Effectiveness of ferrate（VI） preoxidation in enhancing the c

School of Municipal and Environmental Engineering, Harbin Institute of Technology, P.O. Box 2627, Harbin 150090,People’s Republic of China Received 6 August 2001; received in revised form 18 April 2002; accepted 17 May 2002

Abstract
Standard jar tests were conducted to evaluate the effectiveness of ferrate preoxidation in enhancing the coagulation of surface waters. A substantial reduction of residual turbidity after sedimentation and filtration was achieved by ferrate preoxidation in all cases of the investigation of various water qualities at low ferrate dosage (0.5–1.0 mg/l). The enhancement of the coagulation was more obvious when the organic content in the waters were relatively high. r 2002 Published by Elsevier Science Ltd.
Keywords: Potassium ferrate; Preoxidation; Coagulation; Turbidity; Enhanced coagulation
1. Introduction
Preoxidation has been one of the principle means for improving the coagulation process, which is generally aimed at destroying the organic coating on the surface of particles. Traditionally, chlorine has been used to aid the coagulation of waters with a high organic content.
However, the negative effect of using chlorine, resulting from the formation of hazardous by-products, is limiting the use of chlorine as a preoxidant [1].
It has been found that at very low ozone dosages preoxidation has a positive effect on the coagulation of surface water by improving the removal of turbidity, whilst greater doses have a negative effect, resulting in an increase of residual turbidity [2]. Ma et al. [3] reported that permanganate preoxidation (at low permanganate dosage, i.e., 0.5–1.5 mg/l) greatly enhanced the coagulation of surface water as expressed in terms of residual turbidity. Another potential oxidant, potassium ferrate (VI), which has a strong redox potential through the entire pH range (ranging from –2.2V in acid to –0.7V in base)
[4], has received more and more attention recently.
The results of initial studies, in which ferrate preoxidation was applied in alum coagulation of the surface waters, showed that ferrate preoxidation could decrease residual algae concentration [5–7], destroy phenol and coprecipitate heavy metal ions [8], and aid
the removal of turbidity [9,10]. Due to the unique characteristics of combining oxidant and coagulant qualities of ferrate (VI), it is a desirable preoxidant in drinking-water treatment.
2. Materials and methods
2.1. Raw water quality
Three kinds of Chinese surface waters with different characteristics were selected in this study. A summary of the water quality of the three kinds of raw waters is shown in Table 1.
2.2. Experimental procedure
Preoxidation with ferrate followed by aluminium sulphate coagulation–sedimentation was conducted in

a series of 1 l glass beakers in which water samples were agitated with a six-unit stirrer apparatus. Some of the water samples and a certain dosage of potassium ferrate solution were mixed at a speed of 300 rpm for a period of time. Potassium ferrate [K2FeO4] solid was prepared by modifying the method of reaction between OCl
and Fe(OH)3 (gel) in strongly basic media and isolated from the saturated KOH solution [11], and stored in the desiccator. Potassium ferrate solution (0.3 g/l, calculated in K2FeO4) was prepared by dissolving potassium ferrate solid in distilled water just before use in order to minimise the loss of ferrate as a result of rapid decomposition rate in solution. Then, all of the water samples were subjected to coagulation with the addition of specific dose of aluminium sulphate solution (10 g/l), which was prepared by dissolving the analytical reagent (Al2(SO4)3  18H2O, Tianjin chemical Inc., Tianjin, China) in distilled water, at 300 rpm for 1 min. Subsequently, the samples were slowly stirred with the coagulant at 60 rpm for 10 min, and settled for 30 min.
Samples of supernatant after sedimentation were siphoned from 1 cm below the water surface, and filtered with a filter paper (1–2 mm pore size). The residual turbidity of the settled and filtered water was analysed using a turbidity meter (2100A, HACH Chemical
Company, Loveland, USA). The residual iron and manganese concentrations in water were measured using an Atomic Absorption Spectrophotometer equipped with a graphite furnace (AA-670, Shanghai Analytical Instrument Factory, Shanghai, China).
3. Results and discussion
Residual turbidity was used as the principle indicator for the evaluation of the effectiveness of ferrate preoxidation in enhancing the coagulation of surface water. Fig. 1 shows the comparative results of the coagulation of the water from Reservoir Shi with and without
ferrate preoxidation. Alum coagulation has an optimum alum dosage at 70 mg/l, since a higher alum dose resulted in an increase of the residual turbidity of the settled water. Ferrate preoxidation enhanced the coagu

