Design of Enzyme-Based Biosensors for Monitoring Pollutants in Gold Mining Environments
Authors: Adio-Adepoju Aderinsola Airat , Okiei Wesley , Fernandez Carlos , Akinbulu Isaac , Ibrahim Gafar , Ojobe Bukola 
Published: February 2020

Gold is a transition metal which has great economic value, however does not exist alone in the earth crust; but in combination with other metals and non-metals to form compounds. These other elements which could be heavy metals such as lead, cadmium and copper may be highly toxic depending on their concentrations in the gold ore. Mining in itself has many associated risk which could be related to health or environmental degradation. These risks are heightened when mining is done by artisanal miners who have little or no knowledge about safe practices. Artisanal mining is however predominant in the gold rich communities of developing countries such as Nigeria, Ghana, Mali, Senegal etc. Youths engage in illegal mining of gold ore without possible care for the dangers associated with it [1].
Consequences associated with such practices includes environmental pollution, degradation and health hazards such as poisoning outbreaks. A typical example of associated health risk was reported in our previous works [2][3] in which lead (Pb) poisoning outbreaks took the lives of hundreds of children below age five in Zamfara and Kebbi states, Nigeria in 2010 and 2016 respectively [4]. Pb was found to be very high in the mining environments and places where processing of gold ore was carried out. Environmental samples such as water, soil and vegetation on analysis using voltammetry and ICP-OES, gave values of Pb higher than 6000μg/L in water and 13000mg/Kg in soil. This concentration exceeded the WHO permissible limit of 10μg/L in water [5] and 400mg/kg for lead in residential soil [6]. These incidents are very worrisome as they could have been avoided or controlled with the right environmental practices and monitoring. Given the high levels of Pb in the blood of victims which was reported to have caused the deaths, [4] the need for investigation of the environmental matrices became imperative. This is in order to ascertain the actual levels of heavy metals and consequently design a biosensor which could be used onsite to continually monitor the environment.
Lead and copper were detected in samples. Pb was the metal with very high concentration in most samples while copper was within acceptable limit. Enzymatic activity is known to be inhibited by the presence of heavy metals such as lead. Hence, they can be employed in the design of a biosensor for environmental monitoring. Glucose oxidase (enzyme) was immobilized in L-cysteine and coupled to gold electrode with Carbodiimide and this modified electrode (biosensor) was used in the catalysis of different concentrations of glucose standard (0.1mM – 20mM). The linear range was observed to be between 0.1mM – 10mM. To 2.5mM glucose standard solution, Pb standard solutions were successively added while stirring and measurement taken after 10 min of each successive addition. A sharp inhibition due to the presence of Pb was noticed between 5ppb – 20ppb while between 50 ppb – 500 ppb, an onward gradual inhibition continued but at a much slower rate giving almost a plateau. The glucose oxidase inhibition biosensor was optimized for pH, scan rate, temperature and reaction time and characterized using cyclic voltammetry, SEM and FT-IR. The sensitivity to very low concentration of Pb (5 ppb – 20ppb) in the pH range of most samples (6.5 – 7.5) collected from gold mining sites in Niger state, Nigeria makes it promising in its use for analysis of real life samples obtained from this region and in the final development of a portable onsite sensor.

References.

[1] Ogunlesi, M., Okiei, W., Adio-Adepoju, A. and M. Oluboyo, (2017). Electrochemical determination of the levels of cadmium, copper and lead in polluted soil and plant samples from mining areas in Zamfara
[2] Okiei, W., Ogunlesi, M., Adio-Adepoju, A. and M. Oluboyo, (2016). Determination of Copper and Lead in Water Samples from Zamfara State, Nigeria by Linear Sweep Anodic Stripping Voltammetry. Int. J. Electrochem. Sci. 11: 8280-8294, doi 10.20964/2016.10.06
[3] Adio-Adepoju, A.A., Okiei, W.O., Ogunlesi, M.1., Ojobe, B.L., Ibrahim, G.O. and O.S. Sobowale, (2018). Analysis of Surface and Ground Water Samples in the Environs of Gold Mines Linked to Lead Poisoning Incident in Niger State, Nigeria. In T.M. Tundisi, & J.G. Tundisi (Eds.), Water Resources Management (pp. 30-44) São Carlos, SP: editorascienza. ISBN 978-85-5953-031-
[4] MSF MedecinsSansFrontieres. MSF Briefing Paper, (2012). http://www.msf.org/en/article/lead-poisoning-crisis-zamfara-state-nworthern-nigeria
[5] WHO World Health Organization, (2011). Guidelines for drinking water quality, 4th Edition, Incorporating 1st Addendum. WHO publication, Geneva ISBN: 9789241548151
[6] US EPA United States Environmental Protection Agency, (2001). 40 CFR Part 745. Lead; identification of dangerous Levels of Lead; Final Rule. https://www.gpo.gov/fdsys/pkg/FR-2001-01-05/pdf/01-84.pdf

Figure 1