New insight on occurrence of liquid crystal monomers: A class of emerging e-waste pollutants in municipal landfill leachate

Photochemical transformation of liquid crystal monomers in simulated environmental media: Kinetics, mechanism, toxicity variation and QSAR modeling

Liquid crystal monomers (LCMs) are a new class of emerging pollutants with high octanol-water partition coefficients; however, their transformation behavior and associated risk to environments with high organic matter content has rarely been reported. In this study, we investigated the photodegradation kinetics, mechanism, and toxicity variation of 23 LCMs on leaf wax models (e.g., organic solvents methanol and n-hexane). The order of the photolysis rates of these LCMs were biphenylethyne LCMs > phenylbenzoate LCMs > diphenyl/terphenyl LCMs under simulated sunlight, while the phenylcyclohexane LCMs were resistant to photodegradation. The phenylbenzoate and biphenylethyne LCMs mainly undergo direct photolysis, while the diphenyl/terphenyl LCMs mainly undergo self-sensitized photolysis. The main photolysis pathways are the cleavage of ester bonds for phenylbenzoate LCMs, the addition, oxidation and cleavage of alkynyl groups for biphenylethyne LCMs, and the cleavage/oxidation of chains attached to phenyls and the benzene ring opening for diphenyl/terphenyls LCMs. Most photolysis products remained toxic to aquatic organisms to some degree. Additionally, two quantitative structure-activity relationship models for predicting kobs of LCMs in methanol and n-hexane were developed, and employed to predict kobs of 93 LCMs to fill the kobs data gap in systems mimicking leaf surfaces. These results can be helpful for evaluating the fate and risk of LCMs in environments with high content of organic phase.

Fluorinated liquid-crystal monomers in infant formulas and implication for health risk

Fluorinated liquid-crystal monomers (FLCMs), a new class of potential persistent, bioaccumulative and toxic (PBT) emerging pollutants, are extensively utilized in the display panel of various electronic devices. These compounds have been found in various environmental matrixes and dietary. Our previous studies have documented their ubiquitous occurrence in high fat foodstuffs. Infants, a vulnerable group, are more susceptible to the impacts of these pollutants compared to adults. Herein, we provided an assessment of the health risks posed by FLCMs to infants, focusing on their exposure through infant formula. The presence of FLCMs was detected in all infant formulas, with median concentration of 16.5 ng/g dry weight (dw) and the 95th percentile concentration of 65.7 ng/g dw. The most prevalent pollutant in these formulas was 2-fluoro-4-[4'-propyl-1,1'-bi(cyclohexyl)-4-yl] phenyl trifluoromethyl ether (FPrBP), with median and a 95th percentile concentration of 12.2 ng/g dw and 23.8 ng/g dw, accounting for 55.2% to the total FLCMs. Infants aged 0-6 months had the highest estimated daily intakes (EDIs) of FLCMs, with the EDImedian of 267 ng/kg bw/day. FPrBP and 4-[trans-4-(trans-4-Propylcyclohexyl) cyclohexyl]-1-trifluoromethoxybenzene (PCTB) together made up 83.3% of the total EDIs in median exposure scenario of 0-6 months infant. The highest EDI value was 1.30 × 103 ng/kg bw/day, 77.1% of which was attributed to a combination of FPrBP, 4″-ethyl-2'-fluoro-4-propyl-1,1':4',1″-terphenyl (EFPT), 2-[4'-[difluoro(3,4,5-trifluoro-2-methyl-phenoxy)methyl]-3',5'-difluoro-[1,1'-biphenyl]-4-yl]-5-ethyl-tetrahydro-pyran (DTMPMDP), 4-[Difluoro-(3,4,5-trifluoro-2-methyl-phenoxy)-methyl]-3,5-difluoro-4'-propyl-1,1-biphenyl (DTMPMDB), 2,3-difluoro-1-methyl-4-[(trans, trans)-4'-pentyl[1,1'-bicyclohexyl]-4-yl]benzene (DMPBB) and PCTB. It's worth noting that FLCMs have higher exposure risk. Based on the threshold of toxicological concern (TTC) method, the EDImedian of FPrBP (183 ng/kg bw/day) and FPCB (3.27 ng/kg bw/day) were beyond their TTC values (2.5 ng/kg bw/day) in 0-6 months infant, implying their prospective health risk.

A new class of contaminants of concern? A comprehensive review of liquid crystal monomers

Liquid crystal monomers (LCMs) are a class of emerging contaminants of concern predicted to be persistent, bioaccumulative and toxic (PBT). Being one of the key components in liquid crystal displays (LCDs), the disposal of LCD containing devices is closely related to the emission of LCMs into the environment. LCMs have been detected in a wide range of environmental matrices including dust, sediment, soil, sewage leachate, and air, with concentration ranges between 17 and 2121 ng/g found in indoor residential dust. Furthermore, they have been detected on human skin at concentrations up to 2,071,000 ng/m2 and in the serum of e-waste dismantling workers, at concentrations ranging from 3.9 to 276 ng/mL. Despite the far-reaching contamination of these compounds, there is limited knowledge of their environmental behaviour, fate, and toxicity. Model predictions show that 297 of 330 LCMs are persistent and bioaccumulative compounds, with many more indicated as being toxic. However, current knowledge of their physicochemical and PBT properties is largely restricted to theoretical predictions and limited to a small number of experimental toxicity studies. As an emerging class of contaminants of concern, a lack of standardisation between studies was identified as a key challenge to advancing the state of knowledge of these compounds. Not only are harmonised analytical methods for their determination and quantification in environmental media yet to be established, but there is also a need for a universal abbreviation system. To further harmonise the reporting of data on LCMs we propose reporting the sum concentration of ten priority LCMs, selected on the basis detection frequency, toxicity and potential for human exposure. Of the ten priority LCMs five are fluorinated biphenyls and analogues, four are biphenyls/bicyclohexyls and analogues and one is a cyanobiphenyl.

