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Aged Chlorophenol Contaminated Soil's IntegratedTreatment by Ozonation, Soil Washing and BiologicalMethodsP. Haapea & T. TuhkanenPublished online: 11 May 2010.

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Environmental Technology, Vol. 26. pp 811-819© Selper Ltd., 2005

AGED CHLOROPHENOL CONTAMINATED SOIL´SINTEGRATED TREATMENT BY OZONATION, SOIL

WASHING AND BIOLOGICAL METHODS

P. HAAPEA*1 AND T. TUHKANEN2

1Univestiry of Kuopio, Department of Environmental Sciences, P.O. Box 181, FIN-50101 Mikkeli, Finland2Institute of Environmental Engineering and Biotechnology, Tampere University of Technology, P.O. Box 541, FIN-33101

Tampere, Finland,

(Received 2 December 2004; Accepted 3 March 2005)

ABSTRACT

Traditionally soils contaminated by organic compounds have been treated by biological methods, but aged contaminatedsoils usually contain refractory and toxic compounds not any more responsive to biological treatment. By oxidation and soilwashing the biodegradability and bioavailability of these otherwise recalcitrant compounds can be enhanced. The aim of thestudy was to optimize and integrate soil washing, ozonation (+ hydrogen peroxide) and biological treatment for theremediation of old saw mill soil contaminated by chlorophenols (CPs). The integration of different treatment train variablesand the alternation of pHs (3, 7 and 10) and different ozone doses were studied in a laboratory scale. The soil was heavilycontaminated aged soil (> 4000 mgCP kg-1), so more than 99 % of removal should be attained in order to reach the Finnishguideline values. All the individual methods studied were able to degrade/transfer 25 - 95 % of the CPs from the soil matrix.By biological treatment only 25 % of the CPs were degraded. After soil washing, 40- 80 % of the contaminants weretransferred from the soil phase and by ozonation 35 – 95 % decrease of CPs were achieved. All the methods studiedsuccessfully enhanced the biodegradability of the target compound. After biological treatment of ozonated and washedsamples even 93 - 100 % degradation of CPs was obtained. The most effective treatment was the combination of soil washing+ ozonation (in pH 10) followed by biological treatment.

Keywords: Aged soil, chlorophenols, ozonation, biological treatment, soil washing

INTRODUCTION

Chlorophenols have been widely used as woodpreservatives in the Finnish saw industry from the 1930´s tothe 1980´s; the product used was known by the name KY 5.Chlorophenols are also formed e.g. as side products in thepulp industry, burning of non-sorted city waste and duringthe disinfection of wastewater and drinking water. [1.]

The properties of the different CPs vary. Also the pH ofthe environment is important: in acidic conditions CPs are in aneutral form but when pH is higher than the compound acidicconstant value (pKa), an ionic form (phenolate) dominates.Depending on the environmental conditions, CPs may persistfor extended periods of time. Thus they represent potentiallytoxic, long-term source of subsurface contamination. [2-5.]Remediation of hydrophobic pollutants is complicated bysorption to hydrophobic sites of dissolved natural organicmatter (NOM), suspended particulates, soil and sediment. Thesorption makes pollutants less degradable and resistant to

remediation. [6-9.] Also increased contact times decrease theextractability and bioavailability of organic contaminants [10-12].

The efficiency of the bioremediation depends on,besides the bioavailability and sorption, also on toxicity,concentrations of CPs, environmental conditions and thepresence of suitable microorganisms. Many biological systemscan be adapted to degrade CPs, but the degradation is usuallyslow at pristine soil conditions. Various chlorophenols havebeen treated in aerobic conditions by adapted bacteria and bya mixed culture in activated sludge processes [13], in soils andin compost [4,14-17] and in anaerobic conditions due to thedechlorination [18,19]. Biodegradation can be enhanced byusing acclimatised microorganisms and by changing thecircumstances into more favourable, for example byoptimising nutrients, moisture, oxygen, and organic mattercontent [3,4,17,20,21].

Soil washing is a process which uses water or someother washing solution to extract or separate the soil by

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chemical-physical methods into different phases: washed soiland slurry. The fine particles and organic matter, whichcontain most of the contaminants are mainly transferred toslurry fraction. Desorption of chlorophenols from the solidmatter can be increased with suitable pH, surfactants and co-solvents. Soil washing is not a destructive method for CPs, itmainly transfers contaminants from one phase to another andmakes them more available for biological post treatment.[17,22.] The main purpose of soil washing is to minimise thevolume of contaminated fraction that has to be treated [22].

