Activated Carbons for Water Treatment Prepared by Phosphoric Acid

Article pubs.acs.org/IECR

Activated Carbons for Water Treatment Prepared by Phosphoric Acid Activation of Hydrothermally Treated Beer Waste Wenming Hao,† Eva Björkman,†,‡ Malte Lilliestråle,‡ and Niklas Hedin*,† †

Berzelii Centre EXSELENT on Porous Materials and Department of Materials and Environmental Chemistry, Stockholm University, Stockholm, SE-106 91, Sweden ‡ Biokol Lilliestråle & Co KB Sibyllegatan 53, SE-114 43 Stockholm, Sweden S Supporting Information *

ABSTRACT: Activated carbons were produced by chemical activation of hydrothermally carbonized (HTC) beer waste, with phosphoric acid as the activation agent. The activation was optimized within a full factorial design, using the outcome of 19 different experiments. Four different parameters (concentration of the acid, activation time, activation temperature, flow rate) were analyzed with respect to their influence on the median pore size. The concentration of H3PO4 had a strong positive effect on the median pore size. The specific surface areas of these activated carbons were ∼1000 m2/g, which compared well commercially available activated carbons. The activated carbons had mostly large pores with a size of ∼4 nm, and a significant amount of acid surface groups. Scanning electron microscopy (SEM) revealed that the morphology of the HTC beer waste changed significantly after the chemical activation. The capacity to adsorb methylene blue from aqueous solutions was 341 mg/g, for one of the activated carbons at pH 7. A Langmuir model described the uptake of the dye quite well, which suggested a homogeneous adsorption of Methylene Blue (MB).

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INTRODUCTION Powdered activated carbons are currently used for water treatment and purification in batch reactors.1−3 Today’s activated carbons are prepared from a range of different precursors by physical or chemical activation. Typical precursors are coconut shells, peat, coal, petroleum pitch, etc.4 Also agricultural waste is used commercially to produce activated carbons and a wider use of precursors based on waste biomass is actively researched.5 The use of agricultural waste such as rice straw,6 rice husks,7 olive stones,8 walnut shells,9 sugar cane bagasse,6 date stones,10 almond shells,11 corn cobs,12 waste tea,13 waste apricot,14 sawdust,15 cherry stones,16 durian shells,17 herb residues,18 coffee husks,19 and cotton stalks2,20 has been studied. Hydrothermal pretreatment of biowaste can offer new precursors for activated carbons. By such pretreatment, a wet biomass is dehydrated and transformed to a precursor material that is chemically similar to peat or lignite.21 Such a precursor can be further transformed to functional activated carbons. Various agricultural waste such as horse manure, biosludge, grass cutting, beer waste, sawdust, algae, walnut shell, rice husk, sunflower stem, and olive stone, has been sequentially transformed to activated carbons by “hydrothermal carbonization” and physical or chemical activation.21−29 Activated carbons can be prepared by physical activation in CO2 or steam. Such carbons typically have small micropores.30 We and Roman et al. have studied how hydrothermally carbonized (HTC) biomass can be physically activated in CO2 at elevated temperatures.22,23 Chemical activation of HTC biomass with KOH was studied by Sevilla et al.29 Activation with KOH is well-established to render activated carbons with very high specific surface areas.5 Chemical activation with © 2014 American Chemical Society

H3PO4 can also be used to synthesize activated carbons with high specific surface areas.31 An optimization of the activation parameters is preferred when activating HTC biomass. For such optimization, the methods of design of experiments are quite useful. Especially, a factorial design allows the dependencies of several conditions to be studied simultaneously.32 These conditions (called experimental variables) are typically varied between two levels. This systematic approach allows nonlinear dependencies for a variable, and codependencies between variables, to be evaluated in a straightforward manner. The number of experiments needed is 2k for a full factorial design, where k is the number of different variables studied. This method is rather time efficient, when k is not too large. Here, a factorial design is applied to access the important conditions for H3PO4 activation of HTC beer waste. To our best knowledge, H3PO4 activation of HTC biomass has not yet been reported.

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EXPERIMENTAL SECTION Materials. Beer waste, from Ringnes Brewery (a Norwegian brewery in Trondheim, Norway), was hydrothermally transformed by the company “Biokol KB”. For this transformation, first, the semidry beer waste was divided into centimeter-sized pieces and mixed with water, an organic acid, and FeSO4 in an autoclave. After such mixing, the temperature was held at 180− 230 °C for a few hours, at an autogenic pressure of 10−20 bar. Special Issue: Alı ́rio Rodrigues Festschrift Received: Revised: Accepted: Published: 15389

January 31, 2014 April 11, 2014 April 14, 2014 April 14, 2014 dx.doi.org/10.1021/ie5004569 | Ind. Eng. Chem. Res. 2014, 53, 15389−15397

