Low temperature regeneration of activated carbons using microwaves: revising conventional wisdom

Review on microwave immobilization of soil heavy metals: Processes and mechanisms

Soil contamination with heavy metals (HMs) is still a global issue. The maintenance of long-term stability of HMs in soil during immobilization remediation is a challenge. Microwave (MW) technology can promote the immobilization of HMs in the form of crystals and minerals, thus enhancing their resistance of corrosion. This review provides a comprehensive introduction to the basics of MW irradiation through 177 papers, and reviews the research progress of MW involvement in the immobilization of soil HMs in 10 years. The effects of MW parameter settings, absorber/fixative types and soil physicochemical properties on immobilized HMs are investigated. The immobilization mechanisms of HMs are discussed, high-temperature physical encapsulation and chemical stabilization are the two basic mechanisms in the immobilization process. MW has a unique heating method to achieve efficient remediation by shortening remediation time, reducing the activation energy of reactions and promoting the transformation of stabilization products. Finally, the current limitations of MW in the remediation of HMs contaminated soils are systematically discussed and the corresponding proposed solutions are presented which may provide directions for further laboratory studies. There are still serious problems in taking the results obtained in the laboratory to the full scale. Thus, process optimization, scale-up, design and demonstration are strongly desired. In summary, this review may help new researchers to seize the research frontier in MW and can serve as a reference for future development of MW technology in soil remediation.

Conversion of carbon black recovered from waste tires into activated carbon via chemical/microwave methods for efficient removal of heavy metal ions from wastewater

In this research study, recovered carbon black (rCB) was obtained via pyrolysis of waste tires. The obtained rCB was then converted into activated carbon species through both chemical treatment and microwave coupled with chemical treatment as a two-step activation process. The activated carbon obtained from chemical activation was denoted as C-AC, while that obtained from exposure to microwave followed by chemical activation was labeled as MC-AC. These two structures were consequently introduced as sorbents for the removal of cadmium ions from an aqueous solution. The structural characteristics of the introduced adsorbents were confirmed using various techniques, namely X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy, and energy-dispersive X-ray (EDX) spectroscopy. Additionally, textual features of these adsorbents were acquired via both scanning electron microscopy (SEM) and N2 adsorption-desorption BET surface area analyses. These two structures were then introduced for Cd ion adsorption under different operating conditions. Particularly, the effect of pH, contact time, adsorbent dose, and metal ion concentration on the efficiency of adsorption was investigated. The 1maximum adsorption capacity was detected at a pH value of 5.0, a contact time of 30 min, a sorbent dose of 0.4 g L-1, and an initial metal concentration of 50 mg L-1 using MC-AC, which exhibited nearly double the sorption capacity detected for C-AC. Kinetic studies indicated that the process of Cd(ii) adsorption is perfectly described and fitted by the pseudo-second-order model. However, adsorption isotherms for the two adsorbents were found to match the Langmuir model, referring to the occurrence of uniform monolayer adsorption for the metal ions. Thermodynamic analysis demonstrated that the adsorption process was spontaneous and endothermic.

Evaluation of iron-loaded granular activated carbon used as heterogeneous fenton catalyst for degradation of tetracycline

