Activated Carbon from Sugarcane Bagasse Pyrolysis for Heavy Metals Adsorption

Sugarcane bagasse is an agro-industrial waste available in enormous quantities in Egypt. It is rich of organic carbon which makes it a potential feedstock for activated carbon production. This study provides an optimized pyrolysis method for activated carbon production from Sugarcane bagasse. Sugarcane bagasse samples impregnated with sulfuric acid, for 24 h, and carbonized at 500 °C, for two hours, yielded the best activated carbon with a surface area of 431.375 m2/g. The best impregnation ratio was 2.5:1 (sulfuric acid/bagasse). The prepared activated carbon was used for adsorbing heavy metals (Pb, Cd, Mn, Cu, Cr) from Nile Tilapia reused frying oil. It could adsorb 80% of the heavy metals and particularly removed the Cd. The characteristics of the prepared activated carbon are comparable to those recommended for the commercial activated carbon. The production cost of the activated carbon using this method is about 707 $ which is cheaper than the commercial activated carbon by about 40%.


Introduction
Sugarcane bagasse (SCB) is a fibrous material remains after crushing the sugarcane for extracting its juice.For every tone of sugarcane crushed, about 0.3 tons of bagasse is produced (Iwuozor et al. 2022).In Egypt, about 66,000,000 tons of SCB is produced annually from the sugar industry (Ministry of Trade and Industry in Egypt 2022).
The SCB can be used in various industrial applications such as electricity production, paper production as well as furfural production (Pandey et al. 2000;Athira et al. 2020).Not all produced SCB is used in industries, leaving considerable amounts of organic wastes that can be used in other applications.
The SCB is mainly composed of cellulose, hemicellulose and lignin.The cellulose and hemicellulose together consist 55-85% of the SCB.The lignin is less abundant consisting 15-27% of the bagasse (Mahmud and Anannya 2021; Silva et al. 2021).The cellulose is a polysaccharide semicrystalline polymer composed of long chains of D-glucose units linked by glycosidic bonds; likewise the hemicellulose is an amorphous polysaccharide polymer formed of xylose, mannose, and glucose saccharide units (Betancur and Pereira 2010;Kumar et al. 2020).The lignin, in turns, is polyphenolic macromolecule formed by polymerization of aromatic alcohols (Betancur and Pereira 2010).In general, the SCB is formed of 60 to 80% of carbohydrates (Rezende et al. 2011).The carbohydrates are dominated by carbon atoms associated with hydrogen and oxygen which can be treated to create new bio-products including activated carbon (Zeinaly et al. 2017).
Abstract Sugarcane bagasse is an agro-industrial waste available in enormous quantities in Egypt.It is rich of organic carbon which makes it a potential feedstock for activated carbon production.This study provides an optimized pyrolysis method for activated carbon production from Sugarcane bagasse.Sugarcane bagasse samples impregnated with sulfuric acid, for 24 h, and carbonized at 500 °C, for two hours, yielded the best activated carbon with a surface area of 431.375 m 2 /g.The best impregnation ratio was 2.5:1 (sulfuric acid/bagasse).The prepared activated carbon was used for adsorbing heavy metals (Pb, Cd, Mn, Cu, Cr) from Nile Tilapia reused frying oil.It could adsorb 80% of the heavy metals and particularly removed the Cd.The characteristics of the prepared activated carbon are comparable to those recommended for the commercial activated carbon.The production cost of the activated carbon using this method is about 707 $ which is cheaper than the commercial activated carbon by about 40%.
Activated carbon is a carbonaceous material known for its large specific surface area, superior porosity, high physicochemical stability and excellent surface reactivity (Saleem et al. 2019;Nguyen et al. 2021).It is widely employed as a functional material for various applications such as water purification, heavy metals adsorption, gas storage, volatile organic compounds removal, dyes adsorption, and mercury scrubbing (Sharma et al. 2022).
