Activated Carbon Adsorption Research Articles

On the Possibility of Cleaning Producer Gas Laden with Large Quantities of Tars through Using a Simple Fixed-Bed Activated Carbon Adsorption Process

The study presents the results of research on using fixed-bed, activated carbon (AC) adsorbers in the cleaning of heavily tar-laden producer gas from the gasification of biomass. The efficiency of removal of organic compounds as well as the remaining adsorption capacity of the bed were determined using a spectrum of commonly applied diagnostic methods, including qualitative and quantitative analyses of the adsorbed compounds and changes in the pore volume of the bed material (IN, MN). The authors compare these lab quantifications with a simple technique which is based on the analysis of the changes in the position of temperature front in the bed. The main benefit of the latter is the possibility of performing the diagnostics of the bed “online” and using low-cost temperature measurements. The test was performed using a commercially available AC Desotec AIRPEL 10-3 and real producer gas obtained through the gasification of alder chips. For tar, VOC and C2–C5 compounds, the removal efficiencies reached respectively 74.5%-wt., 52.8%-wt., and 85.5%-wt. Obtained results indicate that depending on the final application of the gas, the use of dry adsorption systems is an interesting alternative to the well-established but complicated, cumbersome, and costly wet scrubbers. Moreover, a concept for in situ regeneration of the adsorbent, coupled with direct reforming of the tars, is presented and discussed.

Micropollutants Removal from Water and Activated Carbon Regeneration by Combining Adsorption and Electro-Fenton

The presence of micropollutants in municipal wastewater effluents is a topic of major concern, as these pollutants end up in surface and ground waters. Currently, adsorption on activated carbon (AC) is used to remove these recalcitrant compounds. When saturated, the AC is generally either thermally regenerated or disposed of. Thermal regeneration is expensive and negatively affects the structure and adsorptive properties of the AC.Over the last decade, considerable attention has been directed towards the electrochemical advanced oxidation processes (eAOPs), due to their ability to remove a large range of organic pollutants from water even at very low concentrations [1]. eAOPs rely on the in-situ generation of powerful oxidants, such as HO• by electron transfer at the anode surface. However, these processes are limited by mass transfer and require high energy consumption.In this study, combined adsorption on granular activated carbon (GAC) and electro Fenton process (EF) is applied. The purpose of this coupled process is to overcome the mass transfer issue by pre-concentrating the compounds on AC and proposing a new AC regeneration method. During the EF the AC can directly be used as a cathode. Due to the electrostatic repulsions and oxidation process, the compounds are desorbed from the AC and oxidized by the HO• generated in the bulk. Material and Methods The adsorption cycles and EF experiments were performed in batch mode using deionized water (DW) and simulated wastewater (SWW) with caffeine as a model compound. Similar experimental steps and conditions as described in [2] are followed, using 1g of GAC, with pH=3, Fe2+= 0.15 mM, Na2SO4= 50mM, and a current intensity of 200 mA. To assess the degradation and mineralization of the caffeine, HPLC and TOC measurements were used, respectively. The regeneration efficiency was calculated by comparing the adsorption capacity of the AC after each regeneration cycle with its initial adsorption capacity. Results and Conclusions The efficiency of the process was discussed as regards (i) desorption of organic compounds and regeneration efficiency in DW and SWW, (ii) degradation and mineralization of organic compounds, and (iii) stability of GAC for several successive regeneration cycles. During the regeneration, over 80% of desorbed caffeine was degraded and 75% was mineralized due to the formed HO• by Fenton reaction and at the BDD anode surface (Fig.1). Furthermore, a recovery of the AC adsorption capacity by up to 50 % in PW and 37% in SWW was observed. These regeneration percentages decreased to 25% during the second cycle and then remained stable during the third and the fourth cycle. The incomplete regeneration is most likely due to remaining caffeine molecules at the AC [3], or a blockage of the carbon adsorption sites by formed degradation molecules. Hence, a longer treatment time might lead to higher efficiency. These hypotheses will be taken into consideration in future work to draw a clear conclusion on the regeneration phenomenon. The similar regeneration efficiencies and mineralization rates obtained during the 2nd,3rd, and 4th cycles indicate the stability of the GAC electrode.In conclusion, the results from this study illustrate that combined adsorption and EF is a suitable process for the removal of caffeine and regeneration of AC. The advantage of such a process is that part of the AC can be regenerated while over 75% of the desorbed compounds from the AC are mineralized, thus avoiding toxic effluents. This process is currently implemented as a continuous e-cell for sequential adsorption and EF regeneration.

