Photocatalytic degradation of poly(ethylene oxide) and polyacrylamide

Deciphering polymer degradation chemistry via integrating new database construction into suspect screening analysis

Water-soluble synthetic polymers and their environmental degradation products are overlooked but important industrial pollutants in wastewater. However, the detection of degradation products is limited to bulk solution chemistry and molecular-level analysis remains unreachable. In this work, we assessed the feasibility of current suspect screening and nontarget workflow using liquid chromatography-high resolution mass spectrometry (LC-HRMS) to elucidate molecular level information about polyacrylamide (PAM) and its degraded products by free radicals. Radical chain scission of PAM (10 kDa) using heat-activated persulfate was conducted to simulate hydraulic fracturing conditions in the deep subsurface. We found that the current workflows in the commercial software generated predicted formulae with low accuracy, due to limited capability of peak picking and formula prediction for high mass and charge features. By modeling literature-reported degradation pathways, we constructed a degradation product database of over 463 000 unique formulae, which improved the accuracy of the predicted formula. For the matched features, the ratio of aldehyde/ketone terminating molecule abundance was found to increase over 24 h degradation time, suggesting increasing content of aldehydes by radical-induced oxidative chain scission of PAM. This is contradictory to previously proposed ratios of carbon-centered radical position on polymer backbone initiated by hydroxyl radicals. Using in silico fragmentation of MS1 features, we identified 11 structures with confidence levels 2b and 3 using their MS2 information. This is the first attempt to resolve complex polymer degradation chemistry using HRMS that can advance our ability to detect water-soluble polymer pollutants and their transformation products in environmental samples.

Photochemical fate of nonionic polyacrylamide induced by hydroxyl radicals in the natural water: Mineralization mechanism exploration and half-life time evaluation

Water-soluble polyacrylamide (PAM) compounds have been used extensively in various sectors. The abundance of PAM in the environment raises concerns about its environmental impact. However, the mineralization of PAM in water under natural light irradiation remains insufficiently explored. This study utilizes nonionic PAM (nPAM) as a representative model to investigate both the mechanism and efficiency of nPAM degradation in water when exposed to ultraviolet (UV) light with hydrogen peroxide (H2O2) as the hydroxyl radical source. In the dark or with only UVA irradiation, negligible mineralization of nPAM occurred. In contrast, the presence of hydroxyl radicals (produced by the UVA/H2O2 system) produced 50 % nPAM mineralization over 7 days under our experimental conditions. The corresponding molecular weight (MW) of the nPAM was swiftly reduced from 1.58 ×106 Da to 1.59 ×103 Da in 3 days. Moreover, five carboxylic acids and nitrate ions were identified as the photodegradation intermediates of nPAM. The efficiencies of nPAM photodegradation by the UVA/H2O2 system in different natural waters and environmental conditions were assessed. The rate constant for the reaction between the hydroxyl radical and nPAM was 2.17 ×109 M-unit-1 s-1. The half-lives of nPAM in the sea and continental surface waters were determined to be several years and dozens of days, respectively. The application of UVB obviously accelerated the mineralization of nPAM in ultrapure water (71 % degradation in 7 days). Moreover, mineralization of concentrated nPAM (200 mg/L) in sea water was more efficient when both UVA- and UVB-activated H2O2 were used. Additionally, toxic acrylamide was not generated during nPAM photodegradation. Moreover, the photodegradation intermediates from nPAM were found to be neither acutely nor chronically toxic to aquatic organisms. This comprehensive study sheds light on the photochemical fate of nPAM in natural waters and provides essential insight for practical treatment of PAM in water systems.

