**1. Introduction**

Plastics and synthetic polymers, which are primarily made from petroleum or petroleum derivatives, have become more common over the years. Their attractive properties, such as their durability, lightness, low cost, and plasticity have contributed to the mass production of plastics in various configurations and their wide application in various industries [1,2]. Plastic is both a blessing and a curse because, despite its properties, plastic persists in the environment for a long time and is easily transported into the biosphere. In 2020, end users in Europe, Norway, Switzerland, and Great Britain threw 29.5 million tons of plastic away, with about 23% of this plastic waste being sent to landfills [3].

Plastics are degraded by sunlight, oxygen, heat, mechanical stress, and/or enzymes into smaller particles such as microplastics and nanoplastics through abiotic and/or biotic degradation [4]. This plastic waste is much more toxic compared to macroplastics.

**Citation:** Miroshnichenko, D.; Lebedeva, K.; Cherkashina, A.; Lebedev, V.; Tsereniuk, O.; Krygina, N. Study of Hybrid Modification with Humic Acids of Environmentally Safe Biodegradable Hydrogel Films Based on Hydroxypropyl Methylcellulose. *C* **2022**, *8*, 71. https://doi.org/10.3390/ c8040071

Academic Editors: Indra Pulidindi, Pankaj Sharma, Aharon Gedanken and Dimitrios Kalderis

Received: 14 October 2022 Accepted: 30 November 2022 Published: 3 December 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Numerous studies have reported the role of microplastics as carriers of chemical pollutants, including heavy metals, pesticides, persistent organic pollutants, and persistent bioaccumulative and toxic substances, etc. [5]. Plastic in the soil negatively changes its physical, chemical, and biological properties. Microplastics in the soil can change its porosity, bulk density, and water-holding capacity. The accumulation of microplastics in the soil can also change its biological properties, such as its organic carbon and nitrogen cycling, nutrient transport, and microbial activity [6]. That is why the most relevant direction in industrial polymer materials science today is the receiving of various biodegradable polymers, materials, and composites based on them. Such biodegradable polymers can be broken down abiotically and/or biologically into carbon dioxide, methane, water, and biomass. Such transformations are called biodegradability [7]. Today, there are a large number of biodegradable polymers that implement the principle of "zero waste" during their entire life cycle: «production-use-disposal» [8,9]. The use of a large assortment of environmentally safe biodegradable polymer matrices allows for the obtaining of materials with sufficient strength and heat resistance, which can be processed into various products and parts for various industries. Composite biodegradable materials obtained due to the interaction of chemically different components, most often inorganic and organic, which form a spatial crystal structure that differs from the structures of the original reagents but often inherits the properties of the original components, are called hybrids [10]. Receiving such biodegradable polymer materials and composites allows for a synergistic effect of useful properties for chemically different components in the finished hybrid which leads to area of such materials application expansion.

Several works have shown [11,12] the potential of modifying biodegradable polymer hydrogel materials (BPHMs) with coal, carbon, and graphene oxide derivatives due to their resulting fluorescent ability, photostability, biocompatibility, and large surface areas. Such modified BPHMs are used to receive effective transdermal systems in biomedical applications [13–17]. The most effective modern hydropolymeric microneedle patches are made up of hydroxypropylmethylcellulose [18], hyaluronic acid [19], carboxymethyl cellulose [20], polyvinylpyrrolidone [21], and polylactic glycolic acid [22]. In our opinion, there is great potential for BPHM functional modification based on hydroxypropyl methylcellulose with coal derivatives—humic acids, graphite, graphene, and others. In our previous works, environmentally safe hybrid biodegradable polymer materials based on gelatin [23], polyvinyl alcohol [24], and hydroxypropyl methylcellulose [25], which were modified with humic acids from Ukrainian brown coal [26], were designed and researched. In these works, the antibacterial effect of humic acids in the researched polymers was also established. However, the modification mechanism of hydroxypropyl methylcellulose with humic acids and its impact on the operational characteristics of environmentally safe biodegradable materials with antibacterial action based on them was not determined.

That is why this article studies the hybrid modification of environmentally safe biodegradable hydrogel films based on hydroxypropyl methylcellulose with humic acids. The tasks of this research was to:

