Effect of microplastic size on the adsorption behavior and mechanism of triclosan on polyvinyl chloride

Laboratory simulation of microplastics weathering and its adsorption behaviors in an aqueous environment: A systematic review

Microplastics (MPs) pollution has become a global environmental concern. MPs alone and in combination with pollutants can potentially cause significant harm to organisms and human beings. Weathering of MPs under various environmental stresses increases the uncertainty of their environmental fates. Compared with field surveys, laboratory simulation experiments are appropriate to simplify the research procedures and investigate the mechanisms. In this review, the effects of abrasion, solar radiation, chemical and thermal oxidation, microbial adhesion and colonization, and other environmental factors on the MPs and the relative laboratory simulation methods were summarized and discussed. Photo-oxidation and abrasion are the most appliable methods due to easy operation and adjustable weathering degree. Furthermore, the structural and components changes in weathering process and the applied characterization methods were generalized. In addition, one of important environmental behaviors, adsorption of the weathered MPs towards two typical pollutants was analyzed. Finally, three priorities for research were proposed. This paper conducts systematic summarized of the MPs weathering process and provides a reference for future studies to accurately determine the environmental risks of weathering MPs.

Adsorption of chlorophenols on polyethylene terephthalate microplastics from aqueous environments: Kinetics, mechanisms and influencing factors

Microplastics have received growing attention as carriers of organic pollutants in the water environment. To better understand the contribution of hydrophobic interaction, hydrogen-bonding interaction, π-π interaction and electrostatic interaction on the adsorption of hydrophilic compounds on microplastics and their adsorption behavior in natural waters, polyethylene terephthalate (PET, <150 μm) was used as an adsorbent and 4-chlorophenol (MCP), 2,4-dichlorophenol (DCP) and 2,4,6-trichlorophenol (TCP) were used as adsorbates. The results of batch adsorption experiments showed that chlorophenols (CPs) reached adsorption sites of PET through film diffusion and intra-particle diffusion. pH greatly affected the adsorption capacity. Hydrophobic interaction was the main adsorption mechanism of undissociated CPs on PET. Hydrogen-bonding interaction was also an adsorption mechanism between undissociated CPs and PET, and the contribution of hydrogen-bonding interaction to adsorption decreased with the increase of chlorine content. Meanwhile, the increase of chlorine content was favorable to the hydrophobic interaction between undissociated CPs and PET. However, higher chlorine content CPs with lower pK_a values tended to dissociate at neutral pH condition and resulted in stronger electrostatic repulsion with PET. The increase of solution ionic strength and fulvic acid content negatively affected the adsorption of DCP and TCP on PET, but did not show significant impacts on MCP adsorption. Similarly, the adsorption capacity obtained using Taihu lake water and Bohai seawater as matrices was much lower than that using laboratory water for both DCP and TCP, while the adsorption coefficient (K_d) of MCP remained at approximately 10.6 L/kg to 11.4 L/kg in the three different solution matrices. The K_d values exhibited using natural water matrices consistently followed the order of DCP > MCP > TCP. This study provides insights into the fate of CPs in the presence of microplastics and suggests that the potential risks posed by CPs and microplastics to aqueous ecosystems merit further investigation.

The sorption behaviour of amine micropollutants on polyethylene microplastics - impact of aging and interactions with green seaweed

Microplastics are ubiquitous in the environment. Due to still rising global production, the emission of polymers into the environment and the abundance of microplastics have increased accordingly. Due to the long mineralization processes of microplastics, distribution in all compartments can be found. The hydrophobic surfaces of the particles can sorb chemical pollutants, therefore providing a potential pathway to accumulation by organisms within the food web. However, little is known about how long-term aging and degradation processes of microplastics can affect the sorption behaviours of organic pollutants on the particles. In this study, important industrial additives of emerging environmental concern, such as hydrophobic aromatic amines, were studied in relation to their sorption behaviour on high-density polyethylene and low-density polyethylene microplastics. Diphenylamine (log POW (logarithmic octanol-water partition coefficient) = 3.5) showed strong sorption, carbamazepine (log POW = 2.5) showed moderate sorption, and aniline (log POW = 0.9) showed no detectable sorption behaviour. Artificially aged particles exposed to photochemical aging and long-term mechanical treatment in water were compared to pristine microplastics. While mechanically aged microplastics promoted the sorption of aromatic amines, photochemically aged particles showed a decrease in sorption capacity due to changed surface chemistry. Importantly, the sorption capacity increased with increasing salinity, leading to strong implications for ocean systems, as an elevated uptake of pollutants could occur under marine conditions. Moreover, our study demonstrates that the ecotoxicological effects of diphenylamine on the growth of the seaweed Ulva (sea lettuce, Chlorophyta) were reduced in the presence of microplastics. As the plastic particles withdrew enough contaminants from solution, even toxic levels of diphenylamine (c = 10-4 M) became tolerable for the algae. However, the pollutants initially sorbed on the microplastics can be released again at a later point in the ageing process, thus having delayed pollution potential.

