IIT Bombay ENVIRONMENTAL GEOMECHANICS CE-641 Department of Civil Engineering DR. D. N. SINGH dns@civil.iitb.ac.in www.civil.iitb.ac.in/~dns
IIT Bombay Slide 1 5.9.2009 Lecture No. 11 Lecture Name: Geomaterial Characterization Sub-topics • Specific Surface Area determination • Chemical characterization Pore-solution sampling Corrosion potential Sorption-Desorption • Thermal Characterization • Electrical Characterization Environmental Geomechanics Lecture No. 11 D N Singh
IIT Bombay Slide 2 Specific-surface Area (SSA) Soil-water-contaminant interaction is strongly dependent on SSA SSA is indicative of activity (reactivity) of fine-grained soils A classification scheme based on SSA would help to establish: • Swelling and shrinkage characteristics • Frost heave • Collapse and compressibility • Cation-exchange capacity • Water retention characteristics • Sorption and desorption characteristics These characteristics mainly depend on the grain-size distribution of the soil (i.e., the clay-size fraction) and its mineralogical composition. SSA can capture the combined effect of these factors and hence, can be used for predicting engineering behavior of fine-grained soils. Environmental Geomechanics Lecture No. 11 D N Singh
IIT Bombay Slide 3 Determination of SSA o f fine-grained soils A. gas or vapor adsorption techniques BET nitrogen adsorption water-vapor adsorption B. absorption of the polar liquids and dyes on the soil surface Ethylene glycol (EG) method Ethylene Glycol Monoethyle Ether (EGME) method p-Nitrophenol method Methylene blue (MB) dye method C. application of the state-of-the-art instruments Mercury intrusion porosimetry (MIP) Internal reflectance spectroscopy X-ray diffraction Gas pycnometer Environmental Geomechanics Lecture No. 11 D N Singh
IIT Bombay Slide 4 Specific-surface Area (SSA) Determination (a) Methylene blue (MB) absorption technique 0.5 g of the soil mixed with 50 ml of MB dye of different initial concentrations ( C =5 to 10 4 mg/l), (L/S= 100) The mixture is agitated for 24 h on a mechanical shaker. The solution is filtered using the Whatman filter paper. The filtrate is transferred to microfuge tubes and centrifuged at 10 4 rpm for 30 min. This process helps in separating soil particles and the solution. The clear solution is decanted, collected in polypropylene tubes and stored in a refrigerator (@10 ºC). This process minimizes the precipitation of MB dye and evaporation of the solution. Later, the solution is suitably diluted and analyzed for determining the concentration of MB (i.e., C e with the help of a UV-Spectrophotometer) In order to establish the optimum UV-wavelength, at which the MB dye yields maximum absorbance a , dyes of different concentrations should be tested over a wide range of wavelength ( λ =400 to 800 nm). Develop a calibration curve (i.e., a relationship between C and a , at optimal λ value). This calibration curve can be employed for determining concentration of the MB dye present in a solution. Environmental Geomechanics Lecture No. 11 D N Singh
MB Dye Absorption Method IIT Bombay Slide 5 UV-Spectrophotometer results 3 Absorbance versus wavelength C (mg/l) 4 6 response of MB dye solutions 8 10 2 C =5.4· a a 1 665 0 200 400 600 800 1000 λ ( nm) Relationship between the absorbance and concentration of MB dye solution Environmental Geomechanics Lecture No. 11 D N Singh
MB Dye Absorption Method IIT Bombay Slide 6 C e : amount of MB dye in the solution C si : amount of MB dye initially added to the soil sample C si = C i ⋅ (L/S) C se : that gets adsorbed on the C se = ( C i - C e ) ⋅ (L/S) soil particles after 24 h MW MB : Molecular weight of the MB (=319.87 g/mol) A MB : the area covered by one MB molecule (=13 10 -19 m 2 /molecule) A v : Avagadro’s number (=6.02 10 23 /mol) A = ⋅ ⋅ v S C A MB opt MB MW MB Environmental Geomechanics Lecture No. 11 D N Singh
