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Environ Eng Res > Volume 28(2); 2023 > Article
Offiong, Udo, Inam, Ekanem, Awaka-ama, Uwanta, and Dong: Screening of bio-derived surfactants for soil washing of PAHs: effects of substrate sources and trace metals distribution

Abstract

In this study, we screened the performance of aqueous extracts of Talinum triangulare (AET) and soap solution prepared from Hura crepitans seed oil saponified by aqueous solution of ashed plantain peels (HCS) for removal of naphthalene and phenanthrene from soil. The distribution of trace metals in the soil before and after soil washing was also investigated. The results revealed maximum removal efficiencies after 25 min washing time of 96.25, 96.14, and 25.70% naphthalene, for Tween 80, AET and HCS, respectively. While for phenanthrene, the recorded maximum removal efficiencies after 25 min washing time were: 91.80, 26.00, and 94.0 for Tween 80, AET and HCS, respectively. Based on results from other experiments, AET generally showed slightly lower removal efficiencies compared to the commercial Tween 80 surfactant. Also, the results revealed that the three remediants affected the distribution of trace metals (Cd, Pb, and Zn) in the soil after surfactant washing. Based on the amount of trace metals extracted, the performance of the three remediants are as follows: AET > Tween 80 > HCS. The performance of AET has been attributed to the presence of multiple heteroatomic moieties for trace metals adsorption and moderation of the acidic conditions of the soil.

1. Introduction

Given their persistence, mutagenic and carcinogenic potentials, polycyclic aromatic hydrocarbons (PAHs) contamination are regulated in many countries. Polluted soils such as oil spill sites are often required to be monitored for PAHs during clean-up and remediation exercises. Consequently, many attempts have been launched to remediate PAHs in crude oil contaminated soils. However, most of these attempts are largely impracticable, unsustainable or too expensive to be adopted in low economy countries [1, 2]. The new paradigm is towards the valorisation and utilisation of sustainable materials for cost-effective remediation [3].
Surfactant enhanced soil washing of PAH-contaminated soils have been widely investigated and many reviews published [411]. Due to environmental concerns about synthetic surfactants, researchers are now more focused on naturally derived biodegradable surfactants from plant sources [1215]. Some reports of the application of plant-based surfactants include: solubilization and desorption of hexachlorobenzene (HCB) [16], and soil washing of naphthalene, phenanthrene as well as other PAHs [1719]. In spite of the progress recorded, it is desirable to optimize conditions in order to achieve better efficiency. Some challenges in the selection of surfactants for soil washing protocol include coverage of multi-contaminants systems [19], overcoming negative interactions with soil washing agents (e.g., viscosity and clogging) and common ions effect from the environmental matrix [2022]. Also, given the type of soil and target contaminants, a choice between non-ionic, cationic, anionic, gemini or cosolvent mixtures of surfactants may be required [11, 2329]. In less developed countries, technical setbacks may not allow smooth applications of many of these methods. In the present study, our objective was to valorise wastes from common food and agricultural substrates (waterleaf and plantain peels) into surfactant products (aqueous extracts of waterleaf and saponified oil from Hura crepitans seeds using aqueous solutions of ashed plantain peels).
Plant-sourced precursors such as saponins and oleochemicals have been established to be useful as natural surfactants for soil remediation [18, 19, 30, 31]. Hura crepitans, otherwise known as the sandbox tree, is a tropical tree with many pharmaceutical and ethnobotanical applications [3234]. The seeds of Hura crepitans have oil content within the range of 36–72% wt% [33]. However, the oil is non-edible [35]. As a consequence, the oils have been utilized for several other applications such as preparation of alkyd resins, methyl esters, biodiesel and surface-active materials such as soaps [33, 35, 36]. On the other hand, Talinum triangulare (common name: water leaf) is a tropical perennial herb with high surface-active saponin content [37, 38]. Aqueous extract of Talinum triangulare has been previously screened as candidate for soil washing of crude oil contaminated ultisol [39]. In the present study, we evaluated different substrate sources as plant-derived surfactants for soil washing of organic pollutants and examined the distribution of untargeted trace metals in such systems.

