EVALUATION OF THE CHEMICO-OSMOTIC BEHAVIOUR OF A NATURAL BENTONITE Andrea Dominijanni POLITECNICO DI TORINO, Dipartimento di Ingegneria per l Ambiente, il Territorio e le Geotecnologie. C.o Duca Degli Abruzzi 24, 10129, Torino, Italy andrea.dominijanni@polito.it Sara Puma POLITECNICO DI TORINO, Dipartimento di Ingegneria per l Ambiente, il Territorio e le Geotecnologie. C.o Duca Degli Abruzzi 24, 10129, Torino, Italy ara.puma@polito.it Mario Manaero POLITECNICO DI TORINO, Dipartimento di Ingegneria per l Ambiente, il Territorio e le Geotecnologie. C.o Duca Degli Abruzzi 24, 10129, Torino, Italy mario.manaero@polito.it Summary Te paper preent an introduction to te cemico-omotic penomenon, wic caracterize emi-permeable oil. Subequently te preliminary experimental reult relative to te cemico-omotic beaviour of a natural bentonite, ued in a manufactured geoyntetic clay liner (GCL), are reported. Te reult were obtained at Politecnico di Torino, uing an innovative apparatu, reproduced on te bai of te deign of Gent Univerity. 1. Introduction Clay oil are able to act a emi-permeable membrane tat retrict te paage of olute. Retricted tranport of carged olute (anion and cation) troug te pore of clay oil i due to electrotatic repulion of te ion by te electric field generated by te urface carge of clay particle. Te exitence of membrane beaviour alo reult in cemico-omoi, or te movement of te olution in repone to a olute concentration gradient. Clay membrane barrier, CMB, are oil made barrier tat act a elective membrane tat repreent an innovative anwer to te relevant iue of pollution control and wate management tat concern indutrialied countrie. Te ability of clay to act a CMB i affected, among oter, by te type and amount of clay mineral in te oil. In particular, membrane beaviour i ignificant in clay oil containing an appreciable amount of odium montmorillonite, uc a Na-bentonite (Sackelford et al., 2001). CMB are caracterized by te ability to generate coupled liquid, olute and electrical current fluxe due to te application of ydraulic preure, olute concentration and electrical potential gradient. Te ability of tee barrier to create uitable condition to inder te paage of pollutant more tan te traditional barrier i a conequence of te coupled flux
penomena. Geoyntetic Clay Liner (GCL) are te mot popular manifactured CMB ued in landfill application. GCL generally contain a tin layer of odium bentonite tat enure excellent containment performance in te ort term regarding bot advective and, if coupled wit an attenuation layer, diffuive contaminant tranport. Te extent to wic clay oil act a emi-permeable membrane traditionally a been quantified in term of a reflection or cemico-omotic efficiency coefficient, σ (Staverman, 1952). In cae were σ repreent tre, te cemico-omotic efficiency coefficient a been deignated by ω. Te cemico-omotic efficiency coefficient i one of te parameter of te penomenological ma flux equation tat may derived uing te formalim of te Termodynamic of Irreverible Procee for olution containing a alt uppoed completely diociated, in te abence of electric current (Dominijanni and Manaero, 2005a). Baed on uc an approac, te clay membrane may be conidered a a tranition region or a "dicontinuity" between two omogeneou compartment aving te ame temperature, and te difference in te termodynamic potential acro te membrane tat repreent te driving force reponible for te correponding flow are aumed to be mall. Uing te penomenological parameter propoed by Malui and Sackelford (2002) and Dominijanni and Manaero (2005a), te volumetric flux, q, and te alt ma flux, J, are given by: q k L w P (1a) J (1 )qc nd L * C (1b) Were: q = volumetric flux or Darcy velocity of te olution; k k (n,c ) = ydraulic conductivity depending on te poroity, n, and on te alt concentration, C ; L = tickne of te clay membrane; w = unit weigt of te olvent; P P' P'' = difference of ydraulic preure, P, between te compartment (') and te compartment (''); ( n,c ) = cemico-omotic efficiency coefficient; ( 1 2)RT C = difference of omotic preure; 1 = toiciometric coefficient of te cation; 2 = toiciometric coefficient of te anion; R = univeral ga contant; T = abolute temperature; C C ' C '' = difference of alt concentration, C C1 / 1 C2 / 2, were C 1 i te cation concentration and C 2 i te anion concentration; J J1 / 1 J 2 / 2 = alt ma flux, being J 1 te ma flux of te cation and J 2 te ma flux of te anion; C (C ' C '') / 2 = average alt concentration; n = poroity of clay membrane; * * D D (n,c ) = omotic effective alt diffuion coefficient.
