COMPLEXATION ANALYSIS OF FRESH WATERS BY EQUILIBRIUM DIAFILTRATION
JULIAN LEE*
Resource Geophysics and Geochemistry Division, Geological Survey of Canada, Ottawa, Canada
(Received December 1980)
Abstract--Equilibrium ultrafiltration has been employed to determine the extent of Cu, Ni, Co, Fe, Mn and Zu chelation in oxic waters from mineralized terrains. Concentrations of Cu, Ni and Co were determined in different organic fractions separated by various Amicon ultrafilters. Titrations of fresh water samples with metal ions were made, and "free" metal concentration measured by equilibrium diafiltration. Experiments were made at concentrations and pH at which metals and ligands occur in natural waters. Conditional stability functions ranged from 106.72 at pH 6.5 to 107.07 at pH 7.6 for Ni and 106.85 at pH 6.8 to 106.97 at pH 7.6 for Co. At low metal concentrations ( ~8 × 10 -v M) only one complexing class of major importance for natural water environments was observed. The order of metal complexing ability was found to be Cu > Ni > Co > Zn > Mn with Cu showing a preference towards the higher MW organic fractions.
INTRODUCTION organic models and significantly reduce the extent of adsorption and free metal ion concentration by corn-
In recent years there has been increasing interest in plexation. For example, the solubility of copper in studies stressing the importance of the physicochemi- natural waters (pH 7.5-8.0) in largely controlled by cal state of transition metals in fresh-water systems that of the oxide tenorite. The major inorganic anions and the inadequacy of utilizing total metal determi- (chloride, sulphate, bicarbonate, carbonate and hy- nations for assessment of various geochemical and droxide) form sparingly soluble salts and soluble com- biological transformations (Stiff, 1971; Stumm & plexes with Cu, and in waters which may be desig- Bilinski, 1972; Benes & Steinnes, 1975; Reuter & Per- nated as largely inorganic they play an important role due, 1977; Mann & Deutscher, 1977; Gachter et al., in controlling the geochemical mobility of Cu (Mann 1978; Giesy & Briese, 1977; McCrady & Chapman, & Deutscher, 1977). However, as Vuceta & Morgan 1979). Trace metal aqueous transport, availability of (1978) have demonstated the presence of organic com- heavy metals to aquatic organisms and accumulation plexing agents greatly reduces adsorption of dissolved and remobilization in sediments is very much specia- Cu(II) species. The main aqueous species expected tion dependant. Of particular importance is the extent would be complexes with organic ligands. The extent and nature of complexation of trace metals with the of complexation is dependant on the concentration hydroxyl and carboxyl groups of molecules derived and availability of suitable organic ligands. Vuceta from both autochthonous and allochthonous organic and Morgan have similarily shown that Ni(II), Co(II) sources. Although the nature of this material is and Zn(II) are expected to occur in solution as the complex and ill-defined, it is clear it should exert con- hydrated ion species with an increase in surface area siderable influence on the chemistry of the transition and organic ligand concentration leading to some metals (Jackson, 1975; Reuter & Perdue, 1977; Jack- adsorption and complexation. son et al., 1978). Apart from humic and fulvic acids, A variety of techniques have been employed to which behave as weak acid polyelectrolytes and con- evaluate conditional stability constants for the com- stitute the bulk of the organic material in flesh waters, plexation of trace metals and naturally occurring there may be lesser amounts of small molecular flesh-water organic substances, with the methods weight organic acids, many of which are normal being able to differentiate between free or labile ions metabolites or metabolic intermediates of soil, plant and more stable or kinetically inert complexed forms. and bacterial processes (Steelink, 1977). These acids These techniques include potentiometric titrations, have the potential to react readily with transition ion-exchange, spectrophotometry, electrochemical metals ions and form thermodynamically stable, but measurements, metal-ion specific electrodes, nuclear often kinetically labile adducts. magnetic resonance, electron-spin resonance, dialysis
Organic substances may cause appreciable devi- and others. The use of potentiometry with selective- ations from solubility predictions based solely on in- ion electrodes (Stiff, 1971 ; Buffle et al., 1977; Bresna-
have become increasingly popular. Hovvevcr. their use
113s been limited primarily to Cu. Pb. C’d and other- metals having adequate current responses. without the need of special supporting electrolytes which could alter the equilibria and general chemical nature of the test solution. The reduction of a metal species
on to ;I mercury electrode in anodic stripping voltam-
metry is dependent on the effective diffusion layer
thickness (Davison. 1978) and those complexes with
dissociation rates at least as rapid as the rate of metal transfer to the electrode surface will make substantial contributions to the overall stripping current. This may
result in a low estimation of the complexing capacities for some natural waters. Shuman & Michael (1978) using a rotating disk electrode have established an operational definition for labile and non-labile metal complexes based on kinetic criteria.
