Applicability of a novel and highly effective adsorbent derived from industrial palm oil mill sludge for copper sequestration: Central composite design optimisation and adsorption performance evaluation

Abstract

Palm oil mill sludge (POMS) is a significant industrial waste generated in Malaysian palm oil mills. The re-utilisation of POMS as a value-added adsorbent for copper sequestration is highly desirable as a means for solid waste reduction. Therefore, this research aimed to evaluate the adsorption performance of POMS towards copper pollutant comprehensively. Adsorption attributes of the POMS-copper system were investigated using batch experiments, followed by statistical modelling to establish the simultaneous interactive effects of parameters and to optimise the system. POMS was determined to exhibit 9.72 point of zero charge, with a rough surface containing pore sizes within 1–7 µm and functional groups such as hydroxyl interacted with the copper. The statistically optimised adsorption capacity (15.84 mg g−1) was attained at 0.3 g POMS dosage, pH 4.56, 200 mg L−1 concentration and 60 min. The regeneration ability of POMS was satisfactory up to 5 adsorption-desorption cycles. Equilibrium and kinetic adsorption data were modelled based on the sum of normalised error (SNE) approach. The adsorption equilibrium was well represented by the Langmuir model (SNE: 3.69), with 16.56 mg g−1 Langmuir maximum adsorption capacity at 30 °C. Adsorption kinetics studies revealed that the pseudo-second-order kinetic (SNE: 2.98) was the most appropriate model, with copper ions chemisorbed onto POMS surface via multiple mechanisms such as boundary layer and intraparticle diffusion. The new POMS-copper system was thermodynamically exothermic (ΔH°: −12.72 to −4.25 kJ mol−1) and spontaneous (ΔG°: −19.07 to −11.44 kJ mol−1). Conclusively, POMS was an effective, low cost and eco-friendly adsorbent for the removal of toxic copper from aqueous environment.