Different types of nanomaterials used in biodegradation processes.
Biodegradation is the most viable alternative for numerous health and environmental issues associated with non-biodegradable materials. In recent years, there has been considerable interest in biodegradable nanomaterials due to their relative abundance, environmental benignity, low cost, easy use, and tunable properties. This chapter covers an overview of biodegradation, factors and challenges associated with biodegradation processes, involvement of nanotechnology and nanomaterials in biodegradation, and biodegradable nanomaterials. Furthermore, current chapter extensively is discusses the most recent applications of biodegradable nanomaterials that have recently been explored in the areas of food packaging, energy, environmental remediation, and nanomedicine. Overall, this chapter provides a synopsis of how the involvement of nanotechnology would benefit the process of biodegradation.
- food packaging
- energy storage
- environmental remediation
1. Introduction to biodegradation
Sustainable development is a principle that is implemented to preserve the environment for the future generation while meeting the needs of the present generation. Environmental pollution is considered one of the significant barriers to sustainable development. Therefore, the drive for sustainable development must address environmental pollution by removing pollutants, restoring polluted areas, or using without affecting the unpolluted areas . Biodegradation is identified as a key eco-friendly and economical way of sustainable development, which entails enzymatic degradation or a breakdown of complex organic matter into small molecules in the presence of microorganisms . The microorganisms could also allow the biodegradation of organic matter in the presence of a growth substrate used as the primary source of energy and carbon source, a process called cometabolism . The biodegradation process is an effective alternative for commonly applied waste disposal methods such as incineration and landfilling .
2. Key challenges associated with biodegradation process
Biodegradation may sometimes lead to incomplete mineralization of the total organic content, such as recalcitrant materials leaving unprocessed contaminants behind . This could be due to the complex structure of the materials, higher molecular weight, crosslinking, shape, texture, surface area, and degradation rate . For example, depending on the degree of crystallinity, orientation and packing of polymers, the degradation rate is severely affected. It has been observed that even under the same conditions, the degradation of amorphous regions of polycaprolactone (PCL) by filamentous fungi is much faster than the degradation of crystalline regions of PCL, where the amorphous regions may permit easy access to microbes during the degradation process . Therefore, ensuring the complete or partial degradation of these complex substances to produce harmless products without secondary pollution is extremely important .
Additionally, the microbial biodegradation process also depends on various factors such as nutrient availability, microbe type, substrate properties, and environmental conditions such as pH, moisture content, and redox potential [6, 7, 8]. Generally, the redox potential relies on the presence of the electron acceptors at the active site, such as oxygen, nitrates, manganese oxides, iron oxides, sulfate, and triggering the aerobic or anaerobic biodegradation. Even though many of the microbes prefer physiological pH of 7.4 and a temperature of 37°C for their growth, certain microbes such as fungal species prefer an acidic environment. In contrast, some bacteria prefer relatively high temperatures for their optimal growth. Therefore, not exactly knowing the required growth conditions could be a significant factor contributing to the incomplete degradation of the substrate in some cases .
Furthermore, microbial metabolism in biodegradation is an energy transformation process that is solely governed by the functions of enzymes and the intermediates produced during the reactions . Therefore, proper screening is required to identify the microbes with an inherent set of genes that are capable of degrading the contaminants, the factors and the conditions under which the population of these microbes could increase, and the synergic performance of these microbes with other technologies to establish an environmentally profitable biodegradation platform . It has also been identified that more optimization procedures and scaling up are required for the biodegradation of large contaminated areas .
3. Role of nanotechnology in biodegradation
Biodegradable materials can be considered a preeminent group of materials that could also be called next-generation materials leading to zero global environmental pollution. Owing to this concern, the consumption of biodegradable materials, such as polymers, has increased two to three-fold in a broad spectrum of fields, including agriculture, automotive, packaging, energy and environment, and biomedical. But still, the contribution coming from the biodegradable polymers in said areas only accounts for 3–5% of total polymer consumption . This lower contribution could be mainly due to the poorly addressed issues such as low durability, low performance, and high production cost .
