Chemometric optimisation of cellulose extraction from Hemp: removal of Synthetic Dyes from Aqueous Solutions Using Micro-Cellulose

Abstract

Due to increasing awareness of the environmental impacts and costs of materials used in everyday products, industries are increasingly seeking more ecologically and environmentally friendly materials that are renewable, biodegradable, economically feasible, and have lower energy demands. As a result, recent research has focused on the extraction of natural materials such as cellulose, from plants, including agricultural biomass residue. As a cellulose source, agricultural biomass presents such advantages as being low-cost, widely available, environmentally friendly, and biodegradable. This study investigated the optimisation of cellulose microfibre extraction from Hemp (Cannabis sativa L.) bast fibres by organic acids via response surface methodology (RSM). The goal was to determine appropriate conditions under which organic acids could be applied, in order to replace the commonly used sulphuric acid process, thus providing a greener route for cellulose extraction. Upon extraction, surface treatment by cationisation was then performed in order to demonstrate their application in the remediation of water contaminated by various synthetic dyes.

For the extraction of cellulose microfibres, hemp bast fibres were first subjected to alkali (4 wt% NaOH) and bleaching treatments using acetate buffer in aqueous chlorite. RSM was used to determine combinations of three processing conditions including acid concentration (45 – 64%), hydrolysis time (30 – 90 minutes), and temperature (45 – 65 ℃), using sulfuric acid, formic acid, and maleic acid. A central composite design model with 21 experimental runs was optimised using MODDE 13.1 software. Characterisation of cellulose and cellulose microfibres included surface morphology analysis by scanning electron microscopy (SEM), functional group analysis with Fourier Transform Infrared (FTIR) spectroscopy, crystallinity degree with X-ray diffraction (XRD) analysis and thermal stability analysis with thermogravimetric analysis (TGA). SEM confirmed that hydrolysis produced cellulose microfibres of varying size and morphology. FTIR spectra showed that the main chemical structure of cellulose was not altered during the hydrolysis process. TGA also showed that microfibres with high crystallinity resulted in good thermal stability, which is a favourable property for high temperature applications. The model suitably described the data (R2 =0.99; R2adj = 0.96). Microfibres with an average width of 6.91 μm, degree of crystallinity range between 40% and 75% and good thermal stability were produced with acid hydrolysis processes with assisted ultrasonic treatment. The optimum degree of crystallinity 83.21% was achieved with formic acid concentration of 62 wt%, hydrolysis time of 36 minutes, and hydrolysis temperature of 47 ℃ as predicted by the model. The optimisation results were validated to confirm the accuracy of the model. The data suggests that formic acid can be used as an alternative to sulfuric acid for synthesis of cellulose microfibres from biodegradable hemp waste fibres.

Using hemp plant fibres as a cellulose source, cationised hemp cellulose was synthesized and applied as an adsorbent for the removal of methyl orange (MO), and sunset yellow (SY) from aqueous solutions. For cellulose extraction, the previous method was utilised. Extracted cellulose fibres were functionalised using Glycidytrimethylammonium chloride (GTMAC) to synthesize cationised cellulose (GT-cellulose). Raw plant fibre, bleached cellulose, and GT-cellulose were characterised using SEM, FTIR, XRD, and TGA techniques. SEM showed long finger-like morphologies for all fibres and displayed that cationisation did not endorse any major modifications to the size and shape of the fibres. The FTIR spectra of raw hemp fibres, bleached cellulose, and GT-cellulose displayed functional group attributed to epoxy moieties of GTMAC, confirming cationisation. The crystallinity degree (CrI) of the fibres obtained from hemp bast material was 60%, which was improved following alkali and bleaching treatment extracting cellulose (CrI = 73%). XRD and TGA showed GT-cellulose with lower crystallinity degree and reduced thermal stability. For synthetic dye adsorption studies, the influence of pH, dosage of adsorbent, initial dye concentration, contact time, and temperature were investigated in batch experiments using GT-cellulose as an adsorbent. From the obtained results, the equilibrium processes were best described by Langmuir isotherm model for both dyes of interest, showing a monolayer adsorption. From the kinetic experiments, the adsorption processes for MO and SY dyes followed the pseudo first-order kinetic model indicating that the overall rate of dye adsorption could be governed by one of the reactants. The thermodynamic study showed that the adsorption processes for MO and SY were both endothermic and spontaneous in nature.

Hemp bast fibres can therefore be regarded as a green and sustainable waste material for the preparation of cellulose microfibres with improved crystallinity, enhanced thermal stability and can be cationically- modified to act as an adsorbent for uptake of anionic dyes from aqueous solutions.