RESEARCH ARTICLE

Sericite in the remediation of Cd(II)- andMn(II)-contaminatedwaters: batch and column studies

Seung Mok Lee & Lalhmunsiama & Diwakar Tiwari

Received: 27 August 2013 /Accepted: 28 October 2013 /Published online: 26 November 2013# Springer-Verlag Berlin Heidelberg 2013

Abstract Sericite, a mica-based natural clay was employed inthe remediation of waters contaminated with two importantheavy metal toxic ions, viz. Cd(II) and Mn(II), under batchand column experimentation. The batch reactor studies wereintended to study various physicochemical parameters, viz.effect of sorptive pH, concentration, contact time, and back-ground electrolyte concentrations which helped to deduce themechanism involved at the solid/solution interface. The per-cent uptake of Cd(II) and Mn(II) was increased with increas-ing of the sorptive pH, and almost 100% of these cations wereremoved at pH 10. Equilibrium-state sorption data wasmodeled and fitted well to the Langmuir and Freundlichadsorption isotherms. The kinetic data followed the pseudo-first-order and pseudo-second-order kinetic models. Increas-ing the background electrolyte concentrations by 100 timescaused significant decrease of the uptake of Cd(II) and Mn(II)ions, which inferred that these metal cations were lessadsorbed specifically and predominantly attached with rela-tively weak electrostatic attraction onto the solid surface.Additionally, the fixed-bed column reactor operations werealso performed to assess the suitability of sericite in theattenuation of Cd(II) andMn(II) from aqueous solutions underdynamic conditions. The breakthrough data obtained weresuccessfully utilized to fit into a nonlinear fitting of Thomasequation. The results showed that the naturally and abundantlyavailable sericite could be a potential natural material in the

remediation of aquatic environment contaminated with Cd(II)and Mn(II).

Keywords Sericite . Physicochemical studies . Isotherms .

Fixed-bed . Thomas equation . Cd(II)/Mn(II)

Introduction

An excess release of industrial effluent to natural water bodiescauses heavy pollutant load to the aquatic environment. In linewith this, the contamination of aquatic environment withheavy metal toxic ions is a serious concern because of itspersistency and the level of toxicity occurring towards biolog-ical systems. Various industries, viz. electroplating, metalprocessing, textile, battery manufacturing, tanneries, petro-leum refining, paint, pesticides, pigment manufacturing, print-ing, photographic industries, etc. are known to be culprit ofreleasing excessive amount of toxic metals, ultimately enter-ing into the aquatic environment and leading to significantenvironmental imbalances (Ahmaruzzaman 2011; Reddyet al. 2012). The metal or metalloids of major environmentalconcern are arsenic, cadmium, chromium, cobalt, copper,lead, manganese, mercury, nickel, and zinc (Bhattacharyyaand Gupta 2008). Unlike organic pollutants, heavy metalsare nonbiodegradable and are accumulated in living tissues,causing various diseases or biological disorders (Ngah andHanafiah 2008). Therefore, water contaminated with heavymetal toxic ions requires a proper and adequate treatment priorto its discharge into the aquatic environment.

Cadmium and manganese are known to be potential pol-lutants and showed several biotoxic effects once entered intothe biosystems (Rao et al. 2010; Omri and Benzina 2012). Thepermissible limit for Cd(II) and Mn(II) as demonstrated byWHO in potable water is 0.003 and 0.4 mg/L, respectively(WHO 2004). Cadmium is regarded as a foreign metal to the

Responsible editor: Angeles Blanco

S. M. Lee : LalhmunsiamaDepartment of Environmental Engineering, Kwandong University,Gangneung 210-701, Korea

Lalhmunsiama :D. Tiwari (*)Department of Chemistry, School of Physical Sciences,Mizoram University, Aizawl 796004, Indiae-mail: diw_tiwari@yahoo.com

