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Journal of Hazardous Materials 300 (2015) 546–552

Contents lists available at ScienceDirect

Journal of Hazardous Materials

journa l homepage: www.e lsev ier .com/ locate / jhazmat

itric acid facilitated thermal treatment: An innovative method forhe remediation of mercury contaminated soil

ujun Ma a, Changsheng Peng b, Deyi Hou c, Bin Wu a, Qian Zhang a, Fasheng Li a,ingbao Gu a,∗

State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, ChinaThe Key Lab of Marine Environmental Science and Ecology, Ministry of Education, Ocean University of China, Qingdao 266100, ChinaGeotechnical and Environmental Research Group, Department of Engineering, University of Cambridge, Cambridge CB2 1PZ, UK

i g h l i g h t s

Hg content was reduced to<1.5 mg/kg when treated at 400 ◦Cwith citric acid.The treated soil retained most of itsoriginal soil physicochemical proper-ties.Proton provided by citric acid facili-tates thermal removal of mercury.This thermal treatment method isexpected to reduce energy input by35%.

g r a p h i c a l a b s t r a c t

r t i c l e i n f o

rticle history:eceived 30 April 2015eceived in revised form 15 July 2015ccepted 21 July 2015vailable online 26 July 2015

eywords:

a b s t r a c t

Thermal treatment is a promising technology for the remediation of mercury contaminated soils, butit often requires high energy input at heating temperatures above 600 ◦C, and the treated soil is notsuitable for agricultural reuse. The present study developed a novel method for the thermal treatment ofmercury contaminated soils with the facilitation of citric acid (CA). A CA/Hg molar ratio of 15 was adoptedas the optimum dosage. The mercury concentration in soils was successfully reduced from 134 mg/kgto 1.1 mg/kg when treated at 400 ◦C for 60 min and the treated soil retained most of its original soil

ercuryontaminated soilhermal treatmentitric acid

physiochemical properties. During the treatment process, CA was found to provide an acidic environmentwhich enhanced the volatilization of mercury. This method is expected to reduce energy input by 35%comparing to the traditional thermal treatment method, and lead to agricultural soil reuse, thus providinga greener and more sustainable remediation method for treating mercury contaminated soil in future

.

engineering applications

∗ Corresponding author. Fax: +86 10 84932813.E-mail address: guqb@craes.org.cn (Q. Gu).

ttp://dx.doi.org/10.1016/j.jhazmat.2015.07.055304-3894/© 2015 Elsevier B.V. All rights reserved.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

Mercury is a highly toxic element which affects human nervoussystem, brain, heart, kidneys, and immune systems [1]. During therecent years, mercury pollution is drawing attention from manygovernments, as well as international organizations such as theUnited Nations Environment Programme [2]. The main anthro-

pogenic sources contributing to mercury contamination includemercury mining, gold mining, wood preservation, coal combus-tion, and chlor-alkali industry [3,4]. The US EPA reported that 290

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F. Ma et al. / Journal of Hazard

uperfund sites on the National Priorities List were contaminatedy mercury, and 173 of these 290 sites were soil-contaminatedites [5]. In China, a number of mercury polluted soils have beeneported, the mercury concentrations in some soils are up to 232imes the maximum mercury concentration allowed for soil inhina (1.5 mg/kg, National Standard GB15618-1995) [3], thus rep-esenting a serious threat to human health and the environment.

To clean up mercury contaminated soil, mercury concentrationn the soil should be reduced to a low level (e.g., <1.5 mg/kg). Thevailable remediation technologies for soil contaminated with mer-ury include phytoremediation [6–8], stabilization/solidification9,10], soil washing [11,12], thermal treatment [13–18], andlectrokinetics [19,20]. Stabilization/solidification changes soilroperties dramatically and makes the treated soil unsuitable forgricultural reuse. Soil washing and electrokinetics are generallyot efficient enough to reach an acceptable decontamination level.hermal treatment can achieve an acceptable decontamination

evel when mercury-contaminated soils are treated at tempera-ures above 600 ◦C [13,16]. However, the high heating temperatureequires high energy cost, moreover, dramatic changes in soil prop-rties, including increase in soil pH, dehydration of silicate clayinerals, and reduction in the content of organic carbon, were

bserved when soils were treated at 600 ◦C, thus making the soilsnfavorable for agricultural reuse [14,18]. Therefore, there is a cleareed to develop a method that lowers the temperature at whichercury can be removed to a low level (e.g., <1.5 mg/kg), thus

educing energy requirement and life cycle (i.e. cradle to grave)nvironmental footprint, which aligns well with the ongoing sus-ainable remediation movement [21].

