Characterization of carbonated tricalcium silicate and its sorption capacity for heavy metals: A micron-scale composite adsorbent of active silicate gel and calcite

Available online at www.sciencedirect.com Journal of Hazardous Materials 153 (2008) 775–783 Characterization of carbonated tricalcium cro g Q uan Univ atham logy a ber 2 r 200 Abstract Adsorptio bear adsorbents i n, cha adsorbent composed of silica gel (15.9 w/w%), calcium silicate hydrate gel (8.2 w/w%) and calcite (75.9 w/w%), produced by the accelerated carbonation of tricalcium silicate (C3S, Ca3SiO5). The Ca/Si ratio of calcium silicate hydrate gel (C–S–H) was determined at 0.12 (DTA/TG), 0.17 (29Si solid-state MAS/NMR) and 0.18 (SEM/EDS). The metals-retention capacity for selected Cu(II), Pb(II), Zn(II) and Cr(III) was determined by batch and column sorption experiments utilizing nitrate solutions. The effects of metal ion concentration, pH and contact time on binding ability was investig to be 94.4 m treatment of © 2007 Else Keywords: Tr 1. Introdu Heavy m electroplati require rem the environ human rec heavy met cal separat chemical p not econom standards [ different m ferent pH be achieve ∗ Correspon E-mail ad 0304-3894/$ doi:10.1016/j ated by kinetic and equilibrium adsorption isotherm studies. The adsorption capacity for Pb(II), Cr(III), Zn(II) and Cu(II) was found g/g, 83.0 mg/g, 52.1 mg/g and 31.4 mg/g, respectively. It is concluded that the composite adsorbent has considerable potential for the industrial wastewater containing heavy metals. vier B.V. All rights reserved. icalcium silicate (C3S); Adsorbent; Adsorption capacity; Heavy metal; Wastewater ction etals are widely used by industry, for example, for ng, pigment and paint manufacturing. The metals oval from industrial effluents before discharge into ment to mitigate any impact on plant, animal and eptors. The most common treatment methods for al-bearing wastewater involve physical and chemi- ion techniques. Some conventional methods such as recipitation, ion exchange, and reverse osmosis are ically feasible or cannot satisfy current discharge 1–3]. For example, the minimum solubilities of the etals usually found in polluted water occur at dif- values and maximum removal efficiencies cannot d at a single pH level [4]. Consequently, the metals ding author. Tel.: +86 21 6779 2540; fax: +86 21 6779 2552. dress: [email protected] (Q. Chen). precipitation process is very difficult to control and is usually incapable of meeting discharge limits of 0.1–5 mg/L. Adsorption is very effective for the treatment of wastewater containing low concentration of heavy metals [5–7]. Activated carbon has been the standard adsorbent for the reclamation of municipal and industrial wastewaters, however, it requires regen- eration and its adsorption capacity for heavy metals is usually less than 50 mg/g [8–11]. Furthermore, the adsorption density of inorganic species onto activated carbon surfaces varies sig- nificantly with pH, in accordance with its acid–base behavior [12]. Cation exchange resins have a much higher adsorption capac- ity for heavy metals, ranging from 30 mg/g to 90 mg/g, with the maximum 200 mg/g [2,3,13], but the regeneration of spent resin is also problematic and costly. Therefore, recent developments have largely focused on low-cost adsorbents including natural minerals and industrial or agricultural by-products [5], includ- ing fly ash [14], granulated furnace slag [15,16], carbonaceous material [17,18], metal oxides [19,20], metal hydroxides [4], – see front matter © 2007 Elsevier B.V. All rights reserved. .jhazmat.2007.09.023 capacity for heavy metals: A mi adsorbent of active silicate uanyuan Chen a,∗, Colin D. Hills b, Menghong Y a School of Environmental Science and Engineering, Donghua b School of Science, University of Greenwich, Pembroke, Ch c Department of Materials, Imperial College of Science, Techno