Removal of arsenic ions As (III) and As (V) from simulated aqueous solutions using modified rice husk by manganese oxide

• The modification of rice husk with manganese dioxide showed the ability to remove As (III) and As (V) from aqueous solutions. • Modified rice husk is a viable alternative for the removal of arsenic from contaminated waters • Adsorption of As (III) and As(V) on modified rice husk follows chemisorption mechanisms and is spontaneous and endothermic. The present investigation aimed to remove arsenic ions As (III) and As (V) from simulated aqueous solutions using modified rice husk by manganese oxide. It was possible to demonstrate that rice husk (RH) cannot remove As (III) and As (V) ions; however, the modification of RH with MnO 2 (RH-MnO 2 ) confers to this residue the capacity to remove both As ions. The Response Surface Methodology was used to optimize the removal process, taking into account the factors of pH, initial arsenic concentration, and RH-MnO 2 dosage. As a result, an RH-MnO 2 dosage of 9.6 g/L, pH=5, and an initial concentration of 1 mg/L was obtained for 100% removal of As (III) and (V) ions. The kinetic model that best fitted the removal process was the pseudo-second order kinetic model, which would indicate that the removal process of As (III) and As (V) ions would follow chemisorption mechanisms. The analysis of the adsorption isotherms showed that the adsorption process occurs in monolayer since the process was better adjusted to the mathematical model of the Langmuir isotherm, also, after performing adsorption experiments at 15, 25, and 35°C it was possible to demonstrate that the increase in temperature influences the removal process achieving better values of maximum adsorption capacity ( q m ) at 35°C being these of 0.186 mg/g and 0.133 mg/g for the removal of As (III) and As (V). The thermodynamic study showed that the adsorption process is spontaneous and endothermic. On the other hand, ATR-FTIR analysis indicates that adsorption of As (III) and As (V) ions is evident between 600 and 800 cm -1 , this was also corroborated with EDS analysis. SEM analysis showed that RH-MnO 2 exhibits a more heterogeneous and porous structure than RH unmodified. In conclusion, it was demonstrated that RH-MnO 2 would achieve the remediation of arsenic-contaminated water, being a promising alternative for its scaling up to real conditions.