Water-soluble polymers with fully carbon backbones (C-C) and pendant functional groups such as polyacrylamide, polyacrylic acid, or in the case of this study PVOH, can be degraded through C-C chain scission. This can be achieved by mechanical35,36,37,38,43,44 or chemical14,21,34,43,44 effects. Cavitation as a physical phenomenon includes both types: mechanical effects propagated by shear forces, shock waves, and microjets, all triggered by the collapse of cavitation bubbles, and chemical effects usually associated with &#;OH formation triggered by local extreme temperatures or so-called hotspots. The intensity of the mechanical45 and chemical effects46 formed during cavitation may vary due to the operating conditions of the cavitation device and experimental conditions in terms of the properties and quality of the fluid47. The results of our study clearly show that both AC and HC result in the degradation of PVOH, however differences were observed between the two processes. Although the basic mechanism of cavitation bubble formation and collapse is the same, there is a distinct difference between AC and HC in the behaviour of the random bubble clusters or cavitation clouds. This translates to mechanical and chemical effects of different intensity, which can lead to different results of the processes. We can see that after AC treatment under the investigated experimental conditions, the molar mass characteristics of PVOH do not differ significantly (Figs. 1 and 3). Slightly better PVOH degradation was always achieved at low PVOH concentrations. Results at higher concentrations are comparable except in the case of AC9, where a smaller MMA reduction compared to other AC cases (Fig. 1) could be attributed to the uniqueness of cavitation behaviour at around 60&#;°C48,49,50. We believe that the higher effectiveness of AC at low PVOH concentration could relate to more extended coil conformation of high molar mass PVOH, which facilitates mechanical effects of AC. Our results are in agreement with other studies that used comparable PVOH concentrations of 0.1&#;3% (w/w)36,37,38, concluding that in parallel with a higher PVOH concentration, the viscosity of the solution also increased, which consequently hindered the mechanical effects of AC. As molecules become less mobile and the velocity gradients around the collapsing bubbles are smaller, the degradation decreases in accordance with the viscosity increase38. In addition to concentration, sample temperature is another important parameter affecting the degradation of chemical compounds with AC. As previously reported by Sun et al.23, a lower experimental temperature seems to be beneficial for the degradation of PVOH by AC. Our results (Figs. 1 and 3) suggest a similar outcome, where MMA reduction is consistently higher at 25&#;°C (by a few kDa) compared to 60&#;°C. The observed unfavourable effect of higher temperature on PVOH degradation could be explained by the higher vapour pressure and thus larger and more numerous cavitation bubbles (Fig. 7&#;AC: B compared to A), which has a dampening effect on the intensity of cavitation, leading to lower mechanical degradation of PVOH. A similar degree of degradation obtained at 25&#;°C with the addition of MeOH (Fig. 1: AC3 and AC8) suggests that the degradation of PVOH during AC is predominantly due to mechanical effects, even though the presence of &#;OH was experimentally confirmed by salicylic acid (SA) dosimetry (0.59&#;µg/mL of SA products determined) at these experimental conditions (Fig. 1: AC1*). However, these two experiments only confirmed that &#;OH formed during cavitation do not play a role, but not that they never contribute to the degradation of PVOH. To confirm or deny this, an external oxidant was added to intensify the process. The experiments (Figs. 1 and 3) performed with the addition of H2O2 at 25&#;°C and 60&#;°C again gave PVOH with similar molar mass characteristics as the experiments without the addition, corroborating MeOH experiments and pinpointing mechanical effects as the main mechanism of PVOH degradation. The C-C scission due to mechanical effects is considered to be a non-random process38, in which polymer chains are preferentially cleaved in the middle due to shear forces and shock waves23,36,37,38, and larger molecules are degraded most rapidly38. This can also be deducted from our SEC-RI chromatograms (Fig. 3), where the kinetics of chain scission slows down after the first 5&#;min of treatment, reaching a Mw of about 40&#;kg&#;mol&#;1, whereas the final Mw of 15.3&#;kg&#;mol&#;1 was reached after 60&#;min at 25&#;°C. The continuous decrease in MMAs and ĐM (Fig. 3) shows that the scission of the backbone does not occur randomly but predominantly in the central portion of the chains and does not proceed as fast as in the beginning once a certain Mw is reached (40&#;kg&#;mol&#;1 in our case). The FTIR spectrum recorded for the experiment at 25&#;°C with the addition of H2O2 (Fig. 2a&#;AC7) shows no obvious increase in the intensity of the band due to the carbonyl group and no decrease in the intensity of the bands corresponding to stretching vibrations of O-H and C-O groups, once again pointing to mechanical effects as the main degradation mechanism. When mechanical effects are predominantly responsible for the scission of the main chain, only the C-C bonds of the polymer backbone are broken, resulting in alkyl and allyl functionalized PVOH chains as indicated by 1H NMR.
