Methods for characterization of colloid particles in rain water

Pergamon

.Armosphrnc Enwonmmr

Vol 28. No. 9. pp 1575 15x1. 1994 Elswcr Sc~rncc Ltd Prmkd 1” Great Bnlam 1352 23lO/Y4 S700+000

1352L2310(94)E0025-F

METHODS A.

FOR CHARACTERIZATION OF COLLOID PARTICLES IN RAIN WATER

MALYSCHEW,*

*Tcchnischc Hochschule Hilpcrtstr. 21. D-64295 Chemie. Institut

H.-J. ScHMIDT,t K. G. WElLi’ and P.

HOFFMANN*

Darmstadt. Fachbereich Materialwissenschaft, Fachpebret Chemische Analytik, Darmstadt. Germany: and tTechnische Hochschule Darmstadt. Fachbererch fur Physikalische Chemie. Petersenstr. 10. D-64287 Darmstadt, Germany

Rain abater samples. collected in Darmstadt. were investigated in detail with the aim of &termination of colloids. Besides estahlrshed methods in interface science (hghtscatter sizing and microelectrophoresis). a quartz mrcrobalance was used for the first time in this field. Mainly homodispersed particles wrth a diameter from 75 IO 167 nm. zeta potentral from - 10 to - IS mV and total concentration of 5.X +0.6 ~lg/ - ’ were found.

Abstract

k;r~, IWY!

Sol. afmosphere,

idz\-:

INTRODL

water.

analysis,

CTION

Colloids are of constantly increasing interest to analysts (Hofmann er (I/.. 1991: Lieser. 1991; Lieser et ~1.. 1990: Orlandini er (I/.. 1990) due to their importance in the determination of trace metal ions which are often adsorbed or co-exist with them (Lieser t’r rrl.. 1990: Orlandini er (I/., 1990). Further, the colloid content of the analysed solutions may lead to misinterpretation of analytical data, especially from electroanalytical methods: direct and inverse polarography. potentioand conductometric titration, etc. Elements of interest must thus be quantified in terms of their distribution between colloidal particles and the solution. From a thermodynamical point of view. particles that have a significant contribution of surface energy dilferential dll’s=;,.dS (y=interfacial tension, S=surface area) to the whole differential of internal energy dU =Xdll’, (i corresponds to the different kinds of energy distributions: surface. electrical. thermal. etc.) are defined as colloids (Matijevic. 1974). From this aspect. colloid dimensions are between I and IO3 nm IEdelmann. 1962). But concerning analytical problems, according to the experimental separation treatment (Lieser. 1991). one can observe two strongly distinct ranges: (I) from loids. and (2) from Only connected (Hofmann

2 to 450 450

to

nm 1000 nm-

fine

particles

coarse

a few works deal with with colloid content or trl.. 1991: Lieser,

including

col-

particles.

the analytical of atmospheric 1991). They

problem waters are dedi-

mrcroelectrophoresis,

microbalance.

cated to the distribution of metal ions between colloidcontaining fractions of samples separated by filtration and ultrafiltration. Elemental colloidal concentration is obtained as a difference between the elemental concentrations in the original sample and in the ultrafiltrate. The standard deviations of the obtained results show that these procedures should be augmented by additional, more convenient methods. A characterization of thecolloids is therefore desirable. Environmental studies on dispersed systems have been dedicated to the metal ion analysis in natural waters (Orlandini ef ~11.. 1990; Schnitzer and Khan, 1971; Lieser er ul., 1986). These analyses were made after prefractionation with the help of various filters (similar to Hofmann or (I/.. 1991) plus specific methods (e.g. dialysis). Use of these methods permits the investigation of separated colloids without dissolved ions except, of course. ions in electrical double layer. It could therefore be established that metal ions are often adsorbed on the surface of colloids and concentrated at them in relation to the bulk solution. The nature of colloids in atmospheric water has been only poorly examined and remains undelined. In ground waters they can be organic (humic acids. protein products, etc.). inorganic (silicates. aluminates. ferrites. etc.) and the particles can consist of organic and inorganic matter simultaneously (Stumm and Morgan. I98 I ). The objective of the present study was to examine the possibilities of the determination ofcolloids in rain water samples under natural conditions of extremely low concentration.

