Over 55% of the 28-35 million Bangladeshi that were initially exposed to arsenic concentrations over 50 ppb still regularly drink water exceeding this limit. The two most successful mitigation options - well switching and installation of deep tube wells - have reached 29% and 12% of the population, respectively. Arsenic removal and four other mitigation options have so far each reached less than 1%, but are likely play a larger role in the future.

The applicability of the various mitigation options is highly dependent on the local conditions.

Arsenic removal with naturally present iron, by aeration and removal of the formed iron(III)(hydr)oxide particles with sorbed arsenic by settling or by sand filtration, is the simplest available water treatment method. Sand filters are now widely used in the red river delta of Vietnam and successfully remove arsenic to below 50 ppb in most cases. In Bangladesh, however, water with low iron and high phosphate concentrations makes arsenic removal difficult. Combined research efforts during the past years have lead to a better understanding of the redox-, sorption-, and precipitation reactions that affect arsenic removal and promising arsenic removal units have been developed. For example, zero-valent iron is successfully used to remove arsenic in tens of thousands of households in Bangladesh (SONO-filters) and Nepal (KANCHAN-filters). However, these filters can fail in certain regions, for reasons which are not yet fully understood. In aerated water, corrosion of iron leads to formation of Fe(II) and the subsequent oxidation of Fe(II) leads to oxidation of As(III), formation of iron(III)(hydr)oxides and coprecipitation and sorption of arsenic and phosphate. Under non-aerated water, iron corrosion leads to anoxic conditions that facilitate sulfate reduction (probably biologically mediated) and to arsenic immobilization by reductive pathways (e.g. incorporation of arsenic into iron corrosion products and to formation of iron- and arsenic sulfides). For the optimal performance of zero-valent iron filters, redox conditions have to be controlled, for example by adapting the filter designs to local water compositions.

This presentation will give a brief overview of the relevant processes. It shows that more work is needed to fully understand the important reaction pathways and to further optimize and expand the range of arsenic removal methods. One of the major challenges is the large variability in the concentrations of the most important water constituents that affect the performance and operation time of arsenic removal units.