Remediation of metal contaminated soils in Bangladesh by chemical technology

Abstract

To remediate metal contaminated soils by chemical technologies soil samples were collected from six metal contaminated industrial areas. The Pb concentrations were high (245 and 164 mg kg-1) in soils of battery industries of Madhupur tract (acidic soil) and Ishwardi (calcareous soil), respectively. Soil of tannery industrial area contained high amount of chromium (256 mg kg-1). The saline soil of ship breaking yard, Shitakunda had high amounts of Pb, Cr, Ni, Cu and Zn of 93, 71, 69, 192, 845 mg kg-1, respectively. Other two soil samples were collected from cable industry of Kushtia (calcareous soil) and Chittagong (slightly saline), which contained 196 and 151 mg kg-1 of Cu, respectively. The mobility indexes of metals were found high (above 40%) in calcareous soils. Different synthetic extractants (NH4OAC, CaCl2, HCl, EDTA, DTPA and distilled water) were applied to spiked soils for removal of metal. Among the metals the highest amount of Cd and lowest amount of Cr were removed from soils by these extractants. The metal removal efficiencies of extractants were found higher in light textured soils than that of heavy textured soils. Among the extractants distilled water removed the lowest amount of metals from all soils. Between the salt solutions, buffered NH4OAc extracted comparatively higher amounts of metals than that of CaCl2. The extractants 0.1 M HCl extracted significantly high amount of metals, but the extraction markedly reduced soil pH and also leached a high amount of plant essential elements like Ca, Mg, Fe and Mn. Moreover HCl could not extract much metal from calcareous soils. The chelating agents EDTA and DTPA extracted considerable amounts of metals from all types of soils. These extractants effectively lowered the mobility indexes of metals below 10% in acidic and saline soils and 8 to 29% in calcareous soil. Among the chelating agents, EDTA removed the highest amount of metals from soils, but its use is also problematic one if persists in the environment. Soluble humic substances (HS) might be used instead of synthetic chemicals. Humic substances act as chelating agents for metal ion extraction which are also less expensive, environment friendly, less destructive to soil structure and improve soil properties. Different concentrations of humic and fulvic acids (0.1 to 0.5%) were used for removal of metal from soils. Fulvic acid extracted comparatively higher amount of metals from heavy textured soils of Madhupur, Ishwardi and Kushtia due to its lower molecular structure than that of humic acid. In contrast humic acid leached somewhat higher amount of metals from light textured soil of Hazaribagh and Shitakunda due to having more contact time with soil particles. Humic substances (HS) reduced metal mobility in soils but the values were comparatively higher than that of EDTA, because HS extracted comparatively lower amounts of metals from soils (especially from calcareous soil) compared to EDTA. The extraction capacities of humic substances were significantly increased when combined with small amount of EDTA. Combined HS + EDTA extractants reduced the mobility indexes which were also lower than that of individual HS and EDTA treatments. In heavy textured soils of Madhupur tract and in calcareous soils 0.4 % fulvic acid in combination with 0.04 M EDTA extracted similar amounts of metals as was done by only 0.1 M EDTA solution, whereas humic acid required 0.06 M EDTA. Both humic and fulvic acids required lower amounts of EDTA (0.04 M) to remove metals from light textured soils of Hazaribagh and Shitakunda as was done by EDTA alone. Pot experiments were conducted with red amaranth and rice plants in six different contaminated soils to study the mobility and bioavailability of metals from soil to plant after reclamation with different extractants. Each contaminated soil was treated with only HA, FA, EDTA and also with combined HA + EDTA and FA + EDTA extractants. The contaminated soils treated with these extractants showed reduced metals toxicity and created positive impacts on the growth and yields of plants. In Pb contaminated Madhupur tract soil only 0.4 % FA extractants created almost similar effects on the minimum content of Pb in plants as were obtained with combined FA + EDTA treatment. Soils treated with 0.4 % FA + 0.04 M EDTA extractant yielded the lowest Pb and Cu concentrations in plant materials grown in contaminated calcareous soils of Ishwardi and Kushtia, respectively. The contaminated soil of Hazaribagh treated with only 0.4 % HA and 0.4 % FA was enough to reduce the Cr concentration in both plant materials. In contaminated soil of ship breaking yard the combined application of 0.4 % HA + 0.04 M EDTA and 0.4 % FA + 0.04 M EDTA extractants were equally effective in reducing high concentrations of metals in soil and plant materials. The Cu contaminated soil of Chittagong treated with only 0.4 % HA and 0.4 % FA was enough to reduce Cu concentration in plants. In all pot experiments, soils treated with only 0.1 M EDTA extractants increased metals concentration in labile fraction due to further dissolution of metals from non-labile fraction with time, which enhanced metal uptake by plants. But residual humic substances in soil reduced the mobility of metals by forming immobile HS-metal complex which inhibited metal accumulation in plant.