The activated carbon is commonly produced from wood waste, coal, petroleum residues, peat, lignite, and cellulose waste, which are expensive and nonrenewable materials (Kim et al. 2019;Ouhammou et al. 2019;Saleem et al. 2019).Recently, many studies have investigated the production of activated carbon using low-cost and sustainable alternative precursors for economic feasibility in large-scale production such as rice husk, corn straw, SCB and solid wastes (sludge, food waste, garden waste, etc.) (Yahya et al. 2015;Takdastan et al. 2016;Zhou et al. 2018;Musyoka et al. 2020;Halbus et al. 2021).The activated carbon can be produced using either physical or chemical activation (Moaseri et al. 2017).The physical activation is a thermal treatment which consists of carbonization followed by activation (Tan et al. 2017).In the physical activation, a carbon-rich material is thermochemically carbonized into value-added biochar under temperature in the range 350 to 700 °C followed by exposure to gases (CO 2 and air), or steam, or a mixture of gases and steam at a temperature above 700 °C to yield activated carbon (Sánchez 2011;Lima et al. 2017).However, in the chemical activation (pyrolysis) the raw material is impregnated with strong reagent followed by thermal activation in an inert atmosphere (Saad et al. 2019;Athira et al. 2021).The chemical reagents for impregnation can be acids (e.g., H 2 SO 4 , HCl, H 3 PO 4 , HNO 3 ) or alkalis/bases (e.g., NaOH, KOH, K 2 CO 3 , Ca O) or oxidants (e.g., KMnO 4 , H 2 O 2 ) or salts (ZnCl 2 , K 2 HPO 4 ) (Aworn et al. 2008;Tan et al. 2017).The chemical reagents hinder the formation of tar during the carbonization process which leads to higher activated carbon yield and lower activation temperature than reported in the physical activation method (Ukanwa et al. 2019).Activated carbon obtained by chemical activation has higher surface area and better-developed mesoporosity than that produced by physical activation.The chemical activation process also requires less time compared to the physical activation (Tzvetkov et al. 2016).
Heavy metals are nonbiodegradable and toxic pollutants mainly spread from the industrial wastes (Briffa et al. 2020).Today, various methods are employed to decontaminate the environment from the heavy metals such as adsorption, chemical precipitation, electrodialysis, ultrafiltration, reverse osmosis, oxidation, and ion exchange (Moradihamedani and Abdullah 2017;Chai et al. 2021).The adsorption is the most utilized method for its simplicity and low cost of setup and maintenance (Iwuozor et al. 2021).The activated carbon from agricultural waste has been reported as an effective material for adsorbing chromium (VI) from aqueous solutions as well as adsorbing pollutant gases (Abdulrazak et al. 2016;Somyanonthanakun et al. 2016;Razi et al. 2018;Özsin et al. 2019).
In Egypt and Sudan, the industrial effluents carrying heavy metals are discharged in the Nile River.Accumulation of heavy metals in fish has become a significant hazard worldwide (Toledo et al. 2020).Nile Tilapia (Oreochromis niloticus) is the most consumed fish in Egypt and Sudan and has been found to absorb heavy metals from contaminated water through the gills and skin (Khayatzadeh and Abbasi 2010).Unfortunately, in restaurants fish frying oil is widely reused several times.We hypothesized that heavy metals might defuse from fish into frying oil and back leading to high accumulations in the fried fish.
To the best of our knowledge, there is no work reported on the utilization of activated carbon from SCB for heavy metal adsorption.In this study, we mainly aim at preparing activated carbon from SCB using sulfuric acid impregnation to encourage the localization of the activated carbon production in Egypt.The sulfuric acid, in particular, is a low-cost and recyclable acid.Secondly, we aim at investigating the visibility of utilizing the activated carbon from SCB for removing heavy metals from reused edible oils which are widely used in public food services.

Material and Methods
The experimental flow of this work is depicted in Fig. 1 starting from raw SCB up to the final conclusion.

