Recycling of lead-contaminated EDTA wastewater.
Ethylene diaminetetraacetic acid (EDTA) is one of the chelating agents used in the soil washing process for the decontamination of lead-contaminated soil. Lead-EDTA complexes in the wastewater from the soil washing process must be removed before the wastewater can be safely discharged. This study outlines a method to recycle Pb-EDTA wastewater by substituting the Pb complexed with EDTA with Fe(III) ions at low pH, followed by precipitation of Pb ions with phosphate or sulfate ions. Fe(III) ions complexed with EDTA were then precipitated at high pH using sodium hydroxide. The resulting solution (Fe-precipitated solution) was tested on three lead-contaminated soils. The Fe-precipitated EDTA solution was found to have similar extraction capabilities as fresh EDTA solution. Experimental results showed that the recycled EDTA solution may be recycled several times without losing its extractive power. Recycled EDTA wastewater with phosphate precipitation was found to be slightly more effective than recycled EDTA solution using sulfate precipitation. The recycling procedure may be applied to wastewater generated during soil washing of lead-contaminated soil, resulting in a reduction in wastewater generated and savings in the amount of EDTA used.
Reactions of EDTA Complexes of Fe, Zn, Mn, and Cu with Soils1
The Fe, Zn, Cu, and Mn complexes and the Na salt of EDTA (ethylenediaminetetraacetic acid) were reacted with suspensions of acid, neutral, and calcareous soils. The initial concentration of chelate was 1 × 10⁻⁴M or on a soil weight basis, 11.2, 13.1, 12.7, and 11.0 ppm of Fe, Zn, Cu, and Mn, respectively. Suspensions were aerated and shaken continuously for periods of 2 hours to 30 days. At the termination of each reaction period the pH of the suspensions was measured; following centrifugation, the supernatant solutions were analyzed for Fe, Zn, Cu, Mn, Ca, and Mg by atomic absorption spectrophotometry. The concentration of ¹⁴C-labeled EDTA was determined by liquid scintillation. FeEDTA was stable in soil suspensions of pH 5.7 and 6.1, moderately stable at pH 6.75, and unstable at pH 7.3 and 7.85. This behavior was satisfactorily predicted using formation constants to describe equilibria of EDTA with Fe³⁺, Ca²⁺, and H⁺, with the assumption that the Fe³⁺ concentration was controlled by the solubility of amorphous iron(III) oxides. ZnEDTA and CuEDTA were most stable in suspensions near neutrality. In acid soils Cu and Zn were increasingly displaced by Fe as pH decreased while in calcareous soils these metals were displaced by Ca as pH increased. The loss of Mn from MnEDTA was very rapid in all soils and was essentially complete in less than one day from suspensions of pH 6.1 to 7.85.
DETERMINATION OF CATION EXCHANGE CAPACITY OF CALCAREOUS SOILS AND THEIR PERCENT BASE SATURATION
A method with few inherent errors was developed for the determination of cation-exchange capacity of calcareous soils. In this method calcium was used as index cation and the excess salt removal step was omitted. In addition to this a direct way is proposed for the determination of exchangeable calcium plus magnesium in these soils. The CEC values obtained by the proposed method, were compared with the CEC values obtained by the method of isotopic dilution, using 133Ba, and also with the method of Bower et al. and that of ammonium acetate. The percent base saturation of the samples was calculated when 1 N solution of LiCl, NaCl, KCl, and NH4C1, adjusted to pH 7.0, were used as leaching solutions. Additional variables in this calculation were the number of leachings and also the correction or not for dissolved carbonates and bicarbonates.