Remediation of Anthracene-Contaminated Soil with Sophorolipids-SDBS-Na2sio3 and Treatment of Eluting Wastewater

Home , Anthracene, Calcium hydroxide, Potassium chloride

Article Remediation of Anthracene-Contaminated Soil with Sophorolipids-SDBS-Na2SiO3 and Treatment of Eluting Wastewater

Wei Li 1,2,*, Xiaofeng Wang 1,2, Lixiang Shi 1,2, Xianyuan Du 3 and Zhansheng Wang 3

1 State Key Laboratory of Petroleum Pollution Control, Beijing 102206, China; [email protected] (X.W.); [email protected] (L.S.) 2 College of Environmental Science and Engineering, North China Electric Power University, Beijing 102206, China 3 CNPC Institute of Safety and Environmental Protection Technology, Beijing 102206, China; [email protected] (X.D.); [email protected] (Z.W.) * Correspondence: [email protected]

Received: 29 May 2020; Accepted: 6 July 2020; Published: 4 August 2020

Abstract: The soil pollution of polycyclic aromatic hydrocarbons (PAHs) is serious in China, which not only affects the living and growing environment of plants and animals but also has a great impact on people’s health. The use of hydrophobic organic compounds to make use of surfactant ectopic elution processing is more convenient and cheaper as a repair scheme and can effectively wash out the polycyclic aromatic hydrocarbons in the soil. Therefore, we mixed sophorolipids:sodium dodecylbenzene sulfonate (SDBS):Na2SiO3 according to the mass ratio of 1:15:150. We explored the influencing factors of high and low concentrations of PAH-contaminated soil using a single factor test and four factors at a two-level factorial design. Then, the elution wastewater was treated by ultrasonic oxidation technology and the alkali-activated sodium persulfate technology. The results showed that: (1) In the single factor test, when the elution time is 8 h, the concentration of the compounded surfactant is 1200 mg/L, the particle size is 60 mesh, the concentration of NaCl is 100 mmol/L, and the concentration of KCl is 50 mmol/L, and the effect of the PAH-contaminated soil eluted by the composite surfactant is the best. Externally added NaCl and KCl salt ions have a more obvious promotion effect on the polycyclic aromatic hydrocarbon-contaminated soil; (2) in the interaction experiment, single factor B (elution time) and D (NaCl concentration) have a significant main effect. There is also a certain interaction between factor A (concentration agent concentration) and factor D, factor B, and factor C (KCl concentration); (3) the treatment of anthracene in the eluate by ultrasonic completely mineralizes the organic pollutants by the thermal and chemical effects produced by the ultrasonic cavitation phenomenon, so that the organic pollutants in the eluate are oxidized and degraded into simple environmentally friendly small molecular substances. When the optimal ultrasonic time is 60 min and the ratio of oxidant to activator is 1:2, the removal rate of contaminants in the eluent can reach 63.7%. At the same time, the turbidity of the eluent is significantly lower than that of the liquid after centrifugal separation, indicating that oxidants can not only remove the pollutants in elution water but also remove the residual soil particulate matter; and (4) by comparing the infrared spectrum of the eluted waste liquid before and after oxidation, it can be seen that during the oxidation process, the inner part of eluent waste liquid underwent a ring-opening reaction, and the ring-opening reaction also occurred in the part of the cyclic ester group of the surfactant, which changed from a ring to non-ring.

Keywords: combinational surfactant; anthracene; elution wastewater; ultrasonic oxidation

Water 2020, 12, 2188; doi:10.3390/w12082188 www.mdpi.com/journal/water Water 2020, 12, 2188 2 of 20

1. Introduction Polycyclic aromatic hydrocarbons (PAHs) are persistent organic compounds that are ubiquitous in the environment. They consist of two or more fused benzene rings, such as phenanthrene, anthracene, and benzo[a]pyrene, which are potentially carcinogenic and mutagenic in contaminated soil and water bodies [1]. It has the characteristics of low volatility, low water solubility, and low biodegradability [2–5], which makes the accumulation of polycyclic aromatic hydrocarbon in the soil and groundwater environment increase year by year, resulting in non-negligible pollution. They are generated through natural and man-made means, such as the pyrolysis of organic matter and the use of fossil fuels [6,7]. It has toxicity, mutagenicity, and carcinogenicity, and its toxicity increases with the increase of the molecular weight. Surfactant leaching technology has a good effect on eluting PAHs in soil. There are two main mechanisms for surfactant remediation of oil-contaminated soil: (1) When its critical micelle concentration (CMC) is reached [8–13], surfactant molecules will quickly aggregate and arrange to form an ordered polymer with a certain structure, forming a stable micelle, which can be used to increase the concentration of hydrophobic petroleum hydrocarbon pollutants in the liquid phase through micelle solubilization; and (2) by reducing the interfacial tension between the water–soil, oil–soil, and oil–water, surfactants can improve the migration ability of petroleum hydrocarbon pollutants, promote their desorption from the soil, disperse the pollutants in the soil pores, and make them contact with the surfactant solution to elute. Although the surfactant remediation technology has a high efficiency and short cycle, the high cost and ecological risk of the surfactant limit its wide application to some extent. Choosing suitable surfactants, improving repair efficiency, reducing the amount of surfactants, and reducing the pollution and ecological risks of surfactants on the soil are the key issues facing the current technology. Ji et al. [14] investigated the oxidative remediation performance of activated sodium persulfate (SPS) on polycyclic aromatic hydrocarbon (PAH)-contaminated soil with the aid of solubilization of typical nonionic surfactant Tween 80. Wang et al. [15] selected sodium dodecyl benzene sulfonate, sodium dodecyl sulfate, and sodium dodecyl sulfonate for desorption experiments on contaminated soil, and studied the synergistic solubilizing effect of the anionic surfactant and sodium humate on diesel pollutants in loess. The results showed that sodium humate and three anionic surfactants have a significant solubilization effect on the desorption of diesel in loess. Yang et al. [16] compared the elution effect of SDBS-TX100 and single surfactant on phenanthrene in polluted soil, and found that TX100 and SDBS mixed surfactant with a mass ratio of 1:9 had the best elution effect on phenanthrene in soil, which indicated that the mixed surfactant with an appropriate ratio had higher solubilization and elution efficiency. Wang et al. [17] used the biosurfactants rhamnolipid (Xi’an rege biotechnology Co., Ltd., Xi’an, China) and Tween-80 (Macklin biochemical Co., Ltd., Shanghai, China) to analyze the concentration, leaching time, temperature, and frequency of surfactants for five polycyclic aromatic hydrocarbon (fluoranthene, pyrene, benzo[b]fluoranthene, benzene, and [k] fluoranthene, benzo [α] pyrene)-contaminated soil’s leaching effect. For the selection of surfactants, the potential impact on the environment, the hydrophilic and oil-wet equilibrium (HLB) values, and the availability of the surfactants should be considered [18]. Sophorolipids is a kind of lactone surfactant, which can be 100% degraded in the natural environment, while HLB ranges from 9 to 12 and can be produced in a large scale [19]. The critical micelle concentration (CMC) of sophorolipids ranges from 40 to 100 mg/L, two orders of magnitude lower than chemically synthesized surfactants [20], and remains stable over a wide range of temperatures and salt concentrations. Sun et al. [21] investigated the effects of indigenous microorganisms, inoculated mixed petroleum hydrocarbon-degrading bacteria, and the addition of sophorolipid biological surfactant. The mechanism of oil-containing sludge remediation enhanced by sophorolipid biological surfactant and inoculating petroleum hydrocarbon-degrading bacteria was discussed. Li et al. [22] investigated the influence of the sophorolipid concentration, electrolyte ion strength, and electrolyte acidity on the effect of chrome-contaminated soil electric remediation with sophorolipids. The results showed that the electric current intensity increased significantly in the electric remediation system with sophorolipids Water 2020, 12, 2188 3 of 20 added. With the increase of the ionic strength and acidity of the electrolyte, the removal rate of chromium increased gradually. Sodium dodecylbenzenesulfonate is an anionic surfactant with a high removal rate and low adsorption rate. However, the surfactant exhibits a higher CMC value, which results in a higher concentration of the agent in the washing residues and is more likely to precipitate in the presence of polyvalent cations (Ca2+ and Mg2+)[23,24]. Someone used sodium dodecylbenzenesulfonate to elute the nitrobenzene and benzoic acid in the soil under laboratory and actual contaminated site conditions, respectively. It was found that the surfactant had a better removal rate under the laboratory conditions for the two pollutants, and the maximum elution efficiency at the contaminated site could also reach more than 90% [25]. Ni et al. [26] took sodium dodecyl benzenesulfonate and Tween-80 as the representatives of nionic and non-ionic surfactants, respectively, and studied the influence of mixed surfactants on the interface behavior between phenanthrene and pyrene in soil, as well as the effect and mechanism of strengthening the remediation of phenanthrene and pyrene-contaminated soil. Na2SiO3 affects the pH value of the solution and can react with the acidic components in the oil to form salt, increasing the water solubility of the oil [27,28]. Na2SiO3 belongs to an electrolyte, which can enhance the wetting, emulsification, and solubilization of the surfactant in the washing solution after being added into the solution; it can compress the double electric layer of the micelle, and increase the contact between the surfactant molecules, making more ions enter the micelle. Because cationic surfactant has a low elution efficiency for pollutants, anionic surfactant has a higher CMC, resulting in the higher economic cost, and non-ionic surfactant has a stronger adsorption effect on soil, thus people pay more attention to the solubilization system obtained through compounding. One of the more widely used types of complex is the anionic and non-ionic surfactant complex applications. This may be because the oxygen atoms in the nonionic surfactant molecules are easily combined with the hydrogen ions in the water, so that the nonionic surfactant is positively charged, causing the nonionic surfactant and the anionic surfactant to attract each other to reduce the mixed micelles. Due to the repulsion between the bundles, the critical micelle concentration (CMC) of the system decreases. Anionic surfactants are likely to precipitate in the soil and nonionic surfactants are more likely to adsorb onto the soil fractions [29], which results in a high remediation cost due to the need for more surfactants [30]. It further pollutes the environment. Sorption of nonionic surfactants could be prevented using a combination of nonionic and anionic surfactants [9]. Most of the studies have introduced the solubilization effect of anions and nonionic surfactants. Some studies have shown the effects of soil’s physical and chemical properties and the surfactants themselves on the eluting effect. There is little research on the addition of auxiliaries to anionic and nonionic surfactants. Adding sodium silicate as auxiliaries can prevent surfactants from being adsorbed into the soil solid phase and play an anti-reprecipitation role. However, surfactants only transfer the pollutant from the soil to the aqueous phase and further treatment of this polluted aqueous solution is required. The chemistry of persulfate as the oxidant has been widely studied [31]. In aqueous solution, persulfate salts dissociate to persulfate anions as shown in Equation (1): 2 2 S O − + 2e− 2SO − (1) 2 8 → 4

