Chemosphere 121 (2015) 117–123

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Chemosphere

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Technical Note

Low-concentration tailing and subsequent quicklime-enhancedremediation of volatile chlorinated hydrocarbon-contaminatedsoils by mechanical soil aeration

http://dx.doi.org/10.1016/j.chemosphere.2014.10.0740045-6535/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +86 10 8491 5233; fax: +86 10 8493 2813.E-mail address: ligulax@vip.sina.com (F. Li).

Yan Ma a,b, Xiaoming Du b, Yi Shi b,c, Zhu Xu b, Jidun Fang d, Zheng Li b, Fasheng Li b,⇑a College of Water Sciences, Beijing Normal University, Beijing 100875, Chinab State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, Chinac Beijing University of Science and Technology, Civil and Environment Engineering Department, Beijing 100083, Chinad Shandong Key Laboratory of Eco-Environmental Science for Yellow River Delta, Binzhou University, Binzhou, Shandong Province 256600, China

h i g h l i g h t s

� Tailing behavior of VCHs in mechanical soil aeration was identified.� Quicklime-enhanced remediation could mitigate the tailing of contaminants.� Effect of temperature, soil moisture and permeability on tailing was analyzed.

a r t i c l e i n f o

Article history:Received 21 September 2014Received in revised form 25 October 2014Accepted 28 October 2014Available online 26 November 2014

Handling Editor: O. Hao

Keywords:Mechanical soil aerationVolatile chlorinated hydrocarbon-contaminated soilsLow-concentration tailingQuicklimeEnhanced remediation

a b s t r a c t

Mechanical soil aeration has long been regarded as an effective ex-situ remediation technique and assuitable for remediation of large-scale sites contaminated by volatile organic compounds (VOCs) atlow cost. However, it has been reported that the removal efficiency of VOCs from soil is relatively lowin the late stages of remediation, in association with tailing. Tailing may extend the remediation timerequired; moreover, it typically results in the presence of contaminants residues at levels far exceedingregulations. In this context, the present study aimed to discuss the tailing that occurs during the processof remediation of soils contaminated artificially with volatile chlorinated hydrocarbons (VCHs) and toassess possible quicklime-enhanced removal mechanisms. The results revealed the following conclu-sions. First, temperature and aeration rate can be important controls on both the timing of appearanceof tailing and the levels of residual contaminants. Furthermore, the addition of quicklime to soil duringtailing can reduce the residual concentrations rapidly to below the remedial target values required forsite remediation. Finally, mechanical soil aeration can be enhanced using quicklime, which can improvethe volatilization of VCHs via increasing soil temperature, reducing soil moisture, and enhancing soil per-meability. Our findings give a basic understanding to the elimination of the tailing in the application ofmechanical soil aeration, particularly for VOCs-contaminated soils.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Mechanical soil aeration has been regarded as an effective tech-nique in soil remediation, typically agitating contaminated soilthrough tilling or other means to volatilize contaminants (USEPA,2007); in particular, it has been applied widely as a low-costex-situ remediation technique for sites contaminated by VOCs,particularly in developing countries (Shi et al., 2012). Usually, suchremediation is conducted in a sealed shed using tilling or other

methods to volatilize contaminants, allowing the released gas tobe collected and then treated.

However, traditionally, mechanical soil aeration offers ratherlimited efficiency in removing VOCs from soil in the late stage ofremediation, especially under low-temperature conditions andfor soils with high moisture and clay contents. Tailing is typicallydescribed as a behavior for which further removal of contaminantsmay be achieved only slowly, even if the treatment time isextended, and is associated with the slow, continuous release ofcontaminants over time. A previous study (Shi et al., 2012) demon-strated that the residual concentration did not reach the remedialtarget value until the end of the experiment (i.e., at 615 h) at low