Organic Carbon Content in SCB
The organic carbon content (OCC) in the SCB was analyzed to evaluate the feasibility of utilizing the SCB for producing activated carbon.The OCC was 92.662% in the raw bagasse powder which demonstrates the feasibility of utilizing it for producing activated carbon.

SCB Preparation
SCB was collected from local market and thoroughly washed with fresh tap water for removing residues before exposed to sunlight for three days followed by hot air drying at 110 °C using a laboratory drying oven (99,200-3, STANHOPE-SETA, Chertesy, UK).During the hot air drying, the bagasse was weighed every 30 min to determine the weight loss.Equation (Eq. 1) was used to calculate the total weight loss (TWL) and define the stopping time of the drying process.The stopping time is the time where the weight of the sample remains constant over two consecutive measurements.A digital balance (ATX Series Class L Laboratory Blanca, MARSDEN, Rotherham, UK) was used though out this work.
where W t is the sample weight at time t, W t-1 is the preceding sample weight and t s is the stopping time.The dried SCB was ground using general purpose grinder (TYPE 721, SMC, Cairo, Egypt) and sieved using a laboratory sieve (Funnel/ model No 0.074 -60 mm, JVLAB, China) to obtain SCB powder with particle sizes between 10 and 40 mesh (Chen et al. 2012).

Activation and Pyrolysis of SCB
The SCB powder was activated by impregnation with 70% sulfuric acid for 24 h followed by the pyrolysis process.Two groups of samples were treated each group contains five samples each sample weighs 5 gm.For group one, five different weight ratios of the sulfuric acid were used to activate the samples (0.5:1, 1:1, 1.5:1, 2:1, and 2.5:1 gm acid: gm powder).However, for group two a constant ratio of 1:1 gm acid: gm powder was used to impregnate the samples.These weights of the sulfuric acid were converted to volumes at ASTM.The purpose of the impregnation process (1) is to initiate the porosity in the powder.After 24 h, the sulfuric acid was removed by washing the samples with distilled water.
Pyrolysis was then performed by burning (carbonizing) the samples in a tubular furnace (50KW horizontal, LABMATE, Hertfordshire, UK) for two hours, while passing 30 ml min −1 of pure nitrogen gas (to create an inert medium).For group one, the carbonization temperature was fixed at 500 °C.For group two, each sample was carbonized at a different temperature (300, 400, 500, 600, 700) °C.The pyrolysis by-product ash was removed from the carbonized samples with hydrochloric acid 10% until the PH reached 3 -4.The samples were rewashed with distilled water to remove the hydrochloric acid before they were re-dried at 110 °C for three hours to remove the moisture and obtain prepared activated carbon (PAC).Furthermore, to assess the effect of the activation on the pyrolysis, SCB powder was carbonized without impregnation.Five samples (5 gm each) of the dried SCB powder were carbonized at 300, 400, 500, 600, and 700 °C using the same aforementioned carbonization process.The resultant charcoals were also characterized.

The Characterization Methods
The following physical and chemical methods were utilized to characterize the bagasse powder and the prepared activated carbon.

Bulk Density (BD)
The BD was performed according to the method described by Waring and Kinsella (Wang and Kinsella 1976).BD is defined as weight divided by volume.Samples were placed in a graduated cylinder (50 ml) and packed by gently tapping the cylinder on the benchtop 10 times.The weight and volume of the samples were recorded to calculate the BD.

Moisture Content
The moisture content was characterized according to the method described by Association of Official Agricultural Chemists AOAC (AOAC 2000).Samples of two grams were weighed on pre-dried aluminum dish and placed in a hot air oven at 103 °C overnight (at normal room temperature and 1 atmosphere).The samples were removed from the oven and left to cool for 20 min at room temperature.The samples were then re-weighed, and the moisture content was calculated as a percentage of the sample weight using Eq. 2 as follows where W 1 is the aluminum dish weight, W 2 is the sample and the aluminum dish weight, and W 3 is dried sample and the aluminum dish weight.

