Patent Application
20090159530
The present invention relates to a process for the treatment of water contaminated by apolar compounds based on the use of particular zeolites.
More specifically, the invention relates to a process for the treatment of water contaminated by apolar compounds consisting of halogenated organic solvents and aromatic hydrocarbons which is based on the use of apolar zeolites having structural channels with specific dimensions.
The process according to the invention can be conveniently used for the treatment of contaminated groundwater by the use of a permeable reactive barrier (PRB).
Conventional PRB for the decontamination of water contaminated by halogenated solvents are based on systems using metallic iron and/or granulated activated carbon (GAC).
The first system, functioning for the reducing capacities of the metal, is only active towards reducible substances, such as organo-chlorinated products or metals with a high oxidation number (U.S. Pat. No. 5,266,213, WO 92/19556).
Furthermore, when zero-valent iron is used, there is a reduction in the permeability of the barrier due to encrustations or the precipitation of minerals which derive from the reactions between the ions of the oxidized metal and the substances contained in the groundwater.
The second system is a non-specific absorbent and as such is not very selective with respect to interfering substances present in the water and in particular in groundwater (ions, humic acids, etc.).
If it is used for the production of permeable reactive barriers, it consequently causes exhaustion of the system in short times (Williamson, D. 2000. Construction of a funnel-and-gate treatment system for pesticide-contaminated groundwater. Chemical Oxidation and reactive barriers. Godage B. et al. Eds. In II Intl. Conf. on Remediation of chlorinated and recalcitrant compounds. Monterey, Calif., USA, Battelle Press, Columbus, (2000)), pages 257-264.
Schad, H 2000. Funnel-and-gate at a former manufactured gas plant site in Kalsruhe, Germany: design and construction. In: Chemical Oxidation and reactive barriers.
Godage B. et al. Eds., II Intl. Conf. on Remediation of chlorinated and recalcitrant compounds. Monterey, Calif., USA, Battelle Press, Columbus, (2000), 215-322.
Both systems however prove to be ineffective in removing all the main pollutants often contemporaneously present in contaminated groundwater beneath industrial sites, which frequently consist of apolar compounds such as halogenated solvents and compounds deriving from the oil industry. These are often highly toxic products, some of which are of a carcinogen nature, whose concentration in underground water must respect the strict limits established by the law.
A treatment process of contaminated water has now been found, which allows the above pollutants to be effectively and selectively removed with respect to the mineral salts normally dissolved in water.
An object of the present invention therefore relates to a process for the treatment of water contaminated by apolar compounds which consists in treating the water with one or more apolar zeolites characterized by a silica alumina ratio >50 and by the presence of structural channels having dimensions similar to those of the molecules of the contaminating compounds.
The process according to the invention is particularly effective in removing pollutants consisting of halogenated solvents such as carbon tetrachloride, tetrachloroethylene (PCE), trichloroethylene (TCE), dichloroethylene (DCE), vinylchloride (VC) and aliphatic and/or aromatic compounds deriving from the oil industry such as methyl-terbutylether (MTBE), BTEX (benzene, toluene, ethylbenzene, xylenes), naphthalene, 2-methyl-naphthalene, acenaphthene, phenanthrene.
The process according to the invention can be conveniently used for the decontamination of groundwater by the use of permeable reactive barriers (PRB). In this case, the zeolite forms the active medium of the barrier, placed in situ perpendicular to the flow of the groundwater, which when crossed by the polluted water column allows decontamination by the immobilization of the contaminating species.
The barriers can treat groundwater polluted by chlorinated solvents, cyclic or polycyclic aromatic hydrocarbons and compounds which are particularly resistant both to biodegradation and adsorption such as MTBE or vinyl chloride (VC), with a high selectivity with respect to inorganic interfering products.
Vinyl chloride is considered as being a contaminant which is difficult to eliminate. It is not sufficiently withheld, in fact, by activated carbon and its degradation requires the use of additional structures which involve the use of UV lamps.
The presence of MTBE in groundwater also represents a problem which is difficult to overcome and whose solution justifies the use of relatively costly absorbing materials (Davis et al., J. Env. Eng., 126, page 354, April 2000).