118 Y. Ma et al. / Chemosphere 121 (2015) 117–123

soil temperatures during the tailing phase; further, this study dem-onstrated that the efficiency of remediation was closely related tomany factors, including soil temperature and agitation frequency.Tailing not only necessitates time-consuming remediation activity,but also results in residue contaminant concentrations that areabove the remedial target values. The long duration of tailing isubiquitous in the remediation of soil contaminated with VOCs;such soil is typically treated using processes such as soil vaporextraction, air and steam flushing, and soil ventilation (Nadimet al., 1997; Huang and Goltz, 1999; Sleep and McClure, 2001).The continuous flow of air through soil removes both non-aqueousphase liquids (NAPLs) and the dissolved and sorbed phases into themoving air. In the early stages of vapor extraction, NAPLs areextracted from the contaminated soil (Baehr et al., 1989). Afterthese NAPLs are depleted, removal of the residual contaminantsrequires volatilization from the dissolved aqueous phase togetherwith desorption from the solid surfaces into the dissolved aqueousphase (Armstrong et al., 1994), which is followed by persistentlong-duration tailing (with very small concentration) as vaporextraction continues. To date, it has proven difficult to removeresidual contaminants from soil when tailing occurs; this makesremediation time-consuming and results in low removal efficiency,particularly when the moisture content of soil is high or the satu-rated vapor pressure of contaminants is relatively low (Alvim-Ferraz et al., 2006; Albergaria et al., 2012).

In addition to the in situ remediation techniques mentionedabove, batch experiments and mathematical modeling methodshave been designed to investigate the adsorption and desorptionbehavior of soil contaminants, and have revealed that extendedtailing occurs during the desorption process; thus, remediationefficiency is impaired by low rates of desorption (Johnson et al.,2009; Kempf and Brusseau, 2009; Akyol et al., 2011; Brusseauet al., 2012). Moreover, several researches on the slow desorptionof contaminants have shown that residual contaminants are foundprimarily in micropores, thus inhibiting mass transfer and imped-ing the removal of contaminants (Werth and Reinhard, 1997; Liand Werth, 2001, 2004).

To weaken tailing such as that described above, several meth-ods have been proposed; for example, both heating of soil andapplication of microorganisms have been undertaken to improveremoval efficiency (Malina et al., 1998; Poppendieck et al., 1999).However, these techniques are unsuitable for large-scale applica-tion because of their high costs and the long remediation periods.The reaction of quicklime (CaO, a typical oxide) with water can beinduced to release large amounts of heat, which can be used in siteremediation to evaporate VOCs from soil. CaO is an inexpensiveraw material and its application in remediation processes is rela-tively simple. Moreover, several previous studies have investigatedthe efficiency of CaO remediation. For example, a series of previousstudies investigated the influence of increasing temperature andadding CaO on the removal efficiency of chlorinated hydrocarboncompounds (Koper et al., 1993, 1997; Koper and Klabunde,1997). Others studied the efficiency of CaO during dechlorinationof benzene hexachloride and dioxin when used as one of a combi-nation of mechanical and chemical techniques (Nomura et al.,2005, 2012). Ko et al. (2010, 2011) investigated the influence ofthe proportional addition of CaO on the efficiency and mechanismsof removal of trichloroethylene (TCE). However, few previous stud-ies have investigated tailing associated with VCHs-contaminatedsoil during large-scale remediation and most existing applicationsof CaO mainly target at sandy soil that is relatively permeable. Asfar as accordingly, methods of overcoming tailing in soil remedia-tion involving mechanical soil aeration, the needs to face how toovercome the tailing are required urgently.

The present study aimed to investigate low-concentration tail-ing during the late stage of mechanical soil aeration and to assess

subsequent CaO-enhanced removal mechanisms. 1,2-dichloroeth-ane (1,2-DCA), trichloroethylene, and tetrachloroethylene (PCE)were selected to simulate VCHs-contaminated soils. Clayey siltand silty clay were selected, because they are typical soils collectedfrom a VCHs-contaminated site in northern China. The impacts oftemperature and aeration rates on tailing were also studied, andthe effects of CaO on enhanced remediation were investigated.

