µPIXE study on colloid heterogeneous retention due to colloid /rock
electrostatic interactions
U. Alonso1, T. Missana 1, A. Patelli2, V. Rigato3, J. Ravagnan3
1 CIEMAT. Madrid SPAIN; 2 Universita degli Studi di Padova ITALY;3 INFN-LNL, Padova ITALY
I. INTRODUCTION
The main colloid characteristics (Au, Fe2O3 and CeO2)
are summarized in Table 1.
A full understanding of the colloid-mediated
contaminant transport processes in crystalline rocks
requires the study of the colloid retention on the
heterogeneous rock surface. Both colloids and natural
rocks exhibit a surface charge and considering the colloid
sizes (< 1 µm), colloid retention is expected to be highly
dependent on the rock heterogeneous charge distribution.
Since a single particle will not be affected by the average
rock surface charge but, by that of a certain mineral [1].
In this work, the µPIXE technique is selected to
evaluate the colloid/ rock heterogeneous interactions in
several chemical medium conditions.
II. EXPERIMENTAL SET-UP
The surface charge of colloids and of the major
minerals composing the granite was studied by means of ζ
- potential measurements as function of pH. In Fig. 1A it
can be appreciated that the granite in average and their
major minerals (quartz-plagioclases, feldspars) are
negative the whole pH range, while the micas (biotite,
muscovite) have sometimes positive charge.
(A)
20
Zeta Potential (mV)
0
-10
-20
50
(B)
Au
Fe2O3
Zeta Potential (mV)
40
CeO2
30
20
10
0
-10
-20
-30
-40
0
1
2
3
4
5
6
7
8
9
10
11
pH
FIG. 1: (A) ζ-Potential vs pH measured (A) for granite and
minerals composing it (B) for Au, Fe2O3 and CeO2 colloids.
LNL Annual Report 2004
Colloid Size (nm) Conc. (ppm) pH ζ (mV) vdep (cm/s)
Au
47 ± 1
58
3
≅0
7.2·10-8
Fe2O3
60 ± 3
1000
5
+20
1.5·10-7
Au
47 ± 1
58
6
-35
7.2·10-8
CeO2
50 ± 5
1000
10
-8
5.4·10-8
To detect the granite preferential areas of interaction,
polished granite slices (mm-sized and cm2) were studied
by µPIXE before and after the contact with the colloid
suspensions. Three possible medium conditions were
considered, namely (a) Acid: coexistence of minerals
positive and negatively charged; (b) Mildly-Acid:
coexistence of minerals with neutral and negative charge;
(c) Basic: all minerals negatively charged.
For the µPIXE mapping, performed at LNL (Padova,
Italy), of 2*2 mm2 areas (H+, 2 MeV, I = 0.7 - 1 nA), a 170
µm thick Mylar X-Ray filter was inserted. The lateral
resolution was 8 µm and the detector resolution 155 eV.
III.RESULTS AND DISCUSSION
Moscovite
Biotite
Feldspar
Quartz+Plagioclase
GRANITE
10
TAB. 1: Main characteristics of the studied colloid suspensions.
Acid conditions. Granite sheets, previously analysed by
µPIXE, were contacted separately with the colloids at pH
3. The CeO2 and Fe2O3 colloids show positive charge but
Au colloids have low negative charge. At pH 3, both
favourable (quartz, plagioclase) and unfavourable (micas)
granite minerals for the interaction exist (Figure 1A).
As example, in Fig. 2, the elemental maps of Fe and Au
of an area studied before and after the contact with Au
colloids are presented (whiter areas indicate more
concentration). The Fe map is used for identifying the area.
The natural presence of Au is undetectable, but after the
contact the Au presence is observed, specifically related to
Fe. In fact, it is expected attractive interaction between Au
colloids negatively charged and positive Fe- minerals. No
Au was detected on the negatively charged granite areas.
The same attractive behaviour was observed for Fe2O3 and
CeO2 colloids, both positively charged, that where mainly
detected on feldspars and quartz regions, negatively
charged. Therefore, when favourable and unfavourable
electrostatic regions on the granite surface exist, the
colloid/rock surface interactions are explained just
considering attractive/repulsive electrostatic forces.
Fe before
Au before
Fe after
Au after
FIG. 2: µPIXE maps of Fe and Au content of a 2*2 mm2 granite
area before and after the contact with Au colloids at pH3.
Mildly-acid conditions. At pH 5-6, some granite
minerals (biotite) are neutral but mainly negative (Fig.
1A); Au colloids are negative, Fe2O3 positive and CeO2
almost neutral (Fig. 1B). As example, in Fig. 3 the Fe and
Au presence in a region before (no Au detected) and after
the contact with Au colloids are presented. Again, the Au
is in the Fe regions, as expected by attractive interactions
between negative colloids and positive minerals.
principle the whole granite is unfavourable in terms of
electrostatic interaction. In Fig. 5, the Fe and Ce maps
showed that after the contact and in spite of an expected
repulsive interaction, Ce is still detected on surface,
suggesting that that other mechanisms than pure
electrostatics contribute to colloid deposition. Moreover,
the calculated deposition velocities (vdep in Tab. 2), just
accounting gravitation effects, do not explain that
deposition. Since Ce colloids are not very stable at this pH,
particle aggregation may favour the deposition (filter
rippening phenomenon) [2]. But also, chemical effects, as
surface (co)precipitation, may enhance the colloid/rock
interactions when favourable electrostatic interactions do
not exist. This geochemical contribution to colloid
deposition should be studied and quantified more in detail.
Fe before
Fe after
Fe before
Au before
Fe after
Au after
Ce before
Ce after
FIG. 5: µPIXE maps of Fe and Ce content of a 2*2 mm2 granite
area before and after the contact with CeO2 at pH 10..
IV. CONCLUSIONS
FIG. 3: µPIXE maps of Fe and Au content of a 2*2 mm2 granite
area, before and after the contact with Au colloids at pH 5.
At pH 5, Fe2O3 presents an opposite case to that of Au.
In Fig. 4 it can be appreciated that the Fe2O3 (more Fe in
the region) is mainly detected in not Fe-bearing minerals
(quartz, plagioclases and feldspars). These results reaffirm
that when favourable electrostatic interaction exist, the
colloid deposition occurs mainly on those areas.
The interactions between colloids and the granite
surface were experimentally studied by µPIXE analysing
the presence of colloids on different minerals in several
chemical conditions. Results showed that the colloid/rock
interactions
are
generally
consistent
with
repulsive/attractive interactions. However, even when the
whole surface is unfavourable for a colloid
attraction/deposition, the presence of colloids was also
detected on the surface. This presence could not be only
explained with sedimentation due to gravitation effects, so
it is clear that other mechanisms and geochemical effects
should be experimentally evaluated to get insight in colloid
filtration mechanisms in crystalline rocks.
V. ACKNOWLEDGEMENTS
Fe before
Fe after
FIG. 4: µPIXE maps of Fe content of a 2*2 mm2 granite area
before and after contact with hematite colloids at pH 6.
Basic conditions. At pH 10, granite minerals and the
CeO2 colloids present a negative ζ-potential, so in
LNL Annual Report 2004
This work was supported by LNL and CIEMAT-ENRESA.
[1] J. N. Ryan and M. Elimelech. Colloids Surfaces A107 (1996)
1;
[2] Yao KM, Habibian MT and O'Melia CR. Environmental
Science and Technology 5 (1971) 1105.