Changes of the acute and chronic toxicity of three antimicrobial agents to Daphnia magna in the presence/absence of micro-polystyrene

The effects of microplastics on aquatic organisms are drawing growing attention, but little has been focused on their effects on the toxicity of other chemicals. In this study, we examined the acute and chronic toxicity of micro-polystyrene (5.8 μm dia.), and its effects on the toxicity of three antimicrobial agents (triclosan, triclocarban and methyl-triclosan) to Daphnia magna. Results indicated that polystyrene had a low acute toxicity with an EC₅₀ of 36.5 mg/L. The presence of polystyrene (1 mg/L) did not produce significant effect on the acute toxicity of three chemicals, because the 95% confidence intervals of their EC₅₀ values had a large overlap of 11.3%-48.3%. For the 21 day chronic toxicity, polystyrene alone had significant toxicity with concentrations of at least 2 mg/L, which prolonged the time of the first brood, limited the number of broods, and reduced the total number of neonates. Compared with the chemicals alone, the addition of polystyrene enhanced their reproduction toxicity. Based on the various reproduction indicators, an intrinsic rate of natural increase (r_m) was calculated to assess the rate of population growth. Results suggested that the r_m values of three chemicals decreased in the presence of PS, and further decreased with increasing PS concentrations. Among the three chemicals, methyl-triclosan was the most affected. These results suggested that the presence of microplastics would exacerbate the detrimental influence of pollutants on Daphnia magna.

Aging mechanism of microplastics with UV irradiation and its effects on the adsorption of heavy metals

Microplastics are formed by the degradation of plastic wastes under the action of physicochemical mechanisms in environment, and they are becoming a new type of pollutant that is attractings global attention. However, research on the aging characteristics and mechanism of microplastics is limited. The aging mechanism of Polystyrene (PS) with UV irradiation under different conditions (air, pure water and seawater) and the effect of aging on heavy metal adsorption were studied. The results show that PS have different characteristics with UV irradiation under different conditions, and the aging of PS is the most obvious in air. Based on the 2D-COS analysis, different aging mechanisms were identified under different aging conditions, aging sequence of aged PS functional groups in air and water were clearly definited. An isothermal adsorption model shows that aging can significantly increase the adsorption of heavy metals by PS. The adsorption of heavy metals is also affected by different aging methods. Over all, a 2D-COS analysis was an effective method for understanding the aging process of PS. These results further clarify the aging mechanism of PS, and provides a theoretical basis for the assessment of environmental behavior and ecological risk when microplastics and heavy metals coexist.

The removal of microplastics in the wastewater treatment process and their potential impact on anaerobic digestion due to pollutants association

Microplastics are abundant in municipal wastewater which is mainly from personal care products and laundry. In recent years, great attention has been given to microplastics removal in wastewater treatment. In this article, the study focusing on microplastics in wastewater has been evaluated with VOSviewer. It was found that the major interest was in identification, quantification and pollution of the microplastics in the wastewater, and their transportation and final destination during wastewater treatment processes. The major microplastics and their shapes in wastewater were reviewed. Our evaluation results were consistent with other reported that fibers and fragment were the majority in terms of shape and polyethylene terephthalare (PET), polyethylene (PE), polypropylene (PP), and polystyrene (PS) are the most presented microplastics in wastewater. During wastewater treatment, the removal route of microplastics from wastewater includes settling, adsorption, entrapment, interception, etc. It confirms that microplastics are just simply transferred from wastewater to sludge. It could then bring problems to anaerobic digestion as microplastics are great vector for toxic substances such as antibiotics and persistence organic pollutants. The key to determine the microplastics effect on anaerobic digestion is the desorption behavior of the toxic substances such as antibiotics, persistent organic pollutants and heavy metals from microplastics in digestion condition. Toxic compounds which are commonly presenting in sludge have shown the tendency to release from microplastics. It indicates that microplastics in sludge have great possibility to impact on methane production.