Several oxidation studies with ozone and AdvancedOxidation Processes (AOPs), for the degradation ofchlorophenols have been done to enhance biodegradability forsubsequent biological treatment and to reduce the toxicity. Ingeneral, the by-products formed during oxidation are mainlylow-molecular-weight organic acids or hydroxylatedcompounds, which are more biodegradable, water soluble orless toxic than their parent compounds. [3,5,9,19,23,24.]However, extended ozonation times are not alwaysadvantageous to the biodegradation but can even retard it[3,5]. At higher ozone dozes, biodegradable compounds, bothinitially present and formed as a result of ozonation, competefor the available ozone [21, 25-29]. Therefore it is practical tooptimise the ozone dose so that a higher fraction of dissolvedmatter can be eliminated by usually more economical,biological post treatment. The amount of dissolved humicmatter has been detected to increase during ozonation, whichimplies that ozone broke down the soil matrix and aided thedissolution of particulate and colloidal humic matters fromthe soil surface, which release CPs to the aqueous phase [30].

The aim of the study was to optimise and integrate soilwashing, ozonation and biological treatment in aneconomically feasible way for the remediation of an aged sawmill soil contaminated with chlorophenols. The integration ofthese three different treatment train variables and thealternation of pHs and different ozone doses were studied ina laboratory scale. One of the main targets of this study wasto achieve a degradation of CPs in aged and highlycontaminated soil under the Finnish guideline values. Sincethe characteristics and toxicity of different chlorophenolsvary, there are guideline values for individual CPs. Theselimit values for the four target chlorophenols in soil are: 2,6-DCP = 30 mg kg-1; 2.4.6-TCP= 10 mg kg-1; 2,3,4,6- TeCP andPCP= 4 mg kg-1 [31]. Therefore up to 99 percent removalefficiencies were required from the treatment.

MATERIALS AND METHODS

Description and Characteristics of the Aged CP-contaminatedSoil

The soil was from an old sawmill area situated in themiddle of Finland. A product named KY 5 has been used onthe site for wood preserving for many decades. Regardingcontamination, investigations have been done at the site

during the 1990´s. The highest CP concentration found in soilsample was almost 50 000 mg CP kg-1.

The soil taken for the experiments was morainecontaining high levels of organic matter and it was from adepth of about 1 m under the saw chip layer. Beforeexperiments and sieving, the soil was air- dried in thetemperature about 50 oC. The soil was dried before sieving tomake soil more homogenous due to the lumps containingdifferent fractions of soil particles. Part of the soil was sievedfor the detailed estimation of the soil fractioning and for themeasurement of chlorophenol consentration in each fraction.The mesh sieves used were: 0.063 mm, 0.25 mm, 2 mm, 4 mm,8 mm and 16 mm. Soil sieved with 2 mm mesh was used forthe experiments, because it included the most contaminatedpart of the soil. Iron content of the soil was high, almost 19gFe kg-1. The samples were stored in the freezer in classcontainers covered with aluminium foil.

Experimental Procedures

The schematic diagram of the different treatment trainsfor the CP contaminated soil involving sequential soilwashing, chemical oxidation and biological degradation isrepresented in Fig. 1.

Integrated Soil Washing, Ozonation and Biological Treatment

The pre-screened soil (< 2 mm) was washed in soil:water mixture of 1:2 (w:w) in three different pHs (3, 10 andnot adjusted). The pH of the soil:water mixture, the pH notadjusted, was 5 – 5.3, depending on the amount of water. Arotary shaker (200 rpm) agitated the soil- water –mixture for30 minutes in the glass containers with Teflon caps. Aftermixing, the soil and water phases were separated bydecantation. Most of the detached fines, colloidal material andorganic matter were transferred into the liquid phase.

The laboratory scale ozonation tests were alsoperformed in 3 different pHs for untreated and pre-washedsoil-water mixtures and for the washing water separated aftersoil washing experiments. The amount of pre-washed soil andwashing water added into the container was 200 g andunwashed soil 120 g. Tap water (~20 oC) was added into thecontainer so that the total amount of sample was 1 l.Ozonation was carried out in a glass container (volume 2 l),which was agitated in a vertical shaker (200 rpm). Ozone wasgenerated with a Pacific Ozone Technology generator (model0.1) from synthetic dry air. The influent ozone concentrationwas about 1000 ppm and the flow rate was 5 l min-1. Dasibi1008 analyser was used for measuring the ozoneconcentration in the gas phase from the influent and effluentgas stream. The ozone analyser was connected to a computer,which calculated automatically the amount of fed ozone(mg l-1 of water) and transferred ozone (%). The ozone dosesvs. organic matter were 0.0021 – 0.025 mgO3 mgorg

-1 (25 – 300mg l-1) for unwashed soil and 0.003 – 0.04 mgO3 mgorg

-1 (20 –287 mg l-1) for washed soil and liquid phase.

