Evaluation of the Barrier Effect of Polylactic Acid-Modified Membrane on Odours at the Excavated Soil Interface of a Pesticide-Contaminated Site

Abstract

Polylactic acid (PLA) is a highly promising bio-based polymer that can replace petroleum-based materials. The PLA-modified membrane has been found to effectively block soil odours in laboratory experiments, but its barrier effect at the excavated soil interfaces of actual pesticide sites requires further evaluation. This study investigates the barrier effect of the PLA-modified membrane on odours at the excavated soil interface of a pesticide-contaminated site in Guangdong Province, China. The membrane’s barrier effect on odours was comprehensively evaluated using the static chamber technique with three indicators: diffusion flux, odour concentration, and a health risk index. The results showed that the initial diffusion fluxes of six main odour substances: m- and p-xylene, o-xylene, toluene, ethylbenzene, n-propylbenzene, and cumene were 1.95 × 10⁰, 2.88 × 10⁻¹, 7.27 × 10⁻³, 1.49 × 10⁰, 2.97 × 10⁻³, and 3.89 × 10⁻³ mg/(m²·s) based on the contribution rate. After laying the PLA-modified membrane, the flux reduction rate of all six odour substances was generally >90%. The background odour concentration in the test area was 109.56, and the odour concentration after laying the membrane was <1.12. The initial non-carcinogenic and carcinogenic risk indices of the test area were 3.03 and 1.62 × 10⁻⁴, respectively. After laying the membrane, these indices were <0.05 and <3.78 × 10⁻⁷, respectively, indicating no health risk. Overall, the PLA-modified membrane had a good barrier effect on odours in the on-site application, effectively reducing the diffusion and nuisances of odours, as well as their health risks.

1. Introduction

China is one of the largest pesticide producers in the world. The process of pesticide production involves the emanation of multiple odours that cause many forms of soil pollution [1,2,3]. Over the years, through the effective implementation of the Stockholm Convention and owing to the policy of ‘suppressing the second industry and developing the third industry’, a large number of pesticide production enterprises in cities were shut down or relocated, leaving many sites affected by problems posed by odour [4,5]. During the subsequent soil restoration process of these sites, one main source of odour is the exposed soil interface, which seriously affects the surrounding residents and limits the smooth implementation of the restoration. This problem has become a major challenge in implementing site pollution control and remains the focus of public complaints [6,7,8]. A long-term exposure to certain odours, such as benzene compounds, halogenated hydrocarbons, and sulphides, may cause non-carcinogenic and carcinogenic risks [9].

To date, some research has been carried out on odour control in the pesticide production process using methods such as installing exhaust ventilation systems [10] and dielectric barrier discharge reactors [11]. However, there have been few reports on odour control in pesticide-contaminated sites. Typically, on-site techniques are now used in actual engineering sites to block odours. The most widely implemented covering material is the high-density polyethene (HDPE) membrane. However, the HDPE membrane is not biodegradable, and a large amount of greenhouse gases are emitted during its production, thus, this technique does not align with the globally advocated Sustainable Development Goals [12]. In addition, the HDPE membrane mainly blocks only the odour substances between the membrane and the soil. Once the membrane is opened, the accumulated odours are released, negatively impacting construction personnel and strongly affecting the surrounding environment [13,14]. There is an urgent need for environmentally sustainable materials that can be used as a more effective odour barrier for widespread use in pesticide-contaminated sites.

