Abstract
In this study, a composite system for the sustained release of potassium ferrate(VI) (sustained-release K2FeO4) was prepared and applied for water treatment. The objective of this research was to maximize the effectiveness of K2FeO4 for water treatment by enhancing its stability using diatomite. The sustained-release K2FeO4 was characterized using X-ray diffraction, scanning electron microscopy, and Fourier transform infrared spectroscopy. The results indicated that no new crystal phase was formed during the preparation and some K2FeO4 crystals entered the pores of the diatomite. From K2FeO4 release experiments, we found that the decomposition rate of K2FeO4 was obviously decreased, which greatly improved the contact rate between released K2FeO4 and pollutants. Via degradation of methyl orange, which was used as a model pollutant, the influential factor of K2FeO4 content within the complete sustained-release K2FeO4 system was studied. The optimal K2FeO4 content within the sustained-release K2FeO4 system was approximately 70%. In natural water samples, sustained-release K2FeO4 at a dosage of 0.06 g/L and with a reaction time of 20 minutes removed 36.84% of soluble microbial products and 17.03% of simple aromatic proteins, and these removal rates were better than those observed after traditional chlorine disinfection.
1. Introduction
Potassium ferrate(VI), with the formula K2FeO4, as an environmentally friendly water treatment agent with strong oxidizing power, can not only sterilize natural water but also remove various organic and inorganic compounds [1–3]. In addition, its reduction by-product, Fe(III), is an effective flocculent that can absorb suspended solid and hazardous substances [4, 5]. More importantly, compared with chloric disinfection, no hazardous disinfection by-products (DBPs) are generated in K2FeO4 treatment of drinking water [6–8]. However, to date, K2FeO4 has not yet been widely applied as a replacement for chloric disinfectants in drinking water treatment, mainly due to the poor stability of concentrated K2FeO4 [9, 10]. The reduction potential of K2FeO4 varies from +0.7 V to +2.2 V in basic and acidic solutions, respectively [3]. The ferrate anion decomposes quickly during the treatment process, resulting in low efficiency for water purification.
The stability of K2FeO4 is determined by its purity, the pH value of the solution, and various other factors [11]. Using a hypochlorite oxidation method, Hrostowski and Scott [12] successfully prepared K2FeO4 with a purity as high as 96.9%, which dramatically improved its stability during storage. However, this had little effect on the use of K2FeO4 given that its stability is still poor during the water treatment process. Wagner et al. [11] reduced the decomposition rate of K2FeO4 by controlling the solution pH value. In high pH solution, the concentration of hydrogen ions (H+) is low, leading to only weak catalytic decomposition. However, controlling pH is not commercially viable in practical water treatment processes.
In the present research, K2FeO4 crystals were absorbed into a porous media, diatomite, which has a highly porous structure and high surface area [13], enabling the subsequent slow release of K2FeO4 into water in order to prolong the reaction time and consequently improve the efficiency and purification effect of K2FeO4. This method does not require a specific K2FeO4 purity or solution pH value, and, therefore, the technical difficulties and high cost of K2FeO4 preparation can be drastically reduced for the practical application of K2FeO4 in water purification processes.
2. Materials and Methods
2.1. Chemicals
The following chemicals were of analytical reagent grade and used without further purification: ferric nitrate (Sigma Aldrich Reagent Co., Ltd.), calcium hypochlorite (Shun Chemical Reagent Co., Ltd.), potassium hydroxide (Chongqing Chuandong Reagent Co., Ltd.), and methyl orange (ChengDu Kelong Chemical Co., Ltd.). Diatomite (Chinasun Specialty Products Co., Ltd.) was of chemically pure grade and was used without further purification.
