Preparation of Waste Coffee-grounds Carbon and Study on Phenol Adsorption Ability

LI Huijuan, ZHANG Jinyuan, LI Fengchuan, LUO Shimao, LI Qian, ZHOU Shiping

(Key Laboratory of State Forestry and Grassland Administration on Highly-Efficient Utilization of Forestry Biomass Resources in Southwest China (Southwest Forestry University), Kunming 650224, China)

Abstract: The waste coffee-grounds carbon (WCGC) was prepared with H3PO4 treated using waste coffee-grounds as precursor. Its adsorption ability was studied using phenol as test molecule. The influence of H3PO4 treated, calcined temperature, the initial phenol concentration, the doge of carbon and original pH values on phenol adsorption ability were investigated. Characterization of WCGC was performed by N2 adsorption isotherms, Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and X-ray diffraction (XRD) techniques. First, the second order and Weber-Morris model reaction rate models were used to estimate the WCGC adsorption ability. The results show that the produced WCGC (700 ℃, 2 h) has been graphitized and the layered structure increased BET surface to 435.98 m2/g and adsorption phenol ability. The initial phenol concentration is 50 mg/L, the amount of WCGC (700 ℃, 2 h) is 0.2 g, and the phenol adsorption rate is 97% after 270 min and no intermediate product formation. The adsorption kinetics of the selected WCGC is best fitted by the Weber-Morris model.

Key words: waste coffee-grounds carbon; phenol; adsorption ability; adsorption kinetics and thermodynamics

1 Introduction

Phenolic compounds coming from industrial and agricultural wastewaters originating from coke ovens,petroleum manufacture or paint stripping operations have caused several environmental problems. The wastewater with phenolic compounds is an issue of environmental concern, as they are highly hazardous to humans, wildlife, and aquatic systems, even at very low concentrations[1-4]. It can result in many serious health conditions, including problems with breathing, coma,damage of human vital organs exposure to phenol, or even modifications of DNA in cells[5-6]. Regular AC production is expensive due to the use of non-renewable materials, such as coal, lignite peat or wood. Many studies have focused on the adsorption-photodegradation of phenolic compounds in aqueous solution over different materials[7,8]. Irena Efremenkoet al[9]studied the predicting phenol solute adsorption on activated carbon and found that phenol exhibited a high affinity to hydrophobic adsorption sites on AC surface, and water and phenol molecules were adsorbed on different sites of AC surface. Nanopores of 1-2 nm in diameter present the best adsorption position for phenol.

Biomass is a very rich natural resource on the earth. Nowadays, the uses of bio-based adsorbents,which are formulated by using the natural biocompounds, have gained special attention of the researchers in pollutant water management[10-12]. Songet al[13]prepared the functional activated carbon derived from pyrolysis of rice husk, KOH-activation and EDTA-4Na-modification, and found that its maximum adsorption capacity for phenol increased to 215.27 mg·g-1.The adsorption kinetics of phenol on activated carbon agreed with the pseudo-second-order adsorption model,and the adsorption isotherm followed the Langmuir pattern. Gundogdu Aliet al[14]studied the ability of activated carbon which was produced by chemical activation using zinc chloride from tea industry wastes (TIWAC) to adsorb phenol solution. It was established that phenol adsorption on TIWAC could be better defined by the Langmuir adsorption model and its adsorption capacity was 142.9 mg·g-1.

