A laboratory test of NOM-assisted remediation of arsenic and copper contaminated soils

Soils contaminated by arsenic (As) and copper (Cu) must be remediated because As and Cu are non-degradable and toxic. On moderately contaminated soils, As and Cu may be removed by in-situ plant uptake (phytoremediation), whereas strongly contaminated soils must be removed and cleaned by soil washing at a soil disposal site (ex-situ). Strong bonding of As and Cu to soil solids requires the use of strong complexants to release the elements. Often synthetic chemicals such as EDTA and NTA are used. Since such chemicals are environmentally problematic, their replacement with natural organic matter (NOM) such as humic substances (HS) and citrate might be attractive. To test this possibility, a moderately contaminated calcareous urban soil from a soil depot (called the CRC soil) and a soil from a wood impregnation site (called the CCA soil) highly contaminated with As and Cu were extracted with citrate, HS and NTA at different concentration and pH, and the results compared. Extracted As and Cu were found to increase at increasing extractant concentration but to decrease at increasing pH. The efficiency of the three extractants generally decreased in the order: NTA > ≈citrate > HS but at pH 4, HS extracted the same amounts of Cu as citrate and NTA, and substantial amounts of As. Instead of replacing one contaminant, As/Cu, with another contaminant, synthetic NTA, it is therefore recommended to use HS at pH 4 or citrate for removing As and Cu from strongly contaminated soils and to use HS at neutral pH to enhance in-situ phytoremediation of moderately contaminated soils. Citrate (and NTA) cannot be suggested for enhancement of on-site phytoremediation because of high mobilization rates caused by these extractants, which through leaching and runoff may lead to contamination of recipient waters with As and Cu.