In-Vitro Bioaccessibility Studies and Human Risk Assessment of Potentially Toxic Elements in Contaminated Soils and Vegetables

Author: Odujebe, Fausat Olubusola

Supervisors: Kehinde O. and Aderonke O.

Potentially toxic elements (PTEs) contamination of urban soils is of increasing concern because of food safety issues, potential health risks, and detrimental effects on terrestrial ecosystems. The determination of the total concentration of elements alone is therefore not enough in assessing its potential health risk but the fraction absorbed into the body through oral ingestion. The present study was undertaken to assess the human health risk of potentially toxic elements (As, Cd, Cu, Cr, Mn, Ni, Pb, and Zn) through the oral ingestion of soil and locally grown edible vegetables viz: waterleaf (Talinum triangulare), spinach (Basella alba), pumpkin leaf (Telfairia occidentalis), okro leaf (Abelmolschus esculentus), cockscomb (Celosia argentea) and green leaf (Amaranthus viridis) grown on some contaminated soil samples. The soil samples were obtained from seven locations in Lagos, Nigeria. The sites included MFM (a car park dump site), Orile (a dump site), Katangua (a dump site), Owode (a metal scrap market dump site), Ibafo (a road side dump site), FSS (a dump site) and Control (an uncontaminated soil). Soil and vegetable samples were acid digested using conventional hot plate (STUART C300) and microwave assisted digestion (MARS5, CEM) methods respectively. The quantification of PTE in the soils and vegetables were carried out with Perkin Elmer AAnalyst 200 Flame Atomic Absorption Spectrophotometer (FAAS) and Agilent 7700 Inductively coupled plasma mass spectrometer (ICP-MS) respectively. A range of PTE concentrations were observed from the results obtained for the soil samples used for planting the vegetables. The values ranged between < 0.002 – 20 mgkg-1 for As, 0.01 – 20 mgkg-1 for Cd, 0.1 – 2400 mgkg-1 for Cr, 10 – 14900 mgkg-1 for Cu, 42 – 3000 mgkg-1 for Mn, 1.5 – 1050 mgkg-1 for Ni, 2 – 6200 mgkg-1 for Pb and 98 – 4800 mgkg-1 for Zn, with most soil samples i.e. ORL, KATANG, OWD and FSS having values much higher than soil guideline values. The PTE concentrations in the edible part of the vegetables studied were observed to vary greatly between and within soil sites. Relatively high total concentrations of PTEs (As, Cd, Cr, Cu, Mn, Ni, Pb, and Zn) were found with most of the values obtained higher than recommended tolerable safe limits established by FAO/ WHO for edible vegetables. The values obtained in the vegetable types for individual elements range from 0.0 – 2.2 mg/kg As, 0.0 – 12.5 mg/kg Cd, 0.2 – 12.4 mg/kg Cr, 10.4 – 277 mg/kg Cu, 38.6 – 1680 mg/kg Mn, 0.8 – 13.0 mg/kg Ni, 2.0 – 108 mg/kg Pb and 98.0 – 1040 mg/kg Zn respectively. The estimation of the daily intake rate and health index (HI) showed values obtained higher than recommended intake limits for most of the PTEs studied with values of health index greater than one in most of the vegetables. This suggests some potential risk associated with consumption of the vegetables. The major non-carcinogenic risk contributors were Cd, Cu and Pb while As, Cr, Ni, Zn did not pose any major potential risk through consumption of these vegetables. Bioaccessibility studies using the physiologically based extraction test (PBET) and simplified bioaccessibility extraction test (SBET) showed that PTE were easily solubilised only in the gastric phase while little or no bioaccessibility was observed in the intestinal phase values with concentrations obtained near or below detection limits of the instrument.Re-evaluation of risk using bioaccessibility studies revealed that though the PTEs were present in the gastric phase, the levels bioaccessible through the ingestion of these vegetables were unlikely to pose any major health risk to consumer. The values obtained for the PTE were within the tolerable safe limit for oral ingestion. The research has demonstrated the use of bioaccessibility studies as a more appropriate approach for evaluation of potential human health risks.