Received 12 October 2006; received in revised form 6 Septem Available online 8 Septembe n-based processes are widely used in the treatment of dilute metal- s the subject of continuing interest. This paper examines the preparatio silicate and its sorption n-scale composite el and calcite a , Huanhuan Liu a, Mark Tyrer c ersity, Shanghai 200051, PR China Maritime, Kent ME4 4TB, UK nd Medicine, London SW7 4AZ, UK 007; accepted 6 September 2007 7 ing wastewaters. The development of versatile, low-cost racterization and performance of a micro-scale composite 776 Q. Chen et al. / Journal of Hazardous Materials 153 (2008) 775–783 bentonite [21,22], zeolites [23,24], lignin, biomass and peanut hulls [25–32]. The adsorption capacity for heavy metals of these mater although th 99 mg/g [3 The rete eral calcite (217 mg/g) to Garcia-S the high bin due to meta Porous have been u organic ma ica gel has into siloxan dom three- calcium sil tivity for v comprises ing compos a large num macro-size of the rela structure an present. In orde investigate properties calcium sil of this com solutions a isotherm, k sented and 2. Materia 2.1. Mater The adso hydrate gel silicate (Ca 3:1 molar r Company, p ball mill, p the initial fi was repeate (Fig. 1). Th C3S, as des To ensu acceptable of 10:4 was CO2 at con and relativ saturated N 40 min, fol of carbona Fig. 1. Diffractograms of raw C3S and carbonated C3S. teria dio en g e are igh was n so his m colu een e em r si ium, (100 The u(N 3)2 d wa etho Cha . X- r and o ide ratin nely ground samples ( Zn(II) (185 mg/g) > Cd(II) (16.3 mg/g) according anchez and Alvarez-Ayuso [18]. They concluded that ding capacity particularly for Cr(III) and Zn(II) was ls precipitation rather than adsorption. silica gel (S–H) and its surface-modified products sed as adsorbents for the removal of heavy metals and tter from industrial or natural discharges [35,36]. Sil- usually been ascribed to the condensation of Si(OH)4 e chains, which branch and cross-link to form a ran- dimensional network. Hong and Glasser reported that icate hydrate gel (C–S–H) has a very strong sorp- arious metals [37,38]. Calcium silicate hydrate gel a large and complex ‘family’ of structures of vary- ition [39,40]. Both silica gel and C–S–H gel contain ber of pores, which range in scale from micro- to , and these influence sorptivity. An understanding tionship between metals sorption capacity and the d composition of silica-based sorbents is limited at r to address this deficiency, the following work s the preparation, chemical structure and adsorption of an active silica gel comprised of low Ca/Si ratio icate hydrate gel and calcite. The adsorptive capacity posite for Cu(II), Zn(II), Cr(III) and Pb(II) in nitrate re examined. The results of equilibrium adsorption inetic and column adsorption experiments are pre- discussed below. ls and methods ials rbent composed of low Ca/Si ratio of calcium silicate and calcite was prepared by carbonating tricalcium 3SiO5) or C3S. The C3S had been synthesized from a atio mixture of CaCO3 and SiO2 (Aldrich Chemical urity >98%) by grinding to a fine powder in a ceramic elletising and then sintering at 1500 ◦C for 2 h. After ring, the cycle of grinding, pelletising and sintering d until no free CaO was detected by X-ray diffraction e main peaks observed in Fig. 1 are typical of pure cribed by Taylor [40]. re complete carbonation of C3S was achieved in an time frame, a fresh C3S paste with a solid/liquid ratio placed in a carbonation chamber supplied with pure stant pressure of 0.3 MPa, a temperature of 17 ◦C e humidity of 50%, maintained by a small bath of aCl solution. The paste was exposed to CO2 for lowed by immediate grinding to Q. Chen et al. / Journal of Hazardous Materials 153 (2008) 775–783 777 5◦ and 40◦ 2θ, at scanning rate of 1◦ 2θ/min. The diffractograms were obtained with Diffplus and analyzed using Bruker/AXS