The Ho (pseudo-second-order) kinetics model represented the effect of contact time (kinetics) was represented by the Ho kinetics model. The effect of initial Pb(II) concentration (isotherm) shows that the data fitted well with the Langmuir-b isotherm model indicated the monolayer adsorption of Pb(II) onto homogenous surfaces of the HA-UF with the adsorption capacity of 2.26 × 10–4 mol/g (which is higher than its original HA of 1.12 × 10–4 mol/g). The higher ionic strength will affect decreasing adsorbed Pb(II) at the optimum pH of 5.5. Adsorption behavior of Pb(II) onto HA-UF was influenced by the ionic strength and pH, which were mainly driven by the ion exchange mechanism (EDR = 9.75 kJ/mol). The high stability of HA-UF was shown by 96.8% remaining in solid form at pH 12.4. The success of HA-UF formation was characterized by attenuated total reflection-infrared (ATR-IR), energy dispersive X-Ray (EDX), and X-ray diffraction (XRD). The reaction of the conducted the formation of HA-UF –COOH group of HA with the –NH2 group of UF was evidenced by decreasing total acidity from 549.26 cmol/kg (in HA) to 349.30 cmol/kg (in HA-UF).
In this study, a new adsorbent of humic acid-urea formaldehyde (HA-UF) was synthesized. However, the literature has never written HA modification by UF to improve the adsorbent’s performance. Humic acid (HA) and urea-formaldehyde (UF) have been frequently reported as heavy metal adsorbents.
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