Cleaning heavy metal contaminated soil with soluble humic substances instead of synthetic polycarboxylic acids

Abstract
Soils contaminated with heavy metals constitute a serious and widespread ecological problem but to clean such soils requires
strong chemicals such as polycarboxylates; frequently ethylenediaminetetraacetic acid and nitrilotriacetic acid are used.
However, these compounds are synthetic and toxic and their replacement by natural products such as soluble humic
substances as washing agents for cleaning heavy metal polluted soils would be environmentally very attractive. In fact, such a
replacement seems possible at least on cadmium and copper contaminated soil inasmuch as humic substances, depending
on the concentration, were found to extract up to 45% and 54% of total cadmium and copper from a highly contaminated
calcareous soil. Even though higher amounts of the two metals were extracted by ethylenediaminetetraacetic acid and
nitrilotriacetic acid, the humic substances undoubtedly extracted the most reactive fractions. However, the humic
substances extracted only 4% of total lead and 17% of total nickel, whereas the percentages for the synthetic
polycarboxylates were about 30% for nickel and lead. Ethylenediaminetetraacetic acid and nitrilotriacetic acid may
therefore be replaced by humic substances as washing agents for cadmium, copper and maybe nickel contaminated soils,
whereas they seem unsuited for cleaning lead contaminated soils, at least if the soils are as calcareous as the soil tested.