Adsorbing heavy metals in water with TEMPO-oxidized sugarcane bagasse aerogel

Adsorbing heavy metals in water with TEMPO-oxidized sugarcane bagasse aerogel Student names: Ha Nguyen, Nhat-Ha Pham School: Hanoi-Amsterdam High School for the Gifted City & Country: Hanoi, Vietnam GENIUS Olympiad 2021 Science, Environmental Quality

Table of contents: 3 4 Abstract 4 Question & Hypothesis 4 Variables 5 Background Research (literature search) 6 Materials List 10 Experimental Procedures 13 Results Analysis and Discussions 13 Conclusion 13 Ideas for Future Research 14 Acknowledgement Bibliography

Abstract Every year, there are hundreds of tons of sugarcane bagasse (SCB) extracted from over 40 sugarmill factories in Vietnam. This waste source is known as a raw material rich in cellulose but is currently used ineffectively in our country and causing environmental pollution. Nanocellulose is becoming an important material in the nanoscience industry with valuable properties such as high mechanical strength, high specific surface area, good air resistance, and the development of environmentally friendly materials. Therefore, it is applied in many areas such as medicine, electronics, technology and . In this work , we study the suitability of biomass residues SCB as a new source for nanocellulose fiber production by a low energy consumption method. Firstly, raw material SCB was applied to a soda treatment followed by bleaching with Javel water. The product of this pretreatment referred as SCB pulp.Then, the SCB pulp was converted into nano-size cellulose fiber by subjected to a simple and economical treatment of oxidation using 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) as a selective catalyst. The obtained nanocellulose fibers referred to as ToSCB were characterized in terms of carboxyl content, morphology, and dimension. A cellulose aerogel was fabricated from resulting ToSCB, showing high capacity of adsorption heavy metal ions in aqueous media. Keywords: nanocellulose, sugarcane bagasse, water filtration, aerogel, environmental pollution.

1. Question: Which concentration of sodium hypochlorite (NaClO) as an oxidizer in theTEMPO-mediated oxidation of SCB is able to introduce the highest carboxyl group content in the resulting nanocellulose fiber? 2. Hypothesis: TEMPO-mediated reaction selectively oxidizes the C6 of the D-glucose unit of cellulose chains by substituting the –OH group of that carbon for a COONa group. The TEMPO-mediated reaction selectively oxidizes the C6 of the D-glucose unit of cellulose chains by substituting the –OH group of that carbon for a COONa group. Those anionic carboxylates group resulting in negative charges on the nanocellulose fiber surface, which increase electrostatic repulsions between the nanofibers. Therefore, we hypothesize that the higher carboxyl group content of nanocellulose fiber ToSCB, the higher yield of nanocellulose fibrillation from SCB pulp. 3. Variables: a. Independent variables: NaClO concentration (mmol/g SCB) b. Dependent variables: carboxyl content of obtained nanocellulose (ToSCB) (mmol/g) c. Control variables: temperature, TEMPO concentration and KBr concentration. 4. Background Research (literature search): 4.1. Sugarcane Bagasse Sugarcane is a perennial tropical grass with tall stout jointed stems from which sugar is extracted. The plants are two to six metres tall with stout, jointed, fibrous stalks. Sugarcane has an important role in daily life as ingredients to Vietnam's famous street drink: sugarcane juice, which can be easily found on the sidewalks, markets... It can be cut into cubes and served as comfort food. Besides, it is the only source to produce an indispensable spice to our daily life: sugar. In Vietnam, there are about 40 sugarmill factories around the country. It is a notably profitable exporting product and accounts for nearly 80% of global sugar production. Sugarcane bagasse (SCB) is the dry pulpy fibrous material that remains after crushing sugarcane stalks to extract their juice. Normally, SCB consists of around 42% cellulose, 25% hemicellulose, and 20% lignin. In Vietnam, the production of sugarcane approximates 7,387,610 tons in the 2019/2020 crop. For every 10 tonnes of sugarcane crushed, a sugar factory produces nearly three tonnes of wet bagasse. This means 2,216,283 tons of wet bagasse are produced each harvest season. To sum up, sugarcane bagasse is a highly potential cellulose source in Vietnam as it is plentiful and cheap. However, the inundation and improper dumping of bagasse (e.g: on pavements and near storm drains) are responsible for deteriorating air quality. The dumping of bagasse is causing air and water pollution and is also adding to the solid garbage dump across cities. It is also one of the main reasons behind choking of water bodies, especially tributaries of rivers. Although sugarcane bagasse can be reprocessed as a fuel source for sugar mills or supplied as raw materials to be made into paper, these applications remain limited in operation and generate very low