The zeolites used in the process of the invention are characterized by the presence of structural channels having dimensions ranging from 4.5 to 7.5 Å. Zeolites having structural channels with dimensions ranging from 5 to 7 Å and silica/alumina ratios >200 such as, for example, silicalite, ZSM-5 zeolite, mordenite, are preferably used.
As a result of their selectivity, zeolites have a higher absorption capacity and functioning duration than those of materials currently used in permeable reactive barriers, such as activated carbon.
This is due to the properties of this reactive medium which are based on the dimension of the structural channels, suitably calibrated for organic molecules, and on the high polarity, deriving from high silica/alumina ratios, which excludes any type of interaction with ions or polar compounds.
The zeolite therefore has a selective interaction with molecules of apolar contaminants whereas it completely excludes polar ions and molecules normally present in groundwater together with humic substances, having higher molecular dimensions than those of the structural channels.
Suitable mixtures of particular zeolites, moreover, allow the contemporaneous removal of aliphatic organochlorinated products, aromatic hydrocarbons, polyaromatic hydrocarbons, characteristic components of oil products.
ZSM-5 zeolite and mordenite, with an Si/Al ratio >200, are materials known as molecular sieves or as carriers for catalysts, but their use as active components for the production of PRB has not yet been described in literature.
ZSM-5 zeolite is particularly suitable for aliphatic, halogen-aliphatic and mono-aromatic molecules, such as BTEX and halogen-benzene-derivatives.
Mordenite, on the other hand, is suitable for aromatic molecules with two or more aromatic rings, and halogen- and alkyl-substituted.
The materials, in a quantity of 10 mg, unless otherwise indicated, are incubated in 20 ml of water in a tube with a Teflon plug closed with a metal collar with a minimum headspace to allow stirring; the contaminating compound (up to 100 μl of an aqueous solution at a suitable concentration) is added with a 100 μl syringe; the stirring is carried out in a complete rotation system (powder mixer). At the end of the reaction, after 24 hours, at much higher times, therefore, than the equilibrium times determined for each adsorbent, the mixture is centrifuged for 15′ at 700 rpm to separate the adsorbing material and the non-adsorbed contaminant is determined from its residual concentration in solution. Each determination is carried out at least three times. For each determination the sample and control consisting of liquid and contaminant without adsorbing material are prepared under the same conditions. This procedure is followed for all the contaminants examined.
From 10 mg to 1 g of adsorbing material are left to incubate with 20 ml of water containing from 100 ppb to 5 ppm of contaminant under stirring at room temperature for times varying from 15′ to 48 h. The equilibrium time is considered as being that over which the adsorption has not increased. In studying the effects of the conditions on the adsorption, the quantity of adsorbing material is used which determines the adsorption of at least half of the contaminant put in contact therewith.
Analysis of TCE, PCE, VC, Toluene, MTBE, Naphthalene, 2-methyl-naphthalene, Acenaphthene, Phenanthrene (Solution)
The aqueous solution is extracted with hexane in the ratio 5.666/1 (H2O/hexane), in a tube analogous to the reaction tube; a millilitre of hexane is removed for analysis in GC-ECD, or GC-FID. The control consists of the sample, without the adsorbing material, subjected to the same treatment.
The analysis is carried out from suitable aqueous solutions, measuring the contaminants in the headspace. The system used was GC/MS/DS Mod. MAT/90 of Finnigan; the gaschromatographic column used was a PONA (length 50 m×0.21 I.D. and 0.5 μm of film) of Hewlett-Packard. The flow of the carrier measured at 35° C. proved to be 0.6 ml/min (Helium). 500 μl of the headspace of each sample were injected, removing them with a (heated) gas syringe, from the phial kept for 2 h at 70° C. to reach equilibrium. The mass spectrometer operated in E.I. (electronic impact) at 70 eV and at a resolution of 1500 within the mass range of 30-120 a.m.u. and at a scanning rate which was such as to acquire a spectrum every 0.8 s.
The adsorption is carried out at different concentrations of CaCl2: 5-100 mM; for the pH, solutions were tested at pH 6, 7, 8 obtained with an Na Phosphate 20 mM buffer.
Adsorption Reaction with Real Groundwater
The groundwater of a contaminated site was used. The chemical composition for the inorganic components tested us as follows:
Cations (mg/l)
Iron: 8.6; Nickel: 0.05; Manganese: 1.7; Lead: <0.01; Zinc: <0.8.
