Journal of Environmental Protection, 2011, 2, 592-600
doi:10.4236/jep.2011.25068 Published Online July 2011 (
Copyright © 2011 SciRes. JEP
Metals, Metalloids and Toxicity in Date Palms:
Potential Environmental Impact
John R. Williams1, Avin E. Pillay2
1Williams Analytical Chemistry Consultancy Services, Goole, UK; 2Department of Chemistry, The Petroleum Institute, Abu Dhabi,
United Arab Emirates.
Received February 2nd, 2011; revised March 16th, 2011; accepted April 25th, 2011.
This paper summarizes our studies on metal and metalloid uptake by the date palm, Phoenix dactylifera L., a tree of
considerable importance in arid regions. The typical concentrations of 17 elements in the date palm are summarized
and compared with existing data in the scientific literature. The role and toxicity of these elements are considered. Is-
sues encountered by us during sample collection, pre-treatment and chemical analysis are described. Future studies are
Keywords: Date Palm, Metalloids, Metals, Toxicity
1. Introduction
The date palm, Phoenix dactylifera L., is a tree of con-
siderable agricultural, cultural, economic and scientific
importance in arid regions such as the Middle East. To
quote Al-Shayeb et al. [1]: It occurs over large areas,
usually growing by roadsides in industrial, rural, residen-
tial and agricultural areas even where high levels of pol-
lution exist. It can survive within a wide temperature
range, and will grow in almost any type of soil [2]. It is
an evergreen plant [3]. Dates constitute a substantial part
of the diet in Arab States [4]. There are over 400 differ-
ent cultivars, but only 50 - 60 are grown over large areas
and considered of economic value [5]. Dates are syn-
onymous with the Middle East, but are grown in other
parts of the world such as the United States of America
[6,7]. The date palm has been used as a biomonitor of
pollutants in Saudi Arabia [1], Turkey [8], Kuwait [9]
and Jordan [10].
The fruiting season spans a period of about five mon-
ths, roughly between May and September. With the Fard
cultivar, for example, there are the Kimri (9 weeks after
pollination), Bisir (15 weeks after pollination) and Rutab
(20 weeks after pollination) stages of development. Usu-
ally, it is the Rutab fruit that is harvested and eaten by
humans. The dates are also eaten by insects, birds and
livestock. The leaflets, however, are consumed by insects
and livestock. Consequently, it is vital to monitor toxic
elements in dates and leaflets, and alert of current dan-
gers and potential problems as early as possible.
At this stage, it is useful to introduce some definitions.
In plants, toxicity is defined as the ability of a substance
or element to disrupt the normal functions of a plant [11].
The toxic levels and effects of common elements are well
known. As an illustration, a frequent symptom of metal
toxicity in plants is chlorosis, which is a light green to
yellow coloration of leaves or whole plants [11]. The
data becomes scant for more exotic elements. Accumula-
tion is where metal levels in the plant tissues are greater
than in the soil or when the concentration of metals in the
plant tissues increases with time [12]. Indication is when
the metal levels in the plant reflect or are similar to the
concentrations in the soil [12]. Alternate bearing is when
the palms produce good crops of dates in some years
followed by poor crops [13]. The units of concentration
given in the chemical literature vary widely and the fol-
lowing may be useful to help avoid confusion: 1 ppm = 1
mg·kg–1 = 1 µg·g–1 = 1000 ppb = 1000 ng·g–1 = 1000
µg·kg–1. It is important to note that the concentrations of
elements in plant material and soil given in this paper are
on a dry mass basis, except where stated otherwise.
This paper summarizes the research studies of the two
authors. Our continued research in this area has drawn
attention to the presence and concentration of 17 ele-
ments (13 metals, 3 metalloids and one non-metal) in
Omani date palm tissues and the surrounding Haplic Tor-
riarents soil, and the potential consequences if these ele-
ments reach toxic levels. The efforts of other research
Metals, Metalloids and Toxicity in Date Palms: Potential Environmental Impact593
groups have been included in the paper to put our results
in context and to encourage debate. Chemical analysis
was by inductively coupled plasma-atomic emission spe-
ctroscopy (ICP-AES), except where stated otherwise.
