heavy metal pollution and soil magnetic susceptibility in urban soil of beni mellal city (morocco)

ORIGINAL ARTICLE

Heavy metal pollution and soil magnetic susceptibility in urbansoil of Beni Mellal City (Morocco)

Mohamed El Baghdadi • Ahmed Barakat •

Mohamed Sajieddine • Samir Nadem

Received: 12 November 2010 / Accepted: 9 July 2011 / Published online: 27 July 2011

� Springer-Verlag 2011

Abstract The assessment of anthropogenic impact in the

urban environment can be evaluated according to heavy

metal contents of soils such as Pb, Cu, Zn, Cd and Fe.

These elements have more affinity to establish metallic

bond with ferrous material leading to enhancement of soil

magnetic susceptibility. The objective of this study was to

undertake joint magnetic and geochemical investigations of

road-side urban soil materials to address the environmental

pollution of Beni Mellal city that has been subjected to

environmental stress, due to population overpressure and

related urbanization. Twenty three soils magnetic suscep-

tibility profiles were made along 5 km peripheral national

road (N8) in Beni Mellal. The magnetic survey reported

here for the first time on this City’s topsoils tries to

establish the link between magnetic properties and the

content of heavy metals. High magnetic susceptibility

values and high contents of heavy metals were found near

the paved edge of the road and within the place reserved as

large engine park. Magnetic extracts of highly polluted

areas and unpolluted soil (olive plantation) were analyzed

by SEM coupled with RDX in order to discriminate

anthropogenic magnetic spherules and pedo-lithogenic

magnetite-like minerals. Magnetic mineralogy determined

by Mossbauer spectroscopy suggests the presence of

hematite, magnetite and goethite in highly polluted areas.

The iron oxides and especially goethite are efficient in

incorporating and/or adsorbing foreign ions.

Keywords Magnetic susceptibility � Heavy metal �Mossbauer spectroscopy � Urban pollution � Beni Mellal

Introduction

Roadsides in urban area are commonly polluted by partic-

ulate matter derived mostly from motor vehicle exhaust,

abrasion of tyres, plant and combustion emissions (Petrov-

sky and Ellwood 1999; Gautam et al. 2005a). Deposited near

the roadside, these particulate matters can be used to assess

pollution degree by using magnetic susceptibility of topsoil

close to the road coupled with geochemical content of heavy

metal. The use of magnetic measurements as a proxy for

chemical methods is largely approved because pollutants

and magnetic particles are genetically related (Hanesch and

Scholger 2002; Morton-Bermea et al. 2009). The magnetic

measurements are sufficiently sensible to detect the mag-

netic signal of the minor fraction of ferromagnetic minerals

in the most of the cases with concentration less than 1%

(Chaparro et al. 2006). These magnetic minerals can be of

lithogenic origin, derived from a parent rock from which the

soil was developed, or can be formed in situ by pedogenic

processes. Moreover, significant amount of atmospheric

minerals are deposited on soils contributing to its magnetic

signatures. The occurrence of minerals in the atmosphere

can be either natural as a result of wind erosion, or anthro-

pogenic originating from industrial fly ashes or vehicular

exhausts. Among magnetic minerals, the most important are

M. El Baghdadi (&)

Department of Earth Sciences, Faculte des Sciences et

Techniques, P.O. Box 523, 23000 Beni Mellal, Morocco

e-mail: [email protected]

M. El Baghdadi � A. Barakat � S. Nadem

Laboratoire Georessources et Environnement,

Universite Sultan My Slimane, Faculte des Sciences et

Techniques, B.P. 523, 23000 Beni Mellal, Morocco

M. Sajieddine

Laboratoire de Physique et Mecanique des Materiaux,

Universite Sultan My Slimane, Faculte des Sciences et

Techniques, B.P. 523, 23000 Beni Mellal, Morocco

123

Environ Earth Sci (2012) 66:141–155

DOI 10.1007/s12665-011-1215-5

iron oxides such as magnetite, maghemite, hematite, and

iron sulfides. Numerous studies have described the various

aspects of the magnetic mineralogy of soils (e.g. Maher and

Thompson 1999; Taylor et al. 1987; Schwertmann et al.

2005; Fassbinder and Stanjek 1994; Stanjek et al. 1994;

Dearing 1999; Singer et al. 1996; Jordanova and Jordanova

1999; Gunther et al. 2002; Schneeweiss et al. 2006).

Recently, there is a growing interest in using magnetic

techniques for monitoring environmental pollution (Hay

et al. 1997; Hoffmann et al. 1999; Xie et al. 2001; Chan et al.

2001; Lecoanet et al. 2003; Hu et al. 2007; Yang et al. 2007;

Morton-Bermea et al. 2009; Gautam et al. 2004, 2005a, b;

Zhang et al. 2006; Lu and Bai 2006; Wang et al. 2005; Wang

and Qin 2006).

