Structured carbon foam derived from waste biomass: application to endocrine disruptor adsorption
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In this paper, a novel structured carbon foam has been prepared from argan nut shell (ANS) was developed and applied in bisphenol A (BPA) removal from water. The results showed that the prepared carbon foam remove 93% of BPA (60 mg/L). The BPA equilibrium data obeyed the Liu isotherm, displaying a maximum uptake capacity of 323.0 mg/g at 20 °C. The calculated free enthalpy change (∆H° = − 4.8 kJ/mol) indicated the existence of physical adsorption between BPA and carbon foam. Avrami kinetic model was able to explain the experimental results. From the regeneration tests, we conclude that the prepared carbon foam has a good potential to be used as an economic and efficient adsorbent for BPA removal from contaminated water. Based on these results and the fact that the developed structured carbon foam is very easy to separate from treated water, it can serve as an interesting material for real water treatment applications.
KeywordsEndocrine disruptor Bisphenol A Adsorption Carbon foam Regeneration Water purification
Endocrine-disrupting substances are chemical compounds or mixtures of compounds that alter endocrine system functions of organisms and, as a result, cause adverse effects on the health of living organisms, their offspring, or part of the population (Vos et al. 2000; López-Ramón et al. 2019). Endocrine disruptors (EDs) can, for example, inhibit or stimulate the secretion of hormones, interfere with the hormone receptor (agonist or antagonist effect), and modify the metabolism of these enzymes (Mills and Chichester 2005). The disturbances are caused after prolonged exposure to these compounds even at low concentrations. A large number of pollutants have been identified as EDs. These comprise organic compounds such as alkyphenols, chlorinated pesticides, herbicides, and drug compounds (Singleton and Khan 2003). The type of disturbance caused by these compounds is related to the structural similarity between the compound and the hormone.
Despite the existing water treatment, certain pharmaceuticals and EDs end up to the aquatic environment. Numerous endocrine-disrupting substances or metabolites from their biodegradation are found in the effluents and sewage sludge from wastewater treatment plants (Auriol et al. 2006; Tan et al. 2007). Among these endocrine-disrupting substances found, nonylphenol (NP), bisphenol A (BPA), and triclosan (TCS) are the absolute most regularly identified organic xenobiotics in wastewater treatment areas (Kolpin et al. 2002; Boyd et al. 2003, 2004). These substances are distinguished by the existence of a phenolic group in their chemical structure. The OH-group can improve the adsorption of these substances on materials that contain functional surface OH-groups (Zbair et al., 2018a, b, d).
This research focuses on the elimination of a phenolic EDs, bisphenol A (BPA), from aqueous solution. BPA is acutely toxic to the aquatic organisms from 1 to 10 mg/L concentration range for both fresh water and marine environment (Kang et al. 2006; Aravind et al. 2019). BPA has been detected in several types of water at different concentrations. For example, concentration of 17.2 mg/L has been observed in in hazardous waste landfill leachate (Yamamoto et al. 2001), 12 μg/L in stream water (Liu et al. 2009a), and 3.5–59.8 ng/L in drinking water (Santhi et al. 2012). The solubility of BPA in water is 300 mg/L in normal conditions (Shareef et al. 2006).
Numerous materials such as activated carbons (Asada et al. 2004; Liu et al. 2009b; Zbair et al. 2018a), CoFe2O4/activated carbon (Li et al. 2014), hybrid material (phenyl-mesoporous silica) (Bhatnagar and Anastopoulos 2017), graphene (Wang et al. 2017), and Fe/AC (Arampatzidou et al. 2018) have been propositioned for removal of BPA. Porous adsorbents are promising materials for adsorption of organic pollutants, because of good physiochemical stability, large specific surface area, and well-developed porosity with high pore volume (Agrawal et al. 2016; Jana et al. 2016; Zbair et al. 2018b). It is known that porous carbon materials are thermally stable, chemically inert, and low-cost and display high adsorption capacity for organic pollutants. The earlier research shows that carbon foam is efficient in the removal of copper, arsenic (V), and lead (Burke et al. 2013; Lee et al. 2015; Agrawal et al. 2016). Therefore, in the current work, we decided to apply carbon foam, prepared from waste biomass (argan nut shell) in BPA removal. The aim was to develop a material that can be easily separated from the treated water and increase surface contact solid/liquid. The chemical composition and surface characteristics of the carbon foam were examined. Batch adsorption experiments were conducted to evaluate the BPA uptake, adsorption mechanism, and possibility to regenerate the carbon foam.
