Evaluation of Organoclays for Use in Landfill Liners

Transcription

1 The Open Waste Management Journal, 29, 2, Evaluation of Organolays for Use in Landfill Liners Open Aess Bharat Jhamnani * and S.K. Singh Department of Civil & Environmental Engineering, Delhi College of Engineering, Delhi, India Abstrat: Organolay obtained from two onstituents oo-dimethyl benzyl ammonium hloride, and bentonite was used in this study to prepare five admixtures, by varying the amount of organolay, natural soil, and bentonite. Laboratory experiments were onduted on these admixtures with the objetive of identifiation and assessment of their potential for use in landfill liners. The admixtures had the organolay ranging from 1 to 25%, and bentonite in the range of to 2%. The admixtures prepared were evaluated by onduting laboratory sorption tests and permeability tests for the determination of retardation oeffiient, and hydrauli ondutivity. Permeability tests were onduted using onsolidation permeameter. Total Organi Carbon (TOC) was determined using TOC Analyzer. Admixture with higher perentage of organolay exhibits higher organi sorption apaity. Retardation oeffiient of the admixtures was determined by urve fitting the experimental data of sorption tests to fit to Freundlih (nonlinear) sorption isotherm. One-dimensional transport simulations were performed to evaluate ompletely the effetiveness of these materials for use as landfill liner and to develop design harts for determining required liner thikness. Keywords: Organolay, landfill, groundwater, ontaminant transport, liner thikness. INTRODUCTION Leahates from muniipal solid waste (MSW) and hazardous waste disposal failities ontain a wide range of potential environmental ontaminants. In order to protet underlying groundwater resoures from these ontaminants, waste disposal sites are ommonly lined with ompated lay or a omposite liner onsisting of ompated lay and geomembrane. The design of these liner systems is foused on minimizing the flow rate of leahate through the liner. Generally, ompated soil liner in the thikness of 1 to 1.2m is onsidered adequate to minimize the rate of ontaminant migration by reduing the advetive transport of ontaminants through the liner. However, even with hydrauli ondutivities less than 1-7 m/s, the mass flux of many organi ontaminants aross ompated lay or omposite liners by diffusion an be signifiant [1, 2]. Contaminants an migrate through lay by simple Fikian diffusion at rates that an be signifiant. Diffusion may result in early breakthroughs (less than 3 years) and large ontaminant fluxes in typial lay liners [3-5]. Diffusion involves movement of a hemial speies from a region of high onentration to a region of low onentration. If a liner system is plaed into the position, the only way to redue the effet of diffusion is to redue the onentration gradient by ontaminant retardation. Beause of the inability of omposite landfill liners to impede diffusive transport, enhanement of the sorption apaity of earthen liner materials is onsidered as a means to improve liner performane. The magnitude of organi ontaminant sorption to natural soils and lays is related to the organi solute s aqueous solubility and the organi-arbon ontent of the sorbent [6, 7]. Sorption effiieny of modified lays for organi ontaminants inreases with the inrease in *Address orrespondene to this author at the Department of Civil & Environmental Engineering, Delhi College of Engineering, Delhi, India; madhyabharat@rediffmail.om number of arbon atoms assoiated with Quaternary Ammonium Cations [8]. The natural soils and lays typially have organi-arbon ontents less than.5%. A method of enhaning organi ontaminant sorption and thus minimizing the flux of leahate ontaminants through earthen liners is to amend the liners with materials apable of strongly sorbing organi ontaminants. The replaement of inorgani ations suh as sodium in lay minerals by quaternary ammonium ations through the ation exhange reation redues the hydration of the lay and dereases its surfae negative harge. As a result, the surfae properties of the lay hange signifiantly from being highly hydrophili to inreasingly organophili. The sorption harateristis of lays modified by quaternary ammonium ompounds, alled organolays to ioni and nonioni organi ompounds are mainly due to the partitioning proess or hydrophobi interations [9, 1]. Organolays effetively minimized the impat of phenoli wastes on ement hydration [11]. The