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human body and is basically known as nonessential ion forbiological systems (Amoyaw et al. 2009). It has also beenclassified as a category 1 carcinogen (human carcinogen) bythe International Agency for Research on Cancer (IARC) dueto its potent behavior for lung cancer (Waisberg et al. 2003).Prostate cancer and other hormone-related cancers were alsoassociated with cadmium poisoning (Waalkes and Rehm1994; Akesson et al. 2008). The mechanism of acute cadmiumtoxicity involves the depletion of glutathione and protein-bound sulfhydryl groups, resulting in enhanced productionof reactive oxygen species such as superoxide ion, hydrogenperoxide, and hydroxyl radicals which in turn produces lipidperoxidation and results in DNA damage (Liu et al. 2009). Inaddition, Cd2+ ions replace Zn2+ ions in some enzymes,thereby affecting enzyme activity (Nagarethinam andGurusamy 2005). The other toxic effects of cadmium includekidney damage, renal disorder, high blood pressure, bonefracture, and destruction of red blood cells (Drush 1993). Onthe other hand, manganese is naturally occurring and is anessential trace element for humans, plants, and animals(Paschke et al. 2005; Fraga 2005); however, adverse effectresults from both deficiency and overexposure. The enhancedintake of manganese causes various toxic effects such asgrowth retardation, fever, sexual impotence, muscle fatigue,and blindness (Ahmaruzzaman 2011). A specific study con-ducted on manganese toxicity reveals that manganese mayprovoke peroxidative damage in all the tissues and the gills offish (Vieir et al. 2012). Therefore, there is a need and challengefor the development of efficient and cost-effective methods forthe low-level removal of these two metal ions from aqueoussolutions so as to meet stringent standards set by regulatorybodies.

The commonly employed methods are precipitation, ad-sorption, ion exchange, membrane separation, reverse osmo-sis, etc. (de Lima et al. 2011). Among these methods, adsorp-tion is recognized as a cost-effective and versatile method forthe treatment of wastewaters contaminated with heavy metaltoxic ions (Lalhmunsiama and Lee 2012). Moreover, flexibil-ity in design and operation to produce high-quality treatedeffluent is an additional advantage employing the adsorptionprocess (Fu and Wang 2011). Because of the high input costinvolved with commercial activated carbons, research interestlies in introducing cheaper alternative sorbing materials hav-ing comparable efficiency and suitability which could ulti-mately replace the use of activated carbon in wastewatertreatment methods (Fu and Wang 2011). The selection ofmaterials with natural abundance makes it cost-effective andenvironmentally benign. In this regard, claymaterials receivedgreater interest in recent past because of their low cost, highadsorption capacities, and natural abundance. There are sev-eral classes of clays such as smectites (montmorillonite, sap-onite), mica (illite), kaolinite, serpentine, pylophyllite (talc),vermiculite, and sepiolite (Ahmaruzzaman 2008). Literature

survey reveals that a variety of clay and modified clay materialswas employed and assessed in the removal of organic pollut-ants as well as inorganic pollutants from aqueous solutions.Varied inherent clay properties, viz. high specific surface area,chemical and mechanical stability, surface functional groups,surface properties, low cost, and ion exchange capacities, couldmake it a potential material in the treatment of waste/industrialeffluents (Lee and Tiwari 2012; Vieira et al. 2010). Sericite is amica-based clay mineral, having a compact layered structure,generally recognized as a white fine powder of muscovite form.The interlayer spacing of the (0 0 2) plane is 10 Å (Tiwari et al.2007). Previously, it was utilized in the attenuation of Cu(II)and Pb(II) or even Cs(I) from aqueous solutions, and the resultswere reasonably encouraging (Kim et al. 2013; Tiwari et al.2007). The present investigation, therefore, is an extension ofprevious studies (Kim et al. 2013; Tiwari et al. 2007). Sericite isfurther employed to assess its suitability in the remediation ofCd(II)- and Mn(II)-contaminated waters under batch and col-umn reactor operations.

Materials and methods

Materials

Sericite was obtained from Keumnam deposits, Gagokmyun,Samcheok City, Gangwon province, Korea. The sample wascrushed and sieved to obtain 14–16 mesh size using a me-chanical sieve. No further treatment was done prior to usingthe material for the experiments. The chemicals, viz. manga-nese chloride tetrahydrate, sodium nitrate, nitric acid, andsodium hydroxide were obtained from Duksan PureChemicals Co. Ltd., Korea. Cadmium sulfate was obtainedfrom Kanto Chemical Co. Inc., Japan. The deionized waterwas further purified using a Millipore water purification sys-tem (Milli-Q+) and hence was used for the entireexperimentation.

