Preparation and Characterization of Biochar Sorbents Produced from

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Preparation and Characterization of Biochar Sorbents Produced from Malt Spent Rootlets Ioannis D. Manariotis,† Kalliopi N. Fotopoulou,‡ and Hrissi K. Karapanagioti*,‡ †

Department of Civil Engineering, Environmental Engineering Laboratory and ‡Department of Chemistry, University of Patras, Patras 265 04, Greece ABSTRACT: The present study aims at the preparation and characterization of biochars produced from malt spent rootlets (MSR) under different pyrolysis temperatures. The biochars were characterized for their surface area, microporosity, suspension pH, acid−base behavior, and functional groups on their surface. The highest surface area (340 m2/g) and porosity (0.21 cm3/g) were observed for MSR pyrolized at 800 °C. For the same biochar, 67% of the pore volume corresponds to micropores (1% biochar. Differential Potentiometric Mass Titrations. All differential potentiometric mass titrations (DPMT) took place in a thermostated double-walled Pyrex vessel equipped with a magnetic stirrer and a Perspex lid with holes for the electrodes and N2 gas inlet. The pH during the titration experiments was monitored by an Autoburet System (TIM 800 Radiometer, Copenhagen), which was compiled with Timtalk 8, version 2.0 software. N2 gas was passed into the vessel during the experiment in order to prevent the dilution of atmospheric CO2 that would change the pH. DPMTs were performed for a blank solution and suspensions of a sorbent mass at constant ionic strength of 0.1 M NaNO3 and temperature 25.0 ± 0.1 °C. The aqueous suspensions were equilibrated for 2 h in order to reach an equilibrium pH value. A small amount of a base, 1 mL 0.1 M NaOH, was then added to deprotonate the surface sites rendering the surface negative. After 15 min, a new equilibrium pH value was recorded and noted as initial pH. The suspensions were titrated by adding small volumes of certified volumetric standard 0.1 M HNO3, and pH was recorded as a function of the volume of titrant added to the suspension. The concentration of ions, consumed on the surface H+cons/surf (mol L−1) at each pH value, was determined taking into account the titration curve of the blank solution and the titration curve of the suspension.27,28 Additional calculations have been performed so that ion concentration can be expressed as mol m−2. The initial concentration is expressed as mol L−1 by dividing the ion concentration (mol L−1) with the concentration of the solid in the solution (g L−1) and then

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MATERIALS AND METHODS Material Production. MSR was obtained from the Athenian Brewery S.A. (Patras, Greece). Its chemical composition includes 32% protein, 11% fiber, 8.7% ash, 2.5% reducing sugars, 0.9% nonreducing sugars, 27% starch, 0.02% phytic acid, 0.4% polyphenols, 2% Ca, 1%P, 0.2% K, 0.1% Na, 0.01%, Fe, 0.01% Mg, and 0.01% Zn.23,24 MSR was dried overnight at 50 °C and was sieved into 1.180−0.150 mm. MSR was weighted into quartz or ceramic vessels that were closed with their respective caps. These vessels were custom-made not to allow oxygen to enter the vessels at high temperatures. The vessels were placed in a gradient temperature furnace (LH 60/ 12, Nabertherm GmbH, Germany) at a temperature range between 300 and 900 °C. The mass of the MSR was weighted before and after the pyrolysis process, and the weight loss due to pyrolysis was calculated. The MSR biochars were then tested for their different properties. Material Visualization. The macroscopic structure of biochars was visualized with a JEOL 6300, scanning electron microscope (SEM). The microscope was equipped with spectrometers energy dispersion X-ray (EDS), wavelength dispersion X-ray (WDS), and Cryotrans. The sample surfaces were coated with a layer of carbon by sputtering using a JEOL, JEE-4B, ions spatter. Photomicrographs were taken with a 200, 1000, and 5000-fold magnification. Surface Area and Porosity. The surface area, the pore volume, and the average pore size of the biochars were determined using gas (N2) adsorption−desorption with the Micromeritics TriStar 3000 Analyzer system using the Brunauer, Emmett, and Teller (BET) equation. This instrument can measure surface areas as small as 0.01 m2 g−1. Isotherms with 30 adsorption and 20 desorption points were conducted at liquid nitrogen temperature (77 K). The raw and charred samples were weighted in Micromeritics sample tubes that were then degassed under nitrogen atmosphere for 24 and 1 h at room temperature and 240 °C, respectively. The results were recorded by Micromeritics TriStar 3000 software. To determine the open surface area, relative pressures above 0.3 were used since by this point all the micropores should be filled and only sorption to open surface area should be occurring. N2 isotherms were transformed into t-plots by converting relative pressures into t values by using the De Boer equation as follows:25 B

DOI: 10.1021/acs.iecr.5b02698 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 1. SEM images of MSRs (a, b) before and after pyrolysis at different temperatures (c, d) at 300 °C, (e, f) at 600 °C, and (g, h) at 850 °C. The gray bar under each picture is the scale bar (60 μm for a, c, e, g; 30 μm for b; and 10 μm for d, f, and h).