economic returns. In order to maximise economic value of sugarcane bagasse and exploit its cellulose content, we propose the extraction of nanocellulose from sugarcane bagasse and the optimization of its application in lead adsorption. 4.2. Nanocellulose Nanocellulose is a lightweight material with strong mechanical strength, inexpensive production costs and safe handling compared to synthetic nanoparticles. Thanks to the high specific surface area, broad possibility of surface modification and high mechanical strength, nanocellulose has emerged as a new class of biobased adsorbent with promising potential application in environmental remediation. Currently, treatment methods to produce nanocellulose from lignocellulosic sources can be classified into three categories: mechanical, chemical, ạnd enzymatic. Mechanical treatment involves applying forces through various processes (homogenization, grinding, microfluidization, ultrasonic treatments, ball milling, and cryo crushing) to downsize natural cellulose fibers to nanoscale. This method is very energy-consuming and causes fiber damage, so pretreatments such as enzymatic treatment, alkaline treatment and chemical oxidation are needed. Chemical treatment involves using acids such as sulfuric (most popular), hydrochloric, phosphoric, maleic, hydrobromic, nitric, formic, p-toluenesulfonic to break 1–4 glycosidic bonds of cellulose chains, isolate cellulose nanocrystals and remove a part of the amorphous cellulose. The acid reacts with the surface hydroxyl groups of cellulose to form negatively charged sulfonic groups and a stable gel upon hydrolysis with hydrochloric acid. Enzymatic treatment involves using synthesizing monosaccharides or fermentation to downsize cellulose fibers, which is time-consuming and quite expensive but necessary to make mechanical grinding faster and less energy-guzzling. Depending on the type of treatment, the final nanocellulose properties and costs can vary significantly. Of all the treatments mentioned above, TEMPO-mediated oxidation has been a very popular method, which has been applied to many kinds of plant and biomass to extract nanocellulose. The TEMPO-mediated reaction selectively oxidizes the C6 of the D-glucose unit of cellulose chains by substituting the –OH group of that carbon for a COONa group. Those anionic carboxylates group resulting in negative charges on the nanocellulose fiber surface, which increase electrostatic repulsions between the nanofibers. In this study, we choose TEMPO-oxidation as the sole treatment method to produce nanocellulose from sugarcane bagasse because the experiment can be conducted at room temperature, requires simple equipment, and is affordable (we use a small amount of TEMPO). The nanocellulose obtained from this experiment is going to be tested as a heavy metal adsorbent, particularly with lead ion, to propose an use of nanocellulose fiber in reducing lead poisoning. 4.3. Lead poisoning According to WHO, young children are particularly vulnerable to the toxic effects of lead and can suffer profound and permanent adverse health effects, particularly affecting the development of the brain and nervous system. Lead also causes long-term harm in adults, including increased risk of high blood pressure and kidney damage. Exposure of pregnant women to high levels of lead can cause miscarriage, stillbirth, premature birth and low birth weight. Hence, removing lead from water is crucial for the above mentioned reasons. One way of combating this is to use adsorbents. Adsorbents are highly important in enhancing the removal efficiency of pollutants from wastewater due to their high stability. This paper proposes the experimentation of nanocellulose as adsorbents. Cellulose aerogel obtained from TEMPO-oxidized cellulose is a potential adsorbent because its molecular chain has negative charge carboxyl groups, so it can react with Pb2+ ions in the water environment.

