Liquid-liquid Extraction of Zinc by 3-methyl-quinoxaline-2-thione from Nitrate Medium

The extraction of zinc with 3-methyl-2 (1H)-quinoxalinethione ( ) LH , is investigated in nitrate solution as a function of pHrange 2-6, and analytical concentrations of − 3 NO anions ( ) 3 NO C and extractant ( ) LH C . The extraction mechanism with LH , is determined on the basis of the variation of the distribution coefficient D with pH, 3 NO C and LH C . As a result, the partition equilibrium involves complexing phenomenon, ion-pair formation, and 1H+ exchange reaction. The overall extraction constants are logK01.5 = 5.1, and logK11.5 = -0.6. The extraction complexes are found to be 2 3 2 ) (NO ) Zn(LH + l l , and ) (NO (LH) Zn(L) 3 1 l with l = 1 or 2. The hydrolysis reaction of Zn(II) is evaluated and it is found that it results in the formation of 2aq Zn(OH) and Zn(OH) + species, at pH >5.6. Zinc shows maximum percent extraction (75%) with the pH 4.7 at low organic ligand concentration (0.01M).


INTRODUCTION
Zinc is an important base metal involved in various applications in metallurgical, chemical and textile industries [1][2][3][4][5] .It is mainly recovered from primary sulphide concentrates.A part of zinc is also recovered from different secondary resources such as zinc ash, zinc dross, flue dusts of electric arc Key words: Liquid-liquid Extraction, zinc, 3-methyl-quinoxaline-2-thione, nitrate medium, pH range: 2-6.furnace and brass smelting, automobile shredder scrap, rayon industry sludge etc 7,37 .Wastewater effluents, solid industrial waste and sewage are considered as the main pathways of zinc to aquatic environment 3 .With rising environmental awareness, the presence of heavy metals, in nature is severely controlled 6 .Although zinc in micro amount is an essential oligo-element for a healthy body, zinc excess can be harmful, and causes zinc toxicity 5 .Therefore, zinc is listed indangerous substance, and contamination lists proposed respectively, by European Union Directive and the United States Environmental Protection Agency (USEPA) 6 .The World Health Organisation(WHO) and USEPA recommend respectively,3.0and 5.0 mg/L as maximal acceptable concentrations of zinc in drinking water 6 .The effluent standards are set at 2 mg/L, and 2.5 mg/g, respectively for wastewater 4 , andsolid waste disposal 6 .Reducing the discharge of this element into surface water becomes crucial operation 4,7 .For compliance with strict environmental regulation, developing process for the recovery of zinc from leach solutions, spent solution and effluents becomes increasingly important 8 .Several methods are used to achieve this purposesuch as chemical precipitation, liquid -liquid extraction, ion exchange, and adsorption.Among these methods, liquid-liquid extraction is the most promising process for the separation and recovery of metals from the complex and low metallic containing solutions, for being one of the most economical and practical processes 2,9,10 .Some conventional treatmenttechniques are less efficient, especially when zincconcentration effluentis relatively low 4 .
Recently, there has beenan increasing interest in the development ofchelating ion exchangers with the aminoacid groups to improve the extractability and selectivity of heavy metal ions 19 .Among the chelating ligands, quinoxalines have foundextensive applications in coordination chemistrydue to ease of preparation, and as they can form stable complexes with most transition metal ions 21,22,[38][39] .
To remove zinc more efficiently and economically requires more extensive information on the interaction of this element, with various extractant ligands, and inorganic anions.HNO 3 is suitable for a digestion of concentrated organic samples.The aim of the present study is to examine the extraction of zinc in NO 3 -medium, with 3-methyl-2(1H) quinoxaline-thione which is a bifunctionalextanctant groups.The advantage of this process is that can combine complexing phenomena, ion-pair formation and ion exchange reactions.

Procedure
The organic and aqueous solutions were equilibrated in 20 ml separator funnel with an organic/aqueous ratio of 1:1 (5ml organic solution and 5ml aqueous solution) which is shaken vigorously for 15min.Preliminary tests have shown that equilibrium of extraction is reached in less than 5min.The starting pH of the aqueous phases,is adjusted by addition of drops of LiOH/HClO 4 or KOH/HNO 3 , and measured before and after extraction test.After equilibration, the separator funnel is left standing for at least 10 min for completing phase separation.The aqueous zinc is then analyzed using Zincon (2-carboxy-2'-hydroxy-5'-sulfoformazylbenzene) spectrophotometry method 25,26 .Theextraction experiments are carried out in duplicate at room temperature, as a function of pH, and concentrations of nitrate ionand extractantdesignated thereafteras C NO3 and C LH , respectively.The zinc (II) concentration in the organic phase is obtained by mass balance.The distribution ratio, D, is calculated from the ratio of the equilibrium concentration of Zn(II) in the organic phase to that one in the aqueous phase.
The effect of the initial zinc concentration (C Zn ) on the extraction was studied in the range 0.001M-0.2M (Figure2).It was observed that the LH gives a better extraction for concentrations lower than 10 -3 M Zn (II).