Polylactic acid (PLA) is a completely biodegradable polymer material that is naturally sourced, easily accessible, and has a low cost; moreover, it has good physical and mechanical properties. It is one of the most promising bio-based polymers to replace petroleum-based materials such as HDPE [12,15,16]. However, the conventional PLA membrane has a high gas permeability and a poor functionality in adsorbing and degrading odours, which limits its application as a barrier material [17,18]. By adding nanocomposites such as polybutylene adipate terephthalate/butylene terephthalate and calcium peroxide-loaded organic intercalated bentonite, the mechanical and physical barrier properties of the PLA membrane can be significantly improved such that it adsorbs and degrades odours entering its pores [19,20,21]. In this way, the barrier effect of the PLA-modified membrane on odours has been verified in the laboratory. It has a good barrier effect on typical odour substances such as benzene compounds, chlorinated hydrocarbons, and organic sulphides from pesticide pollution, showing a reduction in the diffusion flux by > 89.8% [22]. However, the barrier effect of the PLA-modified membrane on soil odours at the excavated soil interface of actual sites needs verification.

This study investigates one pesticide-contaminated site in the Guangdong–Hong Kong–Macao Greater Bay Area of China. An excavated soil interface with an area of more than 500 m² was selected for testing. The barrier effect of the PLA-modified membrane material on odours at the excavated soil interface was comprehensively evaluated in terms of its diffusion potential, odour disturbance, and health risks. The results of this study can be expected to serve as a reference for the use of PLA-modified membranes at pesticide-contaminated sites worldwide.

2. Materials and Methods

2.1. Site Overview

The research site is located in Guangzhou Province in southern China (Figure 1a). The main pesticide products present at the site were cyanophos emulsifiable concentrate and trichlorfon, as well as paints, resins, and other coatings. Pesticide production was completely stopped at the site at the end of 1998, and the production unit was relocated in 2015. The areas around the site are considered to be odour-sensitive, which include residential areas and shopping centres (Figure 1b,c). Currently, a plan exists to develop the research site as mixed-use land for commercial and residential uses.

Based on the findings of a previous survey of this site, the exposed strata at the site, from top to bottom, consists of artificial fill (miscellaneous and plain fill), alluvium (mainly sandy clay, with some fine sand and silty clay), eluvium (completely weathered sandy mudstone and highly weathered mudstone), and Lower Palaeozoic bedrock (medium- to weakly weathered mudstone). A restoration process was carried out and divided into 10 layers according to the depth and nature of the soil: 0–1 m (fill), 1–3 m (fill, sandy clay, and completely weathered sandy mudstone), 3–5 m (sandy clay and completely weathered sandy mudstone), 5–7 m (sandy clay and completely weathered sandy mudstone), 7–10 m (sandy clay and completely weathered sandy mudstone), 10–15 m (sandy clay and highly weathered mudstone), 15–17 m (highly weathered mudstone), 17–20 m (highly weathered mudstone), 20–24 m (highly weathered mudstone), and 24–30 m (highly weathered mudstone). Based on the results of the preliminary investigation on soil odours and the site construction conditions, the 1–3 m soil layer within a selected area of 500 m² was selected for testing (Figure 1d). The natural water content (ω), porosity, and vertical and horizontal permeability coefficients of the soil layer were measured to be ω = 24.78%, 44.02%, 3.10 × 10⁻⁵ cm/s, and 2.78 × 10⁻⁵ cm/s, respectively.

2.2. Polylactic Acid-Modified Membrane

The PLA-modified membrane with a width and thickness of 100 cm and 0.2 mm, respectively, was specifically developed for this study [22]. The membrane was spliced with degradable tape. The main raw materials used in the preparation of the modified membrane included PLA, polybutylene adipate terephthalate/butylene terephthalate, calcium peroxide-loaded organic intercalated bentonite, and polymer chain extenders. Preliminary laboratory testing was performed, and the results showed that the elongation at the break of the modified membrane and tensile breaking stress were 3540% and 20.3 MPa, respectively, indicating a high impact strength. The gas transmission rate was 54.7 cm³/(m²·24 h·0.2 MPa), which was much lower than that of the HDPE membrane of the same thickness (132.2 cm³/(m²·24 h·0.2 MPa)). The results of a soil burial biodegradation test showed that the degree of fracture of the modified membrane was very high at 150 days. Finally, there was no need for recycling or disposal, and no secondary pollution was generated during production [18,20].

