Magnetic measurements and heavy metals in atmospheric particulates of anthropogenic origin

The Science of the Total Environment, 33 (1984) 129-139 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands

MAGNETIC MEASUREMENTS AND HEAVY PARTICULATES OF ANTHROPOGENIC ORIGIN A. HUNT, J. JONES, F. OLDFIELD Geography Dept., University of Liverpool,

METALS

P.O. Box 147, Liverpool

129

IN

ATMOSPHERIC

L69 3BX (England)

ABSTRACT Recent investigations have established an apparent link between magnetic mineral and heavy metal concentrations in a variety of environmental contexts. Using a range of rapid, non-destructive magnetic measurements the magnetic components of power The present station fly ash and motor vehicle emissions have been characterised. investigation suggests that such an approach readily facilitates particle source differentiation and identification. INTRODUCTION A number of recent papers have established an apparent link between magnetic mineral and heavy metal concentrations in several different types of aquatic environment. Scoullos et al. (ref. 1) and Scoullos (ref. 2) in their studies of the particulate effluent from a large iron and steel plant discharging into the Elefsis Gulf, Greece, show that the saturation isothermal remanent magnetization (SIRM) of filter paper residues from the waters of the Gulf is roughly proportional to concentrations of particulate iron and zinc. Revitt et al. (ref. 3) document a strong linear relationship between SIRM, and concentrations of lead, zinc and copper measured on filter paper residues from a storm water drainage system in Hendon, N. London. Oldfield et al. (ref. 4) identify a parallel between both concentration dependent and normalized mineral magnetic changes and increased lead and copper concentrations in the recent In this last case the authors interpret the sediments of Newton Mere in Cheshire. apparent link between magentic minerals and heavy metals as an expression of increased atmospheric input over the last century. The growing importance of anthropogenic sources of magnetic minerals in the atmosphere in recent times is confirmed by mineral magnetic studies of ombrotrophic peat profiles from Britain (refs 5, 6) and Finland (ref. 7) where SIRM measurements on moss-increment dated peat profiles provided a chronology of magnetic and henc,e fossil-fuel derived particulate deposition for the 19th and 20th centuries. Although many anthropogenic aerosols are known to have a significant magnetic component (refs. 8, 9), they have nevertheless rarely been examined with regard to their magnetic properties (ref. 10). Even where magnetic separation techiques have

0048-9697/84/$03.00

0 1984 Eisevier Science Publishers. B.V.

130 been used to differentiate between emission categories (refs II, 12) no subsequent magnetic measurements have been carried out either to test the efficiency of the magnetic separation method used or to characterize more fully the magnetic properties of the separated fraction. Whitby and Cantrell (ref. 13) suggest a scheme whereby virtually all the magnetic material generated by all the relevant industrial processes (ref. 14) will be within the ‘coarse particle’ range from c 2pm diameter upwards. This is consistent with all the measurements on magnetic spherules made by Puffer et al. (ref. 9) in the New York area and over the nearby parts of the N. Atlantic, as well as with studies of coal fly-ash by Hansen et al. (ref. 11) and Ondov et al. (ref.15). Keyser et al. (ref. 16) also show that auto-exhaust particles above 1Ottm are rich in iron but that the finer particle mode less than lj.trn contains little or no iron. The relationship between magnetic oxides and heavy metals in fly ash, industrial particulates and auto-emissions is poorly understood, though several authors point to the possibility of close links. Theis and Wirth (ref. 17) note that most metals in the eleven coal-fired fly-ash samples they considered were associated with specific surface oxides of iron, manganese or aluminium. They record copper, chromium, arsenic and zinc as being associated with iron oxides in almost all cases, cadmium and nickel mostly with manganese, and lead with either. Hansen et al. (ref. 11) show that chromium, manganese, cobalt, nickel, zinc and beryllium are all significantly enriched in the ‘magnetic’ fraction of coal fly-ash. Hulett et al. (ref. 13) also show that the first transition group elements (V, Cr, Mn, Fe, Co, Ni, Cu and Zn) are enriched in the magnetic spine1 fraction of fly-ash, irrespective of variations in the type of coal used. Linton et al. (ref. 12) and Olson and Skogerboe (ref. 19) note an association between ‘magnetic iron’ and lead in automobile exhaust particulates sampled on roadways. Hansen et al. (ref. 11) suggest that “Magnetite may also be a hazard to health because of its ability to occlude biologically active transition metal ions such as manganese and nickel by isomorphous substitution . . . . . . and thus act as a slow release carrier agent for toxic elements”. There are many gaps in our knowledge and there is great uncertainty about the extent to which demonstrated magnetic-heavy metal linkages reflect surface association or incorporation into the crystalline matrix of particulate emissions. It is nevertheless reasonable to explore situations in which, in purely empirical terms, the linkage appears to occur or in which the linkage would, if confirmed by further studies, be of major value in both historical and contemporary particulate pollution monitoring and source tracing. The attraction of the magnetic approach lies above all in the nature of the techniques themselves. Portable fluxgate magnetometers and pulse magnetizers, used in conjunction, can detect magnetite concentrations in the range of parts per million using methods of sampling and measurement which are very rapid and non-destructive, adaptable to a wide range of environmental situations and materials, and relatively cheap. Moreover, the methods are almost always compatible with further