Preparation of carbon foam
The argan nut shells (ANS) were collected from the Southwestern Morocco (Taroudant city). First, the raw material was washed with tap water and distilled water. The washed ANS was oven dried at 105 °C for 12 h. After this, the raw material was ground and sieved to a particle size of 200 μm. Then the ANS was pyrolyzed at 600 °C for 2 h under a nitrogen flow (100 mL/min). The employed technique, to produce carbon foam (CF), initially involves the preparation of aqueous slurry from a mixture of pyrolyzed ANS, dispersing agent, structuring agent, and water. Sucrose was used as the both—as the dispersant and the pore promoter (Pradhan and Bhargava 2004). At first, 2.1 g of sucrose was mixed with 10 mL of double distilled water for 1 h. Then, 3 g of pyrolyzed ANS was added step by step to sucrose gel under stirring at 500 rpm until reaching a concentration of 70 wt-% relative to the pyrolyzed ANS. The resulting slurry was stirred for 24 h for homogenization. Then, the slurry was poured into a cylindrical silicone mold and dried firstly at 80 °C for 1 h and then at 105 °C for 24 h to eliminate the surface water and initialize the sucrose consolidation. The molded sample was pyrolyzed under a nitrogen flow of 50 mL/min from the room temperature to 900 °C according to the following cycle: first heating was done with a rate of 0.5 °C/min from room temperature to 500 °C to eliminate sucrose totally (Das 2001). Then the sample was kept at a constant temperature for 1 h followed by heating with a rate of 5 °C/min up to 900 °C, where the temperature was kept constant for 5 h. Finally, cooling was done with the rate of 2 °C/min back to the room temperature.
The N2 adsorption-desorption isotherm of carbon foam was examined using a Micromeritics ASAP 2020 instrument (Micrometrics, Norcross, GA, USA) to determine surface area, pore volume, and pore size distribution. The morphological characteristics of carbon foam were analyzed using a field emission scanning electron microscope ZEISS ULTRA PLUS (ZEISS, Oberkochen, Germany) equipped with an energy-dispersive X-ray spectrometer (EDS) at an accelerating voltage of 15.0 kV. The acidity and basicity measurements of carbon foam were done using temperature-programmed desorption (TPD) of NH3 and CO2 with an AutoChem II 2920 (Micromeritics, Norcross, GA, USA) device. Prior to NH3-TPD analysis, the carbon foam (about 100 mg) was pre-treated with helium (He) at 700 °C for 30 min. Then, carbon foam was cooled to 100 °C followed by adsorption of 15% NH3 in He (at 100 °C) for 60 min and flushing with He for 30 min, in order to eliminate the physiosorbed NH3. The NH3 desorption was carried out from 100 to 700 °C including 10-min constant temperature dwell at the final temperature. The flow rate used was 50 cm3/min and temperature rise was 10 °C/min during the NH3-TPD analysis. After NH3-TPD analysis, the carbon foam was cooled to room temperature prior CO2-TPD. The sample was pre-treated with H2 (30 cm3/min) from room temperature to 500 °C with 10 °C/min for 30 min, then cooled to 50 °C and flushed with Ar with 50 cm3/min for 5 min. The adsorption of 5%CO2/Ar (50 cm3/min) was completed at 50 °C for 60 min then the physiosorbed CO2 was flushed with Ar (50 cm3/min) for 60 min. The thermodesorption of CO2 was carried out under Ar flow (50 cm3/min) from 50 to 700 °C, where temperature was left constant during 10 min. The concentrations of desorbed NH3 and CO2 were analyzed by a thermal conductivity detector (TCD) and the total acidity and basicity of carbon foam were determined by integration of the peak area between 100–700 °C and 40–700 °C, respectively. The PZC (point of zero charge) of carbon foam was determined using the pH drift method. The carbon foam was mixed with 60 mL of 0.01 M NaCl solution. The pH of the starting solutions (2.0 to 12.0) was adjusted using HCl and NaOH. After 24 h, the final pH was measured. The functional surface groups of carbon foam were determined using a Fourier transformed infrared spectroscopy (FTIR–8400S, Shimadzu, Japan).
