Adsorption of cadmium(II) on waste biomaterial

Accepted Manuscript Adsorption of cadmium(II) on waste biomaterial M. Balá ž, Z. Bujň áková, P. Balá ž, A. Zorkovská, Z. Danková, J. Briančin PII: DOI: Reference:

S0021-9797(15)00330-6 http://dx.doi.org/10.1016/j.jcis.2015.03.046 YJCIS 20357

To appear in:

Journal of Colloid and Interface Science

Received Date: Accepted Date:

16 February 2015 25 March 2015

Please cite this article as: M. Balá ž, Z. Bujň áková, P. Balá ž, A. Zorkovská, Z. Danková, J. Briančin, Adsorption of cadmium(II) on waste biomaterial, Journal of Colloid and Interface Science (2015), doi: http://dx.doi.org/ 10.1016/j.jcis.2015.03.046

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Adsorption of cadmium(II) on waste biomaterial M. Baláž, Z. Bujňáková, P. Baláž, A. Zorkovská, Z. Danková and J. Briančin Institute of Geotechnics, Slovak Academy of Sciences, Watsonova 45, 04001 Košice, Slovakia

Abstract Significant increase of the adsorption ability of the eggshell biomaterial towards cadmium was observed upon milling, as is evidenced by the value of maximum monolayer adsorption capacity of 329 mg g-1, which is markedly higher than in the case of most “green“ sorbents. The main driving force of the adsorption was proven to be the presence of aragonite phase as a consequence of phase transformation from calcite occurring during milling. Cadmium is adsorbed in a non-reversible way, as documented by different techniques (desorption tests, XRD and EDX measurements). The optimum pH for cadmium adsorption was 7. The adsorption process was accompanied by the increase of the value of specific surface area. The course of adsorption has been described by Langmuir, Freundlich and Dubinin-Radushkevich isotherms. The adsorption kinetics was evaluated using three models, among which the best correlation coefficients and the best normalized standard deviation values were achieved for the pseudo-second order model and for the intraparticle diffusion model, respectively. Keywords: eggshell; milling; cadmium; adsorption

1 Introduction The water pollution by dissolved heavy metals represents an important environmental issue these days. Many plants discharge significant amounts of heavy metals every day. Therefore, many efforts have been made to purify these wastewaters [1, 2]. Among the heavy metals, cadmium belongs to the most toxic and exhibits significant hazard for human health [3, 4]. Different methods have been used for cadmium removal [5], among which the adsorption has an inevitable place [6, 7]. Various sorbents were used in the past, however, biological and industrial waste materials represent the most interesting group from environmental point of view [8-11]. The eggshell (ES), as a low-cost biosorbent, has been studied extensively [12-18]. Its potential towards adsorption of heavy metals arises from its chemical composition, the main component being calcite (94%) [19], which is an effective sorbent of heavy metals itself [20, 21]. The residual components include MgCO3 (1%), Ca3(PO4)2 (1%) and organic matter (4%) [19]. The industrial ES contains also the eggshell membrane (ESM), which is suitable for a wide variety of applications too [22]. Slight modification of bio-sorbents often leads to a significant increase in their adsorption ability [23-29]. One of these modification methods is mechanical activation (MA) achieved by intensive milling [30, 31]. This process is well-known in the expanding field of mechanochemistry [32-34]. We have already shown the potential of this procedure also in the case of the ES in our previous study [35]. Within this work, we present a detailed study on the adsorption ability of the ES towards cadmium. Although quite recently, a similar paper was published by Flores-Cano, et al. [17], in which the mechanism of adsorption was discussed exhaustively, herewith we report the innovative procedure for significant enhancement of the adsorption ability of the ES by milling. The increase of the adsorption ability as a result of MA is confronted with the basic principles of mechanochemistry and moreover, the considerable attention is devoted to the adsorption kinetics and to the characterization of the product.

2 Experimental 2.1 Materials The raw eggshells were provided by a local canteen in Košice. Cadmium nitrate tetrahydrate, sodium hydroxide and nitric acid (all three purchased from ITES, Slovakia) were used as chemicals without further purification. 2.2 Separation of ESM and mechanical activation The ESM was separated from the ES similarly to our previous work [36]. After subsequent drying and crushing in mixer, the purified ES was subjected to pre-milling, in order to obtain a powder with particles smaller than 160 μm, which were then used for the mechanical activation (MA). When the sample milled for 0 min is mentioned in further text, it means the pre-milled one is discussed. MA was performed in a planetary ball mill Pulverisette 6 (Fritsch, Germany) under the following conditions: mass of ES - 5 grams, loading of the mill - 50 tungsten carbide balls of 10 mm diameter, volume of the milling chamber – 250 cm3, rotation speed of the planet carrier - 500 rpm, milling time 0 - 360 min, atmosphere - air, ballto-powder ratio (BPR) - 70. To make the sample identification more transparent, abbreviations are being used throughout the paper for ES samples milled for different time. They are listed in Table 1. Table 1. Specific surface area values, aragonite content and abbreviations for ES samples milled for different time. Specific surface area (m2 g-1)

Aragonite content (%)

ES0 ES3 ES30

Time of mechanical activation (min) 0 3 30

4.7 14.5 28.4

0 0 1.5

ES60 ES240 ES360

60 240 360

21.7 14.7 10.0

7.5 44.6 54.9

Sample abbreviation

2.3 Characterization methods 2.3.1 X-ray diffractometry. D8 Advance diffractometer (Bruker, Germany) equipped with CuKα radiation (40 kV / 40 mA), secondary graphite monochromator and scintillation detector was used for recording the powder XRD patterns. They have been obtained with steps 0.02˚ and fixed counting time 9 s/step. For the data treatment and analysis the commercial Bruker processing tools have been used, namely the Diffrac plus Eva for the phase identification and the Diffrac plus Topas for the Rietveld analysis and microstructure characterization. The peaks were assigned to particular phases by utilizing JCPDS-PDF2 database. The ratio between calcite and aragonite phase was calculated by comparing the intensities of the most intensive peaks of both phases. 2.3.2 Specific surface area. The value of specific surface area was determined by the low-temperature nitrogen adsorption method using a NOVA 1200e Surface Area & Pore Size Analyzer (Quantachrome Instruments, United Kingdom). The values were calculated using the BET theory. The nitrogen adsorption/desorption isotherms were collected on the same equipment and the pore size distribution was calculated using the Barret-Joyner-Halenda (BJH) method from the desorption isotherm. 2.3.3 Zeta potential. The zeta potential was measured using a Zetasizer Nano ZS (Malvern, United Kingdom). For each measurement, 10 mg of ES was put into 10 mL of distilled water or cadmium nitrate solution with the Cd(II) concentration of 200 mg L-1. The measurement was conducted within the pH range 2-10, which was adjusted by the addition of 0.1M HNO3 or 0.1M NaOH. 2.3.4 Scanning electron microscopy. SEM images were recorded using a MIRA3 FE-SEM microscope (TESCAN, Czech Republic) equipped with an EDX detector.

