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19 September 2022
UWC student pioneers environmentally-friendly synthesis of silica from sugarcane
University of the Western Cape (UWC) PhD student Ntalane Sello Seroka, under the lead of his supervisor Professor Lindiwe Khotseng from the Chemistry Department, has invented a new, environmentally friendly way of extracting silica from sugarcane bagasse ash SCBA.
 
Ntalane Sello Seroka

SCBA recently came under investigation for its high silica content, making it an attractive, alternative, and “green” source for silica extraction. Silica is a group of minerals composed of silicon and oxygen, which is commonly found in the crystalline state and has a wide variety of industrial uses. For example, in the rubber industry it is used as a reinforcing agent, as a cleansing agent in toothpaste, as an anticaking agent in salts and in cosmetics, to name but a few uses.

Sugarcane’s silica content depends on the soil’s silica levels. Plant root systems play an important role in transporting silicic acid from soil to plant shoots where it is deposited as amorphous silica. The transpiration process then enhances the amount of silica deposited in the rest of the plant. The more water is absorbed, the more silica will be deposited. 

Silica content varies among the species, maturity, season, and geography of the plants concerned. Examples of agricultural plants that are known for significant silica levels are rice, wheat, sunflower, bamboo, corn and sorghum. 

Traditional methods of silica production from bagasse and other agricultural solid residues require the use of various hazardous processes and chemicals such as NaOH, KOH, HCl, HNO3 and H2SO4, which are strong bases and acids that cause potential environmental and biological harm. In contrast, organic bases and acids can be easy-to-handle and eco-friendly alternatives. 
 
Silica

The sugar production process involves squeezing the juice from the cane and the fibrous leftover material is called bagasse, making up approximately 30% - 34% of the mass of fresh sugarcane. Bagasse is an example of agricultural solid residue. If not discarded correctly, it may draw pests which could be dangerous to both crop and human health. It is estimated that about 54-million dry tonnes of sugarcane bagasse is produced globally each year.

Bagasse is usually burned by sugar mills to create heat and power. After combustion, SCBA is produced as a by-product. SCBA makes up approximately 2% - 3% of the mass of the bagasse. SCBA cannot be used as a fertiliser since it does not have enough mineral nutrients required for agriculture, and instead, it can increase the presence of heavy metals in the soil. 

Seroka’s first step in the process involves the pre-treatment of SCBA with a non-hazardous, organic amino-acid called L-cysteine hydrochloride monohydrate to remove inorganic impurities. This amino acid has the advantage of being eco-friendly and easy to use, while its use in this process increases the elemental composition of silica to 79% after the leaching process. 

The next step involves extraction with the organic solvent TetraPropylAmmonium Hydroxide (TPAH) to produce a multi-crystalline product silicate which is then burnt in air to remove excess carbon, nitrogen and hydrogen. The resultant product is silica. 

This green method of silica synthesis from SCBA not only produces silica of similar quality as produced by the chemical methods, but the resulting silica could potentially also be used in medicine, pharmaceuticals and cosmetics because of possibly reduced toxicity and reasonable levels of purity compared to traditionally prepared silica. Other applications are found in the rubber and cement industries, and usage as a filler. 

Seroka completed his undergraduate and honours degrees in Chemistry at Sefako Makgatho University (SMU) and his Masters in Nanochemistry (MSc Nanoscience) at the University of Johannesburg (UJ). It was during his MSc that a lecture by Prof Khotseng sparked his interest in nanotechnology and renewable energy. He then joined UWC for his PhD under the supervision of Prof Khotseng.

“I hope to be able to add a valuable knowledge contribution to the rural economy and communities where there is a dearth of information and accessibility to scientific knowledge,” he said.
 
  • UWC would like to connect with collaborators and industry partners that are interested in using or investigating this technology. Interested parties are encouraged to contact the Technology Transfer Office at tto@uwc.ac.za or visit our website at https://www.tto.uwc.ac.za
  • Monique Heystek is a Technology Transfer Specialist at the University of the Western Cape’s Technology Transfer Office.