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Biosorption of chromium VI by free and immobilized Rhizopus arrhizus

Biosorption of chromium (VI) was studied by using non-living free and immobilized biomass of Rhizopus arrhizus at pH 2. A biphasic chromium adsorption pattern was observed in all experimental conditions. Chromium removal rate was slightly more in
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  Biosorption of chromium VI by free and immobilized Rhizopus arrhizus R.S. Prakasham a, *, J.Sheno Merrie b , R. Sheela b , N. Saswathi b , S.V. Ramakrishna a a Biochemical and Environmental Engineering Group, Indian Institute of Chemical Technology, Hyderabad 500 007, India b Biochemicalprocessing and Wastewater Technology Division, Regional Research Laboratory, Trivendram 695 019, India Received 9 March 1998; received in revised form 17 September 1998; accepted 17 September 1998 Abstract Biosorption of chromium (VI) was studied by using non-living free and immobilized biomass of  Rhizopus arrhizus at pH 2. Abiphasic chromium adsorption pattern was observed in all experimental conditions. Chromium removal rate was slightly more infree biomass conditions over immobilized state. Stirred tank reactor studies indicated maximum chromium biosorption at 100 rpmand at 1:10 biomass±liquid ratio. Fluidized bed reactor is more ecient in chromium removal over stirred tank reactor. Immobili-zation of biomaterial has a little eect on chromium biosorption by this R. arrhizus biomass. # 1999 Elsevier Science Ltd. All rightsreserved. Keywords: Bioremediation; Biosorption; Chromium; Heavy metal; Rhizopus arrhizus ; Wastewater treatment 1. Introduction Rapid industrialization and increasing urbanizationincluding technological advancement grossly con-taminating our environment by discharging the heavymetals in the euents causing health hazards to biota.Among all heavy metals, copper, chromium and zincingestion beyond permissible quantities causes variouschronic disorders in human beings (Beszedots, 1983).Potable waters containing more than 0.05 mg/l of chro-mium is considered to be toxic (Vishwanatham, 1997).Therefore, increasing awareness has been growingrapidly over worldwide and one of the oshoot of it istreatment and removal of heavy metals from such eu-ents to a permissible limits before discharging them intostreams and rivers. Towards this direction, several con-ventional wastewater treatment technologies weredeveloped and are used successfully at the large scale, toreduce the hazardous compounds concentration ineuents from higher to lower levels (Verma and Rahal,1996). Application of such traditional treatment techni-ques needs enormous cost and continuous input of che-micals which becomes impracticable and uneconomicaland causes further environment damage. Hence, easy,eective, economic and ecofriendly techniques arerequired for ®ne tuning of euent/wastewater treat-ment.A broad range of bioadsorbent materials like cyano-bacteria and microalgae (Garnham et al., 1993), marinealgae (Leusch et al., 1995), several bacterial species(Brierley, 1990; Brierley and Brierly, 1993), fungi (Tobinet al., 1994), yeast and ®lamentous bacteria (Mat-tuschka et al., 1993) were studied for their potential toremove the heavy metals from the solutions and foundto be optimistic for treatment of wastewater/euents.Each of these biosorbent material is selective in theiradsorption of heavy metals (Niu et al., 1993). Theimmobilization of biomass will provide several advan-tages such as facility to reuse and separation of solidbiomass from the bulk liquid. The process will becomecost eective by reusing the biomass after regeneration.Keeping this in view we have studied the possibility of utilisation of  Rhizopus arrhizus biomass for biosorptionof chromium from the solution under laboratory con-dition and study denoted that, immobilization of bio-material does not change sorption capacity and rapidcontact between sorbent and solute is eective in chro-mium biosorption. 