Enhanced Polyacrylamide Degradation via OH Radical-Initiated Single-Electron Transfer

Hydroxyl (OH) radicals, as common radicals in aqueous environments, play an important role in inducing the degradation reactions of polymers. However, understanding the fundamental mechanisms of radical-induced degradation of polymers at the atomic level remains a formidable challenge. In this study, we employ density functional theory to investigate the geometric and electronic structural properties of polyacrylamide (PAM) in (-CH2CHCONH2-)n (n = 2-6) complexes. Additionally, we explore the degradation mechanism of the n = 4 complex induced by the OH radical. The results indicate that there are three sites for the initial reaction (R1 and R2 are at the ends and R3 is in the middle). The OH radical removes a H atom from the PAM main chain and simultaneously triggers a single-electron-transfer process on the same chain. This process significantly reduces the dissociation energy barrier of the C-C bond in the PAM chain, from ∼90 to ∼20 kcal/mol. Specifically, when the induced reaction occurs at the end of the chain, a series of broken bonds will appear only along the main chain. While it happens in the middle, the broken bonds will exist simultaneously along both the main and side chains. Our results reveal the importance of OH radicals in polymer dissociation, particularly in PAM, and emphasize the degradation mechanism of SET.

Water-Soluble Synthetic Polymers: Their Environmental Emission Relevant Usage, Transport and Transformation, Persistence, and Toxicity

Water-soluble synthetic polymers (WSPs) are distinct from insoluble plastic particles, which are both critical components of synthetic polymers. In the history of human-made macromolecules, WSPs have consistently portrayed a crucial role and served as the ingredients of a variety of products (e.g., flocculants, thickeners, solubilizers, surfactants, etc.) commonly used in human society. However, the environmental exposures and risks of WSPs with different functions remain poorly understood. This paper provides a critical review of the usage, environmental fate, environmental persistence, and biological consequences of multiple types of WSPs in commercial and industrial production. Investigations have identified a wide market of applications and potential environmental threats of various types of WSPs, but we still lack the suitable assessment tools. The effects of physicochemical properties and environmental factors on the environmental distribution as well as the transport and transformation of WSPs are further summarized. Evidence regarding the degradation of WSPs, including mechanical, thermal, hydrolytic, photoinduced, and biological degradation is summarized, and their environmental persistence is discussed. The toxicity data show that some WSPs can cause adverse effects on aquatic species and microbial communities through intrinsic toxicity and physical hazards. This review may serve as a guide for environmental risk assessment to help develop a sustainable path for WSP management.

3D-Printing Biodegradable PU/PAAM/Gel Hydrogel Scaffold with High Flexibility and Self-Adaptibility to Irregular Defects for Nonload-Bearing Bone Regeneration

A three-dimensional (3D) printed biodegradable hydrogel scaffold with a strong self-expanding ability to conform to the contour of irregular bone defects and be closely adjacent to host tissues is reported herein. The scaffold has a triple cross-linked network structure consisting of photo-cross-linked polyacrylamide (PAAM) and polyurethane (PU) as the primary IPN network and chemical cross-linked gelatin (Gel) as the secondary network, which confers the scaffold with good mechanical properties. The addition of PU in the polymerization process of acrylamide (AAM) can improve the ultraviolet (UV) photocuring efficiency of the hydrogel and incorporate abundant hydrogen bonds between the PAAM copolymer chain and the PU chain. The results show that the hydrogel scaffold contains regular structures with smooth morphology, excellent dimensional stability, and uniform aperture. The degradation rate of the hydrogel scaffold is controllable through adjusting cross-linking agents and can be up to about 60% after degradation for 28 days. More importantly, the rapid self-inflating characteristic of the scaffold in water, that is, the volume of hydrogel scaffold can increase to about 8 times that of their own in an hour and can generate a slight compressive stress on the surrounding host tissue, thus stimulating the reconstruction and growth of new bone tissues. The in vitro experiment indicates that the scaffold is nontoxic and biocompatible. The in vivo experiment shows that the PU/PAAM/Gel chemically cross-linked scaffold displays the desirable osteogenic capability. This UV-curable 3D printed self-adaptive and degradable hydrogel scaffold holds great potential for nonload-bearing bone repair.