Subsequently, when persulfate anion is activated, sulfate radical (SO4·−), a non-selective oxidant with a high redox potential (E0 = 2.6V), is produced. This activation can be achieved by the addition of the transition metal, light or heat activation, by the addition of the hydrogen peroxide, or alkaline activation (pH > 10) [18]. Iron activation has the drawback that the pH must be maintained in the acid region to avoid iron precipitation [32], whereas the cost of thermally activated persulfate at the field scale can be unaffordable. Therefore, it is of great interest to develop a low-cost high-efficiency method for the activation of persulfates for organic pollutant degradation [33]. Alkaline activation has gained attention recently, but only few works have studied the remediation of eluting wastewater with polycyclic aromatic hydrocarbons (PAHs) by using this technology [34]. Water 2020, 12, 2188 4 of 20

The eluting eﬀect of sophorolipids-SDBS-Na2SiO3 on anthracene pollutants was studied, and the interaction between exogenous additives and environmental inﬂuencing factors was explained. At the same time, alkali-activated sodium persulfate was used to elute anthracene-contaminated soil wastewater with compound surfactants.

2. Materials and Methods

2.1. Materials

Table1 shows the properties of sophorolipids, SDBS, and Na 2SiO3. The surface tension was determined by the hanging sheet method. The minimum concentration of surfactant molecules to form micelles in solvent is critical micelle concentration, and the surface tension of surfactant molecules does not increase at the same time. The CMC of sophorolipids and SDBS was determined as 40 and 100 mg/L, respectively. Anthracene is a polycyclic aromatic hydrocarbon containing three rings, which is obtained in the ﬁnal stage of distillation of coal tar and can be separated from anthracene oil from coal tar. It is mainly used as luminescent materials, insecticides, fungicides, and gasoline coagulants. It has a non-volatile and special linear molecular structure. Its toxicity can be multiplied under light conditions [35]. Therefore, it is considered as a representative of PAH pollutants. The analysis of polycyclic aromatic hydrocarbon pollution used anthracene. Anthracene was purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). The standard for GC (chromatographically pure) was 99.5%. ≥ Table 1. Physical and chemical properties of anionic and nonionic surfactants.

Surfactant Purity Molecular Formula CMC (mg/L)

Sophorolipids AR C34H56O14 40 SDBS AR C18H29NaO3S 100 Na2SiO3 AR Na2SiO3 -

The mass concentration of surfactant for sophorolipids-SDBS-Na SiO was 10 g L 1. First, 0.12 g 2 3 · − of sophorolipids, 9 g of Na2SiO3, and 0.9 g of SDBS were dissolved in 1 L of pure water solution. After dilution, surfactant solutions of diﬀerent mass concentrations were prepared. Soil samples were taken from the 20–40 cm soil layer in the suburbs of Beijing, China. They were air-dried for 7 days, tapped to break aggregated soil, and homogenized before sieving to remove gravel larger than 2 mm. Selected physical and chemical properties of the soil samples are listed in Table2.

Table 2. Properties of the soil studied.

Organic Matter Moisture Cation Exchange Particle Diameter pH (H O) Salinity (%) 2 Content (%) Content (%) Capacity (cmol/kg) (mm) 8.59 0.75 2.63 7.73 0.55 2

High-concentration polluted soil: First, 80 mg of anthracene were weighed, completely dissolved with acetone solution, and the solution mixed with 200 g of spare soil evenly. Then, the mixture was put in at room temperature, and dried in a fume hood until the acetone was completely volatilized to obtain polluted soil with a high pollution concentration of 400 mg kg 1. · − Low-concentration polluted soil: First, 40 mg of anthracene was weighed, completely dissolved with acetone solution, and the solution was mixed with 200 g of spare soil evenly. Then, at room temperature, the mixture was dried in a fume hood until the acetone was completely volatilized to obtain a polluted soil with a low pollution concentration of 200 mg kg 1. · − Water 2020, 12, 2188 5 of 20

2.2. Soil Washing Using Mixed Surfactants PAH-contaminated soil was washed in a small stirring reactor with a total loading volume of approximately 20 L. The high-concentration polluted soil was washed with a liquid–soil ratio (L/S) of 6:1 at a speed of 160 rpm for 8 h. The low-concentration polluted soil was washed with a liquid–soil ratio (L/S) of 8:1 at a speed of 160 rpm for 8 h. An ultrasonator (Shenzhen Xingde Trade Co., Ltd., Shenzhen, China) was used to extract PAHs from soil samples. PAH concentrations were determined with an HPLC system (Agilent Technologies 1200 Series).

2.3. Eluant Using Alkali-Activated Sodium Persulfate Sodium hydroxide, calcium hydroxide, calcium oxide, and potassium hydroxide were selected as the bases to activate sodium persulfate. Aqueous solutions of sophorolipids, SDBS, Na2SiO3, and their mixtures were respectively put in a 150-mL conical glass ﬂask with a stopper. Excess sodium peroxydisulfate and alkali were added to the ﬂasks, which were then centrifuged at 160 rpm for 24 h. PAH concentrations were determined with an HPLC system (Agilent Technologies 1200 Series).

2.4. Removal Effectiveness Evaluation In order to evaluate the soil eluting efficiency, the anthracene removal ratio was determined as the concentration of residual anthrancene in soil divided by the concentration of original anthrancene in soil: (C0 Ce) 100 Elution efficiency(%) = − × , (2) C0 where C0 (mg/L) and Ce (mg/L) are the initial and equilibrium soil anthracene concentrations, respectively. In the present study, a full 24 factorial design was performed to evaluate the interactions of the eluting time, mixed-surfactant concentration, and the concentration of NaCl and KCl, with the response of the anthracene elution efficiency.