2.3. Collection and Analysis of Gas Samples

Air samples were collected before and after laying the membrane to determine the diffusion potential of odours at the research site. The static chamber technique was used to collect the air samples in order to avoid the interference of other environmental factors on the test results and ensure reliable and complete data for a direct interpretation of the odour diffusion potential. The design of the flux chamber is shown in Figure 2a [23,24]. The test area was divided into 20 grids, and of these, six grids were selected to install sampling points (Figure 1d). Sampling was performed on the 1st, 3rd, 5th, 8th, and 11th days after the membrane was applied. The target air samples were collected at 0, 15, 30, 60, and 120 min after the flux chamber was fastened. Before sampling, one hygrothermograph and one pressure gauge were placed in the box to record the temperature, humidity, and pressure in the flux chamber during the sample collection. The target air sample was collected in a 1 L polytetrafluoroethylene air bag. The air bag was placed in the vacuum sampler and connected to the flux chamber. The target air sample was collected after washing the bag three times (Figure 2b).

The detection of the concentration of odours in gas samples was based on the method prescribed in the Chinese standard ‘Ambient air—Determination of volatile organic compounds—Collected by specially-prepared canisters and analysed by gas chromatography/mass spectrometry’ (HJ 759-2015). The samples entered the automatic sampling system (Nutech 3610, Nutech, Dallas, TX, USA) through the quick connector and were preconcentrated across three traps (Nutech 8910, Nutech, Dallas, TX, USA). Trap 1 had a collection temperature of −30 °C, preheating temperature of 0 °C, resolution temperature of 30 °C, resolution flow of 20 mL/min, resolution time of 4 min, and baking temperature of 180 °C. Trap 2 had a collection temperature of −50 °C, resolution temperature of 210 °C, resolution time of 90 s, baking temperature of 215 °C, and a baking time of 4 min. Trap 3 had a collection temperature of −160 °C and an injection time of 25 s. After preconcentration, the odour samples were transferred to a gas chromatography/mass spectrometer (Agilent 7890A/5975C, Agilent Technologies, Santa Clara, CA, USA) for quantitative analysis. The analytical standard materials were procured from Linde (München, Germany). The chromatographic conditions were as follows: a chromatographic column DB-624UI (small calibre capillary column) was used with the dimensions of 60 m × 0.25 mm × 1.4 μm. The carrier gas used was high-purity helium with a flow rate of 1.0 mL/min. The initial column temperature was maintained for 4 min at 35 °C and was then increased to 150 °C at a rate of 5 °C/min, and then increased again to 210 °C at a rate of 20 °C/min and maintained for 5 min.

2.4. Evaluation Methods

2.4.1. Diffusion Flux Calculation

The emission rate of the soil odours was characterised by the diffusion flux, which was calculated using the following formula [25,26]:

$$f_i=\frac{\Delta m_i}{\Delta t}\times \frac{M}{V_0}\times \frac{P}{P_0}\times \frac{T_0}{T}\times H$$

(1)

where $f_i$ is the flux, (mg/(m²·s)); $\Delta m_i/\Delta t$ is the rate of change of the odour pollutants concentration (mg/(m³·s)); $M$ is the molar mass of the odour pollutants; $P$ is the real-time air pressure in the flux box at the time of the sampling, Pa; $T$ is the real-time temperature in the flux box at the time of the sampling, K; $V_0$, $P_0$, and $T_0$ are the gas molar volume (L/mol), air pressure (Pa), and temperature (K) under the standard conditions, respectively; and $H$ is the height of the flux box, m.

2.4.2. Theoretical Odour Concentration

Assuming that the odours in the breathing zone are fully mixed with the atmosphere and have a uniform concentration, the concentration of odours in the breathing layer of the atmosphere can be calculated using their diffusion flux, as shown in the following formula:

$$C_i=f_i\times \frac{S}{U_{air}\times {\delta }_{air}\times W}$$

(2)

where$C_i$ is the concentration of odours in the breathing layer of the atmosphere in the test area, mg/m³; $S$ is the test area, m²; $W$ is the width of the pollution source, m; $U_{air}$ is the wind speed of the breathing layer, m/s; and ${\delta }_{air}$ is the mixing height of the respiratory layer, m.