New insights into the occurrence and removal of 36 pesticides in pesticide wastewater treatment plants in Korea

This study investigates the occurrence of 36 pesticides—including 33 that were monitored from 2007 to 2019 in Korean rivers—in wastewaters from pesticide manufacturing facilities (PMFs) and in 13 pesticide wastewater treatment plants (PWWTPs). Furthermore, an approach for the effective removal of these pesticides from pesticide wastewater (PW) is proposed. Among the 36 pesticides investigated, 32 were found to be present in the PMF wastewater and PWWTP influents (at concentrations ≤466.8 mg/L). In addition, 24 of the 36 pesticides were detected in PWWTP effluents, indicating that effluents are discharged without the complete removal of many pesticides. Moreover, the PWWTP effluent influences the presence of pesticides in river (confidence interval: ≥ 95%; p < 0.05). Although the chemical oxidation–activated sludge process is frequently recommended for the treatment of PWs, the findings from the current study indicate that an activated sludge–activated carbon adsorption process is more suitable for PW treatment due to its superior removal efficiency, treatment stability, and economic feasibility. Consequently, its application for the treatment of PWs can markedly decrease the levels of pesticides discharged into rivers.

Industrial applications of the composite process “biodegradation+ activated carbon adsorption” in neutralizing the waste gas emissions

In order to absorb and degrade odor gas and volatile organic compounds from sewage plant of refineries, waste gas was treated by the technology of compound “biodegradation and activated carbon adsorption”. Through industrialization and production calibration, emission concentration of pollutants met the GB31571-2015 and GB14554-93, and statistics suggested the cost of utilities was 57.6 yuan per 10000m3, and fixed cost within reasonable boundaries. Therefore, this compounded technology can treat the waste gas from sewage plant of refineries.

Effectiveness and mechanisms of the adsorption of carbendazim from wastewater onto commercial activated carbon.

The wide application of fungicides is becoming one of the main causes of water pollution. Activated carbon (AC) is a frequently-used adsorbent in water treatment. In this work, aiming to obtain a better understanding of fungicides on AC, carbendazim was selected as a model fungicide in water. The effects of AC dosage, adsorption temperature, adsorption time and pH value of solution on carbendazim adsorption by AC were investigated. When the initial concentration of carbendazim was 500mgL-1 and the volume of wastewater was 25mL, the optimum dosage of AC and reaction time was determined to be 0.3g and 150min, respectively. The pH ranging from 3.0 to 10.0 exhibited little effect on the adsorption capability of AC. The higher the adsorption temperature was, the better adsorption capacity was. Adsorption capacity could reach 32.31mgg-1 under the optimal adsorption conditions. The kinetics study reveals that the adsorption of carbendazim occurred on the surface of adsorbent during initial stage. The adsorption data was well fitted by Langmuir adsorption isotherm, indicating that the adsorption process was monolayer adsorption. The thermodynamic experiments confirmed that the adsorption of carbendazim was an endothermic process with the coexistence of physical and chemical adsorption. Because the main components of AC used in this research work is amorphous carbon with low impurity and its surface has not been modified with additional functional groups, the conclusion of the study was easy to be replicated by repeated experiments. Therefore, the findings of this study could guide the adsorption of carbendazim onto the other kinds of AC with high specific surface area, and provide useful information for application of commercial AC in treatment of fungicides wastewater.

LAB-SCALE CONTINUOUS ADSORPTIVE SEPARATION OF Cd (II) ION FROM AQUEOUS EFFLUENT USING MAGNETIC NANOCELLULOSIC ADSORBENT

Haphazard disposal of industrial effluents into the environment has negatively impacted the global water security sector. Some of the accompanying chemical pollutants are reported to be carcinogenic when ingested or inhaled. Cadmium had been linked with several cancers in humans. Batch-wise adsorption technique is the commonly practiced separation procedure due to its lack of ambiguity and the potential for sorption of low concentration pollutants from effluent streams. The frequently used activated carbon adsorbent can be restrictive due to excessive sludge generation. The essence of this research interest is in the development of sustainable, renewable and environmentally friendly adsorbents based on advances in nanotechnology. In this study, 91.31 % recovery of Cd2+ was recorded when 3.0 mg of the solute was treated using novel magnetic nanofibrilated cellulose in fixed bed adsorption process. Key words: Breakthrough curve, Environment, fixed bed, Nanoadsorbent, wastewater