Mercury presents in a variety of forms in soils, including metal-ic mercury, inorganic mercury compounds (chlorides, sulfides,xides, etc.), organic mercury compounds (methyl mercury, ethylercury), and those captured inside pores of the soil particles

e.g. dolomite, pyrite, sphalerite). Mercury speciation has a signif-cant effect on the thermal removal of mercury from soil. Mercurypecies other than Hg(0) and Hg(I) are not volatile when mercury-ontaminated soils are treated at temperatures below 200 ◦C [22].herefore, high residual mercury contents were reported whenuch soil samples were treated at temperatures <200 ◦C [15]. Asemperature increases, other mercury species start to volatilizerom soil. Huang et al. reported that the residual mercury concen-rations were 17 mg/kg and 6 mg/kg when a soil (collected from ahlor-alkali plant, Hg content: 1320 mg/kg) was thermally treatedt 400 ◦C and 550 ◦C for 1 h, respectively [18]. It is reported thatercury species including HgO, HgSO4, and those captured inside

ores of the soil particles release mercury at temperatures above00 ◦C [23,24]. As HgSO4 is decomposed through hydrolysis, thisompound will not be stable under environmental conditions [23].o lower the volatile temperature of these hardly volatile mercurypecies in contaminated soils, some measures should be taken toonvert them to easily volatile species during thermal treatment.n our previous study [25], the mercury content in soil was success-ully reduced to <1.5 mg/kg when treated with FeCl3 at 400 ◦C. Theemoval mechanism involves FeCl3 being partially decomposed toorm HCl at low temperatures. The formed HCl reacts with theseardly volatile metal species to produce mercury chloride com-ounds with low boiling points. However, the added chloride ionay react with organic matters in the soil to form absorbable

rganic halogen and dioxins; high Fe content in soil is also harmfulo vegetation.

As a natural, low-molecular-weight organic acids, citric acidCA) is often used in soil washing to extract heavy metals including

b, Cd, Cu, and Hg due to its chelating ability and acidity [26–28].

n this study, mercury removal from contaminated soil by ther-al treatment with CA was conducted, and then the mechanism

f CA facilitated thermal treatment was carried out. The results

aterials 300 (2015) 546–552 547

obtained in this study are expected to provide an environmentalfriendly technique for treating mercury contaminated soil in futureengineering applications.

2. Materials and methods

2.1. Soil sample preparation

Soil samples were collected from a farm land near a mercurymining area in Tongren, Guizhou Province of China. Soil sampleswere air-dried and grounded. Subsequently, particles with sizeslarger than 2 mm were removed using a standard 10-mesh sieve.Soil samples were then thoroughly mixed. The mixed soil sam-ples were stored properly before sample characterization (shown inSupplementary material (SM)) and thermal decontamination tests.Mercury concentration in the soil was determined to be 134 mg/kg.

2.2. Thermal treatment and mercury determination

Thermal treatment was performed in a laboratory-scale rotarykiln with a mercury vapor gas treatment system. The rotary kiln washeated by electricity. The kiln was 2 m long with an internal diam-eter of 0.2 m. In each test, CA was premixed with soil in a mortarmixer before introduced into the rotary kiln, 1 kg of prepared soilsample with or without CA was used under different conditions. Thetemperature in the kiln was measured by a thermocouple (NiCr-Ni)in the hot zone near the inner wall. Treatment time of soil samplesin the hot zone of the kiln (900 mm) was adjusted by changing thespeed of the kiln. The inclination angle of the kiln was 3.5◦. At theend of the kiln, a collecting container was set up. Mercury vapor gaswould flow into a condenser and condense into liquid mercury at25 ◦C. The liquid mercury was collected in a tank, and the vapor gaswould flow through an activated carbon adsorption tank to furtherremove uncondensed mercury vapor gas. The mercury concentra-tion in the exhaust gas met China Integrated Emission Standard ofAir Pollutants (12 �g/m3, National Standard GB16297-1997).

Mercury contents in soil samples were determined according tothe method of Biester and Scholz [29]. Briefly, mercury contentswere analyzed by cold vapor atomic absorption spectroscopy afterreduction of Hg2+ to Hg0 with SnCl2 solution. Total mercury wasdetermined by digesting 5 g of soil sample in 28 mL of aqua regia for3 h at 160 ◦C. All analyses were conducted in triplicates. The totalHg concentration was found to be within the confidence intervalfor certified values with recoveries in the range of 81–113%. Therelative standard deviation among replicates was <10%. The detec-tion limit for mercury in soil was 1.0 × 10−2 mg/kg dry weight. Thequality control of Hg measurements was confirmed by analyzing0.25 g of standard reference material “SRM 2710a” produced byNIST. Triplicate measurements of the certified reference materialyielded 10.02 ± 0.35 mg/kg, corresponding well with the certifiedvalue 9.88 ± 0.21 mg/kg.

