check this method

http://www.sciencedirect.com/science/article/pii/S1878535210001334

Mmmm, I would not consider lime precipitation (as suggested in the referenced document by Navdeep) as being  a selective method. Selectivity is the result of a very complex competition - most research is based on empirical multi-metal screenings. if you want to manipulate selectivity I would suggest either one of two strategies (i) first do a multi-element screening and perform more in-depth structural analysis of functional groups and surface chemistry of those adsorbents that best reflect the desired selectivity (and continue to work from there) OR (ii) try decreasing competitiveness of elements you would like to remain in solution by adding selective chelates that keep such cations in solution.

There could be competition by the metals for adsorption on the same sites of the adsorbent.  You need to select the suitable adsorbent for the metal you want to remove.

Then work on optimizing the conditions that will help in the removal of the selected metal, whether it is temperature/solution pH or any other condition/parameter that will help in its removal.

Many factors can affect the ions selectivity,among them, pH, temperature, pore size diameter, etc. But many works directed to design an ion memory for the material. For example, the technique named as ion imprinted polymers, which depend on the complexation withe the template ion, polymerization, then leaching the template. This material become has high selectivity or recognition for the applied ion.

The decreased maximum adsorption capacity must have resulted from the competition between the metals for adsorption sites of the adsorbent. Follow the information given by Erik and determine other factors that influence the metal in question from other materials.