Assessment of Concentrations of Heavy Metals in Three Leafy Vegetables Irrigated with Wastewater in Hadnet District, Mekelle, Ethiopia

doi:10.21203/rs.3.rs-2505020/v1

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Assessment of Concentrations of Heavy Metals in Three Leafy Vegetables Irrigated with Wastewater in Hadnet District, Mekelle, Ethiopia

https://doi.org/10.21203/rs.3.rs-2505020/v1

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Objective

Urban and peri-urban agriculture is being promoted as a sector of livelihood and economic activities in many parts of the world including Ethiopia. Mekelle city is one of the Ethiopian cities suitable for urban and peri-urban agriculture for cultivating leafy vegetables using wastewater. The consumption of unprocessed and processed leafy vegetables is also very high in the city. However, the quality of the produces has been of a great public health concern. Thus, this study was aimed to investigate the quality of wastewaters in Hadnet district of Mekelle used to irrigate lettuce, spinach and cauliflower and the safety of the vegetables in terms of accumulation heavy metals.

Result

Wastewater samples collected from four experimental sites in the district were found to have higher concentrations in 4 (i.e., Cd, Cr, Cu, and Mn) of the 10 heavy metals tested than the permissible limit estbalished by the FAO (1985). Interestingly, spring water samples collected from another district called Kallamino, designated as comparison site, also have higher concenterations in 4 (i.e., Al, As, Cd, and Cu) of the 10 heavy metals tested. But the leafy vegetables grown in the least contaminated experimental site was found to have higher concentrations in 6 or 7 of the 10 heavy metals tested. Vegetables grown in the comparison site were nearly clean. The lettuce and spnach had elevated concentration of As and Al, respectively. While the fast growing lettuce has higher accumulation of the heavy metals, the slow growing cauliflower had lower accumulation. Such elevated accumulaion of heavy metals in these vegetables could be due to the presence of higher soil contamination and enahnced bioaccumulation capacity of the plants.

Conclusion

The wastewater used to irrigate vegetable farms in Hadnet district of Mekelle is not safe enough. More importantly, the soils of the farms might have accumulated far more amount of heavy metals. The lettuce, spinach and cauliflower cultivated there are highly contaminated. Thus, the use of wastewater for irrigating urban and peri-urban farms need to be regulated, the suitability of the soils of the farms need to be examined, and consumers of these leafy vegetables need be cautious.

Mekelle city

Heavy metals

Wastewater

Leafy vegetables

Heavy metals often enter into the environment due to anthropogenic activities. The toxicity, persistence, non-biodegradability, and bioaccumulation of the metals are regarded as the most severe environmental problems and human health risks worldwide. Toxic heavy metals enter the human body through the food chain and cause serious health problems. Human exposure to toxic heavy metals causes several health complications that can lead to death such as increased blood acidity, retardation of development, increased incidence of various cancers, interfering with absorption of essential elements, and kidney damage.

Wastewater is a critical medium that carries heavy metals and disposes them into the environment. Wastewaters are discharged from wastewater treatment plants, industries, mining sites, power stations, and many other sites and facilities [1]. With the intensification of urbanization, agricultural activities, and industrialization, the extent of heavy metals accumulation in wastewaters is steadily growing in Ethiopia [2]. Leafy and non-leafy vegetables irrigated with wastewaters often accumulate heavy metals on their surfaces and tissues [1 − 5]. The consumption of such produces contaminated with elevated levels of heavy metals has become a great concern for human health since long ago [3].

Urban and peri-urban agriculture is being promoted as a sector of livelihood and economic activities in many parts of the world including Ethiopia. Mekelle city, Tigrai, Ethiopia − endowed with many rivers and streams − is one of the fast growing cities suitable for urban and peri-urban agriculture. In fact, urban and peri-urban agriculture for growing leafy and non-leafy vegetables is quite common in the city. Moreover, the consumption of unprocessed and processed leafy vegetables is very high. However, the quality of the produces has been of a great concern. Some studies have reported that vegetables grown using waters of some the rivers and streams of the city have excess accumulation of heavy metals (e.g., [3, 6]). Therefore, this study was aimed to investigate the quality of waters from unexplored rivers and streams used for irrigating leafy vegetables and the safety of three leafy vegetables grown there in terms of heavy metal accumulation. Findings of the study will assist consumers to make good choices and take proper precautions when consuming leafy vegetables as well as relevant public entities to implement proper regualtory measures on urban and peri-urban agriculture.