131 conventional

follow-up

consuming

TABLE

studies

analytical

involving

for

example,

destructive

and/or

time-

techniques.

1

Magnetic

Parameters

and

Definitions

Specific

magnetic susceptibility (X) This is the ratio of the magnetization produced in a substance to the intensity the magnetic field to which it is subject. This measure of “magnetizability” often approximately proportional to the volume of ferrimagnetic oxides sample.

of is in a

Saturation isothermal remanent magnetization (SIRM) Isothermal remanent magnetization (IRM) is the magnetization which remains after a sample has been subjected to a magnetic field at room temperature. IRM increases nonlinearly with increasing field strength, and saturates in a high field. This maximum remanent magnetization is known as the saturation isothermal remanent magnetization. Coercivity of saturation isothermal remanent magnetization ((Bo)cR) This is the magnetic field strength required to reduce the IRM to zero after saturation. In this study the field has been determined from a series of IRM measurements. The procedure followed involved growing and measuring the SIRM of a sample and then growing and measuring IRMs in a sequence of increasing reverse fields, so that initially the net IRM decreased at each step and then subsequently increased until saturation in a reverse field took place. The coercivity of remanence is the field at which the SIRM is reduced to zero. (Bo)CR may be regarded as a measure of the stability of magnetization and is effected by mineral type and size rather than mineral concentration. Similar coercivity of remanence profiles are indicative of similar magnetic mineral assemblages and vice versa Anhysteretic remanent magnetisation (ARM) Anhysteretic remanent magnetisation strong alternating field in the presence ARM may be regarded as being sensitive

Magnetic The

magnetic

parameters

susceptibility

isothermal

remanent

parameters.

of are

(magnetite

weakly

will are

not

the

more

be low

used

rarely

reduced in

contribute influenced

present

here

water,

by

diamagnetism.

significantly

iron

are

the

to

the

in (ARM)

concentration of measured

calcium Also,

samples,

paramagnetic

all

the

carbon,

rich

defined

concentration

However,

in which

otherwise

are

magnetization,

(SIRM) to

maghaemite).

by dia-or

paper

remanent

X is proportional

samples,

will

paramagnetism values

and

are

in

magnetizations samples,

ferrimagnetic

abundant,

used

( X ), anhysteretic

In most

concentrations

IRM

of

a of

Parameters

Magnetic

oxides

is produced during the smooth decay of a weak steady field. The magnitude to the smaller grain size.

