Pre-Concentration of Cd ( II ) , Cr ( III ) , Cu ( II ) and Ni ( II ) on a Column Packed with Free Carboxymethylcellulose ( CMCH )

Car bo xi me til ce lu lo se (de ri va do da ce lu lo se) na for ma áci da (CMCH), foi uti li za da como fase es ta ci o ná ria em uma co lu na cro ma to grá fi ca na pré-concentração dos cá ti ons Cd(II), Cr(III), Cu(II) e Ni(II). As con di ções óti mas de pré-concentração são des cri tas (com pri men to da co lu na: 16 cm, diâ me tro in ter no: 0,80 cm, al tu ra da fase es ta ci o ná ria: 12 cm, ve lo ci da de de flu xo: 1.5 mL min). Ve ri fi cou-se que uti li zan do-se 5.0 mL de áci do clo rí dri co 1.0 mol L, tem-se uma re cu pe ra ção ma i or que 95% dos cá ti ons ad sor vi dos na co lu na, con se guin do-se um fa tor de en ri que ci men to de 10 ve zes na con cen tra ção dos cá ti ons Cd(II), Cr(III), Cu(II) e Ni(II), para um vo lu me de 50 mL de so lu ção mis ta 0.50 mg L.


Intro duc ti on
The di rect de ter mi na ti on of tra ce me tals in aque ous solu ti on is dif fi cult and in many ins tan ces it is ne ces sary to use a pre-concentration pro ce du re be fo re the quan tif ying pro ce du re in or der to in cre a se the sen si ti vity of the analy tical met hod 1,2 .
In the last few ye ars ad sorp ti on of me tal ca ti ons on so lid sup ports (mo di fi ed or not) has been stu di ed with spe ci al atten ti on to the pre-concentration of me tals from aque ous or non aque ous me dia [3][4][5][6] .Cel lu lo se pre sents in te res ting proper ti es as a ca ti on ad sor bent ma te ri al, as its sur fa ce can be mo di fi ed by sim ple, low cost che mi cal pro ces ses, per mitting an in cre a se in the ion ex chan ge ca pa city of the cel lu lose 7,8 .This pa per de als with the ap pli ca ti on of free car boxymethylcel lu lo se as a chro ma to grap hic co lumn sta ti o nary pha se, and its uti li za ti on for the ad sorp ti on of Cd(II), Cr(III), Cu(II) and Ni(II) for pre-concentrating the se metals from aque ous so lu ti ons.

Re a gents
The so di um salt of car boxy methylcel lu lo se (Bo nâ mia S/A), was tre a ted with an et ha nol/wa ter mix tu re (80:20 v/v) and sub se quently with a hydroch lo ric acid so lu ti on (20% v/v), to ob ta in the free car boxy methylcel lu lo se (CMCH), which pre sen ted a spe ci fic area of 0.85 ± 0.08 m 2 g -1 (BET) 9 met hod using a MICROMERITICS ASAP-200   Ar ti cle equip ment) with 0.14 ± 0.8 mmol g -1 of car boxy lic groups pre sent on its struc tu re (con duc to me tric ti tra ti on with 0.10 mol L -1 NaOH so lu ti on) 10 .
The hydra ted chlo ri de salts of se ve ral ca ti ons and ot her re a gents were all of p.a. gra de or bet ter.

Adsorp ti on ex pe ri ments
Opti mi sa ti on of ad sorp ti on time The time re qui red for the so lid-liquid system to at ta in the ad sorp ti on equi li bri um was de ter mi ned by pla cing 50 mL of 5.00 x 10 -3 mol L -1 aque ous so lu ti ons of the me tal cati ons in va ri ous flasks and sha king with 0.10 g of CMCH.
At dif fe rent time in ter vals, ali quots of the su per na tant from each flask were ta ken and the me tal ca ti on de ter mi ned by com ple xo me tric ti tra ti on with Na 2 EDTA.
The amount of the me tal ca ti on ( N f ) ad sor bed by the solid pha se was cal cu la ted using the fol lo wing equa ti on: N f = (N i -N s )/m, whe re N i is the ini ti al amount of the me tal ca ti on (mmol), N s is the amount of the me tal ca ti on (mmol) in the su per na tant af ter equi li bri um has been achi e ved and m is the mass in grams of CMCH.