0269-7491/99/$Ðsee front matter # 1999 Elsevier Science Ltd. All rights reserved.PII: S0269-7491(98)00174-2 ENVIRONMENTALPOLLUTION Environmental Pollution 104 (1999) 421±427* Corresponding author. Tel.: 00 91 040 7171852; fax: 91 0407173757; e-mail:  2. Materials and methods 2.1. Organism and growth conditionsR . arrhizus strain NCL 977 was grown in autoclavedliquid media (bacteriological peptone ± 10, sucrose ± 20,KH 2 PO 4 ± 1, NaNO 3 ± 1 and MgSO 4 .7H 2 O ± 0.05 (allin grams per liter) at pH 7.3 at room temperature instatic condition for 5±6 days. 2.2. Biosorbent preparation Heat killed R. arrhizus biomass was washed thor-oughly with distilled water thrice and removed theexcess water present in it by using whatman ®lter paper.The biosorbent was heat dried at 97  C for 6 h in hot airovenandthenmadeintopowderof150 m musingblender. 2.3. Immobilization of biomass Alginate solution of 2% was prepared by dissolving8 g of alginate (sodium salt) in 400 ml of hot distilledwater with constant stirring to avoid formation of lumps. The slurry was cooled to room temperature andequal quantity of powdered biomass was added understirring condition to have a uniform mixture. This mix-ture was extruded as droplets in 50 mM CaCl 2 solution,using peristaltic pump. The gel beads were allowed tocure for 2 h at 4  C and washed thoroughly with distilledwater. Blank alginate beads were prepared similarlywithout having the biosorbent. 2.4. Chromium solution preparation 0.294 g of K 2 Cr 2 O 7 was dissolved in one liter of dis-tilled water and used as stock solution. The concentra-tion of chromium (VI) in the stock solution will be 104mg/l. Calculated quantities of this stock solution wasmeasured and used for further experimental solutionpreparation. 2.5. Synthetic euent preparation Synthetic euent was prepared corresponding to theeuent emanating from the tannery industry using fol-lowing composition (all in g/l): NaHCO 3 ± 2, glucose ± 5,NaCl±10,Na 2 SO 4  ±2.5,Na 2 S±0.1,K 2 Cr 2 O 7  ±0.294. 2.6. Estimation of chromium concentration All experiments were conducted using 100 mg/l chro-mium solution at pH 2. The concentration of unab-sorbed chromium ions in the adsorption media wasdetermined at 540 nm in a spectrophotometer usingdiphenylcarbazide as a complexing agent as per stan-dard methods (American Public Health Association,1986). During measurement the blank bead adsorptionvalues were substracted from immobilised chromiumbiosorption values and represented in the text. 2.7. Reactor studies Experiments were carried out in stirred tank and ¯ui-dized bed reactor, at pH 2 using 100 mg/l chromiumconcentration solution. The biosorbent amount wasvaried to bring solid±liquid ratio of 1:10, 1:16.66 and1:50 in both reactors. In the case of stirred tank reactor,the study was also conducted with variable impellerspeed of 50, 100 and 200 rpm. In ¯uidized bed reactor,¯uidization was attained by employing compressed air.These reactors were continuously run for 4 h and sam-ples were withdrawn every hour and analyzed for ®nalchromium ion concentration. 2.8. Chemicals All chemicals used in this study were analar grade andwere purchased from Qualigens. 3. Results and discussion 3.1. Eect of contact time on adsorption of chromium by free and immobilized biomass of  R. arrhizusThe role of contact time on biosorption of chromiumby R. arrhizus biomass was studied, under shake ¯askconditions at pH 2, at 150 rpm, using 100 mg/l chro-mium ion concentration. Samples were collected at reg-ular intervals and analysed for chromium ionconcentration after ®ltration to remove biosorbent. Forcomparative study, alginate beads without biomass(5%, w/v) alginate beads with biomass (5%, w/v)(solid±liquid ratio of 1:20) and free biomass (250 mg) in100 ml of chromium solution (same amount of biomassin all the cases) were added. The observed results wereshown in Table 1.From the above table, it is evident that alginate beadswithout biomass adsorbed the chromium ion far lessthan that of either free biomass or alginate beads withbiomass, suggesting the role of biomass in the biosorp-tion of chromium from the solution and alginate as suchis not a good absorbent of chromium hence, it can beused as an immobilization material in heavy metalremoval by biomass unlike its role in phosphateremoval (Mallic and Rai, 1994). Critical analysis pat-tern of contact time on role of chromium biosorptionindicated that maximum