Separation and Purification by Crystallization - ACS Publications

Chapter 23

Inclusion of Selenium in Ferrite Crystals Y. Taguchi , M . Ohizumi , K. Aoyama , and K. Katoh 1

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Department of Chemistry and Chemical Engineering, Waste Liquids Treatment Facilities, and Research Institute for Hazards in Snowy Areas, Niigata University, Ikarashi 2-Nocho, Niigata 950-21, Japan

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Downloaded by GEORGETOWN UNIV on August 24, 2015 | http://pubs.acs.org Publication Date: June 1, 1997 | doi: 10.1021/bk-1997-0667.ch023

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Various heavy metals are included in ferrite crystals when a solution containing the metallic ions is ferritized with Fe(II). To determine the capacity of the ferrite as a scavenger for selenium removal from wastewater, artificially prepared wastewater containing selenium was treated using the ferrite method. The final concentration of selenium in the treated water was less than approximately 0.1 mg/l after a wastewater containing selenium at a concentration of less than 400 mg/l was treated. The amount of selenium removed by ferritization was not influenced by the valence state of selenium, Se(IV) or Se(VI). However, differently distributed selenium in the ferrite particles was found by dissolving the ferrite crystals with an acidic solution. Se(IV) is almost uniformly distributed when the wastewater containing Se(IV) was treated, but when water containing Se(VI) was treated, the higher concentration of Se(VI) was near the center of the ferrite particles.

The element selenium is useful for the manufacturing of chemicals, glasses, ceramics, semiconductors, photocells, photosensitive dyes for copiers, etc., but it is a poisonous substance which is detrimental to human health (1). In water, selenium usually exists in the form of selenite(IV) or selenate(VI), the conjugate acids being selenious acid and selenic acid, which chemically are classified as oxoacids. Thermodynamically, selenate is more stable than selenite. Selenium should be removed from wastewater because of its health hazard at values even lower than, e.g., 0.1 mg/1 (2). The removal of selenium is a practical and interesting problem to those who usually apply the ferrite method, a ferritization process, to waste liquids containing heavy metallic ions. Using the process under a given set of operating conditions, some liquids containing selenate or selenite were treated to investigate the mechanism and amount of selenium removed by the ferritization method into the ferrite crystals. Attention was also paid to the effect of ion valency, i.e., four-valent and six-valent, because it was suspected from the inclusion results of arsenic (3) that there could be a difference in the distributions of selenium in © 1997 American Chemical Society

In Separation and Purification by Crystallization; Botsaris, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

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SEPARATION AND PURIFICATION B Y C R Y S T A L L I Z A T I O N

the ferrite particles depending on to its valence. This anticipation was experimentally tested. The aim of this study was to include selenium as an actual impurity in the ferrite crystals under a reaction-crystallization. We let the ferrite crystals play the role of a scavenger as we wanted to produce selenium-free solutions.


Experimental The experimental procedure is shown in Figure 1. The ferritization reaction (4, 5) was carried out using two artificially prepared model solutions of wastewater containing selenium, Se(IV) or Se(VI). For the preparation of the wastewater, Se02 , selenite, tetravalent selenium or K^SeO^ selenate, hexavalent selenium was dissolved. A sample solution of 700 ml was poured into a one-liter glass reactor-vessel, and 35 g of FeSU4 · 7 H 2 O was added to the solution. The temperature of the solution was then raised to 65 °C using an electrical heater with a stirrer. Air was admitted through a distributor immersed in the solution at the rate of 1.0 1/min to start the reaction, and the operation was continued for almost one hour to form the ferrite which included the selenium. The reaction time of one hour is usually necessary to simultaneously remove various heavy metallic ions from the wastewater. During the reaction period, the solution was held at a constant pH of 9 and at a constant temperature of 65 Τλ Using a reflux condenser attached to the reaction-vessel, vapor was condensed and returned to the vessel. This procedure is referred to as the standard ferritization method which is often applied on an industrial plant scale (6). The resultant ferrite crystals were separated from the solution by centrifugation or filtration. Using the separated ferrite crystals, dissolution tests and leaching tests were performed. Results and Discussion Residual Concentration of Selenium after Ferritization In Figure 2, the initial and residual concentrations of selenium during the application of the standard ferritization method are depicted. The initial concentration of selenium was varied from 50 to 500 mg/1. From Figure 2, it can be seen that when the standard ferritization method was applied to the wastewaters containing less than 400 mg/1 selenium, the amounts of selenium could be reduced to a concentration level of 0.1 mg/1. This level was our goal since at this level the treated water can be discharged into the public sewage system (2). A large difference in the residual concentrations between Se(IV) and Se(VI) was not found, whether the wastewater contained Se(IV) or Se(VI). When the solution of 500 mg/1 was treated, the residual concentration exceeded 1.4 mg/1. From the observation of residual concentration changes in iron and selenium during ferritization, as shown in Figures 3-1 and 3-2, their rmnimum values were usually found at almost 40 min after the start of the ferritization reaction. Stopping the reaction at 40


TAGUCHI E T A L .