Effect of var ying the pH
The ef fect of pH on the ad sorp ti on of Cd(II), Cr(III), Cu(II) and Ni(II) was eva lu a ted by var ying the pH over range of 1.0-5.0 with of 0.10 mol L -1 hydroch lo ric acid.

Adsorp ti on isot herms
The ad sorp ti on isot herms were plot ted using the batch met hod 11 .Dif fe rent ali quots of a so lu ti on of me tal ca tions, 5.00 x 10 -3 mol L -1 , were di lu ted in vo lu me tric flasks, pla ced in ther mos ta ted bot tles at 298 K, and sha ken for 60 min with 0.l g of CMCH.The su per na tant from each flask was se pa ra ted, then the amount of me tal ca ti on ad sor bed was de ter mi ned and cal cu la ted using the equa ti on gi ven abo ve.

Effect of ani ons pre sent
The in flu en ce of ani ons on the ad sorp ti on of Cd(II), Cr(III), Cu(II) and Ni(II) was eva lu a ted by ad di ti on of 5 mL ali quots of 1.00 mol L -1 NaCl, NaNO 3 or Na 2 SO 4 to the solu ti on of me tal ca ti ons, in the same con di ti ons pre vi ously des cri bed.

Pre-concentration of me tal ca ti ons
This ex pe ri ment was car ri ed out using a 16 x 0.8 cm i.d.glass co lumn pac ked with 2 g of CMCH (sta ti o nary pha se with 12 cm he ight).Ini ti ally the co lumn was was hed with dis til led wa ter and then 50 mL of an aque ous so lu ti on conta i ning 0.50 mg L -1 of Cd(II), Cr(III), Cu(II) and Ni(II) ca tions (so lu ti on pre pa red by di lu ti on of 5 x 10 -3 mol L -1 so lu ti ons of the se ca ti ons) were per co la ted through the column with a flow-rate of 1.5 mL min -1 .The co lumn was then was hed with 50 mL of dis til led wa ter.Me tal ca ti ons were elu ted using 5, 10, 15, 20 and 25 mL of 0.10 mol L -1 hydroch lo ric acid and de ter mi ned di rectly in this so lu ti on by fla me AAS, using a VARIAN-INTRALAB AA-1475 spec trop ho to me ter.The re so nan ce li nes cor res pon ding to the wave lengths of gre a test sen si ti vity for the ca ti ons Cd(II), Cr(III), Cu(II) and Ni(II) were used.The ex pe riment was car ri ed out in tri pli ca te in or der to de ter mi ne the pre ci si on of the met hod.
Using in the same con di ti ons, ex pe ri ments were car ri ed out by using elu ti on with hydroch lo ric acid at 0.25, 0.5 and 1.0 mol L -1 .

Adsorp ti on ex pe ri ments
Opti mi sa ti on of ad sorp ti on time An im por tant as pect is the rate at which the so lid ion exchan ger ad sorbs me tal ca ti ons from the so lu ti on and that equi li bri um con di ti ons are at ta i ned.Fig. 1 shows plots of N f ver sus time for Cd(II), Cr(III), Cu(II) and Ni(II).For the se and all re ma i ning me tal ca ti ons, the time ne ces sary for the systems to re ach equi li bri um was about 50 min.

Effect of var ying the pH
Fi gu re 2 shows that the ad sorp ti on of me tal ca ti ons decre a ses with the aci dity of the me di um and that the na tu ral pH of the so lu ti ons (pH = 5.0) is op ti mum for ad sorp ti on of the se me tal ca ti ons 11 .Expe ri ments at pH va lu es abo ve the na tu ral pH of the so lu ti ons of the me tal ca ti ons (pH = 5.0) were not car ri ed out be ca u se of the pos si bi lity of hydroly tic pro ces ses 12 .

Adsorp ti on isot herms
The ad sorp ti on isot herms are shown in Fig. 3.In the se isot herms the term N f re fers to the num ber of mmol of metal ca ti ons ad sor bed per gram of CMCH, the term C re pre - sents the fi nal con cen tra ti on (in mmol) of the so lu ti ons con ta i ning me tals af ter the ad sorp ti on ba lan ce is re a ched.After analy zing the ad sorp ti on isot herms, the fol lo wing ma xi mum ca pa city of CMCH ad sorp ti on (N f max in mmol g -1 ) were found: Cd(II) = 0.098; Cr(III) = 0.120; Cu(II) = 0.116; Ni(II) = 0.110.
Com pa ring the va lu es of N f max ob ta i ned with the number of func ti o nal groups (car boxy la te in this case) de ter mined for CMCH, the ma xi mum quan tity of me tals ad sor bed was ve ri fi ed as clo se to the num ber of CMCH func ti o nal groups (0.14 mmol g -1 ).A de cre a se in the pH of the so lu tions of the me tal ca ti ons in di ca tes that af ter the ad sorp ti on ex pe ri ments the ca ti ons Cd(II), Cr(III), Cu(II) and Ni(II) might be at ta ched to the car boxy la te groups of CMCH by ion ex chan ge pro cess.