2.3.5 Adsorption tests. The adsorption ability of the ES was investigated on model solutions of chemically pure Cd(NO3)2.4H2O in distilled water with the desired concentration. The adsorption of cadmium was pursued in the concentration range 50-500 mg L-1. The standard adsorbent concentration was 1 g L-1, although for the experiments investigating the influence of adsorbent concentration on the adsorption ability the adsorbent concentrations in the range 0.1-3 mg L-1 were investigated. The adsorption experiments were performed in Erlenmeyer’s flasks placed on a laboratory shaker for different time (until the equilibrium was reached) at laboratory temperature. The pH was adjusted by the addition of 0.1M HNO3 or 0.1M NaOH into the solution. The solutions were then filtered through a standard filtration paper and the filtrate was analyzed with respect to the content of residual Cd(II) ions using an atomic absorption spectrometer SPECTRAA L40/FS (Varian, Australia). 2.3.6 Desorption tests. The selected sample after the adsorption process was dried and separated into fractions of 25 mg. The samples were then put into 25 mL of distilled water and were mixed for different time (30, 120 and 360 min). pH was not adjusted in the case of the desorption tests. After this process, the suspension was filtered and the filtrate was analyzed in the same way as in the case of adsorption tests.

3 Results and discussion 3.1 Influence of milling on phase composition and specific surface area of ES In Fig. 1, the XRD patterns for the mechanically activated ES samples are presented. After 60 min (ES60), significant peak broadening occurs due to disordering of calcite CaCO3 crystal structure as a result of breaking the particles into smaller ones. This phenomenon is common in mechanochemistry [32] and is connected with the diminishment of particles and strain creation in the milled solids. Moreover, the first traces of aragonite CaCO3 can be detected, which indicate the milling-induced phase transformation. Calcite-aragonite phase transformation observed during the milling is in accordance with the results published on mineral topic in the past [35, 37, 38]. Upon further milling the aragonite content increases, and after 360 min it becomes the dominant phase. This progressive calcite-aragonite phase transformation is also documented in Table 1.

Fig. 1 XRD patterns of ES0, ES60 and ES360 (C- calcite, A- aragonite).

Since the adsorption properties are closely related to the surface properties of solids, the value of specific surface area of the milled samples was analyzed subsequently. The results are given in Table 1. The value of specific surface area increases rapidly at the beginning of the milling process, which corresponds to the so-called Rittinger stage [32]. However, after 360 min of milling a significant decrease of the value of specific surface area is observed. This might be caused by the combination of two effects: (i) intensive agglomeration of particles and (ii) the presence of considerable amount of aragonite, which is a denser phase (ρaragonite = 2.94 g cm-1, ρcalcite = 2.71 g cm-1) and therefore can cause the breakdown of the porous structure of more brittle calcite. 3.2 Influence of pH on adsorption

W With h th he aim m to o op ptimiize the t pH p for f the t adso a orptiion of o Cd(II C I) io ons on o th he ES, E the t mea m sureemen nts o of zeeta pote p entiaal (Z ZP) aat diifferrent pH for mecchan nicaally activ a vateed saamplles, both h in watter and a in cadm mium m sallt so olutiion were w e carried d ou ut. The T cconccenttration of o Cd(III) ion ns in n thee so olutio on for f th hesee meeasu urem mentss waas 20 00 mg m L-1. I Fig. In F 2a, the zetaa po otenttial of the t samp s pless ES S60 and ES360 disperssed in pure p watter is i co omp pared d. Iff thee mo ost n negaativee vaaluess off ZP aree con nsid dered d as a meas m sure of the t suita s abiliity for f the t aadso orptiion of posit p tivelly ch harg ged Cd((II) iionss, it can be ssaid d thaat thee saamplle ES60 0 is more m e su uitab ble for fo th he adsor a rptio on o of Cd(III) at pH 6-7, wh hereeas th he samp s ple E ES3 360 at a lo owerr pH H valluess (pH H 3-5). The T fact, th hat th he ZP Z cu urvees arre no ot siimillar fo or both b mples is a prrooff thaat theere sam aare diffferen nt su urfaace char c rges preesentt at the interfaaces betweeen th he ES E and a watter, dependiing on the millling g tim me. N Nev verth helesss, ZP Z valu v ues are a gen nerallly m moree neegatiive for sam mplee ES S360 0, th hus pred p deterrmin ning g thiis saample to o bee a b betteer Cd(I C II) adsor a rben nt th han ES6 60. It I w will b be show s wn later l r thaat th his h hypo otheesis is corre c ect, nev verth helesss th he samp s ple E ES6 60 was w anal a lysed in n mo ore detaail, as a itt rep presentss a good g d co omprrom mise betw ween faair milli m ing ttimee an nd su ufficciently h high h ad dsorp ption n ab bility y. The T fact f thatt thee ZP P of ES is chan c ngin ng w with millling tim me co ould d alsso be off hig gh in ntereest for f aadso orptiion stud dies deaaling g witth differ d rentt ion ns, aas in ord der to acchiev ve th he maxi m imu um adso a rptio on ccapaacity y forr eacch io on, d diffeeren nt pH H is suittable. Itt seeems thatt it coul c ld be po ossib ble to t “ttunee” th he ad dsorrptio on abilit a ty of thee ES S maateriial tto bee a b betteer so orbeent for f the t corre c espo ondiing ion i by b chan c nging g thee du uration of o millin m ng. I Fig. In F 2b, 2 the t diffe d eren nce in i zeeta pote p entiaal in case off ES60 samp s ple mea m asureed in n waater and d sallt solutio on is ob bviou us. N Neittherr verry lo ow or very v y hig gh pH p is su uitab ble for f the t ES, beccausse att veery low l pH, calciu um from f m thee ES S is beiing d dissolveed,[17] whiile at a hiigheer pH H, Cd(O C OH)2 prrecip pitattes [39] [ . Th he reesults obtai o ined d forr pH H 6 aand 7 were w e thee mo ost p prom misiing, being sligh s htly bettter for pH 7, showin ng laargerr diifferrencee beetweeen zetaa po otenttial befo fore and d aftter the t ssorp ption n. In n casse off ad dsorp ption n at pH 7, zeta z poteentiaal +21.4 4 mV V was w aachieeved d, which w h suggests quite q e hig gh sstabiility of tthe susp penssion n. No prrecip pitattion n of Cd((OH H)2 was w eviidenced forr this pH H. M Moreoveer, the t disssoluttion n of Ca is the t lloweest at a th his pH p [17].

Fig. 2. Depenndencce off zetaa poteentiall of mille m ed ES S on pH: p (a) ( coompaarison of ES600 andd ES360 in i waater, (b) comp c parisoon off ES660 inn H2O and d in Cd(N NO3)2.4H H2O soolutioon.

