Polyvinyl alcohol-alginate ferro photo gels for mercury (II) removal

Abstract

Mercury (Hg(II)) is considered as one of the most noxious heavy metals due to its high toxicity and probability of bioaccumulation in human body. Adsorption is one of the most commonly used technique to treat Hg(II) in wastewater but it requires a two stage process where the toxic Hg(II) is not being converted into nontoxic medium but need to be recovered. Thus photocatalytic process was introduced where the photocatalyst reacts by converting the toxins into toxic-free wastes; henceforth there is no necessity for additional disposal. In this study nano-sized maghemite (γ-Fe2O3) embedded in polyvinyl alcohol (PVA) and alginate matrix was used as photocatalyst to remove Hg(II). Besides being a photocatalyst, the γ-Fe2O3 nanoparticles possess supermagnetism properties which enabled the beads to be easily recovered with the application of an external magnetic field. The influences of pH, initial concentration and photocatalyst dosage on Hg(II) removal were also investigated. The photocatalyst beads were then used for the reduction of Hg(II) in aqueous solution both under sunlight and away from sunlight. The synthesized maghemite nanoparticles were characterized using transmission electron microscopy, x-ray diffraction and vibrating sample magnetometer; and the size distribution of the beads were determined. The current results revealed that 96% of Hg(II) was reduced in four hours under sunlight. However, when the experiment was done in the dark, the percentage of Hg(II) reduction achieved was only 5%. The low reduction rate was due to the minimal absorption activity of Hg(II) onto the beads surface. In addition, the maximum Hg(II) reduction was found at pH 11 whilst the photocatalyst dosage was kept at 16% (v/v). An 8% (v/v) dosage of maghemite nanoparticles loading was found to be capable only to reduce until 67% of Hg(II), while Hg(II) reduction performance was not significantly improved when 24% (v/v) of photocatalyst dosage was used. Excessive addition of catalyst dosage increased the active sites on the beads surface but it also blocked some sunlight illumination as the voluminous load of photocatalyst clogged the reaction region thus reducing the photon availability to be absorbed. At a fixed optimum parameters, it was revealed that increasing the initial concentration of Hg(II) degraded the reduction capability because the photons path length into the solution reduced as concentration of Hg(II) increased. Field emission scanning electron microscopy images and energy dispersive x-ray showed that the beads possessed significant porosity structure that greatly supported mass movement of Hg(II) inside the beads. The maghemite embedded PVA-alginate beads towards reduction of Hg(II) strongly fitted Langmuir- Hinshelwood kinetics model with correlation coefficient, R2 value of 0.9771. In conclusion, this study proved that the γ-Fe2O3-PVA-alginate beads are applicable in reducing and treating Hg(II) in water.