BACKGROUNDBio-treated coking wastewater is difficult to treat further, mainly due to the bio-refractory organic content. Coal gangue is the largest solid waste in China and it contains large amounts of aluminum, iron and silicon. This study attempts to recover the waste in coal gangue as a novel poly-silicic metal coagulant for bio-treated coking wastewater treatment. RESULTSThe poly-silicic metal coagulant was synthesized from coal gangue under different conditions. Fourier transform infrared spectroscopy, X-ray diffraction and scanning electron microscopy confirmed that both amorphous and crystalline phases were present in the samples and new chemical bonds such as Si-O-Al and Si-O-Fe were formed. The results show that the composite coagulant was effective for organic removal from the bio-treated coking wastewater and the optimal Al/Fe/Si molar ratios were Al/Fe = 3 and (Al + Fe)/Si = 13. The removal of organics exhibits a parabolic curve as the coagulant dosage increases. However, the optimal dosage (125 mg L-1) for organic removal was not equal to the isoelectric point (100 mg L-1). Furthermore, SEM images showed that the flocs start to disintegrate at a dosage of 150 mg L-1, leading to deteriorating sludge settling. CONCLUSIONSIt is feasible to fabricate poly-silicic metal coagulants for coking wastewater treatment by using coal gangue as the raw material. The removal of the bio-refractory organics by the composite coagulant was attributed to the combined effects of charge neutralization, bridge adsorption and the net capturing. (c) 2017 Society of Chemical Industry