FeS_x@MOF-808 composite for efficient As(III) removal from wastewater: behavior and mechanism

Arsenic is extremely toxic to humans with water as its carrier. One challenge for arsenic control is the complete elimination of As(III) due to its high toxicity, mobility, and solubility. Herein, an active FeS_x@MOF-808 composite was fabricated to enhance the As(III) removal for wastewater remediation. The FeS_x@MOF-808 showed better As(III) adsorptive performance (Q_e = 73.60 mg/g) compared with Fe₂S₃ (Q_e=12.38 mg/g), MOF-808 (Q_e = 27.85 mg/g), and Fe@MOF-808 (Q_e=34.26 mg/g). This can be attributed to an improved porous structure provided by MOF-808 and abundant reactive sites provided by FeS_x. Calculated by the Langmuir model (R² =0.9965), the maximum adsorption capacity (Q_max) of FeS_x@MOF-808 for As(III) removal at 298 K and pH = 7 was 203.28 ± 6.43 mg/g, which is beyond most of the traditional materials and MOFs. Additionally, FeS_x@MOF-808 exhibited good stability in a wide pH range (1-13). Results also showed that the different Fe/S ratios (1:0-1:8) and FeS_x loading amount (0.00625-0.25 mmol) have effects on the FeS_x@MOF-808 performance. By kinetics studies, XPS, and DFT calculation, the mechanisms for arsenic by FeS_x@MOF-808 were proposed. Multiple reaction mechanisms combine the adsorption by the MOF-808 support, the co-precipitation of iron oxides via hydroxyl (Fe-OH) groups, and most importantly, the precipitation through the break of Fe-S and the bond of As-S.