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Figure 1. The diagram of the different integrated processes studied.

Biodegradation Tests

The respirometric biodegradation tests were done in thedark incubation flasks (WTW BOD Trak Oxitop). The methodprovides also a direct measurement of oxygen consumed bymicroorganisms in a closed vessel in which the inbuiltdatalogger records and calculates BOD readings in every fourhours. The amount of contaminated soil in the incubationflasks was about 10 g. Also 80 g of uncontaminated soiladded. The moisture content was adjusted to 20 % withnutrient water (mixture done according to SFS 5508). Theinoculum was prepared by mixing 8 g of compost frommunicipal composting plant and earlier isolated Pseudomonassp. into a 100 ml of sterile 1 % NaCl –water and added withthe nutrient water. The Psudomonas sp. was isolated from thatsame municipal compost according to the API 20 E test andregenerated every 3 – 4 weeks. The inoculum was added,because the chemical oxidation methods are very strongdisinfectants for microbes. The amount of total microbes(including microbes in contaminated soil + uncontaminatedsoil added) was not detected during these experiments. Thesamples were incubated in the dark at 20 ± 0,5 oC for 12 days.Biodegradation tests for the water samples were done by thesame procedure. The volume of contaminated water was 40ml and the amount of nutrient/ inoculums water added was325 ml. If needed, pH of the samples was adjusted to 6 – 8.The water samples were mixed during incubation with amagnetic stirrer.

Analytical Methods

The amount of total solids (dry matter) and organicmatter in water and soil samples were determined accordingto SFS 3008. Concentration of iron was measured with PerkinElmer Analyst 800 AAS by flame detector after which the

sample was extracted in acidic media. All the glassware usedwas post-washed with acetone.

The amount of CPs was analysed according to themethod developed and accredited by the Finnish NationalPublic Health Institute. Before extraction (NaOH + hexane)2,4,6 –tribromophenol (Supelco) for the internal standard wasadded into the 5/10 g of sample and the mixture was agitatedin ultrasonic bath for 15 minutes in sealed glass bottles. Thehexane phase was separated and the pH of the water solutionwas adjusted < 3 with the sulphuric acid (5 M). The acidicwater phase was extracted three times with diethylether. Theorganic phases were collected to the separate vial anddehydrated with sodium sulphate. On the next day sampleswere filtrated and ether was evaporated in the water path(50 oC). Potassium carbonate and distilled acetic anhydridewas added into the container. After acylation 5 ml of hexanewas added. The hexane phase was separated and stored in thefridge. The analysing was done by GC/MS (Hewlett PackardHP 6890/5972) equipped with HP-5 column (30 m x 0.25 µm x0,25 µm, with 5 % phenyl methyl siloxan film). The feed gas(helium) flow was 1 ml min-1. The splittless injection was donein 50 oC. After injection the oven temperature was increased5 oC min-1 to 180oC and 15oC min-1 to 280 oC. Theconcentrations of chlorophenols were determined bycomparing the chromatograms with 3 different calibrationcurves (Supelco: EPA 8040A Phenol Calibration Mix) and theresponse of the known amount of internal standard.

RESULTS AND DISCUSSION

The Concentrations of the Chlorophenols

The concentration of 3 main chlorophenols (2,4,6-TCP,2,3,4,6- TeCP and PCP) analysed in 5 different soil fractionsare represented in Fig 2. The concentration of of 2,6-DCP are

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Figure 2. The concentrations of chlorophenols in different soil fractions. The percentage amounts of the fraction are shown inthe parentheses.

not shown in this figure because of the low concentration(app. 2.5 ppm). The relative amount of 2,3,4,6 - CP was almost90 % in all fractions. The relative amount of PCP was about 5% and 2,4,6 –TCP less than 3 %. Almost 60 % of thechlorophenols analysed were on the fractions < 2 mm. Eventhough the amount of chlorophenols in the smallest fraction(< 0.063 mm) was the highest, this fraction of soil representsonly 1 % of the total mass of the soil. The relative highestamount of chlorophenols exist in the soil fraction 0.25 – 2 mm,more than 40 % of the soil is this size. The percentage amounts

of soil fractions 8 – 16 mm and > 16 mm were 8 and 7 % (notshown).

