Acid modification of waterworks sludge for dye removals: synthetic and real textile wastewater

This study aims to develop a low-cost granular adsorbent from waterworks sludge for dye removal. The sludge was prepared as non-activated and chemically activated adsorbents using 0.5, 1, 3, and 6 M sulfuric acid (H2SO4). The adsorption capacities of all prepared adsorbents were determined using methylene blue (MB) synthetic solution. Compared to the non-activated sample, the point of zero charge (PZC) of the waterworks sludge decreased after the activation, while the opposite was true for changes in the cation exchange capacity (CEC). Additionally, the acid modifcation helped enhance the surface area and MB adsorption capacity of the acid-activated samples by 1.18–2.44 and 1.40–4.17 times, respectively. The highest maximum MB adsorption capacity in this study was estimated at 10.858 mg/g, for 6 M H2SO4 activated sludge. The non-activated sludge adsorbent had the capacity to be applied for up to 38.3% decolorization of real textile wastewater. The sludge could be regenerated by a simple heating technique, upon which no significant changes on its major surface functional groups were observed. Besides, the MB adsorption capacity of the regenerated sludge was still comparable to that of the original non-activated waterworks sludge even though the breakthrough time of the column was shorter.