Many studies have reported excellent relationships

between v (soil magnetic susceptibility) and the contents of

some heavy metals in street dust or industrial/urban soils

(Hunt et al. 1984; Hay et al. 1997; Kapicka et al. 1999;

Durza 1999; Strzyszcz and Magiera 1998; Xie et al. 2001;

Hoffmann et al. 1999; Matzka and Maher 1999; Knab et al.

2001; Hanesch and Scholger 2002; Muxworthy et al. 2002;

Jordanova et al. 2003; Hu et al. 2007; Yang et al. 2007;

Morton-Bermea et al. 2009). A special link between v and

heavy metal contents in the uppermost layers of lake or

seabed sediment adjacent to industry were also observed

(Chan et al. 2001). Non-destructive and rapid magnetic

techniques seem promising in monitoring soil pollution.

Some recent studies have successfully applied soil vmapping as a tool for preliminary pollution monitoring

(Hoffmann et al. 1999; Boyko et al. 2004) and mapping

areas polluted by industrial emissions (Heller et al. 1998;

Hay et al. 1997; Duan et al. 2009). Measurements of

magnetic susceptibility of soils have been used also in the

loess–paleosol sequences to reveal paleo-climatic changes

in the quaternary period (Maher 1999, 2007, 2009).

Anthropogenic pollution usually has a strong magnetic

signature and magnetic techniques have proven to be

capable of discriminating between different sources of

pollution and to delineate polluted areas.

Iron often occurs as an impurity in fossil fuels during

industrial, domestic, or vehicle combustion; carbon and

organic material are lost by oxidation and the iron forms a

nonvolatile residue, often comprising glassy spherules (due

to melting). These spherules are magnetic, with easily

measurable magnetization levels. It has been shown that

combustion processes simultaneously release hazardous

substances and magnetic particles into the atmosphere. In

addition to these combustion-related particles, vehicles, via

exhaust emissions and abrasion/corrosion of engine and/or

vehicle body material, can generate non-spherical magne-

tite particles (Pandey et al. 2005; Gautam et al. 2005b).

Following the effectiveness of the integration of chem-

istry and magnetic properties in studies of the degree of

pollution of the soil, dust, sediment and land systems

(Petrovsky and Ellwood 1999; Knab et al. 2001; Hanesch

and Scholger 2002), these properties have already been

applied by the authors of the present study to successfully

characterize and quantify the degree of pollution in the

Beni Mellal urban area (El Baghdadi et al. 2010). This

work presents the magnetic susceptibility of soil samples

contaminated with lead, copper, zinc, Fe and cadmium and

non-polluted soil. Mossbauer spectroscopy and Scanning

electron microscope with EDX were performed. Statistical

relationship between magnetic susceptibility and the con-

centration of pollutants is discussed, and the polluted areas

were mapped.

Materials and methods

Study area

The study was performed in peripheral road in Beni Mellal

city. The city is located in the central part of Morocco

(32� 200 N, 6� 210 W) between the High Atlas Mountain and

the vast Tadla plain with altitude of 450–600 m and is

characterized by a semi-arid climate with averaged annual

temperature of 26�C and annual rainfall of 400 mm. The

geology of the region is presented by the Mesozoic limestone

with travertine and Cenozoic phosphate with extremely low

magnetic content (20–50 9 10-5 SI). The road that was used

for this study, named ‘‘20 aout’’, circumvents Beni Mellal

city in the north and the northwest over five kilometers length

(Fig. 1) and is well known as one of the main entrances of the

city with high traffic density. The geology below the road is

mostly alluvial cone conglomerate and agricultural soil. This

geology setting indicates that the magnetic enhancement in

the soil can be attributed to pedological or anthropological

sources, instead of the lithological sources. Considered area

showed, in addition to road traffic, many others anthropo-

logic activities such as residential zone, olive plantation, bus

station, car park for large machine.

Soil magnetic susceptibility measurements

The ‘‘20 aout’’ road that is *5 km long was investigated on

the both sides with a grid of one meter across (10–20 m) the

road and 100 m toward. Magnetic susceptibility expresses

the ease of material to be magnetized and depend on the

concentration of the ferrimagnetic material. The surface

measurements of volume susceptibility were performed

using pocket GF Magnetic susceptibility meter SM20

making access to 90% of measuring signal e.g., concentra-

tion of ferrimagnetic material in the top 20 mm of the land

surface with operating frequency of 10 kHz and sensitivity

of 1 9 10-6 SI units. Investigation measurement counts

142 Environ Earth Sci (2012) 66:141–155

123

were reported as a mean of 10 readings of magnetic sus-

ceptibility in each station together with GPS coordinates. In

order to compare urban and rural soil magnetic suscepti-

bility, two vertical profile distributions were reported in

agricultural site far from any source of pollution. Two other

profiles of urban soil were mapped near the road to check the

fate of magnetic parameters from surface soil to depth soil.