Characterization of the material

Characterization of sericite was performed by scanning elec-tron microscopy (SEM)/energy-dispersive X-ray spectrosco-py (EDX) analysis using a scanning electron microscopemachine (FE-SEM model: SU-70, Hitachi, Japan) equippedwith an EDX system. This could make it possible to study thesurface morphology and elemental composition of sericite.Furthermore, the cation exchange capacity (CEC) of sericitewas obtained by the known USEPA method 9080.

Speciation studies

The speciation studies were performed separately for Cd(II)and Mn(II) using MINEQL+ (version 4.5), a geochemical

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computer simulation program. The input parameters tak-en were the initial concentration of M(II), 10.0 mg/L,and temperature,: 25 °C.

Batch reactor studies

Batch experiments were carried out for various physico-chemical parametric studies, viz. the effect of sorptive pH,initial metal concentration, contact time, dose of the adsor-bents, and background electrolyte concentrations. The ad-sorption of Cd(II) and Mn(II) was investigated by taking0.1 g of sericite in 50.0 mL of sorbate solution having aconstant pH. The solution mixture was equilibrated in anautomatic shaker at 25±1 °C for a prolonged period of ca.12 h in order to obtain a complete equilibrium between thesolid and solution. The equilibrated solutions were thenfiltered using a cellulose mixed ester syringe filter (0.45 μ),and the final pH was measured and reported. The filtrateswere subjected to its bulk sorbate concentrations using anatomic absorption spectrometer (fast sequential atomic ab-sorption spectrometer: model AA240FS, Varian). The sor-bate solution pHwas always adjusted by adding drops of 0.1-mol/L HNO3 or 0.1-mol/L NaOH solutions during the entireexperiment.

Fixed-bed column reactor studies

The column experiments were performed using a glasscolumn having an inner diameter of 1 cm. The columnwas packed with 1.0 g of sericite kept in the middle of thecolumn, and the rest was packed with glass beads. Theinfluent solution of 10.0 mg/L of Cd(II) or Mn(II) having

a constant pH 5.0 was pumped upward from the bottom ofthe column using Acuflow Series II high-pressure liquidchromatograph at a constant flow rate of 1.00 mL/min.Effluent samples were then collected using Spectra/ChromCF-2 fraction collector, filtered with a syringe filter0.45 μ, and the total bulk sorbate concentration wasmeasured using AAS.

Result and discussion

Characterization of the sample

The SEM image shown in Fig. 1 clearly demonstrated thatsericite possessed a very compact structure with an orderedlayer having no meso- or micropores on its surface. Thealuminosilicate layers were forming compact interlayers.The EDX analytical data was obtained and shown graphicallyin Fig. 2. The main chemical composition of sericite is oxy-gen, silicon, carbon, aluminum, and potassium. The atomicpercentage and weight percentage of the elements as obtainedby EDX analysis were shown in Table 1. It was observed thatreasonably high amount of potassium was present in sericitewhich, perhaps, plays a prominent role in the ion exchangeprocess likely to occur with the metal ions. It was reported thattrace amounts of alkali and alkaline earth metals such as Na,Ca, and Mg are also present in sericite which are likely to beexchanged in the sorption process (Tiwari et al. 2007). Fur-ther, the CEC, as determined by the USEPA method, wasfound to be 37.22 meq/100 g of sericite. The significantlyhigh CEC value of sericite inferred it suitable for ion exchangereaction with heavy metal ions in wastewater treatment.

Fig. 1 SEM image of sericite

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Speciation of Cd(II) and Mn(II)

The speciation data obtained by the MINEQL geochemi-cal program for Cd(II) and Mn(II) were presented graph-ically in Fig. 3a, b. The data obtained from this studymade it possible to understand the sorption mechanism ofthese ions onto the solid surface at various pH conditions.Cd(II) speciation showed that up to pH 8.5, it exists asCd2+-soluble cationic species. Beyond pH 8.5, cadmiumturns into insoluble Cd(OH)2(S) species at pH 10.0, andabove this, Cd(OH)2(S) is the only species present in thesolutions. In between, at pH ~9.2, approximately 5 % ofcadmium exists as Cd(OH)+ species. Similarly, speciationof Mn(II) shows that manganese exists as Mn2+ untilpH 9.0, and beyond pH 9.5, manganese starts precipitat-ing as insoluble pyrochroite, i.e., Mn(OH)2(S). The insig-nificant percentage of Mn(OH)+ occurred between pH 8and 11 with a maximum of 6.2 %.

