**6. Regeneration of Sepiolite**

The process of regeneration of modified sepiolite treated with wastewater is to restore most of the adsorption capacity of sepiolite, so that the material can be reused, reducing operational costs and preserving resources, which fitswith green production process philosophy. At present, there are few reports on the adsorption and regeneration of sepiolite, and it is of grea<sup>t</sup> significance to find an economical and efficient method for the treatment of waste water [80,81].

The method of sepiolite regeneration includes: acid regeneration, alkaline regeneration, and salt regeneration. Jia et al. used salt regeneration to regenerate the sepiolite that had adsorbed Zn2+, it shows that the capacity adsorption of regenerated sepiolite is still high although some minor reduction occurs [82]. The adsorption ability of sepiolite pre- and post-regeneration is shown in Table 2.


**Table 2.** The adsorption ability of sepiolite before and after regeneration.

From Table 2, we can see that the regenerated sepiolite maintains good adsorption properties for metals ions. Li et al. [87] used hydrochloric acid, sodium chloride, and sodium hydroxide in three ways to regenerate the sepiolite. It was found that NaOH had the best treatment effect on the regeneration of sepiolite, and the treatment effect was similar with that of water and sodium chloride, with the use of hydrochloric acid being the poorest. Li et al. [88] used two different kinds of acid, salt and alkaline, to study sepiolite regeneration, and showed that the NaOH and nitric acid had the best effect on sepiolite regeneration, but hydrochloric acid and NaOH treatments were not ideal for sepiolite re-use. Yan showed several different methods for the regeneration of sepiolite: two acid regeneration (HNO3, HCl), alkaline regeneration, and salt regeneration. It shows that the removal of metal ions was reduced by 19.72% when the regenerating solution is HNO3, the removal of metal ions was reduced by 16.28% when the regenerating solution is HCl, for NaCl, it was reduced by 5.32% and after five steps, for NaOH, the removal of metal ions was reduced by 6.24% [89]. The results of these trials show that regeneration process is metal specific, which must be considered when developing protocols for full trials and field applications.
