Adsorption isotherms of m-DNB on Na+-, K+- and Cs+-montmorillonite. qe is the amount of m-DNB adsorbed on montmorillonite clay mineral (mmol kg−1), and Ce is the equilibrium concentration of m-DNB (mM). Experimental conditions: 10 mg montmorillonite, m-DNB with initial concentrations ranging from 0.005 to 0.2 mM, reaction medium 10 mL phosphate buffer (5 mM, pH = 7.3).