb-rich water bodies and Pb-contaminated loess soil sites. In the test tube experiments, the Pb immobilization efficiency of above 85% is attained through PbCO₃ and Pb(CO₃)₂(OH)₂ precipitation. Notwithstanding that, in the loess soil column tests, the Pb immobilization efficiency decreases with the increase in depth and could be as low as approximately 40% in the deep ground. PbCO₃ and Pb(CO₃)₂(OH)₂ precipitation has not been detected as the majority of Pb²⁺ combines with -OH (hydroxyl group) when subjected to 500 mg/kg Pb²⁺. The alkaline front promotes the chemisorption of Pb²⁺ with CO₃^2- reducing the depletion of quartz mineral close to the surface. However, OH^- is in shortage in the deep ground retarding the Pb immobilization. The Pb immobilization efficiency thus decreases with the increase in depth. Quartz and albite minerals, when subjected to 16,000 mg/kg Pb²⁺, appear not to intervene in the chemisorption with Pb²⁺ where the chemisorption of Pb²⁺ with CO₃^2- plays a major role in the Pb immobilization. Compared to the nanoscale urease applied to the enzyme-induced carbonate precipitation (EICP) technology, the micrometer scale ureolytic bacteria penetrate into the deep ground with difficulty. The 'size' issue remains to be addressed in near future.

Adsorptive removal of gas phase naphthalene on ordered mesoporous carbon

Adsorptive removal of gas phase low concentration macromolecular organic component, represented by naphthalene, from the enclosed space using ordered mesoporous carbon (OMC) has been studied by molecular simulation and experimental investigation. The simulation results indicated that both adsorption capacity and adsorption stability of the OMCs for naphthalene decreased with the increase of pore sizes from 2 nm to 8 nm. Characterizations showed that the prepared OMCs had the pore structure similar to the simulated OMCs except for the rough surface. In particular, the adsorption performance of the prepared OMCs was significantly lower than that of the simulated OMCs when pore size was 2 nm and 3 nm, which was attributed to the rough inner surface of these adsorbents, blocking the narrow pore channels and significantly reducing the pore volume. OMC with pore size of 4 nm had the highest adsorption amount for naphthalene. The co-adsorption experiments in the presence of both naphthalene and toluene, acetone or water showed the adsorption performance of OMCs for naphthalene were almost unaffected by the presence of low concentration toluene and acetone, as well as high relative humidity.

Enhanced adsorption of perfluorooctanoate (PFOA) onto low oxygen content ordered mesoporous carbon (OMC): Adsorption behaviors and mechanisms

The pollution of perfluorooctanoic acid (PFOA) in water bodies has been a serious threat to environment and human health. Ordered mesoporous carbons (OMCs) with different oxygen contents were prepared and first used for adsorbing PFOA from aqueous solutions. The OMC-900 with a lower oxygen content has a higher PFOA adsorption capacity than the oxygen-rich OMC-700. OMCs require a much shorter time to reach the adsorption equilibrium comparing with other adsorbents reported in literature. The mesopores play an important role in this rapid adsorption kinetics. The pseudo-second-order model better fitted the kinetic data. The multilayers adsorption was proposed for the adsorption of PFOA onto OMCs since the Freundlich isotherm model fits the experimental data well. The micelle or hemi-micelle structures may be formed during the adsorption. Various background salts showed a positive effect on PFOA adsorption due to the salting-out and divalent bridge effects. The humic acid can lead to a discernible reduction in PFOA adsorption by competing for adsorption sites on OMCs. The hydrophobic interaction and electrostatic interaction adsorption mechanisms were proposed and verified by the adsorption data. The high adsorption capacity and fast adsorption kinetics of the OMC make it a potential adsorbent for PFOA removal in engineering applications.

