Which of the following is the LEAST effective method for cadmium removal from drinking water?

The least effective method for removing cadmium from drinking water is activated carbon. This method primarily relies on adsorption, which is effective for many organic compounds and some inorganic substances, but it is generally not very effective for heavy metals like cadmium. Activated carbon's ability to adsorb cadmium can be limited due to the metal's solubility and the nature of the carbon material itself.

In contrast, reverse osmosis, cation exchange, and distillation all have proven effectiveness in removing cadmium from water. Reverse osmosis uses a semi-permeable membrane that filters out contaminants, including heavy metals. Cation exchange involves the exchange of cadmium ions with other cations in a resin, effectively reducing the concentration of cadmium in the water. Distillation, which involves boiling the water and condensing the steam back into liquid, is also capable of removing cadmium, as it evaporates at a temperature higher than that of water.

While activated carbon can provide some reduction of cadmium levels, its efficiency is significantly lower compared to these other methods, making it the least effective choice for this particular application.