White pollution is a global environmental pollution problem caused by the large amount of plastic waste discharged into the environment. As the main type of polymeric foam in plastic products, polyurethane foams (PUFs) have high application demand in many fields.
In recent years, PUFs in different forms have been widely used in environmental applications, such as biological filters for wastewater treatment, microporous adsorbent materials, biocatalyst supports in biotechnology and lightweight packaging materials with high surface area for cell attachment and biofilm formation.
In order to meet the needs of white pollution prevention and control, PUFs in the above applications must be non-toxic bio-based materials and have environmental degradation properties. Therefore, studies on the biodegradation of newly developed biofoam materials and their ecotoxicological effects have great potential to make valuable contributions to solving the world's environmental and resource depletion problems. This study provides a systematic review of the environmental degradation of PUFs, including existing recycling methods of PUFs, new ways to prepare bio-based and biodegradable PUFs, potential pathways for environmental degradation of PUFs, biodegradation behavior of PUFs, and ecotoxicity of PUFs.
Due to the high and growing production of PUFs, handling the increasing amount of PUFs waste has become an increasingly serious problem. Landfilling or incineration are the current mainstream management measures for dealing with PUFs waste, but in most cases it is difficult to achieve resource recycling in this process. The main methods used for PUFs recycling include: 1) physical recycling; 2) chemical recycling; 3) raw material recycling. Physical recycling is a method of preparing new PUFs by cutting and grinding PUFs and mixing them with new materials, but the proportion of mixed recycled PUFs must not exceed 5%. Recyclable polyols or other small molecule compounds obtained through glycolysis, hydrolysis, water-glycolysis and other solvolysis processes can be used for the preparation of new PUFs, but this process requires pretreatment. The pyrolysis recovery of raw materials can convert PUFs into small molecular substances and gases, while acidification treatment requires the participation of oxygen and air, so it does not yet have the characteristics of circular economy. Another way to reduce the environmental harmfulness of PUFs is to synthesize new bio-based or biodegradable PUFs. However, although the raw materials of bio-based PUFs are agricultural or recyclable waste, their products are still 100% similar to petroleum-derived PUFs in terms of properties, characteristics and recycling pathways. In contrast, the stability of biodegradable PUFs decreases after use.
In the natural environment, sunlight exposure mainly mediates the oxidation and photodegradation of PUFs, while humid environment coupled with chemical pollution may lead to the hydrolysis process of PUFs. Proteins secreted by living microorganisms such as bacteria, fungi and algae can also catalyze the degradation process of PUFs, and their degradation rate is affected by abiotic factors such as oxygen, heat, light, humidity and chemical pollutants. The biodegradation of PUFs includes 3 steps: 1) biodeterioration; 2) biofragmentation and 3) absorption/mineralization. Among them, biocorrosion is related to the structural changes, weight and mechanical integrity of PUFs, the biofragmentation process produces small molecular compounds, and the absorption and mineralization processes can be calculated from the increase in microbial biomass on or inside the PUFs surface and respirometry. In addition to environmental factors, the chemical structure and composition, supramolecular composition and macroscopic morphology of PUFs also have an important impact on their biodegradation. Biodegradable additives are often mixed into PUFs formulations to enhance and initiate their biodegradation.
Studies have shown that the biodegradation of PUFs includes two mechanisms: enzymatic oxidation and enzymatic hydrolysis. In current research on the biodegradation of PUFs, esterase and protease activities are typical targets of concern, and urease is the enzyme with the highest PUFs degradation efficiency among many microbial synthetic enzymes. Microorganisms with the ability to degrade PUFs mainly include bacteria Acinetobacter, Bacillus, Corynebacterium, Pseudomonas, Rhodococcus, Staphylococcus or Actinomycetes and fungi such as Aspergillus, Aureobasidium, Chaetomium, Cladosporium, Penicillium and Trichoderma. Microbial-mediated degradation of PUFs mainly produces some macromolecular substances such as oligomers or small molecule substances such as CO2 and H2O.
Microbial-mediated degradation of PUFs can lead to the long-term release and diffusion of harmful pollutants, so it is extremely important to evaluate the ecotoxicity of PUFs. It is an important research method to explore the ecotoxicity of PUFs by adding exogenous degradable substances. These exogenous additives include starch, cellulose derivatives, gluten, etc. Studies have pointed out that adding 10% 2-hydroxyethyl cellulose and 10% cellulose acetate has a better effect on promoting the microbial degradation of PUFs. Given that PUFs are highly toxic to plants, especially seeds, the plant assessment method is also an effective means to judge the ecotoxicity of PUFs. In addition, reliable reference data on the ecotoxicity of PUFs can also be obtained by studying the impact of PUFs on microbial community structure.
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Reference
- Biodegradability and ecotoxicity of polyurethane foams: A review. Critical Reviews in Environmental Science and Technology 52.2 (2022): 157-202.