effects

susceptibility

they

or silica

ferrimagnetic and/or

susceptibility. since

1. the

related

antiferromagnetism

magnetic

and

ferrimagnetic

carbonate where

Table

ARM are

measured

and

132 after the sample has been removed from the field used to grow the remanence. However, both are more strongly affected by magnetic grain size variations and by antiferromagnetic components (e.g. haematite) than is susceptibility. In general, only in sample assemblages where X and SIRM are proportional can SIRM by used as a reliable concentration parameter. The range of normalized parameters used here will vary according to the magnetic mineral and grain size assemblages present. Although using these parameters alone, it is difficult to make quantitative estimates of the relative proportion of different often possible to magnetic mineral phases present in samples, it is nevertheless differentiate between sample sets and to identify magnetic mineral sources (refs 20, 21). HYPOTHESES The work completed so far has been designed to begin to test the following hypotheses:That the pattern of SIRM and Xvariations in recent ombrotrophic peat is related to the concentrations of heavy metals such as lead, copper, zinc and cadmium in the historical record. This can be used as a basis for reconstructing heavy metal deposition histories. That SIRM and X values in recent peat can also be used to provide insight into spatial variations in the cumulative heavy metal loadings received since the beginning of the Industrial Revolution. That where both magnetic minerals and heavy metals have been generated by the same industrial or fossil fuel combustion process, the link between them may persist in a variety of contemporary environments thus allowing measurements of SIRM or X to be used to some extent as rapidly determined surrogates for heavy metal measurements in monitoring programmes. That where variations in normalized magnetic parameters (IRM/SIRM; (Bo)cR; SIRM/ARM; ARM/X ) occur they can provide a basis for particulate source differentiation and identification. The studies designed to address the first three hypotheses form a background and context for the present paper which is largely concerned with the fourth. Jones (ref. 22) has confirmed that SIRM and X values are strongly correlated with the down-profile variations of lead, copper, zinc and cadmium in recent ombrotrophic peats from the northwest Midlands and eastern Scotland. These results point to a direct and persistent association between heavy metal and magnetic components in atmospheric deposition resulting from fossil fuel combustion. Oldfield and Thompson (ref. 23) demonstrate a direct linear correlation between spatial variations in the post 1800 A0 cumulative deposition of magnetic minerals and of lead, copper and nickel onto peat bogs. The

133 sites

studied

from

South

range,

in degree

Lancashire,

northern

shores

of

the

divergent

from

the

mean

where

distinctive With

in the

the

complex

(ref.

23).

soil-derived method

for

Within

the

magnetic

linkages

studies

which in

aerosol

power

carried

station

fly

to

been

used

smelting

measurements,

(ref. by

25)

has

using

and from

developed

the

a

parameters

and

elements

has

received

identification,

been

substantiated

by

(refs

31-33).

However,

the

less

attention,

studies

has

so far

have

appraisal

We have

ash

nickel

referred

although

its

12, 19, 27, 34).

out

careful

direction.

urban

particulates

(refs.

Sudbury

differentiation

and

such

have

urban/industrial

Maxted

source

(refs

require

this

particulate

of toxic

of

least

expected

on leaves.

in a range

established

studies

magnetic

monitoring

deposition

26-30)

used

while

magnetic

from

the

nearby.

profiles

to distinguish

particulates

component

the

have ratios,

Mediterranean

of

peat

are be

are

published

of recent

on deposition 24)

as might

and smelting

to the

scans

(ref.

contemporary

dust

is well

All

the

context

urban

presence

al.

to characterise

of urban

numerous

step

over

rapid above

enrichment

in addition

to

ratios

versa,

manufacture

susceptibility/aluminium

aerosols

considered

et

vice

urbanization,

conurbations,

Metal/magnetic

and

chemical

and

Manchester

Dumfries.

and direction

Chester

development

and

sites,

susceptibility

of distance

magnetic

near

remote

hypothesis,

magnetic

effects

especially

third

to industrial

Merseyside

Firth

in the reflecting

to the

Introduction,

to plot

proximity the

Solway

sources

regard

of

between

to

at source.

chosen

and motor

pointed

to

vehicle

The

heavy

metal-magnetic

present

concentrate,

results

initially,

mineral

represent

on two

a first

source

types:

emissions.

METHODS Analysis

was

Midlands,

supplied

Mersey

Liverpool

with

volume

air

percent

effective 7.0,

provided from

the

sheets

and

and filters

that

samples measured.

were

and

The this obtained

was

Particles

same

sampling

Or

system

from

20

used

used

in the sites

of

the

containers

surrounding

The

impacted

backup

was sheets

measurements. polyethylene

film

measurements particle-sized

power

50

onto

filter

Collection

by a series the

were on a high

impactor

for

of blank

obtaining

River

cascade

impactor.

laboratory

the

m3min-1).

were fibre

all subsequent for

under

West

particulates

the

system

number

to adjust

was

(0.566

glass

in the

mounted

CPM

plastic

on a large

tunnels Tunnel

stages

A’

through ml

two

Station

impactor

the

‘Type

10 out

resuspended Butler

of

successive entering

into

were

the

rate

passing

Power

Mersey cascade

a Gelman

carried

Hall

stage

the

particles

artificially by

and

readings

from

a flow

packed

the

material

four

pm.

