Colour and turbidity removal with reusable magnetite particles—IV

~tater Res. Vol. I'. pp. 141 to 1"'. 19s3 Printed in Great Britain. All rights reserved

01M3-1354 S3 0201al-0"$03.00 0 Cop?right ~ 1983 Pergamon Press Ltd

COLOUR AND TURBIDITY REMOVAL WITH REUSABLE MAGNETITE PARTICLES--IV ALKALI

ACTIVATED MAGNETITE--A NEW COAGULANT-ADSORBENT

SOLID,

REUSABLE

L. O. KOLARIK CSIRO Division of Chemical Technology. P.O. Box 310. South Melbourne 3205, Australia (Receiced March 1981)

Abstract--Alkali treated magnetite (Fe30.t) is shown to be an excellent solid coagulant-adsorbent which is regenerable and reusable. It is effective for the removal of colour and turbidity from water. Alkali treated magnetite can be used alone or in conjunction with another primary inorganic coagulant or organic flocculant. Because of the accelerated kinetics of coagulation and particularly of sedimentation. this new technique is 3-6 times faster than the conventional coagulation-flocculation-sedimentation process. It also appears that filtration of the product water may not be necessary. Regeneration of the spent magnetite is achieved by contacting the used magnetite with 0.l M sodium hydroxide. The development of the process on the laboratory scale using a jar test technique is discussed. INTRODUCTION Attempts to adapt particles of magnetic iron oxide to obtain desirable coagulation-adsorption characteristics were described in the previous articles of this series (Anderson et a L 1980a, b, 1982). Magnetite particles were either encapsulated in a synthetic polymer onto which desirable functional groups were grafted, or treated with a Fe a+ salt to obtain an Fe(OH)3 gel coating which had amphoretic properties. However, either economic or technical difficulties have prevented their commercial application. Natural waters and domestic and industrial effluents are complex, mutticomponent systems. The colloidal fraction is stabilized by electrostatic forces of repulsion existing between particles of the same surface charge, which is usually negative (Hunter & Liss, 1979). The aim of any purification process is to destabilize such particles, generally by adding positively charged species, and to separate the coagulated and flocculated solids from the solution. The conventional process using inorganic coagulants such as alum. or organic polymeric flocculants is reliable and well established, but is a lengthy procedure. Also, filtration is required to remove residual flocs from the product water. The coagulant is, in general, not recovered and recycled. While finely divided particles of sand (Demeter & Galgaczi, 1967), clay, silica or recirculated sludge are sometimes used to improve the quality and settling characteristics of the floc (AWWA, 1971). such particles do not themselves show appreciable surface activity, and their handling, recovery and reuse are difficult. This study shows that by activating the exterior of the magnetite with alkali an extensive positively charged surface is available under mildly acidic conditions. This is the basis for a new magnetic reusable

coagulant-absorbent. Magnetite microparticles show rapid kinetics and settling characteristics and can be easily handled magnetically. The aim of the present study is to discuss the basic principles and applications of this new process under laboratory conditions only. An economic comparison with conventional treatment processes, will be presented in future articles of this series. The results of pilot plant and full scale demonstration plant operation, disposal of the alkaline waste and other details will also be discussed later. The data presented here, especially on the quantities of chemicals used does not necessarily reflect the economics of the process, rather, they were intended to serve as a "'starting point" for further investigations.

MAIN STEPS OF THE PROCESS The process consists of four steps: 1, Premix

Activated magnetite is contacted with water at pH values between pH 4 and 6 for a brief period. For waters containing low turbidity and colour this step is usually sul~cient to produce water of acceptable quality. 2. Coagulant addition and aftermix

If the amounts of impurities exceed certain levels, more magnetite needs to be added to complete the treatment. However, the addition of a coagulant or flocculant is a preferable alternative. After addition of the coagulant, further mixing for a few minutes completes effective attachment of the remaining colloidal particles to the magnetite. The flocculation step, which in the conventional process is accomplished by gentle mixing for prolonged periods, is not required. 3. Separation

When the magnetite particles with attached impurities are magnetized, they agglomerate into "'flocs" and settle our rapidly. Compared with conventional alum-produced 141

142

L.O. KOLARIK

ttocs, the sedimentation rate is approx. 3-4 times faster for nonmagnetizcd and l0 times faster for magnetized Fe30.~ particles.

