Cyclic Organosilicon Compounds in Ambient Air in Guangzhou, Macau and Nanhai, Pearl River Delta

Applied Geochemistry 16 (2001) 1447–1454 www.elsevier.com/locate/apgeochem

Cyclic organosilicon compounds in ambient air in Guangzhou, Macau and Nanhai, Pearl River Delta

X.M. Wang a, S.C. Lee b,*, G.Y. Sheng a, L.Y. Chan b, J.M. Fu a, X.D. Li b, Y.S. Min a, C.Y. Chan b aSKLOG, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China bDepartment of Civil and Structural Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong

Abstract Silicon is present in the Earth’s atmosphere as a consequence of several processes including the release of Si compounds from anthropogenic sources, but little information is available on airborne Si compounds of anthropogenic origins. In this study ambient air samples from Guangzhou, Macau and Nanhai in the Pearl River Delta, South China were collected by sorbent tubes for the determination of concentrations of volatile organosilicon compounds. Samples were analysed by thermal desorption followed by a GC–MSD technique for compound identification and quantitative analysis. Hexamethylcyclotrisiloxane (D3) and octamethylcyclotetrasilo-xane (D4) were found to be the two dominant organosilicon compounds in the air. In Guangzhou, higher total D3 and D4 concentrations were observed in the industrial area, landfill and waste water treatment plant, while the lowest levels occurred in suburb forest. Two types of linear correlation between D3 and D4 were found in Guangzhou samples, indicating different sources of these organosilicon compounds. Samples in Macau and Nanhai showed different D3 and D4 relationships from the samples in Guangzhou. # 2001 Elsevier Science Ltd. All rights reserved.

1. Introduction Due to their unique properties such as thermal stabi- lity, low surface tension and water repellence, silicon Silicon is present in the Earth’s atmosphere as a con- polymers, a dominant product of which is poly dime- sequence of several processes including the suspension thylsiloxane (PDMS) fluids (Fig. 1), have wide use in of silicon containing particles of inorganic and biologi- industrial applications and in consumer products. There cal origin from the Earth’s surface, the release of silicon are many pathways for organosilicon compounds to compounds from anthropogenic sources (e.g. industry enter the atmosphere. Volatile organosilicon compounds and fuel consumption) to the atmosphere and the consisting of –(CH3)2SiO– structural units in linear or introduction of silicon radionuclides from cosmic ray cyclic arrangements are widely used as components of interactions in the upper atmosphere. Little information cosmetic and personal products, and as penetrating oils, is available on the anthropogenic and cosmic sources cleaning agents and lubricants. These applications of compared to the sources of particle suspension. Pelliz- volatile organosilicon compounds lead to their emission zari et al. (1976) reported silane compounds identified at to the atmosphere (Graedel et al., 1986). Study of trans- trace concentrations in air, Graedel (1978) and Graedel formation of PDMSfluids in soil by Buch and Ingeb- et al. (1986) summarized the occurrence of organic sili- rigton (1979) indicated that PDMSderived products are con compounds in the lower atmosphere, these organo- present in the atmosphere. Recent work has shown that silicons were presumed to be derived from industrial ubiquitous PDMShydrolysis in soils generates dime- processes. thylsilanediol and other volatile linear siloxane diols and cyclic dimethylsiloxanes that would enter the atmosphere through volatilization (Lehmann et al., 1994; Carpenter et * Corresponding author. Tel.: +852-2766-6011; fax: +852- al., 1995; Lehmann and Miller, 1996; Varaprath and Leh- 2334-6389. mann, 1997). Another pathway for organosilicon com- E-mail address: [email protected] (S.C. Lee). pounds to enter the atmosphere is thermal decomposition

