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.