A. MALYSCHEW

1576 METHODS,

PROCEDURES

AND

RESLILTS

Sampling methods are extremely critical in the trace analysis of atmospheric waters. Teflon (PTFE) and polyethylene (PE) bottles. flasks and pipettes show the most favourable properties (Hofmann et ul., 1991). PTFE bottles. PE funnels and nylon filter nets were cleaned according to Hofmann et ol. (1991) and employed as rain samplers at a site in the institute garden in Darmstadt. Nylon net (30 /cm mesh) was used in order to prevent the penetration of coarse particles into the sample. The rain water samples were filtered after sampling through a 0.45 jtm pore size cellulose membrane (Millipore) to separate the coarse particles. Part of the filtrate was stored at -20-C for further studies. Another part was given to dialysis with an electric stimulation of ionic transition through immersible CX 30’ Millipore membrane of 2 nm pore size to separate the line particles (including colloids) (Fig. I ). All components of the dialysator contacting sample were cleaned as described by Hofmann et al. (1991). Prior to examination of investigated samples all the following methods were tested using a standard polystyrene latex with particle diameter 0.5 itm and a concentration of 0.01 g/-’ (E. Merck. Darmstadt).

et ol.

Dialysis The principles ofdialysis are well-known and are described in a lot of publications (e.g. Edelmann. 1962). Taking into account the very low ionic and molecular concentrations of the given samples. traditional dialysis cannot be used. For that case electrostimulated dialysis was dcsigncd and used. The peculiarity of this dialysation was that the electrical field was applied parallel to the dtrcction ofdialysis. From time to time the direction of current was changed in order to prevent accumulation ofcolloid particles on the dialysis membranes. as this can lead IO a strong reduction of the membrane permeability. Duration of the electrical treatment in one particular direction was chosen IO be in correlation with ionic mobilities and proposed mobilities ofcolloids. Since ionic mobilities lie in the range of 31510m5 for H’ (II,,,) to 30.10-’ trans(u,in)cmzs-’ V’ for Li’. the time ofelrctrophoretic port (I,,) through the 15 cm long (I) dialysis cell at 5 V (U) is given by equation (I) t,, = I’,‘( Ua). In the given

range for u. I,, is calculated

(1) IO bc from

l.4.10J

up

IO 15~101S.

Because of the essentially mobility ofcolloid particles

larger charges, electrophoretical (see upper horizontal axis on Fig.

-&T---9 Fig. I. Electroanalysis device: I = source of deionized water, 2 = sol chamber. 3 =dialysis membranes, 4 = dialysate chambers. 5 = electrodes. 6 = water sewer. 7 = taps, 8 = commuIalor. 9 = current source.

Characterization

of colloid

particles

1577

0 75

Y5

105

PARTICLE

115

125

SIZE

Fig. 2. Particle

4) is comparable with that of ions (Daniels and Alberty, 1975). But in contrast to the situation for ions. colloid parttcles carry charge of the same sign (otherwise they would flocculate). Taking into account the given time in equation (I). in the dialysis experiment we had to change the current direction 3 times in 24 h. Furthermore, as one can see from Fig. I. the electrode diameter (0=80 mm) is larger than that of the membrane (O= I4 mm). This provides the nonuniformity of the electric held. increasing intensity of electric field inside the sample chamber of the dialysator. Being polarized in the electric held. colloid particles try to stay in the region of larger electric strength. which also limits their mobility. As a result or dialysis a colloid solution (which contains only ions in the electric double layer of colloid particles) is obtained for further research.

The determination of particle size distribution could be perrormed on the isolated colloids. This was done using an Autosizer(Malvern. U.K.) with size range limits from 10 to 5000 nm. In this range thesize distribution of’mineral aerosol particles was established by Schiitz et al. (1992) and Jaenicke (1988). The present examination is based on the quasi-elastic scattering of laser light using photon correlation spectroscopy at a solid angle of 90 (McNeil-Watson. 1988). A distribution curve IS shown in Fig. 2. A relatively narrow distribution with a width or 18 nm at the half-maximum at the mean diameter point 79 nm was obtained. Furthermore the narrow range (75-168 nm) indicates a relative homogeneity of the colloids in the rain water sample.