2.5. Analytical Methods PAHs in the aqueous phases were also measured by HPLC (Agilent, mod. 1200) coupled with an Agilent 1290 Inﬁnity Diode Array Detector. The column used was a ZORBAX Eclipse XDB 80Å C18 in 4.6 150 mm and a 5-µm particle size. The column temperature was 35 C. Analysis was carried out × ◦ under isocratic mode at a ﬂow rate of 1.0 mL min 1, selecting as the mobile phase a mixture of 60% · − acetonitrile and 40% water; the selected injection volume was 10 µL. The operation time of the liquid phase was 18 min. The wavelengths chosen were 254 nm for anthracene. The detection limit was less than 50 ppbs. The calculation formula of anthracene in aqueous solution is as follows:

ρ V ρ = xi × i , (3) i V where ρi is the mass concentration (mg/L) of component i in the sample, ρxi is the mass concentration (mg/L) of component i found from the marking line, Vi is the volume (mL) of the concentrated extract, and V is the volume of the water sample (mL). The calculation formula of anthracene in soil is as follows: ρ V ω = i × , (4) i m W × dm where ωi is the content of component i in the sample (µg/kg), ρi is the concentration of component i calculated from the standard curve (µg/mL), V is the constant volume (mL), m is the sample volume (kg), and Wdm is the dry matter content in the soil sample (%). Water 2020, 12, x FOR PEER REVIEW 6 of 21

ω ρ where i is the content of component i in the sample (µg/kg), i is the concentration of component i calculated from the standard curve (µg/mL), V is the constant volume (mL), m

is the sample volume (kg), and Wdm is the dry matter content in the soil sample (%). Water 2020, 12, 2188 6 of 20 2.6. Data Analysis

2.6. DataThe Analysisfactorial experimental design and statistical analysis were conducted using Design Expert 8.0.6 (Stat-Ease Inc., Minneapolis, MN, USA). The OriginPro 8.5 software (OriginLab Corporation, The factorial experimental design and statistical analysis were conducted using Design Expert Northampton, MA, USA) was used for the elution analysis of the experimental data. 8.0.6 (Stat-Ease Inc., Minneapolis, MN, USA). The OriginPro 8.5 software (OriginLab Corporation, Northampton,3. Results and MA,Discussion USA) was used for the elution analysis of the experimental data. 3. Results and Discussion 3.1. Experiment of Single Surfactant Remediation of Anthracene-Contaminated Soil 3.1. ExperimentAfter configuring of Single different Surfactant concentration Remediation ofgradient Anthracene-Contaminateds of sophorolipids Soiland SDBS solution to elute and Afterrepair configuring petroleum-contaminated different concentration soil, the gradientsresults are of shown sophorolipids in Figure and 1a SDBSand Figure solution 1b. to As elute the andconcentration repair petroleum-contaminated of surfactant increases, soil, the the elutio resultsn areefficiency shown inreaches Figure a1 a,b.certain As thevalue concentration and becomes of surfactantstable. The increases, elution efficiency the elution of esophorolipidfficiency reaches solution a certain can valuereach and67.03%, becomes and the stable. optimal The elutionelution eefficiencyfficiency of of sophorolipid SDBS can solutionreach 76.96%. can reach The 67.03%, elutio andn theratio optimal of surfactants elution effi ciencyto PAHs of SDBS is caninversely reach 76.96%.proportional The elution to the ratiooctanol/water of surfactants partition to PAHs coefficient is inversely of PAHs. proportional Subsequent to the octanolexperiments/water are partition based coeon ffithecient combination of PAHs. Subsequentof these two experimentssurfactants, areand based the additive on the combinationsodium silicate of these(Na2SiO two3) surfactants,is added to the system to improve its elution efficiency. and the additive sodium silicate (Na2SiO3) is added to the system to improve its elution efficiency.

FigureFigure 1.1. Elution eefficiencyfficiency ofof surfactantssurfactants withwith didifferentfferent concentrations.concentrations. (a) ElutionElution eefficiencyfficiency ofof sophorolipidssophorolipids with with di ffdifferenterent concentrations. concentrations. (b) Elution(b) Elution efficiency efficiency of sodium of dodecylbenzenesodium dodecylbenzene sulfonate (SDBS)sulfonate with (SDBS) different with concentrations. different concentrations. 3.2. Effect of Mixed Surfactants on the Single Factor of Anthracene-Contaminated Soil Eluting 3.2. Effect of Mixed Surfactants on the Single Factor of Anthracene-Contaminated Soil Eluting 3.2.1. Effect of Eluting Time 3.2.1. Effect of Eluting Time If the elution time is too short, the surfactant cannot fully contact with the pollutant, resulting in a weakIf elution the elution effect. time When is thetoo elutionshort, the time surfactant reaches acannot certain fully value, contact most ofwith the the soluble pollutant, pollutants resulting will enterin a theweak surfactant elution solution.effect. When On thethe one elution hand, time oil droplets reachesor a surfactantscertain value, that most have beenof the desorbed soluble frompollutants the surface will enter of soil the particles surfactant can solution. be re-adsorbed On the to one the hand, surface oil of droplets particles; or on surfactants the other hand,that have the oil-waterbeen desorbed mixture from may the form surface an emulsion, of soil particles which increases can be re-adsorbed the cost of elution to the andsurface increases of particles; the diffi onculty the ofother subsequent hand, the elution oil-water [36 ,37mixture]. Therefore, may form the an elution emulsion, time shouldwhich increases be selected the within cost of a elution reasonable and range.increases The the low- difficulty and high-concentration of subsequent elution PAH-contaminated [36,37]. Therefore, soil were the repairedelution time with should mixed surfactantsbe selected atwithin different a reasonable shaking times.range. ItThe can low- be seenand high-con clearly fromcentration Figure PAH-contaminated2 that in the PAH-contaminated soil were repaired soil, regardlesswith mixed of surfactants the low-concentration at different contaminatedshaking times. soil It can orthe be high-concentrationseen clearly from Figure contaminated 2 that in soil, the thePAH-contaminated eluting rate reached soil, a maximumregardless at 8of h, whilethe low-concentration the elution efficiency contaminated showed a decreasing soil or trend the afterhigh-concentration 8 h. Therefore, contaminated it is determined soil, that the the eluting optimal rate elutionreached time a maximum of surfactant at 8 elutingh, while and the elutingelution PAH-contaminated soil was 8 h.

Water 2020, 12, x FOR PEER REVIEW 7 of 21 efficiency showed a decreasing trend after 8 h. Therefore, it is determined that the optimal elution

Watertime 2020of surfactant, 12, x FOR PEER eluting REVIEW and eluting PAH-contaminated soil was 8 h. 7 of 21 efficiency showed a decreasing trend after 8 h. Therefore, it is determined that the optimal elution timeWater of2020 surfactant, 12, 2188 eluting and86 eluting PAH-contaminated soil was 8 h. 88 7 of 20

84 87

82 86

8680 8885

s elutingrate (%) 84 87 78 84

8276 8683 80 85 74 82 s elutingrate (%) 7872 8481 76 83 70 80 Low concentration PAH Low 7468 8279 elutingrate (%) concentration PAHs High 0 1020304050 72 81 Eluting time (h) 70 80 Low concentration PAH Low 68 79 elutingrate (%) concentration PAHs High Figure 2. Effect of elution0 time on 1020304050 high- and low-concentration anthracene-contaminated soil. Eluting time (h)

3.2.2. MixedFigure Surfactant 2. Eﬀect ofConcentration elution time on high- and low-concentration anthracene-contaminated soil.

