**7. Summary**

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 organic molecules is usually needed.

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 magnetite core.

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 functional groups determine the selectivity toward certain precious metal cations.

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.

Purification of the recovered metals may be done through well-known processes. Electrochemical process is the best choice of metallurgy. Other methods by the use of chemical reduction could also be selected. In the large scale, we can consider blast furnace combined with a redox reaction. One important point to consider, the use of environmentally friendly reprocessing of the metals must always be prioritized.