2. Metals
2.1. Silver (Ag)
Ag has no known metabolic function and may be highly
toxic towards algae and other organisms [14,15]. It is
more toxic than Pb, but less toxic than Cd [16,17]. Little
is known about the toxic effects of Ag in the date palm,
but attempts have been made to obtain baseline levels in
two contrasting studies. Egyptian dates were found to
contain Ag at a concentration of 4 ppb [18]. Six years
later in a more extensive study [19], Omani dates were
found to contain up to 4.48 µg·g–1 Ag (Table 1), de-
pending on fruit growth stage and the alternate bearing
effect. This three orders of magnitude difference is worth
investigating further. Interestingly, Ag was not detected
in the leaflets, but was present in the soil at an average
concentration of 1.03 µg·g–1 [19].
2.2. Aluminium (Al)
In humans, Al is known to cause brain damage, bone
disease and anemia in patients subjected to haemodialy-
sis using water containing 0.1 - 1 µg·g–1 of Al [20]. Al in
plants at toxic levels causes stunting, dark green leaves,
purpling of stems, death of leaf tips and damage to the
root system [11]. Al has been studied in the date palm
twice. In 1999, Al in Egyptian dates was investigated
[18]. It was found that Al levels were normal and varied
from 33.32 - 43.92 ppm, depending on the sample and
chemical analysis method chosen. In a later study on
Omani date palms, the authors considered the leaflets and
surrounding soil, as well as the dates [19]. It was found
that Al levels in the date palm tissues were, generally,
safe and within literature guidelines [21]. An exception
was Al in dates in the Kimri stage. The upper limit of
166 µg·g–1 was higher than the literature value of 140
µg·g–1 (Table 1).
2.3. Barium (Ba)
Little is known about the toxic effects of the trace ele-
ment, Ba, in plants. However, a concentration of 500
mg·kg–1 in mature leaf tissue is considered excessive [11].
Ba may have been involved in the reduction of conifer-
ous forests in parts of Europe [12]. Ba has been investi-
gated in the date palm in two studies. In 1999, Ba was
found in Egyptian dates at a concentration of 4 ppm [18].
A similar result (Table 1) was obtained six years later for
Omani dates [19]. Interestingly, Ba levels in the Omani
dates increased with time from an average of 0.74 µg·g–1
in the Kimri stage to 2.74 µg·g–1 in the Bisir stage to 12.9
µg·g–1 in the Rutab stage [19]. Previously, it was thought
that plants did not accumulate Ba. As an illustration,
there was relatively little accumulation of Ba by macro-
fungi growing on uranium mill tailings [22]. It was found,
however, that Ba levels in the dates and leaflets were safe
and within the normal range of 10 - 100 mg kg-1 for plant
material [11].
2.4. Beryllium (Be)
Be is one of the more poisonous elements. Its most un-
pleasant effect is berylliosis, and the subsequent lung
fibrosis and pneumonitis, which may develop after a la-
tency period of up to 20 years [23]. Exposure to Be, a
hypersensitising agent, also causes ulcers and skin granu-
lomas. Little is known about the effect of Be in plant
tissues, but a concentration of 0.001 mg·kg–1 is described
as normal [11]. In the two investigations performed on
this metal in the date palm, contrasting results were ob-
tained. In the first study, Be was found in Egyptian dates
at a concentration of 0.4 ppm [18]. This was 400 times
the normal value. In the later investigation in Oman, Be
was not detected in the fruit [19]. Furthermore, it was not
detected in the leaflets either (Table 1), but was present
in the surrounding soil at 0.03 - 0.05 µg·g–1 [19]. This
difference in the results should be investigated further.
