Lead and cadmium interactions in Cynodon nlemfuensis and sandy
soil subjected to treated wastewater application under greenhouse
conditions
Simon Madyiwaa, Moses John Chimbarib,*, Frederik Schuttec
a
Department of Chemical Engineering, University of Pretoria, South Africa
b
c
University Lake Kariba Research Station, Box 48, Kariba, Zimbabwe
Department of Chemical Engineering, University of Pretoria, South Africa
Abstract
Pb and Cd are known to influence each other’s uptake by some plants when the two metals
exist in the soil in significant amounts. This influence may be beneficial if it reduces uptake of
metal by plants but may be detrimental if it increases uptake of the metal. This study was carried
out to investigate the interaction of Pb and Cd in sandy soils and Cynodon nlemfluensis (star
grass). Star grass was grown under greenhouse conditions in 33 fertilized pots containing sandy
soils. Three weeks after planting the grass the pots were randomly assigned to the following
treatments replicated 3 times; a) application of 3 varying concentrations of Pb or Cd in addition
to effluent and sludge, b) application of 3 varying concentrations of combined Pb and Cd in
addition to effluent and sludge, c) application of water and d) application of only effluent and
sludge. Analysis of grass samples was done 45 and 90 days after addition of Pb and Cd to pots
and that of the soil was done 90 days after addition of Pb and Cd to pots. The log normal mean
1
level (in mg/kg) of Pb detected in the soil was 1.75 and that of Cd was 0.057 in mixed treatments
while for single treatments the levels were 1.67 for Pb and 0.03 for Cd. The presence of Cd in the
soil had no effect on the bio-available level of Pb but Pb significantly (p=0.05) increased the bioavailable concentration of Cd. The log normal mean levels of Pb in grass re-growth from mixed
treatment was 1.68 and that of Cd was 0.57 while the values for single treatments were 1.47 for
Pb and 0.31 for Cd. There was no significant change in the level of uptake of Pb between single
treatments and mixed treatments. However, Pb significantly increased uptake of Cd in mixed
treatments compared to single treatments (p>0.05). The results of this study indicate that copresence of Pb and Cd may have the detrimental effect of increasing uptake of Cd in star grass.
Keys words: Pb and Cd interaction, Cynodon nlemfuensis, sandy soils, repeated sewage effluent
and sludge application
*
Corresponding author. Tel.: +263-61-2981/3035; fax: +263-61-3035
E-mail addresses: ulkrs@telco.co.zw; chimbari@telco.co.zw (M.J.Chimbari)
2
1. Introduction
Lead (Pb) and cadmium (Cd) are toxic metals of increasing environmental concern as they
enter the food chain in increasingly significant amounts. Pb is naturally found deep within the
earth and mined together with silver deposits. It is the most common and abundant contaminant
of the environment and human body. If levels in the human body have increased over time and it
is estimated that the human body can take up 1-2mg daily up to a total content of 125-200mg,
which is 500-1000 times more than the levels detected in bones of very old human skeletons
(Haas, 2003). Toxicity of Pb to humans is caused by interference with functions performed by
essential elements such as Ca, Fe, Cu and Zn in various enzymes. Cd is mined as part of Zinc
(Zn) deposits. It is less abundant than Pb but it has entered the air, food and water thus making it
a major environmental concern. Cd is more toxic to humans than lead and increases in content
with age up to 50 years when it is estimated to reach levels of 40mb (Haas, 2003). Toxicity of
Cd is caused by its interference with Zn functions in enzymes.
In Zimbabwe, pasturelands consisting of star grass have been grown on soils on which mixed
treated effluent and sludge has been disposed for over 30 years. Beef cattle are raised on these
pastures. Research has shown that Pb and Cd are taken up by plants from the soil, thereby
making plants potential sources of contamination for humans and animals. Cadmium is readily
transported from the soil to the upper parts of plants (Mengel and Kirkby, 1982) and its transfer
from soils to edible plant parts of agricultural crops is significantly greater than for other heavy
metals except Zn (Moolenar and Lexmond, 1999). According to the same authors, actual plant
uptake in soil-crop ecosystems, not only depends on soil concentrations but also on the
distribution of a chemical element in relation to other chemical species in the soil (also known as
3
speciations) and mechanisms for root entry and translocation to aerial plant parts. Khan and
Frankland (1983) observed that when Cd or Pb caused phytotoxicity Zn levels in radish plants
were close to deficient values. Cd and Pb have been shown to interact positively or negatively in
some plant species. Miller (1977) observed that accumulation of Cd was increased by the
addition of Pb while Cd in the soil reduced uptake of Pb in Zea mays L. (corn). Similarly lead
was observed to increase uptake of Cd in rye and fescue (Carlson and Rolfe, 1979). The addition
of both Pb and Cd increased the levels of both metals in American sycamore over uptake
observed with single metals added (Carlson and Bazzaz, 1977).