To optimize the efficiency of general adsorption-Fenton oxidation treatment, iron-loaded granular activated carbon (Fe-GAC) was prepared, characterized, and used as a catalyst in the heterogeneous Fenton oxidation of tetracycline (TC). Characterization revealed that the Fe(II) was successfully introduced onto the original granular activated carbon (GAC) and diversified the materials' surface morphology and elemental compounds. Under an initial pH of 3.0, the Fe-GAC/Fenton system obtained a maximum removal rate of 92.6%, with hydrogen peroxide (H₂O₂) dosages of 9 mmol g^-1. And the GAC/Fenton without iron supplementation was 89.5%, with H₂O₂ dosages of 8 mmol g^-1. Additionally, the Fe-GAC/Fenton system consumed a lower Fe(II) dosage than GAC/Fenton, with Fe(II)/H₂O₂ molar ratios of 0.007:1 and 0.04:1, respectively. Analysis of total organic carbon demonstrated higher mineralization efficiency in the Fe-GAC/Fenton system (67.2%), which was approximately 1.3 times of GAC/Fenton. Desorption experiments showed that the adsorption and degradation accounted for 19.22% and 80.78% of the total TC removal by GAC/Fenton, and 10.58% and 89.42% in the Fe-GAC/Fenton system, respectively. Electron paramagnetic resonance (EPR) technique and quenching experiments demonstrated that the dominant reactive oxygen species (ROS) in synergistic treatments were hydroxyl (•OH) and hydroxy peroxyl (HO₂•) radicals. In addition, three potential degradation pathways for TC were proposed according to the detected fourteen intermediates. Catalyst regeneration treatments were evaluated over six cycles, and the regeneration was 6.5% higher with the iron-supplemented carbon granules. Overall, the Fe-GAC can be used as an efficient catalyst in practical water treatment, and this study demonstrated a promising method to develop adsorption-Fenton technology.

Optimization of Micro-Pollutants' Removal from Wastewater Using Agricultural Waste-Derived Sustainable Adsorbent

Water pollution due to the discharge of untreated industrial effluents is a serious environmental and public health issue. The presence of organic pollutants such as polycyclic aromatic hydrocarbons (PAHs) causes worldwide concern because of their mutagenic and carcinogenic effects on aquatic life, human beings, and the environment. PAHs are pervasive atmospheric compounds that cause nervous system damage, mental retardation, cancer, and renal kidney diseases. This research presents the first usage of palm kernel shell biochar (PKSB) (obtained from agricultural waste) for PAH removal from industrial wastewater (oil and gas wastewater/produced water). A batch scale study was conducted for the remediation of PAHs and chemical oxygen demand (COD) from produced water. The influence of operating parameters such as biochar dosage, pH, and contact time was optimized and validated using a response surface methodology (RSM). Under optimized conditions, i.e., biochar dosage 2.99 g L⁻¹, pH 4.0, and contact time 208.89 min, 93.16% of PAHs and 97.84% of COD were predicted. However, under optimized conditions of independent variables, 95.34% of PAH and 98.21% of COD removal was obtained in the laboratory. The experimental data were fitted to the empirical second-order model of a suitable degree for the maximum removal of PAHs and COD by the biochar. ANOVA analysis showed a high coefficient of determination value (R² = 0.97) and a reasonable second-order regression prediction. Additionally, the study also showed a comparative analysis of PKSB with previously used agricultural waste biochar for PAH and COD removal. The PKSB showed significantly higher removal efficiency than other types of biochar. The study also provides analysis on the reusability of PKSB for up to four cycles using two different methods. The methods reflected a significantly good performance for PAH and COD removal for up to two cycles. Hence, the study demonstrated a successful application of PKSB as a potential sustainable adsorbent for the removal of micro-pollutants from produced water.

Electrochemical Capacitance of Activated Carbons Regenerated using Thermal and Chemical Activation

Spent activated carbons (SACs) collected from a water treatment plant were regenerated and then adopted as electrochemical material in capacitors. The SACs used in this study were regenerated via two steps, namely thermal and chemical activation. However, during the activation process, the adsorbates were converted into ashes, which caused pore blockage and decreased specific surface area. The regenerated SACs were washed with acid solutions with different levels of acidity (strong: HCl, mild: H3PO4, and weak: H2O2) to remove the ashes. The regenerated SACs washed with HCl exhibited the highest specific surface area, although their capacitance was not the highest. Conversely, the specific surface area of regenerated SACs washed using H3PO4 was slightly lower than that of HCl, but exhibited higher capacitance and electrochemical stability. Although the strong acid removed the generated ashes in the pores efficiently, it could adversely affect their structural stability, which would lead to lower capacitance.

