Cadmium Compounds in Mineral Fertilisers

Codes, laws and publications had to be evaluated for this paper. However, we are not sure whether they were properly assessed in respect of their importance. Nevertheless, it has been established that there have always been cadmium compounds in the food prepared from vegetables. We do not know whether the Cd concentrations were higher or lower a few centuries ago.

Since it is probable that the absorption of Cd compounds by vegetables depends on the cadmium concentrations in the soil, an increase in the Cd content of our food might be due to such a concentration but it is far from reaching a critical level.

In the ERG, statistics show that 76% of the Cd compounds determined do not originate from fertilisers.

It has also been established that values of up to approx. 20 g per ton phosphate rock cannot cause an increase in the Cd content of the soil. If, however, phosphate rock with a higher Cd content is used, a rise in the present concentrations might be the consequence.

According to present findings, the cadmium content of cultivated land has not gone up in spite of the addition of cadmium.

It has hitherto not been established whether cadmium compounds introduced into the land are elutriated or whether they concentrate in deeper layers of the earth crust. Since the Cd content of the earth crust is considerably higher than the Cd quantity introduced into the land, it is difficult to determine it by means of analyses.

Nevertheless, it is necessary that the problem pertaining to the Cd content of fertilisers and the possible remedies be investigated by the fertiliser industry.

According to our findings, the most economical method of reducing the Cd content of fertilisers is to use phosphate rock with a lower Cd concentration.

If phosphate rock were pre-digested, for instance with nitric acid, higher quantities of the Cd compounds contained in the phosphate from various sources could be removed. The solution from the pre-digestion unit could be treated in a special extraction unit to separate further Cd compounds. It would be difficult to purify phosphoric acid e.g. in an extraction process because of all impurities contained in it. It would also be possible to treat the acid with apatites in a precipitation unit where the Cd compounds would concentrate in a salt mixture to be separated. However, this process causes P₂O₅ losses of up to 1%.

None of the processes using ion exchangers are suitable in this case because phosphoric acid contains many other impurities.

Dr H-T Baechle, Hoechst AG, F R Germany

Dr F Wolstein, UHDE GmbH, FR Germany

18 pages, 6 Tables, 26 References.