organic ligands. Because Cd2+complexes of isolated aquatic and soil-derived fulvic acids are much weaker than those of Cu2+ (9, 16, 17), Cd2+ binding would be more affected by excess Ca2+and Mg2+.We see no negative correlations at the 90% confidence level between Cd2+PBL values and inorganic ion properties of the samples because of the small magnitude and range of Cd2+PBL values (Table I). Comparison of New Hampshire and Other Water Sources. Our correlation study results indicate that inorganic constituents like alkalinity and pH influence Cu2+and Cd2+ PBL values more than the DOC and color organic parameters. We can compare our dialysis titration results with two ion selective electrode studies of natural water Cu2+ and Cd2+ binding. McCrady and Chapman ( 4 ) studied natural river water, well water, and artifically reconstituted water. Their river samples, taken in the northwestern U.S., had a higher pH range (7.0-8.5) and higher alkalinities (24-219 mg/L as CaC03) than our samples. They concluded that complexing agents other than simple inorganic species such as OH- and C032- define the titration curve at low Cu2+ levels, but pH and alkalinity dominate the upper titration-curve slope. The samples of Giesy et al. ( 3 ) from southern Maine rivers and lakes had a lower pH range (4.6-6.3) and lower alkalinities (1-30 mg/L as CaC03) than our samples. They concluded that Cu2+ complexation was largely associated with organic matter while Cd2+ binding was chiefly inorganic species controlled. The pH and alkalinity of our samples is intermediate between the low values of Giesy et al. ( 3 ) and the high values of McCrady and Chapman (4). Differences among the three studies are apparently due to the differences in composition of the water samples. In soft, nonalkaline, acidic, and colored Maine water systems, the dissolved organic matter has an appreciable metal-binding influence ( 3 ) .Our geographically nearby water samples taken during an extended rainy period exhibit different metalbinding properties due to the soil run-off loading of the water with inorganic compounds. The metal-ion-binding chemistry of the low-organic-concentration, high-pH, and alkaline northwestern U.S.waters, however, is principally dominated by inorganic species (4). We conclude that concentrations of inorganic and organic species in water samples strongly affect their metal-ion-binding ability.
Acknowledgment
We acknowledge Dr. C. L. Grant for asisstance with the statistical analysis. Literature Cited (1) Tessier, A.; Campbell, P. G. C.; Bisson, M. Anal. Chem. 1979,51, 844-51. (2) Gachter. R.: Davis. J. S.: Mar& A. Enuiron. Sei. Technol. 1978. 12,1416-21. (3) Giesy, J. P., Jr.: Briese, L. A,;Leversee. G. J. Enuiron. Ceol. (NY . ) 1978,2,257-68. (4) McCrady, J. L.; Chapman, G. A. Water Res. 1979, 13, 143-50. (5) Van den Berg, C. M. G.; Kramer, J. R. Anal. Chim. Acta 1979,106, 113-20. (6) Figura, P.; McDuffie, B. Anal. Chem. 1979,52, 120-5. (7) Shuman, M. S.; Cromer, J. L. Enuiron. Sei. Technol. 1979, 13, 543-5. (8) Baccini, P.; Suter, U.; Schweiz. Z. Hydrol. 1979,41, 291-314. (9) Truitt, R. E.; Weber, J. H. Anal. Chem. 1981,53, 337-42. (10) Weber, J. H.; Wilson, S. A. Water Res. 1975,9,1079-84. (11) Guy, R. D.Ph.D. Dissertation, Carleton University, Ottawa, Ontario, Canada, 1976. (12) Truitt, R. E.; Weber, J . H. Anal. Chem. 1979,52,2057-9. (13) “Standard Methods for the Examination of Water and Wastewater”, 14th ed.; American Public Health Association: New York, 1975. (14) Reuter, J. H.; Perdue, E. M. Geochim. Cosmochim. Acta 1977, 41,325-34. (15) Jackson, T. A. Soil Sci. 1975,119,56-64. (16) Bresnahan, W. T.; Grant, C. L.; Weber, J. H. Anal. Chem. 1978, 50, 1675-9. (17) Saar, R. A.; Weber, J. H. Can. J . Chem. 1979,57, 1263-8. (18) Stumm, W.; Morgan, J. J. “Aquatic Chemistry”; Wiley-Interscience: New York, f970; p 270. (19) Baes, C. F., Jr.; Mesmer, R. E. “The Hydrolysis of Cations”; Wiley-Interscience: New York, 1976; pp 267-74. (20) Florence, T . M.; Batley, G. E. CRC Crit. Rev. Anal. Chem. 1980, 9,219-96. (21) Langford, C. H.; Khan, T . R.; Skippen, G. B. Inorg. Nucl. Chem. Lett. 1979,15, 291-5.