Informal E-waste dismantling activities accelerated the releasing of liquid crystal monomers (LCMs) in Pakistan: Occurrence, distribution, and exposure assessment

Liquid crystal monomers (LCMs) are emerging contaminants characterized by their persistence, bioaccumulation potential, and toxicity. They have been observed in several environmental matrices associated with electronic waste (e-waste) dismantling activities, particularly in China. However, there is currently no information on the pollution caused by LCMs in other developing countries, such as Pakistan. In this study, we collected soil samples (n = 59) from e-waste dismantling areas with different functions in Pakistan for quantification analysis of 52 target LCMs. Thirty out of 52 LCMs were detected in the soil samples, with the concentrations ranging from 2.14 to 191 ng/g (median: 16.3 ng/g), suggesting widespread contamination by these emerging contaminants. Fluorinated LCMs (median: 10.4 ng/g, range: 1.27-116 ng/g) were frequently detected and their levels were significantly (P < 0.05) higher than those of non-fluorinated LCMs (median: 6.11 ng/g, range: not detected (ND)-76.7 ng/g). The concentrations and profiles of the observed LCMs in the soil samples from the four functional areas varied. The informal dismantling of e-waste poses a potential exposure risk to adults and infants, with median estimated daily intake (EDI, ng/kg bw/day) values of 0.0420 and 0.1013, respectively. Calculation of the hazard quotient (HQ) suggested that some LCMs (e.g., ETFMBC (1.374) and EDFPB (1.257)) may pose potential health risks to occupational workers and their families. Considering the widespread contamination and risks associated with LCMs, we strongly recommend enhancing e-waste management and regulation in Pakistan.

Analysis of 78 trace liquid crystal monomers in air by gas chromatography coupled with triple quadrupole mass spectrometry

Liquid crystal monomers (LCMs) comprise a class of organic pollutants that have garnered considerable attention because of their dioxin-like toxicity (i.e., modulation of genes) and presence in various environments. However, limited information about the identities, occurrence, and distribution of LCMs has highlighted an urgent need for a high-throughput and sensitive analytical method. In this study, we developed and validated a rapid, simple, sensitive method that involves minimal solvent consumption. The method was applied for the simultaneous detection and identification of 78 LCMs in atmospheric total suspended particulate samples (dae < 100 μm) using gas chromatography coupled with triple quadrupole mass spectrometry. The results showed high degrees of linearity with correlation coefficients >0.995 in the concentration range of 5.0-500 ng/mL. The instrumental detection limits ranged from 0.7 to 5.3 pg, and the method detection limits ranged from 0.1 to 0.9 pg/m3. The accuracy of the method was between 70 % and 130 % for most analytes, and the relative standard deviations of six replicates were <15 % at three levels of spiking (10, 50, and 200 ng/mL). The developed analytical method was applied to analyze real air particulate samples from Beijing, China. Overall, 45 LCMs ranged from 65.5 to 145.7 pg/m3, with a mean concentration of 92.5 pg/m3. Among them, (trans,trans)-4-propyl-4'-ethenyl-1,1'-bicyclohexane (PVB) was the most abundant, with an average concentration of 33.6 pg/m3. The total estimated daily intakes of LCMs for adults and children were 15.6 and 46.6 pg/kg bw/day, respectively. Accordingly, the method described herein is suitable for quantifying LCMs in atmospheric particulate samples. This study will be valuable for investigating LCM environmental occurrence, behaviors, and risk assessments.

Photochemical behaviour and toxicity evolution of phenylbenzoate liquid crystal monomers in water

Liquid crystal monomers (LCMs) are a group of emerging pollutants that pose potential environmental risks because of their ubiquitous occurrence and toxicity. Understanding their environmental transformation is essential for assessing the ecological risk. In this study, we investigated the photochemical transformation kinetics, mechanism, and photo-induced toxicity of three phenylbenzoate LCMs in water. Their apparent photolytic rate constants were within (0.023 - 0.058) min-1, and the half-lives were < 30.0 min, showing lower persistence in water. Dissolved organic matter significantly inhibited their photolysis because of light-shielding effect and quenching of excited triplet states of LCMs. Their photolysis mainly occurred through excited triplet states, and the reactive oxygen species (i.e., ⋅OH, 1O2 and ⋅O2-) contributed to their degradation. The main photolysis pathways were ester bond cleavage, ⋅OH substitution/addition, and defluorination. Experiments and computational simulation revealed that some ·OH addition/substitution products have similar toxicity with LCMs. Additionally, the ∙OH reaction rate constants (kOH) of LCMs were determined to be > 1 × 109 M-1 s-1, evidence for their high reactivity toward ⋅OH. We have further developed reliable methods to estimate kOH of other phenylbenzoate-like LCMs with quantum chemical calculations. These results are useful for understanding the transformation and fate of LCMs in aquatic environments.

Characteristic structures of liquid crystal monomers in EI-MS analysis and the potential application in suspect screening

Liquid crystal monomers (LCMs) are of emerging concern due to their ubiquitous presence in indoor and outdoor environments and their potential negative impacts on human health and ecosystems. Suspect screening approaches have been developed to monitor thousands of LCMs that could enter the environment, but an updated suspect list of LCMs is difficult to maintain given the rapid development of material innovations. To facilitate suspect screening for LCMs, in-silico mass fragmentation model and quantitative structure-activity relationship (QSPR) models were applied to predict electron ionization (EI) mass spectra of LCMs. The in-silico model showed limited predictive power for EI mass spectra, while the QSPR models trained with 437 published mass spectra of LCMs achieved an acceptable absolute error of 12 percentage points in predicting the relative intensity of the molecular ion, but failed to predict the mass-to-charge ratio of the base peak. A total of 41 characteristic structures were identified from an updated suspect list of 1606 LCMs. Multi-phenyl groups form the rigid cores of 85% of LCMs and produce 154 characteristic peaks in EI mass spectra. Monitoring the characteristic structures and fragments of LCMs may help identify new LCMs with the same rigid cores as those in the suspect list.

Liquid crystal monomers (LCMs) in indoor residential dust from Beijing, China: occurrence and human exposure

Liquid crystal monomers (LCMs) are widely used in electronic devices and emerging as an environmental pollutant, while their occurrence in indoor environments is still less studied. In this study, 32 out of 37 target LCMs were detected in indoor residential dust samples (n = 112) from Beijing, China. Concentrations of Σ32LCMs ranged from 17.8 to 197 ng/g, with a median value of 54.7 ng/g. Fluorinated biphenyls and analogs (FBAs) and cyanobiphenyls and analogs (CBAs), with median concentrations of 22.8 and 15.9 ng/g, respectively, were the main kinds of LCMs. Although 32 LCMs can be detected, four monomers with the highest contamination levels contributed to almost 70% of the total LCMs. Spearman correlation analysis found significant correlations among some monomers, which indicated that they might share similar sources in the residential environment. Estimated daily intakes (EDIs) of LCMs via indoor dust for Beijing residents were calculated, and the results showed that dust ingestion and dermal contact were both main intake pathways to LCMs, and younger people may face higher exposure to LCMs. A comparison to the results of China's total diet study showed that EDIs of LCMs via food consumption might be higher than that via dust intake, while health risks caused by exposure of LCMs for the general population, both through food and dust, were insignificant at present.

