Comparison of natural humic substances and synthetic EDTA and NTA as washing agents of a heavy metal polluted soil

Ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic
acid (NTA), and other synthetic polycarboxylic acids have
been shown to possess substantial capacity as washing agents
of heavy metal–polluted soils, but they are environmentally
problematic. Th erefore, a sample of natural soluble humic
substances (HS) was tested as a possible substitute. Th e
effi ciency of HS to extract cadmium (Cd), copper (Cu), and
lead (Pb) from a strongly polluted calcareous urban soil was
compared with that of EDTA and NTA. Th e infl uence of
extractant concentration (25–100 mmol L−1 C), solution/soil
ratio (5–100 L kg−1), and single-step vs. multistep extraction
on heavy metal removal from the soil was investigated.
Th e extracted pools were assessed by sequential extraction.
Ethylenediaminetetraacetic acid and NTA extracted up to
86, 77, and 30% of total soil Cd, Cu, and Pb, respectively,
whereas HS extracted 44, 53, and 4%. Extracted amounts
of Cd, Cu, and Pb increased with increasing extractant
concentration and solution/soil ratio in the range 5 to 100
L kg−1. Single-step extraction removed about the same
amounts of the three metals as multiple-step extraction. Th e
metal-extracted pools of the soil depended on the metal and
on the extractant. Th e overall conclusion is that soluble HS
can replace synthetic EDTA and NTA as washing agents for
Cd- and Cu-polluted soils, whereas HS is not a promising
substitute of EDTA or NTA for cleaning Pb-polluted,
calcareous soils.

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