2.2. Preparation of Sustained-Release K2FeO4
A paste of 15 g of Ca(ClO)2 and 25 mL of 13 mol/L KOH solution were added to a 100 mL beaker. After mild stirring, the mixture was filtered using an 800-eye filtering cloth and then 20 mL saturated KOH solution was added to the filtrate. The mixture was filtered again using 800-eye filtering cloth and 8 g of Fe(NO3)39H2O, which had been ground fully immediately prior to use, was slowly introduced to the filtrate in the ice water bath five times. After 1 hour of continued stirring in an agitator, diatomite was added to the mixture. To achieve samples with different K2FeO4 content, 6.92 g, 11.87 g, 18.46 g, 27.69 g, or 41.54 g of diatomite was added to individual samples. Each mixture was stirred for 5 additional minutes, followed by a 30-minute precipitation step in an ice bath. After filtration of the mixture, the filter cake was collected as the sustained-release K2FeO4 sample. The theoretical weight of K2FeO4 in each sustained-release K2FeO4 sample was 27.69 g. The sustained-release K2FeO4 samples were labeled as Fe80, Fe70, Fe60, Fe50, and Fe40 based on the theoretical mass fraction of K2FeO4 in each sample. For example, Fe80 indicates that the theoretical mass fraction of K2FeO4 in the composite was 80%. When diatomite was not added, the sample contained only K2FeO4.
2.3. Characterization of Sustained-Release K2FeO4
The X-ray diffraction (XRD) patterns of the sustained-release K2FeO4 samples were obtained using a Shimadzu XRD-6000 diffractometer using Cu Kα radiation (40 kV, 30 mA). Data were recorded over the range from 10° to 80° (2θ) with a scan rate of 4° min−1. Fourier transform infrared spectroscopy (FT-IR) patterns were obtained using a Shimadzu IRPrestige-21 infrared spectrometer with the frequency ranging from 500 to 2000 cm−1. Scanning electron microscopy (SEM) images were obtained using a FEI Quanta 450 scanning electron microscope.
2.4. Ferrate Release Rate of Sustained-Release K2FeO4
To control the decomposition rate of Fe(VI), the release rate experiment of sustained-release K2FeO4 was carried out in an ice water bath. First, 0.1 g pure K2FeO4 or a sample of each sustained-release K2FeO4 composite was added to a 250 mL beaker containing 250 mL water. The solution was then agitated at 60 rpm for 30 min in the ice water bath, and, finally, the concentration of Fe(VI) in solution was determined according to the adsorption at 510 nm using a spectrophotometer.
2.5. Degradation of Methyl Orange
Eight test solutions of 20 mg/L methyl orange were prepared by dissolving methyl orange granules in deionized distilled water. Then, 0.125 g Fe80, 0.143 g Fe70, 0.167 g Fe60, 0.200 g Fe50, 0.250 g Fe40, 0.100 g K2FeO4, 0.043 g diatomite, and a simple mixture of 0.100 g K2FeO4 and 0.043 g diatomite were added individually to these test solutions. The theoretical weight of K2FeO4 in all seven K2FeO4-containing samples was 0.100 g. Each test solution was stirred rapidly and then allowed to stand for 20 min. The methyl orange degradation rate was calculated according to the concentration of methyl orange. The concentration of methyl orange was determined by fluorescence spectrophotometry using a Hitachi F-7000 fluorescence spectrophotometer, because methyl orange is a fluorescent substance [14]. The fluorescence intensity varied linearly with methyl orange concentration in the range of 5–20 mg/L, with a correlation coefficient of 0.998. Therefore, the concentration of methyl orange could be calculated using the fluorescence intensity value through a linear regression equation.
2.6. Degradation of Fluorescent Substances in Natural Water
Water samples were collected before and after liquid chlorine disinfection in a water supply network in Shapingba, Chongqing, China for comparative studies. The water in this network was treated for coagulation, sedimentation, and sand filtration before liquid chlorine disinfection. First, 0.015 g Fe70 was added to 250 mL water samples and then the changes in the three-dimensional fluorescence patterns were analyzed after a 20-minute reaction time. The resulting changes in fluorescence in the water samples were compared to those after the liquid chlorine disinfection process. Three-dimensional fluorescence patterns were obtained using a Hitachi F-7000 fluorescence spectrophotometer. The concentrations of soluble microbial products and simple aromatic protein also were determined according to the linear regression method used to determine the methyl orange concentration.
