Cu(II) Adsorption Behavior and Mechanistic Insights for Rice Husk Biochar-Modified Blast-Furnace Slag–Carbide Slag Geopolymer Adsorbents

Developing efficient, low-resource adsorbents from solid wastes is critical for heavy-metal wastewater treatment. Here, solid-waste-based geopolymer adsorbents were prepared from ground granulated blast-furnace slag (GGBFS) and carbide slag (CS) at a CS:GGBFS mass ratio of 15:85, with 0–30 wt% rice husk biochar (BC) as a functional modifier. The effects of pH, contact time, adsorbent dosage, and initial Cu2+ concentration on adsorption were evaluated. The BC-2% adsorbent achieved a maximum adsorption capacity of 208.85 mg/g at pH 4, an adsorbent dosage of 1 g/L, and an initial Cu2+ concentration of 300 mg/L, 48.65% higher than the BC-free control; equilibrium was reached within approximately 80 min. Low BC contents (≤5 wt%) improved adsorption capacity and initial adsorption rate, whereas excessive BC (≥10 wt%) reduced performance. Kinetic and isotherm analyses revealed that Cu2+ uptake followed pseudo-first-order behavior and was better described by the Langmuir and Dubinin–Radushkevich models, indicating a physically dominated process controlled by finite surface sites, pore filling, and multiple mass-transfer steps. Brunauer–Emmett–Teller (BET) and Fourier transform infrared spectroscopy (FTIR) analyses showed that low BC addition increased the specific surface area and enriched hydroxyl and amino groups, thereby improving site accessibility and surface affinity, whereas high BC contents weakened these structural and interfacial advantages. This study offers a practical route for converting industrial and agricultural wastes into low-cost Cu2+ adsorbents.