Received for reuiea September 8,1980. Revised manuscript received January 19,1982. Accepted June 17,1981. This research was p a r tially supported by Office of Water Resources Technology Grant B-004-NH administered by the Water Resources Research Center at the University of New Hampshire.
Structure and Evolution of Fugitive Particles from a Copper Smelter John P. Bradley Department of Chemistry, Arizona State University, Tempe, Arizona 85281
P. Goodman Division of Chemical Physics, C.S.I.R.O., Melbourne, Australia
I. Y. T. Chan and Peter R. Buseck” Departments of Chemistry and Geology, Arizona State University, Tempe, Arizona 85281
Particulate air pollution generated by large industrial operations remains a pressing environmental concern. One important aspect of this problem is the need to be able to identify the source(s) of airborne particles. Since it is now possible to characterize individual particles, the chemical and physical characteristics of single particles may be used to provide information about their sources. In fact, we are finding that in some cases airborne particles carry with them information, not only about their sources, but also about the processes in which they were generated. In this paper we select an important type of industrial source, a copper smelter, and illustrate 1208
Environmental Science & Technology
how a selected airborne particle type can be traced back to a specific stage of copper smelting. In doing this we show that it is also possible to reconstruct aspects of both the chemical and physical evolution of the particles under investigation. There are seven copper smelters in Arizona. Together, they account for more than 50% of U.S. copper production ( I , 2 ) . As part of an ongoing study, we are investigating particulate emissions from the smelters and, where possible, are tracing individual particles back to specific stages of the smelting process. The particles described in this paper were recovered downwind of one of the smelters, within the fallout region of 0013-936X/81/0915-1208$01.25/0
@ 1981 American Chemical Society
Tetrahedrally branched particles have been observed among fugitive emissions from an Arizona copper smelter. Scanning and transmission electron microscopy, employed to investigate their structure and chemistry, has enabled us to trace their origin back to a specific stage of the copper smelting proceas. The particles exhibit a continuous variation in both morphology and chemical composition. This continuum of particle characteristics is subdivided into four particle types: (a) sharply profiled zinc oxide tetrahedra (fourlings) that consist of four single crystals arranged in an approxi-
mately tetrahedral array; ( b ) rounded, carbon-coated zinc oxide tetrahedra; (e) rounded. carbon-coated tetrahedra in which the skeletal zinc oxide core has suffered substantial decomposition; ( d ) well-rounded tetrahedra of carbon in which the zinc oxide corea have undergone complete decomposition. This variation in composition and morphology has enabled us to describe the evolution of the particles and to use them to identify the industrial process where they are generated and released.
