Utilization of Riau Peat-Derived Humic Acid Modified with Urea-Formaldehyde for Ni(II) Adsorption: A Geochemical Approach

Abstract

Nickel contamination in aquatic systems presents serious environmental and geochemical concerns due to its mobility, persistence, and toxicity. This study proposes a geochemically driven approach to develop a nitrogen enriched humic acid based adsorbent modified with urea formaldehyde (HA-UF) to enhance the removal of Ni(II) from aqueous solutions. Humic acid was extracted using an alkaline method and subsequently crosslinked with urea-formaldehyde through a one-step reflux process. The novelty of this study lies in the use of urea-formaldehyde (UF) as a stabilizing agent and in evaluating the effect of ionic strength on Ni(II) adsorption. The characteristic peaks of HA-UF confirmed the formation of C–O–C and C–N functional groups at 1035, 1136, and 1248 cm⁻¹ (ATR-IR), semi-crystalline features at 2θ = 22° and 25° (XRD), and a substantial increase in nitrogen content from 6.85% to 23.46% (EDX). The HA-UF demonstrated enhanced stability (up to pH 12) and exhibited active functional groups (–COOH, –OH, –NH) that are conducive to binding Ni(II), while maintaining improved structural integrity and reduced total acidity.  The adsorption process followed the Langmuir isotherm model, with the adsorption capacity increasing from 1.899 × 10⁻⁴ mol/g for HA to 3.682 × 10⁻⁴ mol/g for HA-UF, indicating a greater availability of active sites after modification. Kinetic analysis showed that the process followed a pseudo second order model, with a rate constant of 2.328 × 10⁻² g/mol·min, suggesting that chemisorption occurred through surface complexation and ion-exchange mechanisms. The calculated adsorption energies (~20–22 kJ mol⁻¹) indicate intermediate-strength interactions dominated by coordination bonding rather than simple physisorption. Increasing ionic strength (NaCl) reduced both the adsorption capacity and rate, likely due to cation competition and compression of the electrical double layer. Overall, HA-UF exhibits reactivity comparable to natural organic matter while providing enhanced structural stability and adsorption performance, highlighting its potential application in wastewater treatment systems under variable pH and ionic strength conditions.