Coffee is one of three world’s major beverage crops, and 1 kg coffee produced 0.9 kg waste coffee grounds (WCG). And WCG are biomass with rich availability. The WCG is composed of approximately 58% carbon. Its skeleton is distributed as a network,and cellulose and lignin are enveloped in the network,thus, it is a natural organic-inorganic composite[15].Using WCG to prepare activated carbon sorbent will greatly reduce the pressure on the environment[16]. Lessaet al[17]prepared the Chitosan/waste coffee-grounds composite for removal of pharmaceutical contaminants. WCG showed a good interaction with the polymeric matrix and good disperse up to 10 wt%.At 5 wt% WCG, the composite exhibited a noticeable enhancement (from 10% to 44%) of the adsorption of pharmaceuticals. Wan[18]combined carbonaceous material obtained from exhausted coffee and used for cobalt(II) and cadmium (II) sorption. The results showed that the obtained carbons had BET specific surface areas of 114.27-390.85 m2/g and pore diameters of 4.19-2.44 nm when the temperature was increased from 600 to 800 ℃. Cobalt and cadmium adsorption by the carbonaceous materials was correlated with the maximum adsorption capacities and specific surface areas of the materials. Activated carbon production takes place in two steps: carbonization and activation carbonization is the pyrolysis of raw material at (500 to 1 000) ℃in N2or air atmosphere to remove no carbonaceous elements. For chemical activation, chemicals such as ZnCl2, H3PO4, H2SO4, HNO3and KOH are used as activating agents[19-20]. Egle Rossonet alprepared activated carbon as the resource of the spent coffee grounds using KOH as activating agent and studied its adsorption capacity on a panel of phenolic compounds compared with those of two commercial powdered activated carbons, and found that the removal efficiency for SCGs-AC was comparable with that of the commercial activated carbons with the highest partition coefficients[21].Among these activation agents, H3PO4has been widely used because of its easy recovery and minimal environmental impacts. The H3PO4-activated carbon has been used in the removal of dyes and heavy metal ions from wastewater.

In this study, waste coffee grounds (WCG) was chemical activated using H3PO4and calcined at different temperatures to obtain waste coffee grounds carbon(WCGC). The phenol molecules adsorption ability from aqueous solution of the obtained carbon was tested by various analytical parameters in terms of equilibrium, kinetics, and thermodynamics, and also the WCGC was characterized by N2adsorption isotherms,FT-IR, SEM and XRD techniques.

2 Experimental

2.1 Preparation of waste coffer grounds carbon

Waste coffee-grounds (WCG) were come from Yunnan Dehong Hogood Coffee Co.Ltd., which were collected and was thoroughly washed with ionized water for several times, and dried at 110 ℃ for 24 h. The dried waste coffee grounds was treated in 100 mL 0.1 mol/L H3PO4aqueous solution under magnetic stirring at room temperature for 4 h. After that, the sample was dried in a furnace at 110 ℃ for 24 h, and the resulting dried waste coffee grounds treated by H3PO4was then put into muffle furnace calcined at different temperature for 2 h to prepare waste coffee-grounds carbon(WCGC).

2.2 Characterization

X-ray diffraction (XRD) data were obtained using a horizontal Rigaku B/Max IIIB powder diffractometer with Cu Kα radiation and a power of 40 kV at 30 mA. The wavelength of the source used was Cu Kα =0.154 06 nm. The line broadening was determined for the corresponding XRD peaks according to Scherrer’s equation:D=Kλ/βcosθ, whereDis the crystal size,Kis the wavelength of the X-ray radiation (λ = 0.154 06 nm), which is usually taken as 0.89, andβis the peak width at the half-maximum height of the sample. Fourier transform infrared (FTIR) spectra were recorded on the Varian 640 spectrometer (USA) operating in the range of 400-4 000 cm-1with a resolution of 4 cm-1and 32 scans. Samples were ground with spectroscopic grade KBr. Transmission electron microscopy (TEM)images of the samples were collected with a Hitachi H-800 transmission electron microscope (Japan), which was equipped with a top-entry holder and ion pumping system, and operated at 200 kV to give a nominal structural resolution of 0.21 nm. Samples were prepared by dipping a 3 mm perforated carbon grid into an ultrasonic dispersion of the oxide powder in ethanol.

2.3 Adsorption activity measurements

2.3.1 Adsorption experiments

100 mL certain concentration aqueous phenol solution was used to study the adsorption efficiency of the prepared WCGCs. Each adsorption experiment was carried out at room temperature. 0.2 g WCGC was added to the beaker containing 100 mL phenol solution under magnetic stirring. At predetermined time intervals aliquots (5 mL) were withdrawn and a vacuum filtration device was used to separate the WCGC, and also the collected filtrate was further treated with a high speed centrifuge. And the absorbance of the phenol solution was analyzed using UV-vis Nicolet 721 spectrometer (USA). After each measurement, the aliquots were returned to the testing reactor.