T conf To c firm m thee ressultss ob btain ned from f m th he zeeta pote p entiaal measu urem mentts, aadsorrptio on of o Cd(II C I) att pH H 4 and a 7 was w p perfform med, witth th he ex xpecctatiion of adso a orptiion aabiliity to t bee lo owerr at lower pH. p Thee ressultss aree preesen nted in Fig. F 3. F From m th he figur f re itt is clea c ar th hat much m h beetterr ressultss weere o obtaained d fo or pH H 7. Moreo M overr, in n thee caase o of pH H 4, 4 affter rreacching g th he maxi m imum adso a orptiion afteer 20 0 min, m the Cd d(II)) ion ns are a obvi o ioussly deso d orbeed, as a evid e enceed b by the t d decrrease off qt valu v ue (aamou unt o of Cd(II C I) ad dsorrbed at a giv ven time t e t). Thee ressultss of this measurremeent aare in i acccorrdan nce w with h thee ressultss of zetaa pottential mea m sureemen nts and a also o with reef. [17], [ , wh here the incrreasse off adssorp ption n abiility y of ES E w with h thee inccreasing g pH H waas allso evide e enceed.

Fig. 3 Comparison of adsorption ability at pH 4 and 7 for ES60, cES60 = 1 g L-1, cCd = 200 mg L-1.

3.3 Influence of initial Cd(II) concentration on adsorption In Fig. 4, the relationship between the Cd(II) uptake and the initial concentration of Cd(II) in the model solutions after two hours of adsorption for sample ES60 is shown. It can be seen, that at lower concentrations, the equilibrium state and complete adsorption is achieved almost immediately, whereas at higher ones, much longer time is needed to reach the equilibrium, and the complete adsorption is not reached at all. Moreover, the higher the initial concentration, the higher the adsorption ability, as is documented by values of qt. For Cd(II) ions concentration 500 mg L-1, time of 120 min was not sufficient to achieve the equilibrium state.

Fig. 4 Influence of initial Cd(II) concentration on adsorption ability of ES60, cES60 = 1 g L-1, pH = 7.

3.4 Influence of adsorbent concentration on adsorption In Fig. 5, the adsorption ability represented by qt is plotted against ES60 concentration. A linear increase can be seen up to the concentration 2 g L-1. In this case, almost complete adsorption was observed within 2 hours. Therefore, further increase of adsorbent concentration could not lead to further linear increase. For the concentration 3 g L-1, complete adsorption was observed within 2 hours.

Fig. 5 Influence of adsorbent concentration on adsorption ability of ES60, cCd = 500 mg L-1, pH = 7.

33.5 Influ I uencce off egggsheell memb m bran ne (E ESM M) prresence on aadsoorptiion O Of part p ticullar inter i rest is the t influ uencce of o ESM E M preesen nce in the t ES on the adssorpttion n abiility y, ass it cou uld alter a r it d defiiniteely. As A was w sho own earrlier [17], E ESM M also ex xhib bited d som me aadso orptiion abillity towardss Cd d(II)) ion ns, howe h everr it w was mu uch low wer th han in casee off ES. Th hereforee wee ex xpectted thatt ES SM-ffree ES willl haave bettter adso a orptiion ability tthan n ES S co ontaiining g ES SM.. Th his expe e ectattion was ex xperimeentallly conf c firmeed, as can c be seen s n in Fig g. 6, in which the t aadso orptiion abillity of o tw wo samp s ples (wiith and a with w houtt ESM) millled for f 5 miin iss com mpaared..

-1 Figg. 6 In nflueence of prresennce of egg gshelll meembraane (ESM ( M) onn the adsoorptioon abbility of ES, E cES 5 mg m L-1, pH H = 7. 7 E = 5 g L , cCd C = 500

33.6 Influ I uencce off milllingg tim me on n ad dsorptioon T Thee adssorp ption n ab bility y off thee ES S tow ward ds cadm c mium m was w stud s died witth reespeect to o th he millin m ng time t e. Th he rresults are a ssum mmarrized d in n Fig. 7 (in n Fiig. 7a, 7 the kin neticc plo ots for all six sam mplees aare com c mpareed, wheereaas in n Fiig. 7 7b the t eequiilibrrium m con ncen ntrattions aree plo otted ag gainst th he tim me of o m milling)..

Fiig. 7 Influuencee of milli m ing tiime on o addsorp ption abiliity off ES:: (a) kinet k tic pllots, (b) qe vs. millling ttime.

A ccan be seen As s n fro om the t figu f ure, tthe adso orpttion abillity has sign nificcanttly in ncreeased with w the t millling tim me. Even E na v very y sho ort mill m ing (3 min) m ) has inccreaased thee adssorp ption n abiility y alm mostt two o-fo old. This T s miight be a co onsequence of the t fform matio on of o m many y freesh active sites s s forr thee intteracction n, whic w ch co ould d be posssiblly reelateed to t th he in ncreased d vaalue of sspeccificc surrfacee area [30]. [ . Th he beest resu r ults w weree ob btain ned for f long g-terrm mec m hanical actiivattion (sam mplees E ES24 40 and a E ES3 360). Allthou ugh the oveeralll adssorp ption n abiility y of thesse tw wo samp s ples is almo a ost equa e al affter reac r ching th he eq quiliibriu um sstatee, th he diifferrence is in the t time t e of reac r ching it.. The saamplle ES24 40 neeedss 4 hour h rs, wher w reas the sam mple ES360 neeeds o only y 3 hour h rs. T The rreasson for f this t beh havio or iss pro obab bly the t larg l er amou a unt of o arrago onitee ph hase in tthe samp s ple ES3 E 360, as iit is disccusssed in i th he neext para p agrap ph.

3 3.7 Infl I luen nce of o sp peciific ssurffacee areea and a amo a ountt of ara agon nite on adso a orpttion n D Durring the millling g of the ES, tw wo main m parrameeterss aree ch hang ging: (i) thee vallue of o speci s ific surffacee areea an nd (ii) ( the t aarag gonite cont c ent. Th he posit p tive infl fluen nce of millling in gen neral on n th he adsor a rptio on abili a ity of tthe ES hass beeen d dem monsstrated earli e ier, how weveer th he quest q tion n wh hich of thesse tw wo p paraametters plaays mor m e signiffican nt ro ole arisees. To T

ssolv ve this queestio on, two o pllots weere elab borated (Fiig. 8), 8 in whiich the equilib brium conc c centtratio on qe, whiich d dem monsstrates th he adsor a rptio on abilit a ty, is i plotted ag gainst th he tw wo param p meters.

