Organic acids for the in situ remediation of soils polluted by heavy metals: Soil flushing in columns

To compare the soil remediation effectiveness of salts of weak organic acids with strong chelating agents, three soils of different textures, all polluted by heavy metals, were washed in a column, at optimum pH, with salts of weak organic acids, namely, citrate, tartarate or oxalate + citrate or chelating agents (EDTA or DTPA). For the clay loam, Cr, Mn, Hg and Pb were removed by citrate and tartarate at levels of 43 to 45, 37 to 41, 91 to 92 and 75%, respectively. EDTA and DTPA effectively leached only Pb after 20 pore volumes. For the loam, citrate leached 98 and 89% of Cd and Pb after 20 pore volumes, respectively, while tartarate leached out 91 and 87% of Cd and Pb. EDTA and DTPA removed 93 to 97% of these metals after 20 pore volumes. For the sandy clay loam, 84 to 91, 73 to 84, 56 to 70 and 72 to 81% of Cd, Cu, Pb and Zn were removed respectively, by citrate and tartarate. EDTA and DTPA removed 93 to 97% of these metals after 20 pore volumes. An in situ soil remediation simulation was also tested using the sandy clay loam in a tub. After 12 hr of retention, the citrate solution washed 81, 82, 73 and 90%, of Cd, Cu, Pb and Zn, respectively, after six pore volumes. EDTA and DTPA effectively removed all heavy metals, except for Hg, but also extracted large quantities of soil nutrients and pollute the soil by being adsorbed on the soil particles. The salts of citrate and tartarate effectively removed the heavy metals from the three polluted soils while leaching little macro-nutrients and improving soil structure. Each soil reached C and B levels of soil-clean-up criteria after 10 to 20 pore volumes and within 10 to 15 hr of flushing.