Photoreduction of m-DNB by indole under sunlight 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. Sunlight exposure was performed from 9 a.m. to 5 p.m. on sunny or partly cloudy days from March 22 to 27, 2014. Error bars are the standard deviations of triplicate analyses.