the smelter plume. A e m l samples containing these particlea were recovered by vacuum filtration directly onto Nuclepore substrates and holey-carbon TEM grids. Sampling episodea were timed to coincide with high sulfur dioxide levels or when the plume had visibly descended over a ground-level sampling station. Particle emissions originate from almost every stage of the smelting process. and, because copper and iron sulfides are important ore constituents. large amounts of sulfur dioxide and associated volatile elements are also released. During a survey of fugitive emissions from one of the smelters, we observed particles having both a distinctive tetrahedral morphology and unusual chemistry. Subsequent investigation has enabled us to trace their origin back to the fire-refining stage of the copper smelting process. In order to discuss the observed continuum of the characteristics exhibited by these particles, we have grouped them into four particle types. Figure la-d shows electron micrographs of each of the parti-
cles, henceforth referred to as types a-d. They exhibit a progressive chemical and morphological variation that includes the following: sharply profiled type a particles that consist of four acicular zinc oxide crystals united at a common juncture (fourlings) and arranged in an approximately tetrahedral array; rounded, carbon-coated zinc oxide tetrahedra (type b); rounded, carbon-coated zinc oxide tetrahedra in which the skeletal zinc oxide fourlings have suffered substantial decomposition (typec); tetrahedral carbonaceous pseudomorphs that have suffered complete loss of their skeletal zinc oxide fourlings (type d ) . Particle agglomerates, formed by fusion of several tetrahedra, are also observed (Figure 2). Particle Analysis X-ray energy-dispersive and wavelength-dispersive spectrometry (EDS and WDS) were employed to investigate the chemical compositions of the particles (16) (Figure 1). Both EDS and WDS indicate a range of compositions that correlates with particle morphologies. Type o particles contain zinc as the major constituent, whereas types b 4 respectively contain progressively decreasing concentrations of zinc. All particle types contain small amounts of sulfur, which may be present as a slvficial sulfate compound ( 3 4 .WDS, employed for lighter elements. indicates that type o particles exhibit little if any evidence of carbon. However, types b-d respectively contain increasing amounts of carbon. Typed particlea invariably contain carbon as the major constituent. The nature of these chemical variations is clarified by transmission electron microscopy (TEM). Type a particles consist of sharply profiled, crystalline tetrahedra. Individual arms yield single crystal selected area electron diffraction (SAED) patterns that can be indexed as zinc oxide. These particles are sensitive to 100-kV electron irradiation. When
I
~lpue 1 . ~ a n 1 m r m o f p h y s i = a 1 m 1ChradaWfaermMted ~~1 by me lebgnedal wllcbs. (co*rmi) E W m mlacqad~of far partic~tvpes,whou,lndikhalmrsngefranO.1lo 1.Ofiminkqth: type a. zinc oxide letraheoal palick (Nuclepore ulbsbale):(ype b. zinc oxide cwe particle within an arophous carbon shell (holey-carbon wbsbale):type c. partially decomposed zinc oxide c a e particle w h i n an amaphaa cahm shell ( w e wbslrale);typed. mtmawws pseudomaph lhal has suflered complere loss of its zinc oxide seed particle (Nuclepne substrate).(Columns ii and iii) The cwresponding chemical variations as indicated by X-ray speclroscopy.
Figure 2.
Scanning electron
micrograph 01 a lelrahedral particle a p
glomerafs. The mows indicate an apparenf reaction 01 lhe particle w i 6 the Nuclepae subslrale. which m y be due lo sulluic acid adsabed onlo I h e particle surface. V O l W 15. NMber
IO. October 1981 1209
the condenser aperture is removed and a strong probe current is focused on the crystals, they beam-damage, acquiring corrugated surfaces and a mosaic structure. Simultaneously the SAED patterns degrade to yield streaked reflections. Type b and c particles are composites of zinc oxide and carbon. Figure 3 shows shadow images of (i) the direct transmitted beam and (ii) a diffracted beam, from an appropriately d e f o c u d SAED pattern (6).obtained from a type b particle. The particle arm indicated by arrow 1 was mosaic in appearance, similar to beam-damaged type a particles, even prior to extended electron irradiation. This mosaic crystal structure, which is typical of both type b and type c particles, is indicative of partial