Removal of Zn (II) ions from aqueous solution using Moringa oleifera Lam.(horseradish tree) biomass

Removal of Zn (II) ions from aqueous solution using Moringa oleifera Lam.(horseradish tree) biomass
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  Removal of Zn(II) ions from aqueous solution using  Moringa oleifera  Lam. (horseradish tree) biomass Haq Nawaz Bhatti*, Beenish Mumtaz, Muhammad Asif Hanif, Raziya Nadeem  Department of Chemistry, University of Agriculture, Faisalabad 38040, Pakistan Received 5 May 2006; received in revised form 11 October 2006; accepted 12 October 2006 Abstract The removal of Zn(II) ions from aqueous solution using pure and chemically pretreated biomass of   Moringa oleifera  was investigated at30  1  8 C in this study. The experimental results explored that the maximum pH (pH max ) for efficient sorption of Zn(II) was 7  0.1 at whichevaluated biosorbent dosage and biosorbent particle size, were 0.5 g/L,  < 0.255 mm, respectively. The cellular Zn(II) concentration increased withthe concentrations of Zn(II) in solution. Pretreatment of   M. oleifera  biomass affected the sorption process and the uptake capacity (mg/g) of biomass for Zn(II) uptake was in following order: NaOH (45.76)  >  H 2 SO 4  (45.00)  >  CTAB (42.80)  >  Ca(OH) 2  (42.60)  >  Triton X-100(42.06)  >  H 3 PO 4  (41.22)  >  Al(OH) 3  (41.06)  >  SDS (40.41)  >  HCl (37.00)  >  non-treated biomass (36.07). There was significant increase inuptake capacity of   M. oleifera  biomass, which suggested that affinity between metal and sorbent can be increased after some sort of pretreatment.Both Langmuir and Freundlich isotherm model fitted well to data of Zn(II) biosorption as represented by high value of their correlation coefficient(i.e.  R 2  1). Kinetic studies revealed that Zn(II) uptake was fast with 90% or more of uptake occurring with in 40 min of contact time and theequilibriumwasreachedin50 minofcontacttime.Thesorptionrateswerebetterdescribedbyasecondorderexpressionthanbyamorecommonlyapplied Lagergren equation. Finally it was concluded that pretreatment of   M. oleifera  biomass can achieve superior Zn(II) uptake capacity incomparison to non-pretreated biomass. # 2007 Published by Elsevier Ltd. Keywords:  Biosorption; Zn(II);  Moringa oleifera ; Pretreatment; Isotherms 1. Introduction Heavy metals are released into the aqueous environmentthrough a variety of sources such as metal smelters, effluentsfrom plastics, textiles, microelectronics, wood preservatives-producing industries, usage of fertilizers and pesticides [1–4].Naturalwatersalsocontaintoxicmetalsdependinguponthebedrock  [5]. Increased consciousness for safeguarding the aqueousenvironment has prompted a search for alternative technologiesfor the removal of toxic metal ions from aqueous solutions.Conventional methods for removing heavy metals includechemical precipitation and ion exchange [6–8]. These becomeinefficientorexpensiveespeciallywhen the concentrationoftheheavy metal ion is low, of the order of 1–100 mg/L. Deadbiological materials are known for their efficient potential toadsorbheavymetalsatlowconcentrationswithveryhighuptakecapacity [9,10]. In this regard, a wide variety of dead biomass isbeing considered as adsorbents of heavy metals for treatment of industrial and domestic wastewaters as well as natural waters,includingdrinkingwater.However,uptakecapacityofpretreatedbiomasses has not been extensively studied till present. Zn(II)maybe foundinwastewaterdischarges fromacidmine drainage(AMD), galvanizing plants, as a leachate from galvanizedstructures and natural ores, and from municipal wastewatertreatment plant discharges. Zn(II) travels through the food chainvia bioaccumulation. Hence, there is significant interestregarding Zn(II) removal from wastewater streams. Traditionalmethods for removal of Zn(II) ions from solution are oftenexpensive and ineffective at low metal concentrations [11].Therefore, there is a need for a cost effective treatment methodthat is capable of removing low concentrations of Zn(II) fromsolution.The  Moringaceae  is a single genus family with 14 knownspecies. Of these  Moringa oleifera  Lam. (syns.  