# Wastewater Treatment Using Alkali-Activated-Based Sorbents Produced from Blast Furnace Slag
## 4 Discussion  
The data shown above pose demands regarding the capture mechanism and nature of the sorbent-ion interaction. Adsorption is reported as the most probable mechanism for metal capture; however, it could be influenced by a number of factors, such as concentration, contact time, stirring rate, temperature, pH, ionic strength, and complexity of the waste stream. Operating conditions are largely influenced not only by the sorbent nature, but also by the adsorbate. A typical example is heavy metal ions, where typical  
Figure 8: Electron paramagnetic resonance (EPR) spectra of monolith geopolymers before (**a**) and after the adsorption of mono-ionic Cu\({}^{2+}\) solutions with concentration (**b**) 4 mg/L; (**c**) 8 mg/L and (**d**) 12 mg/L. The signals of Cu\({}^{2+}\) are evidenced. pH ranges, characteristic of each metal, correspond to no or maximum adsorption or precipitation [13; 36; 37]. It is evident that the knowledge of the behavior of the system sorbent/adsorbate is a key factor to determine operating parameters and, consequently, a successful transfer from small laboratory scale to pilot plant, and finally to larger industrial applications [38]. Different sorption mechanisms were proposed in the literature. For fly ashes, geopolymer, and faujasite, a chemisorption mechanism and a Langmuir adsorption model were proposed for Pb(II) [39]; while in the case of nuclear wastes treatment by geopolymer foams, a homogeneous precipitation of Cs compound on the pores wall of the foam was reported [40]. Concerning clay and organo-clay sorbents, pH modification and related changes in the acid-base condition and equilibria in solution were reported to be responsible for Cu capture in a mixed Cu-La ions solution. Rather than to the nature of the adsorption sites, capture was found to be due to aquo- and hydroxy-copper complexes formation [36], and similar effects were also reported for reduced graphene oxide membranes [12], where solutions of composition similar to those here reported were tested. Despite no effect on metal ions capture of the initial pH being detected, a certain role of the pH trend during the whole adsorption process has to be supposed, considering that its value at the end of the filtration ranged between 11 and 12. Such a strong basic pH is related to Ca ions release in solution during capture, which was found to range between 20 and 300 mg/L, depending on the experiment. Indeed, it is well known that pH influences the nature of the chemical species that are present in a solution. Depending on the pH range, metals can be present in cationic or hydroxy form. Accordingly, pH variations could be considered to explain the capture mechanism. Therefore, to better understand this aspect, speciation in solution at different pH ranges was evaluated for the single ions. For this purpose, equilibria in solution were evaluated by Medusa software, a modeling software that, on the basis of thermodynamic data, is able to provide ranges of existence of each species as a function of pH [12]. A complex scenario was found, where different species were evinced from the calculation in different pH ranges [12]. Medusa software results are summarized in Table 5 as a function of different pH ranges. In detail, for pH < 4, all the ions are present in cationic or charged complex forms, thus soluble. Similarly, for pH > 10, ions are always present in oxide or hydroxide form, thus precipitated. Some differences among the ions can be found at pH values between these two limit situations (i.e., 4-10). In this range, indeed, only Fe is present in a neutral form, which can be precipitated. Medusa results are also in line with literature indications on speciation and precipitation range of the selected ions [41]. By combining all this information with filtration tests results, a reasonable explanation of the geopolymers capture behavior can be attempted.