The profile of chlorophenols in the sample soil differsfrom the original wood preservative mixture. KY-5 containsdifferent amounts of mainly four chlorophenols; 2.4-DCP (1 -4%), 2.4.6-TCP (10 - 30 %), 2.3.4.6- TeCP (60 - 80 %) and PCP (5- 10 %). For the comparison, the relative amounts ofchlorophenols in “fresh” KY 5 –chemical and the relativeamounts of chlorophenols in the aged soil researched in thisstudy are represented in the Figure 3. There is hardly any

Figure 3. The percentage amounts of different chlorophenols in the KY 5 and in the aged soil studied.

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di-chlorophenols left in the aged soil and the relative amountof TCP is more than 30 % smaller than in KY-5. The relativeamount of TeCP is more than 90 % compared to the amount ofKY-5, in which the relative amount of TeCP is 55 %.

Combination of Ozonation and Biological Post Treatment ofSoil

Without any pre-treatment, the degradation ofchlorophenols in soil during biological treatment was only 24%. The amount of chlorophenols in soil phase decreased 24 -96 % during ozonation (Figure 4). 96 % degradation ofchlorophenols was achieved with the ozone dose of 0.025mgO3 mgorg

-1 in pH 10. Ozonation in pH 10 was also the mosteffective method to increase the biodegradability ofchlorophenols in soil. After biological treatment of ozonatedsoil, the amount of chlorophenols remaining was only 4 – 9 %.In other tests the amount of CPs remaining was 22 – 76 % afterozonation and 17 – 46 % after biological treatment of oxidatedsoils. These results are comparable with the literature whichindicates that ozonation enhances biodegradability of organicchemicals by desorbing them and organic matter into themore soluble and bioavailable form [e.g.44;51]. The additionof H2O2 did not enhance either ozonation or biodegradation.

Combination of Soil Washing, Ozonation and BiologicalTreatment

Soil washing extracted about 40 % of the CPs from thesoil and also improved the biodegradability of CPs. Afterbiological incubation of washed soil, the amount of CPsremaining was only 26 % of the initial amount. The relativeamounts of CPs remaining after soil washing, ozonation, andbiological treatment are represented in Fig.5. After ozonation,done in pH 10 and pH not adjusted, the relative amounts ofCPs in the soil phase decreased 55 - 90 % compared to theinitial concentration. Also the ozonation + hydrogen peroxidetreatment was effective.

Ozonation also enhanced the biodegradation of theCPs. The total degradation/transformation of the CPs afterthe three phase treatment train was 98 - 100 % with thebiggest ozone doses (0.04 mgO3 mgorg

-1). Also with thesmallest ozone dose in pH 3 and 10, the amount of CPsremaining in soil was only ~5 % of the initial amount. The O3

+ H2O2 treatment had the negative effect on thebiodegradability of CPs by the biggest ozone dose.

Ozonation and Biological Treatment of Washing Waters

The biodegradation of CPs in washing water separatedin soil washing experiments without ozonation was 24 %. Therelative amounts of CPs in a washing water phase afterozonation and biological treatment can been seen in Fig 6.

Figure 4. The effect of pH and ozone doses for the mass balances of CPs remaining in the liquid and solid phases afterozonation and biological treatment of contaminated soil. The initial concentration was ~4000 mgCPs kg-1.

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Figure 5. The effect of pH and ozone doses for the mass balances of CPs remaining in the liquid and solid phases afterozonation of pre-washed soil:water mixture and after biological treatment. The initial concentration of total CPs was~4000 mgCPs kg-1.

Figure 6. The effect of pH and ozone doses for the mass balances of CPs remaining in the liquid phase after ozonation andafter biological treatment of washing water. The initial concentration of total CPs was about 300 - 640 mg l-1.

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The initial CP concentrations in the washing water varied (280- 640 mg l-1) depending on the efficiency (pH) of the washingprocedure. Ozonation was not so effective against thechlorophenols in water phase than against the chlorophenolsin the soil. It had also only little or no effect on thebiodegradability of CPs. Relatively far more higher ozonedoses were needed to achieve the same degradation level thanduring ozonation of soil. Addition of hydrogen peroxidedecreased the efficiency of ozonation and biodegradation.