problems associated with the above techniques. Di- alysis and ultrafiltration techniques in conjunction
with chromatographic procedures. such as steric ek-
Speciation studies of two principal transition el- ements. Ni and Co. in geochemical and environmen- tal research have been limited because of analvtical
Equilibrium ultrafiltration (Blat1 ct c/i.. I MS1 \I;LS used to study the relative comple\ation atlinttte> of
(‘0, Ni and C‘u tovvard fresh-water organics. pro\ ide
data for assessing conditional stability functtons and to fractionate natural mctalliferous waters according to molecular v\eight and lability of the metal spcc~es.
were, however. carried out as soon as possible to minimize
Fresh-water samples collected from various locatmns (Table I) were immediately tiltered through 0.2 n Amicon macroporous filters and the filtrates transported to the
any possible changes in the solution properties. Measure-
laboratory and stored at 4°C until use. This treatment kept the samples bacteria free and stable with regard to organic
ments of pH, conductivity. absorbance and total metal
carbon content (monitored hy U.V. absorbance at 254 nm)
concentration were made immediatelv orior to each exneri-
and metal concentrations for several months. Experiments
elusion chromatography. have been employed (Man- toura & Riley. 1975: Giesy & Briese, 1977: Means rt
ctl.. 1977). After tiltration through macroporous filters to
remove suspended particulates and bacteria: com- plexing and adsorbing substances in natural waters
may be classified into three broad groups having dif- ferent physical properties based on molecular weight and their reactivity towards metal ions:
ment after equilibration to room temperature (22’C). bon- centrations of Cu. Ni and Co were determined by atomic absorption spectrophotometry with electrothermal atomiz- ation Standard background correction by deuterium lamp was mandatory. No interferences were encountered in any of the samples as shown by comparing peak absorbances from both direct and standard addition measurements. Blanks and background levels were below reportable limits (twice signal to noise ratio) which were Cu. 0.5 x 10 ” g; Co, 0.5 x IO-” g and Ni, 1 x IO- I’ g. Sample locations along with PH. conductivitv. absorbance and metal con-
(i) Colloids: inorganic hydroxides, silicates. organic centrations are reported in Table I.
macromolecules, IO4 < mol. wt < IO”. Molecular weight based separations were achieved using
an Amicon model MMC stirred multi-cell micro-ultrafil-
(ii) True solutes with both complexing and surface active properties; complexes mainly non-labile:
500 < mol. wt < 104. Fulvic and humic acids. (iii) Solutes forming labile complexes both in-
organic ions and organic ligands (amino acids, Krebs cycle and other organic polyhydroxy acids) mol. wt < 500.
This paper deals predominantly with those com- plexing agents categorized in the second group. The three elements selected for this study (Co. Ni, Cu) lie
together in the first row transition metals and as ligand field theory predicts, complexation by ligands further along in the spectrochemical series than H,O
will result in increased stabilization of the metal ion. Natural water fulvic and humic materials provide
tration system with Amicon Diaflo ultrafiltration mem- branes. Each cell may be operated individually, permitting independent sample addition or withdrawal and indepen- dent change of operating mode; i.e. concentration ultratil- tration or constant cell volume diatiltration. For large samples (SO-5OOml) ultrafiltrations were made using a TCF-IO Thin-channel Amicon ultrafiltration system with 5 I. reservoir capacity. Fractionation is facilitated by the selective permeability of the membranes to solutes of given molecular dimensions. The ultrafilters used and their nominal molecular weight retentions are as follows: XM-100 (> lOO,OOO), PM-30 (>30,000), PM-IO (>10.000). UM-IO( > 10.000). DM-5 (> 5000). YM-5 (> 5000) t_JM-2 (> 1000) and UM-05 ( > 500). The molecular weight reten- tion limits assigned to their various membranes arc oper- ational definitions only and depend on the type and shape of the molecular structure.