4, R~Tlencration The used magnetite is separated magnetically from the product water and regenerated with weak solutions of sodium hydroxide before reuse. A similar procedure is used to activate the initial batch of magnetite.

Colour values are adjusted to neutral pH. using colourrpH correlations determined experimentally. Turbidity was measured without filtration with a HACH 2100A turbidimeter and expressed in Nephelometric Turbidity Units (NTU). Iron was determined by atomic adsorption in acidified samples. RESULTS AND DISCUSSION

Clarification studies EXPERIMENTAL

),lagnetite Fe30.~ from Savage River. Tasmania was crushed and cyclonized to obtain particles of 1-5 .urn size. Some experiments were carried out with synthetic F%O.t samples of pigment grade, such as Bayer products 318 M (0.2,urn). 316 (0.4 ttml and 302 T. I0 ml of settled magnetite;water slurry per litre of water {equivalent of 12.5g of dry Fe30.d was used. unless stated otherwise.

4ctication reyenerution The magnetite suspension to be treated is separated from the solution by means of a magnet le.g. flat ceramic type. approx. 1000GI: the remaining water is then decanted. This slurry is then treated with a weak solution of sodium hydroxide and washed 3 times with 100ml of distilled water. Usually 200 ml solution of 0.05-2.0 M sodium hydroxide per 12.5 g of magnetite was used. The effect of temperature ~as also studied. The steps during the regeneration are as follows: (i) regenerant is added to the magnetite slurry and the mixture demagnetized using a commercial demagnetizer (e,g. Eclipse, cat. No. 960. Sheffield, U.K.). Demagnetization causes dispersion of magnetically induced agglomerates to primary particles. The particles are then stirred vigorously with the regenerant for the required time: (ii) the jar is placed on the magnet and the particles separates from the used regenerant. The supernatant liquor is decanted while the magnet keeps the particles in the vessel. Magnetization and demagnetization is also applied during the washing with distilled water. Demagnetization [oscillating of the particles in a.c. field) enhances separation of the attached colloidal matter from the magnetite surface.

Coayulation-adsorption Water treatment tests were carried out with the activated magnetite by contacting it with a sample of the water to be treated. The experiments were performed in I I. square jars using paddle shaped stirrers to provide adequate mixing. Coagulants such as alum. ferric chloride and certain synthetic polyelectrolytes were used in conjunction with the magnetic particles. The jar test technique using coagulant alone was described in detail in the previous study (Anderson et al., 1982). It is important to note that during the coagulationadsorption step magnetite particles should also be demagnetized. When magnetically agglomerated "'flocs" are demagnetized they break up. and hence, a large surface area is available for interaction.

Amtlytical methods After 5 min of settling the supernatant liquor was analysed. Apparent colour was measured without filtration by use of a HACH Comparator calibrated in Pt-Co units. True colour was measured spectrophotometrically after filtration through a 0.45 ~tm filter. The spectrophotometer was calibrated in Pt-Co units. 400 nm.

Effect of particle size. The results obtained earlier using coated magnetite showed that particle size is critical (Anderson et al.. 1982). Very poor coagulation was observed when particles prepared from magnetite > 6 ttm were used. Similar jar tests were performed in this study using magnetite of various particle sizes, treated with sodium hydroxide. Results in Table 1 show the coagulation-adsorption eNciency as a function of particle size, after activation with sodium hydroxide. Magnetite samples were placed in 1 1. of Yarra River (Melbourne) water and stirred at pH 5 for 15 rain, without the addition of a coagulant. The results show the importance of high surface area. This was further demonstrated by comparing the performance of samples of natural I-5 ~m F%O,~ and synthetic pigment grade Fe30,~ (Bayer 318 M) of particle size 0.2~Lm. In the treatment of an underground water at Mirrabooka, near Perth, both samples were used in conjunction with 0.5 mg 1-~ of cationic polyelectrolyte Cyanamid C-573 which was added after 10 min of premix at pH 4. Mixing was continued for a n o t h e r 5 rain. The results in Table 2 show clearly that the magnetite particles used in the clarification process should be as small as possible. The limitation is the slow settling rate of particles < 1 /am in size. Magnetite of particle size 1-5 t~m was used in all further experiments. Amount of magnetic material. To demonstrate the influence of the quantity of magnetite used in the coagulation-adsorption process, Yarra River water was treated with varying a m o u n t s of activated magnetite at pH 4 for 15 min. The results of a series of experiments are shown in Fig. 1. No coagulant was used with magnetite. The o p t i m u m dose of magnetite was 1 0 m l l -~ A blank experiment, i.e. stirring of the water at pH 4 for 15 rain, showed certain colour and turbidity removal (turbidity 9-10 NTU, colour 50 Pt-Co), due to the pH change and autocoagulation. Table l, Effect of particle size on the clarification of Yarra River water Amount of magnetite (ml 1- ~) -

