Koren J. Chem. Eng., 22(4), 599-604 (2005) Cdmium Removl y Juniperus monosperm: the Role of Clcium Oxlte Monohydrte Structure in Brk Eun Woo Shin School of Chemicl Engineering nd Bioengineering, University of Ulsn, Sn 29, Mugeo 2-dong, Nm-gu, Ulsn 680-749, Kore (Received 21 Ferury 2005 ccepted 6 My 2005) Astrct This study suggests tht clcium oxlte monohydrte over Juniperus monosperm rk is n ion-exchngele site for cdmium dsorption on the sis of its cdmium dsorption ehvior nd surfce chrcteriztion. Cdmium dsorption ehvior showed tht juniper rk hd higher cdmium dsorption cpcity (84.6 µmol Cd g 1 t ph 5) thn juniper wood (32.0 µmol Cd g 1 t ph 5), nd tht the se-tretment improved the cdmium dsorption cpcity of only juniper wood. This difference etween juniper rk nd wood might hve originted from different cdmium inding sites. In clcium displcement, the cdmium uptke onto juniper rk ws identicl to the mount of clcium ions displced, which indicted tht clcium plyed n importnt role in cdmium dsorption onto juniper rk. X-ry diffrction (XRD) results gve evidence tht only juniper rk contined the structure of clcium oxlte monohydrte. Furthermore, cdmium dsorption decresed the intensity of the chrcteristic XRD peks for clcium oxlte monohydrte. In diffuse reflectnce infrred Fourier trnsform (DRIFT) spectr, existence of oxlte on juniper rk ws proven gin nd interprettion on IR nd of cronyl groups mtched significntly the cdmium dsorption ehvior. Key words: Clcium Oxlte Monohydrtes, Juniperus monosperm Brk, Cdmium Adsorption INTRODUCTION Juniperus monosperm, forest mteril, hs een pplied to hevy metl removl processes s n dsorent [Min et l., 2004; Shin et l., 2004; Shin nd Rowell, 2005]. Like other iosorents such s pet moss [Crist et l., 1992, 1996], lge [Dvis et l., 2003], nd sugr eet pulp [Reddd et l., 2002], the dsorption cpcity of juniper for cdmium (Cd) removl from wter ws scried to functionl groups (croxylic nd sulfonic cid groups) for ctionexchnge on juniper surfce. Hence, such chemicl tretments s se tretment nd sulfontion hve een used to introduce more hevy metl inding sites onto juniper nd, eventully, to improve hevy metl dsorption cpcity [Min et l., 2004; Shin nd Rowell, 2005]. Mechnisms of hevy metl sorption nd the surfce structure for the inding sites hve een well descried nd fully understood for certin dsorents. For exmple, rown lge contining croxylic nd sulfonic cid groups s key functionl groups for hevy metl inding nd mechnisms for the hevy metl removl process hve een explined with ion-exchnge nd surfce complextion models [Dvis et l., 2003]. In hevy metl uptke experiments, chnges in solution ph s well s relese of light ions resulted from n ion-exchnge of hevy metls [Schiewer nd Volesky, 1995; Crist et l., 1996]. The surfce complextion mechnism ws closely relted to the moleculr structure of the inding sites. In this mechnism, hevy metl inding ws regrded s the complextion of hevy metls to multidentte lignd. The egg-ox model descried steric rrngements of hevy metl coordinted to lginic To whom correspondence should e ddressed. E-mil: ewshin@mil.ulsn.c.kr 599 cid sites; the orienttion of the oxygen toms with respect to COO ws preferle to guluronic cid thn mnnuronic cid [Dvis et l., 2003]. Thus, to the extent of moleculr structure, the interction etween the hevy metl inding sites nd the functionl groups over rown lge hs een well understood. Recent reports on Juniperus monosperm dsorents hve reveled tht hevy metl sequestrtion depended on functionl groups generted y chemicl modifictions [Min et l., 2004; Shin nd Rowell, 2005]. However, unlike rown lge iosorent, detiled understnding the moleculr structure of inding sites on dsorents, which helps cdmium dsorption ehvior, hs not een suggested. In the present study, we demonstrted tht clcium oxlte monohydrte (COM) is n inding structure on Juniperus monosperm rk for cdmium dsorption. The cdmium dsorption ehvior is explined on the sis of the structure of the ion-exchngele