The implication of the interactions between Pb and Cd is that the co-presence of the metals in
the soil may lead to increased uptake of one or the other metal by plants, hence posing an
increased hazard to animals and humans. In a study in which single metals were used to enrich a
sandy soil on which star grass was grown and irrigated using treated sewage, Madyiwa et al
(2002) established that the grass absorbed Pb and Cd way beyond the 40 mg/kg and 1 mg/kg
limits (respectively) recommended for pasture grass (UK Statutory Instrument No. 1412, 1995).
However, the effect of co-presence of the two metals on their bio-availability in the soil and
uptake by star grass was not established. This study was carried out to establish the level of
interaction of Pb and Cd in a sandy soil and star grass subjected to co-presence of high levels of
inorganic metal and repeated sewage effluent and sludge application. The study sought to
establish the influence of (1) Pb on bio-available Cd, (2) Cd on bio-available Pb, (3) Pb on levels
of Cd in star grass and (4) Cd on levels of Pb in star grass
2. Methods and Materials
4
2.1. Description of study site
The study was carried out in a greenhouse at the University of Zimbabwe, using soils taken
from Churu farm, uncontaminated star grass from Domboshava farm and low quality treated
sewage discharged from Firle Sewage Treatment Plant. The treated sewage consisted of effluent
mixed with digested sludge to produce a slurry (3-4% solids). The sewage treatment plant is
owned and operated by the Municipality of Harare, so is the adjacent Firle farm, which provides
grazing to beef cattle that belong to the municipality. Harare City Council has been disposing
low quality treated sewage onto pasturelands on Firle farm for over 30 years. At the time of this
study, the pastures consisted of mixed Pennisetum clandestinum Chiov (kikuyu) and Cynodon
nlemfuensis (star grass). Flooding is used as the disposal method. However, there is no data
available on the application rates of sewage sludge and effluent on land at Firle farm, but based
on annual sewage output and area irrigated, the application rate is about 48 megalitres/ha/year or
126-167t solids/ha/year (Madyiwa et al, 2002).
The portion of Churu farm, from which the soil was sampled into pots, is 30 m away from the
irrigated area and it has the same type of soil as the irrigated area. It has not been subjected to
treated sewage disposal before. The two areas consist of predominantly granitic sandy soils
classified as Haplic Arenosol, in which a sandy top layer (0-30 cm) overlies a sandy loam subsoil
at a depth of 30 - 50 cm (Madyiwa et al, 2002). The average annual rainfall (over a 30-year
period) is 800mm (Department of Meteorological Services, 1977) and the general slope is 3%.
The area from which soils were taken has not been cropped before. It consisted of scant patches
of grass at the time of the study.
2.2. Soil sampling into pots and grass establishment
5
Soil was excavated using picks and shovels, mixed several times into one large heap and
passed through a 10mm sieve to remove large stones and grass debris. The excavation was done
to an average depth of 40 cm and from one continuous area in order to minimise variability. The
soil was then packed into 79l pots by filling the pots in layers consisting of one full shovel and
tamping and shaking the pot several times before addition of the next layer. Every pot was filled
to a depth of 40 cm. Seven star grass stems of 15mm length, each with one node were planted in
the pots. In order to eliminate nutrient deficiency, single super phosphate (Ca(H2PO4)2 + CaSO4),
sulphate of potash (KCl) and ammonium nitrate (NH4NO3) were applied to each pot at 600, 100
and 100 kg/ha respectively. Water was applied to the grass for a period of 3 weeks to establish
the crop before soil enrichment with inorganic Pb and Cd.