study on organolays as waste ontainment barriers for pollutant attenuation showed that they were very attrative alternatives to the onventional lay liners [12, 13]. Inreasing the sorption apaity of the earthen liner effetively redues the ontaminant advetion veloity and the rate of solute diffusion during transient solute transport without requiring an inrease in liner thikness [14, 15]. Although a sorbed ontaminant will eventually desorb from the liner, the mass flux from the bottom of the liner is signifiantly redued when an organobentonite is a omponent of the liner [16]. The objetive of this study was to investigate the potential use of organolays with soil-bentonite admixtures in retarding the movement of ontaminants, while maintaining required permeability speifiations thereby reduing the liner thikness. In order to evaluate the potential effetiveness of sorptive materials as amendments to lay liners, sorption and permeability tests were performed on five admixtures obtained by /9 29 Bentham Open

2 38 The Open Waste Management Journal, 29, Volume 2 Jhamnani and Singh adding varying amount of the organolay to the natural soil, and bentonite. One-dimensional transport modeling was performed to determine how effetive eah of these admixtures would be at retarding the transport of organi ontaminants if used as landfill liner. The required barrier thikness is also determined using design harts developed from a onedimensional ontaminant transport model. MATERIALS & METHODS Liner Materials Three liner materials; natural soil, bentonite lay, organially modified lay, were used in this experimental work. The natural soil seleted to be used in this work was obtained from Delhi, having yellowish olor. The natural soil obtained was oven dried at 13 C for three days, and then passed through a 6 miron sieve after breaking the lods. The soil passing through the sieve was olleted and used in the experimental work. Bentonite lay used was saturated with sodium. Organolay used in the study was obtained by modifying the bentonite by addition of oo dimethyl benzyl ammonium hloride. Small amount of ultra pure water was first added to the bentonite lay. The lay was then added to the oo dimethyl benzyl ammonium hloride with more amount of ultra pure water. Entire ontents were mixed in a pug mill in laboratory. Mixed ontents were kept in oven and a temperature of 12 o C was maintained for 48 hours, followed by grinding and sieving of the ontents. This study involved preparation of five kinds of soil admixtures (M1, M2, M3, M4, M5) using the organolay prepared in laboratory. Organolay, natural soil, and bentonite were mixed in different proportions. These admixtures were subjeted to bath sorption tests for Total Organi Carbon (TOC). Composition of five types of admixtures is shown in Table 2. Perentage of eah onstituent was taken on a dry weight basis. For assessing the organi sorption apaity of admixtures, the parameter Total Organi Carbon (TOC) was used. TOC was determined using TOC analyzer of the make analyti Jena, model multi N/C 21. The gross organi term TOC was seleted to reflet the overall organi removal ability of admixtures. Bhalaswa Landfill Organi sorption tests on admixtures prepared in laboratory were onduted using leahate sample obtained from Bhalaswa landfill site in Delhi. Bhalaswa Landfill site whih is loated in North Eastern part of Delhi ame into operation in The landfill site oupies 4 ares of land that was one used for sugar ane plantation. The total volume available for waste ontainment is about 2.9 million m 3. It is atually reeiving tons of wastes per day and at suh rate, the expeted ative life of the landfill has almost been rossed. However, plans to extend the landfill site are underway. The landfill is publily owned and is managed by Muniipal Corporation of Delhi (MCD). Almost the entire quantity of wastes reeived at the landfill faility an be lassified as muniipal waste. Charateristis of leahate olleted from landfill are shown in Table 1. SORPTION TESTS The TOC onentration in the landfill leahate was found to be 489 mg/l. Bath sorption tests were onduted by the Table 1. Charateristis of Leahate from Bhalaswa Landfill Parameter Conentration (mg/l) Total suspended solids 62 Total dissolved solids 2,31 BOD COD 1,17 Total Organi Carbon 489 Chlorides 6,84 Fluoride.6 Total iron 8.2 Chromium.72 Lead.84 Zin 3.2 Copper.8 Phenoli ompounds.92 Cadmium.11 ph 8.3 addition of 5 4 g of eah air-dried admixtures type