Effect of equilibrium pH
The variation of logD versus pH obtained at C NO3 = 10 -2 M, for various analytical concentrations, LH C , of chelatant are given in As observed, extraction mechanism is less dependent on medium acidity for pH < 3.This process is ion-pair formation involving protonated amine group 20 .
For pH > 3, the extraction mechanism is typical of ion exchange.As a result, logD increases with pH to reach a maximum at pHmax and then decreases as pH continues to rise.Optimum pHmax is found at 4.7, in all explored C LH conditions.
When pH becomes higher than pHmax (4.7)the functional groups with strongly acidic cation exchanger "sulphonate -SO3H" reach its maximum sorption capacity 20 .
On the other hand the ternary amine in position 4 is protonated and sites of bonding to metal ions are limited en ternary amine in position 2 so the extraction is decreased 35 .At pH 5.6 and higher, zinc seems to exhibit a typical hydroxylated species extraction behavior 36 .Similar trend in removal efficiency with pH is reported on the extraction of Zn(II) and analogous divalent metals with organophosphorus extractants 12,27 .The hydrolysis equilibrium can be expressed as: The computation of logD 0 is performed on the basis of corresponding linear equation of logD =f(pH) (Figure 3.), obtained at C LH = 0.01M and pH >5. 6. Figure 4 shows that logy = f(pH) variations are linear with slope (h)varying in the range : 1.3 -1.4.As a result for pH range: 5.6-6, the both first and second hydrolysis reactions are involved, in these conditions.
Taking into consideration the molar fraction (x) of ZN(OH) + specie, the overall hydrolysis equilibrium could be written as K Hov is the overall hydrolysis constant.Based on theexperimental results (Figure 4-2), we obtain logK Hov = -6.55 and x= 0.61.This indicates that Zn(OH) + and Zn(OH) 2aq are the hydrolyzed species formed in this case,and their molar fraction are 61% and 39%, respectively.

Effect of extractant concentration
As shown from logD=f(pH) study, the extraction of Zn(II) with 3-methyl-2 (1H)- quinoxalinethioneisa complex process involving, more than one extraction reaction.In order to have a better understanding of theextraction mechanism,further information is required in this case.For this reason, experimentalanalysis of the logarithmic variation of D with C LH , is undertaken at pH valuesranging from2 to 6.0 (Figure 5.).As found,straight lines with slopesclose to 1.5 and 1.1are obtained in the pH ranges of 1.3 -5.0 and 5.6 -5.9, respectively.So, the stoichiometry coefficient, l= , is not integer as might be expected from theoretical single reaction.Thus, for pH< 5 the extraction of zinc in nitrate medium, by 3-methyl-quinoxaline-2-thione, results in a combination of at least two predominant extracted complexes with1 (l =1) and 2 (l =2) extractant ligands.While for pH ≤ 5.6 the 1:1 metal-ligand complex is the prevailing extracted specie.Previous studies report that the metal: extractant ligand ratio of 1:1, and 1:2 is obtained for the extraction mechanisms of Zn(II) with amine and/ ororganophosphoruschelatants 15,28 .

Effect of nitrate concentration (C NO3 )
The effect of nitrate anion added as NaNO 3 , is examined in 0.01M nitric acid solution (pH=2), for C LH =0.01M and C NO3 ranging from 0.01 to 0.2M.Obtained results are reported in Figure 6.

Extraction reactions of Zn 2+ with 3-methylquinoxaline-2-thione.
It has been reported from previous study that zinc is extracted into chloroform with shift base extractants asboth uncharged chelate complexes, and ion-pairs of charged species with a counter anionin aqueous solution 27 .Taking into account the protonation of amino group at pH <3 20,30 , and neglecting the complexing effect of In the above equations, the onlined entities refer to organic phase andn= 0; 1; 2; etc.The symbol H -n stands both for hydrogen atoms (n< 0) and for OH group (n> 0).