2.3. Experimental treatments
Eleven treatments, each with 3 replicates, were randomized in a block design within the
greenhouse. The treatments comprised (1) a control that was not enriched with inorganic metal
and was irrigated with water only, (2) one treatment that received effluent and sludge mixture but
was not enriched with metal, (3) 3 treatments of Pb enriched by 300, 600 and 1 200 mg/kg
Pb(NO3)2, (4) 3 treatments of Cd enriched by 10, 20 and 40 mg/kg CdS and (5) 3 mixed Pb and
Cd treatments enriched by 300 mg/kg Pb and 10 mg/kg Cd, 600 mg/kg Pb and 20 mg/kg Cd,
1200 mg/kg and 40 mg/kg Cd. All enriched with metal received effluent and sludge application.
The control was referred to as WO and the other treatment that had no metal enrichment was
referred to as ES. Pb treatments were denoted Pb300, Pb600 and Pb1200 respectively, Cd as Cd10,
Cd20 and Cd40 and combined treatments as Pb300Cd10, Pb600Cd20 and Pb1200Cd40. Pb(NO3)2 and
CdS were added to the pots all at once, 3 weeks after planting the grass. Effluent and sludge
irrigation applications were then applied.
6
2.4. Irrigation of grass
A total of 63.5 l of water was applied to each control pot while 23.5 l of treated sewage
supplemented by 40 l of water was applied to the rest of the pots at an irrigation cycle of 3.5
days. This application was intended to satisfy an estimated crop water requirement of 5.5
mm/day at peak demand. The first harvest of grass was collected 45 days after grass
establishment and the second harvest was collected 45 days after the first harvest. The re-growth
received the same irrigation application as the first crop.
2.5.
Sampling and analysis of soil
Soil samples were taken from the pots at depths of 0-10, 10-20, 20-30 and 30-40 cm, using a
soil auger. Sampling was done one day after harvesting grass. After removing plant debris the
samples were air-dried and passed through a 2 mm sieve. Bio-available soil concentrations were
determined using procedures recommended by McGrath and Cegarra (1992).
2.6. Sampling and analysis of grass samples
All the grass in each pot was harvested to constitute a sample for each harvest. The grass was
cut at 5 cm height off the soil surface, washed using de-ionised water, oven dried at 65 0C and
then ground and passed through a 0.1 mm sieve. The samples were then ashed at 550 oC for 16
hours and digested with 25% HCl and concentrated HNO3. After filtration, Pb and Cd were
determined using atomic absorption spectrometry. The samples of the grass that were taken
during planting were subjected to the same metal extraction process prior to determination of
levels of Pb and Cd using atomic absorption spectrometry.
2.7. Sampling and analysis of sewage effluent and sludge samples
Treated sewage was collected from the outlet located at the point of disposal onto the field as
it was being pumped from the plant. Sealed twenty-five litre plastic containers were used to
7
collect, transport to the greenhouse and store treated sewage between every two consecutive
irrigation events. In addition, one sample was collected on each of the 7 occasions when the
treated wastewater was collected for irrigation purposes. The levels of Pb and Cd in treated
sewage samples were determined by atomic absorption spectrometry (Department of
Environment, 1989).
3. Results and data analysis
The measured data on metal concentrations in the soil and grasses was tested for normality
and converted to log10 before analysis.
3.1. Pb and Cd levels in effluent and sludge mixture
The mean concentrations of Pb in treated sewage used to irrigate the first crop and re-growth
were 1.32 mg/l and 1.04 mg/l, respectively. Both levels exceeded the limit of 0.5 mg/l
recommended for irrigation water by the Zimbabwe Statutory instrument 274 (2000). However,
they were below the maximum limit of 5.0 mg/l recommended for irrigation water (Ayers and
Westcot, 1985). Cd was not detected in treated wastewater and water used to supplement treated
sewage.
3.2. Bio-available Pb and Cd content in soils
Lead
A significant p<0.05> increase in bioavailable Pb in soils with single and mixed Pb treatments
was observed as the metal dose increased. Figure 1 shows gradual increase in Pb levels from WO
and ES to treatments amended with inorganic Pb. Log10Pb increased over five-fold from 0.48 in
WO to an average of 2.51 and maximum of 2.80 in enriched single treatments and 2.64 and 2.87
in enriched mixed treatments, respectively. Overall the mean Log10Pb for all treatments was 1.67
8
in single treatments and 1.75 in mixed treatments. Although Pb levels were higher in mixed
treatments compared to single treatments, the differences were statistically insignificant for the
same dose of Pb. Similarly, there was no significant change in the bio-available levels of Pb
between WO and ES.