site. EXPERIMENTAL 1. Mterils Juniper logs were rndomly collected from New Mexico, U.S.A. nd shredded into smll chips t the USDA Forest Products Lortory (FPL) in Mdison, Wisconsin, U.S.A. Juniper rk ws seprted from the chips nd then ground to pss through 3-mm screen using Wiley mill. Juniper wood ws lso seprtely collected nd then ground. The untreted juniper rk nd wood were denoted UJB nd UJW, respectively. For comprison, se-treted juniper rk (BJB) nd wood (BJW) were prepred in this study. A 250- ml liquot of 0.5 M NOH solution ws dded to 1 L-eker contining 10 g of juniper powder. The smples were treted with se-solution y stirring for 24 h. The solutions were decnted nd the smples were continuously wshed with deionized wter until
600 E. W. Shin the ph of the wsh wter reched less thn 8. The se-treted smples were dried in ir t mient temperture for 3 dys. 2. Adsorption Test nd Clcium Displcement Adsorption kinetic experiments were crried out in 1-L solutions with 1.0 g of solid smple nd 0.178 mmol L 1 of initil cdmium concentrtion. The experiments were conducted t two different ph conditions (ph 5.0±0.1 nd ph 3.2±0.1) nd the solution ph ws kept constnt during the experiment y using 0.1 M HNO 3 nd 0.1 M KOH. The suspension ws stirred y mgnetic r nd superntnt ws collected t vrious time intervls during the 2-h experiment. Previous experiments showed tht the 2-h kinetic time ws enough to rech equilirium. Cdmium concentrtion in the filtrte ws determined y inductively coupled plsm - tomic emission spectroscopy (ICP-AES; ULTIMA, Join Yvon Inc.). During the experiment, clcium concentrtions of ech solution smple were mesured y n ICP-AES to clculte the mount of clcium ions displced with cdmium ions. The mount of clcium ions displced ws clculted y sutrcting the mount of clcium ions dissolved from the mount of clcium ions relesed. The clcium dissolved ws the totl clcium ions relesed from the dsorents in the sence of cdmium ions in wter. To mesure the mounts of clcium ions dissolved, the sme kinetic experiments were conducted t the sme ph conditions fter the dsorents were put in deionized wter tht contined no cdmium ions. The clcium ions relesed represented the totl clcium ions relesed from the dsorents in the cdmium dsorption experiments. This vlue ssumed the clcium dissolved plus the clcium displced. The pseudo-second-order eqution ws employed to determine the cdmium dsorption cpcity (q e ) nd the mounts of clcium ions y dt fitting from the kinetic experiments [Ho nd McKy, 1999]. This model ssumed tht dsorption followed the Lngmuir eqution nd showed high correltion coefficients for dt fitting. The kinetic rte equtions cn e written s follows: dq ------ t = k( q e q t ) 2 (1) dt where q t, nd q e re the mounts of cdmium dsored t time t, nd t equilirium (µmol g 1 ), respectively, nd k is the equilirium rte constnt of the second order dsorption (g µmol 1 min 1 ). The eqution cn e integrted nd then rerrnged to e pplied to liner regressions. q e ws determined y clculting the vlues cquired from this liner regression. 3. Chrcteriztions An elementl nlysis for cdmium, clcium, nd sodium ws conducted y ICP-AES (ULTIMA, Join Yvon Inc., Edison, NJ). DRIFT spectr were cquired y using Mttson Glxy 5020 (Mttson Instruments, Mdison, WI) equipped with Hrrick Scientific (Ossining, NY) diffuse reflectnce ccessory. Ech infrred (IR) spectrum ws the verge of 4,000 scns etween 400 nd 4,000 cm 1 (resolution=4 cm 1 ). Prior to nlysis, smples were finely ground with Wiley mill nd sieved with 0.18-mm screen. Ech spectrum ws seline corrected t 400, 840, 2,000, nd 4,000 cm 1 nd normlized ginst the 1,320 cm 1 nd ssocited with the C-H ending mode [Yng et l., 1996]. An X-ry diffrction (XRD) nlysis ws performed to chrcterize the dsorents. In ddition to the prepred dsorents (UJB, UJW, nd BJB), Cd-dsored dsorents were employed in this Tle 1. Cdmium uptke nd clcium displcement of ech dsorent c d Smples ph Q C, D Q C, R Q C, DP Q Cd Q C, DP /Q