Determination of Heavy Metals Concentration in Soil and Leafy Vegetables in Urban Expressway and Peri-urban Road Farms of Lagos State, Nigeria

Abstract

Farmers have long considered urban agriculture as a panacea to urban food insecurity. However, growing food in cities is faced with many challenges like food contamination. Past studies on leafy vegetables focused on vegetables cultivated in urban centres. However, this paper aimed to determine and compare the level of heavy metals concentrations in soil and leafy vegetables in Urban Expressway Farms (UEF) and Peri-urban Road Farms (PRF) in Lagos, Nigeria. These two locations were purposefully selected. Soil Auger was used to drill holes to the required soil depth to collect soil samples. Sample of Amaranthus hybridus, Celosia argentea and Corchorus olitorius were collected using “envelope” principle. Pb, Cd and Mn residues in the samples were digested and determined using Atomic Absorption Spectrophotometer. The collected data were analysed using mean, standard deviation and Analysis of Variance. The mean pH value for UEF soil is 6.5 while PRF soil is 6.6. The highest Pb (0.93±0.06) was recorded in Celosia argentea at UEF. The highest mean concentration of Cd (0.51±0.11) was recorded in Amaranthus hybridus at UEF. The highest concentration for Mn was recorded in Amaranthus hybridus (0.98±0.02) at UEF. The concentrations of Pb in Celosia argentea at UEF 5 m from the expressway and Cd concentration in Amaranthus hybridus at 10 m from expressway were above the WHO/FAO permissible levels in vegetables. Level of heavy metal residue observed in soil and vegetables under this study were mostly influenced by the location of the farm-environment and not mainly the vegetable type. The relative concentrations of Pb and Cd in all the three leafy vegetables raise some serious environmental concerns and need to be monitored regularly to prevent consumption of contaminated leafy vegetables. Farmers are encouraged to cultivate in fields far away from expressway.

Assessment of Heavy Metal Residues in Soil and Vegetables along Urban-peri-urban gradient of Lagos State Nigeria

“…At each location under the study, 5 sub-sampling points at a distance of 5m from each other were identified and from which soil and vegetable samples were collected. The subsamples collected were put into one representative sample (Adewale et al 2022& Baltrėnas et al, 2003.…”

“…The soil sample were collected three times (September 2019, December 2019 and March 2020) from depth of 0-5cm as this was the region where root of the vegetable plants derived nutrients. (Adewale et al, 2022). Soil auger was used for soil sample collection.…”

“…For each sample, 0.25g was transferred into a 50 ml flask and 10 ml concentrated HNO3 (10 ml only) was added to each flask. The digestion of the mixture was done in a fume cupboard overnight and later heated for about an hour (Adewale et al, 2022). The mixture was left to cool for about an hour at room temperature.…”

“…Vegetables supply diets with crude protein, crude fiber and minerals needed by man for healthy living (Houngla et al, 2020). Recently, leafy vegetable (Corchorus olitorius, Amaranthus hybridus and Celosia argentea) production have become an important aspect of social and economic activity in the city of Lagos (Adewale et al, 2022). However, there are possibilities that the production of these vegetables may turn out to affect the local environment in some negative ways due to industrial activities, carbon monoxide from passing vehicles, application of chemical fertilizer, indiscriminate dumping of refuse and other toxic waste around the urban farms (Adewale et al, 2022).…”

Assessment of the concentration of heavy metals in two vegetables in selected urban metropolises (Ilorin and Osogbo), Nigeria

“…Previous studies have identified the accumulation of heavy metals in vegetable crops cultivated near Nigeria's major urban centers (Afonne and Ifediba, 2020;Edogbo et al, 2020;Adewale et al, 2022). For example, a survey conducted in Lagos found that Pb levels in amaranth ranged from 3.2 to 4.8 mg/kg, which is up to 4 times the WHO limit of 1 mg/kg (Uzel, 2015).…”