Comprehensive Characterization of Organic Light-Emitting Materials in Breast Milk by Target and Suspect Screening

Organic light-emitting materials (OLEMs) are emerging contaminants in the environment and have been detected in various environment samples. However, limited information is available regarding their contamination within the human body. Here, we developed a novel QuEChERS (quick, easy, cheap, effective, rugged, and safe) method coupled with triple quadrupole/high-resolution mass spectrometry to determine OLEMs in breast milk samples, employing both target and suspect screening strategies. Our analysis uncovered the presence of seven out of the 39 targeted OLEMs in breast milk samples, comprising five liquid crystal monomers and two OLEMs commonly used in organic light-emitting diode displays. The cumulative concentrations of the seven OLEMs in each breast milk sample ranged from ND to 1.67 × 103 ng/g lipid weight, with a mean and median concentration of 78.76 and 0.71 ng/g lipid weight, respectively, which were higher compared to that of typical organic pollutants such as polychlorinated biphenyls and polybrominated diphenyl ethers. We calculated the estimated daily intake (EDI) rates of OLEMs for infants aged 0-12 months, and the mean EDI rates during lactation were estimated to range from 30.37 to 54.89 ng/kg bw/day. Employing a suspect screening approach, we additionally identified 66 potential OLEMs, and two of them, cholesteryl hydrogen phthalate and cholesteryl benzoate, were further confirmed using pure reference standards. These two substances belong to cholesteric liquid crystal materials and raise concerns about potential endocrine-disrupting effects, as indicated by in silico predictive models. Overall, our present study established a robust method for the identification of OLEMs in breast milk samples, shedding light on their presence in the human body. These findings indicate human exposure to OLEMs that should be further investigated, including their health risks.

Molecular understanding on ultraviolet photolytic degradation of cyano liquid crystal monomers

Cyano liquid crystal monomers (LCMs) are proposed as emerging chemical pollutants with persistent, bioaccumulative, and toxic properties. Herein, five cyano LCMs, including 4-cyano-4'-ethylbiphenyl (2CB), 4-Butyl-4'-cyanobiphenyl (4CB), 4-cyano-4'-ethoxybiphenyl (2OCB), 4-(trans-4-Ethylcyclohexyl)benzonitrile (2CHB) and 4-(trans-4-Vinylcyclohexyl)benzonitrile (2eCHB), were selected to investigate the reaction kinetics and excited state characteristic variations with their molecular structures by ultraviolet (UV) photolysis. Theoretical calculations reveal that the benzene ring, ethoxy and double bond can deeply alter the electron distribution of cyano LCMs. This will affect the exciton separation ability, excitation properties and active sites to electrophilic attack, causing the distinction in photolysis efficiency. Due to the effective charge separation during local excitation (LE) process and the property of being most susceptible to electrophilic attack by 1O2 and O2•-, 2eCHB with double bond exhibits the largest degradation rate. Conversely, the weakest exciton separation of 2OCB with ethoxy during charge transfer (CT) process limits its subsequent sensitized photolysis process. The molecular orbital and fragment contributions to holes and electrons further deepen the understanding of the excited states charge transfer. This study confirmed that the intrinsic molecular structure, chemical nature and existing sites directly defined the excitation and decomposition activity in the UV photolysis of cyano LCMs.

Toxicity Assessment of Environmental Liquid Crystal Monomers: A Bacteriological Investigation on Escherichia coli and Staphylococcus epidermidis

The widespread existence of liquid crystal monomers (LCMs) in various environmental matrices has been demonstrated, yet studies on the toxicological effects of LCMs are considerably scarce and are urgently needed to be conducted to assess the adverse impacts on ecology and human health. Here, we conducted a bacteriological study on two representative human commensal bacteria, Escherichia coli (E. coli) and Staphylococcus epidermidis (S. epidermidis), to investigate the effect of LCMs at human-relevant dosage and maximum environmental concentration on growth, metabolome, enzymatic activity, and mRNA expression. Microbial growth results exhibited that the highest inhibition ratio of LCMs on S. epidermidis reached 33.6% in our set concentration range, while the corresponding data on E. coli was only 14.3%. Additionally, LCMs showed more dose-dependent toxicity to S. epidermidis rather than E. coli. A novel in vivo solid-phase microextraction (SPME) fiber was applied to capture the in vivo metabolites of microorganisms. In vivo metabolomic analyses revealed that dysregulated fatty acid metabolism-related products of both bacteria accounted for >50% of the total number of differential substances, and the results also showed the species-specific and concentration-dependent metabolic dysregulation in LCM-exposed bacteria. The determination of enzymatic activity and mRNA relative expression levels related to oxidative stress confirmed our speculation that the adverse effects were related to the oxidative metabolism of fatty acids. This study complements the gaps in toxicity data for LCMs against bacteria and provides a new and important insight regarding metabolic dysregulation induced by environmental LCMs in human commensal bacteria.

Liquid crystal monomers in soils near the e-waste recycling site and liquid crystal display manufacturer: Exponential decrease with distance

Liquid crystal monomers (LCMs) have been recognized as contaminants of emerging concerns. E-waste recycling sites and liquid crystal displays (LCDs) manufacturers are supposed to be critical sources. However, information regarding LCM contaminations in soils surrounding these sites are currently unavailable. In this study, soil samples were collected from two distinct areas in South China: e-waste recycling area (n = 36) and LCD manufacturer (n = 41), and 60 target LCMs (including 13 biphenyl and analogs (BAs), 10 cyanobiphenyl and analogs (CBAs), and 37 fluorinated biphenyl and analogs (FBAs)) were determined. The concentrations of LCMs in the soils from near the e-waste recycling area (0.32-18 ng/g, average: 4.2 ng/g) were higher than those surrounding the LCD manufacturer (ND - 7.2 ng/g, average: 1.5 ng/g). The compositional profiles of LCMs in soil samples from these two typical point sources were considerably different. The concentrations of FBAs exponentially decreased with distance from the e-waste recycling park, by >90 % within 2 km. The levels of BAs exhibited a similar exponential decrease with distance from the LCD manufacturer. The inventories of LCMs were estimated to be 21.0 kg in the e-waste recycling area and 10.8 kg in the LCD manufacturer area. Remarkably, the inventory of LCMs in soils from e-waste recycling area was one order of magnitude larger than that of hexabromocyclododecanes (HBCDs) in the same region, and 0.2 to 20 times the annual global emissions of LCMs from discarded LCD panels. More studies are required to elucidate the environmental occurrence, behavior, and fate of LCMs in multimedia environment surrounding typical point sources.