2.3. Sequential extraction

To better understand the effect of CA on the thermal decon-tamination of mercury, sequential extractions of soil samplestreated at various temperatures and treatment times with orwithout CA were conducted. Mercury in soil samples before andafter thermal treatment were separated into water-soluble frac-

tion (F1), exchangeable fraction (F2), humic/fulvic fraction (F3),organic/sulfide fraction (F4), and residual fraction (F5) [29]. Theoperational definition of the fractions and the compositions of theextraction solutions are given in Table S1 in SM.

548 F. Ma et al. / Journal of Hazardous Materials 300 (2015) 546–552

0

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20 40 60 80 10 0

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Treati ng ti me (min)

400 oC 600 oC 400 oC, CA

Fig. 1. Effect of treatment time and temperature on decontamination of mercury ins1C

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CA/Hg (molar ratio)

oil samples. The soil samples were thermally treated with or without CA (CA/Hg:5). Dash line represents the maximum mercury concentration allowed for soil inhina (1.5 mg/kg, National Standard GB15618-1995).

. Results and discussion

.1. Thermal treatment without CA

The effect of temperature on the thermal removal of mercuryas investigated in the range of 300–700 ◦C. The heating time was

ept at 30 min. As shown in Table 1, the residual mercury concen-rations in the soil samples decreased as temperatures increased.

ore than 85% of mercury was removed from the contaminatedoil when the temperature was higher than 300 ◦C. As tempera-ure increased from 300 ◦C to 500 ◦C, an additional 10% of mercuryolatilized from the soil. However, the mercury content was noteduced to <1.5 mg/kg until 700 ◦C. These results are consistentith those of a previous study [16].

Fig. 1 shows the effect of treatment time on the thermal removalf mercury. Soil samples were treated at 400 and 600 ◦C withreatment time ranged 30–90 min. Mercury contents graduallyecreased as the treatment time increased. For example, when theoil was treated at 400 ◦C for 30 min, the residual mercury contentas 12.0 mg/kg, when the treatment time was extended to 60 min,

he residual mercury content was reduced to 6.7 mg/kg. It should beoted that the residual mercury contents were similar for 60 min ofreatment (6.7 mg/kg at 400 ◦C and 2.7 mg/kg at 600 ◦C) and 90 minf treatment (6.7 mg/kg at 400 ◦C and 2.5 mg/kg at 600 ◦C). Theseesults indicate that simply increasing treatment time cannot effec-ively reduce residual mercury contents to an acceptable level.

.2. Thermal treatment with CA

To investigate the effect of CA addition on the thermal decon-amination of mercury, soil samples were treated at 400 ◦C for

able 1otal mercury concentrations and mercury removal efficiency as a function of treat-ent temperature. The treatment time was kept at 30 min. The soil samples were

reated without CA.

Temperature (◦C) Hg (mg/kg) Removal efficiency (%)

Untreated 134 ± 8.2 –300 20 ± 3.5 85.1 ± 2.6400 12 ± 1.8 91.0 ± 1.3500 6.5 ± 0.4 95.1 ± 0.3600 3.8 ± 1.3 97.2 ± 1.0700 0.7 ± 0.2 99.5 ± 0.1

Fig. 2. Effect of variable amounts of CA on decontamination of mercury in soil sam-ples. The soil samples (mercury concentration: 134 mg/kg) were treated at 400 ◦Cfor 30 min.

30–90 min with the molar ratio of CA to mercury (CA/Hg) at 15.Mercury removal was greatly enhanced when treated with CA(Fig. 1). The residual mercury content in soil sample treated at400 ◦C for 30 min with CA (2.6 mg/kg) was only 22% of that treatedwithout CA, even lower than that treated at 600 ◦C for 30 min with-out CA. Residual mercury was further removed as the treatmenttime extended. The residual mercury concentration decreased to1.1 mg/kg after 60 min of treatment, reaching the cleanup criterionof 1.5 mg/kg. These results indicate that thermal treatment with CAcan reduce the residual mercury contents to an acceptable level ata relatively low heating temperature (i.e. 400 ◦C).

To investigate CA dosage on the thermal removal of mercury,soil samples were treated at 400 ◦C for 30 min with CA/Hg at 0.5,2, 5, 15, and 50 (Fig. 2). Compared with thermal treatment withoutCA, an initial CA/Hg of 0.5 reduced the residual mercury concentra-tion by 19%, the reduction of residual mercury was enhanced whenCA was added at higher dosages, reaching 78% at an CA/Hg dosageof 15. CA/Hg higher than 15 did not result in additional mercuryremoval, suggesting that CA/Hg molar ratio of 15 was an optimumdosage ratio under the given experimental conditions, resulting inthe maximum reduction of mercury.

3.3. Changes in soil physicochemical properties

In this study, the contents of residual mercury in two soil sam-ples, namely, the soil sample treated at 700 ◦C for 30 min withoutCA and that treated at 400 ◦C for 60 min with CA, were successfullyreduced to levels below the cleanup criteria of 1.5 mg/kg. To evalu-ate whether the treated soils are suitable for reuse on agriculturalland, the changes in soil physiochemical properties of these twosoil samples were studied.