In this context, combining the concept of biodegradation with nanotechnology could be identified as a more systemic and innovative approach to address the current issues with the biodegradation process . Nanotechnology is an evolving branch of science that has diversified its application in many disciplines such as agriculture , healthcare , transportation , electronics , food , water purification [19, 20, 21, 22, 23, 24, 25], and security . Nanotechnology involves manipulating matter in 1–100 nm nanoscale to create materials where at least one of the dimensions of the particles in the nano range . The combined approach of nanotechnology-mediated biodegradation could address a wide range of potential applications in agriculture, food packaging, environmental remediation, and healthcare while accounting for reduced costs and no impact on environmental pollution . Nanomaterials have proven effective as excellent adsorbents, sensors, and catalysts for biodegradation purposes due to their specific surface area and high reactivity .
Furthermore, the presence of nanoparticles and microbes that are actively engaged in the biodegradation process has paved the way in improving growth profiles of microbes by acting as biodegradation enhancers [29, 30, 31, 32]. It has also been observed that the integration of nanotechnology with the enzymatic pathways in the biodegradation process could lead to profound activity and improved reusability of the enzymes . Nanoparticles have also performed as effective sensor systems to detect the utilization of the raw materials and the production of specific products, which provided an inference on the progression of the biodegradation process . Hence, this process of nano-biodegradation would ultimately involve the reduction of accumulating harmful non-biodegradable materials in the environment [34, 35].
3.1 Factors affecting the performance of nanomaterials during biodegradation
3.1.1 Properties of the nanomaterial
The chemical and physical interaction between the nanoparticles and the microbiota during the biodegradation is majorly influenced by the properties of nanomaterials such as size, shape, surface functionalization chemical structure, as they could influence the reactivity and stability of the nanomaterial [36, 37]. In addition, nanomaterials exhibit a quantum effect where less energy is required for associated chemical reactions . Furthermore, the surface plasmon resonance exhibited by certain types of nanomaterials such as gold nanoparticles (Au NPs)  and silver nanoparticles (Ag NPs)  can also be used to detect and identify the contaminants. The smaller size of the nanomaterials also allows them to penetrate deeper into complex organic molecules .
3.1.2 Properties of the microorganisms and culture medium affecting the biodegradation
The performance of the nanomaterials during biodegradation also depends on the type of the organism, such as bacteria, fungi, protozoa and the type of enzymes used for the degradation of the contaminants . Growth conditions such as pH, redox potential, temperature, ionic strength, solubility, presence or absence of electron acceptors in the culture medium also influence the activity and stability of the nanoparticles . Therefore, proper control of the culture medium conditions to obtain a prolonged uninterrupted biodegradation procedure is necessary.
3.2 Types of nanomaterials used in biodegradation
Different types of nanomaterials have been utilized in the biodegradation process. Table 1 summarizes the specific types of nanomaterials and their applications. Commonly used biodegradable nanomaterials include zero-valent metals, metal oxides, metal sulfides, nano clay, nanocomposites, carbon-based nanomaterials, biopolymer-based nanomaterials, and nanofibrous materials (see Figure 1). These nanomaterials can be synthesized using two different ways; one is the laboratory-mediated synthesis of nanoparticles (ex-situ) , and the other one is the in-situ synthesis of nanomaterials inside the microbes . Besides, there could be another lineage of ex-situ synthesized nanomaterials, which are biodegradable in origin and mainly applied in the biomedical field as theragnostic agents . After performing its’ definite action including controlled drug delivery, imaging, implantation, tissue engineering) these nanomaterials undergo natural degradation upon the enzymatic attack inside the living cells .
