Introduction
Remediating diesel contaminated soils deserves various stratagems, such as foam remediation [1, 2], oxidation [3, 4], and bioremediation [5, 6]. Surfactant solutions have been proven to be efficient for remediating petroleum-contaminated soils [7–10], in which the surfactants aid in the desorption of diesel from the soil by facilitating micellar solubilization and directly modifying the contaminant matrix [11–14]. Comparing to anionic [15–17] and cationic [18, 19] surfactants, the nonionic surfactants have been tapped into remediation because their higher solubilizing capacity and cost-effectiveness [20–22], but they may also remain a considerable amount in the remediated soil, causing secondary pollution.
Nonylphenol polyethoxylates (NPEOn), also known as TX (mainly as nonylphenol derivative) or Triton (mainly as octylphenol derivative) in industry, used to be the second abundant nonionic surfactant and presented nice performance in soil washing [2, 23]; it has been banned recently due to their aquatic toxicity and endocrine activity [24–26]. As an alternative, another popular nonionic surfactant Tween 80 or Tween 80 mixed with other anionic surfactants was employed in remediation of diesel-contaminated soil [2, 19, 20, 27].
Recently, a newly emerged, biodegradble and Triton structural-alike surfactant, nonylcyclohexanol ethoxylates (NCEOn) has been found in their similar performance in micellization and emulsification as Triton does [28], and also endows anionic surfactant higher salt tolerance [29]. These findings with NCEOn remind us of the possibility to utilize NCEOn for soil washing.
Therefore, to verify the potential of NCEOn as the alternative of NPEO10 in soil remediation, in this research, a comprehensive comparison study will be conducted on performance of nonylcyclohexanol ethoxylates (NCEOn) on remediating diesel-contaminated soil, along with its complex system blended an anionic surfactant sodium dodecylbenzene sulfonate (SDBS) on diesel-contaminated soil, in the comparison of NPEO10, respectively.
Materials and Methods
2.1. Diesel, Soil, Seeds and Chemicals
Diesel (0# diesel) was purchased from China Petroleum & Chemical Co., Ltd., in Jiangsu, Wuxi, China. The topsoil samples for this study were collected from the campus of Jiangnan University (N31°48′, E120°28′) in Wuxi City, Jiangsu Province of China. Soil was collected at a depth of 10 – 20 cm to minimize the effect of fertilization. Soil samples collected were thoroughly mixed, and any remaining leaves, roots and other debris were taken off. The uncontaminated soil was naturally dried in a fume hood for 14 days to ensure a constant weight, followed by grinding and sieving through 20 mesh.
Nonylphenol polyethoxylate-10 (NPEO10, HLPC purity >90%) was provided by Taixing Lingfei Chemical Technology Co., Ltd., Taixing city, Jiangsu, China. Nonylcyclohexanol ethoxylate-9 (NCEO9, HPLC purity >90%) was prepared in our laboratory. The obtained NCEO9 and NPEO10 were purified to remove PEG and unreacted alcohols or phenols according to the literature [29] with modification. Red bean seeds, soybean seeds and green bean seeds were obtained from Hunan Shaoyang Qingchen Seed Industry Co., Ltd., China. Dichloromethane, methanol, ethyl acetate, cyclohexane, petroleum ether (BP: 60–90°C), sodium dodecylbenzene sulfonate were AR grade (Sinopharm Chemical Reagent Co., Ltd., CN) and used as received.
2.2. Features of the Soil
2.3. Determination of Surface/Interfacial Tension and Surface Activity Parameters of Surfactant Solutions
The surface/interfacial tension were recorded with drop volume and verified with DuNouy Ring method, and the surface activity parameters of the surfactant solutions were calculated as reported.
2.4. Preparation of Diesel-Contaminated Soil and Determination of Actual Contaminant Levels in Soil
Diesel-contaminated soil was prepared by mechanical mixing [13, 27, 30]. 90 g of dried soil was placed in a flask containing diesel (5g or 10g) dissolved in petroleum ether (90 mL), respectively. The dispersion system was mechanically stirred at 200 rpm for 3 h at room temperature. Afterwards, the mixture was placed on a plate and air-dried in a fume hood for 14 days for aging and constant weight [30], followed by passing through a 20-mesh sieve and then stored in a desiccator. Additionally, 10 g diesel and 90 g uncontaminated dry soil mixed with petroleum ether (90 mL) were exposed in hood for 14 days under the same conditions as a blank group, respectively.