Sample collection and analytic methods

Magnetic measurements were performed using perpendic-

ular profile (steep: 1 m) that are each 100 m toward the

road. Sampling locations were selected on the basis of land

use pattern. Roadside soil samples were collected in the

rim of road on both sides at 1, 10 and 20 m from many

locations including olive plantation, residential, commer-

cial and car park for large machine and bus station. At each

sampling point, 2–3 kg of top 2 cm layer of the soil profile

was collected by polyethylene trowel and stored in a plastic

bag. In the laboratory, samples were visibly screened to

remove macroscopic fragment of glass, nails, animal and

plant matter. Samples were then oven-dried at room tem-

perature and fractionated through a-63 lm sieve. We have

chosen to analyze only the \63 lm diameter fraction

because these particles are easily transported in suspension

from automotive exhaust.

Tri-acid mixture (HNO3–HF–HCl) were added to the

beaker containing soil sample and heated at 100–110�C

until the solution became transparent (Allen et al. 1986).

The resulting solution was finally maintained to 50 ml

using deionised water and stored at room temperature for

further analysis of heavy metal (Cd, Zn, Cu and Pb). Metal

concentrations were analyzed by Inductively Coupled

Plasma-Atomique Emission Spectroscopy (ICP-AES) type

ULTIMA2 at UATRS (Centre National pour la Recherche

Scientifiques et Techniques).

Morphological details of individual particles provide

valuable information on the sources of possible pollution

and on the origin of particles. Soil samples and magnetic

extracts (prepared using method described in Hounslow

and Maher 1999) were analyzed using optical microscope

and scanned under scanning electron microscope. Semi-

quantitative analysis of the composition was performed by

energy dispersive X-ray (EDX) detector coupled to the

SEM, set on the backscattered electron mode (BSE).

Mossbauer spectra were collected in a spectrometer with

triangular linear-velocity drive using a 57Fe source in an Rh

matrix. The spectra were analyzed with mere Lorentzian

line profiles or by taking distributions of hyperfine fields

(BHF) into account. Hyperfine parameters such as mag-

netic hyperfine field (BHF), isomer shift (IS) and quadru-

pole splitting (QS) have been determined by the NORMOS

program (Brand 1987), and a-Fe at room temperature was

used to calibrate isomer shifts and velocity scale.

Results and discussion

Distribution of magnetic susceptibility

The magnetic susceptibility profiles of urban soils are shown

in Fig. 2. Soil profiles from roadside areas show an increased

Fig. 1 Satellite photography of

Beni Mellal City surrounded in

the North and the Northwest by

5 km 20 Aout road (arrow)

Environ Earth Sci (2012) 66:141–155 143

123

magnetic susceptibility in its surface horizons (0–10 cm) and

a steeply decreased from top horizons to base horizon sug-

gesting a large contribution of anthropogenic-related ferro-

magnetic minerals. The upper 0–10 cm interval in car park

station exhibits increased magnetic susceptibility three

times higher than those from road side olive plantation soils

and 6–7 times higher than those from agricultural soils far

from road traffic (The magnetic susceptibility value of

background soil is low throughout the profile, ranging from

15 9 10-5 to 114 9 10-5 SI). All of the measured values in

urban topsoil were higher than those of the background

profile (Fig. 3). This indicates that anthropogenic materials

contain a higher portion of magnetic material than the lith-

ogenic and pedogenic background.

Two types of profiles were made to check magnetic

susceptibility variation. The first one was set along 5 km of

the road and the second subsequently 20 m on road bi-sides

(El Baghdadi et al. 2010). The Fig. 4 shows magnetic sus-

ceptibility variations along five kilometers of the road. The

in situ magnetic susceptibility closely to the road ranged

from 67.9 to 577 9 10-5 SI with median value 206 9 10-5

SI and from 17 to 227 9 10-5 SI with median value of

100 9 10-5 SI 70 m far from the paved road. Along the

profile, there is some irregularity in the variation of mag-

netic susceptibility. With the first 500 m (from the East to

the Southwest) in the profile, located close to olive planta-

tion, magnetic susceptibility shows low values between 100

and 200 9 10-5 SI, while closely to large engine park

station, magnetic susceptibility is very high with value

about 400–577 9 10-5 SI. Residential and commercial

area shows medium susceptibility 140–370 9 10-5 SI.

Magnetic susceptibility (MS) variations across road

reported in many cross profiles are illustrated in the Figs. 5,

6, and 7. In the most road cross profile, a relatively high

magnetic susceptibility characterizes the near asphalt-

paved portion of road corridor at distance 0–2 m away

from the edge. Decay of magnetic susceptibility to the low

value occurred within about 5 m of the road edge. How-

ever, four parts could be identified from Northeast to

Southwest along 5 km of the road in respect to the fate of

magnetic susceptibility away from the road relatively to

most anthropologic activities and land use.

