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

Activated carbon is the least effective method for cadmium removal from drinking water because it primarily functions through adsorption, which is more suitable for organic compounds and some micro-pollutants, rather than for heavy metals. While activated carbon can remove certain contaminants, its capacity to effectively bind and remove cadmium specifically is limited when compared to other treatment methods.

In contrast, iron coagulation utilizes the principle of chemical precipitation, where iron ions react with cadmium to form insoluble compounds that can be removed from the water. Lime softening works by increasing the pH, precipitating heavy metals along with hardness, and promoting their removal. Ion exchange, on the other hand, involves the exchange of cadmium ions with less harmful ions, effectively removing cadmium from water with high efficiency.

Thus, when evaluating the various methods based on their efficacy in removing cadmium specifically from drinking water, activated carbon is indeed the least effective approach.