Fenton aging significantly affects the heavy metal adsorption capacity of polystyrene microplastics

Microplastics released into the environment undergo a variety of aging processes, however, information about the influence of aging on the adsorption behavior of microplastics is limited. In order to better understand the effect of aging polystyrene (PS) on the ability to adsorb heavy metal, H₂O₂ and Fenton reagent were used to investigate the aging properties of PS. Aging PS with these two different aging agents at pH = 4 and room temperature for the same time. Physical and chemical characterization indicated that aging caused oxidation of the surface of PS and the formation of surface micro-cracks. Based on the 2D-COS analysis, the aging process of PS functional groups could occur in the following sequence: 1375 (C-OH) > 1739 (C=O) > 1182 cm^-1 (C-O-C) > 1716 (O-C=O). The adsorption experiments for Cd²⁺ with two different concentrations were carried out by PS with different aging time at room temperature. The adsorption data showed that the adsorption capacity of Cd²⁺ was significantly enhanced after aging compared with pristine PS, and the adsorption capacity of PS after Fenton aging treatment is much stronger than that after H₂O₂ aging treatment. The kinetic analysis of the adsorption data indicates that the adsorption process is more consistent with the second-order kinetics than the first-order kinetics, and it is further concluded that the adsorption of Cd²⁺ by PS is a relatively complicated process. According to the fitting results of adsorption isotherms, the adsorption process of pristine PS mainly occurs on the surface, but with the continuous aging, more adsorption sites may be exposed on the surface of PS, so it can be concluded that the adsorption mechanism of Cd²⁺ by PS is the coexistence of physics and chemistry. This study indicates aging microplastics may have a significant impact on the destination and migration of metal contaminants, which deserves to be further concerned.

Interfacial interaction between diverse microplastics and tetracycline by adsorption in an aqueous solution

The accumulation of microplastics in offshore aquaculture waters has gradually become a threat to the survival of marine life, and the combined pollution of microplastics and other pollutants is attracting widespread attention. In this paper, tetracycline (TC) was selected as a typical antibiotic, and its adsorption behavior on the surface of diverse type and different sizes of microplastics was studied to explore their combined pollution in an aqueous solution. The results of isotherm fitting showed that the maximum adsorption capacity and coefficient of polyethylene (PE) were the largest, and the adsorption capacity of PE was the strongest among the three microplastics: polyethylene (PE), polystyrene (PS) and polyvinyl chloride (PVC). With increasing PE particle size, the maximum adsorption capacity and adsorption coefficient of TC showed a significant decreasing trend, with a slight fluctuation in the middle. The presence of Pb²⁺, Cr³⁺, Cd²⁺, and Zn²⁺ markedly enhanced the adsorption of TC to PE, and Cu²⁺ could reduce the adsorption of TC to PE. The presence of chloride ions did not affect the adsorption process, which indicated that the adsorption mechanism between TC and microplastics is mainly an ion exchange mechanism. These results showed that the surface properties of microplastics and the chemical properties of the aqueous solution played an important role in the adsorption of TC. This study provides important scientific guidance and a theoretical basis for the study of the interfacial behavior, migration and transformation of marine microplastics.

Adsorption of triclosan onto different aged polypropylene microplastics: Critical effect of cations

Adsorption of organic contaminants onto aged microplastics (MPs) may be important in understanding their transport potential and ecological risks in aquatic environment. Cations of Na⁺ and Ca²⁺ are common electrolytes in water, which can greatly influence the adsorption behavior of MPs by impacting the electrostatic interaction between MPs and organic contaminants. The results of this study showed that aged isotactic polypropylene (iPP) MPs exhibited higher adsorption capacity to triclosan (TCS) than pristine ones, and the sorption affinity was enhanced with the increase of ionic concentrations. The crucial influence of cations on the adsorption behavior of aged MPs mainly depended on the changed properties of TCS and interactions between MPs and TCS. Salting out effect induced the precipitation of TCS from water and facilitated the partition of TCS onto MPs in high salinity water. Besides, compressing electrostatic double layer of MPs via squeezing out effect and bridging effect between functional groups of aged MPs and contaminants may also be significant factors in the sorption process. These findings will be helpful for understanding the role of cations on the transport of pollutants, the fate of MPs and their associated environmental risks in aquatic ecosystems.