The 0.2 nrn filtered water samples were tirst ultrafiltered through XM-100 Diablo membranes under nitroeen nres-
- c z
such grouns and hence contribute greatlv to the surized at 20 p.s.i. This step assured the removal of any I . - I
movement of these metals in many geochemical en- colloidal material that may have passed through the 0.2 nm
vironments. filter. The system was operated at 40p.s.i. for all suh-
Class (ii) complexes may aggregate to produce class sequent fractionations, with the cells stirred at a speed suf- ficient to oroduce a vortex one-third the samole volume.
(i) colloids. In present studies filtered solutions The volt&e was 5.0 ml, unless otherwise indicated.
remained satisfactorily stable with regard to their The organic macromolecules for binding analysis were
ultrafiltration characteristics over a period of several isolated from the natural waters by Diaflo ultrafiltration
months when stored at 4°C under nitrogen. However and dialysis and the solutions made salt-free by aciditica-
on standing at room temperature or after prolonged tion with nitric acid to pH 2.0 and elution through a strong cation exchanger (Dowex 5 OW-X8) in the H” form. The
storage, colloidal suspensions invariably developed. samples were re-adjusted to the required natural pH
Complexation analysis of fresh waters by equilibrium diafiltration 503
Table 1.
(a) Chemical characteristics of water samples used in study
Ni Co Cu Absorbance
Sample no. Location pH S 254 nm 420 nm (/~g 1- ~)
F1 (swamp) Silver Centre, Ontario 7.80 228 0.240 - - 70 105 8 F2 (swamp) Silver Centre, Ontario 7.36 135 0.140 - - 30 38 - - F3 (spring) Silver Centre, Ontario 7.38 269 0.140 - - 80 - - $3 (swamp) Tant ramar swamp
Location and name NTS Geology and source rocks Metals present
Frontier Mine, Silver Centre, Ontario 41M Proterozoic metasediments, diabase (Huronian) Ni, Co, Cu, As, Ag Tan t ramar Swamp, Sackville, N.B. 21H Pennsylvanian sandstones, conglomerates Cu (S) Thetford Mines, Quebec 21L Cambro-Ordovician serpentinized ultramafics Ni, Co Indian River, Ontario 31F Proterozoic meta-sediments (Grenville) Cu
(usually between pH 6.5 and 8.0) using dilute NaOH. This V' = the apparent void volume of the system; procedure was effective in removing any trace metals from 170 = the average cell volume during the run. the samples. A constant ionic strength was obtained with 1 0 - 2 M N a N O 3 . Experimental plots on semi-log paper were made to
Binding profiles of nickel and cobalt were obtained by examine the diafiltration profiles. By examination of the diafiltering, through an UM-05 membrane, appropriate slope of the semi-log plot (a straight line) it was possible to solutions of the metal nitrate prepared in 1.5 x 1 0 - 4 M determine whether significant metal retardation by mem- CaC12, 4 × 10 -4 M N a H C O 3 and 10 -2 M NaNOa, pH brane rejection had occurred (experimental dilution curve 6.5-8.0, against 5.0 ml of natural organic solutions or sol- without binding). If V0 calculated from the slope is much utions of commercial fulvic acid (Contech E.T.C., Ottawa). larger than the observed value, then rejection of the solute Procedual details and limitations of the equilibrium ultra- may be a cause. A retention factor must then be considered filtration and equilibrium dialysis techniques for binding (Blatt et al., 1968). Membrane retardation and binding of analysis have been described elsewhere (Blatt et al., 1968). nickel and cobalt ions was found to be minimal, but at pH
Absorbance of the filtrate was routinely monitored at above 6.5 the copper ion was significantly retained by 254 nm and the membrane rejected if there was continued membrane binding (possibly because of hydrolysis reac- leakage of organic material. Fractions (2 .5ml )were col- tions). Binding curves for copper were therefore not lected on a fraction collector until the cell effluent concen- obtained by this method. Another membrane, YM-5, tration was equal to the concentration of metal in the became available during the progress of this work and reservoir solution. At this point saturation binding of the received only preliminary examination. Although not as
retentive towards water organics, solute rejection was con- macrosolute had occurred and analysis of the cell content yielded free and bound metal, with the free concentration siderably lessened. equal to the feed solution. The metal in each filtrate was At any point during the ultrafiltration the amount of determined and ultrafiltration or "wash-in" curves con- "bound" metal can be calculated from the following structed. In this paper the term "wash-in" refers to diafil- equation: tration of the diffusable species i.e. the metal ion, into and CML = V~(CB) - V0(C,,) - c',, out of the cell and measured as a function of filtrate volume. The reverse process termed "wash-out" refers to where the diafiltration of a known metal ion concentration in the cell against deionized water or buffer solution containing CML = the total no. of moles of bound metal at any none of the metal species, given value of C¢.
