Calcium is not compatible with phosphate buffer at that pH; it precipitates and the pH is not maintained. Try maleate buffer.

Thank you #AndreiBlasko for response. i will look for maleate buffer.

a maleate buffer solution should work

Maleate buffer may be a bit expensive in real life application. I suggest to use another stable adsorbent , which can work at any pH and buffered solutions, as the ultimate goal of research is to improve environment and to solve ecological problems. You can covalent cross-link Alginate and then it will work at any pH range, moreover I suggest to use Medusa software that will give information about mercury ion formArticle Efficient removal of mercury (II) from water by use of cryog...