Enhanced adsorption of resorcinol onto phosphate functionalized graphene oxide synthesized via Arbuzov Reaction: A proposed mechanism of hydrogen bonding and π-π interactions

Phosphate functionalized graphene oxide (PGO) was successfully prepared by Arbuzov reaction and employed for adsorption of resorcinol from an aqueous phase. The phosphate functional groups were successfully incorporated onto the PGO surface by the formation of P-C bonds as identified by the analysis of FTIR and XPS spectra. The evaluation of adsorption capacity of resorcinol onto PGO exhibited significant improvement of resorcinol removal, achieving an adsorption capacity of 50.25 mg/g in the pH range of 4-7 which was 15 times higher than pristine graphene oxide. The addition of 2.4 M and 5 M NaCl in the adsorption system significantly increased the adsorption capacity towards resorcinol from 50.25 mg/g to 82.10 mg/g and 128.10 mg/g, respectively. Based on kinetics and adsorption isotherm studies, Pseudo-First-Order and Langmuir model are the best model to describe the adsorption process indicating that the adsorption is dominantly controlled by physisorption. The thermodynamic analysis suggested that the adsorption process was the favorable, spontaneous, and endothermic process. Besides, the interplay of hydrogen bonding and π-π interactions is proposed to be the governing physisorption mechanism. The outstanding reusability and better adsorption performance make PGO a promising adsorbent for environmental remediation of resorcinol.

Modification of ordered mesoporous carbon for removal of environmental contaminants from aqueous phase: A review

Contamination of water bodies by potentially toxic elements and organic pollutants has aroused extensive concerns worldwide. Thus it is significant to develop effective adsorbents for removing these contaminants. As a new member of carbonaceous material families (activated carbon, biochar, and graphene), ordered mesoporous carbon (OMC) with larger specific surface area, ordered pore structure, and higher pore volume are being evaluated for their use in contaminant removal. In this paper, modification techniques of OMC were systematically reviewed for the first time. These include nonmetallic doping modification (nitrogen, sulfur, and boron) and the impregnation of nano-metals and metal oxides (iron, copper, cobalt, nickel, magnesium, and rare earth element). Reaction conditions (solution pH, reaction temperature, sorbent dosage, and contact time) are of critical importance for the removal performance of contaminants onto OMC. In addition, the pristine and modified OMC have been investigated for the removal of a range of contaminants, including cationic/anionic toxic elements and organic contaminants (synthetic dye, phenol, and others), and involving different and specific mechanisms of interaction with contaminants. The future research directions of the application of pristine and modified OMC were proposed. Overall, this review can provide sights into the modification techniques of OMC for removal of environmental contaminants.

Employing waste to manage waste: Utilizing waste biomaterials for the elimination of hazardous contaminant [Cr(VI)] from aqueous matrices

Pollution caused due to discharge of toxic and hazardous chemical contaminants from industrial processes is an issue of major environmental concern. Hexavalent chromium [Cr(VI)] is one such known toxic heavy metal contaminant emanated largely from various industrial processes. Since physical-chemical treatment techniques are beset with several problems, there is an increased attention on the use of waste biomaterials/biomass as sorbents for the elimination of heavy metals from aqueous matrices. The main purpose of this study was to evaluate the effectiveness of some low-cost waste biomaterials such as fruit wastes, agricultural and industrial waste/byproducts, waste parts of photosynthetic plants, aquatic plants and fungal biomass collected from different sources for the biosorption of Cr(VI) from aqueous matrices. Amid the tested biomaterials, wood apple shell (WAS) biomass (Limonia acidissima) was found to be highly efficient biosorbent for Cr(VI) sorption. In majority of biomass, it was observed that biosorption of Cr(VI) took place at acidic pH with optimum pH ranging from 2.0 to 5.0. Loading capacity of WAS biomass (29.37 mg/g) was higher than that of conventional adsorbent activated charcoal (26.56 mg/g), which was used as control. Cr(VI) treated biomass (WAS) was characterized using instrumental techniques such as Scanned Electron Microscopy (SEM) and Energy Dispersive X-ray (EDX) confirmed the adsorption of Cr(VI). Boehm titration and FTIR studies were conducted to ascertain the presence of functional groups responsible for Cr(VI) sorption by WAS biomass. The WAS biomass removed Cr(VI) from industrial wastewater with an efficiency of >99.9% thus complying with the statutory limits. Considering the economical aspect, the selected biomass can be viewed as a potential candidate for the elimination of toxic contaminant from wastewater.