Ash Content
The ash content was characterized according to the method described by the AOAC.Samples of two grams were weighed on a clean dry porcelain crucible and placed in a muffle furnace (Tipoforon Z ANO.12803 Get Ran 1002, MXBAO-HENG) at 600 °C for 6 h.Crucible was transferred to a dedicator, cooled for two hours to room temperature, and re-weighed.
The ash content was calculated using Eq.3: where W 1 is both crucible and ash weight, W 2 is the empty crucible weight, and W s is the sample weight.

Surface Area
Samples were tested using a surface area analyzer (Quanta Chrome Nova-1200, Boynton Beach, FFL 33,426, USA).
Nitrogen adsorption experiments at 77°K were conducted to determine the specific surface area.The samples were outgassed overnight at 180 °C before adsorption measurements were performed.The BET model was applied to fit the nitrogen adsorption isotherms and evaluate the specific surface area of the samples.Equation 4 was used to calculate the BET surface area.
where S is the specific surface area, IN is the iodine number, and MBN is methylene blue number.

Analysis of Heavy Metals in Nile tilapia Reused Frying Oil
To assess the adsorption capacity of our PAC, analysis of heavy metals adsorption was performed.Three samples of Nile tilapia reused frying oil (total of 200 g) were collected from three different restaurants specialized in Nile tilapia frying.The three samples were mixed thoroughly at 34 °C & 1 atmosphere and divided into two portions of 100gm each (control and test portion).Heavy metals in the control sample were analyzed using an atomic absorption spectrophotometer (Buck Model 210VGP, Buck Scientific, Constron, UK).We found Pb, Cd, Mn, and Cu with different concentrations. ( The test portion of the reused frying oil was divided into equal samples of 20 gm.Each sample was mixed with an amount of PAC weighed 0.5, 1, 1.5, 2, and 2.5 gm, respectively, and shock for 20 min.The PAC used in this test had the highest BET surface area.The samples were analyzed using the atomic absorption spectrophotometer to determine the concentration of the heavy metals after treated with the PAC.

Characteristics Comparison and Production Cost Estimation
The characteristics of our PAC were compared to recommended characteristics for the commercial activated carbon (CAC).The reference values of the CAC were obtained from the general carbon corporation website (General 2022).Nevertheless, a preliminary production cost was estimated per ton according to the Egyptian market excluding the capital cost of assets.

Results and Discussion
Raw SCB was washed, dried, ground, and sieved before impregnated and carbonized.Several characterization parameters were calculated, and the prepared activated carbon was used to adsorbing heavy metals.Estimated production cost of activated carbon was calculated and compared with the commercial activated carbon cost.

SCB Characterization
The OCC was 92.662% in the raw bagasse powder, and it dropped to 19.121% post the pyrolysis.In addition, the weight loss of the raw SCB during hot air drying was found to decrease over time.Table 1 and Fig. 2 show the weight loss values and trend.The total weight loss was found 7% of the initial weight, and the required time to achieve the optimum drying was 150 min.The moisture content of the raw bagasse has a direct impact on the production cost of the PAC.
Table 6 shows the characterization results of the SCB powder in comparison with the PAC and CAC.The SCB powder was found to have 7.33 ± 0.461% moisture content after drying, average BD of 0.12 ± 0.008 g/cm 3 , and ash content of 0.45 ± 0.028%.The surface area test was not considered for the SCB powder because it was not yet pyrolyzed.

The SCB Charcoal Characterization
The BD, moisture content, ash content, and surface area of the charcoal were analyzed.Focusing on the surface area as the most important parameter, the charcoal was found as nonporous material.Table 2 shows the results of the charcoal characterization.Increasing the carbonization temperature had no effect on the porosity of the charcoal.However, the appearance color of the carbonized samples changed to black with the increase in the temperature degree.Because charcoal has not undergone activation (i.e., impregnation), this shows the importance of activation step in the activated carbon production.