Cadmium
Bio-available soil Cd levels increased as the metal dose increased (Fig 2). However the
increase in Cd in single treatments from log10Cd of 0 in water to an average of 0.03 in enriched
treatments was statistically insignificant. In contrast, the increase of Cd level to an overall
average of 0.057 (or 0.08 for mixed enriched treatments) and maximum of 0.14 in mixed
treatments was significant (p=0.05).
3.3. Pb and Cd metal content in grass
Lead
In both single and mixed metal treatments and the first crop and re-growth, Pb uptake
increased with increase in bio-available soil concentration as in figure 1. There was no
significant difference between the Pb content of the control and ES in both crops. However, there
was a significant increase (p<0.05) in Pb levels as the level of enrichment of the soil increased in
both single and mixed treatments of Pb in both crops. The decline in grass Pb content from an
average value of log10Pb of 2.69 in the first crop to 1.68 in the re-growth was significant
(p<0.05).
There was no significant difference in Pb uptake in both crops between single and mixed
treatments that received the same level of Pb enrichment. This is shown by the small decline in
log10Pb from an average of 2.69 in the single treatment to 2.34 in the mixed treatments in the
9
first crop and 1.68 to 1.47 in the re-growth, respectively. The decrease in Pb content from the
first crop to the re-growth in both single and mixed treatments was significant (p=0.05).
Cadmium
There was a significant (p<0.05) increase in Cd level in grass with increase in soil enrichment
level (figure 2) in both single and mixed treatments and the two grasses. The overall increase in
Cd content in single treatments was however not significant in both crops.
In mixed treatments, there was a rapid increase in Cd content from WO and ES to the rest of
the treatments as dosage levels increased. Mixed treatments had significantly (p=0.05) higher Cd
levels in mixed treatments than single treatments for the same treatment levels. The mean
Log10Cd for single treatments was 0.026 in the first crop and 0.31 in the re-growth while in
mixed treatments it was 0.056 and 0.57 for the first crop and re-growths, respectively. The
highest levels of log10 Cd for single treatments were 0.048 and 0.059 for the first crop and regrowth respectively and for mixed treatments the levels were 0.145 and 1.27 for the first crop
and re-growth respectively.
3.4. Correlations in Pb and Cd
Pb and Cd uptake by the grass re-growths were strongly correlated as shown in fig 3. The
metal contents of mixed treatments were more strongly correlated than single treatments with r 2
= 0.87 and 0.76 for the mixed and single treatments respectively. The slopes of the regression
equations presented in fig 3 suggest that the rate of Cd uptake in mixed treatments was 2.6 times
the rate of uptake in single treatments.
4. Discussion
10
No significant difference in bio-available Pb levels between single and mixed treatments that
received the same dose of metal combined with treated wastewater was observed, suggesting that
Cd did not influence the bio-available level of Pb in soils. In contrast, a significant increase in
bio-available soil Cd from single treatments to mixed treatments was observed indicating that copresence of Pb and Cd in the soil resulted in an increase in bio-available soil concentrations of
Cd.
The increase in Cd content of grass in mixed treatments beyond the levels in single treatments
and the strong correlation between Pb and Cd levels in the mixed treatments indicates increased
accumulation of Cd in star grass due to the co-presence of Pb and Cd. This finding is consistent
with what Carlson and Rolfe (1979),Miller et al (1977) observed rye and corn subjected to copresence of Pb and Cd, respectively.
Under these conditions of high levels of Pb and Cd, the co-presence of Pb and Cd led to a 2.6
fold rate of uptake of Cd above that of single treatment, leading to more than double the level of
accumulation of Cd beyond that in single treatments. This scenario, suggests that the
recommended limit of 1 mg/kg Cd for pasture grass would be reached at a much lower soil bioavailable concentration of Cd under the conditions of co-presence of Pb and Cd than in single Cd
treatments. Madyiwa et al 2003 study demonstrated that phytoextraction of Pb and Cd by star
grass resulted in accumulation of the metals to levels for beyond the recommended maximum
limits for the pasture grass. Thus, enhancement of uptake of Cd due to its co-presence with Pb
may put animals that graze on the pasture to a greater risk.