Microwave regeneration of granular activated carbon saturated with PFAS

This study investigated the regeneration of PFAS-saturated granular activated carbons (GACs) by microwave (MW) irradiation. Two commercially available GACs (bituminous coal based GAC [BCGAC] and lignite coal based GAC [LCGAC]) were saturated with perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) and then irradiated at different MW conditions (applied power = 125 - 500 W, irradiation time = 3 - 12 min). The performance of MW regeneration for PFOS- or PFOA-saturated GACs was assessed by evaluating the variation of GAC adsorption capacity (regeneration efficiency, RE) and weight loss percentage (WL). Moreover, the effect of MW irradiation on GAC textural properties (e.g., surface area and pore volume) was examined through N₂ adsorption isotherms. Additionally, five successive adsorption/regeneration cycles were carried out at the MW operational condition that allowed to reach the target temperature (T>600°C) while minimizing the WL. Both GACs exhibited a strong ability to convert MW irradiation into a rapid temperature increase (~150°C min^-1 at 500 W). The highest values of RE (>90%) for both PFOA- and PFOS-saturated GACs were obtained at MW irradiation conditions that employed short regeneration time (3 min) and optimal temperature. Indeed, the highest RE did not occur at the highest temperatures (>750°C) due to the damage of GAC porous structure, particularly for LCGAC. After five cycles, the observed values of RE (~65%) and a moderate weight loss (<7%) demonstrated the good performance of MW irradiation for regenerating PFOA- and PFOS-saturated BCGAC. The obtained findings pointed out that MW irradiation is a promising alternative regeneration technique for PFAS-saturated GAC.

Palladium nanoparticle loaded β-cyclodextrin monolith as a flow reactor for concentration enrichment and conversion of pollutants based on molecular recognition

This study reports pollutant remediation by a catalyst-loaded, β-cyclodextrin cross-linked polymer monolith. The monolith enabled removal of the pollutant to a residual concentration with no environmental effect and conversion of the adsorbed pollutant into useful compounds with enriched concentration, allowing for the adsorption capacity regeneration.

Aragón-Tobar C, Gámez S, de la Torre E. Reactivation Process of Activated Carbons: Effect on the Mechanical and Adsorptive Properties

Carbon reactivation is a strategy to reduce waste and cost in many industrial processes, for example, effluent treatment, food industry, and hydrometallurgy. In this work, the effect of physical and chemical reactivation of granular activated carbon (AC) was studied. Spent activated carbon (SAC) was obtained from a carbon in pulp (CIP) leaching process for gold extraction. Chemical and physical reactivations were evaluated using several acid-wash procedures (HCl, HNO₃, H₂SO₄) and thermal treatment (650–950 °C) methods, respectively. The effect of the reactivation processes on the mechanical properties was evaluated determining ball pan hardness and normal abrasion in pulp resistance. The effect on the adsorptive properties was evaluated via the iodine number, the gold adsorption value (k expressed in mg Au/g AC), and Brunauer–Emmett–Teller (BET) surface area. Initial characterization of the SAC showed an iodine number of 734 mg I₂/g AC, a k value of 1.37 mg Au/g AC, and a BET surface area of 869 m²/g. The best reactivation results of the SAC were achieved via acid washing with HNO₃ at 20% v/v and 50 °C over 30 min, and a subsequent thermal reactivation at 850 °C over 1 h. The final reactivated carbon had an iodine number of 1199 mg I₂/g AC, a k value of 14.9 mg Au/g AC, and a BET surface area of 1079 m²/g. Acid wash prior to thermal treatment was critical to reactivate the SAC. The reactivation process had a minor impact (<1% change) on the mechanical properties of the AC.