Fig. 1. Dimethylsilanediol and examples of linear and cyclic PDMS. of PDMS, which produces highly volatile cyclic dime- source inputs and distribution of atmospheric organosi- thylsiloxanes (Kleinert and Weschler, 1980). licons. As a category of chemicals, organosilicon compounds A few investigations into indoor organosilicon con- have generated substantial scientific and regulatory interest centrations revealed some interesting results. Shields over the last few years. Octamethylcyclotetrasiloxane (D4), and Weschler (1992) measured volatile organic com- as an important intermediate in the synthesis of silicon pounds at a telephone switch center. Shields et al. (1996) polymers and as a constituent in many personal care and reported the comparison of volatile organic compounds consumer products (deodorants, antiperspirants, skin and in 3 types of UScommercial buildings with different hair care products, etc.), was the first silicon compound occupant densities, they found that D4 and D5 were considered for regulatory review. A number of studies present in higher concentrations when there were more focused on its environmental fate and effects in the aquatic occupants, indicating that they were markers of occu- environment (Kent et al., 1994, 1996; Hobson, 1995; pant density. Table 1 summaries the physical properties Fackler et al., 1995; Mueller et al., 1995, Sousa et al., 1995). of the 3 cyclic organosilicons. Previous studies of the atmospheric chemistry of The present study aims to investigate the ground levels organosilicons emphasized their reaction kinetics with of organosilicon compounds in 3 typical cities (Guangz- OH, NO3 radicals and O3. From the study of Atkinson hou, Macau and Nanhai) situated in the Pearl River (1991), the calculated lifetimes of the selected volatile Delta, South China (Fig. 2). The Pearl River Delta region organic compounds ranged from 10 days for dec- has become China’s most populated and economically amethylcyclopentasiloxane (D5) to 30 days for hexam- developed region since early the 1980s. There are increas- ethylcyclotrisiloxane (D3) in the troposphere due to ing potential emission sources of organosilicons in the chemical reaction with the OH radical. The results sug- region. Moreover, the annual average temperature in the gested that their lifetimes are sufficiently long to undergo region is high (>20C) (see Table 2), thus making it easy long-range transport and to be regionally and globally for volatile organosilicons to enter the atmosphere via distributed. Using a smog chamber–mass spectrometer volatilization. The ambient concentrations, characteristics system, Sommerlade et al. (1993) analyzed the products and possible sources of volatile organosilicon com- and kinetics of the gas-phase reactions of selected orga- pounds in the 3 cities were investigated in this study. nosilicon compounds with OH radicals. Markgraf and Wells (1997) investigated OH radical reaction rate constants and atmospheric reaction products of 3 siloxanes, 2. Methodology and found that some unusual cyclic siloxane products were formed. Although organosilicon compounds in the The basic information for the 3 cities is summarized in atmosphere are considered to be released primarily from Table 2. In the screening of ambient levels of organosili- human activities, there is little data on the concentrations, con compounds in these cities, the chosen sampling X.M. Wang et al. / Applied Geochemistry 16 (2001) 1447–1454 1449

Table 1 Summary of physical properties of selected cyclic organosilicon compoundsa,b

Compound Formula MW mp bp vp Density LogKow (amu) (C) (C) (25C) mm Hg (20C) (g/cm3)

D3 C6H18O3Si3 222 64.5 134 3.53 1.1200 4.47 D4 C8H24O4Si4 296 17.5 176 1.05 0.9561 5.10 D5 C10H30O5Si5 370 À44 210 0.20 0.9985 5.20

a MW, molecular weight; mp, melting point; bp, boiling point; vp, vapor pressure; Kow, octanol–water partition coeﬃcient. b Lide (1995).

Fig. 2. Locations of the 3 cities and sampling areas in the Pearl River Delta, South China.

Table 2 Summary of basic information about the 3 cities

Cities Guangzhou (1996) Macau (1995) Nanhai (1996)

