Integrated treatment of tannery wastewater by coagulation-flocculation and ultrasound-assisted photo-Fenton-like heterogeneous process using a valorized sludge-based catalyst: optimization of operational performance and toxicity assessment.

In this work, a new approach for treating tannery wastewaters (TWWs) was applied, beginning with a coagulation-flocculation pretreatment. Under the optimal operating conditions obtained (5 g L-1 of ferric chloride at pH 7), this process was able to completely remove both turbidity and color present in the effluent, achieving a 73% of chemical organic oxygen (COD) removal. The sludge generated during the coagulation-flocculation (CF) process was valorized and characterized by XRD, FTIR, and SEM-EDX mapping and subsequently applied as a catalyst in an ultrasound-assisted photo-Fenton-like heterogeneous process (SPF). For this purpose, an experimental design based on Box-Behnken planning was employed to optimize the process operating parameters. Under the optimal conditions obtained (initial concentration of H2O2 of 2284 mg L-1, catalyst dosage of 2050 mg L-1 at pH = 3), complete CN removal and a 91% of COD reduction were achieved. Furthermore, phytotoxicity assessments based on germination percentage, germination rate index, and seedling vigor index showed a significant decrease in toxicity, with germination rates of 94% for both wheat and M. sativa seeds. With a total treatment cost of 4.8 $USD m-3 of treated TWW, the sequential treatment with CF followed by a heterogeneous SPF (CF-SPF) using a valorized sludge rich in Fe3+ as a catalyst, proved to be an efficient and viable alternative for the sustainable management of tannery wastewater.