Properties of surfactants on high salt-affected sandy land in enhanced sand fixation: salt tolerance, adsorption isotherms and ecological effect

Abstract

In the present paper surfactant, for the first time, was used to improve the sand fixing ability of emulsion in high salt-affected sandy land. This study started from the analysis of the main components of sand particles from Golmud sandy land, Qinghai province, China, by X-ray diffraction (XRD). Then, two surfactants, sodium dodecyl sulfate (SDS, an anionic surfactant) and Pluronic L35 (L35, a nonionic surfactant), were selected and used to conduct a salt tolerance test before their reaction with sand from a salty desert. The water solubilization method and Fourier transform infrared (FTIR) spectroscopy have been adopted to investigate the salt tolerance of surfactants and their reaction with sand particles, respectively. The adsorption and adsorption characteristics of two surfactants at varied salinities by sand particles have been considered because of its significance for the interaction. In addition, the influence of SDS and L35 on the growth of Escherichia coli (E. coli) and sand microbes were evaluated to understand their ecological effect. The experimental results showed that SDS and L35 can visibly enhance the sand-fixing ability of the emulsion in high salt-affected sandy land. The related mechanism is that, first, SDS and L35 could improve the stability of the emulsion against the salt if the preparation of emulsion uses SDS and L35 as the emulsifying agents; second, SDS and L35 could change the morphology of salt in sand (this experimental result we will report in another paper). The investigation into the influence of SDS and L35 on the growth of E. coli and sand microbes also showed a dependable ecological effect.

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1 Introduction

With the increasing use of low quality water and conventional agriculture practice, soil salinization, poor nutrient availability and desertization are some of the most serious environmental and worldwide socioeconomic problems, and are considered as the major threat to the sustainability of agriculture and economic development.^1–4 It is well known that salinity limits soil fertility because of the accumulation of excess Na⁺ in soil, resulting in the destruction of physical and chemical properties of soil, such as a deficiency in the essential elements nitrogen (N), phosphorus (P), and potassium (K).⁵ The excessively exchangeable sodium leads to the swelling and dispersion of clays as well as the slaking of soil aggregates through the decrease of soil permeability, available water capacity and infiltration rate.⁶ In addition, in the arid zones, intense evaporation tends to accumulate salts in the upper soil profile, especially when it is associated with insufficient leaching or where soluble salts move upward in the soil profile from a water table instead of downward.⁷ Desertization in salt-affected soils has been found in more than 100 countries of the world.^8–17 According to Tòth et al., the total area of salt-affected soil was about one billion hectares, occurring mainly in the arid and semiarid regions of Asia, Australia, Africa and South America.⁹ In China, there were over 99.13 million hectares of salt-affected soils suffering desertization.^10,15

For many years humans around the world have been enforced to consider the control of desertified land caused by salinization. Many scientists have conducted a lot of research to develop a series of measures to reduce or prevent the desertization of salt-affected soil.^18–22 Although numerous financial resources have been used in vegetation restoration and protection of bare saline soil,^23,24 there were lots of limitations in terms of cost, effectiveness and ratio of seed germination from vegetation restoration alone due to adverse wind erosion and poor nutrient availability in high salt-affected sandy land. So, sand fixation under high salt stress was regarded as the key step for saline soil restoration.

Polymeric materials, such as the new sand stabilizers to prevent sand wind erosion, have received much attention.^25–28 The polyurethane (PU) designed by TORAY of Japan was used on the Qinghai salt-affected sandy land of China.^29,30 The experimental results showed that there was excellent sand-fixing capability at the beginning period of using PU; however, with the extension of time, PU began to pulverize and lost sand-fixation performance. Moreover, PU costs too much to be applied on a large-scale. Therefore, the performance and cost became the main consideration for the materials applied in the sand-fixing of high salt-affected sandy land.

Usually, emulsions, especially the emulsions made of acrylate monomers, had a much lower cost and good adhesive. With its peculiar performance, the emulsions became the all-important choice for desertification and soil salinization control. Generally, properties of emulsions were mainly determined by the monomer composition and emulsifying system.³¹ Especially, the influence of surfactant on the mechanical and chemical (e.g. salt tolerance) stabilities of emulsions was often a matter of major concern.