Regeneration of Activated Carbons Spent by Waste Water Treatment Using KOH Chemical Activation

In this study, spent activated carbons (ACs) were collected from a waste water treatment plant (WWTP) in Incheon, South Korea, and regenerated by heat treatment and KOH chemical activation. The specific surface area of spent AC was 680 m2/g, and increased up to 710 m2/g through heat treatment. When the spent AC was activated by the chemical agent potassium hydroxide (KOH), the surface area increased to 1380 m2/g. The chemically activated ACs were also washed with acetic acid (CH3COOH) to compare the effect of ash removal during KOH activation. The low temperature N2 adsorption was utilized to measure the specific surface areas and pore size distributions of regenerated ACs by heat treatment and chemical activation. The functional groups and adsorbed materials on ACs were also analyzed by X-ray photoelectron spectroscopy and X-ray fluorescence. The generated ash was confirmed by proximate analysis and elementary analysis. The regenerated ACs were tested for toluene adsorption, and their capacities were compared with commercial ACs. The toluene adsorption capacity of regenerated ACs was higher than commercial ACs. Therefore, it is a research to create high value-added products using the waste.

Ce-Mn modify Al2O3 adsorbent and the effect on adsorption and regeneration properties

The regeneration of adsorbent by thermal method may transfer the adsorbed organic pollutants into the air phase. Herein, manganese and cerium are loaded on adsorbent as catalyst to mineralize the adsorbed organic pollutants during regeneration. The modified adsorbent is characterized by scanning electron microscope-energy dispersive spectrometer, Brunauer Emmett Teller-Barrett Joyner Halenda, and X-ray photoelectron spectroscopy. Regeneration of modified adsorbent is evaluated by adsorbent yield and mineralization percentage of the absorbed organic pollutants. The results demonstrate that the mineralization percentage of tetracycline increased about 100% after the modification. Besides, the loaded catalyst can significantly reduce the regeneration temperature. Furthermore, cerium can reduce the adverse impact causes by manganese which will decrease the adsorption capacity of adsorbent. Kinetics, equilibrium, thermodynamic, and other adsorption properties of modified adsorbent towards tetracycline are further studied. The theoretical maximum of adsorption capacity calculated by Langmuir model is 196.4 mg g^-1, and the kinetics of adsorption fits pseudo-second-order model. The adsorption of tetracycline on the adsorbent is feasible, spontaneous, and endothermic. The properties of synthesized composite material are stable; the yield of Ce/Mn/Al₂O₃ adsorbent can keep higher than 95% even after five adsorption/regeneration cycles. This research provides another perspective on the design and regeneration of adsorbent, which focus on reducing the secondary pollution during adsorbent regeneration process.

Effect and mechanism of microwave-activated ultraviolet-advanced oxidation technology for adsorbent regeneration

To decrease the secondary pollution of volatile organic compounds (VOCs) during adsorbent regeneration by microwave, electrodeless lamp was added in the microwave field to oxidize VOCs in the gas phase. Ultraviolet has a significant improvement on mineralization of VOCs generated from adsorbate during adsorbent regeneration. However, the mechanism and main influence factors on the degradation of VOCs are not clear. The effect of microwave power, regeneration time, airflow rate, and humidity content on the mineralization of adsorbed tetracycline during adsorbent regeneration was studied. Ozone concentration and ultraviolet irradiation intensity were also measured to analyze the mechanism of the microwave-ultraviolet adsorbent regeneration method. Although the electrodeless lamp adsorbed microwave and competed with the regenerated adsorbent, the mineralization percentage of tetracycline increased about 10% with the presence of electrodeless lamp at the same microwave power supply. Besides, humidity content also takes an important role on enhancing the mineralization of tetracycline. The mineralization of tetracycline in the microwave-ultraviolet field consists of three major parts: pyrolysis, ozone oxidation, and free radical oxidation. More than 50% adsorbed tetracycline can be oxidized into H₂O and CO₂ during regeneration in 5 min. These results support the potential use of electrodeless lamp to treat VOCs in the gas phase to decrease the risk of secondary pollution during adsorbent regeneration.

