The use of a combined process of surfactant-aided soil washing and coagulation for PAH-contaminated soils treatment

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    Soil washingPhenanthreneCoagulation

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    surfactant efciencies were 70% and 30%, respectively. In addition, only the anionic wastewater can be

    s (PAHances ted byounds

    sediments due to properties such as low volatility, low water solu-bility and low biodegradability. These characteristics make it verydifcult to remove this type of compounds from soils [6,7].

    Surfactant-aided soil washing (SASW) is one of the standardtechnologies in the remediation of soils polluted with PAHs[811]. This approach involves washing the soils with an aqueoussurfactant solution with the aim of enhancing the water solubility

    effectiveness of the SASW process with low-permeability soils pol-luted with phenanthrene using three different types of surfactantsas washing reagents (anionic, cationic, and non-ionic) and, sec-ondly, to treat the efuents produced in the SASW process by coag-ulation using iron and aluminium salts.

    2. Materials and methods

    Kaolinite was selected as the model for a clay soil in this work.This material is not reactive and it has low hydraulic conductivity,

    Corresponding author. Tel.: +34 902204100x3411; fax: +34 926295256.

    Separation and Purication Technology 88 (2012) 4651

    Contents lists available at

    i

    .e lE-mail address: Manuel.Rodrigo@uclm.es (M.A. Rodrigo).phobic organic compounds. These substances are formed duringthe incomplete combustion of coal, oil, diesel or gasoline, and otherorganic substances. Normally, these materials enter the environ-ment from several sources: the gases emitted by engines duringcombustion of gasoline and especially of diesel, the combustionof wood or coal, and from sites associated with industries suchas gas production, the renement of oil and the manufacture ofwood. PAHs are an important class of pollutants since some com-pounds have been classed as carcinogenic, mutagenic and terato-genic [25]. Moreover, they are very persistent in soils and

    At present, one of the most relevant technologies for the treat-ment of O/W emulsions is coagulation. This method involvesdestabilization of the micro-drops of organics by the action of achemical reagent (usually an iron or aluminium species), whichleads to the break-up of the emulsion and the formation of twophases that can be easily separated.

    The objective of the work described here was to study the com-bination of a SASW process with a coagulation process to remedi-ate a low-permeability phenanthrene-polluted soil (kaolinite). Inthis study two partial objectives were set: rstly, to compare the1. Introduction

    Polycyclic aromatic hydrocarbonthan 100 chemically different substposed of non-polar molecules formrings [1]. For this reason, these comp1383-5866/$ - see front matter 2011 Elsevier B.V. Adoi:10.1016/j.seppur.2011.11.038satisfactorily treated with this technology, with COD removals greater than 90% achieved. Variation ofpH, zeta-potential and the dose of aluminium required seem to indicate that a charge-neutralizationmechanism is the main process involved in the emulsion break-up obtained in the treatment of aqueoussurfactant wastes. In addition, the effects of surfactant concentration and that of the pH of the soil-wash-ing wastewater seem to have a greater inuence on the performance of the coagulation process.

    2011 Elsevier B.V. All rights reserved.

    s) are a group of morehat are generally com-two or more benzeneare classied as hydro-

    of the PAHs and forming O/W emulsions with micro-drops of PAH.The resulting emulsions can subsequently be treated by a wastewa-ter treatment technology such as: electrochemical oxidation, ozon-ation, biological treatment, etc. [12,13]. In this way PAHs areremoved from the soils and the pollution is transferred to thewash-ing uid.Clayed soilSurfactant

    ionic, and non-ionic) were used as washing agents. The results show that the anionic surfactant is themost effective washing uid because efciencies higher than 90% can be achieved. Non-ionic and cationicThe use of a combined process of surfactfor PAH-contaminated soils treatment

    R. Lpez-Vizcano, C. Sez, P. Caizares, M.A. RodrigDepartment of Chemical Engineering, University of Castilla-La Mancha, Enrique Costa B

    a r t i c l e i n f o

    Article history:Received 15 November 2010Accepted 24 November 2011Available online 13 December 2011

    Keywords:

    a b s t r a c t

    Polycyclic aromatic hydrocand low biodegradability, awork described here involvcoagulation treatment, usisoil. Phenanthrene was sel

    Separation and Pur

    journal homepage: wwwll rights reserved.t-aided soil washing and coagulation

    ng, Av. Camilo Jos Cela, No. 12, 13071 Ciudad Real, Spain

    ons (PAHs) persist in soils due to their low volatility, low water solubilityf which make it difcult to remove this type of compound from soils. Thethe study of a combined surfactant-aided soil washing (SASW) process andiron and aluminium salts, to remediate a low-permeability PAH-pollutedd as the model PAH and three different types of surfactants (anionic, cat-

    SciVerse ScienceDirect

    cation Technology

    sevier .com/locate /seppur

  • tion (containing deionised water and 10 g dm3 of surfactant) were

    PRP% 1 Cf 100 1

    2.3. Coagulation of soil-washing wastewater

    Discontinuous chemical coagulation experiments were carriedout in a standard jar test experimental set-up coaple with a ther-mostatic bath (J.P. SELECTA, Spain). In each experiment, a xedamount of coagulant solutions (134 g/l AlCl36H2O or 240 g/l FeCl3)was added to 150 ml of soil-washing wastewater. This solutionwas vigorously and slowly stirred during 3 and 15 min, respec-tively, and after that it was left during 60 min to allow the sedi-mentation of the solid particles.

