Coffee residue-based activated carbons for phenol removal

The demand for phenol-related products has resulted in the manufacturing of phenol in large scale. One of the major products is phenolic resin, which is produced through polymerization reaction of formaldehyde with phenol. The use of large amounts of phenol in processing may eventually result in the chemical finding its ways to the environment. Phenol is highly poisonous to living beings (Anku et al. 2017). Therefore, the effluent bearing the chemical must be fully treated before it enters the water bodies. Various treatment methods such as coagulation/flocculation, precipitation, membrane filtration, reverse osmosis, ozonation, advanced oxidation processes and ion exchange have been used for the elimination of organic pollutants in wastewater (Shu-Hui & Zaini 2016; Anku et al. 2017; Crini & Lichtfouse 2019). However, these techniques have several limitations, as they involved the use of chemicals and often resulted in secondary pollution.

Among others, adsorption has emerged as a preferred treatment for wastewater because the process is simple and straightforward, inexpensive, consumes low energy, and produces minimum sludge. Activated carbon is a widely used adsorbent in adsorption. It is a material that is produced from carbonaceous precursors, such as coal, coconut shell, wood, and lignite (Ming-Twang et al. 2015).

Coffee is produced everywhere worldwide. Consequently, the leftover of coffee drink; that is, the coffee residue, is abundantly available. Because it has no commercial value, coffee residue could be a sustainable resource of activated carbon for wastewater treatment (Laksaci et al. 2017). Moreover, without proper solid waste management and disposal, the waste material may well lead to other environmental issues.

To date, there is still limited literature available to unlock the true potential of coffee residue as activated carbon, and its two-stage adsorber design in evaluating the efficiency of phenol removal. Castro and co-workers (Castro et al. 2011) reported the removal of phenol at 150 mg/g by coffee grounds activated using potassium carbonate and steam. In a related work, Khenniche & Benissad-Aissani (2010) showed an 85 mg/g phenol removal by zinc chloride-activated coffee residue. Hence, the present work was aimed to establish the adsorptive properties of coffee residue-based activated carbons by phosphoric acid or potassium hydroxide activation so as to enrich the body of present knowledge. The two-stage design would bring useful insight into dosage optimization and practical applications of batch adsorption. The findings and mechanisms of phenol adsorption are discussed.

Coffee residue was obtained from a Starbucks outlet in the Johor state of Malaysia. Potassium hydroxide and phosphoric acid (85%) were supplied by QRec (Asia) and R&M Chemicals, respectively. Phenol (99%) was purchased from Sigma-Aldrich. All chemicals were of analytical grade and were used as received without further purification.

Coffee residue was oven-dried at 100 °C for 24 h. It was mixed with H₃PO₄ or KOH at mass ratio of 1:1. Distilled water was added into the mixture to ensure it was homogeneously stirred for 30 min. The mixture was completely dried in an oven for impregnation prior to activation in a furnace under anoxic environment at 600 °C for 1.5 h. The activated carbons were refluxed with distilled water for 24 h to remove unreacted reagents. The resultant activated carbons were designed as SA and SB for H₃PO₄- and KOH-activated carbons, respectively.

The thermal decomposition profile of materials studied was obtained using a Perkin-Elmer thermogravimetric analyzer (TGA 7) under nitrogen flow of 10 mL/min and heating rate of 15 °C/min. The specific surface area of activated carbons was determined using a Micromeritics surface area analyzer (ASAP 2020) at a liquid nitrogen temperature of 77 K. The surface chemistry of coffee residue and its derived activated carbons was qualitatively determined using a Perkin Elmer FTIR spectrometer (Spectrum One). The surface morphology of activated carbons was characterized using a Hitachi scanning electron microscope (TM3000). The pH coffee residue based activated carbons were measured using a portable pH meter (HI 9813-5, Hanna Instruments). Briefly, 10 g of sample was immersed in 100 mL of distilled water, the mixture was heated for 15 min and allowed to cool at room temperature prior to pH measurement.

