**8. Future recommendation and direction**

of [AuCl<sup>4</sup>

]−

140 Advanced Surface Engineering Research

**7. Summary**

organic molecules is usually needed.

the magnetite core.

ion by magnetite modified with a thiol group and desorption. The desorption is

] −

ion on the adsorbent surface.

done by applying thiourea in HCl solution. The concentration of the thiourea is low. The thiol

Modified magnetic adsorbents have been synthesized and used in the recovery of precious metals from aqueous solutions. Among the magnetic materials, magnetite is studied widely. Surface modification of nanoscale magnetite core is crucial to have a better adsorption capacity, stability, and turnover. The key issues of the magnetic adsorbent include size and shape of the core, choice of surface modification, adsorption capacity, stability, and recyclability. The size of the magnetite core is also better if it is in the nanoscale rather than in micron scale. It will improve the contact between pursued ions and adsorbent surface. The surface modification must have a good affinity toward certain precious metal cations. Many researchers attempt to combine adsorption capability and magnetic properties of the magnetite-based adsorbent for certain metal recovery from the solution. Selective adsorbents are also of interest for separation of precious metals from a complex system such as industrial waste. Adsorption selectivity is highly considered for complex matrices. Magnetite core has low stability in strongly acidic aqueous media. Coating with silica has two advantages, for protection against the acidic environment and a binding site for further functionalization. A suitable modification of the magnetic particles by coating or functionalization using inorganic components or

The synthesis of magnetite as the core material has been established. The use of salts of Fe(II) and Fe(III) with careful stoichiometric calculation is a must. The pH of the magnetic formation should also be controlled, either by the use of sodium hydroxide or ammonia solution. In many cases, ammonia can give better homogeneous particles. It may be better to add a stabilizing agent for reducing aggregation of the magnetite nanoparticle and improve the stability of the colloid. Coating of magnetite with silica has also been well understood. TEOS and TMOS are the main choices for the outer shell of the magnetite, although sodium silicate may work. Silica is a preferable coating since it is resistant to acid and base, which will protect

The final surface modification is functionalization of the silica with ligands that will strongly bind the cations. The end of the modification chain must have a special interaction with the cations, especially through coordination bonds. The functional groups could be an amine, carboxylate, thiol, sulfonate, amide, hydroxyl, and so on. Based on reagent availability, the

The release of the adsorbed metal cations after being concentrated in the adsorbent can be realized using acids and strong complexing agents. The acids are usually not desirable since they can cause the magnetite core to dissolve. Dissolution will damage the structure of the magnetite, which may not be possible to reuse. Complexing agents such as thiourea and EDTA can give a better option to minimize the damage to the magnetite-based adsorbents.

functional groups determine the selectivity toward certain precious metal cations.

group may form a covalent coordination bond with [AuCl<sup>4</sup>

The conventional metal reprocessing uses chemicals that are not environmentally friendly. The magnetite-based adsorbents offer technology that can reduce the application of toxic chemicals. The adsorbents give the possibility to reduce, reuse, and recycle for a few times. The magnetic core of the adsorbent is also readily synthesized with environmentally benign precursors. The coating with silica protects against acid and base media during application and recycle. The silica coating can also facilitate the attachment of the functional groups, which is critical in the modification step.

The current advanced electronic devices utilize the precious metals in their important components. The waste of electronic devices grows rapidly along with an increase in smartphone and PC use. Computer parts like processors, memories, motherboards, hard drives, and CD/DVD drives contain gold and other precious metals such as silver, palladium, and so on. The conventional gold recovery process uses cyanide ions for complex ion formation and electrolysis. The current technology attempts to recover gold and other precious metals from computers' and smartphones' components by utilizing magnetite nanoparticles. The new magnetic materials are effective yet environmentally friendly to recover precious metals. The magnetic adsorbents could also be the future of reclaiming precious metals from the waste of the other industries.

In the magnetic adsorbent development, the magnetite core could be possibly substituted with other oxides of transition metals such as manganese, cobalt, or nickel if they maintain strong magnetic characters. However, silica is the main choice for easy coating of the magnetic core, which also helps protect the magnetic core from dissolution in the acidic and basic media. The presence of the ligands on the surface of the magnetite-silica core-shell is critical for adsorption process. The environmentally safe polymers and simple molecules may be used to facilitate coordination bond with the target cations. The desorption process must be done using suitable solutions. The solution for desorption should leave the adsorbent in good shape for further reuse and turnover. The present technology available for purification of the recovered metals may apply electrochemical, chemical, and thermal processes.