degradation of the zinc oxide crystals. In fact, type c particles exhibit substantial degradation of their crystalline arms. Unlike type o particles, types b and c yield streaked SAED patterns. T h e rounded mass at the tip of one of the branches, indicated by arrow 2, is evidently amorphous (presumably carbon) because, regardless of specimen tilt angle, it does not show up in the diffracted beam shadow image while that particular branch (boxed areas) is strongly diffracting. In Figure 1 the inverse effect is shown; the type b particle shows a sharply profiled zinc oxide crystal protruding from an amorphws carbon shell. Thus, it appears that types b and c were originally type a particles that have subsequently been coated with varying amounts of carbon, together with partial decomposition of their crystalline zinc oxide cores. T y p e d particles occur less frequently than types 0-c. Although well-rounded tetrahedra were observed, type d particles are usually seen to be at various stages of decomposition. SAED patterns obtained from typed particles yield only rings of amorphous carbon. EDS and WDS indicate no evidence of zinc oxide within these particles (Figure 1). Many of the particles exhibit evidence of reaction with the Nuclepore substrate (Figure 2). Carbonaceous particles are a known adsorbant for atmospheric sulfur dioxide and may catalyze the reaction of sulfur dioxide to sulfuric acid ( 4 , 5 ) . As Figure 1 illustrates, all particle types contain minor sulfur. This sulfur may be present on the particle surfaces as a surficial sulfate compound or, in some cases, as sulfate adsorbed onto the amorphous carbon shell. We have been able to duplicate this substrate reaction by preparing a series of carbonaceous particles that were impregnated with a variety of sulfate compounds. Particles were prepared by finely crushing amorphous carbon in a mortar. Solutions of ammonium sulfate, ammonium bisulfate, and sulfuric acid were prepared, over a concentration range of 3-12 M. Amorphous carbon particles were
placed into each solution and allowed to digest. The particles were then removed, dried, and dispersed onto Nuclepore substrates. Particles treated with >9 M sulfuric acid exhibited evidence of reaction with the substrate. Although not conclusive, this observation suggests that sulfuric acid adsorbed onto the particles may indeed be responsible for the substrate reaction. Capper Smelting Process Typical Arizona copper ores contain W o . Following fire refining the product is purified electrolytically to yield a final product containing >99.5% copper. Thus. there are a number of particle-generating processes that occur both prior and subsequent to fire refining. Particulate emissions from these other processes will be discussed in future papers. Discussion Tetrahedrally branched zinc oxide particles (fourlings) can be generated, not only during copper smelting, but also under laboratory conditions. When zinc metal is heated in a crucible, the emerging zinc vapor condenses to form tetrahedrally branched zinc oxide fourlings (7). Each fourling consists of four single zinc oxide crystals united a t a compon juncture and related to one another by twinning on (1122) planes (7, 8).This distinctive zinc oxide morphology thus provides a clue to the origin of the particles under investigation; their probable source is a stage of the copper smelting plocess where zinc vapor is liberated into the atmosphere. Deposition of carbon on zinc oxide occurs during nonferrous smelting p m when a hydrogenhydrocarbon gas mixture is used to reduce zinc oxide to metallic zinc (9. IO).T h e following reactions are important: 2Zn0 + C2Hs = 2Zn(v) + 2CO + 3H2 ZnO + CO = Z n W
~ 3 . 1 n w @ o f a t y p ebpaklttasobservedinmedilfractiondi~s an appropriately d e f o a s d SAED panern: (i) shadow image 01 lhe direct transmined beam showing a type b particle: (ii) shadow image of a dilfracled beam showing a singte branch 01 I h e panicle (boxed w a s in i and ii). The paicle arm irdiilwd bq arrow 1 exhibits a mosaic crystal ssuclure. a feature lypical of both type band lype c panicles. Arrow 2 indicates amorphous material (presumably carbon) at the tip of lhal particle arm
01
+ CO2 ZnO + H2 = Zn(v) + H 2 0 c2ne= 2 c + 3n2 co = c + 'h 0 2
(1)
(2) (3) (4)
(5)
Reduction of zinc oxide by reactions 1-3 has a relatively high reaction rate at loo0 "C and increases rapidly with increasing temperature. However, thermal decomposition of both the hydrocarbon gas and carbon monoxide occurs by
h
