Viewpoint Cite This: Environ. Sci. Technol. XXXX, XXX, XXX−XXX
pubs.acs.org/est
Liquid Crystal Monomers (LCMs): A New Generation of Persistent Bioaccumulative and Toxic (PBT) Compounds? Jianhua Li,† Guanyong Su,*,† Robert J. Letcher,‡ Wanqing Xu,† Mengyun Yang,† and Yayun Zhang† †
Jiangsu Key Laboratory of Chemical Pollution Control and Resources Reuse, School of Environmental and Biological Engineering, Nanjing University of Science and Technology, Nanjing 210094, P. R. China ‡ Ecotoxicology and Wildlife Health Division, Science and Technology Branch Environment and Climate Change Canada, National Wildlife Research Centre, Carleton University, Ottawa, Ontario K1A 0H3, Canada the question as to how many different kinds of LCMs are currently being produced. The result is a product list containing more than 330 LCMs, which were further investigated for their properties in relation to persistence (P), bioaccumulation (B) and toxicity (T) in the environment. LCMs generally have a diphenyl backbone structure and where phenyl ring hydrogen atoms are replaced by various functional groups, that is, cyano, fluorine, chlorine, or bromine (Figure 1B). Using EPI Suite software developed by United States Environmental Protection Agency (USEPA),2,3 the physical-chemical parameter values of these LCMs were predicted and included octanol−water partition coefficients (log Kow) (KOWWIN), air−water partition coefficients (log Kaw), bioconcentration factors (BCF) (BCFWIN), octanol-air partition coefficients (log Koa) (KOAWIN), atmospheric oxidation half-lives (AO1/2) (AOPWIN), and degradation half-lives (t1/2) in water, soil, and sediment (fugacity). Following the PBT criteria proposed by USEPA and European Commission (EC),3 we estimated that more than 90% of 330 investigated LCMs have predicted log Kow ≥ 5 or BCF > 1000, indicating that after entering the environment most of these LCMs have the potential to persist and n the year 2017, the global production of smart phones has bioaccumulate. Approximate 27% of these LCMs were reached 4 billion units. State of the science smart phones and identified as being highly persistent chemicals (sediment halfother display devices such as tablets are designed with large life = 1621 days), and 55% having high potential for long-range colorful display screens. However, have you ever thought about atmospheric transport (LART) (log Kaw ≥ −5 and ≤ −1). It is the chemicals on which these display screens are comprised? worth noting that, of the 46 compounds with log Kaw ≥ −5 and What might the environmental impacts be if these chemicals ≤ −1 and predicted BCF > 5000, no compounds had predicted were to enter the natural environment? AOt1/2 > 2 days, reflecting the predicted instability of the The chemicals in the growing array of smart devises are chemicals toward atmospheric oxidation by OH-radical reactions. comprised of liquid crystal monomers (LCMs), which On the basis of combined criteria including log Kow ≥ 5, BCF transform between liquid and solid phases as a function of > 1000 and biodegradation t1/2 values ≥60 days in water,3 80 of temperature in these liquid crystal displays (LCDs) (Figure these LCM chemicals were selected as probable PB chemicals 1A). With the huge number of LCD devices produced, many (Figure 1C). Of this, 9 LCMs were identified as very persistent such devices end up being discarded and at best end up in eand very accumulative chemicals (vPvBs) (BCF > 5000 and t1/2 waste sites, which raises the spectre of eventually being released ≥ 180 days in water) (Figure 1C). All selected PB chemicals and entering into the natural environment. Current environwere fluorinated (see the example structures in Figure 1B), mental concerns with LCD devices are focused on several toxic reflecting the generally higher PB characteristics of the metals (e.g., lead (Pb), mercury (Hg), hexavalent chromium halogenated compounds.2,3 (Cr6+), cadmium (Cd)) and certain hazardous additives (e.g., The major LCM manufacturing company has declared that polybrominated biphenyls (PBBs), polybrominated diphenyl all the LCMs they currently market have no acutely toxic or ethers (PBDEs)).1 However, there are essentially no published mutagenic properties to rats and aquatic living organisms under reports in the literature on these liquid crystal materials and conditions of short-term and high dose.4 However, we found their presence and impacts in the environment. More fundamentally, we do not know the specific properties of LCMs. An extensive survey was presently conducted and Received: April 3, 2018 covering more than 10 LCM producing industries to address
I
© XXXX American Chemical Society
A
DOI: 10.1021/acs.est.8b01636 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Viewpoint
Environmental Science & Technology
Figure 1. Structure of liquid crystal display (LCD) panels (A), chemical structure of four typical liquid crystal monomers (LCMs; B), and persistence, bioaccumulation and toxicity (PBT) properties of 330 investigated LCMs (C). In Figure 1C, black points represent LCMs that are identified as no PBT compounds (i.e., bioconcentration factor (BCF) < 1000 or degradation half-lives (t1/2) < 60 days in water); red points represent LCMs with moderately PBT properties (i.e., BCF > 1000 and 5000 and t1/2 ≥ 180 days in water).
