Characterization of bio char derived from tapioca skin

Pyrolysis of tapioca skin was conducted to produce bio chars in the range between 500°C–800°C. Surface modification treatment were performed on bio chars by using chemicals within 24 hours at 30°C and hot water within 1 hour to enhance the bio char’s adsorption properties according to surface area, pore volume, pore size, crystallinity structure and functional groups. The samples were characterized by using BET, XRD, FTIR and Methylene Blue adsorption. Based on BET result, it showed the surface area increased as the pyrolysis temperature increased followed by pore volume and pore size for S0. The optimum temperature for SNaOH, SHW and SMeOH was at 600°C, 700°C and 800°C with the surface area of 75.9874, 274.5066 and 351.5531 m2/g respectively compared to S0 while SP3HO4 has the worst result since it felt on macroporous structure. The percentage of MB adsorption was followed the size of bio chars surface area. Based on FTIR result, at temperature 500°C to 700°C, the bio chars still have functional groups while at 800°C, many functional groups were diminished due to high temperature struck on them. XRD result showed all the bio chars were amorphous. In conclusion, the best surface modification treatment was by Methanol followed by hot water and Sodium Hydroxide at temperature of 700°C and 800°C while Ortho-Phosphoric acid was the worst one and was not suitable for bio char’s surface modification for adsorption purpose.


Introduction
In the era of globalization now, world-wide carbon dioxide CO2 emissions from energy uses are increasing, and estimated at 2020, the world will produce 33.8 billion metric tons of CO2 [1]. Regarding this issue, it can be seen the world strive for high and infinite development from developed countries which uses fossil fuel to generate energy for their development. Therefore, this action causes the CO2 gas release to surrounding become increase from year to year. The increases of CO2 emission have contributed to the research in alternative energy, which is from biomass [2]. Biomass energy is abundant, cheap and clean since it trapped solar energy through photosynthesis process to produce sugar and oxygen.
The agriculture waste also known as biomass, which can be converting into value-added products. Waste of agricultural product such as apricot kernel shell, hornbeam sawdust, rice straw, rice bran and rice husk has been widely used in research in order to produce bio char. According to [3], organic biomass which undergoes pyrolysis process has high carbon content is called bio char. As stated by [4], bio char also known as activated carbon. Bio char was widely used as a tool for waste management, sequestration and mitigation of climate change, treatment of waste water, building 2 To whom any correspondence should be addressed. sector, cosmetics industries, metallurgy, food industry, energy production, and last but not least as a support for catalyst development. This can be show bio char has various purposes in human life.
On the other hand, a developed country has improper solid and agricultural waste management including in Malaysia. Since many Small and Medium Industries (SMI) present [5], many agriculture wastes such as tapioca skins were produced which contribute to the increasing of waste and affect the usage area of disposal site.
Other than that, water pollution was one of the problems faced by many countries and need more attention to treated in a safe way with lowest cost. Therefore, tapioca skin can be converting into bio chars due to its ability in treating wastewater. The properties of bio-char depend on types of biomass used and the production condition like temperature, pressure and moisture content. The method of producing bio char by using pyrolysis may remove the carbon dioxide from the air, increase the crops growth and keep the carbon on the soil from returning to the atmosphere.
In a view by [7], bio char is the most effective and widely used method for removal of organic compound such as chlorine, pesticides, volatile compounds, certain metal and many more since it has high content of activated carbon. In wastewater treatment scope, bio char can help in treating water by adsorption process that is by carbon filtering. This can proved that active bio-char filters are most effective in removing of odour, sediments, volatile organic compounds (VOCs), chlorine and taste. According to [8] due to the presence of anions like hydroxyl and carboxyl groups, bio-char acts as cation exchanger. In addition to that, classical graphite structure of carbon in bio-char enables the carbon to connect with neighbouring atoms or atoms from foreign molecules, which increases adsorption capacity. This has been proven by [6], potato peel waste has the ability in removing cobalt ions from synthetic wastewaters (composed of various Co(II) concentrations in distilled water which adjusted previously at different pH values) without any co-existing ions.
Bio char can be used as a material to treat wastewater due to its large surface area and high porosity if it produced by pyrolysis at certain condition. Large surface area of bio-char helps in increasing water holding capacity by combining the impurities with its active sites, cation exchange capacity (CEC) and microbial activity. Therefore, tapioca skin waste produced from SMI can be converted into bio chars in treating wastewater besides can decrease the emission of CO2 in the air. Many other studies have done to produce bio char for treatment of wastewater. However, the search for process to produce bio-char with better yield for absorption properties still face a major challenge in this field.
The main objectives of this study is to prepare bio char from tapioca skin by using pyrolysis at difference temperature (500°C, 600°C, 700°C and 800°C) and to enhance bio char properties for adsorption purpose by using surface modification treatment that is hot water treatment and chemical treatment that is alkali, acid and solvent.