Moringa pterygosperma  Gaertn.) is the most widely known and utilisedspecies. This species is one of the world’s most useful plants. Biochemistry 42 (2007) 547–553* Corresponding author. E-mail address: (H.N. Bhatti).1359-5113/$ – see front matter # 2007 Published by Elsevier Ltd.doi:10.1016/j.procbio.2006.10.009   M. oleifera : a native of the sub-Himalayan regions of north westIndia, is now indigenous to many countries in Africa, Arabia,South East Asia, the Pacific and Caribbean Islands and SouthAmerica,cultivatedforitsleaves,fruits,androotsforavarietyof foodandmedicinalpurposes.Someresearchhasbeenfocusedonthe use of   M. oleifera  seeds and fruits in water purification. It iscommonlyknownasthe‘horseradish’tree(arisingfromthetasteof a condiment prepared from the roots) or ‘drumstick’ tree(arising from the shape of the pods),  M. oleifera  has a host of other country specific vernacular names, an indication of thesignificance of the tree around the world [12–19].The main objectives of this study include identifying thecomparativemaximum theoreticalZn(II) uptakecapacityofthenative and modified  M. oleifera  biomass, determining thereaction kinetics and evaluating the importance of solution pH,particle size, sorbent dose and initial metal concentration onZn(II) uptake. This paper reports the results of an evaluation of the parameters important for the biosorption of Zn(II). 2. Materials and methods 2.1. Reagents All the chemical reagents used in these studies were of analytical grade,includingheptahydratesaltofzincsulphate(ZnSO 4  7H 2 O),conc. H 2 SO 4 ,conc.HCl, conc. H 3 PO 4 , NaOH, Ca(OH) 2 , Al(OH) 3 , Triton X-100, sodium dodecylsulfate (SDS), cetyl trimethyl ammonium bromide (CTAB), conc. HNO 3  andZn(II) atomic absorption spectrometry standard solution (Fluka Chemicals). 2.2. M. oleifera biomass In the present study pod’s biomass of   M. oleifera  (horseradish tree), adeciduous tropical tree to about 20 feet; leaves sub-pinnate; flowers yellow,upper petiole whitish was selected as biosorbent for the removal of Zn(II) fromaqueous solutions.  M. oleifera  biomass used in this work was harvested fromUniversity of Agriculture, Faisalabad, Pakistan, by manually removing thematuredpodsfrom plants.Biomasswas extensively washedwith distilledwaterto remove particulate material from their surface, and oven dried at 60  8 C for72 h[10].Onekilogramofbiomasswassubsampledforuseinthe experiments.In orderto ensurethathomogeneoussampleswere collected, standardsamplingtechniques were applied. Dried biomass was cut, ground using food processor(Moulinex, France) and then sieved through Octagon siever (OCT-DIGITAL4527-01) to obtain adsorbent with homogenous known particle size. Thefraction with mesh size  < 0.255–0.500 mm was selected for use in the sorptiontests. The sieved biomass materials were stored in an air tight plastic containerfor further studies. 2.3. Pretreatment of biomass A 500 g of finely divided biomass was soaked in excess of HCl, H 2 SO 4 ,HNO 3  (acidic pretreatment), NaOH, Ca(OH) 2 , Al(OH) 3  (basic pretreatment),cetyl trimethyl ammonium bromide (CTAB), sodium dodecyl sulphate (SDS)and Triton X-100 (surfactant pretreatment) (1 g of biomass/25 mL of 1 Mreagent) for 24 h. Then suspensions were extensively washed with deionizeddistilled water (DDW) and filtered thoroughly until a pH 7  0.1 of pretreatedbiomass was attained. Finally the resulting biomass was air dried. 2.4. Zn(II) solutions Stock Zn(II) solution (1000 mg/L) was prepared by dissolving 4.398 g of zinc sulphate heptahydrate in 100 mL of DDW and diluting quantitatively to1000 mL using DDW, Zn(II) solutions of different concentrations (25–200 mg/ L) were prepared by adequate dilution of the stock solution with DDW to100 mL. Glassware and polypropylene flasks used were overnight immersed in10% (v/v) HNO 3  and rinsed several times with DDW. 2.5. Batch biosorption studies In all sets of experiments fixed volume of Zn(II) solution (100 mL) wasthoroughly mixed with desired biosorbent dose (0.05, 0.1, 0.2 and 0.3 g) andmesh size ( < 0.255, 0.255–0.355 and 0.355–0.500 mm) at 30  1  8 C and100 rpm up to 24 h. Equilibrium periods of 24 h for sorption experiments wereused to ensure equilibrium conditions. This time was chosen considering theresults of kinetics of metal removal found in literature [11,20,21]. To check theinfluence of pH, initial metal concentration and contact time different condi-tions of pH (3–11), initial metal concentration (50, 100 and 200 mg/L), andcontact time (10–640 min) were investigated during study. The flasks wereplaced on a rotating shaker (PA 250/25 H) with constant shaking. At the end of the experiment, the flasks were removed from the shaker and the solutions wereseparated from the biomass by filtration through filter paper (Whatman no. 40,ashless) followed by centrifugation. For adjusting the pH of the medium 0.1Nsolution of NaOH and HCl were used during study. 