Sodium: 371; Potassium: 12; Magnesium: 60; Calcium: 298; Anions (mg/l)
Carbonates: 475; Chlorides: 2300; Nitrates 13; Nitrites 3; Sulfates 14000.
Table 1 indicates the adsorption data obtained with TCE with different adsorbing materials.
The zeolites examined, silicalite and ZSM-5, gave better performances, much higher than those of GAC. β-zeolite, although characterized by structural channels of 7.5 Å with slightly larger dimensions than those of silicalite and ZSM-5, both with channels of 5 Å, has a silica/aluminum ratio of 70 and therefore lower than both that of ZSM-5, 290, and that of silicalite, infinite.
These ratio values, indicating a higher polarity of β-zeolite, are, together with the different structural channel dimensions, responsible for its different behaviour.
The adsorption kinetics were also determined for silicalite, by measuring the quantity of TCE adsorbed at various times. The following conditions were used in the example:
The adsorbing material, 10 mg, was incubated in 20 ml of water for 1 h in a 20 ml tube with a Teflon plug closed with a metallic collar with a minimum headspace to allow stirring; TCE, about 100 μl of an aqueous solution at a suitable concentration, to give an initial concentration of 300 ppb to the solution to be subjected to absorption, was subsequently added; the stirring was carried out in a mixer.
At the end of the reaction, 2 ml of aqueous phase are removed with a syringe and centrifuged in an Eppendorf phial at 15000 rpm for 5′ to separate the silicalite from the TCE solution.
TCE analysis (solution): the aqueous solution (1 ml) is extracted with hexane (0.5 ml); 100 μl of the extract are removed for analysis in GC-ECD. The control consists of the sample, without the adsorbing material, subjected to the same treatment.
The data at different times are indicated in Table 2.
These dosages seem to indicate very short adsorption times, in the order of 10 minutes or less.
The adsorption isotherm of trichloroethylene on silicalite was obtained from adsorption experiments with initial concentrations within the range of 50 ppb-100 ppm.
The adsorption efficiency under the experimental conditions (10 mg of adsorbing material in 20 ml of water, contaminating at different concentrations) is indicated in
Various adsorption experiments were carried out in the presence of strong concentrations of salts.
No significant effect of the ionic strength on the adsorption of TCE was observed (
The influence of the cations and anions normally present in groundwater was examined using, in the reaction, real polluted groundwater whose content of sulfates was much higher than 10 g/l. 1100 pmm of TCE were added, to 20 ml of groundwater in the laboratory reaction. After incubation with 10 mg of silicalite, the water was analyzed and the quantity of TCE removed from the water proved to be higher than 95%, very clearly demonstrating the adsorption selectivity of the silicalite.
The experimental conditions were the same as those of TCE.
In order to demonstrate the competitive effects of the two compounds TCE and PCE, adsorption tests were effected with a mixture containing both. The results, indicated in Table 4 below, show the absence of inhibition of one contaminant with respect to the other. On increasing the concentration of PCE, in fact, from 250 to 1000 ppb, the adsorption of TCE was not substantially modified.
An examination of aliphatic organo-chlorinated compounds cannot exclude VC which is the most difficult compound to degrade in this group.
In this example, the adsorption capacities of zeolites and GAC were compared.
The results obtained, under analogous conditions to the previous ones, are indicated in Table 5.
Toluene is considered as being the most representative BTEX compound present in fuels, and as such is normally the reference chemical compound of aromatic hydrocarbons. The results obtained at concentrations normally found in contaminated groundwater are indicated in
A comparison between zeolites differing in the adsorption of toluene is provided in Table 6 below.
The results showed that ZSM-5 and silicalite have a comparable behaviour.
In the comparison between silicalite, ZSM-5 and GAC, for the adsorption of contaminated mixtures, organochlorinated and aromatic hydrocarbons, the results indicated in the following table were obtained.
Table 7. Comparison between silicalite, ZSM-5 and GAC in the adsorption of Toluene+PCE+TCE mixtures
Conditions: the same as the previous examples, 20 ml of water containing the contaminants at the concentrations indicated, contact times higher than the equilibrium time.
