Thermally activated attapulgite as an adsorbent for heavy metals

Transcription

1 3rd Intrnational nfrnc on Mdical Scincs and Chmical Enginring (ICMSCE'213) Dc , 213 Bangkok (Thailand) Thrmally activatd attapulgit as an adsorbnt for havy mtals Frman Ntuli 1, Thabo Falayi 2, and Thabani Dlamini 3 Abstract Thrmal activation of attapulgit was invstigatd. Attapulgit calcind at 6 C was usd as an adsorbnt for havy mtals in AMD. Th ffct of solid loading, rsidnc tim and adsorption tmpratur was invstigatd. Activatd attapulgit showd that it could rmov mor than 99% of,, and from AMD. Langmuir isothrm gav th bst fit for th adsorption procss with maximum adsorption capacitis of 8.97,.2,.32 and.32 for,, and rspctivly. Th adsorption of and was thrmodynamically. Th rsults show that thrmally activatd attapulgit has potntial as an adsorbnt for havy mtals from AMD. Kywords Adsorption, attapulgit, havy mtal, isothrm. I. INTRODUCTION cid min drainag (AMD) poss a srious thrat to Ahuman halth, animals and cological systms. It is charactrizd by low ph and high concntrations of havy mtals contaminants, such as 2+, 3+, Mn 2+, Zn 2+, Cd 2+ and Pb 2+. Adsorption using clays is on of th mthods by which to rmdiat th nvironmntal problms associatd with AMD. A numbr of clay minrals hav bn usd as adsorbnts including bntonit kaolinit and attapulgit [1]. Attapulgit is a natural on-dimnsional magnsium aluminum silicat clay minral whos diamtr is btwn 3 and 5 nm [2]. mpard with montmorrilonit and othr minral matrials, th pric of attapulgit is lowr, thrfor offring an altrnativ chap low cost adsorbnt for havy mtals in AMD [3]. Unactivatd attapulgit at 1% (m/v) solid loading has bn shown to rmov at last 93% of,, and whilst rmoving 66% Mn [4]. This rsarch sks to stablish th ffctivnss of thrmally activatd attapulgit as an adsorbnt for havy mtals in AMD. II. EXPERIMENTAL A. Matrial Acid min drainag was collctd from a local gold min sit and analysd to find th concntration of havy and othr lmnts prsnt. Attapulgit was supplid by G&W Bas Frman Ntuli 1 is with th Univrsity of Johannsburg, Chmical Enginring Tchnology Dpartmnt (corrsponding author s phon: ; -mail: fntuli@uj.ac.za ). Thabo Falayi 2 is with th Univrsity of Johannsburg, Chmical Enginring Tchnology Dpartmnt (-mail:tfalayi@gmail.com ). Thabani Dlamini 3 was with th Univrsity of Johannsburg, Chmical Enginring Tchnology Dpartmnt. Minrals. Analytical grads of SO 4.7H 2 O, SO 4.7H 2 O, SO 4.5H 2 O and SO 4.6H 2 O wr supplid by Sigma Aldrich. B. Equipmnt ph was masurd using Mtlr Toldo dual mtr (Svnduo ph/conductivity mtr with a Mtlr Toldo InLab Pro ISM ph lctrod prob). Adsorption xprimnts wr conductd in a thrmostatic shakr (Labotc OrbiShakr). Mtal analysis was achivd using an Atomic Absorption Spctromtr (Thrmo scintific ICE 3 Sris). C. Thrmal activation of attapulgit Attapulgit was calcind in an oxidizing furnac at a tmpratur of 6ºC for 4 hours. D. Fortification of AMD To 1 ml of AMD, appropriat amounts of sulphat salts of and wr addd. Th solution was stirrd at 2 rpm for 1 min and mad up to 1 ml with raw AMD whilst stirring for furthr 2 min. Tabl 1 shows th AMD paramtrs bfor and aftr fortification. E. Effct of solid loading Diffrnt masss of attapulgit (.5,.75, 1, 1.25, 1.5, 2, 3, 3.5, 4 and 5 g) wr addd to 1 diffrnt 25 ml volumtric flasks ach containing 5 ml of fortifid AMD. Th rsulting solutions wr agitatd in a thrmostatic shakr at 2 rpm for 2 h at 25ºC. Aftr 2 h, agitation was stoppd and th solution ph was masurd. Th solutions wr thn subsquntly filtrd using a Buchnr funnl and th filtrat was acid digstd in prparation for AAS mtal analysis. Th amount adsorbd pr unit mass of adsorbnt at quilibrium q is givn by: q = (C - C ) V (1) M Whr C is initial mtal concntration, C quilibrium mtal concntration (mg/l), V volum of solution (L) and M mass of adsorbnt (g). F. Effct rsidnc tim and tmpratur 3 g (6% m/v) of attapulgit was addd to 6 diffrnt volumtric flasks ach containing 5 ml of fortifid AMD. Th rsulting solutions wr agitatd in a thrmostatic shakr at 2 rpm for 15, 3, 45, 6, 9 and 12 min rspctivly at 16