Weight Loss due to Activation and Pyrolysis
The effect of the sulfuric acid ratios and the pyrolysis temperature on the weight loss during the activation and pyrolysis was assessed.The sample weight loss during the impregnation was found to decrease with the increase in the sulfuric acid volume as depicted in Fig. 3.The maximum weight loss was 34% when the acid volume was 1.35 ml (0.5:1 ratio).Generally, the acceptable loss in any chemical process is 30%.Thus we recommend to use 1:1 or higher  sulfuric acid-to-sample ratio to keep the weight loss below 30%.
The weight loss increased with the increase in pyrolysis temperature.Using 700 °C, the weight loss reached 50% of the initial sample weight when the samples were activated with 1:1 sulfuric acid to sample weight.Table 3 shows the weight loss as a result of pyrolysis temperature.The hightemperature degrees decompose the content of the activated sugarcane powder such as CH 4 , H 2 , CO 2 , and tar (López et al. 2002).Selecting the optimum temperature is necessary to save energy and control the weight loss for engineering and economic considerations.

The Surface Area Assessment
The effect of the sulfuric acid ratios and the pyrolysis temperature on the surface area of the PAC was assessed.The effect of the sulfuric acid was analyzed though group one samples.The samples were impregnated with different ratios of the acid and carbonized at a fixed temperature 500 °C.We found that the surface area value was proportional to the ratio of sulfuric acid (the more the sulfuric acid added, the higher surface area).The highest surface area achieved was 431.375 m 2 /g when the acid ratio was 2.5 of the sample weight and the minimum surface area was 196.951 m 2 /g when the acid ratio was 0.5 of the sample weight.This indicates that the acid increases the porosity of sample.To assess the significance of sulfuric acid ratio on the surface area, analysis of variation (ANOVA) test was conducted with a confidence interval of 95%.We found significant differences in the surface area due to the different sulfuric acid ratios.The P-value between all ratios was (P ≤ 0.0001).Table 4 shows the obtained surface areas values in connection to the sulfuric acid ratio.
The effect of the pyrolysis temperature on the surface area was analyzed using the samples in group two where the sulfuric acid ratio to the sample weight was fixed at 1:1.The surface area value was also proportional to the pyrolysis temprature.We found significant differences in the surfae area yielded from the ddifferent pyrolsis temperature (P ≤ 0.0001).The highest surface area achieved was 418.035 m 2 /g at 700 °C pyrolysis temperature, and the minimum surface area was 196.951 m 2 /g at 300 °C.Table 5 shows the obtained surface areas and the pyrolysis temperatures.

The PAC and CAC Comparison
In this comparison, we explicitly considered the PAC with the largest surface area achieved by impregnation with 2.5:1 sulfuric acid to sample and carbonization at 500 °C.Our PAC has shown BD, moisture content, and the ash contents within the recommended ranges for the CAC.However, our highest achieved surface area was 431.375 m 2 /g, which is slightly below the recommended surface area of the CAC (500 m 2 /g).Table 6 shows the comparison between the achieved PAC characteristics and the recommended for Fig. 3 The effect of activation on the weight loss.The sulfuric acid volumes were driven from the acid weight (0.5:1, 1:1, 1.5:1, 2:1, and 2.5:1 gm acid: gm powder)  CAC.It also includes the characteristic of the sugarcane powder.

PAC Heavy Metals Adsorption
Nile Tilapia reused frying oil was analyzed before and after PAC addition.The heavy metals lead, cadmium, manganese and copper were found with different concentrations in the reused oil.The treatment with PAC had a positive effect on reducing the heavy metals concentrations, and the reduction was proportional to the added amount of PAC as shown in Table 7.The addition of 2.5 gm of the PAC could adsorb the existent amount of cadmium completely.ANOVA test has confirmed the treatment with the PAC significantly reduced the heavy metals concentrations (P ≤ 0.001).