Due to the negative charge of sand surface, the cationic poly(vinyl acetate-butyl acrylate-co-2-methylacryloyloxyethyl trimethyl ammonium chloride) (P(VAc-BA-DMC)) copolymer emulsion designed by Xu Meng et al. was used as sand-fixing material.³² After the P(VAc-BA-DMC) was sprayed on the sand surface or mixed with sand, the emulsion showed its cementing ability to aggregate sand particles and formed a crust by neutralizing the negative charge of the sand surface and obtaining the desired mechanical property to resist bigger outside strength without occurrence of cracks. However, they didn’t study its salt resistance. Additionally, the cationic surfactant and DMC monomer in P(VAc-BA-DMC) emulsion had a negative influence on the growth of soil microbes if too much emulsion was sprayed on the sand surface.

In order to improve the salt resistance of the emulsion used for high salt-affected sandy land, the salt tolerance of the surfactant was regarded as the key point for preparing emulsions with a good salt tolerance. In the present work, the surfactants, namely SDS and L35, have been selected and their salt tolerance properties have been intensively studied by spectrophotometric and phase volume methods. Another batch of experiments were carried out to determine the adsorptions of the surfactant on the sand particles (adsorbent) at different salinities. Additionally, the effects of surfactants (SDS and L35) on the growth of E. coli and soil microbes were evaluated.

2 Experimental

2.1 Materials

Nonionic surfactant, Pluronic L35, was supplied by Haian Petrochemical Corp. (Jiangsu, China), and was used as received. Anionic surfactant, sodium dodecyl sulfate (SDS) was purchased from Kelong Chemical Reagent Corporation (Chengdu, China) and was used without further purification. Analytical pure cetyltrimethylammonium bromide (CTAB) procured from Sinopharm Chemical Reagent co., Ltd. (Beijing, China) was used as received. Sodium chloride (NaCl) procured from Guangdong Guanghua Sci-Tech Co., Ltd. (Guangdong, China), was used for the preparation of brine. Deionized water was used in all preparations.

2.2 Experimental procedures

2.2.1 Preparation of sand particles. Sands used for this investigation came from Golmud sandy land, Qinghai province, China. The sands were sieved to get 60 mesh sized particles, and then dried at 353 K for 24 h for experimental purposes.

2.2.2 XRD analysis of sand particles. The sands were ground to prepare powder samples. XRD measurement was carried out with a Rigaku D/max-RB apparatus (Tokyo, Japan) powder diffractometer and image-plate photography using graphite-monochromatized Cu Kα radiation. The data were collected from 10°–90° with a scanning rate of 5(°) min⁻¹ and analysed with the help of the JCPDS files.

2.2.3 FTIR analysis. The apparatus used for measuring the FTIR spectra of the sand particles before and after surfactant treatment, in the range of 450–4000 cm⁻¹, was a PerkinElmer Spectrum version 10.03.07 FTIR spectrometer. The instrument was operated by Spectrum two software supplied by PerkinElmer (USA). For the FTIR analysis, 4 mg of dried sample was mixed with potassium bromide (KBr) (∼300 mg), which was used as a reference standard sample. The mixture was compressed by hydraulic pump to prepare a pallet and the pallet was placed in a desiccator to remove moisture from the sample. The dried sample then was used for experimental purposes.

2.2.4 Salt tolerance analysis of the surfactant. In order to achieve improved sand-fixating ability of an emulsion in high salt-affected sandy land, the salt tolerance of the surfactant becomes an important parameter if the surfactant will be used for preparing the expected emulsion. Meanwhile, the salt tolerance of the surfactant also turns into a criterion for evaluating the effectiveness of sand fixation. A spectrophotometer (721, Shanghai XIPU instrument co., LTD) was used to measure the transmittance of the surfactants’ middle layer aqueous solution with different salinities at 318 K and 600 nm. Additionally, the salt tolerance of the surfactants can also be evaluated by phase volume-fraction diagram.³³ The pseudo-ternary phase diagram for the oil–water-surfactant system was obtained at 318 K.³⁴