adsorption reaches with thecontact time of 2 h under constant shaking conditionand 50% of biosorption of chromium was found in bothconditions, i.e., free and immobilized conditions(Table 1). Further increase in contact time, though 422 R.S. Prakasham et al./Environmental Pollution 104 (1999) 421±427   increased the adsorption rate in either free and immo-bilized conditions, the increase rate is in the range of 10±15%. Immobilized biomass showed slightly lowerbiosorption rate to that of free biomass, indicating thatthe immobilization with alginate has aected either onfree movement of chromium ions to metal sequesteringsites of biomass or masking of active sites of biomaterialin the present study. Such a screening eect of immobi-lization material is observed with biosorption materialwhere upto 40% reduction in heavy metal adsorption isfound under immobilizing condition over free state(Mahan and Holcombe, 1992). The gradual increase inbiosorption of chromium ions by R. arrhizus biomass infree as well as immobilized condition, over 2±8 h con-tact time, was diered and found to be 20% and 13%,respectively, over initial 2 h contact time biosorption(Table 1). This observation further supports the earlierobservation of the role of supporting material in causinghinderance in chromium biosorption process. Theseresults are consistent with the results of uranium uptakeby R. arrhizus , where mass transfer resistance in thenon-biomass layer is noticed (Tsezos et al., 1988).The data presented in Table 1 also depicts that thechromium biosorption with R. arrhizus occurs in twophases: initial fast phase which lasted for 2 h (shortesttime measured) followed by slower second phase whichcontinued till the end of experimental period in free andimmobilized state of biomass. This type of biosorptiontrend relates the two dierent mechanisms of chromiumbiosorption by the R. arrhizus biomass. The faster ®rstphase of chromium biosorption is attributed to the sur-face adsorption due to the action of ion exchange withthe participation of the carboxyl groups of uronic acidspresent in cell structure which are known metal seques-tering sites (Majidi et al., 1990). Moreover, the bioma-terial used here R. arrhizus is known to contain chitinand chitosan in their cell walls, which have been repor-ted to play a role in the metal adsorption (Volesky andHolan, 1995). While the second slower phase mayrepresent diusion of metal ions into the cell debris overa period. These results are at par with the observationsof two phase biosorption of heavy metals in the bioma-terials of algal nature (Crist et al., 1988; Kuyucak andVolesky, 1990).The very slow increase in chromium adsorption byalginate beads without biomass over 8 h contact time is<10%, which may be due to the interaction of glu-couranate composition present in alginate structure(Volesky and Holan, 1995). 3.2. Eect of initial chromium ion concentration onbiosorption The biosorption of chromium ions was carried out atdierent initial chromium ion concentrations rangingfrom 50 to 300 mg/l, at pH 2, at 150 rpm with 8 h of contact time using free and immobilized biomass (thebiomass kept constant in both conditions of experi-ments). The results were presented in Table 2.It can be seen that chromium biosorption pattern isdierent in both free and immobilized biomass condi-tions, and immobilized biomass showed same adsorp-tion capacity over free biomass under similarenvironments. However, the gradual increase in chro-mium uptake quantity by the biomass under increasedinitial chromium ion concentrations, under free andimmobilized conditions, suggest that chromium ioninteraction with metal sequestering sites of biosorbentincreases with increase in initial chromium concentra-tion in solution. Either states of biosorbent, the chro-mium removal eciency was higher at low inletconcentration (50±150 mg/l) and then showed reducedchromium uptake (150±300 mg/l) (Table 2). It is alsoclear from the table that as the chromium ion con-centration increase in the solution, biosorption rate, ineither free and immobilized conditions, reduced in theorder of 20±22% in the ion concentration range of 300± 50 mg/l (Table 2). Such decrease in unit biosorption of metal ion with the enhanced metal concentration indi-cate that the biosorbent is eective in dilute metal solu-tions. These results may be explained by the fact that, atlow chromium concentrations, the ratio of sorptive sur-face of fungal biomass to total chromium availability ishigh, hence, all chromium may be interacted with bio-sorbent and removed. These observations are at parwith the observations made in copper-removal char-acterization by R. arrhizus (Zhou and Ki, 1991). 