2.3.2 Colorimetric estimation of phenol

The quantifications of phenol were analyzed by using colorimetric estimation by Martin[22], and phenol reacts with 4-aminoantipyrine takes the following reaction:

The reagent will turn from yellow to reddish color in the presence of phenol, and has a maximum absorption at 510 nm, which is consistent with the Lambert-Beer’s law. The degradation efficiencies of phenol are calculated by using Eq.(1):

whereC0is initial concentration of phenol, andCtis concentration of phenol at specific time.

3 Results and discussion

3.1 Catalyst characterization

3.1.1 XRD characterization

Fig.1 shows the XRD patterns of WCGC calcined at different temperatures for 2 h. When dried at 110 ℃for 24 h, the diffraction pattern of WCGC exhibited two broad peaks centered at 2θ= 15.9 , 22.1 and 34.5°,which can be associated with cellulosic materials and amorphous hexagonal structure charcoal caused by fat chain in WCGC[23]. When the WCGC was calcined at 300 ℃, the peak intensity at 22.1° was lower than that at 110 ℃, indicating that amorphous carbon is changing. And a new diffraction peak at 2θ=26.2° appeared,which attributed to disordered graphite carbon, mainly (002) crystal plane. And this peak became larger and larger with the calcined temperature until 600 ℃,suggesting the graphite carbon came into being gradually. WCGC calcined at 600 ℃ has diffraction peaks at 2θ=29.2 and 43.5°, which belongs to graphite microcrystalline structure (002) and (100) crystal plane of graphite structure, respectively. And the two peaks intensity increased at 700 ℃ indicating that it formed graphite crystal phase structure more completely and the coffee ground carbon material has been graphitized[24].

It can be seen that there are obvious diffraction peaks changes of WCGC at 110-700 ℃. At low temperature, WCGC was composed of hexagonal structure charcoal caused by fat chain. With the increase of temperature, the fat chain was breakage and the WCGC was changed into a disordered graphite-like layer structure at 300 ℃. And the WCGC has been graphitized at 500 ℃. Literature shows that walnut shell will enter the combustion stage after 420 ℃. The aromatization reaction of the carbonized substance will take place at high temperature to form porous microcrystalline carbon[25].

3.1.2 SEM characterization

The surface properties of materials are also important in explaining reactivity. SEM micrographs for WCGC calcined at different temperatures are presented in Fig.2. The SEM micrograph showed that the porosity and the pore volumes increased with increasing calcined temperature to 400 ℃, which indicated that phosphoric acid and heat treatment contribute to pore formation. The surface textures of the grains were rough in the sample calcined at 110 ℃ and 200 ℃, and the presence of some grooves, loops and macroporous pores were observed. When WCGC was calcined at 700 ℃ for 2 h, it can be seen from Fig.2(d), the sample turned to layered structure and the porous structure still exists. The layered structure of WCGC (700 ℃, 2 h) increases its specific surface area and pore volume and results in its good phenol adsorption. Similar behavior has been reported by Noura A S[26].

3.1.3 FTIR characterization

The FTIR spectra of WCGC calcined at different temperatures (110-700 ℃) are presented in Fig.3. It can be observed that significant changes occurred in the biochars with increasing temperature. For WCGC dried at 110 ℃ for 24 h, the FTIR peaks positioned at 3 463 and 1 640 cm-1corresponds to the hydroxyl stretching and bending vibration of water or hydroxyl group absorbed on the surface of the WCGC. And the bands between 2 936 and 3 100 cm-1are mainly caused by saturated C-H stretching vibration. Band at 2 358 cm-1is the absorption peak of CO2. The bands at 1 692 and 1 557 cm-1are associated with the carbonyl C=O stretching of hemicellulose and chlorogenic acids and stretching vibration of C=C bonds of caffeine. Band at 1 464 cm-1is related to the CH2and CH3bending vibration, and 1 132 cm-1are caused by C-O-H bonds typical of polysaccharides. Peaks at 1 010 and 848 cm-1related to antisymmetric and symmetric vibrations of Si-O-Si were additionally found. Band at 601 cm-1is the vibration peak of the amino group. FTIR characterization results show that the coffee grounds are composed of oxygen-containing functional groups such as carboxyl group, ether group and ester group,and nitrogen-containing functional groups such as benzene ring and N-H group[27].