-

10 I0 10

Particle size of magnetite (/~m) Raw water 15-50 6-15 1-5

Product water Turbidity Colour (NTU) (Pt-Co) 21 17 14 3

28 25 22 4

Colour and turbidit? removal with reusable magnetite particles--IV

The magnetite clarified the water at pH 4 without alum addition. At this pH only traces of iron (<0.1 mg 1- ~ Fe) were detected in unfiltered samples. The untreated magnetite performs marginally better than coagulation induced by stirring of the water at various pH values alone (autocoagulation). This indicates the lack of a significant positive charge on the surface of untreated magnetite. However, at pH 3, increased adsorption was noted. Use in conjunction with alum. The performance of alkali treated magnetite at various doses of alum is shown in Fig. 3. The results of conventional alum treatment and treatment utilizing untreated Fe30, , are shown as well. The advantages of alkali treatment are clearly highlighted. Yarra River water required only 10--15 mg 1-t of alum in conjunction with activated magnetite whereas 3 0 m g l -~ was required when alum alone was used, and the quality of the product was inferior. The performance of untreated magnetite was also inferior to that of activated magnetite. Use in conjunction with cationic polyelectrolyte. The effect of a cationic polyelectrolyte used in conjtanction with activated magnetite is demonstrated in Table 3, where performance of alum and the polyelectrolyte Catoleum A-8101 is compared in the treatment of Yarra River water. The results indicate that polyelectrolyte is an attractive alternative to the inorganic coagulant and warrants further investigation, Optimization of jar test. The removal of colour and turbidity using 10--20mll-' of activated magnetite alone under various conditions, as indicated in Figs 4 and 5, was investigated. Figure 4 shows that the coagulation-adsorption result is strongly influenced by the quality of magnetite used, confirming the importance of the magnetite surface area awtilable. The removal of colour was observed to be a faster process than the removal of turbidity, and was accomplished in approx. 10 rain. Turbidity removal also appears to be more dependent on the quantity of magnetite. This was apparently due to the different size of the colour

Table ~ Effect of particle size on the clarification of Mirrabooka water Amount of magnetite {roll-t)

Particle size of magnetite (um)

-I0 t0

Raw water 1-5 0.2

Product water Turbidity Colour {NTUI " (Pt-Co) 8.8 1.9 0.S

60 35 10

h!fl,eence ql'pH. The effect of pH on performance of activated magnetite was tested using Yarra River water. Figure 2 compares colour and turbidity removal by alkali treated and untreated magnetite with that produced by autocoagulation. The positive charge on alkali treated magnetite increases with decreasing pH (Kolarik et al., 1980). The improved adsorption at pH 3 is ascribed to the increase of positive surface charge on the magnetite as well as to the increase in ionic strength of the solution, causing a compression of the double layer surrounding the magnetite and the colloidal impurities. Raw w a t e r

Turbidity Colour Alkalinity

7

pH 7 . 0 2 4 NTU 7 0 P t - Co 0.5 meq/I

70 i

6

60 c

z •. .'Z--

,5

Residual

50

tubidity

g

Q.

4

40

z

o

3

3o

g

•-o --

2

20

o 3 .'2_

..0 cl

0

o

5

.~"

io

15

Alkali treated magnetite ( m i l l )

Fig. t Turbidity and colot, r removal as a function of the amount of alkali treated magnetite.

