A team of researchers recently published an article in the journal Materials which examined the synthesis of nanostructured carbon (NC) from renewable resources, in particular from agricultural waste.
Study: Nano-structured carbon: its synthesis from renewable agricultural sources and important applications. Image Credit: Matauw/Shutterstock.com
NC materials, such as fullerenes, carbon nanotubes (CNTs), graphene oxide, carbon nanoparticles (NPs), and carbon nanodots, have gained prominence for applications in the fields of environment, biotechnology, energy and biomedical due to their high surface area and excellent thermal resistance. , electrical, mechanical and chemical. NC materials can be synthesized using several strategies, including chemical vapor deposition, microwave methods, and pyrolysis.
The use of cheaper and renewable reagents and agricultural raw materials is increasing for the production of NC to simplify the production process and reduce production costs. In this study, researchers investigated the synthesis of NC using renewable sources. Specifically, the review focused on the synthesis of CNs from agricultural waste using different methods and important applications of CNs.
Various methods such as pyrolysis, microwave, hydrothermal and carbonization for the synthesis of nanocarbon from agricultural waste. Image Credit: Singh, J et al., Materials
Methods used for NC synthesis from agricultural waste
The hydrothermal (HT) method is used to convert agricultural waste into carbon nanomaterials and fuels. HT processes are generally carried out at high pressure in the presence of water. This method is more environmentally friendly and economical for NC synthesis because water as a solvent produces water vapor upon heating to generate high pressure in a closed chamber.
Agricultural waste, such as banana peels, was dried and ground into fine homogeneous particles, then placed in a HT reactor for six hours at 80°C. The product as prepared was washed several times and collected by centrifugation. Finally, the collected samples were dried to obtain banana peel carbon.
Similarly, grape seeds, the waste generated by the wine industry, have been used to produce oil, and the residue left after oil production has been used to produce NC materials. The physicochemical properties of the synthesized carbon can be tuned by varying the concentration of the substrate, the catalyst and the temperature in the HT process.
Microwave hydro-thermal carbonization (MHC) and general carbonization
In the MHC, microwave energy is used to heat the HT unit where carbonization takes place. The MHC method is faster than the normal HT method because microwave heating is a fast, economical and efficient way to induce the carbonization reaction. The rice straw was first chopped and crushed to obtain straw dust of homogeneous size, then placed in digestion tubes with water.
Subsequently, the digestion units were placed in the microwave and the temperature in the microwave was maintained at 230°C to produce hydrochar with various physico-chemical properties. These depended on parameters such as reaction time and temperature. Hydrochar can also be synthesized by placing a powdered rice husk in the HT unit under microwave irradiation.
In general carbonization, the carbonization of agricultural wastes has been carried out using simply closed containers or by chemical pre-treatment in an open fire. Corn husks, wheat straw and rice husk were dried and ground into powder. The powders were chemically activated by adding sodium chloride in a fixed ratio.
Thereafter, the mixture was kept in an open wood fire for half an hour for charring in a low oxygen environment. In the Elsa barrel technique, biochar is produced from agricultural waste such as cassava, peanut-based corn cobs, sawdust, and rice husk. Moreover, graphene oxide has also been obtained from sugar cane bagasse.
SEM image of SCBAC with 70×, 200× (left) and 1500× (right). Image Credit: Singh, J et al., Materials
The microwave irradiation technique is a crucial NC synthesis method due to its low power consumption at different high temperature power modes. The method mainly uses the irradiation frequency to precisely control the duration and temperature of the synthesis process. Various carbon nanostructures with controlled shapes and porosity have been obtained using this method.
For example, microcellulose derived from agro-waste products has been used to synthesize microporous carbon sponges using microwave irradiation. Similarly, a simple microwave irradiation technique was used to produce biochar from waste palm oil, while microwave pyrolysis method was used to synthesize fluorescent carbon dot from natural sesame seeds.
Pyrolysis is a simple thermochemical method commonly used to disintegrate carbon sources into smaller components under anaerobic conditions. Although pyrolysis has traditionally been used for the fractionation of petroleum products, this method is now gaining attention for recycling agricultural waste into environmentally and economically beneficial products, such as CNs.
In pyrolysis, stalks of agricultural waste or wood were first crushed to decrease particle size. Purely crushed quartz silica in the form of a small diameter tube 60 cm long was used in the tube furnace. A vacuum chamber was used to create a vacuum in the tube. Subsequently, the ground agro- or wood rod powder was placed in the tube, and carbon dioxide and nitrogen gases were introduced at temperatures above 500 ohC in the tube furnace. Eventually, biochar was obtained after the process.
Steam pyrolysis and spray pyrolysis are the main types of pyrolysis methods. Steam pyrolysis uses steam at temperatures of 600-700°C to heat the powdered agricultural waste in the quartz tube furnace, while the waste is directly pulverized in the pyrolytic furnace at high temperatures to form carbon particles during spray pyrolysis. Steam pyrolysis can preserve the natural porosity of the waste used for the preparation of NC.
Preparation of NC from different agricultural wastes
CNTs and mesoporous NCs were produced from waste sugarcane and coconut husks, respectively, using the pyrolysis method. Pineapple and date palm wastes were used to produce NC with a continuous three-dimensional (3D) network of homogeneous meso/micropore structure and porous NC, respectively, using the hydrothermal method.
CNTs and nanocarbons were synthesized from rice husk and nicotine tabacum stalks using microwave irradiation method and carbonization method respectively. NC with honeycomb structure was produced from lapsi seed kernel and rubber seed shell using chemical activation method. NCs with a sheet-like structure were synthesized from orange peels using chemical activation with the pyrolysis method.
SEM images of carbonized nanocarbons at 400°C for different time intervals (a) 2 hours, (b) 3h, and (vs) 4h. Image Credit: Singh, J et al., Materials
Application of CNs derived from agricultural waste
Wastewater treatment, biosensors, nanocomposite polymer materials and energy storage devices are the main applications of NCs. Nanocarbons produced from agricultural waste have gained considerable importance due to their non-toxicity, high chemical inertness and low cost. NC derived from pineapple leaf using chemical activating agents such as potassium hydroxide is used as supercapacitors or electrochemical capacitors.
Several carbon-based nanomaterials, such as carbon nanofibers, graphene and CNTs, have been synthesized from different secondary agricultural sources. Nanocarbons functionalized with different organic molecules are suitable for the detection of adsorbates.
Taken together, agricultural wastes could be effectively used to synthesize CNs using different synthesis methods such as HT and simple carbonization, which can reduce the need for natural resources to produce CNs used in different applications and promote sustainability.
Singh, J., Singh, V., Jirimali, H. et al. Nano-structured carbon: its synthesis from renewable agricultural sources and important applications. Materials 2022. https://www.mdpi.com/1996-1944/15/11/3969