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.
Heap Leaching of Lead Contaminated Soil Using Biodegradable Chelator [S,S]-ethylenediamine Disuccinate
The feasibility of heap leaching Pb contaminated soil using biodegradable chelator [S,S]-ethylenediamine disuccinate ([S,S]-EDDS) in concentrations ranging from 10 to 40 mmol kg−1 was assessed in a laboratory study. Treatment with 40 mmol kg−1 [S,S]-EDDS was the most efficient, removing 31.3% of soil Pb and producing a waste washing solution with a peak Pb concentration slightly exceeding 10 mM. The removal of Pb from simulated washing solution containing 10 mM Pb by biodegradation of the Pb-[S,S]-EDDS complex and sorption of the released Pb on a vermiculite/soya meal/Slovakite based permeable bed was completed in 7.5 days. Lead recovery from washing solutions with higher molarity was significantly slower. Lead fractionation and bioavailability were assessed 6-months after soil treatment. Heap leaching removed Pb proportionally from carbonates (approx. 25% Pb) and organic matter (>50% Pb), the two main Pb-bearing soil fractions, and increased the small pool of Pb soluble in the soil solution. The latter had no effect on Pb phytoavailability. Accumulation of Pb in leaves of the reported hyperaccumulator species Thlaspi goesingense was <20 mg kg−1 in all treatments, comparable to the Pb concentration in lettuce (Latuca sativa). Soil treatment with 40 mmol kg−1 [S,S]-EDDS reduced concentrations of Pb orally-accessible in the stomach and intestine phase from initial 234.0 and 52.5 mg kg−1 to 157.8 and 29.7 mg kg−1, respectively, according to Ruby's test. Our results indicate that heap leaching of Pb with [S,S]-EDDS is not a viable remediation option for the tested soil and conditions used.
The Determination of Adsorbed Na, K, Mg and Ca on Sediments Containing CaCO3 and MgCO3
A method for the determination of the cations of Na, K, Mg and Ca adsorbed on clay minerals mixed with CaCO3 and MgCO3 is described. An ethanolic solution of LiCl-CsCl is used to displace the exchangeable cations. Blank determinations performed using either a second ethanolic leach or a second LiCl-CsCl leach, are used to correct for carbonate dissolution. Details of the methods development are given.The method has been tested using mixtures of homoionic forms (Na, K, Mg, Ca) of smectite and kaolinite with either CaCO3 or MgCO3. The smectite and kaolinite were found to have total CECs (with standard deviations) of 765 (4.4) and 39.8 (0.52) mequiv kg−1, respectively. The amount of cation exchanged was found to vary directly with the proportion of clay mineral in the mixture; regression coefficients consistently greater than 0.997 were obtained.Other tests with smectite-CaCO3 mixtures in sea-waters of various salinity vindicated the use of the method with heteroionic forms of smectite. These tests also suggest that the phenomenon of fixation observed in most other studies of clay minerals in estuarine conditions might be redundant. It is contended that there is an urgent need for this suggestion to be tested.