Aerobic Microbial Transformation of Fluorinated Liquid Crystal Monomer: New Pathways and Mechanism

Fluorinated liquid crystal monomers (FLCMs) have been suggested as emerging contaminants, raising global concern due to their frequent occurrence, potential toxic effects, and endurance capacity in the environment. However, the environmental fate of the FLCMs remains unknown. To fill this knowledge gap, we investigated the aerobic microbial transformation mechanisms of an important FLCM, 4-[difluoro(3,4,5-trifluorophenoxy)methyl]-3, 5-difluoro-4'-propylbiphenyl (DTMDPB), using an enrichment culture termed as BG1. Our findings revealed that 67.5 ± 2.1% of the initially added DTMDPB was transformed in 10 days under optimal conditions. A total of 14 microbial transformation products obtained due to a series of reactions (e.g., reductive defluorination, ether bond cleavage, demethylation, oxidative hydroxylation and aromatic ring opening, sulfonation, glucuronidation, O-methylation, and thiolation) were identified. Consortium BG1 harbored essential genes that could transform DTMDPB, such as dehalogenation-related genes [e.g., glutathione S-transferase gene (GST), 2-haloacid dehalogenase gene (2-HAD), nrdB, nuoC, and nuoD]; hydroxylating-related genes hcaC, ubiH, and COQ7; aromatic ring opening-related genes ligB and catE; and methyltransferase genes ubiE and ubiG. Two DTMDPB-degrading strains were isolated, which are affiliated with the genus Sphingopyxis and Agromyces. This study provides a novel insight into the microbial transformation of FLCMs. The findings of this study have important implications for the development of bioremediation strategies aimed at addressing sites contaminated with FLCMs.

Aquatic toxicity, ecological effects, human exposure pathways and health risk assessment of liquid crystal monomers

Liquid crystal monomers (LCMs), one of the key materials for liquid crystal displays, have been considered as emerging pollutants in recent years. However, the environmental behaviors of LCMs have not yet been well investigated. The toxicity data of 1173 LCMs were calculated by integrated computational simulation methods in this study. It showed that 64.6% LCMs exhibited PBT (persistent, bioaccumulative, and toxic) properties. Based on the results, 1173 LCMs were identified as molecules possessing the highest level of acute toxicity to aquatic organisms. Among which, and a human health risk priority control list about LCMs was generated in this study, among which 435 were classified as requiring priority control LCMs. It was confirmed that LCMs could eventually accumulate in the human body along the aquatic food chain or penetrate the bloodstream through the dermis, thereby causing harm to health by identifying the exposure pathways of LCMs in humans. Additionally, the electronegativity of the side chain group of LCMs is the main factor causing toxicity differences; therefore, the LCMs containing halogens presented significant acute and chronic toxic effects. This study provided a more comprehensive understanding of LCMs for the public and scientific strategies for controlling LCMs.

A simulation study of the decomposition, biotoxicity, and transfer of LCMs in LCD panels after being in contact with sulfuric acid and extraction/stripping agents

For indium recycling from LCD panels, the decomposition of 9 commonly used liquid crystal monomers (LCMs) that were in contact with sulfuric acid (i.e., leaching agent) and extraction/stripping agents, has been investigated in the present study. Also their biological toxicity changes and transfer have been studied. The results showed that 7 of the 9 LCMs were decomposed in the sulfuric acid agent, while the reaction time and temperature had no effect on the types of the decomposition products. The maximum decomposition rate was 96% when the concentration of the sulfuric acid was increased to 12 M. The time required for a 100% decomposition of the various LCMs in a 5 M sulfuric acid ranged from 41 h to 150 h. Also, Estimation Programs Interface (EPI) and ECOSAR calculations were used to compare the biotoxicity of the LCMs and the decomposition products. The results from the EPI calculations showed that the biological half-lives of the decomposition products were significantly reduced as compared with the LCMs, from the original highest value of 329.2 days-92.71 days. Furthermore, the ECOSAR calculations showed that the biological toxicity of the decomposition products for aquatic organisms was lower than for the LCMs, but they were still toxic and harmful substances. In addition, the transfer rates of the undecomposed LCMs and decomposition products in different extractants remained above 90%, and reached 100% at most. After stripping with hydrochloric acid, more than 70% of the undecomposed LCMs became enriched in the aqueous solution, while the products were enriched in the extractant.

A review of liquid crystal monomers: Environmental occurrence, degradation, toxicity, and human exposure of an emerging class of E-waste pollutants

Liquid crystal monomers (LCMs) are a class of organic compounds with diphenyl or dicyclohexane as the skeleton structure, which are widely used in the manufacturing of liquid crystal displays. They are recognized as novel organic compounds with persistence, bioaccumulation, toxicity, and potential for long-range transport. LCMs are inevitably released into the environment throughout the life cycle of electronic products, and their presence has been found in various abiotic matrixes (air, dust, sediment, leachate, soil) and biotic matrixes (aquatic organisms, human serum, and human skin wipe). Given that studies on LCMs are still in their infancy, this review comprehensively summarizes the extensive literature data on LCMs and identifies key knowledge gaps and future research needs. The physicochemical properties, production, and usage of LCMs are described. Their environmental distribution, degradation, toxicity, and human exposure are also discussed based on the available data and results. Existing data show that LCMs have large-scale environmental pollution and may pose potential ecological and health risks, but it is still insufficient to accurately assess their risks due to the lack of knowledge on LCMs in many areas, such as global contamination trend, environmental behavior, toxic effects, and human exposure assessment. We believe that future studies of LCMs need to investigate LCMs pollution on a large geographic scale, explore their sources, behavior, and fate in the environment, and assess their potential health hazards to organisms and humans.