Sample Preparation
Dried tapioca skins were cut into small pieces and were compressed into 4 different crucibles to be pyrolyzed in Carbolite Furnace at 500°C, 600°C, 700°C and 800°C within 30 minutes respectively. The samples were collected and were kept in sealed plastic after it was cooled to room temperature.

Surface modification Treatment
Three samples of 1.0 g of bio chars at temperature 500°C were weighted and were put into 100 ml solution of NaOH.H2O, H3PO4 and Methanol respectively. All the three samples were stirred within 24 h at 30°C. Next, the samples were filtered and were dried at 80°C for 24 h. The steps were repeated for bio chars at 600°C, 700°C and 800°C. Similarly, for hot water treatment, 1.0 g of bio char was weighted and was put into 100 ml of hot water at temperature 90°C for 30 minutes. Next, the sample was filtered and dried in oven at 80°C for 24 h. All the samples were kept in desiccator for further used. S0 was stand for untreated bio chars while SMeOH, SNaOH, SHW and SH3PO4 were stand for bio chars treated with Methanol, Sodium Hydroxide, hot water and Ortho-Phosphoric acid respectively. All the methods were adopted from [9].

Characterization
The samples were characterized by using BET, Methylene blue adsorption, XRD and FTIR. BET characterization was performed at 250°C digest temperature within 30 minutes. Based on MB adsorption, 5 different concentrations of MB at 2,4,6,8 and 10 ppm were prepared to create calibration curve. Then, 0.2 g of bio char samples were put into 20 ml of 10 ppm MB solution and were left for 1 h for absorption process. Lastly, the filtrates were filtered before analyzed by using UV-Vis at 645 nm. Other than that, for XRD characterization process, the initial and end angle used was 100 and 700 respectively with 100/min speed. The Voltage used was 20 Watt and the amplitude was 20 mA.

