Vapour-phase concentration profiles and transformation product distribution
Predetermined mass amounts of γ-HCH were placed on glass and carpet coupons to produce surface concentrations. Vapour-phase concentrations, expressed in μg of substance per m3 of vapor in the headspace were determined 96 h later at 20 and 40 °C. In addition to γ-HCH, two of its transformation products were also detected in the vapour phase: α-HCH and PCCH. Both products were not detected in samples of the initial γ-HCH, and it was known that Tedlar bag material would not react with γ-HCH. It was thus concluded that these products were generated in reactions of γ-HCH on surface of the test materials. This correlates with earlier reports on α-γ isomerization of HCH (Malaiyandi and Shah 1984; Walker et al. 1999) and its dechlorination into PCCH (Bhatt et al. 2009; Li et al. 2011).
The release of γ-HCH, α-HCH, and PCCH at 20 °C is shown in Fig. 1a for carpet and Fig. 1b for glass. Accordingly, Figs. 1a and 2b depict respective test data for carpet and glass at 40 °C. For both temperatures, an increase in initial surface concentration of γ-HCH first resulted in a steep increase in vapour concentration. Further increase in the surface concentration had a lesser effect on the vapour concentration which eventually formed plateaus, as the system was approaching the vapour saturation state. The observed plateau concentrations were compared to the saturation concentrations of γ-HCH calculated based on Eq. 1 given by Boehncke et al. (1996). Here, the saturation vapour pressure of γ-HCH P
sat
(Pa) depends on the temperature T (K) between 292 and 326 K as follows:
$$ {P}_{sat}= \exp \left(34.53-\frac{11,754}{T}\right) $$
(1)
This results in P
sat
equal to 3.75 × 10−3 Pa at 20 °C and to 4.87 × 10−2 Pa at 40 °C. P
sat
was then used to calculate the saturation concentrations of γ-lindane in the vapour phase at 20 and 40 °C, using the ideal gas law as follows:
$$ {C}_{sat}=\frac{M\cdot {P}_{sat}}{R\cdot T} $$
(2)
where C
sat
(g/m3) is the saturation vapour concentration, M (g/mol) is the molecular weight of γ-lindane, P
sat
(Pa) is the saturation vapour pressure of γ-lindane, R (8.314 J/K/mol) is the universal gas constant, and T (K) is the temperature. This gives a saturation vapour-phase concentration of 450 μg/m3 at 20 °C and 5,500 μg/m3 at 40 °C. The plateau concentrations were thus below the saturation concentration values calculated from respective vapour pressures at 20 °C (270 μg/m3 observed vs. 450 μg/m3calculated) and at 40 °C (1,250 μg/m3 observed vs. 5,500 μg/m3calculated). The plateau vapour-phase concentration of γ-HCH at 20 °C is approximately a half of TWA value of 500 μg/m3, while at 40 °C is two and a half times higher.
A simple hyperbolic fit (Eq. 3) was applied to approximate the experimental data as follows:
$$ {C}_g=\frac{a_1\cdot C}{a_2+C} $$
(3)
where C
g
(μg/m3) is the concentration of the compound in the vapour phase, a
1 (μg/m3) is the maximum concentration of the compound in the vapour phase, a
2 (mg/cm2) is an empirical constant, and C (mg/cm2) is the initial concentration of γ-lindane on the surface.
Results of the hyperbolic fits are presented in Table 2. The steep slopes of curves for γ-HCH on glass (Figs. 1b and 2b) suggest that the release of γ-HCH from glass is likely driven by the sublimation, which is the mechanism of transfer of γ-HCH from the original solid form into the vapour form (Giustini et al. 1998; Vecchio 2010). In comparison, the effects of the interactions with the building material, such as adsorption/desorption, play a lesser role. The curves for carpet shown in Figs. 1a and 2a have more gradual slopes than those for glass, indicating that more γ-HCH is retained by carpet and less is released in the vapour phase. This can be explained by the fact that the organic carbon-reach polypropylene fibers have a stronger affinity to lindane molecules, compared to the affinity of inorganic glass. This finding correlates with a relative sorption of lindane on different materials reported by Chen and Zhu (2005a, b); lindane is better adsorbed on matrices that have carbon-related functional groups.
