STANFORD – Last January, China’s environmental authorities barely averted the contamination of nearly three million people’s drinking water after a mining company dumped cadmium – a toxic heavy metal used in the manufacture of batteries, paint, solder, and solar cells – into the Longjiang River. To stop the contamination from spreading, the local fire department had to add significant quantities of dissolved aluminum chloride, which binds to cadmium and settles on the river bottom.