A renewable, sustainable and low-cost adsorbent for ibuprofen removal

Ibuprofen (IBP) belongs to the class of non-steroidal anti-inflammatory drugs (NSAIDS). This class of drugs possesses endocrine-disrupting chemicals and is generally referred to as emerging contaminants (Kanakaraju et al. 2018). IBP is safe in its prescribed dosage for effective relief of pains but when larger dosages are administered, it could inflict adverse effects on humans and wildlife at large. Table 1 shows some endocrine-disrupting effects of Ibuprofen. A combination of IBP and diphenhydramine provides the active ingredient of household pain relief drugs with the brand names Advil PM and Motrin PM. Due to the diverse functionality of NSAIDs, they are administered to a wide range of patients suffering from diverse ailments, thereby increasing their presence in the environment owing to the fact that they are not readily biodegraded (Gupta et al. 2013; Qamruzzaman 2015; Ghadir et al. 2020). Reports have shown that the active compounds of endocrine-disrupting chemicals are usually persistent in the environment (Kümmerer 2009; Cherniwchan 2012; Bello et al. 2019) and are introduced through channels such as inappropriate discharge of industrial effluents into the environment and dumping of unused drugs into waste bins and dump sites. Several of these pollutants are usually of organic and inorganic origins with the organic pollutants ranging from Bisphenol A and its derivatives to metals which possess inherent toxicity (Panizza 2000; Mohan et al. 2014; Onwordi et al. 2019). IBP has been specifically reported to have concentrations ranging from 0.018 up to 2.110 μg/L in several water bodies around the world (Dziadkowiec et al. 2017).

Several techniques have been employed in the removal of these endocrine disruptors which are persistent in the environment. These techniques include: membrane technology (Ciardelli et al. 2000; Greenlee et al. 2009), biodegradation, aeration (Panswed & Wongehaisuwan 1986), chlorination (Chamarro 2001), and chemical precipitation. A major setback associated with these techniques is their high cost as well as the difficulties associated with their practicability. Adsorption (Ojedokun & Bello 2016) is a promising means of sequestering unwanted contaminants from the environment with a high level of effectiveness and accuracy (Ntwampe & Bunt 2016; Othman et al. 2017). It is not difficult to execute and quite cost-effective. Unlike activated carbon which can be prepared from readily available agricultural wastes material, remediating the environment through the commercially available activated carbon is usually very expensive (Bello et al. 2020). Therefore, activated carbon prepared from agricultural wastes is usually used instead of the commercially available activated carbon since the precursors are readily available as unwanted materials. By this, unwanted waste materials littering the environment are mopped up, thereby making the environment habitable for man and animals. Different agricultural wastes that have been used include: bean husks, banana stalk, periwinkle shell (Bello et al. 2008), Moringa oleifera seed pod (Maina et al. 2016) mango leaf (Bello et al. 2014)), coconut shell (Bello et al. 2012), rice hull (Mukoko et al. 2015), tea wastes (Ahmaruzzamam & Laxmi 2010), potato peels (Arampatzidou & Deliyanni 2018), Parthenium hysterophorus (Mondal et al. 2016), and palm shell (Pamidimukkala & Soni 2018).

Kola nut is the fruit obtained from the kola tree. It belongs to the family of Malvaceae, genus of Cola; there are two common species: Cola acuminata and Cola nitida. It is an evergreen plant with the nut containing steroids, volatile oils, glycosides, saponins, alkaloids, and tannins (Kanoma et al. 2014). It is a caffeine-containing plant and a cash crop for rural farmers. The husk of the fruit is obtained after the fruit has been broken (Adebayo & Oladele 2012). Kola nut farmers have interest only in the kola fruit, thereby dumping the husk at harvest sites and sometimes burning it. In cases where the husks are dumped on refuse sites, they usually serve as breeding sites for microorganisms due to their high potassium content (Agamuthu 2009; Fabunmi 2018). These have constituted a large amount of undesirable waste and waste products, posing imminent harm to the ecological health of the ecosystem. Harnessing this waste material in remediating contaminated water bodies could be a promising means of achieving a better treatment method (Schwarzenbach et al. 2010). Apart from utilizing kola nut husk in soap production (Osagie & Enyi 2015), poultry feeds, biogas production (Ezekoye et al. 2011), and production of microbial enzymes, a high value can be placed on it by using it for adsorbent preparation (Agarry & Ogunleye 2014). This is an effective way of preparing sustainable adsorbents from waste materials.