SYNTHESIS, CHARACTERIZATION AND EQUILILBRIUM STUDY OF NITRIC ACID MODIFIED ACTIVATED CARBON ADSORBENT FROM BAMBOO CULMS

This study evaluated the synthesis of low cost activated carbon adsorbent from waste bamboo culms collected at a construction site around Auchi Polytechnic, modified with environmentally friendly nitric acid for enhanced pore structures and improved adsorption versatility. The material was characterized by Fourier transform infrared (FTIR), Brunauer-Emmett-Teller (BET), energy dispersive X-ray (EDAX) and nickel adsorption. The FTIR result showed the characteristics O–H stretching of free hydroxyl and strong hydrogen bond group on the activated carbon surface. The BET surface area of the activated carbon displayed high specific surface area of 1.087 ×103 m2/g and 71.41% mesoporosity, despite the lower activation temperature of 600 oC. The maximum adsorption capacity of the activated carbon for nickel was 20.69 mmol/g. Freundlich isotherm model best explained the adsorption behavior, suggesting heterogeneous surface adsorption due to textural properties of the activated carbon or ionic interaction between nickel molecule and the functional groups. Nitric acid modified activated carbon is a promising adsorbent candidate for heavy metal removal. Keywords: Bamboo culms, nitric acid, adsorbent, modified, characteristics.

Simultaneous antibiotic resistance genes reduction and membrane fouling mitigation by a hybrid process of magnetic activated carbon adsorption and ultrafiltration for wastewater reuse

Antibiotic resistance genes (ARGs) and dissolved effluent organic matter (dEfOM) in reclaimed water are potential risks for landscape reuse or ecological water compensation. Adsorption process showed high advantage during advanced wastewater treatment. To improve the recovery of adsorbent during application, magnetic activated carbon (MAC) was successfully prepared by loading ferromagnetic oxide on powder activated carbon and used as pre-treatment for ultrafiltration (UF) to simultaneously reduce dEfOM and ARGs. Meanwhile, the mitigation mechanism of membrane fouling by MAC was further investigated. Results showed that the removal efficiency of dEfOM and ARGs by a hybrid process of MAC-UF was superior to the individual process. dEfOM reduction was more than 60%, and the maximum removal of sulI, sulII, sulIII, tetM, tetQ and tetW reached 3.09, 3.77, 1.96, 4.33, 3.85 and 2.75 log units, respectively. Their reduction was mainly attributed to the surface adsorption of MAC as well as physical retention by UF. Based on correlation analysis, the reduction of tetM, tetQ, tetW were strongly correlated with that of fulvic acids, soluble microbial products and humic acids (p < 0.01). Standard blocking and cake filtration were the dominant membrane fouling mechanisms throughout the filtration process. After MAC pretreatment, membrane fouling was significantly reduced, especially for the irreversible hydraulic resistance (88.9%). In a word, this study provided a new approach for dEfOM and ARGs reduction, and may have important significance for reclaimed water security and UF technology promotion.

Sustainable Proposal for Regulating Organophosphate Pesticides in Wastewater Treatment Plants in South Korea

Organophosphate pesticides (OPs) are highly toxic; their presence in surface waters is a matter of great concern. To the best of our knowledge, OPs in wastewater from agrochemical manufacturing facilities (AMFs) and influents and effluents from agrochemical wastewater treatment plants (AWWTPs) have not been previously investigated. Therefore, we investigated the presence of 8 OPs (5 of which are regulated under the Water Environment Conservation Act (WECA)) in 15 AMFs and 13 AWWTPs detected through surface water monitoring and proposed measures for effectively regulating these OPs in AWWTPs. Five OPs (chlorpyrifos, diazinon, dichlorvos, EPN, and fenitrothion) were detected in the AMF and AWWTP influents; three (methyldemeton, parathion, and phenthoate) were not. Of the five detected OPs, chlorpyrifos, dichlorvos, and fenitrothion are not currently regulated via effluent limitations for WWTPs under WECA; thus, additional regulations are required. The most effective process configuration for the removal of these OPs was biological treatment through activated sludge processes, followed by activated carbon adsorption. In the system, 100% OP removal from the AWWTP influents was observed. This treatment technology can be implemented in AWWTPs to minimize the presence of OPs in surface waters, thereby protecting human health and aquatic life.

