Characterization and performance evaluation of lignin-based bioflocculants for organic micropollutants in adsorptive coagulation-flocculation process

Abstract

The emergence of organic micropollutants in surface water, sewage influent, and sewage effluent has raised public concern. It might lead to the dispersion of antibiotic resistance, which has a massive impact on human health and economic consequences globally. Besides, the increasing demand for environmentally friendly technology in the wastewater treatment process has recently gained considerable attention, especially, towards the application of natural-based coagulants/flocculants. The enormous amount of oil palm empty fruit bunches (OPEFB) produced throughout the years gives the potential for this biomass to be used as a source in the synthesis of lignin-based flocculant. Thus, this study focused on the extraction of lignin from OPEFB using ionic liquid dissolution-alkaline treatment technique and modification of extracted lignin into quaternized lignin (QL). The extracted lignin was characterized using Fourier transform infrared spectroscopy, Gel permeation chromatography and thermogravimetric analysis. The lignin-based flocculant was synthesized using quaternization method with 3-chloro-2-hydroxylpropyl trimethylammonium chloride (CHPTAC). The lignin-based flocculant was characterized using Fourier transform infrared spectroscopy, hydrogen nuclear magnetic resonance spectroscopy and CHNS elemental analysis. The performance of QL in removing turbidity of the kaolin suspension solution was evaluated using a jar test method at various experimental conditions such as the molar ratio of CHPTAC to phenylpropanoid unit of lignin, flocculant dosages, pH values, kaolin concentrations, slow mixing rates and sedimentation times. It was found that the QL10 achieved the highest turbidity removal efficiency (?t) 99.46 ± 0.28 (%) using 5 mg L-1 of flocculant dosage, pH 7, 1000 mg L-1 of kaolin concentration and required 20 min of sedimentation time at all the slow mixing rates tested. Response surface methodology was utilized to find the optimum combination of flocculant dosage, pH of kaolin suspension and kaolin concentration concerning the highest ?t and the lowest sludge volume index (SVI). Results revealed that the optimal conditions were 10 mg L-1 dosage, pH 7 and 1420.45 mg L-1 kaolin concentration to obtain 99.09 % and 5.42 mL g-1 of ?t and SVI respectively. The applicability of lignin-based flocculant in the adsorptive coagulation-flocculation process was evaluated using a jar test to remove oxytetracycline (OTC) as micropollutant from simulated wastewater. Natural zeolite was added during the adsorptive coagulation-flocculation process to improve the removal of OTC. It was found that the removal efficiency of OTC increased up to 85.04 % at pH 9, OTC concentration of 100 µM and 2000 mg L-1 of zeolite dosage. The kinetic of OTC removal was studied at various zeolite dosages and obeyed a pseudo-second order model. The dewatering study showed that the coagulation-flocculation process with 10 mg L-1 QL10 resulted in 0.86 x 1012 m kg-1 of specific resistance to filtration and 92 s of time to filter. In conclusion, lignin-based flocculant was successfully synthesized in this study and OPEFB lignin has shown an ability to be turned into a flocculant for wastewater treatment as well as overcoming the problems of oil palm plantation waste disposal.