11
Acknowledgements
The authors are grateful to the Institute of Water and Sanitation Development for funding this
study and the City of Harare for access to the Firle farm. We would like to extend our many
thanks to the Department of Research and Specialist Services for the use of its facilities for
carrying out chemical analysis.
12
References
Ayers, R. S., Westcot, D. W., 1985. Wastewater quality for agriculture. Irrigation and Drainage
Paper 29, Review 1. FAO. Rome.
Carlson, R. W., Bazzaz, F. A., 1977. Growth reduction in American sycamore (Plantanus
occidentalis L.) caused by Pb-Cd interaction. Environ. Pollution. 12: 243-253.
Carlson, R. W., Rolfe, G. L., 1979. Growth of rye grass and fescue as affected by lead-cadmiumfertiliser interaction. J. Environ. Qual. 8: 348-352.
Department of Meteorological Services, 1977. Mean rainfall in Rhodesia. Rainfall Handbook
Supplement No. 8, Salisbury.
Department of Environment, 1989. Code of practice for agricultural use of sewage sludge.
Department of Environment, London.
Haas, E. M., 2003. Staying healthy with nutrition. The complete guide to diet and nutritional
medicine.
Khan, D. H., Frankland, B., 1983. Effects of cadmium and lead on radish plants with particular
reference to movement of metals through soil profile and plant. Plant and soil. 70:
335-345.
Madyiwa, S. & Chimbari, M.J. (2003) Greenhouse studies on the phyto-extraction of Cynodon
nlemfuensis for lead and cadmium under irrigation with treated waste-water.
Physics and Chemistry of the Earth, 28, 859-867.
Madyiwa, S., Chimbari, M., Nyamangara, J., Bangira, C., 2002. Cumulative effects of sewage
sludge and effluent mixture application on soil properties of a sandy soil under a
mixture of star and kikuyu grasses in Zimbabwe. Phys. Chem. Earth 27, 747-753.
13
McGrath, S. P., Cegarra, J., 1992. Chemical extractability of heavy metals during and after longterm application of sewage sludge to soil. J. Soil Sci. 43, 313-321.
Mengel, K., Kirkby, E. A., 1982. Principles of plant nutrition. International Potash Institute,
Switzerland.
Miller, J. E., Hassett, J.J., Koppe, D. E., 1977. Interaction of lead and cadmium on metal uptake
and growth of corn plants. J. Environ. Qual. 6:18-20.
Moolenar, S. W., Lexmond, T. M., 1999. Heavy metal balances. Part 1. General aspects of
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Industrial Ecology, Volume 2. Number 4.
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20, 24. Zimbabwe.
14
Figures and figure captions
4
3.5
Log(10) Pb concentrations (mg/kg)
3
2.5
2
1.5
1
0.5
0
WO
ES
Pb300
Pb600
Pb1200
Single and mixed Pb treatments
Soil single trt
Soil mixed trt
1st crop, single trt
1st crop, mixed trt
Expon. (Re-growth, mixed trt)
Re-growth, single trt
Expon. (Re-growth, single trt)
Re-growth, mixed trt
Expon. (1st crop, mixed trt)
Expon. (1st crop, single trt)
Fig 1. Log(10) Pb concentrations in soil, first grass crop and re-growth
15
1.4
1.2
1
Log(10) Cd concentrations (mg/kg)
0.8
0.6
0.4
0.2
0
WO
ES
Cd10
Cd20
Cd40
-0.2
-0.4
Single and mixed Cadmium treatments
Soil single trt
Re-growth, single trt
Log. (Re-growth, single trt)
Soil mixed trt
Re-growth, mixed trt
Log. (Re-growth, mixed trt)
1st crop, single trt
Log. (1st crop, mixed trt)
1st crop, mixed trt
Log. (1st crop, single trt)
Fig 2. Log(10) Cd concentrations in soil, first grass crop and re-growth
16
1.6
1.4
y = 1.0128x - 0.6376
R2 = 0.8715
Log(10) Cd concentration (mg/kg)
1.2
1
0.8
y = 0.3879x - 0.2058
R2 = 0.762
0.6
0.4
0.2
0
0
0.5
1
1.5
2
2.5
Log(10) Pb concentration (mg/kg)
Re-growth single metals
Re-growth mixed metals
Linear (Re-growth single metals)
Fig 3. Correlations between Pb and Cd uptake in star grass
17
Linear (Re-growth mixed metals)