2. Materials and Methods

2.1. Preparation of Bio-derived Surfactants

Methods for preparation of aqueous extracts of Talinum triangulare have been previously reported [39]. Matured dry fruits of Hura crepitans were harvested at Uyo Metropolis, Akwa Ibom State, Nigeria. The dry fruits were mechanically broken to remove the seeds. The seeds were air-dried and ground using a manual grinder. The Hura crepitans seed oil was extracted from the ground seeds through solvent extraction using petroleum ether as solvent at a temperature range of 60 to 80°C. Exactly, 500 g of the sample was soaked in petroleum ether, squashed in a borosilicate container, and filtered in order to separate the petroleum ether fraction. The filtrate was kept overnight in a fume cupboard, the settled debris at the bottom was removed by decantation. The filtrate was kept in an oven at the temperature of 70°C to get rid of petroleum ether leaving behind yellow-coloured oil with a pH of 5. Plantain peels were locally sourced and washed with running water to remove dirt, sun-dried for three (3) weeks and burnt to ash using a muffle furnace. The ash formed was cooled and stored in a sealed plastic container. About 500 g of the ash was added to a clean plastic container and 1,000 mL of deionised water added, stirred for 30 min and kept overnight. The resultant solution was filtered using Pall water filtration apparatus with 47 mm, 5-micron glass micro-filter membrane into a clean bottle using a vacuum pump. A pale brown filtrate with pH of 13 was collected and stored in a 250 mL volumetric flask. About 250 mL of the ash solution was added to a stainless-steel pan and boiled to 100°C and then 62 g of the Hura Crepitans seed oil was added and stirred thoroughly. The mixture foamed signifying saponification; the temperature was reduced to 60°C to control the foaming. Stirring continued until the foaming reduced leaving behind a grey soapy viscous liquid.

2.2. Laboratory Soil Washing Experiments

The soil washing experiments were done according to modified methods from the literature [4, 23, 40]. Soil samples were collected from an urban agricultural soil in Uyo, Nigeria. The air-dried homogenized soil samples (approx. 20 g) were added into 30-mL borosilicate round-bottom glass centrifuge tubes. Naphthalene and phenanthrene (AR, 99.9%) were purchased from British Drug House (now Merck, UK), while Tween 80 (extra pure grade) was acquired from Loba Chemie PVT. Ltd (Mumbai, India). Stock solutions of PAHs (naphthalene and phenanthrene) were prepared in volumetric flasks by dissolving appropriate amounts in methanol and stirred using a magnetic stirrer. A measured volume (15 mL) of the PAH stock solution was then added into the centrifuge tubes containing the thoroughly mixed soil samples such that the medium would be saturated. The spiking of the stock solution (concentrations: 0, 25, 50, 75, 100, and 125 mg/L) was done such that on evaporation, it would give a theoretical concentration in g/Kg of soil (0, 0.5, 1.0, 1.5, 2.0, 2.5 mg PAH/g soil). The mixture was then kept for 72 h to allow complete evaporation of methanol. Exactly, 20 mL of surfactant solutions prepared at appropriate concentrations were then added to the soil mixture. For aqueous extracts of waterleaf, a volume-by-volume aqueous concentrations were used (10, 20, 30, 40, and 50% vol/vol). Surfactant solution from saponified Hura Crepitans seed oil prepared using deionised water at different concentrations (5, 10, 15, 20, 30 mg/L). To compare the performance of bio-derived surfactants with a commercial surfactant, Tween 80 was used. Different concentrations (10, 20, 30, 40, 50 mg/L) of Tween 80 was prepared with deionised water from around its critical micellar concentration (CMC) which has been previously determined to be 13.10 mg/L according to Zhang and Zhu [41]. The amounts of soil and the surfactants were chosen so that the glass tubes would be filled up, and then sealed with Teflon-lined screw caps to avoid any loss. The mixtures were then agitated for required period in a thermostated shaker bath maintained at a speed of 120 rpm at room temperature (25°C). Thereafter, the samples were centrifuged at 12,000 g for 15 min. Aliquots (0.5 mL) of the supernatant were withdrawn using syringes, dissolved in 1.5 mL methanol, filtered through 0.22 μm PTFE membrane filters and placed in 1.5 mL vials for subsequent analysis.

2.3. Analysis of PAHs

The concentration of naphthalene and phenanthrene were determined using high performance liquid chromatograph (HPLC) (Shimadzu LC-20AT model) with an ultraviolet (UV) detector and a 5 μm 4.6 × 250 mm Agilent C18 reversed phase column. The following conditions were applied: flow rate of 1.0 mL/min, injection volume of 15 μL, under isocratic flow condition with acetonitrile and water (80:20) as mobile phase. The temperature was maintained at 30°C. The UV wavelength was fixed at 254 nm. The HPLC-UV method was a modification of a suitable method in the literature [42]. Blanks were prepared for all experiments. All aqueous solutions were prepared using deionised water. Experiments were carried out in triplicates.
The HPLC-UV analysis of the standards, the retention times recorded were 4.8 min and 8.2 min for naphthalene and phenanthrene, respectively (Fig. 1). The R2 from the standard calibration curves for both naphthalene and phenanthrene were above 0.99 (Fig. S1).