Figg. 8 Relat R tionsship betw b een aadsorrption abiility and: (a) speci s ific suurfacce areea (S SA); (b) am mounnt of aragonitee; cESS = 1 g L-11, cCdd = 5000 mg L-11, p = 7. pH

F tly, qe was Firs w plottted agaainstt SA. As it caan be b seeen in i F Fig. 8 8a, this t is not a sing gle-valu ued funcction n, su uggeestin ng th hat SA is no ot th he keey param p meteer in n thee ad dsorp ption n prrocess. The T higher SA does d s no ot ineevitaably y briing abou ut th he in ncreease of q e. H How weveer, th he curvee preesen nted d can n be div vided d intto tw wo regio r ons: (i) the low wer part p which is i asssociiated d wiith shor s rter m millling (up to 30 3 min) m ), wh hen the sam mples co onsisst off alm mostt purre caalcitte, (ii) th he uppe u er paart whic w ch iss relaated d to long ger m millling,, wh hen the t arag a gonitte ap ppeaars iin th he saamples. H weveer, if qe is plott How p ted agai a nst the arag goniite conte c ent ((Fig g. 8b b), th he posit p tive corrrelattion beccom mes evide e ent, as the t in the p plott is very v y sim milaar to o thee on ne prreseented d in Fig g. 7b b. This T indiicatees th hat the majjor role r t incrreaseed aadso orptiion aabiliity of o th he m milleed samp s ples is play p yed by b arag a gonitte co onteent. It was w dem monsstratted earli e ier that t aragon nite is i a bettter aadso orbeent for f Cd thaan calci c te [40] [ . Th he diffe d eren nt beehav vior of thee tw wo phase p es was w exp plain ned on thee baasis of d diffeeren nces in theiir crrystaal sttructturess. Itt is inter i restiing thatt no o sig gnifiicantt ch hang ge in n qe valu ue w was obsserveed ffor the t ssam mple milled long ger than t n 240 min, m altho a ough th he in ncreaase in i arago a onitee co onten nt iss quiite pron p noun nced d (fro om 44.6 4 6% for f ssam mple ES2 240 to 57.9 5 9% for f sam s mple ES3 360)). On th he ottherr han nd, iit haas to o bee noted thatt thee up ptakee off Cd is almo a ost ccom mplette in n thee caase of o saamp ple ES24 E 40, so logic l cally y furtheer in ncreaase o of th he arago a onitte co onteent woul w ld not n lead to the t h high her adso orption ability at conccenttration 500 mg g L-1 . Itt is posssiblle th hat if highe h er conc c centrratio on of o Cd(II C I) was w

aappllied, thee difffereencee bettweeen th hesee two o samplles woul w ld be grreateer. I caan be It b co onclludeed th hat th he calci c ite-aaragonitte ph hasee traansfo ormaation n ass a cons c equencee off millling g is ben neficcial for f C Cd(III) adso a orptiion. 33.8 Adso A orpttion isottherm ms T Thee adssorp ption n iso otherrm of o saamp ple ES60 E 0 forr thee rem mov val of o Cd(III) is shown in Fig. F 9. Very V y rap pid iincreeasee of Cd((II) u uptaake in the t area a a of low wer conc c centtratio ons and d a plate p eau at high h her conc c centrratio ons can n be obsserved. Acccord ding to G Gilees, et e al., su uch shap s pe o of th he iso otheerm can n be classsifiied as a H2 H (h high h afffinity isotheerm)) [41 1]. IIn orrderr to deteermiine tthe mosst su uitab ble mod del for f the adsorpttion,, thee Laangm muirr, th he Freun F ndlicch and a the Du ubiniin-R Radu ushk kevicch mode m els w weree stu udieed. The T resu r ults ffor aall th hreee mo odels aree giv ven in T Tablle 2 and are disccusssed belo b ow.

Figg. 9 Adso A orptioon isootherrm off sam mple ES60 E 0, cESS = 1 g L-1 , pH H = 7. 7 T Tablle 2. Paraametters ffor aadsorrptio on off cadm mium m on n sam mplee ES6 60 ob btain ned b by ap pplyiing vario v ous mode m els Laangm muir mod m del

Dub binin n-Raaduscchkeewich h mo odel

F ndlicch m Freun modell

Qm (mg g g-1)

b (m mg g-1)

R2L

KF (m mol g-1)

nF

R2 F

Xm ((moll g-1)

32 28.9 947

2 2.252 2

1

24..289

0.2 221

0.72 20

3 52 39.15

2 kD (mol ( kJJ-2) --1.73 3E-0 02

R2D 0.93 33

33.8.11 Laangm muirr isootheerm.. Th he fiit off Laangm muirr equ uation [42]] waas exam e mineed, in orrderr to obttain the vallue of the t m max ximu um mon m nolay yer adso a orptiion capaacity y Q m:

( (1) w wheere ce an nd qe aree thee eq quilib briu um solutte co onceentraation n an nd th he eq quiliibriu um aadso orptiion capaacity y, reespeectiv vely,, Qm is tthe Lan ngmu uir cons c stan nt rep pressentiing the max ximum mon nolaayerr adssorp ption n cap paciity (amo ( ountt of adssorbeed meta m al io ons p per 1 gram g m off sorrben nt ass a mon nolaayerr) an nd b is thee adsorp ption n eq quiliibriu um cons c stan nt. The T lineearizzed Lan ngmu uir iisoth herm m fo or th he stu udieed saamp ple iss shown n in Fig. 10a. Both B the figu ure and a the corrrelaation n coeefficcientt in Tab ble 2 sho ow tthat ourr exp perim men ntal dataa perfecctly corr c relatte with w this t o saamp ple ES60 E 0 is as high h h as 328 8.9 mg m g-1, model. Qm vallue of w whicch is sig gnifi fican ntly high her com c mpareed to o other natu ural sorb bentts ev ven after theeir mod m dificaation n (seee Tabl T e 3)). It ccan be cconcclud ded from m th he resullts in Table T e 2 thatt meechaanical activ a vatio on iss a morre effec e ctivee meetho od fo or im mprrovin ng the t p prop pertiies of o naatural so orbeents than n theeir mod m dificaation n wiith vario v ous substan nces..

Figg. 100 Linearizzed issotheerms of saample ES S60 bby appplyinng: (aa) Laangm muir mode m el; (b)) Freeundllich mode m el; (c) Du ubininn-Raddushhkevich model m l.

Table 3. Comparison of maximum monolayer adsorption capacities (Qm) of milled ES and other natural sorbents Sorbent

Qm (mg g-1)

Reference

Milled ESM-free eggshell

328.9

this study

Eggshell

3.8

[17]

Bentonite

61.4

[23]

Iron oxide-modified bentonite

63.3

[23]

Natural corncob

1.6

[26]

Citric acid-modified corncob

42.9

[26]

Spent grain

17.3

[9]

Rice husk

8.6

[24]

NaOH-treated rice husk

20.2

[24]

Chitin

14.7

[10]

NaOH-modified pine bark

50.0

[25]

Wood apple shell

32.1

[29]

