Characterization of parameters of sorption isotherm models of cadmium in selected semi-arid region agricultural cambisols and calcisols

Abstract

To characterize the parameters of sorption isotherm models, a study was conducted to (a) correlate fitted parameters of the isotherm models with calculated Cd sorption parameters and (b) evaluate the correlation between Langmuir and Freundlich sorption models parameters. A statistically significant (P ≤ 0.01) negative correlation was observed between Langmuir b and K l (r = −0.69) and a relatively high positive correlation between Langmuir K f and $$ \frac{\text{$\scriptstyle 1$}}{\text{$\scriptstyle n$}} $$ (r = −0.66) indicating a potential for multicollinearity. Langmuir K l showed no significant correlation with calculated total Cd sorbed (TCDsorb.) and desorbed Cd (TCDsorb.) from soils as well as with the amount of Cd sorbed (C s (C i = 30)) and desorbed (C des (C i = 30)) from soil at the highest initial Cd concentrations (30 mg/L). While, b, 1/n, K f, and K SD were positively correlated with TCDsorb. and C s (C i = 30) and negatively correlated with TCDdes. and C des(C i = 30). Results did not confirm the possible indicator of K l as a bonding energy, while parameter b could be considered as a sorption capacity parameter and may provide information regarding bonding energy of sorption process. K f represents an affinity constant, so that, a soil with higher K f may have a higher affinity to Cd. Similarly, 1/n expresses the sorption intensity, implying that a soil with higher 1/n may have a higher Cd sorption intensity. The effect of lower initial concentrations of Cd on fitted parameters was limited by the impact of higher concentrations. The relatively high correlations between parameters of Langmuir and Freundlich models show insights to reduce them to one parameter models.