Liquid crystal monomers in ventilation and air conditioning dust: Indoor characteristics, sources analysis and toxicity assessment

Indoor dust contaminated with liquid crystal monomers (LCMs) released from various commercial liquid crystal display (LCD) screens may pose environmental health risks to humans. This study aimed to investigate the occurrence of 64 LCMs in ventilation and air conditioning filters (VACF) dust, characterize their composition profiles, potential sources, and associations with indoor characteristics, and assess their in vitro toxicity using the human lung bronchial epithelial cells (BEAS-2B). A total of 31 LCMs with concentrations (ΣLCMs) ranging from 43.7 ng/g to 448 ng/g were detected in the collected VACF dust. Additional analysis revealed the potential interactions between indoor environmental conditions and human exposure risks associated with the detected LCMs in VACF dust. The service area and working time of the ventilation and air conditioning system, and the number of indoor LCD screens were positively correlated with the fluorinated ΣLCMs in VACF dust (r = 0.355 ∼ 0.511, p < 0.05), while the associations with the non-fluorinated ΣLCMs were not found (p > 0.05), suggesting different environmental behavior and fates of fluorinated and non-fluorinated LCMs in the indoor environment. Four main indoor sources of LCMs (i.e., computer (37.1%), television (28.3%), Brand A smartphone (21.2%) and Brand S smartphone (13.4%)) were identified by positive matrix factorization-multiple linear regression (PMF-MLR). Exposure to 14 relatively frequently detected LCMs, individually and in the mixture, induced significant oxidative stress in BEAS-2B cells. Among them, non-fluorinated LCMs, specifically 3cH2B and MeP3bcH, caused dominant decreased cell viability. This study provides new insights into the indoor LCMs pollution and the associated potential health risks due to the daily use of electronic devices.

Tunable Fluorine-Functionalized Scholl-Coupled Microporous Polymer for the Selective Adsorption and Ultrasensitive Analysis of Environmental Liquid-Crystal Monomers

Liquid-crystal monomers (LCMs), especially fluorinated biphenyls and analogues (FBAs), are identified to be an emerging generation of persistent organic pollutants. However, there is a dearth of information about their occurrence and distribution in environmental water and lacustrine soil samples. Herein, a series of fluorine-functionalized Scholl-coupled microporous polymers (FSMP-X, X = 1-3) were designed and synthesized for the highly efficient and selective enrichment of FABs. Their hydrophobicity, porosity, chemical stability, and adsorption performance (capacity, rate, and selectivity) were regulated preciously. The best-performing material (FSMP-2) was employed as the on-line fluorous solid-phase extraction (on-line FSPE) adsorbent owing to its high adsorption capacity (313.68 mg g^-1), fast adsorption rate (1.05 g h^-1), and specific selectivity for FBAs. Notably, an enrichment factor of up to 590.2 was obtained for FSMP-2, outperforming commercial C₁₈ (12.6-fold). Also, the underlying adsorption mechanism was uncovered by density functional theory calculations and experiments. Based on this, a novel and automated on-line FSPE-high-performance liquid chromatography method was developed for ultrasensitive (detection limits: 0.0004-0.0150 ng mL^-1) and low matrix effect (73.79-113.3%) determination of LCMs in lake water and lacustrine soils. This study offers new insight into the highly selective quantification of LCMs and the first evidence for their occurrence and distribution in these environmental samples.

Release Behavior of Liquid Crystal Monomers from Waste Smartphone Screens: Occurrence, Distribution, and Mechanistic Modeling

Liquid crystal display (LCD) screens can release many organic pollutants into the indoor environment, including liquid crystal monomers (LCMs), which have been proposed as a novel class of emerging pollutants. Knowing the release pathways and mechanisms of LCMs from various components of LCD screens is important to accurately assess the LCM release and reveal their environmental transport behavior and fate in the ambient environment. A total of 47, 43, and 33 out of 64 target LCMs were detected in three disassembled parts of waste smartphone screens, including the LCM layer (LL), light guide plate (LGP), and screen protector (SP), respectively. Correlation analysis confirmed LL was the source of LCMs detected in LGP and SP. The emission factors of LCMs from waste screen, SP, and LGP parts were estimated as 2.38 × 10^-3, 1.36 × 10^-3, and 1.02 × 10^-3, respectively. A mechanism model was developed to describe the release behaviors of LCMs from waste screens, where three characteristics parameters of released LCMs, including average mass proportion (AP), predicted subcooled vapor pressures (P_L), and octanol-air partitioning coefficients (K_oa), involving coexistence of absorption and adsorption mechanisms, could control the diffusion-partitioning. The released LCMs in LGP could reach diffusion-partition equilibrium more quickly than those in SP, indicating that LCM release could be mainly governed through SP diffusions.

Fluorinated liquid-crystal monomers in paired breast milk and indoor dust: A pilot prospective study

Fluorinated liquid-crystal monomers (FLCMs), one class of emerging persistent, bioaccumulative and toxic (PBT) compounds, are widely used in liquid-crystal displays (LCDs). As a result, they have been found in the environment and serum from occupational workers. However, little is known about their occurrence in non-occupational exposing populations. Herein, we provided an evaluation of the health risks of FLCMs for infants based on breastfeeding exposure and dust ingestion. The detection frequencies (DF) of FLCMs in indoor dust and breast milk was 100 %, with median concentrations of 12.00 ng/g dry weight (dw) and 133.40 ng/g lipid weight (lw), respectively. 1-butoxy-2,3-difluoro-4-(trans-4-propylcyclohexyl)benzene (BDPrB) was the predominant pollutant in indoor dust and human breast milk. Significant positive correlations were observed between the dust concentrations of seven FLCMs including BDPrB, and their breast milk concentrations (r = 0.275-0.660, P < 0.05). Further, associations were also found in some demographic and behavioral factors and concentrations of some FLCMs (P < 0.05). The highest EDI of ∑FLCMs was observed for infants who were < 1 month of age, with a median breast milk intake of 700.35 ng/kg bw/day, in which 1-ethoxy-2,3-difluoro-4-(trans-4-propylcyclohexyl)benzene (EDPrB), BDPrB, and 4'-[(trans, trans)-4'-butyl[1,1'-bicyclohexyl]-4-yl]-3,4-difluoro-1,1'-biphenyl (BBDB) collectively contributed 94.4 % of the total EDIs. Notably, the lactational intake of FLCMs was higher than that of some environmental pollutants (EPs). Overall, our results suggest higher exposure risks for infants and breastfeeding is the predominant exposure route for daily intake of FLCMs for infants.