2.6. Determination of the Zn(II) contents in the solutions The concentration of Zn(II) in the solutions before and after the equilibriumwas determined by flame atomic absorption spectrometry (FAAS), using aPerkin-Elmer AAnalyst 300 atomic absorption spectrometer. 2.7. Metal uptake The Zn(II) uptake was calculated by the simple concentration differencemethod[14,15].Theinitialconcentration C  0 (mg/L)andmetalconcentrationsatvarious time intervals,  C  e , (mg/L) respectively, were determined and the metaluptake q (mgmetaladsorbed/gadsorbent)wascalculatedfromthemassbalanceas follows [23]: q ¼ð C  0  C  e Þ V  1000 w  (1)where V  isthevolumeofthesolutioninmLand w isthemassofthesorbenting. 2.8. Statistical analysis Each experiment was conducted in triplicate to ensure the reproducibility of results. All data represent the mean of three independent experiments. Statis-tical analyses were performed using the statistical functions of Microsoft Excelversion Office Xp (Microsoft Cooperation, USA). 3. Results and discussion 3.1. Influence of initial pH  The equilibrium metal uptake of the  M. oleifera  from Zn(II)solutions(50 mg/L)atvariouspHvaluesisshowninFig.1.Themetal uptake at pH 3 is negligible, thus indicating thepossibility for using this pH effect for metal elution andbiomass regeneration. The results clearly indicated that Zn(II)uptake increases with solution pH. This increase in Zn(II)removal with increasing pH has also been shown by Mameriet al. [24] using fungal biomass. The pH dependence of metaluptake could be related to the functional groups of the biomassand also to solution chemistry. At pH values less than 4 metalsare in their free ionic form and as such the sharp increase inmetal uptake between pH 3 and 5 cannot be described by thechange in metal speciation [25]. This leads to the hypothesisthat the cell wall functional groups and their associated ionic  H.N. Bhatti et al./Process Biochemistry 42 (2007) 547–553 548  state are responsible for the extent of adsorption. Biosorbentmaterials primarily contain weak acidic and basic functionalgroups. It follows from the theory of acid–base equilibria that,in the pH range 2.5–5, the binding of heavy metal cations isdetermined primarily by the state of dissociation of the weak acidic groups. Carboxyl groups (–COOH) are the importantgroups for metal uptake by biological materials [9]. The ionicstates of cell wall functional groups can be used to explain thepH dependence of biosorption. Low pH conditions allowhydrogen and hydronium ions to compete with Zn(II) for metalbinding sites on the biomass, causing poor Zn(II) uptake. Athigher pH values 5–7, there are lower numbers of competinghydrogen ions and more ligands are exposed with negativecharges, resulting in greater Zn(II) sorption. Whereas at pHhigher than 7 precipitation of solution occurs and led thesorption capacity to be reduced. 3.2. Effect of biosorbent dose Increasing the mass of differently pretreated  M. oleifera biomass caused the sorptive capacity,  q , to be reduced(Fig. 2). This effect was also reported by Quek et al. [25] for sago waste for the sorption of lead and copper. In looking atthis effect, it is pertinent to examine the data in relation to thetheoretical maximum, assuming that all of the metal ionswould be sorbed onto the  M. oleifera  biomass. The resultsdemonstrated that the biomass concentration strongly affectedthe amount of metal removed from aqueous solutions.Moreover, as the biomass concentration rises, the maximumbiosorption capacity drops, indicating poorer biomassutilization (lower efficiency) [20]. 3.3. Effect of particle size of biosorbent  The effect of altering the sorbents particle size on thesorption capacity,  q  (mg/g) showed that, there was a moredominant removal of Zn(II) by the smaller particles (Fig. 3).This was most probably due to the increase in the total surfacearea which provided more sorption sites for the metal ions. Theenhanced removal of sorbate by smaller particles has beenreported previously for Ni(II) removal by  Cassia fistula biomass [20]. 