2.2.5 Adsorption isotherms of the surfactants at different salinities. A series of batch experiments were carried out to determine the adsorption isotherms of SDS and L35 on the adsorbent. 8 g of dry sand particles were added to 50 mL surfactant solutions with different concentrations in a 150 mL conical flask, which were oscillated constantly for 24 h at 303 K in a temperature controlled horizontal shaker with a speed of 120 rpm. Then the surfactant solutions were isolated from the treated sand particles by centrifugation. The equilibrium concentration (C_e) of surfactant solutions were determined by two-phase titrations³⁵ and potassium ferrocyanide titration methods.³⁶ The amount of surfactant adsorbed on the adsorbent, Γ (mg g⁻¹), was calculated by a mass balance relation:

Γ = (C₀ − C_e)V/m

where C₀ and C_e are the initial and equilibrium concentrations of the surfactant (mg g⁻¹) respectively, V is the volume of surfactant solution (L), and m is the weight of the sand particles (g) used.

The effects of the NaCl concentrations on the adsorption capacity of the sand particles for SDS and L35 were also investigated.

2.2.6 Microorganism growth. Microorganisms play an important role in sand fertility and plant nutrition; also they could speed up the transformation of the sand to soil. So the growth of soil microbes was a primary parameter for evaluating the effectiveness of sand fixation. E. coli was used to evaluate the effects of the SDS and L35 on microbial growth in the first stage, then, the microorganisms from the sand of Golmud sandy land, Qinghai province were used for examining the ecological effect of SDS and L35. The sand containing 3% NaCl was first treated using SDS and L35 with concentrations of 1.0, 2.0, 3.0, 4.0, and 5.0%, then kept for one month at room temperature before being placed in distilled water to obtain the sand microbes for investigation. A spectrophotometer (721, Shanghai XIPU instrument co., LTD) was used to measure the growth condition of E. coli at 596 nm after the surfactant was introduced into the medium. According to the method proposed by Mathur et al.³⁷ and Lima et al.³⁸ with few modifications, the numbers of soil microbe were estimated, also.

3 Results and discussion

3.1 Characterization of sand particles and their interaction with surfactants

The sand particles treated with the method described in Section 2.2.1 were characterized by XRD study to investigate the components of the sand, and the result is shown in Fig. 1. There is a group of single headed peaks at 21°, 42.56°, 50.22°, 60° and 68.25°, etc., indicating that only one phase was present, which is attributed to the characteristic peaks of silicon oxide according to the JCPDS (file no. 832466). The main peak was obtained at 27.4°.

Fig. 1 XRD analysis of the sand particles.

The FTIR analysis was used to detect the structural changes of sand particles before and after the treatment with SDS and L35. Fig. 2(a) shows the infrared spectrum of untreated sand, with adsorption peaks at 780.82 cm⁻¹ and 1084.29 cm⁻¹ belonging to the symmetric and asymmetric stretching vibration of the Si–O group, respectively; and the adsorption bands at 521.98 cm⁻¹ and 691.57 cm⁻¹ were related to the asymmetric and symmetric bending vibrations of the Si–O group, respectively. The results proved again that the sand samples contained silica as their main composition, as shown in the XRD spectrum. The peak at 3468.43 cm⁻¹ indicated a stretching vibration of the OH group of water molecules, meaning there was moisture in the sand samples; the peaks at 2922.97 cm⁻¹ and 2854.54 cm⁻¹ were the symmetric and asymmetric stretching of the –CH₂ group, respectively.

Fig. 2 FTIR spectra of sand particles before and after surfactant treatment in water: (a) pure sand; (b) treated with SDS; (c) treated with L35.

Fig. 2(b) gives the IR spectrum of sand treated with SDS. The peaks at 2849.18 cm⁻¹ and 2912.83 cm⁻¹ were the symmetric and asymmetric stretching vibration of the –CH₂ group of SDS, the peak at 1381.55 cm⁻¹ came from the stretching vibration of the S=O bond of SDS, and the sharp peak at 2912.83 cm⁻¹ corresponded to the stretching vibration of the alkyl C–H bond of SDS. All the results indicated that SDS was adsorbed to the sand’s surface after its treatment.

