*4.5. Acid Thermal Treatment*

Acid thermal modification combines acid treatment with thermal modification of sepiolite. This approach to removing metal ions from water is widely used, and the adsorption using this acid thermal modified sepiolite is much better than for natural sepiolite, or separate acid modified and thermal treatment sepiolite. The sepiolite was treated with hydrochloric acid solution, and then calcinated at 450 ◦C, and subsequently used to treat Zn2+, Pb2+, Cu2+, and Cd2+ in solution through an ion exchange and surface complex adsorption process [48].

### **5. Examples of the Application of Sepiolite to Potentially Toxic Element Removals from Aqueous Environmental Systems**

It has been reported that sepiolite-based materials can be used to remove a wide range of pollutant elements from water and soils. We focus on reports relating to the key pollution metal elements in Hunan province (Hg, Cd, Pb, and Cr), where most of the research is still at the bench-scale phase.

### *5.1. The Removal Hg2+ from Wastewater*

There are few studies on the removal of mercury ions using sepiolite and modified sepiolite. Those that are available show that modified sepiolite has a significantly improved effect on Hg2+ removal. As shown in Figure 2, the removal of Hg2+ was more than 90% after acid modification, acid thermal modification, and organic modification, compared with the natural sepiolite (about 50%).

**Figure 2.** The removal of Hg by sepiolite and various modified sepiolite products [64–66].

The mechanism of removing Hg2+ using sepiolite and different modified conditions is also different. The reaction mechanism for acid modified sepiolite is the dissolution of the impurities in the sepiolite by the acid, at the same time, the pore area is enlarged, and the acidic hydroxyl group is exposed to contact with the Hg ions [65]. In the case of acid thermal treatment, the mechanism of the reaction with adsorbing ions is mainly due to the combination the approach for both acid and thermal activation. The process of dissolving the impurities in the sepiolite results in improved thermal removal of the structural water in the sepiolite and contact resistance is reduced. The combined effect is better than that of pure acid modification or thermal modification [66]. The reaction of sulfhydryl modified sepiolite for Hg2+ conforms to the pseudo-second-order kinetics equation, and adsorption of Hg2+ onto sepiolite fits Langmuir and Freundlich isotherm models. The regression coefficient (R<sup>2</sup> = 0.994) suggests that Hg2+ adsorption on sepiolite more closely followed the Langmuir model. The sulfhydryl modified sepiolite has a smoother surface, and its internal pores are enlarged, and the increased negative charge is conducive for reaction with metal ions, so as to more effectively remove the Hg2+ [64].

### *5.2. The Removal of Cd from Wastewater*

Studies found that the adsorption reaction of sepiolite with Cd2+ conforms to a pseudo-secondorder kinetics model, and its R<sup>2</sup> is 0.999. It also satisfies the isothermal adsorption model of both Langmuir and Freundlich, and the degree of fit to the Langmuir isothermal model is high (R<sup>2</sup> = 0.999). The saturated adsorption capacity of sepiolite for Cd2+ was 11.48 mg/g, and the saturated adsorption capacity of the acid modified sepiolite for Cd2+ was 13.62 mg/g [67,68]. The study on the adsorption of heavy metal Cd by sepiolite on acid and thermal treatment found that the adsorption of Cd2+ was increased by calcining the sepiolite, the main reason is that the CaO produced by the high temperature roasting makes the liquid alkaline, so that the Cd2+ was removed by precipitation reactions, in the process of acid treatment, the treatment effect of sulfuric acid is better than that of nitric acid and hydrochloric acid, which is due to the precipitation reaction between H2SO4 and Cd2+ [69]. The reaction mechanism of magnetic modified sepiolite to treat Cd2+ showed that the degree of fit to the Langmuir isothermal model was higher than Freundlich model, which indicates that the adsorption reaction of magnetic modified sepiolite and Cd2+ was based on single ion layer surface coverage [70]. Figure 3 shows that the adsorption capacity of Cd2+ was increased 3–17 times after modification, especially for the combined acid thermal modification.