In olive plantation (OLP) area (MS: 61.46–287.25 9

10-5 SI), with low values, the decay of magnetic suscep-

tibility was not monotonous and asymmetric (Fig. 5). High

values can be recorded far from the road and not near the

paved asphalt. This can be related to high amount of

magnetic material originated from pedogenic processes

or to the wind direction. But magnetic extract from samples

Fig. 2 Magnetic susceptibility

vertical profiles in urban soil,

a in the vicinity of road and b in

Bus station and large engine

park

Fig. 3 Magnetic susceptibility vertical profile of the soils from

background and agricultural area at Beni Mellal region. Profiles are

collected far from any source of pollution


123

Fig. 4 Lateral profiles of soil

magnetic susceptibility along

5 km of the road ‘‘20 Aout’’ at

a 0–1 m and b 20 m from the

paved ridge. A North side,

B South side


magnetic susceptibility across

20 Aout road in olive plantation

area. Profiles are asymmetric

and show increase of MS far

away the road. Magnetic

fraction at 20 m from the road

do not contain spherule

morphology


123

of this area (20 m from road) do not show any anthropo-

genic-related spherules-like. So, with low inherited mag-

netic fraction from limestone conglomerates (parent

rocks magnetic susceptibility about 70 9 10-5 SI), the

enhancement of magnetic susceptibility far from the road

edge can be related to some transformation in the course of

pedogenesis. In profile 7, the soil was very compacted and

used as track for the passengers and the animals. Magnetic

susceptibility showed an irregular evolution with zones of

high value and others with low value.

Large engine park (LEP) (Profiles 8–12) with high

traffic density shows the higher recorded values of mag-

netic susceptibility (MS: 16.95–577.6 9 10-5 SI). The

decay of susceptibility away from the road is generally

exponential with respect to the distance especially

for profile A9, A10 and B9 (Fig. 6). The lowest values

occurred at 10–15 m from road and a systematic increase

of susceptibility towards the roads was observed. In profile

A11 and A12, enhancement of susceptibility anomaly was

observed not near the road but 15 m far from the asphalt-

paved edge according to the presence of engine parked in

this area. We infer that the localized, narrow and linear

configuration of susceptibility enhancement parallel to the

road alignment or in the parking area, results from the

deposition of traffic-related particles. These particles

accumulate in the close vicinity of the road or in the park

immediately after discharge into the environment as

described by Petrovsky and Ellwood (1999).

Residential and Commercial place (RCP): The presence

of traditional and modern market involves high density



20 Aout road in large engine

park and high traffic area



20 Aout road in highly pollution

area on both sides of the road

(profile 16) and in residential

activity and olive plantation

(profile 17–23). Profiles are

symmetric and show increase of

MS near the road paved edge

(0–1 m). Distance taken is

1–20 m far away the road


123

traffic in Commercial area. Magnetic susceptibility profiles

(13–16) are asymmetric and shows values enhancement

near and at 5–10 m from the road (profile 15). The profile

16 exhibits symmetric trend of magnetic susceptibility

beside the road and high values are recorded near the road

(Fig. 7).

The susceptibility profiles (17–23) recorded in the last

2 km where residential and olive plantation (ROP) are the

most anthropologic activities showed symmetric and

exponential trends on road bi-sides (Fig. 7). High measured

values close to the asphalt-paved ridge are between 140

and 370 9 10-5 SI and the lowest values are recorded at

18 m far from the road (MS: 33 9 10-5 SI) in residential

and commercial area (profile 16). But, in residential place

associated with olive plantation, lowest values are recorded

at the mid-way in the profiles 10–15 m from the road (MS:

27 9 10-5 SI) and at 20 m some profiles illustrate slight

susceptibility enhancement (A17, A19, A23, B22) while

others show high susceptibility increase (B19). We attri-

bute the effect of increase of magnetic susceptibility in the

topsoils at this distance to the pedogenesis processes and

not to deposition of fly ashes from vehicles emissions since

the petrographic study of magnetic extract did not show

any spherule-like forms designed as the result of exhaust

combustion.

In order to demonstrate surface variation, all measured

magnetic susceptibility along 5-km of 20 aout road are

mapped and reported in the Fig. 8. Compared to the

Fig. 4, less magnetic susceptibility is recorded from road

closely to olive plantation. The highest values occur in

the darkness area in the map with high traffic, large

machine-park station and high commercial activity that

is biased by anthropogenic input of magnetic material

related to vehicular emission. Road sides near residential

place, recreation zone and olive plantation show medium

values.