2.5. Cadmium (Cd)
Cd is highly toxic with long term effects on kidney func-
tion and bone metabolism, and it is carcinogenic [24]. It
occurs in nature in association with Zn minerals. Symp-
toms of Cd toxicity in plants include brown margins to
leaves, chlorosis, reddish veins and curled leaves, and
brown stunted roots [11]. Growing plants acquire Zn and
they also absorb Cd with a similar biochemical function
[11,20,25]. Some of the Cd in plants and fruit are in-
gested by animals and insects. Part of this ingested metal
accumulates in their organs and the rest is excreted.
However, excessive uptake replaces Zn at key enzymatic
sites, causing metabolic disorders [20]. The acceptable
level for Cd in most fruits and some edible leafy plants is
about 50 ng·g–1 [11]. The source of Cd in plants is usu-
ally phosphate fertilisers which contain ionic Cd as a
contaminant, and sewage sludge which is used for soil
enhancement [20,21,23,26]. There have been several
studies on Cd in the date palm. In 1996, a Turkish group
investigated the date palm as a biomonitor of Cd [8].
They found a good correlation between the concentration
of Cd in the surface soil and in washed leaflets. A year
later, the content of Cd in dates was investigated [27]. It
was concluded that the Cd concentration was within the
limits specified by relevant legal standards. In 1999, Cd
was detected in Egyptian dates at concentrations of 15.5 -
Copyright © 2011 SciRes. JEP
Metals, Metalloids and Toxicity in Date Palms: Potential Environmental Impact
Copyright © 2011 SciRes. JEP
Table 1. A summary of the measured and recommended concentrations of 17 elements, as well as their upper limits, in
Omani date palms. D = dates, L = leaflets, [ ] = reference, ND = not detected, NA = not available.
Element Mean measured conc. Upper limit measured conc. Recommended conc.
D 0.60 µg·g–1 [19] D 4.48 µg·g–1 [19] 0.2 mg·kg–1 [11]
Ag L ND [19] L ND [19]
D 15.2 µg·g–1 [19] D 166 µg·g–1 [19] 140 µg·g–1 [21]
Al L 32.2 µg·g–1 [19] L 75.8 µg·g–1 [19]
As L 175 ng g–1 [33] L 800 ng g-1 [33] 80 ng·g–1 [21]
D 3.3 mg·kg–1 [53] D 6.5 mg·kg–1 [53] 10 - 20 ppm [11]
B L 2.5 mg·kg–1 [ 53] L 5.7 mg·kg–1 [53]
D 5.0 µg·g–1 [19] D 21.0 µg·g–1 [19] 10 - 100 mg·kg–1 [11]
Ba L 3.1 µg·g–1 [19] L 11.3 µg·g–1 [19]
Be D, L ND [19] D, L ND [19] 0.001 mg·kg–1 [11]
D 35 ng·g–1 [29] D 225 ng·g–1 [29] 50 ng·g–1 [11]
Cd L 28 ng·g–1 [29] L 125 ng·g–1 [29]
Cr L 2,913 ng·g–1 [33] L 30,300 ng·g–1 [33] 200 ng·g–1 [21]
Ga D,L ND [19] D,L ND [19] 0.1 mg·kg–1 [ 11]
Hg L 37.2 µg·kg–1 [ 37] L 89.8 µg·kg–1 [37] 40 µg·kg–1 [11]
D 0.41 µg·g–1 [19] D 4.83 µg·g–1 [19] NA
La L 0.01 µg·g–1 [19] L 0.08 µg·g–1 [19]
Mo D,L ND [19] D,L ND [19] 0.1 mg·kg–1 [11]
D 0.28 µg·g–1 [49] D 3.98 µg·g–1 [49] 1 mg·kg–1 [11]
Ni L 0.75 µg·g–1 [49] L 2.20 µg·g–1 [49]
D 0.04 µg·g–1 [19] D 1.90 µg·g–1 [19] 0.05 mg·k g–1 [11]
Tl L ND [19] L ND [19]
V D,L ND [19] D,L ND [19] 0.1-1.0 mg·kg–1 [11]
Si D,L ND [19] D,L ND [19] NA
D 0.04 µg·g–1 [19] D 0.28 µg·g–1 [19] 0.05-2.0 mg·kg–1 [11]
Se L 0.03 µg·g–1 [19] L 0.25 µg·g–1 [19]
24.75 ppb, depending on the sample and chemical analy-
sis method [18]. In 2002, Cd was reported at an average
concentration of 5.65 µg·kg–1 fresh weight in Saudi dates
[28]. In the same year, our work in Oman [29] showed
that mature dates (Bisir and Rutab) are safe for human
consumption (generally below 50 ng·g–1), but that levels
of this heavy metal were a cause for concern earlier in the
fruit growing season at the Kimri stage when concentra-
tions as high as 225 ng·g–1 were encountered (Table 1).