3.4. Effect of initial metal concentration TheresultsoftheexperimentareshowninFig.4.Theresultsrevealed maximum metal removal with lower initial concen-trations. The percentage Zn(II) uptake was 74.76% for metalsolutions at 50 mg/L metal. Later, an increase in initial Fig. 1. Effect of pH on the biosorption of Zn(II) by  Moringa oleifera  biomass.Fig. 2. Effect of varying the sorbent dose on the biosorption of Zn(II) by  M.oleifera  biomass.Fig. 3. Effect of different sorbent particle size on biosorption of Zn(II) by  M. oleifera  biomass.Fig.4. EffectofdifferentinitialmetalconcentrationonbiosorptionofZn(II)by  M. oleifera  biomass.  H.N. Bhatti et al./Process Biochemistry 42 (2007) 547–553  549  concentration decreased the percentage binding. Theseobservations can be explained by the fact that at very lowconcentrationsofmetalions,theratioofsorptivesurfaceareatothe total metal ions available is high and thus, there is a greaterchance for metal removal. Thus, at low initial metal ionconcentrations, the removal capacity is higher and vice versatrue for  q  value. When metal ion concentrations are increased,binding sites become more quickly saturated as the amount of biomass concentration remained constant [26]. 3.5. Adsorption isotherm In order to understand the adsorption process the Langmuirand Freundlich isotherms [27], Figs. 5 and 6, respectively, were used to represent the equilibrium relationship for differentinitial Zn(II) concentrations experiment. The Langmuirequation assumes that adsorption is limited to monolayer, itslinearized form can be represented as: C  e q e ¼  1  X  m K  L þ  C  e  X  m (2)where  q e  is the metal ion sorbed (mg/g),  C  e  the equilibriumconcentration of metal ion solution, and  X  m  and  K  L  are theLangmuir constants. The Freundlich equation is an empiricalrelationship describing the adsorption of the solutes from aliquid to solid surface. Linearized form of Freundlich equationis:log q e  ¼ 1 n log C  e þ log K  where  q e  is the metal ion sorbed (mg/g),  C  e  the equilibriumconcentration of metal ion solution (mg/L), and  K   and 1/  n  areconstants.AcomparisonofLangmuirandFreundlichisothermsis tabulated in Table 1. As indicated from Table 1, the coeffi- cients of determination (  R 2 ) of both models were greater than0.9 and were close to one, indicating that both models ade-quately describe the experimental data of theses metal biosorp-tion experiments. These results are in close agreement withearlier reported results [28]. 3.6. Effect of pretreatment on M. oleifera biomass Metal affinity to the biomass can be manipulated bypretreating the biomass with alkalies, acids and surfactants,which may increase the amount of the metal sorbed (Fig. 7)[29–35]. To evaluate the effect of pretreatment of biomass,50 mg/L Zn(II) solution was shaken at 120 rpm with 0.1 g of biosorbent havingsize 0.255 mm at pH 7for 24 h. Pretreatmentof   M. oleifera  biomass with HCl, H 2 SO 4  and H 3 PO 4  presentedincrease in adsorption capacity as compared to pure or non-treated  M. oleifera  biomass. Zn(II) removal of acidicallypretreated biomass was in following order: H 2 SO 4 (90%)  >  H 3 PO 4  (82.44%)  >  HCl (74.21%)  >  non-treated bio-mass (74%). Modification of biomass with acids, apart fromremoval of the mineral matter also resulted in introduction of oxygen surface complexes that change the surface chemistry byincreasing the surface area and porosity of srcinal sample.H 2 SO 4  presented more increase in adsorption capacity ascompared to HCl and H 3 PO 4 . This might be due to solubility of more mineral matter of   M. oleifera  biomass in H 2 SO 4 , whichintroduced more porosity in biomass due to increased cellularmass and resulted in enhancement of Zn(II) uptake capacity of biosorbent [34–36] (Fig. 7). FollowingorderforZn(II)uptakewaspresentedbybasicallypretreated  M. oleifera  biomass: NaOH (91.52%)  >  Ca(OH) 2 (85.20%)  >  Al(OH) 3  (82.08%)  >  non-treated biomass (74%).Removal of surface impurities, rupture of cell-membrane and Fig. 5. Linearlized Langmuir isotherm plot for biosorption of Zn(II) by  M.oleifera  biomass.Fig. 6. Linearlized Freundlich isotherm plot for biosorption of Zn(II) by  M.oliefera  biomass.Table 1Langmuir and Freundlich isotherm parameters for Zn(II) uptake by  M. oleifera  biomassBiosorbent Langmuir isotherm parameters Experimental value,  q max  (mg/g) Freundlich isotherm parameters  X  m  ( q max ) mg/g  K  L  (L/mg)  R 2 q max  (mg/g)  K   (mg/g)  R 