Summary

Concentration of the chlorophenols in soil aftercombinations of treatments in 3 different pHs and withaddition of hydrogen peroxide is represented in Figure 9. Theozone dose in all results presented is the smallest one used(0.002 or 0.003 mgO3 mgorg

-1). The individual methods studiedalone were able to degrade/transfer 25 - 80 % of thechlorophenols from the soil. By biological treatment only 25 %of the CPs degraded. By soil washing about 40 – 60 % of theCPs were transferred into the water phase. By oxidation alone30 – 95 % decrease of the amount of total CPs were achieved.The degradation was better with higher ozone doses.

All of the methods studied enhance thebiodegradability of the target compound. After biologicaltreatment of ozonated samples 93 - 99 % degradation ofchlorophenols were detected. Almost the same level of

degradation was also detected after biological treatment ofwashed soils. The most effective treatment was thecombination of soil washing + ozonation + biologicaltreatment. After this three-phase treatment train even 100 %degradation of the chlorophenols was achieved.

In table 1 is recapitulated the efficiency of biologicaltreatment, soil washing and ozonation for 4 different CPsanalysed. The less chlorinated phenols were morebiodegradable than CPs with 4 or 5 chlorines. After ozonationthe effect of the level of chlorination was not so significant.During ozonation the relative concentration of 2,6-DCPincreased, especially with higher ozone doses. Theassumption is, that these are the degradation by-products ofmore chlorinated phenols. Generally, the removal of thesefour different CPs was more efficient with higher ozonedoses.

DCP and TCP were degraded under the limit valuesalmost by all methods used. The individual methods were notable to degrade PCP and especially TeCP under the limitvalues (4 mgCP kg-1). The combination of the treatments bywhich the limit value of PCP were reached are:

Washing + ozonation (0.04 mgO3 mgorg-1) + biologicaltreatment in pH 3, 10 and not adjusted

Washing + ozonation (0.01 mgO3mgorg-1) + biologicaltreatment in pH 10.

Figure 7. The effect of pH on the concentration of soil chlorophenols in the different combinations of treatments. The ozonedose in all results presented is the smallest one used (0.002 and 0.003 mgO3 mgorg

-1).

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Table 1. The removal of 2.6-DCP, 2,4,6- TCP, 2,3,4,6 –TeCP and PCP after biological treatment, soil washing and ozonation.The relative removal is compared with initial concentration of each compound.

DCP TCP TeCP PCP TotalTreatment method rem. % rem. % rem. % rem. % rem. %Initial 0 0 0 0 0Biological 100 67 13 0 14Soil washing 50 - 70 48 - 76 28 - 67 21 - 54 28-67Biological 100 92-95 61-81 53-63 62-78Ozonation of prewashed soil -35-55 1) 87-97 1) 51-90 2) 75-88 3) 55-90 2)

Biological 100 100 77-99 69-96 75-99Ozonation of soil 25-48 3) 34-87 2) 24-78 2) 23-78 2) 24-78 2)

Biological 100 96-100 68-98 58-89 70-981) relative amount increased with higher O3 doses2) more efficient with higher O3 doses3) not signifigant impact of O3 doses

CONCLUSIONS

The comparison of the profile of composition KY 5 -chemical and aged chlorophenol contaminated soil by thischemical clearly illustrates that di- and tri-chlorophenols aremore degradable and water soluble since they havedisappeared. This also indicates that TeCPs and PCP arerelatively resistant in the natural environmental conditions.Even under the optimal environmental conditions (suitablemoisture, temperature, nutrient and microbes) only 25 % ofthe aged CPs was biologically degraded.

Soil washing with tap water and with pH adjustmentwas effective for the removal of chlorophenols from soilphase. Washing also clearly increased the biodegradability ofthe CPs, especially done in pH 10.

Without prewashing, ozonation decreased 25 – 95 % ofthe remaining CPs in soil. The most part of CPs, especially inpH 10, were transferred into the water phase with smallerozone doses. Ozonation also clearly increased thebiodegradability of CPs. The effect of ozonation for the

biodegradation done in pH 3 was much lower, while therelative amount of CPs in water phase was lower in acidicenvironment. This indicates that ozone breaks the soil matrixand releases CPs into more soluble and biodegradable form.

Soil washing and addition of hydrogen peroxidedecreased the amount of ozone needed. About the same levelof degradation of CPs were achieved for pre-washed soil with8 times lower ozone dose.

According to the results achieved, the following conclusionscan be drawn:

- CPs remaining in aged soil are recalcitrant for biologicaltreatment

- Soil washing increases the efficiency of ozonation anddecreases the amount of ozone needed

- Soil washing and ozonation improve thebiodegradability of the CPs and shorten the treatmenttime

- Biological post-treatment decreases the amount ofozone needed

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