The expression for a "wash-in" binding curve may be c~, = the cumulative no. of moles of metal in filtrate given as: calculated from direct measurement of each frac-
tion or estimated from the area under the ultra- CB Ve - V' filtration curve (plots of C~ vs V~).
In C8 - Ce - 17o (Blat t et al.,19681. Ve, Cn, C,, and 170 are defined as above.
Irradiated solutions, using a water cooled 550W Hg where lamp, were used as organic free blanks. No loss of metal
C8 = the concentration of metal in the buffer occurred with irradiation. solution, mol 1- 1 ; The expression for the complexation of a metal ion, M
C e = the concentration of metal in the filtrate, and a ligand, L, with l : l stoichiometry at a given pH may m o l l - 1 ; be simplified as
Ve = the volume of filtrate, 1.; K = [ML] / ( [M] ILl) (1)
~()4 I~ I1,, ~, I_il
~.herc K is the ~q:~parcnt or conditional stabiiit; iunction, rl,]St I,TS .~NI) I)IS('I;SSlON [ML] the concentration of complex. [M] the concen- tration of uncomplexcd metal ion and [1.] the total con- f:roulionotiml o/ natm~d w~tlcr .sdmp[,~
centration of uncomplexed ligand or binding s~tc -1'o aid in the selection of cxperimental conditions Mass balance equ~.llions LH'C givell as
for subsequent binding studies, natural water samptcs ('M = [ M ] - [ML] 12 were t:ractionated by various Amicon membranes.
C't. - [ I. ] -i. [ML] (3) Diafiltration profiles for nickel in sample T4 using lhe
where C M and C L represent total metal and total /igand "%rash-out'" technique and PM-It). UM-10 and concentration respectively. Substitution for in [L] in (1) UM-05 membranes are shown in Fig. 1. Similar pro- from (3) gives the fraction of unbound to bound metal in files were obtained for Cu and Co. The curves illus- terms of the conditional stability constant and total ligand trate that a wdue of approx. 5 for l,i.. I/o results in the concentration (charges omitted)
remowd of more than 95",, of the dialysable metal, ht [M] 1 [M] the diafiltration "'wash-out" method, free metal is con- = + - . (4)
[ML] K ( ] ('~ tinuously being removed from the cell. The resulting Hence by plotting the titration curve in terms of disequilibrium initiates the dissociation of any kin- [M]//[ML] vs FM]; the slope, C( ~ and y- intercept, eticatly labile complexes regardless of their thermo- (KCL)- *, may be obtained. At the pH of the titrations Co and Ni are expected to be mainly in solution as the hyd- dynamic stability. Therefore, a l though reduced to a rated ions but the proportion of hydrolytic species of low concentrat ion initially, all potentially labile spe- Cu(ll) may not be negligible, particularly at pH > 7.2. cies, assuming low membrane rejection character-
In these experiments metal ions compete with protons istics, will be removed from the cell solution, given and Ca(ll) ions for occupancy of the binding sites. These high I/~,/V o values. That metal retained by the mem- sites are undoubtedly a mixture of different classes of che- lating groups having a gradation of binding affinities. The branes represents either adsorbed or kinetically inert equilibrium function, K, is therefore a weighted average of complexes not dissociated at the experimental ['. ~ all the components and is calculated as a function of pH value of 5 (approx. l0 hr needed to attain this when [Ca 2+], [ML] and Cj. Although interaction between sites l,~) = 5.0 ml). The destruction of organic binding is assumed to be neglible, this may not necessarily be so in ability by u.v. irradiation resulted in a much higher the multisite case. Because of the irregular structure of the fulvic acid macromotecule and the non-identical chemical clearance rate for the diafiltration of the metal, with nature of the functional groups, allosteric and cooperative only ,.3 8 °.o retained after the passage of 5 cell volumes
65, ~" effects may be of importance. These considerations are, through the U M - 0 5 membrane compared with . . . . . . ,, however, beyond the scope of this work. Here. it is in the organic solution. The concentrat ions of Ni. ( 'o assumed that each binding reaction proceeds indepen- dandy to equilibrium and that the bound and free species and Cu (expressed as a percentage of the total) are separated and can be measured accurately without dis- retained by membranes of different nominal pore di- turbing the equilibrium of the system ameter are given in Table 2.