2 3rd Intrnational nfrnc on Mdical Scincs and Chmical Enginring (ICMSCE'213) Dc , 213 Bangkok (Thailand) 25ºC. At th nd of ach tim intrval th agitation was stoppd and th solution ph was masurd. Th solutions wr thn subsquntly filtrd using a Buchnr funnl and th filtrat was acid digstd in prparation for AAS mtal analysis. Th procdur was rpatd at 35ºC and 45ºC to valuat th ffct of tmpratur on adsorption of,, and. G. Stability of mtal loadd activatd attapulgit 5 ml of rvrs osmosis (RO) watr (ph 2) was addd to 3 g of mtal loadd activatd attapulgit. Th rsulting solution was agitatd for 2 h at 25ºC in a thrmostatic shakr at 2 rpm. Aftr 2 h, agitation was stoppd and th solution was thn subsquntly filtrd and th filtrat was acid digstd in prparation for mtal analysis using an AAS. H. R-usability tst 3 g of mtal loadd activatd attapulgit was mixd with 5 ml of frsh fortifid AMD. Th rsulting solution was agitatd for 2 h at 25 C in a thrmostatic shakr at 2 rpm. Aftr 2 h agitation was stoppd and th solution was thn subsquntly filtrd and th filtrat was acid digstd in prparation for mtal analysis using an AAS. This procss was rpatd twic. III. THEORETICAL BACKGROUND Adsorption Isothrms To study th thrmodynamic bhaviour of th adsorption procss, adsorption isothrms wr usd. In ordr to find th adsorption capacitis of th mtals adsorbd by attapulgit, th xprimntal rsults wr analysd using th Langmuir (2) and Frundlich (3) isothrms. Th following linarisd quations wr usd for this purpos C C = q 1 + qm. b q (2) m log q = log K F + 1 logc n (3) Whr C (mg/l) is th mtal concntration in solution at quilibrium, q (mg/g) is th amount of mtal adsorbd pr unit mass of adsorbnt, q m (mg/g) is th maximum adsorption, b (L/g) is a constant rlatd to nthalpy of adsorption, K F and n ar Frundlich quilibrium constants indicativ of th adsorption capacity and adsorption intnsity. Adsorption Kintics To study adsorption kintics th xprimntal data was subjctd to th psudo first ordr plot (4) and psudo scond ordr plot (5), rprsntd by th following quations rspctivly k log( q qt ) = log q ( 1 ) t 2.33 (4) Whr q t (mg/g) is th amount adsorbd at any tim pr unit mass of adsorbnt, k 1 is th rat constant (min -1 ). A plot of log (q -q t ) vs t should b linar if th modl fits th xprimntal data. t q t = t k q q 2 (5) Whr k 2 is th rat constant (g -1 min -1 ). A plot of t/q t vs t should b linar if th modl fits th xprimntal data. Gibbs fr nrgy Gibbs fr nrgy can b approximatd by ( 7) G = RT ln b (7) Whr ΔG is th standard fr nrgy chang (J/mol), R is th univrsal gas constant, J/mol K, T is absolut tmpratur (K) and, b (L/g) is th Langmuir isothrm constant rlatd to nthalpy of adsorption. Equation 8 also givs a linar rlationship of Gibbs fr nrgy and tmpratur. G = H T S (8) A plot of ΔG vrsus tmpratur, T, will b linar and th valus of ΔH and ΔS can b dtrmind from th slop and intrcpt of th plot IV. RESULTS AND DISCUSSION A. AMD charactrization Tabl 1 shows th AMD paramtrs bfor and aftr fortification. Paramtr TABLE 1 AMD PARAMETERS Bfor Fortification Aftr Fortification ph Tmpratur ( C) nductivity(ms/cm) Sulphats (ppm) (ppm) Mn (ppm) (ppm) Blow dtction limit 2.35 (ppm) Blow dtction limit (ppm) Blow dtction limit B. Effct of adsorbnt loading Fig. 1 shows th variation in prcntag of mtal rmovd with incras in activatd attapulgit loading. 17

