Environmental Engineering Reference
In-Depth Information
Operational Procedures
Synthetic effluent with a constant flow rate of 10 L/day was applied with different
metal concentrations: 8.0 mg Zn/L, 1.0 mg Cu/L and 0.4 mg Pb/L. After Zinc
breakthrough (at 156th day), Cu and Pb concentrations were doubled. The hydraulic
load was 0.43 m/day (157 m/year).
The Total Organic Carbon (TOC) concentration related to the acetate, in the
original concentration, was 6.72 mg C/L, with the following contribution: Zn-5.870
mg C/L (87.38%); Cu-0.7553 mg C/L (11.24%) and Pb-0.09266 mg C/L (1.38%).
Zn was the most mobile metal with retention efficiency values decreasing to less
than 50% in a 15, 70 and 110 day period, for columns C1, C2 and C3, respectively.
The evolution of dissolved heavy metal removal efficiencies in the three experi-
mental columns with time can be found in
[6]
.
For Cu and Pb, and after 260 days, the retention efficiencies obtained were
above 70% and 40%, respectively, in Column C1, and above 90% in the sand and
kaolinite media columns.
The different retention efficiencies of Zn observed in the three columns suggest
a direct relation to the filter bed characteristics, since the breakthrough period in
column C2, with a 10% kaolinite filter bed, was almost half the breakthrough
period in column C3, with a 20% kaolinite filter bed. Although other processes may
occur, these results indicate that the cation exchange capacity is relevant and there-
fore sorption processes are the major phenomena.
Desorption Procedures
At the end of the experiment, and for each column, the filtration media were
separated in six layers and numbered from the bottom to the top. Representative
samples of each layer where submitted to chemical desorption. The desorption
experiment was carried out in a 24 h period, using a solution of: 2.5 mL of 0.1 M
NaNO3, 2.5 g air dried filter media and 247.5 mL deionised water.
For each layer, two desorption procedures were applied. The first resulting from
a 24 h mixing at 55 rpm of the effluent (labeled pH6), and the second with the same
procedure but acidified at pH = 2 with HNO3, (labeled pH2). The pH evolution
with time was monitored.
Although other methods are available, in this work the total metals retained in
each layer will be evaluated by the values obtained by desorption at pH2.
The samples obtained after a 24 h mixing were filtered in a 0.45 mm membrane,
acidified with HNO3 to pH < 2 and kept at 4°C before the analytical analysis. An
electronic microscope observation was carried out in order to evaluate the particu-
late heavy metal retention in the filtration membrane.
Results for desorption at pH2 and pH6 for columns C1, C2 and C3, respectively,
are presented in Tables
1
through
3
.