*Percentage of metal remaining in cell after diafiltering 5 cell volumes of deionized water through sample. tOrganics destroyed by u.v. irradiation. Oxidation >97%. ++Some membrane absorption. Recovery only 78%.
As was expected on the basis of chemical binding 0.1 M NaNO3 did not seem to alter the fractionation abilities, the percentage of metal retained in the or- pat terns obtained with deionized water alone. Results ganic fractions increased in the order Cu > Ni > Co. are in general agreement with those of Giesy & Briese Most of the Ni and Cu was retained between the (1977) and Buffle et al. (1978) who showed that most PM-10 and UM-05 membranes, as was the organic of these metals are associated with the smaller tool- carbon. The majori ty of the cobalt passed through the ecular weight organic fractions ( < 10,000). UM-05 membrane indicating its lower binding affin- ity. A significantly high percentage (24.1) of the total Binding of metals in natural water solutions by means of Cu in samples $3 was retained by the PM-10 mem- equlibrium ultrafiltration
brane. This sample was collected from a copper bog Diafiltration profiles of the experimental data for (Boyle, 1977). The retention of the Cu by the PM-10 the binding of nickel and cobalt to various natural membrane may reflect strong binding in large mol- water and fulvic acid solutions are shown in Fig. 2(a) ecular weight porphyrin type complexes (very stable) or simply Cu adsorbed onto large organic macromo- ioo - (o) lecules by electrostatic attraction. It is noteworthy that a l though samples F1 (swamp water from a vein Nickel (CB= 8 . 3 7 x l O - r M )
nickel-cobal t arsenide mineralized area) and T4 / , 1 ° f (ultramafic environment) have much different total or- Io - " ~¢~" ......L.- ganic levels and conductivities (Table 1) the organic carbon fractionated to a similar degree in both / ° 1
• ~ A ~ A ~ X ~ X samples. The PM-10 and DM-5 membranes behaved _ , ~ . . ~ . ~ ~ - × - - - - - - - - - - " similarly whilst the effective molecular weight cut-off ,~ j value of the UM-IO membrane appeared to be con- ',~ I I I t I 1 I siderably less than the nominal value given. The o too (b) results are in accordance with typical solute rejection ~ j°~ characteristics of the membranes given by Amicon Cobol t ( C B = 8.57 x 10 -7 M)
(Amicon Publicat ion No. 448C). Buffle et al. (1978) ~ ~ have reported similar behaviour towards natural Io water organics. ° / ~
Following experiments with waters containing high / ~ x , ~ x ~ - -
concentrat ions of organic material the membranes were slightly coloured, indicating some adsorpt ion of organic material. This was found using the UM-2 and I I I I I I I UM-05 membranes. However at the dilute concen- o 5 ~o ~5 zo 25 30 35
trat ions generally employed adsorpt ion was minimal ve ( mr. I as verified by measurements of absorbance before and r3, UM-2OR, pH 67, r4, pH 6 8;
obs 254 nm 0 9 8 0 X abs 254nm 0 6 6 8 Z~
after diafiltration. As the cell volume is kept constant T4I, 15h u v ierodiofion, 1"4, pH 6-7; during the "wash-our ' . dilution or concentrat ion ob~ 254.m o,o5o , ob~ 25,°~,o329 o
effects are minimized. Fig. 2. Diafiltration (wash-in) binding equilibrium of Co 2÷ The use of borate buffers at pH between 6.5 and and Ni 2+ in natural water solutions using UM-05 mem-
8.0, and the adjustment of ionic strength with brane under nitrogen at 40p.s.i.
~q16 Jl l!\N i II
and (b). It is immediately apparent that thc efli,'ct ol ~H the product momenl coefficients of correlation 0) increasing the ligand concentration produces a lot the calculated regression lines, the lit of the data i,, marked shift of the binding profile to the right and a reasonably good. The plots are linear, at least up |u lingering of the slope. For comparison purposes the the free metal ion concentration equal to that in the "'~ash-in" curve for an irradiated solution of sample reservoir (C~), which indicates only one class of sites T4 (T41) is sho,an. Although more than 97" o of the or sites that are sufficiently simihn chemically and organic material was destroyed by the u.v. treatment therefore indistinguishable by the method employed a small amount of binding was still exhibited as It must be emphasised that the values reported in shown by the magnitude of the calculated rejection Table 3 apply to specilic conditions of pH, ionic coelIicient (R) where: strength and competing calcium and bicarbonate ion
concentration. The type of complex obtained will be a R = 1 - C , , : C ~ , function of the molar ratio metal ion,ligand.