Dietary Exposure and Health Risk of the Emerging Contaminant Fluorinated Liquid-Crystal Monomers

Fluorinated liquid-crystal monomers (FLCMs) widely used in liquid crystal displays are considered to be a new generation of persistent, bioaccumulative, and toxic contaminants. They have been widely detected in the environment. However, little is known about their occurrence in food and human dietary exposure until now. Herein, we provided an evaluation of dietary exposure and health risks of FLCMs in the Chinese adult population based on the fifth and sixth total diet studies (TDSs). The detection frequencies of FLCMs in the two surveys were 90.5 and 99.5%, with concentrations ranging from not detected ∼72.6 μg/kg wet weight (ww) and ND ∼74.7 μg/kg ww, respectively. All TDS samples contained the multiresidue of FLCMs. The mean estimated daily intakes (EDIs) of FLCMs were 172.86 and 163.10 ng/kg bw/day in the fifth and sixth TDS, respectively. Meats, vegetables, and cereals contributed the most to the EDI of FLCMs. According to the threshold of toxicological concern (TTC) method, the EDIs of 1-fluoro-4-[2-(4-propylphenyl)ethynyl]benzene (4.56 and 3.26 ng/kg bw/day) and 2-fluoro-4-[4'-propyl-1,1'-bi(cyclohexyl)-4-yl]phenyl trifluoromethyl ether (3.12 and 3.28 ng/kg bw/day) were above their TTC value (2.5 ng/kg bw/day), suggesting their potential health risk. This is the first comprehensive national dietary exposure assessment of FLCMs.

Tissue-specific uptake and distribution of liquid crystal monomers (LCMs) in mice

Increasing evidence indicated that liquid crystal monomers (LCMs) in liquid crystal displays can be released into the environment, and ubiquitously detected in environmental matrices and even human bodies. Yet databases regarding its uptake and distribution in mammals are lacking. In this study, four LCMs (namely 3dFB, 2OdF3B, 2teFT, and 6OCB) with various physiochemical properties and structures were selected as the target compounds. The LCMs were in vivo and in vitro exposed to mice and rat liver microsomes (RLM). LCMs were found in all mouse tissues, including brain. Pharmacokinetics parameters, C_max-tissue/C_max-blood, ranged from 27.5 to 214, indicating the preferential deposition of LCMs to tissues rather than blood. The LCMs distributed preferentially to lipophilic tissues, and relative mass contribution of LCMs from liver and adipose was 43-98 %. The physicochemical properties (i.e., Kow, molecular weight, and functional groups) had pronounced effect on distribution and accumulation of LCMs. The 2teFT with the highest Kow and molecular weight showed the relatively higher accumulation potential and half elimination time in all the tissues. The 6OCB containing cyano-group was more accumulative than the fluorinated 3dFB with the comparable Kow. In RLM assays, 2teFT and 6OCB were resistant to metabolic degradation. While 3dFB and 2OdF3B underwent rapid degradation with 93.7 % and 72.4 % being metabolized at 360 min. Findings in this study bear significant implications for the biomonitoring and overall risk evaluation of LCMs.

Percutaneous Penetration of Liquid Crystal Monomers (LCMs) by In Vitro Three-Dimensional Human Skin Equivalents: Possible Mechanisms and Implications for Human Dermal Exposure Risks

Liquid crystal monomers (LCMs) are indispensable materials in liquid crystal displays, which have been recognized as emerging persistent, bioaccumulative, and toxic organic pollutants. Occupational and nonoccupational exposure risk assessment suggested that dermal exposure is the primary exposure route for LCMs. However, the bioavailability and possible mechanisms of dermal exposure to LCMs via skin absorption and penetration remain unclear. Herein, we used EpiKutis 3D-Human Skin Equivalents (3D-HSE) to quantitatively assess the percutaneous penetration of nine LCMs, which were detected in e-waste dismantling workers' hand wipes with high detection frequencies. LCMs with higher log K_ow and greater molecular weight (MW) were more difficult to penetrate through the skin. Molecular docking results showed that ABCG2 (an efflux transporter) may be responsible for percutaneous penetration of LCMs. These results suggest that passive diffusion and active efflux transport may be involved in the penetration of LCMs across the skin barrier. Furthermore, the occupational dermal exposure risks evaluated based on the dermal absorption factor suggested the underestimation of the continuous LCMs' health risks via dermal previously.

Liquid Crystal Monomer: A Potential PPARγ Antagonist

Liquid crystal monomers (LCMs) are a large family of artificial ingredients that have been widely used in global liquid crystal display (LCD) industries. As a major constituent in LCDs as well as the end products of e-waste dismantling, LCMs are of growing research interest with regard to their environmental occurrences and biochemical consequences. Many studies have analyzed LCMs in multiple environmental matrices, yet limited research has investigated the toxic effects upon exposure to them. In this study, we combined in silico simulation and in vitro assay validation along with omics integration analysis to achieve a comprehensive toxicity elucidation as well as a systematic mechanism interpretation of LCMs for the first time. Briefly, the high-throughput virtual screen and reporter gene assay revealed that peroxisome proliferator-activated receptor gamma (PPARγ) was significantly antagonized by certain LCMs. Besides, LCMs induced global metabolome and transcriptome dysregulation in HK2 cells. Notably, fatty acid β-oxidation was conspicuously dysregulated, which might be mediated through multiple pathways (IL-17, TNF, and NF-kB), whereas the activation of AMPK and ligand-dependent PPARγ antagonism may play particularly important parts. This study illustrated LCMs as a potential PPARγ antagonist and explored their toxicological mode of action on the trans-omics level, which provided an insightful overview in future chemical risk assessment.

