The Solvent Extraction of Cadmium From Phosphoric Acid Solution By3-Methyl-Quinoxaline-2-Thione in Toluene Diluent

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 effluents, 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  asquinoxaline-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 H₃PO₄ 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 H₃PO₄.

For these reasons the present study aims to carry out cadmium extraction reaction predominating in H₃PO₄ 2.5M.

Material and Methods

Reagents

All reagents used: Cd(NO₃)₂ ,  HCl,  NaOH, and H₃PO₄  are of analytical grade and used without further purification. Stock solutions (10^-3mol/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 HNO₃, and measured before and after extraction test.

Procedure

The organic and aqueous solutions (H₃PO₄ 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 (C₉H₈N₂S) 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 (C₇H₈).

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:

Results and Discussion

Effect of Equilibrium pH

Fig. 1 reproduces the logarithmic variation of D with pH for H₃PO₄ 2.5M, in the pH range of about 1.7 to 7.0, and analytical ligand concentration

and 0.62M. As can be seen, extraction of cadmium increases with increase in pH to reach a maximum value at pH = 2.1, and after a steady stage observed in the pH range 2.1 – 3.4,  logD increases as pH continue to rise. Maxima are found at pH 3.7 and 4.5, in this case. Extraction yield higher than 66% is achieved at

for pH ranging from 2.0 to 5.0. As discussed previously, distinct pH segments with continuous varying slopes suggest that cadmium extraction rises from   different   reactions [7].

Figure 1: Variation   of logD versus pH, at of 0.05, 0.026M, 0.13, and 0.62M, in 2.5M    H3PO4. Click here to View figure

 

Effect of Extractant Concentration 

Fig.2 reproduces the logarithmic variation of D with

at different pH values, for  H₃PO₄  2.5M

Figure 2:  Variation  of  log  D  with  the  LH  analytical  concentration  CLH , at CH3PO 4=  2.5M, for pH value ranging from 1.5 to 6.7.   Click here to View figure

 

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.

Cadmium Extraction from Phosphoric Acid Solutions 

From results obtained previously for Cd(II) removal by 3-methyl-quinoxaline-2-thione [6-7], the extraction equilibrium from phosphoric media may be written in the general forms:

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:

K_{l n} is given by :

and

Assuming in first approximation that

which results in

we obtain:

One can note that n=0 for ion-pair formation.

The nature of C_1n extracted species noted thereafter (l, n) can be obtained from logD=f

variations according to modified slops method:

and

The parameters n and l are varying with experimental conditions. As indicated above there is no predominant extraction reaction, that Cd extracted specie often results in combining two entities [6-7]. Conventional methods of slope analysis can’t be performed successfully in this condition [10; 13].In this case numerical modeling procedure is performed by fitting experiment data to a polynomial equation [7].

Fig. 3 shows plots of  and

and

as a function of pH. As shown, the molar ratio metal: LH: H₃O⁺are varying continuously in extracted entities with aqueous solution acidity. Overall equilibrium constant are obtained from the origin ordinates of log

given by A = logK_1n + n pH.

Figure 3: Variations of l, n, and A versus pH, obtained for H3PO4 2.5M, at CLH = 0.026, 0.052, 0.13 and 0.62M.   Click here to View figure

 

From these results the extraction reaction is found to be influenced by both extractant concentration and aqueous media acidity. The mechanism ofthis reaction involves deprotonation in more acidic condition (pH < 2) and for pH ranging from 3.4 to 4.1. Under mild acidic medium (pH 2.2 – 3.3) recovery proceeds entirely by no ion exchange, in all the cases, and also for

M, at pH > 4.1. While in soft acidic conditions (4.1 < pH < 7), OH^– exchange is prevalent, for

and 0.13M.

Evaluation of Kl, n

Table I show, as example, some overall and predominant extraction constants.

Table I: Equilibrium logarithmic constants (logK_ln) of cadmium extraction by 3-Methyl Quinoxaline-2-thione from H₃PO₄ 2.5M. . (* Calculated constant)

(l ;n)	logKln		(l ; n)	logKln
(2.71; 1.01)	-1.06		(1 ; 1)	2.71*
(2.68; 1)	-0.89		(1 ; 2)	-6.73*
(2.68; 1.83)	-2.58		(1 ; 3)	-8.79*
(2.68; 2.88)	-4.60		(2.74;-0.57)	1.50
(2; 0.04)	0.66		(2.74;-1.71	8.92
(1.6; 0.04)	0.54		(2.74;-2.86	14.08
(1,6; 0.56)	-1,27		(3 ;-0.57)	3.82
(1.6; 2.36)	-7,54		(3 ;-1.7	8.93
(2 ; 1)	0.76*		(3;-2.8)	13.90
(2 ; 2)	-5.63*		(3, 0)	1.23*
(2 ; 3)	-13.66*		(3, 1)	-1.72*
(2, -1)	4.53 *		(3, 2)	-0.73*
(2, -2)	8.12*		(3, 3)	-1.67*
(2, -3)	10.53*		(3,-1)	5.8*
(1.0; 0.0)	0.77		(3,-2)	10.3*
(1.0; 2.2)	-7.12		(3,-3)	14.8*
(1; 0.75)	-2		(1 ;2)	-6.73*
(1; 1.5)	-4.76		(1 ;3)	-8.79*

