Software

Biosorption of metals onto plant biomass: exchange adsorption or surface precipitation?

Description
Biosorption of metals onto plant biomass: exchange adsorption or surface precipitation?
Categories
Published
of 10
All materials on our website are shared by users. If you have any questions about copyright issues, please report us to resolve them. We are always happy to assist you.
Related Documents
Share
Transcript
  Ž . Int. J. Miner. Process. 62 2001 111–120www.elsevier.nl r locate r ijminpro Biosorption of metals onto plant biomass: exchangeadsorption or surface precipitation? Ivo A.H. Schneider  a , Jorge Rubio  b , Ross W. Smith c, ) a Ci Õ il Engineering Department, Uni Õ ersity of Paso Fundo, Paso Fundo RS, Brazil b  Department of Mining Engineering r PPGEMM, Federal Uni Õ ersity of Rio Grande do Sul,Porto Alegre RS, Brazil c  Department of Chemical and Metallurgical Engineering, Uni Õ ersity of Ne Õ ada-Reno, Reno, NV 89557 USA Received 15 August 1999; accepted 17 July 2000 Abstract Heavy metal ions readily adsorb onto the nonliving biomass of many aquaphytes. Further, inmany cases the metal ions can be readily desorbed from the biomass by use of a suitable elutingagent. It has been shown in certain cases, at least, that the biomass can be subjected to a numberof loading and elution cycles without the biomass losing its adsorption capacity. It has beenwidely reported that the adsorption is through a specific ion exchange mechanism and a number of researchers have shown experimental evidence supporting such a mechanism. However, there isalso evidence that the adsorption is through simple surface precipitation of metal hydroxidespecies. The present study examines some of the existing data on adsorption of metal ions ontoaquaphyte biomass and attempts to evaluate which mechanism is the more likely. q 2001 ElsevierScience B.V. All rights reserved. Keywords:  aquaphyte; adsorption; precipitation; heavy metals 1. Introduction Heavy metal ions sorb onto many solid surfaces including the surfaces of living anddead microorganisms and higher plants. A characteristic of the sorption of heavy metalions onto surfaces, both biotic and abiotic surfaces, is that at a set bulk solution heavymetal concentration the sorption is greatest at a pH value that is just slightly more acidic ) Corresponding author. Fax: q 1-775-327-5059. Ž . Ž .  E-mail addresses:  ivoandre@fear.upf.br I.A.H. Schneider , jrubio@vortex.ufrgs.br J. Rubio , Ž . smithrw@scs.unr.edu R.W. Smith .0301-7516 r 01 r $ - see front matter q 2001 Elsevier Science B.V. All rights reserved. Ž . PII: S0301-7516 00 00047-8  ( ) I.A.H. Schneider et al. r  Int. J. Miner. Process. 62 2001 111–120 112 than the pH at which there is bulk precipitation of the metal hydroxide. As pH is furtherraised, once the bulk solubility limit is reached, the sorption is greatly reduced becausethe metal ion is removed from solution by the bulk precipitation.It has long been reported that the sorption of heavy metal ions onto algae, bacteriaand higher plants is through a specific ion exchange mechanism. The sorption is thoughtto involve the replacement of protons, alkali, alkaline earth, or other cations by theheavy metal ions. Evidence presented by various researchers includes evidence that foreach heavy metal ion sorbing an equivalent of protons and r or other metal ions appear in Ž . solution Crist et al., 1991: Schneider et al., 1999 . Further evidence is the fit, or Ž approximate fit, of the sorption data obtained to the Langmuir isotherm Wang, 1995; . Schneider, 1995; Wang et al., 1998, Schneider et al., 1999 . The former is apparentdirect evidence that a replacement phenomenon is taking place. The Langmuir fit isconsidered to be evidence that sorption stops at one monolayer, consistent with specificand strong sorption onto specific sites. Because the exchange reaction between surfacesites and previously adsorbed ions is of only a monolayer or less, there is anaccumulation