The contents of essential and toxic metals in coffee beans and soil in Dale Woreda, Sidama Regional State, Southern Ethiopia

Abstract

Background For developing countries such as Ethiopia, coffee is a commodity of great economic, social, and environmental importance. No detailed investigations have been performed on the contents of essential and toxic metals in coffee beans and soil in this study area. Methods The levels of essential metals (Na, K, Ca, Zn, Mn, Cu, Co, Cr, Ni) and toxic elements (Pb and Cd) were investigated in coffee beans (coffee growing farmland and coffee washed plants) and soil samples (from farmland) using flame atomic absorption spectrometry (FAAS) and flame emission atomic spectroscopy. We selected six (20%) administrative units (kebele) with purposive sampling techniques based on their coffee production capacity in Dale Woreda for soil testing. After coffee sample preparation in a microwave system with HNO3and H2O2 reagents, the accuracy of the optimized procedure was evaluated by analysing the digest of the spiked samples. Soil samples were abridged with a slight revision of the EPA 3050B acid digesting method. ANOVA was used to determine the significant differences in the mean concentration of metal within coffee beans from farmland at the various sampled sites at the p < 0.05 significance level. To correlate the effect of one metal concentration on other metals in the coffee bean samples, Pearson correlation matrices were used. Results Calcium had the highest concentration (1,355 ± 18.02 mg kg−1) of macroelements in soil samples, followed by K (681.43 ± 1.52 mg kg−1). Similarly, Na (111.63 ± 0.35 mg kg−1), Cu (49.96 ± 0.99 mg kg−1), Co (5.43 ± 0.31 mg kg−1), Mn (0.62 ± 0.238 mg kg−1), Ni (0.194 ± 0.01 mg kg−1), and Zn (0.163 ± 0.007 mg kg−1) were detected among the microelements in the soil samples. Pb and Cr were not detected in all soil samples. Potassium (K) was found to have the highest concentration (99.93 ± 0.037 mg kg−1), followed by Ca (17.23 ± 0.36 mg kg−1), among the macroelements in coffee beans from farmers’ farms. Similar to coffee beans from farmland, samples from washed plants also contained the highest K (77.93 ± 0.115 mg kg−1), followed by Ca (4.33 ± 0.035 mg kg−1). Metal levels in coffee bean samples from farmland are in the following order: K>Na>Ca >Mn>Cu> Ni>Zn. Metal levels were found to be K>Na>Ca >Mn>Cu> Zn>Ni in coffee beans from the washed plants. Co, Cr, Pb and Cd were no detected in all coffee bean samples. Except for calcium, potassium and manganese, the levels of metals in coffee beans from farmland and washed plants were not significantly different at the 95% confidence level within a kebele. Conclusions We observed permitted levels of macro- and trace elements in coffee beans from farmlands and washed plants. Only in the soil samples are cadmium concentrations higher than those permitted for agricultural soil recommended by the WHO and FAO. Overall, there is no health danger linked with the use of coffee beans due to detrimental and trace heavy metals.