Tandem production of high-purity sodium glycolate via the dual purification technology of crystallization and active carbon adsorption

High-purity glycolic acid (GA) are highly required for the production of biodegradable polymers, pharmaceuticals, and fine chemicals. For production of bio-polyester, GA purity has emerged as a bottleneck. In this paper, we innovatively propose crystallization to dispose of purity problem and investigated the repurification of sodium glycolate crystals. Compared to traditional chemical techniques, resting cell catalysis with bio-oxidation of ethylene glycol (EG) is a promising method for the green industrial production of GA. Gluconobacter oxydans is one of the most promising strains because of its ability to ferment high GA titer from EG. However, the GA directly from fermentation broth does not meet the high purity requirements for biodegradable materials. Therefore, a tandem process was designed and experimentally verified by combining crystallization and active carbon (AC) adsorption. Sodium glycolate (SG) formation was adopted due to low-cost operation in the biological step and easy electrodialysis of GA. According to the dissociation constants (Kɑ) of glycolate, the fermentation broth was adjusted to pH 9.50 and SG saturates at 60 ℃ for SG crystallization. The macroscopic yellow plate-like crystals 91.1 g of SG with 94 % purity were produced at room temperature (25℃). In the tandem process, AC rapidly removed most pigment-like contaminants in the fermentation broth but adsorbed only a little SG (1 %), indicating selective and effective adsorption. Over 97 % of the color was removed by adding 8 % AC to a saturated solution (188 g/L). Finally, 79.2 g SG with 99 % purity was obtained at the yield of 87 % from 104 g/L SG broth. This tandem process could provide an economically-feasible approach for the industrial bioproduction of high-purity SG and GA derived from EG.

Preparation of Activated Carbon from Co-Pyrolysis Activation of Fly Ash and Biomass

Fly ash from waste incineration and waste poplar bark from furniture manufacturing are domestic wastes. In this study, fly ash and poplar bark were used as raw materials to prepare activated carbon via carbonization, steam activation and reagent modification. The effects of the raw material mixing ratio, carbonization temperature, activator concentration and modifier concentration on the physicochemical properties of the semi-coke and activated carbon were investigated through experiments. The experiment showed that when the carbonization temperature was 300 °C, the steam concentration was 20%, the mixing ratio of the poplar bark and fly ash (B:F) was 5:1, and the modifier was 6% K2CO3 reagent and 9% CaCl2. The prepared activated carbon had a better yield, specific surface area and pore structure, and had an abundant surface functional group structure. This paper points out the direction for the industrial directional production of activated carbon adsorbents with excellent physical and chemical properties, which has practical significance.

Cyclic performance evaluation of CO2 adsorption using polyethylene terephthalate plastic-waste-derived activated carbon

Polyethylene terephthalate (PET) plastic-waste-derived activated carbons have recently been developed and exhibit excellent CO2 adsorption uptake. However, the CO2-adsorption performance of such recycled materials has only been considered on a basic characterization level and has not yet been evaluated in carbon capture cycles, thereby making biased analyses inevitable. Consequently, a whole chain including the material, process, and cycle is essential for comprehensively analyzing and evaluating novel CO2 adsorbents. Therefore, in this study, various CO2-capture cycles using PET plastic-waste-derived activated carbon adsorbents were numerically simulated, the cyclic CO2-adsorption performances were evaluated, and the application scenario was optimized. A methodology for evaluating the cyclic CO2-adsorption performance of PET plastic-waste-derived activated carbon was proposed for CO2 capture. The results suggested that the temperature/vacuum swing adsorption cycle was superior and that its maximum exergy efficiency reached 32.90%.