The mass of colloids in a IO cm’ sample was determined with the help of an in siru quartz crystal microbalance (QCM). The QCM is an extremely sensitivesensor capable of measuring mass changes in the nanogram range (Melroy er (11.. 1986: Deakin and Melroy, 1988: Hepel rt (1/.,1989: Stockel and Schumacher. 1989: Schmidt and Weil, 1991; Schmidt ef 01.. 1994).

The microbalance was equipped with AT-cut crystals with a nominal frequency of 5 MHz (KVG. Neckarbischohheim. Germany) biplanar and circular with a 15 mm diameter. Gold electrodes or 200 nm thickness were deposited onto both sides of the crystals by thermal evaporation (Edwards E306A vacuum coater). In order to improve the adhesion between the crystal and the deposits. a chromtum layer of 5 nm thickness was evaporated onto the quartz prior to the deposition ol the gold. Only one of the gold films was in contact with the solution. The characteristic frequency of the

135

145

155

I05

175

[nm]

size distribution.

quartz oscillator is modified, when material is deposited on its faces. The crystals were operated at their third harmonic, i.e. at a frequency of I5 MHz, and were mounted at a circular hole of a PTFE cell. The geometric area of the working electrode was 0.785 cm’. The counter electrode was a gold film with the same geometric area as the working electrode. The driver circuit consisted of’ a modified Piers-Miller oscillator (Heising. 1946). Frequencies were measured with a “Rohde and Schwarz FEG3” frequency counter. This gives a resolution of I Hz over I s. The binary code digital data of the last three significant digits were converted to a 0-‘IO V analogue signal by a D/A converter and plotted with a 2-channel x/trecorder. The calibration of the microbalance is possible from a cyclic voltammogram in the potential range of monolayer deposition and dissolution of lead on polycrystalline gold substrate surraces. Practical mass sensitivity of the microbalance was measured as 4.6+0.1 ng Hz-’ (Schmidt er al., 1994).

The first attempt to deposit colloid particles on the quartz electrode was not satisfactory because olthe small frequency reduction due to the negligible quantity of deposited colloid particles (only IO Hz with the noise level 2 Hz and whole electrode instability of + I Hz). A simple calculation gives the whole quantity of deposit of 7 + 3 pg / ‘: this error ol about 40% was unsatisfactory and the solution had to be concentrated. This was efiected by ultracentrifugation using a Beckman L8 centrifuge at about 40,000~ for 2 h. A Boltzmann calculation concerning particle distribution in a potential field shows that one quarter orthe liquid from the bottom of the centrifuge tube contains all particles. This Traction was taken by a pipette. The electrodeposition procedure onto the oscillation electrode was repeated. and the function obtained is shown in Fig. 3a. As can be seen, the error is now sufficiently low and the concentration of the deposit can be calculated as 5.75 +0.57pg/-‘.

After sample electrical was very in tested

I week, this experiment was repeated with the same but together with simultaneous recording of the current as function of time (Fig. 3b. c). The result similar (taking into consideration a small difference quantity of’ solution): 5.90+0.6O~cg/-‘.

Surfice

charge

delerminuliwl

via zera sizing

The electrosurlace properties (charge, charge density, electrokinetic potential) of colloid particles can be investigated by electrokinetic methods. Every interface has a particular discrete structure which depends upon the chemical nature of

A. MALYSCHEW

I578 55 P E. z a E x 6 z

MOBILITY (x108)Im2V-1s-~l .P_

L 44_ 3322-

a E

-: ll-’

(a)

i

0

‘I’I*“I,“““‘*‘a 1 2

3

4

5 TIME [h]

6

7

8

9

10

Fig. 4. Electrophoretic --

3 a

48-

G z

x-

u’ z 2E

24-

n: il.