3 3rd Intrnational nfrnc on Mdical Scincs and Chmical Enginring (ICMSCE'213) Dc , 213 Bangkok (Thailand) % Rmovd Adsorbnt Loarding (% m/v) Fig.1 Effct of activatd attapulgit solid loading on adsorption of,, and Mtal rmoval incrasd as th solid loading incrasd raching quilibrium at 6% solid loading. An incras in adsorbnt solid loading is accompanid by a corrsponding incras in th numbr of adsorption sits lading to an incras in mtal uptak [5]. C. Effct of solid loading on th distribution cofficint Fig. 2 shows th variation of distribution cofficint with solid loading. Log (K d ) Adsorbnt Loading (% m/v) Kd Kd Kd Kd D. Effct of rsidnc tim and tmpratur on adsorption of havy mtals Fig. 3 to Fig. 6 shows th ffct of tim and tmpratur on th rmoval of,, and. 1.% Rmovd 99.8% 298 K 99.6% 38 K 318 K 99.4% tim (min) Fig. 3 adsorption at diffrnt tmpraturs and tim intrvals 1% 96% 92% 88% 298 K 84% 8% tim (min) Fig.4 adsorption at diffrnt tmpraturs and tim intrvals 1% Rmovd Rmovd 99%, 298 K 98%, 38 K, 318 K 97% tim (min) Fig. 5 adsorption at diffrnt tmpraturs and tim intrvals 1% Rmovd 99% 98% 97% 96% 95% tim (min), 298 K Fig. 6 adsorption at diffrnt tmpraturs and tim intrvals Fig. 2 Variation distribution cofficint with solid loading Th K d valus for and wr indpndnt of attapulgit solid loading, whilst a dpndnc was noticd for and btwn 2 and 4 % m/v. Thrfor this shows that th adsorption of and wr mor favorabl than that and. This thrfor mans that th activatd attapulgit adsoption prfrnc is of th ordr >>>. Fig. 3 and 4 shows that incras in rsidnc tim incrasd th amount of and adsorbd onto attapulgit. Th prcntag rmovd for aftr 15 min was 99.5 % at T= 298 K showing that th rmoval is spontanous, whilst aftr 12 minuts 1% of was rmovd. Th sam trnd was shown for whrby th prcntag rmoval incrasd from 84% to 98% at th sam tmpratur. Th incras in mtals rcovry with contact tim was du to nough tim bing givn to th systm to rach quilibrium [6]. Thr was no 18

