You can use hyperaccumulating plant

http://www.sciencedirect.com/science/article/pii/S0048969702001651

http://onlinelibrary.wiley.com/doi/10.1046/j.1469-8137.2002.00363.x/pdf

One can use Eichornia crassipes plant as it grows faster and ingress large area within a short time.

Could I methylate arsenate and arseniite with appropriate bacteria or fungi to make As less toxic and more volatile?

Dear Nguyen Hue: following links may help you.

http://www.sciencedirect.com/science/article/pii/S0048969799002697

http://www.bvsde.ops-oms.org/bvsacd/cd51/arsenic-water.pdf

http://revistes.iec.cat/index.php/IM/article/view/4c457c918322b.002

With regards

Dr Dubey