2.4. Soil Washing Efficiency

The efficiency of the removal of PAHs (R) was estimated by determining the ratio of the concentration of PAHs in the supernatant solution (C) and the initial concentration of PAHs in the soil (Ci) according to Eq. (1) [43]:
(1)
R=CCi×100

2.5. Characterization of Bio-derived Surfactants

The prepared surfactant solutions without further purification except filtration, were characterised for functional group determination using Fourier-transform infrared (FTIR) spectroscopy analysis. The FTIR spectra were recorded using Thermo Scientific Nicolet iS5 spectrometer (Thermo Scientific, Waltham, USA).

2.6. Physicochemical and Trace Metals Analysis

Physicochemical parameters of soils and trace metals analyses were carried out using standard analytical protocols according to the American Public Health Association [44]. Physicochemical parameters determined include pH, electrical conductivity, total organic carbon (TOC), cation exchange capacity (CEC), and particle size. Trace metals (Cd, Pb, Zn) were determined after nitric/hydrochloric acid digestion using an atomic absorption spectrometer (AA500 Atomic Absorption, PG Instruments Ltd, USA). The trace metals standards were purchased from Accustandard (New Haven, USA). The R2 for the standard calibration curves of the trace metals were greater than or equal to 0.995.

3. Results and Discussion

3.1. Physicochemical Properties of Soil

The Physicochemical characteristics of the soil used for the present study are presented in Table 1. As expected, the soil is generally classed as ultisols which are characterised by low nutrient and common in tropical regions in both Nigeria and China [3, 45, 46]. The soil belongs to the sandy textural class.

3.2. Functional Group Characterization of Bio-derived Surfactants

Functional group characterization of the prepared bio-derived surfactants is essential to provide insight into their possible chemical properties and behavior. The FTIR spectra of the aqueous extract of Talinum triangulare and saponified Hura crepitans seed oil are presented in Fig. 2. For the aqueous extract of Talinum triangulare (AET), the following key absorption peaks recorded were: 3362.1 cm−1 (O-H bond from alcohols), 1,621.4 cm−1 (C=C from alkenes), 1438.8 cm−1 (C=C from aromatic compounds), 1,304.6 cm−1 (O=C-O-C from aromatic esters), 1084.7 cm−1 (C-O from alcohols or esters), 1,036.2 cm−1 (=C-O-C from aromatic ethers), 898.3 cm−1 (C-H from geminal disubstituted alkenes), and 790.2 cm−1 (C-H from trisubstituted alkenes). On the other hand, the soap prepared from saponification of oil from seeds of Hura crepitans (HCS) recorded the following major absorption peaks: 3,473.9 cm−1 (N-H from amides), 3,011.7 cm−1 (C-H from aromatic compounds), 2,922.2 cm−1, 2,855.1 cm-1, 1,461.1 cm−1, 1,379.1 cm−1, 723.1 cm−1 (C-H from alkanes), 1,744.4 cm−1 (C=O from esters or ketones), 1,237.5 cm−1 (=C-O-C from aromatic ethers), 1,159.2 cm−1 (C-O-C from ethers), 1,095.8 cm−1 (C-O from ethers or alcohols) and 913.2 cm−1 (=C-H from alkenes). Generally, based on the FTIR spectra of the two surfactants, AET recorded more peaks assigned to heteroatomic functional groups than HCS. HCS contained more saturated functional moieties than AET. One major explanation for the differences is that AET is aqueous-based, while HCS is oil-based. Surfactants with multiple heteroatom functionalities usually show superior surface activity [4750]. In addition, previous reports recorded similar absorption peaks for aqueous extracts of Talinum triangulare [14, 51].