Synergetic activation of persulfate by heat and Fe(II)-complexes for hydrolyzed polyacrylamide degradation at high pH condition: Kinetics, mechanism, and application potential for filter cake removal during cementing in CO2 storage wells

The long-term integrity of the interface between cement and formation rock in CO₂-capture and storage wells is crucial to avoid leakage of CO₂ in/along wells. However, the interface can be easily damaged by the filter cake, which is a compressed composite of bentonite, polymers such as hydrolyzed polyacrylamide (HPAM), and barite, on the wellbore rock. Therefore, removing the filter cake during the cementing process by degrading HPAM in an efficient way is essential. In this study, chelated-Fe²⁺ activated potassium persulfate (KPS) was used for HPAM degradation and filter-cake removal. Ethylenediaminetetraacetic acid disodium salt dihydrate (EDTA-2Na) and diethylenetriaminepentaacetic acid (DTPA) were adopted to control the precipitation of Fe²⁺/Fe³⁺. A mixture of 0.4 mM Fe²⁺, 0.8 mM DTPA, and 4 mM KPS at a pH of 10.0 at 70 °C reduced the molecular weight of HPAM significantly from 3.0 × 10⁶ to (3.6-10) × 10⁴ Da. Electron paramagnetic resonance (EPR) analysis suggested that HO was the dominant radical and that SO₄^- and O₂^- were responsible for the degradation. The reactions conformed to continuous distribution kinetics with an activation energy of 38.36 kJ mol^-1. A possible degradation pathway was proposed based on analyses via infrared spectroscopy (IR) and time-of-flight liquid chromatography-mass spectrometry (TOF-LC/MS). >90 wt% of the filter cake was removed by the system. The results suggest that the proposed DTPA-Fe²⁺ activated KPS system exhibits promising potential for in situ degradation of high molecular weight HPAM and for the removal of filter cake in downhole wells.

Hybrid nickel-free graphene/porphyrin rings for photodegradation of emerging pollutants in water

A novel hybrid photoactive material based on graphene foam (G) coupled with porphyrin-based polymers (Porph rings) was formulated by using a time-saving procedure to remove nickel from the final device. Specifically, Porph rings were spin coated onto the G platform with the double function of a visible-light photocatalyst and protective agent during nickel etching. The characterization of G-Porph rings was assessed by Scanning Electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL). The novel material showed photocatalytic ability in degrading different classes of pollutants such as the herbicide 2,4 dichlorophenoxyacetic acid (2,4-D), polyethylene glycol (PEG) as an ingredient of care and health products, and also the methylene blue (MB) dye. UV-Vis spectroscopy, total organic carbon (TOC) and soft mass spectrometry techniques were used to monitor the photocatalytic process. The best performance in terms of photocatalytic efficiency was exhibited versus PEG and MB degradation. Furthermore, to determine the individual contribution of Reactive Oxygen Species (ROS) produced, free radical and hole scavenging tests were also carried out. Finally, a detailed map of the photocatalytic degradation mechanisms was proposed, reporting also the calculation of Porph rings' Highest Occupied Molecular Orbital (HOMO) and Lowest Occupied Molecular Orbital (LUMO) energy level values.

A novel hybrid photoactive material based on graphene foam (G) coupled with porphyrin-based polymers (Porph rings) was formulated by using a time-saving procedure to remove nickel from the final device.

A Study on the Flocculation and Sedimentation of Iron Tailings Slurry Based on the Regulating Behavior of Fe3+

Based on the regulating behavior of Fe3+, flocculation and sedimentation tests of iron tailings slurry flocculated using 2 mg/L anionic polyacrylamide (APAM) were studied, including the tests of Fe3+ dosage, regulating time, and pH. Flocculation–sedimentation tests showed that: a recommended addition of 56 mg/L Fe3+ produced a surprisingly bad flocculation effect; sedimentation ability decreased with the increase of regulating time, however, the regulating ability during the first 1 min was low; Fe3+ displayed a high regulating effect at pH 6–7, and then decreased with the increase of slurry pH. Size analysis demonstrated that the regulating ability of Fe3+ was related to the change of floc size, which increased with the decrease of size. Zeta potential analysis and calculation showed that small concentrations of Fe3+ and certain hydroxyl complex ions (such as Fe(OH)2+ and Fe(OH)2+) adsorbed onto the APAM molecular chains, resulting in a decreased charge density of the APAM molecules, and this electrostatic adsorption was able to make the polymer curl more easily. Fourier Transform Infrared Spectroscopic (FTIR) indicated the APAM on the hematite was slightly degraded into a monomer-like short-chain polymer while adding certain concentration of Fe3+. Scanning Electron Microscope (SEM) imaging showed that the network structure of APAM molecules treated by Fe3+ collapsed, and the APAM adsorption amount on hematite was significantly reduced. Therefore, the effect of Fe3+ on the APAM could be recommended as a new method for desorption and degradation of the APAM contained in the tailings slurry or flocs.

