**4. Modification of Sepiolite**

Despite the wide application of sepiolite in a variety of industrial processes when compared with other sorbent systems, it has relatively low surface acidity, narrow channels, low surface area, and poor thermal stability. This limits some applications of natural sepiolite [44]. The adsorption performance of the "modified" sepiolite can be much better than that of natural sepiolite, and studies have shown that the specific surface area can be increased significantly from 29–87 m2/g [45]. The adsorption and removal capability of magnetic modified sepiolite for the heavy metal Cr (VI) is 10 times that of natural sepiolite [46] and of similar magnitude in the case of Hg2+ for surfactant modified sepiolite [18], but is still 0.5 to 0.3 of modified carbon and synthetic metal organic systems highlighted in the introduction above. When comparing natural sepiolite with a number of modified sepiolite systems to remove Pb2+, it was found that the order of adsorption capacity is as follows: H2O2 modified sepiolite > KNO3 modified sepiolite > natural sepiolite. When the initial Pb2+ concentration is 2.5 mg/L, the adsorption capacity of H2O2 modified sepiolite is twice as much as natural sepiolite [47]. According to other studies, the adsorption of Cr (VI) by activated sepiolite follows the following order: acid activated-mercapto silane organic modified sepiolite > sulphur silane modified sepiolite > acid activated modified sepiolite > natural sepiolite [48]. For the adsorption of Pb2+ thermally modified sepiolite > natural sepiolite > and for the adsorption of Cd2+ thermally modified sepiolite > natural sepiolite [49]. This provides a range of strategies for modification and sources of reactants to enable optimization of technological approach.
