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

Activated carbon is indeed the least effective method for the removal of cadmium from drinking water. This is due to the nature of activated carbon, which primarily adsorbs organic compounds and a limited number of inorganic ions and metals. While it can capture some metal ions, it is not specifically designed for heavy metals like cadmium, leading to lower removal efficiencies.

In contrast, iron coagulation is effective because it utilizes the formation of ferric hydroxide, which can capture and remove cadmium ions through a process called adsorption onto its surface. Lime softening raises the pH of water, facilitating the precipitation of cadmium as insoluble compounds, thereby removing it effectively. Ion exchange is specifically tailored to remove heavy metals like cadmium, as it exchanges cadmium ions for non-toxic ions in the resin, making it highly effective for this purpose.

Therefore, when evaluating the various methods, activated carbon has limitations against cadmium removal, making it the least effective option compared to the others, which possess specific mechanisms and efficiencies for targeting heavy metal contaminants.