Chromium mobility and speciation in a Phragmites australis system used for tannery wastewater treatment
The present study illustrates the distribution, mobility, bioavailability and speciation of total chromium (Cr) in the various strata of a constructed reed bed soil in comparison to unplanted soil under arid climate conditions. The operationally determined speciation of Cr was investigated using a five-step sequential extraction method using CaCl2 (0.1 M) to extract bioavailable Cr. The results indicated that there was a significant accumulation of total Cr, reaching 80% in the surface strata for the planted (PP) and unplanted (NPP) systems. Analysis of variance (ANOVA) showed a significant difference (p<0.05) in Cr accumulation in the surface of the soil between the PP system (48 ± 4 g·kg-1) and the NPP system (39 ± 7 g·kg‐1). The partitioning results indicated that the percentage of Cr in the exchangeable phase was very low. The Cr was mainly associated with the oxidizable phase: 36 ± 10% for the PP system and 54 ± 9% for the NPP system. The residual phase contained 26 ± 3%, 25 ± 2% of the Cr for the PP and NPP systems, respectively. The carbonate fraction represented 19.5 ± 5% for PP system and 10.7 ± 5% for NPP system, whereas the reducible fraction represented 18 ± 6% for the PP system and 10 ± 4% for the NPP system. The sequential extraction results showed that Phragmites australis had the ability to partially transfer Cr from the mobile oxidizable fraction to the less mobile reducible and carbonate fractions. Indeed, Cr was relatively unavailable to the plant but the results did show that Phragmites australis accumulated an appreciable amount of Cr in the roots (1690 mg·kg-1 dry matter). Furthermore, after 13 months, the soil had accumulated high amounts of chromium (94%) with only 5% of the Cr accumulated in Phragmites australis. The presence of plants ensures a sufficient porosity for the percolation of water for treatment, which makes it possible to treat a larger volume of wastewater and to reduce the required soil surface area needed for treatment.