lnuian Journ al or Chemistry Vol. 4);\, Se pt embe r 2006. pp. 20 11 -20 16

Surface modification of granular activated carbon by nitric acid for the enhancement of copper adsorption

D Sat;tpath y"·•:• & G S Natarajan h

" Reg iona l Resea1·ch Laborat ory. 13 huh;lllcsw;Jr 7) I 0 I:\. Oriss;t. Jnd i;1 h Pri yatLl rs hini Inst itu te of En gineer ing anJ Tec hn ology. N~1gpur 440 0 I 9. lnuia

Emai l: dsa t;qxllhy@rrlbhu.n:s .in: J eha I 997 @yahoo. com

Recci1·ed 28 OctoiJN 2005: rt'l'ised II .!ul1· 200()

ConcentratcJ nitric acid. a ve ry powerful ox idizing ;tgent. has been uscJ I'm modifyin g su1·1';1LT oi' l'O ill lllCrci;d g1·;1de carbon. Filt raso rh-400. Scavenging ol· co pper ions l'rom ;1queous so luti ons onto both raw ;1s well as mod ified g1·anul;11· ;1c ti va ted carbo n has bee n carri ed out. Eq ui lib1·iun1 resu lts show th;tt smi'ace modi l.ic1tin n increases the copper ion auso rpti on by 12Yi! in co mpari son to th ;ll o r raw gran ular ;1c tivat cd Girbon. FTIR spcc t1·;1 suggest th;1t oxidation o l· gr;1nul;11· ac ti va tcJ carbon by nit ric ac id results in the rorm;1ti on o r sever;d fun l'l io1d groups at the c trhn11 surface. The p i I studies I'm hmh raw ;tnd ox iJ iseJ gran ul ;1 1· ;Jc ti v~t t eJ carbon show th;1t higher initi;d solution p H resul ts in hi gher coppn inn ;1Lisorptinn . The surface modificatio n wi th nitri c ac id not onl y en h ~1 nces th e copper ausorptio n but also hel ps in rege neration or carbon .

!PC Code: Int. Cl. ' 130 I 120/20: CO I 133 1/08

Current methods fm wastewater treatment inc lude prec ipitati on, coagulati on, notati on, sedimentati on. filtrati on, membrane process, electrochemical techniques . ion exchange, bi o log ical processes etc. A dsorpti on process w ith acti vated carbon has attracted attention bec au se o f it s effecti veness in removal o f di sso l ved heavy metal ions at trace quantiti es. Recentl y clfort s ha ve been made to deve lop new ad smbcnl s and improve cx tsltng ad sorbcnls li ke granular acti vated carbon and granular acti vated carbon modified w ith complex ing agenl sJ. 2

•

iron coaled ~ranular act ivated carbon·', modifi ed chit osan .J and t~ itric acid ox idation o f carbon fiber etc. to minimi se cos t-eflcc ti vcncss o f the process . or these. activated carbon ex hibit good perfmmancc towards Cu2

+ adsorpt ion due to it 's high surface area and the case o f modifi cati on of it's surface by ox idation.

St11·1'ace modificati on o l' acti vated carbon ts recogni zed as an attracti ve approach for the enhance ment o f heavy metal removal. Different heavy metal s ha ve vary ing affinity towards different surface functi onal groups5 It is well known that acti vated carbon surface can be modified by using organic complcx ing agents fo r bcllcr removal o f heavy metal s. Thi s is due to the tendency o f the acti vated carbon to ad smb organi c compounds effecti ve ly f ro m

aqueous so luti ons. Several such cases ha ve been reported in the literaturer'7 However. to impmve the surface reacti v it y o f the acti vated carbon. surl'ace treatment is essential. The vari ous surLtcc treatmen ts arc kn own to initiall y create more steps and pi ts on the surfaccX.') The reacti v it y o f the acti vated carbon in

an ox idi sing atmosphere is grea tl y dependent on in its surface structure.