In a later study in Pakistan using atomic absorption spec-
troscopy, Cd levels in mature dates averaged 27 ng·g–1
[30]. This concentration was similar to what we encoun-
tered two years earlier (Table 1). Most recently, a Saudi
study concluded that Cd levels in dates from around Ri-
yadh were within recommended safe limits [31].
2.6. Chromium (Cr)
Cr is an essential micronutrient for animals, but not for
plants [11]. Cr as Cr(VI) is, however, a known carcino-
gen [24]. At a concentration above 5 mg·kg–1, it is toxic
to plants growing in non-serpentine soils [24]. The
symptoms include chlorosis of new leaves and injured
root growth [11]. Cr in the date palm has been the subject
of several studies by other workers mainly concerned
with the biomonitoring of metal pollution in Saudi Arabia
[1,3,32] and Jordan [10]. In the case of the Saudi studies,
it was discovered that Cr levels varied depending on
whether the date palm tissues were washed or unwashed
and the location. Higher Cr levels were, generally, found
in unwashed plant material from urban areas.
Metals, Metalloids and Toxicity in Date Palms: Potential Environmental Impact595
Similarly, the Jordanian study found that the highest Cr
levels were present in leaflets from palms growing in
urban and industrial areas. One study investigated Cr in
Egyptian dates [18]. They found that Cr levels varied
from 30.2 - 95 ppb, depending on the sample and the
chemical analysis method employed. Our research group
similarly showed that Cr levels in Omani dates were safe
for human consumption [33]. Concentrations in leaflets
were, however, a cause for concern (Table 1). According
to the literature, the permissible mean level of Cr in
plants is 200 ng·g–1 [21]. In the case of Cr, about 45% of
the leaflet samples possessed levels between 250 and 700
ng·g–1, while a significant number produced levels be-
tween 1000 and 5000 ng·g–1 [33]. There was some evi-
dence of Cr accumulation in the leaflets too. This was an
important finding, because insects and other organisms
subsist on the leaves and there is a danger of accumula-
tion of Cr in the food chain.
2.7. Gallium (Ga)
Ga is a trace element [11], but little is known about its
toxic effects in plants. It has been studied in lichen and,
consequently, it is known that background concentrations
in this organism are 0.09 - 0.49 ng·g–1 [34] and elevated
levels are considered to be 0.2 - 2.2 ng·g–1 [35]. A con-
centration of 0.1 mg·kg–1 is a reference value considered
typical in plants [11]. Ga has been studied in the date
palm on two occasions. In 1999, it was found in Egyptian
dates at a concentration of about 0.4 ppm [18]. Six years
later in Oman [19], Ga was found in the surrounding soil
at an average concentration of 1.38 µg·g–1, but was not
detected in the leaflets and fruit (Table 1). The discrep-
ancy between the 1999 and 2005 results should be inves-
tigated further.