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On the other hand, HC treatment under different experimental conditions (Figs. 1 and 4) results in major differences, indicating that another mechanism is mainly responsible for PVOH degradation. Temperature can be excluded as an important degradation parameter, since the results (Fig. 1) at 25&#;°C and 60&#;°C without addition of H2O2 show comparable extent of PVOH degradation. Addition of MeOH (Fig. 1&#;HC3) indicates that &#;OH might play a role in PVOH degradation, especially since the HC1* experiment (Fig. 1) showed that 0.50&#;µg/mL of SA products formed under these conditions. However, the FTIR spectra did not confirm this, as the characteristic band for the carbonyl group was not observed as a result of polymer oxidation (Fig. 2a&#;curves HC1 and HC4). However, this does not exclude the scission of the PVOH chains due to the &#;OH but only that the concentration of PVOH terminal groups bearing a carbonyl group was too low to be detected by FTIR, since the MMA of PVOH are still high at this stage. The addition of H2O2 significantly improved the degree of PVOH degradation at both temperatures, implying that the degradation in these cases is caused by &#;OH radicals in combination with mechanical effects. In studies investigating the degradation of PVOH by various AOP&#;s, it was shown that &#;OH, the common denominator of all, can effectively degrade PVOH and also lead to mineralization34. When &#;OH are responsible for the chemical scission of the C-C backbone, it can occur randomly anywhere along the polymer chain21,34. The mechanism of the scission begins with the abstraction of hydrogen, followed by the formation of carbonyl group or carboxyl group formation from the &#;OH in the side chain finally leading to the formation of short polymer chains after the C-C bonds scission21,23. This can be seen in the FTIR spectra in the case of HC2 and HC5 experiments (Fig. 2a), where raised carbonyl band suggests that &#;OH were responsible for the chain scission. Similar to our study, Prajapat & Gogate44 using HC showed enhanced degradation of polyacrylamide at lower concentrations, higher operating temperature and the addition of H2O2. They suggested that the polymer chains might become shorter due to both, mechanical and chemical effects of cavitation. However, since their FTIR spectra showed no chemical changes in the polymer structure, they concluded that if &#;OH was responsible for breaking the polymer chains, it had no effect on its structure. Even though this study was performed for polyacrylamide and a direct comparison with PVOH is not possible, some general conclusions on the main mechanism involved during cavitation can still be drawn. The higher effectiveness of H2O2 addition in HC compared to AC might also be due to the high turbulence and consequently good mixing, which transports &#;OH formed from H2O2 in the active cavitation zone more effectively into the bulk liquid. Since the entire sample is continuously transported through the cavitation chamber (Fig. 8), the probability of &#;OH encountering a polymer chain is increased. Similarly, Prajapat and Gogate44 concluded that the turbulence generated during HC provides better mixing, which facilitates the transport of &#;OH from H2O2 into the bulk liquid and finally enhance polymer degradation. Our study shows that under the most extreme conditions (HC5t: 60&#;°C and H2O2), the degradation of PVOH reaches 99% in 60&#;min, leading to Mw of 1.6&#;kg&#;mol&#;1 (Fig. 4, HC5t). Additionally, the most pronounced changes in the FTIR spectrum (strong carbonyl band) were also observed for this experiment (Fig. 2a&#;HC5). Similar results were obtained in our previous HC studies, in which the addition of H2O2 at 60&#;°C resulted in the highest removal of pharmaceuticals51, and the cavitation erosion between 30&#;°C and 100&#;°C at comparable HC conditions was most aggressive at about 60&#;°C52. We believe that in HC the combination of H2O2 and higher temperature strongly promotes the degradation of PVOH due to enhanced &#;OH production, pinpointing chemical effects as the prevailing mechanism.