4 3rd Intrnational nfrnc on Mdical Scincs and Chmical Enginring (ICMSCE'213) Dc , 213 Bangkok (Thailand) Elmnt significant diffrnc in mtal rmoval rat at diffrnt tmpraturs from 6 min for and. Fig. 5 and 6 show that incrasing th tmpratur incrasd th prcntag rmoval of th mtals for and and this might b du to an incras in th numbr of ions in solution du to th dissociation of th cation and hydration molcul [7]. E. Adsorption isothrm Th xprimntal data fittd th Langmuir isothrm as th corrlation cofficint for all mtals was abov.9999 whras th corrlation cofficint for th Frundlich isothrm for all mtals wr blow.92. Th Langmuir paramtrs ar shown in Tabl 2. TABLE 2 SUMMARY OF THE FREUNDLICH AND LANGMUIR ISOTHERMS AT DIFFERENT TEMPERATURES Tmpratur (K) Langmuir b (L/mg) q m (mg/g) R F. Adsorption Kintics Th xprimntal data fittd psudo scond ordr kintic modl as th corrlation cofficint for all mtals was abov.999. Th corrlation cofficint for th psudo first ordr kintic modl was blow.91 for all mtals. Th psudo scond ordr kintic modl paramtrs all mtals ar shown in Tabl 3 TABLE 3 PSEUDO SECOND ORDER KINETIC PARAMETERS FOR CU, CO, FE AND NI Elmnt Tmpratur ( o K) Q E (mg/g) K 2 (g/mg.min) G. Adsorption Thrmodynamics Fig. 7 shows th Gibbs fr nrgy plots for,, and. Tabl 4 shows th thrmodynamic paramtrs y = x Gibbs fr Enrgy (ΔG o ) y = x y = x Tmpratur ( o C) Fig. 7 Gibbs fr nrgy plots for,, and R 2 y = x

5 3rd Intrnational nfrnc on Mdical Scincs and Chmical Enginring (ICMSCE'213) Dc , 213 Bangkok (Thailand) Elmnt TABLE 4 THERMODYNAMICS PARAMETERS OF FE, NI, CU, AND CO Tmpratur ( o C) ΔG o (kj/mol.k) ΔH o (J/mol.K) ΔS o (J/mol.K) Th Gibbs fr nrgy of all mtals at all tmpraturs was ngativ (Tabl 4) indicating that th adsorption was spontanous. Th dcras in th ngativ valu of ΔG o with an incras in tmpratur indicatd that th adsorption procss of ths ions onto attapulgit bcam mor favorabl at highr tmpraturs. Th positiv valus of ntropy indicatd a high affinity for mtal ions by activatd attapulgit. H. R-us of mtal loadd activatd attapulgit Fig. 8 shows th variation in mtal rmoval with numbr of cycls. Havy Mtals Rmoval 1% 9% 8% 7% 6% 5% 4% 3% 2% 1% % Cycl Numbr Fig. 8 Variation in mtal rmoval with numbr of cycls. Fig 8 shows that th and rmoval rmaind th sam for all th thr cycls of rus whilst and rmoval dcrasd in ach cycl. dcrasd from 99.4% to 83.4% and thn to 72.6% in th third cycl whilst dcrass from 98.4%, to 6% and thn to 39.5%. Th data suggsts that attapulgit can b r-usd for 3 tratmnt cycls without rgnration. Th dcras in mtal adsorption has to do with a dcras in th numbr of activ adsorption sits. I. Stability of mtal loadd activatd attapulgit Mtal loadd activatd attapulgit showd that it could rtain adsorbd mtals as only 2.2% of was lachd aftr th xprimnts J. Effct of adsorption on lmntal cmposition of activatd attapulgit Tabl 5 shows th XRF analysis of activatd attapulgit Elmnt TABLE 5 XRF ANALYSIS ON ATTAPULGITE Activatd Attapulgit (Mass %) Mtal loadd activatd Attapulgit (mass %) Na Mg Al Si S K Ca Ti Mn Adsorption of,, and rsultd in th incras of th rlativ contnt of ths mtals in mtal loadd activatd attapulgit V. CONCLUSION Activatd attapulgit can b usd as an adsorbnt for,, and from AMD. A 6% solid loading of activatd attapulgit could rmov 99% of all mtals aftr 2 h of agitation. Th adsorption procss was thrmodynamically spontanous and followd Langmuir isothrm modl and psudo scond ordr kintic modl. Activatd attapulgit could b usd in thr cycls without any nd for rgnration. Mtal loadd activatd attapulgit was stabl against furthr laching of mtals. ACKNOWLEDGMENT Th authors acknowldg financial support from th Univrsity of Johannsburg Rsarch mmitt (URC) and G&W Bas Mtals for providing th activatd bntonit. 2

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