 

It is interesting to indicate that l and │n│ represent successively the ligand number involved in extracted specie, and the predominant H⁺or OH^– exchange reactions, contributing to overall equilibrium. Thus the overall constant of extracted complex (l, n), combining for successive (l’, i) , (l’, j), and (l’’, i) (l’’, j) species is given by:

K_l ;n = ( K_{l’; n})^{(1 – x)}(K_{l”; n})^x  (10)

With l’< l <l”, l”= l’ +1, and x = l –l’ ,

and  K_l ;n = ( K_{l; i})^{(1 – y)}(K_{l; j})^y  (11)

with | i | < | n | < | j | and |j |= | i | +1, and y =|n | – |i |

and K_{l; i} = ( K_l’ _{; i} )^{(1 – x)}(K_l’’_{; j} )^x  (12), K_{l; j} = ( K_l’ _{; i} )^{(1 – x)}(K_l’’_{; j} )^x (13)

K_l ;n = ( K_{l’; i})^{(1-x)(1 – y)}(K_{l’; j})^{y(1 – x)°} ( K_{l’’; i}) ^{x(1 – y)} ( K_{l’’; j}) ^{x y} . (14)

This allows to obtain :

logK_l ;n ==(1-x) ( 1 –y) logK_l’ ;i + y(1-x) logK_l’ ;j+ x( 1 –y) logK_l’’;i+ x( y) logK_{l ‘’ ;j} (15)

As example, the overall partition equilibrium (2.68; 1.83) in the pH range 1.9 to 2, involves successive extracted species(2; 1), (2; 2), (3; 1), and (3; 2). Taking into account the protonation of amino group at pH <3 [6-7], and percentage distribution of phosphoric complexes of cadmium versus pH[15], predominant reactions combing, in this case, in overall extraction are as follows:

 

The equilibrium constants for the individual reactions could be obtained from the overall equilibrium constant K_lnaccording to:

As example, in the case of (2.68; 1.83) complex we have:

Conclusion

3-methyl-2 (1H)- Methyl Quinoxaline-2-thione

is used for the extraction for cadmium from phosphoric acid (2.5 M). This extraction is varying with initial organic ligand concentration and pH. The stoichiometry of the extracted Cd(II) complex isvarying between 1:1:0 and 1:3:3:

This stoichiometry was determinedby a modified slope analysis. The extraction reaction is a complex process including ion exchange-ion pair formation,organic and inorganic complexation, H₃PO₄deprotonation and solvatation phenomena. A best recovery of 43 to 60% is achieved in acidic conditions with CLH=0.26M

References

1. JaishankarM., TsetenT., AnbalaganN.,  MathewB. B.:  Toxicol. 2014, 7(2), 60–72.
2. ZhitongY., Jinhui L., HenghuaX. ,Conghai Y.: Procedia Environmental Sciences, 2012,16,  722 – 729.
   CrossRef
3. CiadamidaroL., SchulinR., MadejonP.  , ́ Reiser R., Clucas L., Weber P., Robinson B. :  Environ. Sci. Technol,2013, 47, 4497−4504.
4. HynekD., KrejcovaL., SochorJ., CerneiN., KynickyJ., AdamV., TrnkovaL.  , HubalekJ.  , VrbaR. ,KizekR.:Int. J. Electrochem. Sci., 2012, 7, 1802 – 1819.
5. HubickiZ., KołodyńskaD. : Ion Exchange Technologies.  Maria Curie-SkłodowskaUniversityPoland.2012. 
6. SenhajiS. B., ElyahyaouiA. : Orient. J. Chem, 2015, 31 (3), 1601-1609.
   CrossRef
7. SenhajiS. B., ElyahyaouiA., SaidatiB.,  EssassiE. : IJSR, 2016, 5 (9) , 6-15.
8. Lijuan W.:  B.Sc.Victoria,   University of Technology Victoria, Australia, 1998.
9. RomanovaT.E.  , ShuvaevaO.V.: Chemical Speciation & Bioavailability, 2015,27(3), 139-145.
   CrossRef
10. RiceN.M., SmithM. R.: J. appl. Chern. Biotechnol, 1975, 25, 379-402.
11. StewartJ.A.  : Rare Earth Production and Characterization Studies , University of Tennessee, Knoxville , 2015.
12. NKPOZI, solvent extraction studies,University of Nigeria Nsukka, 2015.
13. Onghena,  BinnemansK.,  pure and industrial chemistry studies,University of Nigeria Nsukka, 2015.
14. Onghena and KoenBinnemans, Ind. Eng. Chem. Res, 2015, 54, 1887−1898.
    CrossRef
15. ElyayaouiA., BoulhassaS., GuillaumontR.: Journal of Radioanalytical and Nuclear Chemistry, 1990, 142(2), 403-415.
    CrossRef

---

This work is licensed under a Creative Commons Attribution 4.0 International License.