Adsorption of Volatile Organic Compounds Onto Biomass-Derived Activated Carbons: Experimental Measurement and Comparison

According to Environmental Protection Agency, volatile organic compounds (VOCs) are defined as “any compound of carbon, excluding carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates, and ammonium carbonate, which participates in atmospheric photochemical reactions” [1]. These organic pollutants possess high vapor pressure at room temperature and directly impact human health and the environment.

With the persistent increase of VOCs and their harmful impact on human health and the ecological environment, stringent emission regulation of the Goteborg protocol was proposed in 1999 [2,3]. Therefore, developing an effective VOCs elimination technique is of great significance and urgent. Consequently, extensive efforts have been made in recent years to develop efficient VOC removal techniques, and many removal techniques have emerged. It can be commonly divided into destruction and recovery methods based on whether the VOCs can be recovered [1]. The recovery methods include adsorption, absorption, membrane separation, and condensation, whereas the destruction techniques include plasma catalysis, incineration, ozone catalytic oxidation, biological degradation, photocatalytic oxidation, etc. Among all these techniques, adsorption is considered as one of the most promising methods to treat VOCs as a highly efficient and economically somehow viable control strategy since it has the potential to recover and reuse both adsorbent and adsorbate. The characteristics of VOCs removal techniques are summarized in Table 1.

The solid porous adsorbent is a crucial element for VOC removal using the adsorption process [4]. Adsorbents such as activated carbon (AC), AC fiber, metal-organic frameworks, and zeolite have been widely studied materials [4,11–15]. Among them, AC is known to have the most potential as a low-cost, high-efficiency, acid/base- and thermo-stability adsorbent for VOCs removal [4]. Literature studies show that VOCs adsorption capacity onto different ACs ranges from 15.9 mg g^–1 to several hundreds of milligrams per gram [4,11–13,16]. The key factors that influence the VOCs adsorption performance onto ACs are physicochemical properties such as large specific surface area, rich porous structure, pore volume, surface chemical functional groups, etc., properties of adsorbates (VOCs) such as molecular size and the polarity, as well as the adsorption conditions such as temperature, moisture, etc.

However, finding the optimal porous activated carbon in terms of cost and its physicochemical properties is crucial for the commercial application of the adsorption technique. Therefore, in this study, two activated carbons (M-AC (mangrove wood derived activated carbon) and WPT-AC (waste palm truck derived activated carbon)) synthesized from waste mangrove wood, and palm trunk biomass precursors are chosen, and four types of VOCs (ethanol, dichloromethane, acetone, and ethyl acetate) adsorption onto them are experimentally investigated at three different temperatures employing the inverse gas chromatography (IGC) technique. The zero uptake adsorption enthalpy and specific entropy of the adsorption are also theoretically computed for all the samples. Here, the first one accounts for the heat generated from the exothermic physical adsorption, whereas the second term specifies the driving force to provide a connection amid the equilibrium and non-equilibrium conditions of an adsorption system [17,18]. After that, these data are compared with the obtained data under identical conditions for a typical commercial activated carbon Maxsorb III made from petroleum coke.

In this study, three types of activated carbons, Maxsorb III, WPT-AC, and M-AC, were used as adsorbent material. Among these samples, Maxsorb III was commercially purchased from the Kansai Coke and Chemical. Co. Ltd, Japan; and M-AC and WPT-AC were synthesized from two biomass sources, namely, mangrove wood and waste plum trunk, respectively. The dried biomasses with their elemental analysis prior to the activation are given in Fig. 1 and Table 2, respectively. At first, the biomass sources were washed, dried, and crushed into smaller pieces. After that, the biomasses were carbonized and then activated with potassium hydro-oxide. After the activation, samples were rewashed again with hydrochloric acid and deionized water to get the final product. The detailed synthesis procedures can be found in the literature provided by Pal [19].

The porous properties of these samples have been investigated in previous studies utilizing the nitrogen adsorption/desorption experiment at 77 K using the volumetric adsorption equipment “3Flex^™ Surface Characterization Analyzer” developed by Micromeritics Instrument Corp., USA. The porous properties of the studied samples are summarized in Table 3. The activated carbons selected in this study have high Brunauer, Emmett, and Teller (BET) surface areas in the range of 3000 m² g⁻¹ with an average pore size residing in the microporous region. The elemental analysis data of the studied activated carbons are furnished in Table 4.

