Enhanced adsorption of roxarsone onto humic acid modified goethite from aqueous solution

The adsorption of roxarsone (ROX) on the humic acid modified goethite (HA-alpha-FeOOH) was evaluated for several potential environmental factors. Results showed that 1) ROX had a higher adsorption capacity onto HA-alpha-FeOOH than unmodified alpha-FeOOH; 2) the adsorption of ROX increased with a decrease in pH; 3) the high ionic strength significantly inhibited the adsorption capacity of HA-alpha-FeOOH; and 4) a higher temperature yielded greater adsorption, since the process for ROX to be adsorbed by HA-alpha-FeOOH was a spontaneous endothermic reaction. The maximum adsorption capacity of ROX was found to be 80.71 mg center dot g(-1), when the temperature was 308 K. Meanwhile, the inhibitory effects of ionic strength and PO43- on the adsorption of ROX onto HA-alpha-FeOOH were enhanced with an increase in concentration. In addition, the adsorption equilibrium data obeyed Langmuir isotherm model and the kinetic data were well described by the pseudo-second-order kinetic model. From the infrared spectra of HA-alpha-FeOOH, it could be deduced that the ROX adsorption onto HA-alpha-FeOOH was achieved via the ion exchange between the arsenic acid and the carboxyl group on adsorbent, as well as the formation of As-O-Fe bond between Fe-O and arsenic acid ions.