Adsorption can be carried out either as batch adsorption or a column adsorption analysis. Experiments that are carried out in the laboratory for the uptake of a minute quantity of adsorbate by adsorbent are referred to as batch adsorption studies. They are usually carried out to create a scheme to follow in large cases or industrial cases of production (Bello et al. 2017a, 2017b). Column analysis is usually conducted by administering a predetermined amount of adsorbent, thereby ensuring that the expected concentration fraction between the adsorbent and adsorbate required for adsorption is achieved (Fang et al. 2017).

To the best of our knowledge, no work has been reported on the use of kola nut husk activated carbon for the sorption of IBP drug from aqueous media. This study is aimed at employing kola nut husk, an agricultural waste, as a precursor material for preparing activated carbon that can efficiently trap IBP, an endocrine-disrupting compound, from aqueous environment.

Kola nut husks were obtained from a farm settlement in Ogbomoso, Oyo State, Nigeria. The kola nut husks were washed with distilled water and air-dried for a few days. The essence of the washing is to remove sands and other interfering dirt. The husks were obtained after the kola nut fruit was removed. The sample was pulverized with a mortar and pestle and then sieved through a 10–120 μm sieve mesh. The product obtained was kept in an airtight container for further use. Thirty grams of the dried kola nut husk was weighed using a weighing balance (Ohaus: Pioneer PA214) and transferred into a crucible. The modification was done by reacting with 1,000 mL of 0.3 M orthophosphoric acid with 50 g of 20 μm coconut husk. The reaction was carried out on an electric hotplate till a slur was formed. After the slurry formation, it was carbonized in a muffle furnace at 500 °C for 3 hours until a char was formed. The char formed was then washed to neutral pH with distilled water after which it was oven-dried to constant weight at 105 °C to remove the moisture content and other volatile adhering material. The sample formed was then stored for further use.

IBP crystals obtained from the chemical store were used as the adsorbate. The physicochemical properties of the adsorbate were investigated to ensure adequate preparation of the adsorbate. One gram of IBP crystals was dissolved in 1,000 mL of ethanol to prepare the stock solution. Working concentrations were further prepared from the stock concentration.

Raw and activated kola nut husks (KNHR and KNHA, respectively) were characterized using physicochemical techniques such as Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM) (Qu 2009), Boehm's titration (Boehm 1966), electron dispersive X-ray (EDX) spectroscopy, and pH of point of zero charge (pH_PZC). FTIR spectra were recorded from 400 to 4,000 cm⁻¹. This was used to identify the different functional groups present in the adsorbents (Bello et al. 2019). The SEM technique was also employed to elucidate the morphological properties of the adsorbents (Ahmad & Alrozi 2011) while the EDX spectroscopy was used to detect the elements that are present in the adsorbent (Ahmad et al. 2015). The point at which the surface of the adsorbent possesses no charge, that is the point when the surface is completely neutral, was identified by the pH_PZC method (Farahani et al. 2011: Mestre et al. 2014).

The effects of contact time and IBP concentration were investigated. IBP solutions (100 mL) with initial concentration of 10, 20, 30, 40 and 50 mg/L were placed into 200 mL conical flasks and arranged accordingly inside the shaker; 0.1 g of KNHA was measured into each conical flask and the flasks were covered with stoppers. The shaker was operated at a speed of 120 rpm at a temperature of 303 K. The effects of solution temperature on the uptake of IBP were studied by adjusting the shaker's temperature to 313 and 323 K respectively (Alkan & Dogan 2003; Hameed et al. 2007; Maria et al. 2019).

Desorption and regeneration studies of KNHA were carried out. This was done by subjecting the adsorbent to water, acid and base treatments. The acids employed were CH₃COOH and HCl, while the bases were NaOH and CH₃COH. The most effective out of these eluents was used for the regeneration experiment.

FTIR analyses were carried out for both the raw and activated samples. They were carried out within the range 400–4,000 cm⁻¹. It was observed that there were appearances and disappearances of some peaks in the spectra. These peaks are indicative of the different functional groups present in the sample (Guedidi et al. 2014). The changes were due to the activation of the raw sample (Bekci et al. 2009; Bello et al. 2015). Table 2 gives comprehensive information about the functional groups while Figure 1(a) and 1(b) are the FTIR spectra for raw and activated kola nut husk samples.

EDX spectroscopy is a technique that serves as a basis for elemental comparison between the activated sample and the raw sample. It was observed that there was an increased amount of carbon in the activated sample (Figure 2(a) and 2(b)). The results obtained from EDX analysis are presented in Table 3. The amount of oxygen in the activated sample is lower than that of the raw sample and some elements present in the raw sample were observed to be absent in the activated sample. These differences observed in the activated and raw samples are largely as a result of the activation process. The increased carbon content is an indication that kola nut husk is a good precursor for preparing activated carbon resulting in a good uptake of IBP due to its ability to catenate with other elements. Okpe et al. (2018) also documented a similar observation.