Figure 2. Effect of elution time on high- and low-concentration anthracene-contaminated−1 soil. 3.2.2.It Mixedcan be Surfactantseen from ConcentrationFigure 3 that the elution efficiency of 1200 mg·L surfactant is the highest, 77.49% for low-concentration PAH-contaminated soil, and 85.27% for high-concentration 3.2.2. MixedIt can be Surfactant seen from Concentration Figure3 that the elution e fficiency of 1200 mg L 1 surfactant is the highest, 77.49% PAH-contaminated soil. As the concentration of surfactant increases,· − the elution efficiency does not for low-concentration PAH-contaminated soil, and 85.27% for high-concentration PAH-contaminated increaseIt can but be decreases.seen from Similarly,Figure 3 that for thepolycyclic elution efficiencyaromatic hydrocarbon-contaminatedof 1200 mg·L−1 surfactant is soils,the highest, as the soil. As the concentration of surfactant increases, the elution efficiency does not increase but decreases. 77.49%concentration for low-concentration increases, the polycyclic PAH-contaminated aromatic hydrocarbons soil, and adsorbed85.27% forby thehigh-concentration surfactants are Similarly, for polycyclic aromatic hydrocarbon-contaminated soils, as the concentration increases, the PAH-contaminatedexcessively depleted, soil. so Asthat the they concentration are re-desorbed of surf intoactant the increases, soil solid the phase. elution Therefore, efficiency selecting does not a polycyclic aromatic hydrocarbons adsorbed by the surfactants are excessively depleted, so that they increasesuitable surfactantbut decreases. concentration Similarly, can for notpolycyclic only improve aromatic the hydrocarbon-contaminated elution efficiency and reduce soils, the aswaste the are re-desorbed into the soil solid phase. Therefore, selecting a suitable surfactant concentration can concentrationof the surfactant increases, but also the reduce polycyclic the surfactant aromatic concentration hydrocarbons in adsorbedthe cleaning by solutionthe surfactants and reduce are not only improve the elution efficiency and reduce the waste of the surfactant but also reduce the excessivelythe subsequent depleted, wastewater so that treatment they are load.re-desorbed Surfacta intont can the reduce soil solid the interfacialphase. Therefore, tension betweenselecting oil a surfactant concentration in the cleaning solution and reduce the subsequent wastewater treatment suitablesubstances surfactant and soil concentration particles, and can make not oilonly substa improvences theroll elutionaway from efficiency the soil and surface, reduce whichthe waste can load. Surfactant can reduce the interfacial tension between oil substances and soil particles, and make ofoccur the whensurfactant the concentrationbut also reduce is thelower surfactant than the concentration CMC [38]; when in the the cleaning concentration solution reaches and reduce or is oil substances roll away from the soil surface, which can occur when the concentration is lower than thehigher subsequent than the wastewaterCMC, surfactant treatment forms load. micelles Surfacta in thent can solution, reduce and the solubilizesinterfacial tensionthe insoluble between diesel oil the CMC [38]; when the concentration reaches or is higher than the CMC, surfactant forms micelles in substancessubstances, andwhich soil are particles, desorbed and from make the oilsoil substa and dincesstributed roll away into thefrom aqueou the soils phase, surface, increasing which canthe the solution, and solubilizes the insoluble diesel substances, which are desorbed from the soil and occurdiesel whenremoval the rate concentration [39,40]. is lower than the CMC [38]; when the concentration reaches or is distributed into the aqueous phase, increasing the diesel removal rate [39,40]. higher than the CMC, surfactant forms micelles in the solution, and solubilizes the insoluble diesel substances, which are desorbed from the soil and distributed into the aqueous phase, increasing the diesel removal rate [39,40]. 78 86

84 76 82

74 78 8680

72 8478 76 8276 70 74 8074

68 72 7872

66 7670 70 7468 Low concentration PAHs eluting rate (%) rate eluting PAHs concentration Low 64 (%) rate eluting PAHs concentration High 68 400 600 800 1000 1200 1400 160072 Mixed-surfactants concentration (mg/L) 66 70

68 Low concentration PAHs eluting rate (%) rate eluting PAHs concentration Low Figure 3. Eﬀect of mixed64 surfactants on high- and low-concentration anthracene-contaminated(%) rate eluting PAHs concentration High soil. 400 600 800 1000 1200 1400 1600 Figure 3. Effect of mixed surfactants on high- and low-concentration anthracene-contaminated soil. 3.2.3. Eﬀect of Ion Concentration Mixed-surfactants concentration (mg/L)

The effect of the NaCl and KCl concentration on the elution experiment was studied. From Figure4, the elution e fficiency of the high-concentration contaminated soil is significantly higher than Figure 3. Effect of mixed surfactants on high- and low-concentration anthracene-contaminated soil. that of the low-concentration contaminated soil. The results show that, when the concentration of

NaCl is 100 mmol/L (Figure4a), the e ﬀect of sophorolipids-SDBS-Na2SiO3 on anthracene is obviously Water 2020, 12, x FOR PEER REVIEW 8 of 21

3.2.3. Effect of Ion Concentration The effect of the NaCl and KCl concentration on the elution experiment was studied. From Figure 4, the elution efficiency of the high-concentration contaminated soil is significantly higher than that of the low-concentration contaminated soil. The results show that, when the concentration ofWater NaCl2020 ,is12 ,100 2188 mmol/L (Figure 4a), the effect of sophorolipids-SDBS-Na2SiO3 on anthracene8 of is 20 obviously increased. For ionic surfactants, when inorganic salts with the same anti ions as surfactants are added (such as Cl− and Na+), the double electron layer will be effectively compressed andincreased. reduce For the ionic mutual surfactants, exclusion when of inorganicsurfactant salts ions. with For the ionic same surfactants, anti ions as surfactantsthe surface are activity added + increases(such as Cl and− and the Na critical), the micelle double electronconcentration layer willdecreases be eﬀectively [41]. When compressed the concentration and reduce theof mutualKCl is 50mmol/Lexclusion of(Figure surfactant 4b), ions.the addition For ionic of surfactants, K+ can promote the surface the remediation activity increases of anthracene and the critical in soil micelle to a certainconcentration extent but decreases it cannot [41 be]. Whenover added. the concentration of KCl is 50 mmol/L (Figure4b), the addition of K+ can promote the remediation of anthracene in soil to a certain extent but it cannot be over added.

83.5 86

83.0 85

82.5 84 82.0

83 81.5

81.0 82

80.5 81

80.0 Low concentration eluting PAHs rate (%) 80 High concentration PAHs eluting rate (%) 0 50 100 150 200 250 300 The concentration of NaCl (mmol/L)

(a)

85 86

85 84

82 82

81 81 Low concentration eluting PAHs rate (%) High concentration PAHs eluting rate (%)

0 50 100 150 200 250 300 The concentration of KCl (mmol/L)

(b)

FigureFigure 4. 4. EffectEffect of of ion ion concentration concentration on on high- high- and and low- low-concentrationconcentration anthracene-contaminated anthracene-contaminated soil. soil. (a) Effect(a)Eff ectof NaCl of NaCl concentration concentration on onhigh- high- and and lo low-concentrationw-concentration anthracene-contaminated anthracene-contaminated soil; soil; (b (b) ) EffectEffect of of KCl KCl concentration concentration on on high- high- and and lo low-concentrationw-concentration anthracene-contaminated soil.

3.2.4. Eﬀect of Granularity The eluting rate of polycyclic aromatic hydrocarbon pollutants in the soil is closely related to the properties of the soil, among which the soil texture is the most aﬀected, followed by the properties of

soil minerals and the content of organic matter. The smaller the soil particles, the larger the speciﬁc surface area of soil is, and the stronger the adsorption of pollution is, so the eluting eﬃciency of pollutants will be greatly suppressed [42,43]. As can be seen from Figure5, as the mesh number increases, the soil particle size is gradually decreasing, and the pollutant of the polycyclic aromatic Water 2020, 12, x FOR PEER REVIEW 9 of 21

3.2.4. Effect of Granularity The eluting rate of polycyclic aromatic hydrocarbon pollutants in the soil is closely related to the properties of the soil, among which the soil texture is the most affected, followed by the properties of soil minerals and the content of organic matter. The smaller the soil particles, the larger the specific surface area of soil is, and the stronger the adsorption of pollution is, so the eluting efficiency of pollutants will be greatly suppressed [42,43]. As can be seen from Figure 5, as Water 2020, 12, 2188 9 of 20 the mesh number increases, the soil particle size is gradually decreasing, and the pollutant of the polycyclic aromatic hydrocarbon in the soil gradually increases. Regardless of the concentration of pollutants,hydrocarbon the in decrease the soil gradually of the soil increases. particle Regardlesssize is beneficial of the concentration to the separation of pollutants, of difficult the decreaseeluting substancesof the soil particlefrom the size surface is beneﬁcial of soil particles, to the separation so as to achieve of diﬃcult the eluting purpose substances of eluting. from the surface of soil particles, so as to achieve the purpose of eluting.

90 85

88 84

86 83

84 82

81 82

80 80 Low concentration eluting PAHs rate (%) High concentration PAHs eluting rate (%)

10 20 30 40 50 60 Granularity(mesh)

Figure 5. Eﬀect of granularity on high- and low-concentration anthracene-contaminated soil. 3.3. EﬀectFigure of Mixed 5. Effect Surfactants of granularity on the on Eluting high- and Interaction low-concentration of PAH-Contaminated anthracene-contaminated Soils soil.