of matter at the solid–solution interface without the creation of a three-di-mensional structure. Thus, the phenomenon is adsorption.Evidence against such a mechanism is the often reported very low sorption activationenergy, which is not consistent with strong sorption onto specific sites, the extremerapidity with which the sorption takes place and the very rapid and easy and rapid Ž . desorption Schneider, 1995; Wang et al., 1998, Schneider et al., 1999 . In addition, itcan often be observed that sorption of heavy metal ions onto biomass is partly in patches Ž . Golab et al., 1991; Golab and Smith, 1992; Bauer, 1999 .An alternate sorption mechanism is the surface precipitation or condensation of heavymetal hydroxides onto the biosurfaces. Such precipitation is possible if there is anaccumulation of the heavy metals within the diffuse part of the electrical double layer.The accumulation will take place if there is a net negative charge on the solid surface.Thus, although the concentration of the metal ion in solution may be much less than thesolubility limit with respect to metal hydroxide and oxide solids, the solubility limit nearthe solid surfaces is exceeded and precipitation onto the solid surface takes place.The concentration of heavy metal ions within the electrical double layer can be Ž . roughly calculated by use of the Boltzmann equation Gaudin and Fuerstenau, 1955 : C   surf   s C   bulk   exp  y  zF   r  RT   1 Ž . Ž . Ž . Ž . i i  fd  Ž . Ž . where  C   surf   s concentration of species  i  at the surface;  C   bulk   s concentration of  i i species  i  in the bulk of the solution;  z s formal charge on the cation;  F  s Faradayconstant;  f   s potential at the Stern plane;  R s gas constant;  T  s absolute temperature. d  In the present paper an attempt is made to evaluate the magnitude of the effect of the Ž . concentration of Cu II within the electrical double layer and determine if the concentra- Ž . tion is sufficient to account for the sorption of Cu II onto a macrophyte. 2. Results and discussion Ž . Fig. 1 shows a log concentration diagram, adapted from Baes and Mesmer 1986 for Ž . Ž . Cu II in equilibrium with solid Cu OH . It can be seen at pH values more acidic than 2  ( ) I.A.H. Schneider et al. r  Int. J. Miner. Process. 62 2001 111–120  113 Ž . Ž . Fig. 1. Logarithm concentration diagram for Cu II in equilibrium with solid Cu OH . 2 pH 7 that the solubility increases rapidly with decrease in pH and that the solubility is y 3.36  y 4 Ž . about 10 mol r L at pH 6.0. If a 1.0 = 10 mol r l Cu II solution is considered, the Ž . bulk solubility limit is reached at about pH 6.3 Fig. 2 from Schneider 1995 , based on Ž . Ž . Ž . data of Baes and Mesmer 1986 , shows Cu II species distribution for a total Cu II y 4 Ž . concentration of 1 = 10 mol r L. Fig. 3 from Schneider et al. 1995 shows the Ž . sorption of Cu II onto  Potomogenten luscens  as a function of pH. The curves of thisfigure should be compared to the curves of Figs. 1 and 2. It is noted that maximum Ž . sorption of Cu II occurs at a pH value slightly more acidic than the pH of bulk  Ž . precipitation of Cu OH . 2 Ž .  y 4 Ž . Fig. 2. Relative concentration of Cu II species in a system containing 1 = 10 mol r l Cu II ; from Schneider Ž . 1995 .  ( ) I.A.H. Schneider et al. r  Int. J. Miner. Process. 62 2001 111–120 114 Ž . Ž .  y 4 Fig. 3. Final concentration of Cu II in solution as a function of pH; total Cu II in the system s 1 = 10 Ž . Ž . mol r l Cu II ; from Schneider 1995 . Ž . Ž . Fig. 4 from Schneider 1995 shows the zeta potential  z   of   P. luscens  as a functionof pH. Using  z   data from this figure and substituting  z   for  f   in the Boltzmann d  equation, it is possible to approximately solve the Boltzmann for the concentration of  Ž . Cu II within the electrical double for the biomass at different pH values. The substitu-tion is necessary since  f   is an inner potential and is not measurable while  z   is an d  outer, measurable, potential. In the calculation, it should be kept in mind that at all pHvalues more basic than the isoelectric point of the plant biomass,  f   will be more d  negative than  z   and, thus, the calculation will always underestimate the concentration of  Ž . Cu II within the electrical double layer.It can be noted from Fig. 1 that the pH value of bulk solution precipitation of  Ž . Ž .  