As the WCGC is calcined at higher different temperature, it can be seen that bands at 3 463, 1 640,2 936, 3 100 cm-1decrease in intensity gradually compared with dried WCGC. It can be concluded that the water adsorbed on the surface loses when increasing temperature. And the CH2and CH3peak decrease with increasing temperature. After carbonization, the C-H bond is destroyed to some extent, and C=C, C=O, C-O,O-H, and -NH2continue to exist in the sample to form aromatic ring species. On the contrary, bands at 3 568,3 415, 3 238 and 1 614 cm-1appeared when calcined at 700 ℃ for 2 h. The peaks located at 3 238 cm-1corresponds to H-P-H stretching and bending vibration from water and 1 614 cm-1comes from C=C vibration of the WCGC. Luoet al[28]studied the FTIR spectra of rice husk residue treated by phosphoric acid (H3PO4)in the preparation of activated carbon and found that bands at 3 384 and 1 600 cm-1shifted with increasing activation temperature. And this phenomenon contributed to the formation of P-containing carbonaceous species in H3PO4-activated. Also in our experiment the two bands shifted. It suggested these two bands at 3 568 and 3 415 cm-1associated to hydroxyl stretching vibration of water or hydroxyl group absorbed on the surface of the WCGC, which splited two peaks from 3 463 cm-1. It could be speculated that the P-containing carbonaceous species might be an important factor for the development of chemical structure in activated carbon, which might be responsible for the phenol adsorption[29].

Table 1 Pore structure properties of WCGCs

3.1.4 Pore structure of WCGCs

The nitrogen sorption isotherms and the corresponding pore size distribution curves of WCGC samples are given in Fig.4(a) and 4(b). And the texture properties surface chemistry of WCGC samples are summarized in Table 1. The nitrogen adsorption-desorption isotherms for the WCGCs at 110 and 700 ℃(Fig.4(a)) are mainly type I. This indicates that the WCGC (110 ℃, 24 h) sample is predominantly microporous. The WCGC (700 ℃, 2 h) changed from type I to nearly type IV, indicating that the small mesopore range becomes larger. As shown in Fig.3(a), the adsorption isotherm of the WCGC (700 ℃, 2 h) has a very wide knee, and the adsorption linearity increases to aP/P0of 0.4, indicating the small mesopores significant increase, which can also be confirmed by the pore size distribution in Fig.4(b). The second part of the isotherm (P/P0> 0.4) consisted of two separated branches,suggesting hysteresis and associated capillary condensation in mesopores. The hysteresis loops were classified as type IV. The pore size distributions (Fig.4(b))showed a sharp peak at 3.38 nm for WCGC calcined at 700 ℃ for 2 h and a weak hillock at 3.15 nm at 110℃ for 24 h. And also the total surface area was less than that at higher temperature. The textural parameters indicated that the surface area rises from 10.85 m2/g for WCGC(110 ℃ , 24 h) to 435.98 m2/g for (700 ℃,2 h) (Table 1). On the other hand, the pore volume increases from 0.026 43 to 0.295 8 mL/g with activating temperature, suggesting that many pores were covered when calcined at 110 ℃ for 24 h and calcined at 700℃ for 2 h could open the pores[30].

3.2 Phenol adsorption on WCGC samples

3.2.1 Calcination temperature

The effect of the calcined temperature (110-800℃) of WCGC on the adsorption of phenol was investigated. As observed in Fig.5, the sample calcined at 110 ℃ for 24 h showed merely 10% adsorption efficiency for phenol. And the phenol adsorption efficiency increased with treatment temperature under the same adsorption conditions. The WCGC (700 ℃, 2 h) can provide 100% phenol removal capacity. At 800 ℃, the phenol adsorption performance will decrease. At low temperatures, the coffee grounds are not completely charred, the pore size is not completely opened, and the organic matter on the coffee grounds is not completely decomposed and volatilized, which can block the pores. In this experiment, 700 ℃ is the super temperature. And from IR spectrum, the superior performance demonstrated at 700 ℃ could be attributed to the increase of adsorption sites on the WCGC and favored the phenol adsorption. And its higher BET surface and volume also gave more adsorption sites.

3.2.2 Effect of WCGC (700 ℃, 2 h) dose on adsorption

of phenol.