3.3.3.3.1. Effect Design of Mixed Scheme Surfactants of Mixed on Surfactantthe Eluting onInteraction Eluting PAH-Contaminatedof PAH-Contaminated Soil Soils

3.3.1. FactorialDesign Scheme design of isMixed a multi-factor Surfactant on and Eluting multi-level PAH-Contaminated cross-grouping Soil design method for a comprehensive test. It can study the effects of multiple levels of two or more factors, and can provideFactorial more experimentaldesign is a informationmulti-factor thanand single multi-level factor experiments.cross-grouping It can design be used method to analyze for thea comprehensivemain effects of alltest. factors It can and study the interactionthe effects betweenof multiple factors levels at allof levels.two or Itmore is comprehensive factors, and can and providebalanced. more The experimental effect of the compound information reagent than concentration, single factor elutingexperiments. time, KCl It can concentration, be used to andanalyze NaCl the main effects of all factors and the interaction between factors at all levels. It is comprehensive concentration on the remediation of anthracene-contaminated soil by the sophorolipids-SDBS-Na2SiO3 andmixed-surfactant balanced. The systemeffect of was the studied compound by using reagent a four-factor concentration, and two-leveleluting time, factor KCl design concentration, scheme to andexplore NaCl the interactionconcentration between on thethe factors. remediation of anthracene-contaminated soil by the sophorolipids-SDBS-NaThe formula of the2SiO model3 mixed-surfactant function was as follows:system was studied by using a four-factor and two-level factor design scheme to explore the interaction between the factors. The formulaLnR = ofb the1 + modelb2 A function+ b3 B was+ b4 as Dfollows:+ b5 A B + b6 A C + b7 A D × × × × × × × × × + b B C + b B C + b C D + b A B C + b A B D + (5) 8LnRbbAbBbDbABbACbAD× ×=+×+×+×+××+××+××9 × × 10 × × 11 × × × 12 × × × b13 A B 12C D 3 4 5 6 7 ×+××+× × × ××+ ××+ ×××+ ×××+ b8BCb 9 BCb 10 CDb 11 ABCb 12 ABD (5) In the formula, R represents the response value; A, B, C, D represents the four factors; AB, AC, bABCD×××× BC, CD, AD, ABC,13 ABD, ABCD represents the interaction between the factors; and bi is the constants matching with the single factor and interaction. The factors and their interactions are expressed in the In the formula, R represents the response value; A, B, C, D represents the four factors; AB, AC, form of code, that is, the high-level code is 1, and the low level is 1. The experimental design and BC, CD, AD, ABC, ABD, ABCD represents the interaction between− the factors; and bi is the levels of independent variables considered in this study are presented in Table3. constants matching with the single factor and interaction. The factors and their interactions are In this experiment, the dependent variable is the elution efficiency (R) of anthracene. The empirical expressed in the form of code, that is, the high-level code is 1, and the low level is −1. The relationships between anthracene removal (R) and four test variables in coded units (A, mixed surfactants concentration; B, effect of eluting time; C, the concentration of NaCl; D, the concentration of KCl) obtained by the application of factor analysis are given by the following models:

LnR = 0.14 + 0.011 A + 0.028 B + 0.029 D + 0.015 A B 0.045 A C + 7.063 10 0.003 A D − × × × × × − × × × − × × (6) + 0.025 B C 0.057 B C + 0.023 C D + 0.015 A B C + 0.018 A B D 0.039 A B C D × × − × × × × × × × × × × − × × × × The Pareto analysis of this model is shown in Figure6a, with a positive correlation between the significance level of the factor and the length of the column. The red and gray line in the Pareto chart Water 2020, 12, 2188 10 of 20 stand for the “Bonferroni limit” and “t-value limit”, respectively, which are two levels of significance on the Pareto chart of effects. The value of the “Bonferroni limit” is a kind of significance indicator calculated based on the input data into Design Expert software in accordance with the Bonferroni test methodology, which is a type of multiple comparison test used in statistical analysis. The “t-value limit” is another kind of the significance indicator based on t-test methodology. The t-value is a test statistic for t-tests that measures the difference between an observed sample statistic and its hypothesized population parameter in units of standard error. It can be seen from the graph that the interaction effect of the oscillation time with the sodium chloride is the most significant factor, followed by the effect of the concentration of the compound medicament and the concentration of the potassium chloride, and finally the interaction effect of the four factors was selected.

Table 3. Experimental design matrix and dependent variables attributed to the factor analysis design.

Mixed-Surfactant 1 1 1 Eluting Time/h KCl/mol L− NaCl/mol L− Concentration/mg L− · · Run · ABCD Coded Actual Coded Actual Coded Actual Coded Actual 1 1 1500 1 12 1 0.05 1 0.1 2 1 1500 1 6 1 0.05 1 0.1 − 3 1 1500 1 6 1 0 1 0 − − − 4 1 1500 1 12 1 0.05 1 0 − 5 1 1500 1 6 1 0 1 0.1 − − 6 1 500 1 12 1 0.05 1 0 − − 7 1 500 1 6 1 0.05 1 0 − − − 8 1 500 1 12 1 0.05 1 0.1 − 9 1 500 1 12 1 0 1 0.1 − − 10 1 500 1 6 1 0 1 0.1 − − − 11 1 500 1 12 1 0 1 0 − − − 12 1 1500 1 12 1 0 1 0.1 − 13 1 1500 1 6 1 0.05 1 0 − − 14 1 1500 1 12 1 0 1 0 − − Water 202015, 12, x FOR PEER1 REVIEW 500 1 6 1 0 1 0 11 of 21 − − − − 16 1 500 1 6 1 0.05 1 0.1 − −

(a) (b)

FigureFigure6. 6. Factorial experimental errorerror analysisanalysis diagram.diagram. ((aa)) ParetoPareto chart,chart, ( (bb)) Normal Normal plot plot of of residuals. residuals

InIn general, general, the the e ﬀeffectect of of the the interaction interaction on on the the elution elution rate rate of of polycyclic polycyclic aromatic aromatic hydrocarbons hydrocarbons is greateris greater than than that that of a of single a single factor. factor. The residualThe residual graph graph of the of model, the model, as shown as shown in Figure in6 Figureb, is evenly 6b, is distributedevenly distributed on both sideson both of the sides straight of the line, straight indicating line, thatindicating the residual that the meets residual the normal meets distribution. the normal distribution. The error analysis of the model was used to evaluate the correlation between the equation of the model and the experimental data. The p value of the model is lower than 0.05, which indicates that this factor has a significant effect on the remediation of anthracene-contaminated soil with compound agents [44]. The error analysis of the experimental model is shown in Table 4, and the p value is less than 0.0018, which indicates that the model is significant. The R2 is 0.99971, which indicates that the regression model has good reliability. Among them, the interaction between the concentration of the compound reagent and the amount of ion is obvious.

Table 4. ANOVA report for factorial model on anthracene removal.

Source Sum of Square df Mean Square F-Value p-Value model 0.167 12 0.014 86.77 0.0018 A 0.002 1 0.002 12.34 0.0391 B 0.012 1 0.012 75.67 0.0032 D 0.013 1 0.013 83.58 0.0028 AB 0.003 1 0.003 21.13 0.0194 AC 0.032 1 0.032 200.37 0.0008 AD 0.001 1 0.001 5.77 0.0957 BC 0.010 1 0.010 60.90 0.0044 BD 0.052 1 0.052 324.18 0.0004 CD 0.008 1 0.008 51.62 0.0056 ABC 0.003 1 0.003 21.72 0.0186 ABD 0.005 1 0.005 30.97 0.0114 ABCD 0.025 1 0.025 152.99 0.0011 Cor 0.167 15 Total

3.3.2. Main Effect Analysis Through Design-Expert analysis, the main effect results of these factors are shown in the Figure 7. According to ANOVA statistics, the order of significance among the single factors is D >

Water 2020, 12, 2188 11 of 20

The error analysis of the model was used to evaluate the correlation between the equation of the model and the experimental data. The p value of the model is lower than 0.05, which indicates that this factor has a significant effect on the remediation of anthracene-contaminated soil with compound agents [44]. The error analysis of the experimental model is shown in Table4, and the p value is less than 0.0018, which indicates that the model is significant. The R2 is 0.99971, which indicates that the regression model has good reliability. Among them, the interaction between the concentration of the compound reagent and the amount of ion is obvious.

Table 4. ANOVA report for factorial model on anthracene removal.

Water3.3.2. 2020 Main, 12, Ex ffFORect PEER Analysis REVIEW 12 of 21 Through Design-Expert analysis, the main effect results of these factors are shown in the Figure7. B > A. The effects of the compound agent concentration (Figure 7a), oscillation time (Figure 7b), and According to ANOVA statistics, the order of significance among the single factors is D > B > A. NaCl (Figure 7c) concentration on the eluting effect of polycyclic aromatic hydrocarbons (PAHs) The effects of the compound agent concentration (Figure7a), oscillation time (Figure7b), and NaCl have a positive and significant effect, while the effect of the KCl concentration on the eluting effect (Figure7c) concentration on the eluting e ffect of polycyclic aromatic hydrocarbons (PAHs) have a is insensitive. The slope of the compound reagent concentration is smaller than the other two positive and significant effect, while the effect of the KCl concentration on the eluting effect is insensitive. factors, which indicates that the slope of the compound reagent concentration has little effect on the The slope of the compound reagent concentration is smaller than the other two factors, which indicates other two factors. that the slope of the compound reagent concentration has little effect on the other two factors.