Combination of granular activated carbon adsorption and electrochemical oxidation processes in methanol medium for benzene removal

• Benzene can be mineralized electrochemically from methanol electrolytes. • Better efficiency of the combined GAC/electrolytic process for removal of benzene. • Formation of anisole opens the possibility of electro-refinery as treatment technology. • Adsorption/electrolysis doesn't reach initially expected efficiency but overcomes water electrolysis. One of the impacts caused by the increase in anthropogenic activities, related to the accelerated urbanization and industrialization, is the increase in emissions of volatile organic compounds (VOC). This type of air pollution has become a matter of great concern nowadays, because of its harmfulness to the environment and human health. Adsorption technologies are one of the most promising methods for decontaminating VOCs from the air because they are simple, inexpensive, and energy efficient. On the other hand, electrochemical treatments have been extensively and successfully studied for water treatment. The combination of an adsorption step followed by an electrochemical treatment, with the regeneration of the adsorbent material, can be an interesting alternative, mainly to overcome the limitations of the treatment of diluted solutions. In this work, the removal of benzene from diluted water solutions were evaluated using a combination of three processes: adsorption, desorption with methanol and electrochemical oxidation with boron-doped diamond electrode (BDD) of the high concentrated solution obtained. This technology is suitable for the recovery of the adsorbent material (granular activated carbon), which can be reused after drying. Two possible concurrent ways to remove benzene from methanol solutions by electrochemical oxidation were found: (1) mineralization and (2) conversion to anisole, a solvent of commercial interest used in the cosmetics industry. The performance of electrochemical treatment for benzene removal from methanol solution was not so different than that in water, but the energy consumption was greater. However, the production of high added-value products is a very interesting finding that will advance the development of more sustainable treatment processes.

Treatment of wastewater from food waste hydrothermal carbonization via Fenton oxidization combined activated carbon adsorption

Treatment of wastewater from food waste hydrothermal carbonization via Fenton oxidization combined activated carbon adsorption

Thermal desorption characteristics of the adsorbate in activated carbon based on a two-dimensional heat and mass transfer model

• A multiphase model of heat and mass transfer in porous media is established. • The multiphase model is validated to simulate the thermal desorption behavior of activated carbon adsorption. • A simplified evaporation rate formula is proposed, and the agreement between the model and the experimental results are verified. • The effect of high thermal conductivity activated carbon on thermal regeneration time is analyzed. • The effects of polar and non-polar adsorbates on model accuracy are analyzed. Thermal desorption of adsorbate in activated carbon is the main part in the thermal regeneration process of spent activated carbon, and previous studies have carried out a series of numerical simulations on it. However, since activated carbon is a porous medium, the heat and mass transfer in activated carbon is different from that in solid. In this paper, a multiphase model of heat and mass transfer in porous media was constructed to study the thermal desorption characteristics of adsorbate in spent activated carbon. In addition, a simplified evaporation rate formula is proposed in this paper, and the agreement between the simulation and the experimental results is verified. When the surroundingtemperature is less than 673.15 K, increasing the surrounding temperature has a significant effect on reducing the thermal desorption time of the adsorbate. Using activated carbon with a thermal conductivity of 5 W / m ∙ K can save about 40% of energy consumption. The model established in this paper can accurately simulate the thermal desorption of adsorbate in activated carbon, which is helpful to deeply understand the mechanism of activated carbon thermal regeneration. In engineering practice, the model can provide technical support for optimizing the thermal regeneration process of spent activated carbon and reducing energy consumption in a targeted manner.

Bael fruit shell–derived activated carbon adsorbent: effect of surface charge of activated carbon and type of pollutants for improved adsorption capacity

Bael fruit shell–derived activated carbon adsorbent: effect of surface charge of activated carbon and type of pollutants for improved adsorption capacity

Methylene Blue Removal Using Activated Carbon Adsorbent from Jengkol Peel: Kinetic and Mass Transfer Studies

Methylene Blue Removal Using Activated Carbon Adsorbent from Jengkol Peel: Kinetic and Mass Transfer Studies

Efficient Removal of Organic Matter from Biotreated Coking Wastewater by Coagulation Combined with Sludge-Based Activated Carbon Adsorption