Fig. 2(c) is the FTIR spectra of sands treated with L35. In this case the C–H stretching vibration of L35 showed at 2928.26 cm⁻¹ instead of 2922.97 cm⁻¹, the shift of the absorption band was due to the fact that the ethoxylated group of L35 was adsorbed by sands in solution. As for the peak at 1881.65 cm⁻¹, it was also because of the adsorption of the ethoxylated group of L35 on sands.

3.2 Salt tolerance of surfactants

The evaluation of the salt tolerance of the polymer emulsion is very important and necessary, because the polymer emulsion will be used as the sand-fixing material in high salt-affected sandy land. As we know, the polymer emulsion is an unstable system, and it is sensitive to salt and many chemicals. The salt tolerance of the polymer emulsion was determined by monomer composition and the surfactant system, therefore, adopting an excellent salt-resistant surfactant was regarded as the key factor for preparing a salt-resistant sand-fixing emulsion. Salt tolerance of the surfactant could be determined by the water solubilization of surfactant with various salinities. Fig. 3 showed the transmittance of middle layer aqueous solutions of SDS and L35 with different salinities. From Fig. 3, we can see that both the SDS and L35 had a good solubility in water with low salinity; correspondingly, the transmittance of the surfactant aqueous solution was similar with water. With the increase in NaCl concentration, the water-solubility of SDS decreased due to the salting out effect. As for L35, the presence of NaCl could destroy the hydrogen bonding between poly(ethylene oxide-b-propylene oxide) chains and water molecules and reduce the hydration degree of L35, so the transmittance of surfactant aqueous solution decreased. And, with the gradual increase of NaCl concentration, at a certain point, a sudden increase of the transmittance of the surfactants’ aqueous solution occurred because the system came to a phase separation. Therefore, a typical salt concentration, at which the minimum transmittance of SDS and L35 aqueous solutions was obtained, is defined as the biggest salt tolerance of SDS and L35, meaning the surfactants will have an excellent salt tolerance below this salt concentration. The biggest salt tolerances of surfactants were found 80 g L⁻¹ for SDS and 365 g L⁻¹ for L35, respectively. Generally, the salt content in saline sandy lands is less than 3%, so these results indicated that both the SDS and L35 could meet the requirement for the preparation of a salt-resistant sand-fixing emulsion.

Fig. 3 Transmittance of the surfactants’ middle layer aqueous solution as a function of electrolyte concentration at 318 K: (a) SDS; (b) L35.

Additionally, the salt tolerance of the surfactants could be evaluated by the phase behavior and phase boundary of microemulsion systems, which was usually formed by the surfactants/sec-butyl alcohol/n-heptane/brine system,³⁹ and its phase behavior could be determined by the water solubilization method with variations of salinity.⁴⁰ The phase state diagrams of the microemulsion based on SDS and L35 are shown in Fig. 4. It was found from the experimental results that the microemulsion phases were changed from Winsor type-I (showed as Fig. 4(a-1)) to Winsor type-II (showed as Fig. 4(a-6)) through a middle phase of Winsor type-III (showed as Fig. 4(a-4)) with a salinity increase at a particular temperature. This change in microemulsion phases could be attributed to the reason that initially at low salinity, surfactants show strong hydrophilic properties and mainly dissolve in a water phase, so only the oil phase and the lower-phase microemulsion can be found at the equilibrium situation of the system. And with the increase of salt concentration, the salt ions start to attract more and more H₂O molecules, which decreases the number of H₂O molecules available to interact with the surfactants. In this condition, the systems exhibit a middle-phase microemulsion. And with the gradual increase of NaCl concentration, the surfactant molecules will precipitate because of the hydrophobic interactions of each other, and then the upper-phase microemulsion and the brine phase occur in the systems at equilibrium. The interfacial film curvature turned from a positive value to zero to a negative one, corresponding to the phase transition from oil–water (O/W) to bi-continuous phase to water–oil (W/O) structure, gradually.^41,42

Fig. 4 Phase behavior of the surfactants at 318 K: (a) SDS; (b) L35.