As shown in Table 2, organic carbon content in the soil sam-ple treated at 400 ◦C was 1.6%, retaining the majority of its organiccarbon (65%), whereas organic carbon content could hardly bedetected in the soil sample treated at 700 ◦C (i.e. <0.02%). Soil pHremained near neutral (pH 7.6) when treated at 400 ◦C, while thattreated at 700 ◦C resulted in a pH that was much basic (pH 10.2).The changes in pH may be attributed to the oxidation of certainelements [30], the dehydration of colloids, the decrease of buffercapacity, and the release of ions from organic matters in the treat-

ment process [14,31]. The soil color (determined in SM) changedslightly from dark brown (10YR 3/3) to brown (10YR 4/3) afterthermal treatment at 400 ◦C, whereas the soil color changed dra-matically to moderate orange (5YR 7/8) when the soil sample was

F. Ma et al. / Journal of Hazardous Materials 300 (2015) 546–552 549

Table 2Soil physiochemical properties before and after thermal treatment.

Soils pH CaCO3 (%) Corg (%) Soilcolor

CEC (cmol/kg) Fed

(g/kg)Mnd

(g/kg)Clay (%) Silt (%) Sand (%)

toetdoaa7m3cidogw7dsiutbcic

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175 C, CA should be decomposed during thermal treatment. There-fore, the acidic environment provided by CA was probably theremoval mechanism during thermal treatment. In order to con-firm this hypothesis, Na3CA, Na2HCA, and NaH2CA were selected

0

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Chloride Add ed

Chloride Removed

Untreated 6.6 ± 0.3 0.6 ± 0.2 2.5 ± 0.3 10YR 3/3400 ◦C, 60 min, with CA 7.6 ± 0.3 0.2 ± 0.1 1.6 ± 0.5 10YR 4/3700 ◦C, 30 min, without CA 10.2 ± 0.4 0.5 ± 0.3 ND 5YR 7/8

reated at 700 ◦C. The observed color change was attributed to thexidation of iron and manganese compounds in a high-temperaturenvironment. It has been shown that Fe-laden minerals in soils tendo transform into hematite when they become completely dehy-rated, which reddens the soil [30]. The removal or pyrolysis ofrganic carbon may also attribute to the color change [32]. Fedmount slightly increased from 5.2 g/kg to 6.1 g/kg when treatedt 400 ◦C, while higher Fed amount was found when treated at00 ◦C which possibly due to the formation of hematite [30]. Ther-al treatment at 400 ◦C caused a few decrease in clay content from

1% to 24% and an increase in sand content from 8% to 20%. Inontrast, the 700 ◦C treated soil sample had a dramatic increasen sand content and decrease in clay content. Similar grain-sizeistributions were found for soil samples treated at 400 ◦C withr without CA (Fig. S1 in SM), the volume percentage of <0.5 �mrains in soil treated 400 ◦C was lower than that in untreated soil,hile <0.5 �m grains disappeared during thermal treatment at

00 ◦C. The aluminum oxides and hydroxides released during clayecomposition might act as cementing agents in the formation ofand-sized particles [32,33]. The cation exchange capacity (CEC)n the 400 ◦C treated soil sample was slightly lower than that ofntreated soil sample, while significantly lower CEC was found inhe 700 ◦C treated soil sample. The decrease in CEC was expectedecause of the oxidation of organic matter and the destruction oflay structures. The dehydration of mineral lattice and the result-ng breakdown of lattice structures were probably also importantauses of CEC reduction [34].

Overall, soil characterization results indicated that the 400 ◦Created soil sample retained most of its original soil physiochemicalroperties, suggesting the treated soil may be suitable for reusen agricultural land. In comparison, the 700 ◦C treated soil samplehowed drastic changes in its physicochemical characteristics andineralogy, which makes it much more difficult to reuse.

.4. Changes in mercury speciation

As shown in Table 3, for thermal treatment without CA, 91% of1, 85% of F2, 86% of F3, 88% of F4, and 93% of F5 were removedhen soil sample was treated at 400 ◦C for 30 min. When treat-ent time was extended to 60 min, 98% of F1, 98% of F2, 93% of F3,

7% of F4, and 94% of F5 were removed, which also demonstrateshat the residual mercury can be further removed (especially F1-4) by extending treatment time. Similar mercury contents in eachraction were found when the time was extended to 90 min. Afterreatment at 600 ◦C or 700 ◦C, mercury was mainly found in F5,uggesting that the residual fraction of mercury was the most ther-ally stable form in soil. For thermal treatment with CA, 99% of

1, 98% of F2, 92% of F3, 97% of F4, and 98% of F5 were removedhen soil sample was treated at 400 ◦C for 30 min. When treat-ent time was extended to 90 min, mercury in F1 and F2 was totally

emoved and mercury in other fractions was further reduced. These

esults indicate that CA addition during thermal treatment not onlyccelerated the volatilization of mercury in the easily removed frac-ion (F1 and F4), but also reduced the volatilization temperature of

ercury in the hardly removed fraction (F5).