3.8.2 Freundlich isotherm. The fit of Freundlich isotherm [43], which is not restricted to the formation of monolayer and assumes that the adsorption occurs on heterogeneous surface of a solid, was also studied. The linearized form of the Freundlich equation is as follows:

ln   ln   ௙ ln ௘

(2)

where cs is the amount of the adsorbed Cd(II) (mol g-1); ce is the equilibrium solution concentration of Cd(II) (mol L-1); Kf (mol g-1) is the Freundlich constant, nf is a constant representing the adsorption intensity of the adsorbent. Kf and nf values can be obtained from the slope and the intercept of the linearized form (2), respectively. The linearized Freundlich isotherm for sample ES60 is shown in Fig. 10b. The obtained correlation coefficient, together with the figure indicates that the Freundlich model is not suitable for the studied system. 3.8.3 Dubinin-Radushkevich isotherm. The Dubinin-Raduskevich (DR) model [44] is generally applied for adsorption mechanism with a Gaussian energy distribution onto a heterogeneous surface. By its application, it is possible to evaluate more closely the adsorption type, since the application of Langmuir and/or Freundlich model do not provide such information. For the analysis, the equation (3) was used:

ln   ln   ௗ ଶ

(3)

where ε is the Polanyi potential, equal to RT ln(1+1/cs), Xm is the adsorption capacity (mol g-1), kd is a constant related to adsorption energy (mol2 kJ-2), T is the temperature (K), and R is the gas constant (kJ mol-1 K-1). Xm and kd values can be obtained from the linear plot ln cs vs. ε2, concretely from the intercept (equal to ln Xm) and slope (equal directly to kd), respectively. The values, together with the correlation coefficient R2D are given in Table 2 and the DR isotherm is shown in Fig. 10c. It is clear, that this model can be also applied for the present system, however its correlation coefficient is lower than in case of Langmuir model. The application of this model enables the possibility of the calculation of the mean free energy of the adsorption (the free energy change when one mol of adsorbate is transferred from infinity in solution to the surface of the adsorbent) from the equation (4) [45]:

E  2k  /

(4)

From the value of E, it is possible to estimate the type of the adsorption. The calculated E value for our system (5.38 kJ mol-1) hints to the physical nature of the adsorption [46]. However, this is in evident contradiction with the results presented later (desorption tests, XRD measurements). Nevertheless, it may refer to the fact that although in a very small part, also physical adsorption can take place. This would be in accordance with Flores-Cano, et al., who stated that the predominant adsorption mechanisms are precipitation, (nonreversible) and ion exchange (reversible) [17] and where the ratio between non-reversible and reversible adsorption was reported to be 68:32 (in percent). However, it has to be noted that in their case the non-milled sample was considered, in which the reversible adsorption should be much more significant, because of the absence of many active sites created during milling. In the milled sample, the role of irreversible adsorption should be more pronounced.

3.9 Kinetics of adsorption The results presented in the previous part were further analyzed from the kinetic point of view. Firstly, it was necessary to find the optimum kinetic model for the studied process. For this purpose, three models were applied on all ES samples: the Lagergen-first-order (LFO, eq. 5) [47], the pseudo-second order (PSO, eq. 6) [48, 49] and the intraparticle diffusion (IPD, eq. 7) [50] models. The kinetic equations are presented in their linearized forms [51]:

ln ଵ   ln  ଵ

೟



 ௞ మమ



ଵ ௤

(5) (6)

    /  C

(7)

where q1 and qt are the amounts of adsorbed Cd(II) ions in mg g-1 at equilibrium and given time t, respectively for LFO adsorption; k1 is the LFO rate constant for the adsorption process (min−1), q2 is the maximum adsorption capacity (mg g-1) for the PSO adsorption; k2 is the PSO rate constant (min−1); kp is the IPD rate constant (mg g-1 min-1/2), and C is the intercept. Firstly, ln (q1-qt) was plotted against t, in order to examine the fit of LFO kinetics. As can be seen from Table 4, very low correlation coefficients were obtained and the calculated value of q1 differs significantly from the experimental value. The LFO model is therefore not suitable for the analysis of the process. Secondly, the fit of PSO kinetics model was evaluated by plotting t/qt vs. t. As it can be seen from Table 4, the model fits almost ideally, as the correlation coefficients are almost equal to 1. The calculated values of q2 are very close to the measured experimental values and also to the maximum monolayer adsorption capacity Qm calculated from the Langmuir model. These results indicate that the adsorption is governed by the PSO equation and the ratelimiting step may be a chemical adsorption [23, 48], which is also in accordance with paper [17]. Thirdly, the plots of qt vs. t1/2 were elaborated, in order to examine the fit of the IPD model. The obtained results are of particular interest, because they differ significantly depending on the sample analyzed. The correlation coefficients suggest that the model exhibits different applicability, depending on the sample. This issue will be discussed in detail later. The obtained results using all three models for the selected samples are summarized in Table 4. Because the correlation coefficients obtained for IPD diffusion model exhibited unexpected behavior and in order to compare the applicability of each model quantitatively, the normalized standard deviation Δq was calculated (see Table 4). It can be calculated as follows [52]:

Δq %  100

∑  ೐ೣ೛  ೎ೌ೗ / ೐ೣ೛ మ 1

(8)

w wheere n is thee num mbeer of daata poin p nts. As A it caan be b seen, thee reesultts neeed to be b discu d usseed more m e deeeply y, beecau use w wheereass thee co orrellatio on co oeffficieents wou uld sugg gestt thee PS SO mode m el to o bee thee mo ost suita s ablee, thee results ob btain ned by u usin ng th he no orm malizzed stand s dard d dev viatiion sugg gestts th he IP PD mod m el. Mor M e lig ght on o th his p prob blem m is shed s d in the folllowiing p partt. Howeeverr, th he LFO L mo odel can n bee deefiniitely y ex xclud ded fro om furth f her con nsideeratiions and itts su uitab bilitty was w iinveestig gated d on nly on o th hree outt of six s sam s mpless. T Table 4. Kine K etic param meterss for adsoorptioon off caddmium m onn eggsshelll obtaainedd by aapplyying varioous m modeels

Tim me off millling g (m min) 0 3 3 30 6 60 2 240 3 360

Lage L ergen n-firsst-orrder (LFO O) m modeel k1 (min ( n-1)

2.3E 2 E-3 2.7E 2 E-3 1.4E 1 E-3

q1 (m mg g-1) 14.5 585 77.2 204 68.9 995

R21

Δq q (% %)

0.8 857 0.9 996 0.3 333

65.19 6 9 74.94 4 3 84.33

P udo-ssecon Pseu nd-o orderr (PS SO) mode m el k2 (g ( mg g-1 min n-1) 2.2E E-3 7.9E E-4 4.2E E-4 2.2E E-4 1.5E E-3 3.1E E-4

q2 (m mg g-1 )

R22

48 8.170 0.99 99 12 27.55 51 1 27 78.55 52 1 32 27.86 69 0.99 99 49 90.19 96 1 48 85.43 37 1

Δq q (%))

34.2 28 26.6 67 22.3 33 38.6 67 23.6 67 17.5 51

Intraapartticlee difffusio on (IP PD) mod del kp (mg g g-1 min n-1/2) 0.4567 1.4 414 1.64 458 2.0614 421 5.04 1.9954

C (m mg/g g)