2. Materials and methods

2.1. Chemicals and reagents

The 1,2-DCA, TCE, and PCE that were used to simulate contam-inated soils were purchased from Sinopharm Chemical Reagent,with purities of 99%, 99%, and 98.5%, respectively. CaO with a pur-ity of 98% was used to treat the artificially contaminated soils, andmethanol (Fisher Scientific, GC Resolv) was used to extract theproducts. The EPA 8260 Internal Standards mix, 8260 SurrogateStandards mix and 8260 VOC (54) mix were used to quantify theproducts.

2.2. Soils

Soils investigated in the study were collected from a contami-nated site in China at which a major chlor-alkali plant has beenoperational for more than 50 yr. All of the main contaminants inthe soil were VCHs. Two soil types were investigated: clayey silt(depth: 2.5–5.5 m) and silty clay (depth: 5.5–8.1 m).

The depollution of the real contaminated soils involved foursteps: (a) removing large particles and plant residues from the soil;(b) drying at room temperature during 20 days; (c) sieving througha #40 sieve (0.42 mm) to obtain a fraction of the soil with uniformphysical properties; (d) drying at 105 �C for 12 h. Properties andcharacteristics of the selected soils are presented in Table 1.

The contaminated soils were made as follows. First, the contam-inant solution was prepared by adding 15 mL of each of the threecontaminants to 250 mL of methanol. Next, 2000 g of sieved soilwas spread evenly on a polyethylene plastic sheet and 400 mL ofhigh-purity water was added. Then, the mixture was stirred toensure a uniform distribution of water content. We added 50 mLcontaminant solution rapidly and covered the mixture with1000 g of sieved soil and 200 mL of high-purity water. The mixturewas immediately wrapped with plastic sheeting and sealed withtape. And then, the inverted contaminated soils were placed inan airtight box at 20 �C for 48 h to ensure that the concentrationof contaminants was even.

2.3. Experimental apparatus

An apparatus was designed to simulate mechanical soil aera-tion. This apparatus allowed removal of contaminants from the soilthrough volatilization induced by mixing, cutting, and flipping ofthe soil by the rotation of coulters in the main body of the appara-tus. The volatile contaminants released were removed by airexchange and then discharged after treating with activated carbon.

The apparatus consisted of a main body, a temperature controlsystem, an automatic control system, and an exhaust gas treatmentsystem. A schematic diagram of the experimental apparatus ispresented in Fig. S1 (see Supporting Material). The main body con-sisted of a motor, inner container, coulters, and supports. The innercontainer was constructed from stainless steel materials, had acapacity of 15 L, and was made airtight to simulate the disposalsites of the contaminated soil. The temperature of these inner linerswas controlled automatically (in the range 0–100 �C) using a ther-mostat within the temperature control system. The operational

Table 1Properties and characteristics of the selected soils.

Soil type Specific surface area (m2 g�1) pH Particle size distribution (%) Organic content (%) Plastic index

<1 (lm) 1–5 (lm) 5–75 (lm) 75–250 (lm)

Clayey silt 16 8.70 7.8 14.4 70.7 5.4 2.4 8.6Silty clay 22 8.47 21.8 13.9 53.6 10.7 3.0 11.2

Y. Ma et al. / Chemosphere 121 (2015) 117–123 119

parameters were set by the automatic control system as follows:speeds of 0–300 rpm; agitation times of 5, 10, 20, and 30 s; and agi-tation intervals of 12, 6, 4, and 2 h. All of these operational param-eters could be controlled both automatically and manually. Theexhaust gas treatment system consisted of a gas-collecting pipeand an activated carbon adsorption device, and all tested exhaustgases were treated before emission.

2.4. Experimental procedures

The contaminated soils were placed into the experimentalapparatus, and the cover was tamped down tightly; then, thepower supply was connected to start the apparatus running. Theexperimental apparatus was then used to agitate the soils at aset temperature and aeration rate to volatilize contaminants fromthe soils.

The experiment included two steps: mechanical soil aerationand enhanced mechanical soil aeration by CaO. In the first step,the VCHs in the soil were removed by physical processes.Conversely, in the second step, CaO was added into the soils tofacilitate the removal of the VCHs from the soils when their con-centrations were low; in this step VCH removal proceeded slowly.Because more toxic by-products were formed when 20% CaO byweight (relative to dry soil) was added to TCE-contaminated soils,and better removal efficiency was achieved without the generationof more toxic substances at the lower ratio (Ko et al., 2010, 2011),the dose of CaO was set to 3%.

