Preparation of activated carbon from biomass and its’ applications in water and gas purification
Due to its versatility and wide range of applications, activated carbon is widely used as contaminant removal media. Recent research have focused on enhancing the effectiveness of activated carbon by modifying their specific properties in order to enable the carbon to develop affinity for certain contaminants. In view of this, a comprehensive list of literatures on chemical, physical and biological modification techniques of activated carbon pertaining to enhancement of contaminant removal from aqueous solutions was compiled and reviewed. Acidic treatment to introduce acidic functional groups onto surface of coal based activated carbon was by far, the most studied technique. It was apparent from the literature survey that the beneficial effects of specific modification techniques on activated carbon adsorption of targeted contaminant species from aqueous solutions were profound, with some studies reported increase of contaminant uptake factors of more than 2. Concurrently, considerable decreases associated with certain contaminant uptakes can also occur depending on the technique used.
Without pure water, it is impossible to survive for any living beings. The ratio of freshwater on our planet is very poor and the demand is increasing with time for the growing population. Furthermore, water is being contaminated by industrial and agricultural activities, pharmaceuticals, technocratic civilization, pesticides, garments, global changes etc. In addition to this, environmental pollution and global warming are swelling due to the greenhouse and harmful gases generated from the dumping and burning of fossil fuel. Addressing these problems, it is necessary to find out the cost-effective and environmental friendly processes to purify the contaminated water and air. Activated carbons (ACs) are one of the best solutions for removing the pollutants from aqueous and atmosphere as it is the carbonaceous materials with a high degree of porosity, well-developed surface area, and distinguished functional groups which are required for elimination of contaminants. The preparations of activated carbon are easy and safe processes, mainly from the pyrolysis or gasification of biomass with heat and/or chemicals. The recycling and regeneration of bituminous coal based activated carbon after use are also essential for resource maintenance and environmental safety. Thus, AC can protect the ecosystem in a double direction by purifying the water and air from the pollutants.
The present paper reviews the efficiency of different types of activation processes of activated carbon from biomass. The adsorption capacity of contaminants and pollutants from water and air has been described elaborately to get pure water with clean air. The recycling/regeneration and handling process of Saw Dust Activated Carbon is also illustrated.
2. Preparation of activated carbon
For the preparations of shell activated carbon, two basic steps are maintained. The first one is the carbonization and the second one is the activation. Carbonization is done through the pyrolysis/gasification at a higher temperature in an inert atmosphere to produce the biochar (Odetoye et al., 2019). In this stage, the carbon content of the carbonaceous substance was prepared by removing the volatile matter through thermal degradation (Radenahmad et al., 2020). The temperature, the heating rate, the nitrogen gas flow rate and the residence time are the significant parameters in this stage. As the achieved biochar reveals low adsorption ability, an activation process is essential to improve the pore volume, the pore diameter and the surface area (X. Yang et al., 2019). In the activation process, initially, the disorganized carbon was eliminated, exposed the lignin to the activating agents and developed the microporous structure. Finally, the existing pores are widened to a large size by burning of the walls between the pores. This raises the intermediate pores and macro-porosity which reduces the volume of micro-pores. Depending on the type of activation, activation can be a process prior to carbonization or subsequent to carbonization for the elimination of deposited tarry substances in biochar that can help to enhance the porosity and to provide high surface areas for the ACs (Ukanwa et al., 2019).