Te cemico-omotic efficiency of a clay membrane i a meaure of it effectivene in cauing ydraulic flow under an omotic gradient (ee Eq.1a) and of it ability to prevent te paage of ion (ee Eq.1b). Te value of are frequently aumed to vary from zero to unity, altoug tere are not termodynamic retriction to uc a variation and, actually, alo low negative value ave been meaured (Kemper and Quirk, 1972). Wen te clay oil doe * * * not exibit membrane beavior, 0 and D D md0, were D i te effective alt diffuion coefficient, m i te matrix tortuoity factor and D 0 i te free-olution alt diffuion coefficient. m repreent te tortuou nature of te actual diffuive patway troug te porou medium due to te geometry of te interconnected pore. Te clay membrane i "ideal" or "perfect" if 1 and D * 0 : in ti cae, te clay membrane completely retrict te movement of te ion pecie. Ti paper report te preliminary reult, obtained at Politecnico di Torino, relative to te cemico-omotic beaviour of a natural bentonite, ued in a manufactured GCL. 2. Material and metod 2.1 Natural odium bentonite Te bentonite ued in ti tudy i a powdered odium bentonite (NaB), provided by Lavioa. Ti bentonite i caracterized by a cation excange capacity (CEC, meaured uing metylene blue adorption metod) equal to 105 meq/100g. In Figure 1 te odium bentonite particle ize ditribution performed by Lavioa i reported. Figure 1. Particle ize ditribution of te powdered NaB. Te mineralogical compoition, evaluated troug te x-ray diffraction analyi performed by Lavioa and own in Figure 2, give a bentonite mainly compoed by mectite (> 98%) and calcite, quartz, mica and gypum in trace. 2.2 Teting olution Sodium olution were prepared wit odium cloride (ACS reagent, purity 99%) uing deionized water (DW). Sodium olution, prepared at different molarity value in te range between 5 mm and 100 mm, were ued to invetigate te effect of monovalent cation on te omotic beavior of natural bentonite.
Figure 2. XRD mineralogical compoition of powdered NaB contained in te tudied GCL. 2.3 Teting Apparatu Te laboratory apparatu for meauring te cemico-omotic efficiency of clay oil in te preence of electrolyte wa ued for teting NaB. Te cemico-omotic tet wa performed by maintaining a contant difference in te NaCl concentration acro te bentonite pecimen and indering te flow of te olution troug te pecimen, wile te cemico-omotic efficiency i derived from te differential preure meaured acro te pecimen and due to te applied concentration gradient. Figure 3 ow a ceme of te laboratory apparatu ued to perform te omotic tet. Te main component of te apparatu are: (1) te omotic cell; (2) te flow-pump ytem, (3) a preure traducer aimed at meauring te differential preure developed acro te pecimen during te tet, (4) te data acquiition ytem. Figure 3. Sceme of te omotic tet apparatu ued during te laboratory activity. In particular, te omotic cell conit of a modified rigid wall permeameter, in wic te top piton and te bae pedetal are equipped wit tree port eac: two of tem enable
circulation of eparate electrolyte olution troug te top and te bae porou tone to etabli a contant concentration difference acro te pecimen; te tird one i intalled in bot te top piton and te bae pedetal to allow for te meaurement of te differential preure acro te pecimen. Te flow-pump ytem, wic conit of a dual-carriage yringe pump and two tainle teel accumulator (Model 33 - Twin yringe pump, produced by Harvard), allow to ave no olution flux troug te pecimen imply infuing and witdrawing te ame volume of olution in eac porou tone at te boundarie of te pecimen. To obtain ti reult te yringe ave to move at te ame rate. 2.3 Teting Metod Since no tandard metod exit to rule te tet, it ad been performed following te metod decribed by Maluiu et al. (2001). In order to meaure te cemico-omotic efficiency coefficient, ω, in te top porou tone te olution, containing a certain concentration of electrolyte (NaCl), i circulated, wile in te bae porou tone te de-ionized water olution i circulated. Te concentration difference acro te pecimen i maintained contant continuouly infuing te two olution at te boundarie of te pecimen. Since te ame volume of olution i infued and witdrawn from te porou tone, te olution flux troug te oil cannot occur. In tee condition ω i calculated uing equation 2: P (2) Applying a concentration gradient acro te bentonite pecimen, a cemico-omotic flux of olution ould occur troug te membrane to balance concentration. If ti flux i indered, a ydraulic preure difference acro te pecimen i induced (ΔP). In equation 2 ΔP i meaured and compared wit te ideal preure (ΔП) tat would occur