Abstract
Hydrous manganese dioxide (HMO) was synthesized by redox reaction of potassium permanganate (KMnO4) and hydrogen peroxide (H2O2), and then freshly synthesized HMO (FSHMO) was used for cadmium (Cd(II)) removal from aqueous solutions. The specific surface area and the maximum adsorption capacity of FSHMO for Cd(II) are 127.46 m2 g−1 and 168.36 mg g−1 at 298 K. The adsorption equilibrium can be well described by the Langmuir isotherm model and the adsorption capacity of FSHMO is higher than most of the average materials. The calculated theromodynamic parameters indicate a spontaneous and endothermic adsorption process. The pseudo-second-order equation gives a good fit for the adsorption process, and the calculated adsorption activation energy (Ea) is 48.68 kJ mol−1. The uptake of Cd(II) by FSHMO is mainly attributed to the chemisorption. The velocities of film diffusion process and mass transfer process of Cd(II) on FSHMO are rapid enough, while the intraparticle diffusion process is the rate-limiting step for the uptake of Cd(II). The speciation of Cd(II) on the surface of FSHMO is close to Cd(OH)2, but there is no formation of the new solid phase of Cd(OH)2. In general, FSHMO is a promising candidate for Cd(II) removal from aqueous solutions.
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