In terms of energy input (Fig. 5) more energy was applied to the samples with HC treatment, pointing to AC as more energy efficient treatment. But in the case of HC with addition of H2O2 and elevated temperature (HC5t) lower molar masses were achieved with less energy, making this treatment as most energy efficient. AC as very localized and energy-intensive process delivers high amount of energy directly under the horn tip, where cavitation occurs. This area represents an active zone (about 125&#;mm3 or 0.25% of the total sample in our study&#;Fig. 6), while the rest of the volume is a passive zone. At the beginning of the treatment, when the C-C chains are the longest, the effectiveness of cavitation (active zone) is most pronounced, which can be deducted from the observed high rate of polymer chain breakage in the first 5&#;min. However, this advantage diminishes with time after a certain chain length is reached. Once most chains break to the limiting length, longer chains are less likely to reach a point of high energy density and encounter cavitation events. The non-uniform distribution of chains of all lengths in the sample and their random mass transfer between the active and passive zones53 leads to the observed decrease in the degradation rate of C-C chains. Similar to our results, others23,38 also observed that polymer chain degradation stops at a certain viscosity. They suggested that below a certain limiting viscosity or molar mass, the polymer chains are too short to be affected by ultrasonic vibrations. A similar analogy can be drawn from cellulose fibre degradation, where Redlinger-Pohn et al.53, suggested that fibres cannot be further shortened by AC after the limiting fibre length of 100&#;nm. On the other hand, in terms of energy density, HC is locally less intense than AC, since the supplied energy is distributed over a larger active zone. In this case the energy input is distributed over &#;mm3, which corresponds to about 1% of the sample in comparison with AC. Therefore, in HC a single cavitation event is less likely to exceed the energy threshold to break a bond of the polymer chain mechanically.
Considering other experimental conditions, the limited AC active volume and non-uniform mixing are also likely the reasons why the results do not show improvement by the addition of H2O2 (Figs. 3 and 5). The &#;OH should form from H2O2 under the influence of AC for the oxidant to have a pronounced effect. However, if the concentration of H2O2 is too high, it may start to act as a scavenger of the formed &#;OH, reducing the effect51. Moreover, the transport of &#;OH into the bulk liquid (passive zone) is slower due to the limited active volume and the possible degradation of the remaining shorter chains by &#;OH is slowed down. A similar effect was observed by Hamad et al.14 in the UV/H2O2 process. The constant decrease in MMAs and ĐM in the case of AC (Fig. 3) also shows that the chain scission occurs predominantly in the central portion of the chains, whereas in the case of HC the degradation mechanism seems to be more complex (Fig. 4). Since the chemical reactions between PVOH and &#;OH take place randomly along the polymer chain14,21,34,43,44, the degradation of polymer chains is independent of chain length. In fact, the greater the number of shorter chains, the greater the probability that each &#;OH formed will encounter a polymer chain and initiate scission. For this reason, the scission process in HC progresses continuously and no limiting chain length was encountered as is the case in AC. We can see that the biggest difference in HC is seen in the ĐM values. When H2O2 is added to the sample, the ĐM value initially increases regardless of temperature and only starts to decrease after 5&#;min, which suggests that scission of PVOH chains by &#;OH occurs randomly, which is in line with the results of Hamad et al.14. Moreover, different degradation rates were observed in the experiments at 25 and 60&#;°C with the addition of H2O2 after 15&#;min. This may be due to two mechanisms: (i) at 60&#;°C, a larger amount &#;OH of H2O2 is generated, which may enter the bulk phase and contribute to the degradation of PVOH, or (ii) the added H2O2 affected the cavitation conditions (Fig. 7F). As can be clearly seen in Fig. 7A, the observed cavitation is different from that in distilled water when PVOH and H2O2 are in solution at 60&#;°C. It cannot be excluded that the addition of H2O2 changed the cavitation dynamics by inducing chemical effects (&#;OH) and enhancing mechanical effects, which then contributed to a better degradation of PVOH.
