Organophosphate flame retardants in total suspended particulates from an urban area of zhengzhou, China: Temporal variations, potential affecting factors, and health risk assessment

Highlights

• Particle concentration in air might be one of the key factors on the temporal variations of OPEs in TSP.

• TSP concentration has significant influence on the concentrations of OPEs with log K_OA between 7.7 and 10.

• No significant correlations were found between TSP concentration and OPEs with log K_OA higher than 12.

• There was still low risk to local residents from the exposure to OPEs in TSP at the current concentration levels.

Abstract

Organophosphate esters (OPEs) are widely used as flame retardants and plasticizers in industry and daily life, but the partition of OPEs to particles is still unclear because of the wide range of their physicochemical properties. In this study, six target OPEs with different vapor pressures (log P_L) were measured from 30 total suspended particulate (TSP) samples collected from an urban area of Zhengzhou from June to November in 2018. The total concentration of OPEs ranged from 0.30 to 3.46 ng/m³, with average concentration of 1.04 ng/m³. Tris (chloroethyl) phosphate (TCEP), tris(2-chloroisopropyl) phosphate (TCPP), and tributyl phosphate (TnBP) were most abundant in TSP, accounting for approximately 86.0% to the total OPEs. The temporal variations showed a specific trend that OPE concentrations in TSP were much higher in autumn than those of summer. Significant positive correlations were observed between TSP concentration in air and the total concentration of OPEs in TSP, with r up to 0.596. Particle concentrations caused major changes on OPE concentrations in TSP with octanol-air partition coefficient (log K_OA) between 7.7 and 10 but had no significant influence on the OPEs with log K_OA higher than 12. Temperature had significant influence on the total and individual OPEs with high vapor pressures (log P_L > −4.0), indicating that log K_OA and log P_L had significant influence on the OPE concentrations in TSP and may be one of the key factors on their temporal variations. Temperature had significant influence on OPE concentrations in TSP due to the strong temperature dependency of log K_OA and log P_L. No significant relationships were found between the wind speed and OPE concentrations in TSP, suggesting that OPEs detected in TSP might be emitted from the local sources. The hazards quotient (HQ) values were 6–8 orders of magnitude lower than 1, indicating that there was a low risk to local residents from the exposure to OPEs in TSP. This study preliminarily illuminates the potential affecting factors on the temporal variations of OPEs in TSP. It would be helpful for investigating the gas-particle partitioning behaviors and human health risks of OPEs in air.

Introduction

Organophosphate esters (OPEs) are widely used as flame retardants and plasticizers nowadays due to the ban of brominated diphenylethers (Stapleton et al., 2011; van der Veen and de Boer, 2012). Chlorinated OPEs, including tris(1,3-dichloro-2-propyl) phosphate (TDCP), tris(1-chloro-2-propyl) phosphate (TCPP), and tris(2-chloroethyl) phosphate (TCEP), are mostly employed as flame retardants (Stevens et al., 2006). Non-chlorinated OPEs, such as triphenyl phosphate (TPhP), tributyl phosphate (TnBP), and tricresyl phosphate (TCrP), are mainly used as plasticizers, anti-foaming, and additives to lacquers, hydraulic fluids, floor polishing, and so on (Kannan and Kishore, 1999). In recent years, OPEs have raised great concern because many of them have been reported as suspect carcinogens, endocrine disrupters, and reproductive toxicant (van der Veen and de Boer, 2012; Wei et al., 2015; Bekele et al., 2018).

Generally, OPEs are physically mixed into the polymer matrix and therefore they are easily released into the environment through abrasion, dissolution, and volatilization (Lai et al., 2015). Hitherto, OPEs have been widely found in the surface water, sediment, urban soil, road dust, indoor air, outdoor environment and biological samples (Reemtsma et al., 2008; van der Veen and de Boer, 2012; Bekele et al., 2018; Li et al., 2018; Wang et al., 2018; Yang et al., 2018; Zeng et al., 2018; Cao et al., 2019; Guo et al., 2019; Li et al., 2019). As a class of semi-volatile compounds, concentrations found for OPEs in air were up to 47 μg/m³ (Green et al., 2008). The occurrence, compositional profiles, and potential sources were investigated for evaluating risks from the exposure to OPEs in air through inhalation, dermal absorption and ingestion (Li et al., 2018; Cao et al., 2019; Yadav et al., 2019). However, there were only a handful of studies on the atmospheric distribution of OPEs in China, even though China has become one of the most important producers in the world with production of OPEs exceeding 70,000 tons as early as 2007 (Ou, 2011). For instance, OPEs were detected at a concentration of 6600–19,400 pg/m³ in the total suspended particulates of an urban city in East China (Ren et al., 2016). The median concentration of OPEs measured in the atmosphere of Bohai and Yellow Seas was 280 pg/m³ during 2015–2016 (Li et al., 2018). High levels of OPEs were detected at a concentration of 531–2180 pg/m³ in the atmosphere of the Beijing-Tianjin-Hebei region (Zhang et al., 2019). Chlorinated OPEs, such as TCPP, TCEP, and TDCP, were found to be the predominant OPEs in the particle matters. Emissions from households, road traffic, and industries were considered to be the potential sources of OPEs in the atmosphere (van der Veen and de Boer, 2012). Previous studies focused on OPEs in the particulate phase since many OPEs have been reported to mainly partition to particle phase in the atmosphere (Carllson et al., 1997; Salamova et al., 2014a, 2014b; Abdollahi et al., 2017). However, owing to the wide range of log P_L from −4.9 to 1.7, the distribution of OPEs may not be limited to the particle phase (Sühring et al., 2016). According to the predictions by OECD POV and LRTP Screening Tool and partitioning models of Junge-Pankow, Harner-Bidleman, the partition of OPEs to particle phase should be governed by their physicochemical properties such as log P_L and log K_OA. OPEs with log K_OA between 10 and 12 or log P_L between −5 and −2 were predicted to partition between gas and particle phase. TSP concentration caused major changes in the partitioning behavior of OPEs with log K_OA between 7 and 12 but had no effects on the partitioning of compounds with log K_OA below 7 or higher than 12 (Sühring et al., 2016; Wang et al., 2017). Therefore, the physicochemical properties of OPEs should be considered for investigating the occurrence and distribution of OPEs in TSP.

Zhengzhou, the capital of Henan province, is facing severe environmental pollution with mean concentrations of PM₁₀ and PM_2.5 up to 132 and 72 μg/m³ in 2017. High particle concentration may have significant influence on atmospheric OPE concentrations, which might be one of the key factors on their temporal variations. This present study aimed to: i) investigate OPE concentrations and compositional profiles in TSP in Zhengzhou city; ii) explore the potential factors on their temporal variations in TSP; iii) investigate the potential sources of OPEs in the sampling area; ⅳ) evaluate the risks to humans from the exposure to OPEs in TSP.