individually into 6 ml of landfill leahate with TOC onentration of 489 mg/l. To maintain the solution at neutral, a buffer solution ontaining.1m potassium dihydrogen phosphate and.1m sodium phosphate was added. The vials were then mixed by subjeting them end-over-end rotation using a tube rotator for 24 h. The supernatant (7 L) was withdrawn, filtered, and plaed into vials. Eah test was repeated thrie. Blanks ontained 6 ml of 489 mg/l of TOC solutions. The blanks in TOC showed equilibrium onentration of 489 ± 24 mg/l, indiating no signifiant loss of TOC in the blanks. For the stok solution of TOC, 1.627g of anhydrous potassium biphthalate (C 8 H 5 KO 4 ) was dissolved in arbon free ultra pure water and diluted to 5 ml (1. ml = 1 mg of organi arbon). Calibration urve was prepared by diluting the stok solutions to over the range of TOC from to 5 mg/l using TOC analyzer. The experimental bath sorption data were fit to the nonlinear Freundlih isotherm S = K f C b using software LabFit, where S is the mass of solute removed from solution per unit mass of soil (mg/g), C is equilibrium onentration (mg/l), and K f and b are the parameters determined from best fitted urve. PHYSICAL AND HYDRAULIC PROPERTIES Optimum Moisture Content (OMC) and Maximum Dry Density (MDD) for five types of admixtures were determined with Standard Protor Test Method in aordane with IS 272 (Part VII). The tests speimens, prepared using admixtures, were allowed to hydrate for 24 hours. The speimens were ompated at varying moisture ontents in a 1 mm diameter mold with a 2.6 Kg weight rammer dropped from a height of 31 mm. The hydrauli ondutivity test was performed using onsolidation permeameter. The speimens were ompated at 3% wet of optimum water in aordane with standard proedures for the onstrution of ompated lay liners for waste ontainment. The ompated

3 Evaluation of Organolays for Use in Landfill Liners The Open Waste Management Journal, 29, Volume 2 39 speimens were extruded from the ompation mold, and tested for permeability in onsolidation permeameter. Table 2. Admixture Compation Properties of Admixtures Perent Component in Admixture Compation Properties B O NS OMC MDD M M M M M B- Bentonite, O- Organolay, NS- Natural Soil. RESULTS & DISCUSSION TOC sorption for the five soil admixtures with different proportions of natural soil, bentonite, and organolay were studied using bath sorption tests. The tests were arried out at ph 7 and temperature 25 ± 1 o C. Fig. (1) shows the sorption isotherms of the TOC from the leahate onto the five types of admixtures. Eah sorption data is average of three values of absorption. All the sorption urves an be seen having initial onvexity and a final upsweep. Suh sorption behavior is indiative of weak adsorbent-adsorbate interation at low onentrations, but one a moleule beomes adsorbed, adsorption of more moleules gets promoted in a ooperative fashion. It an be observed from the figures urves that the TOC sorption follows a pattern dependent on the omposition of soil admixture (Fig. 1). The soil admixture having a higher ontent of organolay has higher sorption apaity for TOC. The admixture M1 whih is omposed of 25% organolay and 75% natural soil has maximum sorption apaity for TOC, and the admixture M3, whih is omposed of 75% natural, 15% bentonite, and 1% organolay has minimum sorption apaity for TOC. Sine the sorption isotherms are nonlinear, the partition oeffiient, K f, is not onstant but is a funtion of the equilibrium onentration of TOC. The variation of partition oeffiient with equilibrium onentration depends on the isotherm shape. The slope of a onvex isotherm at a lower onentration is smaller than that at a higher onentration. It is diffiult to make use of a non uniform partition oeffiient in a one-dimensional ontaminant transport model. An approximation of a representative linear partition oeffiient is therefore essential based on weight-mean method [17]. With the weighted-mean method, the average value of the partition oeffiient, K f, and the retardation fator, R, are estimated using following equations: K p = S C dc = dc K f bc b 1 d = K f C b 1 (1) dc R = 1+ K C b 1 f (2) n where is bulk density of the soil in kg/m 3 and n is soil porosity. The retardation fator alulated at C=.5C is shown in Table 3. The reason for using C=.5C is beause it is more representative of the equilibrium onentration of TOC. The physial meaning of R in this ase an be defined Fig. (1). Sorption Isotherms (TOC).

