Magnetic algae-derived FeS0.66@GA-BC0.33 Composite for Cr(VI) Removal Optimized by Response Surface Methodology

In this study, the adsorption capability of green algae-derived biochar was enhanced by integrating with iron sulfide, creating the FeS 0.66 @GA-BC 0.33 composite. The magnetic FeS 0.66 @GA-BC 0.33 composite exhibited excellent adsorption performance toward Cr(VI) with a maximal adsorption capacity of 423.73 mg/g. Surprisingly, the FeS 0.66 @GA-BC 0.33 composite showed fast adsorption, with an equilibrium time of 60 minutes. Furthermore, batch experiments showed that the maximum removal efficiency of Cr(VI) by FeS 0.66 @GA-BC 0.33 was 92.39% at pH 3, 20 °C, and an adsorbent dose of 0.01 g. The kinetic analysis indicated that the pseudo-second-order model best described the adsorption process, while the isotherm results showed good agreement with both the Langmuir and Freundlich models. A combination of response surface methodology (RSM) and Box–Behnken design (BBD) was used to optimize the adsorption of Cr(VI) onto FeS 0.66 @GA-BC 0.33 . The independent variables were adsorbent dose (5–30 mg), initial Cr(VI) concentration (50–300 mg/L), and contact time (1–120 min). Analysis of variance (ANOVA) showed that the maximum removal efficiency reached 99.09% at an adsorbent dose of 17.5 mg, an initial Cr(VI) concentration of 50 mg/L, and a contact time of 120 min. The Cr(VI) adsorption mechanism was proposed based on the X-ray photoelectron spectroscopy spectra of fresh and used FeS 0.66 @GA-BC 0.33 composites, suggesting the governing reduction reaction, electrostatic interaction, and coordination bond pathways on the adsorption of Cr(VI).