The Solvent Extraction of Cadmium from Phosphoric Acid Solution By 3-methyl-quinoxaline-2-thione in Toluene as Diluent

The solvent extraction of cadmium (II) from phosphoric acid 2.5M by 3-methyl-2 (1H)quinoxaline-thione ( C9H8N2S) is investigated in various experimental conditions. Obtained results show a variation of extraction of Cd with pH, and concentration. The conventional methods of slope analysis can’t be performed successfully for understanding interaction mechanisms. A technique combining slope analysis with the variation of overall extraction constant with pH is performed in this case. There is no predominant removal mechanism, cadmium is extracted as complexes in Cd: : H3O +( OH-) stoichiometry varying between 1:1:0 and 1:3:3. The extraction reaction is a complex process including ion exchange-ion pair formation, organic and inorganic complexation, H3PO4 deprotonation and solvatation phenomena. A best recovery of 43 to 60% is achieved in acidic conditions with CLH = 0.26M.


INTRODUCTION
Due to their harmful effects, heavy metals are a major threat to ecological environment, food safety and agriculture sustainable development.As a result, these elements are not suitable in nature that various measures are undertaken to control, and prevent their presence in environment.Among these heavy metals, cadmium is listed as the seventh most toxic substances [1][2] .Enrichment of Cd in soils is reporte , through the land application of bio solids, industrial eûuents, and Cd-rich phosphate fertilizers 3 .In general, phosphate fertilizers are produced by the acidulation of crushed and powdered phosphate.
Thus the removal of Cd(II) from phosphoric acid which is the intermediary product for the production of all high grade phosphate fertilizers, is of ever-increasing interest.Several methods including precipitation, solvent extraction and ion exchange are proposed in this case.Amine compounds are successfully used for cadmium separation from complex aqueous media.However, in this extraction system, metal extraction is carried out from acidic media, so that the amine in the organic phase is first converted to its salt.To avoid this limitation on the aqueous condition, quaternary ammonium salts, is employed in neutral, and slightly alkaline.As known the sulfhydryl-containing compounds have the ability to chelate heavy metals such cadmium that Chelating exchangers with thiol functional groups are used for Cd(II) removal from aqueous solutions [4][5] .
Recently the use of extractants with a bifunctional groups such as quinoxaline-thione for zinc and cadmium removal are highlighted as potential extractant for metal removal from complex aqueous media [6][7] .
The advantage of this process is that can combine organic and inorganic complexing phenomena, ion-pair formation and ion exchange reactions.Thus, cadmium removal from H 3 PO 4 3.22 M is successfully achieved over wide range of pH, in this case.Understanding interaction mechanisms involved in this case, is essential and allows efficient use of solvent extraction procedure 8 .This interaction is also an actual problem of modern analytical chemistry, which is primordial for environmental sciences, as well as geology, clinical and medicinal chemistry, as discussed previously 9 .
Unfortunately, it is reported that Cd(II) extraction by amines is a variable phenomenon when inorganic complexation is involved in aqueous phase that slope analysis cannot be used to carry out extraction stoechiometry 10 .Similar results are obtained for this metal in phosphoric medium 7 .To determine extracted species, a technique combining slope analysis with the variation of apparent extraction constant with pH is performed, in this case.As shown, cadmium removal is essentially achieved via anion-exchange.Obviously to control extraction process in question, more informations are required for the removal of Cd with analytical concentration variation of H 3 PO 4 .
For these reasons the present study aims to carry out cadmium extraction reaction predominating in H 3 PO 4 2.5M.

Reagents
All reagents used: Cd(NO 3 ) 2 , HCl, NaOH, and H 3 PO 4 are of analytical grade and used without further purification.Stock solutions (10 -3 mol/l) of Cd(II) are prepared by dissolving known amount of cadmium nitrate in adequate volume of 0.1M laboratory phosphoric acid.Initial pH of the aqueous phases, is adjusted using 0.1M KOH or 0.1M HNO 3 , and measured before and after extraction test.

Procedure
The organic and aqueous solutions (H 3 PO 4 2.5M) were equilibrated in 20 ml separator funnel with an organic/aqueous ratio of 1:1 (5 ml organic solution and 5 ml aqueous solution) which is shaken vigorously for 15min.Preliminary tests have shown that equilibrium of extraction is reached in less than 5 min.After equilibration, the separator funnel is left standing for at least 30 min for completing phase separation.After equilibration, the separator funnel is left standing for at least 30 min for completing phase separation.The synthesis of 3-methyl-2 (1H)quinoxaline-thione (C 9 H 8 N 2 S) designated thereafter as , is performed as described previously [6][7] .A solution of synthesized extractant is prepared with dissolution of 0.26 mol/l of in toluene (C 7 H 8 ).
The extraction of cadmium is studied from aqueous phosphoric acid solutions (2.5M) containing 10-4M Cd(II).All the extraction experiments were carried out at room temperature.
The Cd(II) concentrations in the aqueous phase ( ) were determined using Benchtop XRF Spectrometer with X-LabPro Version 4.0 software.while the concentration of cadmium in the organic phase ( ) was calculated from material balance.
The distribution ratio, D, is calculated as follows: ...   As found, these variations are linear with slopes varying in the range of about 1 to 6.The overall stoechiometry extraction reaction is not an integer as could be expected from theoretical single reaction.This indicates a variation of extraction of Cd with pH, and LH concentration.Similar results are observed previously for the extraction of cadmium, zinc and some rare earths by various chelatants 6-7; 10-14 .This shows that at least, two predominant extracted complexes are involved in the removal of Cd (II), in this case.

..(3)
Where X -is aqueous anionic ligand, the onlined entities refer to organic phase, and n = 0;1 ;2, etc….The symbol H -n stands both for hydrogen (n<0) and for OH group (n>0).K l n denotes the constant of equilibrium reaction.The extracted species are given by: ...(4) K l n is given by : ...( 5) and ... (6)   Assuming in first approximation that , which results in we obtain: ... (7)  One can note that n=0 for ion-pair formation.