# Wastewater Treatment Using Alkali-Activated-Based Sorbents Produced from Blast Furnace Slag
## 2 Materials and Methods
### Mine Wastewater Model Solutions  
The obtained porous geopolymer monoliths were then used as a self-supporting filter for the treatment of waste streams, polluted with heavy metals. In particular, a synthetic aqueous solution of composition Cu(II)/Fe(III)/Ni(II)/Mn(II) = 4/4/0.4/1 (mg/L) was pre  
Figure 1: Example of porous monolith at the end of the process.  
pared, which simulated the composition and metal concentrations in the final steps of the purification process of mine wastewater, but still containing heavy metal ions [3; 4]. As ions precursors, nitrates were selected for Ni\({}^{2+}\) (Ni(NO3)2\(\cdot\)6H2O) and Cu\({}^{2+}\) (Cu(NO3)2\(\cdot\)6H2O), while chlorides were used for Fe\({}^{3+}\) (FeCl3\(\cdot\)6H2O) and Mn\({}^{2+}\) (MnCl2\(\cdot\)4H2O), all supplied by Sigma Aldrich (Sigma Aldrich Italy, Milan, Italy) (purity 98-99%). In order to study the capture mechanism and the adsorption selectivity, the capture capacity was firstly analyzed by performing a set of experiments using the four ions in single-ion solutions, then a second set of experiments was performed on multi-ion solutions (Table 3). Single ions concentrations were selected equal to the specific ion concentration in the corresponding multi-ion solution. Measured pH values of the starting solutions are reported in Table 3, while those of the filtered ones were 12-12.5 for all the experiments.  
Operating parameters, such as filtration time, monolith volume, and effect of the initial ion concentration were also analyzed and tuned by filtration experiments using Cu(II) mono-ionic solution. Monolith volumes 54.7 and 87.4 cm\({}^{3}\) and filtration time 5, 10, and 30 min were studied at the fixed Cu initial concentration of 4 mg/L; while the effect of the initial Cu concentration was studied for 4, 8, and 12 mg/L containing solutions at fixed volume of 54.7 cm\({}^{3}\) and a filtration time of 10 min.  
In a typical experiment (Figure 2), 50 mL of the aqueous solutions were poured onto the geopolymeric monolith under vacuum (water pump) at room temperature. The vacuum pump was manually controlled, in order to fix the filtration time at 5, 10, and 30 min.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Mono-Ionic Solution**} & \multicolumn{4}{c}{**Allowed EU Limits**} \\ \cline{2-5}  & **(mmol/L)** & **(mg/L)** & **pH** & **(mg/L)** \\ \hline Cu(II) & 0.063 & 3.97 & 5.3 & 3 \\ Fe(III) & 0.072 & 4.03 & 3.8 & 1–3 \\ Ni(II) & 0.0094 & 0.55 & 5.2 & \textless{}0.1 \\ Mn(II) & 0.019 & 1.05 & 5.3 & \textless{}0.1 \\ \hline \hline \multicolumn{5}{c}{**Multi-Ion Solution**} \\ \hline Cu(II) & 0.063 & 3.97 & & \\ Fe(III) & 0.072 & 4.03 & & \\ Ni(II) & 0.0094 & 0.55 & & \\ Mn(II) & 0.019 & 1.05 & & \\ \hline \hline \end{tabular}
\end{table}
Table 3: Synthetic solutions composition and EU emission limits [1; 2].  
Figure 2: Filtration experimental setup.  
Metal analysis before and after the filtration step was performed by means of inductively coupled plasma--optical emission spectrometry (ICP-OES) technique using a PerkinElmer OPTIMA 7000 DV spectrometer (Perkin Elmer Italia, Milan, Italy).  
The average of three measurements was considered, and the estimated error was within 1%. The capture efficiency was calculated by the difference between the initial ion concentration (C\({}_{0,i}\)) in the starting solution and the residual one after filtration (C\({}_{i,i}\)), according to Equation (1):  
\[\eta_{c}(\%)=\frac{C_{0,i}-C_{f,i}}{C_{0,i}}100 \tag{1}\]  
For all the experiments, the release of Ca ions during filtration was also controlled and measured in the range 20-300 mg/L depending on the experiment.