Rust Could Be the Key to Arsenic-Free Water: Scientific American:
Another interesting technology is described here, published in 2006 (original Science article here). I don't know what stage it's at currently.
The Scientific American report ends as follows: "Given the batch nature of this process, it is unlikely that homes in the developing world can be outfitted with filters placed directly on taps, but getting poison-free water by the tank load is still a step in the right direction." I immediately pictured the technology in conjunction with a water tower and indoor plumbing. Is that what the people we serve would really want?
Is this something about which our Engineers Without Borders colleagues can advise? (I also thought about Plumbers Without Borders - not the first time that idea has arisen, it seems.) Perhaps it's already happening and I just don't know about it. That happens a lot.