Adsorption of roxarsone from aqueous solution by multi-walled carbon nanotubes

Roxarsone, an organoarsenic compound serving as a common feeding additive in poultry industry, brings about potential risk of the toxic inorganic arsenate contamination in ambient environment. Current understanding in the dynamics of roxarsone removal and the determining environmental processes remains unclear, thus restricts the progress in roxarsone-contaminated wastewater treatment. In this study, the adsorption of roxarsone on multi-walled carbon nanotubes (MWCNTs) was investigated. The adsorption of roxarsone on MWCNTs decreased dramatically with increasing pH from 2.0 to 11.7 and decreased significantly with increasing ionic strength from 0 to 1.0 mol/L KCl. It was found that the sorption isotherms of roxarsone on MWCNTs were nonlinear, which can be well described according to the Freundlich and Polanyi-Manes models. Thermodynamic analysis indicates that the adsorption of roxarsone on MWCNTs is an exothermic and spontaneous process. Sorption site energy analysis reveals a distribution of sorption energy and the heterogeneous adsorption sites of roxarsone on MWCNTs. The dynamic adsorption with column shows the potential of the practical application for the roxarsone-contaminated wastewater treatment by MWCNTs. The FTIR analysis indicates that EDA interaction and electrostatic repulsion might be the dominant mechanisms for the adsorption of roxarsone on MWCNTs.