EVA softw internation 2.2.1.2. Th 780 Simult ferential th Samples of tle to less < (5.8 mm di 30 ◦C and of nitrogen 2.2.1.3. So onance (M recorded o with a 7.1 T 59.49 MHz signal resol T1 � 30 s w were repor 2.2.1.4. Sc X-ray spec microanaly equipped w ANINCA, To provide elemental c yses were u the middle different ph 2.2.2. Adso Equilibr determine bent for me the batch so (100–600 m the adsorbe temperatur erwise state for 15 min filtrates we tively coup (Perkin-Elm difference i librium had means with 2.2.3. Colu Continu spex colum of ranging 50 mg/L, a height: 15 cm, BV 32 mL) at a flow rate of 5 mL/min using a peri- staltic pump. Effluent samples were collected from the column regular time intervals for analysis by ICP-AES. Leaching spent materials generated from the continuous flow tion studies were leach tested using the DIN 38414- ching procedure. Twenty grams of each spent adsorbent m) was placed in 200 mL of distilled water (L/S 10) in a bottle and rotated at 30 rpm for 24 h at an ambient tem- re of 17 ◦C. The leachates were then filtered through a m membrane filter prior to analysis by ICP-AES. ults and discussion haracteristics of the composite adsorbent show C3 ilica orph or ca the m el, c lowi H2 ed o t in ing e ) = CC loss 50 ◦C e an es an /w% /w% Fig are and compared with the current version of the al powder data file (ICDD-JCPDS). ermal analysis (DTA/TG). A Stanton Redcroft STA aneous Thermal Analyzer was used to conduct dif- ermal and thermogravimetric analysis (DTA/TG). 20 mg were ground with an agate mortar and pes- 30�m and packed in a rhodium–platinum crucible ameter and 4 mm high) and were examined between 1100 ◦C at a heating rate of 10 ◦C/min under a flow (40 cm3/min). lid-statemagic angle spinning/nuclearmagnetic res- AS/NMR). 29Si solid-state MAS/NMR spectra were n a Varian Infinity Plus-300 spectrometer equipped magnet, at the resonance frequency for 29Si, which is . All samples were spun at a rate of 6 KHz to improve ution. NMR experiments were operated at relaxation ith a relaxation delay of 10 s. 29Si chemical shifts ted relative to tetramethysilane (TMS). anning electron microscopy and energy dispersive troscopy (SEM/EDS). Morphological studies and ses were conducted using a JEOL JSM-640 SEM ith an energy dispersive X-ray spectrometer (LINK LINK ISIS 300) at an accelerating voltage of 15 kV. qualitative and semi-quantitative information on the omposition of the adsorbent matrix, 90 separate anal- sed. The incident beam was intentionally centered in of particle clusters to minimize the influence of the ases on the X-rays generated. rption capacity studies ium adsorption isotherm studies were performed to the relationship between the capacity of the adsor- tal ions and equilibrium metal ion concentrations. In rption studies, 200 mL of each metal nitrate solution g/L heavy metal) was shaken (30 r/min) with 1 g of nt in 250 mL screw-top plastic bottles at an ambient e of 17 ◦C. After a contact time of 60 min (unless oth- d below), the solutions were centrifuged at 3500 rpm and filtered through a 0.45�m membrane filter. The re analyzed in duplicate for using a dual view induc- led plasma-atomic emission spectroscopy, ICP-AES er). The adsorbent loading was determined by the n the ion content before and after the adsorption equi- been established. The results presented are simple a relative deviation of less than 5%. mn adsorption assays ous flow adsorption studies were carried out in a per- n packed with 75 g of the adsorbent with particle size in 0.2–1 mm. The initial solution concentration was nd was passed through the column (diameter: 2 cm, end at 2.2.4. The adsorp S4 lea ( 778 Q. Chen et al. / Journal of Hazardous Materials 153 (2008) 775–783 Table 1 Thermal analysis results of carbonated C3S Weight loss (%) 20–250 ◦C 3.1 250–600 ◦C 4.2 600–850 ◦C 33.4 Composition (%) H2O 7.3 CaCO3 75.9 C–S–H plus SiO2 gel 24.1 