Industrial & Engineering Chemistry Research

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Thermogravimetric analyses (TGA) were performed on the HTC beer waste and the HTC beer waste/H3PO4 mixtures. A Perkin−Elmer Model TAG7 instrument was used for TGA. The mass was recorded on an increased temperature in a flow of N2. Platinum cups were used for the samples; the temperature was raised by 10 °C/min, from 20 °C to 900 °C. Fourier transform infrared (FTIR) spectra were recorded for finely ground powders of HTC beer waste and the activated carbons. A Varian Model 670-IR spectrometer equipped with an attenuated total reflection (ATR) detection device (Goldengate by Specac) and a room-temperature detector (based on deuterated triglycine sulfate) was used. Spectra were recorded in the range of 390−4000 cm−1 with a spectral resolution of 4 cm−1. Acid surface groups were determined by the Boehm titration method.33 Three allots of 300 mg of activated carbon were suspended in 12 mL of aqueous solutions with 0.05 mol/dm3 base. The bases used were NaOH (Sigma−Aldrich), Na2CO3 (Sigma−Aldrich) and NaHCO3 (Sigma). The suspensions were agitated for 24 h, after which the carbons were filtered off and the eluates analyzed. Aliquots of 10 mL of were acidified. Twenty milliliters (20 mL) of aqueous solutions of HCl (0.05 mol/dm3; Sigma−Aldrich) were added to the aliquots for the NaOH and NaHCO3 cases, and 30 mL for the Na2CO3 case. The acidified solutions were back-titrated with aqueous solutions of NaOH (0.05 mol/dm3). The end point at pH 7.0 was determined with a pH electrode (pH 1000L, VWR), and the corresponding volume was used to quantify the amount of acid groups. Carbon dioxide was removed by bubbling N2 into the vials with a needle for 2 h before the titration and continued throughout the titration. The equilibrium uptake of the Methylene Blue (MB) dye was determined by batch adsorption. Several suspensions of the selected activated carbon were prepared from aqueous MB solutions at different initial concentrations (Co). For each suspension, 20 mg of activated carbon was added to an aqueous MB solution (20 mL). The suspensions were stirred at a rate of 500 rpm for 7 h to ensure adsorption equilibrium. After which the activated carbon was filtered off, and the eluate was analyzed with respect to the MB concentration. The equilibrium concentrations of the dye (Ce) were determined by ultraviolet-visible (UV-vis) spectrophotometry. The absorbance at a wavelength of 665 nm was used and attributed to the MB. Standard solutions of MB were used to obtain a calibration curve. The specific amount of MB adsorbed at equilibrium (qe, mg/g) was calculated by using the relation

The resulting slurry of HTC beer waste was pumped into a large vessel and equilibrated for ∼5−6 h at a somewhat elevated temperature. After cooling the HTC beer waste was filtered off. The moisture, ash content, and partial elemental composition of the HTC beer waste were determined by standardized methods. Activation. For each activation, ∼2 g of HTC beer waste was mixed with 8 mL of an aqueous solution of H3PO4 (concentrations of 40−85 wt %) and kept overnight. Subsequentially, the mixtures were dried at a temperature of 100 °C for 2 h. These dried mixtures were activated in a homebuilt vertical steel reactor. The temperature was increased from room temperature to the activation temperature with a rate of 10 °C/min. Activation temperatures of 600−700 °C were used. The HTC beer waste/H3PO4 mixtures were activated for 1−3 h in a flow of N2. Afterward, the reactor, with the activated carbon, was cooled to room temperature. The crude activated carbons were washed thoroughly in hot water until the pH of the eluate stabilized at pH 7. Characterization. With regard to elemental analysis, CHN analyses were carried out by combustion analysis. The amount of P and Fe atoms were determined with a Varian Vista MPX ICP-OES system. Iodine number was measured following the standard test (ASTM Standard D4607.94). The density of activated HTC beer waste was analyzed on a 1 cm3 Micromeritics AccuPyc 1330 Pycnometer by measuring the pressure change of helium. About 1 cm3 of sample was used and 10 measurements were repeated to estimate the uncertainty. Particle size was analyzed by light scattering/diffraction. The measurements were performed on a Malverns Mastersizer 2000 instrument. One hundred thirty eight milligrams (138 mg) of sample was dispersed in 20 mL of distilled water. The sample was extracted in several portions during vigorous magnetic stirring and transferred to the instrument. For each extracted sample, three measurements were performed and an average value was calculated. The adsorption and desorption isotherms for N2 were recorded on the activated carbons at a temperature of −196 °C, using a Micromeritics ASAP 2020 device. Before conducting these adsorption experiments, the samples of activated carbon were degassed under conditions of dynamic vacuum at a temperature of 300 °C for 5 h. The specific surface areas of the activated carbons were determined in the Brunauer−Emmett− Teller (BET) model from the amount of N2 adsorbed at relative pressures of P/P0 = 0.01−0.2. Care was taken to use a pressure range that ensured that the “c” values in the BET model were properly positive. The total pore volumes (Vt) were determined from the uptake at P/P0 = 0.99. The t-plot method was used to determine the micropore volume (Vmic) and the external surface area (Sext). (According IUPAC, micropores are pHPZC, the surface of the adsorbent is negatively charged, which increases the capacity of the adsorption of the positively charged MB molecule on the activated carbon.