5. Materials List: 5.1. Raw ingredients and chemicals: ● Sugarcane bagasse. ● Sodium hydroxide (NaOH). ● Sodium hypochlorite (NaClO). ● Potassium bromide (KBr). ● (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO) ● Lead nitrate Pb(NO3)2 (Pb2+). 5.2. Equipment and apparatus: ● Centrifuge. ● Beakers ● Flasks ● Stirrers. ● Plates. ● Precision scales. 6. Experimental Procedures: 6.1. Nanocellulose production a. Phase 1. Separating cellulose from sugarcane bagasse: Step 1: Boil sugarcane bagasse for 2 hours at 100oC to remove residual sugar in bagasse and eliminate water-soluble compounds. Figure 1. Washing SCB Step 2: Extract cellulose from bagasse by soaking bagasse in NaOH 3% for 4 hours at 100°C (alkaline treatment) and then bleaching it with Javel water at 90°C to remove lignin and hemicellulose. Figure 2. Bleaching SCB b. Phase 2: SCB oxidation reaction to fabricate cellulose nanofibers.

Step 1: SCB pulp was put into a solution including distilled water, TEMPO, KBr. For 1g SCB pulp, add 0.016g TEMPO (0.1mmol), 0.1g NaBr and 80ml water. The mixture was stirred at moderate speed to assure good dispersion of all substances.. Slowly add NaClO solution to the slurry. The volume of NaClO was calculated to add 3, 6, 9 and 12 mmol of NaClO per g of SCB pulp in different batches. . The reaction system was kept at room temperature. The pH of the reaction mixture was kept between 10 and 11 by the addition of a 0.5M NaOH solution. The oxidation reaction was stopped with an ethanol solution when no change in the pH of the system was detected. Step 2: Wash the product with distilled water 4 to 5 times by centrifugation of 8000 rpm. The resulting product in the form of gel-like suspension was then preserved in the refrigerator. The reaction diagram is depicted in Figure 3. Figure 3: Fabrication process of nanocellulose from bagasse 6.2. Producing ToSCB aerogel Figure 4: Production of ToSCB aerogel Step 1: 1.5% and 2% ToSCB gel-like suspension was coagulated with ethanol 99.7% for 2-3 days. Step 2: Wash the gel 3 or 4 times in distilled water and then freeze-dried in a Labconco freeze dryer at 0.120mbar and -54oC. The final product was ToSCB aerogel, which was then tested for the capability of Pb2+ absorption in aqueous media. 6.3. Determination of the carboxyl group content of ToSCB The content of the carboxyl group is determined using sodium bicarbonate method based on ASTM D1926-00 criterion. Prepare chemicals and equipment: ● Standard solution: 0.01N HCl (1)

● HCl Solution 1 + 99 (Dilute 1 volume of concentrated HCl with 99 volumes of water) (2) ● methyl Red indicator solution . (3) ● NaCl + NaHCO3 solution: Dissolve 5.85g NaCl and 0.84g NaHCO3 in 1 liter of water. (4) Procedure : ● Weigh an amount of ToSCB suspension and disperse the sample in HCl solution (1 + 99) at room temperature to 1% consistency ● After 2 hours, wash the sample with distilled water using the vacuum filtration until the drop of filtrate does not change the color of pH test paper. ● Weigh the wet pulp pad and filter paper,transfer it immediately to a 250-mL glass-stoppered Erlenmeyer flask, add 50 mL of the salt solution (4) with a pipet, and shake to obtain a homogeneous slurry. ● Allow the mixture to stand for 1h at room temperature. Filter through a clean, dry, fritted glass funnel, pipe a 25-mL aliquot of the filtrate into an Erlenmeyer flask, and titrate with 0.01 N HCl, using methyl red solution as an indicator. When the first change in color occurs, boil the solution for about 1 min to expel the carbon dioxide and continue the titration to a sharp end point. Record result a . ● Titrate 25 ml of solution (4). Record result b. Calculation ● Content of the carboxyl group (C) (milliequivalents per 100g) in each nanocellulose ToSCB specimen as follows: In which: a: milliliters of 0.01N HCl solution consumed by 25 mL of filtrate (ml) b: milliliters of 0.01N HCl consumed by 25 mL of NaCl-NaHCO3 solution (4) (ml) v: weight of water in the wet pulp pad, (g) G: weight of dry specimen (g). ● Degree of oxidation (DO): In which: C: carboxyl content (mmol/g) 6.4. Morphological characterization and dimension of ToSCB We had the produced nanocellulose fiber ToSCB possessing the highest carboxyl content observed and took a picture by using a field emission scanning electron microscope JEOL JSM-7600F to study its morphological characteristics. Diameters of nanocellulose fiber ToSCB were determined from the FE-SEM picture with support of Image software. 6.5. ToSCB aerogel characterization Density of the aerogel: After being freeze-dried, the aerogel was diced . measure the side of the aerogel cube precisely to determine its volume. The density of the aerogel is calculated as follows:

