Potentially toxic elements in agricultural soil of the Kurdistan region of Iraq and implications for human health.
PhD thesis, University of Nottingham.
A rapidly increasing human population has resulted in the expansion of urban areas and an associated increase in the demand for food . Consequently, urban and peri-urban agriculture (UPA), using waste water as a source for irrigation, is increasingly common around the world, particularly in developing countries. Although UPA plays a crucial role in supplying food to urban populations, the quality of that food may be affected by accumulation of potentially toxic elements (PTEs) in the soil and the food chain.
The aim of this study was to assess potentially toxic element (PTE) contamination in soils and vegetables grown in the Kurdistan region of Iraq. Specific objectives included (i) to measure the total concentrations, mobility and bioavailability of PTEs using isotopic dilution approaches (E- and L-values) and single- and sequential-extraction procedures (ii) to compare these different metrics to determine the best assay for soils in Kurdistan and (iii) to investigate the potential human health risks from consumption of contaminated vegetables by estimating dietary intake and hazard quotients (HQs).
Overall, the concentration of PTEs were noticeably higher in waste disposal areas and progressively lower in urban, peri-urban and rural areas, with the exception of Ni concentrations which were high at all study sites, reflecting the geological background of the region. However, the total metal concentration does not represent the ‘bioavailable’ fraction of an element - chemical extractions such as DTPA, sequential extractions and measurements of isotopically-exchangeable metal may better represent bioavailable PTEs. DTPA-extractable and isotopically-exchangeable metals (E-value) were typically low for Ni (2.89, 0.629 mg kg-1), Cu (4.76, 1.98 mg kg-1), Zn (12.9, 9.92 mg kg-1), Cd (0.144, 0.066 mg kg-1) and Pb (5.03, 2.13 mg kg-1), but the metals were more labile in soils in waste areas compared to urban, peri-urban and remote areas. Sequential extraction suggested that most PTEs were in the residual fraction with the exception of Se, Cd and Pb which were in the organic matter, carbonate and Fe/Mn oxide fractions, respectively.
PTE concentrations in the edible parts of vegetables collected from the study sites varied between vegetable types and site. Higher total concentrations of PTEs were measured in vegetables from the waste areas with decreasing concentrations in urban, semi-urban and rural areas. Concentrations of Ni and Cu exceeded recommended UK and WHO/EU limits (0.5 mg kg-1Fw) in almost all vegetables except for tomato, okra and cucumber for Ni, while limits for As were exceeded in fruit vegetables from waste areas and leafy vegetables in waste, urban and peri-urban areas. Concentrations of Cd were above recommended limits in all vegetables except for cowpea and cucumber at all sites. The concentration of Pb exceeded the limits in okra, cow pea, leek, celery and chard in waste, urban and peri-urban areas.
An experiment growing vegetables in pots under controlled conditions was established to evaluate the ability of plants to accumulate PTEs by estimating the isotopically-exchangeable metal pool in plants (L-values). Results gave similar L-values for different vegetable types ranging from 7.70 mg kg-1 in chard to11.0 mg kg-1 in radish leaf for Ni, 44.2 mg kg-1 in lettuce to 51.8 mg kg-1 in celery for Zn and 0.0912 mg kg-1 in lettuce to 0.0992 mg kg-1 in chard for Cd. The L- values of trace elements in plant roots were higher than those obtained for the shoots in the vegetables. A comparison between DTPA, E- and L-values was made and the results demonstrated that L-values were higher.
Risks to human health were assessed using hazard quotients (HQ) assuming intakes of 0.187 kg FW d-1 for adults and 0.118 kg FW d-1 for children with body weights of 70 and 14.2 kg, respectively. Vegetable consumption showed no risks for adults while HQs ≥ 1 were observed for Ni, As and Cd for children. Although HQs for the vegetables suggest elevated risk for children these are likely to be lower when realistic dietary consumption levels are considered. Thus, more accurate estimates of daily intake of vegetables are recommended for future studies.
Thesis (University of Nottingham only)
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