Chemical extractability of heavy metals during and after long‐term applications of sewage sludge to soil

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Abstract
SUMMARY: Sequential extractions were used on soils from a long‐term experiment treated with either metal‐contaminated sewage sludge or inorganic fertilizers between 1942 and 1961. The four extracts employed were CaCl2, NaOH, EDTA and aqua regia. These showed that large increases in the proportions of Pb, Cu, Zn, Ni and Cd in at least one of the first three fractions occurred during the first 10 years of sewage sludge additions. Cr always remained predominantly in the aqua regia‐soluble fraction. For 30 years after this, including a period of more than 20 years after application of sludges to the field had ceased, there was very little change in the percentage of each metal extracted by each reagent.Although the ‘residual’ (aqua regia‐soluble) and EDTA fractions usually contained the largest amounts of metals in either sludge‐ or fertilizer‐treated soils, there were clear differences between the metals: Pb represented the largest fraction of any metal extracted by EDTA, Cu of any metal extracted by NaOH and Cd of any metal extracted by CaCl2. The same extractions were made of the sewage sludges that were applied to the field, and the distributions of the metals differed from those found in the treated soils. It was particularly apparent that more Pb and Cu was present as the ‘residual’ (aqua regia) fraction in sludges than in the soils.