In which: d: density of the aerogel (g/cm3) m: mass of the aerogel piece (g), determined by a scale of 0.0001 g accuracy V: volume of the aerogel cube (cm3). ● Porosity of the aerogel is calculated as follow: In which: ρ: porosity of aerogel (%) ρa: density of aerogel (g/cm3) ρc: density of cellulose, (=1.53 g/cm3) We had The Brunauer–Emmett–Teller (BET) specific surface area of the aerogel determined by N2 physisorption using a Gemini analyzer (Micromeritics, USA) 6.6. Heavy metal ion (Pb2+) adsorption capacity measurement in aqueous media Step 1: Prepare Pb2+solution: Solutions of Pb2+ were prepared by dissolving amount of Pb(NO3)2 in 1000ml of distilled water so that the concentrations of solution are about 1mmol / l , 2mmol / l, 4mmol / l Step 2: Investigate the Pb2 + ion adsorption process at room temperature: Use a pipette to get 50 ml of Pb (NO3) 2 solution with different concentrations (1mmol / l, 2mmol / l, 4mmol / l) into conical flasks . Add 2 ml buffer solution to stabilize the pH of the solution. Weigh 0.03 g of cellulose Aerogel that needs to be examined and put into the conical flasks. The mixtures were kept under stirring at room temperature for different periods (1 day, 2 days, 3 days). . Figure 5. ToSCB aerogel heavy metal adsorption test After a period of investigation, use a funnel to filter out cellulose aerogel material out of the solution. The heavy metal ion concentrations were measured by atomic absorption spectroscopy AA6800 of

Shimadzu The measured Pb2+ concentrations were then used to calculate the absorption capacity, qe (mg/g) of the adsorbent using the following mass balance equation: qe =V.(Co–Ce)/m The percentage of Pb2+ adsorbed on the adsorbent was calculated by the following equation: pe (%) = 100(Co - Ce)/Co where Co, Ce, V, and m are initial Pb2+ concentration (ppm), Pb2+ concentration at a certain time (ppm), the volume (L) of the solution and weight (g) of adsorbent, respectively. 7. Results Analysis and Discussions: 7.1. Effect of Oxidation Conditions on Carboxylate Content of the Products TEMPO/NaBr/NaClO oxidation applied to cellulose pulps at pH 10 and room temperature is capable of converting significant amounts of C6 primary hydroxyl groups to sodium carboxylates cellulose. The introduction of anionically charged COO- groups promotes strong electrostatic repulsion between cellulose fibrils in water, favoring their defibrillation with mechanical disintegration treatments (Isogai et al., 2011). Table 1 shows associations between the amount of NaClO added as the oxidant and the carboxylate content of the TEMPO-oxidized cellulose. The amount of carboxylate groups formed from the primary hydroxyl groups of cellulose increases with the amount of NaClO added and reaches the highest value at NaClO concentration of 9 mmol/g. The specimen ToSCB9 possesses a carboxylate group content of 1.812 mmol/g, corresponding with degree of oxidation of 0,315 would be observed by FESEM to determine its diameter (the maximum carboxyl group content of TEMPO-oxidized cellulose can be reached is 0,5) Table 1: Associations between NaClO added to the TEMPOmediated oxidation and the carboxylate content of the oxidized cellulose ToSCB Sample NaClO added (mmol/g Carboxylate content Degree of oxidation dried SCB pulp) C(mmol/g) DO SCB pulp - 0.002 - ToSCB 3 3 0.215 0.035 ToSCB 6 6 0.678 0.113 ToSCB 9 9 1.812 0.314 ToSCB 12 12 1.694 0.293