VOCs come from both anthropogenic sources such as exploitation, storage, refining, transport, and usage of fossil fuels and natural sources such as terrestrial and ocean. The emission of VOCs has been increasing intensely owing to the development of industries [4]. VOCs are more than 300 types of carbon-based chemicals, and all generally have a low boiling point, high vapor pressure, and strong reactivity, especially concerning photochemical reactions [4]. It is harmful to humans and the environment. As for human health, alcohols, halogenated VOCs, ketones, and aromatic compounds are toxic and carcinogenic [23]. This study focuses on the henry region adsorption of four different VOCs, namely, ethyl acetate, dichloromethane, acetone, and ethanol. The physiochemical properties of the VOC adsorbates are tabulated in Table 5.

This study includes two major parts: selection of biomass-derived high-grade activated carbons and then using those carbons as adsorbents materials for the studied VOC adsorption. Additionally, the assorted VOC adsorptions onto commercial activated carbon, Maxsorb III, were obtained under similar conditions for comparison purposes. After obtaining the (infinite dilution) experimental adsorption data, the Henry constant of the studied pairs were obtained using Henry’s adsorption isotherm model. Using the values of Henry’s constant at different temperatures, the minimum regeneration temperatures for the activated carbon/VOC pairs were calculated. Additionally, the thermodynamic quantities, namely, adsorption enthalpy and specific entropy at the Henry’s law region, were analyzed. The whole procedure of the workflow is given in Fig. 2.

Adsorbate uptakes onto activated carbons in Henry’s law region were measured by the IGC technique using the inverse gas chromatography-surface energy analyzer (IGC-SEA) (Surface Measurement System Ltd, UK). The IGC-SEA unit is equipped with two sample column holders along with 12 slots for solute reservoirs where the adsorbates are kept in a liquid phase. In this study, we have used four slots for four different VOCs. The experimental setup also includes an inbuilt oven to maintain its temperature (from 20 °C to 150 °C). The IGC-SEA also features a flame ionization detector (FID), mass flow controllers, and a processing unit for controlling and data analysis.

Figure 3 depicts the schematic diagram of the experimental unit. During the experiment, activated carbon samples were placed inside the 3 mm-sized sterilized glass columns. Then, controlled amount of one of the adsorbates with different concentrations was purged at a constant temperature inside the column with the help of carrier gas. In this case, the carrier gas was helium. After adsorption, the adsorbed adsorbate was swept away (desorbed) by the same carrier gas and detected at the outlet using the FID. Using the analysis software provided by the Surface Measurement System Ltd., UK, the respective chromatographs were constructed. The chromatographic data can be invoked to obtain the retention time and retention volume for the respective adsorbate. This unique characteristic of inverse gas chromatography enables it to successfully determine the experimental adsorption uptake in Henry’s region.

Here, n_ads is the mole number of the purged adsorbate, R is the universal gas constant, F is the carrier gas flowrate, and H_Peak and A stand for the peak height and area of the recorded chromatographs.

Here, j is the James–Martin correction factor, and t_R and t₀ stand for the retention time and the dead time, respectively [33,34].

Here, W is the adsorbate loading in kg kg⁻¹, and P is the partial pressure of the adsorbate gas.

Equation (6) is used to calculate the adsorbed phase-specific entropy of ACs/VOCs pairs.

Four VOCs, namely, dichloromethane, acetone, ethyl acetate, and ethanol adsorption, onto three different activated carbons have been experimentally measured in the Henry region at three different temperatures of 30 °C, 50 °C, and 70 °C. For comparing the adsorption capacities, the isotherms obtained at 30 °C are given in Fig. 4. The determination of chromatographs at infinite dilution are quite accurate in IGC-SEA with a deviation of <0.8%. However, the system is also equipped with heaters and mass flowrate controllers which are associated with their respective errors. Therefore, the experimental data are given with a ±5% error bars. The Henry constant values for all the pairs are furnished in Table 6. From the isotherms, it can be concluded that for all the selected VOCs, the Maxsorb III type activated carbons exhibit the highest amount of VOCs loadings except for ethanol’s case. M-AC type activated carbon shows higher ethanol adsorption compared with the other two activated carbons. Additionally, in the case of acetone adsorption, the adsorption uptake onto M-AC has negligible difference with Maxsorb III. Furthermore, in the case of dichloromethane and ethyl acetate, M-AC exhibits comparable VOCs uptakes with Maxsorb III. Nevertheless, in all cases, WPT-AC shows the lowest amount of adsorbate capturing capacity in Henry’s law region.