11 ± 0.2 5.2 ± 0.5 0.13 ± 0.05 31 ± 3 61 ± 5 8 ± 39.8 ± 0.5 6.1 ± 0.5 0.15 ± 0.01 24 ± 5 56 ± 6 20 ± 84.6 ± 0.5 7.2 ± 0.7 0.14 ± 0.04 12 ± 6 55 ± 8 33 ± 9

3.5. Removal mechanism

In this study, hardly volatile mercury species in contaminatedsoils were successfully converted to easily volatile species (metal-lic mercury, Hg2Cl2, HgCl2, etc.) when thermally treated with CA.However, the reaction mechanism of CA and the hardly volatilemercury species is still an open question. In this section, the inter-action of CA and mercury in the soil is discussed.

In thermochemical removal of heavy metals (Cu, Zn, Pb, Cd) fromsewage sludge ash, MgCl2, CaCl2 and gaseous HCl are often used aschloride donors [35–39], the removal mechanism involves formingvolatile heavy metal chlorides. In this study, the chloride concentra-tion was 17.4 mg/kg in untreated soil, the corresponding molar ratioof chloride to mercury in soil was 0.73:1. To investigate whetherthe added CA facilitates the formation of mercury chlorides, thesoil samples were pretreated as follows: (1) washed with ultrapurewater to remove the chloride (Hg concentration: 124 mg/kg); or (2)added the same molar of NaCl as CA. Then the pretreated soil sam-ples were added CA (CA/Hg: 15) and treated at 400 ◦C for 30 min. Asshown in Fig. 3, the residual mercury concentrations were compa-rable in these two soil samples, and also comparable with that in theoriginal treated soil sample. These results suggest that the mercuryremoval mechanism with CA addition was not due to the forma-tion of mercury chlorides. The chloride concentration in treated soilsample was determined to be 16.9 mg/kg, which was similar to thatin untreated soil sample (17.4 mg/kg), further suggesting that mostof the volatile mercury was not in the form of mercury chlorides.

CA is often used in soil washing to extract heavy metals includ-ing Pb, Cd, Cu, and Hg, the removal mechanism is that CA hasa chelating ability for cationic heavy metals and providing pro-tons (H+) [26–28]. Since the decomposition temperature of CA is

◦

Fig. 3. Effect of variable amounts of chloride donors on decontamination of mercuryin soil samples. The soil samples (mercury concentration: 134 mg/kg) were treated at400 ◦C for 30 min with CA (CA/Hg: 15). The soil samples were (1) original; (2) washedto remove chloride; and (3) added the same molar of NaCl as CA, respectively.

550 F. Ma et al. / Journal of Hazardous Materials 300 (2015) 546–552

Table 3Mercury concentrations (mg/kg) in fractions by sequential extraction before and after thermal treatment.

Soils F1 F2 F3 F4 F5

Untreated 5.98 ± 1.35 9.03 ± 2.00 2.04 ± 0.50 6.77 ± 0.41 99.10 ± 1.63Without CA400 ◦C for 30 min 0.54 ± 0.24 1.38 ± 0.32 0.28 ± 0.11 0.84 ± 0.00 7.29 ± 0.21400 ◦C for 60 min 0.11 ± 0.05 0.20 ± 0.18 0.15 ± 0.07 0.21 ± 0.10 5.66 ± 0.28400 ◦C for 90 min 0.09 ± 0.06 0.25 ± 0.12 0.15 ± 0.14 0.24 ± 0.04 5.40 ± 0.21600 ◦C for 30 min ND 0.10 ± 0.05 0.07 ± 0.24 0.09 ± 0.04 3.27 ± 0.15600 ◦C for 60 min ND ND ND ND 2.24 ± 0.55700 ◦C for 30 min ND ND ND ND 0.52 ± 0.13With CA

◦

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400 C for 30 min 0.05 ± 0.02 0.16 ± 0.11

400 ◦C for 60 min ND 0.05 ± 0.01

400 ◦C for 90 min ND ND

s substitutes of CA. As shown in Fig. 4, residual mercury con-ent in the soil was 13.1 mg/kg when treated with Na3CA, whichas comparable with that in soil treated without the additives.

t should be noted that the residual mercury contents in the soilamples were 9.3 mg/kg and 5.6 mg/kg when treated with Na2HCAnd NaH2CA, higher than that in the soil sample treated withA, but lower than that in the soil sample treated with Na3CA.he acidity of CA, NaH2CA, Na2HCA, and Na3CA is in the orderA > NaH2CA > Na2HCA > Na3CA, which was consistent with therder of mercury removal efficiency. These results suggest thathe mercury removal mechanism with CA was providing an acidicnvironment.

