Optimised Silver-Doped Banana Peel Biosorbent for Marble Effluent: Adsorption Properties and Response Surface Methodology

Banana peels, a widely available agro-waste, are rich in lignocellulosic matrices and oxygenated functional groups that enable effective adsorption of metal ions and dyes. Marble industry effluents, containing high suspended solids, turbidity, and dissolved metals, present environmental hazards. This study aimed to synthesise silver-doped banana peel (AgNP-BP) composites and optimise their application for heavy metal removal from marble wastewater. Dried banana peels were pulverised, extracted, and used to synthesise AgNPs via green reduction with 1 mM AgNO₃. The biosorbent was doped with AgNPs, oven-dried, and applied in a laboratory-scale sand filtration system. Effluent filtration was conducted under varying contact times (15–120 min), pH (6.5–9.5), and initial concentrations (25–100 mg/L). Physicochemical parameters and heavy metals (Zn, Mn, Cd, Pb, Ca, Fe) were analysed. Removal efficiency (RE) and adsorption capacity (qₑ) were calculated, and Response Surface Methodology (RSM) optimised process variables. Results showed TDS ranged 198–278 mg/L, EC 430–590 µS/cm, DO 11–11.6 mg/L, BOD 8–15.8 mg/L, and COD 150–245 mg/L after treatment. Zn and Pd were reduced to 1.0–1.95 mg/L and 0.03–0.05 mg/L, while Mn and Cd slightly increased to 0.05–0.125 mg/L and 0.012–0.14 mg/L. Optimal removal occurred at pH 7.5–8.5, contact time 60–120 min, and initial concentrations 25–75 mg/L. Regression models accurately predicted responses, and response surface plots confirmed the significance of linear, quadratic, and interaction terms. In conclusion, AgNP-BP composites effectively reduced Zn and Pd in marble effluents and can be recommended for sustainable, low-cost wastewater treatment, particularly under controlled pH, contact time, and concentration conditions.