Regarding the approach to measuring the odour concentration, the concentration refers to the dilution factor when odour-free air is used to continuously dilute the air sample to the odour detection threshold of the odour sniffer; however, this method is usually time-consuming and expensive. Meanwhile, it can be assumed that most pesticide sites contain substances that are toxic and harmful to the human body, while the standard method of odour discrimination (the olfactory discrimination method) also poses potential health hazards to the odour sniffer. Therefore, estimating the theoretical odour concentration using the odour activity value (OAV) is a faster, more economical, and safer method that has become widely used, and it can also evaluate the odour contribution of a single odour substance [27,28,29]. In addition, considering the sporadic characteristics of odour emission, the average concentration is insufficient to reflect the odour pollution that may be generated at the source; therefore, it is necessary to calculate the theoretical odour concentration in the test area after $C_i$ is converted to the instantaneous peak concentration [30,31]. The formulae are as follows:

$$C_i^p=C_i\times F$$

(3)

$$OA{V}_i=C_i^p/O{T}_i$$

(4)

$$C_{OD}=OA{V}_{SUM}={\displaystyle \sum }_{i=1}^{6}OA{V}_i$$

(5)

where $C_i^p$ is the peak concentration of odours, mg/m³; $F$ is the conversion factor between the peak concentration and the average concentration, with a value of 8, no units; $OA{V}_i$ is the odour activity value of a single odour substance; $C_i$ is the concentration of odours mixed in the atmosphere, mg/m³; $O{T}_i$ is the odour threshold, mg/m³; $C_{OD}$ is the theoretical odour concentration, no units; and $OA{V}_{SUM}$ is the sum of all the odour activity values of the main odours.

2.4.3. Health Risk Assessment

Inhalation is the main exposure route for emitted odour substances [32]. The non-carcinogenic and carcinogenic risks in the test area were calculated according to the USEPA using the following formulas [1,14]:

$$E{C}_i=C_i\times \frac{EF\times ED\times ET}{AT}$$

(6)

$$H{Q}_i=E{C}_i/RF{C}_i$$

(7)

$$HI={\displaystyle \sum }_{i=1}^{6}H{Q}_i$$

(8)

where$E{C}_i$ is the exposure concentration of odours, mg/m³; $H{Q}_i$ is the hazard quotient of a single odour substance; $EF$ is the exposure frequency, 200 d/a; $ED$ is the exposure duration, 20 a; $ET$ is the exposure time, 6 h/d; $AT$ is the average exposure time, non-carcinogenic risk, 24 × 365 × 24 h, carcinogenic risk, 70 × 365 × 24 h; $RFC$ is the non-carcinogenic reference dose, mg/m3; and $HI$is the non-carcinogenic risk, no units. The risk was calculated using:

$$Risk=E{C}_i\times IU{R}_i$$

(9)

where $Risk$ is the carcinogenic risk, no units, and $IU{R}_i$ is the carcinogenic reference dose, mg/m³.

3. Results and Discussion

3.1. Main Odours

The cumulative contribution rate of odours was used to screen the main odour substances in the test area to effectively control and evaluate them. The results are shown in

Table 1. Main odour substances identified in the test area.