To determine actual contaminant levels in the above contaminated soil, diesel was extracted from the contaminated soil using petroleum ether to quantify the absorbed diesel in the soil, according to the literature with modification. 1.0 g of the above air-dried diesel-contaminated soil mixed with 10 ml of petroleum ether was sonicated (KQ2200, Kunshan Ultrasonic Instrument Co., ltd, China) for 20 min, and then shaken at 200 rpm and 45°C for 4 h. The diesel-contaminated soil dispersion was centrifuged at 5000 rpm for 10 min, the supernatant was collected to test diesel content with UV-Vis spectrophotometer at 220 nm. Afterwards, the supernatant was spin-distilled to remove the solvent to obtain the extracted diesel mass for further verification of the UV detection results, which was repeated three times to take the average value. The actual contaminant contents of the two diesel-contaminated soil samples were 4.46 ± 0.26 wt% and 7.52 ± 0.40 wt% after calibration of the storage loss of the original soil and diesel, respectively. The low-boiling components of diesel also volatilized during air-drying (10 g of original diesel in the blank group lost 0.91 wt% during 14 days of air-drying), while the mass loss of uncontaminated soil was 0.06% of soil in addition to the ether.
2.5. Characterization of the Properties of Post-Elution Soil
The moisture and volatile contents of soils were measured using a thermogravimetric analyzer (TGA/1100SF, Mettler Toledo, CH). The soil sample was ground and passed through a 200-mesh sieve (less than 0.074 mm). 4.0 mg of soil was placed in an Al2O3 crucible and heated from 50°C to 600°C at a heating rate of 10°C/min in a nitrogen atmosphere (20 mL/min).
The mineralogical analysis of the soil was carried out using an X-ray diffractometer (D8, BRUKER AXS, Germany). 2.0 g of soil was placed in the center of the sample dish, and the surface was pressed flat. Test parameters: scan speed, 2°/min; scan range, 5°–65°; wavelength, 1.54 Å (Pb/K-α1); tube current, 30 mA; tube voltage, 40 kV; step size, 0.02°.
Zeta potential measurement was carried out with a Zeta potential and nanoparticle size analyzer (ZetaPALS, Nano Brook, USA). Soil colloidal particles smaller than 2 μm were extracted by precipitation. The zeta potential of the surfactant (1.0 wt%)-soil colloidal suspension was determined at different pH at 25°C.
The morphological characteristics of diesel-contaminated soils before and after elution were studied using a scanning electron microscope (S-4800, Hitachi, Japan) with a magnification of 10,000×. The elemental composition of surface of the soil samples at different stages was analyzed by energy dispersive X-ray spectroscopy (EDS).
2.6. Elution of Diesel-Contaminated Soil by Surfactant Solutions
The diesel-contaminated soil was extracted with surfactant solutions (soil:water =1:10, w/v, dry weight basis) with a shaker at 200 rpm and 25°C for 4 h. The suspensions were then centrifuged at 5000 rpm for 10 min, and then repeat the extraction for two more times. The sum of diesel extracted was quantified with UV analysis.
In order to optimize the remediation conditions for the contaminated soils, the influence of various factors on the elution efficiency (EE) of diesel was further investigated. The initial surfactant concentration was set at 0, 0.001, 0.01, 0.1, and 1 wt%, respectively. The suspensions were adjusted to different pH (4, 5.5, 7, 8.5, 10) with 1 M HCl or NaOH, and then shaken at different temperatures (25°C, 45°C, 60°C), respectively. Samples were taken at intervals (0.5 h, 1 h, 2 h, 6 h, 12 h, 24 h) to be centrifuged at 5000 rpm for 10 min. The diesel concentration in the supernatant was measured afterwards. The precipitated soil was air dried to constant weight in a fume hood, and the residual diesel was extracted with petroleum ether to verify the results of the UV analysis. Elution efficiency (EE) of the surfactants on diesel was calculated as Eq. (1):
where Ci (mg/Kg) and Cr (mg/Kg) is the initial and the remaining diesel concentration in the soil, respectively.