Similar to those in soil washing using an acid solution, sev-ral mechanisms may be involved during thermal treatment withA: (1) desorption of mercury cations adsorbed on the surface ofoil particles via ion exchange; (2) dissolution of HgO or HgS intog2+; and (3) dissolution of soil mineral components (e.g., dolomite,e–Mn oxides) which may contain mercury contaminants [40,41].t low pH, the protons (H+) added can react with soil surface sites

layer silicate minerals and/or surface functional groups includingl-OH, Fe-OH, and COOH groups) and enhance desorption of mer-ury cations, forming species that can be easily volatilized [42],hich was confirmed by the decrease of CaCO3 content when

reated with CA (Table 2). The dissolution of soil mineral compo-ents may also cause the reduction of grain size. Moreover, due tohe relatively low boiling point and decomposition temperature,

A and its decomposition products would volatize during thermalreatment, and the dissolved soil mineral components would stillxist in the soil. Therefore, changes in soil properties and structuresere less compared with those in acid washing.

0

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Control CA NaH2CA Na2HCA Na3CA

ig. 4. Effect of different additives on decontamination of mercury in soil samples.he molar ratio of each additive to mercury was 15. The soil samples (mercuryoncentration: 134 mg/kg) were treated at 400 ◦C for 30 min.

0.16 ± 0.04 0.21 ± 0.07 2.08 ± 0.110.07 ± 0.01 0.09 ± 0.02 0.80 ± 0.040.10 ± 0.03 0.11 ± 0.03 0.68 ± 0.04

3.6. Effects of soil pH

Since the mercury removal mechanism with CA was providingan acidic environment, it is reasonable to speculate that soil pH hasa great effect on the removal of mercury in the soil. The influenceof soil pH on the removal of mercury was investigated by adjustingthe soil pH to 3, 7, and 10 and being treated at 400 ◦C for 30 min withor without CA (CA/Hg: 15). As shown in Fig. 5, the residual mercurycontents in soil samples increased with increasing soil pH. At pH3, the residual mercury contents in soil samples were comparablewhen treated with or without CA (the residual mercury contentwas 1.5 mg/kg), possibly due to a relatively small amount of CAwhich changed little of the soil pH (pH 2.9 after addition of CA).At pH 7, the soil pH changed to 5.6 after adding CA before thermaltreatment, which resulted in significantly more mercury removal(2.6 mg/kg residual mercury with CA versus 12 mg/kg residual mer-cury without CA). The residual mercury content was higher thanthat in the soil samples treated at pH 3, which is also consistentwith the hypothesis that enhanced mercury removal was mainlydue to the acidic environment. The mercury removal efficiencies insoil samples of pH 10 were relatively low, also consistent with theabove hypothesis.

3.7. Energy and cost saving

A preliminary cost assessment was conducted to compare the◦ ◦

treatment with CA (400 C for 60 min) to that without CA (700 C

for 30 min). The energy demand was calculated using the specificheat of soil minerals, water, and soil organic matter, as well as heatlosses when maintaining the kiln at high temperatures. The param-

0

5

10

15

20

3

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)

without CA

with CA

7 10Soil pH

Fig. 5. Effect of soil pH on decontamination of mercury in soil samples. The soilsamples were treated at 400 ◦C for 30 min with CA (CA/Hg: 15).

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F. Ma et al. / Journal of Hazard

ters used in the calculation were based on literature and industrialnowledge (see SM). It was estimated that 1.05 × 106 kJ of energy isequired to heat 1 m3 of soil at 400 ◦C for 60 min; and 1.60 × 106 kJf energy is required to heat the soil at 700 ◦C for 30 min. There-ore, CA facilitated thermal treatment can render an energy savingf 35%. Moreover, it is estimated that 1.9 g of CA is required for treat-

ng every 1 kg of soil. Based on market price of CA, it is estimated CAacilitated thermal treatment reduces marginal cost from 57 US$ to8 US$, or 34%. Overall, the preliminary cost assessment suggestshat the CA facilitated thermal treatment provides an energy effi-ient and cost effective method for treating mercury contaminatedoil.

. Conclusions

In this study, a novel method for the thermal treatmentf mercury-contaminated soils with the facilitation of CA waseveloped. A CA/Hg molar ratio of 15 was adopted as the opti-um dosage. The mercury concentration in soils was successfully

educed from 134 mg/kg to 1.1 mg/kg when treated at 400 ◦C for0 min and the treated soil retained most of its original soil phys-

ochemical properties. To achieve the same cleanup level, theraditional thermal treatment method requires heating at 700 ◦Cor 30 min, which would significantly increase soil pH and sandontent, decrease organic carbon content, and change soil color.uring the treatment process, CA was found to provide an acidicnvironment which enhanced the volatilization of mercury. Thehermal treatment method facilitated by CA is expected to reducenergy input by 35% comparing to the traditional thermal treat-ent method, and lead to agricultural soil reuse, thus providing

greener and more sustainable remediation method for treatingercury-contaminated soil in future engineering applications.

cknowledgements

Financial support from the National High Technologyesearch and Development Program of China (863 Program)2013AA06A207) and the State Key Laboratory of Environmen-al Criteria and Risk Assessment (No. SKLECRA 2013FP12) areratefully acknowledged.

