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Coating a dredged sand with recycled ferrihydrites to create a functional material for plant substrate

Flores-Ramírez, Eleonora, Dominik, Peter, Kaupenjohann, Martin
Journal of soils and sediments 2018 v.18 no.2 pp. 534-545
X-ray diffraction, byproducts, calcium carbonate, cities, clay soils, coatings, drainage, drying, ferrihydrite, groundwater, intermediate product, iron, mining, oxides, pH, particle size, peat, phosphates, raw materials, sand, scanning electron microscopy, sludge, vegetation, washing
PURPOSE: Urban greenery provides a series of benefits for the environment and inhabitants of cities. However, the substrate preparation mostly implies the mining and erosion of valuable natural soils (e.g., peat). Purpose-designed substrates, preferably made of waste materials, could avoid the extraction damage. The present work aims at improving the production and lowering the costs of a functional stably coated sand with ferrihydrite. This functional substrate combines the Fe (hydr)oxide sorptive capacities and the fast drainage of sand. Thus, secondary raw materials were tested: a dredged sand and three Fe (hydr)oxides; one from groundwater, an industrial intermediate product, and a mining by-product. MATERIALS AND METHODS: Three Fe (hydr)-oxides were structurally characterized by XRD, XRF analysis, and SSA measurements. Further, amorphous Fe (hydr)oxide concentrations were determined. Sludges of these Fe (hydr)oxides in different concentrations were hand-mixed with a dredged and a mined sand, and dried at 35 °C. The stabilization of the coating was made by heavy shaking (250 rpm) the coated sand with water (3:1 w:w) for 0, 10, and 1000 min, washing and drying at 35 °C afterwards. Thereafter, the effectiveness of this treatment was determined by the Fe concentration and pH of the coated sand, along with the particle size of the detached aggregates during shaking, and the pH in the washing water. The morphology of the coating was observed by scanning electron microscopy. RESULTS AND DISCUSSION: All Fe (hydr)oxides were 2-line ferrihydrites with large SSA, and coated both sands. Only after 1000 min shaking, homogeneous and small ferrihydrite aggregates covered the sands surfaces (verified by SEM and particle size). The impurities of the ferrihydrites affected the stabilization of the coating. Calcium carbonates enhanced the aggregation and reattachment of the Fe aggregates to the sand during shaking, while phosphate reduced the reattachment by stabilizing the aggregates in the suspension. CONCLUSIONS: Two out of three ferrihydrites were suitable to develop a stable coating. To coat dredged sand with both ferrihydrites lowers the cost and production time to obtain a functional substrate. One ferrihydrite has a high pH due to its high CaCO3 content, and sand coated with it may be used as an amendment for acidic clayey soils.