Photoreduction of m-DNB by indole under simulated solar light irradiation as a function of time with (a) no montmorillonite (control experiment) and in the presence of (b) Na+-montmorillonite, (c) K+-montmorillonite, and (d) Cs+-montmorillonite. Experimental conditions: the initial concentrations of m-dinitrobenzene, indole, and clay mineral were 0.0125 mM, 0.5 mM, and 1 g L−1, respectively. pH was controlled at 7.3 with 5 mM phosphate buffer; a 500W xenon lamp was used to simulate solar light irradiation. Error bars are the standard deviations of triplicate analyses.