The corresponding D expression being
For ion-pair equilibrium (8), the extraction constantis:  14) and D is given by: Taking into account the approximations given above, logD can be expressed as: It appears that the distribution coefficient must be obtained in all cases, according to the general equation: m .This result is in consistent with that of logD=f(logC HL ) (Figure 5.) obtained at pHd" 5, showing l=1.5 which is comparable to m=1.4.The origin ordinate of obtained straight line which corresponds, in these conditions, to ion-pair mechanism,leads to logK 01.5 =7.42.
In addition, we have deduced from the experimental data logD=f(logC HL ) (Figure 5), obtained at different pH values the variations of log A=f(pH) at C NO3 =10 -2 M (Figure 7).Straight lines having slopes of 0 and 1 are obtained respectively, at pH < 3 and 3 ≤ pH ≤ 4.7.These results are in agreement with those of Figure 2, indicating that Zn(II) is extracted with 3-methyl-quinoxaline-2thione, essentially, by ion-pair formation in acidic medium (pH < 3), and by 1H + exchangereaction, in low acidic medium (3 ≤ pH ≤ 5).As discussed above, both ion-pair and ionic exchange reactions rise from a combination of at least two predominant reactions with l = 1 and 2.The apparent extraction constants, K n1.5, could be determined by experiment, taking into account that m value.isof 1.4and 0, respectively for pH < 3 and 3 ≤ pH ≤ 5.These constants are evaluated from intercepts of the logA=f(pH) straight lines allowing us to obtainlogK 01.5 = 5.1, and logK 11.5 = -0.6.As found,the log D=f( Taking into consideration the molar fraction (x) of extraction mechanism with 1 chelatant molecule, the overall partition equilibrium involving ion-pair formation, issummarized by the following reaction: Knowing that the positively charged species could be neutralizedby

CONCLUSION
Extraction of zinc from nitrate medium is carried out using 3-methyl-2 (1H)-quinoxalinethione in toluene.The extraction equilibrium of Zn(II) is examined as a function of pH, nitrate and chelate concentrations.The stoechiometryand the stability constants of the extracted Zn 2+ species is postulated based on slope analysis method.Obtained results show that the extraction reaction involves both ion-pair formation and 1H + exchange mechanism.The stoechiometry of the prevailing complexes are found to be 1:1 and 1:2 metal-ligand ratios, in all cases.The molar fraction of these complexes isevaluated.It is foundthat extracted species include 50% of each of identified complexes in all explored conditions.The formation of hydroxylated species is also considered.The formation of Zn(OH) + and Zn(OH) 2aq is shown to take place for pH >5.6, their molar fraction are 61% and 39%, respectively.In addition, extraction of zinc by 3-methyl-2 (1H)quinoxaline-2-thione increases with increasing

.
The extraction mechanism with LH , is determined on the basis of the variation of the distribution coefficient D with pH, 3 NO C and LH C .As a result, the partition equilibrium involves complexing phenomenon, ion-pair formation, and 1H + exchange reaction.The overall extraction constants are logK 01.5 = 5.1, and logK 11.5 = -0.6.The extraction complexes are found to be 1 or 2. The hydrolysis reaction of Zn(II) is evaluated and it is found that it results in the formation of 2aq Zn(OH) and Zn(OH) + species, at pH >5.6.Zinc shows maximum percent extraction (75%) with the pH 4.7 at low organic ligand concentration (0.01M).

Figure 3 .
Figure 3. Similar variations with "S" shape are reported for the extraction of zinc and other divalent metals with some acidic extractants 27 .Distinct pH regions with different slopes characterize the various extracted species, and suggest the change in extraction mechanism.

5 )
a maximum of 97% at C NO3 =0.07M.The plots of logD=f(log exhibits straight lineswith differentslopes, indicating that extraction reaction involves the formation of complexes with various molar ratio, as discussed thereafter.

Fig. 7 :
Fig. 7: Variations of logA = f(pH) For ionic exchange mechanism (9) the extraction constant, K nl is n=0 for pH< 3, and m= 0and n'" 0 in the pH range of 3≤ pH ≤ 5the distribution data on the basis of the general extraction equilibrium allows us to define the nature of the extracted complexes formed according to:

≤
-1.4 and pH ≤ 3 have the same ordinate at the origins,confirming also that m=land n=0, because we have extraction reaction of the predominant Zn 2+ specie by 3-methyl-quinoxaline-2-thione symbolizedas LH in following, combinesion-pair formation and ionic exchange mechanisms.These reactionsare expressed by the general equations