Ca/Si in gel 0.12 Fig. 4. Electr raw materia mately 24.1 to the ther silicate gel equation: Ca/Si = to An elec bonated C3 appeared a grains. The 3�m in siz The compo analysis of average Ca ses was 0.1 DTA/TG. I gel, with a this magnit Table 2 EDS microan Analysis spot 15 spots 63 spots 13 spots The average v F ty of emic id-st /NM pha aw C ral r mbe spec obtai bser hara nce on micrograph of the composite adsorbent (scale bar is 10�m). l (C3S). C–S–H gel and silica gel comprised approxi- % of the carbonated products by mass and according mal analysis. The estimated average Ca/Si ratio of capaci and ch Sol (MAS silicate from r a gene ing nu as Qn sities were o were c resona was 0.12, which was calculated by the following tal Ca − Ca in calcium carbonate total Si (3) tron micrograph showing a fracture surface of car- S is presented in Fig. 4. Silica gel and C–S–H gel s porous fibers whereas calcite appeared as equant porous silica gel aggregates were around 300 nm to e, whereas for calcite, grains were typically Q. Chen et al. / Journal of Hazardous Materials 153 (2008) 775–783 779 Table 3 Silicon species from raw and carbonated C3S based on the 29Si NMR spectra (%) Sample Raw C3S (%) Carbonated C Ca/Si = 2 where Q0, Q the respect in the carbo with the res analyses. It is ther prised silic Q3 domina (7). Q3 spec mC3S + qH →yCaC where m sh Q4 spec mC3S + qH where m sh 3.2. Adsor 3.2.1. pH v For sim gel and calc Heavy met (as heavy m composite The suspen measured ( (Fig. 6). T Pb(II), Zn( pH values gel and aft heavy meta pH in comp de-ionised The kin adsorbent i was 1000:1 Adsorption established was in dyna ite adsorbe 60 min for Fig. 6. Change in suspension pH with time. Sorp Lan tion uir aga 1 qmax qe ( ons ncen is a of t ilarl lg q 1 n lg Ce ); qe ent ( es th In o sorpt Q0 (ppm) Q3 (ppm) Q4 (ppm) −71.9 −74.5 −76.2 −100 −110 30.77 42.31 26.92 0 0 3S (%) 0 0 0 33.96 66.04 Q0 + 1.5Q1 + Q2 + 0.3Q3 = 0.17 (5) 1 , Q2, Q3 and Q4 are the percentages determined for ive species. Note that the Ca/Si ratio of C–S–H gel nated C3S pastes was 0.17, which was in agreement ults obtained from both the DTA/TG and SEM/EDS efore estimated that the carbonated C3S powder com- a gel (15.9%) and C–S–H gel (8.2%), assuming that tes C–S–H and Q4 for S–H, as shown as Eqs. (6) and ies: 2O + CO2 O3 +CaxH(m−2x)(SiO2.5)m·zCa[(OH)2, CO3]·nH2O (6) ould be a large number; n > 0; q > 0; z = 0, 1, 2, 3, etc. ies: 2O + CO2 → yCaCO3 + (SiO2)m·nH2O (7) ould be a large number; and n > 0; q > 0. ption studies ariation of suspensions and adsorption kinetics plicity, the carbonated C3S powder, containing silica ite is referred to as the ‘composite adsorbent’ below. al nitrate solutions with a concentration of 300 mg/L etal ions) were placed in sealed containers and the adsorbent was added at a solid/liquid ratio of 1:200. sion was agitated and the resultant pH values were Philips DW9418 pH meter) at regular time intervals he initial pH of the suspensions containing Cu(II), II) and Cr(III) ranged from 6.0 to 6.5. The suspension rose over time, due to the decalcification of C–S–H er 240 h, reached pH 7.0–7.5. The hydrolysis of the l cations in the nitrate resulted in a lower solution arison to the control suspension (pH 8.5) containing water and composite adsorbent. etics of heavy metals removal by the composite 3.2.2. The adsorp Langm of 1/qe 1 qe = where metal i ion co (mg/g) affinity Sim plot of lg qe = where (mg/L adsorb signifi tively. the ad s shown in Fig. 7. In this experiment, the L/S ratio (1 g/L) and employed 300 mg/L solutions (metal). increased with contact time and an equilibrium was when the concentration of metal in a bulk solution mic balance with that on the surfaces of the compos- nt. Equilibrium conditions were established within all systems investigated. Fig. 7. Ad tion isotherms gmuir and Freundlich equations are used to describe isotherms under constant