2.8. Mercury (Hg)
The health effects of Hg are well-known. Hg is highly
toxic, especially organomercury compounds [24]. Me-
thylmercury, for example, is a neurotoxin and damages
the nervous system [36]. Hg is toxic to plants at concen-
trations greater than 1 mg·kg–1 [24]. Symptoms of Hg
toxicity include severe stunting of seedlings and roots,
leaf chlorosis and browning of leaf points [11]. Several
studies have concentrated on Hg in dates from Egypt [18],
Saudi Arabia [28] and Pakistan [30]. Our study in Oman
focused on total Hg in the leaflets using a cold vapour
atomic absorption method [37]. We found that, in some
cases (Table 1), Hg levels were above the recommended
maximum for plant leaves of 40 µg·kg–1 [11]. This was of
concern, because livestock are fed partly on date palm
leaflets. Additionally, insects eat the leaflets and are
themselves devoured by birds. The possible consequence
in both cases is biomagnification of Hg along the food
2.9. Lanthanum (La)
Some studies have been conducted on the health effects
of La in animals and humans. La3+ inhibits the binding of
calcium ions by muscle microsomes in dogs [38,39]. La
increases the clotting time of normal human plasma in a
dose-dependent manner when clotting was induced either
by thromboplastin or by kaolin in the presence of cepha-
lin and Ca2+ [40]. La3+ increases the energy coupling ca-
pacity of submitochondrial particles [41]. La3+ induces
cell apoptosis [42] and La affects the neurobehavioral
development of rats [43,44]. There have been fewer
studies in plants and just two on La in the date palm. The
first study found La in Egyptian dates in the range 17.75 -
62 ppb, depending on the sample and chemical analysis
method used [18]. They also investigated Ce and Eu, but
these lanthanides went undetected in the dates. During
the later study in Oman, it was discovered that La was
only detected in the dates and leaflets during the Bisir
stage [19]. At that time, La was present in the dates at an
average concentration of 2.45 µg·g–1 and in the leaflets at
0.07 µg·g–1. When all the stages and samples are taken
into account, the average levels decrease (Table 1). La
was present in the soil at a mean level of 0.05 µg·g–1. This
data infers that the fruits were accumulating La during the
Bisir stage. Furthermore, the date palm (through the leaf-
lets during the Bisir stage) could be used as an indicator
species for La in soil. Lanthanides, including La, were
not detected by inductively coupled plasma- mass spec-
troscopy (ICP-MS) in the potable water used to irrigate
the date palms [45]. The large difference in La concentra-
tion in Egyptian and Omani dates should be investigated
further. The remaining lanthanides and all the actinides
are unstudied in the date palm. The actinides have not
received attention, presumably because of their radioac-
tivity and consequent changes in concentration with time.
2.10. Molybdenum (Mo)
Mo is an essential micronutrient for plants and animals
[11]. A high concentration of Mo does not normally af-
fect plants, but can pose a problem for ruminant animals,
particularly dairy cows, that consume plants containing 5
ppm or more of the element [11]. In the one study on Mo
in the date palm, it was found that the element was nei-
ther detected in the leaflets nor the fruit (Table 1) nor the
surrounding soil [19]. The dates and leaflets can be rec-
ommended, therefore, as a livestock feed with respect to
Mo. To alleviate any problems with the date palms due to
Mo deficiency, the trees should be fed the element at a
normal level of 0.1 mg·kg–1 [11].