This study shows that PVOH can be degraded by AC and HC. By manipulating the experimental conditions, a very high degree of PVOH degradation by HC can be achieved. The results indicate that mechanical effects predominate in the case of AC, while chemical effects seem to play the most important role in HC, especially when H2O2 is added. The mechanism responsible for the degradation of PVOH in the case of HC without addition of H2O2 is elusive, and further experiments should be performed to determine which mechanism, chemical or mechanical, is responsible for the observed degradation. Nevertheless, the achieved degradation of PVOH to very short oligomer chains is an important achievement, as low MMA oligomers can be more easily biodegraded in nature. As is indicated by the FTIR spectra in the case of HC, these oligomers are highly oxidized, which is generally a factor that increases reactivity and promotes biodegradation. This study can serve as a starting point for the degradation of other polymers that are perhaps more harmful and difficult to degrade, such as those that constitute microplastics. Another advantage of the very high degradation achieved with HC is that this type of device can be easily scaled up and integrated into current WWT systems as a pre- or post-treatment process, preventing or reducing the negative effects of PVOH in the environment. However, TOC and ecotoxicity studies should be conducted before the technology can be considered safe and used at pilot or industrial scale. These analyses would show in more detail the extent to which cavitation can mineralize PVOH and confirm that no undesirable or even toxic products have been formed.
Earlier this week, Plastic Pollution Coalition and other nonprofit groups joined cleaning products company Blueland to petition the U.S. EPA to urgently study and regulate a type of plastic called polyvinyl alcohol (which is also referred to as &#;PVA&#; or &#;PVOH&#;).
PVA/PVOH has been produced industrially since the s and is used for a wide variety of applications, including fibers for construction supplies; fishing gear; papermaking; cosmetics; industrial sprays, paints, and sealants; textile (clothing) sizing; packaging materials; food additives; pharmaceutical and medical products; and&#;quite commonly&#;film-coated detergent pods and sheets. In light of our petition, corporations that make and sell PVA/PVOH products have doubled down on asserting their material is safe and suggesting there is a &#;debate&#; over the safety of PVA/PVOH. But in reality, enough evidence exists to merit further investigation of the impacts of PVA/PVOH on human and ecological health.
To set the record straight, we&#;ve put together this FAQ backed by science and common sense. Read on to learn the truth about PVA/PVOH.
Conflicting information about PVA/PVOH exists today for two key reasons. Corporations: 1) use greenwashing as a marketing tactic to sell potentially harmful products that we might not otherwise buy, and 2) have control of a scientific environment in which they both shape commonly accepted information and are largely protected from scientific and regulatory scrutiny.
Many highly visible scientific studies that appear favorable of PVA/PVOH, especially those demonstrating apparent &#;degradability,&#; are commonly commissioned by plastic and related industry trade groups seeking to sell PVA/PVOH products. Many cleaning products made with PVA/PVOH are marketed as &#;eco-friendly&#; because they appear to readily &#;dissolve&#; in water&#;seemingly bypassing issues of toxicity and persistence that other plastics face.
Such favorable research findings help corporations sell more PVA/PVOH products, and receive passes from the U.S. EPA, FDA, and other regulatory agencies tasked with protecting human and environmental health. A lack of unbiased information, poor chemical regulatory environment, and corporations&#; drive for profits prevent us from getting a clear picture of what PVA/PVOH is and what it actually does to the environment and our health. And, because it is everywhere, we deserve to know the truth.
Even with decades of research, there is currently no definitive proof that PVA/PVOH is truly biodegradable. In fact, research on the textile industry in particular&#;a major consumer of PVA/PVOH, which is used to &#;size&#; or coat and protect woven fibers during manufacturing&#;suggests that PVA/PVOH plastic does not degrade, even in water; is commonly found in textile wastewater; and due to these concerns, should be replaced in this application with more benign alternatives.
Pure PVA doesn&#;t easily break apart and must be diluted by degradable plant starches and proteins to even appear to dissolve. Think about what happens when you mix salt with water: Just because you can&#;t see all the tiny grains of salt in the water as they dissolve doesn&#;t mean the salt doesn&#;t still exist. PVA/PVOH films like those used in dishwasher and laundry pods and sheets are designed to be similarly soluble in water. PVA/PVOH can be treated in a way (by running its main ingredient, polyvinyl acetate, through a process called hydrolysis) that makes its bonded particles break apart more easily and appear to &#;dissolve&#; in water&#; instead of remaining whole, visible, and separate from water.