Coagulation–adsorption can be effective in the removal of the organic matters remaining in biotreated coking wastewater (BTCW), and cheap and efficient adsorbents benefit the widespread application of this technology. In this study, a sludge-based activated carbon (SAC) was prepared using zinc chloride to activate sludge pyrolysis carbon for the treatment of BTCW with coagulation as the pretreatment process. According to Brunauer-Emmett-Teller (BET) and the scanning electron microscope (SEM) analysis, the SAC exhibited a specific surface area of 710.175 m2/g and well-developed pore structure. The removal characteristics of the organic matter in BTCW were systematically studied. The results show that 76.79% of the COD in the BTCW was removed by coagulation combined with SAC adsorption, and the effluent COD was below the discharge limit (80 mg/L) (GB16171-2012), with the optimal dosages of polyaluminum chloride and SAC being 150 mg/L and 4 g/L, respectively. Compared with a commercial powdered activated carbon (PAC) (48.26%), the SAC achieved a similar COD removal efficiency (47.74%) at a higher adsorption speed. The removal efficiencies of the hydrophobic components (77.27%) and fluorescent components by SAC adsorption were higher than those by PAC adsorption. The SAC also had an excellent removal effect on complex organic compounds and colored substances in the BTCW, as revealed by UV-vis spectra analyses.

Removal of Uremic Solutes from Dialysate by Activated Carbon.

Adsorption of uremic solutes to activated carbon provides a potential means to limit dialysate volumes required for new dialysis systems. The ability of activated carbon to take up uremic solutes has, however, not been adequately assessed. Graded volumes of waste dialysate collected from clinical hemodialysis treatments were passed through activated carbon blocks. Metabolomic analysis assessed the adsorption by activated carbon of a wide range of uremic solutes. Additional experiments tested the ability of the activated carbon to increase the clearance of selected solutes at low dialysate flow rates. Activated carbon initially adsorbed the majority, but not all, of 264 uremic solutes examined. Solute adsorption fell, however, as increasing volumes of dialysate were processed. Moreover, activated carbon added some uremic solutes to the dialysate, including methylguanidine. Activated carbon was particularly effective in adsorbing uremic solutes that bind to plasma proteins. In vitro dialysis experiments showed that introduction of activated carbon into the dialysate stream increased the clearance of the protein-bound solutes indoxyl sulfate and p-cresol sulfate by 77%±12% (mean±SD) and 73%±12%, respectively, at a dialysate flow rate of 200 ml/min, but had a much lesser effect on the clearance of the unbound solute phenylacetylglutamine. Activated carbon adsorbs many but not all uremic solutes. Introduction of activated carbon into the dialysate stream increased the clearance of those solutes that it does adsorb.

The comparison of adsorption characteristics of CO2/H2O and N2/H2O on activated carbon, activated alumina, zeolite 3A and 13X

• The adsorption of CO 2 /H 2 O and N 2 /H 2 O is compared by dynamic experiments. • Effects of flow rate, temperature and steam mole fraction are analyzed. • Activated carbon and activated alumina, zeolite 3A and 13X are compared. • CO 2 /H 2 O adsorption selectivity ranked as: 3A> 13X> alumina> activated carbon. • The thermal wave peak of N 2 /H 2 O is decayed and delayed comparing to CO 2 /H 2 O. The breakthrough curves and adsorption amounts of N 2 /H 2 O and CO 2 /H 2 O on activated carbon, activated alumina, zeolite 3A and zeolite 13X were studied experimentally. The experiments were conducted to investigate the effects of inlet steam mole fraction, flow rate and temperature. The effect of steam mole fraction on the thermal effect of adsorption was also studied. The results show that the breakthrough time and adsorption saturation time of adsorbates are decreased with increasing flow rate and temperature. When steam mole fraction increases, the breakthrough time and adsorption saturation time of adsorbates on zeolite 3A and 13X are decreased, while these of other adsorbents are increased. As steam mole fraction increases from 1% to 2%, the adsorption amount of CO 2 decreases by 2.2–45.4% and that of N 2 decreases by 12.2–19.1%. Zeolite 3A shows a good adsorption selectivity for CO 2 /H 2 O and N 2 /H 2 O. The thermal wave peak time of N 2 /H 2 O is 0.71–8.31 min longer than that of CO 2 /H 2 O, while the peak temperature of N 2 /H 2 O is 2–16 °C smaller than that of CO 2 /H 2 O. As steam mole fraction increases from 1% to 2%, the peak temperature of CO 2 /H 2 O increases by 1.63–11.55 °C and N 2 /H 2 O increases by 2.17–18.17 °C. The thermal wave peak of N 2 /H 2 O and CO 2 /H 2 O on activated carbon adsorption bed is the smallest among these four adsorbents.