## 2. Adsorptive Removal of Contaminants
### Nutrients (Phosphorus and Nitrogen)  
SDB has gained attention for phosphorus and nitrogen removal from wastewater effluents, particularly as a pre-discharge treatment strategy to mitigate nutrient enrichment in receiving water bodies. Its inherent metal constituents (e.g., Fe, Al, and Ca) provide reactive sites that enable phosphate sequestration through ligand exchange and precipitation processes. Higher mesopore volume and exposed mineral phases could effectively bind phosphate. For instance, SDB pyrolyzed at 600 \({}^{\circ}\)C achieved \(\sim\)87% phosphate removal efficiency from municipal wastewater, while lower retention for SDB occurred at 400-500 \({}^{\circ}\)C [27], primarily due to reduced mesopore volume and limited mineral phase exposure. This underscores the pivotal influence of pyrolysis temperature in optimizing adsorption properties. Enhancing SDB with metal-based additives can significantly improve its phosphate adsorption capacity. For example, SDB modified with MgO exhibited high phosphate uptake over a broad pH range, primarily due to the Mg-P mineral complex formation [11]. Impregnating sludge biochar with lanthanum represents another effective approach, as La-modified biochar efficiently removes trace phosphate from water through the formation of stable La-PO\({}_{4}\) complexes [28]. Beyond phosphorus, SDB can also aid nitrogen removal as an adsorbent via cation exchange and surface complexation, with reported capacities ranging from a few mg/g up to tens of mg/g depending on production conditions [29]. One key factor is pyrolysis temperature. SDB produced at \(\sim\)500 \({}^{\circ}\)C was shown to adsorb more NH\({}_{4}\)+ than that produced at 700 \({}^{\circ}\)C, due to better retention of hydrogen-containing functional groups that serve as cation-exchange sites [30]. Various post-processing modifications (e.g., acid/alkali activation or metal impregnation) are another strategy to greatly enhance NH\({}_{4}\)+ uptake. For instance, introducing carboxylate and sodium functional groups raised the capacity from 5 mg/g (unmodified) to 17 mg/g [31]. [PERSON] et al. [32] synthesized an Fe-enriched magnetic SDB (via FeCl\({}_{3}\) infusion and re-pyrolysis at 300-600 \({}^{\circ}\)C) that exhibited the highest NH\({}_{4}\)+ sorption capacity at 400 \({}^{\circ}\)C, with 10.7% greater than the unmodified SDB. The NH\({}_{4}\)+ removal mechanism for such Fe-modified SDB was dominated by ion exchange and electrostatic attraction. In some cases, in situ doping during sludge processing yields excellent results. [PERSON] et al. [33] intensified sludge anaerobic digestion with scrap iron additions, producing an iron-rich biochar at 600 \({}^{\circ}\)C that could remove over 91% of ammonium from the fermentation liquor. Adsorption is also maximized at near-neutral pH, since, under highly alkaline conditions, NH\({}_{4}\)+ is converted to volatile NH\({}_{3}\) (reducing its availability for adsorption) [34]. Furthermore, pre-oxidizing the sludge feedstock can dramatically improve performance. Fenton pre-oxidation of sludge prior to pyrolysis at 400 \({}^{\circ}\)C produced a biochar with exceptionally high NH\({}_{4}\)+ sorption (approximately 222 mg/g) due to increased porosity and surface charge sites [35]. Overall, recent studies confirmed that, while baseline SDB has moderate affinity for NH\({}_{4}\)+, targeted modifications (chemical activation, metal doping, or mineral amendments) can greatly improve its ammonium removal efficiency and capacity. Such improvements are crucial for developing SDB-based technologies to recover nitrogen from waste streams in a circular economy context.