3.3. Changes in clay surface basicity
The TPD patterns revealed the occurrence of at least four basicity strengths, namely weakly (below 100oC), medium (between 100 and 250oC), medium-to-strong (between 300 and 450oC) and strong basicity (between 450 and 550oC). The slight shift of the desorption peak from 60-200oC towards higher temperatures (170-220oC) was noticed from bentonite to NaMt (Fig. S9-a ). This indicates a slight strengthening of the surface basicity, presumably due to the removal of acidic silica phases (quartz, sand, cristobalite). This agrees with the slight increase in the CRC value after bentonite purification from 2101 to 2261mmol.g-1 (Table 1 ) which indicates that bentonite purification induces a slightly higher number of stronger basic sites.