Batch reactor operations

Effect of pH

The pH of the solution is an important parameter in metaluptake from the aqueous solutions as it determines the surfacecharge of the adsorbent, the degree of ionization, and thespeciation of the adsorbates in the aqueous media (El-Ashtoukhya et al. 2008). The effect of pH was studied, vary-ing the sorptive pH from 2.0 to 10.0 for a constant sorbateconcentration of ~10.00 mg/L. The pH edge curves obtainedwere presented as percent removal of the metal ions as afunction of the final pH of the sorptive solution in Fig. 4. Itis observed that increasing the solution pH from 2.0 to 10.0significantly favored the removal of Cd(II) and Mn(II) fromaqueous solutions. Increasing the pH from 2.26 to 9.20, thecorresponding Cd(II) was removed from 4.6 to 100.0 %,respectively. Similarly, the uptake of Mn(II) increased from7.8 to 100.0 % with increasing of the pH from 2.10 to 8.11. It isclear that a significantly low percent uptake occurred for thesetwo metal cations at lower pH values, i.e., around pH 2 ~ 3; thisis explicable by the fact that at these low pH values, both thesurface of sericite and metal ions were carrying net positivelycharged and hence rendered strong repulsive forces (Tiwari et al.2007). Additionally, at these pH values, there could be a strongcompetition between the H+ ions and the metal cations towardthe same active sites of the solid surface, and an excess of H+

ions causes suppression of the possibilities of metal cationsorption onto the solid surface (Gupta et al. 2010, 2011;Lalhmunsiama et al. 2013; Lee et al. 2010; Tiwari et al. 2011).However, a gradual increase in pH from 3.0 to 6.0 slightly

Fig. 2 EDX analytical graph ofsericite

Table 1 Atomic and weight percentage of various elements present insericite obtained by EDX analysis

Elements Atomic percentage Weight percentage

C 13.89 8.32

O 52.91 42.20

Al 7.45 10.02

Si 19.52 27.33

K 6.25 12.13

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increased the percent uptake of these two cations, viz. Cd(II)and Mn(II) from aqueous solutions. This is because of thegradual decrease in H+ ion concentrations in the solution andalso the dissociation of the surface active groups of the

sericite, mainly the silanol group (pH ~2–3, pHPZC ~2.5)(Tiwari et al. 2007). This results in greater electrostatic attrac-tion operative between metal cations and the negativelycharged surface of sericite. This apparently enhances the metaluptake by the solid surface (Jiang et al. 2010). A furtherincrease of pH until pH 7.5 produces a sharp increase inpercent uptake for Cd(II) and Mn(II). It is reported previouslythat in this pH region, the aluminol group is to be dissociated(pHPZC ~ 6.22) (Schindler et al. 1987), and this may causefurther increase in surface negative charge density. Therefore,this may significantly facilitate the electrostatic attraction ofthe metal cations toward the negatively charged solid surface(Kaya and Hakam 2005; Kim et al. 2013; Tiwari et al. 2007),resulting in a significant increase in percent removal of Cd(II)and Mn(II). It is worth mentioning that the metal ions stillcarry positive charge as presented in the cationic form (videspeciation studies, Fig. 3). Beyond pH 8.5, there is a greaterpossibility of metal ions precipitating into its insoluble speciesCd(OH)2(S) and pyrochroite, respectively. Therefore, thecomplete removal of Cd(II) and Mn(II) at a high basic medi-um is, perhaps, a mixed effect of adsorption andcoprecipitation of the metal ions onto the solid surface of thematerials employed (Farley et al. 1985; Lee et al. 2012).