Scanning electron micrographs were utilized to detect pore enlargement of the sample after activation. It was observed that the activated sample possesses well developed prominent pores, unlike the raw sample. Figure 3(a) and 3(b) represent micrographs obtained from raw and activated kola nut husk respectively. The larger pores in the activated samples suggest that IBP trapping from aqueous solution will be more efficient using the activated sample (Rao et al. 2006; Bello et al. 2015). The figure also reveals that there is an increase in the surface area of the adsorbent. Thus, the larger the surface area, the greater the amount of adsorbate removed (Santhi et al. 2010; Fouzia & Abu 2018).

pH_PZC is a resultant parameter that depends on the overall effect exerted by the diverse functional groups present in the KNHA (Bello et al. 2017a, 2017b). In a plot of ΔpH (change in pH) against pH₀ (initial pH), the pH_PZC is the point where the resultant curve passes through the pH₀ region (Figure 4). In the case of KNHA, the value of pH_PZC is 5.32 (Figure 4). This implies that the adsorption of cation is more effective at pH above the pH_PZC while anion adsorption is more effective at pH below the region (Al-Degs et al. 2000). Depending on the medium used for the adsorption process, KNHA will adsorb IBP molecules from aqueous solutions at varying proportions.

The Brunauer–Emmett–Teller (BET) surface area, micropore volume and average pore diameter are reported in Table 4. The average pore diameter of the adsorbent before and after activation was found to be 1.56 and 2.91 nm respectively. This shows that KNHR was microporous while KNHA was mesoporous. The mesopore volume and mesopore contribution of KNHA to the porous texture was higher (>70%) than that of KNHR (<25%) (Table 4). This accounts for the high adsorption efficiency of KNHA. Mesopores act as transporting arteries, and contribute to the adsorption of IBP on the adsorbent. A similar result was obtained in our previous studies of the adsorption of IBP on bean husks and orange peels (Bello et al. 2019, 2020).

Kinetics of IBP uptake by KNHA was studied using four different kinetic models. These are the pseudo-first-order (Lagergren & Svenska 1898), pseudo-second-order (Ho & Mckay 1999), Elovich (Aharoni & Ungarish 1976), and intraparticle diffusion model (Weber & Morris 1963).

Adsorption kinetic data analysis helps in the determination of the rate-determining step of the adsorption process. The pseudo-first-order kinetic model did not provide the best correlation for the adsorption data. From Table 5, it can be seen that the experimental and calculated values of q_e do not agree for the pseudo-first-order kinetic model. The regression coefficient value (R²) for the pseudo-first-order is also lower compared to that of the pseudo-second-order. Similarly, the sum of squared errors (SSE) values obtained for the pseudo-first-order model is high. On the other hand, the pseudo-second-order model exhibited close agreement between the experimental and calculated values of q_e. It also has low SSE values. This suggests that the kinetic data is best correlated with the pseudo-second-order kinetic model. This is indicative that two interacting surfaces were engaged during the adsorption process. Values of the regression co-efficient tending towards unity were also observed for the pseudo-second-order kinetic model. Hanbali et al. (2020) also reported a similar trend of experimental data fitting the pseudo-second-order kinetic model most for the adsorption of IBP onto functionalized carbon nanotubes. In the case of the Elovich model, the desorption constant (β) increased with increasing IBP concentration. This implies that the activation energy of the reaction increased with an increase in concentration. The Elovich constant (α) decreased with an increase in IBP concentration (Table 5).