BET Result Regarding Surface Area, Pore Volume and Pore Size
Based on Figure 1, surface area of bio char samples was increase as the pyrolysis temperature increased. This can be shown surface area for S0 at 500°C, 600°C, 700°C and 800°C was 1.7007, 6.0385, 134.4512 and 206.3966 m 2 /g respectively. The highest S0 surface area was at 800°C. Other than that, for SMeOH, the surface area at 500°C to 800°C was 3.2426, 10.3776, 237.2951 and 351.5531 m 2 /g respectively, compared to S0, SMeOH at 800°C has the highest surface area that was 351.5531 m 2 /g. This can be concluded Methanol solvent has the ability in widen the pores wall from micro-pores to meso-pores. According to [10], the presence of Methanol or other alcohols contributed to mesoporous materials with fibrous, high surface area, large pore volume and open web-like structures. In a view [11], the presence of hydrophilic and the strongly polarity compound with the polarity index of 6.6 of sugar in Methanol solvent has proved its ability to increase the pore size and surface area. Based on Graph 1, SMeOH showed the largest surface area compared to others bio chars.
Similarly, surface area of SHW at 500°C, 600°C, 700°C and 800°C was 2.4024, 2.9233, 274.0566 and 188.2421 m 2 /g respectively. It showed at 700°C, the surface area produced was the highest compared to S0 at 700°C. This can be shown hot water increased the surface area from temperature of 500°C to 700°C compared to S0, while at 800°C, the surface area decreased from 206.3966 to 188.2421 m 2 /g. This proven that hot water has the ability to remove any minerals deposited on the pore's wall. In accordance to [12], bio chars were heterogeneous materials contain high carbon and minerals. Related to the statement, [12] proved hot water vapor penetrated bio char layers during water quenching process. This seen that hot water has an activating effect and expulse minerals out from bio chars in his analysis. In conclusion, hot water has the ability in increasing the surface area, pore volume and pore size of bio chars pyrolyzed at 500°C to 700°C. Briefly, the optimum temperature for SHW was at 700°C since it has the highest surface area compared to bio chars at 800°C.
Other than that, for SNaOH, the surface area at 500°C, 700°C and 800°C was decreased a little compared to the original ones except at temperature 600°C. This clearly seen in Graph 1, the surface area was 2.3235, 75.9874, 88.3366 and 121.7390 m 2 /g respectively. From the observation and comparison between the SNaOH against S0, it can be concluded NaOH was not suitable to modify bio chars compared to the Methanol. In conclusion, the optimum SNaOH was at temperature 600°C compared to the original bio chars.
Lastly, surface areas of SH3PO4 were 1.5183, 1.9945, 7.1621 and 1.2931 m 2 /g for temperature of 500°C to 800°C respectively. Ortho-phosphoric acids were used in treating bio chars since it was a triprotic acid. Triprotic acid was an acid that has highest proton and readily to donate it to reagents, with the three protons. Based on the surface treatment, compared to the S0, SH3PO4 has the lowest surface area and the worst result. The acid was too strong until diminish the surface wall. Hence, it indicated SH3PO4 at temperature 500°C-800°C best as solid fuel as the porosity produced was not in the range for adsorption purposes. In conclusion, the higher the pyrolysis temperature, the higher the surface area of the bio chars. Besides that, it's can be concluded SMeOH has the highest surface area followed by SHW, S0, SNaOH and SH3PO4. Referring to Figure 2, the pore volume also increased according to the pyrolysis temperature that was 0.000572, 0.002743, 0.043207 and 0.062053 cm 3 /g respectively for S0 at temperature 500°C to 800°C. Other than that, pore volume for SMeOH also increased according to the pyrolysis temperature that was 0.001135, 0.003444, 0.071476 and 0.175246 cm 3 /g at 500°C to 800°C respectively. Next, the pore volume for SHW also increased according to the pyrolysis temperature that was 0.000566, 0.000890, 0.082317 cm 3 /g for temperature of 500°C to 700°C compared to the S0 but decrease at 800°C with 0.055122 cm 3 /g. This is due to bio char has reached its resistance temperature at 700°C.
Meanwhile, for the pore volume of SNaOH, the result also showed decreasing in value that was 0.000618, 0.024615, 0.027207 and 0.033994 cm3/g respectively. Last but not least, for SH3PO4, no reading showed since BET only cover the micro-porous and meso-porous structure only. In conclusion based on pore volume size, this can be concluded pore volume was related to the surface area.  at temperature of 800°C lied in micropores type with the pore size of 19.9396 Å. Hence, the small pore diameters showed that narrow and deep pores of bio char was formed.
Other than that, pore type for SHW at temperature of 500°C to 600°C were in mesopores category with the pore size of 118.4134 and 76.7675 Å. Meanwhile, at temperature of 700°C and 800°C, SHW were in micropores category with the pore size of 18.7016 and 19.7218 Å. In addition to that, SNaOH, lied on mesopores category that was 150.4447, 24.6137, 23.2339 and 22.4776 Å for temperature of 500 to 800°C. Based on observation and comparison between SNaOH against the S0, it can be concluded NaOH was not really suitable in modifying bio chars compared to Methanol and hot water.
Lastly, pore size for SP3HO4 were lied on macropores line. This can be concluded, SP3HO4 was not suitable to be used in adsorption purposes in treating waste water but can be used as fuel solid that was charcoal. temperature from 600°C to 800°C and the maximum absorption was at temperature 800°C with 99.50%. Therefore, based on the percentage of the MB absorption by SMeOH, this data related and was supported the surface area data obtained from BET result.
The percentage of MB absorption by SHW was 87.71%, 91.82%, 95.11% and 95.59%. Other studies indicated bio chars treated with hot water possess higher adsorption capacities. However, opposite results were observed in this experiment. At temperature of 500°C to 800°C the percentage of MB absorption showed a little decrement compared to the S0 even though the surface area of the treated one was larger. This can be concluded SHW followed the rule in which the highest surface area that can be yield by SHW was 700°C, since it was the maximum temperature resistance.
Other than that, for percentage of MB absorption by SNaOH, the value was 84.14%, 92.26%, 94.93% and 97.53%. The percentage of the MB absorption was tele with the surface area as shown in Figure 1 1 that was 2.3235, 75.9874, 88.3366 and 121.3790 m 2 /g at temperature of 500°C to 800°C. In addition to that, the percentage of MB absorption by SP3HO4 was 57.11%, 50.79%, 58.42% and 64.12%. The percentage of the MB absorption was also tele with the surface area as shown in Figure 1 that was 1.5183, 1.9945, 7.1621 and 1.2931 m 2 /g.
In conclusion, the bio chars surface area gave a very important impact on the percentage of MB absorption. The result showed every temperature gave different surface area for adsorption purposes. The larger the surface area of bio chars yield the larger percentage of MB absorption. From the analyzed data in Figure 3, the best bio char performance in MB absorption was SMeOH followed by SNaOH and SHW. This can be concluded the optimum temperature for SNaOH was at temperature 600°C, while for SHW and SMeOH was at temperature 700°C and 800°C. The worse bio char in MB absorption was bio char treated by Ortho-Phosphoric acid since it produced smallest surface area resulted too strong acid which destroyed the samples wall. Therefore, the optimum temperature of bio chars in MB absorption was 700°C and 800°C.