3. Results and Discussion
3.1. XRD Analysis of Sustained-Release K2FeO4
The XRD patterns of Fe70, Fe60, K2FeO4, and diatomite samples are shown in Figure 1. It can be seen that diatomite had two sharp diffraction peaks at 2θ = 21.82° and 36.00°. K2FeO4 had only one sharp diffraction peak at 2θ = 29.72°. Other diffraction peaks belonged to the residual crystals of KOH and KNO3 in the samples. The three sharp diffraction peaks of sustained-release K2FeO4 were at 2θ = 21.82°, 29.72°, and 36.00°. Basically, the sharp diffraction peaks of Fe70 and Fe60 coincided with those of K2FeO4 and diatomite. In other words, no new crystal was generated during the preparation. Therefore, the XRD analysis indicates that K2FeO4 and diatomite were combined by physical adsorption, which is beneficial to prevent the loss of K2FeO4 reactivity.
3.2. SEM Analysis of Sustained-Release K2FeO4
Figure 2 shows SEM representative images of Fe70, Fe60, K2FeO4, and diatomite samples. The particle size of K2FeO4 was roughly 2μm. Due to its poor dispersion, an aggregation phenomenon was observed. The particle size of diatomite was approximately 35 μm, and void spaces (i.e., pores) between the particles were obvious. SEM images demonstrated that the voids in the diatomite were filled with K2FeO4 crystals, which even covered the surfaces of the diatomite. The particle size of K2FeO4 in the Fe70 samples was nearly the same as that of the pure K2FeO4 sample. Nevertheless, the dispersion of K2FeO4 particles in the Fe70 samples was much better than that of the pure K2FeO4 samples. Obvious aggregation of K2FeO4 particles was also visible in the Fe60 samples, with K2FeO4 crystals covering the diatomite surfaces. The K2FeO4 crystals covering the diatomite surfaces can dissolve in the water instantly and participate in the reaction, whereas the K2FeO4 crystals contained in the diatomite pores will dissolve slowly and continuously and then react with substances in solution. Therefore, the SEM analysis indicates that a K2FeO4 proportion of 70% in the sustained-release K2FeO4 composites is appropriate for continued release of K2FeO4.
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3.3. FT-IR Analysis of Sustained-Release K2FeO4
Figure 3 shows the transmittance FT-IR spectra of Fe70, Fe60, K2FeO4, and diatomite samples. The IR absorption spectrum of the K2FeO4 sample includes a primary peak at 825.53 cm−1, which can be recognized as the characteristic peak of and is attributed to the stretching vibrations of the Fe–O bond [15, 16]. Other peaks may be the result of impurities in the K2FeO4 samples. The absorption peaks of the diatomite sample at 619.15 cm−1 and 808.17 cm−1 can be assigned to the stretching vibration of SiO–H. Both Fe–O and SiO–H bonds were detected in Fe70 and Fe60 samples, demonstrating the existence of K2FeO4 and diatomite in the composite samples.
3.4. Ferrate Release Rate from Sustained-Release K2FeO4 Samples
Figure 4 shows the changes in Fe(VI) concentration in solution after the addition of various sustained-release K2FeO4 composite samples. The figure shows that when pure K2FeO4 was added to the aqueous solution, the Fe(VI) concentration rapidly increased, reaching the maximum concentration within 2 min before beginning to decrease rapidly and showing complete degradation after 10 min. After addition of the sustained-release K2FeO4 samples in the aqueous solution, the Fe(VI) concentration was maintained at higher levels for longer periods, and the Fe(VI) decomposition rate was slower than that observed for pure K2FeO4. Specifically, for the Fe70, Fe60, Fe50, and Fe40 samples, the Fe(VI) concentration increased within 5 min, and the highest concentration from Fe70 samples reached 99.47 mg/L. These results demonstrate that sustained release of K2FeO4 was achieved from the diatomite pores observed in Figure 2. When sustained-release K2FeO4 was added to the water, K2FeO4 dissolved gradually starting from the outside of the samples and later from the inside of the diatomite pores, releasing Fe(VI) slowly and maintaining a higher concentration of Fe(VI) in solution. Such sustained, continuous release can reduce the decomposition rate of Fe(VI) and thereby improve the probability of interaction between Fe(VI) and pollutants, thus improving the effectiveness of Fe(VI) for water purification.