It is evident from Table 6 that the values of Henry’s constant decrease with increasing temperature, which is expected in the case of physical adsorption. In Fig. 5, Henry’s constants as a function of adsorption temperature are plotted. It can be seen from Fig. 5 that for all the studied pairs, Henry’s constants decreased exponentially with increasing temperatures. Using the exponential fitting equations, some useful information regarding the minimum temperature required for complete regeneration (Henry’s constant is assumed to be 0.001 kg kg⁻¹ kPa⁻¹) can be extracted. However, for practical application, the required regeneration can be obtained with Henry’s constant value of 0.01 kg kg⁻¹ kPa⁻¹. In both cases, the minimum desorption temperatures are calculated and are furnished in Table 7.

From Table 7, it can be seen that for dichloromethane and acetone, M-AC adsorbent would require the lowest regeneration temperature whereas, for ethyl acetate and ethanol, it is WPT-AC. On the other hand, in the case of dichloromethane and acetone, WPT-AC would require the highest amount of heat energy, and for ethanol and ethyl acetate, M-AC would consume the highest energy regarding regeneration. For all the adsorbates, Maxsorb III shows moderate requirements of regeneration temperatures. These regeneration temperatures depend on how the adsorbate molecules interact with the respective adsorbents’ surfaces.

Using the isotherms data, the zero uptake adsorption enthalpy and specific entropy are computed employing Eqs. (4) and (6), respectively. The calculated data are furnished in Table 8. It can be seen that the enthalpy is lowest for WPT-AC/dichloromethane and acetone pairs, whereas the highest is observed for ethyl acetate adsorption onto WPT-AC. It is also noticed that among the three adsorbents/adsorbates pairs, Maxsorb III/ethanol shows the highest adsorption enthalpy and specific entropy. Generally, a larger pore volume for highly porous activated carbons results in more spaces for a single adsorbed molecule. Consequently, reduction in specific entropy is a result of the decreasing interaction among the adsorbed molecules [28].

Two activated carbons, namely, M-AC and WPT-AC made from mangrove wood and waste palm trunk biomass precursors, have been selected as potential adsorbents for VOCs adsorption. The four types of VOCs (ethanol, dichloromethane, acetone, and ethyl acetate) adsorption onto them are measured experimentally in the Henry region at three different temperatures of 30 °C, 50 °C, and 70 °C using the inverse gas chromatography technique. The obtained results are also compared with commercial activated carbon Maxsorb III tested under the same experimental conditions. The zero uptake adsorption enthalpy and specific entropy of the adsorption are theoretically calculated for all the samples. Results show that both activated carbons M-AC and WPT-ACs possess high adsorption performance of selected VOCs adsorption and are comparable with commercial activated carbon. M-AC type activated carbon shows higher ethanol adsorption compared with the other two activated carbons. Additionally, in the case of acetone adsorption, the adsorption uptake onto M-AC has negligible difference with Maxsorb III. Furthermore, in the case of dichloromethane and ethyl acetate, M-AC exhibits comparable VOCs uptakes with Maxsorb III. Nevertheless, in all cases, WPT-AC shows the lowest amount of adsorbate capturing capacity in Henry’s law region. Finally, it can be concluded that the cost-effective biomass-derived activated carbons can be very good alternatives to the high-priced Maxsorb III for adsorption-based VOCs removal systems.

There are no conflicts of interest. This article does not include research in which human participants were involved. Informed consent not applicable. This article does not include any research in which animal participants were involved.

The datasets generated and supporting the findings of this article are obtainable from the corresponding author upon reasonable request.