2.7. Seed Germination
Seed germination experiments were conducted on three common legume seeds (soybean, mung bean, and red bean) to examine the changes in their germination ability on diesel-contaminated soil before and after elution. Seeds were pre-soaked in deionized water for 12 h and then evenly distributed in Petri dishes containing 50 g of diesel-contaminated soil which was treated or untreated with surfactant solution, respectively. Each group of seeds was sprayed with 20 mL of deionized water and incubated in a thermotank at 20°C. An equal volume of deionized water was sprayed to the dishes at 12-hour intervals. The number of germinated seeds and the length of shoots were recorded regularly.
Results and Discussion
3.1. Elution on Diesel-Contaminated Soil with Polyoxyethylene Surfactants NPEO10 and NCEOn
In this experiment, first, the elution efficiency (EE) of diesel-contaminated soil in the presence of NCEOn (n=5, 7, 9, 11) and NPEO10 (1 wt% aqueous solutions) are presented in Fig. 1. The addition of surfactant significantly increases the elution of contaminants. The EE of the blank group (no surfactant in water) was 43.6%, whereas the lowest EE in the presence of surfactant was 80.85%. This phenomenon can be explained by the adsorption of surfactant molecules on the soil surface, which caused the desorption of diesel [19]. Additionally, micelles were formed in the solution when surfactant concentrations exceed their cmc. Diesel molecules can be encapsulated in the hydrophobic cavities of micelles, resulting in a greater solubility in water [33].
Fig. 1 also indicates that for the NCEOn series, there was a slight increase in EE with the adduction number of ethylene oxide until the adduction number reached 9. The rising EE of NCEOn on diesel is also consistent with the increasing cmc value of NCEOn. In addition, benzene moiety in NPEO10 provided the structure similarity to the aromatic hydrocarbons in diesel, consequently endowing NPEO10 higher intermiscibility to diesel. Subsequently, the effect of pH (Fig. 2a), diesel (Fig. 2b), surfactant concentration (Fig. 2c), and the solid-liquid ratio (Fig. 2d) on the EE were investigated.
Taking NCEO9 and NPEO10 for comparison, the EE reached 86.8% at pH 8.5, showing a significant increase of 21.9% compared to the efficiency at pH 4 (Fig. 2a). The main reason for this pH effect may be due to the water solubility of hydrocarbons would be higher in basic solution than in acidic solution. Another reason may be because as the pH increased, the acidic component of diesel reacted with inorganic alkali and generated surface-active substances, which facilitated the dispersion and dissolution of diesel in solution [34, 35]. In addition, the difference of elution efficiency between NCEO9 and NPEO10 is significantly higher at pH lower than 7, but not obvious at basic pH (Fig. 2a). This phenomenon might be caused by diesel elution favored to basic solution, because there was no significant difference in surfactant aggregation was observed in the surfactant solution and the dispersion of elution system. Consequently, pH 8.5 was chosen for the subsequent elution process.
The initial concentration of diesel (Ci) was positively correlated with the final EE (P < 0.01) in the presence of NCEO9 and NPEO10 (Fig. 2b). At Ci = 4.46%, EE of the blank group and the experimental group devoted to 32.1% and 80.5%, respectively, comparing to that of 43.6% and 86.8% at Ci = 7.52%. This result indicates that the nonionic NCEOn could deal with higher diesel concentration in contaminated soil.
Notably, the EE towards diesel is observed to increase as the surfactant concentration increased. This increase became more significant when the concentration exceeded its cmc which is higher than 0.0054% (Fig. 2c). Lower solid-liquid ratio (1:5) significantly bring down the diesel removal, which was attributed to the fact that soil solid particles could not be uniformly dispersed in less solution, and the contact surface between the soil and surfactant was reduced due to particle agglomeration. Diesel removal increased by 21.9% when the solid-liquid ratio was increased to 1:10, at which point the soil particles could be uniformly dispersed in the surfactant solution, and further increasing the solid-liquid ratio (1:20), the diesel on the soil surface tended to be transferred to the surfactant solution (Fig. 2d).
