Influence of digestion method on the pseudototal concentration of potentially toxic elements measured in crude oil contaminated soils

Environmental contamination by hydrocarbons and potentially toxic elements (PTE) has increased in Nigeria because of crude oil exploration and exploitation. When crude oil spills on soil, the PTEs therein may persist for a long time and bioaccumulate,1 causing harm to human health. There is therefore a need to better understand the levels and distribution of crude-oil derived PTE in the environment. However, comparison between results obtained in literature is difficult because different researchers have used different reagents and methods for digestion of crude oil contaminated soils prior to determination of their PTE content.
In the current study, the performance of three acid mixtures – (i) aqua regia, (ii) sequential addition of first HCl and then HNO3 in the ratio 3:1 and (iii) reverse aqua regia – and three digestion methods – block, hotplate and microwave digestion – were compared (all following a pre-digestion of 16 hours’ duration). The test material was a crude oil contaminated soil (COS) obtained by mixing together equal quantities of environmental samples obtained from Bodo and Bomu, two communities in Gokana Local Government Area of Rivers State, Nigeria (average organic matter content 21.8% as estimated by loss-on-ignition). A secondary urban soil reference material, GLA URM, was used as a quality control sample. All digests were analyzed for concentrations of As, Cd, Cr, Cu, Ni, Pb, V and Zn using ICP-MS (Agilent 7700x, Agilent Technologies, Cheshire, UK).
The recoveries of PTE from the GLA URM using microwave digestion with aqua regia were within 12% of target values except for As (132%), Cd (157%) and Cr (121%). For the COS, the pseudototal concentration obtained for As was highest with block digestion while that of Cd was highest with hotplate digestion. For Cr, Cu, Ni, Pb, V and Zn, the pseudototal concentrations were highest when microwave digestion was used (especially with aqua regia) and lowest with block digestion. Overall, this study revealed that microwave digestion using aqua regia was the most efficient method for digestion of crude oil contaminated soil for measurement of PTE.
Acknowledgement: RNV wishes to thank Petroleum Technology Development Fund (PTDF) Nigeria for providing a PhD scholarship.

1. Ekperusi, O. A.; Aigbodion, I. F.; Iloba, B. N.; Okorefe, S., Assessment and bioremediation of heavy metals from crude oil contaminated soil by earthworms. Ethiopian Journal of Environmental Studies and Management 2017, 9 (2). 1036 – 1046