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 dif...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.展开更多
文摘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.