Batch adsorption experiments
A stock solution of BPA (100 ppm) was prepared by dissolving the BPA (Alfa Aesar, 97%) in distillated water. Then, the BPA solutions of desired concentrations were prepared by successive dilutions from their respective stock solution. The effect of solution pH on BPA (15 mg/L) removal was examined at various initial pH values (2.0–12.0) for 2 h, the solutions were adjusted by using 1 M HCl or 1 M NaOH. The adsorption kinetics was studied using two distinctive starting BPA concentrations, 15.0 mg/L and 60.0 mg/L. The adsorption isotherm was examined for BPA concentrations of 10–100 mg/L at different temperatures (20 °C, 30 °C, and 40 °C). After pre-decided times, the samples were taken to assess the remaining BPA concentration in the solution. The BPA concentration was determined using a UV-visible spectrophotometry (SHIMADZU 2450-UV/VIS, JAPAN) at the maximum absorbance wavelength (274 nm). All batch adsorption experiments of BPA were carried out with a mass of 50 mg of carbon foam in 200 mL of BPA solution.
The BPA-laden carbon foam was regenerated using 30 mL of ethanol (Sigma-Aldrich, 99.8%). Ethanol and the used carbon foam were stirred at room temperature for 4 h. At that point, the carbon foam was recovered by filtration and dried at 80 °C. Adsorption and regeneration cycles were repeated 5 times.
Kinetic and equilibrium adsorption parameters
K2 (g/mg−1 min−1)
Results and discussion
Characterization of carbon foam
Isotherm parameters for the adsorption of BPA onto the carbon foam
KF (mg/g) (L/mg)1/n
According to Table 2 and Fig. 5, it is possible to verify that the adsorption capacity of the carbon foam decreases with increasing temperature. The variation of temperature from 20 to 40 °C weakened the adsorption capacity of the carbon foam. The maximum adsorption (QLi) of the carbon foam for BPA was the following: 323.0 mg/g at 20 °C, less than 287.0 mg/g at 30 °C and less than 211.0 mg/g at 40 °C. Moreover, the values of the Liu equilibrium constant (Kg) decreased with increasing temperature indicating exothermic adsorption.
Maximum adsorption capacities of BPA by different adsorbents
Mesoporous carbon (soft templated)
0.01 mg/100 mL
(Libbrecht et al. 2015)
0.05 g/20 mL
(Bele et al. 2016)
0.05 g/20 mL
(Bele et al. 2016)
Carbon nanotubes (CNTs)
(Li et al. 2015)
Montmorillonite modified with DDDMA
(0.2–0.3 g)/40 mL
(Park et al. 2014)
(Tsai et al. 2006a)
Coconut-based activated carbon
(Tsai et al. 2006b)
0.01 g/100 mL
(Xu et al. 2012)
20 mg/100 mL
(Qin et al. 2015)
0.01 g/200 mL
(Zbair et al. 2018b)
Modified biomass–based carbon
(Juhola et al. 2018)
Commercial activated carbon
0.01 g/100 mL
(Libbrecht et al. 2015)
0.05 g/200 mL
Effect of pH on BPA adsorption
Proposed adsorption mechanism
Thermodynamic parameters of the BPA adsorption
ΔS (J/mol K)
In this study, the argan nut shell (ANS) was successfully used as a raw material for producing a structured carbon foam. The prepared foam structure presented a well-developed porous structure with different pore sizes and it had a specific surface area of 435 m2/g. The carbon foam showed 93% removal of BPA in the used experimental conditions. It was also found that the maximum adsorption capacity of BPA on the carbon foam according to the Liu isotherm (QLiu) was 323.0 mg/g at 20 °C. Comparison of this value to the values of the other adsorbents demonstrates a high adsorption capacity of the developed carbon foam. The BPA adsorption kinetics on the carbon foam was best explained by the Avrami fractional model. Determination of thermodynamic parameters showed the adsorption to be exothermic and to take place via physisorption. To summarize, the structured carbon foam prepared from the argan nut shell is an excellent material for the BPA removal due to the following: (1) its easy preparation, (2) low price, (3) re-usability,(4) high adsorption capacity, and (4) facile separation from water, which makes it practical for real water purification applications.
Open access funding provided by University of Oulu including Oulu University Hospital.
The research leading to these results has received funding from the Erasmus+ Global programme between the University of Oulu, Finland, and the University of Chouaïb Doukkali in Morocco. A part of the research was carried out during the NO-WASTE project that received funding from the European Union Seventh Framework Programme (FP / 2007-2013) under the grant agreement no. [PIRSES-GA-2012-317714]. The FESEM analysis was carried out at the Center of Microscopy and Nanotechnology, University of Oulu, Finland.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest
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