## 3 Results
### Heavy Metal Removal Performance
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Metal** & **Initial Concentration (mg L\({}^{-1}\))** & \multicolumn{2}{c}{**Removal Efficiency (\%)**} \\  & & **Module-Scale** & **Kinetic Studies** \\ \hline  & 0.25 & 85.4 & 92.3 \\ Cd & 2.5 & 84.2 & 89.7 \\  & 12.5 & 83.2 & 85.6 \\ \hline  & 0.05 & 92.1 & 95.6 \\ Pb & 0.5 & 89.8 & 93.2 \\  & 2.5 & 85.4 & 90.1 \\ \hline  & 2.50 & 78.5 & 84.2 \\ Cr & 25.0 & 80.2 & 82.8 \\  & 125.0 & 79.8 &presented in Figure 3. The experiments were conducted at two different scales: the 10-day study used modules in 25 L containers with a volume-to-adsorbent ratio of 833:1 (mL) (Figure 2), while the 24 h kinetic study used 10 g biochar in 100 mL vials, providing a ratio of 10:1 (Figure 3). Fluctuations observed in metal concentrations in Figure 3, particularly for Cr\({}^{3+}\) and Zn\({}^{2+}\) at higher concentration levels, reflect the dynamic equilibrium processes occurring during adsorption. These temporary variations are attributed to reversible binding on certain biochar surface sites, where initially adsorbed metals may partially desorb as the system approaches equilibrium. This behavior is consistent with heterogeneous adsorption mechanisms on carbonaceous materials, where binding sites have varying affinities for metal ions. Despite these minor fluctuations, the overall trend shows progressive metal removal throughout the experimental period. Figure 2: Temporal variations in removal efficiency for heavy metals (Cd, Pb, Cr, Cu, and Zn) monitored over a 10-day period under four concentration gradients. (Note: Panels represent, Cd concentrations **(A–D)**; Cr concentrations **(E–H)**; Cu concentrations **(I–L)**; Pb concentrations **(M–P)**; and Zn concentrations **(Q–T)** across treatments T1, T2, T3, and T4, respectively. Treatment concentrations were as follows: T1 (Cd: 12.5, Pb: 2.5, Cr: 125, Cu: 250, Zn: 2500 mg L\({}^{-1}\)), T2 (Cd: 2.5, Pb: 0.5, Cr: 25, Cu: 50, Zn: 500 mg L\({}^{-1}\)), T3 (Cd: 1.25, Pb: 0.25, Cr: 12.50, Cu: 25.00, Zn: 250.00 mg L\({}^{-1}\)), and T4 (Cd: 0.25, Pb: 0.05, Cr: 2.50, Cu: 5.0, Zn: 50 mg L\({}^{-1}\)). Data are presented as mean values \(\pm\) standard error (n = 3). Different lowercase letters (a, b, c) indicate statistically significant differences between sampling intervals within each treatment according to [PERSON]’s multiple range test (\(p<\) 0.05). The notation ‘ab’ indicates values that are not significantly different from either ‘a’ or ‘b’ groups.) For Cd, the concentrations tested ranged from 0.25 to 12.5 mg L\({}^{-1}\). During the 10-day experiment (Figure 2A-D), T4 (0.25 mg L\({}^{-1}\)) achieved the highest removal efficiency at 92.3%, followed by T3 (88.7%), T2 (85.4%), and T1 (83.2%). Temporal analysis revealed that most Cd\({}^{2+}\) ion removal (\(>\)65%) occurred during the first 48 h of treatment, with initial removal rates of 67.8% and 71.2% for the highest (T1) and second-highest (T2) concentration treatments, respectively. In the 24 h kinetic study (Figure 3A-D), Cd demonstrated rapid initial removal of 0.42 mg g\({}^{-1}\) h\({}^{-1}\) during the first 4 h, particularly evident in the steep initial slope of the removal curve. Pb concentrations ranged from 0.05 to 2.5 mg L\({}^{-1}\). As illustrated in Figure 2E-H, Pb exhibited the highest overall removal efficiency among all metals, with T4 (0.05 mg L\({}^{-1}\)) achieving 95.6% removal by day 10. The steep initial slope of the curves shows that the first 72 h accounted for 78.3-85.6% of total Pb\({}^{2+}\) ions removal across all treatments. Even at the highest concentration (T1, 2.5 mg L\({}^{-1}\)), removal efficiency remained above 85%. The short-term kinetics showed initial removal rates of 0.38 mg g\({}^{-1}\) h\({}^{-1}\) in the first 4 h (Figure 3E-H).