Microbial degradation and other environmental aspects of microplastics/plastics

Microplastic (MP) pollution is a significant environmental concern due to the persistence of MPs and their potential adverse effects on biota. Most scientific studies have examined the distribution, ingestion, fate, behavior, amount, and effect of MPs. However, few studies have described the development of methods for the removal and remediation of MPs. Therefore, in this review, we summarize the recent literature regarding the microbial-mediated degradation of MPs and discuss the associated degradation characteristics and mechanisms. Different types and combinations of microorganisms, such as bacteria, fungi, bacterial consortia, and biofilms, that can degrade different MPs are categorized. This article summarizes approximately 50 recent papers. Twelve and 6 papers reported that bacteria and fungi, respectively, can degrade MPs. Nine articles indicated that bacterial consortia have the ability to degrade MPs, and 6 articles found that biofilms can also utilize MPs. Furthermore, to evaluate their associated degradation effects, the corresponding structural changes (i.e., macro size, surface morphology, and functional groups) in MPs after microbial degradation are examined. In addition, MP biodegradation is affected by microbial characteristics and environmental factors; therefore, the environmental factors (i.e., temperature, pH and strain activity) influencing MP degradation and the associated degradation effects (i.e., weight loss, degradation rate, and molecular weight change) are generalized. Furthermore, the mechanisms associated with the microbial-mediated degradation of MPs are briefly discussed. Finally, prospects for the degradation of MPs using microbes and future research directions are envisioned. This review provides the first systematic summary of the microbial-mediated degradation of MPs and provides a reference for future studies investigating effective means of MP pollution control.

Effect of weathering on environmental behavior of microplastics: Properties, sorption and potential risks

Microplastic (MP) pollution is a raising global concern in recent years due to its wide distribution. Additionally, most of the MPs have undergone extensive weathering in the environment, and weathered MPs may exhibit different physicochemical properties from pristine ones. The review reveals the change in physicochemical properties (e.g. size, color, crystallinity, mechanical property and oxygen-containing groups) and the release of additives and MP-derived intermediates (i.e. oligomers and oxygenated compounds) during weathering processes. Weathering further affects the sorption behavior of MPs for environmental pollutants because of the changed crystallinity, specific surface area and oxygen functional groups. The interaction mechanisms of pristine and weathered MPs with pollutants are summarized, and how weathering processes affect sorption behavior is critically revealed. Because of the changed size, color and surface charges, weathered MPs might be ingested by aquatic organisms in different ways from the pristine ones. The detailed effects of weathering on the ingestion of MPs are discussed, and the potential toxicity of leachates from weathering processes is evaluated. In addition, the environmental components (e.g. natural organic matter and salinity) and biofilm correlated to the sorption behavior of MPs are reviewed. As for the knowledge gap, further studies should focus on the long-term weathering of MPs and the relationships between weathering properties and sorption capacities toward pollutants. The potential risks of weathered MPs and leachates on organisms should be explored.

Effective adsorption of diclofenac sodium from neutral aqueous solution by low-cost lignite activated cokes

Activated cokes have attracted great interest inwater treatment to remove organic pollutants due to their low cost and specific textural properties. In this study, adsorptive removal of diclofenac sodium (DCF) from neutral aqueous solution by available lignite activated cokes (LACs) was reported for the first time. Diclofenac sodium could be quickly removed from aqueous solution by LAC-2, with the maximum Langmuir adsorption capacity q_m of 224 mg/g at pH 6.5. Characterization results (including scanning electron microscopy, transmission electron microscopy, elemental analyses, Boehm titrations, N₂ adsorption-desorption isotherms and Fourier transform infrared spectroscopy) and a series of adsorption kinetics, adsorption isotherms model studies revealed that high porosity with developed macro- and micropore structures on LAC-2, as well as high content of phenolic groups, could obviously enhance the DCF adsorption capacity and rate. Moreover, LAC-2 showed high affinity towards DCF at low concentrations, as well as good reusability after three adsorption-desorption cycles. pH effect studies revealed that hydrogen-bonding interaction plays an important role during adsorption, accompanied with certain contribution from electrostatic interaction and π-π interaction. This study indicates the promising potential of LAC-2 as an efficient, low-cost and recyclable material for DCF removal from water bodies.