by dividing by the specific surface area (m2 g−1) for each sample. Origin 6.0 was used for the presentation of the experimentally determined differential potentiometric titration curves as interpolated smoothed curves. Sorption Tests. Sorption tests were performed with selected biochars and aqueous solutions of phenanthrene and HgCl2. Batch studies were performed with a common initial concentration for each pollutant (400 μg L−1 of phenanthrene at pH 7.5 and 100 mg L−1 of Hg(II) at pH 5) and different materials. The solid to liquid ratios were 3 mg solid 120 mL−1 of phenanthrene solution and 3 mg solid 10 mL−1 of Hg(II) solution. The concentrations of phenanthrene and mercury in the aqueous solution were measured after 7 days and 24 h of contact time, respectively. Detailed experimental conditions

and analytical methods have been described in previous publications.9,11,22

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RESULTS AND DISCUSSION

During visualization with SEM, it was observed that the macroscopic properties of MSR did not change with charring at various temperatures (Figure 1). Even at a high temperature such as 850 °C, the macroscopic structures of raw MSR observed in Figure 1a and b were mainly kept unchanged during pyrolysis. Although several macropores can be observed for the pyrolyzed samples, they are not the pores that create significant surface area for the biochars. At the magnifications used (up to 5000), it was not possible to visually observe C


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Biochar Surface Area and Porosity. From Table 1, it is obvious that the surface area of the materials produced is increased by 3 orders of magnitude with pyrolysis at 750 °C. Table 1 also presents the surface area, the open surface area, the pore volume, the micropore volume, and the average pore size of the various biochars developed at different temperatures. The surface area and the pore volume increase with temperature, whereas the average pore sizes decrease. As temperature increases, more pores are formed, and their size is lower. The t-plot analysis reveals that for material pyrolysis temperatures up to 600 °C, most of the surface area is due to macropores, whereas only a low percentage of the surface area for high-temperature biochars is due to macropores (open surface area/total surface area range between 8 and 25%), and no micropores are present. For material pyrolysis temperatures higher than 600 °C, micropore volume is 46−73% of the total pore volume (Table 1). Detailed measurements of pore evolution with temperature are presented in Figure 3. For raw MSR and biochars pyrolyzed

micropores that might have been formed, and no mesopores were observed either. Biochar Development. During pyrolysis, material weight loss increases from 52 to 85% almost linearly with increasing temperature from 300 to 900 °C, respectively (Figure 2). There

Figure 2. Surface area, total carbon (TC), and weight loss for MSR biochars at various pyrolysis temperatures.

are two optimum temperature ranges for MSR pyrolysis. At 350−400 °C, there is an optimum temperature range for producing biochars with the highest carbon content, whereas at 750−850 °C, there is an optimum temperature range for producing biochars with the highest surface areas. Actually, at 800 °C, the surface area reaches a peak value of 340 m2 g−1 and then drops again with increasing temperature (Figure 2; Table 1). Similar behavior with optimum temperature range for the highest surface area has been observed for the production of char from wood with optimum temperature at 700 °C.29 Another study that examined different feedstock and pyrolysis temperatures (250−700 °C) reported that higher pyrolysis temperatures resulted in higher specific surfaces.3 Furthermore, the specific surface area values also depended on the feedstock type, and only with a pecan shell a surface area equal to 222 m2 g−1 was obtained at 700 °C. Similar results were reported by Uchimiya et al.12 who reported that high BET surface areas were obtained at the highest pyrolysis temperatures examined (650−800 °C) and that the surface areas of cottonseed hull chars were lower at the respective pyrolysis temperatures than the wood, pine needle, or grass chars. Antal and Gronli13 reported that the maximum surface areas were typically observed at 500−900 °C. When dairy manure and woodchips were pyrolyzed at temperatures from 300 to 700 °C, the highest specific surface was observed at 700 °C.30

Figure 3. Pore size distribution of MSRs after pyrolysis at different temperatures.

Table 1. Material Properties after Charring at Different Temperatures (Initial MSR Grain Size: 1.18−0.150 mm)a temperature (°C)

BET surface area (m2 g−1)

0 300 350 400 500 600 750 800 850 900

0.18 0.50 ± 0.10 0.05 3.4 5.5 11 ± 4.0 260 ± 42 340 ± 5.8 300 ± 27 230 ± 25

a

t-plot open surface area (m2 g−1)

pore volume (cm3 g−1)

micropore volume (cm3 g−1)

0.000 0.000 0.016 ± 0.002 0.002 0.007 ± 0.001 0.15 ± 0.020 0.21 ± 0.000 0.18 ± 0.010 0.16 ± 0.040

3.9 24 43 25 59

± ± ± ±

2.1 1.0 20 33

average pore size (Å) 204 860 ± 9.2

0.11 ± 0.018 0.14 ± 0.00 0.082 ± 0.067 0.08

170 750 1000 50 49 46 52

± 15 ± ± ± ± ±

220 2.1 1.2 1.7 7.1

The percentage of volume of micropores/pore volume is 73% at 750, 67% at 800, 46% at 850, and 50% at 900 °C. D


