## Materials and Methods
### Physicochemical Characterization of Biochar  
The textural properties of the biochar samples obtained at different pyrolysis temperatures were determined from N\({}_{2}\) adsorption-desorption isotherms at 77 K with a 3 Flex Micromeritic Equipment (see Figures S1-S3 in the Supplementary Material). Prior to analysis, the sample were degassed for 4 h at 573 K using a SmartVacPrep device (Micromerities). The Brunauer-Emmett-Teller (BET) method [21] was used to calculate the specific surface area (S\({}_{\mathrm{BET}}\)), whereas the pore size distribution was determined by the desorption branch using the Barrett-Joyner-Halenda (BJH) method [22]. The total pore volume was determined from the adsorbed quantity at a p/p\({}^{0}\) of 0.99. To study the surface morphology, a sputter-coated gold film was used to increase the signal and resolution of the biochar samples. The surface morphology was examined by scanning electron microscopy (SEM) using a Tabletop Microscope TM3000 (Hitachi, Japan).  
The carbon (C), hydrogen (H), and nitrogen (N) contents of the biochar samples were analyzed before and after the metal adsorption assays using an elemental analyzer (EURO EA 3000 CHNS-O, Eurovector, Italy). With the H and C contents, the H:C molar ratio was calculated. The ash and organic matter contents were obtained following the ASTM D1762-84 and TMECC 05.07-A methods, respectively. Fourier transform infrared spectroscopy (FTIR) was used to characterize the surface functional groups of the raw seaweed biomass and the seaweed biochars. The FTIR measurement was performed by an infrared spectrophotometer (ALPHA FT-IR-ATR model, Bruker, USA), recording the spectral range of 400-4000 cm\({}^{-1}\).  
The cation exchange capacity (CEC) was determined by saturation of the samples with a cation solution (Ca\({}^{2+}\), Mg\({}^{2+}\), Na\({}^{+}\), and K\({}^{+}\)) according to the modified AOAC method [23]. We mixed 5 g of biochar with 50 mL of 1 mol L\({}^{-1}\) CH\({}_{3}\)COONH\({}_{4}\) at pH = 7. The solution was stirred for 30 min at 180 rpm and 25 \({}^{\circ}\)C in a reciprocal shaker. Then, 1 mL of the filtered suspension was mixed with 9 mL of Lanthanum (La) 10 g L\({}^{-1}\) (as a suppressing agent). The final sample was measured by FAAS for Ca\({}^{2+}\) and Mg\({}^{2+}\), and by atomic emission for Na\({}^{+}\) and K\({}^{+}\) with an AA-7000 spectrophotometer (Shimadzu, Japan). The electric conductivity (EC) (Orion Star A212) and pH (pH/ORP meter, Hanna Instruments) of the BCs were measured in 1:20 dilute suspensions of biochar sample/DI water, with an equilibrium time of 1.5 h, according to the Standardized Product Definition and Product Testing Guidelines for Biochar that is Used in Soil (IBI-STD-2.1, 2015; [[http://www.biochar-international.org/characterizationstandard](http://www.biochar-international.org/characterizationstandard)]([http://www.biochar-international.org/characterizationstandard](http://www.biochar-international.org/characterizationstandard)), accessed on February 2021). Finally, the point of zero charge (pH\({}_{\text{pzc}}\)), at which the number of negative sites is equivalent to the number of positive sites (total external and internal charge of the particles is zero), was obtained according to the process described in [24].