RESUMEN
Developing a suitable substrate with high phosphorus (P) sorption capacity, low solubility, and high hydraulic loading for constructed wetlands (CWs) is crucial for their functions. In this study, we used attapulgite and biochar as base materials to prepare a lanthanum/aluminum (La/Al) amended attapulgite/biochar composite as a novel P filter using a one-step drying process and subsequent high-temperature thermal treatments. Results indicated that the solidified poly aluminum chloride (PAC) amended attapulgite/biochar (SAl@AB) has a higher solubility than the solidified La-modified attapulgite/biochar (SLa@AB) and the solidified PAC and La co-modified attapulgite/biochar (SAlLa@AB). Therefore, SAl@AB is not suitable to be used as a substrate for constructed wetlands (CWs). Batch studies indicated that SLa@AB and SAlLa@AB have maximum P sorption capacities of 12.8 mg/g and 21.3 mg/g, respectively. The P sorption rates are higher than those found in most substrates used in constructed wetlands. Additionally, pH and coexisting ions exert minor effects on the P removal performance of SAlLa@AB. Column experiments indicated that longer hydraulic retention time (HRT) favors the removal of influent P. A 120-day column experiment indicated that an average of 95% of the P influent (10 mg P/L) could be removed by the SAlLa@AB with an HRT of 8 h. The P forms analyzed by sequential extraction indicated that P removed by SAlLa@AB occurs through the formation of calcium-bound and Al-bound P fractions, which can account for 68.7% and 18.8% of the total phosphorus, respectively. The formation of lanthanum/aluminum phosphate precipitation was the main P removal mechanism of SAlLa@AB. This was further confirmed by an XPS analysis, showing a formation of La-O-P and Al-O-P inner-sphere complexes after P sorption by SAlLa@AB. The results of this study indicate that SAlLa@AB was a promising substrate for future CWs.
Asunto(s)
Fósforo , Humedales , Aluminio , Carbón Orgánico , Lantano , Compuestos de Magnesio , Fósforo/química , Compuestos de Silicona , Eliminación de Residuos Líquidos/métodosRESUMEN
We tackle the problem of selecting from among a large number of variables those that are "important" for an outcome. We consider situations where groups of variables are also of interest. For example, each variable might be a genetic polymorphism, and we might want to study how a trait depends on variability in genes, segments of DNA that typically contain multiple such polymorphisms. In this context, to discover that a variable is relevant for the outcome implies discovering that the larger entity it represents is also important. To guarantee meaningful results with high chance of replicability, we suggest controlling the rate of false discoveries for findings at the level of individual variables and at the level of groups. Building on the knockoff construction of Barber and Candès [Ann. Statist. 43 (2015) 2055-2085] and the multilayer testing framework of Barber and Ramdas [J. Roy. Statist. Soc. Ser. B 79 (2017) 1247-1268], we introduce the multilayer knockoff filter (MKF). We prove that MKF simultaneously controls the FDR at each resolution and use simulations to show that it incurs little power loss compared to methods that provide guarantees only for the discoveries of individual variables. We apply MKF to analyze a genetic dataset and find that it successfully reduces the number of false gene discoveries without a significant reduction in power.