reactions 4 and 5 . This decomposition presents difficulties since both carbon and carbon-coated material (such as carbon-coated zinc oxide) build up within the reaction vessel and hence lower the overall efficiency of the reduction process. The intimate association of zinc oxide and carbon therefore provides another clue as to the origin of the particles; their probable source must be a reducing environment where a hydrogen/hydrocarbon gas mixture is used as the reducing agent. Type a particles display significant deviations from the expected properties of zinc oxide. For example, our observation that they degrade under 100-kV electron irradiation is not typical of zinc oxide ( I I , 1 2 ) . The apparent anomaly exhibited by type a particles may be due to the presence of impurities within the crystal structures, although, in view of the nature of the environment in which they are generated this is not surprising. The source of zinc is presumably the copper ore since, in addition to pyrite (FeS) and chalcopyrite (CuFeSz), typical Arizona ores also contain wurtzite (ZnS) and sphalerite (ZnS). During smelting the ore separates into two molten phases, a copper-rich matte and an alumina (AlzOs)-fayalite (FezSiOd) slag that floats on the surface of the matte. Zinc concentrates within the matte, but, because of its relatively high volatility (bp 907 “C), the zinc is readily liberated. Consequently, a variety of particle types containing zinc are emitted from all high-temperature stages of the smelting process (13-15). Tetrahedrally branched particles (types a-d) were identified in aerosol samples emitted from the fire-refining stage of the smelting process. Type a zinc oxide tetrahedra are generated above the surface of the copper-rich phase during fire refining. (By the time the molten liquid reaches this stage of the process, it is called blister copper.) Until this point the liquid has been continuously overlain by a slag that prevents direct interaction of zinc vapor with the atmosphere. However, upon charging the refining vessel a liquid/atmosphere interface is established. Consequently, zinc remaining within the refining vessel is liberated directly into the atmosphere to form tetrahedral type a zinc oxide particles. Along with other 01ides, some of these particles accumulate on the surface of the blister copper. During the reduction stage of fire refining, the propane and hydrogen gases generate large quantities of carbon (by thermal decomposition of propane and carbon monoxide) and a 2-3-m flame on the surface of the refining vessel. Deposition of carbon on the type a zinc oxide particles occurs. Furthermore, because the temperature on the surface of the blister copper is -1150 OC, the zinc oxide is readily reduced to zinc vapor, which is then liberated into the atmosphere. Both the degree of carbon coating and zinc oxide reduction depend on the location and the residence time of each particle within the fire refinery; hence the range of particle types that is observed in samples recovered downwind of the smelter (see Figures 1 and 4). On the basis of the preceding discussion, it is possible to postulate a chronological sequence to describe the chemical and physical evolution of the particles under investigation. This sequence is illustrated schematically in Figure 4 and includes the following steps: (i) Tetrahedrally branched zinc oxide fourlings are formed in the fire refinery by condensation of zinc vapor, on or above the surface of the blister copper. (ii) During the reduction event large amounts of carbon are generated by thermal decomposition of propane and carbon monoxide. This carbon is deposited on type a particles to form type b particles. (iii) Some type b particles undergo partial decomposition of their zinc oxide cores, with subsequent loss of zinc as zinc vapor, both as a result of the reducing environment and by thermal degradation. Such particles thus become type c particles. (iv) Those particles that remain
Figure 4. Chemical and physical evolution of a tetrahedral particle. (i) Vapor-phase formation of a type a zinc oxide fourling; (ii) zinc oxide core fourling becomes coated with an amorphous carbon shell (type b particle):(iii) crystalline zinc oxide core undergoes partial degradation (type c particle): (iv) complete degradation of zinc oxide core occurs, leaving an amorphous carbon pseudomorph (type d particle).
within the refining vessel for an extended time suffer complete loss of their zinc oxide cores and then form type d particles.