Description of study area

The study was conducted in two districts of Mekelle city called Hadnet and Kallamino. Mekelle, the capital city of Tigrai, Ethiopia, is located about 200 km southeast of the historic city Aksum (alt.: 1,900 to 2,200 masl; lat.: 13° 30′ 0″ N; long.: 39° 28′ 11″ E). The climate of the city is semi-arid, with mean annual temperature of 16 to 20°C and annual rainfall of 600mm. Hadnet and Kallamino watersheds are about six kilometers apart, separated by a plateau.

Design of the study

The study was conducted in five farmlands. The first four farmlands are located in Hadnet district (Experimental Site) while the fifth one was located in Kallamino (Comparison Site). The farmlands in the Experimental Site (ES) are irrigated with wastewater while that in the Comparison Site (CS) is irrigated with spring water. The farmlands in the ES are located around or at Adi Haqi (ES1), Enda Mikael (ES2), Tsa’eda Agam (ES3) and Fitsum Birhan Eye Hospital (ES4). Thus, the study compared the concentrations of ten heavy metals in the: (a) wastewater and spring water samples, and (b) leafy vegetable samples collected from the least polluted ES and the CS.

Sample collection

Five (5) samples of wastewater were collected from each of the four farms of the ES. Likewise, ten (10) samples of spring water were collected from the farm in the CS. Also, samples of mature cauliflower (Brassica oleracea var. botrytis) (10), lettuce (Lactuca sativa) (10) and spinach (Spinacea oleracea) (10) were collected from ES1 and the CS. While the wastewater samples were collected in clean polyethylene bottles, the vegetable samples were collected in separate sterile plastic bags. The vegetable and wastewater samples were taken to the geology lab in the College of Natural and Computational Sciences, Mekelle University for analyses. The vegetable samples were thoroughly washed with sterile distilled water, air-dried for 1 hr, and put in electric oven at 70 ºC for 24 hrs. Dried samples were ground with mortar and pestle into fine powder, sieved, and kept in polyethylene bags for further analyses.

Analyses of heavy metals in vegetable samples

Samples of vegetables were digested using the wet digestion method developed by Farooq and co-workers [7]. A 1.25g digested vegetable sample was taken into glass beaker and dried at 103°C for 24 hrs. A 25 mL HNO₃ and three boiling chips were put into a destruction tube and heated at 100°C for 1 hr. The sample was subsequently heated at 125°C, 150°C, 175°C, and 200°C for 15 min at each temperature. The solution was concentrated and cooled to about 5 mL. One milliliter of 30% H₂O₂ was added to the destruction tube and kept for 10 min. Then, 3 mL 30% H₂O₂ and 25 mL distilled water were added and boiled for 10 min to dissolve the sample. Finally, the sample was cooled and transferred to a 250 mL volumetric flask, settled for 15 hrs, and the absorbance of the clear supernatant was measured by atomic absorption spectroscopy. The precision and accuracy of the analyses were checked through repeated analysis against NIST standard reference material 1570A.

Analyses of heavy metals in water samples

Each of the wastewater and spring water samples was subjected to the nitric acid digestion method developed by Chauhan [8] as follows. A 50 mL sample was transferred into clean glass beaker and 10 mL conc. HNO₃ was added. The beaker was placed on hot plate at 80°C and let the solution evaporated until it remained 20 mL. Then, the content was cooled and 5 mL conc. HNO₃ was added and heated again on hot plate. A small portion of conc. HNO₃ was cautiously added to the beaker until the solution appeared transparent. After that, distilled water was added to the beaker until the volume of the solution reached 50 mL mark and was filtered through Whatman No. 42 filter paper. Finally, the concentrations of heavy metals were determined using flame atomic absorption spectrophotometer (Varian FAAS-240). As a quality control analyses, glassware was washed with 10% HNO₃ and calibrated using standard solutions of each heavy metal. Precision and accuracy of analyses were checked through repeated analyses against NIST standard reference material RM 1643E.

Data analyses

Acquired data were analyzed using inferential statistical methods with SPSS Version 23.0 software. Mean (± SD) values of heavy metal concentrations of the samples of wastewater, spring water, and leafy vegetables were computed using the analysis of variance and were compared against the values of the permissible limits established by the Food and Agriculture Organization (FAO/1985) [9].