3.3. Adsorption isotherms The study of isotherms indicate the adsorption capa-cities of material for removal of adsorbate from thesolution at constant conditions. Therefore, the dataanalysed for Freundlich isotherm (Freundlich, 1926)and presented in Fig. 1 for free biomass and Fig. 2 forimmobilized biomass. Adsorption data when plottedaccording to Freundlich equation vog e q  vog e K   1 a n log e C  Table 1Eect of contact time on adsorption of chromium by free andimmobilized biomass of  R . arrhizus Time (h) % AdsorptionBlank alginatebeadsAlginate beads withbiomassFree biomass2 2.75 46.50 50.634 5.10 50.85 53.556 5.95 54.90 62.808 6.10 63.54 73.98 R.S. Prakasham et al./Environmental Pollution 104 (1999) 421±427  423  yielded a straight line with a slope of 1/ n and an inter-cept equal to log K  . The adsorption capacities ( K  ) of free and immobilized biomass of  R. arrhizus was foundto be 10.99 and 8.63, respectively. While, intensities of adsorption (1/ n ) with this biosorbent was measured as0.187 for free and 0.23 for immobilized state. Theobserved low biosorption intensity values (<1) sug-gested that, the non-living R. arrhizus biosorbent pos-sess heterogenous surface with identical adsorptionenergy in all sites and the biosorption of chromium islimited to monolayer and the adsorbed metal ion inter-acts only with the active site but not with other. How-ever, these interpretations should be viewed withcaution, as the biosorption and isotherm exhibit anirregular pattern due to: (a) complex nature of sorbentmaterial, (b) presence of varied multiple active sites onbiosorbent surface and (c) change of metallic com-pounds chemistry in a solution; moreover, the Freun-dlich isotherm has not been corrected for variations inenvironmental conditions (Volesky and Holan, 1995). 3.4. Reactor studies The reactor studies are important specially to know theresponse of the biosorption system to various experi-mental conditions and to understand the properties of biomass as well as the parametric sensitivity of themodel to process parameters. Therefore, experimentswere carried out in stirred tank and ¯uidized bed reac-tors and the chromium adsorption rates by immobilizedbiomass of  R. arrhizus were collected and compared. Asa model study for the treatment of industrial euentsby this R. arrhizus biomass, the experiments were alsoconducted with synthetic euent prepared in laboratorywhich has nearly equal composition of leather tanneryand these results were compared with chromium solu-tion observations. 3.5. Stirred tank reactor3.5.1. Eect of impeller speed  Biosorption studies of chromium with R. arrhizus wascarried out in a baed stirred tank rector, at pH 2 using500 ml of 100 mg/l chromium solution. By varying theimpeller speed from 50 to 200 rpm in dierent sets,keeping biomass constant, the eect of contact ratiobetween biosorbent and chromium ions in the solutionupto 4 h has been determined. Samples were collectedevery hour and analysed for chromium ion concentra-tion in solution and the results were represented inTable 3. It can be seen from the table, that the variationin turbulent speed changed the biosorption pattern, Table 2Freundlich adsorption isotherm for free and immobilized biomass of  R. arrhizus Initial [Cr] +6 (mg/l) Residual [Cr] +6 (mg/l) (c) Metal uptake(mg/g) (q) Log c Log q Free Imm. a Free Imm. a Free Imm. a Free Imm.50 0.57 0.79 4.94 4.92 À 0.56 À 0.23 1.59 1.59100 0.55 1.50 9.95 9.85 À 0.60 0.41 2.29 2.28150 5.55 9.08 14.44 14.09 1.71 2.21 2.67 2.64200 23.56 27.34 17.64 17.26 3.16 3.31 2.87 2.84250 34.55 43.04 21.54 20.69 3.54 3.76 3.07 3.03300 69.21 61.16 23.07 23.88 4.23 4.11 3.14 3.17 a Immobilized.Fig. 1. Freundlich adsorption isotherm for chromium (VI) by