As aforementioned, except Triton X-100 [36], another polyoxyethylene surfactant Tween 80 [19] or mixed nonionic surfactants (Span20, Tween20, Tween80, Dehydol LS9) were often applied in soil remediation [2, 32]. It was reported that with 12 soils in China, the maximum sorption of Triton X-100 varied from 1.54 to 15.15 mg/g. The maximum EE of diesel for 12 soils ranged from 62.92 to 90.36%. And with silt-clay, 70% of petroleum hydrocarbons was eluted by 0.5 wt% Tween80 at a solid: liquid ratio of 1:5 in 24h [19]. With initial diesel content less than 2 wt% (12540±599 mg/kg) in soil, the optimum Tween80 dosage is 1.5% (v/v) for soils with 15% bentonite to give an EE of 78% (4000 mg/kg left-over) after first wash at a solid: liquid ratio of 1:10 in 24 h.
In summary, NCEOn (especially NCEO9) provide similar EE on diesel in soil as NPEO10 does regardless the change of elution pH, time, initial diesel content, solid-liquid ration and so on. The EE on diesel in soil of the NCEOn series elevated with the adduction number (n) in NCEOn until the n reached 9, which is also in line with the cmc values of NCEOn. In addition, NCEOn also provided comparable EE compared to the literatures.
3.2. Thermogravimetry Analysis on Diesel-Contaminated Soil Washed with Polyethoxylate Surfactant
The diesel-contaminated soil washed with 1 wt% of polyethoxylate surfactant were analyzed by TGA (Fig. 3). The mass loss in the range of 50°C–105°C could be attributed to evaporation of bound water and volatile components in diesel from the soil. The mass loss that occurred between 120–350°C might be caused by diesel evaporation, thermal oxidation and combustion of carbohydrates and fatty compounds. The weight loss occurring from 350°C to 650°C presents the decomposition of complex structural organic matter, such as aromatic compounds, polyphenols, and other substances [13].
For the uncontaminated soil in Fig. 3a, the final mass loss of uncontaminated soil (4.12%) consists mainly of bound water and organic matter, which was also consistent with the measurements in Table S1. Diesel-contaminated soils showed a significant mass loss in the boiling point interval (150°C–350°C) of diesel. Differences in mass loss of soils before and after washing (Fig. 3a, b) demonstrated the surfactant adsorption on soil, and indicated that NPEO10 adsorbed more to the soil. Differences in mass loss of diesel-soils before and after washing with NCEO9 and NPEO10 (Fig. 3b) further proved the surfactants were effective in removing diesel from contaminated soils. However, it can also result in the loss of nutrients from the soil and a subsequent reduction in soil fertility. Therefore, whether diesel-contaminated soil can be reused after elution needed to be further investigated.
3.3. XRD, Morphology and Surface Element Content of Diesel-Contaminated Soils before and after Elution
The XRD spectra of uncontaminated soil, diesel-contaminated soil, and NCEO9 washed diesel-contaminated soil were shown in Fig. 4. The major mineral in the soil is quartz and other minor mineral fractions include montmorillonite, hematite, silicon carbide, kaolinite, and calcium feldspar, which may contribute to the accumulation of petroleum hydrocarbons in the soil.
It can be seen that there is no crystallographic change in the XRD spectra of the soil before and after contamination and elution, indicating that the soil minerals are neither eroded nor mineralogically altered by the adsorption of diesel or surfactant [37].
The SEM images and EDS analysis were employed to further investigate the morphology of diesel-contaminated soils and the changes in surface element content before and after elution (Fig. S1).