2.11. Nickel (Ni)
Ni is a controversial element. Some believe it to be an
essential element [11], but others class it as an element of
Copyright © 2011 SciRes. JEP
Metals, Metalloids and Toxicity in Date Palms: Potential Environmental Impact
common environmental concern [46], because it is highly
toxic and carcinogenic [24]. Ni is toxic to plants when
soil Ni levels exceed 100 mg·kg–1 and plant tissue Ni
concentrations are above 10 mg·kg–1 [24]. Toxic levels of
this transition metal can cause interveinal chlorosis in
new leaves, stunted foliage, and brown short roots in cer-
tain crops [11]. Several studies have been conducted on
Ni in the date palm. In 1999, Ni was found in Egyptian
dates at a concentration of 0.4 ppm [18]. Furthermore, the
fibre [32], leaflets [47] and fruit [48] of Saudi date palms
have all received attention. More recently, Ni levels in
date palm leaflets from Jordan have been studied [10].
Our work concentrated on Ni in Omani date palms grown
in soil containing elevated levels of Ni [49]. It was found
that, despite the relatively high soil Ni content (a mean of
91 µg·g–1 with a range of 34-153 µg·g–1), the date palms
appeared healthy and did not accumulate Ni in the fruit
and leaflets (Table 1). The Ni content of the fruit and
leaflets was safe (below 4 µg·g–1 in all cases), similar to
the findings of earlier studies, and within international
guidelines [21].
2.12. Thallium (Tl)
Tl is highly toxic to humans [50]. Little is known about
the phytotoxicity of Tl. In the one study conducted on Tl
in the date palm [19], it was found that the element was
present in the fruit, but not detected in the leaflets (Table
1). Tl levels in the fruit averaged 0.47 µg·g–1 in the Kimri
stage, but 1.04 µg·g–1 in the Bisir stage. Tl was not de-
tected in the Rutab stage, which is eaten by humans. Tl
levels in plants are typically 0.05 mg·kg–1 [11], so the Tl
concentration in some of the fruit was high and a cause
for concern. It was found that Tl levels in the fruit were
similar to those in the soil [19], implying that low
Tl-containing soil should be used for date palms grown
for agricultural purposes and that the date palm could act
as an indicator species for this element.
2.13. Vanadium (V)
V is a trace element and believed to be a micronutrient in
animals only [11], although not all scientists share this
view [51]. The toxic effects of V in plants are relatively
unknown. The normal range in plant leaves is 0.1-1.0 mg
kg-1 with a suggested maximum of 2 mg·kg–1 [11]. V has
been investigated twice in the date palm. In the first study,
V was found in Egyptian dates at concentrations of 0.78 -
5.1 ppb, depending on the sample and the chemical
analysis method utilised [18]. In a later study in Oman, V
was not detected in the date palm fruit or leaflets (Table
1) despite being present in the soil at an average concen-
tration of 0.87 µg·g–1 [19]. The contrasting results of the
two studies should be investigated further.
3. Metalloids
Metalloids, also known as semimetals, are eight elements
that exhibit both metallic and non-metallic properties
under certain circumstances [52]. These elements in order
of atomic number are: B, Si, Ge, As, Sb, Te, Po and At.
Some studies have been conducted on metalloids in the
date palm by the authors of this paper. We considered As,
B and Si.
3.1. Arsenic (As)
In humans, As is highly toxic and carcinogenic [24]. It is
phytotoxic when found at greater than 5 mg·kg–1 in plant
tissue [24]. Symptoms of As poisoning in plants include
red-brown necrotic spots on old leaves, yellowing or
browning of roots and depressed tillering [11]. Our re-
search group investigated As in Omani date palm leaflets
[33]. Appreciable levels of As were encountered in the
leaflets (Table 1). This is a cause for concern, because
the leaflets form a significant ingredient in livestock feed
in the date-producing countries of the Middle East. About
70% of the samples were above the documented permis-
sible mean value of 80 ng·g–1 [21]. A year later in Paki-
stan, As was studied in dates using instrumental neutron
activation analysis [30]. It was found that As levels in the
dates were safe.
3.2. Boron (B)
B is an essential element in plants, but not in animals [11].