2.5. Sample collection and analysis

The sampling for analyzing concentration and soil moisturecontent was conducted in two separate experiments, each of whichwas repeated three times. Samples were collected at a fixed timeeach hour for the first 8 h and then every two hours until the24 h, resulting in a total of 17 samples collected during the 24 hexperiment. The concentration at the time the contaminated soilswere placed into the experimental apparatus was selected as theinitial concentration.

Semi-circular drill was used to collect soils. At each samplingpoint, a sample of approximately 5 g was collected for concentra-tion analysis using a hand-held VOC sampling tube, and a250 mL sample was collected for moisture content analysis. TheVCH concentration samples were held in a 40 mL closed glass ves-sel pre-stored with methanol. All samples were stored in a refrig-erator at 4 �C before analysis.

The concentrations of the VCHs in the soil were measured usinga gas chromatograph (GC, USA Agilent Technologies 7890A)equipped with a mass spectrometer (MS, USA Agilent Technologies5975C) in accordance with the USEPA-8260C method. The GC wasfitted with a DB-624 capillary column (60 m � 250 lm � 1.4 lm;Agilent Technologies) and was operated with a helium carrier-gas flow rate of 1.2 mL min�1. The oven temperature was pro-grammed as follows: (i) 40 �C for 2 min; (ii) increased to 200 �Cat a rate of 20 �C min�1; (iii) increased to 250 �C at a rate of10 �C min�1; (iv) maintained at 250 �C for 3 min. Target VOCsdetected according to this method are presented in Table S1 (seeSupporting Material).

3. Results and discussion

3.1. Tailing

After treatment, the majority of the contaminants in the soilswere removed under the various temperature and agitation rates.The results are shown in Fig. 1. Our findings suggested that theremoval of VCHs from soils likely occurred in two stages. In thefirst stage, contaminant concentrations decreased rapidly and theremoval efficiency was relatively high. Conversely, in the secondstage, the contaminants in the soil exhibited a slow removal trend,decreasing at a extremely slow rate. In this context, tailing could beattributed to slow diffusion in water-filled pores, slow intraparticlediffusion, slow desorption from the solid surfaces, or some combi-nation of these processes (Baehr et al., 1989; Armstrong et al.,1994; Li and Werth, 2004; Johnson et al., 2009; Kempf andBrusseau, 2009; Akyol et al., 2011; Brusseau et al., 2012).

In the temperature-controlled experiment (Fig. 1a–c), all threecontaminants decreased rapidly during the first stage (i.e., within7 h of the start of the experiment), although their rate of decreasevaried depending on temperature. The concentrations of 1,2-DCA,TCE, and PCE decreased from 595–600 to 3–44 mg kg�1 (removalefficiency: 93–99.5%), from 796–809 to 9–110 mg kg�1 (removalefficiency: 86–99%), and from 986–996 to 29–162 mg kg�1

(removal efficiency: 84–97%), respectively, during the first stage.Subsequently, the contaminants exhibited extensive low-concen-tration tailing. In the 7–24 h after the start of the experiment, theconcentrations of 1,2-DCA, TCE, and PCE in the soils decreased byonly 2–25, 7–43, and 20–63 mg kg�1, respectively. These changesincreased the associated removal efficiencies by 0.3–4%, 1–5%,and 2–6%, respectively.