Removing emerging e-waste pollutant DTFPB by synchronized oxidation-adsorption Fenton technology

Liquid crystal monomers (LCMs), an emerging group of organic pollutants related to electronic waste, have been frequently detected from various environmental matrices, including landfill leachate. The persistence of LCMs requires robust technology for remediation. The objectives of this study were to evaluate the feasibility, performance and mechanism of the remediation of a typical LCM 4-[difluoro(3,4,5-trifluorophenoxy)methyl]- 3,5-difluoro-4'-propylbiphenyl (DTFPB) via synchronized oxidation-adsorption (SOA) Fenton technology and verify its application in DTFPB-contaminated leachate. The SOA Fenton system could effectively degrade 93.5% of DTFPB and 5.6% of its total organic carbon (TOC_DTFPB) by hydroxyl radical oxidation (molar ratio of Fe²⁺ to H₂O₂ of 1/4 and pH 2.5-3.0) following a pseudo-first-order model under 0.378 h^-1. Additionally, synchronized adsorption of DTFPB and its degradation intermediates by in situ resultant ferric particles via hydrophobic interaction, complexation, and coprecipitation contributed to almost 100% of DTFPB and 33.4% of TOC_DTFPB removal. Three possible degradation pathways involving eight products were proposed, and hydrophobic interactions might drive the adsorption process. It was first confirmed that the SOA Fenton system exhibited good performance in eliminating DTFPB and byproducts from landfill leachate. This study provides new insights into the potential of the Fenton process for the treatment of emerging LCMs contamination in wastewater.

Tissue-specific accumulation, bioaccumulation, and depuration of liquid crystal monomers (LCMs) in adult zebrafish (Danio rerio)

This study investigated the properties of bioaccumulation, tissue-specific accumulation, and depuration of liquid crystal monomers (LCMs) in adult zebrafish (Danio rerio) exposed to a mixture of 39 LCMs for 19 days followed by depuration for 12 days. Tissue-specific accumulation of LCMs was examined at the time point of day 19, and we observed that the distribution of LCMs varied among different tissues with the following order of Σ₃₉LCM concentrations, the intestine > brain > gill > liver > muscle. We also observed that the bioaccumulation potential of LCMs varied among LCM groups with different functional groups, and LCMs with the cyan group were prone to accumulate in zebrafish. Among the 39 target LCMs, nine exhibited bioconcentration factors (BCFs) >1000, indicating their great bioaccumulation potential in aquatic environments. The experimental BCFs of 22 LCMs in the present study were lower than the theoretical values predicted by the Estimation Programs Interface (EPI) Suite software developed by U.S. Environmental Protection Agency (USEPA), suggesting that their bioaccumulation potential might be overestimated by theoretical estimation techniques. Another interesting finding was the significant positive correlation relationship in both sexes of zebrafish (p < 0.01, r² = 0.66 for male; p < 0.01, r² = 0.41 for female) between logBCF_ww and logK_ow values of LCMs. Overall, this study provides fundamental information regarding the bioaccumulation potentials of LCMs, which could be helpful for further investigating the health risks of LCMs in aquatic environments.

Prediction of the Endocrine disruption profile of fluorinated biphenyls and analogues: An in silico study

Fluorinated biphenyls and their analogues (FBAs) are considered new persistent organic pollutants, but their endocrine-disrupting effects are still unknown. To fill this gap, the binding probability of 44 FBAs to different nuclear hormone receptors (NHRs) was predicted using Endocrine Disruptome. And molecular similarity and network toxicology analysis were used to strengthen the docking screening. The docking results showed that FBAs could have high binding potential for various NHRs, such as estrogen receptors β antagonism (ERβ an), liver X receptors α (LXRα), estrogen receptors α (ERα), and liver X receptors β (LXRβ). The similarity analysis found that the degree of overlap of the NHR repertoire was related to the Tanimoto coefficient of FBAs. Network toxicology verified a part of docking screening results and identified endocrine-disrupting pathways worthy of attention. This study found out potential endocrine-disrupting FBAs and their vulnerable, and developed a workflow that would leverage in silico approaches including molecular docking, similarity, and network toxicology for risk prioritization of potential endocrine-disrupting compounds.

Recent advances in carbon-based nanomaterials for the treatment of toxic inorganic pollutants in wastewater

Wastewater contains inorganic pollutants, generated by industrial and domestic sources, such as heavy metals, antibiotics, and chemical pesticides, and these pollutants cause many environmental problems.

Target and suspect analysis of liquid crystal monomers in soil from different urban functional zones

Recent studies have reported the occurrence of liquid crystal monomers (LCMs) in sediment, indoor dust, hand wipes, and human serum samples; however, information regarding their contamination status in soil is currently unavailable. The concentrations of 39 target LCMs were determined in n = 96 surface soil samples collected from five different urban functional zones including agricultural, scenic, industrial, commercial, and residential zones. We observed that 76 of 96 surface soil samples contained at least 19, 13, 16, 19, and 14 of the 39 target LCMs that were detectable in samples from agricultural, scenic, industrial, commercial, and residential zones, respectively. The LCMs in the samples from the agricultural zone exhibited the highest mean concentrations of 12.9 ng/g dry weight (dw), followed by those from commercial (5.23 ng/g dw), residential (3.30 ng/g dw), industrial (2.48 ng/g dw), and scenic zones (0.774 ng/g dw). Furthermore, strong and statistically significant (p < 0.05) correlations were observed for several pairs of LCMs (3cH2B vs. 5bcHdFB in the agricultural zone; 5bcHdFB vs. 2bcHdFB, 5bcHdFB vs. 3cH2B in the commercial zone; 5bcHdFB vs. 2bcHdFB in the industrial zone), indicating that they might have similar commercial applications and sources. Based on a newly established database containing 1173 LCMs, suspect screening was applied to discover other LCMs in these 96 soil samples using gas chromatograph coupled with quadrupole-time-of-flight mass spectrometry (GC-QTOF/MS). We tentatively identified 51 LCM formulas with 69 chemical structures. Collectively, this study provides the first evidence for the occurrence of LCMs in soil samples, and suggests that LCMs could be widely distributed across all five urban functional zones.

Direct evidence on occurrence of emerging liquid crystal monomers in human serum from E-waste dismantling workers: Implication for intake assessment

Liquid crystal monomers (LCMs) are widely used chemicals and ubiquitous emerging organic pollutants in the environment, some of which have persistent, bio-accumulative, and toxic potentials. Elevated levels of LCMs have been found in the e-waste dismantling associated areas. However, information on their internal exposure bio-monitoring is scarce. For the first time, occurrences of LCMs were observed in the serum samples of occupational workers (n = 85) from an e-waste dismantling area in South China. Twenty-nine LCMs were detected in serum samples of the workers, with a median value of 35.2 ng/mL (range: 7.78-276 ng/mL). Eight noticed LCMs were found to have relatively high detection frequencies ranging from 52.9% to 96.5%. The correlation analysis of individual LCMs indicated potential common applications and similar sources to the LCMs in occupational workers. Fluorinated LCMs were identified as the predominant monomers in the workers. Additionally, the estimated daily intake of the LCMs in the occupational workers was significantly higher than those in residents from the reference areas (p < 0.05, Mann-Whitney U Test, median values: 1.46 ng/kg bw/day versus 0.40 ng/kg bw/day), indicating a substantially higher exposure level to e-waste dismantling workers.