Cd UJB 5.0 086.6 168.4 081.8 84.60 0.967 3.2 283.9 311.7 027.8 30.80 0.903 UJW 5.0 025.5 048.9 023.4 32.00 0.731 3.2 053.4 056.5 003.1 09.07 0.342 BJB 5.0 022.5 125.1 102.6 88.60 1.15 BJW 5.0 06.56 033.5 026.9 76.20 0.353 Amount of clcium ions dissolved t equilirium (µmol g 1 ). Amount of clcium ions relesed t equilirium (µmol g 1 ). c Amount of clcium ions displced t equilirium. This ws clculted y sutrcting Q C, D from Q C, R (µmol g 1 ). d Amount of cdmium dsored t equilirium (µmol g 1 ). nlysis for comprison. The dsorents spent in kinetic experiments were used s Cd-dsored dsorents. All XRD ptterns were collected with Scintg PAD V X-ry diffrctometer equipped with CuKα rdition (40 kv, 35 ma) of wvelength 0.154 nm. All XRD ptterns were otined from 10 o to 50 o with scn speed of 1.5 o min 1. RESULTS AND DISCUSSION 1. Cdmium Adsorption Behvior nd Clcium Displcement Tle 1 shows the cdmium uptke nd the clcium displcement of ech dsorent. The metl mounts t equilirium in Tle 1 were cquired y fitting the kinetic dt to the pseudo-second-order eqution. The fitting results showed high correltion coefficients of >0.998. In cdmium uptke, lowering ph of the solutions decresed the dsorption cpcity of UJB s well s of UJW, which ccorded with previous reports [Romero-Gonzlez et l., 2001; Yun et l., 2001; Shin nd Rowell, 2005]. As cdmium ions competed with mny more protons t lower ph conditions, it ws nturl tht the cdmium uptke ws reduced. The juniper rk (UJB) hd higher dsorption cpcities for cdmium (84.6 nd 30.8 µmol Cd g 1 ) under oth ph conditions thn did the juniper wood (UJW: 32.0 nd 9.07 µmol Cd g 1 ). Additionlly, the se-tretment on the juniper rk hd little effect on cdmium cpcity (BJB: 88.6 µmol Cd g 1 ), wheres the se-tretment incresed cdmium uptke on the juniper wood dsorent to 76.2 µmol Cd g 1 (BJW). In previous report [Min et l., 2004], the cdmium dsorption cpcity of juniper fier tht consisted of juniper wood nd rk ws improved y the se-tretment. This result in the previous study ws due to chnges in the surfce structure of juniper wood not juniper rk, s proven in this study. The distinct effects of the se-tretment on juniper wood nd rk s well s differences in the dsorption cpcity etween juniper rk nd wood might indicte difference in the chemicl structures responsile for cdmium dsorption over oth the dsorents. The clcium displcement ehvior of UJB nd UJW ws lso not similr. In Tle 1, the rtios of Q C, DP /Q Cd, representing the molr rtios of clcium ions displced to cdmium uptke, were t lest ove 0.9 for juniper rk (UJB nd BJB), while those of juniper wood (UJW nd BJW) were 0.731, 0.342 nd 0.353, respectively July, 2005
Cdmium Removl y Juniperus monosperm: the Role of Clcium Oxlte Monohydrte Structure in Brk 601 (Tle 1). This indicted tht in juniper rk, over 90% of cdmium uptke ws displced with clcium ions wheres, on verge, 50% of the cdmium uptke ws chieved y ion-exchnge with clcium ions over juniper wood. Accordingly, clcium plyed n importnt role in cdmium dsorption onto the juniper rk. Moreover, the relesed or dissolved clcium ion mounts (Q C, R nd Q C, D )of juniper rk (UJB: 86.6-311.7 µmol g 1 ) were much higher thn those of juniper wood (UJW: 25.5-56.5 µmol g 1 ). Even fter se-wshing, the vlues of BJB (22.5-125.1 µmol g 1 ) were higher thn those of BJW (6.56-33.5 µmol g 1 ). These results were ttriuted to more clcium content in juniper rk. In the results of ICP-AES, the clcium content in UJB ws 945 µmol g 1, fifteen times more thn tht in UJW (61.7 µmol g 1 ), which lso implied tht the role of clcium ions in juniper rk ws importnt for the cdmium uptke. 