Polymer conformation during flow in porous media

Molecular conformations of individual polymers during flow through porous media are directly observed by single-DNA imaging in microfluidics. As the Weissenberg number increases during flow (Wi > 1), we observe two types of elastic instabilities: (a) stationary dead-zone and (b) time-dependant dead-zone washing. When stretched polymer chains enter a dead-zone, they first re-coil and, once inside the dead-zone, they rotate and re-stretch again. The probability distribution of DNA chains under the stretched condition inside the dead-zone is found to be heterogeneous with a broad distribution.

Photoelectrocatalytic degradation of partially hydrolyzed polyacrylamide using a novel particle electrode

Tetrapyrazinoporphyrazines and their metal derivatives (TPyzPzs) are the most widely studied class of phthalocyanines azaanalogues.

On-the-Fly Kinetic Monte Carlo Simulation of Aqueous Phase Advanced Oxidation Processes

We have developed an on-the-fly kinetic Monte Carlo (KMC) model to predict the degradation mechanisms and fates of intermediates and byproducts that are produced during aqueous-phase advanced oxidation processes (AOPs). The on-the-fly KMC model is composed of a reaction pathway generator, a reaction rate constant estimator, a mechanistic reduction module, and a KMC solver. The novelty of this work is that we develop the pathway as we march forward in time rather than developing the pathway before we use the KMC method to solve the equations. As a result, we have fewer reactions to consider, and we have greater computational efficiency. We have verified this on-the-fly KMC model for the degradation of polyacrylamide (PAM) using UV light and titanium dioxide (i.e., UV/TiO2). Using the on-the-fly KMC model, we were able to predict the time-dependent profiles of the average molecular weight for PAM. The model provided detailed and quantitative insights into the time evolution of the molecular weight distribution and reaction mechanism. We also verified our on-the-fly KMC model for the destruction of (1) acetone, (2) trichloroethylene (TCE), and (3) polyethylene glycol (PEG) for the ultraviolet light and hydrogen peroxide AOP. We demonstrated that the on-the-fly KMC model can achieve the same accuracy as the computer-based first-principles KMC (CF-KMC) model, which has already been validated in our earlier work. The on-the-fly KMC is particularly suitable for molecules with large molecular weights (e.g., polymers) because the degradation mechanisms for large molecules can result in hundreds of thousands to even millions of reactions. The ordinary differential equations (ODEs) that describe the degradation pathways cannot be solved using traditional numerical methods, but the KMC can solve these equations.

Computer-based first-principles kinetic Monte Carlo simulation of polyethylene glycol degradation in aqueous phase UV/H2O2 advanced oxidation process

We have developed a computer-based first-principles kinetic Monte Carlo (CF-KMC) model to predict degradation mechanisms and fates of intermediates and byproducts produced from the degradation of polyethylene glycol (PEG) in the presence of hydrogen peroxide (UV/H2O2). The CF-KMC model is composed of a reaction pathway generator, a reaction rate constant estimator, and a KMC solver. The KMC solver is able to solve the predicted pathways successfully without solving ordinary differential equations. The predicted time-dependent profiles of averaged molecular weight, and polydispersitivity index (i.e., the ratio of the weight-averaged molecular weight to the number-averaged molecular weight) for the PEG degradation were validated with experimental observations. These predictions are consistent with the experimental data. The model provided detailed and quantitative insights into the time evolutions of molecular weight distribution and concentration profiles of low molecular weight products and functional groups. Our approach may be useful to predict the fates of degradation products for a wide range of complicated organic contaminants.