Raw water Colour Turbidity Alkalinity

60

pH 7.2 60 Pt-Co 52 NTU 0.6 meq/I

o

o i

60

ck

~Z

50

50 c o

40

o

/ ~

z0

re

IO 5

/----

la3

o

~

/

40

/

//-

30

o

o

O 0

3

4

5 pH

6

7

0

3

4

,5

6

7

pH

Fig. 2. Effect of pH on colour and turbidity removal in the absence of coagulant. Samples stirred at each pH for 15 min and measurements made after 5 min settling. Alkali treated magnetite {e). untreated magnetite (O) and water alone (rq).

l-L~

L. 0. Raw water Colour Turbidity Alkalinity

6O

KOLARIK

pH 7.2 60 PI-Co 52 NTU 0.6mlq/I

50 Z

o I

6O

E

50

,%

¢

>, 40 30

o

zo

"o 5

0

0

5

I0

15

20

25

30

0

5

AI2(S04) 3 16 H=O {rag/I]

I0

15

20

25

30

A12(S04) 3 16 I"LzO(mgll)

Fig, 3. Use of alkali treated magnetite with coagulant. Comparison of performance of alum treatment alone {O)with treated (@)and untreated (©) magnetite. Settling time--alum flocs: 20rain: magnetite: 5 rain.

Table 3. Comparison of cationic polyelectrolyte and alum in conjunctive use with magnetite Amount of magnetite (ml I- z) -10 10

Coagulant Alum A - 8 1 0 1 (mg 1- 1) Raw water 20 ---

1.0

bodies and other suspended solids (clay particles, algae, etc.). A time of 15-20min, depending on the quantity of magnetite used. was normally required to complete the removal Colloids and magnetic particles were mixed in square shaped, I 1. containers using 2.5 × 7.5 cm stirrers. Adequate contact was achieved at stirring speeds of 160-200 rpm (Figs 4 and 5), A stirring speed faster than 200 rpm caused an increase in residual turbidity due to excessively high shear forces (Fig. 5). Colour removal was unaffected. Optimum jar test conditions were obtained when activated magnetite was contacted with water for a short time (premix) prior to coagulant addition, followed by a certain aftermix period to allow attachment of the impurities onto the magnetite surface. The optimum conditions were a 10rain premix at 160 rpm and pH < 6.5, an aftermix period of 4-5 min following addition of the coagulant, and a settling period of 5 rain, Under these conditions satisfactory removal of colour (~<5Pt-Co units) and turbidity (~<1 NTU) was obtained. For a low turbidity, high colour water, the activated magnetite particles could be utilized in either the magnetized or demagnetized form. However, for treatment of high turbidity waters in the absence of alum it was definitely beneficial to use the demagnetized, unflocculated form. The turbidity level could be

Product water Turbidity Colour (NTU) (Pt-Co) 50 l.l 0.5

pH

80 <5

7 5

<5

5

5O 20

o

d;~.~. ~ "N "~lJ ~

R o . *oter pH 7 0 Colour 65 P't - Co Turbidify 32 NTU

I0

0

o

,

,

,

,

,

;

r T

;

;

"~

2O

-~

o

I0

60

80

IOO

120

140

160

Speed ( R.P.M )

Fig. 4. Colour and turbidity removal as a function of magnetite quantity and stirring speed. Alkali treated magnetite contacted with 11. of raw water at pH 4 for 15 min. 10 ml FesO., (O) and 20 ml Fe30.~ (E3).