Effect of concentration

The effect of sorptive concentration was carried out by in-creasing the initial sorbate concentration from 2.0 to 25.0 mg/L for Cd(II) and from 1.0 to 20.0 mg/L for Mn(II) at pH 6.5(final pH). The equilibrium stage percent sorption of Cd(II)and Mn(II) is presented graphically in Fig. 5 as a function ofsorbate concentrations (mg/L). At a low sorptive concentra-tion, an enhanced percentage of adsorption occurred whichgradually decreased with increasing of the initial sorptiveconcentration. It was further noted that increasing the Cd(II)

a

b

Fig. 3 Speciation of a Cd(II) and b Mn(II) as a function of pH (Cd(II) orMn(II) concentration, 10 mg/L)

Fig. 4 pH edge curves in the removal of Cd(II) and Mn(II) by sericite

Fig. 5 Removal of Cd(II) and Mn(II) by sericite at various initial sorbateconcentrations

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initial concentration from 2.00 to 25.28 mg/L caused thepercent uptake of Cd(II) to decrease from 68.00 to 24.21 %,respectively. Similarly, the percent uptake ofMn(II) decreasedfrom 57.33 to 20.45 % with increasing of the Mn(II) concen-tration from 1.50 to 19.56 mg/L, respectively. This is explica-ble by the fact that at low concentration of metal ions, arelatively large number of active sites were available on thesolid surface, but on increasing the sorptive concentration forthe same number of surface active sites (having a constantdose of solid), a relatively lesser number of active sites areavailable for increased number of sorbing ions, resulting inless percent removal of the metal ions at higher sorbate con-centrations (Mishra et al. 1997, 2004; Tiwari et al. 1999).However, increasing the initial sorbate concentration causedthe amount adsorbed for both of these metal cations toincrease.

The concentration dependence data obtained at equilibriumstate (i.e., between solid and solution interfaces) is furtherutilized to perform the adsorption isotherm modeling usingthe known Freundlich adsorption isothermmodel (Yusof et al.2010; Gupta et al. 2005). The Freundlich adsorption equationis taken to its linear form (Eq. 1):

logae ¼ 1

nlogCe þ logK f ð1Þ

where ae and C e are the amount adsorbed (mg/g) and bulksorptive concentration (mg/L) at equilibrium, respectively. K f

and 1/n are the Freundlich constants referring to the adsorption

capacity and adsorption intensity, respectively. The graphs areplotted between log ae versus logCe, and it is observed that thesorption data fitted reasonably well to the linearized form of theFreundlich equation. Therefore, the Freundlich constants alongwith the R2 values are obtained and returned in Table 2. Thefractional values of 1/n (0 < 1/n < 1) obtained for these systemspointed it towards heterogeneous surface structure of the ma-terial employed with an exponential distribution of the activesites (Benes and Majer 1980). Moreover, relatively highervalues of the Freundlich sorption capacity obtained inferredthe strong affinity of sericite towards these two metal cations(Lalhmunsiama et al. 2013). Similar results were reportedpreviously (Gupta et al. 2009).

Similarly, the concentration dependence datawere fitted to thelinearized Langmuir adsorptionmodel (Yusof et al. 2010) (Eq. 2)for the estimation of the maximummonolayer coverage of metalcations onto the solid surface (q0) for the studied concentrationrange. Moreover, the Langmuir adsorption isotherm was pro-posed with the basic assumption that the surface of the solidsorbent materials contained a fixed number of active sites andsaturation of these sites resulted in complete monolayer cover-age; hence, the monolayer sorption capacity could be estimatedwith the isotherm modeling (Li et al. 2009; Mittal et al. 2009)

Ce

q¼ 1

qobþ Ce

qoð2Þ

where q is the amount of adsorbate adsorbed per unit weightof adsorbent (mg/g) at equilibrium; Ce is the equilibrium bulkconcentration (mg/L); qo is the Langmuir monolayer adsorp-tion capacity, i.e., the amount of sorbate required to occupy allavailable active sites in unit mass of solid sample (mg/g ormmol/kg); and b is the Langmuir constant (L/g). C e/q versusCe data was plotted, and a good correlation was obtained forboth ions, i.e., Cd(II) and Mn(II). The estimated values ofLangmuir monolayer adsorption capacity (qo) and Langmuirconstant (b ) along with the R2 values are presented in Table 2.The Langmuir fitting is better than the Freundlich isothermfitting for these two cations. Table 2 clearly revealed that theLangmuir monolayer capacity was found to be higher forCd(II) than Mn(II). The higher values of Langmuir constants(b ) reflected towards the strength and affinity of sericite forCd(II) and Mn(II) (Gupta et al. 2005; Tiwari and Lee 2012).

