Abstract
Acid-base properties of Na-sepiolite and Na-palygorskite were studied by potentiometric titrations at 298 K and two ionic strength I = 0.1 and I = 0.002. Intrinsic constants of deprotonation were calculated by two different methods: a) Stumm method, by extrapolating to zero the function that relates surface charge with the logarithms of apparent acidity constants and b) with MINTEQ program by minimizing the differences between surface H+ concentration data and the values obtained from deprotonation constants proposed according to the Diffuse-Double-Layer Model (DDLM). Hydroxyl groups located at the broken edges of these fibrous clay minerals (SOH) and permanent charge sites (X-) were considered as reactive sites. The determined values of the acid-base constants for Na-sepiolite and Na-palygorskite were intermediate between those for SiO2 and Al2O3, which is in agreement with minerals that contain moderately strong-acidity and weak-acidity surface groups. The SOH groups showed an initial increase (after SOH2+ deprotonation), forming a plateau with a slight de-crease at high pH values (8-9) due to the formation of SO- sites. X- sites ad-sorbed H+, Na+ or Mg2+ ions.
Highlights
Sepiolite and palygorskite are classified as phyllosilicates, because according to the definition of this mineral group [1], they contain a continuous two-dimensional tetrahedral sheet
Intrinsic constants of deprotonation were calculated by two different methods: a) Stumm method, by extrapolating to zero the function that relates surface charge with the logarithms of apparent acidity constants and b) with MINTEQ program by minimizing the differences between surface H+ concentration data and the values obtained from deprotonation constants proposed according to the Diffuse-Double-Layer Model (DDLM)
Intrinsic constants of deprotonation were calculated by two different ways: 1) Stumm method, by extrapolating to zero the function that relates Q with the logarithms of apparent acidity constants
Summary
Sepiolite and palygorskite are classified as phyllosilicates, because according to the definition of this mineral group [1], they contain a continuous two-dimensional tetrahedral sheet. They differ from the other layer silicates in lacking continuous octahedral sheets (Figure 1). Ribbons containing MgO6 (AlO6) octahedral groups and rectangular channels run parallel across to the octahedral sheet [4], the width of the ribbons and the channels differing a little in sepiolite and in palygorskite.
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