Quick Tests for Heavy Metals in Soils

Quick Tests for Heavy Metals in Soils
Diane Wu and Murray B. McBride
Department of Crop and Soil Sciences, Bradfield Hall, Cornell University, Ithaca, NY
In urban environments, total soil concentrations of trace metals such as lead and zinc can
be elevated because of historical contamination. It is important to have estimates of
metals that may be toxic to humans, animals and plants, particularly in recreational and
garden areas. Total soil metals analysis by testing laboratories is relatively expensive, and
management decisions are therefore often based on one or very few soil sample analyses
for an area. Inexpensive tests that could be done by property owners would make it
feasible to more thoroughly test an area and better assess spatial contamination patterns.
To this end, we developed simple and rapid colorimetric tests for soil zinc, copper and
lead. The zinc test is an adaptation of the dithizone method, using a surfactant to permit
color measurement in a single aqueous phase. This test detects 0.1 mg L-1 Zn in acetate
buffer extract from soils. In initial trials, Zn was detected (indicated by pink color) once
soil Zn reached 50-100 mg/kg above background level. The copper test uses EDTA to
extract Cu from soils, with color development involving the Cu-specific cuprizone
complexing agent. This test readily detects 1.0 mg L-1 of Cu in solution, and initial trials
show a positive test (visible blue color) for soils with more than 50 mg kg-1 total Cu. The
lead test uses citrate buffer to extract Pb from soil, and color is developed in the extract
by addition of Na2S, which produces a brown color upon colloidal PbS formation.
Although less sensitive than the other tests, greater than 10 mg L-1 Pb can be detected in
soil extracts. These metal tests can be used semi-quantitatively by visual comparison of
color intensity to standards, or can be made quantitative for extracted metals by
measuring absorbance with a spectrophotometer.