    Table 1Physico-chemical properties of kaolinite.

    Mineralogy (%) Particle size distribution (%)

    Kaolinite 100.00 Gravel 0Fe2O3 0.58 Sand 4TiO2 0.27 Silt 18CaO 0.10 Clay 78K2O 0.75 Specic gravity 2.6SiO2 52.35 Hydraulic conductivity (cm/s) 1 108AI2O3 34.50 Organic content (%) 0Others 11.42 pH 4.9

    R. Lpez-Vizcano et al. / Separation and Purication Technology 88 (2012) 4651 47C0

    Three types of surfactants were used to formulate the soil-washing solutions: sodium dodecyl sulphate (SDS) as a modelanionic surfactant, alkylbenzyldimethylammonium chloridemixed in the reactor for 6 h at a stirring speed of 120 rpm [14]. Thesame tank then acts as a settler (during 24 h) to separate the soilfrom the efuent generated during the soil-washing process. Theseefuents consisted of aqueous mixtures of phenanthrene and sur-factants with a very high COD. The phenanthrene removal percent-age (PRP) was calculated using Eq. (1), where Cf is the outstandingphenanthrene concentration and C0 is the initial concentration insoil samples.

    low cation exchange capacity and no organic content. The physico-chemical properties of this soil are shown in Table 1. Phenanthrene(97%) was selected as a model PAH and this compound was ob-tained from Merck.

    2.1. Preparation of simulated soil

    Samples of polluted soil were prepared by dissolving phenan-threne in acetone and then mixing this phenanthrene/acetonesolution with the kaolinite. The spiked clay was aerated duringone day to favor evaporation of the acetone and in this way thephenanthrene was homogeneously distributed on the clay surface.This method has been described in the literature by differentresearchers [1417].

    2.2. Surfactant-aided soil washing

    Surfactant-aided soil washing tests were carried out in a stirredbench-scale tank operated in discontinuous mode. The tank vol-ume was 2000 cm3. Low-permeability soil (135 g) polluted with500 mg phenanthrene kg1 of soil and 1800 cm3 of washing solu-(ABDMA) as a model cationic surfactant and polyoxyethylenesorbitan monooleate (Tween 80) as a model non-ionic surfactant.The properties of these compounds are given in Table 2.

    Table 2Properties of surfactants.

    Surfactant Type Formula MW

    Tween 80 Non-ionic C64H124O26 1309

    Sodium dodecyl sulfate Anionic C12H25NaO4S 288

    Alkylbenzyldimethylammonium chloride Cationic C21H38CM 3392.4. Analyses

    To quantify the amount of phenanthrene present in the soil, it isused an LS extraction process. It is carried out in Eppendorf tubes(15 ml) and using hexane as solvent (ratio polluted soil/sol-vent = 0.2 w/w). Both phases was vigorously stirred in a VV3VWR multi-tube and subsequent phase separation was acceleratedby the use of a centrifuge rotor angular CENCOM II P-elite. Todetermine the phenanthrene concentration in liquid phase an LLextraction process was used also. It is carried out in bottles(100 ml) and using hexane as solvent also (ratio PHE solution/sol-vent = 0.22 v/v). Both phases were slowly stirred in an orbital sha-ker (100 rpm) during 5 h (operation time required to attain thesteady state).

    In both cases, the concentration of phenanthrene in the result-ing liquid extract phase was determined by UVvisible spectrome-try (Shimadzu UV1603). The characteristic absorbance peaks ofphenanthrene in the UVvisible spectrum were at 346, 338 and330 nm. The absorbance at these three wavelengths was used toquantify the phenanthrene concentration. The standard deviationof this determination is lower than 5%.

    To check the mass balance of the system and to verify the qual-ity of the analytical methodology used, it was used the followingmethodology. Firstly, the total initial mass of the soil was dividedin 4 portions and each portion was subdivided in other 3 portions.Secondly, it was measured the initial PHE concentration of eachportion of soil. To do this, PHE was extracted with hexane (as itwas explained previously). Thirdly, it was carried out the washingprocess of each portion with anionic surfactant in order to removethe PHE from the soi