y 4 Ž . Cu OH for a Cu II solution containing 1.0 = 10 mol r l Cu II is at about pH 6.3. 2 This means that at pH values more basic than this pH, there will be a precipitate of  Ž . Cu OH present in the system. From Fig. 4 it is seen that the zeta potential values for 2 Fig. 4. Zeta potential of   P. luscens  as a function of pH; 1 = 10 y 3 mol r l NaNO added for ionic strength 3 Ž . control; from Schneider 1995 .  ( ) I.A.H. Schneider et al. r  Int. J. Miner. Process. 62 2001 111–120  115Table 1 Ž . Concentration of Cu II within the electrical double layer at selected pH values Ž . pH Concentration of Cu II within double Maximum concentration of soluble Ž Ž . layer mol r l; total bulk concentration Cu II in equilibrium with solid y 4 Ž . . Ž . Ž . Cu II s 1 = 10 mol r l Cu OH mol r l 2 y 1.87 0.64 4.0 10 10 y 1.38  y 1.36 5.0 10 10 y 1.03  y 3.36 6.0 10 10 y 0.86  y 4 Ž . 6.3 10 bulk solubility limit reached 10 y 1.4  y 5.36 Ž . 7.0 10 bulk solubility limit exceeded 10 the  P. luscens  biomass at pH 4, 5, 6 and 7 are approximately y 60,  y 75,  y 85 and Ž .  y 4 y 100 mV, respectively. For a total bulk Cu II concentration of 1.0 = 10 mol r l and Ž . using the Boltzmann equation the concentrations of Cu II within the diffuse part of the Ž . double layer can be roughly calculated. Table 1 shows the amount of Cu II in theelectrical double layer as a function of pH. Thus, it can be seen that at all pH values Ž . more acidic than pH 6.3, the solubility limit of Cu II is exceeded and there should be Ž . precipitation of Cu OH within the double layer. Assuming different values for the total 2 Ž . concentration of Cu II in a system, the pH value at which the double layer concentra- Ž . tion of Cu II approximately equals the solubility limit in the bulk solution can be Ž . determined for these total concentrations of Cu II . This data is presented in Table 2 for Ž .  y 8  y 4 Cu II concentrations from 1 = 10 to 1 = 10 mol r l. Ž . Ž . Ž . Ž . Recent work by Schneider 1995 shows that the sorption of Cu II , Ni II , and Zn IIonto  P. luscens ,  Sal Õ inia herzogii , and  Eichhornia crassipes  can be fitted to aLangmuir isotherm. Furthermore, the sorption of these ions appears to involve an ionexchange mechanism whereby either the ions exchange with protons or with other Ž . cations in equivalent amounts Wang, 1995; Wang et al., 1997; Schneider et al., 1999 .It was deduced that the surface group responsible for the exchange was primarily thecarboxylate group. In addition, this work confirmed previous work that the isoelectric Ž . point iep of all three macrophytes is at about pH 2.0. The p K   of the carboxylate a groups in fatty acids is at pH 4.7, but there is evidence that it is at a more acidic value in Table 2 Ž . The pH value at which the concentration of Cu II aq. species within the double layer equals the maximum Ž . Ž . concentration of Cu II aq. species in the bulk solution as a function of total Cu II in the system Ž . Total concentration of Cu II in system The pH at which the concentration of  Ž . Ž . mol r l Cu II aq. within the double layer equalsthe maximum concentration inthe bulk of the solution y 4 1 = 10 5.0 y 5 1 = 10 5.4 y 6 1 = 10 5.8 y 7 1 = 10 6.3 y 8 1 = 10 6.8
Search
Similar documents
View more...
Related Search
We Need Your Support
Thank you for visiting our website and your interest in our free products and services. We are nonprofit website to share and download documents. To the running of this website, we need your help to support us.

Thanks to everyone for your continued support.

No, Thanks