Abstract
For the first time, speciation of Fe, Mn, Zn, Ni, Cu and Pb was determined along the profiles of 8 constructed wetlands (CWs) consisting of fluviatile sand (Fluv), clinopyroxene-dominated lava sand (Cl-LS) and zeolite-dominated lava sand (Ze-LS), aiming at quantifying metal behaviour in CWs and the impact caused by different filter materials. With the exception of Mn, which underwent reductive dissolution, CWs were sinks for the studied metals. Metal accumulation rates differed in the following order: Ze-LS ≥ Cl-LS > Fluv CWs, reflecting the highest metal adsorption capacity and the lowest hydraulic conductivity of Ze-LS. Sequential extraction data indicated the highest metal mobility (readily mobilised and adsorbed fractions summing up to ~60%) in Fluv CWs, implying a higher risk of metal release into adjacent environments if Fluv from CWs will be improperly disposed after usage. Zinc and Ni were transported into the deeper CW layers to a larger extent than Cu and Pb, reflecting adsorption affinity to all filter materials in the order of Pb > Cu > Zn > Ni. Therefore, understanding metal speciation and mobility in such materials is crucial when they are considered for application as filters in CWs.
Highlights
Constructed wetland (CW) is a green treatment technology, which has been applied for remediation of the waste water of different origins for several decades[1,2,3,4]
Individual metal abundances were in the order Fe > Mn > Zn ≥ Cu ≥ Ni > Pb, and the total metal contents were in the order zeolite-dominated lava sand (Ze-LS) ≥ clinopyroxene-dominated lava sand (Cl-LS) > fluviatile sand (Fluv) CWs, which reflects the background concentrations in the filter materials (Fig. 2)
Calculations based on the difference in concentration of Fe in CW and original filter materials revealed an annual Fe net accumulation of 60.6–137 g m−2 yr−1 in Fluv CWs, 209–795 g m−2 yr−1 in Cl-LS CWs, and 123–560 g m−2 yr−1 in Ze-LS CWs (Table S3–1)
Summary
Constructed wetland (CW) is a green treatment technology, which has been applied for remediation of the waste water of different origins for several decades[1,2,3,4]. The metal removal efficiency varied widely, depending on the metal, the concentration of metals in inlet water[17, 18], operating conditions of CWs (e.g., hydraulic retention time17, 18), and plants growing in CWs19. The metal removal in CWs was highlighted by May and Edwards[11], who reported greater load rate and removal efficiency of metals in CWs compared to natural wetlands. Still, it is not clear how different filter materials may quantitatively influence the removal efficiency of metals and which mechanism is responsible for such differences. The results obained in this study deliver fundamental knowledge as well as valuable experiences, which will serve as reference for the planning and designing of future CWs aimed at removing metals in waste water, especially in developing countries or the BRISC states, where metal production, mining sites and mine wastes are important
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