12-

_I 0

-5

60 H

mobility distributron.

and

i-potential

_,’ :

,/‘.

-

-

/ (W I

Oo

I 3

1

I

I 5

I

I I 7 TIME [h]

I 9

I

I 11

I 13

80 L

I

W

=

er a/

56i --

4ar

,,r0

2

4

6

a

10

12

TIME [h]

Fig. 3. Kinetics of the deposition of colloid particles on quartz electrodes: (a) Frequency decrease as function of time.(b) Frequency decrease as function of time (deposition of a l-week old sample). (c) Current kinetics during the deposition in experiment (b).

the contacting surfaces, In our case the surfaces of the unknown colloid particles are covered with specifically and non-specifically adsorbed ions from solution. Furthermore. the nearest layer of solution is enriched by the ions dissolved/desorbed from the particle surface. Accordingly, a charge appears on the particle surface and the same charge with the opposite sign is non-uniformly distributed in the solution around the particle. In combination an electrical double layer is formed, composing a distribution of electrical potential around the particle. The value of this electrical potential on the shear plane between particle and solution is called zeta-potential. Electrophoretic mobility of particles (velocity in the electrophoretic procedure norrnalised by the electric field strength) is closely connected with their zeta-potential. Thus. zeta-potential characterizes not only the surface structure but also the electrical behaviour of particles. The examination of these properties are the objectives of electrokinetic investigations (Adamson. 1976, pp. 162-189). Electrokinetic measurements were made using a ZetaSizer-3 (Malvern. U.K.). In this device a cylindrical quartz

cell contaimng sample solution is irradiated by IWO similar laser beams crossmg at the so-called stationary level (at a distance of 14.6% ofcell diameter from the quartz surface) in solution. The beam reflected from the particle surface together with a reference beam form an interference pattern in the plane of focus of a photoamplilier. The frequency of light scattered by the particle moving due to an axially applied constant electric held with uniform pulse shape changes according to the Doppler equation and the corrcspondrng interference picture consequently changes. The dcgrcc of change is measured by a special comparator and converted to an electrophoretic velocity. The main electrokinetic property of particles the electrophoretic mobility is the result of dividing electrophoretic velocity by the strength of electric held. Since electrophoretic mobility is tied to zeta-potential by certain equations, a special computer programme sclccts automatically the valid equation depending on electroconductivity of the solution and the value of zeta-potential is obtained. The applied pulse voltage durrng examination was I.50 V and the scanning procedure in the crossing beam mode took a time of about 4 min. Zeta-potential and mobility distribution are given in Fig. 4. Here one can see the almost homocharged dispersion with the average zeta-potential (
DlSCtiSSlON

From the current kinetics shown in Fig. 3c one can obtain the whole charge which was transferred through the cell during the microbalance deposition procedure. Indeed, a lirst jump (by turning on the voltage) and the following quick decrease of current I(t) both take place due to charging of the electrode’s double layer depending upon its active resistance (RI and capacity (C) (Matsumoto er trl., 1982) I(r)-exp[-(RX’)’

‘1.

The next (slow) decrease of the current can tributed to the quantitative reduction of carriers during the so-called electrocleaning suspension of colloids and molecules which are ated through electrolysis and electrophoresis.

(2) be atcharge of the separAfter

Characterization

of colloid

I I h the electrodeposition is finished (see also Fig. 3b) and the current is stabilized. For the quantitative calculations a corresponding picture of the current kinetics in the supernatant is needed which can be obtained using an amperometer with a much better resolution. One can say that the whole charge Q (according to Faraday’s law and by linear approximation of current curve I(T)) is equal to

particles

1579

q=4nm,

< r~=(69.3f21.1)~10”

C

=43.3 f 12.9 acu ( = atomic charge units) (4) where E and Ed are the dielectric constants of supernatant and vacuum. Approximating colloids as spheres. this corresponds to the surface charge density, us, given in equation (5) as = q/( F.S) = 492 f 246 pC m - ’

Q=

I dr=[(l69-105)

nA~39.6~103s]/2=~tC.