S. No.	Malodourous Substances	Concentration (mg/m3)	Olfactory Threshold (mg/m3)	Odour Activity Value	Cumulative Odour Contribution (%)
1	M, p-xylene	2500.00	0.19	13,157.89	63.86
2	N-propyl benzene	57.90	0.02	2895.00	77.91
3	Ethylbenzene	2000.00	0.81	2469.14	89.89
4	Toluene	800.00	1.36	588.24	92.75
6	Cumene	20.00	0.05	400.00	94.69
5	O-xylene	700.00	1.80	388.89	96.58
7	2-Hexone	30.00	0.11	279.55	97.93
8	Styrene	40.00	0.16	250.00	99.15
9	1,2,4-Trimethylbenzene	90.60	0.64	141.56	99.83
10	1,3,5-Trimethylbenzene	31.20	0.91	34.29	100.00
11	Benzene	0.02	2.70	0.01	100.00
	Total			20,604.56

[6]. The odour substances present in the test area included m, p-xylene (OAV = 13,157.89), ethylbenzene (OAV = 2469.14), n-propylbenzene (OAV = 2895.00), toluene (OAV = 588.24), cumene (OAV = 400.00), o-xylene (OAV = 388.89), 2-hexone (OAV = 279.55), styrene, 1,2,4-trimethylbenzene, 1,3,5-trimethylbenzene, and benzene.

The pesticide-contaminated site has a serious odour pollution, and the sum of OAV was about 26 times that of the reported landfill sum of the OAV [33]. The main odour substances with a cumulative odour contribution rate of >95% were m, p-xylene, ethylbenzene, n-propylbenzene, toluene, o-xylene, and cumene, all of which are benzene homologues, whose presence is likely owing to the test area having long been used for cyanophos production, the process for which involves benzene compounds as important raw and auxiliary materials. Any leakage accidents that may have occurred during the production process would have polluted the soil in the test area with benzene compounds. In addition, an underground water tank was also situated near the test area that was once used as a storage tank for liquid waste from paint production, and a small quantity of accumulated liquid was observed in the tank during the field survey. Benzene compounds are important solvents in preparing the colour paste used for paint production, hence any such compounds in the waste liquid may have also caused soil pollution in the test area.

3.2. Diffusion Flux of Odours

The diffusion flux represents the emission rate of odours from the soil to the atmosphere, and it is a direct indicator to quantify and evaluate the diffusion potential and pollution risk of odours in the soil at the site [34]. Figure 3 shows the diffusion flux of odours before and after the placing of the PLA-modified membrane. The background diffusion fluxes of m, p-xylene, o-xylene, toluene, ethylbenzene, n-propylbenzene, and cumene, were relatively high at 3.26 × 10⁻², 4.81 × 10⁻³, 1.21 × 10⁻⁴, 2.49 × 10⁻², 6.48 × 10⁻⁵, and 4.95 × 10⁻⁵ mg/(m²·s), respectively. The diffusion fluxes of the main odours decreased significantly after the membrane was placed. The flux of m, p-xylene was 9.44 × 10⁻⁶–3.61 × 10⁻⁴ mg/(m²·s), and its flux reduction rate was >98.89%. The flux of o-xylene was 6.67 × 10⁻⁶–1.65 × 10⁻⁴ mg/(m²·s), and its flux reduction rate was >96.56%. The flux of toluene was 7.50 × 10⁻⁷–4.07 × 10⁻⁵ mg/(m²·s), and its flux reduction rate was >89.30%, except on the first day, when it was 66.51%. The flux of ethylbenzene ranged from 4.44 × 10⁻⁶ to 4.70 × 10⁻⁵ mg/(m²·s), and its flux reduction rate was > 99.77%. The flux reduction rates of n-propylbenzene and cumene were >92.67%. Overall, the PLA-modified membrane effectively reduced the diffusion flux of odours. The average flux reduction rates of the 1st, 3rd, 5th, 8th, and 11th days in the test area were 92.10, 96.72, 99.39, 96.71, and 99.81, respectively. The diffusion flux of odours generally decreased with time, and the reason for the fluctuation on the 8th day may be related to detection errors. On that day, the concentration of odorous substances in the flux chamber was too low, which is a challenge for determining the concentration [35].

