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The removal of concentrated colour (around 5039 Pt–Co) and chemical oxygen demand (COD; around 4142 mg L−1) from matured landfill leachate through a novel combination of humic acid extraction and coagulation with natural oil palm trunk starch (OPTS) was investigated in this study. Central composite design from response surface methodology of Design Expert-10 software executed the experimental design to correlate experimental factors with desired responses. Analysis of variance developed the quadratic model for four factors (e.g. coagulant dosage, slow mixing speed and time and centrifugation duration) and two responses (% removal of colour, COD). The model confirmed the highest colour (84.96%) and COD (48.84%) removal with a desirability function of 0.836 at the optimum condition of 1.68 g L−1 coagulant dose, 19.11 rpm slow mixing speed, 16.43 minutes for mixing time and 35.75 minutes for centrifugation duration. Better results of correlation coefficient ( R2 = 0.98 and 0.96) and predicted R2 (0.94 and 0.84) indicates the model significance. Electron microscopic images display the amalgamation of flocs through bridging. Fourier transforms infrared spectra confirmed the existence of selected organic groups in OPTS, which eventually signifies the applied method.

Refractory humic substances (HSs), which include humic and fulvic acid as well as hydrophilic portion, are the prime pollutants of stabilized landfill leachate with a concentrated color and chemical oxygen demand (COD). Spectroscopic and microscopic analysis of humic acid remaining in stabilized leachate as a pollutant contributor were conducted in this study. Microfiltration and centrifugation processes were applied to fractionate the humic acid from the HSs of stabilized leachate. The three-stage isolation process recovered a maximum of 1412 ± 2.5 mg/L (Pulau Burung leachate), 1510 ± 1.5 mg/L (Alor Pongsu leachate) at pH 1.5 and 1371 ± 2.5 mg/L (PBLS), and 1451 ± 1.5 mg/L (APLS) of humic acid (about 42% of the total COD concentration) at pH 2.5, which eventually indicates the efficiency of the process. The spectroscopic analysis of isolated humic acid through scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR) significantly indicates the existence of identical elements in the recovered humic acid. The subsequent reduction (around 37%, 36%, and 39%) in ultra-violet absorbance values (UV254, UV280), COD, and color in the humic acid isolated leachate indicates the acid’s significant contribution as a toxic pollutant through aromaticity and conjugated double-bond compounds.

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