As shown in Fig. S1a, the major elements of the uncontaminated soil were Si, Al, O, and C, indicating that it consisted of SiO2 and a small amount of silica-aluminum oxides [38]. Element C was derived from trace organic matter in the soil and from conductive adhesive. For the diesel-contaminated soil (Fig. S1b), the adsorption of alkanes on the soil surface resulted in a significant increase in the percentage of element C to 40.05%. On the other hand, Fig. S1b exhibited a smoother surface than the uncontaminated soil due to its surface being covered with diesel [39]. A significant decrease in the percentage of element C was observed on the surface of the contaminated soil washed with the surfactant solutions (Fig. S1c, Fig. S1d). The presence of unremoved diesel and the adsorbed polyoxyethylene surfactant on the soil surface resulted in a higher surface C content compared to the uncontaminated soil. The polyoxyethylene surfactant was adsorbed to the soil surface through hydrogen bonding and hydrophobic interactions [23, 40, 41].
In summary, although polyoxyethylene nonionic surfactants can effectively remediate diesel-contaminated soils, their adsorption on the soil surface might be prone to secondary contamination. Therefore, first, coupling elution with anionic surfactant will be investigated. Second and after all, the bean germination on the remediated soil will be conducted to verify if the secondary contamination would be harmful for plant.
3.4. Elution on Diesel-Contaminated Soil with Anionic-Nonionic Binary Surfactants
An interesting phenomenon was reported in Tween80 aided elution on diesel contaminated soil. That is, in all the washing tests, the diesel removal efficiencies with lower dose of Tween80 (0.1%) were lower than those in soil with water; based on the calculation method based on the weight loss assay, the author assumed that was because of Tween80 was adsorbed in soil [27]. This surfactant adsorption to soil is prone to secondary soil contamination, if they are difficult to degrade. Therefore, with the expectation of less surfactant adsorption, anionic surfactants with the same charge as the soil surface were proposed to be complexed with the nonionic [2].
Hence, in this experiment, incorporated with anionic surfactant SDBS (sodium dodecyl benzene sulfonate), a binary surfactant system (NCEO9-SDBS) is proposed to verify whether it can reduce surfactant adsorption on the soil surface while maintaining a high EE (Fig. S2). Either SDBS or SDS has been frequently used as model anionic surfactant in diesel elution; while diesel is commonly compromised of alkanes, cycloalkanes and aromatic hydrocarbons. Herein, choosing SDBS instead of SDS is for possible better elution on diesel. For diesel elution, considering the structure similarity of eluent to diesel, SDBS is closer to diesel than SDS.
It was evident that all individual surfactants and combination of SDBS-NCEO9/NPEO10 exhibited similar EE for diesel-contaminated soils. A higher EE was obtained by using NPEO10 in either single surfactant or binary surfactant compared to NCEO9; a very similar phenomenon was observed with SDS and Tween80 [19]. The removal of petroleum hydrocarbons from contaminated soil by SDS and Tween80 reached as high as 79.8% and 73.9%, respectively. The removal efficiency decreased from 75.5% to 68.3% when the mass ratio of SDS and Tween80 was increased from 1:4 to 4:1.
The relationship between the EE and binary-surfactants concentration may be related with their interfacial properties. As shown in Fig. S3 and Table S2, the anionic-nonionic surfactant binary system exhibited surface tension and cmc are similar to those of the nonionic surfactants. Theoretically, the micelles number available to solubilize diesel does not change remarkably under the same surfactant concentration, resulting in a nearly constant EE on diesel. In addition, the interfacial tension (IFT) between diesel and 1 wt% surfactant solution has been measured with a Spinning Drop Video Tensiometer (SVT 20N, Data Physics, Germany) at 25°C. As shown in Fig. S3, compared to the NCEO9-diesel system, NPEO10 exhibited lower interfacial tension with diesel, which explained the higher efficiency of the NPEO10 solution in clearing diesel-contaminated soil. The oil-water interfacial tensions of the two binary surfactant/diesel systems were nearly equal, but NPEO10-SDBS had a lower cmc, resulting in more micelles with same mass of surfactant.
Similarly, the effect of elution conditions on the EE of binary systems was further implemented with SDBS-NCEO9/NPEO10 systems (1:1 in weight) (Fig. 5). It was clear that the effects of pH, surfactant concentration and pollutant concentration on the diesel removal efficiency of the binary system were consistent with that of single surfactant. It has been shown that the binary system can also provide a high efficiency in removing diesel while using the same amount of surfactant. Therefore, its adsorption on the soil surface was further investigated by TGA.