(3)

=(5.6-&2.8),10-‘”

This value corresponds to an average specific charge of 309 C g- ‘. This is too high for the transmitted mass of colloids supposing their stability during the deposition process (Novotny. 1986). But when that destruction ofcolloid particles (because of huge local strength ofelectric field in the place of interfacial contact on the electrode surface) takes place simultaneously with interface contact between particle and electrode, both the high common transmitted charge and another phenomenon can be clarified: “negative declinations” on the current curve (short reduction and immediate returning of the current value from time to time during particle deposition). These short current reductions may be caused by the formation and quick destruction of low conductive layers from depositing colloids. Use of a more precise coulometer would surely lead to a more definite conclusion about charge transfer by colloids. In the present study one is limited to the electrokinetic data (mobility and zeta-potential distributionFig. 4-and size distribution-Fig. 2). Taking into consideration the relation between the particle radius (~1) and its Debye-radius (Y ‘). then KYJ$ I (Henryfactor J(M)= 1.5). We are thus able to obtain an average particle charge (y) according to Helmholtz theory

molm-’

where F = Faraday’s constant and S = particle surface area. From these rough calculations one can see that the quantity of adsorbed ions is very low relative to usual precipitations. The particles measured in this case are negatively charged. Some measurements of single deposited particles using a microprobe at a scanning electron microscope give typical spectra; such a spectrum is given in Fig. 5. These spectra show after background correction elemental distributions of either P and Ca, of Na, or of Si and Ca. which can probably be explained by the minerals apatite, soda and calcium silicate, respectively. Only in one case were Al, Si and Fe found in a particle. representing the group of feldspars. The results derived from the measurements for colloid particles in the special case of the used rain water

are:

mean

radius

79 k I2 nm,

mean

2.0

3.0

4.0

5.0

ENERGY Fig. 5. Energy-dispersive

X-ray

spectrum

zeta-poten-

tial - 11.16f 1.17 mV, total concentration 5.8kO.6 and surface charge density (5.6f 2.8).10- lo P’gf-’ molm-* and permit an exact chemical determination of the colloidal and the molecular dispersed part with the help of analytical procedures shown in the scheme, given in Fig. 6. Thus the content of the fraction of the coarse particles (suspension) can be obtained as a difference between the data of an elemental bulk

Ca

1.0

(5)

of a colloid microscope.

6.0

7.0

6.0

9.0

[key particle

taken

in a scanning

electron

1580

A. MALYSCHEW

et al.

L

:OR CON-

I DATA

ANALYSIS OF FILTRATE AND ULTRAFILTRATE

[Al]

DATA[Af2]

1

COMPLEX ANALYSIS OF DIALYSED

DATA[Auf2]

DATA[Kedl]

COARSE FRACTION: FINE FRACTION:

DATA[Al] DATA[Af2]

EL,ECTROSTl~lS

Ked2

1

I-

SIZING.ZETA-SIZING, QUARTZBALANCEGRAVIMETRY WITH FRACTION DIVIOING AND EXAMINATION , BY EXAFS

DATA[KedS]

- DATA[Af2] - DATA[AufZ]

Fig. 6. Scheme

of the rain

(Al) and the analytical data of a filtrate examination (Al2). The fraction of the fine particles can be estimated as the difference between the results of the filtrate (Af2) and the ultrafiltrate analysis (Auf2). The results can than be compared with analytical data of dialytically treated but not concentrated so called reference sample (Kedl) or with analytical data of this very fraction concentrated, divided and examined by interfacial means (Ked2). analysis

Acknowledgements-The authors are very grateful to K. H. Lieser(Technische Hochschule Darmstadt) for discussing the results as well as to H. P. Weckenmann (E. Merck, Darmstadt) and P. H. Meller (Behringwerke AC. Frankfurt:M.) for the technical and intellectual assistance in size and electrokinetic measurements.