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at http://dx.doi.org/10.1016/j.jhazmat.2015.07.55

eferences

[1] Agency for Toxic Substances and Disease Registry, Toxicological profile formercury, Georgia, 1999. http://www.atsdr.cdc.gov/toxprofiles/tp46.pdf(accessed April 2015).

[2] UNEP Chemicals Branch, Global mercury assessment 2013: Sources,emissions, releases and environmental transport, Geneva, Switzerland, 2013.http://www.unep.org/PDF/PressReleases/GlobalMercuryAssessment2013.pdf(accessed April 2015).

[3] G. Jiang, J. Shi, X. Feng, Mercury pollution in China. An overview of the pastand current sources of the toxic metal, Environ. Sci. Technol. 40 (2006)3673–3678.

[4] J. Wang, X. Feng, C.W. Anderson, Y. Xing, L. Shang, Remediation of mercurycontaminated sites: a review, J. Hazard. Mater. 221 (2012) 1–18.

[5] Office of Superfund Remediation and Technology Innovation, TreatmentTechnologies for Mercury in Soil, Waste, and Water, Washington, DC, 2007.http://www.clu-in.org/download/misc/542r07003.pdf (accessed April 2015).

[6] B. Smolinska, S. Rowe, The potential of Lepidium sativum L. for

phytoextraction of Hg-contaminated soil assisted by thiosulphate, J. Soil.Sediment. 15 (2015) 393–400.

[7] J. Wang, X. Feng, C.W.N. Anderson, G. Qiu, L. Ping, Z. Bao, Ammoniumthiosulphate enhanced phytoextraction from mercury contaminated soil –results from a greenhouse study, J. Hazard. Mater. 186 (2011) 119–127.

[

aterials 300 (2015) 546–552 551

[8] M. Kiyono, Y. Oka, Y. Sone, R. Nakamura, M.H. Sato, K. Sakabe, H. Pan-Hou,Bacterial heavy metal transporter MerC increases mercury accumulation inArabidopsis thaliana, Biochem. Eng. J. 71 (2013) 19–24.

[9] H. Piao, P.L. Bishop, Stabilization of mercury-containing wastes using sulfide,Environ. Pollut. 139 (2006) 498–506.

10] X. Zhang, Q. Wang, S. Zhang, X. Sun, Z. Zhang, Stabilization/solidification (S/S)of mercury-contaminated hazardous wastes using thiol-functionalized zeoliteand Portland cement, J. Hazard. Mater. 168 (2009) 1575–1580.

11] S. Wasay, P. Arnfalk, S. Tokunaga, Remediation of a soil polluted by mercurywith acidic potassium iodide, J. Hazard. Mater. 44 (1995) 93–102.

12] C. Sierra, J.M. Menéndez-Aguado, E. Afif, M. Carrero, J.R. Gallego, Feasibilitystudy on the use of soil washing to remediate the As-Hg contamination at anancient mining and metallurgy area, J. Hazard. Mater. 196 (2011)93–100.

13] M.V. Rose, T.E. Weyand, C.J. Koshinski, Mercury cleanup: the commercialapplication of a new mercury removal/recovery technology, Remed. J 5 (1995)89–101.

14] Y. Roh, N.T. Edwards, S.Y. Lee, C.A. Stiles, S. Armes, J.E. Foss, Thermal-treatedsoil for mercury removal: Soil and phytotoxicity tests, J. Environ. Qual. 29(2000) 415–424.

15] R. Kucharski, U. Zielonka, A. Sas-Nowosielska, J.M. Kuperberg, A.Worsztynowicz, J. Szdzuj, A method of mercury removal from topsoil usinglow-thermal application, Environ. Monit. Assess. 104 (2005) 341–351.

16] T.C. Chang, J.H. Yen, On-site mercury-contaminated soils remediation byusing thermal desorption technology, J. Hazard. Mater. 128 (2006)208–217.

17] A.M. Kunkel, J.J. Seibert, L.J. Elliott, R. Kelley, L.E. Katz, G.A. Pope, Remediationof elemental mercury using in situ thermal desorption, Environ. Sci. Technol.40 (2006) 2384–2389.

18] Y. Huang, Z. Hseu, H. Hsi, Influences of thermal decontamination on mercuryremoval, soil properties, and repartitioning of coexisting heavy metals,Chemosphere 84 (2011) 1244–1249.