were

Hams

peninsula.

of

sub-micron

impactor

Inc at

media

from

Wirral

diameters 1.1

measurements

calculations. save

2.0

collection

ash and

the

operating

to retain

Magnetic

fly

2000

cut-off

3.3,

on

with

an Andersen

sampler,

polyethylene

out

by Dr J D Butler,

linking

collected

are

carried

station

and fly

of

fans. were

ash, Leaf also

134 SAMPLING

STRATEGIES

Two The

road

with

tunnels

Tunnel

is the

older

an enclosed

length

of 3428m.

two

direction.

of traffic, river,

in each

is a twin

twin

lane

tube

traffic.

and

This

continuously traffic The duct.

air

At

into

the

the

has

capacity

of

maximum

with

prevailing

air

The

May

western

and

the

mid-river

approached levels

it

may

the

summer the

1240

figures that

was

pumping

it is fed

along

the

acting

roof

of

and

tube

into

exhaust passes

atmosphere.

a maximum

exhaust

Queensway

has

extraction

capacity

of

to 83,089

m3 min-1.

The

both

tunnels

in

the face.

and

the

capacity

air

kerb

as an

the

into

m3min-1

the

vehicles 1250 for

set

approximately

at

a

72,614 actual

response

locations

to

the

720

samples

sampling

period

per

Kingsway

hour.

per

tubes

(individual

half

this

m

were

traffic

was from taken

densities experienced

hour. tube

density

in Queensway

in Queensway

located

particulate

vehicles

both

up

Sampling

system

months

During

averaging

combined

the

in

of

ventilation

tunnel

here

space

greater

of 1982.

Impactor

two

location.

be assumed

impactor

the

following

sampling,

Ferom

discharged

time

periods

tube.

cascade

with

through

extraction

over

uni-directional during

the

openings

a maximum

boosting vary

in the

June

and

of

road

of 41,884

the

when,

deck.

a slightly

min-1

system

approximately

during

represent

m3

during

In the

supply

With

beneath

east-west

the

tube.

through

subsequently

lanes

carrying

end

the

four

semi-transverse

either

with

Mersey.

carrying

occur

upward

road

is drawn

air

capacities

tube

exit.

from

fresh min-1.

sizing

south

during

be

conditions

particle

Kingsway

air

68,939

exhaust

the

River

under

tubes

operational

at

length

to

both

same

tunnel

the

an additional

and

the plants

station

m3 of

to the

opening

the

direction tunnel

running

conditions

longitudinally

a maximum 46,412

m -

beneath

the

is extracted

intake

intake

duct

along

tunnel

2330

ventilation

a main

ventilation

Kingsway

m3min-1

utilize

under

tube

tunnel,

traffic

transferred

uniformly

is a single

Kingsway

length

are

with

through space

vitiated

- of

passing

in a northeast-southeast

This

The

Kingsway

operates

vehicles

and runs

contra-flow

all vehicles

Queensway

accommodate

tunnel

However,

maintenance,

system.

RESUILTS

traffic

Queensway

river

AND

Queensway’s in Kingsway in Queensway similar

traffic

However,

Kingsway

data

being

unavailable)

experienced

in the

was

and

conducted

data so south

tube. A breakdown seen

in Table

indicative sample.

of

of

some

2.(Bo)GR the

mineralogy

and

typical IRM-100 and

magnetic

results

,T/SIRM domain

status

are of

from

the

normalized the

magnetic

Mersey parameters crystals

Tunnels

can which

present

be are in a

135 TABLE

2

Typical

magnetic

results for the Mersey Tunnels Queensway

Impactor

Kingsway

Stage

Plate 1 (E.c.D. Plate 2 (E.C.D. Plate 3 (E.C.D. Plate 4 (E.c.D. Backup filter

7.0 3.3 2.0 1.1

pm) pm) pm) pm)