Concentrations of heavy metals in the waste and spring waters

Among the four sites of Hadnet district, copper (Cu) was recorded at the highest concentration above the permissible limit of the FAO (1985) at ES2 (1.1140 ± 0.0635 mg/L) followed by ES3 (1.0860 ± 0.0841 mg/L), ES4 (0.9420 ± 0.0390 mg/L), and ES1 (0.6120 ± 0.0249 mg/L) (Table 1). According to the FAO standards, the concentrations of Hg, Pb, Zn, Mn, Fe, and Al in the wastewater were within the safe limit. However, the concentrations of Cu (0.5460 ± 0.0241 to 0.7149 ± 0.0058 mg/L), Cr (0.6120 ± 0.0249 to 1.1140 ± 0.0635 mg/L), Cd (0.0620 ± 0.0045 to 0.1182 ± 0.0176 mg/L), and Mn (0.1246 ± 0.0112 to 0.1602 ± 0.0035 mg/L) were found to be above the permissible limit. On the other hand, the spring water of the Comparison Site (CS) had Al (0.1330 ± 0.0116 mg/L), As (0.0350 ± 0.0053 mg/L), Cr (0.2617 ± 0.0186 mg/L), and Cu (0.3752 ± 0.0191 mg/L) above the safe limit.

Table 1

Concentrations of Heavy Metals in the Waste and Spring Water Samples
Heavy Metals	Concentration (mg/L) (Mean ± SD)					FAO (1985)
Heavy Metals	ES1 (n = 5)	ES2 (n = 5)	ES3 (n = 5)	ES4 (n = 5)	CS (n = 10)	FAO (1985)
Al	0.0013 ± 0.0005	0.0011 ± 0.0002	0.0017 ± 0.0008	0.0010 ± 0.0001	0.1330 ± 0.0116	0.008
As	0.0019 ± 0.0002	0.0018 ± 0.0002	0.0022 ± 0.0012	0.0039 ± 0.0002	0.0350 ± 0.0053	0.004
Cd	0.0800 ± 0.0128	0.1182 ± 0.0176	0.0924 ± 0.0056	0.0620 ± 0.0045	0.2617 ± 0.0186	0.010
Cr	0.5460 ± 0.0478	0.7194 ± 0.0058	0.6300 ± 0.0316	0.5460 ± 0.0241	0.0250 ± 0.0053	0.050
Cu	0.6120 ± 0.0249	1.1140 ± 0.0635	1.0860 ± 0.0841	0.9420 ± 0.0390	0.3752 ± 0.0191	0.050
Fe	0.5520 ± 0.0541	0.6040 ± 0.0336	0.7760 ± 0.0385	0.6240 ± 0.0208	1.5120 ± 0.0148	260.000
Hg	0.0027 ± 0.0005	0.0044 ± 0.0006	0.0009 ± 0.0001	0.0020 ± 0.0001	0.0001 ± 0.0003	0.006
Mn	0.1310 ± 0.0288	0.1246 ± 0.0112	0.1602 ± 0.0035	0.1456 ± 0.0112	0.0128 ± 0.0013	0.100
Pb	0.0012 ± 0.0004	0.0021 ± 0.0002	0.0011 ± 0.0002	0.0021 ± 0.0002	0.0259 ± 0.0336	0.100
Zn	0.6560 ± 0.0513	0.6800 ± 0.0381	0.5760 ± 0.0643	0.7360 ± 0.0297	0.0534 ± 0.0059	2.000
ES: Experimental site; CS: Comparison site; SD: standard deviation; FAO: Food and Agriculture Organization

Concentrations of heavy metals in vegetables irrigated with wastewater

Wastewater acquired from ES1 is relatively less polluted as compared to those acquired from the other three experimental sites. Thus, the investigation into the concentration of heavy metals in the three leafy vegetables was carried out by taking samples from this least polluted site. The three leafy vegetables grown in ES1 were found to have accumulated Al (0.0100 ± 0.0000 mg/L), As (0.0100 ± 0.0000 to 0.0300 ± 0.0000 mg/L), Cd (0.1500 ± 0.0300 to 0.5800 ± 0.0400 mg/L), Cr (1.8500 ± 0.2900 to 3.4900 ± 0.3800), and Fe (434.0000 ± 6.6700 to 503.4000 ± 12.3600) above the safe limited established by the FAO (1985). Moreover, whereas Pb was found above the safe limit in lettuce and spinach, Zn was above the safe limit in lettuce and cauliflower (Table 2). The concentrations of some of the heavy metals is very high (e.g., As, Cd, Cr, and Pb). These imply that the heavy metal accumulation in such leafy vegetables will be much more in the more contaminated sites of ES2, ES3, and ES4.