IIT Bombay Slide 7 (b) Nitrogen gas adsorption technique A surface-area analyzer (Autosorb, Quantachrome, USA) with N 2 as an adsorbate is used. This instrument works on BET adsorption theory Brunauer, S., Emmett, P. H., and Teller, E., “Adsorption of Gases in Multi- molecular Layers,” Journal of the American Chemical Society , Vol. 60, 1938, pp. 309-319. The instrument is calibrated with the help of standard reference material (Alumina) supplied by the manufacturer. 1 g of the air-dried soil is poured into a glass-cell and degassed under vacuum at 100 C, for a period of 24 h. This process helps in minimizing errors incurred due to rise of vapor pressure while adsorption of N 2 takes place. Later, the sample is exposed to N 2 corresponding to different relative pressures P / P 0 . This process ensures optimal adsorption of N 2 . At the end of the test the sample is weighed on a balance of accuracy 0.0001 g. The volume of N 2 adsorbed V a on the sample (at pressure P ) is recorded and adsorption isotherms are developed. Further, the SSA of the soil is determined by employing a single-point BET analyzer (Smartsorb-91). For this purpose, the degassed sample is filled in the sample-holder and is exposed to N 2 . S SBET is obtained with the help of the built-in software. Environmental Geomechanics Lecture No. 11 D N Singh
IIT Bombay Slide 8 Nitrogen Gas Adsorption Technique P: applied pressure P 0 : saturation vapor pressure of N 2 (= 1 atm at 77 ° K) V a is the volume of N 2 adsorbed at P, V Lm is the volume of N 2 required for mono-layer formation as per Langmuir isotherm, b is the parameter related to the maximum amount of N 2 getting adsorbed on the sample, V MBET is the volume of N 2 required to form mono-layer as per multi- point BET isotherm C MBET is a constant, which is proportional to heat of adsorption in first and subsequent adsorbed layers. A mol is the area covered by each N 2 molecule (=16.2 10 -20 m 2 ). V or V = ⋅ Lm MBET S or S A LM MBET mol V mol Environmental Geomechanics Lecture No. 11 D N Singh
IIT Bombay Slide 9 Nitrogen Gas Adsorption Technique Langmuir and multi-point BET adsorption isotherms 50 75 Langmuir Multi-point BET 40 60 Langmuir Isotherm P/(V a (P 0 -P)) (g/cc) (P/P 0 )/V a (g/cc) P 1 P = + 30 45 ⋅ ⋅ ⋅ ⋅ P V b V P P V 0 a Lm 0 0 Lm 20 30 10 15 0 0 0.0 0.1 0.2 0.3 0.4 0.0 0.1 0.2 0.3 0.4 P/P 0 − P 1 C 1 P Multi-point BET isotherm = + ⋅ MBET − ⋅ ⋅ V (P P) V C V C P a 0 MBET MBET MBET MBET 0 Environmental Geomechanics Lecture No. 11 D N Singh
IIT Bombay Slide 10 Mercury Intrusion Porosimetry 0.5 0.4 0.3 V (cc/g) ⋅ Vmax ΔV p ∫ = S ⋅ MIP δ T cos 0.2 Hg 0 0.1 δ =130 to 140 ˚ T Hg = 480 N/m 0.0 = 0.48 dyne/cm 1 2 3 4 5 10 10 10 10 10 p (psi) Environmental Geomechanics Lecture No. 11 D N Singh
IIT Bombay Slide 11 (c) Ethylene Glycol Monoethyl Ether (EGME) absorption method 2 g air-dried soil is spread uniformly on the bottom of a glass dish (40 mm in internal diameter and 20 mm in height) and covered with a perforated watch- glass. These dishes are placed in a vacuum desiccator containing 250 g of P 2 O 5 , which helps in maintaining a constant vapor pressure inside the desiccator. The sample is evacuated by applying vacuum for 2 h and is weighed until it attains almost a constant weight. Later, 6 ml of EGME solution is added to the sample to form a slurry. The slurry is placed in the desiccator over a desiccant (mixture of 100 g CaCl 2 and 20 ml EGME) for 12 h. This helps in maintaining a constant vapor pressure and minimizing the loss of EGME from the monolayer, which forms on the sample and the interlayer spacing of the soil minerals. Initial weight of the slurry along with the glass dish is measured, using the precision balance, and the dish is re-placed in the desiccator for evacuation under vacuum. The glass-dish is taken out of the desiccator, weighed and re-placed in it several times, till a constant weight is attained. Environmental Geomechanics Lecture No. 11 D N Singh
IIT Bombay Slide 12 Ethylene Glycol Monoethyl Ether (EGME) Absorption Method W = 2.8 c S or S Air-dried sample total ext w Degassed sample (at 600 °C) EGME w EGME is the amount of 2.1 EGME required to form monolayer on a square EGME (g/g) meter of Bentonite 1.4 (=2.86 10 -4 g). W External surface area, S ext , of 0.7 the soil is obtained by suppressing its interlayer. For this purpose, the sample is W 0.0 c degassed at 600 ºC for 5 to 6 h under vacuum, prior commencing the EGME test . 0 600 1200 1800 2400 3000 t (min.) Environmental Geomechanics Lecture No. 11 D N Singh
Recommend
More recommend