The salinity, at which the water solubilization of the surfactant is highest, has been generally termed as the “optimal salinity” for the microemulsion system. Usually, the optimal salinity is found in the Winsor type-III region, as in this region the solubilization parameters σ_o, σ_w of oil and water in the microemulsion phase are equal, corresponding to the minimum interfacial tension.^43,44 Based on the experimental data in Fig. 5, the optimal salinities of SDS and L35 obtained by phase behavior were found to be 62 g L⁻¹ and 81 g L⁻¹ for SDS and L35, respectively (see Table 1). Meanwhile, it was also found that the middle-phase microemulsion had the ability to solubilize equal amounts of oil and brine at the optimum salinity. Therefore, we can say with full confidence that the emulsions prepared by SDS and L35 would be stable at the optimum salinity, even though it is much higher than the practical salt content in the sandy land.

Fig. 5 Phase state diagram of the surfactants/sec-butyl alcohol/n-heptane/brine microemulsion at 318 K: (a) SDS; (b) L35.

Table 1 Optimal salinity (S*), initial salinity (S₁), end salinity (S₂), and salinity range (ΔS = S₂ − S₁) for obtaining the middle phase emulsion with SDS or L35 at 318 K

	S* g L^−1	S 1 g L^−1	S 2 g L^−1	ΔS g L^−1
SDS	62	30	90	60
L35	81	60	>130	>70

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The salt tolerance of the surfactant could be evaluated by the salinity range, i.e., ΔS = S₂ − S₁ (S₁ was the initial salinity and S₂ is the end salinity for obtaining the middle phase emulsion). Because the salt content in the saline sandy land was generally below 3%, which is much less than the salinity ranges (ΔS) of SDS and L35, this implies that the SDS and L35 are suitable for the preparation of the emulsions used in high salt-affected sandy land (Table 1).

3.3 Adsorption isotherms of SDS and L35 on sand particles

The adsorption of SDS and L35 on sand directly affected the efficiency of the sand-fixation emulsion in a practical application. Owing to the fact that the above mentioned FTIR spectra could only provide qualitative results of the adsorption of SDS and L35 on sand, the quantitative results of their adsorption onto sand surface were determined in the laboratory, and the Langmuir adsorption isotherm and the Freundlich adsorption isotherm were used to describe the equilibrium adsorption amounts of SDS and L35 on sand particles as well the adsorption mechanism.

The Langmuir equation is related to the amount Γ, of the solid adsorbate adsorbed in the equilibrium liquid concentration at a fixed temperature. The equation was developed by Irving Langmuir⁴⁵ and was expressed in nonlinear form as follows:

where Γ is the amount of adsorbate adsorbed (mg g⁻¹); Γ_max is the maximum amount adsorbed (mg g⁻¹); K_L was the Langmuir equilibrium constant (L mg⁻¹); C_e is the equilibrium aqueous concentration (mg L⁻¹). It is well-known that the Langmuir isotherm is applicable for monolayer adsorption because of the homogeneous surface of a finite number of identical sites. Another important parameter of the Langmuir isotherm model is the term “R_L” which is a nondimensional constant and is called a separation factor or equilibrium parameter, and it is represented by the following equation:⁴⁶
where C₀ (mg L⁻¹) expresses the initial adsorbate concentration in an aqueous solution. K_L (L mg⁻¹) is the Langmuir constant. The R_L parameter is an important sign of the compatibility of adsorption for the given adsorbent–adsorbate pair. There are four possibilities for the R_L value:

in the case of 0 < R_L < 1, adsorption is favorable;

in the case of R_L > 1, adsorption is unfavorable;

R _L = 1 indicates the linearity of adsorption;

in the case of R_L = 0, the adsorption is irreversible.

The values of R_L obtained in this study were between 0.0295 to 0.222, indicating that the SDS and L35 were favorable for adsorption onto the sand surface.

