Reparation of nano-FeS by ultrasonic precipitation for treatment of acidic chromium-containing wastewater

Abstract

Nano-FeS is prone to agglomeration in the treatment of chromium-containing wastewater, and ultrasonic precipitation was used to synthesize nano-FeS to increase its dispersion. The optimization of the preparation method was carried out by single factor method (reaction temperature, Fe/S molar ratio and FeSO₄ dropping flow rate) and response surface methodology. Dynamic experiments were constructed to investigate the long-term remediation effect and water column changes of nano-FeS and its solid particles. The changes of the remediation materials before and after the reaction were observed by SEM, and the mechanism of the remediation of chromium-containing wastewater by nano-FeS prepared by ultrasonication was revealed by XRD. The results showed that the reaction temperature of 12 °C, Fe/S molar ratio of 3.5 and FeSO₄ dropping flow rate of 0.5 mL/s were the best parameters for the preparation of nano-FeS. The nano-FeS has efficient dispersion and well-defined mesoporous structure in the form of needles and whiskers of 40-80 nm. The dynamic experiments showed that the average removal of Cr(VI) and total chromium by nano-FeS and its immobilized particles were 94.97% and 63.51%, 94.93% and 45.76%, respectively. Fe²⁺ and S^2- ionized by the FeS nanoparticles rapidly reduced Cr(VI) to Cr(III). Part of S^2- may reduce Fe³⁺ to Fe²⁺, forming a small iron cycle that gradually decreases with the ion concentration. Cr(III) and Fe²⁺ form Cr(OH)₃ and FeOOH, respectively, with the change of aqueous environment. Another part of S^2- reacts with Cr(III) to form Cr₂S₃ precipitate or is oxidized to singlet sulfur. The FeS nanoparticles change from short rod-shaped to spherical shape. Compared with the conventional chemical precipitation method, the method used in this study is simple, low cost, small particle size and high removal rate per unit.

Figure 1

Dynamic test device system.

Figure 2

(a)-(b) Removal efficiency of Cr(VI) by nano-FeS prepared under different reaction temperatures; (c)-(d) Removal efficiency of Cr(VI) by nano-FeS prepared under different Fe/S molar ratio conditions; (e)-(f) Removal efficiency of Cr(VI) by nano-FeS prepared under different flow rates.

Figure 3

Contour map and RSM diagram of the effect of nano-FeS on the removal rate of Cr(VI) by ultrasonic precipitation.

Figure 4

(a)TEM images of nano FeS;(b) Nano FeS materials nitrogen adsorption–desorption isotherms and Pore-aperture differential distribution curves.

Figure 5

(a) Cr (VI) removal effect;(b) Total chromium removal effect (solution pH: 4, Cr(VI) initial concentration: 50 mg/L).

Figure 6

(a)TFe release, (b) COD release, (c) change of pH, and (d) turbidity analysis (solution initial pH: 4, Cr(VI) initial concentration: 50 mg/L⁾.

Figure 7

SEM images of nano FeS before and after the reaction (a); XRD images of nano FeS before and after the reaction (b).

Figure 8

Mechanism of Cr(VI) reduction by FeS nanoparticles.