lation
performance of the surface water, with a substantial reduction of residual turbidity of the settled water. At low alum dosages (50, 60 mg/l) adopted in the tests, a substantial reduction of residual turbidity was observed (more than 4 NTU) when the water samples
were pretreated by ferrate even at the low dosage of ferrate. The remarkable reduction of residual turbidity after filtration (filtered by 1–2 mm pore size filter papers) was also observed (Fig. 1b) when pretreated with ferrate. Thus, the filtration process accentuated the effects of ferrate preoxidation, indicating that the floc particle size in the process of coagulation with ferrate preoxidation is
larger than those without preoxidation.


When the water was pretreated with ferrate, a certain amount of iron was introduced into the water as the initial form of ferrate; the residual iron concentration in the water is of special concern. Results (Table 2) showed that ferrate preoxidation did not increase the residual iron concentration; in contrast, a remarkable reduction of iron concentration in settled and filtrated water was observed. Filtration did little contribution to manganese removal when the water was coagulated with alum alone, residual manganese after sedimentation and filtration was markedly reduced by the ferrate preoxidation.
Fig. 2 shows the results of the coagulation tests for Songhua River water in summer. The residual turbidity after alum coagulation and sedimentation were relatively low (below 5 NTU) (Fig. 2a). The residual turbidity with ferrate preoxidation after sedimentation was only slightly lower than in the case without ferrate preoxidation (about 2 NTU) at low alum dosage (20 mg/ l), and did not have further reductions with the increase of ferrate dosage. The residual turbidity after filtration was rather low (below 0.5 NTU), and ferrate preoxidation shows no improvement on residual turbidity (Fig. 2b).
The same coagulation test was conducted using Songhua river water in winter (Fig. 3). Songhua River water in winter is the typical surface water with low temperature (21C) and low turbidity (26–28 NTU), containing more organic materials (Permanganate index, as COD, 12.2 mg/l) than that in summer (8 mg/l). The low temperature and low turbidity water is rather difficult to coagulate due to the slower rate of hydrolysis of coagulant and the difficulty in flocculation because of the low concentration of particles in the water.

Ferrate preoxidation enhanced the coagulation of this surface water, with an obvious reduction in the residual turbidity of the settled samples at the whole range of alum dosages, and the residual turbidity decreased further with the increase of ferrate dosage (Fig. 3a).
The enhancement was more obvious in the case of filtrated samples (Fig. 3b). This was consistent with the results for the coagulation of the water from the reservoir Shi, which is similarly difficult to be coagulated by alum alone, and again the experiments indicated that
the floc size in the coagulation process with ferrate preoxidation was larger than when the water coagulated with alum alone.
The experiments carried out on three types of surface waters demonstrated that potassium ferrate preoxidation
had beneficial effects on the performance of coagulation of these waters. It has been shown that the coagulation of organic-rich waters is particularly enhanced by ferrate preoxidation, as demonstrated by a much greater reduction of residual turbidity both in settled and filtrated water when the waters were pretreated by ferrate. Further research is required to make the various physico-chemical mechanism of the above improvements observed known.
4. Conclusion
The laboratory studies using different surface waters with various raw water qualities demonstrated that ferrate preoxidation had significant effect in enhancing the coagulation of surface waters, especially when the waters had high organic content. A remarkable improvement of turbidity removal by ferrate preoxidation on low temperature and low turbidity water was also
achieved.
References
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