2. X-Ry Diffrction XRD ptterns of UJB nd UJW re shown in Fig. 1. The pttern of UJB looked superimposed on the pttern of UJW with severl shrp peks, which implied tht UJB contined unique chemicl structure showing the shrp peks t 2θ=15.0, 24.5, 30.2, 36.1, nd 38.3. Though the pttern seemed too complicted to discover the chemicl structure of the shrp peks, most of the peks could e ssigned to one structure, clcium oxlte monohydrte [Kontoynnis et l., 1997; Yu et l., 2002; Quyng et l., 2004]. On the sis of the literture, the pek t 2θ=30.2 ws chosen s min pek for clcium oxlte monohydrte while the chrcteristic pek for the juniper structure ws exhiited t 2θ=22.7. The XRD ptterns of the juniper rk efore nd fter cdmium uptke re shown in Fig. 2. After cdmium dsorption, the chrcteristic peks of clcium oxlte monohydrte decresed. The heights of the min clcium oxlte monohydrte pek (2θ=30.2) nd juniper pek (2θ=22.7) were mesured to clculte the rtios of the heights (H COM ) tht re proportionl to the mount of clcium oxlte monohydrte in the juniper rk. Tle 2 presents Fig. 2. XRD ptterns of UJB, BJB, Cd-dsored UJB nd Cd-dsored BJB. x nd o mrks represent the chrcteristic peks for clcium oxlte monohydrte nd juniper, respectively. Tle 2. Heights of the min XRD peks of clcium oxlte monohydrte nd juniper nd rtios of the heights Smples H COM H rk H COM UJB 276 339 0.814 BJB 201 300 0.670 Cd-dsored UJB 176 307 0.573 Cd-dsored BJB 150 328 0.457 The height of the pek t 2θ=30.2 The height of the pek t 2θ=22.7 Fig. 1. X-ry diffrction (XRD) ptterns of UJW nd UJB. x nd o mrks represent the chrcteristic peks for clcium oxlte monohydrte nd juniper, respectively. these results clculted. In the cse of UJB, H COM / H rk ws reduced from 0.814 to 0.573 fter cdmium dsorption. For BJB, 0.670 of the rtio went to 0.457 fter cdmium uptke. The decrese in these rtios clerly indicted disppernce of the structure of clcium oxlte monohydrte. We suggest tht this reduction of clcium oxlte monohydrte fter cdmium dsorption is due to the ion-exchnge of clcium in clcium oxlte monohydrte with cdmium ions. The disppernce of clcium oxlte monohydrte could e scried to other fctors. For exmple, the se-tretment could lessen Koren J. Chem. Eng.(Vol. 22, No. 4)
602 E. W. Shin clcium concentrtion in solids, which ws supported y the XRD nd the clcium displcement experiments - decreses in H COM nd the mounts of clcium displced fter the se-tretment. Nonetheless, decrese in the chrcteristic pek of clcium oxlte monohydrte during cdmium dsorption kinetics ws cused minly y the ion-exchnge with cdmium ions. The se-tretment ws likely to reduce more mounts of clcium oxlte monohydrte thn cdmium dsorption ecuse of longer rection time (24 h to 2 h). However, in Tle 2, results showed the opposite tht the reduction in H COM y the se tretment (0.144=0.814-0.670) ws lower thn tht y cdmium uptke (0.241=0.814-0.573). In ddition, even though the soluility (K sp ) of clcium oxlte monohydrte in wter t 25 o C is very low t 2.0 10 9 mol 2 L 2 [Nncolls nd Grdner, 1974], the dissolution rte of clcium oxlte monohydrte ws incresed in the presence of luminum nd iron ions ccording to the report y Petrov et l. [2004]. Similrly, the presence of cdmium ions in solution should fcilitte the dissolution of clcium oxlte monohydrte nd then the interction etween oxlic groups nd cdmium ions cn remove the cdmium ions from solution. Tle 1 exhiits clerly tht cdmium ions in solution promoted the relese of clcium from juniper rk. Consequently, cdmium dsorption reduced the XRD pek intensity of clcium oxlte monohydrte, indicting n ion-exchnge etween cdmium nd clcium. 3. Diffuse Reflectnce Infrred Fourier Trnsform Spectr DRIFT spectr of Juniper monosperm were introduced in previous reports [Min et l., 2004; Shin nd Rowell, 2005]. The chrcteristic IR nds of croxylte (COO ), which re known s hevy metl inding site, nd n oxlte, which is n interesting structure s new inding site in this study, ppered round 1,550-1,800 cm 1 ecuse those contin cronyl group. In ddition, the IR nds in other regions were not useful for nlyzing the functionl groups ccording to previous studies [Min et l., 2004; Shin nd Rowell, 2005]. Therefore, in this study, the IR nds of cronyl groups were displyed nd primrily interpreted. Fig. 3 nd Fig. 4 show the IR nds of cronyl groups for juniper