temperature conditions. isotherm constants may be obtained from the plot inst 1/Ce: + 1 qmaxbCe (8) mg/g) and Ce (mg/L) are the amount of adsorbed per unit mass of adsorbent and the un-adsorbed metal tration in solution at equilibrium, respectively. qmax dsorption capacity and b is a constant related to the he binding sites (L/mg). y, the Freundlich constants may be obtained from the e against lg Ce: Ce + lg KF (9) is the equilibrium concentration of the metal ions the amount of metals adsorbed per unit mass of mg/g); KF (mg/g (L/mg)1/n) and n (dimensionless) e adsorption capacity and adsorption intensity respec- ther words, n gives an indication of how favorable ion process is; KF can be defined as the adsorption sorption kinetics of heavy metals on the composite adsorbent. 780 Q. Chen et al. / Journal of Hazardous Materials 153 (2008) 775–783 Fig. 8. Sorption isotherms for Cu(II), Pb(II), Zn(II) and Cr(III). or distribution coefficient and represents the quantity of metals adsorbed onto adsorbent at the equilibrium concentration. The Freundlich equation describes adsorption onto a heterogeneous surface. The slope of 1/n ranging between 0 and 1 is a mea- sure of surface heterogeneity, with the surface becoming more heterogeneous as its value gets closer to zero. 1/n above one is indicative of cooperative adsorption. The Langmir–Freundlich isotherm is the combination of both equation, and is an empirical model that incorporates the features of both isotherms as given in the following equation: qe = qLF1 + q where KLF the three a constants). Accordi experiment of metals was measured after a contact time of 60 min (Fig. 8). The linear fitting curves obtained from the sorption isotherms of Cu(II), Pb(II), Zn(II) and Cr (III) onto the composite adsor- bent are given in Table 4. The experimental data were further fitted to the Langmuir–Freundlich isotherm equation. The Langmuir–Freundlich isotherm constants were determined by minimizing the error in the experimental data by using the Langmuir EXT model in software Origin 6.0 using the Lang- muir constants as the initial values. The sorption constants for Cu(II), Pb(II), Zn(II) and Cr(III) on the composite adsorbent and regression coefficients (R2) are given in Table 5. From the results shown in Tables 4 and 5, it can be see that 1/n ranges between 0.07 and 0.18 for the Freundlich isotherms, indicat- ing that the surface heterogeneity of the composite adsorbent is very significant, indicating that all the ‘mineral’ compo- nents (silica gel, calcium silicate hydrate gel and calcite) take part in the adsorption reaction. From the regression coeffi- cient values given in Table 5, the very low R2 value of 0.47 shows that it is inappropriate to use the Freundlich isotherm to explain the sorption of Pb(II) ions whereas, the high R2 value of 0.98 for Pb(II) or Zn(II) shows that it is appro- priate to use the Langmuir isotherm. It is obvious that the regression coefficients of fitting to the Langmuir–Freundlich equation are higher than the individual Langmuir and Fre- undlich equations, demonstrating the suitability of the combined on. m th ty of g/g, u(II) lich hat t quen ption Table 4 Adsorption is Metal h isot Cu 7 lg C Zn 7 lg C Cr 8 lg C Pb Table 5 Adsorption co Ion L q Cu 3 Zn 5 Cr 8 Pb 9 Note: qmax in KLF(Ce)1/n LFKLF(Ce)1/n (10) ((L/mg)1/n), qLF (mg/g) and n (dimensionless) are djustable empirical parameters (modified Langmuir ng to the adsorption procedure described in the al section, the dependency of the adsorbed amounts on their equilibrium concentrations in the solution equati Fro capaci 94.4 m and C Freund cated t the se als sor otherms for the metals examined Langmir isotherm Freundlic 1/qe = 0.142/Ce + 0.032 lg qe = 0.0 1/qe = 0.203/Ce + 0.019 lg qe = 0.1 1/qe = 0.049/Ce + 0.012 lg qe = 0.1 1/qe = 0.012/Ce + 0.011 lg qe = 011 lg C nstants for the metals examined angmuir isotherm Freundlich