Conclusions This study illustrates the utility and the strength of individual particle analysis studies for selected environmental problems. By combining a variety of analytical techniques, it is possible to detail specific events in the evolution of airborne particles. The particles described in this paper carry with them evidence linking them not only to their parent copper smelter, but also to the process where they formed, that is, the specific stage of the smelting process where they were generated. Such information is clearly important for assessing the environmental impact and possible methods of abatement of particulate emissions from copper smelters and other industrial sources. Acknowledgment
Transmission electron microscopy was performed at the Facility for High Resolution Electron Microscopy, established with support from the NSF Regional Instrumentation Facilities Program (Grant CHE-7916098). The advice of J. Cowley, G. Aden, M. Germani and the technical assistance of G. Goldman are gratefully acknowledged. Literature Cited (1) Arizona Department of Health Services, “First Annual Report on Arizona Copper Smelter Pollution Control Technology”; State of Arizona, 1977. (2) Arizona Department of Health Services. “Second Annual Report on Arizona Copper Smelter Air Pollution Control Technology”; State of Arizona, 1978. (3) Russell, P. A. In “Proceedings, Carbonaceous Particles in the Atmosphere”; Novakov, T., Ed.; National Science Foundation and Lawrence Berkeley Laboratory, 1979; p 133, (4) Novakov, T.; Chang, S. G.; Harker, A. B. Science 1974, 186, 259. (5) Tartarelli, R.; Davini, P.; Morelli, F.; Corsi, P. Atmos. Enuiron. 1978,12,289. (6) Hirsch, P.; Howie, A,; Nicholson, R. B.; Pashley, D. W.; Whelan, M. J. “Electron Microscopy of Thin Crystals”, 2nd ed; Robert Krieger Publishing Co.: Huntington, NY, 1977; pp 295-316. (7) Cowley, J. M.; Rees, A. L. G.; Spink, J. A. Proc. Phys. Soc., London 1951,64,638. (8) Fuller, M. L. J.Appl. Phys. 1944,15, 164. (9) Downer, H. A. Trans. Am. Znst. Min. Metall. Eng. 1936,121, 636. (10) Fulton, C. H. Trans. Am. Inst. Min. Metall. Eng. 1919, 60, 280.
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(16) Aden, G. D.; Buseck, P. R. In “Microbeam Analysis-1979”; Newbury, D. E., Ed.; San Francisco Press: San Francisco, 1979; p 254.
(11) Iwanaga, H.; Shibata, N.; Suzuki, K.; Takeuchi, S. Philos. Mag. 1977,35,1213. (12) Yoshiie, T.; Iwanaga, H.; Shibata, N.; Ichihara, M.; Takeuchi, S.Philos. Mag., [Part]A 1979,40, 297. (13) Bradley, J. P., Buseck, P. R., in preparation. (14) Germani, M. S.; Small, M.; Zoller, W. H.; Moyers, J. L. Enuiron. Sei. Technol. 1981,15, 299. (15) Small, M.; Germani, M. S.; Small, A. M.; Zoller, W. H.; Moyers, J. L. Enuiron. Sei. Technol. 1981,15, 293.
Received for reuiew September 11,1980. Accepted April 29,1981. This research was supported by grant ATM-8022849 (toP.R.R.)from the Atmospheric Sciences Division of the National Science Foundation.