M etal ions which arc bas ica ll y in the di ssolved state arc tox ic and usc f'ul f'or their commercial value when recovered. M etal s in so lut ion arc dependent on the so lubilit y or their salts and their stabilit y in water. In the present in vesti gati on. coppn metal has been chosen !'or stud y and its sca ve nging on both ra w and modi l'iccl granular acti vated carbon has been in vestigated.

The high concentration o r copper is severely tox ic f. I I Ill H . to mos t o t1c green pants . ypcrcuprc tn ta may

refl ect excess i ve intake o r copper fro m food and water as it may be associat ed w ith sc.ve r~tl ~t c utc and chronic infecti ons like leuke mi ~t , Hodgkin ' s di sease. severe anacmta. hacmoc hromatos is. nwoc ardia l infecti on and hypcrparathy roidi sm 11

. Conccn;rati on ol' copper is very high in Wil son's d iseasc 12

. Copper polluti on can ari se from copper mining and smelting. brass manufacture. petro leum rd incrics. elec troplating and the excess ive use o f Cu-basccl

I . I I' agroc 1emtca s · .

20 12 l i\' Dl/\ N J CI IEM .. SEC/\. SU'TEMf3ER 2006

The main objecti ve o f thi s stud y is to compare the ~td s orption performance of n1tnc ~1cid-modificd

acti vated ca rbon with the unmodified activated ca rbon. Also. the work was carried out to develop a cost-e iTec ti vc process fo r copper ad sorption and al so to minimize the Ltnd di sposa l problem due to rege nerati on of was te carbon.

i\Iatcrials and Methods A di gital spcc tropl1otomctcr (Type- I C16. Sys tronic s

In dia Ltd. ) with mat ched ce ll s o f I em opti cal path length wa s empl oyed fm all absmbancc measurement s. A mec hanical shaker (Re mi. mode l RS-24. Rcmi In strument Ltd .. Mumbai ) prov ided with a timer was used for agitating the so luti ons in ex perimental bottl es . A digital ;JH meter (E ii co. Hyderabad ) was used for maintaining the pH of the so luti on as and when required. All glass wa rcs were 'A' c la ss and we re was hed thoroughl y and rin sed with di still ed water and dri ed in an air ove n before usc.

All reagent s and chemical s used we re or A.R. grade (E Mcrck/S D ri nc) . In the prese nt work . Fil trasorb-400 carbon (M/s Ca lgon Corporation. Pitt sburgh. U.S .A) was used. Cupric chl oride di hydrate from E Merck India Ltd. was used for the preparati on o r copper diet hy I d it hi oc arb a mate sulphate were used in copper and nitric ac id o r the carbon.

metal ion soluti on. Sodium and anh ydrou s sodium

the colorimetri c anal ys is or was used lor the modificati on

Standardisation and charal'llTisalion or Filtrasorh--WO

In the present work. granular acti vated carbon. viz .. Filtrasorb-400. was empl oyed and was processed as follows before usc in the ex periments. The carbon wa s first stirred in boiled di stilled water carefully without an y attrition followed by additi on of cold di stilled wate r. The process was continued until all the fin e ly powdered activated ca rbon was removed and the supernatant liquid became almost colourl ess . It was dried in an air oven for about 4 hours at a temperature of I 08 ± :zoe. It was then transferred to a desiccator containing fu sed calcium chloride where it was allowed to eq uilibrate for almost a fortni ght and is denoted by RAC. The F-400 (RAC) was characteri sed by FT-IR spec tra and also subj ec ted to scanning e lec tron microscope (SEM) studies in order to ascertain the type of pores existing. The Fr-IR spectra were recorded on a Nicolet Magna-IR550 spectrometer seri es II while the SEM studies were carried out on a SEM Cambridoe b

Stereoscan 5250 MK Ill microscope.