isotherm max b R2 n KF R2 1.35 0.23 0.84 14.03 21.38 0.79 2.08 0.10 0.98 5.96 21.67 0.92 2.99 0.25 0.88 5.69 36.04 0.71 4.43 0.09 0.98 8.88 57.56 0.47 (mg/g), b in (L/mg), KF in (mg/g(L/mg)1/n), KLF in (L1/n/mg1/n), qLF in (mg/g). e Langmuir parameter, qmax (mg/g), the adsorption the composite adsorbent at saturation was 83.0 mg/g, 52.1 mg/g and 31.4 mg/g for Cr(III), Pb(II), Zn(II) , respectively. The regression analysis using the and the Langmuir–Freundlich isotherms also indi- he metals adsorption capacity and intensity were in ce: Pb(II) > Cr(III) > Zn(II) > Cu(II). The high met- capacity obtained can be compared to other typical herm Langmir-Freundlich isotherm e + 1.33 q = 27.77×0.001 C 4 e 1+27.77×0.001 C4e e + 1.34 q = 41.28×0.01 C 2.82 e 1+41.28×0.01 C2.82e e + 1.56 q = 78.14×0.16 C 1.47 e 1+78.14×0.16 C1.47e e + 1.76 q = 97.82×0.03 C 1.81 e 1+97.82×0.03 C1.81e Langmuir–Freundlich isotherm qLF KLF n R2 27.77 0.001 0.25 0.95 41.28 0.011 0.36 0.97 78.14 0.160 0.57 0.93 97.82 0.030 0.55 0.95 Q. Chen et al. / Journal of Hazardous Materials 153 (2008) 775–783 781 Table 6 Adsorption capacities for heavy metals Adsorbent Natural zeolit Slag Activated carb Calcite Synthetic tobe Activated mo Peanut Resin Carbon nanot adsorbents area and hi addition, ca [17,18,47], a positive s It is no atoms ten tion and becomes groups [1 or divalen cal adsorp the surface follows: (SiO2)m·nH → (SiO (SiO2)m·nH → (SiO (SiO2)m·nH → (SiO + H2O CaCO3/H+ → CaC CaCO3/OC → CaC CaCO3/OH → CaC Fig. 9. Influence of pH on heavy metal adsorption. me 3 spe tabil ilica diss ortan g pH eme Effe usin n of vy m n co ose w for t after n in l pH spec d ch erve se w cha qmax (mg/g) References e Cu 6.74 [23] Pb 32.26–95.24, Cu 26.21–88.50 [15,16] on Cr(VI) 20 [11] Cr 217, Zn 185, Cd 16.3 [18] rmotite Cd 2.0, Pb 1.99, Zn 1.94 [50] ntmorillonite Cd 33.2, Co 29.7, Cu 32.3, Pb 34.0, Ni 29.5 [51] Pb 30, Cu 10, Cd 6, Zn 10 [33] Pb 200, Cu 85, Cd 50, Zn 90 [13] ube Zn 13.04 [52] in Table 6 and can be attributed to the large surface gh sorption activity of silica gel and C–S–H gel. In lcite can bind heavy metals by chemical adsorption indicating that all the mineral components behave as ynergistic action. w generally accepted that the surface of silicon d to have a complete tetrahedral configura- that in an aqueous medium their free valence saturated with hydroxyl ions, forming silanol 6,37,38,46,48,49]. In the presence of trivalent t heavy metal ions (M3+ or M2+) the chemi- tion mechanisms of heavy metals in solution on s of silica gel and calcite can be illustrated as 2O/SiO2H2++ + M2+ 2)m·nH2O/SiO2M++ + 2H+ (at low pH) (11) 2O/SiOH + MOH+ 2)m·nH2O/SiOM+ + H2O (at mediate pH) (12) 2O/SiO− + MOHx(x−2)− 2)m·nH2O/SiOM(OH)x−1(x−3)− (at high pH) (13) The gel (Q meta-s both s gruent an imp causin enhanc 3.2.3. By solutio on hea solutio bent d values pH 4 As see optima metal cite, an be obs decrea surface + M2+ O3 /M2+ + H+ (at low pH) (14) a + MOH+ O3/M− + Ca2+ + H2O (at mediate pH) (15) − + MOHx(x−2)− O3/OM(OH)x−1(x−3)− + H2O (at high pH) (16) Fig. 10. Brea adsorbent bed chanism governing heavy metal uptake by C–S–H cies) is more complex because of its thermodynamic ity, giving rise to the adsorption characteristics of gel and calcium carbonate. Additionally, the incon- olution of calcium cations in C–S–H gel would play t role in the removal of heavy metals by, for example, to rise, the co-precipitation of double hydroxides and nt of adsorption due to variation in the surface charge. cts of pH on adsorption g CO2 and NaOH to regulate solution pH, the dis- calcite (at low pH) and the influence of initial pH etals removal, was investigated. The initial metals ncentration was 100 mg/L (w/w metal), the adsor- as 1 g/L and the contact time was 60 min. The pH he 100% CO2 saturated adsorbent suspensions were 30 min of contacting time, but pH 5 after 10 min. Fig. 9, the metals uptake was quite high with the values for the metal sorption being 8–10, due to iation, silica gel surface charge, the presence of cal- emical binding forces on the substrate surface. It can d that metals uptake by the composite adsorbent will ith increasing and decreasing pH, due to increasing rges at these pH’s. kthrough curves for heavy metals solutions filtered through the . 