Determination of the uptake of [Pt(NH 3) 4](NO 3) 2 by grass cultivated on a sandy loam soil and by cucumber plants, grown hydroponically

Abstract

Two cultivation experiments were carried out in order to answer the question to what extent platinum can enter the food chain by accumulation in plants, when the platinum is present in a bio-available form: (i) cucumber plants ( Cucumis sativus) were grown hydroponically in nutrient solutions containing [Pt(NH 3) 4](NO 3) 2 (from 0.5 to 50 μg Pt/l solution); and (ii) a water-soluble platinum compound — [Pt(NH 3) 4](NO 3) 2 — was added in increasing amounts to a sandy loam soil (from 0.5 to 50 mg Pt/kg soil) and rye grass ( Lolium perenne) was grown on it. The roots on the one hand and the green plant fractions on the other hand of the cucumber plants and the rye grass were digested using a high-pressure asher. The platinum concentration was determined by means of a quadrupole-based (VG PQ I) or a double focusing sector field ICP-mass spectrometer (Finnigan MAT, Element), depending on the platinum concentration in the sample solution. The detection limit for platinum obtained with the VG PQ I was observed to be 6 ng/l, while with the `Element' the detection limit could be improved to 0.5 ng/l Pt. Accumulation factors were calculated as the ratio of the platinum concentration in the plant to that in the soil or the nutrient solution. The grass grown on spiked soil accumulated platinum only to a slight degree (accumulation factors between 0.008 and 0.032). The hydroponically grown cucumber plants, however, strongly accumulated it (accumulation factors of 11–42 in the shoot and 1700–2100 in the roots). There are three possible causes for the large differences in the accumulation factors: (i) Cucumber plants are dicotyledons; grass, however, is a monocotyledon. Other cultivation experiments already showed that dicotyledons accumulate metals to a higher extent than monocotyledons. (ii) In the grass cultivation experiment, the platinum compound was only added once to the sandy loam soil, namely 2 days before grass was cultivated on it. The nutrient solutions of the cucumber plants were changed twice a week. Consequently, the total amount of platinum that the plants were exposed to during the cultivation of the cucumber plants was higher than during the cultivation of the grass. (iii) Immobilization of the platinum compound in the soil most likely occurred.