Enhanced Pretreatment of Phenols in Coking Wastewater by Fly Ash Adsorption: Performance and Mechanism

The effective removal of phenols from coking wastewater) in the pretreatment process is crucial for reducing the burden on biological unit and enhancing biodegradability. Although adsorption strategy has attracted widespread attention, the high-efficiency and low-cost adsorbents must be developed and proposed in order for the strategy to be competitive. Herein, powdery activated carbon (PAC) and various industrial residues [coal gangue (CG), granulated blast furnace slag (GBFS), and fly ash (FA)] were selected as adsorbents, and results showed that chemical oxygen demand (COD) removal efficiencies of PAC, CG, GBFS, and FA were 47.9%, 30.84%, 27.83%, and 44.15%, respectively. In comparison, FA and PAC had high adsorption capacity for high concentrations of organics, and FA exhibited higher adsorption efficiency for total phenols (52.53%). Moreover, the biodegradability of the wastewater was significantly improved (B/C ratio: 0.35), and the acute toxicity was greatly reduced to 8.3 after FA adsorption. The pseudo-second-order kinetic model and the Sips model could better describe the adsorption process of FA for COD removal. The adsorption mechanism of organics involved hydrogen bonding adsorption and surface complexation. During this adsorption process, FA exhibited a significant effect on the adsorption of organic substances, especially phenols, and the humification degree of wastewater was considerably diminished. Finally, the cost of removing COD per kilogram of FA is only 0.6 ¥. This work presents new idea and strategy for resource recycling and environmental protection, with the aim of providing theoretical support for the effective utilization of FA.