Abstract
Natural water with a high specific ultraviolet absorbance (SUVA) value (4.3) and high vanadium concentration (1.2–1.9 mg/l) was investigated for vanadium(V) removal through sorption using solid sorbents and co-precipitation with a ferric sulphate coagulant. Although there are studies on vanadium removal by sorption and co-precipitation, the removal of vanadium in the presence of natural organic matter has not been adequately addressed. In the sorption method, hexadecyltrimethylammonium bromide (HDTMA) modified clinoptilolite (Cp) and synthetic zeolite (Na-P1), and ferric oxyhydroxide (CFH-12) were tested as sorbents. In the co-precipitation method, vanadium removal may occur simultaneously with the coagulation process of humic substances. Co-precipitation with iron was proven to be more efficient than sorbent-based processes. A ferric sulphate coagulant dosage (as product) of 75 mg/l resulted in a residual vanadium concentration of 70–80 μg/l and vanadium removal closely followed the removal of humic substances. This might be related to the binding of vanadium by the humic substances. Regarding sorbents, 1–2 g/l dosages of Na-P1 (at pH ∼6) and CFH-12 (pH 3.7–4.7) were required to reach a low vanadium concentration. A much higher dosage was required for Cp, which can be explained by the lower loading of HDTMA on the zeolite surface. Na-P1 increased the pH of the treated water whereas CFH-12 decreased the pH. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses revealed that the pH change was due to the leaching of hydroxides and calcite from the Na-P1 and jarosite from the CFH-12. The organo-zeolites exhibited fast kinetics. The surface binding of vanadium and vanadium-humic substances can be regarded as the main mechanism. For CFH-12, the diffusion of vanadium into pores was a significant, albeit slow mechanism, in addition to the sorption on the external surface. The humic substances present in natural water significantly affected the mechanism of vanadium uptake. The XPS data confirmed the strong binding of organic matter on CFH-12 whereas organic matter was only weakly bound to the organo-zeolites.
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