4 4 The Open Waste Management Journal, 29, Volume 2 Jhamnani and Singh as the ratio of the time required for a ontaminant to attain a half-maximum onentration, as ompared with that for a onservative traer. Table 3. Sorption Isotherms for TOC Admixture Freundlih Isotherm R Correlation r 2 M1 7.13x1-9 C M2 1.56x1-1 C M3 2.1x1-13 C M4 5.21x1-11 C M5 1.59x1-9 C It an be observed from Table 3 that admixture M1 with 25% organolay has the larger retardation fator, whih mathes with the trends in sorption isotherms. Similarly, the admixture M3 with minimum perentage of organolay (1% in this ase) has least retardation fator. The physial properties and measured hydrauli ondutivity of eah soil admixture are shown in Table 4. The soil admixtures M1 to M5 have a porosity ranging from.43 to.47. Similar studies arried out showed retardation fators varying between a minimum of 2.74 to a maximum of for a orresponding ontent of modified lay of zero and 2% respetively [14]. The ost of replaing the traditional ompated lay liner would involve the material and tehnology ost, both of whih are expeted to be marginal, making the total ost of substitution to be only marginally higher. However, the additional osts for suh replaement would be offset by the benefits ahieved from in the form of (i) avoidane of remediation ost for leaning up of ontaminated groundwater (ii) environmental benefits ahieved in the form of redued risk of human exposure to ontaminated water, thus avoiding human health ost. Complete ost benefit analysis of the use of organially modified lay in landfill liners is thus a omplex issue, whih an be suitably addressed only by analytial tools and models of environmental eonomis, and is beyond the sope of this paper. Table 4. Physial & Hydrauli Properties of Soil Admixtures Admixture n k M x 1-7 M x 1-8 M x 1-8 M x 1-8 M x Density (kg/m 3 ) n porosity. k Hydrauli ondutivity (m/se). TRANSPORT MODELING SIMULATIONS The experimental results from the sorption and permeability experiments were used as an input data for transport simulations in order to evaluate the potential benefit of using sorptive amendments as a omponent of ompated lay liners. The transport of dissolved ontaminants through porous media an be desribed by equation that desribes mass transport of ontaminants as a result of water movement, hydrodynami dispersion, and sorption. Mass transport of ontaminants represented by Eq. (3) simply states that the inrease in ontaminant onentration within a small region is equal to the aumulation of mass due to advetivediffusive transport minus the derease in mass due to reation/sorption: C t = D h R 2 C z 2 v s C R z where, C = Contaminant onentration at any depth z and time t in the liner; z = depth measured from top of the liner; D h = hydrodynami dispersion oeffiient, m 2 /year; v s = advetive veloity, m/year; R = retardation oeffiient, alulated from sorption data and soil properties. The hydrodynami dispersion oeffiient, D h in Eq.3 is often expressed as sum of the effetive moleular diffusion oeffiient, D e, and mehanial dispersion oeffiient D m ; D h = D e + D m. Previous studies indiate that the hydrodynami dispersion oeffiient an be regarded as the effetive moleular diffusion oeffiient under very low seepage veloity onditions, as is typially observed in lay liners. In other words, diffusion an be the dominant a mass transport proess in lay liners. The ontaminant interation with the liner material is usually aounted for the retardation fator, R, whih represents the relative rate of fluid flow to the transport rate of a reative ontaminant and an be determined using bath sorption tests. For non-reative ontaminants suh as hloride, the retardation fator is unity and the solute is transported at the rate of the advetive veloity. However, for reative ontaminants, the retardation fator is greater than unity and the solute is transported at a rate slower than that of the advetive veloity. For a given values of v s. R and D h, the onentration distribution of a single speies at any time t and spae z an be determined by solving Eq. (3) with a set of boundary onditions. To solve the 1D pollutant transport equation through a low permeability liner at a waste disposal faility, the following boundary and initial onditions an be imposed: Initial ondition: C(z,) = ; (4a) Top of the liner: C(,t) = C, (4b) Bottom of the liner: C ( z,t )= (5) The boundary ondition at the top of the liner is assumed to maintain a onstant onentration. The top of the liner is assumed to have a ontinuous