Population 3,901,840 (urban) 410,500 (peninsular) 7,168 (Taipa) 1,043,186 2,658,668 (non-urban) 3,146 (Coloane) 1,443.6 (urban) 7.0 (peninsular) Area (km2) 4.1 (Taipa) 1152.7 5990.8 (non-urban) 7.2 (Coloane) Population density (person/km2) 2703 (urban) 58,643 (peninsular) 1,748 (Taipa) 905 444 (non-urban) 437 (Coloane) Annual average temperature (C) 21.6 (urban) 23.1 21.8 1450 X.M. Wang et al. / Applied Geochemistry 16 (2001) 1447–1454 times were 9:00 a.m. to 2:00 p.m. at each site on the pling sites. In Taipa the sampling site was in the Uni- same day. Sorbent tubes and passivated canisters were versity of Macau campus, and the sampling site in used for sampling volatile organic compounds (VOCs) Coloane was at the coastal beach. in air (Rudolph et al., 1990; Seeley and Broadway, In Nanhai, 4 sites were selected for a 3 consecutive 1994). In this study, multi-bed sorbent tubes were day sampling from 10 to 12 July 1996. Three of 4 sam- adopted for sampling. The commercial 177.8Â6.35 mm pling sites were located in towns (Dali, Shuitou, Yayao) I.D. multi-bed stainless steel sampling tubes (Tekmar with booming privately owned small factories or manu- Company, USA) were packed with silica gel, carbon- facturers. The other sampling site was near a vegetable sieve and charcoal. Before use, the sampling tubes were growing farmland in the rural area (Tanbian). conditioned by heating at 180C for 24 h while purging By coupling a thermal desorption (TD) system (Tek- with He. After each use or storage, they were recondi- mar 6000 AeroTrap) to a Hewlett Packard 5972 Gas tioned in the same way for 2 h. Samples were all col- Chromatography/Mass Selective Detector (HP 5972 lected at 1.2 m above the ground level. Volatile organic GC/MSD), the analysis of VOCs was performed by a compounds were sampled by drawing air through the TD-GC/MSD technique. A HP-5MS (30 mÂ0.25 mm sampling tube with battery-operated timing pumps. The i.d.Â0.25 mm) capillary column was used with He as the flow rate and sampling time were set to 0.5 l/min and 20 carrier gas and a temperature program of 35C (2 min), min, respectively, for each sample. Each batch of sam- rate 5C/min to 250C (10 min). ples had a sorbent tube as a blank. The whole system, including field sampling and In Guangzhou, intensive sampling was conducted laboratory analytical devices, was considered clean if no covering different ambient environments in the city. target organosilicon compounds were detected when the Samples were collected on 10 July 1996. Thirty-two zero air test was performed. Compound confirmation samples were collected at the roadside in mixed com- and quantification were achieved using prepared stan- mercial and residential areas, 6 samples were collected in dards of known compounds. For determining the con- industrial areas in the east of the city, 12 samples from tents of D3, D4 and D5, m/z 207, m/z 281 and m/z 355 the Datianshan Landfill of Guangzhou in the suburb, 4 were selected as target ions, respectively. samples from the Datansha Wastewater Treatment Plant (the largest in Guangzhou), 18 samples from the suburban areas, and 2 samples from the Conghua For- 3. Results and discussion est Park in the suburbs. Samples in Macau were collected on 20 November The means, standard deviations and ranges of D3 and 1995. Since Macau Peninsular (urban area) has more D4 concentrations in the 3 cities are summarized in than 95% of Macau’s population, 5 roadside sampling Table 3. As D3 and D4 are chemicals with anthropogenic sites in Macau Peninsular, and one sampling site each in origins, their levels in Guangzhou and Macau show sig- Taipa and Coloane were selected. Local monitoring nificant declines with decreasing population density from stations in Macau Peninsular were included in the sam- urban areas to suburban areas. In Nanhai, samples

Table 3 D3 and D4 levels in the 3 cities (mg/m3)a

D3 D4

Sampling site descriptions MeanÆS.D. Range MeanÆS.D. Range

Guangzhou Urban mixed areas (n=32) 2.9Æ3.3 0.0–11.3 0.9Æ0.9 0.0–3.3 Industrial area (n=8) 6.1Æ1.9 1.9–9.3 13.5Æ6.1 6.4–20.5 Landﬁll (n=12) 3.6Æ3.1 0.7–11.2 11.4Æ5.5 2.2–17.5 Wastewater treatment plant (n=4) 2.9Æ2.0 1.1–5.7 10.3Æ5.6 3.0–16.2 Suburban(n=18) 0.5Æ0.3 0.1–1.0 0.4Æ0.4 0.0–1.6 Forest park (n=2) nd nd Macau Peninsular (n=10) 3.2Æ1.1 2.1–5.8 3.0Æ1.0 0.8–4.3 Taipa (n=2) 1.6, 2.0 – 2.4, 2.6 – Coloane (n=2) 0.1, 0.4 – 0.2, 0.3 – Nanhai (n =24) 0.6Æ0.8 0.0–2.3 0.9Æ1.0 0.0–3.5

a S.D., standard deviation; nd, not detected. X.M. Wang et al. / Applied Geochemistry 16 (2001) 1447–1454 1451

