Biosorption of Fe2+ from potable water using natural and modified sugarcane bagasse
Authors:   , Ndebele Nompumelelo Lindi Gelsiah 
Published: July 2023

Even though some metals are crucial for the health and development of human bodies, their presence in higher concentrations is worrisome because it has a detrimental effect on people's health. These heavy metals cause cancer and cannot be broken down by biological processes. The removal of heavy metals from water using traditional techniques; such as reverse osmosis, precipitation, ion exchange; has been the subject of extensive investigation. However, because these processes are so expensive to run, a lot of research is currently focusing on using agricultural biomasses to remove these heavy metals. Dumping of this agricultural waste (sugarcane bagasse) in landfills creates dangers of spontaneous combustion, because of microbial activities. The functionality of circular economy depends on waste resources being utilized to their fullest potential, with almost no production of recoverable waste. In a circular economy, sugarcane bagasse is utilized as a fuel source for the boilers that generate process steam and electricity in the sugar mill facilities. Sugarcane bagasse is used in the manufacturing of paper and paper goods, as well as in the agricultural sector. Stakeholders across the value chain, from product design to waste management, This study fulfils the functionality of the circular economy where it looks at extracting the valuable components of the sugarcane bagasse, then further using the sugarcane bagasse to remove heavy metals from potable water. In this study, the adsorption capacities of unmodified and modified sugarcane bagasse for removing Fe2+ from potable water were investigated in batch experiment studies. Sugarcane bagasse comprises cellulose, hemicellulose and lignin. In order to determine the effect of removing/ extracting each component from the sugarcane bagasse, sugarcane bagasse was pretreated with different concentrations of sodium hydroxide and sulphuric acid, ranging between 0.5 wt% and 2.5 wt%, predominantly used to extract lignin and hemicellulose. A cellulosic structure was left behind after the simultaneous removal of both amorphous components (the lignin and the hemicellulose) using the combined pretreatments of sodium hydroxide and sulfuric acid. The advantages of extracting or eliminating these components came from their high value in many sectors. Lignin is used in the paper business and costs between R11 300 and R17 420 per ton, hemicellulose is used in the pharmaceutical sector and costs between R500 and R1000 per ton, and cellulose is utilized in the textile sector. The concentrations of all chemical pretreatments used on the sugarcane bagasse ranged from 0.5 to 2.5%, with alkaline pretreatments intended to extract lignin, acid pretreatments intended to extract hemicellulose, and combination pretreatments intended to remove both lignin and hemicellulose. While cellulose content increased from 32.02 to 65.65% after sodium hydroxide pretreatment, lignin and hemicellulose content reduced from 22.30 and 24.30% to 7.56% and 13.63%, respectively. Lignin and hemicellulose concentration for the sulphuric acid pretreatment went from 22.30 and 24.30% to 14.90% and 13.63%, respectively, while cellulose content went from 35.02 to 65.65%. After the sugarcane bagasse underwent chemical pretreatments, batch studies were conducted on both the natural and chemically pretreated sugarcane bagasse in order to determine how the removal of lignin, hemicellulose, and cellulose affected the performance of the biosorbents in the biosorption of Fe2+ from drinkable water. To assess the efficacy of natural and modified sugarcane bagasse on the Fe2+ removal, the operational parameters investigated in the batch experiments were initial concentration ranging from 1 to 30 mg/L; pH ranging from 2 to 7, contact time ranging from 5 -100 minutes, and adsorbent dose ranging from 0.2 to 1.4 g. For every variation investigation, one variable was varied