Characterization of lignosulphonate and its changes during purification

Abstract

The world needs an environmentally friendly renewable resource that could satisfy future energy needs and lower the impact on climate change. This need has driven many countries to fund research into biomass and biomass derivates due to their renewability and abundance. This has caused lignosulphonate to become a topic of interest due to its potential benefits of its use. Valorisation of lignosulphonate necessitates the removal of impurities, such as sodium and sulphur, which prevent the use of technical lignosulphonate for high-value applications. The current methods that are used to purify lignosulphonate use expensive ion exchange resins which make it less feasible to incorporate at a larger-scale industrial level. This research study led to the development of a more affordable purification method that can be easily scaled up for commercial application and does not call for expensive machinery or incur high running costs. Three variables were investigated, namely concentration, temperature, and pH, to determine their influence on the purification of lignosulphonate. Whether lignosulphonate underwent any significant changes under different conditions was also investigated. The research study first investigated different purification methods to find the most suitable method for the purification of lignosulphonate. A literature review was conducted of the three most used methods of purifying lignosulphonate, namely ultrafiltration, ion exchange and solvent extraction. Both ultrafiltration and ion exchange methods were found to be effective for purifying lignosulphonate; however, both have disadvantages that solvent extraction can overcome, such as expensive filtration equipment, membrane fouling, expensive resins, and the requirement of regular plant section shutdown. Solvent extraction was selected as the purification method because it showed more potential in being a less expensive process and more easily scaled up for industry. Solvent extraction can be accomplished by two methods, either reflux or Soxhlet extraction with a specific solvent such as ethanol or methanol. To ascertain whether reflux or Soxhlet would be a better choice and whether methanol or ethanol are better solvents, an experimental examination or pre-experimentation was necessary. The pre-experiments were conducted, and it was found that Soxhlet solvent extraction with ethanol as its solvent, resulted in a lower mass loss and significant ash content removal. The above-mentioned investigation was used to construct a modified solvent extraction method. The modified solvent extraction route included two steps: acidification and washing. In the acidification step, hydrochloric acid (HCl) was added to the solution to lower the pH value. The lignosulphonate sample was then washed (Soxhlet extraction method) using ethanol as the solvent to remove the impurities. The next step of the study was to determine the influences of the three variables (concentration, temperature, and pH) on the purification method. To determine the influence of the three variables, it was necessary to characterise the physicochemical properties of lignosulphonate both before and after purification. The characterisation of the sample specifically focused on the ash (proximate analysis*), carbon (elemental analysis*), sodium and sulphur content (inductively coupled plasma analysis*) and was verified by a registered analysis company through an X-ray fluorescence analysis, which included the determination of the molecular weight and number (gel permeation chromatography analysis*) and functional group analyses (Fourier transform infrared spectroscopy*). The assessment of the sodium and sulphur removal, the carbon mass loss and the structural changes led to the discovery of the optimal conditions. The study found that 150 g/L, 25 °C, and a pH of 1 were the best conditions for lignosulphonate purification.