The Freundlich isotherm assumes that at the equilibrium of the adsorption process, if C_e, the concentration of the solute in the solution, is raised to the power 1/n, the amount of the solute adsorbed is Γ. The C_e^1/n/Γ is a constant at a given temperature, and the nonlinear form of the equation will be expressed as:

Γ = K_FC_e^1/n

where K_F (mg g⁻¹) and n are the Freundlich adsorption constants related to adsorption capacity and adsorption intensity, respectively. The Freundlich isotherm has been derived by assuming an exponentially decaying adsorption site energy distribution. The Freundlich isotherm suggests that surfactant adsorption occurs on a heterogeneous surface by multilayer adsorption.

The Freundlich constant (1/n) is related to the adsorption intensity of the adsorbent. When 0.1 < 1/n < 0.5, the adsorption will be favorable; 0.5 < 1/n ≤ 1, it will be easy to adsorb; however, when 1/n > 1, it will be difficult to adsorb.⁴⁷

Fig. 6 shows the adsorption of SDS and L35 on sand surfaces at 303 K. Γ_SDS and Γ_L35, the amount of SDS and L35 adsorbed on the sand particles at equilibrium, depended on the structures and nature of hydrophilic groups of SDS and L35. It was clear that Γ_L35 was considerably higher than Γ_SDS. Fig. 6 indicates that there was a sudden increase in the adsorption isotherm with the increasing concentration of SDS and L35. The sudden increase in the adsorption isotherm may be related to the surface aggregates of SDS and L35, known as the “hemi micelles” of SDS and L35 molecules on the sand surface derived from the lateral interaction of hydrocarbon chains. This lateral attraction force generates an additional driving force to superimpose the existing electrostatic attraction, causing a sharp increase in adsorption. In all cases the increase of adsorption with concentration up to a certain point and then leveling off have been observed.^48,49 The adsorption of SDS at the solid–liquid interface was strongly influenced by the compositions of the sand. Because the XRD and FTIR analyses showed that the main ingredient of the sand was silicate, which made the sand surface negatively charged, therefore the weak interaction took place with the negatively charged head part of SDS, resulting in a not visibly high adsorption capacity of SDS on sand particles. The adsorption of L35 onto sand was based on the weak hydrophobic and hydrogen bond interactions as no positive and negative charges existed in L35, so the adsorption capacity of L35 for sands was considerably higher than that of SDS.

Fig. 6 Adsorption isotherms of the surfactants on the sand surface at 303 K: (a) SDS, (b) L35.

To quantify the adsorption capacity of SDS and L35 on sand, the Langmuir and Freundlich adsorption isotherms have been used to depict the different adsorption models. According to the Fig. 7 and 8, the calculated results from the curves of Langmuir and Freundlich isotherm adsorption for SDS and L35 have been summarized in Table 2. The values of regression coefficient (R²) implied that the adsorption of SDS and L35 on sand surface was well fitted to the Langmuir model.

Fig. 7 Fitting curves of the adsorption of SDS on sand particles at 303 K.

Fig. 8 Fitting curves of the adsorption of L35 on sand particles at 303 K.

Table 2 Adsorption isotherm parameters of SDS and L35

Isotherm models	Parameters	Surfactants
		SDS	L35
Langmuir	Γ max (mg g^−1)	0.897	0.944
	K L × 10^2 (L mg^−1)	1.578	1.648
	R ^2	0.965	0.911
Freundlich	K F (mg g^−1)	0.257	0.334
	1/n	0.178	0.143
	R ^2	0.920	0.831

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3.4 Effect of salt concentration on the adsorption isotherm of surfactants

Adsorption isotherms for SDS and L35 solution at varied salinities were shown in Fig. 9. At the interface between the surfactant and sand particles, there was always an unequal distribution of electrical charges. This unequal charge distribution gave rise to a potential across the interface and formed a so-called electrical double layer.⁵⁰ With the increase of NaCl concentration, the electrical double layer on the surface of adsorbent was compressed, and the electrostatic repulsion between the adsorbed surfactant species decreased, which resulted in the increase of adsorption capacity. The surfactant adsorption capacity increased with the increase in salinity of the system at a constant temperature of 303 K. The phenomenon showed that the adsorption of SDS and L35 on sand particles was favored at high salinity and therefore, the adsorption process was found to be a chemical process with increasing salinity.