35 41 55 30-46 40

-0.74 -0.71 -0.58 -0.75--0.80 -0.65

37 42 49 56 42

-0.74 -0.69 -0.58 -0.50 -0.63

* mT As particle size decreases there appears to be a corresponding reduction in grain size of the magnetic! minerals present in the sample; this can be seen in the increase in the (Ro)CR values down through the impactor. This trend is not followed exactly, Plate 4 for Queensway showing a variety of (Bo)CR values while the backup filter for runs in both tunnels produces values appreciably lower than previous plates. This reversal may result from a very small number of larger magnetic particles being bounced through the system and subsequently influencing the back up plate assemblage. This seems possible as bounce errors are not uncommon in cascade impactors ( ref. 35). The IRM-100mT/SIRM ratio for all particle size ranges from both tunnels indicates the dominance of a ferrimagnetic mineral component in vehicular emissions. All samples are in fact reverse saturated in a field of 250 mT after initial saturation in a field of 800 mT and this is compatible with an insignificant anti-ferromagnetic component. Table 3 summarises the results obtained from the resuspended, particle-sized flyash sample. Additional parameters ARM/X and SIRM/X are included which suggest a consistency of mineralogy across the whole size range. Minor differences on Plate 1 and the backup filter may reflect larger magnetic grains affecting the magnetic response. Comparison with the Mersey Tunnel results shows that the sample sets can be distinguished on the basis of reverse field IRM/SIRM measurements. Figure 1 plots IRM-20 mT/SIRM versus IRM-200 mT/SIRM for the various sets of samples. The range of variation in the lower reverse field ratio is rather small and does not clearly differentiate the sample sets. However, the high reverse field ratio discriminates Moreover, whereas the tunnel between the sample sets rather more effectively. samples are fully reverse saturated between 100 mT and 250 mT the fly ash samples are not, nor are the leaf samples collected

from the power station environs.

136 TABLE

3

Maqnetic

results

Impactor

stage

for

resuspended

fly

ash

-

Plate Plate Plate Plate Backup *

1 2 3 4

(Bo)CR*

(E.C.D. (E.C.D. (E.c.D. (E.C.D. filter

7.0 3.3 2.0 1.1

pm) pm) pm) pm)

40 42 42 42 38

-1OOmT

SIRM

ARM

SIRM

x

x

-0.63 -0.53 -0.55 -0.52 -0.66

227 204 190 176 235

0.43 0.48 0.53 0.52 0.58

mT

DISCUSSION The is

contrast

between

consistent

with

component

in the

The

fly

nature

composition coal

have

and

silicates,

ash

of

the

magnetic

feed

coal.

identified

and

much

fly

ash

higher

fraction

in coal

A number

(ref.

carbonates, type

derived

particulates

illustrated

anti-ferromagnetic

(viz.

in Fig.

1

haematite)

samples.

of the been

mineral

vehicle

a proportionally

36).

oxides

depending

ash

bearing

are

sulphides

These

and

on

fly

of iron

oxyhydroxides

degree

of

is dependent mineral

on

the

compounds

(predominantly

and

iron

and/or

in fly

these

in

pyrite),

sulphates,

coalification

mineral

present

the

clays dominant

original

deposition

environment. The well

formation

of magnetic

understood

pointed

to

framboids undergo

although the

possibility

in coal

and

only

spine1

phase.

their

magnetic

melting

On the

investigating

whilst

the

present

context.

2-801.1m

that

authors

(ref.

frequently

and

designates

be,

in

both

Hansen

magnetite

known

for

more

recently

some

Hansen

ash from postulated.

They

et

original

sources

et al.

between

the

occurrence

of

conclude

that

the

framboids

during

al. (ref.

11)

their

(ref.

is not

Lauf

change

not

phases entirely

38) reports

contain

referred

to

ash.

dimensional

hand,

are

Lauf

particles

phase

been

relationship

in fly

magnetic

spherules

Although

a direct

and

other

have

37) have

pyrite

pyrite

transformation

consider

a silicate

to origin

a for

ash sample.

Studies and,

of

links

ferrospheres

limited

fly

particulates

certain

fact,

an

et al. (ref. and time point

11) finds

haematite (ref. to the

40).

XRD

results

of solid

Hulett

substituted a lack

studies of both

of

(ref.

39) magnetic

are

also

recent

interesting

spheres

in the

of size

identifies

a group

as non-stoichiometric (refs

18,

of

the

form

American and

consider

this

Fe2.3AIO.704 the fly

magnetic forms

of

magnetite 39)

in his samples

British’and

these

comparatively

et al.

ferrite

of haematite

are

glass

He

he regards

phase.

in a variety

presence

their

magnetite.

which

aluminium

ash

existence

precipitated

a spine1

in fly

in accord, the

to as ‘ferrospheres’ them

present

ashes

studies in fly

presence

ash.

has ( ref.