Mesoporous Carbon: A Versatile Material for Scientific Applications

Mesoporous carbon is a promising material having multiple applications. It can act as a catalytic support and can be used in energy storage devices. Moreover, mesoporous carbon controls body’s oral drug delivery system and adsorb poisonous metal from water and various other molecules from an aqueous solution. The accuracy and improved activity of the carbon materials depend on some parameters. The recent breakthrough in the synthesis of mesoporous carbon, with high surface area, large pore-volume, and good thermostability, improves its activity manifold in performing functions. Considering the promising application of mesoporous carbon, it should be broadly illustrated in the literature. This review summarizes the potential application of mesoporous carbon in many scientific disciplines. Moreover, the outlook for further improvement of mesoporous carbon has been demonstrated in detail. Hopefully, it would act as a reference guidebook for researchers about the putative application of mesoporous carbon in multidimensional fields.

Isolation of biosynthetic crystals by microbially induced calcium carbonate precipitation and their utilization for fluoride removal from groundwater

Biosynthetic crystals (BC) were prepared through microbially induced calcium carbonate precipitation (MICP) for fluoride (F^-) removal from the groundwater. Batch experiments were conducted to evaluate the fluoride adsorption capacity and the impacts of critical factors (organic matter, pH, initial fluoride concentration and BC dosage) on defluorination efficiency of BC. The maximum adsorption amount and defluorination efficiency were recorded as 5.10 mg g^-1 and 98.24%, respectively. The adsorption kinetics and isotherms studies showed that pseudo-second-order kinetic model and Freundlich isotherm model were best fitting to the reaction. Adsorption thermodynamic parameters indicated a spontaneous, endothermic and thermodynamically favorable adsorption process. Moreover, the mechanism of F^- removal by BC was further analyzed by SEM, XPS, XRD and FTIR. The method can cope with the problem of applying the external organic substances in MICP, and avoid the microbial safety risk in the effluent. As an economically and environmentally friendly adsorbent, BC can be used for F^- removal from groundwater.

Magnetic phosphorylated chitosan composite as a novel adsorbent for highly effective and selective capture of lead from aqueous solution

Separating and recovering lead from heavy metal contaminated wastewater is crucial for the environment remediation and reutilization of lead resources. Herein, a novel adsorbent, the phosphorylated chitosan-coated magnetic silica nanoparticles (Fe₃O₄@SiO₂@CS-P), was successfully fabricated and applied to highly selective adsorption of lead. Competitive experiments were conducted in a multi-ion solution (7 metal ions coexist) at pH 6.0, Fe₃O₄@SiO₂@CS-P exhibited an excellent selectively for capturing lead with the distribution coefficient (0.75 L g^-1) more ten times than other metal, while Fe₃O₄@SiO₂@CS demonstrated a highly selective adsorption of silver. These implied that phosphorylation of adsorbent not only improves the sorption performance of lead, but also changes the selective adsorption of metal types. Acidity experiments can draw conclusions that Fe₃O₄@SiO₂@CS-P exhibited better acid resistance (with barely any iron leaching) than silica-uncoated adsorbent (Fe₃O₄@CS-P) at pH 1.0. Furthermore, the FTIR and XPS spectra after adsorption suggested that the high adsorption performance and selective capture lead were predominantly controlled by the coordination of the phosphate groups on the surface of the adsorbent. This work shows a broad prospect of developing a series of novel, acid-resistant, good reusable and rapidly separable magnetic materials that can be used to efficiently and selectively capture lead from aqueous solutions.