A Novel Technique for Determining the Adsorption Capacity and Breakthrough Time of Adsorbents Using a Noncontact High-Resolution Microwave Resonator Sensor

A newly developed noncontact high-resolution real-time microwave sensor was used to determine the breakthrough time and adsorption capacity of adsorbents/adsorbates with different dielectric properties. The sensor is a microwave microstrip planar resonator with an enhanced quality factor using a regenerative feedback loop operating at 1.4 GHz and an adjustable quality factor of 200-200000. Beaded activated carbon (BAC, microwave-absorbing) and a polymeric adsorbent (V503, microwave transparent) were completely loaded with 1,2,4-trimethylbenzene (nonpolar) or 2-butoxyethanol (polar). During adsorption, variations in the dielectric properties of the adsorbents were monitored using two microwave parameters; quality factor and resonant frequency. Those parameters were related to adsorption breakthrough time and capacity. Adsorption tests were completed at select relative pressures (0.03, 0.1, 0.2, 0.4, and 0.6) of adsorbates in the influent stream. For all experiments, the difference between the breakthrough time (t_5%) and the settling time of the quality factor variation (time that the quality factor was 0.95 of its final value) was <5%. Additionally, a linear relationship between the final value of the resonant frequency shift and adsorption capacity was observed. The proposed noncontact sensor can be used to determine the breakthrough time and adsorption capacity.

Microwave Reactor Concepts: From Resonant Cavities to Traveling Fields

Microwave chemistry has been investigated for nearly thirty years with many notable results being published on apparent process enhancement due to microwave exposure. Conclusive proof of beneficial microwave-chemical interactions is lacking though, as are design rules for successful implementation of microwave-chemical processing systems. In this chapter, the main cause for this is asserted to be the current absence both of suitable instrumentation for research, and processing equipment that merges chemistry with electromagnetic aspects. Several concepts are presented to show how these challenges may be addressed.

The effects of microwave regeneration on adsorptive performance of functionalized carbon nanotubes

In this study, the microwave regeneration method was applied to investigate the properties and adsorptive performance of functionalized carbon nanotubes (f-CNTs) in different cycles of regeneration/reuse. For this purpose, an organic and hazardous dye (Reactive Blue 19) was chosen as a widely used pollutant. N2 adsorption/desorption isotherms, scanning electron microscopy and Fourier transform infrared spectroscopy were used to characterize f-CNTs during the regeneration/reuse procedure. The morphology, specific surface area and pore volume of f-CNT samples were not significantly altered. However, the functional groups present on the f-CNTs' surface were gradually removed after successive cycles of regeneration/reuse. A sudden decrease of adsorption capacity (about 20%) after the first cycle of regeneration/reuse was attributed to the elimination of functional groups interacting with the dye molecules because of the molecular-level heating. Relatively high regeneration efficiencies (73.30 to 80.16%) proved that the microwave regeneration method was successful. Very high step stripping efficiencies (80.16 to 98.02%) in four cycles of regeneration/reuse demonstrated that the microwave regeneration method could be utilized in consecutive cycles. After four cycles of regeneration/reuse, the CNTs could not be considered as functionalized.

Enhanced Recyclable Magnetized Palm Shell Waste-Based Powdered Activated Carbon for the Removal of Ibuprofen: Insights for Kinetics and Mechanisms

A novel preparation method of magnetized palm shell waste-based powdered activated carbon (MPPAC, avg. size 112 μm) was developed. The prepared MPPAC was assessed by several physicochemical analyses, and batch tests were performed for ibuprofen (IBP) removal. Field emission scanning electron microscopy (FESEM) and N2 gas isotherms revealed that magnetite and maghemite were homogeneous and deposited mostly on the surface of PPAC without a significant clogging effect on the micropores. Isotherm results showed that 3.8% Fe (w/w) impregnated PPAC [MPPAC-Fe(3.8%)] had about 2.2-fold higher maximum sorption capacity (157.3 mg g-1) and a 2.5-fold higher sorption density (0.23 mg m-2) than pristine PPAC. Both Fourier-transform infrared spectroscopy (FTIR) and isotherm data indicated that the high sorption capacity and density of IBP by MPPAC was primarily attributable to donor-acceptor complexes with the C = O group and dispersive π-π interactions with the carbon surface. Based on kinetic and repeated adsorption tests, pore diffusion was the rate-limiting step, and MPPAC-Fe(3.8%) had about 1.9~2.8- and 9.1~15.8-fold higher rate constants than MPPAC-Fe(8.6%) and palm shell-waste granular activated carbon (PGAC, avg. size 621 μm), respectively. MPPAC showed almost eight fold greater re-adsorption capacity than PPAC due to a thermal catalytic effect of magnetite/maghemite.