3.5. Surfactant Absorption in the Eluted Soil
It was reported that Tween80 can be highly adsorbed in soils [42]. Due to the difficulty of quantitative determination of the surfactants in contaminated soil, TGA analysis has been used to estimate the surfactant adsorption [43, 44]. Herein, TGA curves of uncontaminated and contaminated soils washed by anionic-nonionic surfactants are illustrated in Fig. 6. The influence of the surfactant mixture on adsorption and desorption of the surfactants can be revealed from the deference of thermogravimetry between single polyethoxylate (Fig. 3) and SDBS-polyethoxylate (Fig. 6) processed on the uncontaminated and contaminated soils, and the difference in the zeta-potential profiles (Fig. S4).
The results agree with what we proposed, the lines (Fig. 3, Fig. 6) above the line of uncontaminated soil (the dash line) vividly indicate the reduced surfactant absorption comparing to that in individual surfactant-elution; since the mass loss of diesel-contaminated soil eluted by NCEO9-SDBS (4.19%, Fig. 6a) and NPEO10-SDBS (4.86%, Fig. 6a) was lower than that eluted by NCEO9 (6.25%, Fig. 3b) and NPEO10 (7.16%, Fig. 3b). The same phenomenon was observed in the TGA curves of uncontaminated soil samples before and after surfactant washing (Fig. 3a, Fig. 6b). Lower mass loss indicates that less of the surfactant was adsorbed to the soil. So TGA profiles confirmed the expectation that the mixed surfactant can reduce the surfactant adsorption to soil, which might be because that the binary surfactant micelles exhibit negative charges same as that of the soil surface. The stronger electrostatic repulsion compared to the nonionic micelles makes less surfactant to be adsorbed onto the soil surface, which can be further demonstrated with Fig. S4.
The zeta potential of the soil grains in micellar solution was measured to further understand the desorption of diesel from the soils, as well as the adsorption mechanism of surfactant on the soil surface (Fig. S4). The soil grains with anionic surfactant SDBS showed higher absolute values of the zeta potential compared to that in NCEO9/NPEO10 system. There are multiple cationic sites in soil (CEC= 6.95 ± 0.68 cmol kg−1), and the negative charge on soil increased with increasing pH. During the cleaning process of diesel-contaminated soil, the diesel was encapsulated in the hydrophobic nuclei of the micelle molecules, which increased the solubility of the diesel. At the same time, the high pH led to a high negative charge on the soil surface, which increased the repulsive force between the micelles and the soil particles, and the diesel adsorbed on the soil surface can be easily washed away [18].
3.6. Kinetics of Diesel Removal
The time course of diesel removal is shown in Fig. 7. The initial concentration of diesel was 7.52 ± 0.20 wt%. In the kinetic study, diesel was rapidly desorbed from the soil during the first 30 minutes of the elution process and essentially equilibrated within 2h.
The effectiveness of different kinetic equations in fitting the desorption process of different soil pollutants varies widely. In this work, the first-order kinetic equation (Eq. (2)), Elovich equation (Eq. (3)), and parabolic diffusion equation (Eq. (4)) were selected to verify the process of diesel desorption from soil by surfactants.
The first-order kinetic equation is based on the assumption that the desorption process is controlled by a diffusion step, modeled as Eq. (2) [45]:
where S is the amount (mg/g) of pollutant desorbed at time t.
Elovich equation has been widely used for describing soil adsorption/desorption kinetics in non-homogeneous diffusion processes, modeled as Eq. (3) [46]:
The parabolic equation is often used to describe the diffusion process in which ions tend to decrease after desorption on the particle surface reaches an extreme value, modeled as Eq. (4) [47]:
Fitted curves and eigenvalues of kinetic equations for desorption of diesel using NCEO9 were presented in Table S3, respectively. The Elovich equation showed a better fitting correlation with correlation coefficients ranging from 0.94 to 0.98. This suggests that diesel desorption is a non-homogeneous diffusion process. For the parabolic equation (R2 = 0.80–0.87), it can also be used to characterize the desorption process of diesel by surfactants, indicating that the desorption-diffusion process of diesel in soil was mainly influenced by intraparticle diffusion processes. The first-order kinetic equation was a poor fit, with a correlation coefficient ranging from only 0.49 to 0.69. This can be attributed to the fact that certain petroleum pollutants were easily desorbed during the desorption process. However, the remaining pollutants proved to be more challenging to desorb, leading to a gradual decrease in the desorption rate. Consequently, this behavior was not accurately modeled by the first-order kinetic equations.
