Arsenate adsorption, desorption, and re-adsorption on Fe-Mn binary oxides: Impact of co-existing ions

Fe-Mn binary oxides have demonstrated impressive arsenate removal efficiencies as adsorbents. However, their interaction with co-existing ions can lead to arsenate desorption, complicating the remediation process. The dynamics of both desorption and subsequent readsorption of arsenate on these oxides under the influence of various ions remain largely unexplored. This study illuminates these processes through a detailed six-month batch experiment, examining the behavior of arsenate in the presence of both single and multiple competing ions. Our findings demonstrate varied desorption impacts by different ions. Notably, the desorption effects induced by bicarbonate (HCO3-) and phosphate (PO43-) were the most significant, with maximum desorption rates reaching 21.8 % and 20.8 %, respectively. Following desorption, As(V) exhibited varying re-adsorption rates; notably, As(V) desorbed by HCO3- and PO 4 3- achieved nearly complete re-adsorption at rates of 100 % and 97.8 % respectively after 150 days. The dynamics altered substantially in the presence of multiple ions. Specifically, the co-presence of Ca2+and Mg2+significantly reduced the desorption of As(V) by HCO3-, with maximum desorption rates decreasing to 0 % and 2.9 %, respectively. Moreover, the inclusion of silicate (SiO32-) in the system markedly enhanced the re-adsorption of As(V), surpassing rates observed in the presence of HCO3- alone. In contrast, when PO43-and HCO3- were present together, almost no re-adsorption of As(V) occurred, leading to a higher final desorption rate of 27.4 %. These results highlight the complex and significant role that specific ions and their combinations play in the desorption and re-adsorption of As(V) on Fe-Mn binary oxide surfaces. The study emphasizes the importance of considering the types and interactions of co-existing ions in environmental settings when utilizing Fe-Mn binary oxides for efficient arsenic removal.