l'r~paration or nitrk aci d modiried granular ;u:tiYated carbon

The carbon surface was modifi ed \\ith nitri c acid as fo ll ows: The RAC was treated with cone. nitri c ac id at a rati o o r 10 g o f ac ti vated carb ::J n to 100 ml or nitric acid. in a co ni cal l'l ask with a t: ldSS stopper and was kept ove rni ght at room temperature. II wa s boiled the next da y for about half an hou r by ~tdd in g a littl e di stilled wa ter and then cooled. The rt:s idual c~trbon

was se parated by decantati on and ''~! s h ed several times to rid it o f all adherin g acid. T he modifi ed ca rbon (MAC I) was then ag itated " ith copper ion so luti ons fm equilibrium stud y. Art er equilibrium. it was filt e red oil was hed and again subjected to nitri c acid treatment as desc ribed above and added to copper ion soluti on for an other sci of eq uilibriu m stud y. The process was continued for at least three eye les. The mod i li ed ca rbons we re denoted as M f\ C2 ~111d MA C:~ res pccti ve l y.

!'reparation or the solution or ropper ions and it.-; estimation

Standard stock copper( ll ) soluti on was prcp<tred with hydrated cupric ch loride ( E Merck) and titrated again st a standard ().()I M EDTA so luti on. Working standard so luti ons we re prepared oy approprie~t e

diluti on of stock so luti on. Working standard s o f copper( ll ) so luti on in the conce ntrati on ran ue o f 10·4 M was used fm pl ottin g ca libra tion plot s~ The amount of coppcr(ll ) in so luti on was determ ined

I . . II 14 co onmctn ca y .

Adsorption equilibrium studies

All ad sorpti on eq uilibrium experime 1ts were ca rri ed out at fi xed temperature at 25 ± I °C u.> ing a thermostat bat h in batches of fout· to six units at a time. Each batch consisted of a one liter round bottom 8oros il fla sk and a motor impell er assembl y. which enabled vigorous stirring of the solid-liquid sys tem by " two-bladed Te flon stirrer. Actua l experimental run s in volving adsorpti on eq uilibrium studies con :; is ted of lirst wei~hin !2 the req uired quantity of carbon in a wc inhin o

~ ~ b b

bott le and transferring the same to lhc experimental fla sk. The ex perimental soluti on (200 ml) was then ca refull y added. The stirrer was then introd uced and the whole set-up was st irred at th ~ desired bath temperature (25 ±I °C) until equilibri un 1 was attained. Aliquots of 5 ml of soluti on were then ·..v ithdra wn from the flask and anal ysed colori metricall y for copper( II ) concentration. Adsorpti on isotherms were studi ed ~ I t

pH= 5 for the initi a l copper(II) solution.

S;\T;\ I' t\TI IY & i\' ,\T,\1{ ;\ .J ;\ N SU RF;\C I: MOD WI C;\ T ION OF GR ;\f\' LJ L,\1 { ,\CT IV;\ TU) Ct\R ilOi\' 13Y 11 ,\JO , 2() I :1

Kinl'tics ol' ;Hisorplion ol' mppl:r(l l )

Kinetic stu di es o r coppcr( ll ) up1:1~ c hy gyad cs ul CAC w:1 s carri ed nut in :1 nne lite r lb s ~ co nt aining GAC o r give n we ight. The copper(!! ) so lu ti on (-100 ml ) o r ~nown Strength \\'; IS int mduced C:t rci"uiJ y into the ex perimental lb s ~ whi ch w:1s then he ld in the the rmos tat bath at 2) ± I °C. The so luti on in the lb s ~

w:ts stirred at 1000 RPM. The time o r :tdditi on o r c u·bon was noted as time 1 = 0. T ilerc:il"tcr. aliqu ots (2 .) ml ) or tile ex periment al copper( II ) so luti on were \\'ithdrawn rrom the lb s ~ at rcg uJ:1r int e l·\·aJ s by mome nt:t ril y in terrupting the stirrer. The rirst !'our rc:tdin gs we re t : 1~ c n at I) mi nu te time inte rv:il s. the nex t t\\'O readin gs \\'ere ta~ c n :1t :10 minu te intnva ls and the rin:il t\\'0 rc:tdings \\'C re t:I ~C n :It :1 ti me inte rval o l' one hour. The co nce ntrati on o r co pper ( II ) at start. C.,. and ~ 11 time t . C,. in milli grams/! in tile sample \\' :I s es tim:1ted us1ng the sta ndard ccilihr:t ti on curve .

