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High-tech filters to remove arsenic from contaminated water supplies are too expensive for most of the 20 million people affected worldwide. Now there’s a more affordable alternative: a tube filled with volcanic rock. Akira Ohki at Kagoshima University in Japan packed a vertical tube with lumps of volcanic rock called shirasu, abundant in the Kyushu region of Japan. Then he poured contaminated water through it. After a single pass, 85 per cent of the arsenic had been removed. Ohki says the arsenic ions in the solution bind to aluminium hydroxide in the surface of the rock. Shirasu is a plentiful…