Four different equilibrium isotherms; Langmuir, (Langmuir 1918) Freundlich (Freundlich 1906), Temkin (Temkin & Pyzhev 1940), and Dubinin–Radushkevich (DBR) (Dubinin & Radushkevich 1947); were used to correlate the adsorption data to discover the model which gave the best fit. In the case of the Langmuir isotherm, the value of the maximum monolayer capacity uptake of the adsorbent (Q_m), was found to increase with an increase in adsorption temperatures with 39.68 mg/g as the highest value (Table 6). An increase in temperature also increases the mobility of the adsorbate molecule. Mobility increase caused by increased temperature resulted in higher adsorbate removal. The Langmuir constant (K_L) and the dimensionless constant (R_L) were found to increase with increasing temperature. The R_L values for the three temperature ranges were less than 1 suggesting that the adsorption of IBP onto KNHA was favorable (Table 6). In the case of the Freundlich model, the Freundlich adsorption constant (K_F) reduced with increasing adsorption temperatures. K_F value at 303 K (9.818) was observed to be 2.78 times greater than that of 323 K (3.532) (Table 6). High values of K_F indicate that there is very good interaction between KNHA and IBP. The Freundlich coefficient (n_f) values for the three temperatures were observed to be less than 10. The highest value of n_f is 1.032 with a corresponding value of 1/n_f=0.968. This suggests that the adsorption is favorable. The values of R_L and 1/n_f values being less than 1 are indicative of an excellent physical adsorption process. The Langmuir isotherm has R² values that are higher than those of the Freundlich model, with the highest value being 0.994. This is indicative that the Langmuir model best explains the adsorption isotherm. Mestre et al. (2007), Delgado et al. (2015), Puszkarewicz et al. (2017), and Mellah & Harik (2020) reported the same observations for the adsorption isotherm of IBP. In the case of the Temkin model, the value of the constant B can be used to predict the enthalpy of the adsorption experiment. Positive values (11.330, 11.750, and 10.96) were obtained for the three temperatures, which reveals an endothermic process (Table 6). The adsorption equilibrium constant (b_T) was also calculated using the Temkin model. Dubinin–Radushkevich's model is appropriate as a tool for detecting the mean free energy of the adsorption process in terms of porosity (E). E values obtained are less than 8 kJ/mol (1.5, 2.9, and 2.2 kJ/mol respectively); this suggests that the adsorption process is physical in nature, i.e. physisorption (Table 6). This implies that the uptake of IBP from aqueous solution by KNHA is governed by a physical phenomenon. R² value of the isotherm models was used to judge the model with the best fit. The Langmuir model has the highest R² value. Hence, the adsorption experiment can be best illustrated with the Langmuir model. Judging from the value of R², the isotherms considered are rated in this order: The Langmuir > Freundlich > Temkin > DBR. Comparisons of the maximum monolayer adsorption capacities of IBP with other adsorbents are reported in Table 7 .

Temperature was employed as a tool to investigate the adsorption thermodynamic. Thermodynamic parameters of standard enthalpy change (ΔH⁰), standard entropy change (ΔS⁰), and standard Gibbs free energy change (ΔG⁰) were evaluated. They reveal the nature of the reaction, the kind of interaction that exists between the two interacting surfaces and the feasibility of the reaction respectively. Negative values of ΔG⁰ confirm that the reaction is spontaneous. The positive value of the enthalpy change (ΔH⁰) reveals that the reaction is endothermic. Thermodynamic parameters obtained in this study as shown in Table 8 indicated that the adsorption of IBP onto KNHA is endothermic, feasible, and thermodynamically favored.

In view of the cost implication of the adsorbent, regeneration and possible reuse of KNHA was investigated. This was done by subjecting the adsorbent to water, acid and base treatments. The acids employed were CH₃COOH and HCl, while the bases were NaOH and CH₃COH. CH₃COH gave the highest percentage desorbed (95.34%); 0.1 M CH₃COH was used to desorb IBP molecules from KNHA over several cycles (Figure 5). It was observed that the percentage desorption of IBP remained fairly constant without significant change for four cycles (Figure 5). To be concise, 3% decrease in the percentage of IBP adsorbed was observed between the first and fourth cycles (Figure 5). Obviously, the results obtained proved that KNHA desorbed with CH₃COH after each adsorption step is very effective for IBP removal from aqueous solution.

Using adsorption techniques on large scale requires analysis of the cost implications in the preparation of the adsorbent. The net cost of preparing the activated carbon is usually determined by several factors such as cost of the precursor, cost of activating agent, and cost of running necessary appliances. In the case of KNHA, it was prepared from a readily available agricultural waste. Table 9 shows a breakdown of the cost incurred in preparing 1 kg of KNHA in comparison with the commercially available activated carbon (CAC). From the cost evaluation, KNHA was found to be over six times cheaper than CAC.

In this study, the efficiency of kola nut husk activated carbon was studied. KNHA was found to have an adsorption capacity greater than some of the previously reported studies (Table 7). The adsorption study was governed by a physical process, i.e physisorption. The isotherm model that gave the best correlation was Langmuir, and the kinetics of the adsorption process fitted most the pseudo-second-order kinetic model judging from R² values and low SSE values recorded. The KNHA is six times cheaper compared to CAC. Thermodynamic analysis revealed that the adsorption reaction was spontaneous and that the reaction was exothermic in nature. The study also revealed that KNHA is a promising low-cost adsorbent that can be used in removing IBP from wastewater.

The first and corresponding author acknowledges the support offered by The World Academy of Science (TWAS) in the form of grants: Research Grant number: 11–249 RG/CHE/AF/AC_1_UNESCO FR: 3240262674 (2012) and 15–181 RG/CHE/AF/AC_1_: 3240287083 (2015) and LAUTECH 2016 TET Fund Institution Based Research Intervention (TETFUND/DESS/ UNI/OGBOMOSO/RP/VOL. IX) respectively.

All relevant data are included in the paper or its Supplementary Information.