Widespread occurrence of emerging E-waste contaminants - Liquid crystal monomers in sediments of the Pearl River Estuary, China

Liquid crystal monomers (LCMs), commonly used in screens of electronic devices, have recently been identified as a group of emerging chemicals of concern associated with e-waste. They are potentially persistent, bioaccumulative, and toxic substances, and may pose a threat to the marine ecosystem. The Pearl River Estuary (PRE) receives organic contaminants discharged from the Pearl River Delta region, where primitive handling of e-waste is widespread. However, information on the pollution status of LCMs in the PRE is absent. Herein, a rapid and robust analytical method was established using ultrasonic extraction, solid phase extraction cleanup, and GC-Orbitrap-MS analysis. The spatial distribution of 39 target LCMs was investigated in 45 surface sediment samples from the PRE. Ten LCMs were detected, with ΣLCMs ranged from 0.9 to 31.1 ng/g dry weight. Our results demonstrated a widespread occurrence of LCMs in the sediments of the PRE, and a gradient of their contamination from inshore to offshore regions, indicating land-based origins. Our reported ΣLCMs concentrations were relatively higher compared to many other legacy and emerging pollutants found in the same investigated area. Preliminary risk assessment showed 3VbcH, Pe3bcH and tFMeO-3bcHP might be the top 3 risk contributors in the PRE. Further investigation on the ecological impact of LCMs on marine benthic ecosystems, as well as identification of their sources and control measures are warranted.

Emissions of Liquid Crystal Monomers from Obsolete Smartphone Screens in Indoor Settings: Characteristics and Human Exposure Risk

Liquid crystal monomers (LCMs) have been found to accumulate in indoor environments, but the emission kinetics of LCMs from electronic devices are not well understood. Leakage from damaged liquid crystal displays may be an important mechanism for LCMs to enter the environment and become potential health hazards to humans. To address this issue, we conducted chamber experiments to characterize the emissions of LCMs from obsolete smartphone screens and estimated the doses of residential and occupational exposures to LCMs. The emission rates of the detected LCMs were in the ranges of 0.1-7 μg m^-2 h^-1 at 80 °C, 0.05-7 μg m^-2 h^-1 at 60 °C, and 0.002-0.2 μg m^-2 h^-1 at 25 °C. Liquid crystal monomers with large molecular weights and low volatilities tended to accumulate at screen surfaces and were re-emitted at elevated temperatures, leading to high emission rates of heavy LCMs upon thermal treatment. The estimated doses of residential and occupational exposures to individual LCMs were 0.0001-0.009 and 0.007-2 ng kg^-1 d^-1, respectively. As LCMs are potentially carcinogenic based on in silico assessments, LCMs emitted from obsolete smartphones in indoor settings may become human health hazards.

Suspect Screening of Liquid Crystal Monomers (LCMs) in Sediment Using an Established Database Covering 1173 LCMs

Recent studies have suggested that liquid crystal monomers (LCMs) are emerging contaminants in the environment, and knowledge of this class of substances is very rare. Here, we reviewed existing LCM-related documents, i.e., publications and patents, and established a database involving 1173 LCMs. These 1173 LCMs were further calculated for their physicochemical properties, i.e., persistence (P), bioaccumulation (B), long-range transport potential (LRTP), and Arctic contamination and bioaccumulation potential (ACBAP). We found that 476 out of them were P&B chemicals (99% of them were halogenated), and 320 of them could have ACBAP properties (67% of them were halogenated). This LCM database was further applied for suspect screening of LCMs in n = 33 sediment samples by use of gas chromatography coupled to quadrupole time-of-flight mass spectrometry (GC-QTOF/MS). We tentatively identified 26 LCM formulas, which could have 43 chemical structures. Two out of these 43 suspect LCM candidates, 1-butoxy-2,3-difluoro-4-(4-propylcyclohexyl) benzene (3cH4OdFP) and 1-ethoxy-2,3-difluoro-4-(4-pentyl cyclohexyl) benzene (5cH2OdFP), were fully confirmed by a comparison of unique GC and MS characteristics with their authentic standards. Overall, our present study expanded the previous LCM database from 362 to 1173, and 1173 LCMs in this database were calculated for their physicochemical properties. Meanwhile, taking n = 33 sediment samples as an exercise, we successfully developed a suspect screening strategy tailored for LCMs, and this strategy could have promising potential to be extended to other environmental matrices.

Release and Gas-Particle Partitioning Behavior of Liquid Crystal Monomers during the Dismantling of Waste Liquid Crystal Display Panels in E-Waste Recycling Facilities

Liquid crystal monomers (LCMs) are a class of emerging chemical pollutants; however, their release and gas-particle partitioning remain unknown. This study performed the first comprehensive analysis of a wide range of 93 LCMs in the ambient air of liquid crystal display (LCD) dismantling facilities. A total of 53 of the 93 target LCMs were detected in the air samples. The total atmospheric concentrations (gas and particles) of LCMs (∑LCMs) ranged from 68,800 to 385,000 (median of 204,000) pg/m³. Most LCMs were predominant in the gas phase, implying that their atmospheric transport would be mainly governed by gas rather than particle diffusions. Differential distribution patterns of the LCMs were observed due to their different atmospheric partitioning behaviors. Significant linear correlations were found between the gas-particle partitioning coefficients (K_P) and the predicted subcooled vapor pressures (P_L) and octanol-air partitioning coefficients (K_oa) (p < 0.01). Compared with two equilibrium-state models, the experimentally observed particulate fractions (ϕ) fit better with the predicted values based on the Li-Ma-Yang (L-M-Y) steady-state model, and K_oa was identified as a key factor determining the atmospheric fate pathways of LCMs. Our study highlights another new class of chemicals significantly contributing to the chemical mixture in the ambient air at e-waste recycling areas.