A novel method of microwave heating mixed liquid-assisted regeneration of V₂O₅-WO₃/TiO₂ commercial SCR catalysts

An experimental study on the regeneration of deactivated SCR catalysts was carried out using a microwave-assisted method containing three steps of washing with mixed liquid of ethanol and water, impregnating, and drying. After the regeneration treatment, NO conversion at 320 °C increased from 39 to 90% and vanadium content increased by 62.2%, which were much higher than those regenerated by the traditional method. The more impregnated vanadium was due to the fact that the rapid evaporation of mixed liquid inside the catalyst channels led to the enlargement of surface areas by creating more pores on the catalysts. Meanwhile, with the increasing concentrations of ethanol, the heating rate of the mixed liquid increased, and the volume after complete evaporation of the mixed liquid was gradually reduced. Since higher heating rate and lager volume after the liquid evaporation could help to create more pores, therefore, when the volume ratio of ethanol/mixed solution was 20%, the catalyst obtained the maximum specific surface area, which significantly increased to ca. 123% compared with the deactivated catalyst. In addition, the catalyst dried by microwave exhibited better catalytic activity than that dried in conventional oven. Therefore, this method showed great potential in industrial applications.

Using microwave heating to improve the desorption efficiency of high molecular weight VOC from beaded activated carbon

Incomplete regeneration of activated carbon loaded with organic compounds results in heel build-up that reduces the useful life of the adsorbent. In this study, microwave heating was tested as a regeneration method for beaded activated carbon (BAC) loaded with n-dodecane, a high molecular weight volatile organic compound. Energy consumption and desorption efficiency for microwave-heating regeneration were compared with conductive-heating regeneration. The minimum energy needed to completely regenerate the adsorbent (100% desorption efficiency) using microwave regeneration was 6% of that needed with conductive heating regeneration, owing to more rapid heating rates and lower heat loss. Analyses of adsorbent pore size distribution and surface chemistry confirmed that neither heating method altered the physical/chemical properties of the BAC. Additionally, gas chromatography (with flame ionization detector) confirmed that neither regeneration method detectably altered the adsorbate composition during desorption. By demonstrating improvements in energy consumption and desorption efficiency and showing stable adsorbate and adsorbent properties, this paper suggests that microwave heating is an attractive method for activated carbon regeneration particularly when high-affinity VOC adsorbates are present.

Effects of microwave heating on porous structure of regenerated powdered activated carbon used in xylose

The regeneration of spent powdered activated carbons used in xylose decolourization by microwave heating was investigated. Effects of microwave power and microwave heating time on the adsorption capacity of regenerated activated carbons were evaluated. The optimum conditions obtained are as follows: microwave power 800W; microwave heating time 30min. Regenerated activated carbon in this work has high adsorption capacities for the amount of methylene blue of 16 cm3/0.1 g and the iodine number of 1000.06mg/g. The specific surface areas of fresh commercial activated carbon, spent carbon and regenerated activated carbon were calculated according to the Brunauer, Emmett and Teller method, and the pore-size distributions of these carbons were characterized by non-local density functional theory (NLDFT). The results show that the specific surface area and the total pore volume of regenerated activated carbon are 1064 m2/g and 1.181 mL/g, respectively, indicating the feasibility of regeneration of spent powdered activated carbon used in xylose decolourization by microwave heating. The results of surface fractal dimensions also confirm the results of isotherms and NLDFT.