Roxarsone desorption from the surface of goethite by competitive anions, phosphate and hydroxide ions: Significance of the presence of metal ions

- The presence of metal ions enhanced roxarsone adsorption on goethite surface. The effect from Fe3+, Zn2+ and Cu2+ was more pronounced.
- Based on the adsorption/desorption kinetics, phosphate displayed stronger replacement ability than hydroxide ion at any conditions has been confirmed.
- When surface was nearly completely covered, roxarsone had the highest tendency to desorb from the goethite surface.

Aromatic organoarsenical roxarsone (ROX) is a common additive for livestock feed. This arsenic containing pollutant could be discharged into the environment through agricultural application of animal manure, and pose potential threats to both humans and the wider environment. In this study, the influence of pH, competing anions and metal ions on the adsorption and desorption of ROX on goethite were investigated in order to understand their mobility in the environment. Both hydroxide ions and phosphate are common substances in the environment, and both are potential competing anions for ROX. Our results showed the addition of phosphate desorbed more ROX than the addition of hydroxide ions. As pH increased, the effect of phosphate did not show much difference to that of hydroxide ion. The results indicate that the presence of phosphate will greatly increase the mobility of ROX at low pH. Six common metal ions, including Zn2+, Cu2+, Fe3+, Mn2+, Mg2+, and Ca2+, were tested and all spiked metal ions enhanced the stability of ROX adsorption on the surface of goethite, and led to less desorption when phosphate was added. The results demonstrate that metal ions may form complex/surface precipitation with ROX to enhance its adsorption. The effect from Fe3+, Zn2+ and Cu2+ was more pronounced than other metal ions, which might result from the fact that these three metal ions tend to associate with hydroxide ions and decrease the pH. The results of this research may shed light on the environmental fate and transportation of aromatic organoarsenicals in soil.