Fig. 3. D3 and D4 relationships in the 3 cities (Guangzhou, Macau and Nanhai). 1452 X.M. Wang et al. / Applied Geochemistry 16 (2001) 1447–1454 collected in towns also had much higher D3 and D4 con- had relatively higher concentrations than D3. Another tents than samples collected near the rural farmland. type of correlation was observed among samples col- In previous studies, D3, D4 and D5 were among lected in mixed commercial and residential areas. D3 volatile organic compounds detected in indoor air. was found to strongly dominate over D4, the D4 con- Indoor D4 and D5 concentrations were found to be up centrations being only about 30% of the D3 concentra- to 10 mg/m3 with a relatively high level of D5. The out- tions. The dominance of D4 in the industrial area and door D5 concentrations were up to 0.5 mg/m3 (Shields waste treatment facilities could be related with industrial and Weschler, 1992; Shields et al., 1996). In the present application and the use in personal care and consumer study, D5 was only found in trace concentrations in a products. However, the reason for D3 dominance in the few samples, and it was not detected in most samples. mixed areas is unknown since there is little data or The dominant organosilicon compounds detected in this information about its application and environmental study were D3 and D4. Other low molecular-weight occurrence in the city. Previous studies found cyclic organosilicon compounds found in the atmosphere, like PDMSin thermal degradation products of silicon poly- methylsilane and tetramethylsilane (Pellizzari et al., mers (Hunter et al., 1946; Voronkov et al., 1978; Klei- 1976), were not included in the target compounds due to nert and Weschler, 1980), the relative amounts of higher poor recoveries of these highly volatile compounds. membered cyclics gradually decreasing in order of their Nanhai is a newly developed city with a lower popu- size. However, in the mixed commercial and residential lation density, compared with Guangzhou and Macau. areas further investigation is needed to identify sources Observed maximum D3 and D4 concentrations (D3< of the thermal decomposition origin. An alternative 2.3 mg/m3, D4<3.5 mg/m3) in Nanhai were much lower explanation for D3 dominance in the samples might be than those in Guangzhou and Macau. Therefore, popu- the higher volatilization potential of D3 than D4 or lation density can be an important factor for these some consumer products having higher contents of D3. organosilicon compounds due to the use of personal Although the origins of D3 and D4 in the cities are not care and other consumer products. However, industrial fully charaterised, the two obvious types of D3 and D4 activities also contributed to these compounds in the correlation found in Guangzhou could indicate different atmosphere. This was confirmed by the higher con- sources of these cyclic organosilicons in urban areas. centrations of D3 and D4 found in industrial areas in The D3 and D4 concentrations of the samples from Guangzhou. As a contrast to the urban and suburban the urban Macau peninsular were about 10 times higher cases, D3 and D4 were not detected in two samples from than those in the samples collected at the coastal beach the forest in suburban Guangzhou. in Coloane. The total D3 and D4 concentrations of The D3 and D4 concentrations in waste treatment samples collected at the University of Macau in Taipa facilities in Guangzhou were also investigated. The were approximately 70% of the mean total of D3 and results revealed relatively higher D4 levels in these facil- D4 contents in the urban Macau peninsular (Table 3). ities compared to other suburban areas. The Datansha Although the D3 and D4 levels varied with different Wastewater Treatment Plant, with a capacity of 160,000 land use types in Macau, there was no obvious D3 or tons per day, treats wastewater from approximately 500 D4 dominance as found in Guangzhou (Fig. 3). The factories and municipal sewage from about 600,000 mean concentration of D3 (3.2 mg/m3) was similar to dwellers nearby. The use of D4 in personal care and that of D4 (3.0 mg/m3) in Macau peninsular. The mean consumer products, as well as their industrial applica- ratio of D4 to D3 in Macau lay between the two types tion, would eventually result in its discharge to the was- observed in Guangzhou, and D4/D3 ratios showed less tewater treatment plant via sewer systems or wastewater variation when compared to those in Guangzhou. pipelines. D4 is a highly volatile compounds with a The D4 dominance was also found in most samples Henry’s Law constant of more than 17 (Hobson and from Nanhai (Fig. 3), and the mean concentration of Silberhorn, 1995). This property would lead to its ulti- D4 was higher than that of D3 in each sampling site. mate release to the air, and this might be the reason for The mean D3 and D4 concentrations detected in the 3 higher D4 levels detected within the plant. Similarly towns were 0.8Æ0.9 and 1.2Æ1.0 mg/m3, respectively, higher levels of D4 in landfill may be explained by but in the farmland site, mean D3 and D4 concentra- volatilisation of D4 in municipal solid waste. tions were less than 0.1 mg/m3. The mean D4 to D3 The correlation relationships of D3 to D4 levels also ratios, however, was about the same between samples had significant variations, especially in Guangzhou. from the towns (1.5) and the farmland (1.7). 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