Fig. 9 Adsorption isotherms of the surfactants on sand surface at varied salinities of brine at 303 K: (a) SDS; (b) L35.

Table 3 showed the parameters obtained from the two adsorption models. We can see that all the regression coefficients (R²) from the Langmuir model fittings were greater than those from the Freundlich model fittings at corresponding salinities. Therefore, it may be concluded that the Langmuir isotherm model presented a better fit than the Freundlich model when SDS and L35 were used to treat the sand.

Table 3 Adsorption isotherm parameters of SDS at varied salinities

Salinities (wt% NaCl)	Langmuir parameters			Freundlich parameters
	Γ max (mg g^−1)	K L × 10^2 (L mg^−1)	R ^2	K F (mg g^−1)	1/n	R ^2
0	0.897	1.578	0.965	0.257	0.178	0.920
1	0.910	2.059	0.981	0.329	0.144	0.959
2	0.929	2.186	0.971	0.397	0.122	0.878
3	0.948	3.11	0.933	0.459	0.103	0.873

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3.5 Ecological effect of surfactants on sandy land

Microorganisms play a considerable role in sand fertility and plant nutrition, and they could speed up the transformation of the sand to soil. Also, the microorganisms are considered as the most active composition in the land ecological system. So the growth of the soil microbes was a key parameter for evaluating the effectiveness of sandy land’s recovery. The culture medium containing 5 wt% SDS and L35 have been used to test the effect of surfactants on the growth of E. coli. As we know, this concentration of SDS and L35 used was much higher than the dosage for preparing emulsions. The experimental results shown in Fig. 10 revealed that the growth of E. coli was not affected by the SDS and L35 at such a high concentration with the extension of growing time. This result allowed us to strongly believe that the selected SDS and L35 surely could be used to prepare the salt tolerance emulsions with required ecological effect.

Fig. 10 Effects of SDS and L35 on the growth of E. coli: (a) SDS; (b) L35.

The E. coli and microorganisms from the sand were used for examining the ecological effect of SDS and L35. The sand was treated following the procedure described in Section 2.2.6. Table 4 showed the quantity of bacteria from the sand treated with 3.0% of salt and SDS or L35 with different concentrations. As observed, when L35 concentration increased, there were no obvious effects on the quantity of sand bacteria. Although SDS could inhibit the bacteria growth of sand to some extent, there was still large quantity of sand bacteria in the culture. Moreover, addition of 4.0% concentration of SDS and L35 could obtain sand-fixing emulsions with the desired mechanical strength to resist bigger outside stress without cracks. These results indicated that SDS and L35 at the tested concentrations were harmless to the growth of sand microbes, and the emulsions prepared with SDS and L35 will not only provide the anti-erosion but ecological effects when they are used for sand-fixing in high salt-affected sandy land.

Table 4 The quantity of soil bacteria change of sand specimen with spraying different concentrations of surfactant after 30 days

The surfactant concentration (%)	Bacteria (/g) of spraying SDS	Bacteria (/g) of spraying L35
0	7.084 × 10^4	7.084 × 10^4
1	6.635 × 10^4	6.933 × 10^4
2	6.219 × 10^4	6.958 × 10^4
3	5.874 × 10^4	6.691 × 10^4
4	5.663 × 10^4	6.927 × 10^4
5	5.149 × 10^4	6.544 × 10^4

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4. Conclusion

The adsorption of SDS and L35 onto sand particles has been systematically studied. The FTIR and XRD studies showed the presence of silica in the sand which provided the active sites for adsorption of SDS and L35. The salt tolerance of SDS and L35 was improved by the phase behavior of the microemulsions, comprising the surfactants/sec-butyl alcohol/n-heptane/brine with the salinity. The growth of E. coli and soil microbes at a high concentration of SDS and L35 proved the ecological effect of these two surfactants. The adsorption of SDS and L35 on sand particles demonstrated a favored interaction between sand and SDS and L35. All these results indicated that SDS and L35 could enhance the sand-fixing ability of prepared emulsions, which will be used for the ecological restoration of high salt-affected sandy land.

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