(195a) Hydrochars from Lignocellulosic Residues for Green Electronics, Environmental Remediation, Energy and Agricultural Applications

The use of agroindustrial residues for the obtention of high added value materials, compounds and products have gained attention in the last decade. Examples relevant to Ecuadorian reality are related to the banana, cocoa, moringa, mango, and rice crops, among others. For example, the banana, cocoa, and mango agribusinesses in Ecuador are recognized worldwide for having high quality products which are attractive to the consumer. Their waste usually does not enter the value chain of industries and millions of tons of waste are generated annually without relevant industrial reuse. Another agribusiness, such as the moringa agribusiness, has increased exponentially since it was introduced in the coastal region in 2012, it is a recognized superfood [1] and their residues (husks and pressed cotyledons) have been vastly studied for water treatment applications [2]. Rice is another important crop that feeds more than 50% of the world’s population [3] and efforts to use rice husk as source of renewable energy and water treatment have also been made [4]. Regarding blackberry species, this is also of particular interest since it presents high adaptability to new environments, displacing native vegetation that may alter the ecological balance. For example, blackberry is considered an invasive species in the Galapagos Islands and solutions are necessary to be implemented in situ to avoid its propagation. Hence, an extraordinary number of lignocellulosic residues can be found worldwide from which high value-added materials, compounds and fine chemicals can be produced and extracted.

This project focuses on the production of hydrochars from agroindustrial residues. The aforementioned residues were submitted to hydrothermal carbonization treatments under several temperatures and processing times. These carbon-based materials can be chemically treated for improving their adsorptive capabilities. The use of chemical agents such as acid (HCl), base (NaOH, KOH), and metal oxides would increase their specific surface area in conjunction with a surface functionalization [5]. Thus, a faster and selective adsorption process can be performed. On the other hand, HTC can be carried out towards fuel production. Lignocellulosic residues, especially those from the agricultural sector can be used as fuels; however, their direct combustion is not suitable from an environmental point of view. These residues usually show significant contents of fuel-bound nitrogen, which increases NO_X concentrations; high volatile matter contents that promote the release of products of incomplete combustion such as carbon monoxide, volatile organic compounds, and polycyclic aromatic hydrocarbons (PAHs); and high ash contents rising the inorganic particulate matter emissions. In this sense, agro-forest-based hydrochar fuels in form of pellets have been demonstrated to successfully reduce CO and particulate matter emissions in domestic heating stoves [6]. In the case of NO_X emissions, modifying the operating conditions by using catalysts during the HTC process seems to be a proper way to obtain hydrochars showing lower nitrogen contents [7], [8] thus reducing NO_X concentrations during combustion.

It is well understood that lignocellulosic residues are rich in nutrient content and possess their own the chemical/phytochemical composition (identity) and with this in mind, the goal is to suggest which materials and under what conditions are suitable for i. electronic purposes (as dielectric materials or semiconductive ones), ii. soil amendments due to amount of nutrients present in hydrochars, iii. water treatment applications, iv. energy applications such as biofuel production. In parallel, the liquid fractions from the HTC treatments will also be evaluated. For this, materials characterization is performed to all samples via X-ray Diffraction (XRD) (for determination of crystallographic structure), Thermogravimetric Analyses (TGA) (for evaluation of temperature effects), Fourier Transform Infrared (FTIR) spectroscopy (for material composition), Scanning Electron Microscopy (for morphology observation), Brunauer-Emmett-Teller (BET) surface area analysis, Raman spectroscopy (vibrational modes of the samples), and X-ray photoelectron spectroscopy (XPS) (for determination of the chemical state of the surface). In parallel, pellets for electrical characterization using a probe-station connected to a Semiconductor Analyzer is used for the determination of relative permittivity of the samples.