2 1/  n M. oleifera  biomass 52.08 0.150 0.9994 40.99 50.35 0.156 0.9953 0.1223  H.N. Bhatti et al./Process Biochemistry 42 (2007) 547–553 550  exposure of available binding sites for metal bioadsorption afterpretreatment might be the reason for the increase in metalbiosorption. Muraleedharan and Venkobachar [29] showed thatbasic treatment of biomass may destroy autolytic enzymes thatcauseputrefactionofbiomassandremovelipidsandproteinsthatmask reactive sites. Besides this, the pretreatment could releasepolymers such as polysaccharides that have a high affinitytowards certain metal ions [31]. As NaOH is a stronger base, itremoves H + ions from the surface of biomass easily resulting inexcess of negative charge introduced on cellular surface, whichattracts more Zn(II) ions from aqueous solutions.Surfactants are the substances with lyophilic and lyophobicgroups capable of adsorbing at interfaces. The adsorption of heavy metals on to the biomass from solutions can be enhancedin the presence of surfactants due to reduced surface tensionand increased wetting power. In present investigation in case of surfactant pretreatment of   M. oleifera  biomass maximumadsorption capacity was shown by CTAB (85.60%) (cationicsurfactant) followed by Triton X-100 (84.12%) (non-ionicsurfactant) and SDS (80.82%) (anionic surfactant), respec-tively. CTAB is a cationic surfactant and it has greater numberof C-atoms as compared to other tested surfactants, so it mightincrease the volume of the carbon structure by residing on thesurface of biomass carbon and create more positivesites (whichcan be exchanged with Zn(II) ions) by releasing the bromideions. Actually, due to greater number of carbon atoms CABTact as large fiber residing on the biomass surface andsubsequently increase the uptake capacity of biosorbent. Itreleases bromide ions from its surface to attain equilibriumbetween surface of sorbent and solution, which also animportant factor in increasing sorption capacity of biosorbent.H 2 SO 4 , NaOH and CTAB pretreated  M. oleifera  biomass wasselected for sorption kinetics study due to their high adsorptioncapacity in comparison to other pretreatments of biosorbent. 3.7. Biosorption kinetics of Zn(II) A kinetic study with different time intervals with fixedmetal, biosorbent amount and biosorbent particle size wasperformed (Fig. 8). In first few minutes biosorption was sharpprobably due to decrease in pH of solution because of protonreleased by the biosorbent. The rapid initial sorption was likelydue to extra cellular binding and slow sorption phase likelyresulted from intracellular binding [37–39]. The optimumsorption time was 50 min of contact time. Maximumbiosorption of Zn(II) occurred on  M. oleifera  biomass treatedwith NaOH and equilibrium sorption time was 50 min. Acomparison of Zn(II) uptake capacity of   M. oleifera  withpreviously employed biomasses is given in Table 2. 3.8. Kinetic modeling Kinetics of adsorption by any biological material has beenwidely tested by the first order expression given by Lagergrenand pseudo-second order approach [26–30]. The first orderLagergren equation is [38]:log ð q e  q Þ¼  log q e   k  1 ; ads t  2 : 303  The pseudo-second order equation is [26]: t q ¼  1 k  2 ; ads q 2e þ  t q t  where  q e isthe mass of metal adsorbed atequilibrium (mg/g),  q t  the mass of metal at time  t   (min),  k  1,ads  the first order reactionrate constant of adsorption (min  1 ),  k  2,ads  is the pseudo-secondorder rate constant of adsorption (mg/g min). A comparisonbetween Lagergren pseudo-first order and to pseudo-secondorder kinetic models is tabulated in Table 3. The Lagergren first Fig. 7. Effect of pretreatment on  M. oleifera  biomass for the removal of Zn(II)from aqueous solutions.Fig. 8. Zn(II) sorption kinetics onto NaOH pretreated  M. oleifera  biomass.Table 2Comparison of   M. oleifera  with previously used biosorbents for the removal of Zn(II) from aqueous effluentsBiosorbent  X  m  ( q max ) mg/g ReferenceUntreated  M. oleifera  biomass 40.99 Present study  Aspergillus flavus  17.27 [40]Activated sludge 2.50 [41]  Myriophyllum spicatum  15.59 [42] Streptoverticillium cinnamoneum  21.30 [43]Biosolids 36.86 [22]Cork biomass 24.83 [8]Rice bran 0.12 [44]Sewage sludge 27.3 [45]  Botrytis cinerea  biomass 12.98 [46]  H.N. Bhatti et al./Process Biochemistry 42 (2007) 547–553  551
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