## 3 Results
### Heavy Metal Removal Performance  
The removal performance of the ecological concrete-biochar system was evaluated for five heavy metals (Cd\({}^{2+}\), Pb\({}^{2+}\), Cr\({}^{3+}\), Cu\({}^{2+}\), and Zn\({}^{2+}\) ions) through both long-term and short-term experiments. Metal removal rates and efficiencies observed at different time intervals across both experimental scales are summarized (Table 2). The data reveal a three-phase removal process with distinct removal rates at each stage. During the initial 0-4 h period, the module-scale system demonstrated removal rates of 0.18-0.25 mg g\({}^{-1}\) h\({}^{-1}\), while the  
Figure 1: Optimization and fabrication of ecological concrete modules with varying strength grades (C20, C25, C30). (**A**) Fresh concrete mixture showing uniform consistency with optimized watcement ratio (0.36); (**B**) specialized mold design incorporating drainage ports for controlled pore formation and optimal permeability (>1 \(\times\) 10\({}^{-4}\) cm s\({}^{-1}\)); (**C**,**D**) systematic specimen preparation process demonstrating controlled compaction and surface finishing techniques for different strength grades; (**E**) comparative analysis of three strength grade specimens highlighting distinct physical characteristics; (**F**,**G**) optimized C25 specimens (exhibiting ideal pore distribution and surface characteristics for heavy metal removal experiments in 25 L system). kinetic studies showed higher rates of 0.28-0.42 mg g\({}^{-1}\) h\({}^{-1}\), attributable to the differences in biochar-to-solution ratios. This phase accounted for 25-35% and 45-65% of total removal in module-scale and kinetic studies, respectively. The intermediate phase (4-24 h) showed a decline in removal rates to 0.10-0.15 mg g\({}^{-1}\) h\({}^{-1}\) and 0.12-0.18 mg g\({}^{-1}\) h\({}^{-1}\) for the respective systems, while the extended phase (>24 h) exhibited further reduction in rates as available binding sites became increasingly saturated. Heavy metal removal efficiencies at different initial concentrations for both experimental scales show the system's performance (Table 3). The data demonstrate that the system maintains robust performance across two orders of magnitude in concentration ranges. For Cd\({}^{2+}\) ions, removal efficiencies ranged from 83.2% to 92.3%, with higher efficiencies observed at lower concentrations. Pb\({}^{2+}\) ions showed the highest overall removal (85.4-95.6%), while Cr\({}^{3+}\), Cu\({}^{2+}\), and Zn\({}^{2+}\) ions exhibited efficiencies of 78.5-84.2%, 82.4-89.7%, and 73.6-84.8%, respectively. Notably, the kinetic studies consistently showed 5-12% higher removal efficiencies compared to the module-scale experiments, which we attribute to the more favorable biochar-to-solution ratio in the smaller-scale system. The removal performance for five heavy metals was evaluated through both long-term (10-day) and short-term (24 h) experiments across multiple concentration gradients. The long-term study tested four concentration levels: T1 (highest), T2, T3, and T4 (lowest), as shown in Figure 2. Short-term kinetics across five concentration gradients (C1-C5) are \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Time Period** & **Parameter** & **Module-Scale** & **Kinetic Studies** \\ \hline
0–4 h & Removal Rate (mg g\({}^{-1}\) h\({}^{-1}\)) & 0.18–0.25 & 0.28–0.42 \\  & Total Removal (\%) & 25–35 & 45–65 \\
4–24 h & Removal Rate (mg g\({}^{-1}\) h\({}^{-1}\)) & 0.10–0.15 & 0.12–0.18 \\  & Total Removal (\%) & 45–60 & 65–85 \\ \(>\)24 h & Removal Rate (mg g\({}^{-1}\) h\({}^{-1}\)) & 0.05–0.08 & 0.03–0.05 \\  & Total Removal (\%) & 75–85 & 80–95 \\ \hline \hline \end{tabular} Note: module-scale system, 25 L solution, 30 g biochar (833:1 mL g\({}^{-1}\) ratio); and kinetic studies system 100 mL solution, 10 g biochar (10:1 mL g\({}^{-1}\) ratio).  
\end{table} 
Table 2: Analysis of metal removal performance at different time intervals for both experimental scales.