R2p

Δ (%) Δq

Ri

33.71 3 16 8 75 83.57 22 27.80 05 24 47.50 09 34 40.15 59 41 15.87 75

0.29 93 0.51 19 0.70 01 0.95 55 0.67 74 0.55 56

31.6 65 19.7 79 4.6 66 2.2 28 9.8 8 5.8 86

0.34 45 0.42 21 0.213 27 0.22 0.37 73 0.14 47

33.100 Kin neticcs vss. miillin ng B Becausee off som me discr d repaancy y in find f ding the mosst su uitab ble kine k etic mod m del, the t resu r ults obta o ained d fo or thee psseud do-seecon ndo ordeer kiinetiic mode m el an nd th he in ntrap partiicle difffusio on mode m el were w examin ned in i more m detaail. 33.100.1 P Pseud do-ssecon nd orde o er (P PSO)) moodel. Th he PSO kineetic plotts fo or th he siix mille m d saamplles are a g giveen in n Fig g. 11a. As A ccan be seen s n, th he slope of the t plot p ts deecreasess with th he m milliing time t e. Th he plots p s forr thee sam mplees ES24 E 40 and a ES3 E 60 are a p praccticaally iden nticaal. When W n k2 is plott p ted agai a nst tthe mill m ling tim me (seee Fig. F 11b), drramatic deccreasse iss eviiden nced d in the t ffirstt 3 minu m utes.. Th he deecreease becomees leess ssigniificaant as a th he millin m ng time t e pro oceeeds aand afteer 60 0 miin, th he value v e off k2 d doess no ot ch hang ge signiffican ntly.. Giiving th his behav b viou ur in nto rrelattion with h the aragon nite con ntentt, beeing g thee maain drivi d ing fforcce off thee ad dsorp ption n, ass waas ev videenceed eaarlieer, itt is p posssiblee thaat th he in ncreease of arag a gonitte co onteent ““slow ws dow d wn” tthe decrreasse off k2 vallue. Furrtherr examinatiions on pseeudo o-seccond d order kineeticss revealled thatt qt valu v ues calc c culatted ffrom m th he eq quattion difffer from f m th hose exp perim men ntally y ob btain ned very y signiffican ntly at tthe begiinniing o of th he adso a orptiion p proccess. Th hus, Δq valu ues can c be impr i roveed iff qt valu v ues obta o ained d forr thee first 15 5 minut m tes are a omit o tted.. In such h caase, Δq Δ v valu ues for f samp s pless ES S0, ES60 E 0 an nd ES36 E 60 w would bee 17 7%, 9% and d 5% %, reespectively. Ho owev ver, such h ap ppro oach is not n aacceeptable, beccausse th he ussed mod del has h to be b vaalid for the who ole rang ge, not n just j d paart and a mor m reov ver, for the seleected tthe men ntion ned valu ues are stilll quiite high h h com mpaared to Δq Δ valu v es obtai o ined d forr oth her syste s em [53]]. It is aalso imp portaant tto note thatt thee diffferencess beetween Δ Δq and a R2 value v es caan d differ sig gnifiican ntly depe d endiing on o the t eequaation ns ussed [51]].

Fiig. 111 Ploots for pseeudo--secoond-oorderr kineetic mode m el: (a)) lineear plots; (b) k2 vs.. millling time.

3.100.2 Intr I raparticlle diiffussion (IP PD) mod m del. Thee adv vanttagee of app plyin ng th he IPD I model lies in its abili a ity to t prroviide iinfo ormaation n ab bout the mechan nism m off thee adssorp ption n, naameely whet w ther thee intrrapaarticcle d diffu usion n is invo olveed. In I Fig. F 12a,, thee plo ots ffor the t IPD D mo odel forr all six stud died d sam mplees are a given g n. The T mod del is i most m suittablee fo or th he saamplles E ES3 30 an nd ES6 E 60. The T fairr Δq q vallues obttaineed for f these t e saamplles sugg s gest thee rolle off inttraparticcle diffu d usio on. This T is

q quitte un nderrstan ndab ble if wee tak ke in nto acco a ountt thee hig gh SA vaaluess of thesse saamp ples,, wh hich suggestt thee porrouss strructu ure o of th he parti p icless, th hus enab blin ng possiible intrrapaarticlle diffu d usion n. As A th he pores p s arre beeing g creeated du urin ng th he milli m ing p proccess [54 4], in n caase of o th he samp s ples millled forr sho ort time t e (ES S0 and a ES3 3), it i is log gicall thaat th his mod m el iss no ot veery ssuitaablee. Th he samee situ uation iis in n thee casse o of th he saamplle mille m ed fo or lo ong timee (E ES24 40 and a ES3 E 360),, as they y un nderrgo tthe aaggllomerattion stag ge [3 32], in whic w ch th he sp pacees fo or po ossib ble IPD I D aree red duceed. In n Fiig. 12b, the relaation nship p beetweeen e ob kp an nd m milliing timee is shown. At the first vieew, the resu ults difffer signi s ificaantly y fro om thos t btain ned by appl a lying g PS SO m mod del, as more m e or lesss theey exhib bit eexacctly oppo o ositee beehav vior. A very v y sig gnificantt inccrease in n kP vallue is i ob bserrved in ccasee of sam mplee ES S240 0, which w h we arre un nablle to o ex xplaiin at thee mome m ent. How wev ver, it iss in the reg gion in which the t ccalccite : araagon nite ratio is equ ual to t 1, an nd th herefforee posssible ro ole of H Hedv vall effeect can c be draw d wn [55] [ , acccord ding to w whicch iff ph hase tran nsforrmaation n occcurs in the t syste s em, som me prop p ertiees ch hang ge rapid dly whe w n th he raatio of o th hesee phaasess is 1:1.

Figg. 122 Plotts forr intrraparticle diffuusion n kineetic m modeel: (aa) lineear plots; p ; (b) kp vss. millling timee.

A otherr in Ano ntereestin ng pointt is thatt th he pllots do nott paass thro t ugh h thee orrigin n, which w h in ndicaates som me degree of b boun ndarry laayerr con ntro ol an nd th he faact that t the intrrapaarticle diffu d usion n is not the ratee-co ontro ollin ng sttep [50] [ . Beecau use the t iinterrcep pt is possitiv ve fo or alll saamples, the adssorption n pro ocesss caan be b co onsiidereed rapid r d [56 6]. By B appl a lying th he to ools of tthis model, it is po ossib ble to t deeterm min ne th he in nitiall adsorp ption n beehav vior of th he samp s ple. To do this, t , thee equ uatio on (8) ( has h tto be uttilizeed:

( (9) w wheere

is the t initi i al adsor a rptio on facto f or of o th he IP PD m mod del and a te is th he ad dsorrptio on tiime when the t

eequiilibrrium m staate iss reaacheed. T The so-ccalleed chara c acteristiic cu urvee bassed on IPD I o ained d by y plo ottin ng q//qe model can be obta v vs. t/t t e. Theese ccurv ves are a give g en in n Fig g. 13. O Opticcally y, th he Ri vaalue can n be estiimatted from f m the pllot by b su ubtrracting the t v valu ue in n thee place,, wh here it startss to dev viatee fro om the t y-ax y xis (yD) from m 1, so Ri=1-y = yD. The T exacct value v e of Ri can c be 3) by ccalcculatted from f m th he baasic equ uatio on of o th he IP PD mod m del (eq. ( b using u g th he in nterccept from m th he in ntraapartticlee difffusiion p plott C and a equi e ilibrrium m con ncen ntrattion qe aas fo ollow ws[5 56]:

(1 10)

Fig. 13 Characteristic curves of the dimensionless intraparticle diffusion model.

