Determination of Selected Heavy Metals in Soil and Their Impact on Coffee Bean Composition, in Case of West Guji Zone, Ethiopia

Coffee (Coffea arabica L.)is one of the most important agricultural commodities in the world and a major source of income for millions of farmers, with Ethiopia recognized as a center of origin and genetic diversity. Despite the economic importance of specialty coffee produced in the western Guji area, limited information is available on the contamination of coffee-growing soils with heavy metals, the transfer of metals from the soil to the beans and the associated risks to human health. The study measured concentrations of selected heavy metals (Cu, Mn, Zn, Ni, Co, Cr, Cd and Pb) in coffee-growing soils and green coffee beans sampled from the districts of Hambela, Bule Hora and Kercha, and assessed their transport from the soil to the bean. The samples of the soil and coffee beans were digested by an optimized wet acid digestion process and analyzed by Flame Atomic Absorption Spectrometry (FAAS). The analytical method demonstrated excellent linearity (R² = 0.9985-0.9989), good response rates (87-115 mg L⁻¹), and low detection limits (RSD < 5.38 mg). The concentrations of Cu, Mn, Zn, Ni, Co, and Cr in Coffee beans ranged from 20.10 ± 1.30-22.44 ± 1.70, 14.25 ± 1.20-17.14 ± 1.30, 6.74 ± 1.10-8.43 ± 1.10, 1.22 ± 0.20-1.43 ± 0.30, 1.47 ± 0.30-1.49 ± 0.20, and 0.08 ± 0.01-0.92 ± 0.10 mg/kg, respectively, whereas Cd and Pb were below the method detection limits. Soil samples contained comparatively higher metal concentrations, particularly Mn (237.11 ± 3.30-285.44 ± 6.30 mg/kg) and Cu (59.40 ± 2.30-64.40 ± 2.50 mg/kg), However, all values remained below internationally accepted safe limits. The transfer factor values (0.02-0.35) indicated low mobility of metals from the soil to the coffee beans, with Cu being the most mobile of the metals. Metal concentrations in the study areas showed significant differences (p < 0.05) and strong positive correlations between individual metals. The carcinogenic risk (CR) of Cr is 8.7 × 10⁻⁵, which is within the acceptable lifetime risk range as recommended by the USEPA (10-⁶ to 10-4). The levels of the analyzed heavy metals in green Coffee beans were below the recommended safe levels, which mean that coffee produced in the western Guji zone is safe for human consumption as regards heavy metal contamination. These findings provide valuable input for environmental monitoring, sustainable land management, food safety assessment and quality assurance in specialty coffee from Ethiopia.