. of been 10 )

137 -2OmT SIRM

0.3 I

0.4 I

0.5 I

0.6 I

0.7

E.C.D.

-0.5

0

7.0 pm

Kingsway

n

3.3 pm

*

Queensway

0

2.0pm

X

Leaf

0

1.1

*

Hams

*

-0.6

Hall

samples

*

pm

Backup

filter

-0.7 -200mT SIRM

-0.8

//

cc

O'N,

Cl

**

I

l

I

’ 0

\

‘x0.

A

n

Fig. 1 Plot of -ZOmT/SIRM versus -ZOOmT/SlRM Hams Hall fly ash and Mersey Tunnel dusts.

Investigations

of street

__d---

----------

dusts have pointed

-0

.

/

for leaf samples and particle

to vehicle

wear, exhaust products

-1 I I

sized

and

re-entrained dusts as major sources of iron in the urban locale (refs. 27, 29, 41). The association between the magnetic fraction of this iron component and a number of potentially hazardous trace metals is well proven (ref. 27), as is the surface enrichment of magnetic particles from vehicle exhausts in several trace elements (refs. 16, 41). This relation develops from magnetic particles (probably ablation products in the exhaust system) becoming intimately associated with volatile trace metals (notably

138 lead) (refs cooling

12, 16) as they condense

occurs on progression

through

out of the vapour

phase onto larger

particles

as

the exhaust system.

CONCLUSION In so far as the present results are typical, the ‘harder’ magnetic component related to the presence of antiferromagnetic haematite in fly ash provides a readily measurable feature distinguishing it from vehicular emissions which display mainly ferrimagnetic properties. This characterisation of the magnetic fraction of fly ash and of vehicular emissions is potentially important in pollutant tracer studies where the facility to distinguish between them and other sources is valuable. For instance, a major source of material identified in urban dust studies is windblown soil (ref. 27). This component may also be magnetically distinct in that it possesses a finely divided magnetic fraction near the superparamagnetic/single domain boundary which demonstrates magnetic viscosity readily detected by quadrature susceptibility measurements (ref. 42). This demonstrated capacity to identify and differentiate particulate sources, coupled with their rapid and non-destructive nature, suggest that magnetic measurements may contribute significantly to monitoring schemes. ACKNOWLEDGEMENTS The authors would like to express their thanks to Dr. J. Butler of the University of Aston for generously supplying the fly ash and leaf samples used in this analysis. We are indebted to Dr. R. Chester of the University of Liverpool and Dr. G. Eglington of the University of Bristol for the use of air sampling equipment, to Merseyside County Council for permission to sample in the Mersey Tunnels, and the University of Liverpool for financial support. REFERENCES 1. M. Scoullos, F-. Oldfield and R. Thompson, Mar. Pollut. Bull., 10 (1979) 287291. 2. M. Scoullos, Chemical Studies of the Gulf of Elefsis (Greece), Ph.D. thesis, University of Liverpool, 1979. 3. D. M. Revitt, J. 8. Ellis and F. Oldfield, in B. C. Yen (Ed.), Urban Storm Drainage, Proceedings of the 2nd International Conference on Urban Storm Drainage, 1981, in press. 4. F. Oldfield, C. Barnosky, E. B. Leopold and J. P. Smith, Mineral magnetic studies of lake sediments - a brief review, Proceedings of the 3rd International Symposium on Palaeolimnology, 1983, in press. 5. F. Oldfield, R. Thompson and K. E. Barber, Science, 199 (1978) 679-680. 6. F. Oldfield, A. Brown and R. Thompson, Quat. Res., 12 (1979) 326-332. 7. F. Oldfield, K. Tolonen and R. Thompson, Ambio, lb (1981) 185-188. 8. L. J. Dovle. T. L. Hookins and P. R. Betzer. Science. 194 (1976) 1157-1159. 9. J. H. Puffer, E. W. B. Russell and M. R. Rampino, J. Sed. Pet;, 50 (1980) 247256. 10. G. Chaddha and M. S. Seehra, Magnetic components and particle size distribution of coal fly ash, DE-83000576 DOE/MC/14718-1213, 1982. 11. L. D. Hansen, D. Silberman and G. L. Fisher, Environ. Sci. Technol., 15 (1981) 1057-1062.

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