19] C.D. Cox, M.A. Shoesmith, M.M. Ghosh, Electrokinetic remediation of mercurycontaminated soils using iodine/iodide lixiviant, Environ. Sci. Technol. 30(1996) 1933–1938.

20] Z. Shen, J. Zhang, L. Qu, Z. Dong, S. Zheng, W. Wang, A modified EK methodwith an I−/I2 lixiviant assisted and approaching cathodes to remedy mercurycontaminated field soils, Environ. Geol. 57 (2009) 1399–1407.

21] D. Hou, A. Al-Tabbaa, Sustainability: A new imperative in contaminated landremediation, Environ. Sci. Policy 39 (2014) 25–34.

22] C.M. do Valle, G.P. Santana, C.C. Windmoller, Mercury conversion processes inAmazon soils evaluated by thermodesorption analysis, Chemosphere 65(2006) 1966–1975.

23] H. Biester, M. Gosar, G. Müller, Mercury speciation in tailings of the Idrijamercury mine, J. Geochem. Explor 65 (1999) 195–204.

24] A. Navarro, I. Canadas, D. Martinez, J. Rodriguez, J.L. Mendoza, Application ofsolar thermal desorption to remediation of mercury-contaminated soils, Sol.Energy 83 (2009) 1405–1414.

25] F. Ma, Q. Zhang, D. Xu, D. Hou, F. Li, Q. Gu, Mercury removal fromcontaminated soil by thermal treatment with FeCl3 at reduced temperature,Chemosphere 117 (2014) 388–393.

26] G. Wang, S. Zhang, X. Xu, T. Li, Y. Li, O. Deng, G. Gong, Efficiency of nanoscalezero-valent iron on the enhanced low molecular weight organic acid removalPb from contaminated soil, Chemosphere 117 (2014) 617–624.

27] G. Dermont, M. Bergeron, G. Mercier, M. Richer-Laflèche, Soil washing formetal removal: A review of physical/chemical technologies and fieldapplications, J. Hazard. Mater. 152 (2008) 1–31.

28] Y. Jing, Z. He, X. Yang, Effects of pH, organic acids, and competitive cations onmercury desorption in soils, Chemosphere 69 (2007) 1662–1669.

29] H. Biester, C. Scholz, Determination of mercury binding forms incontaminated soils: Mercury pyrolysis versus sequential extractions, Environ.Sci. Technol. 31 (1997) 233–239.

30] U. Schwertmann, J. Friedl, H. Stanjek, From Fe(III) ions to ferrihydrite and thento hematite, J. Colloida Interfaces Sci. 209 (1999) 215–223.

31] S.A. Wills, C.L. Burras, J.A. Sandor, Prediction of soil organic carbon contentusing field and laboratory measurements of soil color, Soil Sci. Soc. Am. J. 71(2007) 380–388.

32] T. Terefe, I. Mariscal-Sancho, F. Peregrina, R. Espejo, Influence of heating onvarious properties of six Mediterranean soils. A laboratory study, Geoderma143 (2008) 273–280.

33] A.L. Ulery, R.C. Graham, Forest fire effects on soil color and texture, Soil Sci.Soc. Am. J. 57 (1993) 135–140.

34] H. Nishita, R.M. Haug, Some physical and chemical characteristics of heatedsoil, Soil Sci. 113 (1972) 422–430.

35] C. Adam, G. Kley, F.G. Simon, Thermal treatment of municipal sewage sludgeaiming at marketable P-fertilisers, Mater. Trans. 48 (2008) 3056–3061.

36] C. Adam, B. Peplinski, M. Michaelis, G. Kley, F.G. Simon, Thermochemicaltreatment of sewage sludge ashes for phosphorus recovery, Waste Manage.29 (2009) 1122–1128.

37] C. Vogel, C. Adam, Heavy metal removal from sewage sludge ash by

Technol. 45 (2011) 7445–7450.38] C. Vogel, R.M. Exner, C. Adam, Heavy metal removal from sewage sludge ash

by thermochemical treatment with polyvinylchloride, Environ. Sci. Technol.47 (2012) 563–567.

5 ous M

[

[

[41] M. Isoyama, S.I. Wada, Remediation of Pb-contaminated soils by washing

52 F. Ma et al. / Journal of Hazard

39] S. Tampouris, N. Papassiopi, I. Paspaliaris, Removal of contaminant metals

from fine grained soils, using agglomeration, chloride solutions and pileleaching techniques, J. Hazard. Mater. 84 (2001) 297–319.

40] S. Kuo, M.S. Lai, C.W. Lin, Influence of solution acidity and CaCl2 concentrationon the removal of heavy metals from metal-contaminated rice soils, Environ.Pollut. 144 (2006) 918–925.

aterials 300 (2015) 546–552

with hydrochloric acid and subsequent immobilization with calcite andallophanic soil, J. Hazard. Mater. 143 (2007) 636–642.