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Magnetic separation of iron and heavy metals from water

J.D. Navratil* and M.T. Shing Tsair**

* Clemson University, Environmental Engineering and Science, 342 Computer Court, Anderson,SC 29625-6510 USA

** Department of Life Sciences and Chemical Technology, Ngee Ann Polytechnic, 535 Clementi RoadSingapore 599489

Abstract A magnetic separation device is being developed for removal of iron and heavy metals from water.The device consists of a column of supported magnetite surrounded by a movable permanent magnet. Themineral magnetite, or synthetically prepared iron ferrite (FeO.Fe 2O 3), is typically supported on variousmaterials to permit adequate water passage through the column. In the presence of an external magnetic

eld, enhanced capacity was observed in using supported magnetite for removal of actinides and heavymetals from wastewater. The enhanced capacity is primarily due to magnetic ltration of colloidal andnanoscale particles along with some complex and ion exchange sorption mechanisms. This paper will reviewsome previous work on the use of magnetite for wastewater treatment and discuss the development andpotential of the magnetic nanoscale ltration/sorption process for water treatment. Recent research resultsare also presented on preliminary experimental studies using the process with water samples containingiron.Keywords Adsorption; ltration; iron; iron ferrite; magnetic separation; water treatment

Introduction

Drinking water quality is being increasingly compromised because of outdated water treat-ment plants and older distribution piping systems. Impurities can add objectionable tasteand odors. Iron is one of the major impurities that is commonly found in many sourcesof water. Iron deposited in the distribution system may promote the growth of micro-organisms leading to high turbidity in drinking water.

The European Committee (EEC, 1980)standard recommends that the iron concentrationin drinking water should be less than 0.2mg/L. In the United Status, the guideline level of iron in drinking water is less than 0.05 mg/L (VEWIN, 1993). The lower level of iron isdesirable in order to avoid problems and increased cost in the maintenance of water distri-bution systems. Iron related problems have become a significant water quality issue to

water supply authorities in Adelaide, Melbourne, the Gold Coast of Queensland and in theurban regions north and south of Sydney (OConnor, 1995). It should be recognized that theproblem is not unique to Australia with, for example, roughly 40 per cent of public watersupplies in the United States exceeding recommended levels of iron (Salem, 1995). Inrecent years there has also been interest in iron removal from well water in the New NileValley in Egypt (Aly, 2000).

This paper briefly reviews a new technique, magnetic filtration/sorption, as a potentialfield device for removing iron from water. The proposed method has the potential to reducecontamination problems inexpensively. Recent studies have demonstrated the magneticenhanced removal of radioactive and heavy metals from water (Boyd et al ., 1986; Kochenand Navratil, 1997; Navratil et al ., 1995 and Cotton and Navratil, 1997). Several suitablesupport materials including glass wool and sand were tested for mixing with magnetite toachieve satisfactory water flow characteristics while retaining the magnetic field enhancedsorption properties of magnetite (Cotton and Navratil, 1997). This paper will review

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selected previous work on the use of magnetite for wastewater treatment and discuss thedevelopment and potential of the magnetic filtration/sorption process for water and waste-water treatment. Recent preliminary research results will also be presented that havefocused on experimental studies with water samples containing iron.

Review of past developmentsMagnetite or iron ferrite has been used to separate a wide variety of substances such as dis-solved metal species, particulate matter, organic and biological materials (Rubio et al .,1996). Magnetic separation and high gradient magnetic separation have been used exten-sively in the processing of minerals and more recently water treatment and environmentalapplication (Davis, 1997).

Magnetite has the ability to remove heavy metals as reported by Kochen and Navratil(1997) who used a magnetic polymer resin for the removal of actinides and other heavymetals from contaminated water. A polyamine epichlorohydrine polymer was synthesizedcontaining magnetite on the surface of the resin beads. In the absence of an external mag-netic field, the iron ferrite readily adsorbed numerous metal species. In the presence of an

external magnetic field, it was observed that there was an increased capacity for theremoval of plutonium-239 and americium-241. These results imply that adsorption proper-ties of magnetite can be used in conjunction with a magnetic field to remove both metal ionsand metal colloids and nanoparticles. Most mechanical filtration systems are only slightlyeffective at removing colloids, so this magnetic enhanced process could offer a substantialimprovement.