Table 2

Concentrations of Heavy Metals in Leafy Vegetables Collected from Experimental Site 1
Heavy Metals	Concentration (Mean ± SD) (mg/L)			FAO (1985)	Levels of Contamination
Heavy Metals	Lettuce (n = 10)	Spinach (n = 10)	Cauliflower (n = 10)	FAO (1985)	Levels of Contamination
Al	0.0100 ± 0.0000	0.0100 ± 0.0000	0.0100 ± 0.0000	0.006	1.67 times
As	0.0300 ± 0.0000	0. 0100 ± 0.0000	0.0100 ± 0.0000	0.001	10.0 − 30.0 times
Cd	0.5800 ± 0.0400	0.1500 ± 0.0300	0.3400 ± 0.0300	0.100	1.50 − 5.80 times
Cr	3.4900 ± 0.3800	2.3100 ± 0.3300	1.8500 ± 0.2900	1.000	1.85 − 3.49 times
Cu	0.8000 ± 0.0800	0.9200 ± 0.2200	1.3200 ± 0.2800	73.000	−
Fe	450.0000 ± 7.9100	503.4000 ± 12.3600	434.0000 ± 6.6700	425.00	1.02 − 1.18 times
Hg	ND	ND	ND	0.010	−
Mn	18.8000 ± 4.9700	17.2000 ± 1.9200	8.0000 ± 2.2400	500.00	−
Pb	0.6600 ± 0.1300	0.6100 ± 0.1100	ND	0.300	2.03 − 2.20 times
Zn	119.0000 ± 3.8100	ND	114.0000 ± 76.2800	100.00	1.14 − 1.19 times
SD: standard deviation; FAO: Food and Agriculture Organization; ND: Not detected

Concentrations of heavy metals in vegetables grown with spring water

With the exception of Al in spinach and As in lettuce, the concentrations of the heavy metals in the vegetables irrigated with spring water at the CS are below the permissible limits (Table 3). Even the concentrations of Al in spinach (0.0080 ± 0.0008 mg/L) and As in lettuce (0.0020 ± 0.0006 mg/L) are only slightly above the safe limits. The concentrations of Al, As, Cd, and Cu were above the permissible limits in the spring water of this site (Table 1).

Table 3

Concentrations of Heavy Metals in the Vegetables Collected from the Comparison Site
Heavy Metals	Concentration (Mean ± SD) (mg/L)			FAO (1985)
Heavy Metals	Lettuce (n = 10)	Spinach (n = 10)	Cauliflower (n = 10)	FAO (1985)
Al	0.0020 ± 0.0004	0.0080 ± 0.0008	0.0020 ± 0.0004	0.006
As	0.0020 ± 0.0006	0.0010 ± 0.0004	ND	0.001
Cd	0.0790 ± 0.0134	0.0580 ± 0.0148	0.0862 ± 0.0062	0.100
Cr	0.9000 ± 0.0381	0.8500 ± 0.0762	0.8400 ± 0.0822	1.000
Cu	ND	ND	ND	73.000
Fe	251.0000 ± 9. 6177	179.8000 ± 10.0349	194.4000 ± 8.8487	425.000
Hg	ND	ND	ND	0.010
Mn	0.9020 ± 0.2056	0.7700 ± 0.1723	0.8780 ± 0.0626	500.000
Pb	0.2670 ± 0.0120	0.0432 ± 0.0160	0.0720 ± 0.0192	0.300
Zn	83.6000 ± 5.5946	ND	ND	100.00
SD: standard deviation; FAO: Food and Agriculture Organization; ND: Not detected

This study has led to three important observations. First, the vegetables grown in farms irrigated with wastewater have higher accumulation of heavy metals than those grown in farms irrigated with spring water. This is quite natural and many studies have come up with similar observations. For example, higher concentrations of heavy metals were observed in radish, spinach, turnip, cauliflower, and carrot grown in farms irrigated with wastewater as compared to those irrigated with spring water [10].