at a time while keeping the other variables constant. The experimental runs done were repeated thrice and average values are reported throughout the study. According to the biosorption results, 1% NaOH was the best performing biosorbent for the alkali-pretreatment. The most effective biosorbent for the acidpretreatment variation was 2.5% H2SO4. The optimal combination for the pretreatment was (0.5% NaOH + 0.5% H2SO4). Regarding initial concentration variations, all biosorbents were most effective at a concentration of 1 mg/L, where natural sugarcane bagasse was able to remove 50% of Fe2+, 1% NaOH was able to remove 99.7% Fe2+, 2.5% H2SO4 removed 75.93% Fe2+, and the combined-pretreated biosorbent of (0.5% NaOH + 0.5% H2SO4) removed 87.17% Fe2+ . The increase in biosorbent dose led to an increase efficiency of the natural and chemically pretreated biosorbents. The highest removal of Fe2+ was obtained at 1 g (both for the natural and for all the pretreated biosorbents), with 32.2% for the natural; 79.04% for the 1% NaOH; 58.79% for the 2.5% H2SO4 and 70.73% for (0.5% NaOH + 0.5% H2SO4). Results of the study also showed that the highest removal of Fe2+ for the pH variation of 2-7 was at pH “6” for both the natural and pretreated biosorbents. For the variation of the agitation speed, the highest Fe2+ removal was at 160 rpm with 52% Fe2+ removal for the natural sugarcane bagasse. The Langmuir and Freundlich adsorption isotherms were used to study the biosorption mechanisms. Good correlation coefficients (R 2 ) of > 0.95 were obtained for both the Langmuir and Freundlich isotherms for both the natural and modified sugarcane bagasse, indicating that the biosorption followed both homogeneous and heterogeneity interaction between Fe2+ ions and active functional groups of the surface and pores of the biosorbents. Biosorption results for the natural sugarcane bagasse best fitted with the Langmuir isotherm with qmax of 0.770 mg/g, R 2 of 0.987 and RL of 0.938. The alkali and acid-pretreated biosorbents favoured both the Langmuir and Freundlich isotherms with R 2 > 0.95; RL < 1 and 1 𝑛 < 1. The highest qmax of 9.199 and 5.743 mg/g was obtained at 1% NaOH and 2.5% H2SO4, respectively. The combined pretreatment fitted best with only the Langmuir isotherm with R 2 of 0.987, the R 2 of the Freundlich isotherm was less than 0.9. The biosorption of Fe2+ followed both the pseudo-first-order and pseudo-second-order kinetic reactions with 𝑞𝑒(𝑒𝑥𝑝) in close proximity to 𝑞𝑒(𝑐𝑎𝑙𝑐) and R 2 > 0.9. These results showed that sugarcane bagasse had great adsorption capacity after removing the valued components, namely, lignin and hemicellulose. Characterization studies, which included FTIR, XRD, BET and SEM, were also carried out on the natural and pretreated bagasse before and after adsorption experiments. FTIR confirmed the existence of carbonyl, hydroxyl and carboxyl functional groups as major groups responsible for the adsorption of Fe2+ onto the natural and pretreated sugarcane bagasse. XRD revealed that the natural structure of the sugarcane bagasse was of native cellulose consisting of both amorphous and crystalline regions; this structure became more crystalline after the chemical pretreatments as the crystallinity index increased from 39.04% to 66.85% at 1% NaOH; 57.47% at 2.5% H2SO4; and 57.92% at (0.5% NaOH + 0.5%H2SO4). The natural sugarcane bagasse structure featured rough surfaces, according to SEM data, and the main constituents were silicon (Si), carbon (C), and oxygen (O). According to the BET data, employing 1% NaOH, 2.5% H2SO4, and (0.5% NaOH + 0.5% H2SO4), respectively, the initial surface area of 0.904 cm3 /g rose to 1.503, 1.233, and 1.376 cm3 /g and the pore size of 56.33 ̊A increased to 99.63, 93.680, and 99.10 ̊A. According to the EDS data, sodium hydroxide pretreatment performed better in terms of adsorption, followed by combined pretreatment and sulphuric acid. The natural sugarcane bagasse, 1% NaOH, 2.5% H2SO4, and (0.5% NaOH + 0.5% H2SO4) were able to biosorb 0.77, 7.89, 1.63, and 3.8% Fe2+, respectively.