7.2. TEMPO-Oxidized Cellulose Fiber morphology characterization The produced ToSCB9 obtained in the form of semi transparent gel-like material as can be seen in figure 3, this result reveals that the cellulose fibers may have dimension of nano-scale as small particles are considered “invisible” when their dimensions are lower than wavelength of visible light. Observation of a dried piece of ToSCB by using FESEM showed a similar result indeed. Cellulose fibers of ToSCB9 specimens are of uniform size and having the diameter varies in a narrow range of 17nm to 26 nm as can be seen in figure 7. Figure 6. FESEM picture of ToSCB Figure 7. ToSCB diameter distribution diagram. 7.3. Overview of the ToSCB aerogel: The TEMPO-oxidized cellulose has great potential as a heavy metal ion adsorbent as the molecular chains contain an amount of negative charged group COO-. Therefore the produced ToSCB was used to fabricate an cellulose aerogel for Pb2+ adsorbent application. Figure 8. SEM images of ToSCB aerogel cross-section

Figure 8 shows the microstructures of the 1.5% and 2% ToSCB aerogel cross-section. Both types of aerogel exhibited a relatively uniform cellular structure with pore sizes typically in the range of 10 to 20 micrometer in the case of 2% ToSCB, but this dimension is much smaller in the case of 1.5% ToSCB aerogel. The pore sizes of 1.5%ToSCB aerogel vary in the range of several micrometers, which benefit specific surface area of an adsorbent. Aerogel characterization results in table 2 showed that the fabricated aerogel having a high porosity, above 95% from both types of aerogel, and very low density range from 0.0342 g / cm3 to 0.0741 g / cm3. The Specific surface area is quite high, reaching 61.1 m2 / g for a sample of 1.5%ToSCB aerogel. Table 2: properties of cellulose aerogel 7.4. Adsorption characterization It is believed that metal ions scavenging in aqueous solutions by porous materials such as aerogels is mainly driven by electrostatic interaction and complexation between the metal ions and the carboxyl groups present in the porous materials. The aerogels carried a lot of carboxyl groups resulting from the TEMPO-oxidation process used to produce the ToSCB. This explains the high metal ion adsorption capacity exhibited by ToSCB aerogel shown in table 3 and figure 9. The Pb2+ absorption rate of our aerogel increased proportionally with the adsorption duration (1-3 days). After 3 days, the adsorption efficiency Pb2+ were as follows Figure 9. Adsorption capacity of 1.5% ToSCB aerogel

Figure 10. Adsorption rate Adsorption rate: Compared with published findings, our aerogel has a similar Pb2+ adsorption capacity. For instance, in the research paper on producing aerogel from PVA/CNF by Qifeng Zheng in 2014, the equilibrium adsorption capacity was 110.6mg/g after 3 days. In another paper by C.Yao F.Wang et.al in 2016, the adsorption capacity in the saturation stage of the aerogel produced from wood pulp was 157.25 mg/g after 5 hours. 8. Conclusion: TEMPO-mediated oxidation is a suitable and effective treatment to isolate nanocellulose fiber from SCB. Obtained ToSCB possesses a uniform diameter of around 21nm without any addition of further treatment. Through TEMPO oxidation, carboxyl groups were successfully attached to the cellulose aerogel l with retention of its nanoporous property. The resulting ToSCB aerogel also proves to be effective in eliminating Pb2+ ions in water. Furthermore, compared with other inorganic adsorbents, this aerogel is more environmentally friendly in the disposal process as it can be biodegradable in the natural environment . 9. Ideas for Future Research: In order to maximise the productivity of the nanocellulose that we extracted, the pH needs to be kept stable during the oxidation since this reaction depends a lot on the pH of the system. While recognising the limitations of our analysis, we have achieved our aims in creating the highly effective nanocellulose from sugarcane bagasse - a pollutant in many nations, as well as . The public and academia would benefit from future research in these aspects: a. It would be helpful if experimentation of the uses of nanocellulose from sugarcane bagasse, from fields such as biomedical and mechanical engineering is performed. b. It would also be helpful if nanocellulose gel is tested on other heavy metals to verify nanocellulose heavy metal adsorption capability. c. Research into commercialisation of nanocellulose & its applications would be beneficial.

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