Waste to treasure: porous manganese oxides derived from the waste liquid for heavy metal ion adsorption

Designing inexpensive and efficient absorbent materials derived from waste is still challenging but of great significance to environmental safety and resource protection. Herein, the modified Hummers’ waste liquid was used as a precursor reactant to synthesize the three types of manganese oxides via a chemical precipitation method. The components, microstructure, and adsorption capacities of the manganese oxides for heavy metal ions were investigated in detail. The results show the manganese oxides have porous structures and different crystal phases as manganese dioxide (I-MO), manganese oxide hydroxide (II-MO), and trimanganese tetraoxide (III-MO), respectively. Compared with II-MO and III-MO, I-MO has a specific surface area and pore volume of 142.27 m²·g⁻¹ and 0.622 cm³·g⁻¹, respectively. The experiments reveal that Ι-MO exhibits better adsorption performance of heavy metal ions than II-MO and III-MO. At 298 K, the maximum adsorption amounts of Pb²⁺, Cd²⁺, and Cu²⁺ on the Ι-MO are 304.15, 175.37, and 74.48 mg·g⁻¹, respectively. The experimental findings closely match both the pseudo-second-order model and the Langmuir model. Moreover, I-MO exhibits a satisfactory adsorption capacity for heavy metal ions even after six repetitive cycles. All of these show that I-MO is a cost-effective adsorbent for heavy metal ions elimination in water.