The two-stage adsorber design is aimed to optimize the activated carbon used in adsorption. The schematic representation of the two-stage batch adsorption process is shown in Figure 1.

Different constraints were set to allow comparison of the amount of activated needed in a single stage adsorber to that of a two-stage adsorber.

Figure 2 shows the thermogravimetric profiles of coffee residue (CR) and activated carbons. Coffee residue exhibits a high weight loss of 81% at 860 °C, with sharp peak centred at 310 °C. In anoxic environment, the weight of coffee residue is stabilized at 600 °C, which is also the activation temperature. The pyrolysis of carbonaceous material normally gives rise to char, oil (tar) and gas phases. A rudimentary porosity is obtained because of the release of volatiles such as hydrogen, oxygen, and nitrogen, leaving a rigid carbon skeleton formed by aromatic structures. Activated carbons display a thermal resistant behaviour with weight loss of 27 and 44% for SB and SA, respectively. A greater weight loss of the latter could be explained by the presence of well-developed porous structure that easily entrapped moisture from the surrounding and collapsed at high temperature. The weight loss at about 100 °C is generally attributed to the elimination of physisorbed moisture, while that at 400 °C corresponds to the release of volatiles and decomposition of the oxygenated surface groups (Laksaci et al. 2017).

Figure 3 shows the nitrogen adsorption-desorption isotherm profiles of activated carbons, and the textural properties are summarized in Table 1. SA displays a Type I isotherm according to IUPAC classification, indicative of the presence of micropores. The shape of hysteresis loop is often assigned to specific pore structure. Most of nitrogen adsorption for SA occurs at P/P_o < 0.4, with hysteresis loop of H4 for the reduced pressures above 0.4. This can be attributed to capillary condensation in narrow slit-shaped mesopores, implying that this activated carbon possesses both micropores and mesopores. SB shows a predominantly Type IV isotherm, the characteristic of mesoporous material. SB also exhibits a hysteresis loop of H4 within the P/P_o range from 0.65 to 1 (Sing 1982). A horizontal adsorption plateau is observed at higher P/P_o, suggesting that the micropores are dominant in activated carbon. The desorption isotherm which shows a deviation from the adsorption isotherm can be explained by the presence of bottleneck or slit pores (Rouquerol et al. 2013).

From Table 1, SA shows a high specific surface of 1,030 m²/g, which is more than 2.5 times that of SB. SA possesses a narrow pore size distribution, located mainly in the microporous domain with D_p of 2.13 nm. Apart from surface area and porosity, the accessibility of reactive centres on the carbon surface is also important for adsorption. The surface is acidic (pH 4.0) for SA and alkaline (pH 9.0) for SB. The acidic character of SA could be attributed to H₃PO₄ activation that caused the deposition of oxygen functional groups, while the surface accumulation of hydroxyl anions renders the basic character to SB (Arampatzidou & Deliyanni 2016). The yield of SB is comparatively small at 17.5% than that of SA, because KOH is a strong dehydrating agent that enhances the carbon burning-off (Hui & Zaini 2015). As coffee residue is readily in powder form, it is easily decomposed, giving low yield and low surface area.

The elemental composition of coffee residue activated carbons is summarized in Table 2. Coffee residue originally contained 52.0% carbon, 7.54% hydrogen, 2.29% of nitrogen and 38.0% of oxygen (by difference). Upon activation, the carbon content increased to 57.3–57.7%, while the hydrogen content decreased to 2.21–3.97%. In KOH activation, the oxygen content increased due to the decomposition of metal hydroxide into oxides within the carbon matrix and the formation of oxygen functional groups on the surface of activated carbon (Hui & Zaini 2015). The sulfur content decreased in SA (0.176%) but increased in SB (0.312%) because of the volatility of this element. The C/H ratio of coffee residue increased upon activation, signifying the formation of rich carbon (graphitic) structure, while the C/N ratio implies that the amount of nitrogen mostly retained after the activation.

