Solid state dehalogenation of PCBs in contaminatedsoil using NaBH4

M. Arestaa,*, P. Caramusciob, L. De Stefanoc, T. Pastored

aMETEA Research Centre, Department of Chemistry, University of Bari, via Celso Ulpiani 27, 70126, Bari, ItalybEnel Produzione Ricerca, Litoranea Salentina Brindisi-Casalabate, localita Cerano, Tuturano 72020, Brindisi, Italy

cNational Research Council—IMM, via Diocleziano 328, 80124, Naples, ItalydINCA, Unit of Bari, METEA Research Centre, via Celso Ulpiani 27, 70126, Bari, Italy

Accepted 9 January 2003

Abstract

In this work we present the results of an experimental study on the abatement of polychlorinated biphenyl (PCBs) in con-

taminated soil using a high energy milling technique, that promotes a reaction only by impact between milling bodies. A sample ofsoil from a controlled landfill was treated with powdered NaBH4 using two different hydride/soil ratios (5 and 2.5% w/w). Theefficiency of the dehalogenation/hydrogenation reaction was studied as a function of the milling time (3.5 up to 30 h). After each

run, the total PCBs content and the production of inorganic chloride were measured. The complete abatement was obtained with astarting PCBs concentration of about 2600 mg/kg. The residual PCBs concentration resulted to be <0.2 mg/kg. The final productsof the treatment were biphenyl and NaCl. Other toxic or hazardous organic by-products were not generated. Boron was found as

boric acid.# 2003 Elsevier Science Ltd. All rights reserved.

1. Introduction

Polychlorobiphenyls (PCBs) are very stable industrialchemicals. They are produced by chlorination(2<nCl<10) of biphenyl (C12H10); the two-ring struc-ture allows the formation of 209 congeners (Hutzingeret al., 1974). The PCB mixtures of congeners are ubi-quitous and persistent pollutants in the global ecosys-tem (Lang, 1992). Their well known physical andchemical stability, along with their excellent dielectricproperties, has led to a widespread industrial applica-tion as insulating materials for the electric industry, incapacitors and transformers. They have also been usedin inks, paints and paper for the printing industry.Although the PCB production was forbidden in 1973 bythe Organization for Economic Co-operation andDevelopment (OECD) and stopped in 1979, since 1929about 950 kt of PCBs were dispersed in the environ-ment. Their source were: accidental leakages or spills

from electric transformers, landfills and illegal waste.PCBs are, thus, common contaminants of industrialsoils, harbour sediments, atmosphere (via incineration)and water bodies. Due to their toxicity even in traceamounts, they represent an environmental hazard. Dif-ferently from air and water, soil represents a complexand heterogeneous matrix, characterized by a low velo-city of contaminant diffusion. Due to their chemicalinertness, despite the research efforts for their abate-ment, the established technologies for their disposal are:incineration and landfill. Incineration may produceadditional toxic compounds, like polychlorinateddibenzo-furans (PCDFs), polychlorinated dibenzo-dioxins (PCDDs) and dioxins. Landfill disposal maygive rise to leacheates into soil and water. Therefore,‘‘innovative technologies’’ for the treatment of PCBs arevery welcome.In this work, we have used high energy milling

(HEM) for the conversion of PCBs. In this technologythe mechanical energy is transferred from the millingbodies to the solid system through shear stresses or com-pression, depending on the device used. A significant partof the milling energy is converted into heat, a minor part

0956-053X/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved.

doi:10.1016/S0956-053X(03)00029-1

Waste Management 23 (2003) 315–319

www.elsevier.com/locate/wasman

* Corresponding author. Tel.: +39-080-5442430; fax: +39-080-

5442429.

E-mail address: aresta@metea.uniba.it (M. Aresta).

is used to induce breaks, stretches and compression atmicro andmacroscopic level or for performing a reaction.The modifications usually induced into the molecularstructure of solids are plastic-reticular stretches andfractures. Besides well established applications at theindustrial level for grinding solids, HEM has beenapplied so far to the synthesis of advanced materials as‘‘metastable alloys’’ and ‘‘nanostructural materials’’ andto more basic processes like ‘‘mechanical alloying’’,‘‘mechanical-synthesis’’, ‘‘solid state vitrification’’ (Tya-gin et al., 1998). Recently, such technology has beenapplied with success to solving some environmentalproblems (Cocco et al., 1999; Hall et al., 1996; Loiselle etal., 1997). Further to previous work (Caramuscio et al.,2000) on the treatment of contaminated soil with sodiumhydride (NaH), in this paper we report the results of ourstudy on the mechanochemical treatment of PCBs con-taminated soils using a ternary hydride, NaBH4.