Other investigators have also reported heavy metal removal with magnetite both in andout of the magnetic field. In 1969, Petkovic and Milonjic (1995) reported the adsorption of cesium on magnetite between the pH of 7.6 and 10.4. It was observed that cesium adsorp-tion increased with pH although the mechanism was not presented. Peak adsorption was

reported at a pH of 10.4. In 1970, Milonjic and Ruvarac also reported the adsorption of cesium (I), cobalt (II) and cerium (II) by magnetite at a pH of 2.2 by batch experiments.Cobalt adsorption was reported at less than 5% w/v although actual data was not enough toverify the results. In addition, work of Boyd et al . (1986) reported the iron ferrite removal inthe pH range of 913. Boyd et al . (1986) also reported the effect of dissolved chemicalspecies on ferrite formation including the use of commercially available magnetite.

In addition, Krumm (1989) described another application of carrier magnetic separation(CMS) for the purification of wastewater containing phosphate, metals, and heavy metaland pigment particles with magnetite and a magnetic carrier. The removal of lead and cop-per ions from diluted solution by sorption into clinoptilolite, together with magnetite, as

reported by Feng et al . (2000) showed that the process was very rapid, effective and yieldedclear solutions with low residual concentrations of heavy metals. In their experimentation,concentration of clinoptilolite fines were used as heavy metal adsorbent, while low dosagesof magnetite fines were used as magnetic carrier materials. In addition, hydrolyzed Fe + 3

was used as a coagulant.

Materials and methodologyFeed solutions of concentration 10 4 , 105 and 10 6 mole/litre (mol/L) were prepared usingiron ammonium sulfate. Feed solutions were sent to columns up flow using a Master Flexconsole drive tubing pump fitted with two Master Flex pumping head. Flow rates were var-

ied 0.1 ml/min to 6 ml/min to determine the adsorption kinetics, i.e. the residence timerequirement. Three columns (of each type) were run in conjunction with a blank, with andwithout 0.5 tesla magnetic field. The columns used were made of plastic droppers 15.7 cmlong with a diameter of 0.3 cm and a plastic filter funnel of diameter 5.3 cm. The magnetite

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and wool added to the columns were of mass percentage 80% and 20% respectively. Thus amass of 0.8 g of magnetite and 0.2 g of wool were used throughout the experimentation.

The volume of feed varied as iron solution was continuously passed through all theecolumns until 100% breakthrough took place. To determine breakthrough capacities, frac-tions of the samples were collected using test tubes of 4 mL volume and transferred to a 25ml volumetric flask containing buffering reagents. Flow rate and pH were measured at eachinterval and samples were analyzed using a spectrophotometer. Sample blanks and period-ically calibration checks ensured sample accuracy and consistency.

The iron concentration was determined colorimetrically using a Spectronic 20 Genesysspectrophotometer equipped with single cell holder, 5 mm disposable cuvets. Ferrous ironfeed solution was made from reagent grade iron ammonium sulfate. Hydroxylaminehydrochloride, sodium acetate, ortho phenanthroline and sulphuric acid were reagent gradematerials. Prior to use, hydroxylamine hydrochloride and sodium acetate were made bydiluting to stock solutions of approximately 5% w/v, and stored in plastic bottles untilrequired. 0.001 mol/l of sulphuric acid was prepared and standardized according to stan-dard method. 0.1% w/v of ortho phenanthroline iron indication solution from Fisher

Scientific USA was used prior to each experiment. Unused solution was refrigerated. Alldilutions were made using distilled, deionization water.

Results and discussionTable 1 shows the summary results of the preliminary column experiments. In comparingthe duplicate data in Table 1, the breakthrough capacities are in good agreement (510%error). Generally, the breakthrough capacity decreases with increasing iron concentrationin the feed and with increasing flow rate, as expected.

