Arsenic oxidation and removal with Manganese oxides in filters

Abstract

The actual European Union (EU) guidelines for allowed concentration of arsenic (As) in drinking water fix a maximum of 10 ?g/L. However, this limit is expected to become more stringent: the recommended health related limitation from the US Natural Resources Defense Council is below 1 ?g/L. In Dutch groundwater treatment plants, the produced drinking water is already far below the EU guidelines due to As removal during sand filtration. On the sand grains, Manganese dioxide (MnO2) is present. MnO2 is able to oxidize As(III) to As(V). This is advantageous because As(III), usually the prevalent specie in groundwater, is uncharged and more difficult to remove. On the other hand, negatively charged As(V) molecules can be easier removed by positively charged iron (Fe) (hydro)oxides in filters. The oxidation of As(III) to As(V) during filtration of groundwater in presence of MnO2 needs to be understood in order to improve this process by adjusting the operational parameters of treatment plants. My research focused on the processes of As(III) oxidation to As(V) by MnO2 at neutral or slightly alkaline pH. I also took into consideration the role of (oxidizing) Fe as inhibitor in As(III) oxidation and as remover of As. I performed laboratory three groups of experiments at the TU Delft Waterlab and at the treatment plant of Vitens in Loosdrecht: • Jar tests using demineralized water with low concentration of As(III) and powders containing MnO2 • Jar tests using demineralized water with low concentration of As(III), powder containing MnO2 and oxidizing Fe(II). • Column tests using groundwater after aeration with As and Fe, and fresh grains containing MnO2 In the three experiment groups it is resulted that: • With lower concentration of purer MnO2 materials, oxidation of As(III) to As(V) was the main process involving As. • As removal in presence of MnO2 material was enhanced by addition of Fe(II) to the system. However, the addition of Fe(II) inhibited As(III) oxidation by MnO2. This inhibition is avoided in experiments with delayed addition of Fe(II), when the As(III) oxidation step is completed. • The MnO2 in the column was active in the oxidation processes of As and Fe. When the free surface MnO2 grains is still largely available, the resulting As concentration was lower than 5 ?g/L. Afterwards, when more grain surface is occupied by Fe and other compounds, As removal decreased. In conclusion, Fe inhibits As(III) oxidation by MnO2, but it is also responsible for As removal. The inhibition processes suggested by the results are competitive oxidation and surface disturbance. The relative importance of these two processes should be studied further. Regarding the operational parameters, tests with a broader pH range can be performed. Moreover, the influence of filter bacteria has to be considered as well.