Table 2 Langmuir andFreundlich constants along withthe R2 values obtained for thesorption of Cd(II) and Mn(II) bysericite

Sample Langmuir Freundlich

qo (mmol/kg) b (L/g) R2 1/n K f (mmol/kg) R2

Cd(II) 30.81 0.341 0.987 0.411 8.75 0.914

Mn(II) 46.63 0.325 0.969 0.525 10.87 0.946

Fig. 6 Effect of contact time on the removal of Cd(II) and Mn(II) bysericite

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Effect of contact time

Time-dependent sorption of Cd(II) and Mn(II) was carried outat pH 6.0 and the initial sorptive concentration of ~10 mg/L.Further, the sorption was calculated in terms of percent re-moval and presented as a function of contact time in Fig. 6. Itis observed that a fast initial uptake of these two cationsoccurred on the sericite, which slowed down with the lapseof time. An apparent sorption equilibrium was achieved with-in the contact of 20min for Cd(II) and 60min for Mn(II) at thesolid/solution interface. Further, increase in contact time af-fected the percent uptake of these cations, i.e., Cd(II) andMn(II). Previous research also showed that the adsorptionequilibrium for these metal ions were attained rapidly by usingdifferent adsorbents, viz. Turkish illitic clay for Cd(II) andmanganese oxide-coated zeolite for Mn(II) (Ozdes et al. 2011;Taffarel and Rubio 2010). The pseudo-first-order kinetic mod-el (Cetinkaya et al. 1999) and pseudo-second-order kineticmodel (Ho 1995, 2006) were used for the experimental dataobtained to clarify the adsorption kinetics of Cd(II) andMn(II)by sericite. The linearized equations used for the pseudo-first-order and pseudo-second-order kinetic models given withEqs. 3 and 4, respectively:

ln qe−qtð Þ ¼ lnqe−k1t ð3Þ

t

qt¼ 1

k2q2eþ t

qeð4Þ

where qe is the maximum adsorption capacity (mg/g) and qt

(mg/g) is the amount of the metal ion adsorbed at time t . k1(min−1) and k2 (g/mg/min) are the adsorption rate constant forthe pseudo-first-order and pseudo-second-order model. Theunknown values such as adsorption capacity (qe) and the rateconstants (k1 and k2) were estimated from the fitting of thestraight lines plotted between ln (qe − qt) versus t and be-tween t

qt against t , respectively for the pseudo-first-orderkinetic and the pseudo-second-order kinetic models. Thefitting results obtained were shown in Table 3. It was observedthat the kinetic data were best fitted to the pseudo-second-order kinetic model with R2 values of 0.999 both for Cd(II)and Mn(II). It was reported earlier that the sorption of divalentmetal cations onto the Sphagnum moss peat followed thesecond-order rate laws and further suggested that the metalcations were bound onto the biosorbent surface by strongchemical forces operating between the sharing or exchangeof valency electrons of peat and divalent metal ions formingthe covalent bonds (Ho 2006; Ho andMcKay 2000). Also, therate of second-order reaction could depend on the sorbate ionconcentration available on the surface and the amount ofsorbate ions sorbed at equilibrium (Ho 2006; Ho andMcKay 2000). Additionally, it was observed that a maximumand significant amount of Cd(II) and Mn(II) was sorbed ontothe sericite within just 5 min of contact time. Beyond that,only insignificant increase in sorption of these ions was ob-served even after 30 min of contact. Therefore, an efficientuptake of Cd(II) and Mn(II) pointed towards the electrostatic

Table 3 Rate constants (k1 andk2) and sorption capacity (qe)along with the R2 values obtainedfrom pseudo-first-order and pseu-do-second-order kinetic models

Sample Pseudo-first-order model Pseudo-second-order model

k1 (min−1) qe (mg/g) R2 k2 (g/mg/min) qe (mg/g) R2

Cd(II) 11.46×10−2 4.455 0.732 57.71×10−2 2.322 0.999

Mn(II) 7.51×10−2 4.925 0.952 68.18×10−2 1.604 0.999

Fig. 7 Effect of sericite dose on the removal of Cd(II) and Mn(II)Fig. 8 Effect of background electrolyte concentrations on the removal ofCd(II) and Mn(II) by sericite

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attraction of metal cations towards the solid surface carryingnet negative charges.