Table 2 Hyperbolic fit parameters for experimental data in Figs. 1 and 2
The hyperbolic fit given by Eq. 3 was also applied to the concentrations of α-HCH and PCCH as a function of the initial surface concentration of γ-HCH. Generally, the curves for α-HCH and PCCH had more gradual slopes than respective curves for γ-HCH. This was likely due to the fact that γ-HCH vapors had to be produced first followed by the formation of by-products in interactions with surface materials. Tichenor et al. (1991) reported that building materials act as sinks for the vapors of organic materials. This would be applicable to both γ-HCH and to its transformation products. The products would be formed and then partially retained by the building material rather than being fully released into air.
As seen in Fig. 1a, b, equilibrium vapour concentration of γ-HCH was similar for both glass and carpet at 20 °C. On the other hand, the equilibrium vapour concentrations of the transformation products were much higher for glass than for carpet. Therefore, the overall toxicity in the vapour phase at 20 °C was higher for glass than for carpet. There are two implications from this finding. First, even if the initial concentration of γ-HCH on the surface is the same, the resulting toxicity of surrounding air may differ for different materials. Second, measuring the concentration of γ-HCH alone to assess the toxicity may not be sufficient as the transformation by-products add to it. Transformation products should be taken into account too. In all tests, except those on glass at 20 °C and the surface concentration of γ-HCH less than 3.4 mg/cm2, PCCH was present in the vapour phase in considerably higher concentrations compared to α-HCH and was therefore the primary transformation product of γ-HCH. For example, the ratio of vapour concentrations of PCCH to α-HCH on carpet at 20 °C (Fig. 1a) and in a range of surface concentrations from 10 to 40 mg/cm2 was approximately between 2 and 8. In comparison, this ratio was approximately between 25 and 60 at 40 °C.
At 20 °C and the initial surface concentration of 40 mg/cm2, the concentration of γ-HCH in the vapour phase reached approximately 60 % of its TWA value of 500 μg/m3, for both carpet (Fig. 1a) and glass (Fig. 1b). As indicated earlier, Swiss occupational standards (Suva 2013) list the same value of 500 μg/m3 for α-HCH as the North American (USA and Canada) TWA for γ-HCH (NIOSH 2010). No occupational standard could be found for PCCH. It was reported that the toxicity of PCCH is significantly less than that of γ-lindane (Bhatt and Kumar 2009); however, when γ-HCH degrades to PCCH, it also produces the toxic hydrogen chloride gas HCl (1 mol of HCl per mole of γ-HCH). Just for estimation purposes in this study, the TWA for γ-HCH was applied total polychlorocyclohydrocarbons, including γ-HCH, α-HCH, and PCCH. At 20 °C, the total concentration of polychlorocyclohydrocarbons in the vapour phase reached approximately 75 and 110 % of the TWA from carpet coupons and glass coupons, respectively. At 40 °C, the concentration of γ-HCH was approximately three times the TWA, in test with both glass and carpet. The concentration of total polychlorocyclohydrocarbons was thus up to five times the TWA and reached approximately 5 % of the Immediately Dangerous to Life or Health (IDLH) level of 50,000 μg/m3. NIOSH (2010) defines IDLH as “any condition that poses an immediate or delayed threat to life or that would cause irreversible adverse health effects or that would interfere with an individual’s ability to escape unaided from a permit space.”
