Coating zirconium oxide-nanocomposite with humic acid for recovery of mercury and chromium in hazardous waste of chemical oxygen demand test

•The humic acid coated nanocomposite HA-HZO-201 shows remarkable adsorption capacity for Cr3+ and Hg2+.•HA-HZO-201 treatment reduces Cr3+ from ∼ 270 to 0.28 mg/L and Hg2+ from ∼1600 to 0.02 mg/L in HWCOD .•The Cr3+ and Hg2+ in HA-HZO-201 are separately desorbed and recycled in ionic form.•HA-HZO-201 is reusable with high removal efficiency after 5 cycles.