Effect of adsorbent dose

In order to optimize the effect of adsorbent dose in the removalof Cd(II) and Mn(II), the experiments were performed byincreasing the dose of the sericite from 1.0 to 10.0 g/L withthe constant initial sorptive concentration of ~10 mg/L andpH 6.0. The results were plotted as percentage removal versusdose of the adsorbents and shown in Fig. 7. The percentremoval of Cd(II) and Mn(II) increased with an increase inadsorbent dosage. This could be explained by the fact that

while increasing the dose of adsorbents, the surface area or thenumber of active sites naturally increases for sorbing species(Amarasinghe and Williams 2007; Singh et al. 2008). Onincreasing the dose of the adsorbents up to 6.0 g/L, about80 % of Cd(II) is removed, and further increase in sorbentdose beyond 6.0 g/L resulted in insignificant change in per-cent sorption of Cd(II). On the other hand, the uptake ofMn(II) increased continuously with increasing of the dose ofthe adsorbents, and finally about 72 % was removed at asericite dose of 10.0 g/L.

Effect of background electrolyte concentration

The background electrolyte concentration-dependent sorptionof metal cations by soil or minerals is usually used to distin-guish between the nonspecific and specific adsorption pro-cess. Therefore, it is an important parameter to study thebinding nature of the metal cations onto the solid surface(Hayes et al. 1988). Outer-sphere complexes were involvedonly with electrostatic or van der Waals interactions and arestrongly affected by the ionic background electrolytes in theaqueous phase, whereas inner-sphere complexes were in-volved with stronger covalent or ionic bonds and are weaklyaffected by background electrolyte concentrations (Sparks1995). Therefore, in order to study the nature of sorption,the background electrolyte concentration was increased from0.001 to 0.1 mol/L NaNO3 (100 times) at pH 6.0, keeping theinitial adsorbate concentration of ~10.0 mg/L constant. Theresults obtained were plotted with percent removal versusbackground electrolyte concentrations and shown in Fig. 8.It was observed that increasing the background electrolyteconcentrations from 0.001 to 0.1 mol/L caused the uptake ofCd(II) and Mn(II) to decrease from 41.06 to 6.76 % and 37.98to 10.72 %, respectively. The significant decrease in percentremoval of these two cations in the presence of backgroundelectrolytes inferred clearly that the Cd(II) and Mn(II) ions arepredominantly sorbed by weak electrostatic or even van derWaals attractive forces and form outer-sphere complexes atthe solid surfaces (Lee et al. 2012). It is also suggested that thecompetition between the cations of the salts and the metal ionsfor the same active adsorption site may impede electrostaticinteraction between the metal ions and the solid surface whichcould further hinder the adsorption of metal ions, resulting inless percent removal of the metal ions (Jiang et al. 2010;Ozdes et al. 2011).

Fig. 9 Removal of Cd(II) and Mn(II) by sericite in a multielementsystem

Fig. 10 Breakthrough curves for the removal of Cd(II) and Mn(II) bysericite

Table 4 Thomas constant (KT) and column loading capacity (q0) ob-tained for Cd(II) and Mn(II) by sericite under dynamic conditions

Sample q0 (mg/g) KT (L/min/mg), ×10−3 Least square sum

Cd(II) 1.425 4.51 8.1×10−2

Mn(II) 0.922 4.29 1.3×10−1

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Competitive sorption of Cd(II) and Mn(II)

The competitive sorption of Cd(II) and Mn(II) by sericite wasstudied by taking the same concentrations of Cd(II) andMn(II) at four different initial concentrations, i.e., 5.0, 10.0,15.0, and 20.0 mg/L, keeping pH 5.0 constant. The resultsobtained are shown in Fig. 9. The figure clearly indicates thatCd(II) removal decreased in competition with the sameamount of Mn(II) and the Mn(II) removal was slightly sup-pressed in the presence of Cd(II) ions. This is due to the factthat the two ions, i.e., Cd(II) and Mn(II) were competing forthe same active sites; therefore, less active sites were availablefor a single metal ion in a multielement system which ulti-mately caused the removal of both cations to decrease com-pared to their removal in single-element systems (Xue et al.2009). Further, it was observed that there is no significantdifference between the amount of Cd(II) and Mn(II) adsorbedin a multielement system, indicating that the two metal ionscompete with almost the same extent for the active sites andthereby conferred that Cd(II) and Mn(II) have comparableaffinities towards sericite (Kosa et al. 2012; Srivastava et al.2005).