Effect of the ani ons pre sent
The in flu en ce of the ani ons pre sent on the pro cess of ad sorp ti on is re la ted to the ir ten dency to as so ci a te with a me tal ca ti on in the sol vent me di um.Ho we ver, Fig. 4 shows that this in flu en ce is rat her small for the con cen tra ti ons of ani ons in the stu di ed ran ge (0.10 mol L -1 of Cl -, NO 3 -and SO 4 2-).The ca ti on-anion in te rac ti on in so lu ti on is a fac tor that could de cre a se the trans fer of the me tal ca ti on from the so lu ti on to the so lid pha se, but this ef fect is of lit tle sig ni fican ce 13 .

Pre-concentration of the me tal ca ti ons
The re sults of the pre-concentration and re co very of the me tal ca ti ons using the co lumn met hod are shown in Ta ble 1.
It can be ob ser ved from Ta ble 1 that with 15-20 mL of 0.10 mol L -1 hydroch lo ric acid, it is pos si ble to elu te al most 100% of the Cd(II) and Cu(II) ca ti ons.On the con trary this was not pos si ble for Cr(III) and Ni(II), who se elu ti on percen ta ge are 66.40% and 93.60%, res pec ti vely, when using 25.0 mL of 0.10 mol L -1 HCl.Using 5.0 mL of HCl with con cen tra ti ons of 0.25 and 0.50 mol L -1 , the re co very is around 100% for Cd(II), Cu(II) and Ni(II) ca ti ons.Me anwhi le, the re cu pe ra ti on of 98% of Cr(III) is pos si ble only when using 25.0 mL of 0.50 mol L -1 HCl, which is un fa vora ble for the pre-concentration of this ca ti on in com pa ri son with the ot her three.Ho we ver ex pe ri ments with 5.0 mL of 1.0 mol L -1 hydroch lo ric acid in di ca te that it is pos si ble to re cu pe ra te al most 100% of all me tal ca ti ons un der con si dera ti on.A ten-fold en rich ment is ob ta i ned.Also com pa ring the to tal quan tity of ad sor bed ca ti ons pre sent on co lumns with CMCH (0.023 mmol) and the ir ma xi mum ca pa city of ad sorp ti on (0.28 mmol g -1 ) for a column with 2.0 g of CMCH, the to tal amount of me tal ca tions is not more than 9% of the ma xi mum ad sorp ti on ca pa city of CMCH.

Con clu si on
CMCH shows high sta bi lity when used for me tal ca ti on pre-concentration from aque ous so lu ti ons, in all pro ba bi lity due to the hydrop hi lic cha rac ter of its cel lu lo se com po nent.
Its usa ge may be also of va lue if em plo yed as the sta ti o nary pha se of a chro ma to grap hic co lumn.In this case ca ti on se para ti on is car ri ed out with dif fe rent elu ting agents.
Ta ble 1. Re co very per cen ta ge of me tal ca ti ons using the co lumn met hod and hydroch lo ric acid so lu ti ons as elu ants.

Fi gu re 1 .
Plots of N f vs. con tact time at 298 K for Cd(II), Cr(III), Cu(II) and Ni(II).

Fi gu re 4 .Fi gu re 2 .Fi gu re 3 .
Influ en ce of ani ons on the ad sorp ti on of Cd(II), Cr(III), Cu(II) and Ni(II) ca ti ons in aque ous so lu ti on at 298 K. Effect of var ying the pH on the ad sorp ti ons of Cd(II), Cr(III), Cu(II) and Ni(II) ca ti ons in aque ous so lu ti ons at 298 K. Adsorp ti on isot herms of Cd(II), Cr(III), Cu(II) and Ni(II) ca ti ons in aque ous so lu ti ons at 298 K.