## 2. Adsorptive Removal of Contaminants  
### Heavy Metals  
Heavy metal (HM), e.g., cadmium, mercury, and chromium, poisoning in wastewater is a major contributor to global health and environmental problems [13]. Adsorption is considered a highly effective and versatile approach for removing both inorganic and organic contaminants from various water sources, including natural water bodies, drinking water, sewage, and industrial effluents. The advantages involve cost efficiency, ease of use, global availability of adsorbents, design flexibility, and environmental sustainability [14]. For heavy metal removal, SDB provides a cost-effective and environmentally friendly solution. Its porous structure has a large surface area for adsorption, and its surface functional groups, including carboxyl, phenolic, and hydroxyl groups, can form chelates with metal ions. The adsorption mechanisms include physical adsorption (metal trapped within the pores) and chemical adsorption (metal ions form complexes with the functional groups of the biochar) (Figure 1). Recent studies have shown that SDB is highly efficient in removing heavy metals, with removal rates typically exceeding 90% under optimal conditions [15; 16]. SDB inherently contains metal oxide minerals and polar groups that confer strong metal sorption. The incorporation of inorganic modifiers, e.g., metal oxides, into SDB can substantially boost its HMs adsorption performance, primarily through improvements in the abundance of reactive functional groups, and introduce additional active binding sites. [17]. For instance, CaO-modified SDB achieved 99.74% Cd\({}^{2+}\) removal, \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Pollutant/Application** &
\begin{tabular}{c} **Sludge-Derived** \\ **Biochar (SDB)** \\ \end{tabular} & **Other Biochar** & **References** \\ \hline Heavy metals & \(\sim\)83\% removal of Pb\({}^{2+}\) at ug/L & \(\sim\)0\% removal in same low-concentration test & \\ (e.g., Pb\({}^{2+}\)) & levels, even at pH 2, due to & (no capacity at pH 2) with wood biochar; heavy & [7; 8] \\  & metal oxide sites & metal sorption capacity ranks SDB \(>\) & \\  & \(\sim\)256 mg/g using & agricultural waste biochar \(>\) wood biochar & \\ Dyes & KOH-activated SDB from & Typical inactivated sludge–rice husk biochar & [9] \\ (e.g., anionic dye) & dyeing sludge & achieves 22–60 mg/g for various dyes & [9] \\  & (high mineral content) & & \\ \multirow{3}{*}{Antibiotics} & SDB modified by NaOH shows & \(\sim\)7–12 mg/g (raw at 400–600 \({}^{\circ}\)C) using corn & \\  & tetracycline antibiotic uptake & straw biochar, improved to \(\sim\)31 mg/g after & [4; 10] \\  & with \(\sim\)379.8 mg/g & Ca(OH)\({}_{2}\) modification & \\  & & & \(<\)1 mg/g P uptake using unmodified & \\  & MgO-loaded SDB achieved & pine/wood biochar: (e.g., \(<\)0.7 mg/g at 50 & \\ Nutrients & maximum phosphate capture & mg/L initial P); activation with CO\({}_{2}\) at 800 \({}^{\circ}\)C & [11; 12] \\  & with \(\sim\)97.45 mg/g & raises wood-based biochar capacity only to & \\  & & \(\sim\)1.2 mg/g in the P removal & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparative performance of sludge-derived biochar (SDB) with biochar derived from other substrates in some typical contaminants in wastewater applications. approximately three times higher than raw SDB under identical conditions. However, while surface activation increases adsorption capacity, it may also reduce material stability over repeated use cycles. Even without modification, SDB can effectively sequester metals such as Cu, Pb, Cr, and Cd through surface complexation and precipitation. Notably, iron-enriched SDB demonstrates exceptional Cr(VI) removal by facilitating its reduction to Cr(III) followed by adsorption [18]. Another effective strategy involves phosphate-based modification, where SDB treated with ammonium polyphosphate exhibits improved Cd adsorption due to the introduction of phosphate functional groups [19].