freebiomass of  R. arrhizus .Fig. 2. Freundlich adsorption isotherm for chromium (VI) biosorp-tion by immobilized R. arrhizus biomass.424 R.S. Prakasham et al./Environmental Pollution 104 (1999) 421±427   over a period of contact time. Percentage of biosorptionincreased in all dierent impeller speed conditions (50± 150 rpm), but chromium removal rate was maximum in100 rpm setup environment, in all contact time inter-vals; indicating impeller speed of 100 rpm is ideal forchromium ion removal by R. arrhizus biomass underthese experimental conditions. The increase in stirrerspeed from 50 to 100 rpm resulted an increase in chro-mium ion removal rate almost 90% within 1 h contacttime. Further increase in contact time, at the aboveconditions, also showed the higher chromium uptakerate by the biomass but in a lesser extent (Table 3). Thusindicating the chromium ion removal from solution by R. arrhizus biomass occurs in more than one mechan-ism. These results are similar to the observations madewith seaweeds, where more than one mechanism isinvolved in biosorption of metals (Kuyucak andVolesky, 1990; Volesky and Holan, 1995). Furtherincrease in impeller speed does not show the corre-sponding chromium removal rate (Table 3). Analysis of above data further indicate that the overall chromiumion removal rate is 1.375 times higher at 100 rpm to thatof 50 rpm depicting the substrate external mass transfercoecient is in proportion to the square root of stirrerspeed (square root of 100/50=1.414) (Sherwood et al.,1975). 3.5.2. Eect of solid±liquid ratio on biosorption of chromium in stirred tank reactor The experiments were conducted as mentioned inprevious section keeping the impeller speed at 100 rpmas constant and by varying the biomass quantity of 50,30 and 10 g in 500 ml of 100 mg/l chromium ion solu-tion, at pH 2 to give a solid±liquid ratio of 1:10, 1:16.66and 1:50, respectively. The observations are presented inTable 4.Maximum percent of biosorption of chromium occur-red at a solid±liquid ratio of 1:10. This increase in chro-mium biosorption was found to be similar in all othersolid±liquid ratios studied with the increase in contacttime from 0 to 4 h. These results suggest that, the solid± liquidratioof1:10,at4hcontacttimeisidealforremovalof chromiumfrom solution by R. arrhizus biomassunderset of experimental conditions. Further decrease in bio-mass±liquid ratio decreased the biosorption percent. Itwas observed that 1:16.66 and 1:50 biomass±liquid ratioshowed corresponding decreased uptake of chromiumions of2and4.5 folds, respectivelyincomparisonto1:10ratio. But increased the unit metal±biosorbent ratio(Fig. 3) at 1 h contact time; suggesting that, at a givenchromium ion concentration, reduction of biomassslightly enhanced the chromium±biosorbent ratio. Theseresults are similar with the metal uptake by other fungalbiomass(Fourest and Roux, 1992).Increase in contact time from 1 to 4 h, enhanced thebiosorption of chromium quantity in all studied solid± liquidratios.Thebiosorptionpercentageincreaseisfoundto be 23% in 1:10, 36% in 1:16.66 and 40% in 1:50 solid± liquid ratios from 1 to4 h contact time (Fig. 3), indicatingbiosorption of chromium by R. arrhizus biomass is notonly by surface adsorption mechanism which is very fast,followed by slower and dierent secondary metal bindingmechanismsasobservedinmarinealgalspecies(KuyucakandVolesky,1989aKuyucakandVolesky,1989b). Table 4Eect of solid±liquid ratio on adsorption of chromium by R. arrhizus in ¯uidized bed reactor from synthetic euentTime (h) Residual [Cr] +6 (mg/l) % Adsorption1:10 1:16.66 1:50 1:10 1:16.66 1:501 26.86 56.55 69.84 73.14 43.45 30.162 21.40 48.14 57.58 78.60 51.86 42.423 14.70 30.42 45.40 85.30 69.58 54.604 5.77 19.42 36.11 94.23 80.58 63.89Table 3Eect of impeller speed on adsorption of chromium by immobilized R. arrhizus Time (h) Impeller speedResidual [Cr] +6 (mg/l) % adsorption50 rpm 100 rpm 150 rpm 50 rpm 100 rpm 150 rpm1 80.85 65.82 71.11 19.15 34.18 28.892 65.85 56.30 62.85 34.15 43.70 37.153 51.03 41.89 44.85 48.97 58.11 55.154 39.27 29.50 35.20 60.73 70.50 64.80 R.S. Prakasham et al./Environmental Pollution 104 (1999) 421±427  425
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