Nature of Bonding between Metallic Ions and Algal Cell Walls Ray H. Crist,” Karl Oberholser, Norman Shank, and Ming Nguyen Messiah College, Grantham, Pennsylvania 17027
Introduction
shrimp, Sunda, Engel, and Thuotte (6) found that mortality decreased with increasing salinity and concentration of the chelating agent NTA; Chakoumakos, Russo, and Thurston (7) showed that for various species of copper the toxicity for cutthroat trout was inversely correlated with water hardness and alkalinity. In our laboratory work has been directed to understanding the nature of the initial process, the interaction of metallic ions with an algal cell wall. Earlier it was shown that the adsorption of copper on algae was a reversible system and could be represented by the Langmuir adsorption isotherm (8).Furthermore, metal ions could displace other metal ions or protons, and a reversible pH titration indicated the existence of labile protons whose loss did not lead to substantial structural changes (9). Though the cell wall composition for the alga Vaucheria s. used here is not known, Siegel and Siegel (10)report a protein content of 16-27% for the V a u c h e r i a group. Amino acids in the proteins could provide such functional groups as
Trace elements enter biological systems through the cell walls of plants or the membranes of animals where as constituents of enzymes within the organism they perform many vital functions. A level of availability in agriculture and nutrition is often of concern ( I ) , while toxic effects may appear at various levels of the elements occurring naturally or resulting from waste discharges to the environment. The toxic effect of trace metals on aquatic organisms frequently is dependent on the species of the metallic ion which in turn may be determined by the pH or the varieties of the complexing agents found in natural waters. Interaction with cell walls or with membranes is the initial process in any biotic action. These exterior surfaces have a common composition of proteins and carbohydrates with which the metallic species could react. Bacteria and diatoms are of particular importance because they have a large surface area, are ubiquitous, and are at the low point in the food chain. Situations that emphasize the interaction process are the following: bacteria are the primary agents in the activated sludge treatment of wastewater where they adsorb the trace elements which through sludge disposal as amendments to soil find their way into food crops (2);for the diatom N i t z s c h i a p y r e n o i d o s a , Nielson and Anderson ( 3 ) found that copper influenced the rate of photosynthesis more than for the alga chlorella while the reverse was found for the growth rate, this being attributed to the excreta of the diatom decreasing the concentration of solution copper; Gross, Pugno, and Dugger ( 4 ) found for chlorella that the pigments were affected and photosynthesis was inhibited after short contact with CuS04 at 10-100 pM; for the alga Microcystis a e r u g i n o s a , Allen, Hall, and Brisbin ( 5 )showed zinc toxicity to be due primarily to Zn2+ and Zn(OH)+ in comparison to chelated species; for the larger organism, grass
The polysaccharides of the cell wall could also provide the amino and carboxyl groups as well as the sulfate. The amino and carboxyl groups, the imidazole of histidine, and the nitrogen and oxygen of the peptide bond could be available for characteristic coordination bonding with metallic ions like Cu2+; such bond formation could be accompanied by displacement of protons dependent in part on the extent of protonation as determined by the pH. Metallic ions could also be electrostatically bonded to unprotonated carboxyl oxygen and sulfate. In earlier unpublished work two types of metalalgal bonds were evidenced by the appearance of two slopes in the Langmuir isotherm plots. With these considerations in mind, it was thought that differences in adsorption for transition elements, e.g., Cu2+ and Zn2+,with their strong coordination tendencies as compared to alkali and alkaline-earth elements might show contrasting bonding character. Also, recent work with colloidal inorganic oxides for anion and cation adsorptions and pH titrations (11-13) provides a possible framework for understanding metallic ion-algal systems. Thus, the charge development on increasing the pH, with a zero point charge at pH -3 (14), would suggest similarities of behavior. However, it must be noted that the well-defined and highly crystalline A1203 and S i 0 2 in aqueous systems are bound to show sub-
w Metallic ions adsorbed by algal cell walls at pH 4.5 ranged from 600 to 100 pmolg-l for Cu2+and Na+, respectively, with a pH dependence in the case of Sr2+of -50 pmol g-l per pH unit. A reproducible pH titration was found which required -1000 pmol of NaOH per gram for pH 3-8. Protons displaced by metal adsorption gave the following ratios for H+ displaced/M2+ adsorbed: 1.2 (Cu2+),0.66 (Zn2+),0.59 (Mg2+), 0.30 (Sr2+),and 0 (Na+). These ratios will vary some with concentration and pH. Ion exchange showed a strength of adsorption in the order Cu2+ > Sr2+ > Zn2+ > Mg2+ > Na+ suggesting a trend from probable covalent to ionic charge bonding. This latter was demonstrated directly by Na+ decreasing adsorption with positive metallic ion complexes and i n c r e a s i n g it with negative ones. Ionic charge bonding was thought to arise from a surface charge generated by increasing pH, and covalent bonding from constitutent proteins.
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Environmental Science 8, Technology
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