Organic lead, removal

In all cases, the results demonstrate that in the presence of sodium chloride in the aqueous phase, the level of organic lead removed from solution is significantly reduced. Figure 2 shows that for complete precipitation of organic lead a ratio, Cr/Cl, of at least 350 is necessary. This is over two orders of magnitude greater than in the absence of sodium chloride. Clearly such an excess of reagent is undesirable in both economic and environmental terms. 

Equations 14.24 to 14.27 can be applied to most sites where soil cleanup regulations are known for metals or organic contaminants. Two examples follow, one for TCE treatment by phytotransformation and another for lead removal by phytoextraction, which demonstrate the use of the design equations. 

The Removal of Organic Lead from Aqueous Effluents by a Combined Chemical Complexing-Solvent Extraction Technique... 

Waste waters containing low concentrations of soluble organic lead in the presence of high concentrations of other diverse ions such as Cl pose a particularly difficult treatment problem. Generally, organic lead exists in solution as the tri- or dialkyl lead chloride species. These salts are not amenable to the conventional methods used to remove inorganic lead, viz., those of pH adjustment followed by settling. The technique of chemical conversion of the... 

The chemical complexing-solvent extraction technique employed in this work involved the formation of a neutral complex in the aqueous phase between trialkyl lead chloride and a dithiocarbamate reagent such as sodium diethyl di-thiocarbamate. The complex was subsequently removed either as a precipitate or by extraction into an organic solvent. The extent of lead removal was traced by analysis of the aqueous phase for residual trialkyl lead using a Pye-Unicam 8000 spectrophotometer. 

The formation of a 1 1 complex was confirmed by examination of the removal of organic lead from the aqueous phase as a function of the reagent-to-organic lead ratio Cr/Cl at a temperature of 30°C. Figure 1 shows that removal of organic lead corresponds to what is calculated. For a molar reagent-to-lead ratio (Cr/Cl = 1), complete removal of organic lead is achieved. This demonstrates that, in the absence of NaCl, the complex is essentially insoluble in the aqueous phase, at least to within the accuracy of analysis, 0.1 ppm. 

Characterization of the complexing reaction was then conducted in the presence of 0.83 m sodium chloride in the aqueous phase over a temperature range 15°-60°C. In addition, removal of Hs PbCl as a function of the re-agent-to-organic lead ratio Cr/Cl in the presence of 5 wt % NaCl was examined at 30° C using in turn three other dithiocarbamate reagents besides that of sodium diethyl dithiocarbamate. 

Reference to Table III shows that in the presence of 0.83 m sodium chloride and for an aqueous-to-organic phase ratio Vaq/Vorg = 1.0, a ratio Cr/Cl of 1.0 is sufficient to remove at least 75% organic lead. For solvents such as benzene and chloroform this ratio is sufficient to achieve complete organic lead extraction. 

Figures 4 and 5 for Vaq/Vorg = 5.0 show that removal of organic lead corresponds closely to that which would be obtained for a theoretical 1 1 complex in the absence of sodium chloride. Variations in extraction efficiency are observed for different solvents, but for all of the solvents employed a ratio Cr/Cl = 1.0 is sufficient to reduce an initial triethyl lead chloride level of 10 ppm to <1 ppm. A comparison of solvents used suggests an approximate order of effectiveness (corresponding to solubility of the organo-lead complex Hs PbSCSN Hs and also the neutral species Hs PbCl0 in the solvent) of the form shown in Table IV.

For ascertaining whether removal of organic lead by chemical com-plexing-solvent extraction was chemical reaction or physical diffusion controlled, stirred cell studies were conducted. 

Because there exists an equilibrium distribution of species in the aqueous phase, removal of certain species will lead, by the law of mass action, to a readjustment of the equilibrium. The concentration of negative species will decrease to compensate for continued loss of the neutral and positive species to the organic phase (loss of the positive species is, of course, caused by complexing). This continued adjustment of equilibrium will be maintained until all of the organic lead has been removed from the aqueous phase if the organic phase is not saturated. 

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