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Industrial & Engineering Chemistry Research below 400 °C, it was not possible to measure the pore size distribution, and thus, it is not presented in Figure 3. However, for biochars pyrolyzed at 400−600 °C (Figure 3), most of the pore volume is found in mesopores (2−50 nm) and micropores (12 g L−1) to the NaNO3 electrolyte solution, a plateau value is observed. Raw MSR suspensions demonstrate a plateau value at pH 5.3. For biochars pyrolyzed at temperatures below and above 600 °C, suspension pH plateau is close to neutral (pH 7.5) and highly alkaline (pH 9.9), respectively. Thus, high-temperature biochars can be added to solutions to produce alkaline suspensions. Another study also reported pH values between 7.59 and 9.77 at pyrolysis temperatures from 400 to 600 °C, respectively.21 There are several possible explanations for this alkaline pH (e.g., the presence of oxides on the surface of the biochar, the removal of acidic groups, chloride ion sorption on the surface, etc.); however, the exact reason for this behavior is under investigation. To better understand the acid−base behavior of MSR biochars, the FTIR spectra of the samples are presented in Figure 5. All samples present a deep and strong peak at 1385 cm−1 that corresponds to (C−H) bond and shallow peaks at 830 (C−H), 1700 (CO), and 1765 (CC) cm−1. Raw MSR demonstrates several peaks that are related to functional groups E


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are anticipated to be good sorbents for organic hydrophobic compounds. Phenanthrene and Mercury Sorption. The sorption capacity of biochars for phenanthrene increases by almost 1 order of magnitude compared to the raw material. However, this is only true for the biochars pyrolyzed at or above 750 °C (Figure 7a). For low pyrolysis temperatures, sorption increases

Figure 7. Sorption of (a) phenanthrene (Co = 400 μg L−1) and (b) mercury (Co = 100 mg L−1 pH 5) by raw MSR and biochars produced at different temperatures after 7 days and 24 h of contact time, respectively. q is the amount of pollutant sorbed per mass of sorbent, and Co is the pollutant initial concentration.

by a maximum factor of 4. Similar trends are observed for mercury sorption (Figure 7b). For mercury, sorption capacity of biochars increases by a maximum factor of 4 and a factor of 6 for low- and high-temperature biochars, respectively. These observations corroborate the results of sorbent characterization suggesting that increased biochar pyrolysis temperatures result in high surface, less functional groups, less surface charge, and thus, better sorbents for hydrophobic organic compounds such as phenanthrene and molecular compounds such as mercury species. In another study, the carbonization temperature strongly influenced the trichloroethylene sorption on biochars produced form soybean stover and peanut shells. The high sorption of biochars produced at 700 °C, compared to 300 °C, was attributed to their aromaticity and low polarity.32 The sorption capacity of phenanthrene by biochars produced from plant residues and animal wastes was slightly higher for biochars produced at 600 °C compared to biochars produced at 400 °C.33 Similarly, the highest sorption of naphthalene by biochar produced from pine wood at temperatures from 150 to 700 °C was observed at 700 °C.34

Figure 6. DPMT curves for raw MSR and biochars produced at different temperatures. The negative values of the y-axis suggest consumption of H+ on the surface and removal from the solution. A negative peak suggests the consumption of hydrogen ions by a functional group on the surface.

that as the pyrolysis temperature is increased, the material surface loses its acid−base behavior and becomes less reactive. This is in agreement with the FTIR spectra that show the loss of functional groups with increasing pyrolysis temperature. The pH of the high-temperature biochars is rather high (9.9) suggesting a reversal of surface charge compared to the raw material at neutral solution pH. At solution pH 7, MSR will be negatively charged (equilibrium MSR pH < 7), whereas the high-temperature biochars will be positively charged (equilibrium biochars pH > 7). However, the low amount of hydrogen ions exchanged with the surface (y-axis in Figure 6) and the lack of functional groups on the surface (Figure 5) suggest that the surface charge is low. This suggests that high-temperature biochars are carbon materials that are nonpolar and, thus, that

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CONCLUSIONS The main conclusions from the present study are as follows: • MSR can be successfully pyrolyzed at different temperatures up to 900 °C. F


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Industrial & Engineering Chemistry Research • As pyrolysis temperature increases, the surface area increases because of the formation of micropores. • MSR contains various functional groups that disappear with increasing pyrolysis temperature. • The pH of the resulting suspensions increases for biochar with increased pyrolysis temperature corroborating the observation of lost functional groups. High surface area biochars based on the results presented in Figure 7 are promising sorbents for both phenanthrene and mercury.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +302610996728. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially funded by “Karatheodori 2009” Program by the University of Patras Research Committee. The authors would like to thank Kamenidou Charoula and George Bokias from the University of Patras and Kyriakos Bourikas from Hellenic Open University for technical assistance and helpful discussions.

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Preparation and Characterization of Biochar Sorbents Produced from

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