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Characterisation and coagulation performance of polysilicate–ferric–zinc

Xia Xu, Rong-Yan Shen, Yu-Ting Liang, Shui-Li Yu, and Li-Ping Wang

School of Environmental & Safety Engineering, Changzhou University, Changzhou 213164, China

 

E-mail: xuxia@cczu.edu.cn

Abstract: A new type of coagulant, polysilicate-ferric-zinc (PSFZn) with different Fe/Zn molar ratios, was synthesised using water glass (industrial grade, w(SiO2) = 21 mass %, ρ = 1.34 × 103 kg m−3, modulus = 3.2), FeSO4 · 7H2O, ZnSO4, and NaClO3 by way of co-polymerisation in the same (Fe + Zn)/Si molar ratio based on polysilicate-ferric (PSF). The effect of the Fe/Zn molar ratios on the morphology and structure was systematically investigated using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD) analysis. Three samples, namely PSFZn4, PSFZn1, and PSFZn0.25, represented Fe/Zn molar ratios of 4, 1, and 0.25, respectively, and were selected for a comparative study while a constant (Fe + Zn)/Si molar ratio equal to 1 was maintained. Accordingly, PSFZn was found to be a complex compound rather than a simple mixture of raw materials. With the decrease in Fe/Zn, a great change occurred in the surface morphology, from a tetrahedral cluster structure to a lamellar structure. The Fe-O and Fe-O-Si bonds were gradually replaced by Zn-O and Zn-O-Si. However, the crystalline peaks were more obvious with the increase in the number of zinc ions; hence the new polymer would be formed from iron, zinc, and polysilicate. In addition, the coagulation performance of PSFZn was investigated using a surface water sample. PSFZn4 exhibited a better coagulation performance than the other PSFZn coagulants. Additionally, the trends in changes in pH with different coagulation times after adding PSFZn were studied relative to PSF and FS. The replacement of zinc ions with iron ions could effectively counter the rapid decrease in pH. The effect of settling time on the coagulation efficiency was also investigated. PSFZn4 exhibited a better settlement performance than PSF and poly aluminium chloride (PAC). Hence, the partial substitution of zinc salt with iron salt not only addresses the inadequacies of iron salt but also improves the coagulation efficiency of zinc salt in water treatment.

Keywords: polysilicate-ferric-zinc (PSFZn) – Fe/Zn molar ratio – morphology and structure – coagulation performance

Full paper is available at www.springerlink.com.

DOI: 10.1515/chempap-2015-0022

 

Chemical Papers 69 (6) 864–871 (2015)

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