Figure 6. Plots of observed values versus the predicted response for the median pore size (Y1) of chemically activated carbons prepared by activation of hydrothermally carbonized beer waste with H3PO4.

Figure 7. (a) N2 adsorption and desorption isotherms and (b) pore size distributions of an activated carbon prepared from hydrothermally carbonized beer waste at 600 °C with H3PO4/carbon ratio of 5 at a flow rate of 48 dm3/h and an activation time of 1 h. The pore size distributions were calculated from the adsorption and desorption branches of the isotherms using the Barrett−Joyner−Halenda (BJH) model.

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CONCLUSION Hydrothermally carbonized beer waste was successfully activated into activated carbons by chemical activation with H3PO4. We expect that such activation can rather straight forwardly be extended to other HTC biomass as well. The concentration of H3PO4 acid influenced the median pore size significantly, and we expect that further tuning of the pore size can be achieved. Further investigation of the effect of low concentrations of H3PO4 could be relevant. A large amount of acid groups were introduced by the phosphoric acid used for the chemical activation. Most of these groups are relatively strong acids. We speculate these activated carbons with large pores and many acid groups could be good adsorbents for large molecules and in particular positively charged ones. Further mechanistic studies of how these phosphoric acid and organic acid groups affect the adsorption of molecules from solutions could be important to the understanding of this type of

the carbon. The phosphonic acid groups were included within these categories; in this context, please note that the three pKa values of H3PO4 are 2.1, 7.2, and 12.7. The activated carbon contained 1.5 mequiv/g acid groups, most of which were relatively strong acids (phosphoric, lactonic, and carboxylic) (see Table 4). As compared with physical activation,44 chemical activation with H3PO4 introduced many more acid groups on the interfaces of the activated carbons. For applications in water purification, we speculate that large pores would be especially beneficial for the removal of somewhat large positive molecules.19 To further study this 15394

dx.doi.org/10.1021/ie5004569 | Ind. Eng. Chem. Res. 2014, 53, 15389−15397


Article

Figure 8. Scanning electron microscopy (SEM) micrographs of (a, b) hydrothermally carbonized (HTC) beer waste and (c, d) activated carbon prepared from HTC beer waste at 600 °C with a H3PO4/carbon ratio of 5, a flow rate of 48 dm3/h and an activation time of 1 h.

Table 4. Surface Functional Groups of the Activated Carbon Prepared from Hydrothermally Carbonized Beer Waste at 600 °C with H3PO4/Carbon Ratio of 5 at a Flow Rate of 48 dm3/h and an Activation Time of 1 h functional groups phenolic lactonic carboxylica total

nCSF (mequiv/g) 0 0.7 0.8 1.5

± ± ± ±

0.2 0.2 0.1 0.1

a

We could not separate the contributions from phosphoric and polyphosphoric acid groups. Figure 10. Adsorbed amount of MB on an activated carbon as a function of time. The activated carbon was prepared from hydrothermally carbonized beer waste at 600 °C with H3PO4/carbon ratio of 5 at a flow rate of 48 dm3/h and an activation time of 1 h. (Initial concentration of MB = 400 mg/L, pH 7.)

Figure 9. Adsorbed amounts of Methylene Blue (MB) (denoted by data points, ■) onto activated carbon prepared from hydrothermally carbonized beer waste at 600 °C with a H3PO4/carbon ratio of 5 at a flow rate of 48 dm3/h and an activation time of 1 h. Regression lines to the Langmuir model (solid line), the Freundlich model (dashed line), and the Temkin model (dotted line) are presented. Figure 11. Adsorbed amount of MB on the activated carbon under different initial pH conditions. The activated carbon was prepared from hydrothermally carbonized beer waste at 600 °C with a H3PO4/ carbon ratio of 5 at a flow rate of 48 dm3/h and an activation time of 1 h. (Initial concentration of MB = 400 mg/L, adsorption time 7 h.)

activated carbons in applications. Such applications could include water purification, but one could also speculate that the phosphoric acid groups could be beneficial to certain electrochemical applications as well. 15395



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ASSOCIATED CONTENT

S Supporting Information *

FTIR spectra of HTC beer waste and a corresponding activated carbon. Yield and textual characteristics of the activated carbons prepared from HTC beer waste. Statistical analysis. Detail of the models for MB adsorption isotherm. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: +46-8-162417/+46-8-152187. E-mail: niklas. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS N.H. acknowledges a grant from the Swedish Energy Agency. REFERENCES

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Activated Carbons for Water Treatment Prepared by Phosphoric Acid

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