|Type of the Nanomaterial||Type of degradation||Biodegradation process||Reference|
|I. Zero valent iron (nZVI)||Microorganism mediated- Organohalide-respiring bacteria (OHRB), sulfate reducing bacteria (SRB) and iron reducing bacteria (IRB)||nZVI provides suitable living conditions for the growth and activity of anaerobic bacteria to degrade organohalides, heavy metals|||
|II. Zirconia (ZrO2)||Microorganism mediated- ||Synthesis of ZrO2 via |||
|III. Silicon dioxide (SiO2)||Microorganism mediated-Indigenous actinomycetes species isolated from the effluent contaminated site||Actinomycetes mediated synthesis of silica and use for adsorption and decolourisation of textile effluent|||
|IV. Iron oxide (Fe3O4)||Microorganism mediated||Enhance the consortium growth that involve in Low-Density Polyethylene (LDPE) degradation|||
|V. Cadmium Zinc sulfide quantum dots (CdZnS QDs)||Microorganism mediated||Immobilization of nanoscale CdZnS QDs in to the extracellular matrix of bacterial biofilms which are later on used as catalysts for the degradation of nitro aromatic compounds|||
|Microorganism mediated- ||Clay/modified clay minerals as effective adsorbents of PAHs/volatile oxygen compounds (VOCs) to trigger the microbial mediated biodegradation|||
|I. Nanocellulose composites||Microorganism mediated- ||Bacteria decorated nanocellulose being used as a scaffold to grow the bacteria as well as to remove Diuron via biodegradation|||
|II. Fe3O4/biochar composites||Microorganism mediated- ||Improve the adsorption capacity of photosynthetic bacteria as well as to improve the efficiency of bioremediation of wastewater|||
|I. Fullerene 60||Microorganism mediated- ||Influence the growth cycle of LDPE, HDPE epoxy and epoxy silicon degrading bacteria and accelerate the polymer biodegradation process of bacterial consortia|||
|II. Carbon nanotubes (CNTs)||Microorganism mediated- ||Immobilization of microbes for bioremediation of heavy metals|||
|I. Alginate beads||Microorganism mediated- ||Improved the bacterial attachment required for oil bioremediation|||
|II. Chitosan beads||Microorganism mediated- ||Remove polycyclic ht by immobilizing the bacteria by improving the degradation rate|||
|I. Polyvinyl alcohol (PVA) and Polyethylene oxide (PEO) nanofibers||Microorganism mediated- ||Provide suitable platforms for preservation of living bacterial cells and direct use for bioremediation of methylene blue|||
|II. Cyclodextrin nanofibers||Microorganism mediated- ||Provide a matrix for the encapsulation of bacteria to perform bioremediation of heavy metals and reactive dyes|||
|I. Polylactic acid (PLA) micelles||Physiological Enzymes mediated||Tumor targeting and efficient drug delivery|||
|II. Polylactic glycolic acid (PLGA) micelles||Physiological Enzymes mediated||Use of thermosensitive and biodegradable triblock copolymer for temperature sensitive drug delivery for liver cancer|||
|III. Polycarprolactone (PCL) nanoparticles||Physiological Enzymes mediated||Biodegradable nanocarriers for therapeutic compounds|||
|IV. Chitosan nanoparticles||Physiological Enzymes mediated||Biodegradable nanocarriers for drug delivery diagnosis and other biological applications|||
|V. Dendrimers||Physiological Enzymes mediated||Biocompatible, biodegradable delivery system against infections and cancer|||
|VI. Liposomes||Physiological Enzymes mediated||Less toxic, biodegradable delivery systems for various diseases|||
However, the selection of the type of nanomaterial relies on the nature of the contaminants and the microorganism that mediates the biodegradation process .
4. Applications of biodegradable nanomaterials
Biodegradable nanomaterials or nanoparticles include two major types: nanomaterials directly synthesized from various biopolymers such as polypeptides, polysaccharides and polynucleotides; and metallic nanoparticles, which are colloidal particles encapsulated inside a polymer matrix. The selection of this biopolymer matrix is based on many factors, including the size of the nanoparticles, degree of biocompatibility and biodegradability, surface properties and functionality and the type of application . These biodegradable nanoparticles are typically in the 10–500 nm size range. Widely used methods for the fabrication of biodegradable nanoparticles include emulsification, solvent evaporation, coprecipitation, desolvation, coacervation, electrospray and electrospinning . Over the past few years, many studies have been conducted in various fields on the preparation and applications of biodegradable nanomaterial. However, the applications in food packaging, energy, environmental remediation, and nanomedicine are discussed in this section.