2. Materials and methods

2.1. The ring mill

The ring mill used in this work was a Fritsch Pulveri-sette 9, equipped with a tempered chrome steel grindingset formed by a disk and two rings with a total mass of3637 g. The rotational rings velocity was 750/1000 rpm,the volumetric capacity 350 ml with an energy con-sumption of 0.6 kWh.

2.2. Soil sample

The granulometry and composition of the soil sample,taken from a controlled landfill for toxic-harmful waste,are reported in Tables 1 and 2. The data in Table 1show that it was a typical sandy soil, with more than90% classified as fine sand. Table 2 gives the total PCB’sconcentration and that of other contaminants.

2.3. Analytical methods

The determination of chlorides was carried out usingthe Method IV.2 (Met. IV.2, 2000).

The extraction of PCBs and the extract clean up wereperformed according to the EPA-3540 and EPA-3620,-3630, -3665 (USEPA SW-846, 1995) methodologies,respectively.The determination of PCBs was carried out according

to the EPA-8082 (USEPA SW-846, 1995) methodology.

3. Experimental

3.1. Pre-conditioning of soil sample

Each soil sample was homogenized and dried at 50 �Cfor 20 h. The sample amount used in each test (100 g)represented about 1/36 of the total weight of themill’s ringand occupied about 2/3 of total mill’s volume. Soil wasmilled prior to the treatment in order to improve the effi-ciency of the solvent extraction process, that depended onthe size of the particles. Three hours of preliminary millingof soil guaranteed a correct extraction, owing to the greatspecific surface of the resulting finely powdered soil.

3.2. Grinding tests

NaBH4 as pure reagent was added to the powderedsoil in the solid state. Two hydride/soil ratios were used,namely: 5:100 (w/w) and 2.5:100. The mixtures werehomogenized by hand and introduced into the ring mill.To verify the PCBs abatement efficiency, samples werewithdrawn and analysed at intervals of time (3.5, 11, 18,23, 30 h) and both total residual PCBs and inorganicchlorides determined.

3.3. Extraction and determination of PCBs

Ground soil samples were subjected to Soxhletextraction (USEPA SW-846, 1995) for 16 h using a 200ml hexane/acetone mixture (1:1 v/v). The extract wasthen purified by absorption on dry Florisil and Silica gelcolumns followed by treatment with concentrated sul-phuric acid. Analyses of the samples were performedwith a HP 6890 gas-chromatograph/electron capturedetector with DB-5 capillary column (30 m�0.2 mm i.d.and 0.33-mm film thickness). As Table 2 shows, millingmade completely available the PCBs present in the soil, asshown by the results of PCBs quantitative determination

Table 1

Soil granulometry characteristics

Granulated diameter (mm)

Incremental fraction detained (%)

4.75

0.5

2

1.4

0.85

2.4

0.425

3.5

0.18

71.3

0.075

18.1

<0.075

2.8

Table 2

Soil composition characteristics

Parameter

Unit Result

Total extracted PCBs

mg/kg 1520

Total extracted PCBs (after 3 h of milling)

mg/kg 2600

BTX

mg/kg <0.1

Total hydrocarbons (C>12)

g/kg 15.88

pH

6.84

316 M. Aresta et al. /Waste Management 23 (2003) 315–319

on the untreated sample and after 3 h of milling of thesame sample. The difference was due to enhancedextraction from finely ground particles. The concen-tration is expressed as comparison with a standardmixture of Aroclor 1254-1260 (1:1). The concentrationof PCBs in samples withdrawn at various milling timesis expressed as residual percentage.

4. Results and discussion

4.1. Reaction of PCBs dechlorination

The general reaction of hydrogenation/dechlorinationof PCBs occurring during the mechanochemical treat-ment is given in Eq. 1.

C12HxCly þMH !Eum

C12H10 þMCl ð1Þ

where:

� C12HxCly=polychlorobiphenyl with 0<x<8and 2<y<10;

� MH=hydride donor compound (NaBH4);� Eum=energy transferred by milling;� C12H10=biphenyl; and� MCl=chloride salts.