**Figure 3.** Adsorption capacity of Cd by sepiolite from solutions [66,68,70,71].

### *5.3. The Removal of Pb from Wastewater*

The adsorption of Pb2+ with sepiolite and modified sepiolite departs from the Langmuir isothermal model, as a result of the precipitation caused by the reaction. The reaction mechanism is not only due in part to the complexation at the ion exchange surface, but also that the Pb2+ will precipitate during the reaction process [68]. The reaction mechanism of sulfhydryl modified sepiolite and Pb2+ fits well with the Freundlich isothermal model, and it also conforms to the pseudo-second-order kinetics, for which the R<sup>2</sup> = 0.9976, and its maximum adsorption capacity for Pb is 97 mg/g [63]. The adsorption of Pb by natural sepiolite and iron oxide-coated sepiolite was found that both fit well with the Freundlich and Langmuir isothermal models, with the degree of fit for the Langmuir model was the best (R<sup>2</sup> = 0.990). It can be seen that the reaction mechanism of sulfhydryl modified sepiolite and natural sepiolite and iron oxide coated sepiolite differ. The introduction of sulfhydryl group and the stable coordination bond with the heavy metal ions in the sulfhydryl group have a good influence

on adsorption, the adsorption effect of iron coating sepiolite on pollutant metals is higher than that for natural sepiolite, which may be due to replacement of the zeolite water in the sepiolite and increase in adsorption sites [72,73]. In addition, the effect of organic modification (dodecyl benzene sulfonic acid sodium and sodium chloride modified sepiolite) on the adsorption of Pb2+ is greater than that of unmodified sepiolite and its mechanism is related to the organic modification providing surface enrichment of macromolecule groups for the adsorption of metal ions. Adsorption isotherm has a good fit to the Langmuir model and pseudo-second-order kinetic equation, the quasi secondary maximum adsorption capacity is 226.8 mg/g [74]. The sepiolite has a good performance for the adsorption of Pb2+ after acid soaking and high temperature calcination, and its removal at 50 mg/L Pb2+ is 80.9%. The reaction has a best fit to the Freundlich adsorption isotherm [75].

### *5.4. The Application of Remove Cr in the Wastewater by Sepiolite*

The mechanism for sepiolite adsorption of Cr (VI) occurs in two stages. Firstly, to remove part of the contamination by surface of physical adsorption, followed by the likely reduction of Cr (VI) to Cr (III) [76,77].

At pH = 2, it was found that amine functionalized natural and acid-activated sepiolites [78] had the best adsorption effect on Cr (VI), and the adsorption capacity was 37 mg/g and 60 mg/g, respectively. The system showed a good fit to the Freundlich isothermal model compared with Langmuir and D–R isothermal models. It shows that in the process of adsorption should be simultaneously to multiple sites. By analyzing the R<sup>2</sup> value for the pseudo-second-order kinetics, the cause of limited adsorption efficiency in chemical adsorption process can be analyzed. In the study of the adsorption treatment of Cr (VI) with sepiolite-supported nanoscale zero-based iron (S-NZVI), the maximum adsorption capacity of S-NZVI for Cr (VI) was 43.86 mg/g, with its response fitting well to the Freundlich isothermal model, the R<sup>2</sup> is greater than for Langmuir isothermal model, which suggested that that is due to the S-NZVI surface heterogeneity the effects on the Cr (VI) removal, the linear relationship between the removal of Cr (VI) and the input of S-NZVI fits to pseudo-first-order kinetics [77]. In the study of the adsorption of Cr on magnetic modified sepiolite, it was found that the reaction is also strongly related to the Freundlich isothermal model. This shows that the magnetic surface of modified sepiolite exhibits heterogeneity, with adsorption between monolayer and multilayer adsorption mechanisms. The reaction has a good fit to pseudo-second-order dynamics (R<sup>2</sup> > 0.99) and the maximum adsorption capacity was 3.6 mg/g. Compared with natural sepiolite, the removal of Cr (VI) by modified sepiolite is much higher than for natural sepiolite [46].

Other studies of natural sepiolite adsorption of Cr3+, Cd2+, and Mn2+ showed the best adsorption effect for Cr3+ was on natural sepiolite, with good fit to the Langmuir isothermal model. The adsorption process is not only the ion exchange, but also the formation of complex and surface adsorption [79].
