Sustainable approaches to minimize arsenic in drinking water and rice in Vietnam
Final Report Abstract
Arsenic (As) contamination in the groundwater is threatening the health of 94 to 220 million people worldwide, especially in low-income regions such as West Bengal, Bangladesh, and Vietnam. Centralized water treatment facilities are insufficient, forcing people in suburban and rural areas to rely on simple household sand filtration systems. Therefore, a comprehensive study on reliability and stability of these sand filters, is required. Based on our previous knowledge based on one sand filter in the Red River delta in Vietnam, we tackled three main aspects regarding household sands filters in this project: i) As removal efficiency of sand filters under different geochemical conditions and the formation of a black MnO2 layer was investigated in order to evaluate if it is feasible to use the black layer as a visual indicator for a good sand filter performance., ii) The role of biological ammonium oxidation and nitrate formation in sand filters was studied and lastly iii) the risk of As remobilization during dumping of used sand filter material into a garden soil was assessed. Our research strategy was a combination of field and lab work, applying a wide range of analytical techniques. The general outcome of the project enabled us to draw several conclusions as follows: i) In agreement with data collected from household sand filters, lab and field-based column experiments achieved stable performances in term of As and Fe elimination with an efficiency of more than 90%. During the stable operational phase, we observed formation of a black layer (MnO2) in both the column setup in the lab as well as in the household sand filters in the field, suggesting that our hypothesis that such a layer can indicate a filter with good As removal efficiency is valid under these conditions. Additionally, MnO2 reduction and dissolution occurred once dissolved oxygen in sand columns was depleted (MnO2 reacted with Fe(II) and As(III) forming As(V) and Fe(III)). ii) Nitrification took place either incomplete or complete in sand filters, by the combination of Nitrosotaleaceae (AOA), Nitrosomonas (AOB) catalyzing ammonium oxidation as well as Nitrospira (NOB) responsible for nitrite oxidation. We observed that ammonium oxidation can be limited by the depletion of dissolved oxygen caused by clogging in the sand columns over prolonged filters operation. iii) Finally, dumping used sand filter material into soil was shown to mobilize As from the sand material into the pore water under reducing conditions such as conditions are occurring during heavy rainfalls and flooding events during the monsoon season. Results from microcosm experiments revealed that the As mobilization mechanisms are linked to the microbial reduction of Fe(III) minerals in the sand filter materials. Moreover, decreasing Fe and As concentrations in the colloidal fraction suggested that colloids may affect and facilitate As mobilization.
Publications
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(2022) Microbial communities contribute to the elimination of As, Fe, Mn and NH4+. Science of the Total Environment 838, 156496
Anh Van Le, Daniel Straub, Britta Planer-Friedrich, Stephan Hug, Sara Kleindienst and Andreas Kappler