with C,, . the concentration in the filtrate and C~n, the There is generally a systematic increase of K ~ith total concentration in the cell. This was 0.16 for nickel pH. If only one type of acidic group is present, a and 0.17 in the case of cobalt. The rejection coefficient 10-fold increase in the stability constant with a one obtained from an experimental dilution curve using unit pH change should be observed. However the de-ionized water only was 0.08. No correction for this results obtained in this study do not follow that was made in the calculation of binding parameters, premise. A comparison of FA binding of nickel ztt Some binding may have been contributed to by the pH 6.8 and 7.6 shows an approx. 2-tkfld increase m the carbonate ion, although at the pH of the experiment derived stability functions. This is a likely reflection of this was assumed to be negligible. An increase in the fact that the fulvic materials of natural waters organic material appears to result in a progressive contain functional groups of a similar nature but in increase in the apparent void volume (V') of the different chemical environments on the molecule, system ( C B / ' C ~ - C~, = 1), indicating the retardation resulting, in classes of sites with a gradation of binding ofdiafiltration. While this may reflect a more complex strengths. Funtional group analyses (Weber & Wil- binding equilibrium, it is more likely to be caused by son, 1975; Choppin & Kullberg, 1978) show that the polarization at the membrane surface which results acidic donor groups of fulvic acid-type organic mol- in increased metal retention. Stirring alone does ecules are probably benzoate, salicylate and pheno- not totally eliminate this effect particularly at high late. However meta l fulvic acid complexing must go pressures and macrosolute concentrations. The ionic beyond the model for the simple phthalic acid or for strength of the supporting electrolyte {10 2 M the salicylic acid bidentate system. NaNOs} was assumed high enough to suppress any The literature reports very few stability constants of Donnan exclusion effects, fulvic acid Co(II) or Ni(ll) complexes for comparison
A comparison of the binding affinity of nickel with purposes. Schnitzer & Hansen (1970) using ion- that of cobalt in like samples shows nickel with the exchange methods have cited wdues for Co(II) FA greater complexing ability. In all fractions the concen- and Ni(II) FA at pH 5.0 of 10 a~ and 104.: respect- tration of bound nickel was greater than that of ively, whilst Malcolm {i972) (cited by Pagenkopf, cobalt. This is reflected in the difference in slope of 1978) obtained a stability constant of l0 f'~' for a the binding profiles of each sample, cobalt FA complex at pH 6.0. Mantoura & Riley
Binding data obtained by the equilibrium uttrafil- (1975) utilizing a gel filtration method (a dynamic tration method a,'e presented in Fig. 3(a) and (b) and equilibrium technique, similar in principle to the equi- the values derived from these and other plots are librium ultraviltration used in this study} gave an summarized in Table 3. As reflected by the magnitude intrinsic stability constant for nickel-lake water humic
Fable 3. Complexation parameters for nickel, cobalt and copper in natural water and fulvic acid solutions
(Ca) (HCO 6 ) C~ ('., (MLI* Metal Sample pH (xl0 '*M)(×I0 '*M) ,:-2s~, {xl0 VM)(x]0 '~M) log k r (xl0 "M)
10 C L =1.47X10 -6 M: log K =,697 . . . . . . . . . . . . &
t,,
L I I I I L I I I I 10 20 30 40 50 60 70 80 90 100
[co2+] x,o-6M Fig. 3. (a) Addition of nickel to natural water and fulvic acid solution by equilibrium ultrafiltration through UM-05 membrane. (b) Addition of cobalt to natural water and fulvic acid solution by equilib-
rium ultrafiltration through UM-05 membrane.
material of 1.38 x 105. This seems low compared to ditions. The similarity in the magnitude of the binding other values particularly as the experiment was at constants for nickel and cobalt is consistant with the pH 8.0; although relatively high metal concentrations results of Schnitzer & Hansen (1970) with the increase were used (5-10 #g ml- 1). It is difficult to make direct in stability "constants" reconciled by the pH differen- comparisons with the values in Table 3, as the stab- tial. A significant increase in Cu binding above pH 5.0 ility "constants" were obtained under differing con- was observed by Bresnahan et al. (1978).
"~()S Jt:l I,\N L I I
Fable 4. Relative binding affinities of some transition metals to various natural ~ater organic fractions determined !~ "'wash-in" diafiltration technique pH 6.5, [Ca] - 1.5 × I0 "* M, [HCO3 ] - 4.0 ,,< 10 "* M, metal reserw}ir concemlalion.