3.3. Surfactant Soil Washing

The effect of washing time was determined and the results are presented in Fig. 3. Generally, removal efficiencies tend to increase with increase in washing time. HCS recorded very low removal efficiencies (< 30%), while AET and Tween 80 recorded approximately 70–96% (Fig. 3). For HCS, as it can be observed in Fig. 3, there was no clear difference between the removal efficiencies of naphthalene and phenanthrene. However, AET tends to remove phenanthrene faster, although naphthalene recorded higher removal efficiency. Given the plateau nature of the graph for Tween 80 data, it seems that equilibrium was achieved earlier due to the fact that naturally derived surfactants possess lower CMCs compared to synthetic surfactants [52]. On the other hand, little changes are observed for the removal efficiencies of HCS over time. This implies that the performance of HCS is not influenced by time. As observed in a previous study, the efficiency of removing contaminants by soil washing increases with increase in washing time, but is later stabilized [7]. According to the study, the time-dependent relationship is caused by rate-limited dissolution and desorption of contaminants.
In an actual contaminated site, it may be necessary to determine the amount of remediant materials required for a certain level of contamination. We measured the removal efficiencies of different concentrations of naphthalene and phenanthrene in soils and the results are presented in Fig. 4. Generally, more contaminants led to higher removal efficiencies. Although beyond 100 mg/Kg the graphs flattened, the result is unusual since the same concentration was used. As explained in a previous study, certain amounts of contaminants could partition into the soil matrix, such that the amount removed depends on the partitioning of the overall concentration of the contaminant at any given time [22]. More so, the present soil used in this study recorded TOC > 0.30 (Table 1). Usually, for soils with organic content > 0.1, PAHs get partitioned into the soil organic matrix [53]. This observation was pronounced in AET and Tween 80 which showed high reactivity during the study. Therefore, to ensure complete removal, it is recommended to carry out soil washing more than once [18]. Mostly, naphthalene, which is of a lower molecular weight, was removed with higher removal efficiencies compared to phenanthrene. This may be due to easier diffusion. The adsorption data was fitted with first and second order pseudo-kinetic models and the results are presented in Fig. S2. From the results, we observed that the pseudo second order show better fitting with R2 > 0.98 for both naphthalene and phenanthrene in all applied surface-active materials. This implies that PAHs-soil-surfactant interactions were beyond physisorption and could be better classified as chemisorption process.
Different surfactant concentrations were used to test the removal efficiencies after 15 min washing time. We observed that the removal efficiencies increased with surfactant concentrations (Fig. 5). In this case, phenanthrene recorded higher removal efficiencies compared to naphthalene. One explanation to this observation could be that phenanthrene with higher octanol-water partition coefficient (log Kow = 4.46) than that of naphthalene (log Kow 3.36) will likely dissolve more in organic phase of the surfactant solutions and gets easily dissolved while some part of naphthalene remains in the aqueous phase [54]. Another explanation is that appreciable amounts of naphthalene could have been lost by evaporation since it is lighter than phenanthrene such that the amounts available for removal was less [55]. Again, higher removal efficiencies were recorded by Tween 80, followed by AET, and lastly HCS. The results demonstrate the superior performance of AET which has been characterized to contain saponins. This implies that saponin-based surfactants may be more suitable for removal of PAHs from contaminated soils compared to oil-based soaps. In a previous study, saponin-based surfactant has been shown to compete favorably with Tween 80 in soil washing of phenanthrene [18]. Other researchers have reported the use of oil alone from plant substrates to achieve removal efficiencies above 80% [53, 56]. When soybean oil was used for removal of anthracene from contaminated soil, it was reported that removal efficiencies decreased with increase in initial anthracene concentration [57]. It seems therefore, that preparation of the soap further introduces additional materials that may lead to high viscosity of the materials and consequent clogging. Furthermore, previous studies have established that light permeability, hydrogen donor capability of soil washing agents significantly influenced their performance for removal of organic pollutants from soils [58]. In this study, AET is more transparent than HCS, which may permit photo-induced attenuation of the contaminants.

3.4. Trace Metals Distribution

The trace metals characteristics of the soil before and after surfactant washing are presented in Table 2. The choice of the three trace metals was based on their usual occurrence in urban agricultural soils. With the exception of zinc, it can be observed from the results that cadmium and lead exceeded the World Health Organization (WHO) allowable levels in agricultural soils (Table 2). Since it is an urban area, the source of contamination may be attributed to anthropogenic activities such as transport-related emissions and atmospheric deposition from industrial processes [5961]. The soil washing experiments significantly affected the distribution of trace metals levels. A cursory visualization of the data reveals that after soil washing, the cadmium levels significantly reduced in all treatments. In the case of lead (Pb) and zinc levels, only treatments with AET and Tween 80 recorded significantly lower values. Generally, the levels of trace metals were lowest in soils treated with AET, followed by Tween 80 and HCS. Tween 80 is a hydrophilic nonionic surfactant that has been used mostly for remediation of organic pollutants [62]. The nonionic nature of Tween 80 may have limited its reactivity with the negatively charged soil matrix [63]. As such, the acidic nature of the soil (Table 1) may not have changed significantly which allowed a significant amount of the trace metals to remain mobile in the soil. On the other hand, AET with a lot of heteroatomic moieties as established from the FTIR spectra (Fig. 2) provides a lot of adsorption sites for metal adsorption and subsequent extraction [64]. Also, it has been established from previous research that AET demonstrates acidic soil pH moderation which may result in immobilization of the trace metals [39]. For HCS, clogging of the soil matrix and weak reactivity may have accounted for the low metal extraction.