The PLA-modified membrane was stable, and it effectively reduced the diffusion flux of the odour substances, likely due to the following reasons: (1) in the production process of the PLA-modified membrane material, nano-structured bentonite is dispersed into the PLA through the layered nanofiller such that the organically intercalated bentonite has a better compatibility and dispersion within the PLA matrix. Under the action of a high shear force, the monolayer peeling of the flakes significantly increases the tortuosity of the passage path of odours and improves the physical barrier ability of the modified membrane to odours, thereby effectively reducing the diffusion flux [36]. (2) The CaO₂ particles loaded on the bentonite layer increase the roughness of the gas transmission path, thereby improving the hydrophobicity of the membrane and reducing the diffusion flux of the odours. (3) CaO₂ has both oxygen-releasing and oxidation properties [37] and it has been used to remove benzene, toluene, and other organic substances from soil and groundwater [37,38,39,40]. The bentonite loaded with CaO₂ can effectively absorb and degrade the odours entering the membrane pores as water vapour, thereby further reducing the diffusion flux of the odours [10,41]. (4) In addition, nanosized calcium peroxide particles were generated between the bentonite-like structures through the in situ reaction process. Under the action of soil water vapour/sprayed water, hydrogen peroxide is slowly released for the continuous degradation of odours within a certain period of time, and the diffusion flux is significantly reduced, while at the same time ensuring the sufficient adsorption sites of the fillers [37].

3.3. Concentrations of Odours

The PLA-modified membrane effectively reduced the diffusion flux of the odour substances. The degree of odour pollution of the test area before and after laying the membrane was characterised in terms of odour concentration and to quantify the odour reduction by the modified membrane. Figure 4 shows that the odour concentration in the test area before the coating was 109.56, with m, p-xylene being the main contributor. The OAV and odour contribution rates were 89.34 and 81.54%, respectively. The flux of ethylbenzene was close to that of m, p-xylene, but its olfactory threshold was four times higher than those of the same substances. The OAV of ethylbenzene was 16.48, and its odour contribution rate was only 15.04%. After the PLA-modified membrane was laid, the OAVs of m, p-xylene decreased from 89.34 to 0.94, 0.99, 0.20, 0.83, and 0.03, respectively, on the 1st, 3rd, 5th, 8th, and 11th days, and that of ethylbenzene decreased to 0.03, 0.04, 0.02, 0.009, and 0.003, respectively.

Calculations show that the degree of odour pollution in the test area was effectively reduced after the PLA-modified membrane was laid. The odour concentration in the test area was below the Chinese standard odour concentration value of 10 [42], indicating that the impact of odours in the test area was effectively reduced. The modified PLA barrier membrane displayed an improved physical barrier performance through the oxidation and adsorption of odours, thereby effectively reducing the deleterious effects of odours and their adverse impacts on the environment and humans during the removal of the membrane.

3.4. Health Risk Level

The main odours present in the research site were benzene homologues. From the perspective of toxicology, benzene homologues pose a variety of threats to human health, and a long-term exposure may cause blood diseases and even cancer [43,44]. Of the six main odours identified in this study, five were non-carcinogenic substances and ethylbenzene was carcinogenic. The risks to human health before and after the membrane was laid in the test area were characterised using the non-carcinogenic (HI) and carcinogenic (Risk) risk indexes, as shown in

Table 2. Non-carcinogenic risks and carcinogenic risks of odour pollutants before and after applying the membrane.