Further analysis revealed that the timing of onset of tailing andthe residual contaminants concentrations varied considerablydepending on the experimental conditions. In particular, higher soiltemperatures and faster aeration rates were associated with earlieronset of tailing and lower residual contaminant concentrations(Fig. 1a–c). For example, for soil temperatures of 5, 10, and 30 �C,the concentration of 1,2-DCA in the soil exhibited inflection pointsat 7, 5, and 3 h after the start of the experiment, respectively. Fol-lowing the onset of tailing, residual concentrations of 1,2-DCA inthe soil decreased from 44 to 19, from 36 to 4, and from 11 to1 mg kg�1 by the end of the experiment for temperatures of 5, 10,and 30 �C, respectively. Similarly, TCE concentration exhibitedinflection points at 7, 6, and 3 h after the start of the experimentand residual concentrations in the soil decreased from 110 to 66,from 77 to 28, and from 19 to 2 mg kg�1 for temperature of 5, 10,and 30 �C, respectively. PCE concentration exhibited inflectionpoints at 7, 6, and 6 h and residual concentrations decreased from162 to 99, from 130 to 67, and from 35 to 9 mg kg�1 by the end ofthe experiments for temperature of 5, 10, and 30 �C, respectively.The remedial target values for 1,2-DCA, TCE, and PCE for this sitewere 0.82, 5.19, and 22 mg kg�1, respectively. Therefore, the con-centration of 1,2-DCA in soil did not meet the remediation require-ments within 24 h, regardless of the temperature. Conversely, TCEand PCE concentrations met the remediation requirements within8 and 12 h, respectively, after the start of the experiment at 30 �C.The results obtained from the aeration-controlled experiment

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eTCE

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3L min-1

Fig. 1. Concentration of contaminants in soil during mechanical soil aeration at different temperature and aeration conditions. (a–c) Temperature-controlled experiment forclayey silt, with aeration rate, agitation interval, speed, and agitation time of 3 L min�1, 2 h, 200 rpm, and 10 s. (d–f) Aeration-controlled experiment for clayey silt, with soiltemperature, agitation interval, speed, and agitation time of 20 �C, 2 h, 200 rpm, and 10 s. Each experiment was repeated three times with the relative standard deviationbelow 10%.

120 Y. Ma et al. / Chemosphere 121 (2015) 117–123

exhibited a similar pattern to those obtained for the temperature-controlled experiment (Fig. 1d–f).

3.2. Effects of CaO-enhanced treatment

In the second step of the experiment, CaO was added into thesoil 24 h after the beginning of the experiment, to enhance theremoval of VCHs from the soils, because the concentrations ofresidual contaminants were low and were decreasing extremelyslowly. Such addition of CaO was able to overcome tailing andreduce the concentrations of residual contaminants in soil effec-tively, base on comparison of concentrations before and after add-ing CaO.

The results demonstrated a rapid decline in the concentrationsof all three contaminants over a relatively short period of time(0.5 h) following the addition of CaO to both clayey silt and siltyclay (Fig. 2). In the clayey silt, 1,2-DCA, TCE, and PCE decreased rap-idly from 3 to 1, from 12 to 3, and from 34 to 2 mg kg�1, respec-tively. Accordingly, the remedial target values were achieved inall cases. The effects of adding CaO to silty clay were similar tothose for the clayey silt, although the contaminant concentrationsfor the silty clay were higher at the tailing stage than those for theclayey silt. This disparity may be attributed to the fact that siltyclay contains a higher ratio of clay, smaller soil particles, and a lar-ger specific surface area, thus improving its adsorption capacity forcontaminants (Ding et al., 1999; Kwok and Loh, 2003). Based onthese results, it is clear that the addition of CaO to soils at the tail-ing stage can mitigate tailing by decreasing the remediation periodand reducing residual contaminants in soils effectively. Theseresults should be directly applicable to field-level mechanical soilaeration operations. Therefore, the technique described here mayhelp to reduce residual contaminant concentrations and achievethe target values required for site remediation, particularly underextreme or adverse conditions such as low-temperature conditionsand for soils with high water and clay contents.

3.3. Mechanisms of CaO-enhanced treatment

3.3.1. Increasing soil temperature to facilitate the desorption andevaporation of contaminants

Quicklime is composed primarily of CaO with minor MgO andcan absorb water to produce Ca(OH)2 and Mg(OH)2 while emittingheat. The heat liberated by hydration of quicklime ranges from 880to 1140 kJ kg�1 depending on the CaO content of the quicklime(Boynton, 1980). In the current study, variation in temperaturewas observed for both soil types after the addition of CaO (Fig. 3).

