Fate of metals (Cr, Ni, Zn and Fe) in the sediment of a constructed wetland.

DI LUCA, G. A.; MAINE, M. A.; SANCHEZ, G.C.; HADAD H. R.; MUFARREGE, M. M.

Barcelona

Simposio; 3rd Wetland Pollutant Dynamics and Control Symposium Wetpol; 2009

Wetpol

INTRODUCTION Although removal efficiencies for treatment wetlands are reported largely, there is a lack of information concerning the long term fate of contaminants. This is because extensive data is only available for a few systems which have been operating for more than 10 years. The studied free water surface wetland was constructed at the Bahco Argentina metallurgic plant and has 6 years of operation. There was a significant increase in metals at the inlet sediment. It is widely recognized that distribution, mobility and bioavailability of heavy metals in the environment depends not only on their total concentration but also on the association form in the solid phase to which they are bound, which are studied through fractionation or speciation. The aim of this work was to study the accumulation and speciation of Cr, Ni, Zn and Fe in the sediment of a wetland constructed for the wastewater treatment at a metallurgical industry.   METHODS Samplings were carried out in August and October 2007, and May 2008. Sediment samples were collected in the inlet and outlet areas of the wetland by duplicate using a 3cm diameter, 10cm long PVC corer. Sediment cores were sliced in situ at the following depth layers: 0-3 (surface); 3-7 (medium) and 7-10 cm. (deep). Redox potential (Eh) and pH of the bulk sediment layers were measured with an Orion pH/mV-meter. Organic matter content (LOI) was determined by loss on ignition at 500ºC for 3h. Each sediment sample was analyzed according to the sequential extraction proposed by Tessier et al. (1979) in order to evaluate the chemical association of metals in sediment (Exchangeable, Bound to Carbonates, Bound to Fe-Mn oxides, Bound to Organic Matter and Residual fractions). Samples were digested with a HClO4:HNO3:HCl (7:5:2) mixture to determine the total concentration of metals. Cr, Ni, Zn and Fe were determined in the extracts and digests by atomic absorption spectrometry (Perkin Elmer, AAnalyst 300). ANOVA analysis (three ways) was performed to evaluate the influence of time, depth and metal fractions. Duncan´s test was used to differentiate means where appropriate. A level of p<0.05 was used in all comparisons. RESULTS AND DISCUSSION At the time the wetland started working, the initial mean concentration of organic matter (LOI) in the sediment was found to be 3.5%, mean Eh was 95 mV and pH was 7.7.  During this study, LOI values ranged from 3 to 5.7%, Eh ranged from -81 to -159 mV and pH from 8.84 to 9.56. For the concentrations of the fractions of the inlet area, samplings were statistically different for the 4 pollutants studied. Surface sediment (0-3 cm) showed the highest Ni and Cr concentrations, whereas deeper sediment accumulated the lowest concentration of Fe (Figure 1). Zn did not show significant differences in concentrations at the different depths (Figure 1). Significant interactions were found among the three factors studied (ANOVA, p<0.05).   Ni was essentially bound to carbonate and residual fractions, which did not show statistically significant differences. Guo et al. (1997) found that Ni was mainly bound to carbonates under reducing sediment conditions (low Eh) while Ni bound to the residual fraction increased as the Eh sediment decreased to low levels. This process was favored by the high alkalinity and high pH found in the wetland inflow.  Cr was mainly associated with oxides. The following fractions were carbonates and residual, presenting statistically significant differences between them. The low Eh of the inflow reduced Cr(VI) to insoluble Cr(III) which precipitated as Cr(OH)n (mostly Cr(OH)3) (Guo et al., 1997). Zn was principally bound to carbonate fraction. Guo et al. (1997) reported that Zn was bound to carbonates when Eh decrease below -170mV. As the sediment Eh of the inflow was lower, the results obtained were the expected values. Fe was accumulated largely in the residual fraction. Sulfate concentration was high at the inflow and this concentration decreased at the outflow. Machemer et al. (1993) found that in anaerobic sediment, where sulfate is not limiting, sulfides formation is the principal mechanism for pollutants removal, and that in general, these sulfides are retained in the residual fraction. The fractionation of the sediment of the outlet area was not shown because no significant differences were found regarding the initial concentrations of wetland sediment. In this area, the 4 metals studied were largely associated with the residual fraction. CONCLUSIONS Pollutants demonstrated different partition patterns. These behaviors were justified by the effluent condition, which is rich in Ca2+ and Fe3+, presents high values of pH and conductivity, which favors CaCO3 and Fe(OOH)n precipitation and the subsequent sorption of pollutants to their surface. The wetland is highly efficient in the retention of the studied metals. Contaminants are retained by the sediment in fractions that will not release them to the water while chemical and environmental conditions of the system are maintained. The exchangeable fraction of all the studied metals, the most labile and readily bio-available, showed in all cases a negligible concentration (less than 5%).