782 Q. Chen et al. / Journal of Hazardous Materials 153 (2008) 775–783 Table 7 Metals concentration in leachates from the spent composite adsorbent Element Concentration Leaching crit 0 * Note: Chi 3.2.4. Con The dat through the metal conc 30 and 80 b with the re the compos adsorption. concentrati to treat hea 3.3. Leach Calcite tact with a the column dure, previ were centr the leachat indicate th leaching of impact (Ta 4. Conclu The ma the binding for the rem main resea The full gel (15.9% size was co posed of ac had a high solution. Examin metals indi Pb(II), 83.0 respectivel 60 min; an metal ions The com removal of of concent the particle the sorbent work. nce orace tainin 76. Tuen stewa . Kur nt tec (200 . Bal roach m wa Babel m con J.S. Y uenti Wang utions –302 Rajesh t for resou Kikuc ivated 06) 1 Moon heav abel onut dising Pagna natura m aqu Dem (II), Z IR-1 . Hu stewa azar . Dim nulat Feng, ne wa (2004 . Zac cite, G Ca Si Cu (mg/L) 93.71 29.49 0.09 eria (mg/L)* – – 2–10 nese standard, HJ/T299-2007, GB175085.3-2007. tinuous flow adsorption a obtained from the percolation of metal solutions composite adsorbent bed are given in Fig. 10. The entrations surpassed the limits (0.1–5 mg/L) between ed volumes (BV) of percolating solutions. In keeping sults obtained in the equilibrium and kinetic studies, ite adsorbent showed the best capacity for Pb(II) ion It was possible to treat 150 BV, obtaining an effluent on of ≤1 mg/L, indicating that the column is suitable vy metal-bearing solutions. ing studies is dissolved from the composite adsorbent in con- cid media. The leaching of the spent adsorbent in experiments used the DIN 38414 leaching proce- ously described, at a S/L ratio of 1:20. The filtrates ifuged and filtered and after measuring the pH, and e was acidified for analysis by ICP-AES. The results at the heavy metal loaded adsorbent exhibited low heavy metals and thus, may have a low environmental ble 7). sions in objective of the present work was to investigate capacity of a cost effective adsorbent with potential oval of metal ions from aqueous waste streams. The rch findings are summarized as follows. y carbonated C3S consisted of calcite (75.9%), silica ) and low Ca/Si ratio C–S–H gel (8.2%). The particle mmonly in the range, 300 nm to 5�m and was com- tive silica gel, C–S–H gel and calcite. The material sorptive capacity for heavy metal ions in aqueous ation of the adsorbent retention capacity for heavy cated that: (1) the binding capacities were 94.4 mg/g mg/g Cr(III), 52.1 mg/g Zn(II) and 31.4 mg/g Cu(II), y; (2) equilibrium conditions were established within Refere [1] J. 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Chiu, Adsorption of Zn(II) from water with purified carbon s, Chem. Eng. Sci. 61 (2006) 1138–1145. Characterization of carbonated tricalcium silicate and its sorption capacity for heavy metals: A micron-scale composite adsorbent of active silicate gel and calcite Introduction Materials and methods Materials Methods Characterisation of the composite absorbent X-ray diffraction (XRD) Thermal analysis (DTA/TG) Solid-state magic angle spinning/nuclear magnetic resonance (MAS/NMR) Scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM/EDS) Adsorption capacity studies Column adsorption assays Leaching Results and discussion Characteristics of the composite adsorbent Adsorption studies pH variation of suspensions and adsorption kinetics Sorption isotherms Effects of pH on adsorption Continuous flow adsorption Leaching studies Conclusions References