Fixed-bed column reactor studies

The fixed-bed column experiments were carried out asstated before, and the breakthrough data were obtainedat initial sorptive concentrations of 10.58 and 10.00 mg/L, respectively, for Cd(II) and Mn(II), keeping the in-fluent pH 5.0 constant. The results are presented graph-ically in Fig. 10. The complete breakthrough occurredfor the throughput volume of 0.1 L for Cd(II) and0.05 L for Mn(II). These results inferred that sericitepossessed significantly high removal capacity for thesemetal ions even under stated dynamic conditions. Thedata obtained were further employed to optimize theloading capacity of the column under dynamic condi-tions using the nonlinear Thomas equation (Eq. 5)(Thomas 1944):

Ce

Co¼ 1

1þ e KT qom − CoVð Þð Þ=Q ð5Þ

where Ce is the Cd(II) or Mn(II) concentration in the effluentsolution (mg/L), C o is the Cd(II) or Mn(II) concentra-tion in the feed solution (mg/L), KT is the Thomas rateconstant (L/min/mg), q o is the maximum amount ofCd(II) or Mn(II) that could be loaded (mg/g) underthe specified column conditions, m is the mass of theadsorbent loaded (g), V is the throughput volume (L),and Q is the flow rate of the pumped sorptive solution(L/min). The Thomas equation was successfully used tofit the breakthrough data and the unknown parameters,

i.e., q o and KT were optimized with the nonlinear leastsquare fitting method. The values are shown in Table 4. Thedata shows that a relatively high amount of Cd(II) andMn(II) was loaded in the column. Cd(II) showed higherloading capacity by sericite compared to Mn(II) which is ina line with batch reactor studies. The lower sorption capac-ity under the dynamic conditions compared to the batchreactor was attributed to the lesser contact time available forthe metal ions to be loaded onto the solid surface (Shahbaziet al. 2011).

Conclusions

The batch and column reactor studies were conducted in theremoval of two important heavy metal toxic ions, viz. Cd(II)and Mn(II) from aquatic environment using the naturallyavailable sericite, a mica-based clay. Batch reactor datashowed that increasing the sorptive concentration, pH, or soliddose favored significantly the removal of these two pollutantsfrom aqueous solutions. The equilibrium-state sorption datafitted well to the Langmuir adsorption isotherm. The kineticdata showed that adsorption process occurred very fast, andthe equilibrium time was achieved within 20 min for Cd(II)and 60 min for Mn(II). Further, the kinetic data fitted well tothe pseudo-second-order kinetic model. The optimum dose ofsericite for the removal of Cd(II) and Mn(II) was found to be6.0 and 10.0 g/L, respectively. Further, a 100-times increase inbackground electrolyte concentrations (NaNO3) caused thepercent uptake of Cd(II) and Mn(II) by sericite to decreasesignificantly, indicating that the sorbing ions were sorbed by aweak electrostatic attraction and formed ‘outer-sphere com-plexes’ at the solid surface. The breakthrough data fitted wellto the Thomas equation; hence, the loading capacity of thesecations was estimated which were found to be 1.425 and0.922 mg/g for Cd(II) and Mn(II), respectively. Sericite wasfound to be a useful and potential natural alternative materialin the remediation of aquatic environment contaminated withCd(II) or Mn(II) pollutants.

Acknowledgments This work was supported by the National ResearchFoundation of Korea (NRF) grant funded by the Korean Government(MEST) (no. 2012R1A2A4A01001539). This research is also supportedby the Korea Ministry of Environmentas “The Converging TechnologyProject” (Vide Proposal No. 2013001450001).

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