4.1 Food packaging
Packaging plays an imperative role in the food industry. The major function of packaging is protecting food from physical damage while handling, transporting and storage. Packaging materials also maintain the food quality by protecting against air, moisture, insects, light, and dust and prevent contamination from chemical and biological sources. Commonly used packaging materials include plastics, metals, paper and paper boards, glass, and other traditional materials. However, food packaging accounts for 50% of petroleum-based plastics . Upon disposal, plastics remain in the environment taking many years to degrade. The fragments of plastics, also known as microplastics, enters the ecosystems via food chains causing growing environmental and health concerns. Therefore, there is a significant interest in the development of environmentally friendly food packing alternatives. Biodegradable nanoparticles have recently been employed for food packaging applications due to their simple synthesis route, non-toxicity, relative abundance, low cost, and eco-friendly nature. Following are recent food packing applications of biodegradable nanoparticles reported.
Starch-based (St) nanocomposite films prepared by incorporating silver (Ag), copper oxide (CuO) and zinc oxide (ZnO) nanoparticles (NPs) were tested for physicomechanical and antimicrobial properties by Peighambardoust
Recent advancement in biodegradable nanomaterials has led to the development of energy-efficient devices including ignition engines, solar cells, supercapacitors, and rechargeable batteries. Current applications of biodegradable polymers in energy-efficient devices are discussed below.
4.3 Environmental remediation
The rapid industrialization and urbanization across the globe have significantly impacted the terrestrial and aquatic environments by releasing harmful industrial effluents, including colored organic dyes, heavy metals, polycyclic aromatic hydrocarbons (PHAs), chlorinated organics and perfluorosurfactants . The release of these toxic substances imposes serious health concerns on all living beings. Biodegradable nanomaterials have recently been considered highly efficient agents for environmental remediation due to their high chemical reactivity, surface properties, catalytic activity, easy synthesis and fabrication, and environmental benignity. This section covers the applications of biodegradable nanoparticles in environmental remediation.
Electrospun and thermally cross-linked poly(vinyl alcohol) (PVA) and konjac glucomannan (KGM)-based biodegradable nanofiber membranes loaded with zinc oxide (ZnO) nanoparticles were prepared by Lv
Biodegradable nanomaterials have been recently investigated in nanomedicine due to their controlled drug release and targeted drug delivery, giving enhanced therapeutic effects and reduced side effects. Biodegradable nanomaterials impose less cytotoxicity on cells. Due to modifying and functionalizing ability, the biodegradable nanoparticles can also improve drug stability and solubility. The vital applications of biodegradable nanoparticles in nanomedicine include drug delivery, cancer therapy, imaging, and antimicrobial activity.
Biodegradation is the naturally occurring degradation of complex substances into simple eco-friendly products by the action of microorganisms and plays an imperative role in sustainable development. One of the significant challenges of biodegradation includes the incomplete breakdown of materials due to the complexity of the materials arising from structure, molecular weight, crosslinking, shape, texture, and surface properties. Other setbacks include the screening and identifying of suitable microbes, nutrients, and environmental conditions. Nanotechnology integrated biodegradation process has recently become an eco-friendly and cost-effective method of diminishing environmental pollutants due to the synergetic effects. The factors including the type of nanomaterials, type of the microorganism, and culture medium directly affect the involvement of nanomaterials in biodegradation. Common types of nanomaterials utilized in biodegradation processes include zero valent metals, oxides, sulfides, nanocomposites, nanoclay, carbon materials, biopolymers, and nanofibers. Biodegradable nanomaterials have been widely applied in food packaging, energy, environmental remediation and nanomedicine.
The authors are grateful for the financial and scientific support rendered by the Faculty of Technology, University of Sri Jayewardenepura.
Conflict of interest
The authors declare no conflict of interest.