Master Thesis

Arsenic, cadmium, and lead in rice from Costa Rica – a market and field survey

Andrea Colina Blanco (09/2018-09/2019)

Support: Britta Planer-Friedrich, Carolin Kerl

The presence of toxic metal(loid)s in paddy soils and rice grains in Costa Rica has not been studied, yet, despite rice being a staple food for this country. Our study shows that Costa Rican paddy soils from the Guanacaste province had metal(loid) contents comparable to data reported for unpolluted soils: 0.4±0.2 mg·kg-1 cadmium (Cd), 4±1 mg·kg-1 lead (Pb), and 26±4 mg·kg-1 arsenic (As). The As is, however, mainly bound to the non-easily mobilizable residual soil fraction, so pore-water concentrations after anoxic soil incubations were low (up to 4.9 μg·L-1). Inorganic As was the dominant species in 18 incubations (up to 57 %), followed by 4 incubations with mainly methylated thioarsenates (up to 59 %) and only 2 incubations with methylated oxyarsenates as dominant species (up to 43 %). Dimethylmonothioarsenate (DMMTA) was present in all treatments. Spiking anoxic soil incubations with sulfate, mimicking Costa Rican fertilization practice, enhanced thiolation of methylated oxyarsenates. Arsenic concentration in field irrigation water was 1 μg·L-1, whereas Cd and Pb concentrations were below detection limit. In the majority of commercial brown, parboiled, and polished as well as field rice samples collected during rainfed and irrigated seasons As, Cd, and Pb were detectable. Inorganic As was the dominant species (up to 91 %), besides DMA (up to 57 %). Small amounts of DMMTA (2 %) were detected in enzymatic extractions. DMMTA currently is not regulated by any food guideline. Inorganic As and Cd contents complied with the European Commission (EC) threshold limits for adults but several of the samples were not safe for preparing baby food. Lead contents in rice grains exceeded the EC limit only during the irrigated season. Differences between the metal(loid) content during the irrigation regimes were only observed for Pb, where all the irrigated samples had higher Pb contents than in the rainfed season.