For each tests, the masses of γ-HCH and its transformation products in the vapour phase were estimated as their concentrations in the vapour phase at equilibrium, as collected in the Tenax tubes, multiplied by the volume of the Tedlar air-sampling bag. The total mass of all the polychlorocyclohydrocarbons in the vapor phase was much smaller than the mass of γ-HCH initially deposited on the surface. Experimental results showed that it never exceeded 0.45 % of the initial mass of γ-HCH. In other words, only a small fraction of the total γ-HCH participated in mass transfer between air and surface. This should, however, be viewed in the context of vapour adsorption by the coupon material. As discussed earlier, the adsorption of organic vapours was likely to be higher on the carpet than on the glass. As shown in Figs. 1 and 2, at lower surface concentrations of γ-HCH of 5 mg/cm2 or less, the difference in vapour concentrations over glass and over carpet was quite visible due to the contribution of adsorption. At higher concentrations, the difference was minimal, if any existed at all. Mass transfer at high initial surface concentration was likely to be dominated by sublimation rather than adsorption/desorption, so the effect of surface materials was diminished.
Release isotherms
In order to estimate vapour concentrations at different surface concentrations and temperatures, Sips isotherm model was used as described by Do (1998). The correlation between the equilibrium concentrations on surface and in vapor phase is given using the following equation:
$$ {C}_s=\frac{C_{sm,0}\cdot \exp \left[\chi \cdot \left(1-\frac{T}{T_0}\right)\right]\cdot {\left\{{b}_0\cdot \exp \left[\frac{Q}{R\cdot {T}_0}\cdot \left(\frac{T}{T_0}-1\right)\right]\cdot {C}_v\right\}}^{\left[{n}_0+\alpha \left(1-\frac{T_0}{T}\right)\right]}}{1+{\left\{{b}_0\cdot \exp \left[\frac{Q}{R\cdot {T}_0}\cdot \left(\frac{T}{T_0}-1\right)\right]\cdot {C}_v\right\}}^{\left[{n}_0+\alpha \left(1-\frac{T_0}{T}\right)\right]}} $$
(4)
where C
s
is the equilibrium surface concentration (mg/cm2), C
sm,0 is the saturation surface concentration (mg/cm2) at the reference temperature T
0,
χ is a dimensionless constant parameter, T is the temperature (K), T
0 is the reference temperature set at 293 K, b
0 is the adsorption constant (m3/μg) at the reference temperature T0, Q is the heat of adsorption (J/mol), R is the universal gas constant equal to 8.314 J/(K mol), C
v
is the equilibrium vapour-phase concentration (μg/m3), n
0 is a dimensionless parameter characterizing the system heterogeneity at the reference temperature T0, and α is a dimensionless constant parameter.
Figure 3 depicts the experimental results and the predicted values of the equilibrium vapour concentrations of total polychlorocyclohydrocarbons as function of the surface concentrations of γ-HCH on carpet (a) and glass (b), respectively. Figure 3 presents the data in the form of adsorption isotherms where the x-axis corresponds to the vapour concentration and the y-axis corresponds to the surface concentration. Since only a very small fraction of γ-HCH participated in mass transfer between the surface and the vapour phase, the initial surface concentration of γ-HCH was used in Fig. 3 as the equilibrium surface concentration of polychlorocyclohydrocarbons.
The Sips isotherm parameters for glass and carpet at 20 and 40 °C are given in Table 3. The calculated heat of adsorption Q was greater for carpet (56,000 J/mol) than for glass (45,000 J/mol). This confirmed that γ-lindane and likely polychlorocyclohydrocarbons in general adsorbed stronger on the organic-based carpet than on inorganic glass. The calculated surface saturation concentration C
sm,0 was also slightly higher on carpet (390 mg/cm2) than on glass (360 mg/cm2). Overall, Fig. 3 demonstrates a good correlation between the experimental data and those calculated using the Sips model.
Table 3 Sips isotherm parameters for total polychlorocyclohexanes
Application of Sips isotherms to assess decontamination requirements
The Sips isotherm model was used to determine the surface concentration (C
s
) corresponding to a vapour-phase concentration (C
v
) equal to the TWA. As indicated in Vapour-phase concentration profiles and transformation product distribution, the TWA value of 500 μg/m3 for γ-lindane was employed as the TWA for total polychlorocyclohydrocarbons as C
v
. The corresponding C
s
values were then calculated at temperatures ranging from 10 to 50 °C using Eq. 4 and parameters from Table 3. The calculated C
s
values for glass and carpet are shown in Fig. 4 as functions of temperature. These C
s
concentrations corresponded to the maximum surface concentrations of respective compounds at which safe working environment was still possible. In other words, if the surface concentrations exceeded C
s
, then the concentration in the vapour phase would exceed the TWA. Surface decontamination would therefore be necessary.
