Efficient removal and long-term sequestration of cadmium from aqueous solution using ferrous sulfide nanoparticles:...

Cadmium (Cd) is one of the most commonly detected toxic heavy metals in the environment. Ferrous sulfide (FeS) nanoparticles were prepared using sodium carboxymethyl cellulose (CMC) as a stabilizer and tested for removal of Cd from aqueous solutions. Effects of CMC concentration, initial Cd concentration, pH, humic acid (HA) and dissolved oxygen were examined. Fully stabilized FeS (100 mg/L) nanoparticles were obtained using 0.01 wt% CMC. Batch kinetic tests showed that the nanoparticles at 100 mg/L as FeS rapidly removed 93% of 1 mg/L Cd within 4 h at pH 7.0, and the kinetic data were well interpreted by a pseudo-second-order rate model with a rate constant of 6.68 g·mg^-1·hr^-1. Sorption isotherm was well simulated by a dual-mode isotherm model with a maximum Langmuir sorption capacity of 497.5 mg/L at pH 7.0. Fourier transform infrared (FTIR) spectroscopy and X-ray powder diffraction (XRD) analyses suggested that chemical precipitation and surface complexation between Cd and FeS were dominant immobilization mechanisms. Increasing pH from 4.0 to 8.0 enhanced Cd removal rate from 73.0% to 98.8%, whereas addition of 3 mg/L HA (as total organic matter) inhibited the removal rate by 2.7% and the presence of molecular oxygen had negligible effect. Increasing NaCl or CaCl₂ from 0 to 10 mM suppressed Cd removal by 10.1% and 27.7%, respectively. The immobilized Cd remained insoluble when aged for 717 days under anoxic or oxic conditions. This study demonstrated that CMC-stabilized FeS nanoparticles can facilitate long-term immobilization of cadmium in contaminated water.