if an ideal (i.e. ω = 1) membrane wa ued. 3. Reult Te omotic tet wa performed uing a powdered odium bentonite pecimen prepared troug te following two tage: (1) a queezing tage, aimed at removing exce oluble alt, naturally contained into te NaB becaue of it formation proce in te marine environment, in wic te pore water i in termodynamic equilibrium wit te eawater; (2) a compaction pae, in wic te queezed ample (oven dried at 100 C) i reydrated wit DW at a water content lower tan te liquid limit value and ten tatically compacted in a compaction mold, allowing exce water releae. After tat, te pecimen wa tranferred to te omotic cell for te omotic tage. Prior to ubjecting te pecimen to a olute concentration gradient, DW wa circulated troug te top piton and bae pedetal to etabli a teady baeline differential preure. Afterward, te ource concentration of NaCl were injected in te top porou tone, wile DW a continued to circulate in te lower porou tone. Te multiple-tage omotic tet (i.e. ource NaCl concentration: 1t tep = 5 mm, 2nd tep = 10 mm, 3rd tep = 20 mm) wa performed at a contant flow rate of 0.05 ml/min. Te reult in term of cemico-omotic efficiency coefficient, derived from te cemicoomotic preure difference, are reported in Figure 4 referred to te average concentration meaured acro te pecimen during te tet. q0
Figure 4. Membrane efficiency veru elaped time referred to te average concentration acro te pecimen. Te cemico-omotic efficiency value meaured at te tady tate i reported in Figure 4 for eac concentration tep. Te teady tate efficiency value tend to decreae wit increaing alt concentration a a conequence of te progreive collape of te diffuive double layer (DDL) urrounding bentonite lamellae. Te reduction of DDL induce (1) an increae in te volume fraction of void available for te paage of te olute and, correpondingly, (2a) an increae in olute diffuion acro te pecimen and (2b) a decreae in te ratio between te meaured cemico-omotic preure difference (ΔP) and te ideal preure (ΔП). In Figure 5 te obtained reult (red croed ymbol) are compared wit te cemicoomotic value founded for bentonite pecimen by Kemper and Rollin (1966), cloed ymbol, and for a GCL by Malui and Sackelford (2002), open ymbol. Figure 5. Cemico-omotic efficiency coefficient a a function of te average alt concentration acro te pecimen and te pecimen poroity (n) for (cloed ymbol) bentonite pecimen (Kemper and Rollin, 1966), (open ymbol) a GCL (Maluiand Sackelford, 2002) and (red croed ymbol) te reult obtained in ti tudy.
Te tet i till ongoing, a lat two tep wit 50 mm and 100 mm ource NaCl concentration are planned. 4. Concluion Te membrane beavior of a natural odium bentonite, ued in a GCL, wa evaluated for different value of NaCl concentration. Te reult of membrane teting a pecimen of powdered NaB, preliminary ubmitted to a queezing tage, aimed at removing exce oluble alt, and a compaction pae, indicate a ignificant value of te cemico-omotic efficiency coefficient. Te trend obtained a a function of te average concentration of alt acro te ample i perfectly comparable wit te reult available in Literature. Te reult ugget tat te tudied odium bentonite and, conequently, te correponding GCL are uitable for wate containment application on te bai of bot (1) te ability to acieve very low value of ydraulic conductivity and (2) te ability to inder te paage of electrolyte pollutant and to generate a volumetric cemico-omotic water flux. Reference Dominijanni, A., and Manaero, M., (2005). Modelling omoi and olute tranport troug clay membrane barrier. Wate Containment and Remediation (ASCE Geotecnical Special Publication No. 47), ASCE, Reton, VA, (CD-ROM). Kemper, W. D. and Rollin, J. B. (1966). Omotic Efficiency Coefficient Acro Compacted Clay, Soil Science Society of America Proceeding, Vol. 30, pp. 529 534. Kemper, W. D., and Quirk, J. P., (1972). Ion mobilitie and electric carge of external clay urface inferred from potential difference and omotic flow. Soil Science Society of America Proceeding, 36(3), 426-433. Malui M.A., Sackelford C.D., Olen H.W. (2001). A laboratory apparatu to meaure cemicoomotic efficiency coefficient for clay oil. Geotecnical Teting Journal, 24 (3), 229-242. Malui, M.A. & Sackelford, C.D. (2002). Cemico-omotic efficiency of a geoyntetic clay liner. Journal of Geotecnical and Geoenvironmental Engineering 128(2): 97-106. Sackelford, C. D., Malui, M. A., and Olen, H. W. (2001). Clay membrane barrier for wate containment. Geotec. New, 19 (2),39 43. Staverman A.J. (1952). Non-equilibrium termodynamic of membrane procee. Tranaction of te Faraday Society, 48 (2), 176-185.