DETERMINATION OF TOTAL CADMIUM IN CALCAREOUS SOILS BY EXTRACTION USING ALIQUAT-336 AND 3-HEPTANONE AFTER AQUA REGIA DIGESTION

Measurement of total soil cadmium (Cd) is difficult due to calcium (Ca ) and other chemicals which cause high background absorbance when trac e levels of Cd are to be determined. When soil Cd is low, even use of deuterium background correction with flame atomic absorption spectrosc opy (AAS) cannot provide accurate Cd results. Use of furnace atomic ab sorption with method of standard additions can circumvent these interf erences, but the cost and time required are substantial. We desired a more rapid, convenient, and reliable alternative to extraction using d ithizone and back-extraction into acid, or to ammonium pyrollidinedith iocarbamate (APDC) which does not require close pH adjustment nor have many sources of potantial contamination. We evaluated analysis of the se complex soil extracts with the method of Viets (1978) which extract s metals from 1N acid solutions using Aliquat-336 in methylisobutyl-ke tone (MIBK). We tested the use of the less toxic and less water solubl e 3-heptanone as an organic solvent alternative to MIBK which can be d irectly analyzed by flame atomic absorption. A series of extraction ex periments were conducted to determine if Cd was extracted from standar d solutions and from total metal digests of calcareous soils into an A liquat-336/3-heptanone solution, and under what conditions extraction was optimum. In the optimum method, Cd was extracted from aqua regia s oil digests by 10% Aliquat-336 in 3-heptanone without addition of asco rbic acid or potassium iodide (KI) used by Viets. Excellent recovery o f Cd was obtained for both standard reference soils and low Cd highly calcareous soils from North Dakota and Minnesota. Addition of ascorbic acid and KI did not increase the efficiency of extraction indicating that the extraction system used was free of ferric-iron [Fe(III)] inte rference. The ion-association complex of Cd remained stable for at lea st 24 hr after extraction, providing a very convenient method to analy ze low levels of total Cd in soils and other geologic materials.