wood nd rk, respectively. In Fig. 3 nd 4, the IR nds relted to the cronyl groups were exhiited t four positions: 1,605, 1,618, round Fig. 4. IR nds of cronyl groups for UJW nd BJW. 1,660, nd round 1,744 cm 1. The nds t 1,660 nd 1,744 cm 1 were ssigned to C=O stretching from the ldehyde/ketone nd ester groups, correspondingly [Chtjigkis et l., 1998; Ppps et l., 1999; Brown et l., 2001; Min et l., 2004; Shin nd Rowell, 2005]. These nds commonly pper over oth juniper wood nd rk lthough the positions of the nds were shifted little. In contrst, UJB nd BJB showed the nd t 1,618 cm 1 which my e ssigned to C=O stretching from the oxlte [Ouyng et l., 2004] while UJW nd BJW the nd ws t 1,605 cm 1 which my e ssigned to C=O stretching from croxylte [Chtjigkis et l., 1998; Frnsioso et l., 1998; Ppps et l., 1999; Zhng nd Kmdem, 2000; Brown et l., 2001; Shin nd Rowell, 2005]. This indictes once gin tht oxlte structure existed only on the juniper rk nd not on the juniper wood. In Fig. 3, the IR nds chnged little fter the se-tretment. The nd of C=O stretching from ester (1,740 cm 1 ) decresed little, ut the other nds were lmost the sme. On the contrry, the se-tretment ffected the pttern of IR nds in Fig. 4. After the se-tretment, the nd t 1,605 cm 1 incresed while the nd t 1,744 cm 1 disppered. This phenomenon is well-known s sponifiction where ester rects with wter under se conditions to produce croxylte group (COO ). According to previous studies [Tiemnn et l., 1999; Min et l., 2004], the intensity of the IR nd of croxylte ws proportionl to the hevy metl uptke, implying tht croxylte ws inding site for hevy metl removl. Therefore, this significntly explins tht the cdmium dsorption cpcity of the juniper wood ws incresed fter the se-tretment nd tht little chnge in the cdmium dsorption cpcity of the Fig. 3. IR nds of cronyl groups for UJB nd BJB. Tle 3. IR nd ssignments Position (cm 1 ) Assignment 1,744 C=O stretching from ester 1,660 C=O stretching from ketone nd ldehyde 1,618 C=O stretching from oxlte 1,605 C=O stretching from croxylte July, 2005
Cdmium Removl y Juniperus monosperm: the Role of Clcium Oxlte Monohydrte Structure in Brk 603 Fig. 5. A proposed mechnism for ion-exchnge etween clcium nd cdmium. juniper rk occurred fter the se-tretment. The cronyl groups ccountle for the cdmium uptke, the croxylte nd the oxlte groups, were not chnged on the juniper rk surfce y the se-tretment, which is significntly consistent with the cdmium dsorption results in Tle 1. A mechnism for the ion-exchnge of clcium oxlte with cdmium ions ws proposed in Fig. 5. Around ph 5, cdmium existed minly in the form of Cd 2+ ions tht cn exchnge clcium ions in clcium oxlte in competition with protons. As seen in Fig. 5, s result of the ion-exchnge, the cdmium ions were trpped in oxlte form nd the clcium ions were relesed into the solution. CONCLUSIONS Bsed on the present study, the following cn e concluded: 1. In cdmium dsorption, UJB showed out two to three times higher dsorption cpcity thn UJW t 0.178 mmol L 1 of initil cdmium concentrtion nd t ph 3.2 nd ph 5. As well, the setretment improved the cdmium dsorption cpcity of UJW, nd not tht of UJB. From the results of the clcium displcement, ionexchnge with clcium ws prevlent in cdmium dsorption on UJB. These results implied tht the mechnism of cdmium dsorption onto UJB would e different thn tht of UJW. 2. XRD nlysis proved tht the clcium oxlte monohydrte structure existed only on UJB. The chrcteristic peks for clcium oxlte monohydrte on UJB were reduced fter cdmium dsorption, which indicted tht clcium oxlte is closely relted to cdmium uptke. 3. DRIFT spectr lso gve evidence tht UJB contined the oxlte structure, nd tht UJW did not. The chnge in IR nds of cronyl groups y the se-tretment mtched well the cdmium dsorption results. 4. Clcium oxlte monohydrte on UJB ws ion-exchngele structure for cdmium inding, which did not exist on UJW. 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