To study the effect of the amount of WCGC (700℃, 2 h) on phenol adsorption, five different amounts ranging between (0.05 and 0.4) mg samples are used for phenol adsorption in solution of 10 mg/L under optimum conditions. A graph of phenol adsorptionversusthe amount of WCGC (700 ℃, 2 h) was plotted from the data obtained (Fig.6). As seen from Fig.6, phenol adsorption percentage increased with an increasing amount of WCGC (700 ℃, 2 h), WCGC (700 ℃, 2 h)adsorbed the phenol rapidly at first 30 min for all dosages, and the adsorption efficiency increased with the amount rising. For the amount of 0.05 g, after adsorbed for 270 min, it could reduce 10 mg/L phenol to 1.52 mg/L and the removal rate was 84.8%. When increasing the WCGC (700 ℃, 2 h) amount to 0.1 g the phenol adsorption ability promoted. With the amount of 0.2 g WCGC (700 ℃, 2 h), it can reach nearly 100% phenol removal. And further increasing the amount of WCGC(700 ℃, 2 h), the phenol abatement increased little.Less amount of WCGC (700 ℃, 2 h) has not reached phenol saturation on adsorbent surface. Increasing the dosage of the sample can increase the phenol removal efficiency. When the amount of the WCGC(700 ℃, 2 h) is enough for phenol adsorption, increase the dose of adsorbent is not necessary. In this experiment, it suggested that 0.2 g WCGC (700 ℃, 2 h) is enough to remove phenol for 100 mL 10 mg/L aqueous solution.

3.2.3 Effect of initial phenol concentration

The efficiency of phenol adsorption on WCGC(700 ℃, 2 h) was investigated (Fig.7). The phenol concentration decreased dramatically in the first 30 min and then decreased to a fixed value when increasing the initial concentration from 5 to 50 mg/L. When the concentration is 10 mg/L, the reaction time is 270 min, and the concentration of the phenol solution reduced to 0.When the phenol concentration is higher, its degradation rate decreases. When the concentration of the phenol was 40 and 50 mg/L, the degradation rate of phenol was 98% and 97%, respectively.

3.2.4 Effect of initial pH on the adsorption of phenol by WCGC (700 ℃, 2 h)

The effect of different pH values (1, 3, 6, 9, and 12) of phenol solution on adsorption efficiency over WCGC (700 ℃, 2 h) are shown in Fig.8.

Solution pH is one of the most important parameters influencing phenol adsorption performance. This result can be seen from the phenol adsorption graph(Fig.8). At low and high pH values, phenol adsorption on WCGC (700 ℃, 2 h) is lower. The optimum pH value was determined as 6.0. When the pH is 1, the phenol concentration is 5.88 mg/L after 270 min adsorption,and the pH is 3, 6, and 9, after adsorbed for 270 min over the WCGC (700 ℃, 2 h), and the phenol concentrations were 0.48, 0, and 0.66 mg/L, respectively.When increasing pH value to 12, the concentration of phenol solution was 2.19 mg/L after 270 min of adsorption. The residual phenol concentration at pH= 6 reaches 0 mg/L after 270 min.

Snoeyinket al[31]first studied the effects of pH on phenol adsorption and they also found that phenol adsorption at low and high pH values are not favorable.The pKa value of phenol is 9.89. At lower pH value ,the concentration of H+is high, and the H+can also be adsorbed with phenol on the surface of WCGC (700 ℃,2 h), which decreases the phenol adsorption. At higher pH, phenol is present mainly in phenolic anion, and also the surface of the WCGC (700 ℃, 2 h) charged negatively, which decreases the affinity of the WCGC(700 ℃, 2 h) to phenol for the electrostatic repulsion.When the phenol solution is neutral, H+is present on the surface of the coffee ground charcoal (but the amount is much lower), and it is neutralized with phenolic hydroxyl groups. Vander Waals force between eand neutral molecules on WCGC (700 ℃, 2 h) promoted the adsorption capacity of coffee ground charcoal.At pH 6.0, which was selected as the optimum, the net charge of the adsorbent is negative. Since phenol is still a protonated structure, adsorbate -adsorbent interaction will be the largest.