4. Conclusions

The removal of trace metals and PAHs from soil using aqueous extracts of Talinum triangulare (AET) and soap solution prepared from Hura crepitans seed oil saponified by aqueous solution of ashed plantain peels (HCS) was investigated. Based on the results obtained, AET recorded removal efficiencies for naphthalene and phenanthrene slightly comparable with commercial Tween 80. Clogging of soil matrix and limited heteroatomic moieties in HCS may have resulted in poor removal efficiencies for naphthalene and phenanthrene. In most cases, the lower molecular weight naphthalene tends to record more removal efficiencies compared to phenanthrene. Similarly, AET recorded better removal of trace metals due to its soil pH moderation ability and availability of more adsorption sites.

Supplementary Information

Acknowledgments

The first author (N.O.O.) acknowledges the Petroleum Technology Development Fund (PTDF) of Nigeria for foreign scholarship award (Award Ref: PTDF/ED/PHD/NPO/5/18).
This project was financially supported by the National Key R&D Program of China (No. 2020YFC1808202) and National Natural Science Foundation of China (Grant No. 42077167).

Notes

Conflict of interests

The authors declare no conflict of interests.

Author Contributions

N.O.O. (Ph.D) visualised and carried out the experiments, literature search, data curation, analysis, interpretation, and wrote the original draft. G.J.U. (Ph.D) carried out some analysis, carried out data interpretation, and contributed to the final manuscript. E.J.I. (Associate Professor) provided resources for the experiments and contributed to the final manuscript. A.N.E. (Ph.D) provided resources for the experiments and contributed to the final manuscript. J.J.A. (Ph.D) provided resources for the experiments and contributed to the final manuscript. E.J.U. (Ph.D) provided resources for the experiments and contributed to the final manuscript. J.D. (Professor) conceptualised and supervised the project, acquired funding, provided resources for the experiments and contributed to the final manuscript. All authors read and approved the final version.

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Fig. 1
HPLC-UV chromatograms for analysed standards of phenanthrene and naphthalene.
/upload/thumbnails/eer-2021-502f1.gif
Fig. 2
FTIR spectra of (a) aqueous extract of Talinum triangulare and (b) saponified Hura crepitans seed oil.
/upload/thumbnails/eer-2021-502f2.gif
Fig. 3
Effect of washing time on removal efficiencies of naphthalene (Naph) and phenanthrene (Phen) using different surface-active agents. Experimental conditions: 100 mg/Kg of contaminants, 30 mg/L of both HCS and Tween 80 and 50% (vol/vol) of AET.
/upload/thumbnails/eer-2021-502f3.gif
Fig. 4
Effect of contaminant dose on removal efficiencies of naphthalene (Naph) and phenanthrene (Phen) using different surface-active agents. Experimental conditions: washing time of 25 min, 30 mg/L of both HCS and Tween 80 and 50 % (vol/vol) of AET.
/upload/thumbnails/eer-2021-502f4.gif
Fig. 5
Effect of surfactant concentration on removal efficiencies of naphthalene (Naph) and phenanthrene (Phen) using different surface-active agents. Experimental conditions: washing time of 15 min, 30 mg/L of both HCS and Tween 80 and 50 % (vol/vol) of AET.
/upload/thumbnails/eer-2021-502f5.gif
Table 1
Physicochemical Properties of Soil
Parameter Values recorded
pH 6.36 ± 3.80
Total organic carbon (TOC) (%) 0.38 ± 0.02
Cation exchange capacity (CEC, cmol/kg) 14.70 ± 2.40
Particle size
Sand (%) 91.10
Silt (%) 1.70
Clay (%) 7.20
Table 2
Trace Metal Levels in Soil before and after Washing Treatments
Trace metals Analytical method Blank Talinum triangulare extract (AET) Hura crepitans soap (HCS) Tween 80 WHO standard*
Cadmium (mg/Kg) APHA 3111 2.19 ± 0.10a 0.20 ± 0.10b 0.74 ± 0.10b 1.59 ± 0.10c 0.003
Lead (mg/Kg) APHA 3111 3.68 ± 0.15a 0.83 ± 0.00b 2.76 ± 0.12a 1.57 ± 0.05c 0.1
Zinc (mg/Kg) APHA 3111 71.11 ± 2.50a 12.80 ± 1.50b 74.14 ± 3.20a 51.85 ± 1.50c 300

* Source: Kinuthia et al. [65]; values with the same letter are not significantly different (p > 0.05); values with different letters are significantly different (p < 0.05)

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