In addition to helping to decide whether the decontamination is required or not, C
s
also helps to estimate the extent of decontamination required to attain the TWA in the air. For example, if the surface concentrations exceed C
s
by a factor of ten, then the required decontamination should reach at least 90 % removal of the contaminant from the surface.
Several hypothetical scenarios for glass and carpet are provided in Table 4. Based on the Sips model Eq. 4 and the parameters in Table 3 determined for glass and carpet, minimum decontamination efficiencies required to revert to a safe environment were estimated based on the TWA. The minimum decontamination efficiency (MDE) was defined as follows:
$$ \mathrm{MDE}=100\cdot \left(1-\frac{C_{TWA}}{C}\right) $$
(5)
where MDE (%) is the minimum decontamination efficiency, C
TWA
(mg/cm2) is the equilibrium surface concentration (C
s
) that corresponds to the equilibrium vapour concentration (C
v
) of the TWA, calculated from Eq. 4, and C (mg/cm2) is the initial surface concentration.
Table 4 Minimum decontamination efficiency (MDE) required for glass and carpet depending on the level of contamination and the temperature
At the same initial surface concentration C, the minimum decontamination efficiency MDE depends only on C
TWA
. Having the same TWA value for total polychlorocyclohydrocarbons, higher C
TWA
values were found for carpet than for glass from Eq. 4 at a given temperature as shown in Fig. 4. A lower MDE was therefore required for carpet compared to that required for glass.
For both carpet and glass, higher decontamination efficiencies would be needed at higher temperatures. This is due to the fact that equilibrium vapour concentrations increase as temperature rises. As a result, if surface decontamination was carried out at a lower temperature and it resulted in a vapour concentration below the TWA, further decontamination could still be required if building temperature was expected to rise. For example, for a surface concentration on carpet equal to 10 mg/cm2, the TWA is attained at 27 °C. This means that when temperature exceeds 27 °C, decontamination would be required. The higher the temperature, the greater the decontamination effort required. At 30 °C, at least 61 % of γ-HCH must be removed from the surface so that the vapour concentration does not exceed TWA. In comparison, the removal should be at least 98 % at 40 °C to stay below TWA. This should be taken into account in cases involving old pesticide storage facilities which are not equipped with ventilation or air conditioning and where temperatures can exceed 40 °C on hot summer days.
Screening tests on different building materials
In addition to glass and carpet, experiments were also carried out on other construction materials including ceiling tiles, painted drywall, ceramic tiles, and vinyl floor tiles at 20 and 40 °C contaminated with 4 mg/cm2 of γ-HCH. The concentrations of γ-lindane, α-lindane, and PCCH were measured in the vapor phase. Similarly to glass and carpet, it was assumed that the 4-day period was sufficient to reach equilibrium on those materials.
As shown in Fig. 5, the concentrations of γ-HCH in the vapour phase were very close for all materials, averaging at 270 μg/m3 at 20 °C and 1,500 μg/m3 at 40 °C. In comparison, the concentrations of transformation products differed more significantly. For example, the vapour concentration of PCCH for ceramic tiles at 40 °C was greater than 5,000 μg/m3, which was more than three times higher than the vapour concentration of γ-HCH. At the same time, the vapor concentrations of PCCH for carpet and vinyl tiles were only about 500 μg/m3. The high vapour-phase concentrations of PCCH detected for ceramic tiles can likely be explained by the porous surface of the tiles and the presence of metals as active sites. The latter can act as mediators of γ-HCH dihaloelimination and the formation of PCCH (Li et al. 2011). Another observation was that while both α-HCH and PCCH were detected in comparable concentrations at 20 °C, the concentrations of PCCH were far greater than those of α-HCH at 40 °C. The action of metals was more profound at a higher temperature which resulted in a major increase in PCCH concentration in the vapour phase.