Relevant results showed that cocoa husk hydrochars (CHH) depict less variability in relative permittivity values with respect to ripe banana peels hydrochars (BPH) with approaching values between 8.25 to 10.29 for HTC temperatures between 150°C to 210°C (Figure 1). Hence, it is concluded that these materials possess dielectric capacities which imply their suitability for consumer electronics applications such as their use in capacitors. Also, according to N₂ adsorption – desorption isotherms, as temperature and time increases the specific surface area represented as m²/g of sample, increase as well. CHH present higher areas at higher temperatures compared to BPH. Surface morphology of the samples show that the BPH particles are slightly larger than CHH particles with and average length of 630 μm (Figure 2). Given that XPS results show the presence of graphitic material in all the samples, the absence of the characteristic (002) reflection may indicate that graphite is in turbostratic form, such that the disordered stacking of the carbon layers would kill the intensity of this peak (Figure 3). The only exception would be the BPH produced at 210°C, where the (002) reflection of graphite is clearly observed through XRD analyses. XPS (Figure 4) reveals the presence of C, O, N, P, K and Ca, being C, the largest contribution due to the presence of lignin and cellulose molecules within the structures. Also, it important to notice that N content was not affected, therefore these hydrochars can be considered as N-doped hydrogenated carbon nanostructures. On top, BPH C1s spectra clearly resembles a more graphitic structural system [9] when compared to the CHH.

These research areas focus on Sustainable Development Goals: 6 ,7, 9, 12 and 13, mainly, to contribute towards ensuring clean water, affordable energy, responsible consumption and production, while promoting innovation to industrial applications with less impacts to climate change. HTC gives greater added value to the agroindustrial residues to develop the Circular Bioeconomy in the country and find new industrial applications with a sustainable approach [1]. Future work comprehends the use of all characterization data as input for AI algorithms that can help predict the evolution of lignocellulosic materials, other properties and even predict the HTC process itself.

[1] A. C. Landázuri et al., “Ripe banana peels and cocoa husk hydrochars as green sustainable ‘low loss’ dielectric materials,” Under Review in Journal of Cleaner Production, 2023.

[2] J. S. Villarreal, J. R. Gándara, D. Navarrete, M. L. Bejarano, and A. C. Landázuri, “Lead (Pb2+) adsorption by means of pristine and prewashed residual Moringa oleifera Lam. seed husk biomass for water treatment applications,” International Journal of Sustainable Engineering, vol. 00, no. 00, pp. 1–13, 2020, doi: 10.1080/19397038.2020.1862350.

[3] D. Portalanza et al., “Potential Impact of Future Climates on Rice Production in Ecuador Determined Using Kobayashi’s ‘Very Simple Model,’” Agriculture (Switzerland), vol. 12, no. 11, Nov. 2022, doi: 10.3390/agriculture12111828.

[4] W. I. Mortada, R. A. Mohamed, A. A. A. Monem, M. M. Awad, and A. F. Hassan, “Effective and Low-Cost Adsorption Procedure for Removing Chemical Oxygen Demand from Wastewater Using Chemically Activated Carbon Derived from Rice Husk,” Separations, vol. 10, no. 1, p. 43, Jan. 2023, doi: 10.3390/separations10010043.

[5] M. S. Soffian, F. Z. Abdul Halim, F. Aziz, M. A．Rahman, M. A. Mohamed Amin, and D. N. Awang Chee, “Carbon-based material derived from biomass waste for wastewater treatment,” Environmental Advances, vol. 9. Elsevier Ltd, Oct. 01, 2022. doi: 10.1016/j.envadv.2022.100259.

[6] H. A. Murillo, L. A. Díaz-Robles, R. E. Santander, and F. A. Cubillos, “Conversion of residual oat husk and pine sawdust by co-hydrothermal carbonization towards biofuel production for pellet stoves,” Ind Crops Prod, vol. 174, Dec. 2021, doi: 10.1016/j.indcrop.2021.114219.

[7] J. Massaya, G. Pickens, B. Mills-Lamptey, and C. J. Chuck, “Enhanced Hydrothermal Carbonization of Spent Coffee Grounds for the Efficient Production of Solid Fuel with Lower Nitrogen Content,” Energy & Fuels, vol. 35, no. 11, pp. 9462–9473, Jun. 2021, doi: 10.1021/acs.energyfuels.1c00870.

[8] B. Zhang, J. Wang, Z. Xu, S. Wu, R. Luque, and H. Zhang, “Sewage sludge valorisation by hydrothermal carbonization: A new method to enhance nitrogen removal in hydrochar catalyzed with Ni–Mg–Al layered double oxides,” J Clean Prod, vol. 386, Feb. 2023, doi: 10.1016/j.jclepro.2023.135880.

[9] A. Fujimoto, Y. Yamada, M. Koinuma, and S. Sato, “Origins of sp3C peaks in C1s X-ray Photoelectron Spectra of Carbon Materials,” Anal Chem, vol. 88, no. 12, pp. 6110–6114, Jun. 2016, doi: 10.1021/acs.analchem.6b01327.