The SEM images of activated carbons showing the heterogeneous surface and porous nature are shown in Figure 4. The cracks, voids, and crevices as observed from the morphology act as the entrance of pore channels for microporous-mesoporous network of activated carbon (Laksaci et al. 2017). The external surface of SA displays some crater-like and non-uniform honeycomb-like pore entrances. They are visible because the specific surface is high (1,030 m²/g). SB exhibits a smooth sponge-like texture as a result of the effect of the dehydrating agent during heat treatment.

Figure 5 shows the FTIR spectra of coffee residue and activated carbons. The spectrum of coffee residue displays multiple peaks of varying intensity, signifying a surface rich in functional groups, while the spectra of activated carbons are simplified, with missing peaks and some of decreasing intensity owing to the decreasing amount of functional groups remaining on the carbon surface upon chemical activation. The broad absorption band at 3,600–3,300 cm⁻¹ indicates the stretching vibration of hydrogen-bonded hydroxyl groups (from carboxyl, phenols or alcohols) and moisture (Laksaci et al. 2017). Two peaks of C–H stretching were observed at 2,922 cm⁻¹ and 2,853 cm⁻¹, correspond to asymmetric and symmetric stretching of alkane compound. The band at 1,740 cm⁻¹ can be attributed to the C = O stretching vibration of acetyl and uronic ester groups (a class of sugar acids containing carbonyl and carboxylic acid) of hemicellulose (Wang et al. 2009). The peak at 1,635 cm⁻¹ signifies the carboxyl linkage derived from xanthine derivatives such as caffeine (Lyman et al. 2003). The FTIR spectra of SA and SB show absorption bands representing the aliphatic C–H stretching in aromatic methoxyl group, and methyl and methylene groups of side chains. The absorption band at 1,028 cm⁻¹ signifies the C–OH bending and CH₂ related modes. The band at around 2,350 cm⁻¹ can be attributed to the stretching of ⁠.

Figure 6 shows the adsorption kinetics of phenol by coffee residue activated carbons, and the kinetic constants are summarized in Table 3. Generally, the removal of phenol displays an increasing trend with increasing contact time to a point of equilibrium, at which the rate of adsorption is equal to the rate of desorption. The adsorption rate is slower at higher concentration, thus a longer contact time would be required to attain the equilibrium. However, the adsorption intensity (slope) increases with concentration because the concentration gradient offers the driving force to overcome the mass transfer resistance for a greater adsorption for the same contact time. The rate of adsorption tends to slow down and subsides to zero as the contact time increases due to insufficient driving force, and repulsion between the adsorbed phenol molecules and that in the bulk solution at equilibrium (Shaid et al. 2019).

From Table 3, both pseudo-kinetic models show a reasonably good fit to the experimental data with small variation between the experimental (q_e,exp) and calculated values. At C_o = 5 mg/L, adsorption occurs mainly by physical diffusion, by which the mass transfer resistance is dominant. While, at C_o = 20 mg/L, the adsorption rate seems to obey the pseudo-second-order model, indicating a process controlled by physiochemical interactions (Shaid et al. 2019). For the concentrations studied, the adsorption increased in the order SA < SB, implying a faster saturation of SB, mainly due to its lower specific surface.

Figure 7 shows the equilibrium adsorption of phenol onto activated carbons, where the isotherm constants are summarized in Table 4. The adsorption capacity gradually increased with increasing concentration to a point of surface saturation. Both activated carbons display a comparable maximum capacity at about 43 mg/g (57% removal), despite the dissimilarity in surface area. This could be explained by the presence of acidic oxygen groups in SA and the basic surface of SB. The acidic oxygen groups dissociate in water to produce the negatively charged surface of SA, while the surface of SB is surrounded by hydroxyl anions. Phenol in water can lose a hydrogen ion to form a more stable phenoxide ion, which carries the negative charge on the oxygen atom and delocalizes around the aromatic ring. This brings about an electrostatic repulsion that interferes with the accumulation of phenol molecules on the active sites. As shown in Figure 6, SA demonstrates a greater degree of repulsion than SB for a lower phenol removal at 20 mg/L, even though the former is high in surface area.