3.5. Degradation of Methyl Orange
Figure 5 presents the removal rate of methyl orange using different treatment agents. It is evident from this figure that the removal rate of the simple mixture roughly equates the sum of the removal rate of K2FeO4 and diatomite. No synergistic effect was observed with the simple mixture of K2FeO4 and diatomite. The removal rates of methyl orange by sustained-release K2FeO4 samples were all higher than that using only K2FeO4. However, considering that the diatomite within the sustained-release K2FeO4 samples could also absorb methyl orange, only the performances of Fe70 and Fe60 samples were better than those of the simple mixture. Specifically, 0.143 g of Fe70 and 0.167 g of Fe60 removed 6.49 mg (removal rate of 64.86%) and 4.11 mg (removal rate of 41.08%) of methyl orange, respectively. The removal efficiency of Fe70 was as 2.2 times that of the simple K2FeO4 and diatomite mixture. Potassium ferrate formed a good Fe(VI) sustained-release structure, allowing Fe(VI) to be slowly released into the solution and thus reducing the decomposition rate of Fe(VI). As a result, the contact probability between Fe(VI) and methyl orange was increased and thus the removal rate of methyl orange was improved. For the simple mixture of K2FeO4 and diatomite, K2FeO4 quickly dissolved in the water, and most of the Fe(VI) failed to react with methyl orange. Therefore, the methyl orange removal rate with the simple mixture was significantly less than the rates achieved by the Fe70 and Fe60 samples. These results are consistent with those from the experiment determining the ferrate release rate from sustained-release K2FeO4 samples (Figure 4).
A possible reason for the reduced methyl orange removal rate at K2FeO4 mass fractions below 60% may be that the dosage of diatomite was high enough to cause an increase in the degradation rate of K2FeO4. When the K2FeO4 mass fraction was above 70%, the crystal shape of the sustained-release K2FeO4 sample was approximately the same as that of the K2FeO4 sample and most of the K2FeO4 crystals failed to be absorbed into the pores of diatomite, which corresponded with a much lower removal rate of methyl orange.
In the process of sample preparation, the actual concentration of K2FeO4 is less than the theoretical value. We tested the purity of the prepared K2FeO4 and found that it was about 97%, which is close to the theoretical value. However, we could not determine the purity of the sustained-release K2FeO4. However, the preparation time of sustained-release K2FeO4 is longer, and thus, the actual concentration of K2FeO4 in the sample may be significantly less than the theoretical value. For the degradation of methyl orange, the degradation effect is better after adding the theory amount K2FeO4. This concluded that the degradation effect is better after adding the real amount of K2FeO4.
3.6. Degradation of Fluorescent Substances in Natural Water
Figure 6 shows the three-dimensional fluorescence patterns of water samples treated by traditional chlorine disinfection in a water network and Fe70. Two fluorescence peaks were observed in these patterns at / = 275 nm/305 nm and / = 225 nm/330 nm. These peaks represent soluble microbial products and simple aromatic protein [17]. After traditional chlorine disinfection, the fluorescence intensity at 275 nm/305 nm was reduced by 11.86%, but the fluorescence intensity at 225 nm/330 nm remained almost unchanged. The intensities of the two fluorescence peaks were reduced by 31.58% and 16.67%, respectively, after a 10 min treatment with Fe70, and by 36.84% and 17.03%, respectively, after a 20 min treatment with Fe70. The results indicate that the released K2FeO4 removed more soluble organic matter from the water samples than liquid chlorine. Thus, treatment with sustained-release K2FeO4 may improve the quality of purified drinking water.
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4. Conclusions
In this study, a novel encapsulation method using hypochlorite oxidation to prepare sustained-release K2FeO4 was presented. The Fe(VI) release experiments demonstrated that diatomite as an encapsulation material affords the slow release of K2FeO4 into solution, which greatly increases the contact probability between released K2FeO4 and pollutants. The optimal mass fraction of K2FeO4 in the sustained-release K2FeO4, among those tested, was found to be 70%. A maximum methyl orange removal rate of 64.86% was achieved with 0.3 g/L Fe70 and a reaction time of 20 min, for an initial methyl orange concentration of 20 mg/L. With an Fe70 dosage of 0.06 g/L and reaction time of 20 min, 36.84% of soluble microbial products and 17.03% of simple aromatic protein were removed from natural water samples.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
Financial support from the National Natural Science Foundation of China (no. 51108483), the Natural Science Foundation Project of CQ CSTC (no. cstcjjA20002), the 111 Project (no. B13041), and the Fundamental Research Funds for the Central Universities (no. 106112013CDJZR210002) is gratefully acknowledged.