XRD analysis
Based on the results at Figure 4, it showed all bio chars has the amorphous structure since at 500°C, all the crystalline structures were diminished due too high temperature penetrate on bio chars surface.

FTIR analysis
FTIR was conducted to determine the functional group presented on bio chars surface. Based on Figure 5, the band at 3348 cm −1 represented the stretching vibrations of the OH groups, which could be attributed to the adsorbed water on the bio char. It showed the intensity and shaped was strong. Other than that, the band at 1569.92 cm -1 represented the stretching variations C=C aromatic. The double bonds appeared as medium to strong absorptions. Based on the C=C bond, it indicated the presence of adjacent carbon in the bio chars. At band 1043.58 cm -1 , it represented the stretching variations of the C-OH. It showed strong intensity of hydroxyl group.  Regarding Figure 6, the band at 1391.76 cm −1 represented the stretching vibrations of NO2 (aliphatic). It belonged to nitro group. It showed the intensity was strong. Other than that, at band 873.39 cm −1 represented C-H bend (meta) aromatics.  Referring to figure 8, the band at 879.12 cm −1 represented the vibrations of C-H bend (para). It showed it belonged to aromatic group. At band 647.09 cm −1 and 618.94 cm −1 represented acetylenic C-H bend. This showed alkynes group present in the bio chars. At the end of the data, it showed the graph was interrupted due to high temperature during pyrolysis until diminish all the functional groups.

Conclusion
Optimum temperature for S0 was at 800°C, while for SMeOH, SHW and SNaOH was at 800°C, 700 and 600°C respectively. The best surface modification treatment for adsorption purpose was by Methanol solvent, followed by hot water and Sodium Hydroxide. The largest surface area and pore volume size was achieved by SMeOH and S0 at 800°C that was 351.5531 m 2 /g and 206.3966 m 2 /g respectively. Meanwhile, surface area for SHW at 700°C and SNaOH at 600°C was 274.5066 m 2 /g and 75.9874 m 2 /g compared to S0. The sample's pore volume also related to the surface area and majority of samples were in microporous and mesoporous line which indicated the bio chars suitable for adsorption purposes. Based on MB adsorption, percentage of MB adsorption by SMeOH was the highest followed by S0, SNaOH and SHW at 800°C that was 99.50, 99.45, 97.53 and 95.59 %.
Lastly, SP3HO4 has the smallest surface area and pore volume compared to S0. The pore size also decreased and lied on macroporous line. Therefore, Ortho-Phosphoric acid was not suitable in modifying bio chars, since it was too strong until break the micropores and mesopores wall. Hence, it indicated the modified bio chars with Ortho-Phosphoric acid at temperature 500°C-800°C best as solid fuel as the porosity produced was not in the range for adsorption purposes.
XRD result showed bio chars have amorphous structure since all the crystallinity has been diminished due to high temperature started at 500°C. FTIR result showed some functional group presents on bio chars surface at 500°C to 700°C. However, it became diminished at temperature 800°C due too strong temperature resistance. In a nutshell, all the bio chars have the ability in adsorption purposes except for SP3HO4.