Fig. 7

a Linear Langmuir isotherm for Cr(VI) adsorption on PSD at 298 K; b Linear Freundlich isotherm for Cr(VI) adsorption on PSD at 298 K; c Linear Temkin-Pyzhev isotherm for Cr(VI) adsorption on PSD at 298 K; d Linear Dubinin–Radushkevich isotherm for Cr(VI) adsorption on PSD at 298 K; e Nonlinear Dubinin–Radushkevich isotherm for Cr(VI) adsorption on PSD at 298 K; f Nonlinear Langmuir, Freundlich, and Temkin–Pyzhev isotherms for Cr(VI) adsorption on PSD at 298 K