Carbon nanotube membranes were fabricated using arrays of millimeter-long multiwalled carbon nanotubes (MWCNTs) with an inner diameter of about 8 nm through a low-cost cast–polishing method. The fabricated carbon nanotube membranes demonstrated a unique ionic rectification phenomenon through two fundamentally different mechanisms: a) the formation of metal ion complexes and b) the precipitation of insoluble lead molybdate inside the carbon nanotubes. These discoveries could be applied to detecting the presence of toxic metal ions in contaminated water resources through a relatively simple approach.