302
N. Narkis et al.
3.1 Aluminium Nitrate as a Primary Flocculant
Figure 1 shows the turbidity removal and the electrophoretic
mobility (EM) curves of Ca-montmorillonite clay mineral
suspension flocculated with aluminium nitrate, at pH 6 and 8.
3
2
J 1
l
~
~
~
..J
~
E Q
0
·1
.2
O.S
0.4
~
..
0.3
0.2
0.1
0.0
10. 5
;a...
. ~ .
~
.... ' ~ . .
~
....
j
~~ ~ .. ~.
.. /I
•
• •
-pH6
• •••• pHS
•
~
10. 4
10"
A1(N03)3 M
Fig. 1. Flocculation and electrophoretic mobility curves for aluminium nitrate at pH 6 and
8
All curves indicate typical behaviour of the aluminium salt as reported by Rebhun
et al. (1968). At pH 6, the flocculation curve shows underdose, good clarification
and overdose regions with zero EM at the good clarification zone and charge
reversal at the overdose. The minimal flocculant dose required for complete
clarification is called the optimal dose. At pH 6 the optimal aluminium nitrate
dose was 10.4 M. Flocculation by Al (III) salts at pH 6 is an adsorptive type. It is a
combined mechanism of partial charge neutralization by the positively charged
polyaluminium hydroxo complexes, such as [Alg(OHhot 4 , and bridging between
the particles (Stumm and Morgan 1962; Rebhun 1966; Narkis 1968; Matijevic
1973). At pH 8 the curve shows underdose and good clarification zones with no