By examined the adsorption capacity for a mixture solution of Cu, Zn and Ni onto goethite-rich soil and birnessite-rich soil, we recorded that the affinity of Zn and Ni in goethite-rich soil always higher than their affinity onto the birnessite-rich soil. On the other hand, the affinity of Cu always higher onto birnessite-rich soil than that of goethite-rich soils. 

Also, why the adsorption capacity of Ni and Zn declined more by using a mixture of the two oxides (Goethite-Birnessite)? the pH of the metal solution was fixed before the experiments at 6.