(a)

Figure 7. Cont.

(b)

Water 2020, 12, x FOR PEER REVIEW 12 of 21

B > A. The effects of the compound agent concentration (Figure 7a), oscillation time (Figure 7b), and NaCl (Figure 7c) concentration on the eluting effect of polycyclic aromatic hydrocarbons (PAHs) have a positive and significant effect, while the effect of the KCl concentration on the eluting effect is insensitive. The slope of the compound reagent concentration is smaller than the other two factors, which indicates that the slope of the compound reagent concentration has little effect on the other two factors.

Water 2020, 12, 2188 12 of 20 (a)

Water 2020, 12, x FOR PEER REVIEW 13 of 21 (b)

(c)

FigureFigure 7.7. Analysis of the mainmain eeffectﬀect ofof thethe factorialfactorial experiment.experiment. (a) RelationshipRelationship betweenbetween elutionelution eefficiencyﬃciency (R) (R) and and surfactant surfactant concentration; concentration; (b) Relationship (b) Relationship between between R and eluting R and time; eluting (c) Relationship time; (c) betweenRelationship R and between NaCl concentration. R and NaCl concentration.

3.3.3.3.3.3. Interaction Analysis ItIt cancan bebe seen seen from from the the graph graph that that the the slope slope of the of twothe straighttwo straight lines islines not is the not same, the whichsame, meanswhich thatmeans there that is athere certain is a interaction certain interaction between thebetween two factors the two on factors the response on the value. response According value. toAccording ANOVA statistics,to ANOVA the statistics, inﬂuence the of interactioninfluence of is interaction BD > AC > isBC BD> >CD AC> > AB.BC The> CD factor > AB. A The and factor the factor A and B arethe obtainedfactor B are at the obtained left to theat the left left ofFigure to the8 lefta, while of Figu a highre 8a, level while of thea high factor level B is of always the factor higher B is than always the lowhigher level than of factorthe low B, andlevel when of factor the compoundB, and when agent the concentrationcompound agent changes concentration from a low changes level to from a high a level,low level the linearto a high slope level, becomes the linear large, slope which becomes indicates large, that which a high indicates level of that factor a high b is beneﬁciallevel of factor to the b elutionis beneficial of the to polycyclic the elution aromatic of the polycyclic hydrocarbons. aromatic hydrocarbons. The factor A and the factor C are also taken at the left to the left of Figure 8b and a higher elution rate can be obtained when the factor A is at a low level and C is at a high level. With the increase of the concentration of factor A, the low factor c is gradually occupying the advantage. As can be seen in Figure 8c,d, the intersection of factor B and factor C on the right, and the slope of the two straight lines is positive, and it has a positive significant effect. However, the slope of the low-level factor D is positive and the slope of the high factor D is negative, indicating that the low level of factor D is dominant after a high level of factor B is reached. The factor C and the factor D can be seen in Figure 8e to have a clear interaction, with the intersection point on the left. The high level of the factor D is always higher than the low level of factor D. When the concentration of factor C changes from a low level to a high level, the linear slope becomes large, indicating that the high level of factor D is beneficial to the elution of the polycyclic aromatic hydrocarbons.

Water 2020, 12, 2188 13 of 20 Water 2020, 12, x FOR PEER REVIEW 14 of 21

(a) (b)

(e)

FigureFigure 8. The8. The interaction interaction between between fourfour testtest variables. (a) ( aThe) The interaction interaction between between surfactant surfactant concentration and eluting time; (b) The interaction between surfactant concentration and KCl concentration and eluting time; (b) The interaction between surfactant concentration and KCl

concentration; (c) The interaction between eluting time and KCl concentration; (d) The interaction between eluting time and NaCl concentration; (e) The interaction between KCl concentration and NaCl concentration. Water 2020, 12, 2188 14 of 20

The factor A and the factor C are also taken at the left to the left of Figure8b and a higher elution rate can be obtained when the factor A is at a low level and C is at a high level. With the increase of the concentration of factor A, the low factor c is gradually occupying the advantage. As can be seen in Figure8c,d, the intersection of factor B and factor C on the right, and the slope of the two straight lines is positive, and it has a positive significant effect. However, the slope of the low-level factor D is positive and the slope of the high factor D is negative, indicating that the low level of factor D is dominant after a high level of factor B is Waterreached. 2020, The12, x FOR factor PEER C andREVIEW the factor D can be seen in Figure8e to have a clear interaction, with15 of the 21 intersection point on the left. The high level of the factor D is always higher than the low level of factorconcentration; D. When the (c concentration) The interaction of between factor C eluting changes time from and a lowKCl levelconcentration; to a high (d level,) The theinteraction linear slope becomesbetween large, eluting indicating time and that NaCl the concentration; high level of factor(e) The D interaction is beneficial between to the KCl elution concentration of the polycyclic and aromaticNaCl hydrocarbons.concentration.

3.4. Removal Removal of of Anthracene Anthracene from Eluent by the Base/SPS Base/SPS

3.4.1. The The Oxidation Oxidation Effects Effects of Different Different Types ofof AlkaliAlkali ActivationActivation onon thethe EluentEluent In thethe experiment, experiment, sodium sodium hydroxide, hydroxide, calcium calcium hydroxide, hydroxide, calcium calcium oxide, and oxide, potassium and potassium hydroxide werehydroxide selected were as theselected bases as to activatethe bases sodium to activate persulfate, sodium and persulfate, the optimal and removal the optimal efficiency removal could efficiencyreach more could than 50%.reach Itmore can bethan seen 50%. from It Figurecan be9 seen that thefrom removal Figure rate9 that of PAHsthe removal in elution rate waste of PAHs liquid in elutionfrom high waste to low liquid is potassium from high hydroxide, to low is calciumpotassium oxide, hydroxide, sodium calcium hydroxide, oxide, and sodium calcium hydroxide, hydroxide, whichand calcium may behydroxide, related to which the dissociation may be related capacity to the of dissociation the substance. capacity Chengdu of the Qisubstance. et al. [33 Chengdu] studied Qithe et e ffal.ect [33] of alkali-activatedstudied the effect peroxic of alkali-activated monosulfate peroxic on the degradationmonosulfate ofon organicthe degradation pollutants, of andorganic the pollutants,results were and consistent. the results Therefore, were potassiumconsistent. hydroxideTherefore, was potassium used as thehydroxide activator was in the used subsequent as the activatoroxidation in experiment. the subsequent oxidation experiment.

FigureFigure 9. TheThe oxidation oxidation effects eﬀects of different diﬀerent types of alkali activation.

The effiefficiencyciency of alkali-activatedof alkali-activated sodium sodium persulfate persulfate has a certain has relationshipa certain relationship with the concentration with the concentrationof added alkali. of Figureadded 10 alkali.shows theFigure analysis 10 shows of base-activated the analysis persulfate of base-activated decomposition persulfate kinetics. decompositionThe pseudo-first-order kinetics. loss The of pseudo-first-order persulfate in solutions loss withof persulfate a range of in base:persulfate solutions with molar a range ratios of is base:persulfateshown in Figure molar 10. As ratios can be is seenshown from in FigureFigure 10 10., after As can adding be seen different from concentrations Figure 10, after of sodiumadding differentpersulfate, concentrations the degradation of sodium rate of persulfate persulfate, increases the degradation with the increaserate of persulfate of the potassium increases hydroxide with the increaseconcentration of the at di ffpotassiumerent times, hydroxide indicating thatconcentration the decomposition at different rate of times, persulfate indicating is a function that of the decompositionalkalinity. OLHA rate S [of37 ]persulfate also reached is a thefunction same conclusionof the alkalinity. by measuring OLHA S the [37] relationship also reached between the same the conclusionultrasonic timeby measuring and ln (C/ C0),the relationship where C is thebetween persulfate the ultrasonic ion concentration time and after ln (C/C0), adding where the oxidizing C is the persulfateagent, and C0ion is concentration the concentration after of persulfateadding the ions oxidizing before oxidant agent, treatment. and C0 Theseis the results concentration also indicate of persulfate ions before oxidant treatment. These results also indicate that alkali-catalyzed hydrolysis of persulfate is the first step of persulfate activation under alkaline conditions and the decomposition rate of persulfate increases with an increasing KOH concentration.