Results and Disc uss io n Cil;lrarleriz;llion ol' F--WO carbon and ox idized 1-'-400 carbon

f-i gure I shows the SEM mi crog r:tph s n r acti v; tt ed ca rbon s;tmp les hcl'me ;111d art e r nit 1ic ac id mod iri ct ti on. The scannin g electron 1111 croscopc images o r the GAC show l;tyc rcd. loose ly packed structure \\' ith m;~n y cav ities. cr:1c~ s. irregular protru sions ~ 1 nd wi dely di spersed pores s ince the S ~lmpl c is bituminous coa l based. Compa ri son o r these minog r;tp hs shows that a l'cw can;il s arc rormed CO nnecting to mi croporcs in the case o r nitri c ;tc id modified c trbon. These act as a ro ute ror the met:il ions to ent er the mi cropmes. Hence . me t:~! ad sorpt ion is CO nsiderab ly hi gher in the Cii Se o r nitri c ac id mod ifi ed carbon as c o 1 np~t rcd to r ~ 1 w activated carbon.

The ca ti on and the ~In i o n cxc h ~ tn gc capaciti es o r carbon mat eri :il depend m:tin ly on the prese nce or surface ru ncti onal gmu ps or co mplexcs 15 Oxidati on is an c llec ti vc met hod ror int mduc ing oxyge n COntain ing runcti ona J groups SUCh as hydi'OX)' (-0 1-1 ). ca rboxy (-COOI-I ) and ca rbonyl (>C=O ) groups on the surface o l' acti vat ed c trbon1r' _ The prese nce o f such groups ma y be establ ishcd by IR spec troscopy. Figure 2 depicts the FT- IR spectra ror the oxidi sed and ra w Qr;tnul ;tr acti vat ed ca rbon in which the pea~ s at I )77 c m·1 and I 195 cm·1 may he ass igned to carbon s ~ e l e t o n . The peaks at 17 12 cm-1

correspond to the >C=O J'un cti onal group . T he sign;il at 1385 cm·1 ma y be attributed to 0 -1-1 bending de formati on in ca rboxy li c ac id. T hi s shows that mainl y ca rbox ylic groups arc rormecl on the ca rbon

l :ig. 1- Sc;lllllillg clec irull lllinugr;qJ!J, lli' L'; 1rhu11 >lll'i':IL'L' . l (a) 1 ~ ;\C :111d (h) f\ 1,\CII .

I S3

18

16

" 1)

:o

0.8

06

li 8J.I JOOO 1000 llOO

V. Clll' J

!Ot\l

J#' .1

/f>HJ C !I 'iJ

Pig . 2 - I·T IR ' pcc lr:1 ol. nil ric :1cid modii'icd F-400 G;\C :111d r:111

F-400 Gi\C. I i\ : F--100 Gi\C ;ll'tl-r nitri c aci d trc:ll llll'llt. :~nd. 13 : raw F--100 Gi\Cj.

2014 INDIAN J CI-IEM .. SEC A. SEPTEMB ER 2006

surface rather than hydmx yl -01-1 groups. These functional groups att ract more copper(II) ions, thereby increasing the copper ad sorpti on.

Adsorption isot herms

The ad sorption isotherm data for the removal of Cu2+ ions by raw as well as modifi ed granular activated carbon up to third cyc les were fitted to the Langmuir and Freundlich models and can be ex pressed in the linea r eq uati ons ( I) and (2) res pec ti ve ly.

- Q II 1/q ,. - 1/Q .b.C.. + II Q

log q,. = log K + B log C..