The temperature increased almost immediately after the addi-tion of CaO, and the highest temperatures obtained for the clayeysilt and silty clay were 28.8 and 25.8 �C, corresponding to respec-tive increases of 9 and 6 �C, respectively. Subsequently, soil tem-perature declined slowly to return to the initial temperaturewithin 120 min. These results demonstrate that the addition ofCaO increases soil temperature considerably; such increases pro-mote desorption and the release of the residual volatile contami-nants retained in soil at the tailing stage.

For organic compounds with medium molecular weight, previ-ous studies have reported that the vapor pressure of a compounddoubles for each 10 �C increase in temperature (Brusturean et al.,2006). The Henry’s law constant for TCE increases by a factor of20 between 10 and 95 �C, which is a dramatic change in volatility(Jury et al., 1987; Heron et al., 1998). Temperature released bythe hydration of CaO can increase the gas-phase concentration ofVCHs through an increase of vapor pressure, and increasing thegas-phase concentrations of VCHs will likely result in removal ofVCHs by volatilization. Moreover, temperatures can provideenough energy to desorb or crack the pollutants (Piña et al.,2002). Increasing temperatures facilitates the desorption of organ-ics in the adsorbed phase and promotes the formation of VOCs inthe vapor phase (Falciglia et al., 2011). During the process ofmechanical soil aeration, intense mechanical agitation and aerationpromote the escape and effective removal of VCHs from soil.

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the remedial target value in this contaminated site

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clayey silt silty clay

1,2-DCA PCE

Fig. 2. Rapid changes of contaminant concentration in soils after adding CaO. The experiment condition was set as aeration rate 3 L min�1, with soil temperature 20 �C, aswell as agitation interval 2 h, speed 200 rpm, and agitation time 10 s. CaO was added after mechanical soil aeration at 24 h since the experiment started, and then theexperiment was continuous for another 8.5 h. Each experiment was repeated three times with the relative standard deviation below 10%.

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clayey siltsilty clay

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Fig. 3. Temperature changes observed in two different soils after the addition ofCaO. The experiment condition was set as aeration rate, initial soil temperature,agitation interval, speed, and agitation time 3 L min�1, 20 �C, 2 h, 200 rpm, and 10 s.Soil temperature was measured by attaching a Pt100 temperature sensor to theinner wall of the reactor (see Supplementary Material Fig. S1). The error of thetemperature measurement was below 1 K.

Y. Ma et al. / Chemosphere 121 (2015) 117–123 121

Therefore, increasing soil temperature by adding CaO and enablingthe volatilization of contaminants should be the key factorsrequired to remove contaminants from soil at the tailing stage.

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Fig. 4. Comparison of soil particle size distribution before and after the addition of Cainterval, speed, and agitation time 3 L min�1, 20 �C, 2 h, 200 rpm, and 10 s. Soil particle(Mastersizer2000). Each experiment was repeated three times with the relative standar

3.3.2. Reduction of soil moisture to improve mass transfer ofcontaminants

In the first stage of mechanical soil aeration, NAPLs weredepleted, such that their concentrations exhibited a roughly expo-nential decay that was followed by persistent long-duration tailingas mechanical soil aeration continued. Several researchers haveattributed this long-duration effluent tail to mass transfer rate lim-itations at the aqueous–air interface (Armstrong et al., 1994;Alvim-Ferraz et al., 2006; Albergaria et al., 2012). Removal of theremaining portion of the compounds requires volatilization fromthe dissolved aqueous phase together with desorption from thesolid surfaces into the dissolved aqueous phase. The effective diffu-sion coefficient of a chemical moving through the air space in soilis four orders of magnitude greater than that for water in soil(Grifoll and Cohen, 1994). Furthermore, increased water in thepore space of soil may result in reduced movement of TCE vaporby blocking the pathway for gas phase diffusion.