The results indicate the absence of inhibition between the different molecules with respect to the absorption sites of the zeolite.
Both silicalite and ZSM-5 seem to be also effective in the adsorption of MTBE, a compound which is difficult to biodegrade and difficult to treat with activated carbon. The comparison is provided in Table 8.
The study of MTBE was further developed with respect to GAC, with different contact times: with silicalite it was 1 h, for carbon 4 h. The results are indicated below.
As shown by the results, the comparison is in favour of the silicalite.
Mixtures of Toluene/MTBE were tested with silicalite to show any possible reciprocal inhibitions; the comparison of silicalite with GAC was then effected on these mixtures of contaminants.
In conclusion, the adsorbing capacities of silicalite were demonstrated, which proved to be very similar to those shown by the commercial product ZSM-5. The fact that these adsorbents have better adsorption characteristics than GAC was confirmed.
Naphthalene was examined as aromatic compound with two condensed rings and adsorption experiments were effected with Silicalite, ZSM-5, MSA, ERS-8, Mordenite, GAC.
Conditions: equilibrium time 24 h, 10 mg of adsorbent, 1 ppm of naphthalene, in 22 ml of water.
Molecules of components of gas oil were also examined, in particular 2-methylnaphthalene, acenaphthene and phenanthrene; the results obtained with Mordenite and with MSA under the conditions of 10 mg of adsorbent in 22 ml of water containing 1 ppm of contaminant are indicated in Table 12 below.
Comparison between a metallic iron barrier and a zeolite, barrier.
(P. G. Tratnyek, T. L. Johnson, M. M. Scherer, G. R. Eykholt, GWMR, Fall 1997, pages 108-114), assuming that the Fe has a reactive surface of 3.5 m2/cm3, i.e. among the highest specified in literature. The concentration trend of the decay products is indicated, in relation to the run in the barrier: tetrachloroethylene (PCE)→trichloroethylene (TCE)→dichloroethylene (DCE)+acetylene and chloroace tylene (AC); dichloroethylene→vinyl chloride (VC)→ethylene. The chloroacetylene degrades rapidly into acetylene and vinyl chloride (
PCE is rapidly decomposed, but the further reaction of its decay products is slower, requiring about two days residence, corresponding to a run of a few metres in the barrier, to obtain the degradation of the last dangerous species of the chain, vinyl chloride. This makes it necessary to have a barrier thickness, under these conditions, of at least 3-5 metres.
For iron barriers, the well-known limits, amply indicated in literature, should be pointed out, according to which the efficacy is acknowledged only for a few groups of reducible products, aliphatic chlorinated products and heavy metals with a high oxidation number, for example Cr+6, U+6; the functioning dependence on the presence of ions which considerably reduce the performances, is also fully described (Dahmke, A., E. Ebert, R. Kober and D. Schafer. 2000. Laboratory and field results of Fe (0) reaction walls—a first resumè. In: Proc. Intl. Conf. Groundwater Research, Rosbijerg et al. Eds. Copenhagen (2000), page 395-396.
Functioning of a zeolite barrier.
Zeolites, even with relatively large particles sizes, thanks to their microporous structure, allow a much more rapid adsorption, with times which can easily be in the order of a second and, consequently, in a run of fraction of cm in a barrier.
The thickness of a zeolite absorbing barrier does not therefore depend on the kinetics, but only on the absorbing capacity of the zeolite itself with respect to the species to be adsorbed.
Correspondingly, the TCE passes with an unaltered concentration. Downstream of this layer, the concentration drops extremely rapidly (
This graph can therefore be used for estimating the thickness required for maintaining the barrier effective for a certain period of time, assuming that the groundwater only contains TCE. If other organic molecules are present, the thicknesses necessary for absorbing these other molecules must be naturally added to that obtained from
One of the strong points of zeolites is that they do not have adsorption inhibition, of one organic molecule on the part of another and, above all, that there is no competition for the adsorption sites on the part of ions up to high concentrations. This is particularly important as, if the material also absorbed ions, it would very rapidly become exhausted as the ions are often hundreds or thousands of times more numerous than the organic molecules.
| Number | Date | Country | Kind |
|---|---|---|---|
| MI201A001362 | Jun 2001 | IT | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10480643 | Jun 2004 | US |
| Child | 12336112 | US |