... ( I )

... (2)

Where C.. is the equi librium concentration of Cu2+ ion (mg/1), q,. is the amount of Cu2+ ion ad sorbed at equilibri um in mg/g. Q0 is the amount adsorbed corresponding to the formati on of a monolaye r on the ad sorbent, b is the Langmuir constant , K and B are Freundlic h constants. From Eq . l above, a plot of 1/q,. versus 1/C.. should be linea r if Langmuir adsorption were operati ve. permitt in g calcu lation of Q0

Howeve r, it mu st be cautioned that the Langmuir equati on may not always be strict ly val id over the range of concentrati ons used in the present work and onl y a mathematica l ana lysis o f the ex perimenta l data would substantiate thi s point. Simi larl y, from Eq. 2. a pl ot of log C,. versus log q,. should be linea r bringing out the va lidity of the Freundlich equat ion over a ran ge of concentrati on. The corre lation coe ffic ient. (R

2) and both the Freund lich and Langmuir constants

are give n in Table I.

Fi gure 3 represents the ad sorpti on isotherm for raw as we ll as nitri c ac id modifi ed granular acti va ted ca rbon eve n up to third cycles. As see n from the ad sorption isotherm , there is a gradual increase in the adsorption capacity of the carbon towa rd s Cue+ ions after each nitric acid ox idation step and the {j 111ar for MAC 2 is a lmost equal to (j11111, for MAC 3 (Fig. 3). The q""" value for RAC, MA C I, MA C 2 and MAC 3 foll ow the order: RA C < MAC I < MAC 2::; MAC 3. These res ults al so support the view that the ad sorpti on capacity of the act iva ted ca rbon is improved by ox idati on. Tabl e I c learly shows th at the eq uili brium adsorption data for different Cue+ ions is best fitted to the Langmuir mode l. The rJ111ar va lue for MAC 3 was found to be 9.24 mg/g as compared to the (j111, 11 va lue for RAC which is 4. 14 mg/g. Thi s impli es that there is an increase in copper ad sorpti on by 125°A,.

negenna tion or granu lar ac tivated rarhon

The most inte restin g part of the prese nt stud y was the regenerati on of the granular activated carbon. The nitric ac id modified granular acti vated carbon. when ag itated with copper( ll ) soluti on till equilibriu m. showed an increase in the ad sorpti on capacity o r carbon. The same carbon was again ox idi sed vvith nitr ic acid when the copper ions we r·e brought into the so lution in form of so luble nitrates and becames ready for fmth er ex peri mentation . The process was repeated up to three cyc les and each time there was an increase in the adsorpti on ca pacity of the ca rbon. However, the Cue+ uptake (q111n, ) for second and third cyc les arc in very close proximity. The ex periment not only helps in the regenerati on of carbon but al so inc reases the ad sorpti on ca pac ity of carbon. Now-a-da ys importance is give n to the regenerati on of granul ar activated carbon becau se of the prob lem of spent carbon . Moreover rege nerati on of the spent carbon would make its use economical and prov ide a so lut ion to the land di sposal problem.

Adsor ption ki neti cs

Figure 4 represents the copper( II ) ad sorpti on kineti cs on ra w and mod ified granular acti vated carbon up to three cyc les. Figme 4 shows that ad sorpti on initiall y fo ll ows a fast rapid step followed

Table I - Freu ndli ch and Langmuir Isotherm constant s !"or the studied systc rns

Fr·cund li ch Isotherm L tn!!muir Isotherm

l< :c K f"\ ' N- Q() h

RAC 0.990) 0.4 18 1 0.2788 0.982) 4.14:19 0.4<) MAC I 0.92 19 0 )9 11 0 . .\468 0.9928 7.4:17 1 0. 8 1 M AC 2 0.92:19 0 6797 0 . .\049 0.9942 7.7942 1.98 MAC 3 0 937.1 0.7 199 0 . .\847 0.98:18 9.242 1 1.68

9 0000

80000 • • • 7 0000 • • • • • • 60000 • • • • • • • • • • 50000 • "' • r:T • 4 0000 ~. xx X

I • X X 30000 X X

2 0000 •.tx X X

1 0000 j ooooo l

0 0000 1 0000 2.0000 3 0000 4 0000 5 0000 6 0000

Ce

Fig. 3 - Adsorption iso therm l·or raw and rnodi l"icd granular

act ivated carbon s !"or diiTercnt co ncc n tr~l!ion o l" copper ion s. [ • .