In the present study, the soil water content in soils was found todecrease considerably after the addition of CaO: soil moisturedecreased from 18–21% to 15–18% for both the clayey silt andthe silty clay. This likely occurred primarily because the CaO wasconverted to Ca(OH)2 and Mg(OH)2, both of which continue toadsorb water while dissolving in the ionic state, producing energyto heat the soil and promoting the evaporation and escape of con-taminants from the soil. Soil moisture influences the mass transferof dispersed contaminants to the gas phase: it reduces the air-filled

10 1 0.1 0.0 1 1E-3e size (mm)

me after the addition of lime

clayey silt

O. The experiment condition was set as aeration rate, soil temperature, agitationsize distributions of the samples was measured using a laser particle size analyzerd deviation below 10%.

122 Y. Ma et al. / Chemosphere 121 (2015) 117–123

pores of the soil and acts as a barrier between the contaminant andthe soil matrix. In soils with low moisture, the soil porosityincreases, facilitating the movement of air into soil, consequentlyincreasing the effective diffusion coefficients of the contaminantand promoting the mass transfer of contaminants in soil (Parkeret al., 1987; Cabbar and Bostanci, 2001; Albergaria et al., 2012).Thus, the addition of CaO can effectively reduce soil water content,promote the mass transfer of contaminants in soil, and increase thechannels available for diffusion. All of these factors combine tofacilitate the evaporation and escape of contaminants, helping tomitigate the tailing phenomenon.

3.3.3. Increasing soil particle size and permeability to facilitate thetransport of gas contaminants

The addition of CaO significantly modifies soil texture. After theaddition of CaO, soils typically undergo carbonation, gelling, crys-tallization, and other processes that change their particle sizeand chemical composition (Boardman et al., 2001). The effects ofCaO on soil physical properties are complex, and many differentinteractions can occur. For example, the addition of CaO can induceincrease in pH and ionic strength, thereby decreasing the repulsiveforce between soil particles and increasing porosity, thus favoringflocculation and soil aggregation (Haynes and Naidu, 1998; ChangChien et al., 2010). Fig. 4 illustrates changes in the composition ofsoil particles observed in response to adding CaO. In particular, theparticle size distribution curve exhibits a considerable leftwardshift, indicating clear changes in the particle distribution of the soiland an overall coarsening of soil particles.

Typically, the composition of soil particles influences the per-meability of soil. In particular, the coarser the soil particles, thehigher the permeability coefficient and porosity of the soil. Thishigher permeability favors the transport of gas by diffusionthrough soil. Moreover, the coarsening of soil particles acts todecrease the specific surface area, thus reducing its ability toadsorb contaminants (Ding et al., 1999). Therefore, increasing par-ticle size and soil permeability should facilitate the transport ofcontaminant gases. This process further helps to explain how theaddition of CaO enhances remediation.

4. Conclusions

A remediation technique involving mechanical soil aeration forsoils contaminated by VCHs was shown to mitigate the occurrenceof tailing during the late stage of treatment. Moreover, both thetime of onset of tailing and the residual levels of contaminants werefound to vary depending on experimental conditions. In particular,higher soil temperatures and faster aeration rates led to earlieronset of tailing and lower residual concentrations of contaminants.

Adding CaO during the late stage of site remediation was foundto mitigate tailing. This process reduced the residual concentra-tions of contaminants in soil, strengthened the remediation effects,and lowered the costs of remediation.

CaO acts to enhance remediation effects in three ways: itincreases soil temperature, facilitating the desorption and volatili-zation of contaminants; it lowers the soil moisture, promoting themass transfer of contaminants; and it coarsens soil particle size,increasing the permeability of soil and promoting the diffusion ofcontaminant gases.

The enhancement of site remediation achieved by the additionof CaO at the tailing stage shortens the remediation period; suchenhancement should be achievable for a wide range of applica-tions, in conjunction with either mechanical soil aeration or otherremediation techniques. Thus, CaO enhance remediation can beconsidered an economical and practical technique that will be use-ful for field applications in environmental engineering.

Acknowledgments

The present work is supported by National EnvironmentalProtection Public Welfare Projects (201109017 and 201409047).We are also grateful to Dr. Shijie Wang for his valuable suggestions.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.chemosphere.2014.10.074.

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