The second important observation is that the leafy vegetables grown in ES1 have higher levels of accumulation in six or seven of the 10 heavy metals than the wastewater used to irrigate it. This implies that the wastewater is neither the direct nor the only source of the heavy metals for the vegetables. The soil, as continuously exposed to contaminated wastewater, can accumulate heavy metals over a course of time much more than the flowing wastewater. The physical and chemical properties and compositions of the soils such as pH, organic matter content, cation exchange capacity, nutrient balance, and metal permissibility can also lead to the accumulation of heavy metals in soils [11 − 13].

Moreover, despite the concentrations of many of the heavy metals in the wastewater are below the safe limit, what has reached the vegetables can be accumulated in their tissues to far greater levels. The bioaccumulation of heavy metals in vegetables is affected by many factors such as climate, atmospheric depositions, heavy metals concentrations in soil, the nature of the soils of the vegetable farms, and the degree of maturity of the plants at the time of harvest [14, 15]. Therefore, enhanced levels of heavy metals can be accumulated in the leafy vegetables through the soil and by way of bioaccumulation. In line with this, the higher accumulation of the heavy metals despite their relatively lower concentration in the wastewater can also be due to their higher transfer factor. The heavy metals of Cd, Cr, Fe, Zn, and Pb, detected in the vegetables above the permissible level, have high transfer factor from the soil and water to the vegetables [16, 17].

The third important observation is the difference in the accumulation of the heavy metals in the vegetables. The bioaccumulation of heavy metals of lettuce is higher than spinach and cauliflower and the bioaccumulation of cauliflower is lower than both the lettuce and spinach. This variations may be accounted to differences in morphologies and anatomies, absorption and assimilation capacities, and sensitivities to the heavy metals of the vegetables [13, 18 − 20]. The findings of the present study are similar with previous ones that demonstrated that leafy vegetables that grow quickly with high transpiration and absorption rates bio-accumulate higher concentrations of heavy metals quickly [21].

The three vegetables grown in the experimental sites at Hadnet district of Mekelle city provided us with important indications that the soils of the farms are accumulating heavy metals. The observation that the fast growing lettuce has higher bioaccumulation capacity implies that other fast growing produces cultivated in farms irrigated with wastewater can have similar contamination levels. Thus, urban and peri-urban agriculture in Mekelle city for growing leafy vegetables has to be monitored not to use wastewater for irrigation. Likewise, the soils of the farms growing such vegetables need to be examined if they are suitable. Furthermore, consumers need to be cautious in their consumption of vegetables growing in urban and peri-urban farms.

Aluminum

Alt.

Altitude

Arsenic

Cadmium

Chromium

Control site

Copper

East

Experimental site

FAO

Food and Agriculture Organization

Iron

gram

H₂O₂

Hydrogen peroxide

Mercury

HNO₃

Nitric acid

Lat.

Latitude

Long.

Longitude

masl

meters above sea level

mg/L

milligram per Liter

Min

Minutes

milliliter

Manganese

North

Not detected

NIST

National Institute of Standards and Technology

Lead

Standard deviation

SPSS

statistical package for social science

Zink

Acknowledgements

The authors acknowledge the administrators and technicians of the geology lab of the College of Natural and Computational Sciences for allowing us to carry out the laboratory studies.

Authors contributions

HTT, DBS, and GG were involved in conceiving, designing, and planning of the study, HTT carried out the collection and preparation of water and vegetable samples, the determination of heavy metal contents and data collection, the organization and analyses of the data, and the drafting of the manuscript, GG reviewed the draft manuscript and developed the final draft, DBS reviewed the manuscript for content, language, and style, and readied it for publication.

Funding

This study was supported by Mekelle University, Mekelle, Ethiopia (Grant No.: CRPO/MIT/005/08).

Availability of data

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interests

No competing interests to disclose.

Authors details

¹Department of Biological and Chemical and Biological Engineering, Mekelle Institute of Technology, Mekelle University, PO Box 231/1632, Mekelle, Ethiopia.

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No competing interests reported.

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Assessment of Concentrations of Heavy Metals in Three Leafy Vegetables Irrigated with Wastewater in Hadnet District, Mekelle, Ethiopia

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Abstract

Objective

Result

Conclusion

Introduction

Materials And Methods

Results

Discussion

Conclusion

Abbreviations

Declarations

References

Additional Declarations

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