Based on the Ri values, the systems can be divided into groups depending on their initial adsorption behavior. Ri values for all six milled samples are given in Table 4. All values fall into the range 0.1 < Ri < 0.5, which is characteristic for strongly initial adsorption- zone 3 [56]. The influence of milling can be noticed also in case of this parameter. Generally, it can be said that the parameter decreases with time of milling. It follows that the role of initial adsorption becomes more evident with the increasing milling time. The value for sample ES360 almost approaches the region of completely initial adsorption [56]. This is in agreement with the creation of more active sites during milling. The fact that the lowest value was obtained for sample ES360 is, again, due to the presence of considerable amount of aragonite. However the values obtained for samples ES3 and ES240 do not stick to the trend about decrease of Ri during milling. The former probably because there is no sign of aragonite in the sample, and the latter might be also a consequence of already mentioned Hedvall effect [55].

3.10.3 Kinetics conclusion. Although the discrepancy among the two methods of acquiring the best-fitting model remains still unsolved for the present case, according to [51], Δq method may be a better method than using R2. We do not strictly stick to this hypothesis, however the suitability of the IPD model for the samples with the highest values of SA is undeniable. Nevertheless, the almost ideal fit of PSO model cannot be overlooked and at the first view, it should definitely be considered more suitable. In general, it can be concluded that it is always advantageous to use both methods when discussing the most suitable model for the description of adsorption kinetics, because as was shown in this paper, the great R2 values do not have to consequently mean the agreement with the experimental data.

3.11 Desorption experiments After the adsorption of Cd(II) at initial concentration 200 mg L-1 using the sample ES60 (pH = 7, cES60 = 1 g L-1), the desorption experiments were performed. For each experiment, the Cd-laden sample was put into 25 ml distilled water in order to check the stability of the system. The experiment ran for 6 hours and the data were collected after 30 min, 120 min and 360 min and in all cases, the residual concentration of Cd(II) was below 0.25 mg L-1. This means that Cd(II) is adsorbed strongly on the ES and there is no danger of its leakage into water again. These results are different from those achieved by Flores-Cano, et al. [17], however the fact that in our case milled sample was used has to be stressed out again. It can be concluded that by introducing the operation of milling, we have transformed the reversible Cd(II) adsorbent into an irreversible one.

3.12 Characterization of product after adsorption The sample ES60 before and after adsorption process was characterized by nitrogen adsorption, in order to examine the changes dealing with the surface properties taking place during the adsorption process. The BET surface area of the adsorption product was measured and surprisingly its value was higher (30.3 m2 g-1) than that of the sample before the adsorption (21.7 m2 g-1). A possible explanation for this could be that the milled material is able to relax in the liquid environment during the adsorption process and that the bigger agglomerates

fform med durring the millling g aree desinteegraated into o sm malleer particcles. Ev ven if i th he diisinttegration is nott inv volv ved, Cd((II) iionss cou uld be b possi p ibly y adssorbed on o th he su urfaace of o ag gglo omerrated d paarticlles and a creaate new n , porrouss strructu ure. T The analyssis of o nitro n ogen n ad dsorp ption issoth herm ms on o Fig. F 14aa prrovid des info orm matio on abou a ut pore typ pes in the t m mateerials. The T shaapes of the isottherms do not n difffer ssigniificaantly y. T The pres p sencce off meesop porees in n bo oth samp s ples is d docu umeented d by y thee hy ysterresiss loo op. The T e shaape of the t iisoth herm ms in n th he arrea o of reelatiive pres p ssurees aroun a nd 1 su uggests aalso o thee prresen nce of maccrop poress. T The adso orption isottherrm of o th he samp s ple afteer th he Cd(II C I) adsorrptio on p praccticaally ccopiies the t deso d orptiion isoth i herm m off thee sam mplee befforee Cd d(II) adsorpttion. Th he faact, tthat the isottherm m off thee sam mple affter C Cd(III) adso a orptiion is i lo ocateed in n thee graaph abo ove the t one o of th he samp s ple befo b ore Cd(I C II) ad dsorrptio on, docu d umeents the t high h her v valu ue off speciffic su urfaace area a a in case c e of the form mer sam mplee. Neve N erth helesss, th he fo orceed cllosu ure of o th he hy ysteeresiis lo oop rresu ulting frrom tensile streengtth efffectt (T TSE)) [57 7] in n caase o of th he ssamp ple after a r Cd d(II)) ad dsorp ption n is mo ore ssigniificaant tthan n in casse off samplle beeforre ad dsorrptio on, whic w ch furth f her cconffirm ms th he laargeer vaalue of specificc su urfacce arrea of the t ssam mple afteer ad dsorp ption. T porre siize distr The d ribution ns caalcullated d fro om the desorpttion isottherrm of o saamplle ES60 E 0 beeforee and d affter the t Cd((II) aadso orptiion werre co omp pared d (F Fig. 14b). No N siignificaant diffe d erencce in n po ore size s s caan bee ob bserv ved,, thu us co onfiirming the t o obseervaation ns made m e fro om th he nitro n ogen n adssorp ption n iso otherrms. Th he maxim m mum m arroun nd 2 nm m is the t p proo of off thee preesen nce o of very v smaall meso m oporres. How weveer as waas conclludeed frrom the com mparrison off thee ressultss obttaineed from fr m thee porre siize d distrribu ution ns caalculated fro om the adsorpttion isottherms (nott sho own n herre), it iss parrtly a co onseequeencee also off thee TS SE. S Slig ght d diffeerencce b betw ween n thee two o saamplles can c be obse o erved d in n thee reg gion n of pore p e rad diuss (RP) beetweeen 3 an nd 10 1 nm. n IIn th his area a a, more m porres are a pres p ent in case c mple beffore adssorpttion n. Th hesee ressultss sug ggesst th hat sligh s ht ch hang ges of sam d dealling witth th he micro m ostru ucturre of thee ES S occur duriing Cd( C II) adso a orptiion.

Figg. 14 4 Texxturall anaalysiss of samplles beforee andd afteer adssorpttion : (a) nitro n ogen adsorptioon annd dessorpttion isoth i hermss; (b)) poree sizee disstribuutionss.

W h th With he aim a to obsserv ve iff an ny morp m phologiical chaangees take plaace durring thee ad dsorp ption proce p ess, SE EM m micrrogrraph hs beforre (F Fig. 15a) and a afteer (F Fig. 15b b) th he C Cd(III) aadso orptiion processs aree co omp pared d. No N signi s ificaant d diffeeren nce in i th he morp m pholo ogy can n be observeed betw b een the two o sam mplees, altho a ough h thee saamplle affter Cd(II) adso a orptiion sseem ms to t bee sliightlly more m e agg greg gated d. Itt is poss p siblee thaat up pon dry ying afteer th he ad dsorrptio on, aggr a regaates with w h difffereent aand mo ore poro p ous stru ucturre could d bee forrmeed, tthus resultin ng in i highe h er value v e off SA com mpaared to the t agg glom merattes b befo ore Cd(I C II) adsor a rptio on.

