Sustainable remediation of nickel (II) in aqueous solutions using waste orange peel biomass as a bioadsorbent

A low-cost adsorbent for Ni(II) uptake was developed using orange peel waste after pretreatment with ethanol/water at 40 °C for 6 h. Various experimental factors such as pH, adsorbent dosage, and contact time were investigated, with optimal conditions determined to be pH 5, an orange peel dose of 2.5 g/L, and a contact time of 90 min. The characterization of the adsorbent was analyzed. The linear and nonlinear isotherm and kinetic models were investigated. The Langmuir model suggested chemisorption with a 19.42 mg/g capacity, following pseudo-second-order kinetics. The thermodynamic analysis revealed that the uptake was exothermic (ΔH° < 0), feasible, and spontaneous (ΔG° < 0). Regeneration studies (NaCl, NaOH, HCl, HNO₃), interference studies (Al³⁺, Cd²⁺, Ca²⁺, Na⁺, mixed solution), and application studies with real water samples (wastewater, drainage water, tap water, bottled water) were also conducted. A possible uptake mechanism was suggested. Utilizing ethanol/water-pretreated orange peels (EOP) addresses organic biomass waste disposal and offers a cost-effective, readily available solution for heavy metal removal.