## 4 Discussion
### Metal-Specific Removal Patterns  
Individual metal removal patterns reveal complex behaviors influenced by ionic properties, concentration ranges, and environmental conditions [29]. Pb demonstrated exceptional removal characteristics, achieving 95.6% efficiency at 0.05 mg L\({}^{-1}\) and maintaining 90.1% efficiency even at 2.5 mg L\({}^{-1}\), consistent with findings regarding preferential binding to calcium-silicate-hydrate surfaces [30]. The rapid initial binding of Pb\({}^{2+}\) ions can be attributed to their optimal ionic radius (1.19 A) and high electronegativity, facilitating strong surface complexation as predicted by theoretical models [31].  
Cd\({}^{2+}\) ions exhibited concentration-dependent efficiency ranging from 83.2% to 92.3%, correlating strongly with recent molecular modeling studies [32]. At lower concentrations (0.25 mg L\({}^{-1}\)), Cd\({}^{2+}\) removal reached 92.3% efficiency, while higher concentrations (12.5 mg L\({}^{-1}\)) showed 83.2% removal, supporting the competitive binding mechanism proposed by [PERSON] et al. (2024) [33]. The removal mechanism involves both surface precipitation and pore diffusion [33], with pH-dependent speciation playing a crucial role in removal efficiency.  
Cr demonstrated remarkable consistency across concentration ranges (78.5-84.2% efficiency), supporting the multi-oxidation state removal mechanism proposed by [PERSON] and [PERSON] (2020) [23]. The consistent performance across pH ranges (5-8) indicates robust removal mechanisms independent of solution chemistry. Cu\({}^{2+}\) and Zn\({}^{2+}\) ions removal patterns exhibited distinct characteristics reflecting their different ionic properties [34]. Cu showed strong correlation with concrete porosity, supporting the internal surface area dependency model [18], achieving removal efficiencies between 82.4% and 89.7% across all concentrations tested (5.0-250 mg L\({}^{-1}\)). The removal mechanism involves both surface complexation and pore-filling processes, with the biochar providing additional binding sites through its oxygen-containing functional groups. Zn\({}^{2+}\) ions removal efficiencies (73.6-84.8%) were slightly lower but still significant, with notable pH sensitivity corresponding with theoretical predictions regarding metal speciation effects [35].  
At the experimental pH of 7.0 \(\pm\) 0.2, the predominant metal species present in our system were determined through speciation modeling using Visual MINTEQ software 4.0. Pb\({}^{2+}\) exists primarily (>85%) as free Pb\({}^{2+}\) ions with minor fractions of PbOH\({}^{+}\) and Pb(OH)\({}_{2}\). Cd\({}^{2+}\) predominantly exists as free Cd\({}^{2+}\) ions (>90%) with minimal CdOH\({}^{+}\) formation. Cr\({}^{3+}\) speciation is more complex, with approximately 65% as Cr(OH)\({}_{2}\)\({}^{+}\), 25% as Cr(OH)\({}_{3}\), and 10% as other hydroxy complexes. Cu\({}^{2+}\) exists as a mixture of free Cu\({}^{2+}\) ions (-60%), CuOH\({}^{+}\) (-25%), and Cu(OH)\({}_{2}\) (-15%). Zn\({}^{2+}\) is present mostly as free Zn\({}^{2+}\) ions (-80%) with ZnOH\({}^{+}\) as the secondary species. This speciation profile explains the differential removal efficiencies observed, as the free divalent metal ions typically demonstrate stronger interactions with the oxygen-containing functional groups on biochar surfaces compared to their hydroxy complexes.