**2. Technologies and materials**

According to the literature, the methodologies that have been studied for sulfur removal in water are membrane filtration, adsorption, ion exchange, bioreactors, and constructed wetlands. **Table 1** specifies the main characteristics of each technology or material, grouped together in three categories: physical, chemical, and biological procedures.

The Technology Readiness Level (TRL) is an index that helps measure or qualify the maturity of the technological components of a system. Here, this analysis is used to determine the level of application in which each investigation was carried out, what does it consist of, if it was at the laboratory level, pilot scale or even at the industrial level. The scale is established from level 1 (passage from scientific research to applied research) to level 9 (technology in its final form). In **Figure 1**, the level of maturity based on the information presented in each study is shown. As can be seen, most of



*a Adsorption is both a chemical and a physical process, however, for purposes of the criteria for ordering the table, it is included in physical.*

**Table 1.** *Summary of analyzed methods for sulfate removal.*

**Figure 1.** *TRL of each methodology analyzed. \* AC (Activated Carbon).*

the studies fall in level 4 (validation in laboratory environment) or 5 (validation in relevant environment, in this case, prototype) [23].

In order to understand how the experiments were made and to compare between studies, it is important to determine the characteristics and parameters used. **Table 2** lists the reports found related to the topic where at least two of the following parameters are available: maximum input concentration, % removal, pH, and/or T.




#### **Table 2.**

*Summary of relevant parameters depicted in each study for sulfate removal reviewed.*

#### **Figure 2.**

*Maximum adsorption capacity and surface area reported for different tested materials. \* AC (Activated Carbon).*


When not available, "N/R" is reported.

As can be seen, within the physical treatments, nine of them are by adsorption, which requires an independent analysis of the information. The analysis between different adsorption materials and a comparison between the sulfate adsorption capacity for each material tested can be seen in **Figure 2**.