and infinite replenishment of solute. This is onsistent with the fat that there is a virtually infinite soure of mass in the waste and that any loss by transfer downward through the liner an be immediately replaed. The onstant onentration ondition is onservative in design, as it does not aount for the redution in the strength of the leahate generated from a landfill for many years. For the boundary ondition at the bottom of the liner, it is assumed that the liner has an infinite thikness, where the exit boundary ondition is defined as zero ontaminant onentration for all times, whih implies zero diffusive flux at the liner exit. The initial ondition means that the onen- (3)

5 Evaluation of Organolays for Use in Landfill Liners The Open Waste Management Journal, 29, Volume 2 41 Maximu um Relative Conentration Fig. (2). Design hart for liner thikness. M1 M2 M3 M Liner Thikness (m) M1 M2 M3 M4 M5 tration of the ontaminant is zero at any depth in the liner at time zero, whih means that the liner may be assumed to be free of ontaminant at the initial time. Transport simulations were arried out using values of v s. and R determined in this study. For landfill liners, where mass transport is diffusion dominated; Dh D e and may be taken as.2 m 2 /year for MSW landfills [18]. Total time period of simulation was taken as 5 years aounting for the first 1 15 years for waste reeiving at landfill site and the remaining for post-losure Eq. (3) representing transport of ontaminants was solved using finite differene method implemented in MatLab 7.. Simulation results were used for determining the maximum relative onentration at any depth within the liner and plotting the same. Simulations were arried for five types of admixtures. The results of simulation plotted in the form of graphs between maximum relative onentrations with respet to depth from the top of liner, an be used as design hart. Suh hart an be utilized for the determination of liner thikness, if the maximum permissible relative onentration after a ertain period of time is defined based on pollution prevention riteria in a region. Fig. (2) presets a maximum relative onentration for a traveling time of 5 years for different admixtures. It is obvious that the required thikness is highly dependent on the retardation fator and hydrauli ondutivity of the speimen. The required thikness for the admixture M1 is the least, as it has the highest retardation fator. Similarly, the required thikness for admixture M3 is the largest sine it has the lowest retardation fator. Therefore, the only way to redue the hemial flux is to improve the retardation fator of the soil material. The rule of thumb is that the larger the retardation fator of a liner material is, the lesser the liner thikness. This, in turn, saves more spae for landfill disposal of solid wastes. CONCLUSIONS In this study, both experimental work and simulation modeling were onduted. Experimental data on five soil admixtures, prepared by varying the ontent of organolay and bentonite in natural soil, show that replaing 15% of bentonite with organolay inreases the retardation fator from 4.2 to However, the presene of bentonite in soil speimen is also neessary, as it ould redue hydrauli ondutivity of admixtures. A design hart was developed by solving 1D ontaminant transport and mass transport equation by using finite differene tehnique. The hart provides a logial method for estimating the required thikness of a liner having omposition based on these admixtures. The liner material with higher proportion of organolay signifiantly redues the required thikness beause of the high sorption apaity. REFERENCES [1] C. D. Shakelford, Laboratory diffusion testing for waste disposal-a review, Journal of Contaminant Hydrology, Vol. 7, pp , February [2] J. K. Park, and M. Nibras, Mass flux of organi hemials through polyethylene geomembranes, Water Environment Researh, Vol. 65, pp , Marh [3] R. L. Johnson, J. A. Cherry, and J. F. Pankow, Diffusive ontaminant transport in natural lay: A field example and impliations for lay-lined waste disposal sites, Environmental Siene Tehnology, Vol. 23, pp , Marh [4] H. V. Mott, and W. J. Weber, Sorption of low moleular weight organi ontaminants by fly ash: Considerations for the enhanement of utoff barrier performane, Environmental Siene Tehnology, Vol. 26, pp , June [5] R. W. Gullik, Effets of sorbent addition on the transport of inorgani and organi hemials in soil-bentonite utoff wall on-

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