Photochemical degradation study of polyvinyl acetate paints used in artworks by Py-GC/MS

Photochemical degradation of commercial polyvinyl acetate (PVAc) homopolymer and PVAc paints mixed with burnt umber, cobalt blue, cadmium red dark, nickel azo yellow and titanium white commonly used for artworks were studied by pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) and Fourier transform infrared spectroscopy-attenuated total reflectance (FTIR-ATR). Py-GC/MS with single-shot technique was used for the characterization of the thermal degradation of PVAc at different temperatures, while the double-shot technique of Py-GC/MS was used to reveal the differences in the specimens before and after UV ageing, including the changes of detectable amounts of deacetylation product - acetic acid and plasticizers such as diethyl phthalate (DEP). Furthermore, the relative concentration of the pyrolysis products of the paint samples could be measured and compared in the second step of the double-shot Py-GC/MS, which are highly dependent on the presence of pigments and the ageing status of PVAc paints.

Chemical degradation of polyacrylamide by advanced oxidation processes

This paper presents the results obtained from the oxidation of polyacrylamide (PAM) by the UV/H2O2, Fenton, UV/Fenton, visible light/Fenton, visible light/Fenton/C2O4(2-), UV/Fenton/C2O4(2-), visible light/Fenton/C4H4O6(2-) and UV/Fenton/C4H4O6(2-) processes. Degradation efficiency for PAM had the following order: UV/Fenton/C4H4O6(2-) > UV/Fenton/C2O4(2-) > visible light/Fenton/C4H4O6(2-) > visible light/Fenton/C2O4(2-) > UV/Fenton > visible light/Fenton > UV/H2O2 > Fenton. The addition of tartrate had a positive effect on chemical oxygen demand (COD) removal. Increasing the concentrations of reagents promoted the oxidation of PAM by the UV/Fenton/C4H4O6(2-) process. The presence of NaCl led to large decreases in the COD removal in the PAM solution.

Heterogeneous photo-Fenton degradation of polyacrylamide in aqueous solution over Fe(III)-SiO(2) catalyst

This article presents preparation, characterization and evaluation of heterogeneous Fe(III)-SiO(2) catalysts for the photo-Fenton degradation of polyacrylamide (PAM) in aqueous solution. Fe(III)-SiO(2) catalysts are prepared by impregnation method with two iron salts as precursors, namely Fe(NO(3))(3) and FeSO(4), and are characterized by Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) methods. The irradiated Fe(III)-SiO(2) is complexed with 1,10-phenanthroline, then is measured by UV-vis-diffuse reflectance spectroscopy (UV-vis-DRS) and XPS to confirm the oxidation state of Fe in solid state. By investigating the photo-Fenton degradation of PAM in aqueous solution, the results indicate that Fe(III)-SiO(2) catalysts exhibit an excellent photocatalytic activity in the degradation of PAM. Moreover, the precursor species and the OH(-)/Fe mole ratio affect the photocatalytic activity of Fe(III)-SiO(2) catalysts to a certain extent. Finally, the amount of Fe ions leaching from the Fe(III)-SiO(2) catalysts is much low.

Acrylamide release resulting from sunlight irradiation of aqueous polyacrylamide/iron mixtures

Linear anionic polyacrylamide (PAM) has been used in irrigation practices as a flocculating agent to minimize water losses through seepage in earthen canals. The stability of PAM is of concern because of the possibility of acrylamide (AMD) monomer release during environmental weathering. Aqueous solutions of commercial PAM mixed with ferric sulfate, subjected to simulated and natural sunlight irradiation, showed polymer chain scission and release of the AMD monomer. At acid/neutral pH, the amount of AMD released was directly related to the concentration of ferric ion and the irradiation time. At alkaline pH (approximately 8), PAM/Fe(3+) mixtures were stable under irradiation. PAM chain scission involved the hydroxyl radical, but specific AMD release appeared to require PAM-bound iron. Low iron concentrations and alkaline pH of irrigation water would limit AMD release. Residual monomer in PAM can contribute AMD to irrigation water, but concentrations would remain below the U.S. EPA drinking water standard of 0.5 ppb.