MACJ: • · MAC2: .A. . M!\( "1: x. RAC I

SAT;\PATH Y & NAT A RAJ AN: SU RFAC E MODI FICATI O OF GR AN I_AR ACT IVATED CA RBON 13Y I I NO, 20 I )

C'l 7 c, • • • 6 • E • • • • • c 5 • • • • . Q ... ... ...

4 ... ... e. • . ... 0 3 • ... VI X X X X "0 2 ... X X 111 X

+ X

N:J

u 0 •--- ' ---

0 100 200 300

Time, min

Fig. -1 - Adso rption kine tics ror raw and modiricd granu lar activated carbons ror dirre rcn t copper ions co ncent rat ion. I +. MAC 3 ICuJ0 = 29.5-1 mg/1: • · 1\ti!\C 2 [Cu io = 24.82 mg/1: .A. . MAC I ICulo = 19 .75 mg/1: x . RAC !Cu !0 = 9.62 mg/IJ.

by a second slow step. Ad sorpti on kinetics of ra w granular act ivated carbon is much slower than that o f modified granular acti vated carbon.

Ex perimental results show that amount of copper ions ad sorbed in modi fi cd carbon was larger as compared to that on ra w carbon ove r a parti cul ar time period. Thi s is due to surface modification o f the carbon by nitric acid which prod uces more sur face functi ona l groups and hence more copper ions arc easil y allachcd to it. making the ad sorption process fa ster. Fi gure 4 suggests that mos t o f the copper( ! I) ions are adsorbed during the first one and ha l f hours. V ery small amount or copper ions \.vas ad sorbed in the nex t two hours leading to equi l ibrium.

The ad sorpti on rate became slow subsequentl y due to the longer diffusion range o f the Cu2

+ ions ( ionic radiu s = 0.072 nm) into the inner cav ities and inter­layers o f the oxidi sed activat ed carbon. Such slow diffusion would lead to a smaller increase in the ad sorpti on curve at the later stage.

Thus, it can be assumed that the ad sorpti on o f copper ions on acti vated carbon fo ll ows mainl y the three consecuti ve steps o f ex ternal diffusion. intraparticl c diffusion and finall y ad sorption reacti on. One or more o r these steps can control the ad sorpti on kinetic s. In mos t cases . the •td sorpti on reaction s occur rapid I y. Therefore, the first two steps are norma II y important in the kineti cs . There arc several theoreti cal models available to describe the ad sorpti on kineti cs 17

_1x.

!\ simplifi ed intrapanicular diffusion model was al so employed to eluc idate the ad sorpti on mechani sms. It was first deve loped by W eber and M orri s and has been used in ad sorption kinet ic

studi es . It is assumed that the ad sorpti on is controll ed by three stages: the first sharp linear stage being a rapid external diffusion and surface adsorpti on. the second linear stage being a gradual ad sorpti on stage . where the intraparti c le diffusion is rate limited: and the final stage being the equilibrium stage where the intraparticle eli ffu sion starts to dec rease clue to low concentrati on in soluti on phase as we ll as less availab le ad sorpti on sites . Out o f the above th ree steps, the first one is the rate determining step and hence fo ll ows first ord er kinetic s.

Erred or Jl II on copper adsorption

Elec trokineti c studi es ha ve shown that ac ti vated carbons w ith a predominance o f ba sic runcti on•tl groups ex hibit a positi ve functi onal group as opposed to a negati ve surface potential for carbon wi th predominance o f acidic fun cti onal group

20 However.

the sur face charge o f both types of carbon can be modified by changing the pl--1 o f the ex ternal so luti ons. Therefore, thi s parameter must be an important vari abl e aflcc ting the ad sorpti on process o r meta l ions. The pH o f the ex ternal so luti on not onl y a!Tcc ts the surface charge o f the ad sorbent , but •tl so the degree o f ioni sati on ~nd spec iati on o f ad sorba tc

21.