Heavy metal concentrations

Environmental heavy metal contamination, especially by

lead in soil and sediment, has become increasingly recog-

nized over the last 40 or more years as a significant

problem in public health (Kim et al. 2007). Urban

anthropogenic particulates are enriched in toxic trace

metals (including Fe, Pb, Zn, Ba, Mn, Cd and Cr) and,

almost invariably, magnetic particles (Maher 2009). The

correlation between magnetic concentration related

parameters and heavy metals content reveals a causal

relation between ferrimagnetic oxide and heavy metals in

urban topsoils. This relationship could be due to that fact

that heavy metal elements are incorporated into lattice

structure of the ferrimagnetics during combustion process

or are adsorbed onto surface of pre-present ferrimagnetics

in the environments.

The concentrations of Cd, Cu, Zn, Fe and Pb in different

area along the road are listed in Table 1. The ranges of metal

contents measured near the road are as follows: Cd:

0.511–2.154 ppm (mean: 0.93 ppm); Cu: 30.32–62.9 ppm

(mean: 54.25 ppm); Zn: 95.6–207 ppm (mean: 160 ppm);

Fe: 1.9–4.8% (mean: 3.12%); Pb: 66.9–559.65 ppm (mean:

304 ppm), and the mean magnetic susceptibility value of

analyzed samples is 258.50 9 10-5 SI whereas ranges of

metal contents at 20 m far from the road are Cd:

0.159–1.954 ppm (mean: 0.73 ppm); Cu: 21–69.6 ppm

(mean: 40.81 ppm); Zn: 65.778–225.049 ppm (mean:

127 ppm); Fe: 1.87–4.87% (mean: 3.61%); Pb: 19–114 ppm

Fig. 8 Detailed magnetic

susceptibility map showing

highly polluted area along

studied road


123

(mean: 69.44 ppm); and the mean magnetic susceptibility

value of analyzed samples is 123.21 9 10-5 SI.

The content of Cd, Cu, Zn, Fe and Pb in the studied areas

is significantly higher than those of background soils in Beni

Mellal region. The average concentrations of Cd, Cu, Zn,

and Fe in urban topsoils were one to three times higher than

the mean values of rural soils; but these of Pb is six time

higher. Large standard deviation of these metals implies a

great heterogeneity of heavy metals contamination. The

mean concentrations of Cd, Cu, Zn, Fe and Pb in four areas

along 20 aout road are shown in Fig. 9. Metal concentrations

plotted were higher in roadside soils (black box) and much

higher large engine park (LEP) and residential and com-

mercial place (RCP) than those in olive plantation (OLP)

and olive and residential place (ORP). The above results

indicate that urban topsoil have been polluted by heavy

metals of Cd, Cu, Zn, Fe and Pb, which is considered as the

result of the gradual accumulation of various pollution

sources over time, including industrial emission, automobile

exhaust, and atmospheric particulates.

Heavy metal and magnetic susceptibility relation

The magnetic fractions of the urban topsoils are highly

enriched in heavy metals. Many previous studies have

reported the strong relationships between magnetic sus-

ceptibility and heavy metals in polluted soils (Hu et al.

2007; Hay et al. 1997; Knab et al. 2001; Wang and Qin

2006; Chaparro et al. 2008; Bijaksana and Huliselan 2010).

Table 1 lists the Pearson’s correlation coefficients

between the concentration of heavy metals and magnetic

susceptibility in studied district. Also, Fig. 9 shows

enhancement of heavy metal content in LEP and RCP as

well as magnetic susceptibility. Behavior of heavy metal in

Table 1 Minimum and

maximum values, mean,

standard deviation (SD), median

for heavy metal concentration

(mg kg-1) in urban soils and

correlation coefficient with

magnetic susceptibility. In

brackets, number of analyses is

mentioned

Cd Cu Fe % Pb Zn MS (10-5 SI)

Olive plantation (13)

Moyenne 0.871 38.26 3.71 132.19 123.71 137.56

Max 2.154 67.99 4.88 295.76 186.92 287.25

Min 0.218 23.98 2.34 19.08 60.43 37.89

SD 0.67 11.54 0.83 111.45 35.76 72.03

Median 0.65 34.76 3.87 54.69 122.68 126.56

Large engine park (9)

Moyenne 1.145 67.53 3.59 296.69 206.96 353.13

Max 1.568 72.98 4.88 559.65 267.07 577.00

Min 0.342 62.91 2.24 110.04 167.88 135.75

SD 0.38 3.42 0.94 161.26 29.18 143.59

Median 1.23 66.98 3.82 265.39 203.43 388.45

Residential and commercial place (9)

Moyenne 1.009 61.05 3.37 244.85 163.72 194.36

Max 1.943 81.76 4.88 450.12 221.66 367.85

Min 0.456 21.07 1.88 63.76 74.37 33.18

SD 0.57 22.38 1.17 130.12 50.61 129.79

Median 0.85 74.98 3.64 276.92 183.04 200.60

Residential and olive plantation (12)

Moyenne 0.458 35.24 2.72 168.34 108.09 187.27

Max 0.786 45.82 4.35 342.74 137.25 357.42

Min 0.159 25.17 1.74 19.53 65.78 76.05

SD 0.22 6.51 0.79 111.99 23.95 85.57

Median 0.39 34.28 2.60 167.60 113.44 187.47

Background soil (10)