F 15 Fig. 1 Sccanniing ellectroon microg m graphhs off sam mple ES60 E 0: (a)) befoore adsor a rptionn; (b)) afteer adssorpttion.

T conffirm To m thee prresen nce of cadm miu um in i th he prod p uct, ED DX aanallysiss off thee graainss preesen nted in Fig.. 15 5b was w p perfform med. Thee resultiing spectru um is in n Fig g. 16 6. The T resu r ults conf c firm m thee preesen nce of o th he elem e mentss fro om whiich the t eeggsshelll is form med d an nd allso ssign nificaant amo ountt of cad dmiu um (22. ( 2%)) is evid dencced, thu us co onfirrmin ng tthe ssucccessfful aadso orptiion.

Fiig. 166 ED DX sppectru um of o sam mple ES660 aftter ad dsorpption.

I order In o r to exaamin ne th he phas p se co omp posittion n of the pro oducct affter Cd((II) adso a orptiion, thee XR RD meaasureemeents of ssam mpless ES S60 and d ES360 0 beffore and d aftter the t aadso orptiion proccesss weere perfo p ormed. Thee ressultin ng XRD X D paatterrns are a g giveen in n Fiig. 17. Thee difffereencees in n th he patterrns befo ore and d aftter the t adso orpttion pro ocesss arre ob bvio ous, as in the t ssam mpless beeforee ad dsorrptio on, the peaaks of both h caalcitte and a arag goniite phas p ses can n bee ob bserv ved,, wh hile aftter the t aadso orptiion, a siiginficiaant d decrreasee in casse off thee peaaks of arago a onitte ph hasee tak kes plac p e an nd more m eoveer, a new w ph hase of o otav vite CdC CO3 cou uld be b id dentiified d.

Fiig. 177 XR RD paatterss beffore (red ( lline) and afterr (blaack liine) adsor a rptionn: (a)) ES660; (bb) ES S3600

of ssam I case In c mple ES6 60, the arag goniite peak p ks allmost co omp pleteely disa d appeared d fro om the t sam mple afteer adsorrptio on. A Alth houg gh th he m majo or caalcitte peeakss aree mainttaineed in n th he m measu ured d 2 theta t a raange, theeir inten i nsity y haas deecreeased d. The T rresu ulting produ uct afteer th he Cd(I C II) adso a orptiion, acccord ding to the sem mi-q quan ntitattive anaalyssis o of th he XRD X D data, is

composed of 64% calcite (JCPDS 85-1108), 33.5% otavite (JCPDS 42-1342) and 2.5% aragonite (JCPDS 71-2392). Because calcite and otavite phases crystallize in the trigonal crystal system with rhombohedral symmetry (space group R-3c), the corresponding peaks in the XRD spectra strongly overlap. The resolved crystallite size is 17-19 nm for both components. In case of sample ES360 (Fig. 17b), the resulting product, according to the semi-quantitative analysis of the XRD data, is composed of: 45% calcite, 42% otavite and 13% aragonite. The resolved crystallite size is on the nanoscale, calcite and aragonite around 10 nm, otavite 19 nm. These results differ from the XRD results of Flores-Cano, et al. [17], who evidenced the formation of solid solution of (Ca,Cd)CO3 via slight shift of the calcite peaks in the spectra. The presence of otavite in the adsorption product in our case gives unambiguous evidence of the irreversible character of adsorption, as the adsorbed metal is stored in the form of new phase. As the amount of aragonite decreased very significantly during the adsorption, while calcite phase is much less affected, it can be stated that Cd(II) ions are adsorbed preferentially on aragonite. Nevertheless, also calcite phase is included in the adsorption process, as there is still a small amount of aragonite phase maintained after the adsorption while the intensity of calcite peaks has slightly decreased. Moreover, also ES samples which possess no aragonite (e.g. ES3) exhibit significant adsorption ability.

Conclusions In this paper it was demonstrated that by milling the eggshell biomaterial, it is possible to increase its adsorption ability towards cadmium significantly. It was found that the optimum pH for adsorption of Cd(II) is 7. The main driving force of adsorption is the formation of aragonite phase during milling. The adsorption process can be accurately described by the Langmuir model. The calculated maximum monolayer adsorption capacity is 329 mg g-1, which is by far the highest among the compared natural and modified adsorbents. The adsorption kinetics was further characterized by both the pseudo-second-order kinetic model and the intraparticle diffusion model. Although the former exhibited better correlation coefficients, the latter is more suitable according to better Δq values. The eggshell biomaterial exhibits significant initial adsorption behaviour. Almost no cadmium was detected in water after the desorption experiments, which is the proof that the adsorption process is irreversible. This was further confirmed by XRD measurements, in which the formation of new phase otavite CdCO3 was observed. During the adsorption, aragonite was more affected than calcite. The adsorption process was accompanied by increase of the value of specific surface area. The obtained results could be of significant interest also from the environmental point of view, since a simple treatment of abundant waste biomaterial can dramatically increase its ability to effectively adsorb Cd(II) from wastewaters and moreover, by milling the ES, it is possible to completely suppress Cd leakage after the adsorption process. The results suggest that it should be possible to “tune” the adsorption ability of the ES by changing the milling time, in order to make it suitable for the adsorption of different ions.

List of abbreviations ε- Polanyi potential (mol g-1) BPR- ball-to-powder ratio b- adsorption equilibrium constant in Langmuir equation C- intercept from the intraparticle diffusion model equation ce- equilibrium solution concentration of cadmium (mol L-1) cs- amount of the adsorbed cadmium (mol g-1) E- free energy of adsorption ES- eggshell ESM- eggshell membrane IPD- intraparticle diffusion

k1- Lagergen-first-order rate constant for the adsorption process (min−1) k2- pseudo-second-order rate constant (min−1) kd- constant related to adsorption energy in Dubinin-Radushkevich equation (mol2 kJ-2) Kf- Freundlich constant kp- IPD rate constant (mg g-1 min-1/2) LFO- Lagergen-first-order MA- mechanical activation n- number of datapoints nf- constant representing the adsorption intensity of the adsorbent in the Freundlich equation PSO- pseudo-second-order Δq- normalized standard deviation q1- amount of Cd(II) adsorbed at equilibrium for Lagergen-first-order adsorption (mg g-1) q2- maximum adsorption capacity for Cd(II) for pseudo-second-order adsorption (mg g-1) qe- amount of Cd (II) adsorbed at equilibrium (mg g-1) qcal- calculated value of adsorbed Cd(II) at equilibrium (mg g-1) qexp- experimental value of adsorbed Cd(II) at equilibrium (mg g-1) Qm- maximum monolayer adsorption capacity (mg g-1) qt- amount of Cd(II) adsorbed at given time t (mg g-1) Ri- initial adsorption factor of the IPD model te- time of equilibrium Xm - adsorption capacity (mol g-1)

Acknowledgements This work was supported by the Slovak Grant Agency (projects VEGA 2/0027/14 and 2/0114/13) and European Regional Development Fund (project nanoCEXmat II, ITMS 26220120035). The support of Centre of Excellence of Slovak Academy of Sciences (CFNT-MVEP) is also gratefully acknowledged.

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