REFERENCES Adamson A. W. (1976) P/r~sico/ Clremisrr~ c?/Sur/acrs, pp. 345-346. J. Wiley. New York. Daniels F. and Alberty R. A. (1975) Physical Chrmisrr.v. pp. 183-210. J Wiley, New York. Deakin M. and Melroy 0. (1988) Underpotential metal deposition on gold monitored in-situ with a quartz microbalance. J. electroanal. Chem. 239, 321-332. Edelmann K. (1962) Lehrhuch der Kolloidchemir. pp. 120-123. VEB Deutscher Verlag der Wissenschaften. Berlin. Heising R. A. (1946) Quarrz Cry.sru/sjor Elecrric Circuits. pp. 36-37. Van Nostrand, New York.

water

examination.

Hepel M.. Kanige K. and Bruckenstein S. (1989) In-silu underpotential depositron study of lead on silver using the electrochemical quartz crystal mrcrobalance. J. elrctronul. chrm. 266. 409-423. Hofmann H.. Hoffmann P. and Lieser K. H. (1991) Transition metals in atmospheric aqueous samples, analytical determination and speciation. Fresenius J. anal~r. Chem. 340, 591-597. Jaenicke R. (19X8).Aerosol physics and chemistry. In Merroroloyy P/1ysicu/ und Ckmiral Properties q/Air. LandoltBornstein New Series V;4b (edited by Fischer G.). pp. 39 I-457. Springer, Berlin, Lieser K. H. (1991) Kolloide in der Analytischen Chemie. GI T Fuc~hxrrschrj/i .jiir das Luhorutorium 35, 583-591. Lieser K. H., Gleitsmann B., Peschke S. and Steinkopf T. (1986) Colloid formation and sorption of radionuclides in natural systems. Radiochimica Acra 40, 39-47. Lieser K. H.. Ament A., Hill R.. Singh R. N., Sting1 U. and Thybusch B. (1990) Colloids in groundwater and their influence on Migration of trace elements and redionuelides. Radiochimica Acra 49, 83-100. Matijevic E. (1974) Kolloide: die Welt der vernachllssigten Dimensionen. In Konzepre der Kolloidchemie (edited by SteinkopR J.). pp. 116-129. D. Steinkop/T, Darmstadt. Matsumoto K., Kutowy 0. and Capes C. E. (1982) Some theoretical aspects of the electrophoretic sedimentation of particles. Powder Techno/. 31, 197-203. McNeil-Watson F. K. (1988) New instruments for particle size analysis. In Particle Size Analysis (edited by Loyd L.). J. Wiley, London, Melroy 0.. Kanazawa K., Gordon J. G. and Buttry D. (1986) Direct Determination of the mass of an underpotentially deposited monolayer of lead on gold. Lanymuir 2. 697-708. Novotny V. (19X6) Contribution of colloid particles in elec-

Characterization troconductivity of suspensions. Coll~~ids Su~/i~rs 21. 219-133. Orlandini K. A., Penrose W. R., Harvey B. R.. Lovett M. B. and Findlay M. W. (1990) Colloidal behaviour ofactinides in an oligotrophic lake. hrir. .%i. Trch~~ol. 24, 706-712. Schmtzer M. and Khan S. U. (1972) Humic Suhsrcrnces it1 r/jr Ewirotmr~u. Dekker. Stuttgart. Schmidt H.-J. and Weil K. G. (1991) Electrochemical lead deposition from highly diluted solutions. Drclrema-Monoyrapllirtl 124, p, 249. VCH Verlaggesellschaft, Weinheim. Schmidt H.-J.. Pittermann U. and Wcil K. G. (1994~ (in preparation).

of colloid

particles

1581

Schiitz L., Maser R. and HofTmann P. (1992) Physical and chemical parameters of marine background and Sahara aerosols over the North Atlantic Ocean (unpublished results). Stiickel W. and Schumacher R. (1989) Electrochemical in-situ investigations on polycrystalline gold electrodes with oscillating quartz crystals: deposition and bulk diffusion of palladium. Err. Bunsenyes. Phys. Chem. 93, 606-617. Slumm W. and Morgan J.. J. (1981) Aquofic Chemistry, pp. 431-447. J. Wiley, New York.