Moyenne 0.31 20.82 1.69 73.52 34.75 53.88

Max 0.36 21.87 1.89 92.43 42.09 88.60

Min 0.23 19.68 1.33 55.32 29.55 31.30

SD 0.05 1.04 0.24 15.56 5.59 27.15

Median 0.32 20.96 1.79 76.96 31.46 38.20

Correlation coefficient with MS 0.438 0.699 0.645 0.927 0.681 1


123

soil and their relation to magnetic fraction are illustrated in

Fig. 10. Cd shows slightly negative correlation with soil

magnetic susceptibility whereas Pb shows high positive

correlation with susceptibility values. Zn and Cu exhibit

moderate positive correlation with magnetic susceptibility.

The possible cause for the imperfect correlation between vand some heavy metals could be due to some grain size

variations of magnetic particles since larger particles multi-

domain exhibit higher magnetic susceptibility than smaller

single domain particles, while adsorption of heavy metals

is greater on finer particles (Petrovsky et al. 2001).

Source of magnetic particles and heavy metals

contamination

Magnetic minerals in urban soils may be either inherited from

the parent rocks (lithogenic origin), formed during pedogen-

esis and/or may stem from anthropogenic activities (second-

ary ferromagnetic materials) (Magiera and Strzyszcz 2000;

Yang et al. 2007; Boyko et al. 2004; Hanesch and Scholger

2005). The lithogenic influence on the topsoil magnetic sus-

ceptibility can be excluded in all measured sites by the

significant enhancement of the topsoil susceptibility, which is

about four and two times higher than that of the subsoil.

The anthropogenic particles are likely to have come

from three sources (Lu and Bai 2006): (1) emissions from

fossil-fuel combustion processes (fly ash); (2) other parti-

cles from vehicles, building and road surface (e.g. brake

lining dust, exhaust particulates); (3) exotic materials (e.g.

metallic fragments, slag and building materials) incorpo-

rated into disturbed soil surfaces. All three sources con-

tributed to the increase of the magnetic susceptibility of the

urban topsoils. However, soil analyzed samples reveal that

heavy metals have high contents near road edge and low

contents far from the paved-asphalt except for some loca-

tions (Bus and large engine park) where increase is

observed far from the paved-asphalt.

As mentioned above, Magnetic susceptibility is more

enhanced near asphalt ridge except for some profile B10

and A11 which is more pronounced at 10–20 m far away

the road in Car and large engine park. Analyzed samples

show an increase in the contents of heavy metal in parking

zone according to enhancement of magnetic susceptibility.

The Pb in automobile exhaust was known to be associated

Fig. 9 Box-plots of

concentration of Cd, Cu, Zn, Fe

and Pb, and magnetic

susceptibility in urban topsoils.

Grey box shows values near

roadside and the black one at

20 m far away the roadside.

Vertical bars represent the

standard deviation. (OLP olive

plantation, LEP large engine

park, RCP residential and

commercial place, ROPresidential and olive place). The

dash lines mark the background

values of heavy metal

concentration of soils in

agricultural area far from urban

effects


123

with magnetic particles. Therefore, a strong correlation of

this metal with magnetic susceptibility is expected. Some

magnetic susceptibility enhancement observed in olive

plantation area is caused by the production of magnetic

fractions by pedogenic processes and not linked to vehicle

exhaust. Microscopic analysis of magnetic mineral shows

an absence of spherule form and abundance of octahedral

magnetite (see below). Furthermore, the contents of heavy

metal in profile A5 are very low such as Pb (19 ppm).

It was also found that Zn, Pb, Cd and Cu contents in the

magnetic fractions of the 20 Aout road topsoils are sig-

nificantly higher than those in the agricultural topsoils far

from any source of pollution. Heavy metal measured in

agricultural topsoil are Zn (34.75 ppm), Pb (73.52 ppm),

Cd (0.314 ppm) and Cu (20.82 ppm). We expect that

vehicular emissions that contain magnetic particles and

produce many magnetic aerosols (Hay et al. 1997; Shu

et al. 2001) in the urban environment of Beni Mellal are the

most important source of pollution. Leaded petrol is the

most probable source for the Pb enrichment, while tyre

wear may be the main source for Zn and Cd in these

samples (Andersson et al. 2010).

Optic microscopy of the magnetic extract from the soil

closing the paved-asphalt road and soil in large engine park

reveals abundance of magnetic grains having spherical

morphology with different diameters. These types are

related to anthropogenic inputs, and formed by combustion

processes. Spherules of diameter 2–40 lm, were abundant

in the dust from road surface as well as from road-side

soils, but absent in the soil samples from sites distant from

roads. Therefore, the most likely source of these spherules

is the road traffic. These spherules are inferred to be

produced in exhausts of vehicles, as shown in the Tubingen

study, although some of the larger particles, with diameter

of tens of microns, may be too large to be explained in this

way (Knab et al. 2001).

