**2. Experimental**

#### **2.1 Reagents**

All the reagents that were used were of analytical grade. The water used for the preparation of solutions in the experiment was deionized. The solutions to be evaluated were prepared by dissolution of salts of nickel (Ni(NO3)2 6H2O), cobalt (Co(NO3)2⋅6H2O), and copper (Cu(NO3)2) in concentrations from 0 to 1,000 ppm.

#### **2.2 Preparation of the adsorbent**

Brushite natural (nDCPD) was obtained from bovine bone, which was washed with hot water several times to remove tissue debris, and then it was dried at 353 K for 24 h. Next, the bones were crushed and sieved to obtain a particle size of 150 mesh (104 μm). Then, the powder obtained was treated with HCl and NaOH solution, 10<sup>−</sup><sup>2</sup> M, respectively, using a ratio of 30% w/v. Finally nDCPD was stored until its use [1].

### **2.3 Characterization of natural brushite**

The X-ray diffraction patterns (XRD) were obtained in a Rigaku diffractometer (Ultima IV). The Fourier transform infrared studies of the samples were performed in an IR 100 Analyzer spectrophotometer (PerkinElmer), in a range of 400–4000 cm<sup>−</sup><sup>1</sup> . The scanning electron microscopy (SEM) images were obtained in a JOEL equipment (6510-Plus).