Time (d)	Target HQ						Target Risk
	m, p-Xylene	o-Xylene	Toluene	Ethylbenzene	n-Propylbenzene	Isopropylbenzene	Ethylbenzene
0	2.48 × 100	3.66 × 10−1	1.84 × 10−4	1.89 × 10−1	1.23 × 10−3	9.42 × 10−4	1.62 × 10−4
1	2.60 × 10−2	9.67 × 10−3	6.19 × 10−5	3.58 × 10−4	3.74 × 10−5	6.56 × 10−5	3.07 × 10−7
3	2.74 × 10−2	1.26 × 10−2	1.97 × 10−5	4.41 × 10−4	1.90 × 10−5	2.48 × 10−5	3.78 × 10−7
5	5.45 × 10−3	6.98 × 10−4	3.42 × 10−6	1.94 × 10−4	3.28 × 10−6	9.51 × 10−6	1.66 × 10−7
8	2.31 × 10−2	5.76 × 10−3	1.10 × 10−5	1.04 × 10−4	4.76 × 10−5	6.89 × 10−5	8.88 × 10−8
11	7.19 × 10−4	5.07 × 10−4	1.14 × 10−6	3.38 × 10−5	1.59 × 10−6	2.01 × 10−6	2.90 × 10−8

. The HQ value of m, p-xylene was 2.48. The non-carcinogenic risk was 3.03, which exceeded the limit specified by USEPA (non-carcinogenic risk = 1). The risk posed by ethylbenzene was 1.62 × 10⁻⁴. When the carcinogenic risk is <10⁻⁶, it is within the acceptable range for the human body. A value between 10⁻⁶ and 10⁻⁴ indicates a potential carcinogenic risk, and a value of >10⁻⁴ indicates a high carcinogenic risk. The carcinogenic risk of the test area before laying the membrane was >10⁻⁴, indicating a high carcinogenic risk [9,45,46]. The hazards caused by carcinogenic and non-carcinogenic odours in the test area were significantly reduced after starting the experiment. The HQ of m, p-xylene on the 1st, 3rd, 5th, 8th, and 11th days decreased to 2.60 × 10⁻², 2.74 × 10⁻², 5.45 × 10⁻³, 2.31 × 10⁻², and 7.19 × 10⁻⁴, respectively. The non-carcinogenic risk values on the 1st, 3rd, 5th, 8th, and 11th days in the test area were 3.62 × 10⁻², 4.05 × 10⁻², 6.36 × 10⁻³, 2.91 × 10⁻², and 1.26 × 10⁻³, respectively. The carcinogenic risk posed by ethylbenzene reduced to 3.07 × 10⁻⁷, 3.78 × 10⁻⁷, 1.66 × 10⁻⁷, 8.88 × 10⁻⁸, and 2.90 × 10⁻⁸ on the 1st, 3rd, 5th, 8th, and 11th days, respectively (Table 2).

Overall, the PLA-modified membrane effectively reduced the health risk of odour, and the non-carcinogenic risk in the test area after laying the membrane was <1, indicating that there was no non-carcinogenic health risk to the human body. The carcinogenic risk was <10⁻⁶, indicating that the carcinogenic risk was within the acceptable range for the human body.

4. Conclusions

In this study, the static chamber technique was used to evaluate the barrier effect of the sustainable PLA-modified membrane on soil odours in an actual pesticide-contaminated site by evaluating the diffusion potential, odour nuisance, and health risk and using the indicators of the diffusion flux, odour concentration, and health risk index. The diffusion flux (1.04 × 10⁻² mg/(m²·s)) and odour concentration (109.56) in the test area before laying the membrane were high, causing odour nuisances to the site and surrounding areas, with non-carcinogenic (HI = 3.03) and carcinogenic (Risk = 1.62 × 10⁻⁴) risks. The laying of the PLA-modified membrane effectively reduced the diffusion potential of odour substances, odour nuisances, and health risks. The flux reduction rates of the six main odour substances that were determined based on the cumulative odour contribution rate were >90%. The average diffusion flux reduction rates in the test area on the 1st, 3rd, 5th, 8th, and 11th days were 92.10, 96.72, 99.39, 96.71, and 99.81%, respectively, and the overall average diffusion flux reduction rate was 94.66%. The odour concentration was reduced to <1.12, which is lower than the standard value (10). The HI in the test area was reduced to <0.05 and the risk was reduced to <3.78 × 10⁻⁷; thus, the non-carcinogenic and carcinogenic risks were reduced to within an acceptable range for the human body.