Ultimate removal of refractory organics in coking wastewater by integrated sustainable oxidation and adsorption processes

Traditional Fenton oxidation shows low efficiency in treating coking wastewater. A high dosage of iron reagents is commonly required while the improvement in efficiency remains limited with significant iron sludge production. Here, we designed and demonstrated an integrated sustainable hydroxyl radical (•OH) oxidation and hydrolyzed iron adsorption process. Hydroxylamine was introduced to effectively accelerate the Fe3+ reduction, enabling sustained generation of •OH and thus enhancing oxidation of refractory organics. Crucially, the mean oxidation number of carbon changed from -0.8 to +1.2, revealing that sustainable •OH oxidation transforms hydrophobic organics to hydroxylated and carboxylated intermediates (such as maleic and tartaric acids). X-ray photoelectron spectroscopy, Fourier-transform infrared analyses confirmed that the oxygen-containing groups from sustainable depth oxidation improved the interaction between modified organic pollutants and hydrolyzed iron flocs. As a result, the adsorption efficiency of hydrolyzed iron for dissolved organic matter is significantly enhanced, increasing from 10.2 % to 42.4 %. The integrated sustainable oxidation and adsorption processes decrease hard chemical oxygen demand (COD) from 106 to 11 mg/L, achieving 88.6 % removal with low iron sludge yield of 0.06 kg/m3. This work provides an efficient strategy towards solving the major challenges of hard COD removal in wastewater.