*3.3.3. Effect of electronegativity and atomic weigh of metals*

In multi-metal solution, the electronegativity and atomic weight of metals can also have an important role in the biosorption process and efficiency. Biosorption experiments showed that when mixed, Ni(II) and Zn(II) sorption by wheat straw presented different performances, revealing a competition between both metals for the actives sites present on the biosorbent surface and a higher preference for Zn(II) rather than Ni(II) [50]. These results are easily justified taking into consideration the more appealing physical characteristics of Zn(II): lower electronegativity and higher atomic weight of Zn(II). The oxygen-containing group present on the wheat straw (negative sites) repels Ni(II) more than Zn(II), making it more difficult to be sorbed.
