NaOH-Activated Tofu Waste Adsorbent for Pb(II) and Cu(II) Adsorption: Kinetic and Isotherm Studies

The study was focused on developing adsorbents from tofu waste (TW) for the adsorption of Pb(II) and Cu(II). FTIR analysis showed that NaOH activation of TW affected the transmittance of active sites. XRD analysis identified increasing crystallinity after NaOH treatment. SEM analysis demonstrated larger pores with higher NaOH concentrations, reducing adsorbent size. The adsorption capacity reached a maximum equilibrium of 99.86 mg/g and 87.81 mg/g for Pb(II) and Cu(II), respectively, using TW activated by 0.6 M NaOH. Both Pb(II) and Cu(II) adsorptions followed a linearized pseudo second-order kinetics model, with capacities of 100.00 mg/g and 87.719 mg/g, respectively. The Langmuir isotherm provided the best fit, yielding maximum overall adsorption capacities of 77.519 mg/g for Pb(II) and 91.743 mg/g for Cu(II) at the initial concentrations being 9.89–499.51 mg/L and 10.08–499.92 mg/L for Pb(II) and Cu(II), respectively. Brunauer–Emmett–Teller study results showed that the saturation capacities and total pore volumes were 50.505 mg/g and 49.500 L/mg, respectively, for Pb(II), and 84.388 mg/g and 237.000 L/mg, respectively, for Cu(II).