The removal o r copper( ll ) ions at dillercnt initi al

fJH o f soluti on for both ra w and nitri c ac id modi f ied acti vated carbons was studi ed. Data show th•tt the copper ad sorpti on by ra w and nitri c ac id modi fied carbon is strongly pH dependent. There is a sharp increase in the removal o f copper at pH = 4-5. At pf-1<2, almost no Cu2

+ ions we re ad sorbed on raw granular acti vated carbon due to c lec trost•t t ic repul sion between smface o f the acti vated carbon and

'+ '+ the Cu- ions. As the pH value o f the Cu- ion

so lut ion increased, the removal effic iency incre•tsed graduall y and attained a value o f 50% at pH 6.0. T hi s is because the copper ions are now ab le to repl ace hydrogen ions on the carbon surface, g iv ing ri se to

I . I d . I') en1anccmcnt tilt l C a sorpt! Oll .

One o f the poss ible path ways to met•tl sorpti on by acti vated carbons can be desc ribed by the l"o ll ow ing reaction :

M 1.+ + 11RH ¢=> R11 M IZ·nl + 11 T-1 +

w here M is a metal ion, R-H is an acidi c functi on•Ji group, 11 is the number o f functi on groups ( l igand s) interactin g w ith one metal ion. and Z is the charge on

~ "l ()

the meta l 10 11 - .

:2016 INDI AN .I C II EM .. S EC t\ . S EPTEMB ER 2006

According to the surface co mpl ex formati on (SCF) theory, the increase in metal removal as pH increases c~1n be ex plained on the basis o f the decrease in competition between proton and metal species for the surface sites. and by the decrease in pos iti ve surface charge, which res ults in a lowe r coul ombic repul sion o f the sorbing metal ~~ .

Ac idi c fun cti onal groups on the surface o f the carbon act as ion-exchange sites for metal s. fn order to enable the sorpti on or cati ons, the feed soluti on pH ha s to be c loser to th~1t of flKa values o f functi onal groups. The pH beha viour o f the copper ion sorpti on sugges ts that the re is a weak acidi c group (groups) in the carbon responsibl e for the ion uptake. The ca rboxy li c group is the li ke ly candidate s ince it typi call y start s to di ssociate at pH values 4-6 in weak ly ac idic acti vated carbons2

' . S ince nitric ac id modifi ed acti vated carbons contain more ca rboxy lic groups. on its surface. it ha s a tendency to remove more copper ions whi ch is ~d so evident from FT-IR spectra .

The present stud y shows that nitri c ac id ox idati on e llccti ve ly generated signifi cant amount of ac idi c fun cti onal groups on granular acti vat ed carbon surfaces. The ac idi c capac iti es of acti vated carbon increased continuously with subsequent nitri c ac id oxidation as seen from the ad sorpti on isotherm curves. The experimental result s indi cated that nitri c ac id ox idati on was ve ry significant in improv ing the copper ion ad sorpti on capabiliti es o f granular acti vated carbon due to ch~1n ge s in the chemica l properti es of acti vated carbon induced by ox idati on. The copper ion ad sorpti on for modifi ed carbon was a lmost doubl e the amount o f copper ion ad sorption for ra w granular acti vated carbon. The ad sorpti on kin etic s or rnodificd c1rbon was much fa ster than th~1t of unmodifi ed carbon. The initial pH of copper ion soluti on affec ts both ra w and modifi ed granular acti vated carbon. The copper ion re moval increases rapidl y with increase in the initial ;JH o f metal ion so luti on and is app licable for a wide range o f pH and can be useful in treating wastewater with a wider pH range . The most important aspect o f the present stud y was the regenerati on o f the granular acti vated carbon which makes the process cost-ciTec ti ve. Setting aside the current cos t-effec ti veness o f the process of using granular acti vated carbon as ad sorbcnts, such

modifi ed acti vated carbon s arc e ffec ti ve and potential cand idates for the replacement of acti vated carbon as an ad sorbent in wate r treatment.

Acknowledgement The authors are thankful to the Direc tor. Reg ional

Research Laboratory, Bhubancswar for hi s kind permi ss ion to publi sh the work and to the Direc tor. LIT, Na gpur for prov iding faciliti es to carry out the present vvork .

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