Cost Analysis of Activated Carbon Production from SCB and PAC Discussion
Activated carbon production cost was calculated according to the Egyptian local market prices.The capital cost and the VAT were ignored in this estimation and recommended to be included in the future.The total cost for producing a ton of activated carbon from SCB is about 707USD.Table 8 shows the breakdown of the cost estimation.The SCB is an excellent feedstock for the activated carbon production.The calorific of the bagasse is only 54$ per ton based on its value in the electricity production.By utilizing the SCB in the activated carbon production, we expect to leverage its economic value beyond 200$ per ton.
The global interest on the activated carbon applications is annually increasing which necessitates the development of activated carbon from sustainable, reliable, cheaper and ecofriendly resources (Karagöz et al. 2008).The SCB is a potential resource for producing activated carbon and is available in huge quantities in many countries.The utilization of SCB not only guarantees the production of activated carbon but also supports the management of the industrial waste and the environment protection.The resultant BET surface area is dependent on number of processing factors including the ratios and concentrations of activation acid, the pyrolysis temperature and the processing time (Karagöz et al. 2008).The selection of the optimum processing values is relatively flexible.However, by selecting 500 °C, for the pyrolysis, and 2.5:1 weight/ weight sulfuric acid to bagasse powder, for activation, we could achieve a surface area of 431.375 m 2 /gm and overall weight loss of 40%.This surface area is comparable to the recommended for CAC; however, we believe it can be further increased by allowing more activation time or using higher sulfuric acid concentration or adding more amount of sulfuric acid.The higher concentration or more amount of acid leads to the formation of more pores in the structure of the bagasse powder which yields activated carbon with higher surface area.Our PAC could significantly adsorb heavy metals from Nile Tilapia reused frying oil achieving a reduction of 80% of the heavy metals concentrations.We think this adsorption performance can be improved if the contact time between the PAC and sample is prolonged or more amount of PAC is added.Karagöz et al. recommended long contact time (1440 min) to achieve maximum adsorption capacities for methylene blue onto waste biomass based activated carbon (Karagöz et al. 2008).
Such application of our PAC in the food service keeps it safer for human.The heavy metals are allowed within specified limits for human intake.For instance, lead is allowed in the range of 20 -280 micro g/day for adults and 10 -275 micro g/day for children and cadmium in the range of 15-50 micro g/day for adults and 2-25 micro g/ day for children.The copper and manganese are essential for human but toxic when taken in excess concentrations (Amin et al. 2014).We hereby advice the inclusion of activated carbon in public fish frying process for adsorbing heavy metals.
Scaling up the production of activated carbon from SCB is achievable since the raw materials are available.The nitrogen is an abundant gas, sulfuric acid is one of the cheapest acids in the market, and the tubular furnace can be designed locally.

Conclusion
Activated carbon prepared from SCB pyrolysis showed a maximum BET surface area of 431.375 m 2 /g which is comparable to CAC.Our PAC significantly reduced the heavy metals concentrations in Nile tilapia reused frying oil and in particularly could completely remove cadmium from the sample allowing a safe reuse of the frying oil.The estimated production cost of PAC is 707 $/ton compared to 1500 $/ ton for CAC, hence saving about 800 $/ton.Therefore, we recommend the utilization of SCB in the activated carbon production.

Table 1
The sample drying rate over 150 min.The weight in grams and the percentage of the weight loss

Table 2
SCB charcoal characteristics

Table 4
Effect of sulfuric acid on the surface area (SD: Standard Deviation)

Table 5
Effect of the pyrolysis temperature on the surface area

Table 8
(Zhao et al. 2011)t estimation.The electricity prices in the table are adopted from (Egyptian Electricity Corporation 20,220), and the electricity quantities are from the experiments except the pyrolysis is from(Zhao et al. 2011)