Fig. 3. Adsorption of total organic C (TOC) by sample clays (see Table 1) from 100 mg L–1 solutions of xanthan at room temperature (18°C) as a function of Ca(NO3)2 concentration at pH (a) 4 and (b) 7. Values are averages of 1 to 10 replications. Error bars are standard deviations of the means.