3.2.5 Liquid UV characterization

The changes in the UV spectrum of phenol by WCGC (700 ℃, 2 h) adsorption process are shown in Fig.9. The spectrum of phenol in the UV region exhibits bands with peak absorbance at 203 and 268 nm. The decrease of the two peaks in this figure indicates a rapid removal of phenol. The absorption peak decreased gradually after 30 min and reached equilibrium at 90 min. During adsorption process, the WCGC (700 ℃, 2 h) adsorbed the phenol rapidly in the first 90 min and then reached saturation. There is no other absorption peak in the process. It can be inferred that no by-products are formed, or it was in very small amount and difficult to detect.

3.2.6 Adsorption kinetics

Kinetic parameters can provide important information on the designing and modeling of the adsorption process. Therefore, WCGC(700 ℃,2 h) was investigated by a kinetic study. Experimental data were modeled using the pseudo-first order and the pseudo-second order model and the Weber-Morris model (Table 2) as shown in Eqs.(1) to (3).

wheretis the adsorption time, min;Qis the adsorption amount,Qeis the equilibrium adsorption amount,Qtis the adsorption amount at timet(mg•g-1).K1is the pseudo first-order adsorption rate constant,(min-1).k2is the pseudo-secondary adsorption rate constant, (g/(mg•min)-1).kipis the internal diffusion rate constant, (mg•L-1•min1/2); C is a constant involving thickness and boundary layer. The adsorption characterization data of the WCGC(700 ℃, 2 h), the adsorption amountQat each moment can be calculated for the kinetic equation, as shown in Table 2.

Table 2 Kinetic model constants and correlation coefficients for the phenol adsorption on WCGC(700 ℃, 2 h)

If intraparticle diffusion (IPD) occurs, the plot ofQtagainstt1/2yields a straight line withkpias the slope andCias the intercept. If the plot for all points passes through the zero, IPD is the only rate-limiting process.Otherwise, the plot may present multi-linearity, indicat-ing that some other mechanism, together with IPD, is involved as well. In general, the rate of uptake might be limited by the size of adsorbate molecule, the adsorbate concentration, the affinity for the adsorbent and the diffusion coefficient of the adsorbate in the bulk phase[32].It can be seen from Table 3 that the Weber-Morris model correlation coefficient of phenol adsorption on coffee slag carbon is nearly equal to 1(R2= 0.990 8). The above results indicate that the Weber-Morris model can be used to describe the adsorption process of phenol on coffee ground carbon.

Table 3 Weber-Morris model constants and correlation coefficients for the phenol adsorption over WCGC(700 ℃, 2 h)

3.2.7 Adsorption thermodynamics

The adsorption thermodynamic parameters mainly include the Gibbs free energy change (△Gθ, kJ/mol)and the adsorption potential (E, kJ/mol). The two parameters can be calculated by the following two formulas:

In the formula:kdis the partition coefficient,kd=qe/ce, (L/mg);Ris the gas molar constant, 8.314 J/(mol • K);Tis the absolute temperature, (K).ceis the phenol concentration at equilibrium; c0is the initial concentration of phenol. The temperatureTis 298.15 K. After calculation, the final result is △Gθ=-5.696 kJ/mol;E=7.536 kJ/mol. △Gθis -20-0 kJ/mol. In addition, the absolute ΔGθvalue of each test sample was less than 40 kJ/mol, which indicated that the adsorption process was controlled by the physical adsorption mechanism[33].

4 Conclusions

In this study, the waste coffee-grounds carbon(WCGC) was prepared from the agricultural waste of coffee-grounds (WCG) processing using a H3PO4treated method and used as an adsorbent to remove phenol from aqueous solution. Results showed that the phenol adsorbed on WCGC process was dependent on the solution pH, calcined temperature, the amount of WCGC, and initial phenol concentration. Three kinetic models were used to adjust the adsorption and the best fit was the Weber-Morris kinetic model. The adsorption process was controlled by the physical adsorption mechanism. The BET surface of the produced WCGC(700 ℃, 2 h) is 435.98 m2/g, the total pore volume is 0.30 m2/g, and the average pore size is 3.96 nm. The coffee ground carbon material has been graphitized and the layered structure of WCGC (700 ℃, 2 h) increases its specific surface area and pore volume and results in its good phenol adsorption.