The milling energy had a fundamental role in thePCB’s dechlorination process. In fact, no reaction tookplace if solids were simply mixed or ground by hand.The transferred energy caused the nucleophilic sub-stitution of Cl� with H� to occur.Biphenyl and NaCl were the final dechlorination pro-

ducts isolated from the reaction mixture. We have alsoinvestigated the fate of boron. By using the XRD tech-nique boron could not be found in treated soil because itsconcentration was below the sensitivity of the method.Conversely, if soil was washed with water, boron wasextracted and speciated as boric acid (Met. XVI.2, 2000)in concentrated waters. Anyway, the added boron couldnot be quantitatively recovered from soil in this way. Themilling equipment is now under adaptation for the gas-phase might be analysed in order to check if volatileboron compounds are formed during HEM.

4.2. Progressive dechlorination of PCBs during themechanochemical treatment

The progressive abatement of PCBs in soil mixed withNaBH4 is clearly shown in Figs. 1 and 2, where the chro-matograms obtained at different milling times are repor-ted. A careful analysis of the chromatograms at varioustimes did show that every PCBs peak is reduced increas-ing the milling time, even if a greater abatement efficiencywas obtained for high chlorinated congeners with respectto lower ones. The residual concentration was lower than

Fig. 1. Chromatogram B shows the PCBs abatement after milling

the contaminated soil (Chromatogram A) for 18 h with NaBH4

(at 5% w/w).

Fig. 2. Chromatogram B shows the PCBs abatement after milling

the contaminated soil (Chromatogram A) for 23 h with NaBH4 (at

2.5% w/w).

M. Aresta et al. /Waste Management 23 (2003) 315–319 317

0.2 mg/kg. Figs. 1A and 2A give the PCBs concentrationin the soil milled without addition of hydride. Figs. 1Band 2B, show the results for soil ground for 18 and 23 h,respectively, after the addition of NaBH4.

4.3. PCBs and chloride trends as a function of themilling time

Fig. 3 represents the total PCBs and the inorganicchloride trend as a function of milling time when thecontaminated soil was treated with 5% of NaBH4. The

contaminants concentration decreased while the chlo-ride concentration increased following the same expo-nential trend. After 18 h the PCBs concentrationdecreased from 2600 mg/kg to practically zero and theinorganic chloride concentration increased from 60 to430 mg/kg. Fig. 4 shows the results obtained addingNaBH4 at 2.5% w/w ratio to the contaminated soil.Comparing Figs. 3 and 4, it is evident that the millingtime for a total abatement of PCBs decreases withincreasing the NaBH4 amount. However, in both cases,ca. 90% of PCBs dechlorination took place during the

Fig. 3. Total PCBs and chlorides concentration in the soil sample added with 5% of NaBH4 (w/w) as function of the milling time.

Fig. 4. Total PCBs and chlorides concentration in the soil sample added with 2.5% of NaBH4 (w/w) as function of the milling time.

318 M. Aresta et al. /Waste Management 23 (2003) 315–319

first 11 h of treatment. A better view of the differences isreported in Fig. 5, that compares the PCBs abatementefficiency, when 5 and 2.5% NaBH4 are used. With 5%w/w NaBH4 the dehalogenation reaction was just a bitfaster within the first 15 h. In both cases, the totalabatement of PCBs was observed after 23 h.

5. Conclusions

As extension of previous studies (Caramuscio et al.,2000) on mechanochemical treatments, in this paper wereport that the detoxification of PCBs contaminated soilis possible using the earlier mentioned technology. Wehave demonstrated that the efficiency of PCBs extractiondepends on the soil’s specific surface. Three hours ofpreliminary milling of the neat soil can assure a completeextraction of PCBs. NaBH4, applied at different con-centrations (5 and 2.5% w/w), achieves the total abate-ment of PCBs within a time of 18 and 23 h, respectively.As comparison, the Italian law (D.M. n.471, 1999)

recently enforced fixes at 5 mg/kg the maximum PCBsconcentration limit for decontaminated soil for indus-trial reuse. The technology we have described achievesmuch better results, the residual concentration remain-ing below 0.2 mg/kg. This study has also demonstratedthat, unlike other conventional treatments used, noother toxic or hazardous organic by-products are gen-erated during the mechanochemical process. Moreover,boron is found as boric acid in the soil.For its intrinsic simplicity (confined and controlled

conditions, low cost, clean process with no emissions)this kind of treatment appears to be very attractive as aremedial technology.

Acknowledgements

This work was supported by MIUR, Project P6-L488, INCA Consortium. The cooperation with thelaboratories of ENEL Production Research Centre inBrindisi is acknowledged.

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M. Aresta et al. /Waste Management 23 (2003) 315–319 319