The presence of a magnetic field does play a part in the adsorption of iron onto mag-netite. As shown in Table 1, for a concentration of 10 5 mol/l, the column surrounded by a

0.5 tesla magnetic field resulted in a higher iron breakthrough capacity. From this result, inagreement with reported results, we can conclude that magnetic field does affect theremoval of colloidal ferric hydroxide particles along with the adsorption of iron fromsolution.

When sea sand was added as part of the supporting material for the column, the rate of removal of iron was also affected. As we can see in Table 1 for the same concentration andflow rate, the breakthrough capacities were higher. Sea sand does not only reduce channel-ing but also improve the surface area in contact between magnetite and the colloids present.Thus we can also conclude that sea sand is a useful support material that permits adequatewater flow through the column of magnetite.

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Table 1 Column breakthrough capacity tests

Feed iron(II) Column type (mol/l) Flow rate 10% 50% Magnetic

concentration (ml/min) breakthrough breakthrough eld

(mol/l) vol feed (ml) vol feed (ml)

2.75 10 4 Magnetite and glass wool 0.8 < 12 ml 15 Yes2.75 10 4 Magnetite and glass wool 0.8 12 20 Yes

2.75 10 4 Magnetite and glass wool 2 4 14 Yes

2.75

104

Magnetite and glass wool 2 10 20 Yes2.75 10 4 Sea sand, wool and magnetite 0.7 15 20 Yes2.75 10 4 Sea sand, wool and magnetite 0.7 < 12 18 Yes2.75 10 5 Magnetite and glass wool 0.75 28 40 No2.75 10 5 Magnetite and glass wool 0.75 24 76 Yes

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ConclusionsThis work provided a quantitative framework for assessing the utility of applied magneticfields for creating an attractive magnetic force between the adsorbent (magnetite) and theadsorbate (iron). The use of magnetite adsorbent material in a fixed bed model surroundedby a high gradient magnetic field was shown to be efficienct for the removal of iron fromwastewater. The above experimentation proved that iron could be adsorbed by magnetiteprovided with the correct type of support material and flow rate.

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Chemical Thermodynamics , 25 (12), 14431445.Aly, H. (2000). Egyptian Atomic Energy Authority, Cairo Egypt, private communication.Boyd, T.E., Cusick, M.J. and Navratil, J.D. (1986). Chapter 6: Ferrite Use in Science and Technology,

Recent Developments in Separat ion Science Volume (III), CRC Press, inc, Boca Ration, Fl.Cotton, G.B. and Navratil, J.D. (1997). Proceedings of Waste Management Conference .Davis, J. (1997). Removal Iron and manganese from nature water Diversity Water Technology, Incm

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L229/11L229/23.Ellis, D., Bouchard, C. and Lantagne, G. (2000). Removal of iron and manganese from groundwater by oxi-

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Kochen, R.L. and Navratil, J.D. (1997). Removal of Radioactive Materials and Heavy metals from WaterUsing Magnetic Resin, United States Patent 5,595666.

Milonjic, S.K. and Ruvarac, A. (1970). Adsorption of Cs + , Co 2+ and Ce 3+ From Acid Aqueous Solutions onNatural Magnetite, Bullet in of the Boris Kidric Institute of Nuclear Sciences , Vol. 21, Chemistry, No. 3P/462.

Navratil, J.D., Kochen R.L. and Ritter, J.A. (1995). Magnetic Swing Adsorption Process, Proceedings of Waste Management Symposia 1995, Tucson, AZ.

OConnor, J.T. (1995). Iron and Manganese; Water Quality and Treatment A Handbook of Public Water Supplies , Chapt 11, pp. 378396.

Petkovic, D.M. and Milonjic, S.K. (1969). Adsorption of Cesium From Basic Water Solutions on NaturalMagnetite, Bullet in of the Boris Kidric Institute of Nuclear Sciences , Vol. 20, Chemistry, No. 3 P/449.

Rubio, J., Schneider, L.A.H. and Allaga, W. (1996). New Processes for heavy metals separation from wastewater streams in the mining industry, Proc. Clean Technology for Mining Industry , Santiago, pp. 8598.

Salem, F.B. (1995). Determination of Iron in Water Samples, Reviews in Analyt ical Chemistry , 14 (1),5973.

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