The use of artificial neural network (ANN) for modeling adsorption of sunset yellow onto neodymium modified ordered mesoporous carbon

Discharging coloring products in water bodies has degraded water quality irreversibly over the past several decades. Order mesoporous carbon (OMC) was modified by embedding neodymium(III) chloride on the surface of OMC to enhance the adsorptive removal towards these contaminants. This paper represents an artificial neural network (ANN) based approach for modeling the adsorption process of sunset yellow onto neodymium modified OMC (OMC-Nd) in batch adsorption experiments. Neodymium modified OMC was characterized using N₂ adsorption-desorption isotherm, TEM micrographs, FT-IR and XPS spectra analysis techniques. 2.5 wt% Nd loaded OMC was selected as the final adsorbent for further experiments because OMC-2.5Nd showed highest removal efficiency of 93%. The ANN model was trained and validated with the adsorption experiments data where initial concentration, reaction time, and adsorbent dosage were selected as the variables for the batch study, whereas the removal efficiency was considered as the output. The ANN model was first developed using a three-layer back propagation network with the optimum structure of 3-6-1. The model employed tangent sigmoid transfer function as input in the hidden layer whereas a linear transfer function was used in the output layer. The comparison between modeled data and experimental data provided high degree of correlation (R² = 0.9832) which indicated the applicability of ANN model for describing the adsorption process with reasonable accuracy.

Enhanced Pb(II) adsorption onto functionalized ordered mesoporous carbon (OMC) from aqueous solutions: the important role of surface property and adsorption mechanism

Functionalized ordered mesoporous carbon (MOMC-NP) was synthesized by chemical modification using HNO₃ and H₃PO₄ to enhance Pb(II) adsorption. The phosphate functional group represented by P-O-C bonding onto the surface of OMC was verified by FT-IR and XPS. Batch adsorption experiments revealed the improvement of adsorption capacity by 39 times over the virgin OMC. Moreover, the Pb(II) adsorption results provided excellent fits to Langmuir model and pseudo-second-order kinetic model. The adsorption mechanism of Pb(II) onto MOMC-NP revealed the formation of metal complexes with carboxyl, hydroxyl, and phosphate groups through ion exchange reactions and hydrogen bondings. The calculated activation energy was 22.09 kJ/mol, suggesting that Pb(II) adsorption was a chemisorption. At pH>pH_pzc, the main Pb(II) existing species of Pb(II) and Pb(OH)⁺ combine with the carboxyl, hydroxyl, and phosphate functional groups via electrostatic interactions and hydrogen bonding. All these findings demonstrated that MOMC-NP could be a useful and potential adsorbent for adsorptive removal of Pb(II). Graphical abstract.

The effects of carbon disulfide driven functionalization on graphene oxide for enhanced Pb(II) adsorption: Investigation of adsorption mechanism

The surface properties of graphene oxide (GO) have been identified as the key effects on the adsorption of Pb(II) from aqueous solutions in this study. This study reveals the effect of the surface reactivity of GO via Carbon Disulfide (CS₂) functionalization for Pb(II) adsorption. After successfully preparing CS₂ functionalized GO (GOCS), the specific techniques were applied to investigate Pb(II) adsorption onto GOCS. Results indicated that the new sulfur-containing functional groups incorporated onto GOCS significantly enhanced Pb(II) adsorption capacity on GOCS than that of GO, achieving an improvement of 31% in maximum adsorption capacity increasing from 292.8 to 383.4 mg g^-1. The equilibrium adsorption capacity for GOCS was 280.2 mg g^-1 having an improvement of 83.2% over that of 152.97 mg g^-1 for GO at the same initial concentration of 150 mg L^-1 under the optimal pH of 5.7. Moreover, the results of adsorption experiments showed an excellent fit to the Langmuir and Pseudo-Second-Order models indicating the monolayer and chemical adsorption, respectively. The mechanism for Pb(II) adsorption on GOCS was proposed as the coordination, electrostatic interactions, cation-pi interactions, and Lewis acid-base interactions. The regeneration study showed that GOCS had an appreciable reusability for Pb(II) adsorption with the adsorption capacity of 208.92 mg g^-1 after five regeneration cycles. In summary, GOCS has been proved to be a novel, useful, and potentially economic adsorbent for the high-efficiency removal of Pb(II) from aqueous solutions.