Insights into cadmium adsorption characteristics and mechanisms by new granular alginate hydrogels reinforced with biochar: Important role of cation exchange

Green biochar-alginate composite (BAB) was synthesized by integrating biochar derived from peanut shells (PS700) with Na-alginate using a Ca²⁺ cross-linker. A hydrogel bead (SAB) was prepared using sodium alginate without biochar. The materials were characterized by the SEM–EDS, XRD, FTIR, XPS, TGA–DTG, and BET-analyzer techniques. They were applied as adsorbents to adsorb Cd²⁺ ions from aqueous solutions in batch adsorption experiments. XRD data confirmed that BAB exhibited a crystalline structure, with 2θ peaks corresponding to PS700 (∼24° and 42°) and SAB (∼13°). SEM images suggested PS700 was successfully encapsulated within alginate beads. BAB microsphere had a large specific surface area (353.2 m²/g) and dominant micropore (1.86 nm). Adsorption results indicated the Langmuir maximum adsorption capacities of BAB, SAB, and PS700 were 230.0, 125.6, and 92.3 mg/g. Cation exchange was a primary Cd adsorption mechanism by BAB that was determined by a qualitative study (%Ca decreasing from 2.43% to 1.27% after adsorption) and a quantitative experiment (cation exchange ratio between Ca exchange and Cd adsorption: q_exchanged/q_e = 0.7). The XPS spectrum for C 1s (binding energy peak at ∼282 eV) and Ca 2p (∼348 eV) highlighted that Ca in the Ca–C bond of BAB was remarkably replaced by Cd ions in solution to form the Cd–C bond in the Cd-laden BAB. The O 1s spectrum (peak at ∼532 eV) demonstrated the presence of surface complexation and electron-sharing phenomenon between the ionized carboxyl groups in BAB and Cd. Cd–π interaction was observed at 405.96 eV in the Cd 3d spectrum. Pore filling and van der Waals force played a minor role in adsorption mechanisms. The calculated costs (10⁻³ US$/g) for adsorbing 1 g of cadmium per the maximum adsorption capacity of PS700, SAB, and BAB were 2.69, 18.79–129.8, and 5.72–36.4, respectively. The innovative biochar-reinforced hydrogel beads can be an effective and green material for water treatment.