BACKGROUNDMagnetite is widely used as a green and cheap mineral material to adsorb heavy metals in water samples, but it has the disadvantages of low adsorption capacity, poor selectivity and easy reunion. Surface modification of magnetite could overcome these problems and improve its adsorption properties.

OBJECTIVESTo enhance the adsorption properties of magnetite by using a humic acid modifier.

METHODSThe morphology and structure of magnetite and humic acid-modified magnetite were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy and X-ray photoelectron spectroscopy. The effect of conditions such as optimal pH and adsorption time on Pb²⁺ and Cd²⁺ adsorption rate were investigated by static equilibrium experiment.

RESULTSCarboxyl and hydroxyl groups in humic acid were successfully absorbed onto the surface of magnetite. At room temperature, the initial pH of the solution had little effect on the adsorption rate of Pb²⁺, but had a greater influence on Cd²⁺. When pH=7, the adsorption rates of Pb²⁺ and Cd²⁺ reached 95%. The optimal adsorption equilibrium time of Pb²⁺ and Cd²⁺ with initial mass concentration of 10mg/L was 360 minutes, and the adsorption process accords with the quasi-second-order kinetic equation. The order of competitive adsorption was Pb²⁺ followed by Cd²⁺. The Langmuir isothermal adsorption model yielded maximum adsorption capacity of Pb²⁺ and Cd²⁺ of 39.27mg/g and 28.95mg/g, respectively.

CONCLUSIONSThe adsorption capacity of humic acid-modified magnetite was higher than that of magnetite, indicating that the ability of humic acid-modified magnetite was enhanced to adsorb Pb²⁺ and Cd²⁺ in water samples.