## 3 Results and Discussion
### Economic and Environmental Assessment  
An economical pre-evaluation was tentatively conducted to evaluate the role of the activation process and citric acid modification on the adsorption efficiency. Table 7 summarizes the costs associated with the adsorption of lead and acetaminophen using char materials, expressed as EUR per kilogram of the contaminants retained, and the preparation cost of each analyzed material. The yield during the synthesis, as shown in Table 7, \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Elements** & & **Char** & **char-CA-1.0** & **AC** & **AC-CA-1.0** \\ \hline \multirow{8}{*}{Major elements (mg L\({}^{-1}\))} & Al & 0.95 & 0.60 & 0.03 & 3.08 \\  & Ca & 44.2 & 3.04 & 83.88 & 16.44 \\  & Fe & 0.03 & 0.54 & \textless{}0.01 & 4.12 \\  & K & 0.18 & 0.97 & 0.59 & 3.34 \\ \cline{1-1}  & Mg & 12.19 & 1.05 & 19.46 & 4.73 \\ \cline{1-1}  & Na & 0.08 & 5.46 & \textless{}0.01 & 22.57 \\ \cline{1-1}  & P & 11.49 & 0.41 & 122.50 & 3.49 \\ \cline{1-1}  & S & 1.50 & 1.91 & 5.60 & 19.05 \\ \cline{1-1}  & Si & 11.18 & 1.28 & 53.16 & 38.97 \\ \cline{1-1}  & Zn & 1.18 & 1.97 & \textgreater{}0.01 & 0.657 \\ \hline \multirow{8}{*}{Minor elements (\(\mu\)g L\({}^{-1}\))} & As & 2.6 & 6.6 & 0.6 & 4.2 \\ \cline{1-1}  & Ba & 739.5 & 401.9 & 2.4 & 217.3 \\ \cline{1-1}  & Cd & 1.8 & 65.0 & 0.9 & 10.2 \\ \cline{1-1}  & Co & 10.2 & 134.3 & 19.9 & 18.3 \\ \cline{1-1}  & Cr & 12.0 & 64.7 & 3.6 & 111.0 \\ \cline{1-1}  & Cu & 116.5 & 37.6 & 91.8 & 213.1 \\ \cline{1-1}  & Ga & 18.9 & 20.4 & 0.1 & 11.0 \\ \cline{1-1}  & Li & 28.1 & 43.6 & 25.5 & 10.7 \\ \cline{1-1}  & Mnr elements & Mn & 91.6 & 115.1 & 10.7 & 114.9 \\ \cline{1-1}  & Mo & 0.1 & 82.3 & 0.2 & 77.9 \\ \cline{1-1}  & Ni & 116.5 & 156.3 & 91.8 & 67.7 \\ \cline{1-1}  & Pb & 140.1 & 4.1 & 12.4 & 1.2 \\ \cline{1-1}  & Rb & 13.4 & 16.0 & 8.0 & 13.7 \\ \cline{1-1}  & Sb & 0.4 & 470.4 & 0.0 & 2587.4 \\ \cline{1-1}  & Se & 3.3 & 4.1 & 5.4 & 1.5 \\ \cline{1-1}  & Sr & 311.9 & 255.5 & 22.5 & 108.6 \\ \cline{1-1}  & Ti & 2.5 & 194.0 & 45.0 & 2448.1 \\ \cline{1-1}  & V & 2.5 & 4.1 & 21.7 & 4.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Leached metals into aqueous solution by selected samples. Solid-to-water ratio (\(m/v\)) 1:10.  
was accounted for in the cost estimation. Briefly, the activation with Na2 CO3 led to a 31.7% yield and the post-treatment with citric acid did not substantially alter this value. The untreated char shows moderate adsorption costs for both lead (2214.0 EUR/kg) and acetaminophen (1046.3 EUR/kg). In contrast, the application of Na2 CO3 activation reduced these costs, with lead adsorption costs dropping to 1169.7 EUR/kg and acetaminophen to 582.4 EUR/kg. This cost reduction is the consequence of the enhanced adsorption capacity achieved after activation, making the process more economically viable for both contaminants. The citric acid modification of the non-activated char increased the costs of lead (5008.9 EUR/kg) and acetaminophen (5029.0 EUR/kg) removal. This suggests that citric acid alone may hinder the adsorption efficiency, leading to higher costs per unit of contaminant retained. Conversely, with the citric acid functionalization of the activated char at low CA dosage (0.5 M), the costs decrease to 1506.8 EUR/kg for lead and 906.9 EUR/kg for acetaminophen, demonstrating that a strategic combination of treatments can enhance the adsorption efficiency while maintaining low costs. At high citric acid concentration (1.5 M), the costs rise, especially for lead adsorption (2493.9 EUR/kg), indicating that excessive citric acid modification diminishes economic feasibility for certain applications. In addition, the preparation costs of each material vary between 4.7 and 25.1 EUR/kg. Although the results found are a preliminary estimation based on laboratory-scale consumption and do not include some aspects to be considered at a large scale, they are like those found by other researchers.