In humans, B appears to offer protection from prostate
cancer and an intake of 3.0 mg·day–1 is considered bene-
ficial, but large amounts are toxic [52]. In plants, an ex-
cess of B (50 - 200 ppm) causes leaf tips to become yel-
low, followed by necrosis [11]. Leaves eventually assume
a scorched appearance and prematurely fall off. B has
been studied in the date palm [53]. Levels of B were
generally 1 - 6 mg·kg–1 in the dates and leaflets (Table 1),
and 3 - 8 mg·kg–1 in the surrounding soil. Normal B lev-
els in plant material are 10 - 20 ppm [11], so the palms
were B deficient. Low levels of B (below 3.5 mg·kg–1)
were associated with palms that produced a poor yield of
fruit. Furthermore, K/B ratios of greater than 2500 were
found in these trees. It is recommended that the date
palms be fed a B supplement in future.
3.3. Silicon (Si)
Si has been described as essential for some plants and
often beneficial for others [54]. Concentrations of soluble
Si range from 1 - 40 mg·l–1 in soil water [55]. It has been
studied in the date palm [19]. No Si was detected in the
leaflets and fruit (Table 1). It was, however, found in the
surrounding soil at a concentration of 3.83 µg·g–1. Con-
sequently, the date palm does not take up Si to any great
opyright © 2011 SciRes. JEP
Metals, Metalloids and Toxicity in Date Palms: Potential Environmental Impact597
extent and it is not an element of concern from a toxicity
point of view.
4. Non-Metals
The only non-metal considered by the authors was sele-
nium (Se). It is an essential micronutrient for animals, but
highly poisonous at greater concentrations [24]. At levels
greater than 4 µg·g–1, Se can be toxic as it prevents proper
bone formation in animals [55]. In plant tissue, Se is
phytotoxic at concentrations exceeding 5 mg·kg–1 [24].
Se toxicity manifests itself as interveinal chlorosis or
black spots at a content of about 4 ppm, and complete
bleaching or yellowing of younger leaves at higher Se
concentration, and pinkish spots on roots [11]. Se has
been investigated in Pakistani date palm fruits using in-
strumental neutron activation analysis [30]. It was found
that the average concentration of Se in the dates was 75
ng·g–1. This is within the normal range for plants, which
is 0.05 - 2.0 mg·kg–1 [11]. A year later in Oman, Se was
not only studied in the dates, but also the leaflets and
surrounding soil [19]. It was discovered that Se was
found in the fruit and leaflets (Table 1), but only during
the Kimri stage. Se levels in the leaflets and fruit, when
detected, were within the normal range for plants (Table
1). Consequently, Se levels in the date palm are not a
cause for concern yet. Interestingly, Se was not detected
in the surrounding soil, implying that the date palm was
accumulating the element during the Kimri stage.
5. Problems and Remedial Measures
Several issues arose during our studies, mainly around
sample collection, pre-treatment and chemical analysis.
5.1. Sample Collection
Samples should be collected from date palms all of the
same age, because elemental uptake varies with age (El
Mardi, personal communication). A sampling site should
be chosen that is not sprayed with pesticides, which often
contain metals such as Hg and As [24]. Pesticide use
could produce misleading metal content results, since the
leaflets and fruits are washed (removing surface pesticide)
before chemical analysis, but the soil is not. It is particu-
larly important to collect a representative sample for ac-
curate results. Consequently, four strands of fruit, each
from a different direction (north, south, east and west),
were taken from a date palm and combined to form one
sample [56]. Similarly, four leaflets per date palm from
the median position along the rachis (Figure 1) were
picked and united. Furthermore, three plugs of soil
around the base of each palm were taken and mixed to-
Soil samples were needed in connection with accumu-
lation and indication studies. The soil often contained
Figure 1. Date palm leaflet showing the median portion of
the rachis.
many small stones. This made soil sample collection dif-
ficult, because it prevented the Auger soil sampler (Fig-
ure 2) going deeper into the soil once a stone was struck.