Inclusion of Selenium in Ferrite Crystals

SAMPLE SOLN. (700 ml, 50-1000 mg-Se/l) 1) Se(IV): SeOzlNtoO 2) Se(Vl): teSeCUlNHaO

ADDITION OF FeS04 7H 0 (35g), ADJUSTMENT OF pH, HEATING (65^)


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REACTION; ADMITTANCE OF AIR (1 l/min), MAINTAINING pH CLOSE TO 9, ONE HOUR SEPARATION OF FERRITE FROM THE SOLN. BY CENTRIFUGATION OR FILTRATION (N0.5C)

FERRITE CRYSTALS

I FILTRATE I

Figure 1. Ferritization procedure.

Figure 2. Residual concentration of total selenium in the treated water after ferritization.


SEPARATION AND PURIFICATION

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CRYSTALLIZATION


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Figure 3-2. Concentration changes of Se(IV), Se(VI) and iron during ferritization of Se(VI) soin.


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TAGUCHI E T A L .

Inclusion of Selenium in Ferrite Crystals

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min enabled the residual concentration of selenium to be less than 0.1 mg/1 for the treatment of the 500 mg-Se/1 solution. Distribution of Selenium in Ferrite Crystals Some dissolution tests of the resultant ferrite crystals were performed to investigate how selenium was distributed in the crystals. About 10 g of ferrite crystals which were usually obtained from each ferritization, were dissolved. They were gradually dissolved with several acidic solutions from the surface to the center. The dissolution operation was repeated six times in order to thoroughly dissolve the residual ferrite crystals. The distributions of selenium in the crystals formed with Se(IV) and Se(VT) were found as shown in Figures 4-1 and 4-2, respectively. The reduced radius, r/R, on the abscissa was determined by the amount of dissolved iron. The ratio of Se/Fe on the vertical axis was the average value calculated from the amounts of iron and selenium dissolved in each acidic solution. Different distributions of selenium were observed between the crystals formed with selenite and the crystals formed with selenate. From Figure 4-1, almost uniformly distributed Se(IV) is recognized. Near the surface, Se(IV) is slightly higher than that near the center. Near the center, a small amount of Se(VI) was also detected. The partial oxidation of Se(IV) to Se(VI) was considered to be due to the following reaction with oxygen in air. 2Se0 2- + 0 3

— 2Se0 2-

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As ferritization proceeds, a part of the resultant ferrite F e 0 also acts as a oxidizing 3

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agent according to: Se03 " + F e 0 + 3 H 0 — Se0 2- + 3Fe(OH) These reactions spontaneously occur according to the oxidation-reduction potential in basic solution (7, 8). From Figure 4-2, Se(VI) was found concentrated around the center. It decreased linearly with the radius, and approached a constant ratio of 0.003 near the surface. At the reduced radius between 0.25 and 0.4, a very small amount of reduced selenium, Se(IV), was detected. The amount was almost less than the detectable limit, but reduction reaction below might occur. The reaction rate of this reduction of Se(VI) was considered to be very slow. Se0 2- + 2Fe(OH) + H 0 — Se0 2- + 2Fe(OH) The smaller amount near the surface is desirable for the inclusion of selenium, because even if the ferrite surface were subject to dissolution, the amount of selenium dissolved would be lower. 2

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In the former crystals, selenite was almost uniformly distributed in the ferrite crystals, but in the latter, selenate was concentrated near the center of the crystals. The reason why Se(VI) was included more deeply or more tightly into the ferrite crystals is not presently understood. However, it is possible to speculate that one of the reasons may be due to its ionic radius and ionic valence. The radius of Se(VI) is smaller than


SEPARATION AND PURIFICATION BY C R Y S T A L L I Z A T I O N

0.03

Ο


LL •— ω CO 0.02 Q) CO

jQkey Se ratio Ο Se(IV)/Fe Se(VI)/Fe

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C

ο ço 0.01

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Initial concn. of Se(IV):100 mg/l

α) ϋ ο ϋ 0.2

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r/R[-] Figure 4-1. Distribution of selenium in the ferrite crystals formed from Se(IV) soin.

0.03 key

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Φ CO

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Se ratio Se(IV)/Fe Se(VI)/Fe

Initial concn. of Se(VI):100 mg/l

c ο

2 o.oi c

Separation and Purification by Crystallization - ACS Publications

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