IC B) = 501~gl ~. Results expressed as total metal retained by membrane in the cell minus Cu at equilibrium: ( ' {'~
Abs Membrane (~> Sample pH 254 nm used ('u NI (,ttg i t} Zll \'llt
T3, UM-20 R 6.5 0.988 UM-05 184 176 83 Y M-05 122 108 54 69 15
The effect of competition for binding sites by various functional groups, or with the same molecular weight
transition metals freactions, it does seem that the behaviour of Ni, Cu
The absolute concentrations of metals retained by and Co towards natural water organics is in general UM-05 and YM-5 membranes after diafiltration of a similar, but small differences are evident. Copper in mixed metal buffer at pH 6.5 through a cell contain- particular has a higher selectivity towards larger ing various samples of natural waters are reported in organic macromolecules. Table 4. For the lower molecular weight organics It has been assumed that transition metals bind to (UM-05) the general order of retention is natural water organics through bonding with a high Cu > Ni > Co > Zn > Mn and for the higher mol- degree of covalent character. Comparative studies ecular weight fraction (YM-5) the order is slightly with model metal-salicylic acid and metal-phthalic changed with Cu > Ni > Zn -~ Co ,> Mn. As total acid chelates indicate that this form of bonding pre- organic concentration increases there appears to be a dominates. E.S.R. studies (McBride, 1978) have shown relative increase in Cu, Ni and Zn binding towards the Cu 2+ strongly immobilized in humic acid by the YM-5 retained fraction. A notable observation bonding to oxygen donor ligands but a low degree of though, is the very low cell retention of Mn in the covalency is apparent and electrostatic attraction is YM-5 fraction. It has been suggested (McBride, 1978 inferred. Organic matter is classed as a "'weak-field" and Langford, personal communication) that the exchanger and its ability to replace the weak axial Mn(H20)6 z+ ion retains its inner hydration sphere bonded waters of the copper ion (due to Jahn-Teller upon adsorption and is adsorbed at the organic sur- distortion) is greater than for other metals where this face by electrostatic attraction rather than substitu- distortion is not so great and no unique ligand axis tion of the waters by suitable ligands. The results tend exists. This may account for the greater affinity of Cu, to show however that for Mn and Co the lower tool- compared with other metals, towards the higher mol- ecular weight organics are relatively more important ecular weight organic components, with electrostatic in retention of the metal ion. No retention of any of attraction overcoming the weakly held H 2 0 ligands the metals occurred when the u.v. irradiated sample in the axial positions. T41 was placed in the cells and a "wash-in" experi- The condiiional stability functions for Co and Ni ment carried out. This tends to substantiate the im- are of similar magnitude but Ni has a much higher portance of the organics in controlling metal reten- complexing capacity. This indicates that chemical dif- tion. Although the experiment was carried out in a ferences exist between binding sites and metal ion af- nitrogen saturated atmosphere, MnO2(s) may have finity varies towards organic fractions of varying con- been adsorbed on the membrane surfaces, centrations. While only Ni(lI} is present in natural
The predominant complexing agent in natural waters, in the presence of complexing agents, particu- waters, fulvic acid, is primarily a polyelectrolyte and larly nitrogen donors, the oxidation of Co(ll) to is more inhomogeneous than most other naturally Co(III) is favourable, with increased stability of the occurring polymers. Because of its complexity and Co(II) complexes in basic media. Ligand exchange conformational irregularity it cannot be assumed that kinetics for Co(III) are slow compared to CoIII} and any two of the donor groups are chemically identical. Ni(II) (Cotton & Wilkinson, 1966). Some Co{Ill) may Therefore any experimentally observed equilibrium occur naturally in waters as cyanocobalamin parameters are based on a weighted average. It (4-187pgml ' ; Beck, 1978). should be emphasized that the binding parameter (k) Binding studies of this work have been can'ied out is in no way a constant and will vary depending on at low ,ugl 1 concentrations (1-50,ug1-~). The inter- the concentration of binding sites (these are a function pretation of geochemical data at such levels is of of the total dissolved organic matter) and the equilib- prime importance to geochemists seeking explana- rium concentration of metal. Although it cannot be tions of anomalous metal concentrations (often only a assumed that different cations all react with the same difference of a few gg l - I from background) and to
Complexation analysis of fresh waters by equilibrium diafiltration 509
environmental scientists examining the effects of Buffle J., Deladoey P. & Haerdi W. (1978) The use of ultra- increased trace metal pollution. Many studies have filtration for the separation and fractionation of organic been made involving much higher metal levels ligands in fresh waters. Analytica chim. Acta 101,
339-357. ( > 1 #g m l - 1) and the extrapolat ion of conclusions to Butfle J., Greter F. L. & Haerdi W. (1977) Measurement of lower levels may not be valid. This paper has shown complexation properties of humic fulvic acids in natural that trace levels of some transi t ion metals will corn- waters with lead and copper ion-selective electrodes. pete with each other for the available complexation Analyt. Chem. 49, 216-222.