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The EPA currently lists PVA/PVOH on its Safer Choice and Safer Chemical Ingredients lists. But a close look at research on PVA/PVOH and the EPA&#;s generous criteria for &#;safe&#; standards shows that PVA/PVOH is not verifiably nor consistently biodegradable. According to EPA standards, if 60% of a substance has degraded into carbon dioxide and water in 28 days, it passes its OECD 301 standard, and can be called &#;readily biodegradable&#;&#;even though it is not necessarily clear what happens to the remaining mass and chemistry of this substance when it is diluted with water.
In the case of PVA/PVOH, the research recently cited by the American Cleaning Institute shows a wide range of performance when it comes to PVA&#;s supposed degradation in water only. However, a close look reveals flaws in the study ACI cites: It is industry driven, small in size, brief in duration, and lastly, does not actually define what it means by &#;biodegradable.&#;
As other research indicates, PVA/PVOH itself is not actually biodegradable by the common definition of the word. PVA/PVOH, while synthetic and made of fossil fuels, is commonly mixed with degradable additives, such as plant starches and proteins, or even nanosized clays and metals. The presence of these other substances and particles helps cleave apart intact PVA/PVOH plastic, helping it seem to disappear when it makes contact with water.
PVA/PVOH films like detergent pods and sheets are especially prone to causing pollution because they are discharged directly into your home&#;s wastewater system. From there, polluted water is sent back into the ground through your septic, cesspool, or sewer system (and in this case, usually to a sewage treatment plant). Because tiny plastic particles are extremely hard to capture, water could thereby remain polluted by PVA/PVOH and other plastics even after treatment&#;endangering our planet&#;s waterways and our health.
According to the EPA, and corporations and industries that make and sell PVA/PVOH, this plastic is currently labeled as &#;safe,&#; largely due to its apparent biodegradability. Yet there is a serious lack of unbiased, dedicated research on the human and environmental health effects of PVA: Health-related research on PVA/PVOH has almost exclusively been conducted on nonhuman animals such as rodents and dogs by PVA/PVOH producers. What little research has been done on humans shows concerning links to inflammation and irritation, especially when PVA/PVOH exposure occurs over a prolonged period.
Despite clearly missing data on PVA/PVOH&#;s short-term and long-term human health and environmental effects, makers of this material advise on their own safety information sheets that their employees avoid breathing in, touching, consuming, and otherwise coming into contact with PVA/PVOH. With PVA/PVOH detected in human breast milk, it appears this plastic has the potential to accumulate in the environment or at least in human bodies where it could cause adverse health effects. Bioaccumulation of PVA has also been observed in carp fish over a short-term period. To prevent potential harm, much more research must be done to understand PVA/PVOH&#;s full range of effects on people, other animals, and the environment.
What&#;s more, PVA/PVOH and all other plastics are disproportionately produced in facilities intentionally placed in underserved low-income, rural, and BIPOC communities. This intentional poisoning of specific communities results in widespread health and environmental injustices, which the EPA has recently agreed to double down on. Allowing for continued production of all plastics&#;including PVA/PVOH&#;undermines the EPA&#;s initiative to seriously address injustice in the US. Producers of plastics release large quantities of greenhouse gases during manufacturing, in addition to toxic chemicals known to cause human health problems. PVA/PVOH production releases potent methanol gas, and often butyraldehyde (PVA/PVOH can be combined with this chemical to make polyvinyl butyral [PVB], another type of plastic, in the same facilities as PVA/PVOH).
Healthy, eco-friendly alternatives to plastic PVA/PVOH pods and sheets exist.
With our petition, we are emphasizing that there is currently a lack of accurate, unbiased information on PVA/PVOH and clear reasons to be concerned about its effects on human and environmental health.
The plastics industry, and the petrochemical industry which fuels it, has long tapped into misinformation campaigns to mislead the public and regulatory bodies, enabling widespread production and pollution of plastic across our planet and in our bodies. It is ironic that the ACI has called our common-sense request to the EPA &#;misinformation,&#; while it continues to lean on clearly biased industry-produced research to make its case for continued production (and pollution) of PVA/PVOH and other plastics.
We are calling on the EPA to take swift and urgent action to study the full ecological and health impacts of PVA/PVOH to best protect people and our planet from potential harm.
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