There was no choice, but to remove the sampler from the
ground and start again. Consequently, a patient approach
had to be adopted. Considering that there were typically
12 palms in the study, this was arduous work in the heat
of a Middle Eastern summer where outside temperatures
can reach 50˚C.
Bearing in mind the heat and the physical demands of
sample collection, it is advisable to obtain assistance
when collecting the large number of samples needed.
This usually involves employing local laborers. Since
they are not scientifically trained, they need close super-
vision to ensure that they are collecting samples correctly.
In addition, the wages of the laborers add to the overall
cost of the project and this should be borne in mind when
applying for research grants.
5.2. Sample Pre-Treatment
The large number of collected samples presents problems
in the laboratory at the pre-treatment stage before chemi-
cal analysis. Dozens of soil, leaflet, and fruit samples
need to be digested or ashed. This may prove prohibitive
for final year undergraduate project students, who have
other commitments. Consequently, a postgraduate stu-
dent (MSc or PhD) or postdoctoral researcher is recom-
mended for the labor-intensive work involved.
Digestion and ashing methods should be tested on
samples of known concentration such as certified refe-
rence materials (CRMs) to ascertain the efficiency of the
procedure. At the time of writing, there are no date palm
CRMs. Consequently, other plant CRMs have to be used
instead [37]. This could be a problem, because the date
Copyright © 2011 SciRes. JEP
Metals, Metalloids and Toxicity in Date Palms: Potential Environmental Impact
Figure 2. Soil sampler used in the study.
Figure 3. Schematic diagram of the ICP-MS instrument.
palm is adapted for arid conditions, for example, it has
tough skin on the fruit and leaflets. Other plants may not
have these natural modifications and may behave diffe-
rently during pre-treatment. In addition, some elements
form volatile species during pre-treatment and may not
be recovered quantitatively [57]. In such cases, a closed
vessel is recommended during, for example, digestion.
5.3. Chemical Analysis of Samples
The chemical analysis of the samples can be challenging
too. As stated previously, ICP-AES was used for the bulk
of the chemical analysis by the authors. In some cases,
for example, Be, Ga, Mo, Se, Si, Tl and V, the elements
were not detected in some samples. It is possible that
these elements were there, but present at concentrations
below the detection limit of ICP-AES. Consequently, it is
recommended that ICP-MS (Figure 3) be used in any
future studies as the detection limit is superior [57].
5.4. Other Issues
Contamination of samples by the laboratory glassware
can be a problem. Metals (as ions) from previous studies
can adsorb to the glass surface and can contaminate new
samples [57]. This contamination can be avoided by
rinsing the glassware with dilute nitric acid (1.54 M) be-
6. Future Work
There is considerable potential for further work. On sev-
eral occasions, for example with Ag, Be, Ga, La and V,
markedly different findings have been reported by other
research groups. The reasons for these differences should
be determined by further investigation. Studies could be
extended to other as yet unexplored elements, for exam-
ple, the majority of the lanthanides, the five remaining
metalloids, most of the transition metals and the Group 5
metal Bi. At present, more metals are unstudied than
studied. Highly sensitive ICP-MS is the chemical analy-
sis method of choice. There is a lack of toxicity data in
plants for many elements. Further studies are needed to
correct this problem. Our studies have concentrated on
relatively clean sites. Research can be extended to more
contaminated venues. The type of soil could be a factor in
elemental uptake by the date palm. Our studies have fo-
cused on Haplic Torriarents soil. Further work could be
performed on other soil types. The Fard cultivar of the
date palm was considered by the authors. Research
should be extended to other cultivars, since what happens
in one variety does not guarantee the same behavior in a
different one.
7. Acknowledgements
The authors would like to thank the students, staff and
faculty of Sultan Qaboos University, Sultanate of Oman
opyright © 2011 SciRes. JEP
Metals, Metalloids and Toxicity in Date Palms: Potential Environmental Impact599
for their assistance.
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