Choppin G. R. & Kullberg L. (1978) Protonation thermo- sites even in the presence of several orders of magni- dynamics of humic acid. J. inorg, nucl. Chem. 40, tude higher concentrat ions of calcium and other in- 651-654. organic cations. Pagenkopf (1978) has suggested that Cotton F. A. & Wilkinson G. (1966) Advanced Inorganic the enhanced carrying capacity in natural waters due Chemistry. Interscience, London.
Davison W. (1978) Defining the electroanalytically to organic complexat ion may be greater than that measured species in a natural water sample. J. Electrana- predicted for a totally inorganic model, by an order of lyt. Chem. 87, 395 404. magnitude. Gachter R., Davis J. S. & Mares A. (1978) Regulation of
In terms of the mobility of metals, the presence of copper availability to Phytoplankton by macromolecules natural water organics would postpone the precipi- in lake water. Envir. Sci. Technol. 12, 1417-1421.
Giesy J. P. & Briese L. A. (1977) Metals associated with tat ion of these ions by hydrolysis to higher pH values organic carbon extracted from Okefenokee swamp than would be the case in purely inorganic situations, water. Chem. Geol. 20, 109-120. This is of particular importance in the geochemistry Giesy J. P., Briese L. A. & Leversee G. J. (1978) Metal of Cu. binding capacity of selected Maine surface waters. Envir.
Besides obtaining quanti tat ive information on the Geol. 2, 257-268. Jackson K. S., Jonasson I. R. & Skippen G. B. (1978) The
extent of metal chelation in natural waters, it would nature of metals-sediment-water interactions in fresh- be of help in geochemical interpretat ions if the lability water bodies, with emphasis on the role of organic of complexes were assessed and the variation in con- matter. Earth Sci. Rev. 14, 97 146. centrat ion of strong and weak organic complexing Jackson T. A. (1975) Humic matter in natural waters and
sediments. Soil Sci. 119, 56-64. sites in different waters considered. These parameters Lee J. (1979) A scheme for the separation and characteriz- have impor tan t implications regarding the dispersion ation of possible metal-organic species in natural waters: of trace metals along drainage systems from unknown Some preliminary data. Geol. Surv. Can. Part A Paper and known sources. Not only equilibrium concen- 79 1A, 121-125.
Mann A. W. & Beutscher R. L. (1977) Solution geochemis- t rat ions but kinetic availability of soluble metal may try of copper in water containing carbonate, sulphate
influence the deposition or remobil izat ion of metals and chloride ions. Chem. Geol. 19, 253-265. such as Ni, Co and Cu in water-sediment organic Mantoura R. F. C. & Riley J. P. (1975) The use of gel interactions, filtration in the study of metal binding by humic acids
and related compounds. Analytica chim. Acta 78, 193-200.
Acknowledyements--The author acknowledges the signifi- McBride M. B. (1978) Transition metal bonding in humic cant contribution of ideas by Dr I. R. Jonasson and other acid: An ESR study. Soil. Sci. 126, 20(~208. members of staff in the Geochemistry Subdivision, Geo- McCrady J. K. & Chapman G. A. (1979) Determination of logical Survey of Canada, Ottawa. Recognition is also copper complexing capacity of natural river water, well given to the analytical staff for their Co-operation and as- water and artificially reconstituted water. Water Res. 13, sistance in the laboratory studies. 143 150.
This work was carried out under the auspices of a Means J. L., Crerar D. A. & Amster J. L. (1977) Appli- National Research Council of Canada Post-doctoral Fel- cation of gel filtration chromatography to evaluation of lowship and funded by the Department of Energy, Mines organic-metallic interactions in natural waters. Limnol. and Resources, Ottawa. Oceanogr. 22, 957-965.
O'Shea T. A. & Mancy K. H. (1976) Characterization of trace metal-organic interactions by anodic stripping vol-
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