Water 2020, 12, 2188 15 of 20 Water 2020, 12, x FOR PEER REVIEW 16 of 21 thatWater alkali-catalyzed 2020, 12, x FOR PEER hydrolysis REVIEW of persulfate is the ﬁrst step of persulfate activation under alkaline16 of 21 conditions and the decomposition rate of persulfate increases with an increasing KOH concentration. 0.2

0.2 0.1

0.1 0.0 ） 0.0 -0.1 C/C0 ）（ ln

-0.1 C/C0 -0.2 （ ln 2:1 KOH:sodium persulfate -0.2 1:1 KOH:sodium persulfate -0.3 1:2 KOH:sodium persulfate 1:4 KOH:sodium persulfate 2:1 KOH:sodium persulfate 1:1 KOH:sodium persulfate -0.3 0 1:2 20KOH:sodium persulfate 40 60 80 100 120 1:4 KOH:sodium persulfate Ultrasonic time (min) 0 20 40 60 80 100 120 Ultrasonic time (min) Figure 10. The relationship between ultrasonic time and ln(C/C0).

Figure 10. The relationship between ultrasonic time and ln(C/C0). 3.4.2. Effect of DifferentFigure Ultrasound 10. The relationship Times on betweenthe Base ultr of asonicSodium time Persulfate and ln(C/C0). 3.4.2. Effect of Different Ultrasound Times on the Base of Sodium Persulfate This mixed surfactant solution only transfers contaminants from the soil to the aqueous phase. 3.4.2. Effect of Different Ultrasound Times on the Base of Sodium Persulfate FromThis Figure mixed 11, surfactant we can solutionconclude only that transfers in differen contaminantst ultrasonic fromtime theranges, soil to the the removal aqueous effect phase. of Fromalkali-activatedThis Figure mixed 11, wesodiumsurfactant can concludepersulfate solution that ononly inanthracene transfers different co pollutants ultrasonicntaminants in time from eluent ranges, the increased soil the to the removal from aqueous 28.41% effect phase. of to alkali-activated61.77%From Figure in the range11, sodium we of can persulfate20–60 conclude min. on With anthracenethat the in increase differen pollutants oft ultrasonicthe intime, eluent the time increasedremoval ranges, efficiency from the 28.41%removal of anthracene to effect 61.77% of intendsalkali-activated the range to be ofgentle 20–60 sodium or min. even Withpersulfate decreases. the increase on Due anthracene to of the the ac time,tion pollutants theof the removal ultrasonic in eluent efficiency wave, increased of anthracene anthracene from pollutants28.41% tends to to bedissolved61.77% gentle in or thein even eluentrange decreases. ofare 20–60 dissociated Due min. to theWith from action the theincrease of thesoil. ultrasonic ofAfter the time,60 wave,min, the the anthraceneremoval eluting efficiency pollutantsefficiency of anthracene dissolvedno longer inincreases,tends eluent to are be which dissociatedgentle may or even be from due decreases. the to soil.the existenceDue After to 60 the min,of ac a tion thelarge elutingof number the ultrasonic effi ciencyof surfactants wave, no longer anthracene in increases,the waste pollutants whicheluent, maywhichdissolved be duehas ina to certain theeluent existence competitive are dissociated of a large effect numberfrom on the the ofsulfate soil. surfactants Afterradical 60 in[45], min, the so waste theit will eluting eluent, further whichefficiency affect has the no a removal certain longer competitiveefficiencyincreases, ofwhich eff pollutantsect on may the be sulfatein duethe waste radicalto the eluent. existence [45], so it of will a furtherlarge number affect the of removalsurfactants effi ciencyin the ofwaste pollutants eluent, inwhich the waste has a eluent. certain competitive effect on the sulfate radical [45], so it will further affect the removal efficiency of pollutants in the waste eluent.

Figure 11. Effect of different ultrasound times on the base of sodium persulfate (SPS). Figure 11. Effect of different ultrasound times on the base of sodium persulfate (SPS). 3.4.3. Effect of Different Activation Ratios on the Base of Sodium Persulfate

3.4.3.Oxidant Effect /Figureofpollutant Different 11. Effect ratios Activation of were different selected Ra ultrasoundtios accordingon the times Base to on previousof theSodium base studies of Persulfate sodium in the persulfate literature (SPS). with alkaline persulfate in diesel and PAHs abatement [46,47]. The molar ratio of activator/oxidant in the literature Oxidant/pollutant ratios were selected according to previous studies in the literature with ranges3.4.3. Effect from 2of to Different 6 [35]. As Activation shown in Ra Figuretios on 12 the, it Base was shownof Sodium that Persulfate the ratio of oxidant to activating alkaline persulfate in diesel and PAHs abatement [46,47]. The molar ratio of activator/oxidant in the literatureOxidant/pollutant ranges from 2 ratiosto 6 [35]. were As selected shown inaccordin Figure g12, to itprevious was shown studies that thein theratio literature of oxidant with to alkaline persulfate in diesel and PAHs abatement [46,47]. The molar ratio of activator/oxidant in the literature ranges from 2 to 6 [35]. As shown in Figure 12, it was shown that the ratio of oxidant to

Water 2020, 12, x FOR PEER REVIEW 17 of 21

Water 2020, 12, 2188 16 of 20 activatingWater 2020 ,agent 12, x FOR is 1:2PEER when REVIEW sodium persulfate is activated by alkali, which is consistent with17 the of 21 above research contents and ensures that it can be carried out in an alkaline environment. activating agent is 1:2 when sodium persulfate is activated by alkali, which is consistent with the agent is 1:2 when sodium persulfate is activated by alkali, which is consistent with the above research above research contents and ensures that it can be carried out in an alkaline environment. contents and ensures that it can be carried out in an alkaline environment.

Figure 12. Effect of different activation ratios on the base of SPS. Figure 12. Effect of different activation ratios on the base of SPS. 3.4.4. Effect of TurbidityFigure of Eluent 12. Effect of different activation ratios on the base of SPS. 3.4.4. Effect of Turbidity of Eluent After centrifugation, the turbidity value of the soil contaminated by PAHs eluted with complex 3.4.4. Effect of Turbidity of Eluent surfactantsAfter centrifugation,was about 200–400 the turbidity nephelometric value ofturbidity the soil unit contaminated (NTU), which by PAHs was relatively eluted with high. complex This issurfactants becauseAfter of wascentrifugation, the about soil remains 200–400 the in nephelometricturbidity the aqueous value turbidity solution,of the soil unit or contaminated when (NTU), the which centrifugal by was PAHs relatively solutioneluted high.with is poured complex This is out,becausesurfactants some of theof was the soil about fine remains 200–400particle in the nephelometricsuspensions aqueous solution, fl turbidityow out or when withunit the(NTU),the centrifugal washing which was solutionsolution relatively is[48,49]. poured high. The out,This solubilizationsomeis because of the fineof and the particle soilemulsification remains suspensions in ofthe flowthe aqueous surfactant out with solution, the during washing or when the solution washingthe centrifugal [48 process,49]. The solution results solubilization isin poured the presenceandout, emulsification some of oil-in-water, of the of fine the dispersed surfactantparticle oilysuspensions during substances, the washing flow and out processmicelles with results inthe the washing inliquid the presencephase. solution In of the oil-in-water,[48,49]. filtration The process,dispersedsolubilization there oily substances, areand different emulsification and filtration micelles of instates,the the surfactant liquid small phase. soil during Inparticles the the filtration washingare intercepted, process, process there andresults are ditheff erentin oil the substancesfiltrationpresence states, andof oil-in-water, macromolecular small soil particles dispersed micelles are oily intercepted, attachedsubstances, andto the and the particles oilmicelles substances may in the also and liquid be macromolecular intercepted. phase. In the This micellesfiltration is an unavoidableattachedprocess, to there theproblem particles are indifferent the may process also filtration be of intercepted. operation, states, sosmall This it will issoil an have unavoidableparticles a certain are impact problemintercepted, on inthe the turbidityand process the of ofoil theoperation,substances water. As so andshown it will macromolecularhave in Figure a certain 13a, micelles impactafter the on attached elution the turbidity wasteto the solution ofparticles the water. wasmay Astreated also shown be with intercepted. in alkali-activated Figure 13 Thisa, after is an oxidant,theunavoidable elution the waste turbidity problem solution changed in wasthe process treated significantly withof operation, alkali-activated and decreased so it will oxidant, tohave below a the certain turbidity100 NTU.impact changed For on thethe significantly resultsturbidity of of differentandthedecreased water. activation As toshown below ratios, in 100 Figure as NTU. shown 13a, For afterin the Figure resultsthe elution 13b, of di that ffwasteerent is, solution activationthe turbidity was ratios, treated of aseluent shown with waste alkali-activated in Figure solution 13b, beforethatoxidant, is, and the turbiditytheafter turbidity alkali of eluentaddition changed waste significantly significantly solution before changed and and decreased afterfrom alkali346–440 to additionbelow NTU 100 significantlyto NTU.39–72 ForNTU changed the under results fromthe of condition346–440different NTU of activation different to 39–72 ratios,alkali NTU underconcentrations,as shown the conditionin Figure indica of 13b,ting diff erentthat that is, alkalithe the turbidity concentrations,turbidity of of the eluent solution indicating waste changed thatsolution the significantlyturbiditybefore and of theunder after solution alkalidifferent changedaddition alkali significantly significantlyactivations and underchanged treatment diff erentfrom times. alkali346–440 activations NTU to and39–72 treatment NTU under times. the condition of different alkali concentrations, indicating that the turbidity of the solution changed significantly under different alkali activations and treatment times.

