Wednesday, 20 May 2015

Chromium in crops... by Elliott Hamilton

Elliott Hamilton (right) sampling with partners from CBU and ZARI
Chromium, the 22nd most abundant element in our Earth's crust, takes many different forms. Some of these forms, or 'species', pose a risk to human health so it's important to know where they're created and how they move through the soil into crops. Elliott Hamilton's fieldwork in Africa is focused on the mechanics of agricultural practices and the knock-on impact of soil-crop transfer of harmful Chromium species....  

In November 2014, barely a month into my part-time PhD, I visited Africa for the first time with Michael Watts and Murray Lark to undertake fieldwork in the Copperbelt region of Zambia.

Sample logging
The aim of my PhD research is to improve the understanding of chromium species transfer into crops from soils in close proximity to mine tailings in the Copperbelt region of Zambia and whether agricultural practices, such as liming or organic reincorporation have an impact on the bioavailability of these different chemical forms.

Chromium exists in the environment as trivalent (Cr(III)) and hexavalent (Cr(VI)) forms. Cr(III) plays a role in the metabolism of carbohydrate, fat and protein. Cr(VI) is released primarily through industrial processes such as wood preservation and leather tanning, it is toxic and a known carcinogen. Cr(VI) is also more mobile in the environment, with common soil parameters such as pH and soil organic carbon (SOC) dictating species distribution and bioavailability.

The Black Mountain, Kitwe
As a result of large-scale exploitation of rich copper deposits within the Copperbelt region of Zambia since the turn of the 20th Century, a number of contaminant exposure studies on mine tailing and metal smelters and the subsequent risk to human health have been undertaken (Ettler 2014; Ikenaka 2010; Ndilila 2014).

These studies focused predominantly on the metals mined, such as copper, cobalt and zinc, with little attention devoted to chromium and its species. The vicinity of Kitwe was chosen for a preliminary sampling campaign undertaken in November 2014 to establish chromium concentrations at a number of sites in close proximity to mine tailings, with a view to carrying out crop trials and species kinetics experiments in parallel with experiments undertaken during a recently funded RS-DFID project (see Michael Watts blog "Geochemisty brings societal benefits ...").

The fieldwork was carried out with partners from the Zambian Agricultural Research Institute (ZARI), Copperbelt University (CBU) and the University of Zambia (UNZA). Four locations were sampled, within half a mile of a tailings heap or smelter. To understand variation at different spatial scales, a nested sampling design was implemented (discussed in more detail in Murray’s blog “Random variables and field sampling”). The laboratory analysis of the soils is now complete; once the data have been interpreted in the context of the sample design I’ll have a better understanding of the spatial variation to plan future sampling and experimental plots with greater efficiency.

A smelter in Kalulushi
The next few months will involve the development and optimisation of analytical methodology for extracting and quantifying the different chromium species in solid matrices, and the use of pot experiments to investigate the physical chemistry dictating the transfer of chromium species from soil to crop; the results of these will ultimately lead to a better understanding of the impact of agricultural practices on soil-crop transfer of Cr(VI). These are the references used in the blog...

Ettler, V., M. Vítková, et al. (2014). "Dust from Zambian smelters: mineralogy and contaminant bioaccessibility." Environmental Geochemistry and Health 36(5): 919-933.
Ikenaka, Y., S. Nakayama, et al. (2010). "Heavy metal contamination of soil and sediment in Zambia." African Journal of Environmental Science and Technology 4(11).
Ndilila, W., A. C. Callan, et al. (2014). "Environmental and toenail metals concentrations in copper mining and non mining communities in Zambia." International Journal of Hygiene and Environmental Health 217(1): 62-69.

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