From Table 4, the Langmuir model fits the equilibrium data well with R² > 0.94, and proximity between the constant, q_m and the experimental data, implying the monolayer coverage of phenol molecules onto the homogeneous surface of activated carbon. Nonetheless, a slight deviation of model lines from the experimental data at higher concentration, as visualized in Figure 7, indicates that the removal of phenol could be governed by multilayer stacking of molecules, surpassing the restriction of electrostatic repulsion due to the high concentration gradient. In this work, the maximum phenol capacity of 43 mg/g by coffee residue-based activated carbons is comparable and sufficiently higher than that of organo-montmorillonite (50.1 m²/g, 3.6 mg/g) (Wang et al. 2017), activated semi-coke (347 m²/g, 42.8 mg/g) (Gao et al. 2017), pine fruit shell char (228 m²/g, 31.5 mg/g) (Mohammed et al. 2018), zeolite composite (872 m²/g, 40.3 mg/g) (Cheng et al. 2016) and waste tyre activated carbon (237 m²/g, 48 mg/g) (Shaid et al. 2019). Undeniably, some activated carbons from egg shell (113 m²/g, 192 mg/g) (Giraldo & Moreno-Piraján 2014) and rubber seed shell (852 m²/g, 302 mg/g) (Yan et al. 2019) recorded a superior removal of phenol due to favourable texture and surface charge for adsorption.

A two-stage adsorber design was proposed for SB using the Langmuir model to minimize the amount of activated carbon required for phenol adsorption. For any volume of phenol to be treated, the two-stage batch adsorber is able to reduce nearly 14% of adsorbent dosage as compared to the single-stage adsorber. The required amounts of SB to reduce the effluent concentrations to 0.5–10 mg/g, and to achieve the performance of 80–99%, are visualized in Figure 8.

In Figure 8(a), C₁ is lower for a greater performance because of low equilibrium to compensate the remaining removal in the second stage. In other words, the amount of SB used in the first stage is higher for a higher target performance. As the phenol removal increased, the amount of SB also increased with C_o. From Figure 8(b), to achieve a 99.9% phenol removal, the minimum amounts of SB required for a two-stage adsorber are 90.6 and 547 g for C_o = 50 m/L and 350 mg/L, respectively. In Figure 8(c), the efficiency for the second stage is always lower than that of the first stage, as C₁ is lower. Likewise, the efficiency for the first stage is significant for high removal performance using high C_o (Hijab et al. 2020). At high C₂, the efficiency of the second stage is minimal. However, the gap of efficiency in the second stage is broadened when the target performance is high, and the influent concentration is low. Therefore, adding a second stage even for low C₂ is essential to achieve the desired removal efficiency with minimum dosage of activated carbon.

Activated carbons were prepared from coffee residue using H₃PO₄ or KOH activation. The H₃PO₄-activated carbon exhibits a higher specific surface of 1,030 m²/g. Nonetheless, both activated carbons demonstrate a similar phenol removal at 43 mg/g (57% removal) due to possible electrostatic repulsion. From the kinetics point of view, the KOH-activated carbon displays a less restricted barrier for a better phenol adsorption at C_o = 20 mg/L. The phenol adsorption could be governed by physicochemical interactions. In two-stage adsorber design, the efficiency of the first stage should be high enough to permit high performance for high phenol concentration, while the second stage is necessary to accomplish the process with minimum dosage of activated carbon. Coffee residue-based activated carbon could be useful for the removal of phenol in wastewater treatment.

This work is financially supported by UTMShine Signature Grant No. 07G80.

The authors declare no conflict of interest.

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