A comparative study on the removal of dyes from wastewater by nascent-state manganese dioxide and ferric hydroxide under acidic conditions

Abstract

Nascent-state manganese dioxide (NSMD) and ferric hydroxide (NSFH) are the main intermediates formed during potassium permanganate and Fenton oxidation processes, respectively, which play an important role in the removal of organic contaminants. This study investigates the removal efficiency in 38 synthetic dye wastewaters by NSMD and NSFH. Results showed that the average removal rates (R_exp) of dyes by NSMD and NSFH were 83.58 ± 5.96% and 43.11 ± 4.17%, respectively. SEM, FT-IR, TGA, XRD and BET analyses revealed that NSMD had a larger specific surface area and more abundant surface hydroxyl groups, resulting in a stronger adsorption capacity than that of NSFH. Besides, the degradation of dyes by NSMD was a combination of adsorption and oxidation; NSFH involved only in the adsorption effect, and thus NSMD had higher removal efficiency than that of NSFH. Additionally, to better illustrate the internal factors that affect the removal efficiency, correlation analyses between R_exp and 28 molecular descriptors, as well as quantitative structure–activity relationship (QSAR) model analysis using R_exp as dependent variables and molecular descriptors as independent variables, were conducted. The optimal QSAR models for NSMD and NSFH systems were R_pre = 1.829–0.460BO_x + 0.017μ − 1.248q(CH⁺)_x + 0.024E_LUMO and R_pre = −0.949 + 0.001MM − 0.017μ + 4.674f(0)_x − 0.072E_HOMO − 0.413q(C⁻)_n + 0.034N_N, respectively, which suggested that the maximum value of bond order (BO_x) representing oxidation capacity and the relative molecular mass (MM) representing adsorption capacity are the dominant factors affecting the dye removal of NSMD and NSFH, respectively. These findings not only reveal the differences and characteristics of dye removal using NSMD and NSFH, but also provide a rapid way to evaluate the removal efficiency of the two systems.