■
that all of the investigated LCMs were predicted to meet criteria of chronic (long-term) toxicity to fish (fish chronic toxicity values (ChV) < 10 mg/L) estimated by the ECOSAR program (the Ecological Structure Activity Relationship Class Program v1.11) (Figure 1C). Since the amounts of LCMs in electronic devices are quite small with approximately 0.6 mg/ cm2 in a typical LCD, data is therefore important on the effects of LCMs in organisms exposed to low concentrations or doses. In fact, there are some recent reports on LCMs that are under suspicion of being an environmental health hazard. Two LCMs, 4-n-penty-4′-cyanobiphenyls (5CB) and the eutectic mixture of biphenyls and terphenyls (E7), were shown to be toxic to various bacteria.5 An et al. demonstrated the LCMs can cause pollutant-induced stress in exposed freshwater catfish at low doses.4 Collectively, our viewpoint is that some LCMs, especially fluorinated ones, are predicted to satisfy the screening criteria to be categorized as PBT chemicals, although to our knowledge no environmental studies are known. The ever expanding production and use of devises with liquid crystal screens, as well as the predicted PBT properties of LCMs, should render a serious pause for concern and for there to be quick action taken to understand the environmental behavior, fate and adverse effects of LCMs.
■
REFERENCES
(1) European Commission. Directive 2002/95/EC of the European Parliament and of the Council of 27 January 2003 on the restriction of the use of certain hazardous substances in electrical and electronic equipment. Off. J. Eur. Union 2003, L37, 19−23. (2) Howard, P. H.; Muir, D. C. G. Identifying new persistent and bioaccumulative organics among chemicals in commerce. Environ. Sci. Technol. 2010, 44, 2277−2285. (3) Muir, D. C. G.; Howard, P. H. Are there other persistent organic pollutants? A challenge for environmental chemists. Environ. Sci. Technol. 2006, 40, 7157−7166. (4) An, R.; Li, Y. D.; Niu, X. J.; Yu, H. T. Responses of antioxidant enzymes in catfish exposed to liquid crystals from E-waste. Int. J. Environ. Res. Public Health 2008, 5, 99−103. (5) Woolverton, C. J.; Gustely, E.; Li, L. F.; Lavrentovich, O. D. Liquid crystal effects on bacterial viability. Liq. Cryst. 2005, 32, 417− 423.
AUTHOR INFORMATION
Corresponding Author
*Phone: 86-13951763661; e-mail:
[email protected]. ORCID
Guanyong Su: 0000-0003-4356-5194 Robert J. Letcher: 0000-0002-8232-8565 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS Prof. G. Su was supported by Natural Science Foundation of Jiangsu Province (Grant No. BK20170830), and “the Fundamental Research Funds for the Central Universities” (Grant No. 30917011305). B
DOI: 10.1021/acs.est.8b01636 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