FigureFigure 13. 13. ChangesChanges in in the the turbidity turbidity of of anthracene anthracene elution elution wastewater. wastewater. (a (a) )Turbidity Turbidity of of anthracene anthracene elutionelution wastewater wastewater in in different diﬀerent ultrasonic ultrasonic times; times; (b (b) )Turbidity Turbidity of of anthracene anthracene elution elution wastewater wastewater in in differentdiFigureﬀerent rations rations13. Changes of of oxidant oxidant in the and and turbidity activator. activator. of anthracene elution wastewater. (a) Turbidity of anthracene elution wastewater in different ultrasonic times; (b) Turbidity of anthracene elution wastewater in different rations of oxidant and activator.

Water 2020, 12, x FOR PEER REVIEW 18 of 21

3.4.5. Results and Analysis of Elution Waste Liquid by Infrared Spectrum The infrared spectra of eluent without oxidant treatment is shown in Figure 14a. Here, 3183 Watercm−1 2020is ,the12, 2188C-H stretching vibration absorption peak (-C≡C-H), 1276 cm−1 is the C-O 17(ester) of 20 absorption peak, and 692 cm−1 is the C-H surface bending vibration absorption peak of aromatic hydrocarbon. As shown in Figure 14b, the infrared spectrum after oxidation treatment increased by 3.4.5. Results and Analysis of Elution Waste Liquid by Infrared Spectrum 1726 cm−1 as ester (non-cyclic), 1037 cm−1 as the absorption peak of bending vibration in the C-H 1 plane,The 918 infrared cm−1 as spectra the absorption of eluent withoutpeak of oxidantbending treatment vibration is outside shown the in Figure C-H plane, 14a. Here, and 3183724 as cm the− 1 isabsorption the C-H stretchingpeak of bending vibration vibration absorption outside peak the (-C C-HC-H), plane. 1276 cm− is the C-O (ester) absorption 1 ≡ peak,Anthracene and 692 cm is− ais thick the C-Hcyclic surface aromatic bending hydrocarbon vibration containing absorption three peak rings. of aromatic Through hydrocarbon. the analysis 1 Asof infrared shown in spectroscopy, Figure 14b, the the infrared inner part spectrum of eluent after waste oxidation liquidtreatment underwent increased a ring-opening by 1726 reaction, cm− as 1 1 esterand the (non-cyclic), ring-opening 1037 cmreaction− as thealso absorption occurred peakin the of part bending of the vibration cyclic ester in the group C-H plane, of the 918 surfactant, cm− as thewhich absorption changed peak from of a bendingring to non-ring. vibration The outside results the show C-H that plane, the andoxidant 724 ashad the a significant absorption effect peak on of bendingthe removal vibration of theoutside elution thewaste C-H liquid plane. containing surfactant and anthracene.

51.6

43.0

34.4

25.8

3500 3000 2500 2000 1500 1000 500 0 Wave/cm-1

(a)

3500 3000 2500 2000 1500 1000 500 0 -1 Wave/cm (b)

FigureFigure 14.14. InfraredInfrared spectraspectra beforebefore andand afterafter oxidationoxidation treatment. (a)) The infraredinfrared spectraspectra ofof eluenteluent withoutwithout oxidantoxidant treatment;treatment; ((bb)) TheThe infraredinfrared spectra spectra of of eluent eluent after after oxidant oxidant treatment. treatment.

4. ConclusionsAnthracene is a thick cyclic aromatic hydrocarbon containing three rings. Through the analysis of infrared spectroscopy, the inner part of eluent waste liquid underwent a ring-opening reaction, and the ring-openingThe use of surfactants reaction also to repair occurred PAH-contaminated in the part of the soil cyclic is esteran economical group of theand surfactant, efficient method which changedwith environmental from a ring sustainabili to non-ring.ty. The The resultspresent show study that showed the oxidant that the had sophorolipids-SDBS-Na a significant effect on2SiO the3 removalcould remove of the anthracene elution waste from liquid soil containingto a certain surfactant extent. Alkali-activated and anthracene. sodium persulfate also has a good effect on the removal of anthracene pollutants from eluting wastewater. 4. Conclusions (1) In the single factor experiment, the best concentration of sophora and SDBS was 40 and 100 Themg/L, use respectively, of surfactants that to repairis to PAH-contaminatedsay, the removal efficiency soil is an economicalwas best at and the e fficriticalcient methodmicelle with environmentalconcentration. sustainability.At the same Thetime, present the additive study showed sodium that silicate the sophorolipids-SDBS-Na can react with the 2acidicSiO3 couldcomponents remove anthracene of oil to from form soil salt, to which a certain increases extent. Alkali-activatedthe water solubility sodium of oil persulfate and improves also has the a goodremoval effect on efficiency the removal of the of anthracene composite pollutantssurfactant. from eluting wastewater. (2) When the elution time was 8 h, the concentration of the compounded surfactant was 1200 (1) In the single factor experiment, the best concentration of sophora and SDBS was 40 and 100 mg/L, mg/L, the particle size was 60 mesh, the concentration of NaCl was 100 mmol/L, and the respectively, that is to say, the removal efficiency was best at the critical micelle concentration. At the same time, the additive sodium silicate can react with the acidic components of oil to form salt, which increases the water solubility of oil and improves the removal efficiency of the composite surfactant. Water 2020, 12, 2188 18 of 20

(2) When the elution time was 8 h, the concentration of the compounded surfactant was 1200 mg/L, the particle size was 60 mesh, the concentration of NaCl was 100 mmol/L, and the concentration of KCl was 50 mmol/L, thus the effect of the PAH-contaminated soil eluted by the composite surfactant was the best; externally added NaCl and KCl salt ions had a more obvious promotion effect on the polycyclic aromatic hydrocarbon-contaminated soil. (3) In the factorial design experiment, the residual plot of the regression equation conformed to the normal distribution, and the p value of the error analysis was less than 0.0018, which showed that the regression equation simulation was significant. It can be seen from the Pareto analysis chart that single factor B (elution time) and D (NaCl concentration) have a significant main effect. There is also a certain interaction between factor A (concentration agent concentration) and factor D, factor B, and factor C (KCl concentration). (4) The ultrasonic oxidation technology was used, combined with alkali-activated sodium persulfate, to explore the influence of the ultrasonic time, activation ratio, turbidity, and other factors on the oxidation effect. When the optimal ultrasonic time was 60 min and the ratio of oxidant to activator was 1:2, the removal rate of contaminants in the eluent could reach 63.7%. At the same time, the turbidity of the eluent was significantly lower than that of the liquid after centrifugal separation, indicating that oxidants can not only remove the pollutants in elution water but also remove the residual soil particulate matter. (5) By comparing the infrared spectrum of the eluted waste liquid before and after oxidation, it was seen that during the oxidation process, the inner part of eluent waste liquid underwent a ring-opening reaction, and the ring-opening reaction also occurred in the part of the cyclic ester group of the surfactant, which changed from a ring to non-ring.

Through the eluting of soil pollutants and the removal of pollutants from wastewater, organic matter can be mostly removed. This complete removal process is suitable for the field of surfactant eluting soil and polluted wastewater. However, there are still some deficiencies in the current research, such as: (1) The elution effect of the composite surfactant studied in this paper on PAHs in the actual contaminated site was not measured, and the subsequent research should start from the actual contaminated site to determine the elution effect; (2) some basic water quality parameters should be measured after the treatment of the elution wastewater, so that the removal method is more practical; and (3) the removal mechanism of PAH eluting wastewater should be further studied.

Author Contributions: Conceptualization, L.S. and X.W.; methodology, X.W.; software, X.W. and L.S.; validation, X.W., and W.L.; formal analysis, X.W.; investigation, X.W.; resources, X.W.; data curation, X.W. and L.S.; writing—Original draft preparation, X.W.; writing—Review and editing, X.W. and L.S.; visualization, X.W.; supervision, W.L.; project administration, X.D. and Z.W.; funding acquisition, W.L. All authors have read and agreed to the published version of the manuscript. Funding: This Study was funded by the National Key Research and Development Plan, grant number (2018YFE0196000). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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