Solidification/stabilization of metal polluted sediment of Krivaja river
Authors: Dalmacija Milena , Dalmacija Bozo , Krcmar Dejan , Prica Miljana , Rajic Ljiljana , Roncevic Srdjan , Gavrilovic Olivera 
Published: December 2011

The Krivaja River is the longest natural water body (109 km) that flows
completely within the borders of Serbian province of Vojvodina. In the
absence of national legislation, the sediment quality was assessed in
accordance with the Dutch classification methodology. It was found that the
river sediment is highly contaminated with copper and zinc (192 mg kg-1 and
1218 mg kg-1 respectively), and as such is an extreme risk to the environment
and human health. The solidification/stabilization (S/S) treatment with local
clay, that has high capacity of cation exchange 70.2 meq/100 g and specific
surface area of 630 m2 g-1, was employed for remediation of the contaminated
sediment. The sequential extraction procedure showed that the copper and zinc
have medium risk for the environment, with the percentage in the carbonate
fraction of 18 and 22% respectively. The results of sequential extraction are
not in full agreement with the results of pseudo-total metal concentration in
the sediment, which only confirms that the total metal concentration is not
sufficient to define the real danger to the environment. Based on the
pseudo-total metals concentration, the sediment is of Class 4 (Dutch
standards). However, judging from the results of sequential extraction, the
metals show medium risk. Obviously, these results have to be taken into
account in the assessment of the sediment quality, remediation procedures and
sediment disposal in general. After the treatment, the proportion of these
two metals in the first fraction is significantly reduced (Cu less than 2%,
Zn 10%) in most of samples. In order to determine the long-term behavior of
S/S mixtures, leaching tests were conducted in accordance with semi-dynamic
ANS diffusion test for 90 days. The results indicated that clay can
effectively immobilize Cu and Zn: the cumulative leached fraction of copper
in mixtures with clay was in the range from 0.001% (mixture with 80% clay) to
0.15% (mixture with 10% clay), and the cumulative leached fraction of zinc in
the range of 0.06% (mixture with 80% clay) to 0.10% (mixture with 10% clay).
The diffusion coefficients, ranging from 1.5?10-12 cm2s-1 to 3.7?10-14
cm2s-1, showed an effective immobilization of both metals which suggests that
these metals are practically immobilized in S/S mixtures even if the level of
clay is low. From the point of LX values, all S/S mixtures can be used for
the controlled utilization, because the leaching indices ranged from 11.8 to
13.4 for both metals. In all clay and sediment mixtures the dominant leaching
mechanism is diffusion (slope values of 0.35 to 0.60), which once again
confirms the effectiveness of the applied treatment and can be expected that
only a very small amount of contaminants would leach into the environment
over time. Future research is heading in the direction of troubleshooting the
accumulation of used natural adsorbent after treatment, the possibility of
permanent disposal, its regeneration or the possibility of its application as
construction material.