CoIour and turbidity removal with reusable magnetite particles--IV

145

large quantity of magnetite is expensive, unless a source of cheap heat is available. Thus it was decided o,. Raw water pH 7.0 v to use 200ml of0.1M NaOH at 20:C over a period Colou r 4 5 P t - Co c of 10 rain per 10 ml {12.5 g dry) of magnetite slurry in Turbidity 3 4 N T U all further experiments as a standard reactivation pro¢x o cedure. No significant deterioration in the coagulaour tion-adsorption efficiency on prolonged reuse was obo o co served. l 1 I l , I Regeneration with lime. Regeneration with lime was investigated as a cheaper alternative to sodium hydroxide. However, contact of the magnetite particles with a solution of calcium hydroxide had a deleterious effect on performance in the coagulation-adsorption step. During the lime treatment, some turbidity e Turbidity particles but no colour substances, were released from the magnetite surface. The magnetite surface, satug g rated with impurities, lost its coagulation-adsorption properties. Regeneration with acid. The use of a cheap sulfuric 3 I .~ I 1 1 acid in the regeneration step was also investigated as 0 I00 150 200 2'50 300 a possible economic proposition. It was found that periodical treatment (every fifth reuse of otherwise Speed (R.P.M.) untreated magnetite) using H,SO+ solution at pH 2 Fig. 5. Colour and turbidity removal as a function of stir- was ineffective. However, a solution of hydrochloric ring speed. Alkali treated magnetite (t0mll-Vl contacted acid at pH 2.5 was partially effective if used in every with l I. of raw water at pH 4 for 15 rain. cycle but was inferior to alkali treatment. When more concentrated sulfuric acid was used some improvement was observed, but the effect did not approach that of alkali treatment, as shown in reduced from 30 to 7 NTU (magnetized) or 2 NTU (demagnetized), and colour from 65 to 15 units (mag- Table 4. Colour removal does not depend greatly on surface netized) or 5 units (demagnetized) at pH 4. In the treatment, suggesting that both electrostatic interacpresence of 20 mg 1- 1 of alum at pH 5, high performtion and chemical reaction of the magnetite with the ance was obtained using either form, with the product humic substances are taking place. This technique water having a turbidity of 0.9 (magnetized) or 0.6 could be advantageous for treatment of low turbidity, NTU (demagnetized), and colour <5 units in both high colour waters. caSes. In "'standard" comparative jar tests the magnetite Two step regeneration with alkali and acid particles were used in the demagnetized form. The effect of combined alkali/acid regeneration on Regeneration studies the coagulation-adsorption characteristics of magneAfter the clarification step, the surface of the mag- tite was investigated. Colour and turbidity removal netite is saturated with attached impurities and co- using this material was monitored and compared with agulant. To renew its coagulation-adsorption capacity acid or alkali reactivated samples. Under the same the surface must be cleaned; a cheap, effective reacti- experimental conditions the alkali/acid treated magvation procedure is crucial to the economic viability netite always performed more efficiently. The results in Table 4 indicate that acid reactivation is usually of the process. Regeneration with caustic soda. Studies have shown less effective than alkali and that the most effective that the regeneration efficiency, expressed in terms of treatment was with combined alkali/acid. This is in accord with the results of our e[ectrophoretic study of turbidity and colour removal from Yara River water, magnetite surface characteristics (Kolarik et al., 1980). depends on alkali concentration, temperature and The aim of regeneration is therefore to clean the contact time with the magnetite. The reactivation effect was studied over a range of conditions using magnetite surface effectively and economically. Samples were used in conjunction with the cationic 0,0%2 M NaOH at ambient and elevated temperapolyelectrolyte Catoleum A-8101 and the results are tures (up to 60"C). It was shown that the same activation could be achieved, for instance, by treatment of averaged for a number of consecutive runs. Regeneration with alkali and oxidant. Magnetite, magnetite with 200 ml of 0.05 M NaOH at 6WC over FeO-Fe203, being a mixed oxide contains both Fe 2+ a period of 10min as with the same volume of 0.5 M NaOH at 6WC over 5 rain. However, comparable and Fe 3+ in its inverse spinel structure (Wells, 1962). results were obtained using 0.1 M NaOH at ambient Under acidic conditions the surface thus may contain a mixture of Fe 2+ and Fe a+ ions. Under alkaline temperature over a period of 10 min. Heating up of a o ¢..9 i

[46

L.O. KOLARIK Table 4. Clarification of Yarra River Water at pH 5.5 as a function of regenerant Amount of magnetite Iml 1- t) -

Amount of A-8101 [mg 1- ~1

-

10 10 10 10 I0

-0.3 0.3 0.3 0.3 0.3

Regeneration procedure t200 mI regenerant,'10 min) Raw water 0.05 M H_,SO.~ 0.2 M H,SO.~ 0.5 M H,SO.L 0.1 M NaOH 0.1 M NaOH followed by 0.2 M H,SO~