Mossbauer spectroscopy

Magnetic fractions of two topsoils samples, one near the

road (A9) and the other 20 m far from the paved edge (A7)

in the district were separated and measured by Mossbauer

spectroscopy. The room temperature spectra are displayed

in Fig. 11. The fitted parameters of the samples are pre-

sented in Tables 2 and 3, respectively. The spectrum

observed in highly polluted soil (A9) is composed of four

sextets (S1, S2, S3, and S4) with corresponding hyperfine

field of 494, 449, 469 and 366 kOe, respectively and two

doublets D1 and D2 inferred to weakly magnetic compo-

nents. S1 sextet exhibits an isomer shift value about 0.35

and correspond to Fe3? in a-Fe2O3 (hematite) with relative

abundance (RA) about 39%. The magnetite is represented

by octahedral Fe2.5? site (S2, RA: 20.3%) and tetrahedral

Fe3? site (S3, RA: 4.5%). The particularity of this sample

is the appearance of the sextet S4 with RA about 3.7%

which can be attributed to the goethite. Spectra contain also

two doublet D1 and D2 subspectra which can be attributed

to two different behaviors: presence of paramagnetic iron

silicate minerals or partly to the abundance of superpara-

magnetic (effect associated to decrease of volume of par-

ticle) iron (oxyhydr)-oxides.

The spectrum of the sample A7 shows only three sextets

(S1, S2 and S3) with corresponding hyperfine field of 507,

Fig. 10 Magnetic susceptibility

versus heavy metal

concentrations (Zn, Cu, Cd, Pb)

in urban topsoils. R2 represents

the correlation number


123

451 and 490 kOe, respectively, and two doublets D1 and

D2. The hematite is represented with the same relative

abundance S1 (RA: 39%) but the magnetite is more

abundant with the total representation S2 and S3 about

46.2%. For more detail, scanning and transmission electron

microscopy linked with energy dispersive spectroscopy and

X-ray diffraction will be performed to these samples to

determine all magnetic phases and further substitutions that

can occur during formation or deposition.

The collapsed hyperfine field BHF is a typical effect

resulting from magnetic relaxation and usually found for

natural goethite (Barrero et al. 1996). For a well-crystal-

lized goethite the BHF at room temperature is around

380 kOe (Vandenberghe et al. 2000) and for sample A9, it

was found to be a distribution of magnetic hyperfine fields

with BHFmax = 366 kOe. The decrease of hyperfine BHF

observed in sample A9 could be associated to several

effects such as a wide distribution of goethite particle sizes,

structural defects, like vacancies and/or isomorphic Al-for-

Fe substitution and also small particle size (Berquo et al.

2007; Murad and Cashion 2004).

Scanning electron microscopy (SEM)

SEM observations were conducted on magnetic particles

separated from the soil samples. The chemical composi-

tions of the grain surfaces were also determined using EDX

analysis (Table 4). Based on the morphology, the magnetic

materials observed can be grouped into three types: ferro-

oxyde spherules, lead fragments and titano-magnetite

euhedral particles (Fig. 12):

• The most frequently observed magnetic materials

throughout the study area especially in high polluted

places (LEP, RCP), were spherules with diameters of

5–250 lm, which were identified as iron-oxides

(Fig. 12a, b). These Spherules are commonly associ-

ated with Mn, Cr, V, La, Ti and sometimes coated by

clays (Fig. 12c). In general, the chemical composition

of the isolated spherules is close to that of pure

magnetite.

• The lead (Pb) with high content occurs as anhedral

fragments associated with iron and chromium. Analytic

Fig. 11 Mossbauer spectra of urban topsoils, A91- in lagre engine

park and A7- at 20 m far away the road

Table 2 Mossbauer parameters

of the sample A7 (20 m from

the road edge) at room

temperature

Site BHF (kOe) IS (mm s-1) QS (mm s-1) % Phase

S1 507 0.36 0 39.5 Hematite (aFe2O3)

S2 451 0.57 0 10.9 Magnetite Fe2.5?

S3 490 0.33 0 35.3 Magnetite Fe3?

D1 - 0.267 2.82 3.3

D2 - 0.26 0.55 11.1

Table 3 Mossbauer parameters

of the sample A9 (large engine

park) at room temperature

Site BHF (kOe) IS (mm s-1) DEQ (mm s-1) Aire Phase

S1 494 0.35 0 39.0 Hematite

S2 449 0.58 0 20.3 Magnetite Fe2.5?

S3 469 0.34 0 4.5 Magnetite Fe3?