3.7. Seed Germination in Diesel-Soil before and after Remediation with NCEO9
Bean plants have been used as indicator organisms for environmental risk assessment and phytotoxicity experiments to predict the potential toxic effects of contaminants [32]. The classic Triton has been used for many years in huge scale including application in soil remediation, no harmful effect on seed germination or plant growth was reported for Triton; the concern on Triton toxicity is not about its effect on plant but on human procreation. But NCEO9 is new, it has to be verified if NCEO9 could provide remediation on the contaminated soil by seed germination or plant growth. It was reported that Tween-20, NP-9.5, and Triton X-100 at 5% (v/v) dosage was applied for removing diesel (0.8 mg/g soil initially) from soil [48], where NP-9.5 was found to be the most efficient surfactant in improvement of geotechnical properties. Hence given that fact that the NCEO9-SDBS did not performed much better than NCEO9 did but remained less on the soil, we targeted the seed germination in diesel-soil before and after remediation with NCEO9, using soybean, mung bean and red bean.
The impact of diesel-contaminated soil on seed germination was evaluated before and after elution (Fig. 8). In control groups (diesel contaminated soil), seed germination of soybean, mung bean and red bean were 27.78%, 66.67%, and 0 respectively, indicating that germination of the three seeds was significantly inhibited in the contaminated soil; and red bean is a so sensitive specie for diesel contaminated soil. After treatment with NCEO9 solution, the germination rate was increased to 96.67%, 88.89%, and 96.67%, respectively. Despite the negative effects of nutrient loss and surfactant adsorption in the soil on seed germination [49, 50], the removal of diesel fuel essentially restored the planting capacity of the soil.
Furthermore, the sprouted seeds also showed large differences in bud length after 7 and 14 days of growth, respectively (Fig. S5). Plant growth was inhibited in the contaminated soil, whereas a noticeable improvement in growth rate was observed in the surfactant-washed soil. Therefore, remediating diesel contaminated soil with NCEO9 could be an alternative of Triton for green and efficient remediation of oil-contaminated soils.
Conclusions
A comprehensive and comparative study was conducted to reveal the insight performance of polyethoxylate surfactant NCEOn and NPEOn on remediating diesel-contaminated soil, and hence to verify if NCEOn could be an alternative of NPEOn in this aspect.
It is found that the soil washing with NCEOn can effectively remove diesel from contaminated soils with a removal efficiency up to 86.8% with surfactant concentration 1 wt%, pH 8.5, 25°C in 24 h towards an initial diesel concentration of 7.52 ± 0.20 wt%. The result is comparable to the EE of NPEO10 (89.6%) under the same conditions.
Since NCEO9/NPEO10 might be adsorbed to the soil surface during cleaning and poses a risk of secondary contamination, according to SEM-EDS and TGA results. Comparative study was carried out with the washing with NCEO9-SDBS system, which maintains a high EE up to 86.6% while reducing surfactant adsorption on the soil surface by increasing the electrostatic repulsion between micelles and the soil surface. The desorption kinetics of diesel in contaminated soil can be described using Elovich equation with correlation coefficients ranging from 0.95 to 0.98, indicating the desorption of diesel from soil promoted by surfactants is primarily a non-homogeneous diffusion process. Moreover, diesel-contaminated soils resulted in 27.78%, 66.67%, and 0 of germination and growth inhibition on soybean, mung bean, and red bean, respectively. In contrast, the germination of the NCEO9-eluted contaminated soil on the three seeds was increased to 96.67%, 88.89%, and 96.67%, respectively, and the growth of the plants was significantly enhanced. Overall, the use of more environmentally friendly NCEO9 instead of NPEO10 in diesel soil remediation has been shown to be feasible and of great practical importance.
PDF Links
PubReader
Full text via DOI
Download Citation 
Supplement
Print 