conditions with oxygen present the surface is assumed to be oxidized (Stumm. 1961). The conditions on the surface effect the properties of the interface and thus the coagulation-adsorption characteristics of the particles. The effect of oxidant was therefore investigated by blowing air into the alkali-magnetite mixture in one sample and by using hydrogen peroxide in conjtmction with alkali in another. The effect was monitored and the results expressed in terms of efficiency of colour and turbidity removal from Yarra River water. Results of a number of consecutive jar tests showed that the aeration of a 0.1 M N a O H solution containing magnetite for I0 min did not inftuence performance. The rest, It was comparable to that for magnetite activated with 0.1 M N a O H exposed to air. It appears that a sufficient quantity of oxygen entered the mixture during stirring, and forced aeration did not improve the situation. Oxidation with a strong oxidant such as H20., improved coagulation-adsorption properties of magnetite. Excellent colour and turbidity removal (product water turbidity 1 NTU, colour 10 Pt-Co) was recorded when 1 0 m l l - I of magnetite was treated with 4 0 m l 0.1 M N a O H and 5 - 1 0 m l of 30~o H202 for 10min. Under the same conditions, activation with 0.2 M N a O H had to be used to obtain similar results. The oxidizing action of HzO2 was not studied in detail because the process would be uneconomical in practice. However, it could be assumed that the surface of the magnetite as well as the attached organic m~,terial was oxidized. Treatment with H 2 0 2 could thus influence both the surface characteristics of magnetite and the regeneration efficiency, due to an easier separation of the degraded organic macromolecules from magnetite. Other oxidants could perhaps be used if costs permitted.

CONCLUSIONS The interaction of activated magnetite with the colloidal impurities present in water is an interracial phenomenon. Thus, a knowledge of the magnetitewater interface with respect to its behaviour in the process is important. The effect of various pretreatments of magnetite surface and the basic chemistry of

Product ~',ater Turbidity Colour INTLI I Pt-Cm -'2 20 15 10 1-2
30 5 5 5 <5 <5

the process have been discussed elsewhere (Kolarik et al., 1980). The same principles concerning metal hydroxides (Anderson et al., 1982) can be applied to metal oxides in contact with an aqueous medium. Below a certain pH value (pH 6.5 for magnetite), surface is positively charged and may interact with the negatively charged waterborne impurities (Kolarik, 19801. Conversely, above this pH the surface acquires a negative charge and the attached impurities can be stripped off. This phenomenon forms the basis of the clarification and regeneration steps of the process. It also appears that any oxide having such properties can be used in water treatment. The choice of magnetite gives the added advantage of accelerated sedimentation of the magnetically agglomerated particles. Regeneration of the magnetite particles can be carried out with 0.1 M sodium hydroxide at ambient temperature over 10 min. Vigorous stirring must be provided. Under these conditions no significant degradation in coagulation-adsorption efficiency of activated magnetite was observed in prolonged, repeated reuse. Current research is focused on further optimization studies and process development work.

Acknowledgements--The author is indebted to Drs B. A. Bolton. D. R. Dixon, A. J. Priestley, and D. E. Weiss and to Messrs W. G. C. Raper and N. J. Anderson for their advice and helpful discussion during the course of this study and in the preparation of this article. The technical assistance of Mrs M. Skukies is also appreciated.

REFERENCES

American Water Works Association (197l) Water Quality and Treatment, Third Edition, pp. 94-97. McGraw-Hill, New York. Anderson N. J., Kolarik L. O., Swinton E. A. & Weiss D. E. (1982) Colour and turbidity removal with reusable magnetic particles--Ill. Immobilized metal hydroxide gels. Water Res. 16, 1327-1334. Anderson N. J., Bolto B. A., Eldridge R. J_ Kolarik L. O. & Swinton E. A. (1980a) Colour and turbidity removal with reusable magnetic particles--ll. Coagulation with magnetic polymer composites. Water Res. 14, 967-973. Anderson N. J, Eldridge R. J., Kolarik L. O.. Swinton E. A. & Weiss D. E. {1980b1 Colour and turbidity removal with reusable magnetic particles--I. Use of magnetic

Colour and turbidity removal with reusable magnetite particles~IV cation exchange resins to introduce aluminium ions. ~Varer Res. 14. 959-966. Demeter L. & Galgaczi D. (19671 U,S. Patent 3 350 302. Hunter K. A. & Liss P, S. (1979~ The surface charge of suspended particles in estuarine and coastal waters. .Yature 282, 823-825. Kolarik L. O,. Dixon D. R,, Freeman P. A., Furlong D. N. & Healey T. W. (19801 Effects of pretreatments on the

147

surface characteristics of a natural magnetite. Fine particles processing. Proceedings of the International Symposium on Fine Particles Processin 9, Chap, 34, Vol. 1. pp. 652-665. Stumm W. (1961) Oxidation of ferrous iron. Ind. E~qnq Chem. 53(2), 143-146, Wells A. F. (1962) Strnctnral [norqanic Chemistry. Third Edition, pp, 490---492, Clarendon Press, Oxford,