S4 366 0.37 0 3.7 Goetite

D1 0 – 2.05 6.2

D2 0 – 0.65 26.3


123

formula shows presence of lead dioxide (PbO2) that can

be released to the environment from battery material

or by combustive oxidation of lead contained in fossil

fuel (Fig. 12d). Notably, Pb appeared to be associated

also with the smallest spherules, \5 lm. The angular

particles were Fe rich, some particles also containing S,

Al, K and Ca

• Angular magnetic particles mostly have grain sizes of

50–100 lm (Fig. 12e), with euhedral particles form.

EDX analyses showed that the angular particles were

titanium-iron-oxide and aluminosilicate, respectively.

Euhedral titanium-iron-oxides with silicates (clays)

occurred only in the samples from olive plantation soil

far away from road emissions. For the titanium-bearing

grains, accompanying rock magnetic results (Table 4)

suggest that the iron-titanium combination is probably

due to titanomagnetite (Perkins 1996). These iron-

oxides with silicates (Si, Al, K, Ca) are likely to be

natural in origin (Lithogenic/pedogenic), as described

for the magnetic extracts obtained from topsoils in main

localities (Kim et al. 2007; Hounslow and Maher 1996;

Maher et al. 2003). For instance, Maher et al. (2003)

also described the higher magnetic susceptibility values

in topsoils than in the deeper soil horizons, which were

ascribed to the formation of pedogenic ferrimagnetic

particles in topsoils. Thus, it is proposed that the

angular iron-oxide particles accompanied with silicate

minerals were likely formed by natural processes, such

as pedogenesis or weathering (Fig 12e).

Conclusion

Compared with the background, magnetic signals of the

urban topsoils in Beni Mellal are extremely enhanced; while

these of the agricultural topsoils are only slightly increased.

This study has successfully delineated the spatiotemporal

pollution features in Beni Mellal city. Magnetic suscepti-

bility measurements on ground and soils along 5-km along

‘20 Aout’ road is used to quantify the degree and the effect

of environmental pollution related to vehicular emission.

The high magnetic susceptibility values occur at 0–1 m

from the paved road and excessively recorded in area

reserved as park for large engine. Road out sided by olive

plantation shows enhancement of magnetic susceptibility

far from the road in soil which magnetic signal is more

pronounced by pedogenic processes. Microscopy reveals

abundance of anthropogenic magnetic spherules of which

the latter are derived from vehicular motor emission. Heavy

metals are well correlated to magnetic susceptibility such as

Pb, Cu and Zn. This indicates that the traffic density in Beni

Mellal has a strong influence on metal pollution of its

roadside soil dusts. Magnetic mineralogy determined by

Mossbauer spectroscopy suggests the presence of hematite,

Table 4 The chemical compositions of the grain surfaces determined using EDX analysis

Spherules Fragments Titanomagnetite

A9-1_1.3 A17-1_2.1 A17-1_2.3a A17-1_2.3.1 A9-1_1 B4_5.4

Elements (a) (b) (c) (c) (d) (e)

Fe 37.23 42.43 54.91 42.04 5.73 25.43

Mg – – – – – 3.99

Mn 0.58 0.18 3.19 1.49 – 0.84

Ti 0.56 0.27 1.73 0.59 – 16.05

Ca – 0.27 2.18 0.94 1.97 –

Si 0.79 0.5 20.54 11.31 – 3.69

Al – 0.65 2.51 1.98 – 3.32

K – 0.22 0.62 0.36 – 0.19

Pb – – – – 26.41 –

Sr – – – – 2.41 –

Cr 0.4 – – – – –

Co – – – – 1.19 –

La 0.39 – – – – –

V 0.2 – 0.63 – – –

O 59.85 55.48 13.7 41.28 62.29 46.5

Total 100 100 100.01 99.99 100 100.01

Letters referred to SEM images in Fig. 12a EDX spectra of spherules coated clay


123

Fig. 12 Scanning electronic

miocroscopy (SEM) results for

representative samples roadside

soils and resulted EDX spectra.

a, b Spherules consisting of

magnetite, titanomagnetite and

hematite. c Magnetite spherule

with clay coated surface. d Pb

occurring as clast debris.

e octahedral titanomagnetite

inherited from pedogenic

processes in olive plantation.

The X-ray results show

particulate chemical

composition


123

magnetite and goethite in highly polluted areas. The iron

oxides and especially goethite are efficient in incorporating

and/or adsorbing foreign ions. The adsorption of metals by

goethite plays a very important role, because it affects their

mobility and bioavailability and it is a very relevant topic

for environmental studies. Some authors report the incor-

poration of several metals like Ni, Zn, Pb, and Cu in goe-

thite (Trivedi et al. 2001; Berquo et al. 2007). Hence,

magnetic method is a useful technique to detect pollution

related to vehicular emissions.

Acknowledgements The authors acknowledge funding for this

project provided by the Centre National de la Recherche Scientifique

et Technique of Morocco (UATRS service).

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heavy metal pollution and soil magnetic susceptibility in urban soil of beni mellal city (morocco)

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